Windy Hills Observatory
Clear Sky Chart
History

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

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	11% 6% 3 6% 4 4 4 3 5% 7% 47%	126361
	Night	11% 6% 2 6% 4 3 4 3 5% 7% 50%	124230
Transparency	Day	36% 5% 11% 26% 19% 3	129985
	Night	34% 5% 11% 26% 20% 4	128669
Seeing	Day	13% 10% 30% 29% 15% 3	17096
	Night	13% 9% 33% 34% 11% 1	26922

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	14% 8% 3 10% 5% 3 4 3 5% 7% 37%	4077
	11th to 20th	Day	7% 9% 4 10% 6% 6% 5% 3 4 6% 41%	4080
	21st to end	Day	11% 12% 5% 7% 4 3 4 2 4 5% 43%	4488
February	1st to 10th	Day	13% 11% 4 8% 4 4 4 3 4 6% 41%	1800
	11th to 20th	Day	10% 11% 4 6% 4 4 4 3 5% 8% 41%	1800
	21st to end	Day	8% 10% 3 6% 3 4 4 3 6% 6% 47%	1665
March	1st to 10th	Day	12% 7% 2 5% 3 3 3 3 3 5% 54%	4936
	11th to 20th	Day	11% 6% 1 4 3 3 4 3 5% 6% 55%	4980
	21st to end	Day	12% 9% 5% 7% 5% 5% 4 4 5% 6% 38%	5462
April	1st to 10th	Day	13% 7% 3 5% 3 3 3 3 5% 6% 48%	2260
	11th to 20th	Day	13% 7% 3 6% 4 4 4 4 5% 7% 42%	2403
	21st to end	Day	19% 8% 2 7% 4 4 4 3 5% 7% 38%	2600
May	1st to 10th	Day	15% 7% 2 6% 4 3 3 3 5% 6% 46%	5862
	11th to 20th	Day	13% 6% 2 6% 3 3 4 3 5% 7% 48%	5862
	21st to end	Day	19% 6% 2 5% 3 4 4 4 6% 6% 41%	6434
June	1st to 10th	Day	6% 4 2 5% 4 5% 5% 5% 7% 10% 50%	2665
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 49%	2786
	21st to end	Day	6% 4 2 6% 4 4 4 4 6% 10% 51%	2600
July	1st to 10th	Day	9% 5% 2 5% 4 5% 4 4 6% 8% 47%	5900
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 7% 10% 46%	5900
	21st to end	Day	7% 4 2 5% 5% 4 5% 4 7% 8% 48%	6422
August	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 6% 8% 53%	4234
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	4171
	21st to end	Day	6% 4 2 5% 3 4 4 4 5% 9% 55%	4494
September	1st to 10th	Day	11% 4 2 5% 3 4 3 3 5% 6% 53%	2331
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 6% 6% 49%	2200
	21st to end	Day	9% 5% 2 5% 3 4 4 4 5% 6% 55%	2200
October	1st to 10th	Day	11% 5% 1 5% 3 3 3 3 5% 7% 54%	2035
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 4 4 59%	2000
	21st to end	Day	16% 6% 2 4 3 2 3 3 4 6% 51%	2200
November	1st to 10th	Day	8% 4 2 5% 4 4 4 3 4 6% 58%	1974
	11th to 20th	Day	11% 5% 2 4 3 3 3 2 4 5% 57%	1863
