Zooplankton(about 1 1/2 pints) of sea water.

Zooplankton are tiny animals the size of the letters in this sentence. They eat plant plankton and get eaten by larger animals.

When a lot of water vapor is in the air, we say the air is humid. Scientists frequently use the term relative humidity, which describes how much water vapor is in the air at a particular temperature compared with how much water the air at that same temperature is able to hold. An easy way to understand this is to imagine that the air is a towel. If you spill a glass of water, you can wipe up the water with a towel. But the towel probably could absorb more than just a glassful of water. Perhaps it could hold water spilled from 5 or 10 glasses before becoming completely soaked. The amount of water vapor actually present in the air is often only a fraction of the total amount that the air can hold, so relative humidity is expressed as a percentage. When the relative humidity is 100 percent, the air is saturated. Like a towel totally soaked with water, the air can hold no more moisture. When the relative humidity is 100 percent and the air is saturated, evaporation and precipitation are in a state of balance. As moisture precipitates, the amount of evaporation increases to reach a balanced state again.

Water vapor in the air is called humidity. Because molecules of water vapor are so small they can't be seen, people who study humidity have developed creative ways to measure the amount of vapor in the air.
Probably the first person to think of an instrument to measure the vapor content of the air was Leonardo da Vinci, a man who was born in Italy in the 15th century. He placed a small wad of dry cotton on one side of a balance scale. Then he placed an object of exactly the same weight as the wad of cotton on the other side of the scale. As the dry cotton absorbed water vapor from the air, it became heavier and the balance pan lowered. The difference between the two weights was the measure of the humidity.
Now scientists use an instrument called a psychrometer to measure relative humidity. A psychrometer is made of two thermometers that are fastened next to one another. The bulb of one thermometer is wrapped in material that is soaked with purified water. To begin measuring relative humidity, a person whirls the psychrometer around until the thermometer with the wet material reaches a steady temperature, which is always lower than the temperature on the dry bulb. The actual air temperature is measured by the thermometer with the dry bulb. The difference between the two temperatures is called the wet-bulb depression and is the result of the evaporation of water from the material. Scientists mark the dry-bulb temperature and the wet-bulb depression on two charts, called Psychrometric Tables, to calculate relative humidity and dew point temperature.

Scientists also measure humidity with an instrument called a hair hygrometer. Materials such as wood, cotton, skin, and hair absorb moisture from the air. Human hair gets longer as it absorbs water, increasing its length by about 21/2 percent over a relative humidity range of 0 to 100 percent.
On a hair hygrometer, strands of hair are attached to a pointer on a mechanical dial that has been specially marked to indicate relative humidity. The pointer moves as the hair lengthens or shortens. If a written record of relative humidity is needed, a hair hygrometer can be connected to a hygrograph, which has a clock-driven pen that marks a continuous line on graph paper.

The amount of water vapor in the air determines how comfortable we feel on a given day. One way the human body releases heat is by sweating. Evaporation of sweat from our skin helps us cool down on hot days. We feel cooler because during the evaporation process, water molecules require energy to change into vapor. The kind of energy they use is heat energy taken from the water molecules' environment, which in this case is our skin. If the air already contains a lot of water vapor, however, sweat does not evaporate quickly, and we continue to feel hot and uncomfortable.

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