Chapter 6 Review: Air Pressure and Winds

List as many ways as you can of determining wind direction and speed.

trees, flags, wet finger, wind instruments

What is considered standard sea-level atmospheric pressure?

1013.25 mb @ 650F or 29.92 in Hg

If the clouds overhead are moving from north to south, would the upper-level center of low pressure be to the east or west of you?

• With your back to the wind, the low is on the left and the high is on the right. • East

How does an aneroid barometer measure atmospheric pressure? How does it differ from a mercurial?

• An aneroid contains a silphon cell that has a partial vacuum in it. Any changes in pressure outside of the cell causes the cell to expand or contract. • It differs from a mercurial in that it does not have any liquid.

What do Newton's first and second laws of motion tell us?

• Law 1: a body in motion tends to stay in motion, a body at rest tends to stay at rest unless acted upon by an equal and opposing force. • Law 2: F = ma

What does the Coriolis force do to moving air in the Northern Hemisphere? Southern Hemisphere?

• Northern Hemisphere - causes the wind to veer to the right. • Southern Hemisphere - causes the wind to veer to the left.

How does sea-level pressure differ from station pressure? Can the two ever be the same?

• Sea level pressure is pressure reduced to sea level. • Station pressure is the pressure of the station at it's particular elevation. To convert it to sea level, apply an altitude correction. • The two can be the same if the station altitude is zero.

Since there is always an upward-directed pressure gradient force, why doesn't air rush off into space?

• The vertical motion is not strong enough to exceed the escape velocity imposed by gravity. • Called hydrostatic equilibrium

How does a mercurial barometer work?

• Tube filled with mercury, inverted and placed in a pot of mercury. • Weight of air pressing down on pot increases or decreases, moving the mercury up or down the tube.

Explain how each of the following influences the Coriolis force: • Wind speed • Latitude

• Wind speed - the stronger the wind, the greater the deflection • Latitude - zero at the equator to a maximum at the poles

Below is a list of instruments. Describe how each one is able to measure wind speed, wind direction, or both.

• Wind vane - direction only; points into wind • Cup anemometer - speed only; cups spin and speed is registered • Aerovane - direction and speed; points into wind, spinning propeller measures speed. • Radiosonde - calculated geometrically from speed and direction of balloon • Satellite - measures movement of clouds • Wind profiler - measures vertical profile of wind speed and direction from movement of rain drops.

What does a steep (or strong) pressure gradient mean? How would it appear on a surface map?

A steep pressure gradient means a big change in pressure over a short distance. It would show up on the surface map with the isobars drawn close together.

How does a gentle (or weak) pressure gradient appear on a surface map?

A weak pressure gradient appears on the map as isobars spaced far apart.

Describe how the wind blows around high-pressure areas and low-pressure areas aloft and near the surface in: • Northern Hemisphere • Southern Hemisphere

Aloft - parallel to isobars • NH Lo - counterclockwise • NH Hi - clockwise • SH Lo - clockwise • SH Hi -- counterclockwise Surface - across isobars • NH Lo - counterclockwise into • NH Hi - clockwise out of • SH Lo - clockwise into • SH Hi - counterclockwise out of

Explain why, on a map, closely spaced isobars (or contours) indicate strong winds, and widely spaced isobars (or contours) indicate weak winds.

Closely spaced isobars means a strong pressure gradient. This results in a stronger pressure gradient force and hence a stronger wind speed. Since F=ma, the "a" will be bigger if the slope or gradient is greater - gravity will pull it down hill faster.

On an upper-level map, is cold air aloft generally associated with low or high pressure? What about warm air aloft?

Cold air aloft is associated with a low pressure, warm air aloft is associated with a high pressure.

Why is the decrease of air pressure with increasing altitude more rapid when the air is cold?

Cold air is dense, hence the molecules crowd closer to the surface.

On a surface map, why do surface winds tend to cross the isobars and flow from higher pressure toward lower pressure?

Friction

What is a geostrophic wind? On an upper level chart, how does it blow?

Geostrophic wind is a wind blowing parallel to the isobars. Geostrophic winds blow parallel to the isobars.

Would a sea-level pressure of 1040 mb be considered high or low pressure?

High

Why will Denver, Colorado, always have a lower station pressure than Chicago, Illinois?

It is at an altitude of 1 mile.

What are isobars? In what increment are they usually drawn on a surface weather map?

Lines of equal pressure. They are drawn for every 4 mb.

What is the name of the force that initially sets the air in motion, and, hence, causes the wind to blow?

PGF

Why do upper-level winds in the middle latitudes of both hemispheres generally blow from west to east?

PGF

What are the forces that affect the horizontal movement of air?

PGF, coriolis, and friction if in the boundary layer.

An upper wind direction is reported as 225o. From what compass direction is the wind blowing?

SW

What might cause the air pressure to change at the bottom of an air column?

The column is heated and the air is now lighter than the surrounding air, so it rises causing the pressure to drop at the surface.

Why does atmospheric pressure always decrease with increasing altitude?

There are fewer molecules, hence the pressure is less.

How does zonal flow differ from meridional flow?

Zonal flow is wind blowing from west to east. Meridional winds blow from north to south or along the Meridians (longitude lines).