Windy Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windy Hill Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	12% 7% 2 7% 5% 5% 5% 4 6% 7% 40%	136497
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 44%	136348
Transparency	Day	27% 37% 8% 12% 12% 5%	140108
	Night	27% 36% 7% 12% 12% 6%	140817
Seeing	Day	14% 9% 22% 25% 21% 9%	17185
	Night	15% 8% 24% 29% 20% 4	26858

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	17% 9% 3 8% 4 3 3 3 4 6% 41%	5289
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 4 5% 53%	5310
	21st to end	Day	22% 10% 3 8% 4 4 4 3 4 4 36%	5844
February	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 3 4 5% 40%	2100
	11th to 20th	Day	18% 8% 4 7% 4 4 4 3 4 6% 40%	2048
	21st to end	Day	17% 8% 3 8% 4 4 4 2 5% 7% 37%	1730
March	1st to 10th	Day	19% 9% 3 8% 5% 5% 4 3 4 6% 33%	5035
	11th to 20th	Day	14% 6% 2 6% 4 5% 4 3 5% 6% 45%	5090
	21st to end	Day	13% 8% 3 7% 6% 5% 4 3 4 5% 41%	5720
April	1st to 10th	Day	10% 7% 3 6% 5% 5% 5% 5% 5% 7% 41%	2520
	11th to 20th	Day	14% 7% 2 7% 4 5% 4 4 5% 8% 41%	2494
	21st to end	Day	10% 5% 2 6% 5% 6% 5% 4 7% 8% 43%	2520
May	1st to 10th	Day	20% 8% 2 7% 4 5% 5% 4 5% 7% 32%	7021
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 6% 9% 39%	7031
	21st to end	Day	10% 7% 3 8% 5% 6% 6% 6% 7% 10% 32%	7736
June	1st to 10th	Day	9% 5% 3 8% 6% 7% 6% 6% 8% 10% 32%	2717
	11th to 20th	Day	7% 6% 2 8% 7% 6% 7% 6% 8% 10% 34%	2730
	21st to end	Day	7% 6% 2 8% 8% 7% 7% 6% 8% 11% 31%	2730
July	1st to 10th	Day	5% 5% 3 8% 7% 8% 9% 7% 9% 11% 28%	6030
	11th to 20th	Day	6% 5% 2 7% 7% 7% 8% 6% 9% 11% 32%	5966
	21st to end	Day	8% 7% 3 9% 5% 7% 7% 5% 7% 11% 30%	6269
August	1st to 10th	Day	3 3 2 4 4 5% 5% 4 5% 7% 58%	3905
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 5% 8% 10% 45%	3849
	21st to end	Day	10% 6% 3 7% 6% 6% 6% 4 6% 7% 37%	4312
September	1st to 10th	Day	10% 6% 3 7% 6% 5% 6% 4 8% 8% 37%	2400
	11th to 20th	Day	11% 6% 3 7% 5% 5% 6% 5% 6% 8% 40%	2400
	21st to end	Day	10% 5% 2 6% 4 4 3 4 5% 7% 51%	2182
October	1st to 10th	Day	7% 4 2 5% 4 5% 3 4 5% 6% 55%	2000
	11th to 20th	Day	8% 5% 2 5% 3 3 3 3 4 6% 59%	2000
	21st to end	Day	11% 4 2 5% 4 3 3 3 4 6% 57%	2200
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 5% 6% 53%	2058
	11th to 20th	Day	9% 6% 2 5% 4 4 3 3 5% 5% 53%	2100
	21st to end	Day	14% 6% 2 6% 3 4 4 3 5% 6% 45%	1931
December	1st to 10th	Day	17% 9% 3 6% 4 4 4 4 4 5% 41%	3020
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 3 5% 6% 43%	3623
	21st to end	Day	15% 7% 2 6% 3 4 4 3 5% 5% 47%	4587
January	1st to 10th	Night	20% 8% 3 5% 3 3 3 3 4 5% 43%	7634
	11th to 20th	Night	15% 7% 3 5% 3 3 3 2 4 5% 49%	7680
	21st to end	Night	25% 9% 3 6% 4 4 3 3 4 5% 35%	8364
February	1st to 10th	Night	15% 8% 3 5% 4 4 5% 3 5% 5% 43%	2520
	11th to 20th	Night	20% 9% 3 6% 3 3 3 3 4 6% 40%	2458
