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All living creatures on the earth
and all material objects on it

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are subject to the pull of one great force:
The force of gravity.

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Were that to be suspended, even for a moment,
the most extraordinary things would happen.

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I, for example, would suddenly float into the air
because I at the moment...

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...am flying in an aircraft on a very special course
which in effect cancels out the effect of gravity.

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So I float easily through the air.

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Our plane is climbing and diving
as though it were on a giant roller coaster,

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and as it goes over the crest of its climb, it really
lifts you out of your seat and keeps you there.

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If there were no gravity on earth,
seas would rise from their beds

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just as this water lifts out of its cup
and disintegrates into droplets.

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Nothing would remain where it was placed.
There would be no up and no down.

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There would no longer be the sense
of earthly order that we take so much for granted.

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Some creatures have overcome the force
of gravity sufficiently to enable them to fly,

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but the only ones that match
this total freedom in the air that I have now

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are those that are so small
that they are, in effect, weightless.

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And there are more of them...
both plant and animal...

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...than you might think.

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The force of gravity holds the clouds
around the earth and the air in which they float.

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You can't touch air,
it's invisible and all-pervasive,

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so it's easy to forget that it has real substance.

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But it's only by exploiting the presence of air
that seeds, insects, birds and man

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are able to overcome gravity
and float above the earth's surface.

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Dandelion seeds rise
because a puff of air carries them up

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and they fall slowly
because their parachutes catch the air beneath.

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A tuft of fluff will serve the same purpose.

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Milkweed and cotton grass,
willowherb and thistles,

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all provide their seeds with downy floats.

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These delay the fall of the seeds for so long
that currents in the air, winds,

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can carry them for hundreds of miles
from their parents.

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Seeds like these have crossed the widest oceans
and landed on the loneliest islands.

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Pollen grains are so small,
they don't even need fluff to keep in the air.

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The microscopic roughness
of their surface is enough.

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Spores, shot out from a puffball

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and shed in tens of millions
from the gills of fungi, are smaller still.

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The merest breath of air
sweeps them away like smoke.

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The gossamer,
that sometimes carpets the meadows,

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is the animal equivalent of downy seeds.

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It's produced
by thousand upon thousand of tiny spiders.

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The young of many species of spider,
soon after they hatch,

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climb to the top of grass stems
or onto the tiny pinnacles of stones

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and lift their abdomens upwards.

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Then, from the spinnerets at the tip,
they produce a thread of finest silk.

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As it lengthens and the wind catches it,
the spiderling turns,

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grabs the thread with its forelegs
and away it goes.

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Only the tiniest and the lightest of animals
and plants can defy gravity in this way.

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Many seeds are far too heavy to be lifted
by the breeze, no matter how downy they are.

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But if they are produced at the top of a tall tree
they can exploit the pull of gravity.

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These, hanging in the jungle of Venezuela,
grow wings.

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The wing is so shaped and weighted,
with the seed at one end,

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that as it falls through the air, it spins.

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This protracted fall gives the breeze a chance
to deflect the seeds sideways

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so that they will land some distance away
from the parent tree.

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The seed is functioning
like the blade of a helicopter.

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Its wing is so shaped that as it sweeps round,

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it puts pressure on the air below
and reduces pressure just above

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so that the seed hangs in the air
much longer than it would otherwise do.

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Sycamore seeds spin and glide in the same way.

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And animals glide too.

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The flying frog of Central America
has a parachute on each foot,

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formed by the web of skin between its toes.

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So one jump from a high branch
is enough to carry it from one tree to another.

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In South-East Asia lives a gecko
that not only has a parachute on each foot,

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but flanges on its body and tail.

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Another lizard glides through the forests

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by extending even bigger wings of skin
from its flanks supported by elongated ribs.

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And the best glider of all: A flying squirrel.

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Its huge cloak of floppy skin
sometimes serves as a simple parachute.

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But in horizontal flight
it does more than just trap air beneath it.

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As air passes over the front edge,
it's deflected slightly upwards,

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creating a slight reduction
in the air pressure on the upper surface,

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like on an aircraft wing
or the spinning blade of a sycamore seed,

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so the squirrel creates a little lift
and floats through the air.

