|
Weather Tips
By Pierre Martin
Will the sky be clear tonight?
It is about sunset, and you go look outside only to see a beautiful crystal
clear blue sky. You want to go out stargazing tonight but how do you know
how good the conditions will be a few hours later? ...For a preview of
things to come, look at aircraft contrails (the trail of vapor left behind
from the hot exhaust of high altitude jet engines - usually above 30,000
feet). If an aircraft leaves behind a contrail that dissipates completely
within a few seconds, the atmosphere is dry. You will likely have an
excellent night ahead. On the other hand if the contrail persist into a long
line that seems to get thicker across the sky, a weather system is probably
approaching. You can expect some serious cloud cover in less than a few
hours. Contrails can even trigger the formation of natural clouds.
You will often hear amateur astronomers talk about the weather, and for a
very good reason! As we are planing for an evening of observing, many of us
will have our eyes glued on radar and satellite imagery, and listen to the
latest forecasts. Others will carefully observe the daytime sky for anything
that will help them determine how good the sky will be later on. The weather
can be a very critical factor to someone debating whether he/she should head
off for a 2 hours drive to a remote location. Weather forecasting is tricky.
How many times did we get fooled into thinking we had a very promising night
ahead of us with a "clear" forecast, only to be unexpectedly clouded over
after sunset. After a number of years of observing, and regularly watching
the daytime sky, I eventually learned a few simple things that can help make
my own predictions (at least I hope). I will try and include a simple
weather tip regularly, that everyone can try out and see for themselves if
it helps. To start, you will find on the web, countless weather links and
radar/satellite imagery. It's important to get the ones that show a high
enough resolution to pick up the cirrus (thin high altitude clouds) as well
as low level fog/haze. Try:
http://wwwghcc.msfc.nasa.gov/GOES/
Some forecasters don't always show you the thin layer of clouds that can
ruin a night of observing. After all, the public is only interested in
seeing the "general" weather picture without getting into too much details.
Meteorologists who don't carefully look at the different levels of humidity,
and moisture in the atmosphere may insist that it is clear, even though it
is obviously overcast to an observer.
What is the definition of "clear sky" to meteorologists? My feeling is that
there are no standard definition to the term, shared equally by all
meteorologists. To many forecasters, if some stars and the Moon can shine
through clouds, then it is considered a "clear sky". Astronomers on the
other hand will call such a night cloudy or overcast (..and lousy!). Quite
often, frustrated observers will pack up their telescope and head back home.
Stargazers picture a clear sky as completely FREE of all types of cloud
cover and fog.
To help solve this problem, Allan Rahill of the Canadian Meteorological
Centre (CMC) came up with forecasts specifically geared for astronomy. Allan
is an accomplished amateur astronomer himself. His detailed models show how
the weather will look like up to 48 hours into the future!!! Isn't that
cool, or what!!! These weather maps are available in visible clouds,
infrared and transparency. In my experience, the visible and especially the
transparency and the most useful. They are often very accurate models at
least 90% of the time! You will have to convert UTC time to local on all
these maps. See...
http://www.cmc.ec.gc.ca/cmc/htmls/mainpage.html
CMC's weather may be extremely useful but as mentioned previously, you must
remember to convert the UTC time back to local. CMC's maps are also large in
size. Most of the time, you're simply interested in knowing if it's going to
be clear at your observing site. You don't really care what the weather is
predicted to be elsewhere. To help solve these problems, Attilla Danko
created a really nifty feature called the "Clear Sky Clock". This lets the
user view CMC's predictions by using tiny squares representing the sky's
opacity, transparency and darkness. With the Clear Sky Clock, you can now
tell with one quick glance how the sky conditions will be in the next few
hours... and it really works!!!! View all the Clear Sky Clocks for a number
of OAOG observing sites, (Be sure to click on one of the graphics to see the
larger sized version, and explanations about it). The CMC and "Clear Sky
Clock" combined together are in my view one of the finest weather predicting
tools available for the amateur astronomer. I certainly highly recommend
them, so be sure to bookmark these links!
Need to know the current conditions for Ottawa, as well as the four day
forecast by Environment Canada? Try the following link. It also provides
data for the past 24 hours, as well as sunrise and sunset times...
http://weatheroffice.ec.gc.ca/city/pages/on-118_metric_e.html
You might also be interested in this link. It's a not often linked-to
satellite image that has lots of detail for the Ottawa area...
http://weather.unisys.com/satellite/sat_ir_ne.html
This animated imagery looks like a normal (somewhat fuzzy) quality weather
picture at first glance. However, it does a lot more by predicting North
America cloudcover for the following 48hrs!!! You can control the time in
UTC by moving the slider on the bottom of the image. This is an alternative
weather projection in case CMC happens to be down.
http://cimss.ssec.wisc.edu/model/daily/xsau.mpg |