ESCI 106 – Weather and Climate Lecture 7

1. ESCI 106 – Weather and ClimateLecture 7 9-29-2011 Jennifer D. Small 

2. Weather Fact of the Day: September 29 • 1938: Five tornadoes struck Charleston, South Carolina, killing 32 people and causing $2 million in damages

3. National Watches and Warnings

4. “ Chapter 7- Circulation of the Atmosphere”…Con’t

5. Monsoons • The greatest seasonal change in Earth’s global circulation is the MONSOON. • Monsoon does NOT mean rainy season. • Refers to a wind system that exhibits a pronounced seasonal reversal in direction. • Winter : Winds blow off the continents (dry) • Summer: Winds blow from the sea towards the land (moist) • The Summer Monsoon brings all the rain.

6. Asian Monsoon • A seasonal reversal in weather patterns • An alternation between two types of weather patters • Ex: India – Wet hot summer, dry cool(ish) winter • A seasonal reversal of wind also • F SUMMER MONSOON WINTER MONSOON H L H L Down sloping air = No clouds H COLD L L H Hot Indian Continent Warm Ocean Warm Ocean

7. Asian Monsoon Siberian High SUMMER MONSOON WINTER MONSOON

8. North American Monsoon • Weaker than the Asian Monsoon • Hot temperatures over the desert cause an intense low to form • Moisture for summer precipitation comes from Gulf of Mexico and California

9. Westerlies • General west to east flow of air aloft • Due to the effects of the PGF and Coriolis Force • Wind speeds increase with height until the tropopause then decrease.

10. Jet Streams • Weaker than the Asian Monsoon • Hot temperatures over the desert cause an intense low to form • Moisture for summer precipitation comes from Gulf of Mexico and California

11. Jet Streams • High winds, high in the atmosphere • Location of jet stream influences local weather • Region of the upper atmosphere where a narrow band of air moving REALLY fast • LARGE temperature constasts • Wind speeds of Jet Stream • Winter 120-240 mph • Summer ~100 mph • Height (altitude) • ~7 to 12 km above the surface

12. Jet Streams • Polar Jet Stream (Mid-Latitude) • Where the Polar Front is located • Also called the mid-latitude jet stream • Where the cold polar easterlies interact with the warm westerlies • It’s a Geostrophic Wind since it’s high up in the atmosphere • Meanders west to east • Can exceed 500 km an hour (300 mph) • In winter on average it travels at 125 kph (75 mph) • In summer ~65 kph

13. Jet Streams • Subtropical Jet Stream • Semi-permanent jet that exists over the subtropics • Is mainly a wintertime phenomenon. • Due to the weak summertime temperature gradient, the subtropical jet is relatively weak during the summer. • Slower than the polar jet. • Still travels west to east • Usually at about 25 N • Altitude of 13 km.

14. Waves in the Westerlies • The Jet Streams do not travel in a straight line • They follow wavy paths at long wave lengths, these waves are called Rossby Waves. • The waves move slowly • Has seasonal changes in strength and location

15. Global Winds and Ocean Currents • Winds are the driving force for ocean currents • A relationship exists between ocean and atmosphere circulation.

16. El Nino / Southern Oscillation • Ocean phenomenon that occurs in the Equatorial Pacific • As the southeast trade winds decrease in strength (weaken) the warm water can make it farther across the Pacific towards South American

17. El Nino / Southern Oscillation • Named El Nino because it usually starts during Christmas. • Usually happen every 3-7 years.

18. El Nino / Southern Oscillation • Marked by abnormal weather patterns • Especially for Equador and Peru • Cold-nutrient rich water is blocked by the warm water. • In California we have increase severe winter storms. • Strongest ones in recent years: • 1982-83 and 1997-98 Santa Cruz, County San Francisco, County

19. La Nina • The opposite of an El Nino • When you have colder than average temperatures in the Pacific.

20. “ Chapter 8- Air Masses

21. What is an air mass? • Air Mass • really big blob of air with similar properties • Usually 1600 km (1000 miles) across • Several km thick • Change in weather when one air moves out and a new air moves in • Also known as a FRONTAL PASSAGE • Brings changes in T, P, RH and Winds • Area between different air masses is known as FRONT

22. How is an airmass formed? • Air masses form when they move over large regions that exhibit very similar properties • Just land or water • Has to stay put for a while • Be stationary or slow moving…. Like anti-cyclones • If more heat is added than escapes an air mass, then it warms up • If more moisture is added than precipitates, then its humidity increases • Obviously, the opposites are also true.

23. Air Mass Source Regions • Polar and tropical regions tend to exhibit such properties and therefore are good source regions. • The mid-latitudes tend to be strongly varies and therefore are not good source regions

24. Types of Air Masses • Polar (P) air masses form over northern Canada and the Arctic. • The tend to be COLD. • Tropical (T) air masses form to the south of the United States. • These tend to be WARM. • Maritime (m) Air masses form over ocean (either the Pacific, North Atlantic, subtopical Pacific, or Gulf of Mexico). • These tend to be HUMID. • Continental (c) air masses form over land (northern Canada or northern Mexico). • These tend to be DRY. • Air masses are designated by two-letter combinations describing whether they are m or c, P or T. • Describes basic info about T and RH.

25. Air Mass Designations • All the possible combinations are: • mP: N Pacific or N Atlantic • mT: Subtropical Pacific or Gulf of Mexico • cP: Northern Canada • cT: Northern Mexico and South West USA

26. cP (continental polar) • Also continental Arctic (cA) • Generally cold and dry • During winter, cooled by the land, creating stable air mass. • Mostly cloud free • In summer, warmer and more humid than in winter • Tend to remain farther north and therefore do not influence as much of North American as during winter

27. cP (continental polar) • Land warms the air mass slightly, so can be slightly unsettled, resulting in fair weather cumulus. • Related to lake effect snows around the Great Lakes • Can affect large regions of central and northeastern US during winter, much less during the summer.

28. mP (maritime polar) • Because systems move W to E, generally mP from N Pacific is more influential. • In general, this is where CA gets our bad weather from

29. mP (maritime polar) • Humid and cool • Generates low level stratus clouds, which maintain instability through cloud-top radiative cooling • This leads to somewhat unpleasant showers in Pacific NW, and to much of our bad winter weather.

30. mP (maritime polar) • Upon orographic lifting, get heavy snow and rain (in the Sierras) • Tend to remain farther N during the summer (recall the jet stream average latitude in summer is 50N, the US/Canada border).

31. mT (maritime tropical) • Hot and dry, forming over the desert regions of North Mexico and South West US. • Intense surface heating creates very unstable air, but generally cloud-free because of extremely dry conditions • If however, some moisture is acquired, intense thunderstorm can develop. • Occasionally bring drought to the Plains states.

32. cT (continental tropical) • Warm and moist • Can be unstable because of high heating from warm waters  perfect for precipitation.

33. cT (continental tropical) • In summer, land is even warmer creating even more instability, often triggering thunderstorms. • In winter, often warmer than underlying land (more stable), bringing widespread fog and low level precipitation. • Strongly influences southeastern US much of the year. During summer, influence extends further north.