The Atmosphere

Air is a mixture of gases and water vapour. A sample air contains oxygen, nitrogen, argon, carbon dioxide, and traces of hydrogen and inert gases. Air also contains a variable amount of water vapour and impurities such as dust, minute salt particles, etc.

Science divides the atmosphere into five main layers. They are known as the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The upper limit of the troposphere is called the tropopause.

Atmospheric stability is dependent on lapse rates – the rate of temperature decrease. The dry adiabatic lapse rate is the rate of cooling of dry air rising without any transfer of heat. If the air continues to rise, condensation of the water vapour will occur and the latent heat of condensation of the water will offset the adiabatic cooling.

Three possible conditions can be clearly distinguished: 1) absolute stability, 2) absolute instability, 3) conditional instability.

Atmospheric pressure is defined as the force exerted, by the weight of the atmosphere on a unit area. A line can be drawn connecting equal values of pressure. Such a line is called an isobar. Isobar on a weather map shows some areas of relatively low pressure and other areas of relatively high pressure. The four types of pressure areas are: 1) low depression, or cyclone, 2) high, or anticyclone, 3) trough, 4) ridge.

The instruments commonly used in the measurement of atmospheric pressure are: 1) the mercury barometer, 2) aneroid barometer, 3) the barograph.

One of the most important factors of the atmosphere is moisture, which is found in three states: solid, liquid, and gaseous. As a solid it takes the form of snow, hail, sleet, frost, ice-crystal clouds, and ice crystal fog. As a liquid, pilots encounter it as dew, drizzle, rain and fog. In the gaseous state, water forms an invisible vapour and water vapour is the basis for all clouds.

Relative humidity is usually measured with a hair hydrometer or sling psychrometer.

Wind is moving air. In simplest terms wind is caused by three things: 1) different regions of pressure, 2) the earth’s rotation, and 3) friction.

Clouds are countless millions of minute water droplets or ice particles. These microscopic specks have a central core of dust, ash, salt, or smoke (called nuclei).

Clouds may be classified according to appearance, the way they are formed or the height of their bases. There are special clouds which are of interest to aviators-tornadoes, waterspoute, contrails, snowstorms and dust storms.

The study of air masses, and more particularly their interaction, is called synoptic meteorology.

Front of frontal zone is defined as an interface or transition zone marking the boundary between two different air masses. There are several types of front with which the aviation should be familiar: the polar front, the cold front (both slow moving and fast moving), the warn front, the stationary front, and the occluded front.

The birth of new fronts, the regeneration of old, weak, and decaying fronts is called frontogenesis. The opposite process, the decaying and dissipation of existing fronts is called frontolysis.

Answer the questions.

1. What does air contain?

2. What does atmospheric stability depend on?

3. Could you name possible atmospheric conditions?

4. How many states of moisture are there in nature?

5. What forms does solid moisture take?

6. What liquid forms of moisture can pilots encounter?

7. What is cloud? What does cloud consist of?

8. What special clouds should be of interest to aviators?

9. How is the birth of new fronts called?

10.What does meteorology study?

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