Meteorology 302.01 Spring 2004

Class Notes for 2/9

Bruce Lichti & Steven Loftus

Assignments: Sheets and Williams: Preface; Chapter 1
Williams, Up through Page 36; 63-71; Chapter 9, 139-147; Chapter 12

Housekeeping: All old links are moved to “Completed Class Discussion Topic Links” for reference. Ongoing links will stay at the top level.

Reviewed “Storm Prediction Center” (Convective outlook) showing that to the right of the brown arrow we would expect to have thunderstorms.

Defined:

Synoptic Chart: A chart summarizing the weather conditions at a particular date and time, a “snapshot”.

Synoptic Meteorology: A field seeking to understand and predict weather patterns.

Watch: Meteorological conditions are favorable for the given hazardous weather event to occur across a general (specified) area and during a broad (specified) time frame. Persons in the watch area should be aware of developing hazardous conditions and should be prepared to take immediate protective action should the conditions develop.

Warning: The given hazardous weather event has been observed and is approaching your area. Take immediate (specified) action.

Stratus: Commonly known as fog, derived from the Latin word for layered. Usually thin layers of a few hundred feet or less at or near ground level. The temperature of the water droplets in fog is roughly equal to the temperature of the ocean they came from.

Cirrus: Latin for streak, thin, high, feathery clouds. Can be mistaken for cumulonimbus by infrared satellite imagery.

Satellite Imagery

It takes 15 to 20 minutes to produce an image, regardless of resolution. Top left portion of the picture is 20 minutes older than the bottom right area.

Visual Imagery: Records the reflection of the sun’s light off of clouds, earth, and ocean. The thicker the cloud the brighter the reflection. Cannot be used during night, images are usually black and white, extremely limited in identifying cloud type, and cannot see precipitation or wind (though wind can be inferred by cloud movement) If you can see lakes in the then you’re looking at snow through a clear sky, not clouds.

Infrared Imagery: Reads the temperature of anything greater than 0 degrees K (-273 degrees C). It only registers the temperature at the top of a cloud, so the higher or taller a cloud is, the colder it looks to the IR camera. Colder clouds usually indicate taller clouds, which are more likely to cause precipitation, however, cirrus clouds can be mistaken for cumulonimbus.

Mid-Latitude clouds move from east to west.

Air moves counter-clockwise around a low pressure area in the northern hemisphere.

Dew Point Temperature (DPT)

Specifically, the point at which water vapor in the atmosphere would condense. It is a direct measurement of the amount of water vapor in the atmosphere, the more H2O, the higher the DPT. DPT is generally over the ocean and near the coast. The closer DPT is to actual temperature, the higher the relative humidity, when Temp = DPT, the relative humidity is 100%. Warmer air can hold more water than colder air.

Atmospheric Pressure

The three-digit number in the upper right of station reports. It is in a shorthand, to reverse the shorthand, simply write millibars (mb) after the number, add a decimal point before the 3^rd digit, and add either a 10 or a 9 to the front of the number, whichever would cause the final number to fit between 1050 and 960 (the range of observed values at sea level). For example, 893 would become 989.3 mb, 023 would become 1002.3 mb, etc.

Average pressure at sea level is approx. 14 pounds per square inch (psi) = 1000 millibars.

Differences in atmospheric pressure is the major cause of wind (but not the only one):

“on a given level surface, in the absence of other effect, air tends to move from regions of higher pressure to areas of lower pressure.”