1. Meteorology Pressure

2. Reference
From the Ground Up Chapter 6.3: Pressure Pages

3. Introduction
Air pressure differences around the Earth cause many different pressure systems which in turn cause different weather phenomena.
Pilots need to recognize these pressure systems in order to know what weather will result.

4. Outline Atmospheric Pressure Pressure Systems Pressure Gradient
Coriolis Force

5. Atmospheric Pressure
Pressure measured with a Mercury Barometer, expressed in inches of mercury (“ Hg)
Standard pressure = 29.92” Hg = 1 bar = mb (millibars) = hPa (hectopascals) = kPa (kilopascals)
Station Pressure (actual pressure at observing station) is corrected to Mean Sea Level (MSL) Pressure to become Altimeter Setting

6. Altimeter Pressure Error
Low to high pressure, altimeter reads lower
“Low to high, watch the sky” H L
Intended Path
Actual Path

7. Altimeter Pressure Error
High to low pressure, altimeter reads higher
“High to low, look out below” H L
Intended Path
Actual Path

8. Pressure Systems
Weather maps show areas of pressure with different symbols/lines
Lines that join places of equal pressure are Isobars
Isobars in hectopascals, in intervals of 4, above and below 1000 hPa

9. Low Pressure Areas Low Pressure Areas Secondary Low
Called cyclone, depression or low
Pressure lowest in centre
Wind flows counterclockwise, inwards towards centre, then up
Inward airflow called Convergence
Creates unstable, poor weather
Secondary Low
Smaller low within a low
Trough (of Low Pressure)
Elongated U-shaped are of low pressure

10. Low Pressure Areas
Example of hurricane (low pressure system)

11. High Pressure Areas High Pressure Area Ridge (of High Pressure) Col
Called anticyclone or high
Pressure highest in centre
Wind flows down through centre, clockwise, then outwards
Outward airflow called Divergence
Creates stable, good weather
Ridge (of High Pressure)
Elongated U-shaped are of high pressure
Col
Neutral area between two highs and two lows

12. Pressure Gradient
Air moves from high pressure to low pressure (creating wind)
Pressure gradient is rate of change of pressure over distance at right angles to isobars
Closer isobars = steeper pressure gradient = faster winds

13. Coriolis Force
Created by Earth’s rotation, causes air to move parallel to isobars
In Northern Hemisphere, air spirals clockwise into low and counterclockwise out of high
Buys Ballot’s Law
If you stand with your back to the wind, low pressure area is on your left
On ground, Surface Friction slows down air and reduces coriolis force

14. Next Lesson
4.4 – Meteorology Winds From the Ground Up Chapter 6.4: Pages