Corkill Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Corkill Astronomical Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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 3 5% 7% 48%	132348
	Night	13% 6% 2 4 3 3 3 3 4 6% 52%	130822
Transparency	Day	35% 4 9% 20% 19% 12%	136179
	Night	33% 4 10% 19% 20% 14%	135055
Seeing	Day	15% 8% 25% 30% 18% 4	16761
	Night	16% 7% 26% 35% 14% 1	26793

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	11% 6% 2 5% 3 3 3 2 4 5% 56%	5289
	11th to 20th	Day	12% 7% 2 4 2 2 3 2 4 5% 57%	5385
	21st to end	Day	21% 7% 2 5% 3 3 3 3 3 6% 46%	6468
February	1st to 10th	Day	14% 8% 2 5% 3 3 3 3 4 5% 49%	2100
	11th to 20th	Day	16% 7% 2 5% 4 3 3 3 5% 5% 48%	2049
	21st to end	Day	15% 5% 2 5% 2 2 2 3 4 5% 53%	1730
March	1st to 10th	Day	13% 6% 2 4 3 3 3 2 4 6% 55%	4821
	11th to 20th	Day	14% 8% 2 5% 3 3 2 2 3 5% 53%	5090
	21st to end	Day	14% 7% 2 5% 3 3 3 3 5% 7% 48%	5583
April	1st to 10th	Day	14% 8% 3 5% 4 3 3 3 5% 6% 45%	2310
	11th to 20th	Day	18% 5% 2 6% 4 3 3 3 5% 6% 43%	2420
	21st to end	Day	15% 8% 2 6% 4 3 4 3 5% 5% 45%	2586
May	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 5% 7% 37%	5992
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	5941
	21st to end	Day	14% 8% 3 6% 5% 4 5% 5% 6% 8% 35%	6434
June	1st to 10th	Day	6% 5% 2 5% 5% 4 6% 3 6% 10% 49%	2600
	11th to 20th	Day	8% 6% 2 5% 4 4 5% 4 7% 8% 47%	2600
	21st to end	Day	8% 5% 2 6% 7% 5% 6% 5% 6% 9% 41%	2600
July	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	5900
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 6% 9% 46%	5900
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 8% 10% 45%	6422
August	1st to 10th	Day	1 2 1 3 3 3 3 3 5% 8% 68%	3980
	11th to 20th	Day	4 3 1 4 3 4 3 3 6% 10% 59%	3846
	21st to end	Day	5% 4 2 5% 5% 4 5% 4 7% 10% 49%	4251
September	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 3 5% 7% 42%	2200
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 3 6% 8% 43%	2200
	21st to end	Day	9% 5% 2 7% 4 4 5% 3 5% 9% 48%	2200
October	1st to 10th	Day	6% 4 1 6% 3 5% 5% 4 4 7% 55%	2200
	11th to 20th	Day	13% 5% 2 5% 3 2 4 3 4 6% 53%	2146
	21st to end	Day	12% 4 1 5% 3 3 3 2 3 4 58%	2200
November	1st to 10th	Day	14% 7% 2 6% 4 3 3 2 5% 7% 47%	2027
	11th to 20th	Day	10% 7% 1 5% 4 4 3 4 4 5% 52%	1890
	21st to end	Day	16% 8% 3 5% 4 2 4 3 3 5% 47%	1890
December	1st to 10th	Day	24% 9% 3 6% 3 2 2 2 4 6% 40%	3030
	11th to 20th	Day	15% 9% 2 5% 4 3 4 3 5% 6% 45%	3481
	21st to end	Day	12% 7% 2 4 2 2 2 2 3 3 60%	4587
January	1st to 10th	Night	12% 5% 2 5% 3 3 3 2 3 5% 58%	7634
	11th to 20th	Night	10% 4 1 3 2 2 2 2 5% 5% 65%	7648
	21st to end	Night	26% 7% 2 4 3 3 2 2 3 4 43%	7746
February	1st to 10th	Night	15% 6% 2 4 3 2 2 3 4 5% 53%	2520
	11th to 20th	Night	16% 7% 2 4 3 3 3 2 5% 5% 50%	2458
	21st to end	Night	15% 7% 2 5% 3 3 3 3 5% 5% 50%	2076
