Final Output Nat Sci 2

8/3/2019 Final Output Nat Sci 2

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WEATHER

Weatheris the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm

or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the

stratosphere. Weather refers; generally, to day-to-day temperature and precipitation activity,

whereas climate is the term for the average atmospheric conditions over longer periods of time.

When used without qualification, "weather" is understood to be the weather ofEarth.

I. AIR MASSESDifferences in air pressure at different places on the Earth create wind patterns. The region

around the equator receives much more solar energy than the regions at the poles. Because warm

air rises and cold air sinks, the heated equatorial air rises. As the air rises, it creates a low-pressure

belt. Conversely, cold air near the poles sinks creating high-pressure belts.

Air moves from areas of high pressure to areas of

low pressure. Therefore, there is a worldwide movement of

surface air from the poles toward the equator. At higher

altitudes the cooler air returns from the equator toward the

poles. Temperature and pressure differences on the Earths

surface alter this general wind pattern. However, these

conditions create three wind cells in the Northern

hemisphere and three in the Southern hemisphere. The Earths rotation also influences the wind

pattern by causing the deflection of winds called the Coriolis Effect. The deflection influences the

direction of the prevailing winds.

In areas where air pressure differences are small, air can remain relatively stationary. If the

air remains stationary or moves slowly over a uniform surface, it takes on the characteristics
http://en.wikipedia.org/wiki/Tropospherehttp://en.wikipedia.org/wiki/Stratospherehttp://en.wikipedia.org/wiki/Climatehttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Climatehttp://en.wikipedia.org/wiki/Stratospherehttp://en.wikipedia.org/wiki/Troposphere


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temperature and humidity of that region. Such a large body of air one with uniform temperature

and moisture content is called an air mass.

TYPES OF AIR MASSES

Air masses are classified according to their source regions. The source regions also

determine the temperature and the humidity of the air mass.

Types of Air Mass Label Formation Area Description

Polar P Polar areas Cold air

Tropical T Tropical areas Warm air

Maritime m Ocean Moist air

Continental c Land Dry air

**These types of air masses also come in combination. (eg. Maritime polar --mP)

An air mass may remain over its source region for days or weeks. Eventually, it will move

into other regions because of the overall wind pattern.

II. FRONTSWhen two air masses meet, temperature differences usually keep the air masses separate.

The air of a cool air mass is dense and does not mix with the less dense air of a warm air mass. Thus,

a definite boundary, called afront, usually forms between air masses. A typical front is about

100km long, but some fronts maybe several thousand kilometers long. Changes in the weather

usually take place along the various types of fronts.

TYPES OF FRONTS

In order for a front to form, one air mass must collide with another air mass. The kind of

front that forms depends upon how the air masses are moving. When a cold air mass overtakes a


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warm air mass, a cold frontis formed. The moving cold air lifts the warm air. If the warm air is

moist, clouds will form. Large cumulus and cumulonimbus clouds typically form along fast-moving

cold front. Storms created along a cold front are usually short-lived and violent. A long line of heavy

thunderstorms, called squall line, may occur and advance just ahead of a fast-moving cold front. A

slow-moving cold front lifts the warm air ahead of it more slowly than does a fast-moving front. For

that reason, a slow-moving cold front produces less-concentrated cloudiness and precipitation.

A warm air mass that overtakes a cooler air mass

produces a warm front. The less dense warm air rises

over the cooler air. The slope of a warm front is gradual.

Because of this gentle slope, clouds may extend far ahead

of the surface location, or base of the front. A distinct

pattern of clouds precedes the approaching base of a warm

front. At the beginning of the pattern are cirrus clouds.

Behind the cirrus clouds are cirrostratus clouds followed by altostratus, then nimbostratus, and,

finally, stratus clouds at the base of the front. A warm front generally produces heavy precipitation

over a large area. A warm front may produce violent storms if the body of warm air advancing over

the cooler air is very moist.

