Weather Patterns & Forecasting

Weather- the conditions of the atmosphere at a certain place and time.

Weather is often associated with pressure systems

Air pressure is the weight of the molecules in the air

Pressure Systems

High-pressure system- a large body of circulating air that has high pressure at it’s center and lower pressure on the outside.

Occurs when air masses move apart.

High-pressure air at the center sinks and moves toward low pressure areas. Brings clear skies and “nice” weather

Winds rotate clockwise around high pressure

Symbol:

High Pressure Systems

Low pressure system- a large body of circulating air that has low pressure at it’s center and higher pressure on the outside.

Occurs when air masses come together. Air in the center of the system rises, and the water vapor in

it condenses and forms clouds. Bring cloudy skies and showers Winds rotate counter-clockwise around low pressure

Symbol:

Low Pressure Systems

Air masses- large bodies of air that have specific temperature and moisture characteristics. forms when a high pressure system stays in

one area for a few days. Changes in weather are caused by the

movement and interaction of air masses.

Air Masses

4 kinds of Air masses: Maritime (m)- originates over a body of water Continental (c) -originates over land Polar (P) -originates over a cold area Tropical (T) -originates over a warm area

The characteristics of an air mass are determined by where it is formed.

Air Masses

Air Mass Combinations

cP = continental polar = cool and dry cT = continental tropical = warm and dry mP = maritime polar = cool and wet mT = maritime tropical = warm and wet

If an air mass forms over land, what type of air mass is it?

What words would you use to describe an air mass that formed over water near the equator?

WRITING RESPONSE

If an air mass forms over land, what type of air mass is it?Air masses that form over land are called continental air masses

How would you describe an air mass that formed over water near the equator?An air mass that formed over water and near the equator could be described as maritime tropical

WRITING RESPONSE

A weather front is the boundary between two air masses. They occur where air masses come together 4 kinds of weather fronts:

Cold front Warm front Stationary front Occluded front

Weather Fronts

Cold air mass meets a warm air mass

Pushes the warm air up, lifting the clouds

Results in heavy rains or thunderstorms and cooler temperatures

Symbol:

Cold Fronts

Warm air mass meets a cold air mass

Warm air rises above cold air and gradually replaces it

Results in clear weather and warmer temperatures

Symbol:

Warm Fronts

Cold & Warm Fronts

Cold Front Warm Front

Cold air mass meets a warm air mass, but there is not enough force for the warm air to be lifted

Results in days of cloudiness and constant rain

Symbol:

Stationary Fronts

Warm air mass gets caught above two cold air masses that converge

Results in large amounts of rain and cooler temperatures

Symbol:

Occluded Fronts

Stationary & Occluded Fronts

Stationary Front Occluded Front

WRITNG RESPONSE

What type of front is being modeled in the diagram? Justify your answer

WRITNG RESPONSE

What type of front is being modeled in the diagram? Justify your answerThe front being modeled by the diagram is a Cold front because a cold air mass is moving toward a warm air mass.

Meteorology – the study of weather

Meteorologists( scientists who predict weather) measure weather variables before making a forecast.

A surface report describes a set of these weather measurements made on Earth’s surface.

These measurements include: Temperature Air pressure Humidity Precipitation Wind speed and direction

Weather Forecasting

Temperature is the measure of the average kinetic energy of molecules in the air.

Molecules in warm air move faster than molecules in cooler air.

Thermometers are used to measure temperature.

Temperature

Air pressure is the measure of the force exerted by air molecules as they push down on objects—including you—with a force that we call the “barometric pressure.”

Air pressure decreases as altitude increases.

Barometers -respond to differences in air pressure

Air Pressure

Humidity is the amount of water vapor in the air

When air is saturated, it holds the maximum amount of water vapor possible at that temperature.

Sling Psychrometers and Hygrometers are used to measure relative humidity.

Humidity

When air near the ground becomes saturated, the water vapor condenses into a liquid and forms dew.