March	1st to 10th	Night	14% 8% 2 5% 3 3 3 3 5% 6% 48%	5261
	11th to 20th	Night	13% 8% 2 5% 3 3 3 2 4 5% 53%	5090
	21st to end	Night	16% 10% 2 4 3 3 3 3 4 5% 48%	5588
April	1st to 10th	Night	14% 7% 2 5% 3 3 4 2 5% 5% 49%	2310
	11th to 20th	Night	19% 7% 2 5% 4 3 3 2 4 5% 46%	2144
	21st to end	Night	20% 8% 2 5% 4 2 2 2 3 4 46%	2032
May	1st to 10th	Night	24% 9% 3 5% 3 3 3 2 4 6% 39%	4143
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 3 5% 7% 51%	4107
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 10% 39%	4443
June	1st to 10th	Night	5% 4 1 3 3 3 4 4 5% 10% 58%	1800
	11th to 20th	Night	6% 5% 2 4 3 3 3 3 4 9% 58%	1800
	21st to end	Night	6% 6% 2 5% 4 3 3 4 6% 9% 53%	1800
July	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 6% 9% 41%	4078
	11th to 20th	Night	5% 4 2 5% 4 4 4 3 6% 10% 52%	4077
	21st to end	Night	8% 6% 2 5% 4 4 4 4 6% 9% 48%	4449
August	1st to 10th	Night	1 2 1 3 2 2 3 2 4 8% 72%	3175
	11th to 20th	Night	3 3 1 3 2 3 3 3 6% 12% 61%	3213
	21st to end	Night	6% 5% 2 5% 3 3 4 3 7% 10% 53%	3654
September	1st to 10th	Night	10% 5% 3 6% 4 5% 4 3 5% 7% 50%	2200
	11th to 20th	Night	10% 6% 3 5% 4 4 4 3 6% 8% 46%	2200
	21st to end	Night	7% 6% 2 5% 4 3 4 4 5% 7% 53%	2200
October	1st to 10th	Night	6% 5% 1 4 3 2 3 2 4 6% 62%	2200
	11th to 20th	Night	13% 4 1 4 3 3 3 3 5% 6% 56%	2231
	21st to end	Night	15% 4 2 4 2 2 2 2 2 4 62%	2640
November	1st to 10th	Night	14% 7% 2 4 3 3 3 2 4 5% 55%	2486
	11th to 20th	Night	10% 7% 2 4 2 3 3 2 5% 6% 57%	2730
	21st to end	Night	18% 6% 1 4 3 3 2 3 5% 6% 49%	2730
December	1st to 10th	Night	24% 7% 2 5% 3 3 3 2 4 5% 41%	4380
	11th to 20th	Night	19% 9% 2 5% 3 2 3 2 3 4 49%	4946
	21st to end	Night	16% 6% 2 3 2 2 2 1 2 4 58%	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	32% 3 5% 21% 26% 14%	5290
	11th to 20th	Day	33% 1 4 15% 26% 22%	5385
	21st to end	Day	41% 3 5% 14% 25% 12%	6468
February	1st to 10th	Day	39% 4 5% 12% 22% 18%	2100
	11th to 20th	Day	39% 3 5% 19% 18% 14%	2059
	21st to end	Day	33% 3 7% 17% 23% 17%	1730
March	1st to 10th	Day	33% 2 5% 16% 17% 27%	4821
	11th to 20th	Day	36% 3 5% 9% 15% 32%	5090
	21st to end	Day	37% 3 8% 18% 19% 15%	5585
April	1st to 10th	Day	40% 5% 8% 16% 18% 13%	2310
	11th to 20th	Day	42% 4 7% 16% 18% 12%	2420
	21st to end	Day	41% 5% 8% 18% 17% 11%	2586
May	1st to 10th	Day	46% 5% 8% 18% 17% 6%	6030
	11th to 20th	Day	34% 5% 10% 17% 22% 11%	5945
	21st to end	Day	44% 7% 9% 24% 13% 3	6447
June	1st to 10th	Day	31% 6% 12% 28% 19% 3	2600
	11th to 20th	Day	33% 7% 11% 24% 20% 5%	2600
	21st to end	Day	39% 7% 12% 23% 15% 4	2600
July	1st to 10th	Day	45% 6% 16% 27% 6%	5836
	11th to 20th	Day	33% 6% 16% 30% 14% 2	5831
	21st to end	Day	31% 7% 17% 26% 18% 1	6240
August	1st to 10th	Day	15% 5% 11% 41% 28% 1	3966
	11th to 20th	Day	22% 4 23% 38% 11% 2	3882