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All those creatures are gliders.

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Some can control to some extent
the direction in which they glide,

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but none of them can climb in the air
except with the help of rising air currents,

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like the breezes
which sweep up these downs in southern England,

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carrying with them whole populations
of seeds and spores and spiders.

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But there are no such breezes
down below the grass stems.

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Down there, if creatures want to climb into the air
they have to have true powered flight.

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The most demanding moment is at take-off.

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The insect has to haul itself into the air
by sheer unaided muscle power.

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The downward sweep of the wings produces
greater pressure in the air beneath than above,

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so, in a slightly different way
from the cloak of the squirrel,

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beating wings also create lift,
and the insect is sucked upwards.

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Bigger insects, like grasshoppers,
boost their take-off with a powerful spring.

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Birds are even bigger and heavier.

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For them, too, getting into the air is
the most energetic and demanding part of flying.

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They also use their well-muscled legs
to assist their labouring wings.

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They jump
even before their wings begin their downbeat.

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But really big birds, to get airborne,
have to generate the extra lift

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by increasing the speed of air
streaming over their wings,

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so they get up a lot of speed
on the ground or over water,

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just as an aircraft does, before they can take off

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Once in the air,
a whole new environment is open to them,

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and flying animals of all kinds
exploit it to the full.

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Damsel flies catch their food in the air,
mate in the air and even fight in the air.

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As males squabble over territory, they flutter
their patterned wings in an aggressive display.

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This hawkmoth lays its eggs on flowers
while it's still flying,

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for it's too heavy to land on them.

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It feeds by hovering in front of a blossom
and sucking out the nectar

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with a tube-like proboscis as thin as thread.

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One of the smallest of all birds, the bee
hummingbird, even smaller than a hawkmoth,

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is equally skilled,
beating its wings 80 times a second

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to keep itself stationary in the air
as it drinks from the flowers.

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Bird wings are more versatile
than those of insects,

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for their feathers fit so closely alongside
one another and slide so easily past each other

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that the bird can change
the shape and size of its wing

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while maintaining its air-deflecting surface,

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so the wing can be spread wide
on the downstroke,

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and then, on the upstroke, be made small
to offer less resistance to the air.

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This kestrel is maintaining a steady position
in the sky, relative to the ground,

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by facing into the wind and flying with such
accuracy that it exactly matches the wind speed.

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The reduction of air pressure,
creating lift on the surface of the wings,

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can be seen quite clearly,
for it sucks up the smaller feathers.

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The albatross also habitually gets lift
by gliding into the wind,

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and the reduction in pressure produced as
the air blows over the wings ruffles its feathers.

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When it wants to travel against the wind,
it drops down close to the surface of the water,

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where the roughness of the waves
slows down the wind blowing over them.

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Albatrosses spend most of their lives in the air.

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Occasionally, for a minute or so,
they alight on the water to collect food.

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Once every year or so they come to
their nesting grounds to meet their mates again,

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greeting one another
with a charming courtship dance.

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It's difficult to appreciate how big these birds a
when you see them gliding over the ocean.

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It's only when you come to one of their nesting
sites that you really see how big they are.

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When they open these wings,
they are 11 feet across,

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the biggest wingspan of any bird.

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Long, narrow wings are the most efficient shape
for uninterrupted gliding,

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and no bird glides better than the albatross,

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but such wings are hard to flap
fast enough to give take-off,

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so many species of albatross nest on the edge
of cliffs, where they can just fall into the air.

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Cliffs are much favoured by gliders,

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for the wind from the sea striking the cliff face
is deflected upwards,

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and an albatross can hang on it.

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If it wants to fly slower
and prevent itself from being swept away

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or carried too high by a sudden gust,

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it uses its tail and webbed feet as air breaks,

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and reduces its lift by pulling in its wings,
so making their surface smaller.

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With such techniques,
an albatross will glide all day above a line of cl

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travelling effortlessly
along this highway in the sky.

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Land birds also exploit the air currents
above cliffs in the same way.

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This is the coast of Paracas in Peru.

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As the day wears on, the sun heats up
these desert sands, causing rising air,

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and that in turn sucks in cold air from the sea,
often bringing mists with it.

