Notes (P. 1 of 10)

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

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.

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.

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.

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. The troposphere is where all of Earth's weather occurs. It contains approximately 80% of the total mass of the atmosphere.

The boundary between the troposphere and the stratosphere is called the "tropopause," located at an altitude of around (5 mi} in the winter to around (8 mi) in the summer and (11-12 mi) in the deep tropics. When you see the top of a thunderstorm flatten out into an anvil cloud, it is usually because the updrafts in the storm have reached the tropopause where the surrounding air is warmer than the cloudy air in the storm. This causes the cloudy air to stop rising and flattens the top.

A jet stream is a wind current, which moves around the Earth capturing and changing anything in its path. A jet stream forms high in the upper troposphere between two air masses of very different temperatures. The greater the temperature difference between the air masses, the faster the wind blows in the jet stream. This “river” of air has wind speeds that can reach up to 643.7 km/hr. (400 mph), several hundred km wide, and 3.2 km (2 mi) in depth.

Jet streams usually form in the winter when there is a greater contrast in temperature between cold air masses over continents and warm air masses over the ocean. Colder air is denser than warmer air. There is an air pressure difference between them at any altitude. Since it is horizontal air pressure differences that cause wind, this leads to very strong winds. The strongest jet stream winds occur between air masses having the largest temperature differences over the deepest layer of the troposphere.

Even though the wind "tries" to flow from high pressure to low pressure, the turning of the Earth causes the air flow to turn to the right (in the Northern Hemisphere), so the jet stream flows around the air masses rather than directly from one to the other. The jet stream does not cause weather conditions of a certain type to occur. Its existence is instead the result of certain weather conditions such as a large temperature contrast between two air masses.

Jet streams change in form almost every day, sometimes only slightly and at other times very drastically. A stream of air must be over (56 mph) to be considered a jet stream. Jet streams are independent, but they can create or aid and become part of a minor or major weather pattern or event. Project the animated image for a 5 day movement of the jet stream to show the moving river of air from the following site: http://www.pbs.org/wgbh/nova/vanished/jetstr_five.html

Notes (p. 6 of 10) Air Masses The lower atmosphere or troposphere is not one uniform and consistent mass of air. Large pockets of moving air commonly form that are separate from the surrounding atmosphere. These pockets of air are called air masses. Two common properties of air masses are moisture content and temperature. These masses move over the Earth’s surface, sometimes for thousands of kilometers, and slowly change.

The best source regions for air masses are large flat areas where air can be still long enough to take on the characteristics of the surface below. Moist air masses form over water (maritime) and dry air masses (continental) form over land.

The system by which air masses are classified reflects the fact that certain locations on the planet have the topographic and atmospheric conditions that favor air mass development. This system uses two letters to designate an air mass. One letter, written in upper case, indicates the approximate latitude, and indirectly temperature, of the region: A for arctic; P for polar; E for equatorial; T for tropical. The first two terms (arctic and polar) refer to cold, dry air masses. The polar air masses are not as cold as the arctic masses because the form further south. The second two terms (equatorial and tropical) are warm air masses with equatorial being the warmer of the two.

A second letter, written in lower case (but before the capital letter), indicates whether the air mass forms over land or sea and gives an idea of the relative amount of moisture in the air mass. The two designations are c for continental (land) air mass and m for maritime (water) air mass.

The major air masses are: Continental Arctic (cA): Extremely cold temperatures and very little moisture. These usually originate north of the Arctic Circle and are mostly a winter air mass Continental polar (cP): Cold and dry but not as cold as Arctic air masses. These air masses bring clear and pleasant weather during the summer to the North. Maritime polar (mP): Cool and moist. They usually bring cloudy, damp weather to the USA. They form over the northern Atlantic and Pacific oceans. Maritime tropical (mT): Warm temperatures with lots of moisture. Originate over the warm waters of the southern Atlantic Ocean and the Gulf of Mexico. Maritime tropical air masses are responsible for the hot, humid days of summer across the South and the East. Continental Tropical (cT): Hot and very dry. They usually form over the Desert Southwest and northern Mexico during summer. They can bring record heat to the Plains and the Mississippi Valley during summer.

The diagram shows 2 land masses. Mass #1 formed either in the cP or cA area. Mass #2 formed in the mT area. When the two masses meet, a front is formed and the weather will change.

Notes (p. 7 of 10)

Winds Wind moves excess heat around the Earth. Wind is caused by air flowing from high pressure to low pressure. Where air is moving downward, high pressure develops. Where air is rising, atmospheric pressure is low. Since the Earth is rotating, the air does not flow directly from high to low pressure, but it is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (Coriolis effect). This causes the global winds to flow mostly around the high and low pressure areas. The Coriolis effect is commonly referred to as a "force," although it is not really a force. Forces can be measured and are related to mass and acceleration. High

60° N Low

High 30° N Low Equator 30° S High Low 60° S

High

Movement of air and moisture in the atmosphere is powered by heat from the Sun. Sea water evaporates and the warm moist air rises from the equator. This process transfers heat from the equator towards the poles. At the same time, dry cool air drops at the middle latitudes and is pulled back to the equator. Huge convection cells transfer heat around the Earth.

