1. The Atmosphere
© Lisa Michalek

2. Structure of the Atmosphere
The atmosphere is divided into four layers
Troposphere
The lowest layer is where most of the weather occurs
Stratosphere
Mesosphere
thermosphere

3. Weather
The short-term (a few hours or days) condition of the atmosphere at a given location
Temperature, sky conditions, precipitation, atmospheric pressure, humidity, wind speed, and wind direction
Meteorologists are scientists who study and predict the weather

4. Air Temperature
In the daily cycle, temperature is usually lowest in the early morning and warmest at mid-afternoon
In the season cycle, winters are generally cold, while summers tend to be hot
Short term factors such as cloud cover and regional weather systems affect temperatures
Clouds reduce daytime temperature by reflecting sunlight back into space
At night, clouds help hold heat energy to Earth

5. Air Temperature Measured with a thermometer
A bulb that contains liquid that expands into a narrow, calibrated neck when it is heated and moves down the neck when the temperature decreases
When meteorologists record official air temperature, the thermometer is kept in a special weather shelter to protect the instruments from direct sunlight

6. Temperature Scales
A temperature of zero on the Fahrenheit scale is the temperature of a mixture of equal parts ice, water, and salt
The freezing point of water is what sets the zero point on the Celsius (centigrade) scale
The point at which all particle motion stops is defined as zero on the Kelvin scale

7. Air Pressure Is caused by the weight of the atmosphere
Above each square inch of Earth’s surface is a column of air the weighs 14.7 pounds

8. Measuring Air Pressure
A barometer is an instrument used to measure air pressure using the dense liquid metal mercury
Meteorologists measure air pressure in millibars
Standard sea level pressure is millibars
On a weather map, isobars connect places that have the same air pressure

9. Air Pressure Factors If air is cooled, it contracts and becomes denser
This causes pressure to rise
If air is heated, it expands and becomes less dense
This causes pressure to fall
Humid air is lighter than dry air
This is because water molecules are lighter than the gasses they displace in the air

10. Moisture in the Atmosphere
When the air is holding as much moisture as it can, the air is saturated
The air’s ability to hold water vapor depends upon the temperature
The warmer the air, the more moisture the air can hold
The dew point is the temperature to which the air must be cooled to become saturated
If the temperature falls below the dew point, condensation occurs as water vapor changes to liquid water

11. Measuring Moisture in the Atmosphere
Meteorologists use a sling psychrometer and a dew-point temperature table to determine the dew point
The psychrometer consists of two thermometers mounted side by side which can be swung through the air
One thermometer measures the air temperature
The bulb of the other thermometer is covered by a wet cloth
As the thermometers are swung through the air, evaporated cooling causes the wet-bulb thermometer to register a lower temperature
When you subtract the wet-bulb temperature from the dry-bulb temperature, you can use the dew-point table to determine the dew point

12. Psychrometer

13. Relative Humidity
Compares how much moisture the air is actually holding with how much moisture it could hold if the air were saturated
It is expressed as a percent of saturation
Air is saturated if it is holding all the moisture it can hold at its present temperature
Determined with a psychrometer and a relative humidity table

14. The Wind Wind is heat flow by convection within the atmosphere
Winds are the result of uneven heating of the Earth’s surface
This uneven heating causes differences in air pressure to develop

15. The Wind
Winds always blow from areas of high pressure to areas of low pressure
Winds blow fastest where the gradient in air pressure is greatest, where the isobars are close together

16. Measuring the Wind
To measure the wind, you need to determine both the wind speed and the wind direction
Wind speed is measured with an anemometer
The cups catch the wind, causing it to spin
Wind direction is indicated by a wind vane, which points into the wind

17. The Coriolis Effect The Earth’s rotation causes winds to curve
to the right in the Northern Hemisphere
to the left in the Southern Hemisphere

18. The Coriolis Effect
Winds move in a clockwise outward spiral around high-pressure systems
Winds move in a counterclockwise inward spiral around low-pressure systems

19. The Coriolis Effect

20. Zones of Convergence and Divergence
Rising warm, moist air at the center of the low causes winds and air masses to blow into the low-pressure system
The rising air cools, which causes cloud formation and precipitation
The descending air turns a high-pressure system into a single mass of cool, dry air that spreads across the surface of Earth

21. Cloud Formation
Clouds form when rising air is cooled below its dew point
Tiny particles called condensation nuclei in the air allow a cloud to form

22. Precipitation Rain and snow are the most common forms of precipitation
Drizzle is small raindrops that fall slowly
Sleet is a partially frozen mixture of rain and snow that occurs when the temperature is just above freezing
Hail is in the form of ice balls, which usually occurs in violent thunderstorms
Hailstones begin as snowflakes that start to melt and gather more moisture as they fall

23. Precipitation