Champaign/Urbana Astronomical Society Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Champaign/Urbana Astronomical Society Observatory clear sky chart from 2003-08-01 to 2005-11-07.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	17% 5% 5% 6% 5% 8% 36%	8728
	Night	21% 5% 5% 7% 37%	8444
Cloud Cover	Day	17% 5% 5% 6% 5% 8% 35%	8727
	Night	21% 5% 5% 7% 36%	8446
Transparency	Day	18% 16% 39% 22%	4739
	Night	16% 14% 42% 23%	4501
Seeing	Day	7% 23% 28% 28% 11%	578
	Night	11% 28% 36% 21%	2485

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	31% 10% 10% 5% 6% 18%	502
	Night	29% 6% 7% 5% 6% 6% 22%	851
February	Day	24% 8% 9% 5% 6% 27%	559
	Night	27% 8% 7% 5% 6% 29%	695
March	Day	21% 6% 6% 6% 5% 6% 6% 29%	672
	Night	24% 8% 5% 6% 6% 5% 28%	694
April	Day	17% 5% 7% 5% 7% 37%	726
	Night	23% 5% 42%	594
May	Day	16% 6% 5% 11% 37%	830
	Night	14% 5% 5% 5% 5% 5% 11% 40%	523
June	Day	11% 5% 8% 6% 9% 36%	840
	Night	9% 5% 5% 6% 6% 11% 42%	476
July	Day	12% 9% 5% 6% 10% 36%	866
	Night	13% 6% 6% 5% 8% 41%	496
August	Day	13% 7% 5% 7% 9% 39%	1145
	Night	11% 7% 5% 7% 46%	895
September	Day	9% 6% 5% 8% 51%	1012
	Night	11% 7% 58%	930
October	Day	20% 6% 5% 5% 5% 36%	614
	Night	23% 5% 5% 7% 35%	747
November	Day	36% 6% 5% 5% 27%	473
	Night	41% 5% 27%	694
December	Day	24% 10% 7% 5% 35%	489
	Night	27% 9% 5% 5% 5% 28%	849

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	30% 9% 11% 6% 19%	502
	Night	29% 6% 7% 5% 6% 7% 22%	851
February	Day	24% 9% 9% 5% 6% 27%	559
	Night	26% 8% 7% 5% 6% 30%	695
March	Day	21% 5% 6% 5% 6% 5% 8% 28%	672
	Night	25% 7% 7% 5% 5% 28%	694
April	Day	17% 5% 7% 7% 37%	726
	Night	23% 5% 5% 42%	594
May	Day	15% 6% 6% 11% 37%	830
	Night	13% 5% 6% 5% 5% 11% 40%	523
June	Day	10% 6% 8% 6% 9% 36%	840
	Night	7% 6% 5% 6% 7% 11% 41%	477
July	Day	11% 8% 6% 6% 10% 35%	863
	Night	13% 5% 6% 6% 9% 40%	496
August	Day	12% 7% 5% 6% 9% 39%	1146
	Night	11% 7% 6% 7% 46%	895
September	Day	8% 6% 5% 8% 51%	1012
	Night	11% 7% 57%	931
October	Day	21% 6% 5% 6% 6% 35%	614
	Night	22% 5% 6% 36%	747
November	Day	36% 6% 5% 5% 27%	474
	Night	41% 5% 27%	694
December	Day	23% 10% 6% 5% 5% 35%	489
	Night	27% 9% 5% 5% 28%	849

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	52% 13% 28% 5%	138
	Night	31% 21% 35% 11%	294
February	Day	17% 14% 43% 21%	216
	Night	14% 15% 52% 16%	273
March	Day	20% 20% 50% 9%	294
	Night	21% 13% 46% 18%	273
April	Day	20% 13% 46% 15%	388
	Night	14% 10% 46% 25%	328
May	Day	23% 21% 39% 15%	468
	Night	26% 15% 42% 14%	311
June	Day	18% 12% 45% 23%	472
	Night	14% 16% 43% 23%	285
July	Day	20% 14% 40% 23%	478
	Night	22% 13% 34% 25%	268
August	Day	17% 26% 36% 17%	643
	Night	17% 15% 40% 24%	556
September	Day	8% 12% 28% 40% 9%	698
	Night	5% 11% 36% 35% 11%	687
October	Day	14% 12% 40% 27%	509
	Night	10% 14% 38% 31% 5%	589
November	Day	18% 12% 46% 21%	216
	Night	15% 6% 55% 21%	296
December	Day	28% 18% 31% 21%	219
	Night	23% 21% 44% 10%	341

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Night	14% 50% 26% 7%	226
February	Night	10% 46% 35% 7%	176
March	Day	26% 52% 15% 5%	19
	Night	20% 39% 31% 7%	173
April	Day	7% 36% 38% 17%	52
	Night	13% 30% 41% 13%	145
May	Day	18% 39% 19% 21%	82
	Night	8% 32% 47% 10%	170
June	Day	29% 26% 28% 10%	111
	Night	19% 47% 28%	172
July	Day	18% 33% 32% 11%	112
	Night	8% 7% 49% 32%	168
August	Day	5% 10% 23% 34% 26%	96
	Night	11% 30% 45% 8%	259
September	Day	10% 32% 34% 18%	83
	Night	5% 14% 35% 38% 6%	291
October	Day	17% 26% 39% 13%	23
	Night	13% 22% 37% 24%	319
November	Night	18% 37% 34% 8%	166
December	Night	20% 37% 32% 9%	220

Page generated at 2008-05-07 18:24:10. Version ab11