The atmosphere

CHAPTER 18

ENERGY BALANCE IN THE ATMOSPHERE

Prepared by: Serene Grace A. Lacea BSEd 3- Physical Science University of Bohol

QUICK FACTSAtmosphere- a blanket of gases that wraps around a planet or any other object in space.

Earth’s Atmosphere is AirAir contains more Nitrogen than any other gas. Nitrogen makes up 78 percent of the air. Oxygen, the gas that is most important for keeping you alive, makes up 21 percent.

The Weight of Air

You cannot feel the weight of air, but all the air in the atmosphere presses downward. This weight is called atmospheric pressure.

Atmospheric pressure depends on how much gas is in the atmosphere. The higher you go, the less air there is and the lower the atmospheric pressure gets.

A Layer Cake of AirEarth’s atmosphere extends about 6000 miles (9600 kilometers) above the surface of the Earth.

The atmosphere has several layers:

Troposphere

ENERGY BALANCE IN THE ATMOSPHEREHow glorious a greeting the sun gives the mountains! -John Muir

Sections:18.1 Incoming Solar Radiation18.2 The Radiation Balance18.3 Energy Storage and Transfer18.4 Temperature Changes with Latitude and Season18.5 Temperature Changes with Geography

Weather is the state of the atmosphere at a given place and time, temperature, wind, cloudiness, humidity, and precipitation are all components of weather. The weather changes frequently, from day to day or even from hour to hour.

Climate is the characteristic weather of a region, particularly the temperature and precipitation, averaged over several decades.

18.1 INCOMING SOLAR RADIATION

Solar energy streams from the Sun in all directions, and Earth receives only one two-billionth of the total solar output. However, even this tiny fraction warms Earth’s surface and makes it habitable.

The space between Earth and Sun is nearly empty. How does sunlight travel through a vacuum?

Light is unique; it behaves as a wave and a particle simultaneously.

Particles of light are called photons. In a vacuum, photons travel only at one speed, the speed of light, never faster and never slower.

The speed of light is 3 x 108 meters per second.

Light also behaves as an electrical and magnetic wave, called electromagnetic radiation.

The terms used to describe a light wave are identical to those used for water, sound, and other types of waves.

Its wavelength is the distance between successive crests.

The frequency of a wave is the number of complete wave cycles, from crest to crest, that pass by any point in a second.

The electromagnetic spectrum is the entire range of electromagnetic radiation from very-long-wavelength (low frequency) radiation to very-short-wavelength (high frequency) radiation.

The visible light of the electromagnetic spectrum

Visible light is only a tiny portion (about one-millionth of 1 percent) of the electromagnetic spectrum.

ABSORPTION AND EMISSION

Absorption of radiation is the process that occurs when energy is absorbed: the energy of a photon is converted to electrical, chemical, vibrational, or heat energy, and the photon disappears.


Excited state is a state of physical energy higher than the lowest energy level (or ground state) of an electron in an atom or molecule.


Emission of radiation is the process that occurs when energy, in the form of a photon, is emitted as an electron falls out of an excited state, with the equivalent loss of energy from the emitting substance.

REFLECTION

Reflection occurs when a light ray hits a surface and bounces off.

If the light cannot pass through the surface, it bounces off or reflects.

REFLECTION

Most surfaces- including your body- absorb some light and reflect some light. Mirrors, however, reflects almost all the light that hits them. The metallic coating of the back causes the reflection.

The reflectivity of a surface is referred to its albedo (from the Latin for “whiteness”) and is often expressed as a percentage.

REFLECTION

A mirror reflects nearly 100 percent of the light that strikes it and has an albedo close to 100 percent.

Snowfields and glaciers have high albedos and reflect 80 to 90 percent of sunlight. Clouds have the second-highest albedo and reflect 50 to 55 percent of sunlight.

On the other hand, city buildings and dark pavement have albedos of only 10 to 15 percent.

REFLECTION

Forests, with independent surfaces of dark leaves, have an even lower albedo of about 5 percent. The oceans, which cover about two-thirds of Earth’s surface, also have a low albedo.

As a result, they absorb considerable solar energy and strongly affect Earth’s radiation balance. Thus the temperature balance of the atmosphere is profoundly affected by the albedos of the hydrosphere, geosphere, and biosphere.

