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Measuring weather
Surface observations - humidity
One of the great features that makes weather interesting and complex is the presence of water vapor in the atmosphere. Humidity is a measure of water vapor, a gas, in the atmosphere. It can be measured in an absolute sense. For example, this portion of the atmosphere has 3 grams of water vapor per kilogram of dry air.
It can also be measured in a relative sense. For example, this portion of the atmosphere has 50% relative humidity.
Humidity instruments, known as hygrometers, take various forms. Some use
humidity sensitive materials and others use temperature and
condensation/evaporation to measure humidity. In fact the ASOS instrument you
see here also uses the temperature sensor and is known as a joint
"hygrothermometer." It measures the
dew point temperature
by chilling a mirror until condensation forms on it. Condensation is detected
by measuring reflection of infrared light off of the mirror, and a resistance
thermometer inside the mirror reports the temperature at which condensation
forms. This dew point temperature is an absolute measure of water vapor
in the atmosphere, but is often reported as relative humidity, relative to the
air temperature.
Next page
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in situ, surface observations, humidity
Links and resources
Surface observations - humidity
One of the great features that makes weather interesting and complex is the presence of water vapor in the atmosphere. Humidity is a measure of water vapor, a gas, in the atmosphere. It can be measured in an absolute sense. For example, this portion of the atmosphere has 3 grams of water vapor per kilogram of dry air.
It can also be measured in a relative sense. For example, this portion of the atmosphere has 50% relative humidity.
Humidity instruments, known as hygrometers, take various forms. Some use
humidity sensitive materials and others use temperature and
condensation/evaporation to measure humidity. In fact the ASOS instrument you
see here also uses the temperature sensor and is known as a joint
"hygrothermometer." It measures the
dew point temperature
by chilling a mirror until condensation forms on it. Condensation is detected
by measuring reflection of infrared light off of the mirror, and a resistance
thermometer inside the mirror reports the temperature at which condensation
forms. This dew point temperature is an absolute measure of water vapor
in the atmosphere, but is often reported as relative humidity, relative to the
air temperature.
Next page
->
in situ, surface observations, humidityLinks and resources
