Why Do We Have Weather?

An IntroductionTake good notes! There will be

a quiz at the end of this presentation.

WHY DO WE HAVE THE WEATHER ?

Just think about it! Why is there wind? Why does it blow

from one direction one day and another the next?

Why is it rainy one day and dry the next?

How come it’s cold in the winter? How can we have hail in the summer? What causes snow and freezing rain?

Let’s take a look at the weather picture and why we have

weather!

If we were to pick one term to help explain why we have weather, what do you think would be a good word?

You might pick heat or sun….but another good choice would be

Convection

Convection is the transfer of heat,

usually in gases or liquids.

After the atmosphere is warmed by radiation and conduction, the heat is transferred

throughout the atmosphere by convection. Since warmed air has

more space between the molecules, it’s less dense and rises

Cooled air is more dense and tends to sink

In general, air near the equator tends to rise and air near the poles tends to sink

Take a look at this!

Notice the band of clouds around the equator ?

This is the ITCZ or inter tropical convergence zone

Why do you think there is this band of clouds near the

equator?

Did you figure it out? Warm, moist air in the topics rises Cold air can hold less moisture than

warm air As the moist air rises, it condenses

and forms clouds!More on this later

Consequences of Rotation: the Coriolis effect

The Weather Highways The rotation of the

earth creates the Coriolis effect.

The Coriolis effect causes the air and water to be deflected to the right north of the equator.

This creates global weather highways

The Westerlies Because of our latitude,

most of our weather comes from the west

Looking at the weather map, what type of weather might we expect?

What type of weather might we expect in a few days?

Let’s break for a short review1.Transfer of heat in liquids or

gases_____2. _____ air is dense and tends to sink.3. Band of clouds found around the

equator______4. Cold air holds _____ moisture than

warm air5. The Coriolis effect causes the air and

water to be deflected to the _____ of the equator

How did you do?1. CONVECTION

2. COLD

3. ITCZ

4. LESS

5. RIGHT

Now What? Ok, so we know that the weather

moves around on these highways and that warm air rises and cold air sinks.

But why is it sunny one day, and rainy the next?

Let’s take another look at the weather map

Notice that there are H’s and L’s on the map

There are also blue lines with spikes and red lines with half circles

Let’s take a closer look!

High Pressure Areas When cooler air

sinks and is warmed, the air can hold more moisture

This usually means sunny skies

Winds tend to move clockwise around a high

Low Pressure Areas When warm air rises

and is cooled, the air can not hold as much moisture

Often, these areas are associated with precipitation and stormy weather

Winds tend to move counter clockwise around the low

So, if you see a big H on the weather map over the area you

live, you can expect fair weather

When you see a big L in your area, there will probably be

stormy weather

These highs and lows move or less along the jet stream and bring us our weather

changes

Fronts and Air Masses An air mass is a large body of air whose temperature and moisture are

fairly similar at a given altitude Fronts are boundaries separating

different air masses

There are four different air masses that affect the United States

The Air Masses cP( continental polar) : cold, dry stable cT( continental tropical) : hot, dry,

stable air aloft, unstable at the surface mP( maritime polar) : cool, moist,

unstable mT( maritime tropical) : warm, moist,

unstable

This map shows the air mass source regions and there paths

Ok, now we see the difference in the air masses

Let’s look at the different fronts and their impact on weather

Can you see the four different types of fronts on the map?

Warm Fronts A warm front is

warm air displacing cool air diagram

Shallow leading edge warm air must “overrun” cold air

These are usually slow moving

Cold Fronts Cold air advances

into region of warm air

Intensity of precipitation greater, but short lived

Clearing conditions after front passes

Usually approaches from W or NW

Stationary Fronts

Surface positions of the front do not move

Often a region of clouds

Occluded Fronts Cold front

overtakes warm front

Often found close to the low pressure center

Ready for a little quiz?You’ll need a sheet of paper

Write your answers as we go

Here we go!

1. Winds in a low

pressure system move _____ around the low LL

2. What type of

front can be found close to point D ?

3. Which of these

fronts would you expect to have greater precipitation, but be short lived as the front passes?

4. Give the name

of the air mass that would have the following characteristics:

cool, moist, unstable

5.That important weather word that refers to the transfer of

heat

6.In general, air near the equator

tend to_____ ( rise or fall )

7.

It causes air and water to be deflected to the right north

of the equator

8.Which of the weather highways

usuallycontrols our weather

9.Warm air holds ( more or less )

moisture than cold air

10. If there is a big

H on the weather map

where you live, would you

expect fair or stormy weather

HH

How did you do?Let’s check the answers!

Answers 1. Counterclockwise 8. Westerlies 2. Cold 9. More 3. Cold 10. Fair 4. Maritime polar (mP) 5. Convection 6. Rise 7. Coriolis