Metrology 302.01

Class notes for 2-20-04

Jennifer Lee

Diego Vallejo

We started the lecture by differentiating satellite images from radar images. Weather satellites detect clouds and the temperature of clouds. Visible satellite image is usually black and white, while infrared imagery is indicated by color. Both of these images are limited in detecting rain clouds because they do not tell us which clouds have precipitation.

Doppler radar is used around the U.S to detect precipitation in rain clouds and tell us how fast a rain clouds are moving and where they are moving to. The radar uses sound waves and microwaves, the same principle as radio signals, to reflect messages back to the radar. This website http://tornado.sfsu.edu/geosciences/classes/m302/RadarBasic/radarops.gif shows how the radar detects objects in the atmosphere; the signal is then reflected back to the radar. The radar has a 75-100 mile radius but only works in line of sight (signal can be obstructed by objects).

It can detect any solid object in the atmosphere such as rain drops, snow crystals, and hail stones, essentially anything that is the size of a rain drop. It emits electro-magnetic waves of radiation while it spins 360 degrees. You can see a drawing of this at http://tornado.sfsu.edu/geosciences/classes/m200/Radar.jpg .

One of the draw backs of this invention may be its psychological impact on its operator. During a sever tornado or thunderstorm, the operator must man the radar, even if their family is in danger, until the conditions at the station become too dangerous. When this occurs, the operator must move to a special reinforced room located inside the station for safety. Studies have suggested the emotional distress suffered during instances like these is damaging to the operator.

Another draw back associated with the Doppler radar is its application in mountainous areas such as the Rockies. The radar is unable to detect conditions through mountain peaks because it operates on line of sight. To avoid this complication, a station may be placed on top of a mountain; however, it may emit a signal over an area of precipitation, possibly missing the severe weather conditions. Human safety issues have limited the radar to a degree of tilt of one half inch on above of the horizontal.

http://tornado.sfsu.edu/geosciences/classes/m302/RadarBasic/doppler.jpg This image shows one of the many radar stations around the United States, all of whom are identical in their floor plan. Each satellite has a protective covering surrounding it. The cover conceals the radars ‘unpleasant appearance from the communities surrounding it, as well as keeps the elements from affecting it, such as birds, insects, and weather conditions.

If you look in the national Doppler radar site, (http://tornado.sfsu.edu/geosciences/classes/m302/RadarBasic/doppler.jpg) you will see the many in stations in concentrated in the mid and eastern part of the United Sates. This is due to the fact that these areas are most likely to experience severe weather. You can click on any weather station to see the enlarge image of the forecast. The different colors indicate the intensity of the precipitation that is detected; loops created by multiple signals can be used to predict the direction and speed of an area of precipitation.

In class today we were introduced to three types of fronts. Fronts are boundaries that divide warmer air from colder air.

Warm fronts are semi-circles placed on a line. They are filled in and colored red. A warm air front develops where warm air and cold air collide. In warm fronts, warm air is replacing colder air. The lighter, warmer air slides over the heavier cold air, creating a boundary. The warm air moves in the direction of the shapes.

Cold fronts are triangles placed on a line. They are filled in and colored blue. In this front, the cold air dominates the boundary where the warm and cold airs meet. The cold air moves in the direction of the shapes.

A dry line is an area designated by brown, hollow semi-circle. A dry line separates warmer from colder air but also indicates dry from moist air. The hollow semi-circles point away from the dry air. Dry lines usually move toward the east and can cause severe thunderstorms.

Hurricane names

· Names are assigned to each hurricane so that it raises public awareness.

· Names replaced the old system of letter/number designations which were confusing and hard to keep track of.

· Names are designated each year for storms in alphabetical order.