	21st to end	Day	15% 7% 2 5% 3 3 3 2 4 5% 51%	1635
December	1st to 10th	Day	6% 4 2 5% 4 3 3 3 5% 6% 59%	2610
	11th to 20th	Day	9% 7% 3 7% 5% 4 3 3 3 5% 51%	3008
	21st to end	Day	19% 13% 8% 10% 5% 4 3 3 4 5% 27%	4424
January	1st to 10th	Night	11% 9% 4 14% 6% 3 3 3 5% 6% 36%	5888
	11th to 20th	Night	7% 7% 4 8% 4 6% 5% 4 5% 5% 46%	5900
	21st to end	Night	13% 10% 4 8% 4 3 4 3 4 6% 43%	6490
February	1st to 10th	Night	10% 12% 3 6% 5% 5% 4 3 5% 7% 40%	2600
	11th to 20th	Night	11% 9% 3 8% 4 4 4 4 4 6% 45%	2503
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 5% 51%	1965
March	1st to 10th	Night	14% 6% 2 5% 2 3 3 2 3 5% 55%	4919
	11th to 20th	Night	8% 5% 2 4 3 3 4 3 5% 8% 57%	4980
	21st to end	Night	14% 8% 3 6% 4 4 4 4 6% 7% 40%	5467
April	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 5% 6% 52%	2140
	11th to 20th	Night	13% 7% 2 6% 3 4 4 3 4 7% 47%	1944
	21st to end	Night	17% 6% 3 6% 3 4 4 3 6% 6% 43%	1800
May	1st to 10th	Night	15% 7% 2 5% 4 3 3 2 4 8% 47%	4053
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 7% 52%	4053
	21st to end	Night	18% 5% 2 4 3 2 3 3 4 6% 50%	4443
June	1st to 10th	Night	5% 4 1 4 3 4 3 3 6% 9% 57%	1722
	11th to 20th	Night	6% 4 2 4 3 4 5% 4 7% 9% 53%	1600
	21st to end	Night	4 3 1 4 3 3 4 3 6% 10% 59%	1798
July	1st to 10th	Night	8% 5% 2 5% 4 3 4 3 6% 7% 53%	4078
	11th to 20th	Night	5% 4 1 5% 4 4 5% 4 6% 10% 53%	4077
	21st to end	Night	8% 5% 2 5% 3 4 4 3 6% 11% 49%	4449
August	1st to 10th	Night	2 2 1 4 3 4 5% 4 7% 10% 58%	2926
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 5% 7% 51%	2888
	21st to end	Night	5% 4 2 5% 3 3 3 3 6% 7% 59%	3407
September	1st to 10th	Night	10% 4 2 4 3 4 3 2 5% 6% 56%	2065
	11th to 20th	Night	8% 4 1 4 3 4 4 3 5% 8% 54%	2200
	21st to end	Night	7% 4 2 4 4 3 3 3 4 5% 60%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 2 4 3 4 5% 56%	2344
	11th to 20th	Night	12% 4 1 4 3 3 3 2 4 6% 58%	2400
	21st to end	Night	15% 7% 2 4 2 3 3 2 3 5% 53%	2640
November	1st to 10th	Night	9% 5% 1 4 3 3 3 2 4 5% 61%	2539
	11th to 20th	Night	10% 5% 1 3 3 3 3 4 4 6% 57%	2692
	21st to end	Night	12% 7% 2 4 3 3 3 2 5% 6% 54%	2760
December	1st to 10th	Night	6% 6% 2 4 3 3 3 3 4 6% 59%	4580
	11th to 20th	Night	11% 6% 3 6% 4 4 4 4 5% 6% 48%	5199
	21st to end	Night	19% 13% 5% 9% 4 3 3 2 4 4 33%	6521

