Windham Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windham Hill Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	12% 6% 2 5% 4 4 4 4 6% 8% 46%	127161
	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	124113
Transparency	Day	35% 6% 11% 21% 19% 8%	130839
	Night	33% 5% 10% 20% 21% 11%	128498
Seeing	Day	13% 8% 21% 27% 23% 8%	16462
	Night	14% 8% 26% 33% 17% 2	26335

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 8% 3 5% 3 3 3 3 4 6% 48%	4077
	11th to 20th	Day	14% 6% 2 4 2 2 3 2 4 8% 53%	4080
	21st to end	Day	27% 9% 3 5% 3 2 2 2 2 4 41%	4488
February	1st to 10th	Day	21% 8% 2 6% 3 3 3 2 4 5% 42%	1900
	11th to 20th	Day	20% 9% 2 7% 3 4 3 3 3 6% 40%	1949
	21st to end	Day	22% 7% 2 5% 3 3 3 2 4 5% 44%	1763
March	1st to 10th	Day	17% 9% 3 6% 4 4 4 3 5% 6% 41%	4936
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 5% 6% 49%	4980
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 6% 41%	5462
April	1st to 10th	Day	14% 8% 3 7% 4 5% 4 4 5% 6% 40%	2200
	11th to 20th	Day	15% 7% 2 5% 5% 5% 4 4 5% 7% 40%	2309
	21st to end	Day	12% 7% 2 6% 4 5% 4 4 5% 6% 44%	2400
May	1st to 10th	Day	19% 8% 3 7% 4 4 4 3 5% 8% 34%	5816
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 4 6% 7% 42%	5862
	21st to end	Day	11% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	6434
June	1st to 10th	Day	6% 4 2 6% 6% 5% 5% 5% 7% 10% 45%	2600
	11th to 20th	Day	5% 4 2 4 5% 5% 5% 5% 7% 10% 48%	2600
	21st to end	Day	5% 5% 2 5% 6% 6% 7% 5% 8% 11% 41%	2600
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 4 8% 12% 38%	5900
	11th to 20th	Day	4 4 2 5% 4 5% 5% 4 8% 12% 47%	5900
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 6% 8% 43%	6422
August	1st to 10th	Day	2 1 1 2 2 2 3 3 5% 8% 70%	4234
	11th to 20th	Day	3 2 1 3 3 2 4 4 7% 12% 60%	4138
	21st to end	Day	5% 3 2 5% 5% 5% 6% 5% 8% 11% 45%	4165
September	1st to 10th	Day	7% 6% 2 6% 6% 5% 5% 4 5% 8% 47%	2200
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 4 6% 8% 46%	2200
	21st to end	Day	6% 4 1 4 4 4 4 4 7% 7% 54%	2200
October	1st to 10th	Day	5% 3 2 5% 3 3 4 3 5% 6% 61%	2105
	11th to 20th	Day	12% 4 2 3 3 4 3 3 4 5% 58%	2000
	21st to end	Day	14% 5% 2 3 3 3 3 3 3 4 58%	2200
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 4 6% 51%	2058
	11th to 20th	Day	11% 6% 2 4 3 3 3 3 4 5% 54%	2100
	21st to end	Day	16% 8% 2 7% 3 4 4 3 5% 5% 45%	1911
December	1st to 10th	Day	20% 6% 2 4 3 3 3 2 4 5% 47%	3030
	11th to 20th	Day	19% 8% 2 6% 4 3 3 3 4 6% 41%	3454
	21st to end	Day	15% 8% 3 6% 4 3 4 2 4 5% 46%	4488
January	1st to 10th	Night	17% 6% 2 5% 3 2 2 2 3 4 53%	5888
	11th to 20th	Night	13% 5% 1 4 2 2 2 3 4 5% 59%	5900
	21st to end	Night	27% 8% 2 5% 3 2 2 2 3 5% 39%	6490
February	1st to 10th	Night	18% 8% 3 6% 4 3 3 3 3 4 46%	2515
	11th to 20th	Night	20% 8% 3 5% 3 3 3 2 3 4 45%	2338
	21st to end	Night	21% 6% 2 5% 3 3 3 2 4 4 47%	1865
