WEATHER

María Jesús Camposlearningfromgeography.wikispaces.com

WeatherIt’s the state of the atmosphere in a certain place at a specific time. It is studied by Meteorology.Meteorological phenomena combine in the atmosphere to give rise to weather.

Meteorological Phenomena

Weather

Temperature

Atmospheric Pressure and

WindPrecipitation

Air Masses

Temperature

The sun’s energy heats up some parts of the Earth more than others.

Temperature is how warm the air is thanks to solar radiation.

Factors that modify

temperature

Latitude

Altitude

Proximity to oceans

LATITUDE: HOW LATITUDE MODIFIES TEMPERATURE?

Because of the tilt of the Earth, solar radiation hits the Earth directly on the Equator and at a less angle on the Poles.

Thus, temperatures close to the Equator are much higher than in the Poles.

These differences in temperature create a restless movement of air and water in great swirling currents to distribute heat energy from the sun across the planet.

When air in one region is warmer than the surrounding air, it becomes less dense and begins to rise, drawing more air underneath.

Elsewhere, cooler denser air sinks, pushing air outward to flow along the surface and complete the cycle.

ALTITUDE: HOW ALTITUDE MODIFIES TEMPERATURE?

At higher altitudes the air is thinner and loses heat. Every 1000 metres temperature falls 6ºC.

PROXIMITY TO OCEANS: HOW DOES IT MODIFY TEMPERATURE?

Water stores more heat than continents so temperatures are often mild in coastal areas.

Map of Isotherms

Temperature is measured with a thermometer and expressed in degrees centigrade (ºC)

An isotherm is a line that connects points of equal temperature at a given date or time on a map.

Warm area: The highest temperatures (over 20ºC) are registered between the Tropics.

Cold area: The lowest temperatures (less than (0º) are registered close to the Poles.

Temperate area: zone in between in which temperatures go from 0ºC to 20ºC.

Meteorological Phenomena

Weather

Temperature

Atmospheric Pressure and

WindPrecipitation

Air Masses

Atmospheric Pressure It is the weight of air

on the Earth’s surface. The average atmospheric pressure at sea level is 1013 millibars (mb).

Air pressure depends on the density of the air, that is on how close together the molecules of the air are.

Factors that modify

atmospheric pressure

Temperature

Altitude

HOW DOES TEMPERATURE MODIFY ATMOSPHERIC PRESSURE?

In cold air, the molecules are more closely packed together than in warm air, so cold air is more dense than warm air.

Thus, hot air weighs less than cold air.

Since warm air is less dense and creates less air pressure, it will rise.

As cold air is denser and creates greater air pressure, it will sink.

When warm air rises, cooler air will often move in to replace it. So wind often moves from areas where it’s colder to areas where it’s warmer.

When warm air rises it originates a low pressure area or depression.

As warm air is rising it takes with him water vapour that cools and condenses originating clouds. That’s why usually, depressions are accompanied by precipitation.

As cold air is heavier it sinks originating high pressure areas or anticyclones. It is a drier air so it usually brings dry weather.

HOW DOES ALTITUDE MODIFY ATMOSPHERIC PRESSURE?

Air lower in the atmosphere is more dense than air above, so air pressure down low is greater than air pressure higher up.

Meteorological Phenomena

Weather

Temperature

Atmospheric Pressure and

WindPrecipitation

Air Masses

Air Masses In the lower layers of

the atmosphere, air separates into air masses which differ in temperature and humidity:

Close to the poles there are cold air masses.

Close to the Equator there are warm air masses.

Continental air masses are dry.

Maritime air masses are moist.

The boundaries that separate air masses are called fronts.

Weather in the fronts is usually unstable.

There are: Cold fronts Warm fronts

A cold front appears when a cooler mass of air replaces at ground level a warmer mass of air.

As a cold front moves into an area, the heavier cool air pushes under the lighter warm air that is replacing. The warm air becomes cooler as it rises. If the rising is humid enough, the water vapor it contains will condense into clouds an precipitation may fall.

As the cold front moves, warm, moist unstable air is usually replaced by cold, dry stable air.

A warm front appears when a warmer mass of air moves over a cooler mass of air.

Warm air at the surface pushes above the cool air mass, making clouds and storms. Warm fronts bring stormy weather.

A warm front typically replaces cool dry air with warm moist air.

Meteorological Phenomena

Weather

Temperature

Atmospheric Pressure and

WindPrecipitation

Air Masses

Wind It is the movement of air masses. Wind circulates between areas with different

pressures. It always circulate from the high pressure area to the low pressure area.

The greater the difference between the high and low pressure or the shorter the distance between the high and low pressure areas, the faster the wind will blow.

Wind also blows faster if there’s nothing in its way, so winds are usually stronger over oceans or flat ground.

Although wind blows from areas of high pressure to areas of low pressure, it doesn’t blow in a straight. This is because the Earth is rotating.

In the northern hemisphere, the spin of the Earth causes the winds to curve to the right (to the left in the southern hemisphere). This is called the coriolis effect.

So in the northern hemisphere, winds blow clockwise around an area of high pressure and counter-clockwise around low pressure.

TYPES OF WINDS:

Trade winds: they always blow form the Tropics to the Equator.

Seasonal winds: like the monsoons which carry heavy rain from the Indian Ocean to the continent in summer, and from the land back to the ocean in winter.

Sea breezes: wind blows from the land to the sea during the day and the other way down during the night.

Local winds: cierzo (Aragon), levante (Comunidad Valenciana, Andalucía)

Precipitation Is the amount of water

that falls in a place.

It can fall as rain, snow or hail.

It is caused by the condensation of water vapour on the atmosphere. When water vapour condenses, it transforms into water drops. When drops get so heavy that the atmosphere can not hold them, they fall to the ground attracted by gravity.

Map of Isohyets

Precipitation is measured with a rain gauge or pluviometer and expressed in millimetres (mm)

An isohyet is a line that connects points with the same amount of precipitation at given date or time on a map.

Developed by María Jesús CamposChusteacherWikiteacher