METEOROLOGY — FORECASTING 257 
in which the units of measurement used are the same 
as above. The rate at which M increases with altitude 
is given by 
dM (dn, 1 . 
an (5; : i) wy 
which in the standard atmosphere is 
dM 
—— = —=() 15 
ah 0.039 + 0.157 
= 0.118 M unit per meter 
= 0.036 Mf unit per foot. 
Psychrometric Nomogram. Radiation Laboratory 
(MIT) has developed a nomogram with which to 
compute the modified index of refraction from the 
necessary meteorological parameters. This chart is 
known as the psychrometric nomogram. (See p. 255.) 
Mrrroronocican TERMS 
Absolute Humidity. The mass of water vapor pres- 
ent in a unit volume of air is known as the absolute 
humidity of the air. It is another way of expressing 
the water vapor density. 
Specific Humidity. The specific humidity of moist 
air is the ratio of the weight of water vapor mixed 
with the air to the weight of the. moist air. If p is the 
barometric pressure and e is the partial pressure of 
the water vapor, then the specific humidity is given by 
q = 622 
e 
p — 0.377e BEB o 
Mixing Ratio. The ratio of the mass of water vapor 
mixed with unit mass of perfectly dry air is known 
as the mixing ratio and may be expressed as 
e 
= 622 —— kg. 
w Reet g 
Relative Humidity. The ratio of the actual water 
vapor pressure to the saturation vapor pressure at the 
same temperature is known as the relative humidity 
of moist air. If e and e, are the respective vapor pres- 
sures, then (in per cent) the relative humidity is 
expressed as 
RH = = x 100. 
Wet Bulb Temperature. The lowest temperature 
to which a wetted ventilated thermometer can he 
brought by evaporation is called the wet bulb tempera- 
ture. It is not strictly an air temperature. 
Air Mass. An extensive body of air which approxi- 
mates horizontal homogeneity is known as an air 
mass. The four principal types are illustrated by the 
accompanying table. 
Moisture Thermal 
Source classifi- _classifi- 
region _ cation cation Name Symbol 
Land Dry Hot Tropical continental cT 
Cold Polar continental cP 
Oceanic Wet Hot Tropical maritime mT 
Cold Polar maritime mP 
Front. The surface of separation between dissimilar 
air masses is known as a frontal surface. On a surface 
weather map a “front” is the intersection of this sur- 
face with the surface of the earth. 
Dry Adiabatic Lapse Rate. When dry air ascends 
so as to expand adiabatically, it is said to cool at the 
dry adiabatic lapse rate (5.5 F per 1,000 ft or 1 C 
per 100 m). There must be no condensation or evapora- 
tion of associated water vapor during the process. 
Subsidence. An extensive sinking process, result- 
ing in dynamically heated air and an increase in 
stability, most frequently observed in anticyclones, is 
known as subsidence. 
InstRucTIoNS ror Usr or Nomocram 
This nomogram (Figure 23) may be used to com- 
pute M when temperature is expressed in degrees 
Fahrenheit or centigrade, humidity in terms of wet 
bulb (degrees Fahrenheit or degrees centigrade), dew 
point (degrees centigrade), or vapor pressure (milli- 
bars), and height in feet or meters. Place a straight- 
edge so as to align the temperature on scale 1 with the 
wet bulb temperature on scale 3 (or with the dew point 
or vapor pressure on scale 4). The point at which the 
straightedge intersects scale 6 indicates the value 
of the modified index uncorrected for height. Pivot 
the straightedge at this point (on scale 6) so that 
it crosses scale 2 at the desired height. Then read the 
value of M where the straightedge crosses scale 5. 
A 
