Chapter 17 – Meteorology

Why can’t meteorologists always predict the weather correctly?

I. Weather changes are caused by the following atmospheric conditions:

A. Air masses- large bodies of air covering thousands of square kilometers. Four major types of air masses affect weather in the United States:

1. Maritime tropical- warm, moist air that forms over the ocean near the equator bringing hot, humid air in the summer

2. Maritime polar- forms over the Pacific ocean in both winter and summer, bringing cooler temperatures to Eastern states in the summer and heavy snow and colder temperatures in the winter

3. Continental tropical- forms over land in Mexico during the summer, bringing dry, hot air to southwestern states

4. Continental polar- forms over land in Northern Canada bringing cold temperatures to the US during the winter

B. Fronts- the place where two different air masses meet. Fronts are usually scenes of dramatic weather changes and sometimes violent storms. The changes that fronts bring depend on the season and location. There are four types of fronts:

1. Cold front- a mass of cold air meets and pushes under a mass of warm air. The warm air is pushed up creating storms.

2. Warm front- a mass of warm air overtakes a cold air mass and moves over it creating rain and showers followed by hot, humid weather

3. Occluded front- a cold front overtakes a warm front or cold air overtakes a cold front producing less extreme weather

4. Stationary front- a warm air mass meets a cold air mass and no movement occurs, causing rain to stay in an area for many days

That was a dust mite.

II. Storms (also called extratropical cyclones) are violent disturbances in the atmosphere. Storms draw their energy from temperature differences, which makes the boundary between warm and cold air (fronts) a favored path for storms.

A. Rainstorms- form when a warm front meets a cold front and nimbostratus clouds develop. In the summer, steady rainfall develops and in the winter, heavy snowfall

Rainstorm over salt flats

B. Thunderstorms- occur when a cold front meets a warm front producing cumulonimbus clouds. These systems need unstable air, warm air near the ground and cold air aloft to form.

1. Lightning is a common component of thunderstorms. It is caused when the attraction between positive (builds on ground) and negative (clouds) charges becomes strong enough to overcome the air’s high resistance to electrical flow

2. Thunder results as the tremendous energy of lightning flashes is turned into heat and then into sound waves. (Start counting when you see a lightning flash to determine how far away it is. If you hear the thunder in 5 seconds the lightning’s a mile away, 10 second 2 miles away)

Mimicry

3. Single-cell thunderstorms are the simplest kind although the life cycle of other kinds are essentially the same. The following steps explain the birth, life and death of a thunderstorm:

a. Warm humid air (called updrafts) rises from the ground

b. The air cools to its dew point, condensation begins to form a cloud.

c. Ice crystals or water droplets grow big enough to overcome the updraft and begin falling, dragging down the air.

d. The falling precipitation and air being dragged down form downdrafts.

e. Updrafts continue feeding warm, humid air into the storm. Existence of both updrafts and downdrafts make this the storm’s most violent stage

Single Cell Thunderstorm

f. Downdrafts grow, choking off updrafts.

g. With its supply of humid air cut, the storm begins dying and rain tapers off.

4. Multicell storms, which are clusters of single cell storms, are the most common kind of thunderstorms.

5. Supercell thunderstorms, sometimes called steady-state thunderstorms, are the most dangerous and may last for hours. These are the storms that produce dangerous tornadoes. These storms are driven by mesocyclones (a rotating column of rising air) which supply the spin that strong tornadoes require.

Review Questions 1. What atmospheric conditions cause changes in our weather?

Air masses and fronts

2. Explain how air masses and fronts are related As air masses meet, fronts are created.

3. Why do we usually experience our worst thunderstorms in spring and summer months?

Because storms are caused by great temperature variations. They draw their energy from heat.

III. Cyclones and Anticyclones

A. A cyclone is an area of low pressure that contains rising warm air. Cooler air moves in to take the place of warm air and the currents begin to spin.

1. Hurricanes are examples of powerful cyclones that form over tropical oceans. Winds spiraling around move in a counterclockwise direction in the Northern Hemisphere.

Hurricane Katrina

Katrina

Katrina

Flood damage from Katrina

2. Ingredients that a hurricane needs:

a. Warm ocean water (above 80 degrees Fahrenheit)

b. Converging winds near the surface.

c. Unstable rising air.

d. Air up to 18,000 feet needs to be humid as it’s pulled into the storm.

e. Winds not created by the storm, should be coming from nearly the same direction and at close to the same speeds at all altitudes to ensure the storm continues.

f. Upper atmosphere high-pressure area helps to pump away air rising in the storm

3. A hurricanes spinning system of rising air creates a doughnut-shaped wall of strong winds (74 mph or more), clouds and rainfall. Inside the wall is the calm eye of the hurricane.

4. Storm surge- this is the large dome of water that causes flooding, claiming more victims than wind.

Hurricane Hugo 1989

Track of Hugo

B. Anticyclones- a high pressure area that contains cold, dry air. In the Northern hemisphere these winds move in a clockwise direction. Weather caused by anticyclones is usually fair, clear, and dry

Review Questions

1. What atmospheric conditions are needed to produce a hurricane?

Very warm ocean

Converging winds

Unstable rising air

2. What is the difference between cyclones and anticyclones?

Cyclones usually cause very destructive weather.

Anticyclones are characterized by fair weather.

