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What is weather?

Last updated over 3 years ago
22 questions
Objectives:

Students can explain the roles of thermal convection, Earth's rotation, changes in air pressure in the formation of weather.

Students can explain the resulting weather when different types of fronts move into an area.
Weather

Rain and dull clouds, windy blue skies, cold snow, and sticky heat are very different conditions, yet they are all weather.
Weather is the mix of events that happen each day in our atmosphere. Weather is different in different parts of the world and changes over minutes, hours, days and weeks. Most weather happens in the troposphere, the part of Earth’s atmosphere that is closest to the ground.
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Weather happens in the...

Air Pressure and Weather

The weather events happening in an area are controlled by changes in air pressure. Air pressure is caused by the weight of the huge numbers of air molecules that make up the atmosphere. Typically, when air pressure is high there skies are clear and blue. The high pressure causes air to flow down and fan out when it gets near the ground, preventing clouds from forming. When air pressure is low, air flows together and then upward where it converges, rising, cooling, and forming clouds. Remember to bring an umbrella with you on low pressure days because those clouds might cause rain or other types of precipitation.
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Weather is controlled by changes in air

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Air pressure is caused by

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When air pressure is high...

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When air pressure is low...

The Highs and Lows of Air Pressure



Air near the surface flows down and away in a high pressure system (left) and air flows up and together at a low pressure system (right). NESTA
Standing on the ground and looking up, you are looking through the atmosphere. It might not look like anything is there, especially if there are no clouds in the sky. But what you don’t see is air – lots of it. We live at the bottom of the atmosphere, and the weight of all the air above us is called air pressure. Above every square inch on the surface of the Earth is 14.7 pounds of air. That means air exerts 14.7 pounds per square inch (psi) of pressure at Earth’s surface. High in the atmosphere, air pressure decreases. With fewer air molecules above, there is less pressure from the weight of the air above.
Pressure varies from day to day at the Earth’s surface - the bottom of the atmosphere. This is, in part, because the Earth is not equally heated by the Sun. Areas where the air is warmed often have lower pressure because the warm air rises. These areas are called low pressure systems. Places where the air pressure is high, are called high pressure systems.
A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses, forming clouds and often precipitation. Because of Earth’s spin and the Coriolis effect, winds of a low pressure system swirl counterclockwise north of the equator and clockwise south of the equator. This is called cyclonic flow. On weather maps, a low pressure system is labeled with red L.
A high pressure system has higher pressure at its center than the areas around it. Winds blow away from high pressure. Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air is blown outward. On a weather map, you may notice a blue H, denoting the location of a high pressure system.
How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors in weather stations measure air pressure. These sensors are able to make continuous measurements of pressure over time. In the past, barometers were used and measured how much air pushed on a fluid, such as mercury. Historically, measurements of air pressure were described as “inches of mercury.” Today, meteorologists use millibars (mb) to describe air pressure.
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Describe a low pressure system

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Describe a high pressure system

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How do we measure air pressure?

Wind

Wind is air moving from a place that has higher pressure to one that has lower pressure.
Let’s say that you blow up a balloon but don’t tie a knot at the open end. You just pinch it closed with your fingers. Then, you relax your fingers a little and let some air flow out of the balloon. Have you ever thought about why the air comes out of the balloon instead of going in?
Air comes out because the pressure inside the balloon is higher than the pressure outside the balloon. Air molecules will spread out, spacing themselves far apart if they have the room. Air pressure isn’t exactly the same everywhere at the Earth’s surface because the Sun heats some places more than others. In the places where air is warmed most, it rises. This leaves a little less air near the Earth’s surface than in surrounding areas that didn’t get as warm. There is still air everywhere, but the air molecules are more widely spaced in some areas and more tightly packed in others. The places with more air molecules have higher pressure. The areas with fewer air molecules have lower pressure. Air whooshes into the area with lower pressure. That’s wind. So when you let the air out of a balloon, you are making a little bit of wind.
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What causes wind?

