GEOG220 Lecture Notes - Lecture 20: Coriolis Force, Northern Hemisphere, Hydrostatic Equilibrium
28 Jul 2018
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04.19.2018
Winds at high altitudes…
- The PGF and the Coriolis Force working together… (no friction)
- If you’re up a 1000m or above in the atmosphere, you don’t need to worry about friction
anymore.
- The wind that results from a balance of CF (Coriolis Force) and the PGF is called the
Geostrophic Wind. In the northern Hemisphere the geostrophic wind blows with lower
pressure to the left and higher pressure to the right of the wind direction.
Upper-Level Weather Maps
- Stuff on Map
o Black lines –mb heights
▪ All have an extra 0 at the end of the number
o Colors – wind speeds
o Wind symbols
- Solid Lines on upper-level maps are called height lines. These lines tell you how high
you need to go in the atmosphere to find a particular pressure (850 mb, 500 mb, 250 mb,
etc.) These lines are equivalent to isobars (lines of constant pressure).
o 500 mb are the pressure that we will be looking at the most in this class.
- At upper levels of the atmosphere the winds blow parallel to the lines of constant height
(constant pressure). Therefore, it is very easy to deduce the direction and the relative
speed of the winds just by looking at an upper level chart.
o These upper-level winds are Geostrophic Winds!!
- In an area where the upper-level winds blow primarily from the west toward the east, we
say there is a zonal flow.
o When the upper level winds blow in large, looping, meanders in northeast,
southeast directions the flow is called meridional.
Winds near the earth’s surface…
- Enter friction
o At upper levels of the atmosphere there is no friction so you get geostrophic
balance and geostrophic winds….
- Near the earth’s surface you must also take into account frictional forces. Thus, you
cannot have geostrophic winds. Instead, you get a balance between PGF, CF, and
Friction… Surface winds
- Surface winds
o Near the surface, friction reduces the wind speed
o If the wind speed is reduced, the Coriolis force is weaker.
o If the CF is weaker, it can no longer balance the PGF and the winds will cross
the isobars in the direction of the PGF (toward low pressure).
o The PGF will ultimately be balanced by the sum of the Fric and CF
▪ At the surface winds cross the isobars toward low pressure and away from
high pressure.
o The angle at which the winds cross the isobars depends upon the roughness of
the earth’s surface. The rougher the surface, the slower the wind speed for a
given PGF and the large the crossing angle.
Horizontal Winds and Vertical Motions
Document Summary
The pgf and the coriolis force working together (no friction) If you"re up a 1000m or above in the atmosphere, you don"t need to worry about friction anymore. The wind that results from a balance of cf (coriolis force) and the pgf is called the. In the northern hemisphere the geostrophic wind blows with lower pressure to the left and higher pressure to the right of the wind direction. Stuff on map: black lines mb heights, colors wind speeds, wind symbols, all have an extra 0 at the end of the number. Solid lines on upper-level maps are called height lines. These lines tell you how high you need to go in the atmosphere to find a particular pressure (850 mb, 500 mb, 250 mb, etc. ) These lines are equivalent to isobars (lines of constant pressure): 500 mb are the pressure that we will be looking at the most in this class.
