GEOG 131 Lab 10 - Mid-latitude cyclones

1 Name: Lab Day and Time: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ LAB 10: WEATHER MAPS AND MID-LATITUDE CYCLONES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ LEARNING OBJECTIVES This lab focuses on weather maps, mid-latitude cyclones, and weather forecasting. By the end of this week's lab, you will be able to: • Read and interpret weather maps. • Apply weather forecasting techniques. • Describe midlatitude cyclone development. • Distinguish between the influences of different air masses using weather conditions as clues. See the reference sheet on the last page. It will help! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This exercise will give you experience with interpreting weather maps and using them to make weather forecasts. It includes a sequence of five daily weather maps covering the conterminous U.S. for March 11 – 15, 1993. Each map represents the situation at 7:00 AM Eastern Standard Time on the date given. These maps are from the “ Daily Weather Maps ” series published by the National Oceanic and Atmospheric Administration and the National Weather Service. The large map at the top of each page shows several types of information. Surface barometric pressure is represented by isobars spaced at 4 mb intervals. High- and low-pressure centers are marked and their associated cold, warm, stationary, and occluded fronts are represented by the thick dashed or decorated lines. The small black boxes that contain a white X represent the position of the center of a low-pressure system 6, 12, 18, and 24 hours prior to the time the map was drawn (7:00 AM). The groups of small numbers surrounding a small circular symbol represent the data gathered at individual weather stations (see keys on pages 8-9). Areas of precipitation are indicated by shading. Areas that dipped below 32° F on the previous day are outlined by a thin dashed line. Areas that went below 0° F are surrounded by a thin dash/dot line. The map at the lower left of each page shows contour lines that represent the height above the surface at which barometric pressure reaches 500 mb. These heights are measured in decameters (1 decameter = 10 meters) and the contour interval is 6 decameters. High- and low-pressure centers aloft are labeled. The “arrows” are shown as flying with the upper -level wind and the wind speed is indicated by the number of attached flags and barbs. Isotherms (the thin dashed lines) represent the temperature in degrees Celsius at the 500 mb level. The small map at the right center represents the maximum and minimum temperatures at weather stations for the previous day. Maximums are taken from the 7 AM to 7 PM period while minimums are taken between 7 PM and 7 AM. The map at the bottom right represents precipitation amounts and areas for the past 24 hours. Trace amounts of precipitation are represented by the abbreviation “ T. ” The shaded areas have received some form of precipitation

2 and the amount is given to the nearest hundredth of an inch (snow is converted to depth of liquid water for this). The dashed lines represent the depth of snow on the ground at 7:00 AM. The plastic models of midlatitude cyclones will help you visualize the Storm of the Century in three dimensions. Look at one as you work through the activities. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ MAP FOR THURSDAY, MARCH 11 On this map, the low-pressure center that later developed into the Storm of the Century is not yet visible. In the past 24 hours, a cyclonic storm moved out into the Atlantic while a moderately strong high-pressure system has moved south from central Canada into the northern Plains states. 1. Although the Storm of the Century low-pressure center is not yet visible, there are a few subtle hints that one might be starting to form around Brownsville, Corpus Christi, and Laredo, TX. What do the surface wind patterns appear to indicate in this vicinity? 2. Locate the weather station data for Knoxville and list the conditions for 7 AM. • Temperature: Dew point temperature: • Cloud cover: Pressure change and trend: • Wind direction: Present weather: 3. Has there been any precipitation in Knoxville in the last 24 hours? If so, how much? a. Has Chattanooga received any precipitation? If so, how much? 4. Find the weather stations in Mobile, AL and Tampa, FL. Using data for temperature and dew point given at these stations, roughly compare the relative humidity in these two locations. Which is higher? a. Explain why two cities located on the Gulf of Mexico would have such different relative humidity values. Hint: think about air masses. MAP FOR FRIDAY, MARCH 12 By 7 AM on March 12, the surface low to the east of Brownsville, TX has become well developed (isobars have begun to close around the low-pressure center and there are cold and warm fronts forming). Because of the initial location of this cyclone and the process by which it was formed, it is termed a West Texas Gulf Cyclone. Some other low-pressure systems are

