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 2025-01-22.

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%	105218
	Night	13% 6% 2 4 3 3 3 3 4 6% 52%	104300
Transparency	Day	35% 5% 9% 21% 20% 11%	109088
	Night	33% 4 10% 20% 20% 13%	108557
Seeing	Day	15% 8% 24% 30% 18% 4	15034
	Night	16% 7% 26% 35% 14% 1	25101

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% 8% 2 4 3 3 3 2 3 5% 55%	2937
	11th to 20th	Day	13% 7% 2 3 2 2 2 2 3 5% 57%	2975
	21st to end	Day	15% 7% 1 5% 3 3 3 3 4 8% 48%	3497
February	1st to 10th	Day	14% 8% 2 5% 3 4 3 3 4 5% 49%	1900
	11th to 20th	Day	16% 7% 2 5% 4 3 3 3 5% 4 47%	1849
	21st to end	Day	15% 5% 2 5% 2 2 3 3 4 5% 52%	1570
March	1st to 10th	Day	16% 8% 2 5% 4 3 3 3 4 5% 46%	3281
	11th to 20th	Day	20% 11% 3 8% 4 4 3 2 4 5% 35%	3480
	21st to end	Day	14% 6% 2 5% 3 3 3 3 6% 7% 47%	3814
April	1st to 10th	Day	13% 7% 3 5% 4 3 3 4 5% 6% 46%	2090
	11th to 20th	Day	18% 5% 2 6% 4 3 3 3 5% 6% 44%	2187
	21st to end	Day	15% 8% 2 6% 4 3 4 3 4 5% 47%	2340
May	1st to 10th	Day	22% 9% 3 6% 4 4 4 3 5% 6% 35%	4082
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 5% 6% 44%	4082
	21st to end	Day	13% 7% 2 6% 5% 4 6% 5% 6% 9% 38%	4477
June	1st to 10th	Day	6% 5% 2 5% 5% 4 5% 3 6% 9% 49%	2470
	11th to 20th	Day	7% 6% 2 5% 4 5% 5% 3 7% 9% 47%	2470
	21st to end	Day	9% 5% 2 6% 7% 5% 6% 5% 6% 8% 40%	2470
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 8% 46%	4120
	11th to 20th	Day	5% 4 2 4 4 4 5% 5% 6% 10% 52%	4120
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 8% 10% 40%	4465
August	1st to 10th	Day	1 2 1 3 3 3 3 3 5% 8% 68%	3858
	11th to 20th	Day	4 3 1 4 3 4 3 3 6% 10% 59%	3726
	21st to end	Day	5% 4 2 5% 4 4 5% 4 7% 10% 49%	4121
September	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 3 5% 7% 41%	2090
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 3 6% 7% 42%	2090
	21st to end	Day	9% 5% 2 7% 4 4 5% 3 5% 8% 47%	2090
October	1st to 10th	Day	6% 4 1 6% 4 4 5% 4 4 7% 53%	2090
	11th to 20th	Day	14% 5% 2 5% 3 2 4 3 4 6% 52%	2039
	21st to end	Day	13% 4 1 5% 3 3 2 2 3 4 57%	2090
November	1st to 10th	Day	15% 8% 2 6% 4 3 3 2 5% 7% 45%	1929
	11th to 20th	Day	11% 7% 2 5% 4 4 3 4 4 5% 52%	1800
	21st to end	Day	15% 8% 3 5% 4 2 4 3 4 5% 48%	1800
December	1st to 10th	Day	24% 9% 3 6% 3 2 2 2 4 6% 40%	2940
	11th to 20th	Day	15% 9% 2 5% 4 3 4 3 5% 6% 45%	3391
	21st to end	Day	12% 7% 2 4 2 2 2 2 3 3 61%	4488
January	1st to 10th	Night	14% 5% 3 5% 2 3 3 2 3 5% 55%	4238
	11th to 20th	Night	12% 5% 1 3 2 2 2 2 3 4 67%	4245
	21st to end	Night	18% 8% 3 5% 4 3 2 2 4 6% 45%	4192
February	1st to 10th	Night	16% 6% 2 4 3 2 2 3 4 5% 52%	2280
	11th to 20th	Night	16% 7% 2 4 3 3 3 2 5% 5% 51%	2218
	21st to end	Night	16% 7% 2 5% 3 3 3 3 5% 6% 49%	1884
March	1st to 10th	Night	15% 9% 2 6% 4 3 4 3 5% 6% 45%	3576
	11th to 20th	Night	19% 12% 2 7% 4 4 4 3 4 4 36%	3480
	21st to end	Night	16% 8% 2 4 3 3 4 3 4 6% 47%	3817
April	1st to 10th	Night	13% 7% 2 5% 3 3 4 2 5% 5% 51%	2090
	11th to 20th	Night	18% 7% 2 5% 4 3 3 2 4 5% 47%	1938
	21st to end	Night	18% 7% 2 5% 4 2 2 2 4 4 49%	1838
May	1st to 10th	Night	25% 10% 2 4 2 2 2 2 4 6% 39%	2823
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 6% 50%	2823
	21st to end	Night	15% 6% 2 5% 3 3 4 3 5% 8% 46%	3090
June	1st to 10th	Night	5% 4 1 3 3 3 4 4 5% 10% 59%	1710