SCATTERING

Atmospheric gases, water droplets, and dust particles scatter sunlight in all directions.

The amount of scattering is inversely proportional to the wavelength of light. Short-wavelength blue light, therefore, scatters more than longer-wavelength red light.

SCATTERING

The Sun emits light of all wavelengths, which combine to make up white light. Consequently, in space the Sun appears white.

The sky appears blue from Earth’s surface because the blue component of sunlight scatters more than other frequencies and colors the atmosphere.

The Sun appears yellow from Earth because yellow is the color of white light with most of the blue light removed.

18.2 THE RADIATION BALANCE

Of all the sunlight that reaches Earth, 50 percent is absorbed, scattered, or reflected by clouds and atmosphere, 3 percent is reflected by Earth’s surface, and 47 percent is absorbed by Earth’s surface. The absorbed radiation warms rocks, soil, and water.

If Earth absorbs radiant energy from the Sun, why doesn’t Earth’s surface get hotter and hotter until the oceans boil and the rocks melt?


Rocks, soil, and water reemit virtually all the energy they absorb. Most solar energy that reaches Earth is short-wavelength, visible, and ultraviolet radiation.

Earth’s surface absorbs this radiation and then reemits the energy mostly as long-wavelength, invisible, infrared radiation.

Some of this infrared heat escapes directly into space, but some is absorbed by the atmosphere.


If Earth had no atmosphere, radiant heat loss would be so rapid that Earth’s surface would cool drastically at night. Earth remains warm at night because the atmosphere absorbs and retains much of the radiation emitted by the ground.

If the atmosphere were to absorb even more of the long-wavelength radiant heat from Earth, the atmosphere and Earth’s surface would become warm. This warming process is called the greenhouse effect.

18.3 ENERGY STORAGE AND TRANSFER: THE DRIVING MECHANISMS FOR WEATHER AND CLIMATE

HEAT AND TEMPERATURE

All matter consists of atoms and molecules that are in constant motion. They fly through space, they rotate, and they vibrate.

The temperature, or measure of heat in a substance, is proportional to the average speed of the atoms or molecules in a sample.

In contrast, heat is a measure of the total energy in a sample. It is related to the average energy of every molecule multiplied by the total number of molecules.

HEAT TRANSPORT BY CONDUCTION AND CONVECTION

Conduction is the transport of heat by direct collisions among atoms or molecules.

Ex: If you place a metal frying pan on the stove, the handle gets hot, even though it is not in contact with the burner, because the metal conducts heat from the bottom of the pan to the handle.

HEAT TRANSPORT BY CONDUCTION AND CONVECTION

Convection is the upward and downward flow of fluid material in response to heating and cooling. It involves the transport of heat by the movement of currents. Convection occurs readily in liquids and gases.

CHANGES OF STATE

Most substances can exist in three states: solid, liquid, and gas. At Earth’s surface, many substances commonly exist in only one state.

Latent heat (stored heat) is the energy released or absorbed when a substance changes from one state to another.

HEAT STORAGE

If you place a pan of water and a rock outside on a hot summer day, the rock becomes hotter than the water.

Why is the rock hotter?

1. Specific Heat is the amount of energy needed to raise the temperature of 1 gram of material by 1°C.

2. Rock absorbs heat only at its surface, and the heat travels slowly through the rock.

3. Evaporation is a cooling process.

18.4 TEMPERATURE CHANGES WITH LATITUDE AND SEASON

TEMPERATURE CHANGES WITH LATITUDE

The region near the equator is warm throughout the year, whereas polar regions are cold and ice-bound even in summer.

The equator receives the most concentrated radiation. The Sun strikes the rest of the globe at an angle and thus radiation is less concentrated at higher latitudes.

TEMPERATURE CHANGES WITH LATITUDE

Because the equator receives the most concentrated solar energy, it is generally warm throughout the year.

Average atmospheric temperature becomes progressively cooler poleward. But the average temperature does not change steadily with latitude, because many other factors- such as winds, ocean currents, albedo, and proximity to the oceans- also affect atmospheric temperature in any given region.

THE SEASONS

Earth circles the Sun in a planar orbit, while simultaneously spinning on its axis. This axis is tilted at 23.5° from a line drawn perpendicular to the orbital plane.

