The atmosphere and the Weather

Water Vapor in the Atmosphere

A "Parcel of Air" is an iminaginary body of air about the size of a large balloon that is used to explain the behavior of air. We will describe what is meant by the relative humidity and dew point temperature of air in a parcel. The parcel concept is used because we often would like to know what will happen to air as it moves in the atmosphere, and air tends to move together in blobs about the size of parcels (not molecule by molecule). The parcel concept will be extemely important in describing cloud and thunderstorm formation. For one thing, we will want to keep track of the relative humidity in an air parcels as it moves up and down in the atmosphere.

Relative Humidity

The relative humidity in an air parcel is defined as the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at a particular temperature

water vapor content

Relative humidity =

water vapor capacity

For example, air with a 50 percent relative humidity actually contains one-half the amount of water vapor required for saturation.

Air with a 100 percent relative humidity is said to be saturated because it is filled to capacity with water vapor.

One way to compute relative humidity is to take the ratio of the actual vapor pressure in a parcel (measured) with the saturation vapor pressure at the parcel temperature, or RH = e/e_s. However, following meteorological convention, we will use something called mixing ratio instead of vapor pressure. The definitions of mixing ratio, saturation mixing ratio, and its relationship to relative humidity are provided in this WORD document humidity notes. The document also contains examples for relative humidity calculations.

Most people use Relative humidity to describe the water vapor content of the air, but it is widely misunderstood. Relative humdity in itself does not indicate the actual amount of water vapor in the air because it depends on temperature. For example, an air parcel at a temperature of 10°C with RH = 100% contains less water vapor than an air parcel at a temperature of 25°C with RH = 50%. You should be able to convince yourself of this by using the table of saturation mixing ratios.

Dew point

The dew point temperature represents the temperature to which air would have to be cooled (with no change in moisture content) for saturation to occur. If you know the actual mixing ratio in an air parcel, you can use the table of saturation mixing ratios to get the dew point temperature.

The dew point temperature does indicate the actual vapor content of the air: the higher the dew point, the more water vapor in the air.

The difference between air temperature and dew point temperature can indicate whether the relative humidity is low or high.

when the air and the dew point temperatures are far apart, the relative humidity is low
when the air and the dew point temperatures are close to the same value, the relative humidity is high
when the air and the dew point temperatures are the same, the air is saturated and the relative humidity is 100 percent.

Near the ground surface, we are often interested in how much net evaporation (evaporation minus condensation) will occur. The rate of net evaporation depends on:

The relative humidity. The lower the relative humidity or the larger the difference between the air temperature and the dew point temperature, the faster the rate of net evaporation.
The wind speed. Wind moves the moist air away from the water surface and replaces it with drier air. The higher the wind speed, the faster the rate of net evaporation.

Usually, unless a weather front passes, the dew point temperature will not change much during the course of the day. Assuming that the dew point temperature does not change during the day, what time of day should you water outside plants to best conserve water? ANSWER: In the morning around the time of the low temperature. This is typically when the relative humidity will be the highest. When relative humidity is high, the net rate of evaporation will be the slowest, allowing more liquid water to be absorbed into the soil and plants rather than evaporating.

Normally, near the Earth's surface, the relative humidity is less than 100%, therefore, net evaporation is occurring. To convince yourself, leave out an uncovered glass of water. The water will eventually evaporate. Once evaporated from the surface, water vapor moves around with the rest of the air. Occasionally, there may be net condensation occurring near the Earth's surface. If air near the ground is cooled below its original dew point temerature, fog will result. Fog is nothing more than a cloud at ground level. We will talk more about clouds next week.

Beside fog, are there ever situations near the Earth's surface when net condensation occurs? ANSWER: Yes, dew and frost.

When an object becomes colder than the dew point temperature, the air in contact with that object becomes colder than the dew point temperature, resulting in net condensation:

Dew if the object is warmer than 0° Celsius
Frost if the object is colder than 0° C. NOTE: frost forms by the process of deposition (water vapor --> ice). It is not frozen dew.

An everyday example is condensation that happens on the outside of a cold can of soda or a glass of ice water. The water that you see came from the air as water vapor condensed into liquid. It tells you that the can must be colder than the dew point temperature of the air. Here in the desert, where the dew point temperature is often well below 0° C, you will not always get condensation on the sides of a cold can of soda. But in a more humid climate where the dew point is higher, you will see this type of condensation more often.

A couple of links: current dew point temperature and relative humidity on the Univ. of Arizona campus.

U.S. map of current dew-point temperature.

Summary for how Water Behaves Based on Relative Humidity

If RH = 100% (Td = T), there will be no net evaporation or condensation, since the rate of evaporation is equal to the rate of condensation.
If RH < 100% (Td < T), any liquid water present will evaporate with time, since the rate of evaporation is greater than the rate of condensation.
If RH > 100% (Td > T), water vapor will condense to liquid water until RH falls back to 100%, since the rate of condensation is greater than the rate of evaporation. When this occurs, it is only a temporary situation until sufficient water vapor condenses out of the air.