Where convection cells come together, bands of prevailing winds and jet streams are formed. Prevailing winds are winds that blow mostly from a single general direction over a particular point on Earth's surface. The prevailing winds are named for the direction from they blow.

Polar Easterlies 60° N Westerlies

30° N Easterlies (Trade Winds) Equitorial Doldrums Equator Easterlies (Trade Winds) 30° S Westerlies 60° S Polar Easterlies

Clouds that are being circulated by the prevailing winds can be seen in satellite views on TV weather reports.

Local winds are characteristic of particular geographical regions. They change directions they blow from and may be short term winds. These winds influence the local weather.

Notes (p. 8 of 10) Gyres

A gyre (pronounced /jī(ə)r/ like gyrate) is another name for a swirling vortex. Gyres are large mounds of water with flowing water around them that are often 1,000’s of km across. There is nothing to stop wind from blowing across water for long distances and time. A wind blowing for 10 hours across the open ocean will cause the water at the surface to flow at about 2% of the speed of the wind. This causes water to pile up in the direction the wind is blowing, forming a hill or mound of water. At the same time, the force of gravity tends to pull the water down the hill or mound of water. The Coriolis effect, caused by the Earth’s rotation, causes the water to move to the right in the Northern Hemisphere and to the left in the southern Hemisphere. This deflected water is forced around the gyre. The combination of the effects of gravity, the blowing wind, and Coriolis effect produce large circular currents in all oceans.

The major gyres of the ocean include: North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean gyres. Many other smaller gyres exist in the ocean too. One of the largest ocean gyres, the North Pacific gyre, is home to an area called the Great Pacific Garbage Patch. This area, located between California and Hawaii, is estimated to cover an area about twice the size of Texas. It contains approximately 3 million tons of plastic litter that has made its way to the ocean.

Ocean atmosphere interactions affect climate change and how water, energy, nutrients, or pollutants move through or get trapped in different parts of the Earth system.

Major Gyres and Direction of Flow

1. North Pacific: clockwise

2. South Pacific: counter- 5 clockwise 1 3. Indian Ocean: counterclockwise

4. South Atlantic: counter- 3 2 4 clockwise

5. North Atlantic: clockwise

Note: This projection has the Pacific Ocean as its center.

Notes (p. 9 of 10)

Ocean Currents

Oceans have a slower, much longer-lasting effect on climate than winds. Ocean currents move more slowly than winds and retain more heat than the atmosphere. And while winds influence ocean currents, oceans also influence winds and help to determine their direction and speed. This is an important interaction between the oceans and the atmosphere.

Ocean currents are basically driven by the Sun and the rotation of the Earth. The sun affects the ocean by heating up the atmosphere to create winds. Winds move the surface layer of the ocean. The Sun also changes the density of the surface water by changing its temperature and/or its salinity. If water cools or becomes saltier due to evaporation, it becomes more dense. This causes this dense water to sink, displacing water that is less dense.

The rotation of the Earth creates the Coriolis force. Water and wind in the Northern Hemisphere tend to move to the right and to the left in the Southern Hemisphere. The effect increases away from the equator.

Surface Currents

There are two types of ocean currents. The types of currents that are labeled on the map below are surface currents. The arrows indicate both the direction of the currents due to the Coriolis Effect and the temperature of the areas of the currents. Surface currents are driven by the wind. Notice that the surface currents around the areas where gyres are formed. These currents involve about 10% of all the water in the ocean and go to a depth of 0.4 km (1/4 mile).

Notes (p. 10 of 10) Gulf Stream There are about 29 different surface currents around the globe. One of the important surface currents is the Gulf Stream. This current is strong and swift. It originates at the tip of Florida and follows the eastern U.S. coastline and Newfoundland before it crosses the Atlantic Ocean. The Gulf Stream influences the climate along its path. Both northern and western Europe would have a much cooler climate if it were not for the effect of the Gulf Stream. This current is also part of the North American Gyre.

Deep Water Currents

The second type of current is called deep water currents. These currents involve 90% of the oceans water. Deep water has a temperature of 3° C (37.4° F). These currents are more dense, have a higher salinity composition, and are colder than surface currents. A change in the density of ocean water at the surface causes the currents. More dense water sinks. These changes cause specific water masses, which have defined temperature and salinity characteristics. These masses can be tracked for long distances.

The biggest source of deep water that sinks is from the North Atlantic. This cold, salty water can be dense enough to sink below less dense material. If it is denser than all the other water, it may sink to the sea floor. The North Atlantic water sinks until it reaches a level of equal density; then it spreads out.

This process breaks the deep ocean into horizontal layers with each deepening layer having more dense water in it. The water that sinks in the North Atlantic flows all the way past the equator into the Southern Hemisphere. The water then flows past Antarctica and into the Pacific and Indian Oceans. Here some of the deep waters are warmed and so rise again to the surface. This cycle of ocean water circulation from the surface to the deep ocean and back to the surface again is called conveyor belt cycling.