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DataStr= eme Atmosphere Current Weather Studies 6A:

CLOUDS, TEMPERATURE, AND AIR PRESSURE

After completing the introductory portion of Investigation 6A in the = Investigations Manual, use the WeatherCycler provided in the Study Guide to answer the following questions.

1= .      Apply the hand-twist model of a low-pressure center to simulate vertical air moti= ons. The motion of your palm is [(downward)= (upward)]. Such atmospheric vertical motion leads to cloud [(formation)(dissipation)]. The WeatherCycler shows stations under the influence of a low-pressure system are generally <= b>[(clear or partly cloudy)(mostly cloudy or overcast)].

2= .      Apply the hand-twist model of a high-pressure center. The motion of your palm is = [(downward)(upward)]. Such atmospheric vertical motion leads to cloud [(formation)(dissipation)]. The WeatherCycler shows stations = under the influence of a high-pressure system are generally [(clear or partly cloudy)(mostly cloudy or ov= ercast)].

Clouds are conglomerations of tiny water droplets (or ice particles) that formed from condensation (or deposition) of water vapor. This requires air = to have a relative humidity of 100%, meaning saturation. Therefore, atmospheric processes that lead to saturation above Earth's surface form the clouds that are so prevalent in the sky. The first part of Investigation 6A in the W= eather Studies Investigations Manual demonstrated how an air parcel containing water vapor, rising through the atmosphere would expand and could eventually cool to the dewpoint.

3= .      Image 1 is the surface weather map for 00Z 11 OCT 2009, Saturday evening. At = map time several weather systems were bringing clouds and patches of precipitat= ion to many areas of the country. An extensive frontal system was stretched alo= ng the Southeast and Gulf of Mexico coastal region producing clouds and precipitation. This was the remnants of a system that brought heavy rains and flooding to the central U.S. during midweek. Remaining scattered precipitation was over the northern Plains and western Great Lakes from a passing Alberta clipper-type low-pressure system. Finally, a stationary front lay over the = West and along the eastern slopes of the Rockies in Colorado. Wind flows and terrain to the western or "back-side" of the cold high-pressure system marked by= the "H" centered in Neb= raska that followed the clipper were also creating ris= ing motions leading to clouds and precipitation. As noted in the Monday, 12 Oct= ober 2009 Daily Weather Summary locally heavy snowfalls accompanied the cold Hig= h, a decidedly "unfair" weather event. North Platte, Nebraska received almost 18 inches to set a monthly snow record. The snow in Denver to the wes= t caused a postponement of the baseball championship series game on Saturday evening= , a stunning event for the conclusions of our "summer pastime".

The sky cover condition reported at Denver, in northeastern Colorado, was [(clear= )(partly cloudy)(overcast)= ].

4= .      The wind direction at Denver was from the [(northeast)(southeast)(northwest)(southwest)].

5= .      This direction [(was)(was not<= /u>)] consistent with the counterclockwise flow pattern around a Low designated by the L west of Denver and clockwise around = the H of the elongated High over central Nebraska.

6= .      The air flow, as shown by the wind barb at Denver, was toward the mountains to the west forcing the air upward, resulti= ng in [(frontal)(orographic)] lifting of the air.

7= .      Image 2 is the Stüve diagram from Denver, Colorado (DNR) rawinsonde observation at 00Z on 11 OCT 2= 009 (091011/0000), the same time as the Image 1 surface map. Denver, the "mile high city", had a surface air pressure of 840 mb. The temperature and dewpoint= reported in the radiosonde text data (not shown= ) for Denver were –3.5 °C (26 °F) and –6.3 °C (21°F),= respectively. This difference of temperature and dewpoint tel= ls us that the air at the ground in = Denver [(was)(was not)]<= /b> saturated. This is further seen on the Stüve by the two separate heavy curves representing temperatures and the dewpoints near the surface.

8= .      Air rising above the surface at De= nver was cooling, and the temperature became equal to the d= ewpoint at about [(825)(790)] mb.

9= .      This equality of temperature and dewpont indicated t= hat the air [(was)(was not)] saturated at that level. Based on this equality we can assume the base of t= he cloud over Denver<= /st1:City> to be at that level.

1= 0.  For t= he DNR sounding, selected pressure/altitude correspondences are given in the table below: [The altitudes of the pressure levels are from the Upper - Air, Text Data for the DNR sounding, not shown.]

 

Pressu= re Level

Altitu= de

 

646 mb

3658 m

 

700 mb

3023 m

 

720 mb

2807 m

 

786 mb

2134 m

surface

840 mb

1625 m

1= 1.  Tempe= ratures in the atmosphere (and within any clouds present) above Denver were below freezing (less than 0 °C) throughout the layer from the surface (1625 m) upward. Precipitation falling through this sub-freezing air would be [(<= i>rain)(snow)].

1= 2.  Takin= g the bottom of the cloud to be at the altitude where the temperature and dewpoint first became equal (saturated air =3D cloud), and the top of the cloud to be the altitude where= they cease to be identical, the vertical extent of the cloud was about [(<= u>510)(890)(2000)] m.

1= 3.  Image 3 is the Stüve diagram from the Denver, Colorado (DNR) rawinsonde observation at 12Z on= 11 OCT 2009 (091011/1200), twelve hours following the Image 2 sounding.= Air at the rawinsonde site near Denver had a temperature of –3.3 = °C and dewpoint of –6.8 °C. They were, respectively, within a degree of the values 12 hours earlier. At 12Z, the surface air near Denver [(was)(was not)]<= /b> saturated.

1= 4.  Air r= ising from the surface at Denver was expanding and cooling as shown by the temperature profile. The temperat= ure [(did)(did not)] approximately equal the = dewpoint at a level of about 780 mb.

1= 5.  There= fore, we conclude there [(was)(= was not)] lifting of the air to produce saturation at 780 mb over = Denver and clouds were present.

1= 6.  Compa= ring the lower levels of the soundings at Denver at 00Z and 12Z, the surface temperatures, dewpoints, cloud base levels, ver= tical cloud extents and cloud top levels were [(similar)(v= ery different)].

1= 7.  Compa= re the surface winds at the radiosonde site (the lowest symbol) of the vertical wind columns to the right of the temperature grids.= The surface winds at both times were directed generally toward the [(= west or northwest)(east or southeast)], producing the orographic lifting mechanism. The= two soundings explain why the snowfall was so persistent!

Although Denver is located within sight= of the Rocky Mountains, North Pla= tte is on the "high plains" some distance from the mountains. The ris= ing terrain westward often proves sufficient for orographi= c lifting of the air under appropriate air flow and temperature patterns to produce significant snows even this early in the fall season. The Wednesday Current Weather Study 6B will consider further details of rising air motion= s.

Place the answers to Current Weather Studies activities 6A and 6B on the= CWS Answer Form (provided from the DataStreme Atmos= phere website on Wednesdays).

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