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As this cold air hits the cliffs,
so it's deflected upwards,

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providing just the sort of conditions
that soaring birds need.

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The condor, one of the heaviest of all flying bird

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Yet its skill in soaring is so consummate

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that it can remain in the air for hours
with scarcely a wingbeat,

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sustained entirely by those air currents
swept upwards by the cliffs.

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And something else
produces columns of rising air: Heat.

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When we turn on these burners,
they will create a current of rising air so powerful

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that it'll lift this balloon,
this basket and us up into the sky.

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We're in Africa, floating
over the great game plains of the Serengeti.

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I'm now about 100 feet up
and kept up entirely by hot air.

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But gas burners aren't the only things
which produce rising currents of hot air.

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The sun, as it rises, heats up the landscape,

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but all parts of the landscape
don't react in the same way.

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Some parts absorb the heat. Other parts,
bare slopes of grass or patches of rock,

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reflect the heat, and that causes
those uprising currents of air, the thermals.

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That's a moment those big birds down there
are waiting for.

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They are vultures,
and at the moment they're grounded.

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They're big birds with large wings, so large
that beating them is a very laborious business,

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and the vultures don't do so unnecessarily.

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At this time in the morning, they don't try
to battle against gravity and climb high,

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but flap from one low tree to another.

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They're waiting for the land to heat up
and the thermals to form.

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But we have our own thermal,
created by our burner, and up we go.

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This bird begins to follow us.

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An outcrop of rock is already warming

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and providing it with the thermal it needs
for effortless flight.

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And now the vultures are beginning
to come up here to join me.

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They will use the thermals to provide them
with an observation post in the sky

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from which they can scan the plains below,

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and I'm getting the same kind of view as they are,
and it's a very, very exciting one.

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Below me must be
the biggest concentration of meat on the hoof

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to be found anywhere in the world: Wildebeest.

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Last night or in the early dawn, somewhere,

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lions or hyenas or hunting dogs will have killed.

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The vultures, several thousand feet up in the sky,
quickly spot a kill or deduce its presence

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from the behaviour of birds
in a neighbouring thermal,

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and when they do, they swiftly glide down to it.

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Once one bird finds a carcass,
dozens arrive within a few minutes.

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These are tearing apart
the body of a wildebeest calf.

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Most of these are medium-sized vultures:
Ruppell's griffon and white-back.

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But among them is the biggest and most powerful
of African vultures: The lappet-faced.

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With a heavy load of meat,
the vultures won't fly far, to a nearby tree,

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to perch and digest and wait for tomorrow's
thermals to carry them effortlessly aloft again.

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But all the sustenance
has not yet been extracted from the carcass.

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In the African mountains,
as well as in Asia and Europe,

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lives a species of vulture with
a very specialised diet indeed: The lammergeier.

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It feeds, though it sounds extraordinary,
not only on marrow but on the bones themselves,

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and to do so,
it has developed a special technique.

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First it brings bones from a carcass to a special
workshop which several birds may share.

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A patch of bare rock near the top edge of a cliff.

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It chooses a cliff top
so that when it takes off again with a heavy bone,

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it has the least difficulty in getting into the air.

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Now it has to gain height.

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And this is why it chooses a patch of bare rock
for its operations.

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So that the bone will land so heavily that it cracks.

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One drop, however, may not be enough.

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White-collared ravens
often hang about the scene of operations.

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The ravens are starting to learn the technique
but haven't mastered it.

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They tend to drop their bones on grass,
where they don't break.

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The lammergeier eats the splinters of bone,
impossibly spiky though they appear to be.

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Some birds exploit the force of gravity by dropping
not their food but themselves from the sky.

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The pied kingfisher hovers
as it searches the water beneath.

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Terns dive with such speed, they can strike fish
several feet beneath the surface,

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pulling back their wings at the last moment
so as to get a clean entry into the water.

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Gannets do the same thing.

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During the nesting season,
concentrated in their colonies,

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huge flocks set out on fishing trips, and when
they find a shoal of fish near the surface,

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they subject it to an aerial bombardment
of devastating intensity.

