WINDS and WEATHER

The Creation of Wind

Wind blows in response to horizontal changes in pressure. Although we are usually most interested in the wind itself, it turns out that it is often easier to forecast the wind (and even to estimate its spped once we are just above the earth's surface) by considering the pressure. So we will first look at pressure and then at wind.

Pressure and Pressure Distributions

Atmospheric pressure can be thought of as the weight of the air pressing down from above. The amount of air above a place varies with time, and varies from place to place. There are two main causes of this:

1 - Thermal causes - cold air tends to be associated with high pressure, hot air with low pressure

2 - Dynamic causes - descending air tends to be associated with high pressure, ascending air with low.

A complex series of interactions, involving global energy exchanges, water circulation, earth rotation and the distribution of land and ocean, has led to the general pattern of pressure across the globe. The patterns in turn influence the wind, which has an impact on the patterns.

Pressure patterns are depicted on these - and other similar - maps, using isobars, lines joining places of equal atmospheric pressure.

Forces Creating Wind

Wind speed and direction is controlled by a series of forces:

1 - Pressure gradient force - this is the true 'driving force' of the wind. It is created by horizontal variations in pressure and starts to drive the wind from regions of high pressure to regions of low pressure. However, the other forces intervene to alter this simple relationship.

2 - Coriolis force - this is an apparent force, a result of the earth's rotation. Some distance about the earth's surface the air doesn't 'know' that the earth is rotating and so the air carries on in a straight line in space, the earth rotating under it. To us, observers fixed to the rotating earth, it looks as if the air flow is curved. A force is needed to reconcile the two different perspectives, Coriolis does it. The needed reconciliation decreases as we get closer to the Equator and as we get closer to the surface of the earth.

Poleward of 30^o N & S the full Coriolis effect is felt. Away from the surface there are only the pressure gradient and Coriolis forces acting to move and deflect the air. The result is the geostrophic wind. This is parallel to isobars.

Equatorward of 30^o N & S only a partial Coriolis effect is felt. While, once again, away from the surface there are only the pressure gradient and Coriolis forces acting, the result now is that the air flows, at an angle to the isobars, towards low pressure. The major way we see this result is through the Trade Winds, discussed below.

3 - Frictional Force - near the earth's surface friction slows down the wind. The result is that the wind blows, as with the Trade Winds, at an angle across the isobars towards low pressure.

With the frictional force included, we have the surface wind, the one we feel each day. The speed is lower and the direction is somewhat different from that of the upper level winds (which we might think of as the ones which bring in our weather).

Sometimes we can see this effect, when the upper clouds are moving in a different direction from that of the surface wind - although there are numerous other reasons why the wind direction may change with altitude.

4 - Centrifugal force - whenever the isobars are curved, the centrifugal force acts, tending to decrease the speed below that of the geostrophic wind when we have counter-clockwise (cyclonic) motion, increase it above geostrophic with clockwise (anticyclonic) motion.

The often slight wind speed differences arising from curved isobars can be very important when considering vertical motions. When discussing clouds we talked about various ways in which air can be forced to rise, but we did not really consider the causes of some of them - especially the convergence and frontal situations. Convergence near the surface, for example, can be a result of divergence aloft, that divergence being caused by a change in wind speed because the air is flowing around isobars which curve first in one direction, then in another.

Mid-latitude Weather

N.B. This deals with the 'theoretical' aspects, with some references to North Carolina. However, we shall deal with North Carolina's weather and climate in class 16.

Note that we are ignoring the weather and climate of a great deal of the world by concentrating just on the mid-latitudes. In many ways this can give a misleading picture, since all parts of the atmosphere are interconnected. We just note that interconnection here.

The schematic on the right emphasizes the interconnection - it links with the global distribution of pressure (and to a great extent, with that of temperature and precipitation)

Mid-latitude Weather: In many ways we can think of the mid-latitude atmosphere as a river of air moving from west to east, with flexible 'banks' at about 30^oN and 60^oN. Like a river, there are areas, which may vary with time and location, with relatively calm conditions, or even stagnant pools. In the atmosphere these form the source regions for air masses. Away from these is the main current of the air, meandering like a river, but being more flexible in that there are no fixed banks. This we here simply call The Westerlies. It is composed of several elements, the meandering main current being variously (and interchangeably) called the long waves, planetary waves, or Rossby waves. Associated with these are the jet streams, the Polar Front, and many cyclonic storms.

Air Masses

An air mass is a region of air with horizontally uniform temperatures. This also implies (for reasons which need not concern us here), that there is also horizontally uniform humidity, that winds are generally light, and that the wind does not change direction much wiht height. Although these are by no means 'rigid' blocks of air, it is often useful to think of then as behaving something like a block - perhaps as a block of Jello.

Air masses form over source regions, areas where there are relatively calm conditions for several days, allowing the air sitting above the surface to take on the characteristic of that surface. Theoretically, anywhere on earth can be a source region, but there are in fact a limited number of areas which are favorable for air mass development.

General Character

North Carolina Frequency

Common in winter

Rare

Rather rare

Dominates summer

Common in winter

The Westerlies

As the name implies, the wind is generally from the west, although this is not going to be true on all days, and the direction itself must be treated in very general terms.

Jet Streams

A jet stream is a thin, flat ribbon of very fast moving air - usually several thousand kilometers long, a few kilometers wide and a few meters deep. Because it is a fast airstream embedded in an environment which is much slower, it tends to swing from side to side - or even split into several streams.

There are several types of jet stream, the most important for us is the Polar Front Jet, which occurs above the Polar Front:

Polar Front

A Front is a boundary between to blocks of air of different temperature. IN general the air of the warmer block rises over the cooler one, often giving cloud and possible rain. (We met fronts earlier, when considering ways in which air can rise) The Polar Front is a major example of a front, separating two air masses, one Tropical, one Polar. Commonly it extends for a thousand miles or so across the surface of the earth, separating warm from cold air.

Depressions

A depression - also known as a wave cyclone - is an area of low pressure with counter-clockwise winds. Converging and ascending air is common, giving clouds and rain.

For North Carolina these depressions often form on or in association with the Polar Front somewhere west of the Mississippi River and move into our area from the west.

Fronts are usually associated with a depression. When they arrive from the west a warm front - where warm air REPLACES cold air- arrives first. There is usually a gentle rise of the warm air up the frontal surface, giving horizontal (or very slightly "tilted") clouds - all diagrams of fronts greatly exaggerate the steepness of the slope of the frontal surface. Gentle rain the the characteristic of the warm front. Once this has passed there is likely to be a period of warm air and maybe cloudless skies, before the cold front approaches from the west. This is often indicated by a line of towering thunderclouds, heralding a squall line associated with the front. Thunder, and short intense showers, are not uncommon with this front. It passes, and cool cloudless air returns.

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