8. High Level Winds

Question 1

Pressure in a warm column of air compared to cold (surface and aloft)

Answer 1

Whilst surface pressure is the same, pressure aloft is higher in column of warm air compared to cold at any given height

Question 2

Winds due to temperature differential at high level (“Thermal Component”)

Answer 2

Wind blows from higher to lower pressure, but is turned by Coriolis force and starts to flow parallel with the temperature divide (wind blows along isotherms). Buys Ballots Law then applies again (wind from the back = low pressure to the left)

Question 3

Direction and relationship of low and high level winds (Northern Hemisphere, back to wind)

Answer 3

• Low level winds blow along isobars (low pressure on left)
• High level thermal component wind blows parallel to isotherms (low temperature on left)
• Upper wind blows parallel to contour lines (isohypses) (low contour height on left)

Question 4

Isohypses (what does it mean)

Answer 4

Lines joining points of equal pressure height

Question 5

Global distribution of high level winds (3 flows, summer)

Answer 5

• Around the ITCZ: Easterly winds
• Moving away from equator, westerly winds (northerly air is colder than equatorial air, Buys Ballots law applies = with your back to the wind low pressure on left in northern hemisphere)
• Around the poles: Easterly winds

Question 6

Global distribution of winds in the winter compared to summer (what happens to westerly winds at mid latitudes)

Answer 6

Air at mid latitudes is much colder, no temperature differential to polar air = westerly winds continue all the way up to north pole (no easterly winds around the north pole)

Question 7

what are the four main air blocks from the equator to the poles?

Answer 7

Tropical, Subtropical (30-50 degrees lat), Polar, Arctic (winter only)

Question 8

Where do jet streams form?

Answer 8

Air mass boundaries aloft where significant temperature differentials exist. Jet stream forms in the warm air (overlapping the cold air) around or just above the tropopause level

Question 9

Height and speed of polar front jet stream compared to subtropical jetstream

Answer 9

Polar front: 30,000ft, reaches 200kts, Subtropical: 40,000 ft, 100kts

Question 10

What is the equatorial jetstream?

Answer 10

Summer, Indian subcontinent heats up so much that upper air getks warm as well creating temperatures differential with colder air over the sea, leading to the only easterly jetstream (warmer air unusually to the north in the northern hemisphere)

Question 11

Low level jetstream

Answer 11

Can form at 5,000ft if there is a marked temperature difference (e.g. central US cooling at night), rare and not recognised officially as jetstream

Question 12

Typical dimensions of a jetstream (length, wiidth, depth) in nm/ft

Answer 12

Length: 1,000nm
Width: 100nm
Depth: 10,000ft

Question 13

Typical dimensions of a jetstream (length, wiidth, depth) in nm/ft

Answer 13

Length: 1,000nm
Width: 100nm
Depth: 10,000ft

Question 14

Where is maximum CAT found in jet stream?

Answer 14

In the warm air, on the cold side of the core, level and slightly below the core

Question 15

Change in OAT when crossing a above, at or below jetstream from cold into warm sector

Answer 15

Above: gradual increase
At jet stream level: no market change in OAT
Below: spike in OAT as aircraft crosses from cold to warm sector

Question 16

Clouds associated with jetstreams

Answer 16

Generally. jet streams sit in clear air, however cirrus type cloud can form if upper air is moist through turbulence