Rain

-Humidity
-Measuring Humidity
-What does relative humidity tell you?
-Effects of Water Vapour in the air
-How Frost forms
-Dew point

Humidity is a term describing the amount of water vapor or moisture being carried by air. Measurement can be either absolute or relative; absolute measurement deals with the weight per unit of volume of water vapor whereas relative measurement deals with the condition of saturation, usually stated in percent. At 100% RH, the air is completely saturated with water and no evaporation is taking place.

Measuring Humidity

The measure of humidity used most often is 'relative humidity'. Relative Humidity is a relative measurement. The total amount of water vapor that air can hold is dependent upon its pressure and temperature.

Meteorologists use various ways of describing how much water vapour is in the air. One way is to make use of a 'saturation amounts' chart. 'Saturation amount' means the amount of water vapour needed to saturate air increases as the air's temperature increases.

The chart shows how many cubic inches of water vapour are needed to saturate a cubic yard of air at various temperatures. It has been converted from metric units that all scientists use. It ignores slight changes in water density associated with temperature changes. It assumes that a cubic centimeter of water is one gram.

From the chart, it is surprising that how little water is in even the most humid air. If you could squeeze all the water out of a cubic yard of saturated 95¢XF air, you would have only 1.85 cubic inches of water-about one-eighth of a measuring cup!

What does relative humidity tell you?

The 'saturation amounts' chart also helps explain relative humidity means.Imagine that the temperature now is 86¢XF, with 0.6 cubic inches of water vapour per cubic yard of air.

The chart shows that 86¢XF air needs 1.42 cubic inches of vapour per cubic yard to be saturated. Divide the actual vapour in the air (0.6 cubic inches) by the amount the air could hold (1.42 cubic inches) and multiply by 100. This gives you the relative humidity, which is 42 percent.

Relative humidity depends not only on how much water vapour is in the air, but also on the air's temperature. This explains why relative humidities are higher at night, when the air is cooler, than during the day even though the amount of water vapour in the air doesn't change.

Effects of Water Vapour in the air

Understanding how water acts as it evaporates into the air or condenses out of the air helps explain why bathroom mirror fogs up during a hot shower. It also explains how dew, frost, fog and clouds form.

A bathroom mirror fogs up because some of the hot water spraying from the shower evaporates into the bathroom's air, increasing its humidity, and therefore the air's dew-point temperature. The mirror's surface is cooler than the dew point of the now-humid air in the bathroom. Some of the vapour in the air that touches the mirror condenses onto it, making tiny water drops.

Dew has formed on the mirror. If the shower is hot enough and the air is already humid enough, a light fog might form in the room. This is because the vapour condenses into tiny drops that float in the air.

Dew or frost and fog are more likely to form on clear nights than cloudy nights. The Earth is always radiating away infrared energy into the atmosphere. After the sun goes down, solar energy is no longer warming the ground, but the infrared energy keeps on sending heat upwards. The ground cools. When it's cloudy the clouds absorb infrared energy from the Earth and radiate it back down. On clear nights infrared energy is lost to space; the ground becomes cooler. The cold ground cools the air next to it. If the air cools to its dew point, dew, frost or fog form.

How frost forms

Water doesn't always turn to ice when it drops below 32¢XF. At temperatures from 32¢XF to around 0¢XF water vapour condenses as dew and then turns to ice and frost. After that, vapour molecules begin sublimating directly from the air onto the frost. Sometimes dew condenses above 32¢XF, but then the air turns colder, freezing the air. This frozen dew creates solid ice drops or a glaze of clear ice. Frost forms inside windows when the glass cools to the frost point of air inside the house or between the panes of a double window.

Dew point

Dew point is a temperature at which air in the atmosphere becomes saturated with water vapor and starts to condense. Any lowering of temperature below the dew point results in condensation of some of the water pressure present. Relative Humidity at the dew point is always 100%.

When water vapour either condenses into liquid or sublimates into ice, it releases latent heat and warms the air a little. This means that overnight as the air cools to its dew point, condensation will begin slowing the temperature fall. As a result the air is not likely to get colder than its initial dew point anytime during the night.

Of course the air doesn't always cool to the dew point. Also, a mass of new cold air could move in during the night making the temperature plunge after the front passes and colder, drier air arrives. But if no fronts are expected to arrive overnight, the afternoon's dew point gives you an idea of what minimum temperature to expect that night.

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