CWS 5A

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AIR PRESSURE CHANGE

Near the end of last week an expansive low-pressure system with its accompanying fronts slowly crossed the central and eastern U.S. As= noted in the Monday, 5 October 2009 Daily Weather Summary, locally heavy rains accompanied its passage. This passing system brought corresponding pressure= and weather changes to places in its path.

1= . Image 1 is the surface weather map for 12Z 01 OCT 2009 (7 AM CDT Thursday morning). At map time, the storm system was curved across the breadth of the central U.S. mainly fr= om North Dakota to central Texas. It was marked by one low-pressu= re center in west-central ND and another in southeastern Nebraska as depicted by the Ls. The low-pressure centers were linked by an occluded front while the Nebraska Low was located at the juncture of the occluded, a stationary front to the south and a cold front that curved to the southwest.

The station model for D= allas, Texas is located in northeastern Texas. The weath= er conditions at Dallas at map time were: temperature 76 °F, dewpoint 70 °F, cloudy skies, coded pressure value "079" (partially obscu= red by the frontal symbol) and winds from the south at about 15 knots. At map t= ime the cold front was shown stretched across the Texas panhandle to the northwest and t= he stationary front was to the east. From the frontal symbols and the map locations shown, at 12Z the cold front [(had already)(had not yet)] passed Dallas' location.

2= .      Compare the temperatures at Oklahoma City and thos= e of central and southern Texas to those at <= st1:City w:st=3D"on">Amarillo, TX in t= he northern panhandle and Albuque= rque, New Mexico. The air ahead o= f (to the southeast) the front was [(colder)= (warmer)].

3= .      Also compare the dewpoints at those same stations ah= ead of and behind the front. The air ahead of the front was [(more)(less)] humid.

4= .      Compare the wind directions at the stations ahead of the front to those behind the front. The wind directions shift from being generally [(southerly = to being northwesterly)(northwesterly= to being southerly)] as the front passes.

5= .      Image 2 is the meteogram for Dallas-Ft. Worth, TX= (DFW) for the 24-hour period from 1100Z 01 OCT 2009 (091001/1100 in the top heading of the meteogram) to 1100Z 02 OCT 2009.= Meteograms for current weather conditions for the pre= ceding 24-hour period at selected stations can be obtained fro= m the course website (Surface section, Meteogr= am for Selected Cities). On the Image 2 meteogram, draw a vertical line on the meteogram at the 12= Z 01 OCT 2009 time (unlabelled tick mark) corresponding to the Image 1 surface weather map time (Z times along the lower graph axis). Label this time as "Im 1".

The weather conditions at Dallas on the Image 1 surface weather m= ap [(were)(were not)] the same as those depic= ted on the Image 2 meteogram at that time.

6= .      From 11Z to 22Z on 01 OCT, wind directions at Dallas were generally from the [(south)(west)(north)].

7= .      From 11Z to 16Z, the air pressure at Dallas was relatively steady. After 16Z it generally fell very slowly until reachi= ng its lowest value of the meteogram period. Durin= g the 22Z to 11Z 02 OCT time period, the pressure [(fell)(rose)], first rapidly and then more gradually.

8= .      From 11Z to 17Z on 01 OCT, the temperature changed little. The temperature rose following 17Z, reaching its maximum at about 21Z and then declined slightly until 23Z (from noon to 6 PM local time). Following 23Z the temperature = [(fell)(rose)], first rapidly for an hour = then more gradually.

9= .      The dewpoint was relatively steady from 11Z to 23Z = on 01 OCT, then it [(fell)(rose= )] until about 07Z on 02 OCT.

1= 0. Wind directions shifted most significantly between [(14Z and 15Z on 01 = OCT)(22Z and 23Z on 01 OCT)(05Z and 0= 6Z on 02 OCT)].

1= 1. Based= on these changes in patterns of temperature, dewpoint, wind direction, pressure, one can conclude that the frontal system probably passed Dallas during the hour or so following [(15Z)= (22Z)(05Z)].

1= 2. Image 3 is the surface weather map for 00Z 02 OCT 2009, twelve hours following the Image 1 map. On the Image 2 meteogram, draw= a vertical line on the meteogram at the 00Z 02 OC= T 2009 time (unlabelled tick mark) corresponding to the Image 3 surface weather map time. Label this time as "Im 3".

Note the temperature, de= wpoint, sky cover, wind direction and pressure at Dallas at 00Z 02 OCT. These values [(were)(were not)] the same as those shown on the 00Z, Image 3 surface map.<= /p>
1= 3. From = the cold front's position shown on the Image 3, 00Z 02 OCT map, the front at 00= Z [(had already)(had not yet)] passed Dallas' location.

1= 4. This = Image 3 map depiction of frontal location [(was)(was not)] consistent with the changes in weather conditions and the pressure pattern shown on the Image 2 meteogram= .

By comparing the weather conditions at Oklahoma City between maps, the changes experienced are cle= ar. While temperatures aren't greatly different, at 12Z on 01 OCT, early mornin= g, it was already 74 °F and very humid. By 00Z, evening it was a little co= oler at 71 °F but with low humidity. Skies had cleared and a brisk northwest= erly wind foretold a period of fair weather. The air pressure changes accompanyi= ng the passing frontal system harken back to the e= arly forecasting with a "weather glass" (liquid barometer) - falling pressures predicting stormy weather and rising pressures presaging fair weather.

You can acquire meteograms for the NWS stati= on closest to you if not among those on the course website, (or to other locat= ions around the U.S. and world) by going to: http://vortex.plymouth.edu/statlog.html. [This is the "click here" link on the bottom of the Meteograms for Selected Cities in the U.S.<= /i> page from the course website Surface data section.]

Critical Thinking and Diversity Component: The Week 5 Critical Thinking/Diversity Component, located in the Learning Files section of the course homepage, applies the skill of testing models and the attributes of = an objective learner to the critical thinking activity of relating local weath= er changes to large-scale weather systems and to the diversity component, which examines crop freeze protection strategies.

If directed by your instructor, place the answers to Investigation 5A and Current Weather Studies 5A on the A Answer Form linked from the AMS Weather Studies website.

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