Sometimes when two air masses meet, neither is displaced. The two air masses moves

parallel to the front between them. The front formed between the air masses is called astationary

front, because it does not move. The weather around a stationary front is similar to that produced

by a warm front.

An occluded frontformed when a fast-moving cold front overtakes a warm front, lifting the

warm air completely off the ground. The advancing cold front then comes into contact with cool air

that develops beneath the lifted warm air. The warm front is completely cut off, or occluded from


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the ground by colder air. The warm air is held parallel to the ground in the upper levels of the

atmosphere.

POLAR FRONTS AND WAVE CYCLONES

The boundary at which the cold polar air meets the warmer air of the middle latitudes is

called apolar front. A polar front circles the Earth about 40 to 60 latitude in each hemisphere.

The polar fronts move closer to the equator in the winter and back toward the poles in the summer.

Waves often develop along the polar fronts. A wave is a bend formed in a cold front or

stationary front. These waves are similar to the waves that moving air produces when passing over

a body of water. However, they are much larger, often hundreds of kilometers in length.

The waves along the boundary of a polar front are

the beginnings of low-pressure storm centers called wave

cyclones. A wave cyclone consists of a very large body of

airup to 2,500 km in diameter. Its winds blow in

circular paths around the low-pressure region at the center. Wave cyclones form not only along

polar fronts but also along other cold or stationary fronts, and they influence weather in the middle

latitudes particularly.

STAGES OF A WAVE CYCLONE

In the first stage of a typical wave cyclone, there is a stationary front between a warm air

mass and cold air mass. At this stage, the winds usually move parallel to the front. However, the

winds on one side of the front blow in the opposite direction from the winds on the other side. A

wave develops along the stationary front as cold air that is moving toward the equator pushes into

the warm air, forming a cold front. At the same time, warm air that is moving pole ward pushes into

the cold air, forming a warm front. The result of this air movement is a slow-moving warm front


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and a fast-moving cold front turning around a central point. Clouds and precipitation spread along

both the warm and cold fronts.

A low-pressure region develops at the central point where the two fronts come together.

The less dense warm air is lifted as it meets the cold air along the warm front. It is also pushed up

by the advancing cold air behind the cold front. As a result of these lifting actions, an occluded front

develops. The occluded front usually brings the storm to its highest intensity, with winds spinning

around the central low-pressure region. Eventually, the wave cyclone uses up the moisture and

energy from the warm air and dies down.

A wave cyclone usually takes one to three days to develop. During this time, the air masses

are in motion, and the disturbance moves with them. A specific sequence of cloud formation is

associated with the development of a wave cyclone.

ANTICYCLONES

The air of an anticyclone moves outward from a center of high-pressure, unlike a wave

cyclone, which moves inward toward a low-pressure center. Because of the Coriolis effect, the

circulation of air around an anticyclone is clockwise in the Northern hemisphere. In general,

whereas wave cyclones bring cloudy, stormy weather, anticyclones bring fair weather, since the

sinking air does not promote cloud formation.

HURRICANES

A severe tropical storm, with winds as strong as 240

km/hr is called a hurricane. Hurricanes develop over warm,

tropical oceans near the equator. Like the wave cyclones of

the middle latitudes, hurricane winds spiral in toward the


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storm center. Seldom more than 700 km in diameter, hurricanes are much smaller than wave

cyclones but much more powerful. These whirling storms are the most violent storms occur on the

earth.

A hurricane begins when very warm, moist air over the ocean rises rapidly. When moisture

in the rising warm air condenses, a large amount of energy in the form of latent heat is released.

This heat increases the force of the rising air. Moist tropical air continues to be drawn into the

column of rising air, releasing more heat and sustaining the process. The entire storm system spins

as a result of the Coriolis effect.

