Windy Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windy Sky Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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% 2 6% 4 4 4 3 5% 7% 41%	130549
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	130256
Transparency	Day	40% 7% 13% 23% 16% 2	134090
	Night	42% 5% 11% 22% 16% 3	134802
Seeing	Day	20% 13% 30% 23% 11% 3	16649
	Night	21% 10% 30% 28% 10% 1	25827

By Month

Percentage Cloud Cover NE (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	12% 7% 3 6% 4 4 4 4 5% 5% 47%	4661
	11th to 20th	Day	15% 8% 2 6% 4 3 4 3 4 5% 47%	4650
	21st to end	Day	15% 8% 3 7% 3 4 4 3 4 6% 43%	5364
February	1st to 10th	Day	16% 7% 2 7% 4 4 4 3 5% 6% 41%	1981
	11th to 20th	Day	15% 6% 3 6% 3 4 4 2 5% 5% 46%	1998
	21st to end	Day	18% 9% 3 6% 4 4 4 2 5% 6% 39%	1650
March	1st to 10th	Day	20% 8% 2 5% 3 3 3 3 4 5% 43%	4476
	11th to 20th	Day	11% 8% 3 6% 3 3 5% 4 6% 8% 43%	4667
	21st to end	Day	18% 9% 2 6% 3 4 4 3 5% 6% 40%	5970
April	1st to 10th	Day	18% 8% 2 6% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 5% 6% 43%	2374
	21st to end	Day	19% 6% 2 4 3 3 3 4 6% 6% 42%	2489
May	1st to 10th	Day	28% 9% 2 5% 3 3 4 3 6% 9% 29%	5886
	11th to 20th	Day	22% 10% 3 6% 3 3 3 3 4 6% 36%	6061
	21st to end	Day	20% 6% 2 6% 4 3 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	17% 6% 2 5% 4 4 4 4 7% 9% 39%	2800
	11th to 20th	Day	15% 7% 3 4 4 4 5% 5% 6% 8% 40%	2800
	21st to end	Day	11% 7% 2 5% 5% 3 4 4 7% 10% 43%	2800
July	1st to 10th	Day	13% 7% 3 7% 4 4 5% 4 6% 10% 36%	6360
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 5% 8% 10% 35%	6360
	21st to end	Day	14% 6% 2 5% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	17% 7% 3 6% 4 5% 4 4 6% 8% 36%	4322
	11th to 20th	Day	11% 6% 3 7% 4 4 4 3 6% 8% 44%	3850
	21st to end	Day	7% 4 2 5% 4 4 5% 4 7% 10% 48%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 5% 7% 52%	2260
	21st to end	Day	19% 6% 3 7% 4 3 4 3 5% 6% 39%	2200
October	1st to 10th	Day	20% 6% 1 5% 4 2 3 3 5% 7% 45%	2173
	11th to 20th	Day	15% 6% 1 6% 4 3 4 3 6% 7% 46%	2000
	21st to end	Day	18% 5% 2 6% 4 4 3 4 5% 6% 43%	1980
November	1st to 10th	Day	13% 6% 2 5% 5% 4 3 2 4 6% 50%	1851
	11th to 20th	Day	15% 6% 2 4 4 3 4 3 4 7% 48%	1890
	21st to end	Day	16% 7% 2 5% 4 4 3 4 5% 6% 44%	1890
December	1st to 10th	Day	14% 7% 2 5% 4 4 4 3 5% 8% 44%	2685
	11th to 20th	Day	23% 7% 2 3 3 2 2 2 4 5% 47%	3066
	21st to end	Day	17% 10% 4 7% 4 4 4 2 4 6% 38%	4048
January	1st to 10th	Night	14% 8% 2 6% 3 3 3 3 4 6% 49%	8234
	11th to 20th	Night	16% 8% 3 7% 4 3 3 3 4 5% 46%	8235
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 5% 41%	8799
February	1st to 10th	Night	16% 9% 3 7% 4 4 4 3 5% 6% 41%	2637
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 6% 41%	2399
	21st to end	Night	20% 7% 2 5% 4 4 3 3 4 6% 41%	1980
