Blue Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Ridge 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	14% 5% 5% 6% 5% 7% 39%	12229
	Night	15% 5% 5% 6% 43%	12162
Cloud Cover	Day	14% 5% 5% 6% 5% 7% 39%	12248
	Night	15% 5% 5% 5% 6% 43%	12185
Transparency	Day	10% 18% 26% 18%	8919
	Night	8% 16% 27% 20%	9079
Seeing	Day	5% 20% 27% 23% 13%	740
	Night	10% 27% 34% 16%	3628

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	16% 6% 5% 6% 42%	833
	Night	17% 6% 5% 5% 5% 6% 37%	1207
February	Day	13% 6% 5% 6% 5% 5% 7% 37%	764
	Night	14% 7% 5% 45%	1026
March	Day	13% 5% 6% 5% 43%	999
	Night	15% 6% 47%	998
April	Day	15% 5% 6% 6% 6% 5% 6% 35%	1079
	Night	20% 5% 6% 6% 38%	901
May	Day	15% 5% 6% 5% 6% 8% 39%	1231
	Night	11% 5% 5% 8% 45%	814
June	Day	10% 6% 6% 8% 5% 6% 8% 34%	1251
	Night	11% 8% 6% 9% 44%	718
July	Day	14% 5% 6% 5% 9% 6% 7% 8% 31%	1222
	Night	8% 5% 7% 6% 9% 9% 38%	819
August	Day	12% 5% 6% 7% 5% 8% 10% 33%	1162
	Night	10% 6% 7% 5% 7% 5% 5% 8% 36%	870
September	Day	15% 7% 5% 8% 42%	1025
	Night	16% 5% 50%	952
October	Day	15% 5% 54%	923
	Night	15% 5% 5% 53%	1111
November	Day	16% 5% 6% 5% 5% 5% 42%	993
	Night	16% 5% 5% 6% 7% 39%	1452
December	Day	17% 6% 5% 8% 41%	747
	Night	16% 5% 5% 6% 42%	1294

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	16% 7% 5% 5% 5% 41%	832
	Night	17% 6% 5% 5% 5% 6% 37%	1208
February	Day	12% 7% 7% 5% 5% 6% 38%	762
	Night	14% 6% 46%	1017
March	Day	13% 5% 6% 5% 6% 42%	1022
	Night	15% 5% 5% 46%	1020
April	Day	15% 6% 7% 6% 5% 5% 6% 35%	1078
	Night	20% 5% 5% 6% 6% 38%	901
May	Day	15% 5% 6% 6% 8% 39%	1230
	Night	11% 6% 5% 9% 44%	814
June	Day	11% 5% 6% 8% 5% 6% 8% 34%	1260
	Night	11% 7% 5% 9% 43%	720
July	Day	13% 5% 6% 5% 8% 5% 7% 9% 30%	1222
	Night	8% 6% 6% 9% 10% 37%	820
August	Day	12% 5% 6% 7% 6% 7% 10% 34%	1157
	Night	10% 5% 7% 6% 7% 5% 5% 9% 35%	872
September	Day	15% 6% 5% 8% 41%	1024
	Night	16% 5% 50%	953
October	Day	16% 5% 53%	923
	Night	15% 6% 53%	1111
November	Day	16% 6% 5% 5% 5% 43%	991
	Night	16% 5% 6% 5% 7% 39%	1451
December	Day	16% 7% 8% 41%	747
	Night	16% 5% 6% 6% 42%	1298

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	9% 12% 25% 26%	553
	Night	7% 16% 27% 23%	817
February	Day	7% 23% 24% 17%	558
	Night	8% 16% 27% 25%	758
March	Day	11% 17% 29% 16% 6%	724
	Night	8% 20% 31% 16%	741
April	Day	10% 17% 19% 15% 9%	774
	Night	9% 19% 17% 20% 8%	644
May	Day	13% 18% 28% 19%	896
	Night	12% 20% 32% 16%	620
June	Day	14% 30% 26% 11%	816
	Night	14% 22% 33% 13%	493
July	Day	13% 16% 25% 14%	649
	Night	13% 23% 23% 14%	471
August	Day	9% 15% 27% 16%	967
	Night	10% 15% 24% 16%	722
September	Day	6% 13% 21% 25%	832
	Night	6% 10% 21% 30% 5%	804
October	Day	5% 10% 31% 23% 5%	811
	Night	5% 10% 30% 24% 5%	970
November	Day	10% 19% 25% 16%	779
	Night	7% 14% 30% 18%	1088
December	Day	7% 19% 28% 16%	560
	Night	8% 17% 27% 20%	951

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	20% 39% 28%	330
February	Day	15% 26% 36% 10%	19
	Night	19% 45% 26%	291
March	Day	11% 35% 34% 11%	85
	Night	17% 39% 31%	342
April	Day	8% 21% 28% 20% 6%	82
	Night	16% 31% 27% 12%	325
May	Day	31% 26% 16% 11%	90
	Night	28% 34% 22%	249
June	Day	20% 28% 32% 17%	80
	Night	9% 53% 28%	159
July	Day	13% 23% 31% 21%	76
	Night	8% 44% 35%	224
August	Day	5% 21% 31% 30%	88
	Night	10% 37% 34%	294
September	Day	8% 20% 29% 25%	79
	Night	14% 35% 34%	323
October	Day	5% 20% 29% 24% 9%	86
	Night	9% 23% 34% 20%	345
November	Day	5% 25% 32% 18%	55
	Night	14% 33% 31% 8%	406
December	Night	8% 33% 40% 7%	340

Page generated at 2008-11-27 13:47:37. Version ab11