Dew Point.
All air contains water vapour of varying quantities. The
dew point indicates the amount of moisture in the air.
The higher the dew
point, the higher the moisture content of the air at a given temperature.
Conversely, the dew
point of humid air will be higher than the dew point of
dry air.
Dew point temperature is defined as the temperature to
which the air would have to cool (at constant pressure and
constant water
vapour content) in order to reach saturation. A state of saturation exists when
the air is holding
the maximum amount of water vapour possible at the
existing temperature and pressure.
Condensation of water vapour begins when the temperature
of air is lowered to its dew point and beyond.
The dew point, like other
measures of humidity, can be calculated from readings taken by a hygrometer.
Wind Chill.
Wind chill is the apparent temperature felt on exposed
skin due to the combination of air temperature and wind speed.
At higher
temperatures wind chill is considered less important but the wind chill
temperature is always lower than
the air temperature.
Humidity.
Humidity is the amount of water vapour in an air sample.
Relative humidity is the most frequently encountered
measurement of humidity because it is regularly used in
weather forecasts.
It’s an important part of weather reports because it indicates the likelihood of
precipitation
(dew, or fog).
Higher relative humidity also makes it feel hotter
outside in the summer because it reduces the effectiveness of
sweating to
cool the body by preventing the evaporation of perspiration from the skin. This
effect is calculated
in a heat index table. Warmer air has more thermal
energy than cooler air; thus more water molecules can evaporate
and stay in
the air in a vapour state rather than a liquid state. This may be why people say
that warmer air "holds"
more moisture — in warmer air, there is more energy
for more water molecules to hold themselves in the air (and
overcome hydrogen
bonds which seek to pull water molecules together).
Barometer.
A Barometer is a devise used to measure atmospheric pressure in a given area.
High pressure in a region indicates fair weather while low pressure indicates that storms are more likely.
Simultaneous barometric readings from across a network
of weather stations allow maps of atmospheric pressure to be produced.
Isobars (Lines)
drawn on such a map link sites with the same pressure and give a contour map of
areas of high and low
pressure. Localised high atmospheric pressure
acts as a barrier to approaching weather systems and so diverts their
course. Low atmospheric pressure, on the other hand, represents the path of
least resistance for a weather system
making it more likely that low
pressure will be associated with increased storm activities.
If the barometer is falling then bad weather or some form
of precipitation (Rain, fog or mist) may fall, however if
the barometer is
rising then there could be nice weather or no precipitation.