Grade 8 Science Unit: 08 Lesson: 01 Notes (p. 1 of 10) Convection In terms of weather, convection always involves rising air. It usually refers to moist convection where the excess water vapor in rising air condenses to form a cloud. The heat released through this condensation can help to keep the convection process going by warming the air further and making it rise still higher. The additional rising causes more water vapor to condense, and the cycle repeats. Convection can also be dry. This occurs on a sunny day over the desert or in more humid regions early in the day before clouds form. The sun warms the ground, and convective air currents help to remove the excess heat from the surface. Dry convection isn’t easy to see because clouds don’t form. All of the air rising through convection must be balanced by an equal amount of sinking air elsewhere. Clouds represent vertical circulation systems involving rising air where the visible cloud forms and sinking air around the cloud. Convection (both dry and moist) help to make the Earth a tolerable place to live by removing excess heat from the surface, which is where most of the solar energy is absorbed by the Earth and transporting it high into the atmosphere. It has been calculated that, without convection, the average surface air temperature on the Earth would be about 51.7° C (125° F) rather than the current 15° C (59° F). In wind patterns that will be studied later, huge convection cells form at different latitudes. These convection cells drive global wind patterns that affect climate. The atmosphere is in constant motion. Moist Convection: Air flowing Dry Convection: Air flowing upward causes a cloud to form. upward no clouds form. Warm Water or Land Warm Land ©2011, TESCCC 12/07/11 page 1 of 10 Grade 8 Science Unit: 08 Lesson: 01 Notes (p. 2 of 10) Humidity The water from the cotton ball represents humidity. Humidity is how much water vapor is present in the air. On days that humidity is high, the air may feel wet or sticky. On days when the humidity is low, the air may feel dry and crisp. When the fan blows on the two thermometers, the air it produces causes the water on the cotton ball to evaporate quickly. This evaporation causes the temperature on the thermometer with the cotton ball to decrease more rapidly than the thermometer without the cotton ball. Cold air is heavier than warm air, and this is the basis for much of what we call weather. Cold air is denser than warm air because the molecules are packed closer together. The amount of water vapor in the air also affects the density of the air. The more water vapor that is in the air the less dense the air becomes. That is why cold, dry air is much heavier than warm, humid air. The cold, dry air tends to move downward while the warm, humid air tends to rise. There are two different ways to express humidity. Both these terms are used in weather broadcasts. Relative humidity is a measure of the amount of water vapor actually in the air compared to the amount of water vapor that could be in the air at a given temperature and pressure. If the air has half the amount of water vapor it could have, then the relative humidity is 50%. When it is raining or snowing, the maximum amount of water vapor is in the air, and the relative humidity will be at 100%. A relative humidity above 80% will feel humid especially in mild or warm air. A relative humidity below 30% will feel dry. The dew point is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor condenses into liquid water. The dew point is always lower than (or equal to) the air temperature. If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%. While relative humidity is a relative measure of how humid the air is, the dew point temperature is an absolute measure of how much water vapor is in the air. Relative humidity is measured with a psychrometer. A psychrometer is a system of 2 thermometers, one with a wet material around the bulb and the other bulb is bare. This piece of equipment can be hung in a place where the wind can blow on it or can have a handle attached to it. If the psychrometer is held by this handle and swung through the air, it is called a sling psychrometer. ©2011, TESCCC 12/07/11 page 2 of 10 Grade 8 Science Unit: 08 Lesson: 01 Notes (p. 3 of 10) Air Pressure Atmospheric pressure is the amount of force pressing down on everything at the surface of an area by all the air above that area. When atmospheric pressure of an area is lower than normal, there are fewer air molecules above an area. If an area has a higher than normal atmospheric pressure, there are more air molecules in the atmosphere above it. Because the Earth is warmer at the equator than at the poles, differences in pressure occur. Air moves north and south to try to equalize the pressure difference created by the temperature difference. Pressure variations on the Earth surface causes wind and can affect the weather. High Pressure High pressure areas are usually caused by air masses that are being cooled. As the air mass cools, it contracts and allows surrounding air to fill in the space. This process increases the total amount of atmosphere above the area, which increases the pressure the air mass is pushing down with. The difference in pressure between the high pressure area and the surrounding areas with lower pressure causes wind. High pressure areas have a tendency to seek out low pressure areas causing gusts. Because of the rotation of the Earth, the wind is deflected to the right in the Northern Hemisphere. This causes wind to flow in a clockwise direction around a high pressure zone. Weather associated with high pressure areas is dry conditions, light winds, and fair skies. The symbol for high pressure on a weather map is a capital letter H in blue. Low Pressure When air warms, it expands. The air becomes lighter and rises. This usually happens along the boundary between warm and cold air masses. The air flows between them try to equalize the difference in temperature by the colder air flowing under the warmer air mass and the warmer air flowing over the colder air mass. The winds flow in a counterclockwise. The symbol for low pressure on a weather map is a capital letter L in red. Pressure Area and Fronts A high pressure area generally moves toward a low pressure area. When the two air masses of different pressures and temperatures meet, they will form a front, which will cause local weather changes. ©2011, TESCCC 12/07/11 page 3 of 10 Grade 8 Science Unit: 08 Lesson: 01 Notes (p. 4 of 10) Fronts Fronts are the boundaries between two air masses with different temperatures and pressures. Fronts bring changes in weather. If cold air is moving toward warm air, it is called a cold front. Colder air forces the warm air higher into the atmosphere. This warm air that is pushed up cools and forms clouds. This causes rain and thunderstorms to develop along a cold front. Cold fronts can move up to twice as fast as warm fronts and can produce sharper changes in weather. Cold fronts are usually associated with low-pressure areas. On a weather map, cold fronts are shown as a blue line with triangles lined up on the front edge of the line. The triangles point in the direction the front is moving. If warm air is moving toward cold air, it is called a warm front. Warm, moist air slides over the cold, dense air. This also causes clouds to form, but many miles ahead of the front. As the front approaches and passes it can cause steady rain or snow to fall. After all of this happens, the sky becomes clear and the temperature starts to rise. Warm fronts move more slowly than cold fronts do. On a weather map, warm fronts are shown as a red line with half circles on it. The half circles face the direction the front is traveling. If neither air mass is moving very much, the result is a stationary front. A stationary front is a boundary between two different air masses, neither of which is strong enough to replace the other. A stationary front tends to remain essentially in the same area for long periods of time. A wide variety of weather can be found along a stationary front. Often though, you will have clouds and long periods of precipitation, On a weather map, this is shown by an inter-playing series of blue triangles pointing one direction and red half circles pointing the other. ©2011, TESCCC 12/07/11 page 4 of 10 Grade 8 Science Unit: 08 Lesson: 01 Notes (p. 5 of 10) Jet Stream To understand about the jet stream, an explanation of part of the layers if the Earth’s atmosphere is first necessary. The troposphere is the lowest major atmospheric layer extending from the Earth's surface up to the bottom of the stratosphere.