Weather refers to the state of the atmosphere at a specific time and place. Weather describes...

• Weather refers to the state of the atmosphere at a specific time and place.

• Weather describes conditions such as air pressure, wind, temperature, and the amount of moisture in the air.

Weather Factors

What is weather?What is weather?

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• The Sun’s heat evaporates water into the atmosphere forming clouds; water returns to Earth as rain or snow; the Sun also heats the air.

• Air is made up of molecules that are always moving randomly, even when there’s no wind.

Air Temperatures


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• Temperature is a measure of air molecule movement.

• Energy from the Sun causes air molecules to move rapidly; the temperature is high, and it feels warm.

• When less of the Sun’s energy reaches air molecules, they move less rapidly and it feels cold. The temperature is lower.

• Air moving in a specific direction is called wind.

Wind


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• As the Sun heats the air, it expands, becomes less dense, rises, and has low atmospheric pressure.

• Cooler air is denser and tends to sink, causing high atmospheric pressure.

Wind


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Wind results because air moves from regions of high pressure to regions of low pressure.

• Wind direction can be measured using a wind vane which has an arrow that points in the direction the wind is blowing from. The name of the wind tells you the direction the wind is blowing from. For

Wind


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example, an east wind blows from east to west.

• Wind speed can be measured using an anemometer.

• Anemometers have rotating cups that spin faster when the wind is strong.


11 Wind

• Heat evaporates water into the atmosphere.

Humidity


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• Water vapor molecules fit into spaces between the air molecules.

• The amount of water vapor present in the air is called humidity.

• Air doesn‘t always contain the same amount of water vapor.

Humidity


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• Warm air can hold more water vapor than cool air, tending to make warm air more humid.

• Cooler air holds less water vapor than warm air, tending to make it less humid.

Humidity


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• Air is said to be saturated when it holds the maximum amount of water vapor possible at that temperature.

• Relative humidity is a measure of the amount of water vapor present in the air compared to what it can hold at a specific temperature.

Relative Humidity


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• If you hear a weather forecaster say that the relative humidity is 50 percent, it means that the air contains 50 percent of the water vapor needed for the air to be saturated.

• Air at 25°C is saturated when it contains 22 g of water vapor per cubic meter of air.

Relative Humidity


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• The relative humidity is 100 percent.

Relative Humidity


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• If air at 25°C contains 11 g of water vapor per cubic meter, what is the relative humidity? (50 %)

• When the temperature drops, less water vapor can be present in air.

Dew Point


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• The water vapor in air will condense to a liquid or form ice crystals.

• The temperature at which air is saturated and condensation forms is the dew point.

• When air near the ground cools to its dew point, water vapor condenses and forms dew.

Dew Point


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• If temperatures are near or below 0°C, frost will form instead of dew.

• Clouds form as warm, moist air is forced upward and cools.

Forming Clouds


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• At a certain height, the air cools to the dew point and condensation begins. Water vapor condenses on tiny particles in the air (dust, smoke, salt) to form droplets of liquid water. These droplets remain suspended in the air.

• Billions of these droplets form a cloud.

Forming Clouds


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• Clouds are classified mainly by shape and height.

Classifying Clouds


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• The shape and height of clouds vary with temperature, pressure, and the amount of water vapor in the atmosphere.

• The three main cloud types are stratus, cumulus, and cirrus.

Shape - Stratus


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• Clouds that form smooth, even sheets or layers in the sky are called stratus clouds.

• These clouds form low in the sky and often cover the entire sky.

• When air is cooled to its dew point near the ground, it forms a stratus cloud called fog.

Shape - Cumulus


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• They sometimes tower to great heights and can be associated with fair weather or thunderstorms.

• Cumulus clouds are masses of puffy, white clouds, often with flat bases.

Shape - Cirrus


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• Cirrus clouds are associated with fair weather, but they can indicate approaching storms.

• Cirrus clouds are high, thin, white, feathery clouds made of ice crystals.

• Some prefixes describe the height of the cloud base.

Height


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• The prefix cirro- describes high clouds, alto- describes middle-elevation clouds, and strato- refers to clouds at low elevations.

• Clouds associated with rain or snow often have the suffix or prefix nimbus attached to their name. The term nimbus is Latin for “dark rain cloud.”

