Weather Map Worksheet

GSCI-122 OL – Weather Maps (rev. 10/07/21) 1 Weather Maps INTRODUCTION Air masses are large bodies of air that take on the characteristics of the area in which they originate. Air masses originating over Canada are cold and dry, whereas air masses originating over the Gulf of Mexico are warm and humid. An air mass is fairly homogeneous in temperature and pressure across its expanse, but it shows considerable vertical variation. In general, the pressure and temperature of an air mass decrease with increasing elevation. Because of unequal heat received by Earth’s surface from the Sun, the temperature changes; the density of the air is thereby caused to change, and thus the pressure. Such changes coupled with Earth’s rotation bring about movement of the air masses over the surface of Earth. The boundary between two air masses is called a front. If a cold air mass is moving into a warm area, the front is labeled a cold front. If warm air is moving into a cold region, the front is called a warm front. The term weather refers to the day-to-day variations in the general physical properties of the lower part of the atmosphere, known as the troposphere this region varies from 7 to 11 miles in height. The physical properties that play an active role in our daily lives are temperature, pressure, humidity, precipitation, wind, and extent of cloud formation. The U.S. National Weather Service is the federal organization that provides national weather information. It is part of the National Oceanic and Atmospheric Administration (NOAA— pronounced “Noah”), which was created within the U.S. Department of Commerce in 1970. Under NOAA, the National Weather Service reports the weather of the United States and its possessions, provides weather forecasts to the general public, issues warnings against tornadoes, hurricanes, floods, other weather hazards, and records the climate of the United States. To help analyze the data taken at the various weather stations, maps and charts are prepared that display the weather picture. A weather Map taken on Thursday, February 19, 2004 shows some of the important symbols used to represent the weather in these charts. Since these charts present a synopsis of the weather data, they are referred to as synoptic weather charts. There are a great variety of weather maps with various presentations. However, the most common are the daily weather maps issued by the U.S. National Weather Service. A set consists of a surface weather map, a 500-millibar height contour map, a map of the highest and lowest temperatures, and one showing the precipitation areas and amounts. The Surface Weather map presents station data and analysis for 7 A.M. EST for a particular day. This map shows the major frontal systems and the high- and low-pressure areas at the constant surface level. 1. ISOBARS. Every weather map is marked by a series of either closed or endless “contour” lines called isobars. Each Isobar connects points of equal atmospheric pressure. Notice the small numbers such as 1008, 1012, 1016, 1020, etc., on the isobars of figures. These numbers represent a measurement of the atmospheric pressure in millibars. Standard

GSCI-122 OL – Weather Maps (rev. 10/07/21) 2 Atmospheric Pressure as measured by the barometer is either 760 millimeters or 30 (29. 921) Inches of mercury. Either of these units is equivalent to 1013.25 millibars. 2. Barometric Pressure on station models. Barometric pressures can vary and sea-level ranges can be from 960.0 to 1040.0. (The lowest recorded is 870.0 inside a hurricane and 1080.0 in the Siberian desert.) As a space saving tool the barometric pressure is recorded as three digits on station models indicating the last two digits and the decimal (without the decimal). Thus, 960.5 would be indicated as 605 and 1040.2 would be indicated as 402. In general if the pressure is 501 or higher you would place a 9 in front to convert it back into millibars. (ex. 678 would be 967.8 mb). If the value is 499 or lower, you would place a 10 in front of the number. (ex. 213 would be converted to 1021.3 mb) 3. HIGHS and LOWS. By using the Isobars one is able to determine those geographical areas affected by atmospheric pressure “higher” or “lower” than the standard or normal pressure of 1013 millibars. 4. FRONTS. The boundary between two different air masses is called a front. There are four different kinds of fronts and each is represented by different symbolism. A cold front is the boundary of relatively cold air of polar origin advancing into an area occupied by warmer air. A warm front is the boundary of relatively warm air advancing into an area occupied by colder air. An occluded front is the line along which a cold front has overtaken a warm front at the ground. A stationary front is the boundary between two air masses, neither of which shows a tendency at the time of observation to advance into either the warm or the cold area. Front symbols are given below. The side of the line on which the symbols are placed indicates the direction of movement The most outstanding features shown on a daily weather map (see Experiment 3) are the warm and cold fronts. In addition to the long black lines with their rounded or sharp-pointed projections that symbolize a warm or cold front, there are plain dark lines with open ends, and others that form complete circles. These lines, called isobars, are lines of equal pressure, and their central areas indicate regions of high or low pressure. A study of the weather map will show many facts concerning the weather elements associated with these pressure cells. The first noticeable feature is the way the isobars are plotted on the map. Note the unit of measurement and the difference between any two isobars drawn on the map. It is clearly seen that reporting stations seldom report these particular values of pressure; the isobars are drawn between the known values indicated at the stations. Note how the pressure varies as one approaches the center of the high- or low-pressure cell. Also note the fact that no two isobars ever cross. Why? LEARNING OBJECTIVES After completing this experiment, you should be able to do the following:

GSCI-122 OL – Weather Maps (rev. 10/07/21) 3 1. Define air mass, warm front, cold front, isobar, isotherm, and wind. 2. Read weather information from surface weather maps issued by the National Weather Service. 3. Plot station reports similar to those found on Surface Weather maps. 4. Define high-pressure cell, low-pressure cell, and isobars. 5. Identify and explain the physical characteristics of weather associated with high-pressure and low-pressure cells. 6. Plot pressure values on a map and draw lines of equal pressures (isobars) on the map. APPARATUS This Laboratory Guide and daily weather maps (weekly series) from the National Weather Service. PROCEDURE: Part 1 Please watch the video How to... Read and interpret pressure on a weather model. Using Table 1 and Figure 1 for reference, create a station model form the information given below for the three cities (A, B, and C). Please refer to the introduction item 2 to ensure you record the pressure correctly. City A City B City C Air temperature 62°F 88°F 32°F Air dew point 60°F 52°F 26°F Air pressure 1022.6 mb 987.4 mb 1042.8 mb Wind direction SW NW Wind magnitude 24 knots Calm 10 knots Cloud coverage 0.75 No clouds 0.5 Present weather Continuous rain Intermittent fall of snowflakes, moderate City A City B City C 62 60 62 . . . · d 226 88 874 * 428 52

GSCI-122 OL – Weather Maps (rev. 10/07/21) 4 Selected Weather Symbols Table 1 – Selected Weather Symbols