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The Webfooted Astronomer - May 2000

 

Minutes: Northwest Weather

by Leslie Irizarry

Announcements

The managing director for Discovery Park is holding another star party on May 11. Astronomy Day Open House at the UW is on May 13, from 2 p.m. to 6 p.m. People are needed to staff a booth and do solar observing, weather permitting. Let Randy know if you are willing to participate in this event.

Long-time SAS Member Walt Coggin has passed away. There was a memorial service Saturday, April 22.

Pat Lewis said that May 12 is an excellent opportunity for a Lunar Observers meeting. Pat and Joanne will be away; however, if anyone wishes to host this meeting, see Pat. Astrophotography meeting will be held on May 21 at Keith Allred's house. Telescope Makers will be at Peter Hirtle's May 20. A New Member's Meeting will be at Karl Schroeder's on May 6.

If anyone wants any changes made to the annual roster , please contact Steve Schonberger.

Dark Skies Northwest

Karl Schroeder spoke about our section in the International Dark Sky Association (IDA). This section, Dark Skies Northwest, of which the SAS is a member, started because of concern regarding the spotlight on the Space Needle. Being a section of the IDA enables SAS members to use the IDA's name and logo. The section's immediate goal is to educate the members, the club, and the public. The first project is a Star Count Survey, which will be done by Project ASTRO, other clubs, and the public. A selected constellation will be looked at, and the stars will be counted. The object of the IDA is not to get rid of light, but to control the wasted light.

Puget Sound Weather

Jeff Renner got his B.S. here at the UW. He has been on KING 5 since the 70s. He grew up in the Midwest, and has long been interested in astronomy as an amateur. Jeff's talk concerned how weather patterns in the Pacific Northwest can affect observing, how to predict changes in the weather, sky phenomena related to atmospheric conditions, and how some areas within Washington state provide more suitable observing conditions than other areas.

Atmospheric Conditions
Cirrus clouds act like thousands of little prisms. They create a halo around the moon. If the halo is a wide ring, it suggests moisture in the atmosphere. A corona is high up in the atmosphere. Watch and see if it thickens or lowers. The corona means that the moisture content is increasing; cloud cover is beginning to form.

High clouds are cirrus—16,000 feet or higher. Altostratus clouds are somewhat lower, and straight stratus clouds are 6,000 feet or lower. Look for this progression from high to low. Stratus clouds are associated with a warm front. Cold fronts are associated with cumulus clouds, due to instability in the atmosphere. The sky can become covered with cumulus clouds very rapidly; however, they can dissipate at night.

Lightning safety
To determine how far away lightening is, count the seconds between a flash of lightening and a peal of thunder, and divide by 5. Do this several times to see if the lightning is getting closer or farther away. If you are caught in a lightning storm, get away from metal objects. It is best to be 5-10 miles away from lightning, as it can travel at angles and not just vertically. If you feel a tingling sensation, it's because the lightning sends out a "step leader." Move and get out of the vicinity. Get away from isolated big trees. A forest is okay. If you are caught out in the open, get as low as you can. Don't spread out, however, crouch down instead. If you are in a metal shed or automobile, you should be okay.

If clouds are thickening and lowering, and winds have picked up in velocity, it indicates that some system is moving in. Low pressure is generally associated with bad weather. Winds converge; air rises and cools.

A sustained decrease in air pressure manifests itself by a windshift to southern or southeasterly wind. If you are observing westerly winds, this suggests higher pressure.

The most valuable indicator is a change in pressure. A barometer/altimeter is handy to have. Keep track of the pressure every 3 degrees. Here's the Three Degree Pressure Fall Guidance:

..04": monitor as usual
..04-.06": watch sky and winds closely
..06 to .08": SCA likely. Check every hour.
..08" or more: gale. Check hourly.

It's beneficial to get a sense of the jet stream. The weather patterns vary, depending on whether it comes all the way from the tropics vs. from the north Pacific Ocean.

In the Puget Sound region, we have a convergence zone: the wind (SW to NW) at the coast hits the Olympic Mountains and splits. It then converges between Paine Field and SeaTac, and between Stevens Pass and Snoqualmie Pass. This causes increased chance of rain. So for good observing, go south of Snoqualmie or north of Stevens!

There can be secondary convergence zones. There is one by Seattle and either side of Vancouver Island. The ocean-mountain interaction creates localized patterns.

Temperature
Dew point is the temperature at which the air becomes saturated. If the temperature and dew point are within 7 degrees of each other, there will be fog or low clouds. There is a rule of sevens. If the wind speed is less than 7 knots, fog will form. If greater than 7, there will be some low clouds.

Stability of the atmosphere: the more unstable, the more shifting from air currents. If temperature decreases with altitude, the more unstable it is. Conversely, if temperature remains relatively static and the higher the altitude, the more stable the weather is.

Resources
For weather satellite imagery and radar imagery, see http://www.atmos.washington.edu. It includes the Profiler, which gives you temperature and winds. The wind velocity reads out at different altitudes.

For local radar and reports from the 70 schools in Western Washington, see http://www.king5.com. This site gives you better coverage than the FAA.

Jeff Renner' Mountain Weather Field Guide by Jeff Renner.

Northwest Mountain Weather: Understanding and Forecasting for the Backcountry User by Jeff Renner.

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