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# ES 5.2 PPT

## by Ryan Cooper on Jan 04, 2011

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## ES 5.2 PPTPresentation Transcript

• 5.2WATER IN THEATMOSPHERE
• 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.