March	1st to 10th	Night	18% 9% 3 5% 4 4 3 3 4 5% 42%	4919
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 49%	4980
	21st to end	Night	17% 9% 3 6% 4 3 3 2 4 5% 45%	5467
April	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 5% 8% 47%	2200
	11th to 20th	Night	13% 6% 2 5% 3 3 3 4 5% 7% 47%	2037
	21st to end	Night	13% 6% 2 5% 4 3 3 3 6% 6% 50%	2000
May	1st to 10th	Night	16% 11% 3 7% 4 4 4 4 6% 8% 35%	4080
	11th to 20th	Night	10% 6% 3 8% 4 5% 5% 4 6% 7% 42%	4053
	21st to end	Night	10% 6% 2 7% 4 4 5% 4 7% 10% 41%	4443
June	1st to 10th	Night	6% 3 1 5% 3 4 3 3 6% 10% 56%	1800
	11th to 20th	Night	5% 2 1 3 3 2 3 4 6% 10% 62%	1800
	21st to end	Night	5% 4 2 5% 4 4 3 4 6% 10% 53%	1800
July	1st to 10th	Night	5% 4 2 6% 5% 5% 6% 4 8% 12% 41%	4078
	11th to 20th	Night	2 3 1 4 3 3 4 4 7% 12% 57%	4077
	21st to end	Night	6% 6% 2 5% 3 4 5% 4 7% 9% 48%	4449
August	1st to 10th	Night	1 1 2 1 1 2 2 4 9% 76%	2926
	11th to 20th	Night	2 2 1 3 3 3 5% 3 6% 11% 60%	2920
	21st to end	Night	4 3 1 4 4 4 5% 5% 7% 10% 52%	3714
September	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 6% 8% 51%	2200
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 5% 7% 50%	2200
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 8% 60%	2200
October	1st to 10th	Night	5% 3 1 3 2 3 3 2 4 7% 67%	2272
	11th to 20th	Night	11% 4 1 3 2 2 2 2 3 5% 62%	2400
	21st to end	Night	13% 4 2 3 3 3 2 2 3 4 61%	2640
November	1st to 10th	Night	11% 5% 2 4 4 3 3 3 4 6% 57%	2460
	11th to 20th	Night	9% 6% 2 5% 3 4 3 3 4 6% 55%	2520
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 6% 48%	2704
December	1st to 10th	Night	19% 7% 2 6% 3 2 2 3 4 5% 45%	4380
	11th to 20th	Night	17% 9% 3 6% 4 3 3 2 3 5% 45%	4908
	21st to end	Night	15% 6% 2 5% 3 3 3 2 3 4 54%	6490

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	38% 3 6% 25% 23% 5%	4078
	11th to 20th	Day	33% 3 4 21% 29% 12%	4080
	21st to end	Day	51% 2 4 12% 18% 14%	4488
February	1st to 10th	Day	46% 3 5% 14% 21% 10%	1900
	11th to 20th	Day	48% 4 7% 15% 18% 9%	1959
	21st to end	Day	43% 4 6% 16% 17% 13%	1763
March	1st to 10th	Day	44% 5% 6% 12% 21% 12%	4936
	11th to 20th	Day	36% 4 7% 12% 19% 22%	4980
	21st to end	Day	43% 5% 9% 20% 18% 5%	5464
April	1st to 10th	Day	45% 5% 9% 18% 14% 10%	2200
	11th to 20th	Day	43% 5% 9% 15% 18% 9%	2309
	21st to end	Day	39% 6% 9% 19% 22% 5%	2400
May	1st to 10th	Day	47% 6% 11% 22% 12% 3	5854
	11th to 20th	Day	40% 7% 12% 16% 24% 2	5866
	21st to end	Day	41% 9% 12% 22% 15% 2	6447
June	1st to 10th	Day	31% 8% 15% 29% 14% 2	2600
	11th to 20th	Day	28% 8% 14% 29% 17% 4	2600
	21st to end	Day	33% 10% 15% 25% 16% 2	2600
July	1st to 10th	Day	35% 9% 28% 24% 5%	5836
	11th to 20th	Day	27% 9% 21% 31% 13%	5831
	21st to end	Day	36% 8% 16% 28% 12% 1	6240
August	1st to 10th	Day	12% 5% 14% 40% 28% 1	4220
	11th to 20th	Day	17% 6% 26% 39% 10% 1	4177
