How Can You Predict Weather Changes?

Transcription

1 How Can You Predict Weather Changes?

2 How to Predict Weather Meteorologists make weather predictions daily. They use the same processes that you will. They collect weather data. They plot the data on maps. Then they study the maps to look for patterns. To predict the weather, meteorologists pay a lot of attention to fronts. Fronts are the places in the atmosphere where air masses are colliding. You will be able to better predict weather after you know more about how fronts move and the weather conditions they cause.

3 Using Fronts to Predict Weather Changes Fronts occur when air masses with different pressures collide. Air masses with different pressures generate winds and those winds affect weather. Along the line of a front the weather is changing. When you see a line representing a front on a weather map, it is showing you the edges of the air masses that are colliding.

4 Warm Front The leading edge of an advancing mass of warm air. What will happen to the temperature? What will happen to the air pressure?

5 Warm Front Because the warm air is less dense, it rises up and over the cold air mass. As the warm air rises in the troposphere, the temperature of the rising air decreases. The warm air cools, and moisture in the air condenses, changing from water vapor to liquid water. The condensing water forms clouds that can produce precipitation. In a given location, the precipitation can last from 12 to 24 hours. The front then generally moves eastward. The storm is followed by a period of warmer, more humid weather as the low-pressure warm air mass moves in.

6 Cold Front The leading edge of an advancing mass of cold air. What will happen to the temperature? What will happen to the air pressure?

7 Cold Front The denser cold air forces the warm air mass upward. This forms towering clouds. Cold fronts often cause thunderstorms which can be brief but severe. After the cold front passes, the air will be much cooler and drier than it was before the front passed through because the high-pressure cold air mass is continuing to move across the region.

8 Stationary Front The edges of a pair of air masses, neither of which is strong enough to replace the other. What will happen to the temperature? What will happen to the air pressure?

9 Stationary Front A stationary front occurs when a warm front or a cold front stops moving. This can happen if two air masses collide and neither air mass is strong enough to push the other air mass along. While the boundary between the two air masses may stay in place, the air in the areas may continue moving. This will happen if the wind on each side of the front blows parallel to the boundary line between the warm and cold air masses. In this case, the wind does not change the location of the front.

10 Occluded Front When a cold front catches up to a warm front. What will happen to the temperature? What will happen to the air pressure?

11 Occluded Front As the warm air is lifted, cold air moves in to replace it. The two cold air masses meet, one from the warm front, the other from the cold front. This situation can cause several days of cloudiness and light precipitation. The weather at an occluded front is generally similar to that of a warm front, but it is calmer.

12 How do fronts move? Knowing where a front is located may allow you to predict a few days of weather in that location. But to predict more than a few days of weather, and to predict the paths of storms, you need to know how fronts are moving. Global winds are causing air masses and fronts to move. For example, because global winds generally blow from west to east over the United States, fronts moving across the United States also generally travel from west to east. However, fronts can travel in any direction when local conditions change. The jet stream generally moves faster across the United States in winter. For that reason, fronts tend to move faster in winter, so winter storms moving west to east do not tend to last as long in a given area. Generally, cold fronts can move faster than warm fronts. This is why the weather changes more suddenly when a cold front passes through than when a warm front passes through. The speed and direction in which a front moves is also affected by geography. High mountains can block the pathway of a front and slow it down because of the energy it takes for the air masses to climb up the mountain. Mountains also can deflect a front toward the north or south.

13 Pressure Systems 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.

14 Low Pressure Systems Areas where air is warmed often have lower pressure because the warm air rises and are called low 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 too. 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.

15 High Pressure Systems A high pressure system has higher pressure at its center than the areas around it. Wind blows away from high pressure. Air from higher in the atmosphere sinks down to fill the space left as air blew outward. Since the air is not rising, high pressure systems cause clear, sunny skies. Winds of a high pressure system swirl in the opposite direction as a low pressure system - clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. On a weather map the location of a high pressure system is labeled with a blue H.

16 Answer on a sheet of paper 1. How are cold fronts and warm fronts alike? How are they different? 2. Why does an occluded front have at least three air masses? 3. Suppose a cold front is moving east, but stops. What kind of front has it become? What kind of front will it become if it starts moving west? 4. Suppose you are in Minneapolis, Minnesota, in summer, and a cold front is 500 km (310 mi) to the west in Rapid City, South Dakota. What would you need to know to predict the weather in Minneapolis over the next five days? 5. At which time of year, summer or winter, do fronts move faster across the United States? How does this affect how long a storm will last in a given location? 6. Which type of front moves fastest? Which is the slowest-moving type of front? 7. In what directions can fronts move? 8. Under what circumstances would the direction in which a front is moving change?