Mount Wilson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Wilson 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	6% 5% 70%	12287
	Night	6% 5% 70%	12139
Transparency	Day	5% 9% 17% 30% 29%	10554
	Night	6% 11% 16% 27% 30%	10589
Seeing	Day	16% 41% 38%	821
	Night	19% 37% 29% 7%	3957

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	14% 63%	834
	Night	11% 64%	1202
February	Day	15% 8% 51%	809
	Night	11% 6% 7% 54%	970
March	Day	11% 5% 7% 54%	998
	Night	8% 9% 60%	1040
April	Day	11% 6% 57%	1077
	Night	10% 5% 66%	899
May	Day	5% 5% 74%	1190
	Night	7% 77%	855
June	Day	82%	1231
	Night	5% 85%	748
July	Day	5% 5% 72%	1238
	Night	6% 5% 71%	803
August	Day	85%	1108
	Night	84%	929
September	Day	85%	1023
	Night	85%	952
October	Day	77%	954
	Night	80%	1078
November	Day	6% 5% 66%	1003
	Night	6% 67%	1443
December	Day	9% 5% 9% 59%	822
	Night	8% 6% 62%	1220

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	11% 17% 21% 28%	726
	Night	11% 13% 22% 33%	1062
February	Day	9% 11% 22% 34% 12%	631
	Night	7% 13% 16% 33% 18%	787
March	Day	8% 13% 25% 28% 19%	755
	Night	7% 14% 20% 34% 18%	831
April	Day	9% 17% 24% 31% 13%	828
	Night	10% 14% 23% 28% 19%	698
May	Day	7% 15% 22% 33% 13%	1102
	Night	14% 15% 22% 24% 16%	791
June	Day	6% 14% 32% 40%	1139
	Night	8% 9% 14% 28% 38%	701
July	Day	9% 12% 27% 40%	824
	Night	6% 10% 12% 25% 38%	527
August	Day	6% 12% 37% 36%	1076
	Night	7% 14% 33% 36%	901
September	Day	9% 34% 43%	1008
	Night	5% 10% 29% 43%	931
October	Day	6% 15% 20% 46%	879
	Night	6% 13% 20% 48%	1015
November	Day	8% 18% 27% 29%	888
	Night	12% 16% 26% 30%	1327
December	Day	11% 13% 23% 25% 20%	698
	Night	7% 11% 20% 29% 22%	1018

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	30% 44% 13%	52
	Night	6% 28% 42% 9%	361
February	Day	24% 36% 32%	77
	Night	6% 20% 40% 22%	305
March	Day	29% 42% 23%	84
	Night	7% 30% 39% 18%	361
April	Day	31% 44% 23%	85
	Night	29% 39% 21%	343
May	Day	19% 36% 38%	92
	Night	22% 37% 31% 5%	294
June	Day	6% 45% 46%	90
	Night	12% 36% 43% 7%	196
July	Day	20% 69% 5%	79
	Night	18% 60% 16%	221
August	Day	39% 59%	89
	Night	17% 55% 21%	356
September	Day	54% 39%	88
	Night	6% 32% 44% 15%	350
October	Day	21% 44% 30%	85
	Night	17% 42% 25% 7%	366
November	Night	21% 40% 28%	443
December	Night	6% 22% 49% 14%	361

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