A fully developed hurricane consists of a series of thick cloud bands spiraling upward into

the center of the storm. Precipitation is very heavy. Winds increase in velocity toward the center, or

eye of the storm, reaching speeds up to 160 km/hr. The eye, itself, however, is a region of calm,

clear air. The winds of a hurricane usually last 9 to 12 days. During this time, the hurricane moves

with the prevailing winds. Hurricanes that form over the Pacific Ocean are called typhoons.

THUNDERSTORMS

A severe accompanied by thunder, lightning, and strong winds is called a thunderstorm.

Thunderstorms often occur when a small section of air in a warm,

moist mT air mass is heated and rises. High surface temperature

on the ground influences the rising of this warm, moist air. For

that reason, thunderstorms occur most commonly in the late

afternoon or early evening. Mountains or cold air in an occluded front also may cause the air to rise.

A thunderstorm develops in three distinct stages. The first stage of a thunderstorm is called

the cumulus stage. At this stage, warm mist air rises until the water vapor within condenses and

forms a cumulus cloud approximately 7.5 km high. In the next stage, called the mature stage, the


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rising warm, moist air swells higher and higher. The cumulus cloud grows until it becomes a

cumulonimbus cloud. The cloud reaches a height of approximately 12.2 km, where the top spreads

out in an anvil shape. Heavy showers of ice crystals, water drops, and occasionally, hailstones fall

from the cloud. This precipitation produces a generalized cooling effect. While some of the air

continues to rise, much of the air is dragged downward by the falling ice and raindrops, causing a

strong downdraft. During thefinal stage, the strong downdraft stops air currents from rising. The

thunderstorm dies out as the supply of water vapor in the air decreases.

During a thunderstorm, clouds discharge alectricity in the form of lightning. The released

electricity heats the air, causing it to expand rapidly. The rapid expansion and collapse of air

produces the loud noise known as thunder. For lightning to occur, the clouds must have areas with

distinct electrical charges. The upper part of the cloud usually carries a positive charge, while the

lower part carries both positive and negative charges. Lighting occurs as a huge spark that travels

between the two parts of the cloud when the difference in their electrical charges becomes great.

TORNADOES

The smallest, most violent, and shortest-

lived severe storm is a tornado. A tornado is a

whirling, funnel-shaped cyclone. A tornado forms

when a thunderstorm meets high-altitude,

horizontal winds. These winds cause the rising air

in the thunderstorm to rotate. One of the storm

clouds may develop a narrow, funnel-shaped, rapidly spinning extnsion. The extension reaches

downward and amy or may not actually touch the ground. If the tip of the funnel does touch the

ground, it generally moves in wandering path faster than a person can run. Frequently, the funnel


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rises and touches down again a short distance away. The tornado generally covers a path not more

than 100m wide. Usually, everything in that path is destroyed.

The destructive power of a tornado is due to the speed of the winds whirling within the

funnel. Although the speed of these winds has never been measured, their great destructive power

indicates that they may reach speeds up to 480 km/hr. Most of the injuries caused by tornadoes

occur when people are trapped in collapsing buildings or are struck by objects flung by the wind. If

you are outside during a tornado, you should immediately find a low plae in the ground, lie down,

and cover your head.

Tornadoes occur in many locations. Tornadoes that occur over the ocean are called

waterspouts. Waterspouts are usually smaller and less powerful than tornadoes occuring over

land.

III. WEATHER INSTRUMENTSWeather observations are based, in part, on measurements of atmospheric pressure,

humidity, and precipitation. There are barometers which measures atmospheric pressure,

psychrometers and hair hygrometers which are used to measure relative humidity, and rain

gauges which are used to measure precipitation. Weather observations are also based on

measurements of temperature and winds, which are made by special instruments.

MEASURING AIR TEMPERATURE(THERMOMETER)

1. Ordinary thermometerA common type of thermometer that uses a liquidusually mercury or alcohol

sealed in a glass tube. A rise in temperature causes the liquid to contract and move

down the tube. A scale marked on the glass tube indicates the temperature.


