Meteorology
Pressure
Lesson 4.3
Sep 2012
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”
L
H
Intended Path
Altimeter Pressure Error
• High to low pressure, altimeter reads higher
• “High to low, look out below”
L
Intended Path
H
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