1. ESCI 106 – Weather and Climate Lecture 7
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 Summer Monsoon brings all the rain.
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. Down sloping air = No clouds
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 Height (altitude) 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 mph
Summer ~100 mph
Height (altitude)
~7 to 12 km above the surface

12. Jet Streams Polar Jet Stream (Mid-Latitude) Meanders west to east
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 Slower than the polar jet.
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:
and
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.