Sep 2012

Lesson 4.3

Meteorology

Pressure

Reference

From the Ground Up

Chapter 6.3:

Pressure

Pages 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”

HLIntended 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 – Meteorology

Winds

From the Ground Up

Chapter 6.4:

Winds

Pages 130 - 136