|RELATIVE HUMIDITY IN THE HOME
METEOROLOGIST JEFF HABY
Relative humidity is a measure
of the amount of moisture in the air compared to what the maximum
amount of moisture in the air could be. The maximum amount of moisture that can be in the air depends on
the temperature. As air warms it can have more moisture. The actual amount of moisture in the
air is going to depend on the weather. If the flow is from a warm ocean then the amount of moisture
in the air will tend to increase. If the flow is from a dry area such as a desert or semiarid area then
the moisture in the air will tend to decrease. When it rains the amount of moisture in the
The weather inside the home can be much different than that outside when it comes to relative humidity.
This is because temperatures in the home can be much different than the temperature outside. This is
because of that important relationship between temperature and how much maximum moisture can be in the air. Let us
look at two situations: when it is very hot outside and when it is very cold.
When it is very hot outside then air conditioning is needed. Air conditioning will raise the
relative humidity in the home compared to what it is outside. The relative humidity increases when
the temperature and dewpoint get closer together. Suppose the temperature outside is 105 F with a
dewpoint of 50 F. The difference between the temperature and dewpoint outside is 55. Now bring that
air inside and cool it. After the air is cooled the temperature can be brought down to a comfortable
72 F while the dewpoint is still 50 F. Since the temperature is closer to the dewpoint, the relative humidity
will increase. In humid climates, such as when the temperature is 95 F outside with a dewpoint of 70 F, when
that air is brought inside and cooled the relative humidity can be very high. The excessive humidity can lead to
problems such as mold. When the humidity is too high then some moisture can be taken out of the
air using a dehumidifier.
When it is very cold outside then heating is needed. Heating can dramatically decrease the
relative humidity in the home compared to what it is outside. The temperature and dewpoint spread
dramatically apart when air is heated. Suppose the temperature outside is 35 F with a dewpoint of 20 F. When
that air is brought inside and heated the temperature can increase to a comfortable 72 F while the dewpoint
is still 20 F. Even though there is the same amount of moisture in the air outside and inside, it will
be much drier inside because the rate of evaporation is much higher when the relative humidity is
lower. Static and dry skin will be problems in the home when the relative humidity is low. Many people
in dry climates use a humidifier to add moisture to the air, especially in winter.