102_Lab_3 with maps rotated.pdf

Weather Lab 3 Materials textbook straight edge calculator Objectives • to examine the nature, causes and patterns of weather, i.e. , air pressure and winds • to relate air pressure and winds Read Geosystems , pp. 145-159; 163-164; 208-209; 216-217 Introduction Air Pressure and Wind Air pressure anywhere on earth simply means the number of atmospheric molecules in that area. Air molecules always exert pressure on each other and move in directions to where there is more space for them. During their transfer to their new space, they generate wind in the process (that is what you feel when it is windy outdoors). Wind direction and speed are generated by four forces: (1) gravity, (2) pressure gradient force, (3) Coriolis force, and (4) friction. In this lab we are interested in wind at earth’s surface where it affects our daily lives. Part I Surface Winds, Pressure Systems Six North American weather maps are used in this lab to show weather data over three days, at 12-hour intervals. Map 1 represents 7:30pm, Eastern Standard Time (E) on April 13; Map 6 represents 7:30am (E) April 16. On weather maps, each weather station is located where people can collect data on a continual basis, so a station is located usually at a city or large community. Each weather station on the map shows most of the symbols below, with the center of the circle showing the precise location of the weather station. The weather station data have fixed positions around the center (Fig. 1). Figure 1. Example of weather station symbols. The symbols shown are used in this lab. 36 46 135 -16 wind speed air pressure pressure tendency cloud cover temperature wind direction

Fig. 1 shows a simplified weather station set of symbols. Below is an explanation of the symbols. cloud cover - the circle is increasingly black with increasing cloud cover. See Geosystems , GIA 8.1, p. 218 for all cloud cover symbols wind direction - a line outside of the circle center, and connected to the circle, lines up in the direction the wind is blowing from . The wind always blows toward the circle, e.g. (a) no wind, (b) north west (NW) wind, (c) east (E) wind. North is always at the top of the map. wind speed - these are the ‘feathers’ on the wind direction line. See the scale of feathers in Geosystems , Fig. 6.12, p. 159. On the maps in this lab, wind speeds are shown in miles per hour (m.p.h.). In this course, you will not be asked to convert miles/kilometers/knots, either in lab or on a lab exam. temperature - values are positioned at the upper left of the circle. Temperatures on the maps are in °F. You will not be asked to convert °F/°C, either in lab or on a lab exam. air pressure - values are positioned at the upper left of the circle. The air pressure example (Fig. 1) says the air pressure is ‘135’. However, the value is actually 1013.5 millibars (mb). If the entire 1013.5 value was documented, the map would be congested with information. When possible, unnecessary information is removed in the following manner. First, the decimal point is removed, leaving the number as 10135. Then the 10 is removed. Why? Normal air pressure on earth ranges from 980 to 1050 mb. Therefore, only a number ‘10’ could be in front of the 135 to know the correct air pressure. pressure tendency - values are positioned to the immediate right of the circle. The pressure tendency is the air pressure change since three hours before the current readings were taken. The same convention for air pressure documentation applied to pressure tendency documentation. In the example (Fig. 1), -016 means the air pressure lowered by 1.6 mb during the previous three hours. The decimal point is removed; the negative sign means the air pressure lowered. A ‘+’ symbol would mean the air pressure increased since the three hours preceding the current readings. Another essential map symbol - lines of equal air pressure are called isobars . They are drawn by connecting points of equal air pressure. In the following exercises use the weather maps 1-6, showing weather stations, high and low pressure systems and areas of precipitation to answer the questions. 37 1/10 cloud cover 5/10 cloud cover 7-8/10 cloud cover completely overcast clear sky (a) (b) (c) isobar 1012 1008 1004

Map 1 1. Look at the weather station wind directions. Generally, how does the wind blow in relation to individual isobars? It blows ________________ to the isobars. [Question 1 in mêskanâs] \\ parallel perpendicular diagonally (circle one) 2. Look for two pressure areas: [Question 2 in mêskanâs] • high pressure area over western Canada • low pressure area over northern Quebec and the U.S. east coast. a) How do the winds blow around a high? b) How do the winds blow around a low? 3. Give the wind direction (e.g. NE, SW) for weather stations below [Questions 3-5 in mêskanâs] • Edmonton __________ • Armstrong, Ont. (north of Lake Superior) __________ • Gander, Nfld. and Labrador __________ 4. For stations below, how does the wind blow relative to the nearby: [Questions 5-8 in mêskanâs] 3. isobars: perpendicular or diagonally or parallel (circle one answer) 4. high or low pressure centers: in toward or away from (circle one answer) • Edmonton 1) ____________________ 2) ____________________ • Armstrong 1) ____________________ 2) ____________________ • Gander 1) ____________________ 2) ____________________ 5. For the stations above, explain why surface wind does not blow along the pressure gradient, i.e. perpendicular to the isobars. [Question 9 in mêskanâs] 6. Overcast skies are more prevalent near __________ pressure areas. [Question 10 in mêskanâs] low or high (circle one) 7. Strong winds are more prevalent near __________ pressure areas. [Question 11 in mêskanâs] low or high (circle one) 38

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Map 2 8. Give the wind speed for [Questions 12-13 in mêskanâs] • Williston, North Dakota (south of Regina, Sask.) __________ • Windsor, Ontario __________ 9. The wind speed at Williston is less than at Windsor. Why? [Question 14 in mêskanâs] 10. Give the wind speed for [Questions 15-16 in mêskanâs] • Saskatoon __________ • Halifax __________ 11. The wind speed at Saskatoon is less than at Halifax. Why? [Question 17 in mêskanâs] 12. Why is the temperature at Norman Wells, NWT (37°F) warmer than at Louisville, Kentucky (29°F) (south of Lake Michigan), although Normal Wells is at a much higher latitude? [Question 18 in mêskanâs] 40

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