Connections METR100-01 4 DEC2009

Last class we presented the horizontal Pressure Gradient Force, which is the driver for all winds. We only touched on the cause of pressure gradients and did not investigate what happens near the surface of the Earth. These subjects will be covered in today’s class.

All of this week’s presentations may be found at my websitehttp://funnel.sfsu.edu/students/frankv/www/m100/

?First, however, a thought question about a vertical column of air: Earth’s gravity is acting on each parcel of air. Why don’t they fall to the ground?

?

Air is compressible. An air parcel develops pressure due to the weight of all the air molecules on top of it. As we showed two classes ago, air pressure must increase with decreasing altitude. This results in a vertical pressure gradient, and that is what overcomes the force of gravity.

The vertical forces on an air parcel are more balanced than the horizontal forces as evidenced by the fact that up and down drafts are usually much slower than horizontal wind velocities and that the extent of the troposphere is only around 10 km (6 mi) or so.

So gravity is the cause of vertical pressure gradients (which are much greater than horizontal pressure gradients.) What causes horizontal pressure gradients?

?B&W Slides cut and pasted from a Metr100-02 concept map handout by Dr. Dempsey

Divergence

&

convergence

aloft

lead to:

•The fastest winds tend to occur where the PG is largest, which is also where the isobaric surfaces slope the most steeply (which is where height contours on an isobaric map are closest together).

•The height of an isobaric surface aloft depends on the (average) temperature below that isobaric surface.

–Colder air in the lower troposphere creates lower heights (lower pressure) aloft.

–Warmer air in the lower troposphere creates higher heights (and higher pressure) aloft.

Wind and Pressure Patterns

• On horizontal surfaces (such as at sea level), pressure varies from place to place.– Maps with isobars drawn on them help us visualize

the spatial pressure pattern.• Pressure differences between places create a net force—

the pressure-gradient (PG) force--on air, pushing toward lower pressure. The PG force pushes air into motion.– The strength of the PG force is greater where the

pressure gradient (PG) is larger.– On a weather map, the spacing of isobars allows us

to tell about the relative size of the PG and hence the PG force.

•Once air is moving, the rotation of the earth affects the motion by apparently trying to deflect it. We account for this effect by inventing a Coriolis force.

–The Coriolis force is stronger when the wind is faster.

–The Coriolis force pushes on moving objects (including air) to their right in the N. Hem. and to their left in the S. Hem., but not at all at the equator.

When the travel time of an air parcel is short (for instance local thunderstorm, land/sea breeze, or water flowing down a drain) Coriolis Force may be neglected, however it cannot when the time scale is in hours. The concept map to the right is modified for the Coriolis force.

•Together, the PG force and Coriolis force tend to drive the wind close to geostrophic balance.

–A wind where the balance is exactly achieved is the geostrophic wind.

–The observed winds aloft are usually close to the geostrophic wind.

•Winds aloft tend to blow toward the east, northeast, or southeast.

Friction with the Earth’s surface• Near the earth’s surface, friction is a third

important force (within the “friction layer”). – Friction opposes the wind, trying to slow it

down.– Friction is larger over land than over water.

(Land is “rougher” than water.)• The 3-way combination of PG force, Coriolis

force, and friction drives winds across isobars at an angle.

•As a result, surface winds tend to converge into low-pressure areas and diverge out of high-pressure areas. (We don’t see this aloft, though!)

•As a result, air tends to move upward out of surface low-pressure areas and sink (subside) into surface high-pressure areas.

•Regardless of the combination of forces acting on air, winds tend to be faster where the PG (and hence PG force) is stronger.

Cold

Core

Low

(Height

variation

expanded)

Cold

Core

High

weakens

Warm

Core

Low

weakens

Warm

Core

High

strengthens