Making Local Weather Prediction to Choose an Astronomical Observing Dark Site for the night

This is not that scientific so if you have a better way, please leave a comment on how you do it. I have a few people I observe with on a regular basis, and one or two of them have asked me how I get my weather down to know what site to go to. To be honest, I think a large part is luck. I observe enough that I am just bound to hit some really good nights of seeing. However, I do tinker with the weather and I am finding it does help in knowing local conditions and the impact of a weather system on local conditions. So this is how I do it and it seems to work for me.

First thing is I watch the local weather and compare the forecasts online and then I go to the National Weather Service website located here and I click on the Utah Zone Forecast that will take me here. Next I look at the zones I am interested in observing in, for example the Salt Lake and Tooele Valleys is one for home and for SPOC or anything in the northern Tooele Valley or SLC area. Another is Great Salt Lake Desert and Mountains including Wendover . . . Snowville which I use for Lakeside, The Knolls and other observing areas along or near I-80 west bound. For the Wolf Creek Pass location I use the Western Uinta Mountains including . . . Hanna, since Hanna is close to Wolf Creek Pass though lower in elevation. I look over the forecast and determine what day I am prepared to go and observe and then I start watching conditions on the Mesonet station near that observing spot.

Now, I know like this week, we had a storm on Thursday, a rather large storm and thus the readings for Thursday and Friday at an observing site will be off when compared to the actual tonight. I do KNOW, as I have kept a log, that 86.47% of the time, 2 days after we have a cold front come through, we get some of our best viewing conditions. Not all the time, but is is a rather good rule of thumb. The jet stream though can negate this rule though if it parks overhead. If a storm comes through, and then the jet stream is bumped north before the muck settles back in, seeing and transparency will be good to excellent depending on the time of the year.

Here is a link to the Mesonet and I'll share how I use it. Once the map loads, on the far middle right are some buttons you can click and check for RH and Dew Point. I check those and reload the map, and then zoom in to a station near my observing site. I then watch conditions and since dew can be an issue for some people I observe with, I watch the dew point (don't worry about it with my dobs). I then check the Clear Sky Clock and the NWS site and if both predict a temperature well above what I am observing for the dew point, I know I don't have to worry about dew. Humidity may increase and impact viewing, but it won't dew or frost up on the equipment. To check temperature I use the NWS forecast and the AccuWeather forecast. I provided the link above for the NWS forecast and for AccuWeather, I use their astronomy forecast which should be in that link and then click on details for that day. By using the two, I find the temperature is in that range and if that is above the dew point, I'm good to go.

For example. Lets say I wanted to go to Pit n Pole. Tonight, the day after a storm, I would not go at all because I know humidity at that site gets REALLY high after a storm (and in the fall and spring). So I will look at tomorrow as a possibility. I go to the Mesonet and find the Pony Express Mesostation which is close to the observing site. I notice the the relative humidity right now is 45%, high for 9:35p.m. and I make a note of that. The Dew point for right now is 32 degrees F, with a current temperature of 53 degrees F. Right there confirms that humidity will be high at that site tonight (the day after a storm). Temperature will continue to fall and humidity will rise. The forecast for that area is a low between 38 degrees F and 44 degrees F tonight, with a forecast low tomorrow of around 50 degrees F (which I will drop since this is a Camp Floyd/Cedar Fort forecast and I know that Pit n Pole is colder than those two sites); so I drop it by 8 degrees to 42 degrees F tomorrow. With a dew point that should hold around 32 degrees F tomorrow, observing at Pit n Pole will be okay, but humidity will be high. If you have a dob and want to observe, no problem. Imaging, you might want a different site.

If I want to confirm my prediction, I can bring up the Clear Sky Clock for Camp Floyd. I see that CSC is predicting 50% to 60% humidity from say 1:00a.m. til 6:00a.m., rising to 65% humidity right after 6:00a.m. So my method and predictions work. I will say though that sometimes I predict something other than CSC and because I am using local information, conditions and experience, I am usually on with what I forecast; often CSC says no to viewing and everything I read and know say go,
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so I go and about 80% of the time I am right.

So, though this is for local Salt Lake City area, it can be used in most of Utah and the west where the Mesonet and the other information is available. In the end, I usually only spend about 20 minutes over a couple of days prior to my observing and that is mainly so I pick the right site. In the end, I think what matters is that you and I are getting out, observing and enjoying the hobby, and the more often you do that, the better you'll be at knowing where to go to observe if you leave home. Sometimes home, even with its light pollution, is the finest place to view from. I hope to see you out at some of the observing sites as you bring your scope out to view the stars, planets, objects and deep sky objects that are out there. Good luck, and keep the wonder of the heavens alive within you.

Here are the links again, all in one place:

National Weather Service Zone Forecast (click on Utah Zone Forecasts at the top)

Mesonet Weather Stations

AccuWeather Astronomy Forecast

(to change a location on AccuWeather simply go to the search above, hover your mouse cursor in there and then move down to browse by state, and then find the nearest city in your state in the alphabetical list/drop down menu).

CRWS Jet Stream

(note, tonight, September 10, 2010 it is right over Utah and that should be impacting seeing).

Clear Sky Clock (set to West Jordan, click on the right for Utah, List for a list of other locations in Utah; also more are in the U.S. and Canada, just look around for your location).

Pick your favorite local weather stations, I use KSL and FoxNews13.

1 comment:

  1. Hi Jay, I use a combination of the BBC weather website, Accuweather, Metcheck and The Weather Outlook to get an overall picture of what the weather's going to do.
    As I mostly observe from the backyard, I also look out the window to see what the sky is doing.