Rain- or Snow-Producing Clouds


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• The water content of these clouds is so high that little sunlight can pass through them.

When a cumulus cloud grows into a thunderstorm, it is called a cumulonimbus cloud.

Rain- or Snow-Producing Clouds


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Nimbostratus clouds are flat, layered clouds that cover the sky and bring long, steady rain or snowfall.


11 Classifying Clouds

• Precipitation is water falling from clouds.

Precipitation


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• Precipitation occurs when water droplets in clouds combine and grow large enough to fall to Earth.

• Air temperature determines whether water forms rain, snow, sleet, or hail—the four main types of precipitation.

Precipitation


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• Drops of water falling in temperatures above freezing fall as rain.

• Snow forms when the air temperature is freezing.

• Sleet forms when raindrops pass through a layer of freezing air near Earth’s surface, forming ice pellets.

Precipitation


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• Hail is precipitation in the form of lumps of ice.


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• Hail forms in cumulonimbus clouds of a thunderstorm when water freezes in layers around a small nucleus of ice.

Precipitation

Weather PatternsWeather Patterns

22Weather Changes

Because air and moisture move in the atmosphere, weather constantly changes.

• An air mass is a large body of air that hasproperties similar to the part of Earth’s surface over which it develops.

Weather Changes—Air Masses

• Six major air masses affect weather in the Unites States.


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Weather Changes—Air Masses

• Air masses can be classified according to temperature and humidity.

• They can be warm or cold, and moist or dry.


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Highs and Lows

• Weather has high- and low-pressure systems.


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• Stormy weather is associated with low pressure areas.

• Fair weather is associated with high pressure areas.

• Air pressure is measured using a barometer.

Fronts• A boundary between two different air

masses is called a front. This is an area where two air masses meet and do not mix.


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• Clouds, precipitation, and storms sometimes occur at frontal boundaries.

• There are four types of fronts: • Cold front• Warm front• Occluded front• Stationary front


22Fronts - Cold front

• A cold front occurs when a cold air mass moves under a warm air mass.

• Cold fronts move quickly, so they can cause abrupt weather changes, including thunderstorms.

• After a cold front passes, cool, dry air moves in, often bringing clear skies and cooler temperatures.


22Fronts - Warm front

• A warm front occurs when a warm air mass moves over a cold air mass.

• Warm fronts move more slowly than cold fronts, so the weather may be rainy or foggy for several days.

• After a warm front passes through an area, the weather is likely to be warmer and more humid.


22Fronts - Occluded front

• An occluded front involes three air masses. It occurs when a warm air mass is caught between two cooler air masses and pushed upward.

• At an occluded front, the temperature near the ground becomes cooler. The rising warm air may cause the weather to turn cloudy and rainy or snowy.


22Fronts - Stationary front

• A stationary front occurs when a cold air mass and a warm air mass meet and neither has enough force to move the other. The air masses and their boundaries stop advancing.

• Stationary fronts may bring many days of clouds and precipitation.

Fronts


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What type of weather is brought by each front?

Weather Observations

• A meteorologist is a person who studies and predicts the weather.

• Meteorologists take measurements of temperature, air pressure, winds, humidity, and precipitation.

Weather ForecastsWeather Forecasts

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Weather Observations


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• Computers, weather satellites, Doppler radar, and instruments attached to balloons are used to gather data.

NOAA-18 Weather satellite in orbit

Weather balloon launch

Weather Maps

• Meteorologists at the National Weather Service use the information provided by weather instruments to make weather maps.


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• Weather maps are used to make predictions about future weather patterns.

• However, meteorologists cannot always predict the weather exactly because conditions can change rapidly.

• Meteorologists record the weather data on a map by usingsymbols. This forms a station model, which shows weather conditions at a specific location.

Weather Maps - Station Models


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Weather Maps

• Weather maps have lines called isotherms that connect locations of equal temperature.


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Weather Maps


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• An isobar is a line drawn to connect points of equal atmospheric pressure.

• On a weather map, pressure areas are drawn as circles with the word High or Low in the middle of the circle.

Weather Maps


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• Fronts are drawn as lines and symbols.

• In general, weather fronts move from west to east.

Weather Maps


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Fig. 19, p. 472


Weather Maps33

Weather refers to the state of the atmosphere at a specific time and place. Weather describes...

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