1. Air masses and Fronts

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

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

4. 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

6. 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

7. 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

9. 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.

10. 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

11. 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

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

13. 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

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

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

16. 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

17. 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.

18. 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.

19. Review question
What type of front can be found close to point D ?

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

21. Review question
Give the name of the air mass that would have the following characteristics:
cool, moist, unstable

22. Review question
That important weather word that refers to the transfer of heat

23. 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

24. Take a look at this Notice the band of clouds around the equator ?
This is the ITCZ or inter tropical convergence zone

25. Why do you think there is this band of clouds near the equator?

26. 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!

27. 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?

28. 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

29. 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

30. 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

31. 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

32. 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.

33. 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

34. 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

35. Global Winds
TRADEWINDS
Equatorial doldrums (Low P)
TRADEWINDS

36. 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

37. 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

38. Short Review 7. Warm air holds ( more or less ) moisture than cold air
1.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 air
5. 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 usually controls our weather ?

39. 2. COLD 3. ITCZ 4. LESS 5. RIGHT 6. FAIR 7. MORE 8. WESTERLIES
1. CONVECTION
2. COLD
3. ITCZ
4. LESS
5. RIGHT
6. FAIR
7. MORE
8. WESTERLIES