Changing Forms of Water Weather is affected by the amount of water in the air. ◦ The heat energy that is absorbed or released by a substance during a phase change is called latent heat. When liquid water evaporates, the water absorbs energy from the environment, which becomes potential energy between the molecules. When water vapor changes back into a liquid through the process of condensation, the energy is released to the surrounding air.
Humidity As water evaporates, it becomes water vapor, or moisture in the air, which is invisible. The amount of water vapor in the air is called humidity. ◦ When water evaporates, the humidity of the air is increasing. ◦ As the temperature of the air increases, the air’s ability to hold water vapor also increases. ◦ Recall, warm air holds MORE water vapor than cool air.
Absolute Humidity Absolute Humidity is the mass of water vapor contained in a given volume of air. ◦ A measure of the actual amount of water vapor in the air. Calculated by using the following formula: mass of water vapor _____________________________Absolute Humidity ___(grams) air (cubic meters volume of= m3) ◦ However, as air moves, its volume changes as a result of temperature and pressure changes.
Relative Humidity Relative humidity (RH) is a ratio of the actual water vapor content of the air to the amount of water vapor needed to reach saturation at a certain temperature. ◦ More common way to express the humidity; uses the following formula: actual water vapor content _____________________________ x 100Relative Humidity (RH) % (g/kg) ___ saturation water vapor content= (g/kg) When air holds all of the water it can at a given temperature, it is said to be saturated, with a relative humidity of 100% and condensation will likely result.
Relative Humidity Relative Humidity example: Air at 20 C is saturated when it contains 14 g/kg of water vapor. What is the relative humidity of a volume of air that is 20 C and contains 10 g/kg of water vapor? 10 71 % x = Relative g/kg 100 Humidity 14
Relative Humidity 2 factors affecting the relative humidity (RH) include: ◦ Amount of water vapor At constant temperature and pressure, the more water vapor there is in the air, the higher the relative humidity. ◦ Temperature At constant water vapor content in the air, increased temperature leads to lower relative humidity and decreased temperature leads to higher relative humidity.
Measuring Humidity A psychrometer is an instrument used to measure relative humidity made of two thermometers; dry-bulb and wet-bulb (covered with a damp cloth). ◦ The difference between the readings indicates the amount of water vapor in the air. The larger the difference, the less water vapor in the air, therefore lower humidity. The smaller the difference, the more water vapor in the air, therefore higher humidity.
Condensation Recall, condensation is the process by which a gas, such as water vapor, becomes a liquid. ◦ Before this can occur however, the air must be saturated RH of 100%! ◦ Air can become saturated: When water vapor is added through evaporation. When air cools to the dew point, which is the temperature a gas condenses into a liquid.
Cloud Formation A cloud is a collection small water droplets or ice crystals in the air. ◦ Form as warm air rises and cools, condenses onto tiny particles of gas and dust in the atmosphere. ◦ Classified based on shape and altitude.
Cloud Formation For water vapor to condense and form a cloud, a solid surface on which condensation can take place must be available. ◦ The troposphere contains millions of particles of ice, salt, dust, smoke, and other particles serving as solid surfaces. ◦ These suspended particles providing a surface necessary for water vapor to condense are called condensation nuclei.
Stratus Clouds Stratus clouds are clouds forming in layers and have a flat, uniform base beginning to form at low altitudes. Cover large areas of the sky and often block out the sun. ◦ Form where a layer of warm, moist air lies above a layer of cool air. Overlying warm air cools to the dew point, creating a cloud. ◦ The prefix -nimbo and the suffix –nimbus mean “rain”. Nimbostratus clouds are dark stratus clouds usually producing heavy, continuous rainfall.
Cumulus Clouds Puffy, white clouds tending to have flat bottoms are called cumulus clouds. Form when warm, moist air rises and cools. ◦ Normally indicate fair weather. ◦ If they get larger, they can produce thunderstorms cumulonimbus clouds.
Cirrus Clouds Thin, feathery clouds found at high altitudes are called cirrus clouds. ◦ Made of ice crystals due to low temperatures at higher altitudes in the troposphere. ◦ Form as a result of strong winds. ◦ If they get thicker, cirrus clouds indicate a change in weather is coming.
Clouds and Altitude As mentioned previously, clouds are classified on their altitude as well as their form, or shape. ◦ The prefix cirro-, is used to describe clouds forming at higher altitudes. Normally made of ice crystals. ◦ The prefix alto-, is used to describe clouds forming a middle altitudes. Can be made of both water droplets and ice crystals. ◦ There is no prefix for clouds forming at low altitudes. Made of water droplets.
Fog Fog is water vapor that has condensed very near the surface of the Earth because air close to the ground has cooled. ◦ A type formed from the nightly cooling of Earth is known as radiation fog. Layer of air in contact with the ground becomes chilled to below the dew point. ◦ Another type is advection fog, which forms when warm, moist air moves across a cold surface. Common along coasts.
Precipitation When water from the air (condensed clouds) returns to the Earth’s surface, it is known as precipitation. ◦ 5 major forms include: Rain (most common) Begins as a drop smaller than a period at the end of a sentence. Snow Forms when temperatures are so cold, water vapor changes directly to solid and falls to the ground as a single crystal or snowflake. Sleet Forms when rain falls through a layer of freezing air. The rain freezes in the air, producing falling ice. Freezing Rain Occurs when cold water droplets freeze when they make contact with Earth’s surface. Hail Balls or lumps of ice falling directly from clouds. Forms in cumulonimbus clouds. Common during thunderstorms and tornadoes.