The North Pole tilts toward the Sun in summer and away from it in winter. June 21 is the summer solstice in the Northern Hemisphere because at this time, the North Pole leans the full 23.5° toward the Sun.

THE SEASONS

As a result, sunlight strikes Earth from directly overhead at a latitude 23.5° north of the Equator. This latitude is called the Tropic of Cancer.

If you stood on the Tropic of Cancer at noon on June 21, you would cast no shadow. June is warm in the Northern Hemisphere for two reasons:1. When the Sun is high in the sky, sunlight is more

concentrated than it is in winter.2. When the North Pole is tilted toward the Sun, it receives

24 hours of daylight

THE SEASONS

Polar regions are called lands of the midnight Sun because the Sun never sets in the summertime. Below the Arctic Circle the Sun sets in the summer, but the days are always loner than they are in winter.

When it is summer in the Northern Hemisphere, the South Pole tilts away from the Sun and thus the Southern Hemisphere receives low-intensity sunlight and has short days. June 21 is the first day of winter in the Southern Hemisphere. Six months later, on December 21 or 22, the seasons are reversed.

THE SEASONS

The North Pole tilts away from the Sun, giving rise to the winter solstice in the Northern Hemisphere, while it is summer in the Southern Hemisphere.

On this day, sunlight strikes Earth directly overhead at the Tropic of Capricorn, latitude 23.5° south. At the North pole, the Sun never rises and it is continuously dark, while the South Pole is bathed in continuous daylight.

THE SEASONS

On March 21 and September 22, Earth’s axis lies at right angles to a line drawn between earth and the Sun. as a result, the poles are not tilted toward or away from the Sun and the Sun shines directly overhead at the equator at noon.

In the Northern Hemisphere, March 21 is the first day of spring and September 22 is the first day of autumn, whereas the seasons are reversed in the Southern Hemisphere.

THE SEASONS

On the first days of spring and autumn, every portion of the globe receives 12 hours of direct sunlight and 12 hours of darkness.

For this reason, March 21 and September 22 are called equinoxes, meaning equal nights.

THE SEASONS

18.5 TEMPERATURE CHANGES WITH GEOGRAPHY

Even though all locations at a given latitude receive equal amounts of solar radiation, some places have cooler climates than others at the same latitude.

Lines called isotherms connect areas of the same average temperature

18.5 TEMPERATURE CHANGES WITH GEOGRAPHY

ALTITUDE

At higher elevations in the troposphere, the atmosphere is thinner and absorbs less energy. In addition, lower parts of the troposphere have absorbed much of the heat radiating from earth’s surface.

Consequently, temperature decreases with elevation.

OCEAN EFFECTS

Land heats more quickly in summer and cools more quickly in winter than ocean surfaces do. As a result, continental interiors show greater seasonal extremes of temperature than coastal regions.

WIND DIRECTION

Winds carry heat from region to region just as ocean currents do.

CLOUD COVER AND ALBEDO

During the day, clouds reflect sunlight and cool the surface. In contrast, clouds have the opposite effect and warm the surface during the night.

After the Sun sets, Earth cools because radiant heat from soil and rock escape into the air. Clouds act as an insulating blanket by absorbing outgoing radiation and reradiating some of it back downward.

Thus, cloudy nights are generally warmer than clear nights.

CHAPTER 19

MOISTURE, CLOUDS, AND WEATHER

MOISTURE, CLOUDS, AND WEATHERSunshine is delicious, rains is refreshing, wind braces us up, snow is exhilarating; there is really no such thing as bad weather, only different kinds of good weather, - John Ruskin

Sections:19.1 Moisture in Air19.2 Cooling and Condensation19.3 Rising Air and Precipitation19.4 Types of Clouds19.5 Fog19.6 Pressure and Wind19.7 Fronts and Frontal Weather 19.8 How Earth’s Surface Features Affect Weather19.9 Thunderstorms19.10 Tornadoes and Tropical Cyclones19.11 Hurricane Katrina19.12 El Niño

19.1 MOISTURE IN AIR

Precipitation occurs only when there is moisture in the air. Therefore to understand precipitation, we must first understand how moisture collects in the atmosphere and how it behaves.

HUMIDITY

It is the amount of water vapor

in the air.

