Copyright © McGraw-Hill Education.Key Concepts• What is weather?• What variables are usedto describe weather?• How is weather relatedto the water cycle?What is weather?Weather is the atmospheric conditions, along with short-termchanges, of a certain place at a certain time. Have you ever beencaught in a rainstorm on what began as a sunny day? If so, youknow that weather can change quickly. It can also stay the samefor days.Weather VariablesVariables are things that can change. Temperature and rainfallare two of the variables used to describe weather. Meteorologistsare scientists who study and predict weather. They use severalvariables that describe a variety of atmospheric conditions. Thesevariables include air temperature, air pressure, wind speed anddirection, humidity, cloud coverage, and precipitation.Air TemperatureAir temperature is a measure of the average kinetic energy ofmolecules in the air. Kinetic energy is the energy an object hasbecause it is moving. When the temperature is high, moleculeshave a high kinetic energy. Therefore, molecules in warm air movefaster than molecules in cold air. Air temperatures vary with thetime of day, season, location, and altitude.What do you think? Read the two statements below and decidewhether you agree or disagree with them. Place an A in the Beforecolumn if you agree with the statement or a D if you disagree. Afteryou’ve read this lesson, reread the statements to see if you havechanged your mind.Before Statement After1. Weather is the long-term average ofatmospheric patterns of an area.2. All clouds are at the same altitude within theatmosphere.Summarize What YouRead After you read eachparagraph, write asentence or two in yourown words describing whatyou read. Use yoursentences to review thelesson.Key ConceptCheck1. Define What isweather?Describing WeatherWeather LESSON 1CHAPTER 6Reading Essentials Weather 121These variables include airtemperature, air pressure,wind speed and w-HillEducation.direction, humidity, cloudcoverage, and precipitation. Copyright © McGraw-Hill Education.Air PressureAir pressure is the pressurethat a column of air exerts on theair, or surface, below it. Look atthe figure. Air pressure decreasesas altitude increases. Therefore, airpressure is greater at low altitudesthan it is at high altitudes.Air pressure is measured withan instrument called a barometer.Air pressure is measured inmillibars (mb). The termbarometric pressure means “airpressure.” Knowing the barometricpressure of different areas helps meteorologists predict theweather.WindWind is created as air moves from areas of high pressure toareas of low pressure. Wind direction is given as the direction fromwhich the wind is blowing. For example, winds that blow fromwest to east are called westerlies. Meteorologists measure windspeed using an instrument called an anemometer (a nuh MAHmuh tur).HumidityThe amount of water vapor in the air is called humidity(hyew MIH duh tee). Humidity can be measured in grams percubic meter of air (g/m3). When the humidity is high, there is morewater vapor in the air. On a day with high humidity, your skinmight feel sticky because sweat might not evaporate quickly fromyour skin.Relative HumidityA sponge can absorb water. When it becomes full, it cannotabsorb any more water. In the same way, air can hold only acertain amount of gaseous water vapor. When air is saturated, itholds as much water vapor as possible. Temperature determineshow much water vapor air can contain. Warm air can contain morewater vapor than cold air can. Relative humidity is the amount ofwater vapor present in the air compared to the maximumamount of water vapor the air could contain at that temperature.Relative humidity is measured using a psychrometer. It is stated asa percent. A relative humidity of 50 percent means that theamount of water vapor in the air is one-half of the maximum theair can hold at that temperature.Visual Check2. Apply What happensto air pressure as altitudedecreases?Reading Check3. Name What instrumentmeasures air pressure?Reading Check4. Compare humidity andrelative humidity.122 Weather Reading EssentialsA relative humidity of 50 percentmeans that theamount of water vapor in the air isone-half of the maximum theair can hold at that temperature.They get closer togetherbarometer Copyright © McGraw-Hill Education.Dew PointWhen a sponge becomes saturated with water, the water startsto drip from the sponge. Likewise, when air becomes saturatedwith water vapor, the water vapor condenses and forms waterdroplets. When air near the ground is saturated, water vaporcondenses into a liquid. If the temperature is above 0°C, dewforms. If the temperature is below 0°C, ice crystals, or frost, form.Higher in the atmosphere, saturated air forms clouds.When the temperature decreases, air can hold less moisture.Eventually, the air becomes saturated, condensation occurs, anddew forms. The dew point is the temperature at which air issaturated and condensation can occur.Clouds and FogThink about what happens when you exhale warm air on a coldday. The warm air you exhale cools. If it reaches its dew point, youcan see the water vapor condense into a foggy cloud in front ofyour face. This also happens when warm air containing watervapor cools as it rises in the atmosphere. When the cooling airreaches its dew point, water vapor condenses on small particles inthe air and forms droplets. The