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But the ace of dive-bombers, which can reach
at least 80 miles an hour in a dive,

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00:30:26,860 --> 00:30:28,570
is the peregrine falcon.

215
00:30:32,740 --> 00:30:36,670
It patrols the skies,
high above the flight path of other birds.

216
00:30:36,920 --> 00:30:40,000
When it has selected its victim, it folds its wing

217
00:30:40,210 --> 00:30:45,010
steering almost entirely with its tail,
and hurtles downwards.

218
00:31:44,980 --> 00:31:48,030
The talons are brought forward for the strike

219
00:31:48,200 --> 00:31:52,530
and to make last-second adjustments
to the accuracy of its final run.

220
00:32:06,050 --> 00:32:07,760
A hunter of the night.

221
00:32:08,170 --> 00:32:15,220
Owls, this is a barn owl, don't rely on speed
like the peregrine, but on a slow, silent approach

222
00:32:17,810 --> 00:32:22,270
Their flight feathers have special
soft edges to them which serve as silencers.

223
00:32:22,810 --> 00:32:26,820
Their wings are large
and support the bird so easily

224
00:32:26,980 --> 00:32:29,820
that there's no need for any noisy flapping,

225
00:32:30,030 --> 00:32:34,240
and the owl can waft its way in silence
through the trees.

226
00:32:38,910 --> 00:32:44,170
Although owls hunt after dark, they find their way
with their large, sensitive eyes,

227
00:32:44,330 --> 00:32:52,180
and, because their flight is virtually soundless,
they can listen for the squeak of voles and mice.

228
00:32:54,510 --> 00:33:00,180
But on the darkest nights, even an owl can't see,
and it seldom ventures into the air.

229
00:33:00,430 --> 00:33:03,150
Such nights belong to bats.

230
00:33:05,480 --> 00:33:09,030
They are able to navigate
without the aid of vision.

231
00:33:09,230 --> 00:33:15,950
Instead they use sonar, squeaking ultrasonically
and guiding themselves by the reflected echoes.

232
00:33:32,800 --> 00:33:38,350
They do this so skilfully
that they can pluck a flying moth from the air.

233
00:34:12,670 --> 00:34:16,930
It's been known for a long time
that bats use sounds in this way,

234
00:34:17,140 --> 00:34:24,180
but it's less well known that one or two birds
have, independently, evolved the same technique.

235
00:34:26,020 --> 00:34:29,650
This cave in Venezuela
is the home of one of them.

236
00:34:41,370 --> 00:34:45,410
These, flying all around me, are oilbirds.

237
00:34:45,910 --> 00:34:50,790
Most of the noise that they're making
is nothing to do with navigation.

238
00:34:51,000 --> 00:34:55,420
It's their alarm calls.
They're alarmed by the brightness of my light.

239
00:34:55,840 --> 00:34:59,140
So what I'm going to do
is to put on a deep-red filter.

240
00:34:59,390 --> 00:35:03,850
That will disturb them less,
but it will enable us to watch them

241
00:35:04,020 --> 00:35:08,730
with a special electronic device
called an image intensifier.

242
00:35:14,280 --> 00:35:19,030
They're big, relations of the nightjars,
and about the size of pigeons.

243
00:35:19,320 --> 00:35:24,750
Their nests are compiled from their droppings
and bits of regurgitated food.

244
00:35:26,460 --> 00:35:30,880
When their alarm calls subside,
you can hear the clicks by which they navigate.

245
00:35:31,840 --> 00:35:38,590
These calls are lower in frequency than
the signals of bats, and they're less accurate,

246
00:35:38,800 --> 00:35:42,470
so the oilbirds can't detect objects
much smaller than a foot across.

247
00:35:43,010 --> 00:35:47,430
That's quite good enough to prevent the birds
crashing into the cave walls or one another.

248
00:36:08,080 --> 00:36:10,710
Their favourite food is the fruit of a jungle tree

249
00:36:10,960 --> 00:36:14,630
and the cave floor is covered
by a soggy carpet of seeds.

250
00:36:14,920 --> 00:36:19,340
Many germinate, though in the dark
they can't develop chlorophyll,

251
00:36:19,550 --> 00:36:23,470
and they remain pallid, leggy seedlings
which soon die.