	21st to end	Day	29% 12% 20% 25% 12% 2	4164
September	1st to 10th	Day	33% 7% 13% 21% 17% 9%	2200
	11th to 20th	Day	35% 7% 14% 21% 15% 7%	2200
	21st to end	Day	26% 7% 11% 22% 19% 14%	2200
October	1st to 10th	Day	15% 3 6% 13% 23% 40%	3430
	11th to 20th	Day	18% 3 5% 19% 33% 21%	3240
	21st to end	Day	25% 4 5% 20% 34% 12%	3538
November	1st to 10th	Day	35% 5% 8% 18% 24% 10%	2058
	11th to 20th	Day	33% 4 8% 19% 23% 14%	2100
	21st to end	Day	43% 4 7% 18% 19% 10%	1911
December	1st to 10th	Day	40% 3 8% 17% 24% 7%	3030
	11th to 20th	Day	44% 4 7% 12% 23% 10%	3452
	21st to end	Day	42% 3 6% 13% 28% 8%	4488
January	1st to 10th	Night	36% 3 5% 21% 26% 8%	5888
	11th to 20th	Night	29% 3 5% 17% 33% 13%	5900
	21st to end	Night	50% 2 4 12% 17% 15%	6490
February	1st to 10th	Night	43% 4 5% 13% 19% 16%	2515
	11th to 20th	Night	44% 3 7% 15% 19% 13%	2338
	21st to end	Night	42% 4 7% 14% 17% 16%	1865
March	1st to 10th	Night	45% 4 6% 12% 21% 13%	4919
	11th to 20th	Night	36% 5% 7% 15% 15% 22%	4980
	21st to end	Night	43% 4 8% 16% 20% 9%	5467
April	1st to 10th	Night	37% 6% 8% 18% 18% 12%	2200
	11th to 20th	Night	34% 6% 11% 19% 18% 13%	2037
	21st to end	Night	35% 5% 10% 18% 25% 7%	2000
May	1st to 10th	Night	46% 6% 11% 22% 12% 3	4107
	11th to 20th	Night	39% 7% 12% 18% 22% 2	4054
	21st to end	Night	36% 8% 15% 27% 12% 3	4452
June	1st to 10th	Night	23% 8% 17% 30% 20% 2	1800
	11th to 20th	Night	18% 6% 18% 33% 20% 5%	1800
	21st to end	Night	26% 8% 16% 28% 19% 3	1800
July	1st to 10th	Night	33% 9% 28% 26% 4	4035
	11th to 20th	Night	20% 7% 25% 36% 12%	4026
	21st to end	Night	30% 8% 20% 28% 12% 1	4320
August	1st to 10th	Night	7% 4 15% 36% 34% 3	2916
	11th to 20th	Night	18% 6% 23% 41% 10% 2	2947
	21st to end	Night	25% 11% 21% 28% 13% 2	3714
September	1st to 10th	Night	30% 7% 16% 22% 17% 9%	2200
	11th to 20th	Night	33% 7% 15% 20% 17% 7%	2200
	21st to end	Night	23% 6% 12% 23% 23% 14%	2200
October	1st to 10th	Night	12% 3 7% 16% 19% 43%	3714
	11th to 20th	Night	16% 2 3 20% 33% 25%	3899
	21st to end	Night	22% 3 5% 19% 34% 17%	4252
November	1st to 10th	Night	30% 4 9% 20% 24% 13%	2460
	11th to 20th	Night	31% 4 8% 20% 23% 15%	2520
	21st to end	Night	39% 4 8% 17% 21% 12%	2704
December	1st to 10th	Night	42% 3 5% 18% 24% 8%	4380
	11th to 20th	Night	44% 3 7% 10% 23% 13%	4909
	21st to end	Night	35% 3 6% 15% 30% 11%	6490

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	24% 17% 34% 19% 6% 2	300
	11th to 20th	Day	23% 15% 37% 19% 5% 1	300
	21st to end	Day	24% 18% 36% 15% 7%	330
February	1st to 10th	Day	25% 18% 34% 19% 4	300
	11th to 20th	Day	32% 20% 31% 14% 3	300
	21st to end	Day	29% 22% 37% 9% 3	249
March	1st to 10th	Day	25% 17% 34% 17% 6%	300
	11th to 20th	Day	20% 23% 38% 15% 4	300
	21st to end	Day	19% 16% 38% 19% 7%	330
April	1st to 10th	Day	23% 15% 40% 17% 5%	300
	11th to 20th	Day	21% 17% 32% 19% 10% 1	427