Absolute humidity, is the mass of water vapor in a given volume of air, expressed in grams per cubic meter (g/m3).

Relative humidity is the amount of water vapor in air relative to the maximum it can hold at a given temperature. It is expressed a s a percentage.

HUMIDITY

Relative humidity (%) = x 100

If air contains half as much water vapor as it can hold, its relative humidity is 50 percent. Suppose that air at 25°C contains 11.5 g/m3 of water vapor. Since air at that temperature can hold 23 g/m3, it is carrying half of its maximum, and the relative humidity is

11.5g/23g x 100 = 50%.

HUMIDITY

When relative humidity reaches 100 percent, the air is saturated. The temperature at which saturation occurs is the dew point.

If saturated air cools below the dew point, some of the water vapor may condense into liquid droplets.

SUPERSATURATION AND SUPERCOOLING

When the relative humidity reaches 100 percent, water vapor condenses quickly onto solid surfaces such as rocks, soil, and airborne particles.

Airborne particles such as dust, smoke, and pollen are abundant in the lower atmosphere. Consequently, water vapor may condense easily at the dew point in the lower atmosphere, and there the relative humidity rarely exceeds 100 percent.


In the clear, particulate-free air high in the troposphere, condensation occurs so slowly that for all practical purposes it does not happen.

As a result, the air commonly cools below its dew point but water remains as vapor. In that case, the relative humidity rises above 100 percent, and the air reaches a point of supersaturation.


Similarly, liquid water does not always freeze at its freezing point. Small droplets can remain liquid in a cloud even when the temperature is -40°C. Such water has undergone supercooling.

19.2 COOLING AND CONDENSATION

Moisture condenses to form water droplets or ice crystals when moist air cools below its dew point. Clouds and fog are visible concentrations of this airborne water and ice.

Three atmospheric processes cool air to its dew point and cause condensation:1. Air cools when it loses heat by radiation2. Air cools by contact with a cool surface such as water,

ice, rock, soil, or vegetation3. Air cools when it rises.

RADIATION COOLING

The atmosphere, rocks, soil, and water absorb the Sun’s heat during the day and then radiate some of this heat back out toward space at night. As a result of heat lost by radiation, air, land, and water become cooler at night, and condensation may occur.

CONTACT COOLING: DEW AND FROST

Heat water on a stove until it boils and then hold a cool drinking glass in the clear air just above the steam.

Water droplets will condense on the surface of the glass because the glass cools the hot, moist air to its dew point.

The same effect occurs in a house on a cold day. water droplets or ice crystals appear on windows as warm, moist, indoor air cools on the glass.

CONTACT COOLING: DEW AND FROST

In some regions, the air on a typical summer evening is warm and humid. After the Sun sets, plants, houses, windows, and most other objects lose heat by radiation and therefore become cool.

During the night, water vapor condenses on the cool objects. This condensation is called dew. If the dew point is below freezing, frost forms. Frost is not frozen dew, but ice crystals formed directly from vapor.

COOLING OF RISING AIR

Radiation and contact cooling close to Earth’s surface form dew, frost, and some types of fog. However, clouds and precipitation normally form at higher elevations where the air is not cooled by direct contact with the ground. Almost all cloud formation and precipitation occur when air cools as it rises.


Work and heat are both forms of energy. Work can be converted to heat or heat can be converted to work, but energy is never lost.

Variations in temperature caused by compression or expansion of gas are called adiabatic temperature changes. Adiabatic means without gain or loss of heat.


During adiabatic warming, air warms up because work is done on it, not because heat is added. During adiabatic cooling, air cools because it performs work, not because heat is removed.

Air pressure decreases with elevation. When dense surface air rises, it expands because the atmosphere around it is now of lower density. Rising air performs work to expand, and therefore it cools adiabatically.


Dry air cools by 10°C for every 1,000 meters it rises. This cooling rate is called dry adiabatic lapse rate. Thus, if dry air were to rise from sea level to 9,000 meters, it would cool by 90°C.

The wet adiabatic lapse rate is the rate at which rising moist air cools adiabatically after it has reached its dew point and condensation has begun- varying depending on moisture content from 5°C to 9°C for every 1,000 meters it rises.

