Gale Warning in PDF file 
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The temperature of the air is measured by thermometers. Meteorological thermometers are glass tubes, containing either mercury or alcohol. They respond to temperature changes by corresponding changes in the length of the liquid column. The temperatures of melting ice and of boiling water are designated 32˚ and 212˚, respectively, on the Fahrenheit scale; 0˚ and 100˚ on the centigrade scale; 273˚ and 373˚ on the absolute scale. |
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Registering thermometers, set to the current temperature at regular intervals are time, are used to ascertain the highest and the lowest temperatures occurring between the intervals. Continuous records of air temperature are obtained by the use of thermographs. To make certain that the thermometer or thermograph assumes the temperature of the air, it is screened from direct and reflected heat as much as possible by being exposed in a shelter with louvered sides and open to the wind in all directions. |
Knowledge of temperature averages, extremes, annual and irregular variations is essential in describing the weather and climate of a region. |
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The pressure of the air is continuously variable, and its variations bear a close and causal relation to the other weather elements. A mercury barometer measures the pressure of the air in terms of the length of the column of mercury supported by the air pressure. The length is expressed in inches or millimeters; or the force corresponding to the weight of the mercury column is expressed directly in millibars. A mercury barometer must be corrected for the temperature of the mercury, the value of gravity, and instrumental errors. An aneroid barometer measures the pressure of the air directly by the compressive effect upon an elastic metal box. A barograph is an aneroid barometer that writes a continuous record of air pressure upon a drum rotated by a clock. |
 Barometer readings are reduced to sea level by calculating what the pressure at a given station of known elevation would be if it were at sea level. Records show that there are seasonal, diurnal, and irregular changes in pressure. |
Altimeters are aneroid barometers graduated to read in elevations instead of pressures. The pressure of the air decreases with increase of elevation above the surface of the earth, roughly at the geometric ratio of 1/30th of its value at any elevation for each 900 feet increase in elevation. For a more accurate relation of pressure to elevation, variations in the temperature and humidity of the air and in the force of gravity must be considered. Barometer readings are reduced to sea level by calculating what the pressure at a given station of known elevation would be if it were at sea level. Records show that there are seasonal, diurnal, and irregular changes in pressure.Altimeters are aneroid barometers graduated to read in elevations instead of pressures. The pressure of the air decreases with increase of elevation above the surface of the earth, roughly at the geometric ratio of 1/30th of its value at any elevation for each 900 feet increase in elevation. For a more accurate relation of pressure to elevation, variations in the temperature and humidity of the air and in the force of gravity must be considered. |
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The direction and velocity of air movement are important elements of weather. Wind vanes and anemometers are used in observing and recording these elements. Both direction and velocity of wind have a great, but irregular, variability from day to day, and also show many fluctuations in periods of a few seconds. The gustiness and turbulence evidenced by these minor variations are due in large part to irregularities of the surface over which they are moving, but in part to other causes. Turbulence is greater over land than over oceans and increases with velocity increases with velocity but decreases with elevation, although the velocity increases on the average up to about 1,600 feet. |
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