	21st to end	Night	20% 8% 3 5% 4 4 4 3 5% 5% 40%	2076
March	1st to 10th	Night	19% 11% 3 6% 3 4 4 3 5% 6% 37%	5080
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	5090
	21st to end	Night	17% 8% 2 6% 5% 4 4 3 4 5% 43%	5448
April	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 6% 8% 43%	2100
	11th to 20th	Night	12% 6% 2 5% 4 4 5% 4 5% 7% 46%	2075
	21st to end	Night	10% 5% 2 5% 4 3 4 4 6% 7% 51%	2100
May	1st to 10th	Night	22% 7% 3 7% 4 3 3 3 5% 7% 37%	5832
	11th to 20th	Night	14% 7% 3 6% 4 3 4 4 6% 8% 41%	5829
	21st to end	Night	13% 8% 3 8% 5% 5% 6% 5% 7% 9% 32%	6415
June	1st to 10th	Night	8% 5% 2 7% 5% 4 5% 4 7% 11% 42%	1896
	11th to 20th	Night	4 4 2 6% 4 4 6% 5% 9% 10% 47%	1890
	21st to end	Night	5% 5% 2 6% 6% 5% 5% 6% 9% 12% 39%	1890
July	1st to 10th	Night	4 5% 2 8% 6% 6% 7% 6% 9% 12% 34%	4168
	11th to 20th	Night	4 4 2 5% 4 6% 7% 6% 9% 14% 39%	4122
	21st to end	Night	6% 6% 3 7% 6% 5% 7% 6% 8% 11% 35%	4592
August	1st to 10th	Night	2 2 1 3 3 3 4 3 7% 10% 63%	3244
	11th to 20th	Night	3 3 1 6% 5% 5% 5% 5% 8% 13% 46%	3200
	21st to end	Night	7% 6% 3 7% 5% 6% 6% 5% 7% 8% 43%	3586
September	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 4 7% 9% 43%	2000
	11th to 20th	Night	8% 6% 2 5% 5% 4 5% 5% 6% 10% 45%	2000
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 7% 52%	2192
October	1st to 10th	Night	7% 4 1 4 3 3 3 3 5% 7% 59%	2400
	11th to 20th	Night	8% 4 2 4 3 3 3 3 4 5% 60%	2400
	21st to end	Night	11% 6% 2 4 3 3 2 3 4 5% 57%	2640
November	1st to 10th	Night	12% 4 2 4 4 4 3 2 5% 6% 56%	2460
	11th to 20th	Night	13% 5% 1 4 3 3 3 3 4 5% 56%	2520
	21st to end	Night	14% 6% 2 5% 4 3 4 3 4 6% 48%	2679
December	1st to 10th	Night	20% 8% 2 6% 4 3 3 2 3 5% 44%	4380
	11th to 20th	Night	12% 11% 3 7% 4 4 3 3 5% 5% 44%	4755
	21st to end	Night	19% 10% 2 5% 3 3 2 2 3 4 46%	6633

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% 21% 5% 16% 18% 2	5290
	11th to 20th	Day	27% 21% 4 14% 26% 7%	5310
	21st to end	Day	43% 22% 3 7% 12% 12%	5844
February	1st to 10th	Day	19% 59% 3 7% 8% 5%	2100
	11th to 20th	Day	22% 59% 3 8% 8% 2	2058
	21st to end	Day	22% 59% 3 3 8% 6%	1730
March	1st to 10th	Day	40% 29% 4 9% 12% 6%	5035
	11th to 20th	Day	29% 29% 4 11% 13% 15%	5090
	21st to end	Day	35% 29% 8% 11% 13% 6%	5722
April	1st to 10th	Day	19% 59% 3 8% 8% 3	2520
	11th to 20th	Day	19% 60% 4 7% 7% 3	2494
	21st to end	Day	17% 60% 6% 8% 6% 3	2520
May	1st to 10th	Day	41% 26% 7% 14% 8% 3	7056
	11th to 20th	Day	32% 25% 12% 15% 14% 2	7035
	21st to end	Day	35% 27% 12% 16% 8% 1	7749
June	1st to 10th	Day	19% 61% 5% 7% 8% 1	2717
	11th to 20th	Day	22% 62% 6% 6% 4 1	2730
	21st to end	Day	19% 61% 9% 8% 3	2730
July	1st to 10th	Day	31% 32% 18% 15% 5%	5966
	11th to 20th	Day	30% 32% 24% 13% 1	5897
	21st to end	Day	33% 30% 12% 20% 4 1	6089
August	1st to 10th	Day	10% 37% 6% 25% 21% 1	3893
	11th to 20th	Day	15% 39% 15% 23% 8% 1	3885
