Sierra Stars Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sierra Stars 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	5% 8% 61%	12278
	Night	5% 9% 62%	12152
Transparency	Day	10% 16% 22% 24% 17%	10256
	Night	11% 17% 25% 24% 13%	10383
Seeing	Day	27% 41% 21%	794
	Night	9% 26% 33% 22% 6%	3790

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	6% 8% 10% 12% 42%	782
	Night	5% 9% 15% 46%	1254
February	Day	7% 5% 8% 13% 46%	782
	Night	5% 6% 12% 54%	997
March	Day	7% 5% 10% 13% 45%	1006
	Night	5% 6% 9% 13% 52%	1033
April	Day	5% 5% 6% 6% 8% 50%	1077
	Night	5% 5% 8% 9% 51%	899
May	Day	5% 5% 7% 58%	1247
	Night	9% 64%	798
June	Day	6% 73%	1260
	Night	5% 75%	720
July	Day	73%	1290
	Night	74%	751
August	Day	82%	1131
	Night	84%	906
September	Day	80%	1039
	Night	85%	936
October	Day	5% 66%	921
	Night	75%	1112
November	Day	5% 6% 10% 58%	999
	Night	11% 64%	1446
December	Day	5% 5% 6% 8% 14% 41%	744
	Night	5% 7% 6% 9% 16% 42%	1300

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	18% 32% 25% 9%	625
	Night	14% 28% 29% 15%	1067
February	Day	16% 23% 31% 13%	647
	Night	16% 24% 34% 10% 6%	808
March	Day	18% 30% 28% 14%	768
	Night	16% 31% 31% 11%	839
April	Day	16% 28% 28% 17%	775
	Night	18% 29% 24% 20%	661
May	Day	9% 15% 32% 22% 7%	1080
	Night	7% 16% 37% 20% 8%	706
June	Day	5% 12% 18% 36% 24%	1111
	Night	7% 9% 20% 34% 24%	645
July	Day	6% 14% 25% 45%	861
	Night	5% 11% 33% 43%	505
August	Day	6% 32% 48%	1087
	Night	5% 10% 37% 40%	887
September	Day	7% 18% 33% 30%	1016
	Night	6% 16% 37% 30%	927
October	Day	7% 11% 22% 34% 11%	863
	Night	5% 11% 29% 37% 7%	1048
November	Day	12% 20% 23% 25% 6%	877
	Night	14% 17% 30% 20% 7%	1301
December	Day	19% 22% 29% 11%	546
	Night	21% 24% 22% 13%	989

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	13% 47% 21% 10%	38
	Night	13% 35% 31% 13%	366
February	Day	26% 26% 30% 15%	73
	Night	13% 24% 30% 20% 8%	309
March	Day	29% 42% 21%	89
	Night	14% 35% 31% 14%	367
April	Day	38% 34% 16%	84
	Night	12% 33% 34% 12% 5%	345
May	Day	31% 39% 18%	86
	Night	6% 21% 39% 27%	237
June	Day	35% 49% 12%	87
	Night	20% 49% 26%	169
July	Day	24% 50% 24%	78
	Night	9% 46% 39%	166
August	Day	29% 57% 11%	89
	Night	21% 38% 31%	335
September	Day	14% 42% 35%	85
	Night	16% 30% 36% 12%	349
October	Day	25% 28% 29% 8%	85
	Night	6% 21% 32% 24% 12%	361
November	Night	10% 24% 30% 22% 9%	430
December	Night	15% 35% 28% 12% 7%	356

Page generated at 2008-11-27 10:47:41. Version ab11