The heating of the earth, as we have seen in Session 2, is uneven. As parts of the earth are heating, causing the air near the surface to warm, expand, and rise, other areas are cooling. The region vacated by warm rising air becomes an empty pocket in the atmosphere. This empty space is an area of low pressure or low density because there are few molecules left to occupy the space. Air, like all other fluids, naturally moves from areas of high pressure to low pressure, thus the air near the empty pocket rushes in to fill the void. This movement of air from high pressure to low pressure is what we call the wind.
Two important measurements of the wind are the direction and speed of the wind. Wind speed is generally recorded in knots (kn) or nautical miles per hour where one knot is equal to 1.15 mi/hr. For making computations, however, knots must be converted to meters per second (m/s). One knot is equal to 0.51 m/s. Wind directions are not given in reference to the direction in which they are blowing, but rather the direction from which they are blowing. A westerly wind blows from west to east. A northerly wind blows from north to south. The simplest way to measure the wind direction and speed at the earth's surface is with wind cups and vanes. The vane gives the direction while the cup catches the wind and rotates giving an indication of speed. Wind speeds and directions at the surface and aloft help us to predict where and how fast weather systems will move and pollutants will be transported. We can also infer from the winds whether pollutants will be mixed or deposited. Light winds cannot keep heavy particles aloft while even the slightest breeze may keep lighter particles suspended indefinitely.
In this brief overview of temperature, moisture, pressure, and wind, we have seen how each of these parameters play an important role in predicting weather phenomenon as well as the transport, dispersion, and deposition of pollutants. We have also seen how these properties of the atmosphere can influence each other. For example, heating at the surface causes convection or vertical motion by altering the temperature and pressure of the air at the surface. This rising air produces horizontal motion (wind) as air rushes in to fill the the void left by the rising air. As the rising air cools, the condensation of water vapor releases heat which, in turn, increases the instability of the surrounding air, and so on.
While the parameters we have discussed so far can be directly measured, there are many others that we can calculate from them which provide us with even greater detail of the atmosphere. These will be the topic of our next section.
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