	21st to end	Day	28% 38% 9% 10% 14% 1	4311
September	1st to 10th	Day	17% 59% 7% 8% 6% 2	2400
	11th to 20th	Day	14% 58% 6% 11% 9% 2	2400
	21st to end	Day	14% 58% 6% 12% 7% 3	2182
October	1st to 10th	Day	13% 36% 4 8% 12% 26%	3260
	11th to 20th	Day	6% 35% 4 11% 17% 26%	3240
	21st to end	Day	13% 35% 2 8% 28% 14%	3538
November	1st to 10th	Day	19% 54% 4 9% 9% 5%	2058
	11th to 20th	Day	18% 54% 4 8% 11% 6%	2100
	21st to end	Day	18% 55% 4 9% 11% 4	1931
December	1st to 10th	Day	32% 35% 3 8% 17% 4	3020
	11th to 20th	Day	26% 32% 6% 10% 17% 9%	3621
	21st to end	Day	30% 27% 4 15% 21% 3	4587
January	1st to 10th	Night	37% 21% 7% 15% 16% 5%	7635
	11th to 20th	Night	31% 21% 3 11% 23% 11%	7680
	21st to end	Night	44% 22% 3 10% 13% 7%	8364
February	1st to 10th	Night	17% 59% 3 7% 10% 4	2520
	11th to 20th	Night	21% 58% 2 8% 8% 3	2459
	21st to end	Night	20% 59% 3 6% 6% 6%	2076
March	1st to 10th	Night	38% 30% 6% 6% 14% 6%	5080
	11th to 20th	Night	28% 30% 4 11% 12% 15%	5090
	21st to end	Night	34% 29% 5% 14% 13% 5%	5448
April	1st to 10th	Night	18% 60% 3 6% 8% 4	2100
	11th to 20th	Night	17% 60% 4 9% 7% 3	2075
	21st to end	Night	15% 60% 6% 8% 8% 3	2100
May	1st to 10th	Night	41% 25% 9% 12% 11% 2	5862
	11th to 20th	Night	33% 25% 12% 17% 10% 2	5830
	21st to end	Night	38% 27% 12% 16% 6% 1	6427
June	1st to 10th	Night	14% 61% 6% 10% 8% 1	1896
	11th to 20th	Night	15% 62% 9% 9% 4 1	1890
	21st to end	Night	16% 60% 10% 10% 3	1890
July	1st to 10th	Night	29% 33% 21% 13% 4	4125
	11th to 20th	Night	26% 34% 30% 9% 1	4071
	21st to end	Night	32% 32% 12% 20% 4	4460
August	1st to 10th	Night	8% 37% 9% 23% 22% 1	3232
	11th to 20th	Night	15% 40% 17% 24% 4 1	3230
	21st to end	Night	26% 39% 8% 12% 12% 3	3586
September	1st to 10th	Night	14% 60% 8% 9% 7% 3	2000
	11th to 20th	Night	12% 59% 8% 12% 7% 3	2000
	21st to end	Night	15% 58% 6% 12% 7% 3	2192
October	1st to 10th	Night	13% 36% 5% 8% 14% 24%	3920
	11th to 20th	Night	7% 35% 2 14% 17% 25%	3899
	21st to end	Night	12% 35% 3 6% 28% 15%	4252
November	1st to 10th	Night	20% 54% 5% 7% 10% 5%	2460
	11th to 20th	Night	18% 54% 3 8% 11% 7%	2520
	21st to end	Night	17% 54% 4 9% 12% 5%	2679
December	1st to 10th	Night	33% 34% 2 10% 18% 2	4380
	11th to 20th	Night	28% 32% 5% 11% 16% 8%	4756
	21st to end	Night	33% 26% 4 14% 18% 5%	6633

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	23% 25% 33% 15% 3 1	330
	11th to 20th	Day	22% 19% 40% 14% 5%	330
	21st to end	Day	31% 22% 35% 10% 2	363
February	1st to 10th	Day	23% 21% 37% 15% 3 1	330
	11th to 20th	Day	37% 18% 29% 11% 4 1	330
	21st to end	Day	25% 29% 38% 7% 2	273
March	1st to 10th	Day	25% 19% 38% 15% 3	330
	11th to 20th	Day	27% 20% 37% 13% 3	330
	21st to end	Day	18% 21% 31% 25% 5%	423
April	1st to 10th	Day	20% 15% 35% 22% 7% 1	540
	11th to 20th	Day	23% 16% 32% 19% 10% 2	532
	21st to end	Day	12% 12% 32% 30% 11% 3	540
