Humidity and Dew Point in Atmospheric Sciences
Explore the significance of humidity in the atmosphere, the role of water vapor in energy transfer, and the concept of dew point as a measure of water vapor content. Learn how air's ability to hold water vapor changes with temperature, leading to condensation and saturation. Discover the practical implications of these concepts in weather and comfort.
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Presentation Transcript
Atmospheric Sciences 101 Humidity
Air is mostly composed of Nitrogen (N Air is mostly composed of Nitrogen (N2 2, 78%) and Oxygen (O Oxygen (O2 2, 21%), with the next largest component , 21%), with the next largest component being water vapor (H being water vapor (H2 20, ~0 0, ~0- -4%) , 78%) and 4%)
Humidity Humidity is a measure of the water vapor content of the air Water vapor plays a critical role in the atmosphere Associated with clouds and precipitation Very important way of moving energy in the atmosphere E.g., energy going into evaporating water vapor in the tropics, which condenses and release energy in the midlatitudes Has a big influence on our comfort and how we cool when warm
Some basic water vapor concepts Some basic water vapor concepts H2o vapor is a DRY gas, just like O2 and N2. It is not wet Warm air can hold more water vapor than dry air. The amount of water vapor air can hold increases rapidly with temperature
An Experiment Start with a sample of air with some water vapor Start cooling it down. It will progressively be able to hold less and less water vapor Eventually it will only be able to hold the water vapor it started with. This is called saturation. If one cools the air down further, some of the moisture will begin condensing out.
Dew Point (T Dew Point (Td d) ) The temperature to which must be cooled (at constant pressure) in order for it to become saturated. Measured in C or F A measure of water vapor in the air. Why? The more water vapor there is in the air, the less you have to cool it down! So more water vapor means higher dew point.
Dew Point Dew Point Typical dew point in Seattle during summer is ~ 50F In humid, Washington DC in summer: 65- 75F Reported at most airport stations around the world.
Relative Humidity Varies During the Day/Night Relative Humidity Varies During the Day/Night Lower during the day when temperatures are warm Higher at night when temperatures are cool
How do we measure humidity? Use devices How do we measure humidity? Use devices called hygrometers. called hygrometers. In the old days, hair hygrometers
Then moved to Then moved to psychrometers thermometers, one with a wet bulb thermometers, one with a wet bulb psychrometers, with two , with two
Psychrometer Psychrometer Measures both a dry bulb and wet bulb temperature The difference between the two can be used to calculate humidity, either relative humidity or dew point
More Recently Electronic More Recently Electronic Psychrometers Psychrometers/Hygrometers Available /Hygrometers Available Some use solid state devices Others chilled mirrors
