Earth Science - Weather - Quick Guides | Earth Science

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Study GuideEarth Science–Weather1. Moisture in the AtmosphereMost of Earth’s weather happens in thetroposphere, thelowest layer of the atmosphere. Becauseall weather processes take place here, the troposphere is often called the“weather sphere.”1.1Why the Troposphere Is the Weather LayerThe main reason the troposphere is the weather sphere is the presence ofwater vapor:•Water vapor is the source ofclouds, rain, snow, and storms.•This layer containsnearly all of the atmosphere’s moisture.As a result, changes in temperature and air movement in the troposphere strongly affect dailyweather.1.2What Happens Above the Troposphere?At the top of the troposphere is a boundary called thetropopause.•Above the tropopause, water vapor is essentiallyabsent.•Without water vapor, weather processes do not occur in the higher layers of the atmosphere.

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Study Guide1.3Atmosphere Layers and Their PropertiesEach layer of the atmosphere has different:•Temperatures•Pressures•Amounts of water vaporThese differences help explain why weather is limited to the troposphere. (A figure usually shows howtemperature, pressure, and water vapor change from one atmospheric layer to another.)1.4Measurement of Water in the AirWater vapor is always present in the air, but the amount can change. Scientists use differentmeasurements to describehow much water vapor is in the atmosphere.1.5Specific HumidityOne way to measure moisture is calledspecific humidity.•It tells ushow much water vapor is actually in the air.•Warm air can hold more water vapor than cold air.When air holdsas much water vapor as it possibly can, it is said to besaturated.•As temperature increases, air’s capacity to hold water also increases.•In fact, this capacitydoubles for about every 11°C rise in temperature.1.6Relative HumidityThe measurement most commonly used isrelative humidity.•Relative humidity compareshow much water vapor is in the airtohow much it can holdat that temperature.•It is always expressed as apercentage.

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Study GuideFor example:•100% relative humiditymeans the air isfully saturated.•Lower percentages mean the air is holding less moisture than it could.1.7Finding Relative HumidityThere aretwo main waysto measure relative humidity.Method 1: Using a HygrometerAhygrometermeasures humidity using a simple but clever idea:•It uses apiece of hair attached to a pointer.•When humidityincreases, the hairstretches, moving the pointer.•When humiditydecreases, the hairshrinks, and the pointer moves back.This is why humid days are sometimes called“bad-hair days.”Method 2: Using Two Thermometers (Wet-Bulb and Dry-Bulb)Another method usestwo thermometersand achart, all based on theCelsius scale.If temperature readings come from astation model, they must be converted fromFahrenheit toCelsius.Station models showsurface weather observationsfor a specific city.Here’s how the two thermometers work:•Dry-bulb thermometeroMeasures theactual air temperature.•Wet-bulb thermometeroHas awet wickwrapped around the bulb.oAs waterevaporatesfrom the wick, it takes energy with it.oThis causes the wet-bulb thermometer tocool down.

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Study Guide1.8Wet-Bulb DepressionThe difference between the dry-bulb and wet-bulb temperatures is called thewet-bulb depression.Important points to remember:•Thewet-bulb temperature is always lower than or equal tothe dry-bulb temperature.•If both temperatures are thesame, the air issaturated, and the relative humidity is100%.•Higher humidity→ less evaporation →smaller temperature difference.•Lower humidity→ more evaporation →larger temperature difference.Thewet-bulb depression, along with the dry-bulb temperature, is used with achartto find therelative humidity.

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Study Guide1.9Dew PointTo measure moisture in the air using wet-bulb and dry-bulb thermometers, scientists use aninstrument called apsychrometer.•Asling psychrometeris a handheld version that is spun in the air to get accurate readings.1.10What Is the Dew Point?Thedew pointis thetemperature at which water vapor in the air changes into liquid water.•When air cools to its dew point, condensation begins.•If the dew point isbelow 0°C, it is called thefrost point.Dew point and frost point are important because they help meteorologists decide when to issuefrostand freeze warnings, which are especially important for agriculture and safety.

