300 Weather

Kindergarten
1. Earth Sciences Topics
  1. The Earth is composed of land, air, and water. As a basis for understanding this concept, students know:
    1. changes in weather occur from day to day and over seasons, affecting the Earth and its inhabitants.

Grade 1
1. Earth Sciences Topics
  1. Weather can be observed, measured, and described. As the basis for understanding this concept, students know:
    1. the weather changes from day to day, but trends in temperature or of rain (or snow) tend to be predictable during a season.

Grade 5
1. Earth Sciences Topics
  1. Water on Earth moves between the oceans and land through the processes of evaporation and condensation. As the basis for understanding this concept, students know:
    1. water moves in the air from one place to another in the form of clouds or fog, which are tiny droplets of water or ice, and falls to the Earth as rain, hail, sleet, or snow.
  2. Energy from the sun heats the Earth unevenly, causing air movements resulting in changing weather patterns. As the basis for understanding this concept, students know:
    1. causes and effects of different types of severe weather.
    2. how to use weather maps and weather forecasts to predict local weather, and that prediction depends on many changing variables.

Grade 6
1. Energy in the Earth System
  1. Many phenomena on the Earth’s surface are affected by the transfer of energy through radiation and convection curents. As the basis for understanding this concept, students know:
    1. differences in pressure, heat, air movement, and humidity result in changes of weather.

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II. Goals of this module -
To learn how and why water changes state from ice (solid) to water (liquid) and to water vapor (gas).
To learn the definitions of and proper spelling for scientific terms that are used commonly in newspapers, magazines, and books, and on television and radio programs to explain weather and climate phenomena.
To learn the causes and effects of various types of weather and climate.

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III. Definitions -
Weather - The state of the atmosphere at a particular place for a short period of time.
Climate - A generalization of the weather conditions at a particular place over a long period of time.
All the weather and climate phenomena on the Earth are a result of the effect of the sun's heat energy on the Earth’s atmosphere.

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IV. Changes of state in water -
Latent heat - heat energy absorbed or released in the process of changing state; temperature does not change. Also known as heat of transformation.
Solid (ice) to liquid (water) = melting - requires that the molecules speed up; 80 calories per gram of latent heat absorbed.
Liquid (water) to solid (ice) = freezing - requires that the molecules slow down; 80 calories per gram of latent heat released.
Liquid (water) to gas (water vapor) = evaporation - requires that the molecules speed up; 600 calories per gram of latent heat absorbed.
Gas (water vapor) to liquid (water) = condensation - requires that the molecules slow down; 600 calories per gram of latent heat released.
Solid (ice) to gas (water vapor) = sublimation - requires that the molecules speed up; 680 calories per gram of latent heat absorbed.
Gas (water vapor) to solid (ice) = deposition - requires that the molecules slow down; 680 calories per gram of latent heat released.

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V. Humidity -
Humidity - The amount of water vapor in the air.
Relative humidity - The percent of water vapor in the air at a certain temperature compared to the potential water vapor capacity of the air at that temperature.
Saturation - When the air at a certain temperature can no longer hold any more water vapor it is said to be saturated. The relative humidity is 100% at this point.
Saturation occurs when the number of water molecules changing from liquid to gas (evaporating) is exactly balanced by the number changing from gas to liquid (condensing). As the temperature of the air increases, its capacity to hold water vapor increases.
Relative humidity can be changed, therefore, either by adding water vapor to or subtracting water vapor from the air or by raising or lowering the temperature of the air.
Dew point - The temperature of the air when it reaches saturation.
Fog and Dew - When the earth radiates heat at night, the temperature drops. If the temperature at ground level goes below the dew point, water vapor is forced to condense out of the air and fog and/or dew will be formed.

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VI. Dry adiabatic temperature change -
The word adiabatic refers to a change in volume.
If a volume of air decreases (is compressed), the molecules speed up, so the temperature increases, even though no heat has been added.
If a volume of air increases (expands), the molecules slow down, so the temperature decreases, even though no heat has been taken away.
If unsaturated (dry) air rises through the atmosphere, it will expand due to reduction in pressure and cool.
If unsaturated (dry) air descends through the atmosphere, it will be compressed due to increase in pressure and heat up.
The dry adiabatic lapse rate is equal to 10°C of temperature change for every 1,000 meters of altitude change (= 5.5°F/1,000 ft).