19.3 RISING AIR AND PRECIPITATION

Three mechanisms cause air to rise and cool:

1. Orographic lifting2. Frontal wedging3. Convection-convergence

OROGRAPHIC LIFTING-lifting of air that occurs when air flows over a mountain.

FRONTAL WEDGING- A process by which a moving mass of cool, dense air encounters a mass of warm, less-dense air; the cool, dense air slides under the warm air mass, forcing the warm air upward to create a weather front.

CONVECTION-CONVERGENCEConvection is the upward, and downward, and horizontal flow of fluids in response to heating and cooling.

If one portion of the atmosphere becomes warmer than the surrounding air, the warm air expands, becomes less dense, and rises.

CONVECTION-CONVERGENCE

CONVECTIVE PROCESSES AND CLOUDSAir is generally warmest at Earth’s surface and cools with elevation throughout the troposphere. The rate at which air that is neither rising nor falling cools with elevation is called normal lapse rate.

The average normal lapse rate is 6°C for every 1000 meters of elevation.

CONVECTIVE PROCESSES AND CLOUDSThe normal lapse rate is variable. It is greatest near Earth’s surface and decreases with altitude. The normal lapse rate also varies with latitude, the time of day, and the seasons.

Unstable air is a parcel of warm, moist air that rises rapidly, ascends to high elevations, and leads to formation of towering clouds and heavy rainfall.

Stable air is a parcel of warm, dry air that does not rise rapidly, does not ascend to high elevations, and does not lead to cloud formation and precipitation.

19.4 TYPES OF CLOUDS

Cloud names are based on the shape and altitude of the clouds.

CIRRUS- Latin for “wisp of hair”

- Wispy, high-altitude clouds composed of ice

crystals.

STRATUS- Latin for “layer”

- Horizontally layered

clouds that spread out into a broad sheet, usually

creating dark, overcast skies.

CUMULUS- Latin for “heap” or “pile”

- Fluffy white clouds with flat bottoms and billowy

tops.

STRATOCUMULUS- Low, sheet like clouds with some vertical structure.

CUMULONIMBUS- Towering storm clouds that form in columns and produce

intense rain, thunder, lightning, and sometimes hail.

NIMBOSTRATUS- Stratus clouds from which rain or snow falls.

ALTOSTRATUS- High altitude stratus clouds.

TYPES OF PRECIPITATION Rain

The droplets in a cloud are small, about 0.01 millimeter in diameter.

In still air, such a droplet would require 48 hours to fall from a cloud 1000 meters above Earth. But these tiny droplets never reach Earth because they evaporate faster than they fall.

Rain

If the air temperature in a cloud is above freezing, the tiny droplets may collide and coalesce.

If two droplets collide, they merge to become one large drop.

If the droplets in a cloud grow large enough, they fall as drizzle or light rain.

About 1 million cloud droplets must combine to form an average-size raindrop.

Rain

Part of the reason for this is that the temperature in clouds is commonly below freezing, but another factor also favors ice formation.

In many clouds, water vapor initially forms ice crystals rather than condensing as tiny droplets of super cooled water.

As air cools toward its dew point, all the vapor forms ice crystals rather than super cooled water droplets.

Rain

The tiny ice crystals then grow larger as more water vapor condenses on them, until they are large enough to fall.

The ice then melts to form raindrops as it falls through warm layers of air.

Snow, Sleet, and Glaze

When the temperature in a cloud is below freezing, the cloud is composed of ice crystals rather than water droplets.

If the temperature near the ground is also below freezing, the crystals remain frozen and fall as snow.


In contrast, if raindrops form in a warm cloud and fall through a layer of cold air at lower elevation, the drops freeze and fall as small spheres of ice called sleet.


Sometimes the freezing zone near the ground is so thin that raindrops do not have to freeze before they reach Earth.

However, when they land on subfreezing surfaces, they form a coating of ice called glaze.

Hail

Large ice globules varying from 5 millimeters to a record 14 centimeters in diameter that fall from cumulonimbus clouds.

Hail

A 500-gram hailstone crashing to Earth at 160 kilometers per hour can shatter windows, dent car roofs, and kill people and livestock.

19.5 FOG

- A cloud that forms at or vey close to ground level.- Advection fog forms when warm, moist air from the sea

blows onto cooler land, where the air cools and water vapor condenses at ground level.