Air Masses



Air is not the same everywhere. In North America, for example, cold and dry air covering thousands of miles flows south from the Arctic, especially in winter, and warm moist air flows north from the Gulf of Mexico. These different types air are called air masses.
An air mass is like a team whose players are all wearing the same uniform. In this case, the players are air, not people. And the uniforms that they wear are the similar characteristics such as temperature and humidity. Like sports teams, when two air masses come together, there is often turbulence. The turbulence of the two air masses moving together can cause clouds and thunderstorms to form. The border between two air masses at the Earth’s surface is called weather front.
Air masses are given a two-part name that describes the humidity and temperature characteristics of the region where they form. The first part of an air mass’ name describes its humidity. Air masses that form over the ocean, called maritime air masses, are more humid than those that form over land, called continental air masses. The second part of the name describes the temperature of the air mass, which depends on the latitude where it formed. Air masses that form near the equator or in the tropics (equatorial or tropical air masses) are warmer than air masses that form in polar areas or uin the Arctic (polar or arctic air masses).
The word that describes humidity (maritime or continental) is paired with the word that describes temperature (equatorial, tropical, polar or arctic).  For example, if an air mass forms over a tropical ocean, it is called maritime tropical. If an air mass forms over land in the far north it is called continental polar.
An air mass can change as it moves into different environments. For example, if a continental polar air mass moves into warmer areas and over the ocean the air will warm and moisture may evaporate from the ocean surface into the air, adding humidity.
Cold and warm air masses usually come together in middle latitudes areas such as the United States, where they form weather fronts and can produce massive storms.
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In your own words, what is an air mass?

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Match the term to the definition

Draggable itemCorresponding Item
Equatorial
A warm air mass that formed near the equator
Continental
a cold air mass that formed near the poles
Polar
a humid air mass that formed over a body of water
Maritime
a dry air mass that formed over land

A Global Look at Moving Air: Atmospheric Circulation



Air in the atmosphere moves around the world in a pattern called global atmospheric circulation. Credit: UCAR
Even with disruptions like weather fronts and storms, there is a consistent pattern to how air moves around our planet’s atmosphere. This pattern, called atmospheric circulation, is caused because the Sun heats the Earth more at the equator than at the poles. It's also affected by the spin of the Earth.
In the tropics, near the equator, warm air rises. When it gets about 10-15 km (6-9 miles) above the Earth surface it starts to flow away from the equator and towards the poles. Air that rose just north of the equator flows north. Air that rose just south of the equator flows south. When the air cools, it drops back to the ground, flows back towards the Equator, and warm again. The, now, warmed air rises again, and the pattern repeats. This pattern, known as convection, happens on a global scale. It also happens on a small scale within individual storms.
But because Earth is spinning, the air that moves north and south from the equator also turns with the spin of the Earth. Air going north turns to the right. Air traveling south turns to the left. The power of Earth’s spin to turn flowing air is known as the Coriolis Effect. If the Earth didn’t spin, there would be just one large convection cell between the equator and the North Pole and one large convection cell between the equator and the South Pole. But because the Earth does spin, convection is divided into three cells north of the equator and three south of the equator.
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Why does all of the air on Earth move around?

Weather Fronts


When a front passes over an area, it means a change in the weather. Many fronts cause weather events such as rain, thunderstorms, gusty winds, and tornadoes. At a cold front, there may be dramatic thunderstorms. At a warm front, there may be low stratus clouds. Usually, the skies clear once the front has passed. A weather front is a transition zone between two different air masses at the Earth's surface. Each air mass has unique temperature and humidity characteristics. Often there is turbulence at a front, which is the borderline where two different air masses come together. The turbulence can cause clouds and storms.

Instead of causing clouds and storms, some fronts just cause a change in temperature. However, some storm fronts start Earth's largest storms. Tropical waves are fronts that develop in the tropical Atlantic Ocean off the coast of Africa. These fronts can develop into tropical storms or hurricanes if conditions allow.
Fronts move across the Earth's surface over multiple days. The direction of movement is often guided by high winds, such as Jet Streams.  Landforms like mountains can also change the path of a front.
There are four different types of weather fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.
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After reading the passage, what is a weather front? Do not google this and copy and paste the answer. Read the passage and describe in your own words.

