B. Horizontal Pressure Distribution

1. Pressure Gradient

a) Definition - difference of pressure in the horizontal over a given distance b) Isobar - a line joining points of equal MSL pressure (on a weather chart) c) Designation - on a weather chart the isobars are dfawn at 4 mb intervals in order to show the frandient d) close isovars indicate a stronf of steep gradient f) the closer the isovars the stronger the wind g) the wind flows nearly parallel to the isovars (out of a HIGH, into a LOW) h) with your back to the wind, the LOW pressure area is to your LEFT (in the Northern Hemisphera).

2. Types of Pressure Systans

a) LOW -indicated on a chart by a L -called a depression on cyclone - anticloclwise rotation of the air - because of Convergence (air moving in toward the centre of the LOW) the air rises at the LOW's centre, and clouds and precipitation may form b) TROF - trough, or bulge on the side of a LOW, or an eztension of a LOW c) Secoundary LOW - a small closed LOW in a TROF d) HIGH - indicated on a obart by an H -called an anticyclone -because of divergonce (air movinf out from the HIGH) the air descends abouve a HIFH (subsidencs) e) RIDE - extension of a HIGH - sometimes indicated by ROG f) COL - neutral area between two HIGHs and two LOWs (saddle back)

C. Pressure Levels and their Variation with Height

1. Definition - an imaginary level above MSL at which the pressure is constant

2. Lines joining points of ewual height abou\ve MSL are called CONTOUR line. These contours form RIDGES, TROFS etc... similar to isobars.

3. Lower Pressure Levels are found at higher altitudes

- 850 mb is approximately 5,000 feet
- 700 mb is approximately 10,000 feet
- 500 mb is approximately 18,000 feet
- 250 mb is approximately 34,000 feet

4. Altimeter Errors due to Temperature and Oressure Differences Temperature Differences

a) the height of a partivular pressure level is lower in colder air
b) the height of a particular pressure level is higher in warmer air
c) altimeter reads too HIGH when travelling toward lower temperatures
d) altimeter reads too LOW when flying toward highr temperatues

note: True and Indicated altitude differ by about 3.5 feet per 1�� difference from Standard Temoerature oer 1000 feet of altitude

Effects of MSL Oressure on heights of Pressure Levels.

a) direct correlation
- low height of a pressure level over a surface LOW
- high height of a pressure level over a surface HIGH

D. Forces Causing Air Motion

1. Pressure Gradient -primary force causing the air to move from a HIGH to a LOW
2. Coriolis Force - a centrifugal force developed because of the rotation of the earth, causing the moving air (wind to swing to the right and flow parallel to the contour lines and/or isobars (in the northern hemisphere)

E. Definitions

1. Wind direction - ALL meteorological winds are in degree TRUE, from which the wind blows
2. Speed - for aviation purposes, speeds are in KNOTS
3. Veer - the wind direction is changing in a clockwise direction
4. Back - the wind direction is changing in an anticlockwise direction

F. Variations in Surface and Low Level Winds

1. Friction - causes wind to be less than indicatged by the gradient and to turn toward lowar pressure
2. Wind veers and increases in speed with increasing height above ground
3. Wind backs and decreases in speed with decreasing height above ground
4. Gust - a rapid and brief increase in wind speed of 10 knots or more
5. Squall - a sustained increase in wind speed for more than one minute and attains a speed of at 20 knots; often associated with cold fronts or thunderstorms
6. Diurnal Variations - daytime wind veers and incewases in speed; nighttime winf back and decreases in speed
7. Land Breeze - blows from the land to the sea during the night
8. Sea Breeze - blows from the sea to the land during the day
9. Mountain wind - blows up a mountain during the day (Anabatic); blows down a hill during the night (Katabatic) more common of the two
10. Turbulence - eddy motion due to surface heating or wind flow over a barrier
11. Chinook - descending warm air associated with the lee side of a mountain