Air masses and Fronts

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 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 bigHon 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?

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