	21st to end	Day	16% 13% 32% 29% 8% 1	500
May	1st to 10th	Day	17% 11% 35% 29% 8% 1	683
	11th to 20th	Day	15% 8% 28% 30% 16% 3	725
	21st to end	Day	12% 7% 25% 33% 18% 4	789
June	1st to 10th	Day	6% 2 17% 38% 27% 10%	730
	11th to 20th	Day	6% 2 16% 41% 28% 7%	730
	21st to end	Day	7% 2 12% 32% 35% 12%	730
July	1st to 10th	Day	8% 2 8% 30% 36% 16%	721
	11th to 20th	Day	4 1 6% 34% 41% 15%	718
	21st to end	Day	6% 1 4 28% 48% 13%	794
August	1st to 10th	Day	4 1 4 25% 52% 15%	726
	11th to 20th	Day	6% 1 3 28% 47% 15%	695
	21st to end	Day	8% 2 3 21% 42% 24%	501
September	1st to 10th	Day	5% 1 8% 30% 34% 22%	330
	11th to 20th	Day	5% 2 8% 26% 38% 21%	330
	21st to end	Day	8% 2 11% 31% 32% 17%	330
October	1st to 10th	Day	4 3 16% 37% 33% 7%	330
	11th to 20th	Day	8% 7% 25% 33% 20% 7%	330
	21st to end	Day	21% 12% 25% 23% 14% 5%	363
November	1st to 10th	Day	17% 7% 25% 31% 17% 3	330
	11th to 20th	Day	16% 10% 30% 30% 12% 3	330
	21st to end	Day	19% 15% 32% 22% 10% 1	330
December	1st to 10th	Day	21% 9% 37% 20% 10% 4	330
	11th to 20th	Day	27% 15% 30% 20% 7% 1	321
	21st to end	Day	22% 15% 35% 22% 5% 1	330
January	1st to 10th	Night	19% 13% 35% 27% 7%	1000
	11th to 20th	Night	22% 13% 32% 26% 7%	1000
	21st to end	Night	22% 11% 34% 28% 5%	1100
February	1st to 10th	Night	23% 13% 34% 26% 3	915
	11th to 20th	Night	26% 13% 36% 22% 3	784
	21st to end	Night	25% 15% 36% 21% 2	545
March	1st to 10th	Night	23% 12% 36% 25% 4	600
	11th to 20th	Night	17% 12% 42% 24% 6%	600
	21st to end	Night	14% 15% 41% 25% 5%	660
April	1st to 10th	Night	15% 11% 42% 26% 6%	600
	11th to 20th	Night	13% 14% 37% 27% 8%	593
	21st to end	Night	14% 10% 42% 24% 9% 1	600
May	1st to 10th	Night	12% 11% 41% 29% 7% 1	600
	11th to 20th	Night	14% 7% 32% 32% 15%	591
	21st to end	Night	7% 6% 29% 40% 17% 2	650
June	1st to 10th	Night	7% 2 15% 43% 31% 3	600
	11th to 20th	Night	4 2 21% 50% 21% 2	600
	21st to end	Night	5% 1 13% 45% 30% 6%	600
July	1st to 10th	Night	4 1 12% 43% 34% 6%	585
	11th to 20th	Night	3 1 8% 51% 33% 5%	582
	21st to end	Night	4 2 8% 45% 37% 4	657
August	1st to 10th	Night	3 1 6% 46% 41% 4	597
	11th to 20th	Night	5% 1 4 46% 38% 5%	594
	21st to end	Night	4 2 7% 39% 42% 6%	660
September	1st to 10th	Night	7% 1 8% 42% 37% 5%	600
	11th to 20th	Night	11% 2 6% 37% 40% 4	600
	21st to end	Night	8% 1 11% 39% 37% 5%	600
October	1st to 10th	Night	6% 2 14% 45% 29% 4	672
	11th to 20th	Night	12% 5% 23% 39% 19% 2	800
	21st to end	Night	14% 9% 28% 31% 18% 2	880
November	1st to 10th	Night	13% 6% 25% 35% 19% 1	820
	11th to 20th	Night	11% 10% 29% 37% 12%	840
	21st to end	Night	18% 11% 30% 26% 13% 1	1024
December	1st to 10th	Night	20% 8% 33% 29% 10%	1050
	11th to 20th	Night	23% 11% 29% 30% 6%	1036
	21st to end	Night	20% 11% 31% 30% 7%	1100

Page generated at 2025-12-15 00:13:57. Version ac07

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