Earth image Basic Introduction to Weather

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  1. Identify the three basic cloud types, and the type of weather associated with each.
  2. Discuss atmospheric pressure and how it is measured.
  3. Be able to define and identify types of air masses.
  4. Compare the four major types of air masses and how they create fronts.
  5. Be able to define and identify types of air masses and fronts.
  6. Describe the weather associated with a cold front.
  7. Describe the weather associated with a warm front.
  8. Discuss the difference between absolute humidity and relative humidity.
  9. Describe the dew point and the processes which happen at that temperature.
  10. Discuss how the dew point is determined.
  11. Discuss the measurement of relative humidity and dew point.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S1E1. Students will observe, measure, and communicate weather data to see patterns in weather and climate.
  • S1E1a. Identify different types of weather and the characteristics of each type.
  • S1E1b. Investigate weather by observing, measuring with simple weather instruments (thermometer, wind vane, rain gauge), and recording weather data (temperature, precipitation, sky conditions, and weather events) in a periodic journal or on a calendar seasonally.
  • S1E1c. Correlate weather data (temperature, precipitation, sky conditions, and weather events) to seasonal changes.

  • S4E3. Students will differentiate between the states of water and how they relate to the water cycle and weather.
  • S4e3c. Investigate how clouds are formed.
  • S4e3e. Investigate different forms of precipitation and sky conditions. (rain, snow, sleet, hail, clouds, and fog).

  • S4E4. Students will analyze weather charts/maps and collect weather data to predict weather events and infer patterns and seasonal changes.
  • S4E4a. Identify weather instruments and explain how each is used in gathering weather data and making forecasts (thermometer, rain gauge, barometer, wind vane, anemometer).
  • S4E4b. Using a weather map, identify the fronts, temperature, and precipitation and use the information to interpret the weather conditions.
  • S4E4c. Use observations and records of weather conditions to predict weather patterns throughout the year.
  • S4E4d. Differentiate between weather and climate.

  • S6E4a. Demonstrate that land and water absorb and lose heat at different rates and explain the resulting effects on weather patterns.
  • S6E4b. Relate unequal heating of land and water surfaces to form large global wind systems and weather events such as tornados and thunderstorms.
  • S6E6a. Explain the role of the sun as the major source of energy and its relationship to wind and water energy.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • Because the earth turns daily on an axis that is tilted relative to the plane of the earth's yearly orbit around the sun, sunlight falls more intensely on different parts of the earth during the year. The difference in heating of the earth's surface produces the planet's seasons and weather patterns.
  • The cycling of water in and out of the atmosphere plays an important role in determining climatic patterns. Water evaporates from the surface of the earth, rises and cools, condenses into rain or snow, and falls again to the surface. The water falling on land collects in rivers and lakes, soil, and porous layers of rock, and much of it flows back into the ocean.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Clouds, formed by the condensation of water vapor, affect weather and climate.
  • Global patterns of atmospheric movement influence local weather. Oceans have a major effect on climate, because water in the oceans holds a large amount of heat.
  • The sun is the major source of energy for phenomena on the earth's surface, such as growth of plants, winds, ocean currents, and the water cycle. Seasons result from variations in the amount of the sun's energy hitting the surface, due to the tilt of the earth's rotation on its axis and the length of the day.
 


Hillcrest Orchards, Ellijay, GA
Photo courtesy of Miranda Gore.

Meteorology

Meteorology is the study of the atmosphere and its phenomena.

Meteorologists are scientists who study the atmosphere and its phenomena. Many also interpret weather maps and make forecasts.

Weather and Climate

Weather includes daily changes in precipitation, barometric pressure, temperature, and wind conditions in a given location. Weather can change quickly.

Climate is the average of the weather in a given place over a long period of time.

Clouds

The three main types of clouds are:

  1. Cirrus clouds

    Cirrus clouds are wispy in appearance, and resemble horsetails (they are sometimes called mares' tails). They are among the highest clouds, forming at elevations of 25,000 feet and above, where the temperatures are far below freezing. Cirrus clouds are formed almost entirely of tiny ice crystals.

  2. Cumulus clouds

    Cumulus clouds are "fair weather" clouds and are unlikely to produce precipitation. They form in warm air on sunny days. Cumulus clouds can form at almost any altitude, with bases sometimes as high as 14,000 feet.

