Ch 13 S1Water in the Atmosphere

Essential Questions Ch12 S1How does the water cycle work?What is humidity and how is it measured? What is relative humidity?How do clouds form?What are the three main types of clouds? Can you tell about weather conditions by looking at clouds?

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Water CycleAs the sun heats up the land and oceans, the amount of water in the atmosphere changes.

Water is always moving between the atmosphere and Earths surface.

The movement of water between the atmosphere and Earths surface is called the WATER CYCLE..

Water vapor enters the atmosphere in two ways.Water vapor enters the air by evaporation from the oceans and other bodies of water.Evaporation** is the process by which water molecules in liquid water escape into the air as water vapor

Water vapor is also added to the air by living things through transpiration. Transpiration**Water enters the roots of the plants and rises to the leaves and is released as water vapor.

Once water vapor is in the atmosphere, some of it condenses (molecules of water vapor becomes a liquid) to form clouds.Precipitation falls from the clouds toward the surface.The water then runs off the surface or moves through the ground, back into the lakes, streams, and eventually the oceans.

How is the quantity of water vapor in the atmosphere measured?It is measured by the humidity in the air.Humidity is a measure of the amount of water vapor in the air.

Weather reports usually refer to the water vapor in the air as relative humidity. Relative humidity - is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold at a particular temperature.

Relative Humidity The relative humidity tells us how full the air is at the time of measurement.

For example, 90% relative humidity means that at that moment the air is holding 90% of the maximum amount of water it could. See page 433 for more detail

Measuring relative humidity is done with an instrument called a psychrometer.PsychrometerHas two thermometersOne wet bulb thermometerOne dry bulb thermometer

How a Psychrometer WorksThe wet bulb thermometer has a cloth covering that is moistened with water.When the psychrometer is slung or spun by the handle, air blows over both thermometers.Because the wet bulb thermometer is cooled by evaporation, its reading drops below that of the dry bulb thermometer.

If the relative humidity is high, the water on the wet bulb evaporates slowly, and the temperature on the wet bulb does not change much.If the relative humidity is low, the water on the wet bulb evaporates rapidly, and the temperature on the wet bulb drops.The relative humidity can be found by comparing the temperatures of the wet bulb and dry bulb thermometers.

See page 434 analyzing data

How Clouds FormClouds form when warm, moist air rises and cools. Water vapor condenses on tiny particles to form liquid water or ice crystals.- Water in the Atmosphere

Clouds form when water vapor in the air condenses to form liquid water or ice crystals.

Condensation is the process of water vapor in the air becoming liquid water.

Two conditions are required for condensation: cooling of the air and the presence of particles in the air.

Cold air holds less water vapor than warm air. As air cold, the amount of water vapor it can hold decreases. The water vapor condenses into tiny droplets of water or ice crystals. The temperature at which condensation begins is called the dew point.

Dew point

Tiny particles must be present in the atmosphere for the water to have a surface on which to condense. Particles:Salt crystalsDust from soilSmokeWater vapor can also condense on solid surfaces such as blades of grass or window panes. Liquid water that condenses from the air onto a cooler surface is called dew. Ice that has been deposited on a surface that is below freezing is called frost.

Types of CloudsScientists classify clouds into three main types based on their shape: cirrus, cumulus, and stratus. Clouds are further classified by their altitude.- Water in the Atmosphere

Cirrus comes from a word that means a curl of hairWispy, feathery or fibrous clouds are called cirrus clouds. Cirrus clouds form at very high altitudes. (temp. are low) Cirrus clouds are usually made of ice crystals. They often indicate that rain or snow will fall within several hours.

Cirrus clouds that have feathery hooked ends are called mares tails. Cirrocumulus clouds which look like rows of cotton balls or fish scales are called mackerel sky. Often indicate a storm is coming.

Cirrus or Cirrocumulus?

Cumulus clouds are fluffy and white. They form at altitudes of 2 to 18 kilometers. Cumulus clouds that are not very tall usually indicate fair weather. Cumulus means heap or mass

Cumulus clouds are common on sunny days .Fair weather cumulusTowering clouds with flat tops often produce thunderstorms. Cumulonimbus suffix nimbus means rain

StratusForm in flat layersStrato means spread outUsually cover most of the skyAre uniform dull, gray colorAs stratus clouds thicken they may produce drizzle, rain, or snow. Then they are called nimbostratus.

Nimbostratus or Stratus?

Middle Level CloudsAlto means highTwo main types Altocumulus and AltostratusHigher than regular cumulus and stratus clouds but lower than cirrus and other high clouds

altocumulusaltostratus

FogClouds that form at or near the groundOften forms when the ground cools at night after a warm humid dayGround cools the air just above the ground to the airs dew pointHeat of the morning sun burns the fog off as its water droplets evaporateMore common in areas near water or low lying marshy areasCan form in mountainous areas as the warm air moves up the slopes and cools

Answering E.Q. QuestionsQuestionAnswerHow does the water cycle work?Water evaporates from the surface, condenses to form clouds, and falls to Earth as rain or snow.What is relative humidity?The percentage of water vapor in the air compared to the maximum amount air can hold at that temperatureHow do clouds form?Water in the air condenses on tiny particles in the air to form liquid water or crystals.Can you determine weather conditions by looking at clouds?Yes; each type of cloud is associated with a particular type of weather.- Water in the Atmosphere

Identify CloudsSTRATUSNIMBOSTRATUSCUMULUSCUMULONIMBUSCIRRUSFOG

Sheets of low, grey clouds that bring light snow, rain, or drizzle.

Thicker layer than stratus clouds that completely block out the sun. They cause steady rain or snow.

White and puffy clouds that usually mean good weather.

Dark, towering clouds that are also called thunderheads. These clouds produce heavy rain, thunder, and lighting.

Thin, featherlike clouds that are made of ice crystals high in the atmosphere. Usually means a change in the weather is coming.

A cloud in contact with the ground.