1. 7th Grade Science Weather Unit
Weather and Climate
7th Grade Science
Weather Unit

2. Weather and Climate Overview
Weather VERSUS Climate
Weather Variables
Temperature
Humidity
Cloud Types
Precipitation
Air Masses
Types of Fronts
Pressure
Storms

3. Weather VERSUS Climate
Weather includes the daily and hourly conditions in the atmosphere.
Climate includes the average weather of an area over many years.

4. Weather Variables
Weather variables include many things like temperature, cloud coverage, and precipitation that produce our current weather conditions.
Some variables affect one another, some do not.
Can you think of two variables that influence one another?

5. Temperature
Temperature is defined as “A measure of the ability of a substance to transfer heat energy to another. The temperature of a substance is closely related to the average kinetic energy (energy of motion) of its molecules.”
Measured in degrees Celsius or Fahrenheit.

6. Humidity Humidity is the amount of water vapor in the air.
Relative humidity is the amount of water in the air compared to how much water the air can hold at a give time.
Relative humidity can be found by using a sling psychrometer.

7. Cloud Types Clouds are formed by droplets of water.
Luke Howard first classified clouds in 1803.
His system of classification used Latin terms such as:
-cumulus = heap (puffy clouds)
-cirrus = lock of hair (thin, wispy clouds)
-stratus = layer (layers of clouds)
-nimbo = rainy or stormy (produces precipitation)

8. Cloud Types
Low Clouds include those without an “Alto” or “Cirro”. (Ex. Stratus, Stratocumulus)
Middle Clouds include those the “Alto” as part of their name. (Ex. Altocumulus)
High Clouds include those that have “Cirro” as part of their name. (Ex. Cirrus, Cirrocumulus)
Exception to the cloud naming rule: Nimbostratus extend through more than two layers)

9. Stratus
Stratus clouds and formed when cooled air is holding as much water as possible.
These clouds appear as broad, flat clouds that appear blanket-like.
Type of precipitation:

10. Cumulus Big, puffy clouds describe cumulus clouds.
They are caused by warm air rising.
Precipitation: NONE

11. Nimbostratus Clouds that are in sheets or layers.
Hail, sleet, rain, or snow can fall from these clouds.
Lightning is not present.

12. Cirrus
Clouds that are “wispy” and light. These clouds forms when the weather is cold.
These clouds usually do not produce precipitation.

13. Cumulonimbus
Large, towering clouds that produce thunderstorms are called cumulonimbus.
Thunderstorms also contain lightning, rain, and strong winds.

14. Precipitation
Any form of water from a cloud is considered precipitation.
The type of precipitation depends upon the air temperature.
Cloud temperatures below 0ºC can cause snow if the air temperature as the water falls is below 0ºC.
Sleet can form if rain falling comes into contact with cold air.
Rain and Drizzle can form if the air temperature is above 0°C.
Hail forms as raindrops are bounced back and forth. Constant freezing causes layers to form.

15. Picture of Hail

16. Air Masses
An air mass is a “large body of air with uniform temperature and humidity.”
Air takes on the characteristics of the land or water that it is over.
Examples include a cold air mass from Canada and warm moist air mass from the Gulf of Mexico.
Air masses near poles = cold; air masses near equator = warm

17. Types of Fronts Definition of Front: A boundary between two air masses
Cold
Warm
Stationary
Occluded
Fronts usually move across the US from west to east.

18. Cold Front
A cold front occurs when cold air pushes underneath warmer air.
This pushes of the warm air up causes cumulonimbus clouds to form and heavy rains.
Cold fronts move quickly.
Following a cold front is cold, dry weather.

19. Warm Front
A warm front occurs when a warm air mass pushes into a cold air mass.
Cirrus clouds are formed as warm air is lifted.
As the warm front continues to move, gentle rain may occur if stratus clouds are present.

20. Stationary Front
A stationary front is a warm or cold front that has stopped moving.
When moving across a stationary front air temperature and winds shift.
Once the air masses begin to move again and the front advances it will become a WARM or a COLD front.

21. Occluded Front
An occluded front occurs when a cold front catches up to and overtakes a warm front.
Usually stormy weather near cold front and as the occluded front forms the winds shift dramatically.

22. High (H) Pressure http://ww2010.atmos.uiuc.edu/
The center of a high pressure system has the highest pressure and pressure decreases as one move away from the center.
Winds move in a clockwise direction in the northern hemisphere.
Winds generally move from high to low pressure.
High pressure usually means little cloud coverage and fair weather.

23. Low (L) Pressure
The center of a low pressure system has the lowest pressure and pressure increases as one move away from the center.
Winds move in a counterclockwise direction in the northern hemisphere.
Fronts rotate around a low pressure system.
Weather can be poor with rain, storms, and even tornadoes.

24. Tornado
Tornadoes normally form when warm, humid air moving east runs into a cold front.
Tornadoes move counterclockwise in the Northern Hemisphere.
A tornado over warm water is called a waterspout.
Tornadoes rated F1 (weak) to F5 (very strong)

25. Thunderstorm Formed when warm, moist air is lifted upward.
Cumulonimbus clouds form producing lightning and rain (and sometimes hail).

26. Lightning
Lightning is formed when water droplets moving within a cloud become electrically charged.
Usually the upper part of a cloud is positive (+), the lower part is (-).
Lightning charges between clouds or between clouds and the earth’s surface is big enough an electric discharge occurs.

27. Thunder Lightning causes a quick and rapid expansion of air molecules.
This causes a sound, we call thunder.
Since sound waves travel more slowly than light waves, we SEE lightning and then HEAR thunder.
The closer we are to the storm, the more closely we see the lightning and hear the thunder.

28. Hurricane
A tropical cycle or a rotation like a tornado over warm, humid waters.
Hurricanes are named each year alternating men’s and women’s names.
Floods, high winds , landslides, and mudslides can accompany hurricanes.
Hurricanes are rated Category 1-5.

29. WARNING VERSUS WATCH VERSUS ADVISORY
From the National Weather Service:
-WARNING: Issued when a weather event is occurring or has a high probability to occur.
-WATCH: Occurs when the risk of a weather event has increased. The time and location are still uncertain.
-ADVISORY: Issued for less serious occurrences. Caution should still be taken.

30. Sources www.dictionary.com
Science Plus: Science and Technology BLUE BOOK
Pictures: -
-scijinks.jpl.nasa.gov - - - - - - - - -
-Hong Kong Observatory