252
00:36:23,930 --> 00:36:28,480
The fruits are too small
for the oilbirds to locate with their clicks,

253
00:36:28,770 --> 00:36:32,480
but out in the moonlit forest, where the trees grow

254
00:36:32,730 --> 00:36:35,320
there's enough light
for the birds to find them by eye.

255
00:36:39,360 --> 00:36:42,950
The mastery of the air
and the strength to remain in flight for days

256
00:36:43,160 --> 00:36:48,080
has enabled birds to become
the greatest of all animal travellers.

257
00:36:49,620 --> 00:36:52,790
In the skies above Panama
every October and November,

258
00:36:53,000 --> 00:36:55,670
there is a great aerial traffic jam.

259
00:36:55,920 --> 00:37:00,720
Hawks and turkey vultures,
fleeing from the winter in North America,

260
00:37:00,970 --> 00:37:04,180
are on their way
to spend a few months in the south.

261
00:37:05,470 --> 00:37:09,930
As the day warms up, they find the thermals
in which they can spiral upwards,

262
00:37:10,140 --> 00:37:14,770
to give them the altitude they need
to make the day's flight with the least effort.

263
00:37:20,780 --> 00:37:23,450
These long journeys require a lot of fuel.

264
00:37:23,740 --> 00:37:27,490
Big birds, like hawks,
can draw it from their body tissues.

265
00:37:29,330 --> 00:37:35,000
But north-east of Panama, across the Caribbean,
on the Atlantic coast of the United States,

266
00:37:35,250 --> 00:37:40,920
smaller wading birds, sandpipers and phalaropes,
are preparing for their journey.

267
00:37:41,470 --> 00:37:44,260
They must put on fat before they start off,

268
00:37:44,510 --> 00:37:50,350
and they find food in the quantities they need
in the rich waters of the Bay of Fundy.

269
00:38:15,250 --> 00:38:21,510
In a few days of intensive feeding, each tiny bird
will increase its weight by half as much again,

270
00:38:21,880 --> 00:38:26,300
and they need all that fat,
for they are about to travel across the ocean,

271
00:38:26,470 --> 00:38:29,100
and then they can't feed at all.

272
00:38:50,780 --> 00:38:56,540
On the other side of the Atlantic, migration route
also run predominantly north and south,

273
00:38:56,750 --> 00:39:00,670
as birds move back and forth
to get the best of the changing seasons.

274
00:39:02,130 --> 00:39:06,470
In Scandinavia, every autumn
great numbers make their way south.

275
00:39:07,180 --> 00:39:11,350
Most land birds
prefer to keep their flights over water short,

276
00:39:11,600 --> 00:39:17,100
and huge flocks assemble on the shores of
the narrow straits between Sweden and Denmark

277
00:39:17,270 --> 00:39:19,480
to make the crossing into southern Europe.

278
00:39:22,480 --> 00:39:25,690
Small birds often fly in parties, close to the water.

279
00:39:35,660 --> 00:39:38,710
Buzzards, experts at soaring and gliding,

280
00:39:38,920 --> 00:39:43,040
use the thermals to climb so high
that they cover the distance

281
00:39:43,210 --> 00:39:46,340
in what amounts to one long, shallow glide.

282
00:39:49,010 --> 00:39:55,270
Red-breasted geese spend their summer
much farther east in the tundra of western Siberia

283
00:39:55,560 --> 00:39:57,680
They too move south in the autumn.

284
00:40:12,700 --> 00:40:18,830
Their journey is almost entirely over land,
so they're able to stop each night to refuel.

285
00:40:37,680 --> 00:40:42,730
After several weeks, they reach
their wintering grounds south of the Caspian Sea,

286
00:40:42,900 --> 00:40:46,190
many of them on the marshes
of the Danube delta.

287
00:40:52,450 --> 00:40:57,580
Birds are not the only creatures
to make these immense transcontinental flights.

288
00:40:57,790 --> 00:41:03,080
Almost unbelievably, a few small,
seemingly frail creatures do so as well.

289
00:41:03,830 --> 00:41:11,220
Insects, flying with just as steadfast a purpose,
achieve journeys as long as many migrating birds.

