Ch 11 - Upper Winds And Jet Streams

Question 1

Wind Speed and Heights

Answer 1

The winds at high levels (up to the tropopause) are generally much faster than those on the surface;

• Pressure differential between columns of different temp ai increases with altitude
• The reduction in air density increases the wind speed

Question 2

Thermal Wind

Answer 2

Flows from areas of higher temperature to lower temperature

Relatively high pressure above the warm air so that pressure decreases more slowly

Relatively low pressure above cold air so air moves from warmer air to colder air

In the northern hemisphere, with your back to the upper wind, the cold air is on your left

Question 3

Contour Charts and Thickness Charts

Answer 3

These chart the height (or thickness) of pressure levels

Heigh variations are plotted as count our lines referenced to MSL (vertical distance of a pressure level from MSL)

Heigh or thickness is given in Decameters (dams)

If the pressure level is at a higher altitude, we assume relatively high pressure exists.

The upper wind blows approximately parallel to the contours

Question 4

Global Upper Wind Patterns

Answer 4

Driven by temperature differences so we should expect to find the strongest winds at the boundaries between warm and cold air masses.

Relatively stable global pattern of upper winds

Question 5

Westerly Flows

Answer 5

In the northern hemisphere, with your back to the wind, the cold and low pressure air is on your left

Therefore the global wind pattern is westerly.

Long haul flights from west to east try to take maximum advantage of tailwinds.

Question 6

Easterly Flows

Answer 6

Generally, around the equatorial regions (23.5N - 23.5S), the weather patterns are moved by the easterlies, east to west

Question 7

Jet Stream

Answer 7

A jet stream is a narrow band of airflow with a velocity greater than 60kt

Occurs at the boundary between warm and cold air masses with large temperature differences, just below the tropopause

Strongest and sharpest differences of temp is found at the boundary between the polar and tropical air so the strongest jet streams are found in the mid latitudes

2000 miles long
200 minutes wide
2 miles deep

Approximate depth ratio of 1:100

Question 8

The 4 Notable Jet Streams

Answer 8

The sub-tropical jet stream

The polar front jet stream

The (seasonal) arctic front jet stream

The (seasonal) Tropical Easterly jet stream

Question 9

The Sub Tropical Jet Stream

Answer 9

Permanent and occur in both hemispheres

Move with the heat equator; December (25-40 degrees N/S) and June (40-45 N/S)

Approximately 200hPa level (40,000ft) but height varies in season

During winter, jet streams become faster due to the larger temp differences

Question 10

The Polar Front Jet Stream

Answer 10

Permanent and found in both hemispheres

Temp difference between the Warmer Ferrel cell and the much colder Polar Cell (40-65N, 50S) northwards in June and July, southwards Dec and Jan

Don’t quite circumnavigate the globe, frequently start and stop over land masses

Can lead to the presence of streaky or streaked cirrus clouds

Jet core roughly 300hPa (30,000ft) but frequently found lower in winter

Question 11

The Arctic Front Jet Stream

Answer 11

A seasonal jet stream found only in the northern hemisphere - not very common

Between the boundary of the warm polar maritime air and the colder arctic air. Only forms in winter in the North Atlantic ocea and is typically positioned around 60N

Core varies between 300/400hPa (30,000-20,000ft) and is muc lower than the subtropical and polar jets. Can be a transient feature found over large continents during arctic air outbreaks or arctic vortex outbreaks

Question 12

The Tropical Easterly Jet Stream

Answer 12

Or equatorial easterly jet, is a seasonal jet stream found only in the northern hemisphere

It occurs during summer when a sufficient temp difference occurs between hot west Africa and the very hot Indian sub continent and their relatively cooler oceans to the south.

Between 10 and 20N only. Runs from the south chine sea westwards across southern India, Ethiopia and the sub saraha

Typically 150hPa (45,000ft)

Question 13

CAT (Clear Air Turbulence)

Answer 13

CAT occurs whenever there is a rapid change of wind speed and or wind direction

Isotachs (lines of equal wind speed) show how much the wind speed changes either side of the jet stream

Most pronounced change occurs on the cold side of the jet stream where the isotachs are closest

If you encounter CAT, descend and move further into the warmer air

CAT zones are marked with dashed line adjacent to the colder side of the jet - CAT 5 = severe turbulence where two jet streams are flowing in opposite directions