WET-BULB TEMPERATURE
The wet-bulb temperature is the lowest temperature
an object may be cooled to by the process of
evaporation. It is read directly from the wet-bulb
thermometer on an electric psychrometer, sling
psychrometer, or rotor psychrometer. Water
evaporating from the moistened wick on the wet-bulb
thermometer bulb cools the thermometer bulb and
lowers the temperature reading. The cooling effect of
the evaporation from the bulb is inversely proportional
to the amount of water vapor present in the air: the more
water vapor present, the less moisture will evaporate
from the moistened wick, and the less cooling of the
thermometer bulb will occur. From the dry- and wet-
bulb readings, the dew-point temperature and humidity
values may be calculated. The automatic weather
observation systems do not provide a wet-bulb
temperature, but automatically process equivalent
measurements to compute dew-point temperature.
TF =
9
TD
10
DEW-POINT TEMPERATURE
The dew-point temperature is the temperature a
parcel of air must be cooled to in order to reach
saturation.
Cooling past the dew-point temperature
normally results in condensation or precipitation.
Changes in temperature do not alter an air-parcel's dew-
point temperature; therefore, dew-point temperature is
termed a conservative property. The extraction or
addition of moisture, however, from or to an air parcel
will respectively decrease or increase the dew-point
temperature.
Dew-point temperature is calculated from the dry-
bulb temperature and the wet-bulb depression by using
the CP-165/UM psychrometric computer. The wet-
bulb depression is the difference between the dry-bulb
temperature and the wet-bulb temperature. Dew-point
temperature is automatically calculated by the
automatic weather systems.
Many calculations that you will be using call for a
dew-point depression as a value. The dew-point
depression is the difference between the air temperature
and the dew-point temperature, expressed as a positive
number. For example, if the air temperature is 78°F and
the dew-point temperature is 67.5°F, the dew-point
depression is 10.5°F.
FROST-POINT TEMPERATURE
The frost-point temperature is the temperature,
below freezing, that a parcel of air must be cooled to in
1-41
order to reach saturation. Cooling past the frost-point
temperature normally results in sublimation of ice
crystals from the air. The frost-point temperature is
occasionally referred to as "the dew-point temperature
with respect to ice."
Calculations using the "low"
temperature side of the CP-165/UM psychrometric
computer refer to use of the "Ti" scale when the wet-
bulb thermometer wick is frozen, and to the "DP" scale
if the wet-bulb wick is not frozen. Both scales calculate
a dew-point temperature with respect to liquid water,
and not a frost-point temperature.
The frost-point
temperature may be approximated by the following
formula:
where TF is the frost-point temperature, and TD is the
dew-point temperature.
As an observer, you may be asked to calculate frost-
point temperatures, especially when working with a
Skew T, Log P diagram. Although the frost-point
temperature is not usually computed for a surface
aviation weather observation, we have introduced it at
this point because it is so closely related to the dew-
point temperature. Relative humidity, and other
humidity computations derived from temperature and
dew-point temperature readings are covered later in this
chapter.
SEA SURFACE TEMPERATURE
Another temperature reading in shipboard weather
observations is the sea surface temperature. It is
supposed to reflect the temperature of the upper few
inches of the sea surface. On some ships with OA
divisions, installed sensors automatically measure this
value. There are three other acceptable methods for
obtaining a sea-surface temperature reading: the
bucket temperature method; by expendable
bathythermograph; and by use of the seawater injection
temperature. The sea-surface temperature reading must
be accurate since it is a major input into many undersea
warfare (USW) acoustic products.
Bucket Temperature
The bucket temperature method is by far the most
accurate, yet is also the most work intensive. In this
method, a sample of seawater is obtained by casting a
lightweight bucket or coffee can with a strong line
attached over the side of the ship and retrieving a water
sample. This should be done as near to the bow of the
