Sunday, August 6, 2017

Reading- and dealing with- the weather

The last few posts have dealt with traffic issues, getting to the eclipse path, and escaping from the clouds. Now, we pull these topics all together and help you learn about how to read the weather in the days leading up to, and including, eclipse day.

Links (with some explanations):

(all links with eclipsophile in them are courtesy of Jay Anderson, former meteorologist with Environment Canada)

http://eclipsophile.com/real-time-weather/ - this one is a quick links site, useful for quick loading on eclipse day)

http://eclipsophile.com/eclipsewx/ - basically the same links as the above (and a few more), just with explanations on how to use them. Good to study well before eclipse day.

From Jay, as posted to the eclipse chasers discussion group: “I draw your attention to spotwx.com, which provides site-specific graphical forecasts – click on a map or type in a community name (or lat/long) and you will get access to up to a half-dozen Canadian and US computer models for the site. You can easily compare the models to see how they agree ahead of the magic day (up to 10 days out). For areal forecasts, you can look at SkippySky or the College of DuPage (which has access to several models, great satellite coverage, and is my favourite storm-chase resource). Both CoD and spotwx offer ensemble forecast outputs; these are created by running a model many times with slightly different starting conditions to give a range of values for the outputs: temperature, dewpoint, cloud cover, wind and so on.”

Also, as you’re looking at cloud cover percentage forecasts, such as the ones on http://www.skippysky.com.au/NorthAmerica/ and http://clearoutside.com/ (linked in the above, under “numerical forecasts”)- that cloud cover  percentage could mean different things (using 50% as an example):

1.    That one side of the sky is completely clear, and the other side of the sky is overcast or mostly cloudy. If the side of the sky that is clear is where the sun will be at eclipse time, then you may be in good shape. Watch the timing of that forecast, though- it could mean that the overcast is moving in! Here are the approximate directions and altitudes of the sun, during totality, from different locations:

·       Oregon/Idaho: 45-50 degrees up in the E/SE

·       Wyoming/Nebraska: 55-60 degrees up in the S/SE

·       Kansas/Missouri/Illinois: 60-65 degrees up in the South

·       Tennessee/Georgia/North and South Carolina: 60 degrees up in the south

2.    It could also mean that the clouds are covering the sky in all directions, but the clouds are likely small and scattered.

3.    You may have scattered clouds in one area of the sky, but perhaps a thunderstorm or 2 in another part of the sky. ­

Articles (also by Jay Anderson):

 
http://www.kasonline.org/primefocus/2017/PF0817.pdf (weather article starts on page 14, but the other articles are interesting too!)

Reading satellite photos; identifying types of clouds:

See https://cimss.ssec.wisc.edu/satmet/modules/4_clouds/clouds-1.html#tag on how to read satellite photos and learn about how different types of clouds appear on them. See http://www.metoffice.gov.uk/learning/clouds/cloud-spotting-guide for more on identifying clouds.

Please note that most of the types of clouds on the metoffice link above will moderately to significantly degrade the view if they are in front of the Sun during totality. The exception to this is cirrus clouds- interference from them may be slight- but ONLY if they are very thin and wispy.

Of special note: fair weather cumulus clouds have a good chance to dissipate- partially or completely- before totality. These clouds look like popcorn, both from the ground and on the satellite photos. See the image below, for example. These types of clouds need heat to form, so the temperature drop helps. This assumes that there aren’t too many of them, and that the clouds have not built up to the point that they are producing precipitation. For example, I would think that most of the clouds in the image below would dissipate, but not necessarily all of them.
From Jay: “Typically, small convective clouds with their “roots” on the ground will dissipate as the shadow approaches as the ground cools far more than the atmosphere and their forcing is cut off.” 

Of course, if the clouds have built up to point that they are causing precipitation, then you'll want to avoid those- they are not likely to dissipate. See this image:




You'll want to avoid the main body of the storm (of course!) but also the thick cirrus (anvil) that blows out ahead of the storm. These will show up as bright white on satellite photos (see slide 8 on the cimms link above). Also note from the picture that each storm may be rather small: unless the storms have merged together, you may only need to drive about 20-30 miles to get the sun in the clear. Remember, though, don't drive yourself outside of that totality path: make sure you have detailed digital- and print- maps! 

There is a concern about clouds forming as the temperature drops- the most common is stratus, and fog. If stratus or fog is in your area as the eclipse begins, but starts to go away during the partial phases, it may reform as you get closer to totality. This is especially worrisome on the Oregon coast. Timing is important. From Jay: “Fog may reform if it is only a short time elapsed since it first dissipated (Baja 1991 but not Egypt 2006). If an hour or two has passed, the moisture is likely distributed through a greater depth of the atmosphere and will not reform in the duration of an eclipse. Of course, if a persistent fog bank lies nearby (offshore?), all bets are off.” 

Remember that local topography can play a role in influencing cloud cover. More from Jay: “Having said that, convective clouds of sufficient depth or amount (broken cloudiness) may also spread out into a layer of stratus, though I think that is more likely in mountain environments (i.e. 1988 in the Philippines, 1991 in Baja (see http://astropix.com/html/l_story/baja91.html - courtesy of Jerry Lodriguss).” (do note that both of those mountain ranges are surrounded by water- lots of moisture available.) A mountain range does not have to be very tall or large in size to produce local cloud cover.

Also, broad valleys can possibly reduce cloud cover, if you are on the leeward side of a mountain range. This is assuming that a frontal system or low pressure system is not over that area.

Speaking of that, you’re mainly trying to avoid cold and warm fronts, low pressure systems, and storms. The mid and high level clouds associated with weather systems are generally unaffected by eclipse cooling.

Smoke

If you are under the smoke from a forest or grass fire, your view of the eclipsed sun during totality will be compromised. The farther you are away from the thickest smoke, the less the interference. If you are under thick smoke, the corona may be seen as a pinkish color (it is normally whitish). Of course, if you're that close to the thickest smoke, then your safety may be a concern. See the "forest fires and smoke" links on the eclipsophile sites above.

Summary

The key phrase is: be as flexible to moving as you can reasonably and safely be. Take a look at these links and tips (and the last few posts here.) The final decision on what you will do on eclipse day will come on that day, or maybe the day before, but NOW is the time to discuss with your group what you may possibly do in these scenarios (both long range and short range).

Of course, I hope the entire totality path is clear and that writing this has been a complete waste of time!

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