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Thunderstorms form when what type of front moves into an area?

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Air masses have their own _______ and _______ characteristics.

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When a front moves into an area, which of the following can occur?

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What can impact the movement of a front?

Cold Front


A cold front forms when a cold air mass pushes into a warmer air mass. Cold fronts can produce dramatic changes in the weather. They move fast, up to twice as fast as a warm front. As a cold front moves into an area, the heavier (more dense) cool air pushes under the lighter (less dense) warm air, causing it to rise up into the troposphere. Lifted warm air ahead of the front produces cumulus or cumulonimbus clouds and thunderstorms.
As the cold front passes, winds become gusty. There is a sudden drop in temperature, and also heavy rain, sometimes with hail, thunder, and lightning. Atmospheric pressure changes from falling to rising at the front. After a cold front moves through your area, you may notice that the temperature is cooler, the rain has stopped, and the cumulus clouds are replaced by stratus and stratocumulus clouds or clear skies.
On weather maps, a cold front is represented by a solid blue line with filled-in triangles along it, like in the map. The triangles are like arrowheads pointing in the direction that the front is moving. Notice on the map that temperatures at the ground level change from warm to cold as you cross the front line.
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Explain what happens when a cold front moves into an area. You can use pictures, words or both.

Warm Front

A warm front forms when a warm air mass pushes into a cooler air mass, shown in the image to the right (A). Warm fronts often bring stormy weather as the warm air mass at the surface rises above the cool air mass, making clouds and storms. Warm fronts move more slowly than cold fronts because it is more difficult for the warm air to push the cold, dense air across the Earth's surface. Warm fronts often form on the east side of low-pressure systems where warmer air from the south is pushed north.
You will often see high clouds like cirrus, cirrostratus, and middle clouds like altostratus ahead of a warm front. These clouds form in the warm air that is high above the cool air. As the front passes over an area, the clouds become lower, and rain is likely. There can be thunderstorms around the warm front if the air is unstable.
On weather maps, the surface location of a warm front is represented by a solid red line with red, filled-in semicircles along it, like in the map on the right (B). The semicircles indicate the direction that the front is moving. They are on the side of the line where the front is moving.
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Explain what happens when a what happens when a cold front moves into an area. You can use pictures, words or both.

Stationary Front

A stationary front forms when a cold front or warm front stops moving. This happens when two masses of air are pushing against each other, but neither is powerful enough to move the other. Winds blowing parallel to the front instead of perpendicular can help it stay in place.
A stationary front may stay put for days. If the wind direction changes, the front will start moving again, becoming either a cold or warm front. Or the front may break apart.
Because a stationary front marks the boundary between two air masses, there are often differences in air temperature and wind on opposite sides of it. The weather is often cloudy along a stationary front, and rain or snow often falls, especially if the front is in an area of low atmospheric pressure.
On a weather map, a stationary front is shown as alternating red semicircles and blue triangles. Notice how the blue triangles point in one direction, and the red semicircles point in the opposite direction.
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Explain what happens when a what happens when a stationary front moves into an area. You can use pictures, words or both.

Occluded Front

Sometimes a cold front follows right behind a warm front. A warm air mass pushes into a colder air mass (the warm front), and then another cold air mass pushes into the warm air mass (the cold front). Because cold fronts move faster, the cold front is likely to overtake the warm front. This is known as an occluded front.
At an occluded front, the cold air mass from the cold front meets the cool air that was ahead of the warm front. The warm air rises as these air masses come together. Occluded fronts usually form around areas of low atmospheric pressure.
There is often precipitation along an occluded front from cumulonimbus or nimbostratus clouds. Wind changes direction as the front passes and the temperature either warms or cools. After the front passes, the sky is usually clearer, and the air is drier.
On a weather map, shown above, an occluded front looks like a purple line with alternating triangles and semicircles pointing in the direction that the front is moving. It ends at a low pressure area shown with a large ‘L’ on the map, begins at the other end when cold and warm fronts connect.
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Explain what happens when a what happens when an occluded front moves into an area. You can use pictures, words or both.

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What questions do you have about what you learned today? What do you need us to spend more time next class figuring out?