March	1st to 10th	Night	20% 9% 3 5% 3 3 3 3 4 5% 43%	5364
	11th to 20th	Night	16% 10% 3 6% 3 4 4 3 5% 5% 42%	5274
	21st to end	Night	19% 9% 3 6% 4 4 4 3 5% 5% 38%	4962
April	1st to 10th	Night	20% 7% 2 6% 5% 5% 4 3 4 6% 38%	2000
	11th to 20th	Night	19% 8% 2 6% 3 3 4 3 5% 7% 41%	1975
	21st to end	Night	19% 7% 2 6% 4 3 3 3 5% 7% 41%	1910
May	1st to 10th	Night	25% 11% 4 7% 4 4 4 3 5% 7% 27%	4054
	11th to 20th	Night	23% 11% 3 7% 3 3 3 4 5% 6% 33%	3860
	21st to end	Night	23% 9% 3 6% 4 3 4 3 5% 7% 32%	3926
June	1st to 10th	Night	18% 8% 2 5% 5% 3 4 3 5% 8% 39%	1600
	11th to 20th	Night	16% 7% 2 5% 4 3 4 4 6% 10% 38%	1600
	21st to end	Night	11% 7% 2 6% 6% 5% 5% 5% 6% 9% 38%	1600
July	1st to 10th	Night	15% 11% 3 8% 5% 4 4 3 6% 8% 33%	3600
	11th to 20th	Night	19% 10% 3 7% 5% 4 4 4 6% 8% 30%	3600
	21st to end	Night	15% 7% 3 6% 4 4 4 3 6% 8% 41%	3926
August	1st to 10th	Night	23% 6% 3 6% 4 5% 4 4 7% 8% 31%	2778
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 4 7% 8% 40%	3197
	21st to end	Night	8% 5% 1 5% 4 3 4 4 5% 8% 53%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 5% 4 4 5% 7% 43%	2000
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 5% 6% 52%	2138
	21st to end	Night	19% 6% 2 6% 4 4 4 3 5% 6% 40%	2200
October	1st to 10th	Night	18% 8% 2 7% 4 3 4 2 4 7% 42%	2224
	11th to 20th	Night	16% 5% 2 6% 4 3 3 3 5% 6% 49%	2400
	21st to end	Night	19% 7% 2 7% 4 3 3 3 4 6% 42%	2860
November	1st to 10th	Night	12% 6% 3 5% 5% 4 3 3 5% 6% 49%	2667
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 5% 47%	2730
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 7% 46%	2730
December	1st to 10th	Night	15% 7% 2 6% 4 3 3 2 4 5% 49%	4713
	11th to 20th	Night	25% 7% 2 4 3 3 3 2 3 4 43%	5348
	21st to end	Night	18% 8% 3 6% 3 4 4 3 4 5% 42%	7150

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	37% 4 11% 29% 18% 1	4662
	11th to 20th	Day	39% 4 10% 23% 18% 6%	4680
	21st to end	Day	41% 5% 12% 25% 16% 1	5364
February	1st to 10th	Day	44% 5% 15% 22% 13% 1	1981
	11th to 20th	Day	41% 4 10% 26% 16% 2	2007
	21st to end	Day	45% 5% 11% 21% 14% 3	1650
March	1st to 10th	Day	41% 5% 12% 20% 14% 8%	4479
	11th to 20th	Day	36% 7% 15% 27% 14% 1	4667
	21st to end	Day	45% 4 9% 15% 22% 5%	5970
April	1st to 10th	Day	42% 6% 12% 20% 17% 4	2400
	11th to 20th	Day	40% 6% 13% 22% 17% 3	2374
	21st to end	Day	41% 6% 10% 22% 18% 4	2489
May	1st to 10th	Day	52% 9% 16% 14% 9% 1	5897
	11th to 20th	Day	49% 6% 12% 19% 13% 2	6065
	21st to end	Day	45% 9% 11% 22% 12% 1	6949
June	1st to 10th	Day	41% 8% 13% 21% 16% 1	2800
	11th to 20th	Day	40% 8% 15% 24% 13% 1	2800
	21st to end	Day	35% 8% 17% 24% 15% 1	2800
July	1st to 10th	Day	44% 10% 11% 19% 14% 1	6290
	11th to 20th	Day	43% 8% 19% 21% 9% 1	6285
	21st to end	Day	38% 7% 12% 24% 18% 1	6728
August	1st to 10th	Day	43% 8% 17% 22% 9%	4308
	11th to 20th	Day	36% 7% 14% 32% 10% 1	3886