	21st to end	Day	29% 8% 18% 24% 19% 2	4250
September	1st to 10th	Day	43% 6% 9% 18% 17% 7%	2200
	11th to 20th	Day	37% 8% 11% 21% 17% 5%	2200
	21st to end	Day	33% 6% 10% 20% 18% 12%	2200
October	1st to 10th	Day	18% 4 6% 12% 27% 33%	3590
	11th to 20th	Day	27% 3 7% 18% 23% 22%	3477
	21st to end	Day	28% 3 5% 16% 35% 13%	3538
November	1st to 10th	Day	38% 3 8% 19% 21% 12%	2027
	11th to 20th	Day	35% 4 7% 17% 20% 18%	1890
	21st to end	Day	41% 3 5% 16% 21% 14%	1890
December	1st to 10th	Day	48% 2 5% 17% 17% 10%	3030
	11th to 20th	Day	40% 4 7% 13% 21% 14%	3479
	21st to end	Day	30% 2 4 15% 29% 20%	4587
January	1st to 10th	Night	31% 2 6% 17% 28% 16%	7635
	11th to 20th	Night	24% 1 3 17% 29% 25%	7648
	21st to end	Night	46% 2 3 10% 23% 16%	7746
February	1st to 10th	Night	35% 3 7% 13% 22% 21%	2520
	11th to 20th	Night	36% 3 7% 18% 20% 16%	2458
	21st to end	Night	37% 3 6% 12% 23% 18%	2076
March	1st to 10th	Night	36% 3 6% 18% 18% 19%	5261
	11th to 20th	Night	35% 4 5% 11% 16% 29%	5090
	21st to end	Night	39% 3 7% 18% 17% 15%	5588
April	1st to 10th	Night	37% 4 10% 17% 15% 17%	2310
	11th to 20th	Night	41% 5% 8% 14% 16% 16%	2144
	21st to end	Night	43% 4 9% 17% 17% 11%	2032
May	1st to 10th	Night	48% 4 7% 17% 13% 10%	4170
	11th to 20th	Night	33% 5% 11% 19% 19% 13%	4108
	21st to end	Night	40% 5% 14% 26% 11% 4	4452
June	1st to 10th	Night	22% 6% 18% 33% 18% 3	1800
	11th to 20th	Night	25% 5% 15% 28% 22% 5%	1800
	21st to end	Night	28% 6% 18% 28% 16% 3	1800
July	1st to 10th	Night	39% 8% 22% 27% 4	4035
	11th to 20th	Night	28% 5% 22% 31% 13% 1	4026
	21st to end	Night	32% 6% 20% 26% 13% 3	4320
August	1st to 10th	Night	14% 3 15% 46% 22% 1	3165
	11th to 20th	Night	18% 5% 30% 35% 9% 2	3243
	21st to end	Night	27% 10% 20% 25% 17% 2	3654
September	1st to 10th	Night	34% 6% 13% 21% 19% 8%	2200
	11th to 20th	Night	34% 8% 16% 21% 15% 6%	2200
	21st to end	Night	31% 6% 11% 21% 18% 12%	2200
October	1st to 10th	Night	16% 3 8% 14% 24% 35%	3590
	11th to 20th	Night	24% 3 9% 16% 30% 19%	3626
	21st to end	Night	27% 2 5% 18% 34% 15%	4252
November	1st to 10th	Night	33% 4 7% 18% 24% 14%	2486
	11th to 20th	Night	29% 4 8% 21% 21% 18%	2730
	21st to end	Night	37% 4 8% 16% 21% 15%	2730
December	1st to 10th	Night	46% 3 7% 15% 15% 13%	4380
	11th to 20th	Night	42% 2 5% 11% 21% 18%	4947
	21st to end	Night	33% 2 4 15% 28% 18%	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% 13% 38% 19% 6% 1	330
	11th to 20th	Day	25% 13% 34% 22% 5% 1	330
	21st to end	Day	23% 16% 38% 18% 6%	363
February	1st to 10th	Day	25% 12% 33% 22% 7% 1	330
	11th to 20th	Day	24% 18% 35% 19% 3 1	330
	21st to end	Day	22% 19% 40% 16% 3	273
March	1st to 10th	Day	25% 18% 38% 17% 2	330
	11th to 20th	Day	17% 23% 38% 17% 5%	330
	21st to end	Day	19% 17% 39% 20% 4 1	363
April	1st to 10th	Day	25% 18% 38% 17% 2	330
	11th to 20th	Day	21% 19% 38% 18% 3	458
