Weather is defined as the state of the atmosphere at a specific time and place, and is determined by factors such as temperature, wind, humidity, precipitation, and air pressure. Changes in weather are caused by the movement of air masses and fronts as warmer or colder air moves into an area. Meteorologists study weather patterns and use tools like weather maps, reports from weather stations, isotherms, and isobars to make forecasts and predict upcoming conditions and severe weather threats.

1. Weather Notes

2. Section One- What is Weather?

Weather is the state of the atmosphere at a specific time and place.
Includes such conditions as air pressure, wind, temperatures, and
moisture in the air.
The Sun’s heat evaporates water into the atmosphere forming clouds and
returning the water to Earth as rain or snow; the Sun also heats air
Temperature is a measure of air molecule movement
• The Sun’s energy causes air molecules to move rapidly;
temperatures are high and it feels warm
• When less of the Sun’s energy reaches air molecules, they move
less rapidly and it feels cold

3. Section One- What is Weather?
Wind--air moving in a specific direction
• As the Sun heats air, it expands, becomes less dense, rises, and has low
atmospheric pressure
• Cooler air is denser and sinks, causing high atmospheric pressure
• Air moves from high pressure areas to low pressure areas, causing wind
Humidity--the amount of water vapor in the air
• Warmer air can hold more water vapor, tending to make it more humid
• Relative humidity--the amount of water vapor in the air compared to
what it can hold at a specific temperature
• When air cools, it can’t hold as much water vapor, so the water vapor
condenses to a liquid or forms ice crystals
• Dew point--the temperature at which air is saturated and condensation
forms

4. Section One- What is Weather?

Clouds from as warm air is forced upward and cools. Then the
water vapor condenses in tiny droplets that remain suspended in
the air
The shape and height of clouds vary with temperature, pressure and the
water vapor in the atmosphere
Shape
• Stratus--smooth, even sheets or layers at low altitudes
• Cumulus--puffy, white clouds, often with flat bases
• Cirrus--high, thin, white, feathery clouds made of ice crystals

5. Section One- What is Weather?
Height
• Cirro--high clouds
• Alto--middle-elevation clouds
• Strato--low clouds
Nimbus clouds are dark and so full of water that sunlight can’t penetrate
them

Precipitation- water falling from clouds
When water droplets in clouds combine and grow large enough,
precipitation falls to Earth
Air temperature determines whether the droplets form rain, snow, sleet,
or hail

6. Section Two- Weather Patterns

Because air and moisture move in the atmosphere, weather
constantly changes
Air mass--a large body of air with properties like the part of Earth’s
surface over which it forms
Stormy weather is associated with low pressure areas
Fair weather is associated with high pressure areas
Air pressure is measured by a barometer

Front--a boundary between two different air masses
Clouds, precipitation, and storms occur at frontal boundaries
Cold front--where colder air advances under warm air
Warm front--where warmer air advances under colder air
Occluded front--involves three air masses of different temperatures
Stationary front--air masses and their boundaries stop advancing

7. Section Two- Weather Patterns

Severe weather
Thunderstorms occur along warm, moist air masses and at fronts
• Warm, moist air is forced rapidly upward, where it cools and
condensates
• Strong updrafts of warm air and sinking, rain-cooled air cause
strong winds
Lightning
• Movements of air inside a storm cloud causes parts of the cloud to
become opppositely charged
• Current flows between the regions of opposite electrical charge,
forming a lightning bolt

8. Section Two- Weather Patterns
Thunder--lighting superheats the air, causing it to expand rapidly and
then contract, forming sound waves
Tornado--a violent, whirling wind that moves in a narrow path over land
Hurricane--a large swirling, low-pressure system that forms over tropical
oceans
Blizzard--a winter storm with strong winds, cold temperatures, and low
visibility, that lasts more than three hours

Severe weather safety
The National Weather Service watch means conditions are favorable for
severe weather to develop
A warning means that severe weather conditions already exist

9. Section Three- Weather Forecasts

Metorologists study and predict the weather

The National Weather Service makes weather maps
Station models show weather conditions at a specific location
Isotherms are lines on a weather map connecting points of equal
temperature
Isobars are lines on a weather map connecting points of equal
atmospheric pressure
Weather fronts move from west to east

10. Section Three- Weather Forecasts

Metorologists study and predict the weather

The National Weather Service makes weather maps
Station models show weather conditions at a specific location
Isotherms are lines on a weather map connecting points of equal
temperature
Isobars are lines on a weather map connecting points of equal
atmospheric pressure
Weather fronts move from west to east