It the temperature is below 0C, ice crystals referred to as frost form.

The temperature at which saturation occurs is called the DEW POINT.

Dew Point

Rain is easy to measure—all you need is an “open tube with a ruler”—a RAIN GAUGE.

“Tipping bucket” rain gauges can measure rainfall outside and connect to show a display inside.

Measuring Precipitation

Measuring Snowfall- Measuring how much snow falls is trickier than

measuring rain, because wind can carry it away or pile it up in drifts.

Meter sticks or rulers in carefully selected locations are used to record how deep the snow lies.

Meteorologists may also melt snow to find its “water equivalent”—what it would have been had it fallen as rain.

Measuring Precipitation

When air moves—”wind”—we feel both its direction and speed (force.)

Wind direction is measured with a “vane”—an arrow or other shape that points into the wind.

Speed is measured with an anemometer. The faster the cups spin, the

faster the wind.

Wind Direction and Speed

An upper –air report describes wind, temperature, and humidity conditions above Earth’s surface.

Weather balloons with an attached instrument package called a radiosonde are released into the atmosphere to measure weather conditions above the surface

Radiosondes contain: Temperature sensor Relative humidity sensor Barometer Radio transmitter Battery Parachute String to attach to balloon

Weather Conditions above the Surface

Weather satellites provide even larger views of weather systems They provide weather information by measuring the

radiation given off by Earth and by taking photographs.

Infrared Satellite images provide information about cloud temperature & height

Doppler radar- a special form of radar that can be used to detect precipitation and approximate wind speed.

Weather Satellites & Radar

This color-enhanced image shows a storm passing Des Moines, Iowa.

Heavier precipitation is shown by yellows and greens.

http://radar.weather.gov/radar/latest/DS.p19r0/si.kdmx.shtml

Here is a satellite image for the same time. ”X” shows the area where the storm in the radar image was

located

X

A station model displays many weather measurements for a specific location

It appears on weather maps.

The next set of slides will explain how to read one.

Weather Maps

Air temperature is shown in Fahrenheit degrees by the number to the upper left of the circle.

Temperature

75

Pressure—measured in mb (millibars)—is given by the number to the upper right of the circle For example, suppose it’s 1019.4 mb Only the 10s/1s/0.1s are shown—so it is represented as 194

Pressures below 1000 mb would start with high numbers, such as 964 for 996.4 mb

Pressure

194

The dew point temperature in Fahrenheit degrees is shown to the lower left of the circle.

Dew Point Temperature

57

Wind direction is show by an “arrow” going into the circle. The example shows wind blowing from the lower right, so this is a southeast wind.

Each long “feather” represents 10 mph and each short “feather 5, so the wind speed is 25 mph.

Wind Direction and Speed

The amount of the circle that is filled in indicates how much of the sky is covered by clouds.

It may range from “clear” (left) to “overcast” (right).

Cloud Cover

no clouds

about 10%

about 25%

about 40%

about 50%

about 60%

about 75%

about 80%

completely overcast

sky obscured

When rain, snow, or other forms of precipitation, lightning, and special weather conditions exist, these are shown by symbols to the left of the circle between air temperature and dew point temperature

Present Weather

thunderstorm

Samples of present weather symbols.

fog

haze

thunderstorm

, drizzle

lightning

rain

* snow

slight rain showers

. ice or snow pellets

Try to interpret this station model

1. Temperature: 45 o F2. Pressure: 1004.5 mb3. Dew Point: 29 o F4. Clouds: overcast5. Wind: from southeast at 15 mph6. Precipitation: light rain

1

2

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3

5

Weather maps have isobars which are symbols made up of lines that connect places that have equal air

pressure.

Isotherms – are lines that connect places that have the same temperature.

Fronts- are represented as lines with symbols on them.

Modern weather forecasts are made with the help of computers.

Computer Models are detailed computer programs that solve a set of complex mathematical formulas. The formulas predict temperature, winds, precipitation, and types of clouds.

Predicting The Weather