290
00:41:11,550 --> 00:41:14,970
In South America, in a high valley in Mexico,

291
00:41:15,140 --> 00:41:20,600
hundreds of thousands of monarch butterflies
roost in just a few special trees.

292
00:41:28,020 --> 00:41:34,410
They hatched in the autumn woods of North
America and have flown 2,000 miles to hibernate.

293
00:41:34,700 --> 00:41:40,290
They won't feed here, but they're spared
the lethal frosts and snows farther north.

294
00:41:40,580 --> 00:41:44,460
In spring they will set off back,
travelling ten miles a day,

295
00:41:44,620 --> 00:41:47,750
feeding, courting and laying eggs as they go.

296
00:41:48,130 --> 00:41:53,260
But only a few will live long enough to reach
the northern woods where they were hatched.

297
00:41:56,390 --> 00:42:00,100
The world is criss-crossed
by the flight paths of animal migrants.

298
00:42:00,350 --> 00:42:06,150
In the Americas, nearly all pass through Panama.
A few hardy travellers cross the Caribbean.

299
00:42:07,060 --> 00:42:12,650
On the other side of the world there's more land,
and birds and insects have more routes,

300
00:42:12,820 --> 00:42:17,490
travelling north and south
but also east and west between Asia and Africa.

301
00:42:19,370 --> 00:42:23,160
Although the journeys
may be thousands of miles long,

302
00:42:23,460 --> 00:42:28,380
the earth's wrapping of air
is less than six miles deep.

303
00:42:28,960 --> 00:42:32,460
On rare occasions the gases
from which it's formed become visible.

304
00:42:32,630 --> 00:42:38,350
Subatomic particles from space, attracted
to the poles by the earth's magnetic field,

305
00:42:38,510 --> 00:42:44,060
energise the gases of the atmosphere
so that they glow and form shifting veils of light

306
00:42:44,230 --> 00:42:46,350
the aurora borealis.

307
00:42:49,940 --> 00:42:52,480
The atmosphere is not composed entirely of gas

308
00:42:52,650 --> 00:42:56,610
and at certain times
you can see evidence of other things.

309
00:42:57,450 --> 00:43:04,160
Dust particles are scattered through its lower
layers, and when the sun shines across the earth,

310
00:43:04,330 --> 00:43:07,080
they scatter its white light, turning it red.

311
00:43:08,040 --> 00:43:14,260
Minute droplets of water, being translucent,
act like tiny prisms and produce a rainbow,

312
00:43:14,550 --> 00:43:19,010
and at high altitudes
tiny ice crystals create a similar effect.

313
00:43:21,220 --> 00:43:27,730
Up away from the earth, the gases become
thinner and the temperature becomes colder.

314
00:43:40,700 --> 00:43:45,660
The balloon taking us to these heights
must be bigger than that we used in Africa

315
00:43:45,830 --> 00:43:51,710
for, as we climb, we will require a greater volume
of the rarefied air to give us the necessary lift.

316
00:43:52,920 --> 00:43:59,130
A rubber bladder, sealed with a cork, gives us a
rough idea of the drop in pressure as we ascend.

317
00:44:07,930 --> 00:44:16,230
We are now at 8,000 feet, and you might think
that no living creature would come as high as this

318
00:44:16,490 --> 00:44:19,030
except perhaps some rather foolhardy men.

319
00:44:19,280 --> 00:44:24,370
But no. Some small creatures
are swept up as high as this

320
00:44:24,580 --> 00:44:28,040
by the convection currents
rising from the surface of the ground,

321
00:44:28,290 --> 00:44:35,250
and we're going to try and catch some
using this rather curious machine.

322
00:44:36,090 --> 00:44:43,970
Inside there's a fan which will suck in air
through this end when I turn it on here,

323
00:44:44,300 --> 00:44:47,100
and I'll lower it over the side
to see what we catch.

324
00:44:55,820 --> 00:45:01,950
And now we're going to go higher still
and it's going to get very, very cold,

325
00:45:02,160 --> 00:45:05,280
so I shall need all this warm clothing I've got,

326
00:45:05,490 --> 00:45:10,960
but, perhaps even more seriously,
the oxygen is going to get thinner and thinner,

327
00:45:11,160 --> 00:45:18,920
and so I shall have to put on this mask in order
to breathe oxygen as we go higher and higher.

