1. Global Patterns & Relative Humidity
Chapter 5 Lessons 3 & 6
Global Patterns & Relative Humidity

2. How do global patterns affect local weather?
Patterns That Affect Temperature:
Temperature is affected by latitude, altitude, distance from large bodies of water, and ocean currents.
Areas near the equator are warmer than areas far from the equator. The reason is that the sun’s rays hit Earth’s surface more directly at the equator than at the poles. At the poles, the same amount of solar radiation is spread over a large area, bringing less warmth.
Latitude is the distance from the equator, measured in degrees.

3. How do global patterns affect local weather?
The area near the equator, between about 23.5 degrees north and 23.5 degrees south latitude is called the tropical zone.
These polar zones extend from about 66.5 degrees to 90 degrees north and 66.5 to 90 degrees south latitudes.
Between the tropical zone, and the polar zone are the temperate zones.

4. How do global patterns affect local weather?
In the case of high mountains, altitude is a more important climate factor than latitude. In the troposphere, temperature decreases about 6.5 degrees Celsius for every 1 kilometer increase in altitude.
As a result, mountainous areas are cooler than the lower areas around them.
The oceans greatly moderate, or make less extreme, the temperatures of nearby land. Water heats up about five times more slowly than land. It also cools down more slowly. Therefore, winds off the ocean often prevent extremes of hot and cold in coastal regions.

5. How do global patterns affect local weather?
Areas along the coasts are influenced by ocean currents – streams of water within the ocean that move in regular patterns.
Some warm ocean currents move from the tropics toward the poles. This warm ocean water warms the air above it. The warmed air then moves over nearby land, affecting weather patterns.
Cold currents also affect weather patterns by bringing cold water from the polar zones toward the equator. A cold current brings cool air.

6. How do global patterns affect local weather?
Patterns That Affect Precipitation:
The main factors that affect precipitation are prevailing winds, the presence of mountains, jet streams, and seasonal winds.
Weather patterns depend on the movement of huge bodies of air called air masses. Prevailing winds – the winds that usually blow in one direction in a region – move air masses around.
Air masses can be warm or cool, dry or humid. If there is more water vapor in the air mass, more precipitation might fall. The amount of water vapor in a prevailing wind depends on where the wind blows from. Winds that blow inland from oceans or large lakes carry more water vapor than winds that blow from over land.

7. How do global patterns affect local weather?
A mountain range in the path of prevailing winds can influence where precipitation falls. When humid winds blow from the ocean toward coastal mountains, they are forced to rise. The rising air cools and its water vapor condenses, forming clouds. Rain or snow falls on the windward side of the mountains, the side the wind hits.

8. How do global patterns affect local weather?
About kilometers above Earth’s surface are jet streams, narrow bands of high-speed winds. These winds blow west to east and occur along the boundary of warm and cold air masses. The greater the temperature difference between the air masses, the greater difference there is in air pressure. This causes winds to blow faster.

9. How do global patterns affect local weather?
In parts of the world, a seasonal change in wind patterns can affect rainfall. Sea and land breezes over a large region that change direction with the seasons are called monsoons.
A wind blows inland from the ocean all summer. This air blowing from the ocean is warm and humid. As the humid air rises over the land, the air cools. The cooling air causes water vapor to condense into clouds, producing heavy rains.
In winter, the land becomes colder than the ocean. A wind blows from the land to the ocean. These winds carry little moisture.

10. What is relative humidity and how is it measured?
Water exists as a solid, a liquid, and a gas in the air. The gaseous form of water is water vapor. Humidity is a measure of the amount of water vapor in the air.
Relative Humidity
Weather reports usually refer to the water vapor in the air as relative humidity. Relative Humidity is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold at a particular temperature.
The ability of air to hold water vapor depends on its temperature. Warm air can hold more water vapor than cold air.

11. What is relative humidity and how is it measured?

12. What is relative humidity and how is it measured?
Measuring Relative Humidity:
Relative humidity can be measured with an instrument called a psychrometer.
A sling psychrometer has two thermometers, a wet- bulb thermometer and a dry-bulb thermometer.
When the psychrometer is “slung”, or spun, air blows over the thermometers. Because the wet-bulb thermometer is cooled by evaporation, its reading drops.

13. What is relative humidity and how is it measured?
If the relative humidity is high, the water on the wet bulb evaporates slowly, and the wet bulb temperature does not change much. If the relative humidity is low, the water on the wet-bulb evaporates rapidly and the wet-bulb temperature drops by a large amount. The relative humidity can be found by comparing the temperatures of the wet-bulb and dry-bulb thermometers.