Post by mrjamie on Jan 1, 2008 10:56:35 GMT -6
As most of you have guessed I am into astronomy. One of the tools we use to help forecast the weather is the clear sky clock.
www.cleardarksky.com/csk/prov/Louisiana_clocks.html
The link above will show all the clock in the state of Louisiana, along with a preview of the weather for the next 48 hours. 24 hours out they do have a good success rate.
For us it is not just a clear sky that is needed but a steady sky. We look for two things, transparency and seeing.
Transparency
Here 'transparency' means just what astronomers mean by the word: the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in the air. It is somewhat independant of the cloud cover forecast in that there can be isolated clouds in a transparent air mass, and poor transparency can occur when there is very little cloud.
Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clusters and planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poor transparency.
Seeing
The line, labeled Seeing, forecasts astronomical seeing. (It's an experimental forecast.) Excellent seeing means at high magnification you will see fine detail on planets. In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies is probably undiminished. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. This forecast attempts to predict turbulence and temperature differences that affect seeing for all altitudes.
Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It's because seeing is not very related to the water vapor content of the air.
The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range. There are some more details in CMC's seeing forecast page.
There are gaps in the line of seeing blocks because CMC's seeing model does not consider daytime heating, so the forecast is only available for the night. Seeing is forecast for 3-hour blocks, so triples of seeing blocks will show the same color. A white block on the seeing line means that there was too much cloud (>80% cover) to calculate it.
Note also that you may observe worse seeing though your telescope than what a perfect seeing forecast would predict. That is because tube currents and ground seeing mimic true atmospheric seeing. You may also observe better seeing than predicted here when observing with an instrument smaller than 11 inches.
in the graph above the first line is cloud cover. dark blue means no clouds.
the second line is the transparency which are dark blue to light blue.
If the third line would have come across you would see the seeing is bad tonight. For me most of the time I use an 8 inch telescope, I can get away with some bad seeing but tonight it looks to be to bad for planets and a few other objects.
Since the moon does not rise to after midnight tonight would give me about 5 hours of a very good night and I could be in for around midnight or 1am before it gets to cold. Tonight would be used to viewing deep sky objects, galaxy or open or globular clusters. Along with a host of other objects.
Jamie
www.cleardarksky.com/csk/prov/Louisiana_clocks.html
The link above will show all the clock in the state of Louisiana, along with a preview of the weather for the next 48 hours. 24 hours out they do have a good success rate.
For us it is not just a clear sky that is needed but a steady sky. We look for two things, transparency and seeing.
Transparency
Here 'transparency' means just what astronomers mean by the word: the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in the air. It is somewhat independant of the cloud cover forecast in that there can be isolated clouds in a transparent air mass, and poor transparency can occur when there is very little cloud.
Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clusters and planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poor transparency.
Seeing
The line, labeled Seeing, forecasts astronomical seeing. (It's an experimental forecast.) Excellent seeing means at high magnification you will see fine detail on planets. In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies is probably undiminished. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. This forecast attempts to predict turbulence and temperature differences that affect seeing for all altitudes.
Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It's because seeing is not very related to the water vapor content of the air.
The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range. There are some more details in CMC's seeing forecast page.
There are gaps in the line of seeing blocks because CMC's seeing model does not consider daytime heating, so the forecast is only available for the night. Seeing is forecast for 3-hour blocks, so triples of seeing blocks will show the same color. A white block on the seeing line means that there was too much cloud (>80% cover) to calculate it.
Note also that you may observe worse seeing though your telescope than what a perfect seeing forecast would predict. That is because tube currents and ground seeing mimic true atmospheric seeing. You may also observe better seeing than predicted here when observing with an instrument smaller than 11 inches.
in the graph above the first line is cloud cover. dark blue means no clouds.
the second line is the transparency which are dark blue to light blue.
If the third line would have come across you would see the seeing is bad tonight. For me most of the time I use an 8 inch telescope, I can get away with some bad seeing but tonight it looks to be to bad for planets and a few other objects.
Since the moon does not rise to after midnight tonight would give me about 5 hours of a very good night and I could be in for around midnight or 1am before it gets to cold. Tonight would be used to viewing deep sky objects, galaxy or open or globular clusters. Along with a host of other objects.
Jamie