1. Unit 2 Lesson 1 Influences on Weather

2. How does the water cycle affect weather?
Unit 2 Lesson 1 Influences on Weather
How does the water cycle affect weather?
Weather is the condition of Earth’s atmosphere at a certain time and place.
The water cycle is the continuous movement of water among the atmosphere, land, oceans, and living things.
Evaporation, condensation, and precipitation in the water cycle are also parts of weather.
Weather is described by factors such as temperature, humidity, precipitation, air pressure, wind, and visibility. 2

3. What is humidity and how is it measured?
Unit 2 Lesson 1 Influences on Weather
What is humidity and how is it measured?
Humidity is the amount of water vapor in the air. As more water evaporates, humidity of the air increases.
Relative humidity is the amount of water vapor in the air compared to the amount of water vapor needed to reach saturation. It is measured with a psychrometer.
When air is saturated, the rates of evaporation and condensation are equal. 3

5. What is humidity and how is it measured?
Unit 2 Lesson 1 Influences on Weather
What is humidity and how is it measured?
Dew point is the temperature at which more condensation than evaporation occurs.
When air temperature drops below the dew point, water vapor condenses to form dew, fog, and clouds.
When an object’s temperature drops below the air surrounding it, dew will form on the object. 5

7. Unit 2 Lesson 1 Influences on Weather
What is precipitation?
Precipitation is any form of water that falls to Earth’s surface from the clouds. It includes rain, snow, hail, and sleet.
Inside a cloud, water droplets collide forming larger droplets. When they become too heavy, they fall as rain.
Snow forms when air temperatures are low enough to turn water vapor directly into ice in the clouds.
When balls or lumps of ice fall from clouds during thunderstorms, it is called hail.
Sleet forms when rain falls through a layer of freezing air, producing falling ice. 7

8. Unit 2 Lesson 1 Influences on Weather
What is air pressure?
Air pressure is the force of air molecules pushing on an area.
A barometer is used to measure air pressure.
Air pressure and density decrease with altitude.
Air pressure and density will also increase with cooler air and decrease with warmer air. 8

10. Unit 2 Lesson 1 Influences on Weather
What is air pressure?
Why do mountain climbers sometimes need extra oxygen to breathe at the top of a mountain? 10

11. What is wind and how is it measured?
Unit 2 Lesson 1 Influences on Weather
What is wind and how is it measured?
Wind is air that moves horizontally, or parallel to the ground.
Differences in air pressure sets air in motion. Over a short distances, air moves from higher pressure to lower pressure. This is felt as wind.
An anemometer is used to measure wind speed.
A wind vane or wind sock is used to measure wind direction. 11

12. What are pressure systems and how do they interact?
Unit 2 Lesson 1 Influences on Weather
What are pressure systems and how do they interact?
Areas of different air pressure cause changes in the weather.
In a high-pressure system, air slowly sinks down and spreads outward.
In a low-pressure system, air rises and cools. 12

14. What are pressure systems and how do they interact?
Unit 4 Lesson 3 What Influences Weather?
What are pressure systems and how do they interact?
A high-pressure system can form a low-pressure system. . 14

15. How do different pressure systems affect us?
Unit 2 Lesson 1 Influences on Weather
How do different pressure systems affect us?
Sinking air in a high-pressure system generally produces clear skies and calm air or gentle breezes.
Rising air in a low-pressure system generally produces clouds and rain. 15

16. How do air masses affect weather?
Unit 2 Lesson 1 Influences on Weather
How do air masses affect weather?
An air mass is a large volume of air in which temperature and moisture content are nearly the same throughout.
An air mass forms when air remains over a region for many days and takes on the temperature and humidity of the land below it. 16

18. Unit 2 Lesson 1 Influences on Weather
Where do fronts form?
A boundary, called a front, forms between air masses with differing densities.
Fronts cause a change in weather as they pass.
The temperature and moisture content of the air masses that meet and their movement relative to each other determine the type of front formed. 18

19. Unit 2 Lesson 1 Influences on Weather
Where do fronts form?
Cold fronts form as a cold air mass pushes an existing warm air mass up. 19

20. Unit 2 Lesson 1 Influences on Weather
Where do fronts form?
Warm fronts form as a warm air mass slides up over a retreating cold air mass. 20

21. Unit 2 Lesson 1 Influences on Weather
Where do fronts form?
Stationary fronts form when a cold air mass and a warm air mass remain in one place. 21

24. How do global wind patterns affect local weather?
Unit 2 Lesson 1 Influences on Weather
How do global wind patterns affect local weather?
Winds are caused by changes in pressure, which is due to unequal heating.
Wind speed and direction are affected by local winds. Globally, there is an overall movement of surface air from the poles toward the equator.
Winds bring different air masses to a region, which affects the weather. 24

26. How do jet streams affect weather?
Unit 2 Lesson 1 Influences on Weather
How do jet streams affect weather?
Jet streams are long-distance winds that travel above global winds for thousands of kilometers.
A jet stream affects temperatures and precipitation patterns.
The polar jet stream travels further south in the winter and further north in the summer. 26

27. How do jet streams affect weather?
Unit 2 Lesson 1 Influences on Weather
How do jet streams affect weather?
Each hemisphere usually has two main jet streams. 27

28. Ocean Effects How do ocean currents influence weather?
Unit 2 Lesson 1 Influences on Weather
Ocean Effects
How do ocean currents influence weather?
Surface currents in the ocean are caused by wind.
Energy is transferred to the atmosphere by the movement of warm and cold currents.
Hurricanes and monsoons are affected by ocean currents. 28

29. How do ocean currents influence weather?
Unit 2 Lesson 1 Influences on Weather
How do ocean currents influence weather?
As currents flow, they warm or cool the atmosphere above, affecting local temperatures.
Coastal areas may have lower summer temperatures than inland areas because of cold-water currents.
Coastal areas may have higher winter temperatures than inland areas because of warm-water currents. 29

31. How do ocean currents influence weather?
Unit 2 Lesson 1 Influences on Weather
How do ocean currents influence weather?
Why is San Diego cooler than El Centro in the summer? 31

32. How do ocean currents influence weather?
Unit 2 Lesson 1 Influences on Weather
How do ocean currents influence weather?
Why is Lillehammer colder than Bergen in the winter months? 32