There is often a lot of water in the air. It is in the gas state and is called water vapor. Relative humidity compares the amount of water vapor in the air to how much water vapor the air can hold. It is expressed in a percent. 100% means that it can not hold any more. 50% only means it is half full and new air can be added.
Hygrometers measure relative humidity. One type of hygrometer is called the psychrometer. It is simply two thermometers. One is regular and the other has a wet cloth wrapped around the bulb. If the air can hold more water, then the water on the cloth will evaporate. This will change the temperature because when water evaporates, it takes energy. This means there is less average kinetic energy and (you guessed it!) a lower temperature. By determining the difference between the wet bulb and dry bulb thermometers, scientists can figure out the relative humidity. The table below is used to help them figure it out.
Notice that if there is no difference, the humidity is 100%. This is because none of the water evaporated because the air above it cannot hold any more water.
Effects of Humidity
Humidity affects the weather because the water in the air eventually comes back down to earth in many weather events like thunderstorms, hurricanes, monsoons, and common rain. Scientists an use the humidity to try to predict if one of these storms are possible in the near future.
Have you ever seen shapes in clouds? Ever wonder where they come from? Clouds form when water condenses meaning that it changes from water vapor to a liquid. This can only occur when the air holds more water vapor than it can hold. This doesn't necessarily mean that there has to be more water vapor in the air. It can also happens if the air cools rapidly. If it cools below what is called the dew point, then the water vapor can condense. This cooling can happen by mixing with colder air, rising, or at night when the sun is not out. The water needs something to condense around. This is the condensation nucleus. This can be something like dust.
Types of Clouds
Clouds are classified mainly according to height. Cirrus clouds are really high up, made of ice crystals, and appear thin or feathery. Stratus clouds are low sheets. Cumulus clouds are formed vertically. They appear fluffy. Other clouds are combinations of these clouds. If they have "alto" in it, it usually means they are high and "nimbus" or "nimbo" means rain.
Precipitation is any form of rain that falls to the ground. We have all seen rain and sometimes even played in the snow. These are two forms of precipitation. Precipitation can also be hail, sleet.
How does precipitation occur? Well, after water condenses, water droplets can combine with other droplets until they get very big. When they are too big and heavy, they fall from the sky.
Where does Precipitation Occur?
Precipitation often occurs where cold air and warm air meet. This is called a front. A cold front happens when cold air pushes warm air out of the way, and a warm front occurs when warm air pushes cold air out of the way. In both cases, warm air rises over the cold air because it is less dense and cools rapidly. This forms a cloud where precipitation can occur.
Other places where air rises like this are on the sides of mountains facing the wind and where the global winds meet at the equator pushing each other up.
Places where little precipitation occurs are places where air sinks. This can be the side of a mountain facing away from the wind or places where the global winds blow away from each other.
This map shows the places of the Earth where precipitation is most and least common.
Weather 4 Kids
by Zachary Gold