Page:Handbook of Meteorology.djvu/266

254 The cubic centimeter is the unit of volume. 1 cc = 0.0000353 cu ft = 0.061 cu in.

The unit of density is numerically the same as the specific gravity. It is the rate of 1 gram per cubic centimeter.

The unit of velocity is the velocity of 1 centimeter per second; 1 cm per second = 0.0328 foot per second = 0.0224 m. per hour.

The unit of acceleration is the rate of a unit of velocity per second or 1 centimeter per second, per second.

The unit of force, 1 dyne, is the force which imparts to 1 gram a velocity of 1 centimeter per second, per second. It is sometimes expressed in the term poundals; 1 dyne = 0.0000722 poundal. The acceleration of gravity of a falling body varies in different latitudes because the earth is spheroidal and not globular in form. In latitude 45°, the latitude to which results commonly are reduced, the acceleration is 980.621 centimeters per second, per second. If the relative value at latitude 45° be taken as a unit, the relative value at the equator will be 0.9974; at the pole 1.0027.

The unit of pressure is 1 dyne per square centimeter. The megadyne is a more convenient unit and generally is used. The megadyne is commonly called 1 bar. Barometric pressure is read in kilobars and its subdivisions in millibars. 1 bar = 1000 millibars = 0.001 kilobar. 1 kilobar = 0.03386 inch; 1 inch = 33.864 millibars; 1 millimeter = 1.333 millibars. The barometric pressure at 29.53 inches =1000 millibars, which is equivalent to an altitude of 338 feet above mean sea level. The dyne may be conceived as the pressure upon the hand—that is, the weight—of a piece of very thin tissue paper 1 centimeter square.

The unit of rainfall is 1 millimeter. For all practical purposes, the reduction of hundredths of an inch in rainfall to millimeters of rainfall is effected, by moving the decimal point two places to the right and dividing by 4. Thus, if the catchment of a storm is 2.40 inches; the division gives 60 millimeters. A more accurate result will be obtained by using 3.94 as the divisor. Multiplying by .04 reduces millimeters of rainfall to inches of rainfall.

The practical unit of wind velocity is 1 meter per second—that is, 100 times the C. G. S. unit.

The practical unit of wind force is 1 kilodyne per unit of area.