to describe how precipitation

happens at what are called

cold fronts and warm fronts.

These are weather systems

that develop wherever masses

of cold air collide

with masses of warm air.

And the name actually comes

from war where armies would

meet along a battle line

called a front, and one way

to think about the difference

between cold fronts

and warm fronts is to think

about which mass

of air is going

to win the battle.

So, we'll start

with a cold front.

This is a situation

that happens

where cold air moves

in like a big wedge underneath

warm air.

Cold air is always heavier

than warm air

and so it's denser

and it's going to tend

to slide under the warm air

and push the warm air upward.

Anytime you have air rising

upward, it's going to expand

and cool and lose its ability

to hold moisture

and this will give you a band

of clouds and precipitation

along a cold front.

On weather maps,

if you watch the weather

on the news,

you can see cold fronts marked

by lines of blue teeth

that point

in the direction the cold air

is moving.

Along cold fronts,

you typically have very

intense local thunderstorms

and rainstorms.

So, it might pour rain

for a short time

but then quickly--usually

within an hour

or so--the front will have

moved on.

So, cold front is

where the cold air pushes

underneath the warm air

and you have high rainfall

and usually

intense thunderstorms.

Now, the warm front has

in common a couple of things

with the cold front.

One is that the warm air still

goes up on top

of the cold air,

but in this case the warm air

is actually pushing the cold

air out of the picture,

so the warm air is winning

and that's how you can

remember if it's a warm front.

The warm air pushes

against the cold air squeezing

it out like imagine squeezing

a watermelon seed

between your fingers,

it kind of squeezes out,

right?

And as the air pushes,

it also rises

up over the cold air.

Rising air again loses its

ability to hold moisture

and you tend

to get clouds and rain.

Along warm fronts you

typically have more mild,

more mellow, more spread

out rainstorms rather

than really local

intense thunderstorms.

And if you're looking

at the weather on the news,

they mark warm fronts on maps

with red half moons that point

in the direction

that the warm air is pushing

the cold air.

So, that's cold fronts

and warm fronts,

major precipitation belts

in many places on the Earth.