ELSEC 765 - Temperature, Humidex & Heat Index; Humidity; Atmospheric Pressure

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Temperature, Humidex & Heat Index
Temperature can be measured in degrees Centigrade or Fahrenheit. To change the
units press Set\Units\Temperature.
Humidex & Heat Index
In hot weather conditions the 765 displays values for the Humidex (Hmdx) and the
Heat Index (HI). These are index numbers used by meteorologists to describe how hot
the weather feels to the average person by combining the effect of heat and humidity.
Heat Index is normally used in USA, Humidex in Canada. These are displayed when
the T button is pressed, but only if the temperature is high enough and the calculation
is valid.
Humidity
Humidity is measured as % relative humidity (%RH). This is the fraction of the
maximum amount of water that the air can hold at the current temperature and
pressure.
In general if the temperature is reduced the amount of water it can hold gets less. So if
the temperature of some air is reduced its humidity rises, and at some point the air will
not be able to hold the water it has and water will start to condense out (form
mist/cloud or drops of water on a surface), the relative humidity has reached 100%.
The temperature at which this happens is the dewpoint. The 765 displays the dewpoint
with the humidity.
The temperature/dewpoint difference is often used as a measure of the likelihood of
condensation (fog) occurring, particularly in meteorology. If the temperature is only
one or two degrees above the dewpoint in the evening then fog is likely as the
temperature falls during the night.
The UK National Trust tries to keep the indoor relative humidity between 50% and
65%, aiming for 58%.
Also shown in later versions of the 765 with a pressure sensor is the Absolute
Humidity in g/m³
Atmospheric Pressure
Later versions of the 765 (V13 hardware or greater) have a pressure sensor. The
values for this are displayed by pressing the RH and T buttons at the same time. The
main reading is the atmospheric pressure in Hectopascals (hPa).
Shown below this are:
Temperature (T)
Relative humidity (RH)
Local air density (kg/m³)
Density altitude in feet (DA)