Transcript Class14 (June 28)

UPPER-LEVEL WINDS
Atmospheric pressure, temperature and winds at surface
Weather conditions
Atmospheric pressure and elevation
Winds at upper level (500mb = ~5600 m of elevation or 18000 feet)
1 knot = 1 “nautical” mile/h
1 knot = 1.15 MPH
1 knot = 1.85 km/h
Height and pressure at upper level (500mb = ~5600 m of elevation)
Winds at upper level (200mb = ~11800 m of elevation)
WINDS ALOFT
How does air move at higher elevations?
The gradient force INCREASES with altitude
How does pressure changes with elevation?
stronger winds
800mb
colder
warmer
850mb H4
800mb
850mb
H
L
H3
900mb
H2
900mb
Low
Height
L
H
950mb
H
L
POLE (90°)
950mb
High
Height
H1
Sea level
30° latitude
Pressure decreases with elevation:
SLOWER in WARMER air
FASTER in COLD air
Height (of pressure):
HIGH in WARMER air
LOW in COLDER air
Figure 5.17, Page 139
500mb
GEOSTROPHIC WIND
When the Coriolis force balances the gradient force.
The speed and direction of wind remain constant
It takes place at upper levels
geostrophic wind
GLOBAL CIRCULATION AT UPPER LEVELS
•Weak equatorial easterlies (from ~ 25° to poles)
•Tropical high pressure belts (15° -20°)
•Upper air westerlies (from ~ 25° to poles, see undulations)
•Polar low
(See Fig 5.19)
ROSSBY WAVES
•Undulations of upper air westerlies
Formation:
1.
Waves arise in region of contact between cold polar air
and warm tropical air (Polar front)
ROSSBY WAVES FORMATION
2. Warm air pushes pole ward and a tongue of
cold air is moved to south (undulation
development)
3. Waves are strongly developed. Cold air are
“troughs” of low pressure
4. Waves are pinched off, forming cyclones of
cold air
ROSSBY WAVES
•They are important for poleward heat transport
•Reason for variable weather in midlatitudes
JET STREAMS
• Regions at high elevation with strong wind streams
•According to the World Meteorological Organization we can call a
“jet stream” any speed exceeding:
67mph, 58 knots, 108 km/h
• The core of jet streams reaches ~102mph
• They take place where atmospheric pressure gradients are strong
The greater the contrast in
temperature, the stronger
the jet streams blow
JET STREAM
1.
Polar jet stream
Follows the edge of Rossby waves.
Found at 10-12 km elevation (33,000-40,000 ft)
Wind speed: 75-125 m/s (170-280 mi/hr)
2. Subtropical jet stream
In the subtropical latitude zone
Speed 100-110 m/s
3. Tropical easterly jet stream
In summer season, over Asia, India, Africa, only in Northern
Hemisphere
Figure 5.21
JET STREAMS
•Jet streams tend to be weaker in summer than in winter
•Jet streams are less intense in the Southern Hemisphere due
to smaller land masses
•Strongest jet stream have been recorded during winter over
Southern Japan (speeds up to 310 mph)
Wind at 200 mb (~12 km or 8 miles)
Atmospheric pressure and temperature at surface (summer)
Atmospheric pressure and winds at surface
Winds at upper level (500mb = ~5600 m of elevation)
http://weather.unisys.com
Winds at upper level (200mb = ~11800 m of elevation)