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2. Bimetal thermometerThis thermometer consists of a bar made up of

two strips, each of a different metal. The metals,

such as brass and iron, expand different amounts

when heated. The bar will curve when heated and

straighten when cooled. An instrument called

thermograph measures temperature changes by

recording the movement of the bar. In

thermograph, a pen is attached to the bar. The

pen point is placed against a chart on a rotating drum. As the drum rotates, the

bending and straightening of the bar, and thus the temperature changes, and

recorded on the chart.

3. Electrical thermometer

As the temperature rises, the electric current

flowing through certain materials increases. The

electric current flowing through these materials

is translated into temperature readings. This type

of thermometer is useful especially when an

observer cannot be present.

MEASURING WIND SPEED AND DIRECTION

1. AnemometerThis instrument measures wind speed. A

typical anemometer consists of small


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cups attached by spokes to a shaft that rotates freely. The wind pushes against the

cups, causing them to rotate. This rotation triggers an alectrical signal that registers

the wind speedin m/sec, mph, or knotson a dial.

2. Wind vaneThis instrument measures the wind direction. It is like an

arrow with a large tail. The wind vane turns freely on a pole

as the tail catches the wind. Thus, the arrows always points

into the wind. A wind is described according to the direction

from which it comes (example: westerly) and in degrees.

MEASURING UPPER ATMOSPHERIC CONDITIONS

1. RadiosondeIt is an instrument used by meteorologists to

investigate weather conditions in the upper

atmosphere. Carried aloft by a helium-filled balloon, a

radiosonde measures relative humidity, air pressure,

and air temperature. These measurements are sent out

by a radio signal. A special radio on earth receives and records the information. The

path of the balloon is also tracked to determine the direction and speed of high-

altitude winds. When the balloon reaches an

extremely high altitude, it bursts and the radiosonde is

parachuted back to earth.

2. RadarIt is another instrument used to measure data in the

upper atmosphere. It is an electronic device that sends

or transmits a pulse of radio waves in the form of beam. Objects that cross the beam


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reflect it back to the transmitter, which receives the returning beams. Radar can

detect objects that are too small or too far away for the human eye.

FORECASTING THE WEATHER

Predicting the weather has challenged people for thousands of years. People of early

civilizations attributed control of weather conditions such as wind, rain, and thunder to gods. More

than 4,000 years ago, people attempted to forecast the weather using positions of the star as their

basis.

Modern weather forecasting began with the invention of more sophisticated scientific

instruments, such as the thermometer and the barometer. The invention of the telegraph in 1844

enabled meteorologists to share information about weather conditions quickly, and it led to the

development of national weather services.

For more information about weather forecasting in Philippines, please visit Philippine

atmospheric, geophysical and astronomical services administration website at

http://www.pagasa.dost.gov.ph/ .
http://www.pagasa.dost.gov.ph/http://www.pagasa.dost.gov.ph/http://www.pagasa.dost.gov.ph/


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WEATHER MAP SYMBOLS

WEATHER MAP (EXAMPLE)


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TYPES OF FORECAST

1. Daily Weather ForecastDaily weather forecasts are updated every 24 hours. This is done daily.

2. Long-term Weather ForecastLong-term weather forecast can be made for the expected wether for the next few

days (usually 2-3 days).

CONTROLLING THE WEATHER

Scientists are currently investigating methods of controlling rain, hail, hurricanes, cyclones,

and lightning. So far, the most successful method has been the production of rain by cloud seeding.

In this process, freezing nuclei are added to upper cooled clouds, causing rain to fall.

Hurricanes have also been seeded with freezing

nuclei, and some meteorologists think that this process may

reduce the intensity of a storm. Attempts to control middle-

latitude cyclones in this way have not been successful,

however. Meteorologists know so little about the structures of

tornadoes that they have not yet tried to control these storms.

