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Sunset Astronomical Society
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Astronomical Society 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	16% 7% 5% 6% 5% 6% 5% 5% 7% 31%	12235
	Night	19% 8% 6% 5% 5% 5% 6% 31%	12154
Cloud Cover	Day	15% 8% 5% 6% 5% 5% 5% 5% 7% 31%	12253
	Night	19% 9% 6% 5% 5% 6% 30%	12177
Transparency	Day	8% 20% 25% 12%	8189
	Night	7% 19% 24% 13%	8014
Seeing	Day	8% 24% 30% 21% 8%	766
	Night	10% 32% 33% 11%	3575

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	24% 10% 10% 9% 7% 8% 5% 5% 9%	747
	Night	29% 11% 8% 8% 5% 5% 5% 14%	1294
February	Day	17% 14% 11% 9% 6% 6% 6% 5% 15%	776
	Night	19% 12% 7% 7% 5% 6% 5% 8% 20%	1015
March	Day	17% 11% 6% 7% 6% 6% 7% 24%	970
	Night	17% 9% 6% 6% 6% 6% 31%	1026
April	Day	15% 8% 5% 5% 5% 6% 37%	1128
	Night	21% 5% 5% 6% 37%	851
May	Day	16% 5% 5% 6% 6% 9% 35%	1231
	Night	17% 5% 5% 10% 38%	815
June	Day	11% 6% 7% 5% 7% 10% 37%	1299
	Night	11% 5% 5% 6% 6% 5% 7% 39%	667
July	Day	6% 5% 6% 6% 6% 6% 9% 41%	1296
	Night	7% 6% 5% 6% 9% 46%	747
August	Day	12% 5% 5% 5% 6% 44%	1190
	Night	10% 5% 6% 51%	845
September	Day	16% 5% 5% 6% 9% 39%	1005
	Night	16% 5% 6% 46%	969
October	Day	18% 6% 6% 6% 6% 7% 32%	911
	Night	20% 5% 5% 5% 5% 36%	1122
November	Day	22% 14% 6% 6% 6% 7% 19%	986
	Night	23% 13% 6% 5% 6% 5% 5% 22%	1459
December	Day	23% 14% 11% 8% 5% 17%	696
	Night	27% 16% 10% 6% 5% 5% 5% 12%	1344

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	23% 10% 10% 10% 6% 8% 5% 5% 10%	746
	Night	28% 11% 8% 8% 5% 6% 14%	1294
February	Day	16% 14% 10% 9% 6% 6% 5% 16%	774
	Night	19% 12% 8% 7% 6% 5% 8% 20%	1006
March	Day	17% 11% 7% 7% 5% 6% 7% 23%	991
	Night	18% 9% 8% 5% 5% 5% 30%	1050
April	Day	16% 8% 5% 5% 5% 7% 36%	1127
	Night	21% 6% 5% 6% 37%	851
May	Day	16% 7% 5% 5% 9% 35%	1230
	Night	17% 5% 5% 11% 38%	815
June	Day	11% 5% 6% 5% 6% 9% 38%	1309
	Night	10% 5% 5% 5% 5% 6% 8% 39%	668
July	Day	6% 5% 6% 6% 6% 6% 9% 41%	1296
	Night	7% 6% 6% 9% 47%	748
August	Day	12% 6% 5% 5% 6% 45%	1185
	Night	10% 5% 6% 51%	846
September	Day	17% 5% 5% 9% 39%	1004
	Night	15% 5% 7% 46%	970
October	Day	17% 7% 5% 6% 5% 7% 32%	911
	Night	19% 7% 5% 5% 5% 36%	1122
November	Day	22% 15% 6% 7% 6% 6% 19%	984
	Night	22% 14% 5% 6% 5% 5% 5% 22%	1459
December	Day	22% 14% 11% 8% 5% 17%	696
	Night	27% 14% 12% 6% 5% 12%	1348

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	12% 18% 12%	363
	Night	9% 23% 14%	678
February	Day	10% 18% 18% 5%	419
	Night	11% 17% 20% 6%	619
March	Day	9% 28% 23%	580
	Night	6% 25% 26% 9%	632
April	Day	8% 19% 32% 18%	736
	Night	5% 18% 28% 23%	563
May	Day	9% 23% 29% 15%	839
	Night	6% 18% 32% 21%	573
June	Day	9% 24% 31% 13%	966
	Night	7% 22% 27% 17%	471
July	Day	8% 22% 26% 14% 5%	724
	Night	9% 23% 34% 15%	431
August	Day	14% 28% 20%	1017
	Night	13% 28% 27%	724
September	Day	19% 30% 14%	818
	Night	5% 16% 33% 16%	787
October	Day	6% 15% 21% 16%	712
	Night	5% 13% 28% 15%	897
November	Day	8% 18% 16% 8%	648
	Night	8% 23% 17% 6%	969
December	Day	13% 27% 15%	367
	Night	11% 23% 12%	670

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	Night	15% 40% 26%	378
February	Night	14% 43% 24% 5%	288
March	Day	9% 38% 29% 11%	54
	Night	11% 40% 29% 7%	335
April	Day	9% 28% 36% 17%	75
	Night	11% 34% 36% 8%	237
May	Day	16% 28% 19% 24% 7%	104
	Night	6% 31% 39% 14%	248
June	Day	10% 29% 26% 21% 7%	165
	Night	6% 20% 42% 24%	195
July	Day	5% 25% 33% 19% 11%	148
	Night	21% 48% 19%	233
August	Day	6% 37% 24% 16%	86
	Night	18% 48% 22%	257
September	Day	18% 31% 27% 8%	83
	Night	5% 18% 38% 22%	307
October	Day	13% 17% 31% 19%	51
	Night	10% 31% 33% 11%	336
November	Night	14% 33% 28% 6%	392
December	Night	14% 41% 24%	369

Page generated at 2008-11-27 13:47:42. Version ab11