|INSIDE RH AS A FUNCTION OF OUTSIDE WEATHER
METEOROLOGIST JEFF HABY
Although the thermostat in the home can be used to give the same temperature year round,
it can still feel much different in the home when comparing the summer to the winter. This is
because moisture also influences how you feel.
In the winter if temperatures are cold outside, the amount of
moisture in the air will
be fairly low. Cold air does not have
the capacity to evaporate or sublimate as much moisture into the air as compared
to when temperatures are warm. The
moisture potential in the air increases exponentially as temperature increases. This winter
air is brought into the home heater and warmed. The warming process vastly changes the
relative humidity of the air.
Because of the exponential relationship, this warmed air will only have a fraction of the possible amount
of moisture that could be in the air at this warmer temperature. The result leads to static electricity, very
dry skin and the
drier working fast. Suppose the outside temperature and dewpoint are 24 F. Air with this dewpoint
at sea level will have a mixing ratio of about 2.7 g/kg. Since the temperature is equal to the dewpoint the relative
is 100%. Now this air is brought inside and warmed to 72 F. Air at 72 F can have a maximum possible sea level
mixing ratio of about 17 g/kg. This produces a relative humidity of (2.7/17)*100% = 16%. Air that feels saturated outside
feels bone dry once it it heated. In both cases the amount of moisture in the air is the same; What changes is
the potential for adding more moisture to the air. When there is a huge potential to add more moisture to the
air, it will feel bone dry because it is sucking moisture away from your skin. The
evaporative cooling effect
can also make an inside temperature of 72 F in the winter feel colder than an inside
temperature of 72 F in the summer.
In the summer the opposite effect takes place with relative humidity. When air is cooled the relative
humidity increases. This keeps
the relative humidity in the home higher than it is outside. Air conditioning units have to remove water from the
system in cases where the air is being chilled below the dewpoint. Suppose the temperature is 95 F with a dewpoint
of 70 F outside. If this air is chilled below 70 F, condensation will occur.
The air used to cool the home will have a high relative humidity once it is brought inside and cooled,
especially when the dewpoint is high outside.
To counteract these problems, people will use humidifiers in the winter and dehumidifiers in the summer.