ERTH 260 In-class Exercise 3 Key: Weather Map Features Introduction to Boundaries

Transcription

1 San Francisco State University Name Department of Earth & Climate Sciences Spring 2018 ERTH 260 In-class Exercise 3 Key: Weather Map Features Introduction to Boundaries 100 points Due Wednesday 14 February ) Here is the surface weather plot for the United States for 1200 UTC 7 February The arrows are streamlines that show the sense of the windflow observed at ground level over the south central United States. The colored lines with symbols are boundaries a sharp division between air of certain characteristics and air of different characteristics. (30 pts) a) On a separate sheet, describe the general characteristics (general wind directions and temperatures) of the air in the region of the two blue streamlines over Kansas/Oklahoma/Nebraska/eastern Colorado).

2 Generally the winds are westerly northwesterly or northwesterly with temperatures in the 30s and 40s F. b) On a separate sheet, describe the general characteristics (general wind directions and temperatures) of the air in the region of the two red streamlines over eastern Texas/Arkansas/Louisiana. Generally the winds are southwesterly with temperatures in the middle and upper 60s. c) On a separate sheet, describe the relative amount of water vapor of the air in the region of the brown streamline over west Texas) compared to that over central and eastern Texas. Generally, the dew points in the area of the brown streamline are quite low compared to the dew points in the area of central and eastern Texas. Thus, we can infer lower amounts of water vapor in the area of the brown streamline and higher amounts in the area of the red streamline. 2) Describe how the information shown on the surface chart above corroborates the fact that a warm front was located in the position shown over the eastern part of the United States. (10 pts) Air north of the warm front has temperature in the 40s or high 30s with easterly or northeasterly winds. Air south of the warm front has temperatures in the 60s with southwesterly winds. Along the line warmer air is colliding with colder air, hence it is a warm front. 3) Here's the meteogram for Dallas-Fort Worth International Airport (KDFW) for the 24 hours from 04 UTC 7 Feb to 04 UTC 8 Feb 2017, showing the run of weather observations for the 24 hours ending 0400 UTC.

3 a) On a separate sheet, briefly discuss the nature (not the reason) of the change of the temperature, dew point temperature, wind direction changed from 10 UTC to 17 UTC. (20 pts) Between 10 and 17 UTC the temperature stayed the same except for a slight increase in the last two hours. However, in that time increment, dew point temperatures fell from around 60F to 33F and the wind direction shifted from SW to WNW. b) Given our discussion in class and your examination of the surface chart above, on a separate sheet, now discuss the reason for the change you observed in (1). (20 points) The surface analysis shows a dry line just west of Dallas at 12 UTC. The scallops on the line indicates that the dry line, and the dry air west of it, was moving eastward. Thus, Dallas should experience a fall in dew point temperatures and a wind shift to westerly. That is exactly what the meteogram shown above indicates did happen.

4 4) Here s a closeup surface chart (called a subsynoptic plot) with some streamlines and the position of the dry line at 1700 UTC 7 February. Note the weather stations at A and at B. On the basis of what we discussed in class, on a separate sheet discuss how the weather data at the two stations corroborates the position of the dry line analyzed on the chart and direction in which the scallops on the line are pointing. (20 points) Station A is west of the dry line and station B east of it. Since the dry line separates warm moist air on the east from warm dry air on the west, both stations should be experiencing warm temperatures. But station B should be in the moist air with high dew point. The temperature of 77F is the same at both places, but Station A has a dew point of 36, and Station B a dew point of 62. This is exactly what one should expect at these two places given their position relative to the dry line. Also, the symbols for the dry line indicate that the dry air should be moving eastward. The northwest winds observed at Station A are consistent with that.

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