328
00:45:45,240 --> 00:45:51,040
And now an indication of our height
can come from this balloon.

329
00:45:51,250 --> 00:45:55,920
Before it had those corners to it
and now it's swollen quite considerably,

330
00:45:56,080 --> 00:46:01,760
so the pressure here is really considerably lower
than it was when we were on the ground.

331
00:46:07,550 --> 00:46:12,640
We are now getting on for four miles
above the surface of the earth.

332
00:46:13,230 --> 00:46:20,070
It certainly looks very far away.
And it's shrouded beneath a pall of clouds.

333
00:46:20,530 --> 00:46:27,490
And we're getting very close
to the outermost frontier of life on earth.

334
00:46:28,330 --> 00:46:35,000
It's very cold and I certainly wouldn't be able
to talk at all if I hadn't got this oxygen,

335
00:46:35,250 --> 00:46:42,130
so conditions here are really
very much more severe than you might imagine

336
00:46:42,300 --> 00:46:48,260
when you sit in your aircraft
flying comfortably from one continent to another.

337
00:46:48,680 --> 00:46:52,680
But let's see what we've caught...

338
00:46:53,770 --> 00:46:56,730
in our apparatus.

339
00:47:00,860 --> 00:47:01,940
Turn it off.

340
00:47:04,150 --> 00:47:05,360
And...

341
00:47:08,370 --> 00:47:10,160
...take off the end.

342
00:47:18,420 --> 00:47:19,540
Well...

343
00:47:22,090 --> 00:47:27,510
We certainly haven't caught anything large.

344
00:47:29,720 --> 00:47:35,060
But if we examine this mesh,
when we get down to earth, with a microscope,

345
00:47:35,350 --> 00:47:43,360
it's very likely that, at the very least, we shall
have some pollen grains and spores of fungus.

346
00:47:44,650 --> 00:47:48,240
But bigger creatures are found at these heights

347
00:47:48,740 --> 00:47:53,660
and I've some of them here, in this phial,
that were caught here.

348
00:47:55,500 --> 00:48:01,590
I'll pour them out on a dish
to get a better look at them.

349
00:48:08,010 --> 00:48:14,350
There are tiny spiders that must have sailed up
hanging from their threads of gossamer.

350
00:48:15,140 --> 00:48:22,480
And winged aphids. At these altitudes
they can be carried halfway around the world

351
00:48:22,730 --> 00:48:25,280
and, amazingly, be frozen solid,

352
00:48:25,440 --> 00:48:29,610
and yet revive when they fall to lower altitudes.

353
00:48:30,950 --> 00:48:37,700
But now we are very close
to the top of our environment,

354
00:48:39,040 --> 00:48:45,380
for all the weather goes on
within these five brief miles,

355
00:48:45,590 --> 00:48:50,550
the envelope of atmosphere
that wraps round the world.

356
00:48:50,800 --> 00:48:54,390
It's here that the weather is manufactured.

357
00:48:56,010 --> 00:49:01,100
Molecules of water, evaporating in the heat
of the sun from the surface of the sea and lakes,

358
00:49:01,190 --> 00:49:03,480
or breathed out by plants as vapour,

359
00:49:03,650 --> 00:49:09,030
rise up from the land
and cool and condense into clouds of droplets.

360
00:49:09,860 --> 00:49:15,200
Driven by the winds, the clouds evaporate
and condense, form and re-form.

361
00:49:36,260 --> 00:49:41,140
The summit of Mount Everest
is less than six miles above the sea,

362
00:49:41,350 --> 00:49:43,810
yet few clouds ever sail much above it.

363
00:49:45,560 --> 00:49:50,530
The earth, as it spins,
creates vast eddies within the atmosphere.

364
00:49:50,990 --> 00:49:54,570
If they become intense,
they will develop into hurricanes.

