The AtmosphereThe Atmosphere

© Lisa Michalek

Structure of the AtmosphereStructure of the Atmosphere

The atmosphere is divided into four layersThe atmosphere is divided into four layers– Troposphere Troposphere

The lowest layer The lowest layer is where most of the is where most of the weather occursweather occurs

– StratosphereStratosphere– MesosphereMesosphere– thermospherethermosphere

WeatherWeather The short-term (a few hours or days) The short-term (a few hours or days)

condition of the atmosphere at a given condition of the atmosphere at a given locationlocation– Temperature, sky conditions, Temperature, sky conditions,

precipitation, atmospheric precipitation, atmospheric pressure, humidity, wind pressure, humidity, wind speed, and wind directionspeed, and wind direction

MeteorologistsMeteorologists are are scientists who study scientists who study and predict the weatherand predict the weather

Air TemperatureAir Temperature In the daily cycle, temperature In the daily cycle, temperature

is usually lowest in the early is usually lowest in the early morning and warmest at mid-afternoonmorning and warmest at mid-afternoon

In the season cycle, winters are generally In the season cycle, winters are generally cold, while summers tend to be hotcold, while summers tend to be hot

Short term factors such as cloud cover and Short term factors such as cloud cover and regional weather systems affect regional weather systems affect temperaturestemperatures– Clouds reduce daytime temperature by reflecting Clouds reduce daytime temperature by reflecting

sunlight back into spacesunlight back into space– At night, clouds help hold heat energy to EarthAt night, clouds help hold heat energy to Earth

Air TemperatureAir Temperature Measured with a Measured with a thermometerthermometer

– A bulb that contains liquid that A bulb that contains liquid that expands into a narrow, expands into a narrow, calibrated neck when it is calibrated neck when it is heated and moves down the heated and moves down the neck when the temperature neck when the temperature decreasesdecreases

When meteorologists record When meteorologists record official air temperature, the official air temperature, the thermometer is kept in a special thermometer is kept in a special weather shelter to protect the weather shelter to protect the instruments from direct sunlightinstruments from direct sunlight

Temperature ScalesTemperature Scales A temperature of zero A temperature of zero

on the on the FahrenheitFahrenheit scale is the scale is the temperature of a temperature of a mixture of equal parts mixture of equal parts ice, water, and saltice, water, and salt

The freezing point of The freezing point of water is what sets the water is what sets the zero point on the zero point on the CelsiusCelsius (centigrade) (centigrade) scalescale

The point at which all The point at which all particle motion stops particle motion stops is defined as zero on is defined as zero on the the KelvinKelvin scale scale

Air PressureAir Pressure

Is caused by the weight of the atmosphereIs caused by the weight of the atmosphere Above each square inch of Earth’s surface Above each square inch of Earth’s surface

is a column of air the weighs 14.7 poundsis a column of air the weighs 14.7 pounds

Measuring Air PressureMeasuring Air Pressure

A A barometerbarometer is an instrument used to is an instrument used to measure air pressure using the dense liquid measure air pressure using the dense liquid metal mercurymetal mercury

Meteorologists measure Meteorologists measure air pressure in air pressure in millibarsmillibars

Standard sea level pressure Standard sea level pressure is 1013.2 millibarsis 1013.2 millibars

On a weather map, On a weather map, isobarsisobars connect places that have the connect places that have the same air pressuresame air pressure

Air Pressure FactorsAir Pressure Factors If air is cooled, it contracts and becomes If air is cooled, it contracts and becomes

denserdenser– This causes pressure to riseThis causes pressure to rise

If air is heated, it expands and becomes less If air is heated, it expands and becomes less densedense– This causes pressure to fallThis causes pressure to fall

Humid air is lighter than dry airHumid air is lighter than dry air– This is because water This is because water

molecules are lighter molecules are lighter than the gasses they than the gasses they displace in the airdisplace in the air

Moisture in the AtmosphereMoisture in the Atmosphere

When the air is holding as much moisture as When the air is holding as much moisture as it can, the air is it can, the air is saturatedsaturated– The air’s ability to hold water vapor depends The air’s ability to hold water vapor depends

upon the temperatureupon the temperature– The warmer the air, the more moisture the air The warmer the air, the more moisture the air

can holdcan hold The The dew pointdew point is the temperature is the temperature

to which the air must be cooled to to which the air must be cooled to become saturatedbecome saturated– If the temperature falls below the If the temperature falls below the

dew point, condensation occurs as dew point, condensation occurs as water vapor changes to liquid waterwater vapor changes to liquid water

