Unit 4 – Atmospheric Processes
Winds…
 Earth’s atmospheric circulation is an important
transfer mechanism for both energy and mass
 The imbalance between equatorial energy surpluses
and polar energy deficits is partly resolved
 Wind – the horizontal motion of air across Earth’s
surface
 Differences in air pressure (density) between one
location and another produce wind
 Names for the direction from which they originate
Driving Forces within the
Atmosphere
 Gravitational force
 Pressure gradient force
 Coriolis force
 Frictional force
Earth’s Gravitational Force
 Earth’s gravitational force on the atmosphere is
practically uniform, equally compressing the
atmosphere near the ground worldwide
 Density decreases as altitude
increases
Pressure Gradient Force
 High- and low-pressure areas exist in the atmosphere
because of uneven heating of the Earth’s surface
 A pressure gradient is the difference in atmospheric
pressure between areas of higher pressure and lower
pressure
 A steep gradient causes faster air movement from a high-
pressure area to a low-pressure area.
 High and low pressure areas are caused by differences
in surface heating and cooling.
 Land versus water.
 Different albedos and energy absorbing abilities of land,
vegetation and human built surfaces
Isobars
 Isobars are lines of equal atmospheric pressure drawn on
a map.
 Widely spaced isobars display a gradual pressure gradient.
 Gradual pressure gradients imply light winds.
 Closely spaced isobars display a steep pressure gradient.
 Steep pressure gradients imply strong winds.
Coriolis Force
 Deflects from a straight path
any object that flies or flows
across Earth’s surface
 Wind, airplane, ocean currents
 Deflection to the right in the Northern Hemisphere; to
the left in Southern Hemisphere
 The strength of the Coriolis force strengthens with an
increase in latitude.
Friction Force
 Drags on the wind as it moves across surfaces; up to 500m
 Varies with surface texture, wind speed, time or day and year,
and atmospheric conditions
 Since surface friction decreases wind speed, it reduces the
effect of the Coriolis force
 Anticyclone
 Winds in the Northern Hemisphere spiral out from a high-
pressure area clockwise
 Cyclone
 Spiral into a low-pressure area counterclockwise
 In the Southern Hemisphere these circulation patterns are
reversed
When the “forces” combine…
 When the gravitational force, pressure gradient force, Coriolis
force and frictional force combine we see the development of
‘predictable’ global wind patterns.
Air Masses
 Air masses are bodies of air that develop over large
areas of the Earth’s surface
 Two conditions needed:
1.
2.
Air must stay over the source region for a long period
of time so that it can take on the characteristics of that
region
Air must be stable
 Usually form where air is descending back to Earth
 These high-pressure cells line up roughly over the polar
circles and the Tropics
Your task
 Look at the defining characteristics of the 6 major air masses.
 This will be done by completing the “Characteristics of Air Masses”
chart
 You will then look at cloud formation along fronts
 Complete the “cloud development in fronts” diagram and associated
questions on handout
 Additional questions:
1.
2.
3.
4.
5.
Explain a mid-latitude storm.
Why are storms more common in the mid-latitudes than
elsewhere?
Explain the differences between continental and maritime
air masses.
Explain the role of convections currents and the rotation of the
Earth in the development of a storm.
Explain how studying jet streams could help long-range
weather forecasting.
 All info is found in Chapter 16, pages 262-268