	21st to end	Day	28% 10% 14% 29% 18% 1	4312
September	1st to 10th	Day	37% 7% 14% 22% 19% 2	2400
	11th to 20th	Day	32% 5% 13% 25% 23% 2	2260
	21st to end	Day	46% 6% 10% 18% 18% 2	2200
October	1st to 10th	Day	33% 5% 13% 23% 21% 5%	3537
	11th to 20th	Day	26% 5% 11% 23% 31% 3	3240
	21st to end	Day	31% 5% 15% 27% 20% 2	3180
November	1st to 10th	Day	37% 4 14% 23% 20% 2	1851
	11th to 20th	Day	37% 5% 12% 22% 20% 3	1890
	21st to end	Day	40% 5% 15% 28% 10% 2	1890
December	1st to 10th	Day	40% 5% 15% 28% 12% 1	2685
	11th to 20th	Day	42% 4 13% 21% 17% 3	3066
	21st to end	Day	46% 6% 9% 22% 15% 3	4048
January	1st to 10th	Night	38% 4 11% 25% 21% 2	8234
	11th to 20th	Night	42% 4 11% 22% 16% 5%	8280
	21st to end	Night	45% 4 9% 21% 18% 4	8799
February	1st to 10th	Night	44% 6% 13% 23% 12% 2	2637
	11th to 20th	Night	44% 5% 9% 23% 15% 4	2401
	21st to end	Night	45% 4 9% 22% 16% 4	1980
March	1st to 10th	Night	45% 4 11% 20% 13% 8%	5374
	11th to 20th	Night	43% 6% 12% 24% 11% 4	5275
	21st to end	Night	47% 5% 7% 14% 22% 5%	4962
April	1st to 10th	Night	46% 6% 10% 19% 16% 3	2000
	11th to 20th	Night	43% 6% 10% 19% 20% 2	1975
	21st to end	Night	44% 4 10% 21% 17% 5%	1910
May	1st to 10th	Night	55% 7% 13% 13% 9% 2	4060
	11th to 20th	Night	52% 6% 12% 15% 12% 3	3861
	21st to end	Night	50% 7% 12% 20% 10% 1	3927
June	1st to 10th	Night	44% 7% 10% 19% 18% 2	1600
	11th to 20th	Night	40% 7% 13% 25% 13% 1	1600
	21st to end	Night	40% 8% 13% 25% 12% 2	1600
July	1st to 10th	Night	49% 7% 11% 21% 11% 1	3560
	11th to 20th	Night	49% 8% 13% 20% 9% 1	3552
	21st to end	Night	41% 6% 13% 24% 15% 1	3809
August	1st to 10th	Night	49% 8% 16% 17% 9% 1	2768
	11th to 20th	Night	38% 7% 15% 29% 9%	3227
	21st to end	Night	28% 6% 12% 34% 18% 1	3586
September	1st to 10th	Night	41% 6% 10% 25% 17% 2	2000
	11th to 20th	Night	34% 5% 10% 26% 23% 3	2138
	21st to end	Night	45% 6% 9% 20% 18% 2	2200
October	1st to 10th	Night	40% 4 11% 24% 18% 3	3640
	11th to 20th	Night	26% 4 10% 26% 30% 4	3899
	21st to end	Night	32% 5% 13% 27% 21% 2	4610
November	1st to 10th	Night	37% 6% 12% 24% 18% 4	2667
	11th to 20th	Night	40% 5% 9% 22% 19% 5%	2730
	21st to end	Night	38% 5% 14% 29% 13% 1	2730
December	1st to 10th	Night	39% 4 13% 28% 13% 3	4713
	11th to 20th	Night	46% 4 9% 24% 15% 2	5348
	21st to end	Night	43% 5% 11% 21% 17% 3	7150

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	25% 22% 34% 14% 5%	220
	11th to 20th	Day	26% 29% 30% 11% 4	220
	21st to end	Day	27% 20% 34% 14% 6%	286
February	1st to 10th	Day	25% 20% 36% 15% 4	421
	11th to 20th	Day	26% 18% 38% 15% 2	516
	21st to end	Day	27% 20% 33% 16% 4	414
March	1st to 10th	Day	23% 21% 32% 19% 5%	500
	11th to 20th	Day	23% 17% 37% 18% 5% 1	500
	21st to end	Day	27% 18% 33% 15% 5% 1	550
April	1st to 10th	Day	24% 20% 31% 16% 7% 1	500
	11th to 20th	Day	25% 16% 33% 16% 9% 1	492
	21st to end	Day	26% 13% 34% 19% 5% 2	500
