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 2009-12-29.

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% 5% 69%	16528
	Night	6% 5% 70%	16424
Transparency	Day	5% 8% 17% 29% 27%	14793
	Night	5% 9% 16% 27% 29%	14880
Seeing	Day	16% 42% 36%	1103
	Night	18% 36% 29% 7%	5388

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	13% 65%	1105
	Night	10% 67%	1587
February	Day	18% 7% 47%	1089
	Night	14% 5% 7% 52%	1306
March	Day	11% 5% 7% 56%	1333
	Night	7% 8% 62%	1387
April	Day	9% 6% 61%	1414
	Night	8% 5% 66%	1189
May	Day	6% 73%	1576
	Night	7% 75%	1122
June	Day	6% 72%	1631
	Night	6% 77%	997
July	Day	75%	1637
	Night	5% 73%	1052
August	Day	84%	1480
	Night	83%	1239
September	Day	86%	1353
	Night	86%	1249
October	Day	5% 78%	1264
	Night	81%	1428
November	Day	5% 68%	1300
	Night	5% 69%	1872
December	Day	11% 8% 56%	1346
	Night	8% 6% 58%	1996

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	9% 14% 19% 34%	997
	Night	10% 12% 20% 38%	1448
February	Day	7% 9% 20% 26% 12%	911
	Night	6% 11% 16% 27% 18%	1123
March	Day	6% 12% 24% 28% 17%	1081
	Night	6% 12% 23% 31% 17%	1178
April	Day	7% 13% 24% 30% 14%	1163
	Night	8% 11% 22% 27% 19%	988
May	Day	7% 14% 21% 35% 11%	1495
	Night	12% 14% 21% 28% 14%	1063
June	Day	5% 6% 13% 26% 34%	1539
	Night	8% 13% 15% 21% 32%	950
July	Day	7% 16% 30% 37%	1223
	Night	9% 15% 27% 35%	776
August	Day	6% 11% 38% 36%	1448
	Night	7% 13% 36% 33%	1211
September	Day	10% 38% 38%	1338
	Night	5% 11% 33% 39%	1228
October	Day	5% 14% 27% 41%	1191
	Night	5% 12% 25% 45%	1365
November	Day	7% 17% 26% 31%	1185
	Night	10% 16% 24% 33%	1756
December	Day	8% 10% 20% 24% 17%	1222
	Night	6% 8% 18% 27% 20%	1794

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	27% 42% 15%	70
	Night	6% 30% 42% 9%	481
February	Day	25% 37% 25%	105
	Night	6% 21% 40% 18%	417
March	Day	29% 41% 23%	115
	Night	6% 29% 38% 20%	485
April	Day	32% 47% 19%	112
	Night	5% 26% 40% 22%	459
May	Day	15% 42% 38%	123
	Night	20% 38% 32% 5%	387
June	Day	10% 42% 40%	120
	Night	12% 38% 40% 5%	263
July	Day	26% 66%	109
	Night	17% 63% 14%	296
August	Day	40% 55%	120
	Night	19% 55% 19%	480
September	Day	54% 40%	117
	Night	5% 28% 46% 16%	462
October	Day	20% 45% 27%	112
	Night	18% 40% 26% 7%	486
November	Night	19% 41% 28%	575
December	Night	7% 22% 45% 14%	597

Page generated at 2009-12-30 02:20:35. Version ab11