Attempts have also been made to control lightning. The release of large quantities of ions

near the ground can modify the electrical properties of small cumulus clouds. However, it is no

known whether this method would affect the electrical properties of large clouds. Seeding of

potential lightning storms with silver-iodide nuclei has seemed to modify the occurrence of

lightning, although no conclusive results have been obtained.


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CLIMATE

The average weather conditions of a region, or the weather patterns that occur over many

years, are referred to as climate. Usually, scientists describe climate in terms of the average

monthly and yearly temperatures and the average amount of precipitation. Average temperatures

are calculated by adding two or more temperature readings and dividing by the number of

readings. For example, the average daily temperature is calculated by averaging the high and low

temperatures of the day. The monthly average is determined by averaging the daily averages. The

yearly average temperature is calculated by averaging the monthly averages.

Another way scientists describe temperature is by indicating the temperature range.

Temperature range is between the highest and lowest temperatures of a day or month. The yearly

temperature range is the difference between the highest and lowest monthly averages.

Another major weather condition, precipitation, is described as the average precipitation a

region receives in a year. However, average yearly precipitation alone does not accurately describe

climate.

An accurate description of climate must include several factors that influence both

temperature and precipitation. These factors include latitude, heat absorption and release, and

topography.

I.FACTORS THAT AFFECT CLIMATE

1. LATITUDEA major influence on the climate of a region is its latitude, or distance from the equator.

Latitude determines the amount solar energy received by and the prevailing wind patterns of the

region.


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SOLAR ENERGY

The amount of solar energy that the earth receives depends on two factors. They are the

angle at which the rays of the sun strike the earth and the number of hours of daylight. The angle at

which the suns rays strike a region is determined by its latitudes and by the tilt ofthe earths axis.

At the equator, the rays always strike the earth at almost 90 angle.

In equatorial regions, both day and night are about 12 hours long throughout the year. The

result is steady high temperatures year-round and a yearly temperature range of only 3C os 4C in

most areas. There are no summers or wintersonly dry or rainy seasons.

At higher latitudes the suns rays strikes the earth at an angle lower than 90, the rays do

not heat the earth as much because their energy is spread over a wider area. Thus, average yearly

temperatures in these locations are lower than those at the equator. Also, the lengths of the days

and the nights vary.

In Polar Regions, the sun sets for only a few hours each day in the summer and rises for only

a few hours each day in the winter. Thus, the annual temperature range is very large, but the daily

temperature range is very small.

WIND PATTERNS

Latitude also determines global wind belts that affect

a region and thus the general direction of the wind in any

particular location. Winds affect many weather conditions,

such as humidity, precipitation, temperature, and cloud cover.

Hence, regions with different prevailing winds often have

different climates. The global wind pattern is also influenced


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by storms and local weather.

Within the different global wind belts are various regions of low and high-pressure. In the

equatorial belt of average low-pressurethe doldrumsthe air rises continuously and thus loses

moisture. As a result, the regions within this latitude range of low-pressure generally receive heavy

precipitation. The amount of rainfall is most abundant in a belt around the equator and decreases

steadily with increasing latitude. In the areas around 20 to 30 north and south latitudethe

subtropical highsthe air is mostly dry and sinking, and little precipitation occurs.

Closer to the poles, beginning at around 45 to 55 latitude, is a belt of low average

precipitation. In these regionsthe subpolar lowswarm tropical air meets cold polar air and

wave cyclones frequently develop. At latitudes above 50 to 55, average precipitation decreases in

the cold, dry polar air masses.

With the changing seasons, the global wind pattern shifts in north-south direction. As the

wind and pressure belts shift, the belts of precipitation associated with them also shift.

2. HEAT ABSORPTION AND RELEASEThe way solar energy strikes the earth and is absorbed or deflected also influences the

surface temperature. Land heats faster and to a higher temperature than water does. One reason

for this difference is that the land surface is solid and basically unmoving, while the water surface is

liquid and continuously changing. Waves, currents, and other movements continuously replace

warm surface water with cooler water from ocean depths. This action prevents the surface

temperature of water from increasing rapidly. The surface temperature of the land, on the other

hand, can continue to increase as more solar energy is received.