Radiation fog occurs when Earth’s surface and the air near the surface cool by radiation during the night, and water vapor in the air condenses because it cools below its dew point.

Radiation fog is particularly common in areas where the air is polluted because water vapor condenses rapidly on the tiny particles suspended in the air.

Evaporation fog forms when air is cooled by evaporation from a body of water, commonly a lake or river and typically in late fall or early winter when the air is cool but the water is still warm.

The water evaporates, but the vapor cools and condenses o fog.

Upslope fog forms when air cools as it rises along a land surface. Upslope fogs occur both on gradually sloping plains and on steep mountains.

19.6 PRESSURE AND WIND

Warm air is less dense than cold air. Thus, warm air exerts a relatively low atmospheric pressure and cold air exerts a relatively high atmospheric pressure.

Warm air rises because it is less dense than the surrounding cool air. Air rises slowly above a typical low-pressure region, at a rate of about 1 kilometer per day.

In contrast, if air in the upper atmosphere cools, it becomes denser than the air beneath it and sinks.

19.6 PRESSURE AND WIND

19.6 PRESSURE AND WINDAir must flow inward over Earth’s surface toward a low-pressure region to replace a rising air mass. But a sinking air mass displaces surface air, pushing it outward from a high-pressure region.

Thus vertical airflow in both high- and low-pressure regions is accompanied by horizontal airflow, called wind.

Winds near Earth’s surface always flow away from a region of high pressure and toward a low-pressure region.

Ultimately, all wind is caused by the pressure differences resulting from unequal heating of Earth’s atmosphere.

PRESSURE GRADIENT

Wind speed is determined by the magnitude of the pressure differences over distance, called the pressure gradient. Thus wind blows rapidly if a large pressure difference exists over a short distance.

A steep pressure gradient is analogous to a steep hill. Wind flows rapidly across a steep pressure gradient. To create a pressure-gradient map, air pressure is measured at hundreds of different weather stations. Points of equal pressure are connected by map lines called isobars.

PRESSURE GRADIENT

A steep pressure gradient is shown by closely spaced isobars, whereas a weak pressure gradient is indicated by widely spaced isobars.

Pressure gradients change daily or sometimes hourly, as high- and low-pressure zones move. Therefore, maps are updated frequently.

CORIOLIS EFFECT

The Coriolis effect, caused by Earth’s spin, deflects ocean currents.

The Coriolis effect similarly deflects winds. In the Northern Hemisphere wind is deflected toward the right, and in the Southern Hemisphere, to the left.

The Coriolis effect alters wind direction but not its speed.

FRICTION

Rising and falling air generates wind both along Earth’s surface and at higher elevations. Surface winds are affected by friction with Earth’s surface, whereas high-altitude winds are not.

As a result, wind speed normally increases with elevation.

CYCLONES AND ANTICYCLONES

If Earth did not spin, wind would flow directly across the isobars. However Earth does spin, and the Coriolis effect deflects wind to the right.

This rightward deflection creates a counterclockwise vortex near the center of the low-pressure region.

Such a low-pressure region with its accompanying surface wind is called a cyclone.

CYCLONES AND ANTICYCLONESThe opposite mechanism forms an anticyclone around a high-pressure region.

When descending air reaches the surface, it spreads out in all directions. In the Northern Hemisphere the Coriolis effect deflects the diverging winds of an anticyclone to the right, forming a pinwheel pattern, with the wind spiraling clockwise.

In the Southern Hemisphere, the Coriolis effect deflects winds leftward and creates a counterclockwise spiral.

PRESSURE CHANGES AND WEATHER

As explained earlier, wind blows to any difference in pressure. However, low pressure generally brings clouds and precipitation with the wind, and sunny days predominate during high pressure.

Rising air forms a region of low pressure. But rising air also cools adiabatically. If the cooling is sufficient, clouds form and rain or snow may fall.

PRESSURE CHANGES AND WEATHERLow barometric pressure is an indication of wet weather. When cool air sinks, it is compressed and the pressure rises.

In addition, sinking air is heated adiabatically. Because warm air can hold more water vapor than cold air can, the sinking air absorbs moisture, and thus clouds generally do not form over a high-pressure region.

Fair, dry weather generally accompanies high pressure.

