Sep 2012

Lesson 4.3

Meteorology

Pressure

Reference

From the Ground UpChapter 6.3:PressurePages 127 - 130

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.

Outline• Atmospheric Pressure• Pressure Systems• Pressure Gradient• Coriolis Force

Atmospheric Pressure• Pressure measured with a Mercury Barometer,

expressed in inches of mercury (“ Hg)

• Standard pressure= 29.92” Hg= 1 bar = 1013.2 mb (millibars)= 1013.2 hPa (hectopascals)= 101.3 kPa (kilopascals)

• Station Pressure (actual pressure at observing station) is corrected to Mean Sea Level (MSL) Pressure to become Altimeter Setting

Altimeter Pressure Error• Low to high pressure, altimeter reads lower• “Low to high, watch the sky”

HL

Intended Path

Actual Path

Altimeter Pressure Error• High to low pressure, altimeter reads higher• “High to low, look out below”

HL Intended Path

Actual Path

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

Low Pressure Areas• Low Pressure Areas

– 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

Low Pressure Areas• Example of hurricane (low pressure system)

High Pressure Areas• High Pressure Area

– 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

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

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

Next Lesson

4.4 – MeteorologyWinds

From the Ground UpChapter 6.4:WindsPages 130 - 136