What is humidity?

Humidity refers to the amount—or saturation—of water vapor in the air. Depending on air temperature and pressure, the air can contain differing amounts of humidity before the vapor turns into actual precipitation.

What is the difference between absolute humidity and relative humidity?

Absolute humidity is the actual amount of water that is mixed in with the air. It is measured in milligrams per liter. Relative humidity is expressed as a percentage and refers to the actual humidity divided by the maximum water vapor content possible at a given air temperature and pressure. For example: if a liter of air at 98°F (37°C) at one atmosphere pressure contains 44 grams (1.5 ounces) of water vapor, and the actual water content is 11 grams, then the relative humidity would be 25 percent (11/44 x 100 = 25%).

Can the relative humidity ever be higher than 100 percent?

No. It was once theorized that when clouds are in a state called "supersaturation" it was possible to have a relative humidity of slightly over 100 percent, but this has since been proven false.

What is indoor humidity?

Because we now live in homes with environments controlled by air conditioning, furnaces, and fans, the humidity inside is often very different from what the weather is like outside. For instance, in the winter many homes are very dry inside, which increases static electricity and causes uncomfortably dry skin. Overly humid air within a home can lead to the development of mold and mildew, some of which can be a risk to your health. Also, air that is too dry or too moist can cause structural damage to a house; overly moist wood, too, encourages pests such as termites to chew wooden supports and flooring. A comfortable humidity level for most people is between 30 and 60 percent

What is a hygrometer?

A hygrometer is an instrument that measures the humidity. Italian artist and inventor Leonardo da Vinci (1452-1519) is often credited with constructing the first hygrometer; his design was later improved upon by Francesco Folli (1624-1685) in 1664, and Swiss physicist and geologist

Horace Benedict de Saussure (1740-1799) designed the first mechanical hygrometers. There are basically two types of hygrometers: 1) dry and wet bulb psychrometers, and 2) mechanical hygrometers. The first type uses dry and wet bulb thermometers to compare temperature changes resulting from humidity; the second type uses either organic material (blonde human hairs), which expand or contract based on humidity levels, or semiconductors made of lithium chloride or other substances whose degrees of electrical resistance change according to humidity.

Hygrometers like this one have been used for hundreds of years to measure humidity.

Hygrometers like this one have been used for hundreds of years to measure humidity.

Who invented the dew point hygrometer?

The dew point hygrometer (a type of dry and wet bulb psychrometer) was invented in 1820 by John Frederic Daniell (1790-1845). It consisted of two thin glass bulbs joined by a glass tube. One bulb held a thermometer and was filled with ether; the other was empty. As the air in the empty bulb cooled, the ether would condense on the thermometer, indicating the dew point temperature. Variations of Daniell's device are still used today, including cooled-mirror hygrometers that measure dew point when condensation forms on a mirror.

Is the relative humidity always 100 percent during times when it is raining or snowing?

While the formation of rain and snow within a cloud requires humidity to reach a saturation point, by the time the precipitation reaches lower elevations and the ground the rain, hail, or snow can exist within air that has a lower humidity.

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