19.7 FRONTS AND FRONTAL WEATHER

An air mass is a large body of air with approximately uniform temperature and humidity at any given altitude.

Typically, an air mass is 1500 kilometers or more across and several kilometers thick. Because air acquires both heat and moisture from Earth’s surface, an air mass is classified by its place of origin.

Temperature can be either polar (cold) or tropical (warm).


Air masses move and collide. The boundary between a warmer air mass and a colder one is a front.

When two air masses collide, one of the air masses is forced to rise, which often results in cloudiness and precipitation.

Frontal weather patterns are determined by the types of air masses that collide and their relative speeds and directions.


Symbols commonly used in weather maps.

WARM FRONTS AND COLD FRONTSA warm front forms when moving warm air collides with a stationary or slower-moving cold air mass. The moving warm air mass rises over the denser cold air, cools adiabatically, and the cooling generates clouds and precipitation.

WARM FRONTS AND COLD FRONTSA cold front forms when moving cold air collides with stationary or slower-moving warm air. The dense cold air distorts into a blunt wedge and pushes under the warmer air, creating a narrow band of violet weather commonly accompanied by cumulus and cumulonimbus clouds.

OCCLUDED FRONT

A front that forms when a faster-moving cold air mass traps a warm air mass against a second mass of cold air. Precipitation occurs along both frontal boundaries, resulting in a large zone of inclement weather.

STATIONARY FRONT

A front at the boundary between two stationary air masses of different temperatures.

THE LIFE CYCLE OF A MIDLATITUDE CYCLONE

A cyclone is a low-pressure system with rotating winds. Most cyclones in the middle latitudes of the Northern Hemisphere develop along a front between polar and tropical air masses.

The storm often starts with winds blowing in opposite directions along a stationary front between the two air masses.

Storm tracks are paths repeatedly followed by storms.

19.8 HOW THE EARTH’S SURFACE FEATURES AFFECT WEATHER Earth’s surface features- including mountain ranges, rainforests, proximity to the sea, and uneven heating and cooling continents- can create conditions that affect the weather of a region.

MOUNTAIN RANGES AND RAIN-SHADOW DESERTS

Air rises in a process called orographic lifting when it flows over a mountain range. As the air rises, it cools adiabatically, and water vapor may condense into clouds that produce rain or snow.

These conditions create abundant precipitation on the windward side and the crest of the range. When the air passes over the crest onto the leeward (downward) side, it sinks. The air has already lost much of its moisture. In addition, it warms adiabatically as it falls, absorbing moisture and creating a rain-shadow desert on the leeward side of the range.

FORESTS AND WEATHER

Forests cool the air. Large quantities of water evaporate from leaf surfaces in the process called transpiration, and evaporation cools the surrounding air.

FORESTS AND WEATHER

Forests shade the soil from the hot sun, and tree roots and litter retain moisture. Forests soils remain moist long after the rain dissipates. Evaporation from soil litter combines with transpiration cooling from leaf surfaces to maintain relatively cool temperatures during times when there is no rain.

FORESTS AND WEATHER

In another feedback mechanism: Forests cool the air. Cool air promotes rainfall. Rainfall supports forests.

In today’s tropical rainforests, local rainfall has decreased by as much as 50 percent when the forests were cut and replaced by farmland or pasture.

When the rainfall decreases, wildfires ravage the boundary between the logged area and the remaining virgin forests.

More forest is destroyed, establishing a negative feedback mechanism of increasing drought, fire, and forest loss.

SEA AND LAND BREEZES

Sea and land breezes are caused by uneven heating and cooling of land and water.

Land surfaces heat up faster then adjacent bodies of water and cool more quickly.

If land and sea are nearly the same temperature on a summer morning, during the day the land warms and heats the air above it.

SEA AND LAND BREEZES

Hot air rises over the land, producing a local low-pressure area. Cooler air from the sea flows inland to replace the rising air.

At night, land cools faster than the sea, and descending air creates a local high-pressure area over the land. Then the winds reverse, the breezes blow from the shore out toward the sea.

MONSOONS

A monsoon is a seasonal wind and weather system caused by uneven heating and cooling of continents and adjacent oceans.

Just as the sea and land breezes reverse direction with day and night, monsoons reverse direction with the seasons.