    Cumulus clouds are clouds of vertical development and may grow upwards dramatically under certain circumstances. The updrafts may be caused by heating of the air by the ground surface (common in the eastern and midwestern US), the action of a cold front, or to temperature differences between land and ocean (common in Florida and the southeastern US).

    The vertical air curents results in towering clouds with an anvil head on top called cumulonimbus clouds. The anvil head forms in the tropopause (about 6 - 8 miles up). These clouds are sometimes referred to as "thunderheads". They can produce heavy rain, thunder and lightning, and sometimes hail. They occur chiefly in summer.

    Cumulonimbus cloud

    Another type of cumulus cloud is the altocumulus cloud, which sometimes resembles fish scales. They sometimes have dark, shadowed undersides.

    Altocumulus clouds

  3. Stratus clouds

    Stratus clouds are low clouds, ranging in height from near the earth's surface up to 6,500 feet. Stratus clouds form flat layers or uniform sheets. Only a fine drizzle can form from stratus clouds because there is no vertical development.

Atmospheric Pressure (Air Pressure)

The air is composed of billions of molecules that are constantly moving in all directions, bouncing off whatever they encounter.

These collisions determine the air pressure of a mass of air.  The more collisions, the greater the air pressure.

The speed at which the air molecules move depends on the temperature.

When an air mass is heated, air molecules move faster, causing them to push outward, and the air mass expands.

The expansion causes a drop in density, and the air mass becomes lighter than its surroundings. This causes the air mass to rise.

If the air parcel is cooled, the process is reversed and the air tends to sink.  As warm air rises, it cools and spreads.  Once it cools, it starts to sink back to Earth.

The movement of air from high-pressure to low-pressure areas produces wind.

Pressure Gradients

The difference in air pressure over a horizontal distance is called the pressure gradient.

The greater the pressure difference, the stronger the winds blowing from the high-pressure area toward the low-pressure area.

Measuring Air Pressure

Air pressure is measured using a barometer.

Air pressure is generally measured in hectopascals (formerly known as millibars).

The air pressure at ground level generally tends to vary between 980 and 1040 hectopascals.

Air Pressure and Weather

As air rises to create an area of low pressure, water vapor tends to condense and form clouds.

Low pressure is generally associated with cloudy skies and wet weather.

Sinking air associated with high pressure areas generally means that no condensation takes place.

High pressure is generally associated with clear skies and sunny conditions.

Air Masses

An air mass is a large body of air with relatively uniform temperature and moisture characteristics. The Sun’s heat causes air masses to form and circulate in the atmosphere. This movement creates differences in air pressure, which in turn, creates winds. (See course notes on Wind.)

Air moves both horizontally (because of differences in pressure) and vertically because it is forced to rise mechanically (such as when it encounters a mountain range), or because of changes in buoyancy. Buoyancy is controlled by differences in density.

When air is heated, the air expands, the density decreases, and the air rises.

When air is cooled, the air, the air becomes more dense and sinks.

Heating the surface below a parcel of air causes the air to expand. With fewer molecules in a given area, the number of collisions between air molecules decreases, and the air pressure decreases. When the parcel of air is being cooled at the surface, the air pressure increases as the volume of air decreases. If the two air parcels are adjacent to each other, the high-pressure air will push sideways against the low-pressure air and the air will move sideways from areas of high to areas of low pressure.

Pressure differences among air masses are typically related to distribution of surface temperatures.

If an air mass is heated until its density is lower than that of its surroundings, the lower density air will rise.

If an air mass is cooled until its density is higher than that of the underlying air, it will sink.

Air masses form mostly in tropical or polar regions. When air masses move from their regions of origin, they bring heat waves and cold spells. There are four main types of air masses that control the weather in the U.S.:

  1. Continental polar air masses- form over Alaska and northwestern Canada. This air is typically dry and quite cold. Generally fair weather accompanies this air mass.
     

  2. Maritime polar air masses- build up over the northern oceans. These are not as cold as continental air masses but are more humid. The chilly, damp weather that is typical of the Pacific northwest is a result of these air masses.
     

  3. Maritime tropical air masses- form over the warm water of the Gulf of Mexico and the Caribbean Sea. These air masses are typically warm and humid. Georgia's warm, humid summers are the result of this type of air mass.
     

  4. Continental tropical air masses- form over the deserts of the Southwest and Mexico. These air masses give the Southwest its warm, dry climate, but they do not move into other parts of the United States as often as the maritime tropical air masses or polar air masses. This is also the type of air mass that controls the climate in the Middle East.