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Study Guide1.11Finding the Dew PointThe dew-point temperature is found in a way similar to finding relative humidity:1.Measure thedry-bulb temperature(air temperature).2.Measure thewet-bulb temperature.3.Find thewet-bulb depression(the difference between the two temperatures).4.Use thedry-bulb temperatureand thewet-bulb depressionwith achart(like the oneshown in the table) to determine thedew point.1.12Condensation and Fog FormationWhen water vapor comes intodirect contact with a cooler surface, it can condense onto thatsurface.Fogforms when air cools to its dew point near the ground. There are different types of fog:•Advection fogoForms whenwarm, moist air moves over a cold surface.oThis cooling causes condensation and fog.oIt can also happen whencooler air moves over a warmer surface.•Ground (radiation) fogoForms at night due toradiational cooling.oCommon inhumid valleysand nearrivers and lakes.1.13CloudsAcloud forms when air cools to its dew-point temperaturehigher in the atmosphere.•As airrises, it expands and cools.•Once it reaches the dew point, condensation begins and clouds form.•If the temperature isabove 0°C, clouds are made ofwater droplets.•If the temperature isbelow 0°C, clouds containice crystals, snow crystals, andsupercooled water.

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Study Guide1.14Types of CloudsClouds are grouped intothree main categories:•Cirrus cloudsoVeryhigh-altitude cloudsoMade ofice crystalsoThin, wispy, and feathery in appearance•Stratus cloudsoLow-level,layeredcloudsoForm sheet-like blankets across the sky•Cumulus cloudsoPuffy,cotton-likecloudsoForm fromstrong vertical rising of airMany clouds arecombinationsof these basic types.1.15Cloud Name CluesCloud names often includeprefixes or suffixesthat give more information:•Altomeanshigh altitude•Nimbusmeans the cloud is associated withrain or precipitation1.16Cloud DevelopmentClouds form and grow because of howair moves, cools, and changes density. Let’s break thisprocess down step by step.

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Study Guide1.17Rising Air and CoolingWhen aparcel of air rises, it begins tocool. This happens because:•Air pressuredecreases with height.•As pressure drops, the airexpands, and expanding air cools.Therate of coolingdepends on how much moisture the air contains.1.18Adiabatic Lapse Rates•Dry aircools at a rate of1°C for every 100 metersit rises.This is called thedry adiabatic lapse rate.•When the air contains moisture, the cooling rate slows to0.6°C per 100 meters.This is known as themoist adiabatic lapse rate.The difference between these rates is caused by thehigh specific heat of water, which allows moistair to retain heat longer.

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Study Guide1.19Buoyancy and Vertical Cloud GrowthWhen air near the surface isheated, it becomes:•Warmer than the surrounding air•Less dense•More buoyant, meaning it risesThis buoyancy explains why clouds appear tofloat in the sky.As rising air continues to cool, clouds can growvertically. In strong conditions, this process can leadto the formation ofcumulonimbus clouds.•Cumulonimbus cloudsarethunderstorm clouds•They are often associated withheavy rain, hail, strong winds, and tornadoes•These clouds form in anunstable air mass, where air movement is driven by densitydifferences1.20Clouds in Stable Air MassesNot all clouds form from strong vertical motion.In astable air mass, clouds can form when air is:•Forced upward by land features, such asmountains•Cooled by radiational cooling, especially when warm air mixes with a cooler layerThese clouds are usuallylayered clouds, such as stratus clouds.1.21Cloud Base and Condensation LevelSome clouds have aflat baseand rise upward on top.•Thebase of the cloudforms where theair temperature equals the dew-pointtemperature.•This level is called thecondensation level.

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Study Guide1.22Finding the Cloud Base HeightThe height of the cloud base, also called thelifting condensation level (LCL), can be found in twoways.Method 1: Using a Formula1.Find the difference between theair temperatureand thedew pointat the surface.2.Divide that difference by0.8°C(the rate at which dew point approaches air temperature per100 meters).3.Multiply the result by100to find thecloud base height in meters.Method 2: Using a Chart•Plot theair temperaturealong thesolid lines.•Plot thedew-point temperaturealong thedashed lines.•Where the two lines meet, read across to the side labeled“Altitude.”•This value gives thecloud base height in kilometers.1.23Anvil-Shaped Cloud TopsAs clouds continue to rise, they eventually reach a height where:•Thecloud temperature equals the surrounding air temperature•The cloud isno longer buoyantAt this point, the cloud spreads outward instead of upward, creating theflat, anvil-shaped topscommonly seen on large thunderstorm clouds.1.24PrecipitationPrecipitationincludes all the ways water falls from the atmosphere to Earth, such as rain, snow, andhail. For precipitation to form, the atmosphere must meettwo important conditions.

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