IV. Meteorology and Predicting the Weather

A. Meteorology is the study of Earth’s atmosphere, weather and climate.

B. Meteorologists use information from various sources before predicting the weather. The following are used to help meteorologists around the world:

1. Local weather observers and weather stations- observations taken near the Earth's surface using instruments (e.g. barometers, thermometers, anemometers and rain gauges) and visual observations (e.g. cloud and weather type). These surface observations are made at approved sites over the land and from ships at sea. Standard types of instruments are used and observations are usually made at least every three hours, and in many cases hourly.

2. Weather balloons- from approximately 500 stations around the globe. These balloons are launched at the same time every day, 7 a.m. and 7 p.m. Eastern Standard Time. These balloons rise more than 15 miles into the atmosphere and contain radiosondes, which measure air pressure, temperature and humidity. The balloons are also tracked to measure wind speeds and direction.

3. Satellites- provide cloud-top images, ocean and land temperatures, upper-air temperatures, humidity, wind speeds, and locations of invisible water vapor. In addition, they can monitor the sun, pick up emergency signals from downed aircraft and ships, and relay data from weather instruments on land and at sea.

a. Geostationary satellites remain at fixed points (their speed matches the Earth’s rotation) and orbit west-to-east at 22,238 miles above the equator. GOES stands for Geostationary Operational Environmental Satellite.

b. Polar satellites orbit at lower altitudes (200 miles) traveling north to south over the poles

c. In North America, images from satellites are sent every half hour.

4. Radars- radar measures reflected sound waves. Using Doppler radar, meteorologists can get a picture of precipitation that allows them to track a storm's progress over time.

As sound waves are given off by the radar they will come in contact with raindrops and other kinds of precipitation and reflect waves back to the radar antenna. If the rain is moving away, fewer sound waves at a lower frequency are sent back.

If the rain is moving towards the radar, more sound waves at a higher frequency are sent back. This data is then converted into maps.

5. Computer generated models- these models use mathematical equations to help predict the weather. The models are only calculated by acquiring vast amounts of information from satellites, radars, weather balloons and weather stations. This data is then used by supercomputers to calculate what the atmosphere will do at specific places over a large area and from the surface to the top of the atmosphere.

That was a yellow bellied sap sucker

C. Meteorologists must use data from many sources in order to get the most accurate picture of the weather.

That was a Vicuna

D. The World Meteorological Organization (WMO) allows for international exchange of weather data. This data is then sent to World Meteorological centers (in US, Washington D.C.) for analysis. Then charts, maps, and forecasts are sent to regional and local weather service offices.

E. Scientists soon realized after using computer models that a tiny difference in the numbers fed into the computer created drastic differences in the results. Therefore, meteorologists use several computer models to try to predict weather conditions on a daily basis.

F. Meteorologists use current and historic data to predict the weather, but atmospheric conditions can change quickly, causing local weather to be different than predicted.

Review Questions 1. What sources of information do meteorologists use to predict the weather?

Doppler, local people, weather balloons, computer models, satellites.

2. Why do meteorologists use several computer models to forecast weather?

Because weather can change instantly and its best to have as many sources as possible.

V. Weather Maps

A. Weather data gathered from the various sources mentioned above are used to prepare daily weather maps.

B. Information on weather maps is usually recorded using numbers and symbols to represent wind speed, cloud cover, precipitation, fronts, and areas of high and low pressure.

B. Information on weather maps is usually recorded using numbers and symbols to represent wind speed, cloud cover, precipitation, fronts, and areas of high and low pressure.

D. To represent a warm front on a weather map use a line with half circles pointing the direction of its movement. Use a line with triangles pointing the direction of its movement to represent a cold front.

E. On some weather maps you may see lines called isotherms or isobars.

1. Isotherms- lines that connect locations with the same temperature

2. Isobars- lines that connect locations with the same air pressure

VI. Controlling the Weather

A. Efforts to control the weather have centered around rainmaking, because it is so important for the growth of crops.

B. In 1946, General Electric discovered cloud seeding in its research laboratory.

C. Cloud seeding has been the only successful weather modification project in the past. It encourages clouds to form growing water droplets that will become heavy enough to fall as rain or snow. Steps in cloud seeding:

1. Airplanes fly through clouds spreading dry ice or silver iodide throughout the cloud.

2. The silver iodide causes ice crystals to form and grow from surrounding water vapor and water liquid.

3. The ice crystals grow until they are large enough to begin falling as rain or snow.

C. The latest research into controlling the weather is based on the sensitivity of the atmosphere. Scientists understand that the atmosphere is chaotic and difficult to predict. Knowing this, scientists believe that “just right perturbations” to the atmosphere may allow them to have some control over the weather. These “perturbations” would be possible with sensors, satellites, aircraft contrails (essentially cirrus clouds), solar reflectors, space solar power generators, wind turbines, and massive computer technology.

D. In the future, controlling the weather could lead to no droughts, no tornadoes, no snow storms during rush hour, and controlling the path of a hurricane.

E. The problem with weather experiments is there are no guarantees that weather changing efforts are working or if it is simply Mother Nature at work causing changes.

F. There are also many ethical and legal issues when considering the possibilities of controlling the weather in the future.

Review Questions 1. What information is displayed on a station circle?

Cloud cover, temp, air pressure, wind speed and wind direction

2. Do you think that scientists should be able to control the weather? Why or why not?