May	1st to 10th	Day	30% 15% 31% 16% 6% 1	503
	11th to 20th	Day	20% 13% 35% 23% 8% 1	527
	21st to end	Day	20% 9% 26% 25% 16% 4	573
June	1st to 10th	Day	18% 7% 31% 26% 14% 3	530
	11th to 20th	Day	17% 9% 29% 28% 13% 3	530
	21st to end	Day	16% 8% 25% 32% 16% 4	530
July	1st to 10th	Day	14% 6% 23% 34% 18% 6%	525
	11th to 20th	Day	11% 4 24% 37% 17% 7%	524
	21st to end	Day	13% 7% 26% 34% 17% 3	576
August	1st to 10th	Day	13% 5% 25% 33% 19% 6%	526
	11th to 20th	Day	13% 4 22% 36% 17% 8%	519
	21st to end	Day	10% 6% 27% 32% 16% 8%	583
September	1st to 10th	Day	18% 6% 25% 26% 18% 6%	530
	11th to 20th	Day	14% 5% 28% 28% 15% 10%	530
	21st to end	Day	22% 7% 29% 24% 13% 5%	530
October	1st to 10th	Day	23% 9% 24% 20% 15% 8%	530
	11th to 20th	Day	16% 16% 30% 23% 12% 3	530
	21st to end	Day	22% 12% 33% 20% 11% 2	462
November	1st to 10th	Day	17% 15% 28% 27% 11% 2	425
	11th to 20th	Day	22% 19% 30% 20% 8%	430
	21st to end	Day	21% 18% 29% 23% 8% 1	430
December	1st to 10th	Day	24% 19% 34% 17% 6%	225
	11th to 20th	Day	33% 19% 32% 12% 3	220
	21st to end	Day	28% 17% 33% 15% 6% 1	242
January	1st to 10th	Night	20% 16% 37% 22% 5%	840
	11th to 20th	Night	27% 16% 31% 22% 4	840
	21st to end	Night	21% 15% 38% 23% 3	924
February	1st to 10th	Night	21% 14% 38% 24% 3	840
	11th to 20th	Night	26% 15% 39% 18% 2	804
	21st to end	Night	25% 16% 33% 22% 4	660
March	1st to 10th	Night	23% 13% 37% 21% 6%	800
	11th to 20th	Night	22% 12% 38% 23% 5% 1	800
	21st to end	Night	22% 13% 38% 19% 6%	880
April	1st to 10th	Night	23% 12% 37% 23% 5%	800
	11th to 20th	Night	25% 9% 32% 27% 7%	791
	21st to end	Night	23% 8% 34% 29% 6% 1	710
May	1st to 10th	Night	25% 9% 31% 26% 9% 2	600
	11th to 20th	Night	24% 8% 35% 26% 7%	523
	21st to end	Night	21% 4 21% 37% 16% 1	433
June	1st to 10th	Night	20% 5% 23% 36% 15% 1	400
	11th to 20th	Night	20% 4 19% 41% 16% 1	400
	21st to end	Night	12% 4 25% 44% 15% 1	400
July	1st to 10th	Night	13% 6% 20% 41% 19% 2	390
	11th to 20th	Night	15% 5% 14% 43% 22% 1	388
	21st to end	Night	15% 4 18% 43% 18% 1	438
August	1st to 10th	Night	9% 4 18% 45% 23% 1	512
	11th to 20th	Night	14% 3 16% 40% 24% 3	790
	21st to end	Night	14% 4 16% 38% 24% 4	880
September	1st to 10th	Night	17% 3 24% 32% 20% 4	800
	11th to 20th	Night	16% 4 20% 32% 21% 7%	800
	21st to end	Night	23% 5% 22% 31% 17% 3	800
October	1st to 10th	Night	22% 7% 25% 25% 18% 4	800
	11th to 20th	Night	21% 10% 25% 31% 11% 1	800
	21st to end	Night	25% 8% 31% 26% 10%	880
November	1st to 10th	Night	16% 10% 32% 28% 12% 1	820
	11th to 20th	Night	22% 14% 31% 23% 9% 1	840
	21st to end	Night	19% 16% 31% 26% 7% 1	840
December	1st to 10th	Night	24% 15% 31% 22% 6% 1	840
	11th to 20th	Night	25% 14% 33% 23% 5%	840
	21st to end	Night	25% 13% 34% 23% 4	924

Page generated at 2026-02-13 01:42:11. Version ac07

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