	21st to end	Day	21% 15% 42% 18% 3	606
May	1st to 10th	Day	19% 13% 41% 23% 5%	753
	11th to 20th	Day	20% 10% 33% 29% 7% 1	756
	21st to end	Day	20% 7% 34% 29% 10%	789
June	1st to 10th	Day	8% 5% 21% 44% 20% 2	730
	11th to 20th	Day	11% 2 22% 46% 17% 2	730
	21st to end	Day	9% 2 16% 39% 27% 7%	730
July	1st to 10th	Day	14% 3 12% 36% 28% 8%	721
	11th to 20th	Day	7% 1 7% 43% 35% 7%	718
	21st to end	Day	7% 1 7% 38% 38% 9%	794
August	1st to 10th	Day	3 6% 39% 39% 12%	576
	11th to 20th	Day	8% 6% 36% 39% 11%	517
	21st to end	Day	7% 1 7% 30% 39% 16%	548
September	1st to 10th	Day	10% 2 10% 29% 37% 12%	330
	11th to 20th	Day	6% 1 5% 35% 43% 10%	330
	21st to end	Day	9% 2 9% 34% 36% 11%	330
October	1st to 10th	Day	5% 2 17% 37% 31% 8%	330
	11th to 20th	Day	12% 5% 21% 39% 20% 4	330
	21st to end	Day	18% 13% 26% 29% 12% 2	363
November	1st to 10th	Day	19% 7% 30% 27% 14% 2	330
	11th to 20th	Day	18% 12% 26% 28% 11% 4	330
	21st to end	Day	23% 18% 33% 16% 9% 2	330
December	1st to 10th	Day	20% 9% 32% 28% 9% 2	330
	11th to 20th	Day	20% 15% 33% 21% 10% 1	330
	21st to end	Day	20% 13% 34% 26% 8% 1	363
January	1st to 10th	Night	17% 10% 31% 33% 9%	1050
	11th to 20th	Night	20% 11% 32% 31% 6%	1044
	21st to end	Night	23% 11% 30% 29% 7%	924
February	1st to 10th	Night	22% 11% 30% 31% 6%	840
	11th to 20th	Night	23% 13% 34% 26% 4	824
	21st to end	Night	22% 10% 43% 23% 1	692
March	1st to 10th	Night	24% 13% 35% 25% 3	737
	11th to 20th	Night	20% 11% 36% 28% 5%	630
	21st to end	Night	18% 11% 40% 27% 4	693
April	1st to 10th	Night	16% 14% 42% 25% 3	630
	11th to 20th	Night	22% 13% 38% 24% 3	623
	21st to end	Night	22% 11% 41% 20% 4	630
May	1st to 10th	Night	19% 12% 40% 26% 4	630
	11th to 20th	Night	15% 8% 36% 33% 7% 1	609
	21st to end	Night	17% 7% 35% 33% 8%	650
June	1st to 10th	Night	6% 2 28% 46% 18% 1	600
	11th to 20th	Night	8% 2 33% 46% 11%	600
	21st to end	Night	8% 3 22% 45% 21% 2	600
July	1st to 10th	Night	11% 3 11% 49% 24% 3	585
	11th to 20th	Night	6% 1 13% 51% 28% 2	582
	21st to end	Night	6% 1 10% 51% 28% 3	657
August	1st to 10th	Night	2 1 9% 52% 35% 2	597
	11th to 20th	Night	5% 1 7% 51% 34% 2	594
	21st to end	Night	7% 2 7% 41% 38% 5%	660
September	1st to 10th	Night	11% 1 9% 41% 35% 4	600
	11th to 20th	Night	12% 1 7% 40% 38% 2	600
	21st to end	Night	9% 2 13% 41% 31% 4	600
October	1st to 10th	Night	9% 1 13% 44% 31% 2	600
	11th to 20th	Night	14% 5% 20% 40% 19% 2	631
	21st to end	Night	16% 8% 25% 29% 20% 1	880
November	1st to 10th	Night	17% 6% 22% 37% 18% 1	846
	11th to 20th	Night	14% 7% 25% 38% 15% 1	1050
	21st to end	Night	22% 9% 27% 30% 11%	1050
December	1st to 10th	Night	20% 7% 28% 36% 9%	1050
	11th to 20th	Night	23% 10% 29% 30% 8%	1050
	21st to end	Night	18% 11% 28% 34% 8%	1155

Page generated at 2026-05-14 00:58:54. Version ac07

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