4 b. How fast has the center moved in Nautical MPH? (Use the distance scale for 30° N) 2. Where is the cold front located? a. How have wind speeds and directions at the weather stations in Florida been affected by the passing of this front? b. The linear feature located to the east of the cold front is called a squall line. What is happening there? 3. Locate the weather station data for Knoxville and list the conditions for 7 AM. • Temperature: Dew point temperature: • Cloud cover: Pressure change and trend: • Wind direction: Present weather: 4. Has there been any precipitation in Knoxville in the last 24 hours? If so, how much? Notice the symbols for both snow and a thunderstorm that are listed for the Knoxville station. This is an unusual situation, which is also one of Weather Channel meteorologist Jim Cantore’s favorite weather phenomena , called thundersnow . 5. Locate the weather station for Jackson, MS and list the conditions for 7 AM. • Temperature: Dew point temperature: • Cloud cover: Pressure change and trend: • Wind direction: Present weather: 6. Locate the weather station data for Charleston, SC and list the conditions for 7 AM. • Temperature: Dew point temperature: • Cloud cover: Pressure change and trend: • Wind direction: Present weather:

5 7. The weather stations in Knoxville, Jackson, and Charleston have conditions representative of the air masses that influence them. Three interacting airmasses now make up the storm: • A cP air mass that has been humidified. • A dry cP air mass. • An mT air mass. a. Which air mass is influencing: • Charleston? • Knoxville? • Jackson? b. Which of the three air masses is probably the source for most of the precipitation shown on the March 13 map? Explain your answer. 8. How would you describe the temperature and sky conditions recorded at the weather stations in Nebraska, Kansas, and Oklahoma? 9. Bearing in mind the position of the upper air trough (map on lower left) and the surface low (main map) in the southeast corner of the trough for both March 12 and 13, what process discussed in class might have played a part in the initiation of this cyclonic storm? 10. Using your knowledge of the tendency for a low-pressure system to move in the direction of the greatest pressure decrease, analyze the 3-hour pressure change supplied in the station data and predict the direction in which you expect the storm center to move. a. If the storm travels in the predicted direction and at the same speed that you calculated earlier, where do you expect to see the storm center located on the March 14 map? 11. What do you expect to happen to Knoxville’s temperature and wind direction in the next 24 hours? Why? a. What should happen to temperature and wind direction at Charleston, SC during the same period? Why?

7 The figure above is from http://www.hammondmap.com . Map symbols to the left and on the next page are adapted from Lutgens and Tarbuck (1992) The Atmosphere (5 th ed), Englewood Cliffs, New Jersey: Prentice Hall.

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10 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ POSSIBLE EXAM QUESTIONS 1. In the Northern Hemisphere, surface winds spiral in which direction around an area of low pressure? 2. You are viewing a weather map and see a strong storm to the west of your location. What measurement would you look at to get an idea if the storm was going to affect your location? a. Dew point temperature b. Wind direction c. Pressure change d. Cloud cover ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

11 WEATHER CONDITIONS ASSOCIATED WITH MID-LATITUDE CYCLONES IN THE NORTHERN HEMISPHERE 1. Ahead of the warm front (cool air sector): • Air Mass: cool temperatures and moderate humidity. • Winds: easterly direction (NE, E, SE). • Clouds: lowering and thickening clouds in a steady progression from cirrus (Ci) to cirrostratus (Cs) to altostratus (As) to stratus (St) and rain-producing nimbus (Ns). • Precipitation: light, steady precipitation of long duration across a wide area ahead of the warm front. 2. Passage of the warm front (changing weather conditions): • Temperature: rises significantly. • Winds: shift to SE or S. • Clouds and Precipitation: rain ends and skies clear. 3. Between the warm and cold fronts (warm air sector): • Air Mass: mT air (maritime Tropical), warm and humid. • Winds: southerly flow (S, SW). • Clouds: developing cumulus (Cu), cumulonimbus (Cb) clouds are found along and just ahead of the cold front. • Precipitation: possible thunderstorms along or just ahead of the cold front, precipitation is of greater intensity heavy but of short duration, scattered over the area in a narrow band. 4. Passage of the cold front (changing weather conditions): • Temperature: sharp drop in temperature. • Winds: abrupt shift to W or NW. • Clouds and Precipitation: clearing skies and ending precipitation. 5. Behind the cold front (cold air sector): • Air Mass: cP air (continental Polar), cold and dry. • Winds: northwesterly winds (N, NW, W). • Clouds: clear skies or fair-weather cumulus. • Precipitation: fair weather. • Pressure: high pressure cell moving in. 6. Conditions north of the low-pressure center: • Temperature: slowly falling. • Winds: shifting slowly from NE to N to NW (backing wind shift). • Clouds: low overcast of stratus (St) and nimbus (Ns). • Precipitation: continuous light precipitation accompanied by fog and drizzle, possibility of heavy snow in winter.