365
00:49:54,870 --> 00:49:58,950
From a satellite
22,500 miles away from the earth,

366
00:49:59,200 --> 00:50:03,710
the build-up and dissipation
of these huge storms over 15 days

367
00:50:03,870 --> 00:50:08,000
can be seen with pictures taken every hour
and run continuously.

368
00:50:11,300 --> 00:50:15,760
Away to the east of Brazil in the Atlantic,
a hurricane is forming.

369
00:50:17,600 --> 00:50:21,350
As it spins, it moves west across the Caribbean.

370
00:50:26,480 --> 00:50:32,150
Northwards it goes towards Florida, while
up in the north, air sweeping over North America

371
00:50:32,320 --> 00:50:37,990
moves across the Atlantic towards Europe
in another immense, swirling storm.

372
00:50:46,210 --> 00:50:51,670
Other disturbances in the atmosphere are caused
when the sun builds up gigantic thermals

373
00:50:51,880 --> 00:50:54,510
in a sky already loaded with moisture.

374
00:50:54,880 --> 00:51:00,640
As the air is driven upwards, the tops of
the towering clouds burgeon with fearsome speed.

375
00:51:01,560 --> 00:51:06,310
The water molecules within the clouds
condense to form bigger and bigger droplets,

376
00:51:06,690 --> 00:51:11,980
but the speed of the rising air is now so great
that it keeps them suspended within the cloud.

377
00:51:14,360 --> 00:51:18,530
Eventually, the droplets become so big
that they cannot be supported,

378
00:51:18,740 --> 00:51:20,870
and they fall as torrential rain.

379
00:51:21,280 --> 00:51:26,420
The molecules of gas surging upwards
create a build-up of electricity

380
00:51:26,580 --> 00:51:30,420
that eventually becomes so great,
it discharges down to earth.

381
00:51:34,460 --> 00:51:38,640
The water droplets
may have been carried so high that they freeze

382
00:51:38,840 --> 00:51:42,010
and eventually tumble out of the cloud as hail.

383
00:52:01,200 --> 00:52:05,910
If the storm is really intense,
they may rise and fall several times.

384
00:52:06,160 --> 00:52:12,040
In the lower parts of the cloud, the ice
forms relatively slowly and is clear and black.

385
00:52:12,250 --> 00:52:16,380
But when they get to the top again,
the ice forms quickly,

386
00:52:16,550 --> 00:52:19,430
trapping air bubbles,
which makes the ice look white.

387
00:52:19,680 --> 00:52:26,270
So big hailstones may be banded, like an onion,
with alternate rings of black and white ice.

388
00:52:43,910 --> 00:52:50,370
Really big hailstones are often a sign that a truly
devastating storm is about to strike the earth.

389
00:52:52,670 --> 00:52:57,010
A strong, high-altitude wind,
linked with a severe storm such as this,

390
00:52:57,170 --> 00:53:01,930
may vacuum up lower-level air,
increasing the updraught dramatically,

391
00:53:02,180 --> 00:53:05,720
and beginning a spiral motion
in part of the storm.

392
00:53:06,180 --> 00:53:11,440
If these converging winds are powerful enough,
the vortex at the centre of this great whirl

393
00:53:11,600 --> 00:53:17,440
reaches down to the surface of the earth
as a suction funnel, a tornado.

394
00:53:50,520 --> 00:53:55,560
Winds up to 300 miles an hour
devastate the land, tearing things apart,

395
00:53:55,730 --> 00:54:01,700
ripping the roofs from buildings, sweeping
animals and trees and sometimes even people

396
00:54:01,860 --> 00:54:04,910
high into the sky and throwing them down.

397
00:54:05,870 --> 00:54:09,910
When it strikes the land,
it's seldom more than 500 yards across,

398
00:54:10,080 --> 00:54:17,750
but it lashes the earth with the most powerful
and destructive of all atmospheric forces.

399
00:54:50,740 --> 00:54:54,660
Storms like that may bring death and destruction,

400
00:54:54,870 --> 00:54:58,590
but they also bring life,
because the rain that comes from them,

401
00:54:58,790 --> 00:55:05,260
distilled by the sun from the surface of the ocean
is fresh water, salt-free,

402
00:55:05,470 --> 00:55:10,760
and that is something
that all life on land must have.