droplets block and reflect light.This makes them visible as clouds.Clouds are water droplets or ice crystals suspended in theatmosphere. Clouds can have different shapes. Clouds can form atdifferent altitudes within the atmosphere. Read the table belowthat describes different types of clouds. As clouds move, water andthermal energy are transported from one location to another.Recall that clouds reflect some of the Sun’s incoming radiation.A cloud that forms near Earth’s surface is called fog. Fog is asuspension of water droplets or ice crystals close to or at Earth’ssurface. Fog reduces visibility. Visibility is the distance a person cansee into the atmosphere.Types of CloudsStratus Clouds• flat, white, andlayered• altitude up to2000 mCumulus Clouds• fluffy, heaped, orpiled up• 2,000 to 6,000 maltitudeCirrus Clouds• wispy• above 6,000 mVisual Check5. Illustrate Look outthe window and find acloud. Sketch the shape ofthe cloud in the spaceabove. Then highlight in thetable the type of cloud youdrew.Reading Check6. Describe What is fog?Make a two-tab book anduse it to collect informationabout the similarities anddifferences between cloudsand fog.Reading Essentials Weather 123OFog is asuspension of water droplets orice crystals close to or at Earth’ssurface. Fog reduces visibility. Copyright © McGraw-Hill Education.Visual Check7. Compare What is thedifference between snowand sleet?Key ConceptCheck8. Identify Whatvariables are used todescribe weather?PrecipitationDroplets in clouds form around small solid particles in theatmosphere. These particles might be dust, salt, or smoke.Precipitation occurs when cloud droplets combine and becomelarge enough to fall to Earth’s surface. Precipitation is water inliquid or solid form that falls from the atmosphere. Types ofprecipitation—rain, snow, sleet, and hail—are shown in the figureabove. Rain is precipitation that reaches Earth’s surface as dropletsof water. Snow is precipitation that reaches Earth’s surface assolid, frozen crystals of water. Sleet may start out as snow. Thesnow melts into rain as it passes through a layer of warm air andrefreezes when it passes through a layer of below-freezingair. Other times, sleet is just freezing rain. Hail reaches Earth’ssurface as large ice pellets. Hail starts as a small piece of ice that isrepeatedly caught in an updraft within a cloud. A layer of ice isadded with each lifting. When it becomes too heavy, it falls toEarth.The Water CyclePrecipitation is an important process in the water cycle, shownat the top of next page. Evaporation and condensation are alsoimportant processes in the water cycle. The water cycle is theseries of natural processes in which water continually movesamong oceans, land, and the atmosphere.Types of PrecipitationRainSleetSnowHail124 Weather Reading EssentialsTypes ofprecipitation—rain, snow, sleet,and hail—are shown in the figureabove. Rain is precipitation thatreaches Earth’s surface asdroplets of water. Snow isprecipitation that reaches Earth’ssurface assolid, frozen crystals of water.Sleet may start out as snow.Hail starts as a small piece of ice that isrepeatedly caught in an updraft within acloud. A layer of ice is added with eachlifting. When it becomes too heavy, it fallstoEarth. Copyright © McGraw-Hill Education.Most water vapor enters the atmosphere through evaporation.Water vapor forms as water is heated at the ocean’s surface. Watervapor cools as it rises in the atmosphere. The cooled water vaporcondenses back into liquid. Clouds form from droplets of liquidand solid water. Clouds produce precipitation. The precipitationfalls to Earth’s surface and later evaporates, continuing the cycle.V isual Check9. Identify Circle thename of the process inwhich liquid water changesinto water vapor.Key ConceptCheck10. Describe How isweather related to thewater cycle?Reading Essentials Weather 125oClouds produceprecipitation. Theprecipitation falls to Earth’ssurface and laterevaporates, continuing thecycle. Copyright © McGraw-Hill Education.Mini Glossary1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains howrelative humidity and dew point are related.2. Complete the graphic of the water cycle by writing the terms below in the correct order. You maybegin with any process.air pressure: the pressure that a column of airexerts on the air, or surface, below itdew point: the temperature at which air is fullysaturated because of decreasing temperatureswhile holding the amount of moisture constanthumidity (hyew MIH duh tee): the amount ofwater vapor in the airprecipitation: water, in liquid or solid form, that fallsfrom the atmosphererelative humidity: the amount of water vaporpresent in the air compared to the maximumamount of water vapor the air could contain atthat temperaturewater cycle: the series of natural processes inwhich water continually moves among oceans,land, and the atmosphereweather: the atmospheric conditions, along withshort-term changes, of a certain place at a certaintimecloud formationcondensationevaporationprecipitationThe Water Cycle c.b.d.a.Reread the statements at the beginning of thelesson. Fill in the After column with an A if youagree with the statement or a D if you dis-agree. Did you change your mind?What do you thinkEND OFLESSONLog on to ConnectED.mcgraw-hill.comand access your textbook to find thislesson’s resources.126 Weather Reading EssentialsAll of it has to do with the weather