May	1st to 10th	Day	19% 13% 29% 27% 11% 2	543
	11th to 20th	Day	13% 7% 28% 27% 17% 7%	567
	21st to end	Day	8% 5% 24% 33% 21% 8%	617
June	1st to 10th	Day	10% 4 18% 32% 27% 9%	767
	11th to 20th	Day	9% 3 16% 36% 27% 8%	780
	21st to end	Day	10% 2 13% 27% 30% 17%	780
July	1st to 10th	Day	6% 3 8% 26% 41% 17%	771
	11th to 20th	Day	6% 2 7% 24% 41% 21%	744
	21st to end	Day	9% 3 7% 25% 41% 15%	729
August	1st to 10th	Day	5% 2 6% 21% 40% 26%	526
	11th to 20th	Day	8% 2 4 16% 45% 25%	519
	21st to end	Day	11% 4 8% 26% 28% 23%	583
September	1st to 10th	Day	10% 3 11% 28% 31% 16%	530
	11th to 20th	Day	11% 3 12% 25% 34% 14%	530
	21st to end	Day	12% 3 12% 31% 31% 12%	512
October	1st to 10th	Day	6% 7% 25% 39% 17% 5%	330
	11th to 20th	Day	9% 5% 27% 34% 20% 5%	330
	21st to end	Day	18% 10% 25% 28% 14% 4	363
November	1st to 10th	Day	15% 6% 33% 31% 12% 2	330
	11th to 20th	Day	14% 12% 34% 28% 12%	330
	21st to end	Day	17% 18% 37% 19% 6% 3	330
December	1st to 10th	Day	23% 15% 34% 22% 5% 1	330
	11th to 20th	Day	19% 15% 38% 22% 5% 1	330
	21st to end	Day	25% 17% 32% 20% 5% 1	363
January	1st to 10th	Night	24% 15% 33% 23% 5%	1050
	11th to 20th	Night	22% 16% 32% 23% 7%	1050
	21st to end	Night	26% 14% 33% 23% 5%	1143
February	1st to 10th	Night	20% 17% 38% 23% 4	840
	11th to 20th	Night	28% 16% 29% 22% 5%	823
	21st to end	Night	23% 22% 33% 18% 3	692
March	1st to 10th	Night	19% 16% 34% 25% 5%	657
	11th to 20th	Night	17% 14% 35% 27% 6%	630
	21st to end	Night	17% 12% 39% 26% 7%	693
April	1st to 10th	Night	19% 13% 35% 25% 8% 1	630
	11th to 20th	Night	17% 10% 37% 26% 11%	623
	21st to end	Night	11% 9% 36% 30% 13% 1	630
May	1st to 10th	Night	14% 5% 30% 37% 13% 1	630
	11th to 20th	Night	12% 5% 26% 35% 21% 1	621
	21st to end	Night	8% 4 18% 40% 25% 5%	683
June	1st to 10th	Night	7% 3 14% 38% 33% 4	630
	11th to 20th	Night	4 1 12% 39% 37% 7%	630
	21st to end	Night	5% 2 8% 32% 38% 15%	630
July	1st to 10th	Night	5% 2 5% 27% 47% 15%	615
	11th to 20th	Night	6% 2 4 26% 45% 18%	597
	21st to end	Night	6% 1 6% 26% 49% 12%	657
August	1st to 10th	Night	3 1 5% 22% 53% 17%	597
	11th to 20th	Night	5% 1 6% 22% 49% 18%	594
	21st to end	Night	10% 2 6% 27% 39% 16%	660
September	1st to 10th	Night	9% 2 7% 29% 42% 12%	600
	11th to 20th	Night	10% 2 9% 29% 42% 7%	600
	21st to end	Night	10% 3 14% 32% 36% 5%	780
October	1st to 10th	Night	9% 2 19% 36% 31% 3	800
	11th to 20th	Night	10% 5% 25% 36% 22% 3	800
	21st to end	Night	16% 7% 21% 34% 20% 2	880
November	1st to 10th	Night	14% 6% 22% 34% 21% 2	820
	11th to 20th	Night	15% 8% 28% 34% 13% 1	840
	21st to end	Night	18% 10% 32% 28% 12% 1	840
December	1st to 10th	Night	22% 11% 29% 28% 9%	840
	11th to 20th	Night	20% 11% 33% 29% 7%	898
	21st to end	Night	27% 13% 28% 26% 6%	1155

Page generated at 2026-07-13 02:14:47. Version ac07

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