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If you've ever looked at the
weather forecast,

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you might be familiar with a
percentage.

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It's the probability of
precipitation.

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For example, sixty percent
rain means that when similar

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forecasts have occurred in
the past, only sixty percent

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actually rained.

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Uncertainties can sometimes
leave us unprepared for storms

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and floods.

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But predictions improve as
we monitor storms in greater
detail

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and now NASA's
newest precipitation satellite

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GPM is measuring the individual
water drops that make up a
storm.

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GPM's the first time
we've been able to get this type

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of information on
a global scale.

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To understand why this matters,
it helps to know how weather
predictions are made.

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First, computer programs
known as weather models turn the

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processes in the atmosphere into
math equations,

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governed by the laws of physics.

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These equations receive
measurements of the land,
sea, and air 

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to get a view of the
atmosphere's current state

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then predicts how it
will change over time.

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To ensure predictions are
correct, equations need

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to represent the
atmosphere accurately.

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But one thing that's uncertain
in the equations is how

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precipitation is
structured within storms.

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A storm is made up of water
drops of different sizes but
without

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knowing these measurements many
weather models operate under an

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assumption. What they do
instead is have some assumption

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about how that's distributed.
So as an example, for every one

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hundred small drops, there would
be ten medium sized drops or

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just one large drop.
Assumptions are made because

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researchers have only studied
drops in isolated areas.

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Now for the first time, GPM
is measuring the size and

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distribution of
drops around the world.

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If we go into a storm we'll
see varying drops sizes labeled

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different colors.

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Near the top, there 
are many small drops
around .5,1 and

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2 millimeters. As they
fall to the middle of
the storm, some drops

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bump into others
causing them to 
grow in size.

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Right at the bottom, 
drops can grow around
four, five and six millimeters.

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This 3-D mosaic of
water drops is called
drop size distribution.

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It shows a high concentration of
small drops colored in blues and

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greens near the top and
lower concentrations of big

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drops colored in reds and
yellows near the bottom. 

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A storm with a higher ratio of
red and yellow will contain more

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water than a higher ratio of
blue and green. Without knowing

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the relationship or ratio of
those large drops to the smaller
or medium-sized drops. 

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We can have a big error in how
much

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rain we know fell and that can
have some big implications for

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knowing long term accumulations
which can help for flash flood
prediction.

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Not only does it give a more
accurate measurement of rainfall

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but drop sizes also give insight
into the winds within a storm.

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Thunderstorms have a lot of wind
associated with them and we all

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know this.

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But the strength of that wind
actually depends on the size of

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the drops that are falling
from that storm in some ways.

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Because a storm with small drops
will have more evaporation,

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which cools the air more,
which creates stronger winds.

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We've never been able to see
how water droplet sizes vary

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globally until now.

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So what causes them to
vary in different places?

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One factor is the temperature of
the environment drops grow in.

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In the mid-latitudes a lot
of those raindrops actually

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originated as snowflakes or even
hailstones and snowflakes can

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grow a lot larger
than cloud droplets can.

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So you have these big snowflakes
that then melt into big

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raindrops.

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In contrast, over the oceans and
in the tropics, they tend to be

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smaller and the reason is
because smaller raindrops tend

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to originate from clouds that
don't have any ice in them.

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It's worth noting,
however, these
measurements are only a

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small part of the
equation. The drop
size distribution is one of

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many factors that determines how
big a storm will grow, how long

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it will last and how much
rain it will ultimately produce.

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As GPM improves our
understanding of precipitation

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from space, that information
will be vital in improving

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weather models and thus helping
us better predict and prepare

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for our weather.