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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 2003-08-01 to 2005-11-07.

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	20% 6% 5% 5% 6% 5% 5% 7% 29%	8723
	Night	20% 7% 6% 6% 5% 7% 30%	8445
Transparency	Day	26% 20% 36% 14%	4083
	Night	24% 19% 38% 16%	4042
Seeing	Day	9% 26% 29% 23% 11%	582
	Night	13% 33% 31% 19%	2560

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	28% 8% 9% 6% 7% 5% 5% 18%	494
	Night	30% 8% 7% 5% 6% 20%	859
February	Day	26% 11% 10% 6% 5% 8% 5% 5% 16%	546
	Night	24% 11% 8% 5% 5% 5% 6% 21%	707
March	Day	29% 11% 7% 6% 5% 6% 7% 15%	662
	Night	22% 12% 7% 7% 6% 6% 6% 19%	702
April	Day	18% 5% 7% 6% 7% 34%	754
	Night	17% 7% 5% 5% 5% 6% 7% 36%	568
May	Day	23% 7% 6% 5% 5% 6% 6% 5% 7% 24%	817
	Night	17% 6% 5% 5% 7% 5% 5% 8% 31%	535
June	Day	14% 5% 6% 5% 6% 9% 36%	872
	Night	12% 5% 6% 6% 48%	441
July	Day	13% 9% 5% 7% 11% 31%	862
	Night	14% 5% 6% 5% 8% 39%	500
August	Day	14% 5% 5% 5% 6% 6% 6% 7% 36%	1199
	Night	9% 6% 7% 7% 10% 39%	843
September	Day	9% 7% 5% 6% 7% 47%	987
	Night	8% 7% 6% 6% 7% 53%	955
October	Day	22% 7% 6% 5% 5% 7% 8% 24%	611
	Night	21% 7% 6% 5% 5% 7% 32%	751
November	Day	33% 8% 5% 5% 5% 6% 21%	464
	Night	33% 6% 5% 6% 9% 16%	702
December	Day	34% 9% 8% 5% 5% 20%	455
	Night	25% 11% 8% 7% 5% 7% 5% 5% 16%	882

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	44% 24% 30%	126
	Night	39% 15% 36% 8%	249
February	Day	47% 24% 17% 9%	151
	Night	35% 27% 33%	224
March	Day	43% 26% 26%	189
	Night	30% 22% 42% 5%	240
April	Day	24% 17% 35% 20%	370
	Night	20% 9% 34% 32%	293
May	Day	27% 31% 31% 8%	316
	Night	27% 20% 41% 11%	250
June	Day	24% 13% 46% 14%	455
	Night	19% 10% 48% 20%	274
July	Day	22% 16% 41% 16%	437
	Night	19% 14% 44% 18%	276
August	Day	22% 24% 37% 15%	633
	Night	18% 21% 36% 23%	479
September	Day	18% 16% 38% 25%	650
	Night	12% 19% 39% 24%	701
October	Day	24% 23% 40% 10%	411
	Night	27% 22% 39% 9%	516
November	Day	40% 25% 29%	208
	Night	32% 28% 29% 8%	306
December	Day	26% 22% 35% 16%	137
	Night	38% 16% 35% 9%	234

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	22% 44% 27%	226
February	Night	18% 38% 28% 12%	180
March	Day	15% 34% 25% 25%	32
	Night	22% 39% 25% 10%	211
April	Day	12% 25% 31% 27%	48
	Night	15% 34% 36% 14%	157
May	Day	14% 34% 29% 14% 7%	64
	Night	13% 28% 44% 13%	162
June	Day	10% 34% 37% 12% 5%	108
	Night	7% 30% 39% 22%	139
July	Day	9% 22% 33% 21% 12%	110
	Night	5% 22% 40% 29%	165
August	Day	24% 22% 22% 26%	86
	Night	25% 33% 31% 5%	263
September	Day	14% 27% 34% 19%	83
	Night	25% 31% 36%	316
October	Day	13% 23% 19% 43%	51
	Night	14% 32% 31% 21%	326
November	Night	18% 41% 29% 9%	184
December	Night	24% 40% 23% 10%	231

Page generated at 2008-09-02 02:07:33. Version ab11