Download CHAPTER 6 – Air Pressure and Winds

CHAPTER 6 – Air Pressure
and Winds
Section 1: Pressure
Why is wind important?
 December 19, 1980, was a cool day in Lynn,
Massachusetts, but not cool enough to dampen
the spirits of more that 2000 people who gathered
in Central Square – all hoping to catch at least one
of the 1500 dollar bills that would be dropped from
a small airplane at noon. Right on schedule, the
aircraft circled the city and dumped the money
onto the people below. However, to the dismay of
the onlookers, a westerly wind caught the currency
before it reached the ground and carried it out
over the cold Atlantic Ocean. Had the pilot or
sponsoring company examined the weather charts
beforehand, they might have been able to predict
that the wind would ruin their advertising scheme.
Atmospheric Pressure
Air Pressure – the mass of air above a given level.
 A model of the atmosphere
where air density remains
constant with height. The
air pressure at the surface
is related to the number of
molecules above. When
air of the same
temperature is stuffed into
the column, the surface air
pressure rises. When air is
removed from the column,
the surface pressure falls.
Atmospheric Pressure
 Two cities located at the
same elevation with
identical surface
pressures.
 The same number of
molecules (same mass of
air) is in each column
above both cities.
 Air pressure remains
constant, while the air
above City 1 cools and the
air above City 2 warms.
Atmospheric Pressure
 As the air in column 1
cools, the molecules move
more slowly and crowd
closer together – the air
becomes more dense.
 In the warm air above City
2, the molecules move
faster and spread farther
apart – the air becomes
less dense.
 It takes a shorter column
of cold, more dense air to
exert the same surface
pressure as a taller column
of warm, less dense air.
Atmospheric Pressure
 There are more molecules
above the letter H in the
warm column than above
the letter L in the cold
column.
 Warm air aloft is normally
associated with high
atmospheric pressure, and
cold air aloft is associated
with low atmospheric
pressure.
 The horizontal difference
in temperature creates a
horizontal difference in
pressure called the
pressure gradient force.
Measuring Air Pressure
 Barometer – an
instrument that detects
and measures
pressure changes.
 Since we measure
atmospheric pressure
with a barometer, it is
also referred to as
barometric pressure.
Measuring Air Pressure
 Mercury Barometer –
consists of a long
glass tube open at one
end and closed at the
other.
 Aneroid Barometer –
contains a small,
flexible metal box that
expands or contracts
depending on the
external air pressure.
Pressure Readings
 Station Pressure – the
barometer reading at a
particular location and
elevation.
 Sea-Level Pressure – an
adjusted reading of the
pressure at one elevation
compared with another.
The station pressure
observations are normally
adjusted to a level of mean
sea level.
Pressure Readings
 Isobars – lines connecting points of equal
pressure. Drawn on a surface weather map as
solid black lines at intervals of 4 mb, with 1000 mb
being the base value.
Surface and Upper-Air Charts
 Surface map shows areas of high and low pressure and
arrows indicating wind direction. The wind blows across
the isobars toward regions of lower pressure.
 Anticyclones – area of
high pressure; indicated by
the large blue H at the
center of high pressure.
Winds blow clockwise and
outward from the center.
 Mid-Latitude Cyclones –
area of low pressure;
indicated by the large red
L at the center of low
pressure. Winds blow
counter-clockwise and
inward toward the center.
Surface and Upper-Air Charts
 The upper-air map is a
constant pressure chart
because it is constructed
to show height variations
along a constant pressure
surface.
 This particular isobaric
map shows height
variations at a pressure
level of 500 mb.
 Contour lines of low height
represent a region of lower
pressure, and contour
lines of high height
represent a region of
higher pressure.
Surface and Upper-Air Charts
 Notice the dashed red
lines, isotherms – lines of
equal temperature, coincide
with the contour lines.
 Ridges are elongated
highs where the air is
warmer.
 Troughs are elongated
lows where the air is
colder.
 The winds on the 500-mb
chart tend to flow parallel
to the contour lines in a
wavy west-to-east
direction.
Isobaric Maps
 In the warmer air, a
pressure reading of
500 mb is found at a
higher level, while in
the colder air, 500 mb
is observed at a much
lower level.
 Each contour line tells
us the elevation above
sea level at which we
would obtain a
pressure reading of
500 mb.
Questions for Review
p. 164-5
#’s 1-4, 7-10, 12-15
Due Thursday,
th
January 7