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 only a certain amount of water vapor. If more and more water vapor is added to the air, the saturation
point eventually will be reached and some of the water vapor will condense, or become liquid. The
condensation takes the form of dew, rain, or other precipitation. Relative humidity is the ratio of the
amount of water vapor in the air to the total amount that the air can hold at the saturation point, or 100
percent humidity.
(2) The warmer the air is, the more water vapor it will hold. Consequently, cooling a volume of air will
reduce its capacity to hold water vapor. If the cooling is continued, the dew point (the temperature at
which moisture suspended in the atmosphere will begin to form dew) will be reached, and the water vapor
will condense. Readings taken from a psychrometer are used to compute relative humidity and dew point.
(3) There are two types of instruments used aboard ship to determine relative humidity and the dew
point. These two instruments look different, and a different method is used to get a reading, but both
instruments will give you the same results.
(a) A hygrometer consists of two thermometers mounted vertically in a ventilated case or box. One
thermometer, known as the dry bulb, has a mercury bulb exposed directly to the air. The other
thermometer, known as the wet bulb, has a bulb covered with muslin. In use, the muslin is stretched
tightly around the bulb and kept moist by a wick immersed in a small cup filled with water. The wick
consists of a few threads of cotton long enough to allow 2 or 3 inches of it to be coiled in the cup. The
muslin is kept thoroughly moist, but not dripping, at all times.
(b) A sling psychrometer also consists of two thermometers. They are mounted together on a single
strip of material and fitted with a swivel link and handle.
1 One thermometer is mounted a little lower than the other is, and has its bulb covered with
muslin. When the muslin covering is thoroughly moistened and the thermometer well ventilated,
evaporation will cool the bulb of the thermometer, causing it to show a lower reading than the other
thermometer. With the sling psychrometer, ventilation is caused by twirling the thermometers by using the
handle and swivel link. The dry-bulb temperature is the reading shown by the uncovered thermometer,
and the wet bulb temperature is shown by the muslin-covered thermometer.
2 The dry-bulb thermometer records the temperature of the free air. The wet-bulb thermometer
records what is known as the temperature of evaporation, which is always less than the temperature of
free air.
NOTE
The difference between the temperature readings of the dry-bulb and the wet-bulb shows how
near the air is to a state of saturation.
(4) When the wet and dry-bulb temperatures are known, the relative humidity of the atmosphere may
be found by referring to the table for determining relative humidity. The table may be readily understood
by reviewing the following example.
Assume the temperature of the air (dry-bulb) is 60 and the temperature of evaporation (wet-bulb) is 56;
the difference is 4. Look in the column headed "Temperatures of the air;" find 60 and follow the same
horizontal line across to the column headed "4 ." Here the figure "78" will be found. This means that the
air is 78 percent saturated with water vapor. The amount of water vapor present in the atmosphere is 78
percent of the total amount it could carry at the given temperature (60). The total amount, or saturation,
is represented by 100. Any increase in the amount of vapor beyond this point would show in the form of
mist or rain. The relative humidity over the ocean's surface is generally about 90 percent. Because of this
increased moisture, the relative humidity at sea level is normally higher than that cited in the above
example.
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