Land and water also absorb and release heat at different rates. The specific heat of water is

higher than that of land.Specific heatis the amount of heat needed to raise the temperature of 1g

of a substance to 1C. A given mass of water requires more heat than does of the same mass of land


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to increase its temperature the same number of degrees. Even if not in motion, water warms more

slowly than land does. Water also releases heat more slowly than land does.

The average temperatures of land and water at the same latitude vary also because of

differences in the loss of heat through evaporation. Evaporation affects water surfaces much more

than it does to land surfaces.

OCEAN CURRENTS

The amount of heat absorbed or released by the air is influenced by the temperature of

ocean currents with which air comes in contact. If the winds consistently blow toward the shore,

currents have a stronger effect on air masses.

SEASONAL WINDS

Heat differences between the land and the oceans sometimes cause winds to shift

seasonally in certain regions. During the summer, the land heats more quickly than the oceans do. If

a low-pressure center develops over the land, warm air rises and is replaced by cool air from the

ocean. Thus the wind moves landward. During the winter, the land loses heat more quickly than

ocean does, and the cool air flows away from the land. Thus the wind moves seaward. Such seasonal

winds are called monsoons. They are strongest over the large landmasses near the equator.

3. TOPOGRAPHYThe topography, or shape of the land, also influences climate. The altitude, or height above

sea level, produces distinct temperature changes. Average temperature decreases as altitude

increases in the troposphere. For every 100 m increase in altitude, the average temperature

decreases 1C. Even along the equator, for example, the peaks of high mountains are cold enough to

be covered with snow.

Mountains influence the temperature and moisture content of passing air masses. When a

moving air mass enters a mountainous region, it rises and cools. As the air rises, it loses most of its

moisture through precipitation. As the air descends on the other side of mountain, it is warmed


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about 1C every 100 m. Air flowing down mountain slopes, therefore, is usually warm and dry. One

such wind is thefoehna warm and dry wind that flow down the northern slopes of the Alps

Mountains. Similar warm winds that flow down the eastern slopes of the Rocky Mountains are

called Chinooks. A Chinook can raise the air temperature very rapidly in a short period of time. In

1990, a Chinook raised the temperature in a small town of Montana 17C in three minutes.

Some winds that blow down mountain slopes are not warm. These winds are so cold that

they remain cold even after heating. The mistralwhich blows from the North down to the Alps to

the Mediterranean Sea, is a stormy, cold wind. It is sometimes strong enough to knock over

chimneys. Another cold northern wind is bora, blows from the mountains of Yugoslavia to the

Adriatic Sea.

II. CLIMATE ZONES

A geographic region that has a predictable temperature range and other predictable

weather conditions is called climate zone. The earth has three major climate zones. The warm zone

immediately around the equator is the zone oftropical climates. Tropical climates have an average

monthly temperature of at least 18`C, even during the coldest month of the year. Tropical climates

are influenced by the continental and maritime tropical air masses, which develop close to the

equator.

At the other extreme are thepolar climates. In these regions the average monthly

temperature is never higher than 10C. Continental and maritime polar air masses originate in

these areas.

Between the tropical and polar climates zones is the zone of temperate climates, or middle-

latitude climates. The average monthly temperature of these climates is no warmer than 18C in

the coldest month and no cooler than 10C in the warmest month. In the middle-latitude climate

zones, the weather changes often because both tropical and polar air masses move across these


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regions. The middle-latitude climate zones are also frequently exposed to cyclonic storms, with

strong winds and heavy rains that are produced along the polar front.

