Background: What is Weather?

Weather Maps Background: What is Weather? Weather is the day-to-day state of the atmosphere. The interaction of three important factors result in weather systems: air temperature, air pressure, and the amount of moisture in the air (relative humidity.) All of the other things that we associate with weather conditions, like how much and the type of precipitation that occurs, wind direction and speed, and cloud cover, are caused by the interactions between temperature, pressure, and humidity in the atmosphere. In the Engage activity, you were introduced to instruments used by a meteorologist (a scientist who studies weather) as shown on the chart below: Weather Factor Instrument Used for Measurement Air Temperature Thermometer Air Pressure Barometer Relative Humidity Sling Psychrometer Wind Speed and Direction Anemometer and Wind Sock or Weather Vane Precipitation Rain Gauge Pictures of Instruments Of course more sophisticated digital instrumentation can be used to measure these weather factors. Weather satellites and balloons, airplanes, and ocean buoys are examples of instrumentation used to record weather data from all around the globe. Data collected for temperature, pressure, humidity, wind speed, and the type and amount of precipitation is used to study weather systems and make predictions about weather conditions. These factors are different all around the world, but they cause world-wide patterns that can affect the local weather. In these Explore activities, you will learn about factors that determine the weather. You will also see how they are demonstrated using a weather map. 1 Journal. Complete the Background questions in your Student 1

Part I: Air Lift You have learned that radiant energy is absorbed by Earth s waters and landmasses. The absorption of the radiant energy will not occur equally because of the varying surfaces on Earth. This causes global changes in temperature patterns. The equatorial region receives more direct radiant energy as compared to other areas on the planet, so convention currents result. These convection currents cause global wind patterns with the warmer air rising and cooler air sinking. This recurring pattern of uneven heating will happen at other latitudes causing convection in these locations. The global winds formed from this influence weather all over Earth. Differences in air temperatures will cause differences in air pressure resulting in the formation of wind. The pressure differences can cause local to global winds. Air is matter. It is made of atoms, has mass, and has volume. Therefore, the matter in the atmosphere is actually pulled down on Earth s surface due to the force of gravity and causes pressure. The more matter present in an area, the higher the air pressure. The less matter present in an area, the lower the air pressure. Human bodies are able to handle air pressure although the only time you might notice it is when your ears pop as they adjust to a change in atmospheric pressure. This can occur when you go to higher elevations where you have less air pressure. Your body self-regulates to the different weight of the air above you. Weather throughout Earth also changes, depending on the atmospheric pressure. In this activity, you investigate the meaning of air pressure. 1. Take a flat board, and lay the board on a table so that 16 inches of the board sticks out from the table. 2. Lay a large sheet of chart paper, open and flat, on top of the part of the board on the table. Smooth out the paper, and make it as flat as possible. 3. Predict what will happen if you hit the part of the board sticking out from the table. Write your prediction in your Student Journal. 4. Hit the board, and record the results in your Student Journal. 5. Use a yardstick to find the surface area of the chart paper. Use the value to complete Part I of your Student Journal. 6. Lastly, observe the current air pressure on the classroom barometer, and record it in your Student Journal. Complete Part I in your Student Journal. 2

Part II: High and Low Air Pressure Systems In the last activity, you learned that air pressure results from the force of gravity that pulls the atmosphere toward Earth s surface. A HIGH pressure area has more air pressure because the atmosphere above is denser. Dense, high pressure systems are represented on weather maps with a capital H. A LOW pressure area has lower air pressure because the atmosphere above is less dense. Less dense, low pressure systems are represented on weather maps with a capital L. Wind Movement and Conditions in Low Pressure Areas Air is pulled into a LOW pressure area and lifts higher into the atmosphere. When the air rises to higher altitudes it cools. Low pressure systems in the Northern Hemisphere have a counterclockwise circulation pattern. The cooling causes any moisture in the air to condense and form clouds with possible precipitation. Low pressure areas are associated with stormy weather conditions. The more moisture in the air that spirals upwards in a low pressure system, the more likely it is that precipitation will occur as rain, sleet, or snow. Please continue to the next page. 3

Part II: High and Low Air Pressure Systems, continued Wind Movement and Conditions in High Pressure Areas Air sinks in HIGH pressure areas and spreads out slowly. The spreading air tends to move toward neighboring areas of lower pressures. High pressure areas are associated with fair weather and mostly clear skies. Most high pressure systems are large and move slowly. High pressure systems in the Northern Hemisphere have a clockwise rotation. Complete Part II in your Student Journal. 4

Part II: High and Low Air Pressure Systems, continued Look at the map set below. On the left is a satellite image of Texas that shows cloud cover. The center of the high pressure system and the low pressure system are indicated on the image and the corresponding outline map with L and H. H L H L Satellite View of Texas Map View of Texas Part III: Air Masses and Fronts Wind is a result of temperature and pressure differences. Wind is generated within a low pressure system as the air spirals upward and within a high pressure system as denser air sinks. Wind also results when air moves from a HIGH pressure area toward a LOW pressure area. Molecules of air move directionally from areas of HIGH to LOW pressure, just as thermal energy always transfers from substances of high temperatures to low temperatures, never in the opposite direction. Complete Part II in your Student Journal. 5

