Humidity and Condensation Water is unique because it is the only substance that commonly exists in all three states of matter. Depending upon temperature, water can be a solid, a liquid or a gas. Water is in a solid state at temperatures of 0 o C or below, appearing as ice, snow, hail and ice crystals.

Water is in a liquid state between 0 o C and 100 o C, present as rain and cloud droplets. At 100 o C or above, water evaporates and enters the atmosphere as water vapor, an invisible gas. The bubbles in boiling water are an example of water vapor. Clouds and steam are liquid droplets, not gas.

Although you can't see water vapor, sometimes you can feel it. The more water vapor the air contains, the more humid the air feels. Water often changes state in the atmosphere. Changing from one state to another requires energy to either be absorbed or given off. Condensation occurs when water vapor becomes a liquid - this creates dew, fog and clouds.

The opposite of condensation is evaporation. While condensation releases heat, evaporation absorbs heat. So condensation slows down the rate at which air cools. Evaporation, by contrast, is a cooling process. After you get out of a swimming pool you may feel chilly, because the water molecules on your skin are stealing heat from your body as they evaporate.

Frost forms by deposition when water vapor condenses as a solid. Snow often disappears as much through sublimation as by evaporation. Sublimation is when water changes directly from a solid to a gas, without becoming a liquid.

The amount of water vapor present in the air varies widely. The actual amount of water vapor in the air at a given time and place is called the specific humidity. It is expressed as the number of grams of water vapor per kilogram of air. On a humid summer day, for example, the humidity may be about 20 grams per kilogram.

There is a limit to the amount of water vapor that can be present in the air. Imagine a fish tank with a glass lid. Some water molecules have enough energy to to escape from the surface and become water vapor. Other water vapor molecules lose energy and return to the liquid state though condensation. When there is so much water vapor in the air that the rate of condensation equals the rate of evaporation, the air is saturated.

If any more water evaporates into saturated air, an equal amount will condense. This explains why water droplets may form on the lid of the fish tank. These drops confirm that as water continues to evaporate, an equal amount of water condenses from the saturated air.

The amount of water vapor present in saturated air depends upon the temperature of the air. The warmer the air, the more water vapor it can contain. The water vapor capacity of air roughly doubles for every rise in air temperature of about 11 o C blnFromSearch=1&productcode=US

Relative Humidity When meteorologists refer to the relative humidity, they are telling us how near the air is to its maximum capacity for holding water. Relative humidity compares the actual amount of water vapor that is present in air with the maximum amount of water vapor that can be present at a given temperature and pressure. It is usually stated as a percentage. Saturated air has a relative humidity of 100%; air that contains no water vapor has a relative humidity of 0%. mSearch=1&productcode=US

To calculate the relative humidity of a kilogram of air, divide its specific humidity by its maximum capacity. If the air holds 11 grams of water vapor per kg of air and it can contain at most 22 grams per kg, then it holds half of the water vapor it can contain. Thus, its relative humidity is 50%.

Measuring humidity Humidity is typically measured with a psychrometer - an instrument that works on the principle that evaporation causes cooling. A psychrometer consists of two thermometers. One is a dry-bulb thermometer that shows the air temperature. The other is a wet-bulb thermometer that has a water-soaked wick wrapped around its bulb.

The wet-bulb thermometer usually has a lower temperature because water evaporating from the wick cools the wet bulb. The drier the air, the greater the difference in the readings. If both the wet-bulb and the dry-bulb thermometers read the same, this shows that no water is evaporating from the wet bulb, and the air must be saturated. Relative humidity can be determined by using a psychrometer along with a table like the one on the next page.

Condensation At night, the air cools rapidly. Its ability to contain water vapor diminishes. The air becomes saturated. If the air continues to cool past the point of saturation, condensation occurs. The water vapor may condense into droplets, forming clouds or fog. If the water vapor condenses on a surface, such as grass, it's called dew. The temperature at which saturation occurs and condensation begins is called the dew point.

The dew point is a measure of the amount of water vapor in the air. The more water vapor the air contains, the less the air has to cool in order for condensation to start, so the higher the dew point.

Cooling and Condensation Two conditions are necessary for water vapor to condense. 1 - There must be material for water vapor to condense onto and, 2 - Air must cool to or below its dew point. When fog or clouds form, the water vapor is condensing on tiny particles called condensation nuclei. Condensation nuclei are usually substances such as salt, sulfate particles or nitrate particles.

The salts usually come from ocean water evaporating. The sulfates and nitrates come from natural sources and from the burning of fossil fuels. These particles are so small that one puff of smoke contains millions of condensation nuclei.

The cooling of the air occurs in the following ways: - contact with a colder surface. - radiation of heat - mixing with colder air - expansion as it rises

Even when air is cooled below its dew point, condensation into fog or clouds may not occur if there are no condensation nuclei available. Dew and frost form when moist air contacts a colder surface. Fog forms when air cools through contact and mixing.

Formation of Dew and Frost When air cools to its dew point through contact with a colder surface, water vapor condenses directly onto that surface. If the temperature is above 0 o C, dew forms. If the air temperature is below 0 o C, the water vapor becomes frost through deposition. This type of frost is often called a "killing frost" because it causes liquid in the cells of some plants to freeze. As liquid water freezes, it expands, bursting the cell walls and killing the plants.

Formation of Fog Fog forms when a cold surface cools the warmer moist air above it. As water vapor condenses in the air, tiny droplets fill the air and form fog. Each droplet is centered around a condensation nucleus. The droplets are so tiny that they fall slowly. The slightest air movement keeps them suspended in the air. At very cold temperatures, the fog may consist of tiny ice crystals.

Radiation Fog - forms when the night sky is clear and the ground loses heat rapidly through radiation. As the ground cools, light winds mix the cooled bottom air with the warmer air just above it. Eventually, the whole layer cools to its dew point.

The radiation fog occurs at ground level and is cooler than the air above it. This arrangement of cold air beneath warm air is called a temperature inversion.

Radiation fogs are common in humid valleys near rivers or lakes. They are most frequent in the late fall and in winter. These fogs are thickest in the early morning and are "burned away" by the later morning sunshine.

Advection Fog - forms when warm, moist air blows over a cool surface. In the northern US and southern Canada, advection fogs form when warm, moist southerly winds blow over snow-covered ground.