1. TROPICAL CLIMATESEach principal climate zone has specific average temperatures. However, these are several

different types of climates within these zones because of differences in the amount of precipitation.

a) RAIN-FOREST CLIMATEThe warm, humid regions within 5 to 10 on

either side of the equator are covered with

dense vegetation called rain forests. For that

reason the climates of these regions are

known as rain-forest climates. The moist,

rising easterly wind produces an annual

rainfall that is usually greater than 250 cm.

b) TROPICAL-DESERT CLIMATEWarm, dry weather conditions

occur in regions about 2.570 km

north and south of the equator.

The northern boundary lies

along the Tropic of Cancer at

23.5 north of the equator, and

the southern boundary lies along the Tropic of Capricorn at 23.5 south of the

equator. These areas have tropical desert climates. Tropical-desert climates are

influenced by the dry, sinking air masses of the subtropical highs. They include some


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of the earths driest deserts. Annual rainfall of tropical-desert climates is less than

25cm.

2. POLAR CLIMATESClimates that are influenced by polar air massespolar climatesoccur in the regions

located between 55 north latitude and the Arctic Circle. These are two types of polar

climates: the subarctic climate and the tundra climate.

a) SUBARCTIC CLIMATEAll the land across North America,

Europe, and Asia that lies between 55

and 65 north latitude, including most of

Alaska, has a subarctic climate. Dry

continental polar air masses control this

climate. In subarctic climates, the yearly

precipitation is only 25 cm to 50 cm, which is just a little more than falls on tropical

deserts. Winters are severe and summers are short. The subarctic climates have a

yearly temperature range tha5t is usually large. In fact, the largest yearly

temperature range on earth 61C that was recorded in subarctic Yakutsk,

Siberia. In places with subarctic climates, the vegetation consist mostly of spares

forest of pine, fir, spruce, and other cone-bearing trees.

b) TUNDRA CLIMATEA Tundra has no trees; its ground is

covered with mosses, lichens, and

small flowering plants. It also has

large expenses of rocky land with no

vegetation. The yearly temperature


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range is not as great as that of subarctic climates. This is because areas with tundra

climates are near the ocean, which holds heat during the winter, the warmest

monthsJuly and August have average temperatures of 4C. Only about 25 cm of

precipitation are received in a year, mostly as snow.

The areas of tundra climates that are located between 60 latitude and the poles are

calledpolar deserts. Polar deserts have very dry, cold air and receive little

precipitation. Parts of polar moist because they are covered with ice, but dry desert

like conditions are more common.

3. MIDDLE-LATITUDE CLIMATESNorth American stretches from Central America, in the tropical climate zone, to northern

Canada and Alaska, in the polar climate zone. However, the United states and most of

Canada lie in the middle latitudes.

The climates of the various parts of the United States differ greatly. Cyclonic storms

bring most of the precipitation that the falls unevenly across these two countries , they have

several different types of middle-latitude climates.

a) MARINE WEST-COAST CLIMATEBetween 40 and 60 north latitude, the northwestern coastline of the United States

has the marine west-coastline climate. This land is in the bell ofprevailing

westerly winds of cool, moist maritime polar air. As these winds move east from the

Pacific Ocean, the west coast receives precipitation. The northwestern coast of

Washington and Oregon and northern California, where mountains block the

movement of moist air toward the east, receive a great deal of moisture. The

average yearly precipitation is 50 cm to 75 cm. The average temperature is

relatively cool 20C in summer and relatively mild 7Cin winter. The yearly


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temperature range is 13C. Most regions with marine west-coast climates are

covered with dense forest cone bearing trees.b) MEDITERRANEAN CLIMATE

Regions along the southern pacific coast of United States have a climate like that if

the northern coast of the Mediterranean climate. Regions with a Mediterranean

climate are located between regions with a tropical desert climate and those with a

marine west-coast climate. They generally lie between 30 and 40 north latitude.