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VII. Wet adiabatic temperature change -
When dry air rises far enough, its temperature will eventually decrease below the dew point and water will begin to condense. When water condenses, latent heat is released. This release of latent heat slows down the rate of cooling that is due to adiabatic expansion.
The wet adiabatic lapse rate varies from 5°C/1,000 m (2.5°F/1,000 ft) for air with high humidity to 9°C/1,000 m (4.5°F/1,000 ft) for air with low humidity.

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VIII. Stability of air -
Stable air - When the environmental lapse rate is less than the wet adiabatic rate, air will not move upward.
Unstable air - When the environmental lapse rate is greater than the dry adiabatic rate, air will move upward.

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IX. Processes that lift air -

Orographic lifting - When a mass of moving air approaches a mountain range, it is forced to rise up over the mountain range, thus decreasing its temperature.

Frontal wedging - When one mass of moving air approaches a second mass of air of different density, the lighter of the two air masses is forced to rise up over the other one, thus decreasing the temperature of the mass that is being lifted.

Convergence - When two masses of air of equal density approach each other, the two masses will rise together, thus decreasing the overall temperature of the two masses.

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X. Clouds -
When the temperature of an air mass drops below the dew point, water will begin to condense on microscopic dust, smoke, and salt particles that are suspended in the air. These are called condensation nuclei. The very tiny water droplets in a cloud are so small that they remain suspended.
Cloud classification -

Cumulus - globular cloud masses, usually at low altitudes.

Altocumulus - middle altitude, globular clouds.

Cirrocumulus - high altitude, globular clouds.

Cumulonimbus - towering clouds that produce rain.

Stratus - sheet-like clouds that cover most of the sky at low altitudes.

Altostratus - middle altitude sheet clouds.

Cirrostratus - high altitude sheet clouds.

Nimbostratus - low cloud sheets that produce rain.

Stratocumulus - low altitude mixture of cumulus and stratus clouds.

Cirrus - high altitude veil-like, wispy clouds.

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XI. Precipitation -
Rain - Tiny water droplets that condense in the cloud move around and collide with other drops and coalesce into drops that are big enough to fall.
Snow - Forms when deposition occurs below 0°C; ice crystals form instead of water droplets and grow until they are heavy enough to fall.
Hail - Water drops that form in cumulonimbus clouds are moved upward by updrafts in the clouds and frozen before falling to earth.
Freezing rain - Water drops encounter freezing temperatures just before landing and turn to ice upon colliding with solid objects.
Rime - Fog droplets freeze onto surface objects
Measurement of precipitation -
Rain falling into the receiving portion of a rain gauge is funneled into a cylinder that has a cross-sectional area of 1/10 the area of the receiver, so the amount of rainfall is magnified 10 times, which allows for accurate measurement to the nearest 0.025 cm (0.01 inch).

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XII. Air masses -
Air masses are immense bodies of air, 1,000 miles or more across, that are characterized by a similarity of temperature and moisture at any given altitude.
Continental polar - Originate over continental polar areas, so are cold and dry. Produce snow after passing over the Great Lakes.
Maritime polar - Originate over polar oceans, so are cold and wet. Produce Pacific Coast storms and the "nor'easter" storms of New England by orographic lifting.
Continental tropical - Originate over continental tropical areas, so are warm and dry. Do not produce storms.
Maritime tropical - Originate over tropical oceans, so are warm and humid. Produce warm storms in the southwestern and southeastern United States.

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XIII. Fronts -
Fronts are the boundaries that separate two air masses of different temperature, the trailing one moving faster than the leading one.
Cold front -
The trailing cold air mass pushes under the leading warm air mass.
Lifting of the warm air over the cold air causes condensation and rain.
Slope of the front is steeper than a warm front.
Cumulonimbus clouds form just behind the front.
Heavy rain lasts for a short time.

Warm front -
The trailing warm air mass rides up over the leading cold air mass.
Lifting of the warm air over the cold air causes condensation and rain.
Slope of the front is gentle.
Cirrus clouds form 600 miles ahead of the front, followed by cirrostratus and altostratus clouds.
Stratus and nimbostratus clouds cause rain around 300 miles ahead of the front.
Light rain lasts for many hours.

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XIV. Middle-latitude cyclone -
A low pressure area that circulates in a counterclockwise direction in the northern hemisphere and pulls a cold air mass down from the north to create a cold front and a warm air mass up from the south to create a warm front.
The two fronts meet at the low pressure center of the cyclone.
Such a cyclone will move slowly across the United States in an eastward direction.
It passes over a given region in 2 to 4 days.
Air converges toward the low pressure center of the cyclone and rises to create rain in the low pressure area.
Rain also falls in association with the two frontal wedges.
Rising air in the low pressure area joins the jet stream in the upper troposphere.
Surface air into the cyclone is fed by diverging air from a nearby anticyclone.

