Reading a Weather Map

Transcription

1 Name Reading a Weather Map by Cindy Grigg

2 Answer the following questions BEFORE you read this book. It is okay if you do not know as much as you thought. Do the best you can! 1. What kinds of information are shown on a weather map? 2. What do some symbols mean on a weather map? 3. Why do we want to predict the weather? Weather maps show what is happening in Earth's atmosphere at a given time. Many different things can affect weather. That's why weather maps have so many different symbols. At first glance, a weather map may look very confusing! Each symbol, letter, number, and line means something different. 1 2 Reading a Weather Map

3 A capital "H" shows where there is high air pressure. A capital "L" shows where there is low air pressure. Air pressure is measured with a barometer. The unit of measurement is usually millibars (mb). In general, weather will get better when pressure increases. It will worsen when pressure decreases. High pressure regions usually have dry weather. Low pressure regions often get precipitation. On this map, the first things you might notice are the capital letters. Look at the red "L" on North Carolina. The number next to it is 996 mb (millibars). The blue "H" on Minnesota shows the air pressure there is 1008 mb. Can you guess now what the letters stand for? Isobars are lines on a weather map. Isobars connect places with the same air pressures. Differences in air pressure and temperatures cause winds. Surface winds usually flow from regions of high pressure to regions of low pressure. Winds flow clockwise around high pressure. 3 4 Reading a Weather Map

4 coming out from the black circle is the wind barb. It shows the last two: wind speed and direction. The numbers and symbols on this surface map show the data recorded at each official reporting weather station. The red curving line with an arrow shows winds blowing counterclockwise around a low pressure system. The symbol xx (between the numbers 21 and 17, upper left) means that it is snowing at that weather station. Each weather station reports the current temperature (at the top left weather station: number on top, 21), the dew point temperature (number on bottom, 17), the last three digits of the air pressure in millibars ( mb), amount of cloud cover (black circle - sky completely cloud covered), current precipitation (xx means snow), and the wind direction and speed. The line 5 6 Reading a Weather Map

5 Wind barbs point in the direction the wind is blowing from. A weatherman might say, "Generally westerly winds prevail across the state today." He means that the winds are blowing from the west. This wind barb shows the wind is blowing from the northeast. The white circle shows that the sky is clear with no cloud cover. Wind speed is given in knots. A knot is a nautical mile per hour. (One knot is equal to 1.15 miles per hour.) Each short horizontal line represents 5 knots. A long line represents 10 knots. A triangle on the barb represents 50 knots. You must add all the barbs to get the total wind speed. If a station circle is shown without barbs, the winds are calm. 7 8 Reading a Weather Map

6 masses (purple color) form over land in the far north. They bring even colder air southward. Air masses are very large pockets of air. Their movements bring changes in the weather. They are named for where they form. An air mass that forms over land is called continental. There are two types of continental air masses. Continental tropical air masses are hot and dry. The symbol for this kind of air mass is ct. They often form over the deserts of the Southwest (orange color in this picture). Continental polar (cp) air masses form over Alaska and Canada (dark blue color in picture). In the winter, they carry very cold air southward. In summer, they are welcomed for bringing respite from hot, humid weather. Continental arctic (ca) air 9 10 Reading a Weather Map

7 Air masses that form over oceans are called maritime air masses. If they form over the Caribbean Sea or the Gulf of Mexico, they are called maritime tropical (green in picture). The symbol for them is mt. These air masses bring warm, humid weather with them. A maritime polar (mp) air mass usually forms over the cold waters of the North Pacific (light blue color). They bring heavy rain to the west coast, especially in winter. Air masses are usually called polar, arctic, or tropical. Maritime air masses form over oceans. They usually carry a lot of moisture. Continental air masses form over land. They usually have drier air. When two different air masses meet, a front is formed. There are four different kinds of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts. Fronts can stretch for miles or cover only a small region. They always bring changes in the weather. A blue line with triangles shows a cold front on the weather map. The triangles point in the direction the front is headed Reading a Weather Map

8 Cold fronts hold cold, dense air. When a cold front meets a warmer air mass, it forces the warm air up along a steep front. The warm air rises and cools. Clouds form when the water in the cooled air condenses. Rain, snow, and thunderstorms can crop up ahead of the front. The precipitation usually lasts only a little while. Cold fronts can cause violent weather, including tornadoes. A red line with half circles on the map shows a warm front. A warm front forms when a large mass of warm air meets another front with colder air. The warm air advances, while the cold air retreats. Thick clouds and rain are common. Rain may last one to two days. The circles point in the direction the front is headed Reading a Weather Map

9 A stationary front is shown on the map as a blue line with blue triangles on one side and red half circles on the other. It looks like a mixture of a warm front and a cold front. Sometimes there are some clouds and a little bit of precipitation behind the edge of the front. A stationary front happens when two air masses meet that have similar temperatures and pressures. "Stationary" means still or motionless. Stationary fronts stand still or move very slowly. An occluded front is shown on a weather map as a purple line with alternating triangles and half circles on the same side Reading a Weather Map

10 Occluded fronts form when a large, cold air mass moves in fast and overtakes a warm or stationary front. The warm air is squeezed upward between the two air masses. A wide variety of weather can be found along an occluded front. Thunderstorms and tornadoes are possible. Tall cumulonimbus clouds like these can form when the warm air is forced high into the atmosphere. Usually, though, an occluded front is connected with a drying of the air mass. Occluded fronts bring changes in temperatures and wind direction. Precipitation can be shown with various symbols Reading a Weather Map

11 Answer the following questions AFTER you have completed this book. 1. List some symbols you might find on a weather map. Put a caption by each one that explains what the symbol means. Weather maps show a lot of information! Knowing some of the symbols will help you understand some of the information that helps predict what weather is coming your way. Picture credits: NOAA (National Oceanic and Atmospheric Administration) 2. Differences in air pressure and temperatures cause what? 3. What are the four kinds of fronts described in this book? National Weather Service Reading a Weather Map

12 4. What do the letters "H" and "L" stand for on a weather map? 5. Write a newspaper article to inform the newspaper's readers about some of the symbols found on a weather map. Use the facts you learned from this book. 6. Imagine that you are the person who reports observations at a weather station. Write a description in words of what weather data you have at a certain time. Include temperature, dew point temperature, wind speed and direction, barometer reading in millibars, cloud cover, and precipitation (if any). Then draw the symbols to report the same information. Reading a Weather Map Reading a Weather Map