050 02 00 00 WIND

Wind is the result of differences in atmospheric pressure and causes air to move horizontally. Other factors play a part such as the rotation of the earth which can change the relative speed of the wind.

Done.

Ok

It is a vector, i.e. speed and direction.

When there is a pressure differential, a force is exists from high to low pressure. It acts at right angles to the isobars.

The Coriolis force is the rotation of the Earth apparently deflecting the wind to the right in the northern hemisphere and to the left in the southern.

The formula is:

Coriolis Force = 2 Ω ρ V sin θ

Ω = Angular rotation of the Earth

ρ = Density

V = Wind speed

θ = Latitude

It is proportional to both wind speed and latitude. At the equator sin 0° = 0 and therefore no Coriolis force there. Equally, there is no Coriolis force when the wind velocity is zero. Anything multiplied by zero is of course….

 

The geostrophic wind blows parallel to the isobars when the pressure gradient force and Coriolis for are in balance.

Anti-clockwise around a low in the north. Clockwise in the south.

With little or no Coriolis force at the equator (only really with high winds) it is all PGF. Between 10° latitude, the geostrophic scale does not work. This is the same when the isobars are very close, PFG is much more than Coriolis.

There is additional centrifugal force when flowing around curved isobars. The gradient wind is the geostrophic wind modified by the centrifugal force. This opposes the PGF around a low reducing wind speed which in turn reduces the Coriolis effect thereby reducing total wind speed. Around a high the centrifugal force now compliments the PGF which increases Coriolis and therefore total wind speed.

Slack and back – northern hemisphere

Slack and veer – Southern hemisphere

Due to surface friction the wind at the surface slows down and therefore reduces the Coriolis effect.

 

The diurnal temperature change affects the surface wind.

Rising thermal currents during the day mix with the faster upper layers thereby increasing surface wind.

During the cooler night the upwards currents are less, no mixing, slower wind.

Ok

Geostrophic wind scale.

 

Remark: Approximate value for variation of wind in the friction layer (values to be used in examinations) Type of landscape Wind speed in friction layer in % of the geostrophic wind The wind in the friction layer blows across the isobars towards the low pressure. Angle between wind direction and isobars.
over water ca 70 % ca 10°
over land ca 50 % ca 30°
WMO No 266

 

 

Convergence – mostly caused by rising or sinking air drawing in air from either side, i.e bottom of a low and the top of a high.

 

 

Convergence at the surface and divergence aloft create low pressure areas and cause air to rise, possibly creating clouds and rain and all the things convection brings us. It can be quite strong, increasing PGF and therefore wind.

 

Divergence at the surface and convergence aloft creates high pressure, dispersing clouds and rain, and conversely can calm the wind down.

 

 

Drives global weather patterns. Heating at the equator causes rising air and spreads out when it his the isothermal bit at the tropopause. Coriolis makes it head north in the north and south in the south before it cools and sinks – these are the Hadley cells. At the surface, some air returns round the Hadley and some goes the other way into the Ferrel cell. This happens again and forms the polar cell.

 

 

It goes low high low high, between the cells. The low pressure area created at the equator is a strong area of convergence, the ITCZ, then there is the sub-tropical highs at the next convergence zone, then the traveling lows (the polar front depressions).

The surface wind blowing into the ITCZ are the trade winds. NE in the northern and SE in the southern Hemisphere.

As the equator moves with the seasons, so do the cells and their pressure patterns.

 

I think this is referring to jet streams, the strongest jet stream is the polar jet because of the largest temperature differential.

Starting from the equator or rather, at the equator there is the Tropical easterly jet (seasonal).

Next, north and south is the sub tropical westerly jets (permanent). Move with the heat equator. (Stronger in winter)

Then the westerly polar front jets, then; arctic front jet (seasonal)

Anabatic flows up terrain, caused by heating. Good for thermals and connective fluffy clouds.

Katabatic flows down. Caused by cooling, can cause fog and a temperature inversion above

Very local, some given names because the always exist.

When the wind is constricted by a valley is speeds the air flow and wind up…

Temperature differential between land and sea which reverses during the night.

 

Sea breezes during the day – from sea to land.

Land breeze during the night – from land to sea, generally weaker.

The same principle as high jet streams. A low level inversion at night acts like a low tropopause where the high level jets sit.

 

Wind speed at mountain height >15-20 its increasing with height.

Wind direction within 30° of the perpendicular to the mountain range.

Marked stability,  an inversion or isothermal layer so the wind doesn’t keep ascending.

Undulating air above and downwind of mountains

Cap clouds over the mountain, lenticular clouds and rotor clouds.

Vertical air movement can exceed 2000’/min and if you’re being thrown around like that, your wings could fall off!

Can exist up to the tropopause but with decreasing severity.

Turbulence: Disturbed air which causes a sudden change of speed and direction, generally affects altitude not altitude, of course if a different attitude remains altitude will change.

Gust: Increase in windspeed, can change in direction. Short lived and over a small area.

Just have

Convective: An area of convection restricts the horizontal flow of the wind creating turbulence on the other side – similar to a physical object in the way.

Mechanical: mountains, buildings, any large enough solid object.

Orographic: Same I think.

There….said it. relief is the difference between high and low elevations.

Sudden severe turbulence, cloudless areas, hence, “clear”. High altitude. Don’t fly in it, init.

When air changes velocity abruptly.

Found on the cold side of a jet streams, and along the centre line of a trough (Refer to Subject 050 09 02 02) 

There, said it…

Again, said it.

Relatively narrow snake like winds faster than 60 its, formed where there is a large air temperature differential, i.e between the cells.

OK

2nm high and 200nm wide and up to 2000 nm long.

Arctic front jet:

• Height: 300 – 400 hPA (20-30 000′)
• Speed:
• Seasonal speed:
• Geographical Location: Only in northern hemisphere 60°N.
• Seasonal Occurrence:  Not very common, winter.
• Seasonal Movement:

 

Polar front jet:

• Height: 300 hPa 30 000′
• Speed:
• Seasonal speed: Faster in winter.
• Geographical Location: Both hemispheres between ferrel and polar cell. 40-65°N 50°S.
• Seasonal Occurrence:  Permanent.
• Seasonal Movement: North in summer, south in winter.

Sub tropical jet:

• Height: 200hPa 40 000′
• Speed:
• Seasonal speed: Faster in winter due larger temperature difference.
• Geographical Location: Both hemispheres between Hadley and ferrel. 25-40° in winter. 40-45° in summer.
• Seasonal Occurrence: Permanent.
• Seasonal Movement: Lower in winter due lower tropopause.

Tropical Easterly jet:

• Height: 150 hPa 45 000′
• Speed:
• Seasonal speed:
• Geographical Location: 10-20°N
• Seasonal Occurrence: Only in northern hemisphere in summer.
• Seasonal Movement:

Top corner of the warmer air mass just below the tropopause..

Cold air side.

Also Strong jets, curved jets, above and to the lee of high mountains and developing and rapidly moving jets.