Transcript ESCI 106 – Weather and Climate Lecture 1

ESCI 106 – Weather and Climate
Lecture 7
9-29-2011
Jennifer D. Small 
Weather Fact of the Day: September 29
1938: Five tornadoes struck Charleston,
South Carolina, killing 32 people and
causing $2 million in damages
National Watches and Warnings
“ Chapter 7- Circulation of the
Atmosphere”…Con’t
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.
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
SUMMER MONSOON
F
H
WINTER MONSOON
L
COLD H
L
H
L
H
Down sloping air
= No clouds
L
Hot Indian Continent
Warm Ocean
Warm Ocean
Asian Monsoon
SUMMER MONSOON
Siberian High
WINTER MONSOON
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
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.
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
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
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
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.
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
Global Winds and Ocean Currents
 Winds are the driving force for ocean currents
 A relationship exists between ocean and atmosphere
circulation.
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
El Nino / Southern Oscillation
 Named El Nino because it
usually starts during Christmas.
 Usually happen every 3-7
years.
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.
Santa Cruz, County
 Strongest ones in recent years:
 1982-83 and 1997-98
San Francisco, County
La Nina
 The opposite of an El Nino
 When you have colder than
average temperatures in the
Pacific.
“ Chapter 8- Air Masses
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
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.
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
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.
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
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
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.
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
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.
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).
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.
cT (continental tropical)
Warm and moist
Can be unstable because of high heating
from warm waters  perfect for
precipitation.
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.