Whilst ambient temperature is the most commonly presented weather temperature there ar several other forms of
temperatures that are commonly quoted to describe the current weather:
Popularly referred to as wind chill factor, Wind Chill is the perceived decrease in air temperature felt by
the body on exposed skin due to the flow of cold air and is more commonly used in regions where there is
a risk of frost-bite exposure
Wind chill temperatures are always lower than the air temperature for values where the formula is valid.
Where Twc is the wind chill index, based on the Celsius temperature scale,
Ta is the air temperature in degrees Celsius (°C), and V is the
wind speed at 10 metres standard anemometer height, in kilometres per hour (km/h).
When the apparent temperature is higher than the air temperature, the heat index is used instead.
The heat index (HI) or humiture or humidex is an index that combines air temperature and relative humidity
in an attempt to determine the human-perceived equivalent temperature — how hot it feels. The result is also
known as the "felt air temperature" or "apparent temperature". For example, when the temperature is 32 °C
with very high humidity, the heat index can be about 41 °C.
C1 = -41.3216667, C2 =3.26324648, C3 =5.63518404,
C4 =-0.357943801, C5 =-3.12176486 10-2,
C6 =-3.04539833 10-2, C7 =5.60972904 10-3,
C8 =1.35819481 10-3, C9 =-9.08520988 10-6
The above formula for approximate HI is only valid for temperatures above 27°C with a relative humidity
of 40% or higher.
Physical Effects of Heat Index
|27 to 32 °C
Physical activity could lead to heat cramps.
|32 to 40 °C
Possible heat cramps and exhaustion.
Physical activity could lead to heat stroke.
|40 to 54 °C
Likely heat cramps and exhaustion.
Probable heat stroke.
|above 54 °C
Imminent heat stroke.
The dew point is the temperature below which the water vapour in a volume of humid air at a given constant
barometric pressure will condense into liquid water at the same rate at which it evaporates. Condensed water is
called dew when it forms on a solid surface.
The dew point is a water-to-air saturation temperature and is associated with the relative humidity.
A high relative humidity indicates that the dew point is closer to the current air temperature so that at a
relative humidity of 100% the dew point is equal to the current temperature and that the air is fully saturated
with water. When the dew point remains constant and temperature increases, relative humidity decreases
The, saturated vapour pressure Ps in Pascals can be calculated from the temperature T (°C) using the formula below:
Reference: Tetens, O., 1930: Uber einige meteorologische Begriffe. Zeitschrift fur Geophysik,
Vol. 6:297. There are more accurate formulæ given in standard reference works, such as the
Smithsonian Tables, but this version is adequate for all but high accuracy laboratory studies.
The next step is to calculate the temperature at which Pa would be the saturation vapour pressure. Using
the relationship beteen Pa, Ps and Relative Humidity (RH) we can equate the Pa
pressure to the vapour pressure at the dew point (Td)
Rearranging we abtain the following relationship for dew point
An alternative formula for calculating dew point: Set x = (1 - 0.01 x RH) where RH is the relative humidity expressed as a percent (a number between 1 and 100). eg If the relative humidity is 38 percent, x = 0.62.
DPD = (14.55 + 0.114T)x + ((2.5 + 0.007T)x)^3 + (15.9 + 0.117T)x^14
where T is the temperature in degrees Celsius.
This calculation yields the difference between the temperature and dew point in degrees Celsius.
Finally, compute the dew point:-
TD = T - DPD.
The answer is in degrees Celsius.
The psychrometer, or wet and dry bulb thermometer
The psychrometer is the nearest to an absolute method of measuring RH that
the conservator ever needs. It is more reliable than electronic devices,
because it depends on the calibration of thermometers or temperature sensors,
which are much more reliable than electrical RH sensors. The only limitation
to the psychrometer is that it is difficult to use in confined spaces (not
because it needs to be whirled around but because it releases water vapour).
The psychrometer, or wet and dry bulb thermometer, responds to the
RH of the air in this way:
Unsaturated air evaporates water from the wet wick. The heat required
to evaporate the water into the air stream is taken from the air stream,
which cools in contact with the wet surface, thus cooling the thermometer
beneath it. An equilibrium wet surface temperature is reached which is
very roughly half way between ambient temperature and dew point temperature.
The air's potential to absorb water is proportional to the difference
between the mole fraction, ma, of water vapour in the ambient
air and the mole fraction, mw, of water vapour in the saturated
air at the wet surface. It is this capacity to carry away water vapour
which drives the temperature down to tw, the wet thermometer
temperature, from the ambient temperature ta :
( mw - ma) = B( ta- tw)
B is a constant, whose numerical value can be derived theoretically
by some rather complicated physics (see the reference below).
The water vapour concentration is expressed here as mole fraction in
air, rather than as vapour pressure. Air is involved in the psychrometric
equation, because it brings the heat required to evaporate water from the
wet surface. The constant B is therefore dependent on total air pressure,
P. However the mole fraction, m, is simply the ratio of vapour pressure
p to total pressure P: p/P. The air pressure is the same for both
ambient air and air in contact with the wet surface, so the constant B
can be modified to a new value, A, which incorporates the pressure, allowing
the molar fractions to be replaced by the corresponding vapour pressures:
pw - pa= A* ( ta- tw)
The relative humidity (as already defined) is the ratio of pa,
the actual water vapour pressure of the air, to ps, the saturation
water vapour pressure at ambient temperature.
RH% = 100 *pa/ ps = 100 *( pw
- ( ta- tw) * 63) / ps
When the wet thermometer is frozen the constant changes to 56
The psychrometric constant is taken from: R.G.Wylie & T.Lalas,
"Accurate psychrometer coefficients for wet and ice covered cylinders
in laminar transverse air streams", in Moisture and Humidity 1985,
published by the Instrument Society of America, pp 37 - 56. These values
are slightly lower than those in general use.
To calculate a wet-bub temperature the best way is to calculate the dew point
then use a Skew-T diagram.A blank Skew-T diagram can be found here at this link:
For finding the wet-bulb temperature, first find the elevation of your
location. Next, at the elevation of your location, plot the air temperature (in
degrees Celsius) and the dewpoint temperature on the chart. Take the air
temperature up the dry adiabat line and the dewpoint temperature up the
theta line until they meet. At the point where they meet, come back down
the moist (or wet) adiabat to the elevation of your station. This will be the
For information on how to read and understand a Skew-T diagram, see the
following link :