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	46% 5% 11% 26% 11% 1	4078
	11th to 20th	Day	45% 4 11% 20% 16% 3	4080
	21st to end	Day	46% 4 12% 19% 18% 2	4488
February	1st to 10th	Day	46% 5% 13% 22% 11% 3	1800
	11th to 20th	Day	43% 5% 11% 24% 14% 3	1809
	21st to end	Day	37% 4 11% 25% 19% 4	1665
March	1st to 10th	Day	35% 3 9% 20% 23% 10%	4936
	11th to 20th	Day	30% 4 9% 21% 28% 8%	4980
	21st to end	Day	45% 6% 14% 27% 6% 1	5464
April	1st to 10th	Day	38% 4 11% 23% 20% 3	2260
	11th to 20th	Day	42% 5% 11% 22% 18% 1	2403
	21st to end	Day	47% 5% 10% 26% 11% 1	2600
May	1st to 10th	Day	40% 6% 9% 25% 19% 1	5900
	11th to 20th	Day	35% 6% 10% 22% 23% 4	5866
	21st to end	Day	41% 7% 9% 27% 14% 1	6447
June	1st to 10th	Day	27% 9% 16% 28% 19% 2	2665
	11th to 20th	Day	32% 7% 13% 29% 18% 2	2786
	21st to end	Day	29% 7% 14% 28% 17% 5%	2600
July	1st to 10th	Day	33% 7% 11% 27% 20% 3	5836
	11th to 20th	Day	33% 7% 12% 28% 18% 2	5831
	21st to end	Day	30% 8% 13% 33% 14% 2	6240
August	1st to 10th	Day	28% 7% 11% 27% 22% 4	4220
	11th to 20th	Day	36% 6% 14% 26% 17% 1	4210
	21st to end	Day	26% 6% 16% 29% 17% 6%	4493
September	1st to 10th	Day	31% 5% 12% 26% 20% 7%	2331
	11th to 20th	Day	34% 6% 10% 26% 21% 3	2200
	21st to end	Day	30% 5% 9% 28% 25% 3	2200
October	1st to 10th	Day	19% 3 7% 22% 42% 7%	3308
	11th to 20th	Day	20% 3 5% 29% 34% 10%	3240
	21st to end	Day	31% 4 9% 28% 25% 4	3538
November	1st to 10th	Day	30% 4 12% 30% 20% 5%	1974
	11th to 20th	Day	32% 4 8% 29% 23% 4	1863
	21st to end	Day	37% 4 10% 26% 21% 2	1635
December	1st to 10th	Day	26% 4 12% 40% 17% 1	2610
	11th to 20th	Day	37% 4 12% 33% 13% 1	3005
	21st to end	Day	61% 4 9% 18% 7% 1	4424
January	1st to 10th	Night	50% 4 11% 23% 11% 1	5888
	11th to 20th	Night	39% 4 11% 24% 19% 3	5900
	21st to end	Night	45% 3 11% 17% 18% 5%	6490
February	1st to 10th	Night	45% 5% 11% 22% 15% 3	2600
	11th to 20th	Night	41% 4 11% 24% 16% 3	2504
	21st to end	Night	36% 4 9% 23% 21% 7%	1965
March	1st to 10th	Night	34% 3 6% 26% 26% 5%	4919
	11th to 20th	Night	26% 4 8% 21% 33% 9%	4980
	21st to end	Night	42% 6% 14% 27% 8% 2	5467
April	1st to 10th	Night	34% 4 11% 23% 22% 5%	2140
	11th to 20th	Night	37% 6% 9% 26% 19% 4	1944
	21st to end	Night	42% 6% 11% 25% 15% 2	1800
May	1st to 10th	Night	38% 6% 10% 26% 18% 2	4080
	11th to 20th	Night	33% 5% 9% 26% 20% 6%	4054
	21st to end	Night	36% 4 12% 28% 19% 1	4452
June	1st to 10th	Night	24% 7% 16% 33% 17% 3	1722
	11th to 20th	Night	27% 7% 12% 32% 19% 3	1600
	21st to end	Night	20% 6% 14% 33% 22% 4	1798
July	1st to 10th	Night	29% 6% 13% 24% 23% 4	4035
	11th to 20th	Night	26% 6% 13% 31% 22% 2	4026
	21st to end	Night	28% 7% 14% 35% 15% 1	4320
August	1st to 10th	Night	19% 7% 17% 27% 23% 6%	2916
	11th to 20th	Night	32% 7% 12% 27% 20% 2	2915
	21st to end	Night	24% 5% 13% 34% 18% 5%	3407
September	1st to 10th	Night	30% 4 9% 27% 21% 8%	2065
	11th to 20th	Night	29% 4 11% 27% 24% 5%	2200
	21st to end	Night	27% 4 8% 25% 31% 4	2200
October	1st to 10th	Night	21% 3 5% 19% 46% 6%	3838
	11th to 20th	Night	21% 2 5% 29% 30% 13%	3899
	21st to end	Night	31% 3 7% 29% 27% 3	4252
November	1st to 10th	Night	26% 4 11% 29% 24% 6%	2539
	11th to 20th	Night	29% 4 10% 28% 24% 5%	2692
	21st to end	Night	33% 3 10% 30% 20% 3	2760
December	1st to 10th	Night	27% 4 13% 35% 19% 1	4580
	11th to 20th	Night	36% 4 12% 30% 15% 2	5201
	21st to end	Night	55% 4 9% 23% 8% 1	6521

