A Simple Method
to Measure
the Dew Point Temperature
FP003 Quick Answer
R. L. Snyder, Biometeorology Specialist
Department of Land, Air and
The dew point temperature is defined as the temperature at which the
air
becomes saturated with water vapor when the air is cooled by removing
sensible
heat. It is very important in meteorology because it is directly
related to the
amount of water vapor in the air and it can be used to determine other
variables (e.g., vapor pressure, relative humidity, wet bulb
temperature, and
vapor pressure deficit) that are often used in agriculture. In
addition, the
dew point measured during nighttime is often a good first guess for the
next
morning's minimum temperature. Consequently, it is extremely important
for
freeze protection of crops. The definition of the dew point temperature
is
given in the figure below.
Nighttime Energy Balance and Inversions
During nighttime, there is a net loss of long wave radiation from the
surface
to the sky. This causes the surface to cool and sensible heat, which is
measured
with a thermometer, is convected downward from the air to the surface
to
partially replace the heat loss. However, the surface cools faster than
the air
above and this usually leads to an inversion (i.e., the temperature
increases
with height). Also, heat is conducted upward through the soil to
partially
replace the heat lost to radiation. The net radiation is relatively
constant
during the night, but the volume of air supplying heat to the surface
increases
during the night and so the net rate of heat loss of air near the
surface
decreases with time during the night. Similarly the net rate of heat
loss of
the soil layer near the surface decreases with time because heat is
being
transferred from deeper in the soil. As a result, the air temperature
drops
fast immediately after sunset, but the rate of temperature drop slows
near
dawn.
Sensible and Latent Heat
Gaseous water molecules consist of one oxygen and two hydrogen atoms.
When in a
liquid state, the water molecules form strong hydrogen bonds between
the
molecules. In order to evaporate water, heat must be supplied to break
the
hydrogen bonds and allow individual molecules to break off as a gas.
This is
what happens when water is boiled. The heat supplied by your stove
breaks
hydrogen bonds and the liquid water is rapidly evaporated. Therefore,
heat must
be supplied to break hydrogen bonds and evaporate water. If air is the
source
of the heat (called sensible heat), then the air temperature will drop
and the
energy is stored as chemical energy in the water molecules (called
latent
heat). When water vapor condenses into liquid water, the hydrogen bonds
form
again and release latent heat, which increases the sensible heat and
causes the
air temperature to rise. This exchange between latent and sensible heat
is one
of the most important factors controlling our climate and environment. Much of the heat transfer on a global scale results from
water
evaporating at the surface near the equator and condensing into clouds
and
moving poleward to redistribute the energy.
The main point is that sensible heat is removed from the air and the
temperature drops when evaporation is occurring and latent heat is
converted to
sensible heat and the temperature rises when condensation occurs.
Dew Formation
When the surface temperature reaches the dew point temperature, dew
will form.
Note that dew does not fall from the sky. Water vapor, like other
gases, moves
at sonic speeds and continually strikes the surface. When the surface
temperature is at the dew point, more water molecules will condense
onto the
surface than evaporate from the surface. Hence, dew forms when the
number of
water molecules striking the surface and forming hydrogen bonds with
other
water molecules is bigger than the number of molecules breaking
hydrogen bonds
and separating off as a gas.
The Ice-Water Can Method to Measure Dew Point
Based on the definition, a simple method to measure the dew point
temperature
involves cooling a surface until water vapor begins to condense on the
surface.
This is the principle used in a chilled mirror hygrometer, which is
used to
measure the dew point. Unfortunately, a chilled mirror uses complicated
electronics to measure the dew point and; therefore, it is expensive. A
simple,
inexpensive method involves using a shinny can, a thermometer, and ice
water as
shown below.
During low dew point, freezing conditions, it is sometimes difficult to
get the
can cold enough for condensation to occur. Adding salt to the liquid
ice
mixture will help to melt the ice and cool the water. When the
temperature is
well below freezing, sometimes frost rather than dew will form on the
outside
of the can. When this occurs, you have measured the "frost point"
rather than the dew point temperature. For the same vapor pressure, the
frost
point will be slightly higher than the dew point temperature. However,
for most
agricultural operations, there is little difference and they can be
used
interchangeably.
Conversions
The equations for converting humidity expressions are typically given
using the
metric system and degrees Celsius. To convert from degrees Fahrenheit (oF) to degrees Celsius (oC),
use the
following:
To convert from the dew point temperature (Td) in oC
to other expressions for humidity, first calculate the vapor pressure (e).
(kPa)
If you measured the frost point temperature (Tf)
in oC, then use the following:
(kPa)
To calculate the saturation vapor pressure (es)
at air temperature (T) in oC, use the following:>
(kPa)
Relative humidity (RH) is calculated as:
(%)
Vapor pressure deficit (VPD) is calculated as:
(kPa)
Last updated June 22, 2000