Air masses and Fronts

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

What causes weather?• Water vapor (moisture)• Composition of atmosphere• Solar heating• Air pressure • Wind

What is the distribution of Insolation?• Insolation: incident solar radiation• Tropics receive more concentrated

insolation due to Earth’s curvature• Tropics receive 2.5X more than the poles

What is an air mass?• Because air and moisture move in the

atmosphere, weather is constantly changing• Air pressure – measured by barometer• An air mass is a large body of air whose

temperature and moisture are fairly similar at a given altitude; properties like the part of

Earth’s surface over which it formed

• There are 5 different air masses that affect the United States

How do we classify air masses?• Classified via moisture and temperature• 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• A (artic):same as cP but colder

What is air mass modification?

• As air masses move from their source region, their temperature and moisture characteristic change with the regions they come in contact with.

What is a front?

• Fronts are boundaries separating different air masses of two different densities caused by temperature, pressure and humidity.• clouds, precipitation, and storms occur at

frontal boundaries

How do warm fronts work?• A warm front is warm air displacing cool air • Shallow leading edge warm air must “overrun”

cold air• These are usually slow moving• Hot, humid weather to follow

How does a warm front look on a weather map?A red-line with solid semi-circles

How do cold fronts work?• Cold air advances into region of warm air• Intensity of precipitation greater, but short lived• Clearing conditions after front passes. Fair and cool

weather• Usually approaches from W or NW

How does a cold front look on a weather map?• Solid blue line with triangles

How does a stationary front form?• Surface positions of the front do not move• Often a region of clouds but no precipitation

How does a stationary front look on a weather map?• When the front starts moving again it returns to either being a cold or warm front• Red semi-circles and blue triangles

How do occluded fronts form?• Cold front overtakes warm front• Involves three air masses of different

temperatures• Often found close to the low pressure center• Strong winds and precipitation on both sides of

the front.

How does a occluded front look on a weather map?• This occurrence usually results in

storms over an area

• In U.S., the colder air usually lies to the west• Purple semi circles and triangles.

Review question

• What type of front can be found close to point D ?

Review question

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

Review question

• Give the name of the air mass that would have the following characteristics:• cool, moist, unstable

Review question

• That important weather word that refers to the transfer of heat

What are pressure systems?• Created by air molecules, product of motion, size and number.• 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 tropics rises• Cold air can hold less moisture than warm air• As the moist air rises, it condenses and forms

clouds!

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

What is air pressure?

• Air weight that varies over Earth’s surface• Warmer air is less dense and exerts

less pressure • Cooler air is more dense and exerts

more pressure

How does a high pressure system form?• 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

in the Northern hemisphere.• Anti-cyclones • Diverging and descending winds

How do low pressure systems form?• 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 in the northern hemisphere. • Rising and converging air

What to expect on a weather map?• If you see a big L in your area you should expect stormy

weather and if you see a big H in your area expect clear, sunny skies.

What else affects weather?• Wind: Movement of air from one temperature or pressure

area to another• Different areas of Earth receive different amounts of the

Sun’s energy• Equator’s warm air, being less dense, is pushed

upward by denser, colder air• Poles’ cold air, being more dense, sinks and moves

along Earth’s surface• CORIOLIS EFFECT: spinning of the Earth

causes moving air to turn to the right in the northern hemisphere and to the left in the southern hemisphere

How do global wind patterns affect weather?• Wind patterns, caused by convection currents combined with

the Coriolis effect, of Earth that affect the world’s weather• Near equator, very little wind and daily rain patterns

called the doldrums• Surface winds:• Between equator and 30 degrees N and S latitude

are steady trade winds• Between 30 and 60 degrees N and S latitude, the

westerlies blow in opposite direction from the trade winds. Affect weather in the United States.• The polar easterlies blow from northeast to

southwest near the north pole and from southeast to northwest near the south pole

Equatorial doldrums (Low P)

TRADEWINDS

TRADEWINDS

Global Winds

How does the Coriolis effect create 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

What is a jet stream?

• An irregular, concentrated band of wind occurring at several different locations that supports surface weather conditions.• Occur at the boundaries between wind zones.• polar jet stream and subtropical jet stream

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

6. If there is a big H on the weather map where you live, would you expect fair or stormy weather?

7. Warm air holds ( more or less ) moisture than cold air

8. Which of the weather highways usuallycontrols our weather ?

1. CONVECTION

2. COLD

3. ITCZ

4. LESS

5. RIGHT

6. FAIR

7. MORE

8. WESTERLIES