Fronts

The boundary between two air masses of different temperature and density is called a front.

Differences in densities are most often the result of differences in temperatures. Fronts usually separate air masses with contrasting temperatures. Because air masses of different temperatures often differ in their humidities, fronts also generally separate air masses with different humidities as well.

Because air masses have both horizontal as well as vertical movement, the upward extension of a front is called a frontal surface or frontal zone.

Cold Fronts

When cold, dry, stable polar air displaces warm, moist, unstable tropical air, the moving boundary is called a cold front. At the front, the cold, dense air wedges under the warm air, forcing it upwards.

Cold air is more dense than warm air and therefore, a cold front will tend to push under warmer air as it advances. As the warm, moist tropical air is forced upwards, it will cool. Water vapor will condense to form clouds and precipitation will occur.

Cold fronts generate strong convective currents and are accompanied by violent surface winds and destructive storms. A relatively narrow band of thunderstorms at the front produces heavy showers and gusty winds. Behind the front, the air cools quickly.

Warm Fronts

If the advancing air mass is warmer that the local air, the boundary is called a warm front. Warm fronts are a zone where warm, tropical air replaces cold, polar air.

Warm air is less dense than cold air and the warm front will rise up over the cooler air. This produces clouds and precipitation well in advance of the front's surface boundary. Gradual uplifting associated with warm fronts do not tend to produce the violent storms that are associated with cold fronts.

Warm fronts are associated with a variety of clouds at different levels and precipitation.

Other types of fronts

Stationary front- A front with essentially no movement.  Surface winds tend to blow parallel to the front, but in opposite directions on either side of the front.

Occluded front- when a cold front catches up to and overtakes a warm front.

Humidity

Humidity is the amount of water vapor in the air. The amount of water vapor that the air can hold depends upon the temperature of the air. As the air temperature increases, the volume of water vapor the air can hold increases. Absolute humidity is a measure of the amount of water vapor that a given volume of air at a certain temperature can hold.

A more useful measure of humidity is expressed as relative humidity. Relative humidity is the amount of water vapor in the air, expressed as a percentage of the amount required to saturate the air at the current temperature.

In other words, relative humidity is the ratio of the water vapor content (amount of water vapor actually in the air) compared to the water vapor capacity (maximum amount of water vapor the air can hold), at that particular temperature.

Relative humidity (%) = Water vapor content x 100%
________________________
Water vapor capacity

Saturated air at a given temperature can be referred to as 100 percent relative humidity.

If we say that the relative humidity is 75 percent, it means that the air mass is holding only three-quarters of its capacity for its current temperature.

If the amount of water vapor stays constant as the temperature rises, the relative humidity drops because warmer air is capable of holding more water vapor (but there is no actual change in the absolute amount of water vapor present).

Dew point

As an air mass cools, it can hold less and less water vapor.

If it cools down enough, it reaches a point where the water vapor present in the air mass represents the amount needed to saturate an air mass at the lower temperature.

The temperature at which saturation occurs is the dew point temperature.

Condensation will begin to form if the temperature drops below the dew point and dew will form. The dew point, or the temperature at which condensation occurs, depends on the amount of water vapor in the air.

Measuring relative humidity

The relative humidity and dew point temperature of the air can be determined by making measurements with a psychrometer or a hygrometer, and reading a table of numbers.

The psychrometer consists of two thermometers mounted side-by-side, one of which has a wick (or piece of wet cloth) around its bulb. The psychrometer is spun around through the air, during which time the water begins to evaporate from the wick, cooling one of the thermometers. (This is why it is sometimes called a "sling psychrometer".) Comparing the temperatures on the wet and dry bulb thermometers, the relative humidity and dew point are determined by using tables (see textbook and lab manual).

With a hygrometer, relative humidity can be measured directly without the use of a table.

Weather Maps

The first features to look for on a weather map are the high and low pressure systems (usually labeled with an H or an L in the center of the system).

Weather maps show isobars and isotherms.

The wind direction is given by arrows.

Weather maps also show fronts, which mark the boundary between air masses of different temperatures.

Locating fronts on weather maps

The following criteria are used to locate a front on a surface weather map:

 

Return to Earth & Space Science page

Return to Georgia Geoscience Online

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created April 2, 2005
Modified April 30, 2005
Image added December 29, 2006
Updated August 20 and 23, 2008