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XV. Thunderstorms -
Produced in the afternoon or early evening when warm, moist air rises and begins to condense.
Release of latent heat from condensation further warms the air, making it more buoyant and unstable.
Velocity of rising air may reach 60 miles per hour judging by the size of hailstones that are suspended in the cloud.
Finally the updrafts cannot support the rain drops and hail stones any more, so they begin to fall, dragging gusty downdrafts of wind with them.
Within about an hour, downdrafts dominate and the storm dissipates.

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XVI. Tornado -
A rapidly spiraling, narrow funnel cloud that touches down on the surface of the earth. Over the ocean it is called a water spout.
Diameter is about 400 meters, forward velocity is about 30 miles per hour, and the touchdown path averages about 6 miles in length.
Low pressure in the center of the tornado is as much as 10% lower than on the outside.
Wind speed, therefore, can approach 300 miles per hour.
Tornado watch means that conditions are favorable for tornadoes.
Tornado warning means that funnel clouds have been spotted.
Tornado forecasting has been enhanced by the use of Doppler radar which can detect motion in clouds.

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XVII. Hurricanes - (Click on the small picture to see a satellite movie of a hurricane.)
Gigantic, fast-spiraling cyclones; the greatest storms on earth. Average diameter is about 375 miles.
Pressure drop in the center of the storm (the eye) may be as much as 60 millibars. Wind speeds will go up to 160 miles per hour.
Hurricanes form over the ocean where the water temperature is 80°F or more. Ocean temperature is this high only between 5°N and 30°N latitude.
A hurricane begins in the northern trade winds belt as a normal cyclone that is at first referred to as a tropical depression, becomes a tropical storm when the wind is over 38 miles per hour, and evolves into a hurricane when the wind passes 74 miles per hour.
Hurricanes will dissipate over land because of the lack of warm, moist air.
Hurricane destruction -
Wind damage - Force of the wind blows things over and away.
Storm surge - Force of the wind pushes a large bulge of ocean water ahead of it, which raises sea level as much as 10 feet. In addition, the waves can reach a height of 50 feet. The result is coastal flooding from the ocean.
Inland flooding - Intense rain falling on the land due to the hurricane causes flooding of the inland rivers.

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XVIII. Special attributes of atmospheric circulation -
The two subtropical high zones are where air is descending and diverging, so the climate is warm and dry.
The two polar front zones are where air is converging and rising, so warm air masses from the mid-latitudes are meeting cold air masses from the poles and the climate is stormy.
The monsoons of India are created by high pressure over Asia in the winter due to the continent being colder than the ocean and producing strong northeast winds, and low pressure over Asia in the summer due to the continent being warmer than the ocean and producing strong southwest winds.
Land and sea breezes - Air is heated over the land during the day and rises, causing an onshore sea breeze. Air is cooled over the land during the night and descends, causing an offshore land breeze.
Valley and mountain breezes - Air is heated over the mountain slopes during the day and rises, causing an up-valley valley breeze. Air is cooled over the mountain slopes during the night and descends, causing a down-valley mountain breeze.
Santa Ana wind - High pressure over the Mojave Desert and low pressure over the Pacific Ocean causes a warm, northeast wind.

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XIX. Possible essay questions -
Explain in detail why fog or dew form. Be sure to define any meteorological terms that you use in your answer.
Explain in detail the cause of the dry adiabatic temperature change that occurs with a change in altitude.
Explain in detail the cause of the wet adiabatic temperature change that occurs with a change in altitude.
Describe and illustrate the three processes that cause air to be lifted upward.
Describe how a rain gauge is constructed and tell what is done with the measurements to secure a correct rainfall value.
Describe the characteristics of a cold front and illustrate one with a cross section.
Describe the characteristics of a warm front and illustrate one with a cross section.
Describe the characteristics of a mid-latitude cyclone and illustrate one with a map view and a cross section.
Describe in detail the origin of a thunder storm.
Describe and contrast the characteristics of a tornado and an hurricane.
Describe the cause of the monsoons of India.
Describe the cause of land and sea breezes.
Describe the cause of mountain and valley breezes.
Describe the cause of the Santa Ana winds in southern California.

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XX. Practice Questions
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