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	19% 15% 34% 24% 8%	300
	11th to 20th	Day	19% 18% 40% 20% 3	300
	21st to end	Day	12% 11% 49% 25% 3	330
February	1st to 10th	Day	19% 20% 40% 17% 3	300
	11th to 20th	Day	17% 20% 43% 13% 6%	300
	21st to end	Day	12% 23% 44% 16% 4	249
March	1st to 10th	Day	16% 19% 41% 20% 3 1	300
	11th to 20th	Day	24% 14% 36% 21% 5%	300
	21st to end	Day	26% 20% 40% 11% 2 1	330
April	1st to 10th	Day	23% 18% 33% 23% 4	360
	11th to 20th	Day	18% 16% 39% 22% 4 1	521
	21st to end	Day	22% 15% 36% 18% 8% 1	700
May	1st to 10th	Day	17% 14% 36% 24% 9% 1	703
	11th to 20th	Day	18% 10% 29% 28% 13% 2	725
	21st to end	Day	19% 9% 31% 25% 13% 3	789
June	1st to 10th	Day	5% 6% 24% 33% 26% 6%	730
	11th to 20th	Day	7% 4 29% 37% 18% 5%	730
	21st to end	Day	6% 7% 33% 40% 11% 2	730
July	1st to 10th	Day	9% 3 18% 32% 30% 8%	721
	11th to 20th	Day	4 3 17% 41% 27% 7%	718
	21st to end	Day	7% 3 18% 39% 27% 7%	794
August	1st to 10th	Day	7% 2 19% 40% 27% 4	726
	11th to 20th	Day	8% 2 19% 42% 25% 5%	715
	21st to end	Day	11% 2 14% 34% 31% 8%	700
September	1st to 10th	Day	12% 2 18% 37% 26% 5%	461
	11th to 20th	Day	5% 4 24% 36% 26% 5%	330
	21st to end	Day	15% 6% 24% 34% 17% 5%	330
October	1st to 10th	Day	15% 5% 29% 30% 15% 5%	330
	11th to 20th	Day	8% 16% 43% 21% 11% 1	330
	21st to end	Day	21% 15% 35% 22% 6% 1	363
November	1st to 10th	Day	14% 12% 39% 26% 8% 1	330
	11th to 20th	Day	15% 16% 37% 23% 8% 1	321
	21st to end	Day	21% 10% 37% 23% 9% 1	300
December	1st to 10th	Day	7% 15% 41% 28% 7% 2	300
	11th to 20th	Day	18% 14% 39% 21% 7%	300
	21st to end	Day	20% 21% 35% 20% 4	330
January	1st to 10th	Night	12% 12% 38% 31% 7%	1000
	11th to 20th	Night	14% 13% 36% 31% 5% 1	1000
	21st to end	Night	14% 11% 41% 30% 4	1100
February	1st to 10th	Night	17% 13% 39% 27% 3	1000
	11th to 20th	Night	15% 14% 39% 29% 3	949
	21st to end	Night	14% 13% 42% 26% 5%	645
March	1st to 10th	Night	14% 12% 43% 26% 5%	600
	11th to 20th	Night	18% 11% 37% 28% 5% 1	600
	21st to end	Night	22% 13% 40% 22% 4	660
April	1st to 10th	Night	13% 13% 44% 24% 6%	600
	11th to 20th	Night	16% 15% 39% 26% 5%	593
	21st to end	Night	20% 10% 39% 25% 6% 1	600
May	1st to 10th	Night	17% 9% 34% 34% 6% 1	600
	11th to 20th	Night	15% 8% 30% 35% 12%	591
	21st to end	Night	12% 7% 32% 34% 14% 1	650
June	1st to 10th	Night	6% 3 25% 43% 23% 1	600
	11th to 20th	Night	7% 3 29% 44% 17% 1	600
	21st to end	Night	4 5% 33% 45% 13% 1	600
July	1st to 10th	Night	8% 2 16% 45% 28% 1	585
	11th to 20th	Night	2 2 18% 52% 25% 1	582
	21st to end	Night	7% 1 16% 47% 27% 1	657
August	1st to 10th	Night	3 1 18% 52% 25% 2	597
	11th to 20th	Night	6% 1 16% 48% 26% 3	594
	21st to end	Night	6% 2 15% 44% 31% 2	660
September	1st to 10th	Night	13% 2 21% 42% 21% 2	600
	11th to 20th	Night	9% 2 22% 44% 21% 2	600
	21st to end	Night	8% 4 24% 44% 20% 1	600
October	1st to 10th	Night	13% 6% 29% 33% 18% 1	744
	11th to 20th	Night	13% 9% 40% 30% 8%	800
	21st to end	Night	19% 11% 35% 27% 9%	880
November	1st to 10th	Night	11% 9% 34% 34% 11%	899
	11th to 20th	Night	12% 12% 39% 30% 6%	1036
	21st to end	Night	16% 11% 35% 31% 7%	1000
December	1st to 10th	Night	13% 11% 38% 32% 6% 1	1000
	11th to 20th	Night	18% 9% 39% 29% 5%	1000
	21st to end	Night	18% 13% 37% 28% 4	1100

Page generated at 2025-11-14 23:55:55. Version ac07

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