Spot Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spot Observatory clear sky chart from 2005-11-05 to 2008-11-20.

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	14% 5% 5% 5% 8% 40%	12220
	Night	13% 5% 5% 5% 7% 43%	12069
Transparency	Day	6% 16% 27% 22%	8914
	Night	6% 15% 26% 24% 5%	8935
Seeing	Day	5% 18% 30% 29% 10%	701
	Night	11% 28% 31% 18%	3584

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	18% 7% 5% 5% 6% 38%	805
	Night	21% 7% 5% 5% 5% 35%	1234
February	Day	20% 8% 5% 6% 5% 33%	809
	Night	20% 9% 5% 6% 6% 34%	970
March	Day	14% 6% 6% 6% 5% 5% 8% 34%	1001
	Night	11% 7% 5% 6% 7% 9% 36%	1044
April	Day	14% 5% 5% 7% 44%	1077
	Night	17% 7% 48%	899
May	Day	14% 5% 5% 5% 8% 42%	1204
	Night	10% 5% 5% 8% 46%	841
June	Day	9% 6% 6% 12% 44%	1245
	Night	6% 6% 9% 10% 50%	734
July	Day	9% 5% 6% 7% 10% 5% 5% 11% 30%	1272
	Night	6% 5% 8% 5% 8% 11% 39%	771
August	Day	8% 5% 6% 7% 13% 40%	1102
	Night	5% 5% 5% 5% 7% 10% 47%	930
September	Day	13% 5% 5% 6% 5% 8% 39%	1019
	Night	12% 6% 5% 7% 5% 7% 42%	955
October	Day	14% 55%	944
	Night	14% 6% 56%	1090
November	Day	18% 7% 5% 46%	937
	Night	15% 5% 5% 5% 48%	1363
December	Day	23% 7% 6% 43%	805
	Night	19% 6% 5% 5% 5% 5% 41%	1238

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	13% 34% 23%	532
	Night	16% 22% 29%	767
February	Day	22% 23% 17%	526
	Night	6% 18% 30% 14%	649
March	Day	8% 24% 25% 15%	675
	Night	8% 23% 32% 11%	723
April	Day	5% 12% 31% 26% 6%	794
	Night	15% 28% 32% 5%	654
May	Day	7% 20% 27% 20%	871
	Night	9% 17% 24% 25% 5%	619
June	Day	8% 17% 30% 26%	920
	Night	12% 19% 29% 25%	581
July	Day	9% 19% 22% 15% 7%	674
	Night	9% 20% 23% 21% 8%	444
August	Day	7% 17% 26% 16% 5%	948
	Night	8% 16% 28% 19%	821
September	Day	8% 10% 19% 24%	874
	Night	6% 10% 18% 27% 7%	788
October	Day	9% 25% 31% 5%	826
	Night	8% 26% 32% 8%	946
November	Day	5% 11% 29% 26% 6%	695
	Night	12% 29% 22% 10%	1051
December	Day	20% 33% 17%	579
	Night	6% 16% 28% 22%	892

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	26% 39% 21%	363
February	Night	23% 47% 15%	263
March	Day	10% 37% 37% 8%	37
	Night	19% 40% 29%	312
April	Day	15% 30% 30% 18%	83
	Night	17% 34% 30% 9%	243
May	Day	8% 19% 25% 21% 15%	84
	Night	9% 26% 32% 22% 5%	270
June	Day	5% 22% 32% 24% 12%	154
	Night	15% 43% 34%	253
July	Day	5% 11% 34% 31% 12%	130
	Night	12% 41% 39%	240
August	Day	22% 52% 14%	89
	Night	6% 36% 44% 8%	265
September	Day	14% 28% 35% 7%	82
	Night	19% 31% 33%	298
October	Day	16% 35% 35% 7%	42
	Night	7% 23% 30% 25%	340
November	Night	12% 27% 33% 11%	387
December	Night	11% 37% 32% 6%	350

Page generated at 2008-11-20 23:15:42. Version ab11