Blackrock Ranger Station
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackrock Ranger Station 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	7% 64%	12279
	Night	5% 5% 7% 64%	12149
Transparency	Day	6% 11% 20% 28% 26%	10743
	Night	7% 11% 21% 30% 22%	10719
Seeing	Day	28% 41% 25%	780
	Night	10% 26% 31% 23% 6%	3898

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	5% 8% 8% 56%	794
	Night	5% 7% 58%	1241
February	Day	5% 6% 7% 5% 9% 8% 44%	809
	Night	5% 8% 10% 8% 47%	970
March	Day	5% 7% 8% 6% 6% 50%	1000
	Night	6% 5% 7% 7% 51%	1039
April	Day	5% 7% 59%	1077
	Night	10% 6% 59%	899
May	Day	7% 66%	1199
	Night	7% 69%	846
June	Day	5% 8% 74%	1260
	Night	8% 75%	720
July	Day	5% 7% 62%	1275
	Night	7% 66%	766
August	Day	83%	1108
	Night	87%	929
September	Day	79%	1026
	Night	86%	949
October	Day	6% 72%	940
	Night	76%	1092
November	Day	5% 7% 66%	1003
	Night	6% 5% 10% 60%	1443
December	Day	7% 6% 5% 11% 52%	788
	Night	6% 8% 8% 10% 51%	1255

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	9% 20% 26% 25% 5%	702
	Night	7% 16% 18% 32% 10%	1128
February	Day	12% 19% 34% 17%	668
	Night	15% 19% 27% 20% 9%	818
March	Day	16% 15% 30% 24% 8%	799
	Night	14% 14% 29% 27% 9%	850
April	Day	10% 17% 31% 26% 10%	888
	Night	10% 13% 34% 29% 9%	739
May	Day	5% 11% 24% 39% 11%	1098
	Night	10% 29% 37% 12%	782
June	Day	5% 13% 32% 44%	1184
	Night	6% 15% 34% 39%	669
July	Day	6% 7% 11% 17% 48%	847
	Night	6% 8% 10% 17% 50%	514
August	Day	5% 25% 59%	1086
	Night	7% 25% 58%	912
September	Day	11% 33% 44%	1013
	Night	9% 40% 41%	930
October	Day	6% 15% 37% 29%	900
	Night	5% 18% 38% 28%	1037
November	Day	5% 13% 29% 27% 14%	914
	Night	8% 14% 24% 29% 15%	1323
December	Day	12% 21% 31% 22% 6%	644
	Night	12% 22% 29% 23% 7%	1017

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	Day	42% 31% 10%	38
	Night	15% 31% 30% 13%	361
February	Day	28% 30% 28%	73
	Night	14% 28% 25% 18% 8%	312
March	Day	39% 41% 11%	86
	Night	20% 35% 26% 11%	364
April	Day	59% 25% 13%	76
	Night	19% 37% 23% 16%	348
May	Day	29% 40% 22%	88
	Night	13% 27% 36% 18%	281
June	Day	24% 54% 19%	87
	Night	23% 45% 24%	168
July	Day	10% 46% 41%	79
	Night	7% 44% 40% 6%	186
August	Day	11% 56% 30%	88
	Night	10% 38% 40% 8%	356
September	Day	19% 40% 35%	87
	Night	14% 24% 42% 13%	350
October	Day	30% 34% 28%	78
	Night	7% 28% 28% 25% 9%	364
November	Night	9% 22% 35% 22% 7%	442
December	Night	13% 37% 27% 15%	366

Page generated at 2008-11-27 10:47:51. Version ab11