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George Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the George Observatory clear sky chart from 2005-11-05 to 2008-11-27.

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	13% 5% 6% 6% 7% 5% 5% 8% 33%	12277
	Night	11% 5% 5% 8% 45%	12157
Transparency	Day	10% 14% 20% 18% 8%	8504
	Night	8% 18% 22% 19% 12%	9183
Seeing	Day	14% 32% 29% 15%	636
	Night	6% 21% 33% 28%	3653

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	26% 6% 6% 35%	833
	Night	27% 6% 37%	1207
February	Day	20% 6% 5% 42%	825
	Night	17% 5% 5% 47%	956
March	Day	14% 7% 7% 5% 5% 7% 36%	1022
	Night	12% 6% 5% 7% 43%	1021
April	Day	11% 7% 7% 5% 7% 5% 5% 6% 36%	1037
	Night	13% 5% 6% 6% 9% 39%	939
May	Day	10% 6% 6% 7% 8% 5% 6% 10% 30%	1184
	Night	7% 5% 5% 5% 6% 11% 45%	860
June	Day	6% 7% 8% 13% 7% 8% 10% 26%	1170
	Night	6% 6% 8% 12% 47%	810
July	Day	13% 7% 7% 8% 9% 5% 10% 7% 7% 9% 14%	1209
	Night	7% 5% 8% 6% 8% 13% 33%	834
August	Day	8% 5% 9% 7% 5% 10% 7% 7% 10% 23%	1134
	Night	5% 7% 7% 12% 45%	897
September	Day	15% 7% 7% 6% 5% 5% 8% 29%	987
	Night	9% 5% 5% 5% 6% 10% 41%	990
October	Day	11% 5% 5% 6% 48%	935
	Night	9% 6% 57%	1096
November	Day	10% 5% 5% 6% 7% 45%	1104
	Night	8% 5% 5% 5% 56%	1339
December	Day	15% 6% 5% 5% 5% 40%	837
	Night	16% 6% 5% 5% 5% 5% 6% 41%	1208

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	5% 7% 15% 27% 13%	554
	Night	6% 16% 24% 18%	795
February	Day	6% 8% 22% 28% 14%	555
	Night	6% 11% 17% 30% 20%	690
March	Day	8% 18% 22% 18% 10%	677
	Night	8% 20% 22% 19% 14%	712
April	Day	11% 14% 22% 18% 13%	695
	Night	8% 20% 21% 19% 12%	663
May	Day	13% 18% 22% 21%	783
	Night	9% 21% 30% 19%	648
June	Day	15% 23% 20% 19%	771
	Night	11% 31% 24% 20%	637
July	Day	18% 21% 22%	527
	Night	17% 36% 23%	433
August	Day	15% 17% 19%	908
	Night	14% 31% 27%	803
September	Day	9% 14% 15% 14% 7%	807
	Night	9% 20% 20% 13% 7%	871
October	Day	9% 15% 28% 19%	758
	Night	5% 8% 21% 24% 25%	916
November	Day	7% 9% 25% 19% 15%	901
	Night	6% 15% 25% 22% 13%	1174
December	Day	8% 10% 23% 27% 10%	568
	Night	8% 14% 19% 24% 14%	841

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	13% 31% 31% 11%	399
February	Night	11% 41% 35% 6%	277
March	Night	12% 32% 39% 10%	257
April	Day	8% 18% 35% 29%	48
	Night	9% 32% 45% 7%	253
May	Day	5% 29% 40% 18%	152
	Night	26% 47% 17%	273
June	Day	8% 36% 36% 13%	169
	Night	6% 39% 47% 6%	252
July	Day	11% 30% 27% 24%	150
	Night	16% 58% 17%	236
August	Day	19% 39% 27%	117
	Night	9% 67% 16%	264
September	Night	8% 20% 52% 10%	258
October	Night	5% 13% 32% 37% 6%	334
November	Night	5% 19% 36% 29%	438
December	Night	10% 32% 36% 15%	412

Page generated at 2008-11-27 12:47:45. Version ab11