In the summertime the continents become warmer than the sea. Warm air rises over land, creating a large low-pressure area and drawing moisture-laden maritime air inland. When the moist air rises as it flows over the land, clouds form and heavy monsoon rains fall.

MONSOONS

In winter, the process is reversed. The land cools below the sea temperature, and as a result, air descends over land, producing dry, continental, high pressure.

At the same time, air rises over the ocean and the prevailing winds blow from land to sea.

19.9 THUNDERSTORMSAn estimated 16 million thunderstorms occur every year, and at any given moment about 2000 thunderstorms are in progress over different parts of Earth.

19.9 THUNDERSTORMS

A single bolt of lightning can involve several hundred million volts of energy and for a few seconds produces as much power as a nuclear power plant.

It heats the surrounding air to 25000°C or more, much hotter than the surface of the Sun. The heated air expands instantaneously to create a shock wave that we hear as thunder.

All thunderstorms develop when warm, moist air rises, forming cumulus clouds that develop into towering cumulonimbus clouds.

19.9 THUNDERSTORMS

Different conditions cause these local regions of rising air:

1. Wind convergence. Where the two air masses converge, the moist air rises rapidly to create a thunderstorm.

2. Convection. Thunderstorms also form in continental interiors during the spring or summer, when afternoon sunshine heats the ground and generates cells of rising, moist air.

19.9 THUNDERSTORMS

Different conditions cause these local regions of rising air:

3. Orographic lifting. Moist air rises as it flows over hills and mountain ranges, commonly generating mountain thunderstorms.

4. Frontal thunderstorms. Thunderstorms commonly occur along frontal boundaries, particularly at cold fronts.

LIGHTNINGLightning is an intense discharge of electricity that occurs when the buildup of static electricity overwhelms the insulating properties of air.

In 1752 Benjamin Franklin showed that lightning is an electrical spark. He suggested that charges separate within cumulonimbus clouds and build until a bolt of lightning jumps from the cloud.

LIGHTNINGTwo hypotheses for the origin of lightning:

1. Friction between intense winds and ice particles generates charge separation.

2. Charged particles are produced from above by cosmic rays and below by interactions with the ground.

lightning.html

19.10 TORNADOES AND TROPICAL CYCLONES

Tornado

A small, intense, short-lived, funnel-shaped storm that protrudes from the base of a cumulonimbus cloud.

Tropical cyclone

A broad, circular storm with intense low

pressure that forms over warm oceans.

TORNADOThe base of the funnel can be from 2 meters to 3 kilometers in diameter. Some tornadoes remain suspended in air while others touch the ground.

Tornadoes are the most violent of all storms.

Tornadoes are most likely to occur when large differences in temperature and moisture exist between the two air masses and the boundary between them is sharp.

tornado.html

TROPICAL CYCLONES

Less intense than a tornado but much larger and longer-lived. Tropical cyclones average 600 kilometers in diameter and persist for days or weeks.

If the wind exceeds 120 kilometers per hour, a tropical cyclones is called a hurricane in North America and the Caribbean, a typhoon in the western Pacific, and a cyclone in the Indian Ocean.

TROPICAL CYCLONES

The low atmospheric pressure created by a tropical cyclone can raise the sea surface by several meters. As a tropical cyclone strikes a shore, strong onshore winds combine with the abnormally high water level created by low pressure to create a storm surge that floods coastal areas.

hurricane.html

TROPICAL CYCLONES

Tropical cyclones form only over warm oceans, never over cold oceans or land. Thus, moist, warm air is crucial to the development of this type of storm.

The center of the storm is a region of vertical airflow, called the eye. In the outer, and larger, part of the eye, the air that has been rushing inward spirals upward. In the inner eye, air sinks. Thus, the horizontal wind speed in the eye is reduced to near zero. A typical eye is only 20 kilometers in diameter.

19.11 HURRICANE KATRINA

- Tropical cyclone that struck the southeastern United States in late August 2005. The hurricane and its aftermath claimed more than 1,800 lives, and it ranked as the costliest natural disaster in U.S. history.

19.12 EL NIÑO

-a periodic change in the currents of the Pacific Ocean that occurs every five to eight years and brings unusually warm water to the coast of northern South America. It often leads to severe climate disruption to countries in and beside the Pacific..

The atmosphere

Science

Transcript of The atmosphere