1. Dynamics in Earth’s Atmosphere
Weather
Dynamics in Earth’s Atmosphere

2. What Drives Air Motion?
Warm air rises and cools as it expands. Cool air falls to fill the space left by warm air. This sets up a vertical convection current.
Particles exert a force as they move and this force per unit area is called pressure.

3. Pressure in the atmosphere
This is a contour
display of pressures
at the surface.
Blues represent
lower pressures
while reds
indicate higher
pressures.

4. Where would the winds be highest on this date?

6. Global Heating and Air Circulation
Unequal heating of the Earth causes general air circulation.
Global regions receive more direct radiation and therefore have higher temperatures.
The warmer equatorial air rises and moves toward the polar regions (cooling as it goes). The colder polar air sinks and is drawn toward the equator.

8. Global Heating and Air Circulation
The fact that the Earth rotates complicates the circulation. Free-moving objects appear to deviate from straight-line paths as the Earth spins. This deflection due to Earth’s rotation is called the Coriolis effect. The effect is greatest near the poles and non-existent at the equator.

10. The Coriolis effect The Coriolis effect
Is a result of Earth’s rotation
Causes moving objects to follow curved paths:
In Northern Hemisphere, curvature is to right
In Southern Hemisphere, curvature is to left
Changes with latitude:
No Coriolis effect at Equator
Maximum Coriolis effect at poles

11. A merry-go-round as an example of the Coriolis effect
To an observer above the merry-go-round, objects travel straight
To an observer on the merry-go-round, objects follow curved paths
Figure 6-8

12. The Coriolis effect on Earth
As Earth rotates, different latitudes travel at different speeds
The change in speed with latitude causes the Coriolis effect
Figure 6-9a

13. Missile paths demonstrate the Coriolis effect
Two missiles are fired toward a target in the Northern Hemisphere
Both missiles curve to the right
Figure 6-9b

14. Global Heating and Air Circulation
Because the Earth rotates, the air circulation develops three cells in each hemisphere. These cells redistribute heat across Earth’s surface and generate weather systems.

15. Global Winds
The three cell circulation generates global wind patterns. The general winds include polar easterlies, westerlies, northeast or southeast trade winds, and doldrums (at the equator).
Wind direction is always designated as the direction from which the wind blows.

16. Also see Hewitt Figure 26.18

17. Wind Directions
In the Northern Hemisphere, upper level winds move clockwise around a high pressure system and counter-clockwise around a low pressure system.
Wind directions around high and low pressure systems are reversed in the Southern Hemisphere.

18. Pressure contour plot (blue
is lower pressure, red is higher)
Plot of wind direction-
Note that the winds are
moving counter-clockwise
around the low and clockwise
around the high region.

19. Upper Atmospheric Circulation
In the upper troposphere “rivers” of rapidly moving air circle the Earth. These high speed winds are referred to a jet streams.
The polar jet stream is a result of a temperature gradient at the polar front where warm tropical air meets cold polar air.

20. Upper Atmospheric Circulation
The subtropical jet stream forms when warm tropical air is moved from the equator to the poles, generating a sharp temperature gradient along the subtropical front.

21. The polar front and the two jet streams have an
influence on the movement of weather systems
across the globe.

22. Air masses are steered by the jet stream.

23. Jet Streams
The polar front is strongest during the winter and weakest during the summer.
During major cold outbreaks, the polar front dives south over the country. Consequently, the polar jet stream also dives south in response. The polar jet stream often aids in the development of storms and it also tends to steer the storms.

24. Jet Streams
The subtropical jet stream tends to develop during the winter season. The subtropical jet stream can also help develop and steer storms and disturbances.

25. Local Weather
Local weather systems depend on the movement of the polar front (jet streams), seasonal conditions (temperature variations), and local topography (mountains, bodies of water…).