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.
Apply the hand-twist model of a low-pressure center to simulate vertical air motions. 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 [(clear or partly cloudy)(mostly cloudy or overcast)].
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 overcast)].
Clouds are conglomerations of tiny water droplets (and/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 prevalent in the sky. The first part of Investigation 6A in the Weather Studies Investigations Manual demonstrated how an air parcel containing water vapor, rising through the atmosphere would expand and could eventually cool to the dewpoint.
Image 1 is the surface weather map for 00Z 7 MAR 2010, Saturday evening. At map time generally clear or only partly cloudy conditions were prevalent over the eastern half of the U.S. where a fair weather High dominated. Two main weather systems were bringing clouds and areas of precipitation to regions of the country. One relatively weak low-pressure system with its accompanying warm and cold fronts was centered in the middle of the country. A second more complex set of low-pressure centers represented an expansive storm system that was impacting the West Coast. A long sequence of storm systems has been hitting the West coast from southern California to Washington state. While wintertime is the season where most precipitation falls along the coast, this pattern of storms bringing heavy precipitation is more closely linked to the El Niño conditions that have existed in the central Pacific Ocean this winter.
The portion of the western storm system most directly impacting the coast at map time was a low-pressure center located just west of Los Angeles with an occluded and then cold front curving near the coast before swinging southwestward into the Pacific. Radar echo shadings showed that there [(was)(was not)] precipitation occurring along the southern California coast.
The sky cover condition reported at Los Angeles, on the coast just east of the Low at the end of the occluded front, was [(clear)(partly cloudy)(overcast)]. The sky cover at San Diego to the south was obscured by the numerical label for the pressure system value, but was presumably the same.
The wind direction at San Diego (the wind arrow is seen below the pressure value box) was from the [(north)(east)(west)(south)].
This direction [(was)(was not)] consistent with the counterclockwise flow pattern around the Low associated with the frontal system.
With the front approaching the southern California coast, there would be [(frontal)(orographic)] lifting of the air along the frontal surface.
The air flow shown by the wind arrows at San Diego and Los Angeles, was along the coast and partially toward the mountains to the east that parallel the coast forcing the air upward, resulting in [(frontal)(orographic)] lifting of the air. Thus lifting of the air had multiple causes.
The temperature at San Diego was reported as 58 °F and the dewpoint as 47 °F. With this difference between temperature and dewpoint, the air at the surface in San Diego [(was)(was not)] saturated.
Image 2 is the Stüve diagram from San Diego, California (NKX) rawinsonde observation at 00Z on 7 MAR 2010 (100307/0000), the same time as the Image 1 surface map. The temperature curve is the heavy plotted curve to the right and dewpoint is the curve to the left, respectively. The difference of temperature and dewpoint curves at the surface (lowest plotted level) tells us that the air at the ground in San Diego [(was)(was not)] saturated.
Air rising above the surface at San Diego was cooling due to expansion. The temperature became within about a degree of the dewpoint at about [(1000)(900)] mb. (Because of sensor behavior, temperatures may not always equal dewpoints on radiosonde profiles where air is saturated. However, meteorologists consider that when the values are within a few degrees saturation and clouds may exist in the area.)
This near equality of temperature and dewpont readings indicated that the air probably [(was)(was not)] saturated at that level. Based on this evidence we can assume the base of the cloud over San Diego to be at about that level.
Cloud conditions would exist where the temperature and dewpoint were reported as nearly equal. If they separate by several degrees at a higher level, we would assume the air [(was still)(was no longer)] saturated.
Taking the bottom of the cloud to be at the altitude where the temperature and dewpoint are nearly equal (900 mb), and the top of the cloud to be the altitude where they are separated by several degrees, the top of existing cloud conditions in the lower troposphere over San Diego would be near [(800)(730)(700)] mb.
From upper air map information (not shown), the 900 mb level occurred at about 1000 m. The pressure level at which temperatures and dewpoints began separating by several degrees occurred at about 2500 m. Therefore, the vertical extent of the cloud between these two altitudes over San Diego was about [(1000)(1500)(2500)] m.
Many of the cities along the West Coast are especially scenic because they are near the ocean but still within sight of the coastal mountains. The meteorological "price" to be paid for these topographical attractions are the influences they have on the weather. The moisture supplied by the ocean and the storm systems that come at some times of the year encounter the rising terrain. This often proves sufficient for frontal and orographic lifting of the air leading to precipitation. The Wednesday Current Weather Study 6B will consider further details of rising air motions.
Place the answers to Current Weather Studies activities 6A and 6B on the CWS Answer Form (provided from the DataStreme Atmosphere website on Wednesdays).
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