These regions have dry summers and wet winters. In the summer, the dry air of the

subtropical highs blows over the regions. In the winter this pressure belt shifts

southward, bringing cyclonic storms to the region. Almost all of the yearly rainfall

an average of 25 cmfalls during the mild winter months. The yearly temperature

range is smallonly 7Ceith average summer temperatures of 21C and average

winter temperatures of 14C

c) MIDDLE-LATITUDE DESERT AND STEPPES CLIMATESTwo types of middle-latitude climates are found between 35 and 50 north latitude

in the interiors of Asia and North America. They are the middle-latitude-desert

and the middle-latitude-steppes climates. Much of the land in the far western

United States, other than the coast and mountains, has a middle-latitude-desert

climate. Little precipitationless than 25 cmfall annually in these deserts. Unlike

tropical desert, middle latitude deserts have a winter season that may be quite cold

and a summer season that ranges from warm to very hot. The vegetation of middle-

latitude deserts consists of widely scattered drought-resistant shrubs and cactus.

From the western united states, the dry air moves eastward. When it reaches the

central part of the continent, it begins to pick up moisture from the tropical air

moving northward


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At elevation of 1200 m to 2100 m, steppes are gradually replace barren

western deserts. Steppes received 25 cm to 50 cm of rain a year, which supports a

dense growth of grasses. The yearly temperature range is high-24C.The average

summer temperature is 23C and the average winter temperature,-1C.

HUMID CONTINENTAL CLIMATE

The areas farther inland from the steppes and extending to the east coast in north

America and Asia have a humid continental climate. Areas with this type of climate are

subject to cold, dry polar air masses and warm humid as tropical air masses move north.

Winters are commonly very cold as polar air masses move south. When this air mass is

meeting, weather condition may change rapidly and violently. Seasonal changes are

great, with yearly temperature ranges as high as 30C.Average summer temperatures

are as high as 25C: and average winter temperatures, as low as -5C. Yearly average

precipitation, mostly from cyclonic storms and summer thunderstorms, is at least 75

cm. dense forests of hardwood and softwood trees are found in humid continental

climate.

d) HUMID SUBTROPICAL CLIMATEThe southeastern coast of continents located at around 30 north or south latitude

have a humid tropical climate. In the summer, moist, tropical air masses move north

across this region. These air masses usually bring warm, humid weather. The

tropical air often brings heavy rains also. In the winter, continental polar air masses

moving from inland regions may bring brief but intense cold. For example,

Charleston, South Carolina, has an average summer temperature of 27C and an

average winter temperature of 10C. However, the temperature occasionally has

plunged to15C in Charleston. The yearly temperature range in humid subtropical


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climate is a relatively small 17C.Annual precipitation in a humid subtropical

climate is between 75 cm and 165 cm. The land in a humid subtropical climate is

usually covered with a dense growth of grasses and trees.

4. LOCAL CLIMATEThe climate in any particular place maybe influenced by local conditions as well as by the

major factor that has been discussed. The elevation of the land is the most important factor

affecting local weather conditions.

Large lakes and forests also influence local climates. Lakes, like oceans, moderate

temperature. They can also cause an increase in precipitation on the shore farthest from the

prevailing wind. For example, the eastern shore o9f lake Michigan generally has more

moderate temperatures, more cloudiness, and higher precipitation then the western shore

doest. Forests affect local climate by reducing the speed of the wind and by increasing the

humidity.

Cities are climate zones. In a city the average temperature is 1C to 2C higher than that in

surrounding rural areas. There are several reasons for this phenomenon. Because cities

contain far less vegetation than rural areas do, less transpirations occurs, and therefore,

more solar energy is available to heat the air. In addition, at night the air over cities is

warmed by radiation from the materials in streets and buildings that have been heated

during the day. Heavy traffic and some of the energy use for heating, lighting and industry

may also raise the air temperature in cities. More precipitation falls within cities and in

areas cross by winds that have blown over the cities than in rural areas. Dust, smoke, and

other pollutant, carried into clouds by rising warm city air, form nuclei around which

raindrops condense.

SOURCES: Earth Science book in the library for the facts, Internet for the pictures

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