Sierra Remote Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sierra Remote Observatories 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	9% 6% 63%	12276
	Night	9% 5% 63%	12154
Transparency	Day	13% 25% 30% 16%	9840
	Night	15% 24% 28% 17%	9989
Seeing	Day	18% 35% 37%	800
	Night	9% 34% 34% 12%	3730

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	17% 7% 7% 47%	790
	Night	14% 6% 6% 53%	1247
February	Day	19% 9% 5% 5% 7% 39%	796
	Night	18% 5% 5% 5% 8% 42%	983
March	Day	15% 9% 5% 5% 6% 40%	1005
	Night	12% 5% 7% 51%	1034
April	Day	16% 5% 5% 7% 42%	1077
	Night	17% 5% 5% 51%	899
May	Day	9% 7% 65%	1221
	Night	5% 5% 69%	823
June	Day	6% 76%	1260
	Night	5% 78%	720
July	Day	6% 76%	1282
	Night	6% 73%	760
August	Day	91%	1116
	Night	90%	921
September	Day	87%	1035
	Night	89%	940
October	Day	6% 5% 68%	931
	Night	6% 76%	1101
November	Day	9% 5% 5% 56%	1003
	Night	9% 7% 60%	1443
December	Day	19% 8% 42%	760
	Night	18% 8% 46%	1283

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	8% 17% 29% 16% 6%	627
	Night	5% 17% 23% 25% 7%	1064
February	Day	6% 18% 37% 11% 6%	530
	Night	6% 20% 34% 12% 9%	712
March	Day	9% 26% 28% 23%	643
	Night	6% 22% 37% 22%	743
April	Day	7% 27% 35% 20%	660
	Night	6% 28% 35% 22%	614
May	Day	20% 39% 23%	1040
	Night	18% 36% 27% 6%	715
June	Day	13% 20% 43% 19%	1109
	Night	13% 16% 45% 20%	643
July	Day	5% 19% 31% 33%	878
	Night	5% 17% 33% 36%	502
August	Day	8% 50% 36%	1087
	Night	9% 43% 40%	904
September	Day	20% 42% 26%	1024
	Night	19% 41% 29%	919
October	Day	9% 21% 29% 21%	861
	Night	8% 22% 28% 25%	1046
November	Day	6% 15% 28% 23% 11%	838
	Night	18% 24% 25% 15%	1211
December	Day	8% 25% 30% 15%	543
	Night	29% 26% 17% 7%	916

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	28% 33% 17% 6%	45
	Night	5% 18% 41% 16% 7%	355
February	Day	21% 23% 23% 17%	69
	Night	9% 40% 25% 12%	285
March	Day	29% 46% 17%	84
	Night	6% 12% 45% 24% 8%	341
April	Day	36% 36% 21%	79
	Night	8% 15% 38% 26% 9%	319
May	Day	18% 36% 33%	90
	Night	11% 26% 44% 10%	261
June	Day	17% 34% 43%	90
	Night	5% 28% 50% 14%	169
July	Day	5% 26% 63%	79
	Night	11% 59% 25%	180
August	Day	39% 57%	89
	Night	18% 66% 13%	348
September	Day	12% 36% 48%	88
	Night	29% 46% 17%	350
October	Day	16% 35% 35% 8%	87
	Night	10% 34% 29% 18%	361
November	Night	9% 38% 30% 12%	428
December	Night	8% 9% 48% 18% 9%	333

Page generated at 2008-11-27 10:47:49. Version ab11