Measuring Moisture Measuring Moisture in the Atmospherein the Atmosphere

Meteorologists use a sling Meteorologists use a sling psychrometerpsychrometer and a and a dew-point temperature table to determine the dew dew-point temperature table to determine the dew pointpoint– The psychrometer consists of two thermometers mounted The psychrometer consists of two thermometers mounted

side by side which can be swung through the airside by side which can be swung through the air– One thermometer measures the air temperatureOne thermometer measures the air temperature– The bulb of the other thermometer is covered by a wet The bulb of the other thermometer is covered by a wet

clothcloth– As the thermometers are swung through the air, As the thermometers are swung through the air,

evaporated cooling causes the wet-bulb thermometer to evaporated cooling causes the wet-bulb thermometer to register a lower temperatureregister a lower temperature

– When you subtract the wet-bulb temperature from the dry-When you subtract the wet-bulb temperature from the dry-bulb temperature, you can use the dew-point table to bulb temperature, you can use the dew-point table to determine the dew pointdetermine the dew point

PsychrometerPsychrometer

Relative HumidityRelative Humidity

Compares how much moisture the air is Compares how much moisture the air is actually holding with how much moisture it actually holding with how much moisture it could hold if the air were saturatedcould hold if the air were saturated– It is expressed as a percent of saturationIt is expressed as a percent of saturation

Air is saturated if it is holding all the moisture Air is saturated if it is holding all the moisture it can hold at its present temperatureit can hold at its present temperature

Determined with a psychrometer and a Determined with a psychrometer and a relative humidity tablerelative humidity table

The WindThe Wind

Wind is heat flow by convection within Wind is heat flow by convection within the atmospherethe atmosphere

Winds are the result of uneven heating of Winds are the result of uneven heating of the Earth’s surfacethe Earth’s surface– This uneven heating This uneven heating

causes differences causes differences in air pressure in air pressure to developto develop

The WindThe Wind

Winds always blow from areas of high Winds always blow from areas of high pressure to areas of low pressurepressure to areas of low pressure– Winds blow fastest where the gradient in air Winds blow fastest where the gradient in air

pressure is greatest, where the isobars are close pressure is greatest, where the isobars are close togethertogether

Measuring the WindMeasuring the Wind

To measure the wind, you need to determine To measure the wind, you need to determine both the wind speed and the wind directionboth the wind speed and the wind direction

Wind speed is measured with an Wind speed is measured with an anemometeranemometer– The cups catch the wind, The cups catch the wind,

causing it to spincausing it to spin

Wind direction is indicated Wind direction is indicated by a wind vane, which by a wind vane, which points points intointo the wind the wind

The Coriolis EffectThe Coriolis Effect

The Earth’s The Earth’s rotation causes rotation causes winds to curve winds to curve – to the to the rightright in in

the the Northern Northern HemisphereHemisphere

– to the to the leftleft in in the the Southern Southern HemisphereHemisphere

The Coriolis EffectThe Coriolis Effect

Winds move in a clockwise outward spiral Winds move in a clockwise outward spiral around high-pressure systemsaround high-pressure systems

Winds move in a counterclockwise inward Winds move in a counterclockwise inward spiral around low-pressure systemsspiral around low-pressure systems

The Coriolis EffectThe Coriolis Effect

Zones of Zones of Convergence and DivergenceConvergence and Divergence

Rising warm, moist air at the center of the Rising warm, moist air at the center of the low causes winds and air masses to blow low causes winds and air masses to blow into the low-pressure systeminto the low-pressure system

The rising air cools, which causes cloud The rising air cools, which causes cloud formation and precipitationformation and precipitation

The descending air turns a high-pressure The descending air turns a high-pressure system into a single mass of cool, dry air system into a single mass of cool, dry air that spreads across the surface of Earththat spreads across the surface of Earth

Cloud FormationCloud Formation

Clouds form when rising air is cooled below Clouds form when rising air is cooled below its dew pointits dew point

Tiny particles Tiny particles called called condensationcondensation nucleinuclei in the in the air allow a air allow a cloud to formcloud to form

PrecipitationPrecipitation Rain and snow are the most common forms Rain and snow are the most common forms

of precipitationof precipitation Drizzle is small raindrops that fall slowlyDrizzle is small raindrops that fall slowly Sleet is a partially frozen mixture of rain and Sleet is a partially frozen mixture of rain and

snow that occurs when the temperature is snow that occurs when the temperature is just above freezingjust above freezing

Hail is in the form of ice balls, which usually Hail is in the form of ice balls, which usually occurs in violent thunderstormsoccurs in violent thunderstorms– Hailstones begin as snowflakes that start to Hailstones begin as snowflakes that start to

melt and gather more moisture as they fallmelt and gather more moisture as they fall

PrecipitationPrecipitation