Part III: Air Masses and Fronts Wind is a result of temperature and pressure differences. Wind is generated within a low pressure system as the air spirals upward and within a high pressure system as denser air sinks. Wind also results when air moves from a HIGH pressure area toward a LOW pressure area. Molecules of air move directionally from areas of HIGH to LOW pressure, just as thermal energy always transfers from substances of high temperatures to low temperatures, never in the opposite direction. Global High and Low Pressure Areas Photo courtesy of NOAA Because Earth's shape is spherical, not flat, the surface heats unevenly and causes global wind belts. The area near the equator receives the most direct rays from the Sun. This area gets warmer and causes surface air to rise and form a low-pressure zone. At about 30 north and south, the air sinks toward the surface and forms a set of subtropical highs which are large bands of high surface pressure. The Coriolis effect forces these convection cells of sinking air to deflect in both hemispheres causing two bands of global wind movement called the northeast and southeast Trade Winds (convection cell 1 on the image). Another set of convection cells begins at about 60 north and south, where surface air rises, then moves toward and sinks at about about 30 north and south. The Coriolis effect forces these sets of sinking air in the convection cells to deflect in both hemispheres causing two bands of global wind movement called Westerlies (convection cell 2 on the the image.) A final set of convection cells begin at about 60 north and south where warmer air rises, then cools and sinks as it moves toward the poles. The Coriolis effect forces these sets of sinking air in the convection cells to deflect in both hemispheres causing two bands of global wind movement called Polar Easterlies (convection cell 3 in the image.) These three sets of convective wind movements are semi-permanent and strongly influence the climate and daily weather in the regions in which they blow. Continue to the next page. 6

Part III: Air Masses and Fronts, continued What is an air mass? An air mass can be huge, as big as the size of a continent, and these can be thought of as almost being on a global level. Temperature and moisture content are nearly the same throughout the entire air mass. Air masses tend to form in high pressure areas, where stable conditions allows the air mass to remain in one location long enough to take on the characteristics of the surface below. For example, when an air mass forms over an ocean, water evaporates into the atmosphere, and the air becomes moist. When an air mass forms over land, the air is typically dry. Air masses that form over the warm surfaces of tropical latitudes are warmer than those that form over surfaces nearer to the poles. So, air masses can take on a combination of characteristics: cool or warm, dry or moist. Typical air masses that form and affect weather patterns in the United States are shown on the map below. Air masses that form over ocean surfaces are labeled as Maritime, and those that form over land surfaces are labeled Continental on the map. Con$nue to the next page. 7

Part III: Air Masses and Fronts, continued These large air masses move slowly away from where they formed. Global wind patterns and the jet stream (narrow bands of very fast moving wind located in the highest part of the atmosphere) affect the movement of these air masses. An air mass has nearly uniform moisture content and temperature throughout, so when it moves to a new area, it brings along its moisture and temperature characteristics. Over time, as the air mass moves over new surfaces and blends with other air masses, the original characteristics can change. Boundaries between air masses are called fronts; they are zones of transition between the very different characteristics of the two air masses. Fronts extend from low pressure systems because air flows into a low pressure area. Satellite images show the cloud cover that forms around the low pressure systems and along the edge where the two air masses meet. Weather conditions can get interesting at fronts due to interactions between different temperatures, pressure changes, and moisture content of the air masses. There are four kinds of fronts: 1. A warm air mass pushes a cooler air mass a warm front 2. A cool air mass pushes a warmer air mass a cold front 3. A warm air mass and a cool air mass both push against each other a stationary front 4. A warm air mass can get trapped between two cooler air masses an occluded front Your teacher will demonstrate interactions between fluids with different temperature characteristics. Although you will view liquids, gases in the atmosphere behave the same way based on temperature differences. Complete the Part III questions based on the demonstration of front boundaries in the Student Journal before moving on to Part IV: Four Types of Fronts. Part IV: Four Types of Fronts As you saw in the demonstration, the interaction at frontal boundaries, or where two air masses meet, can be turbulent. The characteristics of the two air masses determine actual weather conditions; however, there are four types of front boundaries that produce typical weather conditions. Complete Part III in your Student Journal. Continue to the next page. 8

Part IV: Four Types of Fronts, continued Cold Front When a denser, cooler air mass pushes a warmer air mass, the boundary is called a cold front. Cold fronts are usually fast moving and push the warmer, less dense air up quickly to higher altitudes where it cools, and the moisture condenses, producing heavy cloud cover and short, but intense precipitation until the front passes. Notice the heavy cloud band line in the satellite image and the corresponding map symbol line on the outline map. The map symbol is a line with triangles, and the triangles point toward the direction of the front movement. Satellite View of Texas Map View of Texas Continue to the next page. 9

Part IV: Four Types of Fronts, continued Warm Front When a warm air mass pushes a dense, cooler air mass, the boundary is called a warm front. A warm front moves more slowly than a cold front. At the boundary, the warm air slides slowly up and over the dense cool air to higher altitudes where it cools, and the moisture present condenses, often producing light scattered clouds and showers. Weather conditions may remain cloudy with scattered precipitation for several days due to these slow moving fronts. Notice the scattered cloud bands in the satellite image and the corresponding map symbol line on the outline map. The map symbol is a line with half circles, and the circles point toward the direction of the front movement. Satellite View of Texas Map View of Texas Continue to the next page. 10

Part IV: Four Types of Fronts, continued Stationary Front As the name implies, these fronts typically do not move one way or the other. The two air masses stall, and at the boundary line where the warm and cool air meet, moisture in the warm air mass condenses causing precipitation and cloud cover. These conditions can remain in place for several days until one of the two air masses begins to move forward. The map symbol is a line with alternating half circles and triangles on the opposite side of the line. Occluded Front An occluded front is the most complex of the four types of boundaries. Warm, moist air becomes trapped between two cooler air masses. Since the warm air is less dense, it rises, and the two cooler masses move in underneath. The moisture in the warm air mass cools and condenses at higher altitudes, producing clouds and precipitation. The map symbol is a line with alternating half circles and triangles on the same side of the line, and they point toward the direction of the front movement. Complete Part VI and the Reflections and Conclusions in your Student Journal. 11