	11th to 20th	Night	5% 5% 2 4 3 3 3 3 5% 9% 59%	1710
	21st to end	Night	6% 6% 2 5% 3 3 3 4 6% 9% 53%	1710
July	1st to 10th	Night	6% 4 1 4 4 4 4 3 6% 11% 53%	2850
	11th to 20th	Night	4 3 2 4 4 4 4 3 6% 10% 56%	2850
	21st to end	Night	11% 8% 2 6% 4 4 4 4 6% 9% 44%	3098
August	1st to 10th	Night	1 2 1 3 2 2 3 2 4 9% 72%	3077
	11th to 20th	Night	3 3 1 3 2 3 3 3 6% 12% 61%	3113
	21st to end	Night	6% 5% 2 4 3 3 4 3 7% 10% 53%	3542
September	1st to 10th	Night	10% 5% 3 5% 4 5% 4 3 5% 7% 50%	2090
	11th to 20th	Night	10% 7% 3 6% 4 4 4 3 5% 8% 46%	2090
	21st to end	Night	7% 6% 2 5% 4 3 4 3 5% 7% 53%	2090
October	1st to 10th	Night	7% 6% 1 4 3 2 3 2 4 6% 61%	2090
	11th to 20th	Night	14% 4 1 4 3 3 3 3 5% 5% 55%	2119
	21st to end	Night	15% 4 2 4 2 2 2 2 2 4 62%	2508
November	1st to 10th	Night	15% 7% 2 4 3 3 3 2 4 5% 53%	2365
	11th to 20th	Night	10% 7% 2 4 3 3 3 2 5% 6% 57%	2600
	21st to end	Night	17% 6% 1 4 3 3 2 3 5% 6% 49%	2600
December	1st to 10th	Night	24% 7% 2 5% 3 3 3 3 4 5% 41%	4250
	11th to 20th	Night	19% 9% 2 4 3 2 2 2 3 4 48%	4816
	21st to end	Night	16% 6% 2 3 2 2 2 1 2 4 58%	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	33% 2 4 17% 24% 20%	2938
	11th to 20th	Day	32% 2 5% 14% 30% 18%	2975
	21st to end	Day	37% 4 6% 14% 29% 11%	3497
February	1st to 10th	Day	39% 4 6% 12% 22% 17%	1900
	11th to 20th	Day	40% 3 5% 17% 18% 16%	1859
	21st to end	Day	34% 4 7% 17% 21% 17%	1570
March	1st to 10th	Day	42% 3 7% 12% 21% 16%	3281
	11th to 20th	Day	52% 4 7% 10% 17% 10%	3480
	21st to end	Day	36% 4 10% 17% 17% 16%	3814
April	1st to 10th	Day	39% 5% 8% 16% 19% 13%	2090
	11th to 20th	Day	41% 4 7% 17% 19% 12%	2187
	21st to end	Day	40% 5% 6% 18% 19% 12%	2340
May	1st to 10th	Day	49% 6% 9% 13% 16% 7%	4120
	11th to 20th	Day	39% 6% 7% 18% 20% 9%	4086
	21st to end	Day	39% 8% 11% 27% 12% 3	4489
June	1st to 10th	Day	31% 7% 12% 27% 20% 3	2470
	11th to 20th	Day	33% 7% 11% 24% 18% 6%	2470
	21st to end	Day	40% 7% 11% 22% 16% 4	2470
July	1st to 10th	Day	33% 7% 16% 36% 7% 1	4056
	11th to 20th	Day	27% 6% 16% 32% 17% 2	4051
	21st to end	Day	34% 8% 17% 29% 12% 1	4325
August	1st to 10th	Day	15% 5% 11% 41% 28% 1	3844
	11th to 20th	Day	22% 4 22% 38% 12% 2	3762
	21st to end	Day	29% 8% 18% 24% 19% 2	4120
September	1st to 10th	Day	43% 6% 9% 18% 16% 8%	2090
	11th to 20th	Day	38% 8% 11% 20% 17% 5%	2090
	21st to end	Day	33% 6% 10% 20% 18% 12%	2090
October	1st to 10th	Day	19% 4 6% 12% 26% 34%	3480
	11th to 20th	Day	28% 3 7% 17% 22% 22%	3370
	21st to end	Day	28% 3 5% 16% 35% 13%	3428
November	1st to 10th	Day	40% 3 8% 19% 19% 11%	1929
	11th to 20th	Day	35% 4 7% 17% 20% 18%	1800
	21st to end	Day	40% 4 5% 16% 21% 15%	1800
December	1st to 10th	Day	49% 2 5% 18% 16% 10%	2940
	11th to 20th	Day	40% 4 8% 13% 21% 14%	3389
	21st to end	Day	30% 2 4 15% 29% 20%	4488
January	1st to 10th	Night	34% 2 4 13% 25% 22%	4238
	11th to 20th	Night	25% 2 3 18% 26% 26%	4245
	21st to end	Night	42% 2 5% 12% 26% 13%	4192
February	1st to 10th	Night	36% 3 7% 12% 22% 20%	2280
	11th to 20th	Night	37% 3 6% 17% 20% 17%	2218
	21st to end	Night	37% 3 7% 12% 22% 18%	1884
March	1st to 10th	Night	41% 4 7% 14% 21% 13%	3576
	11th to 20th	Night	50% 5% 8% 11% 14% 12%	3480
	21st to end	Night	39% 4 9% 16% 15% 16%	3817
April	1st to 10th	Night	36% 4 10% 18% 16% 17%	2090
	11th to 20th	Night	40% 5% 8% 14% 17% 17%	1938
	21st to end	Night	40% 4 8% 18% 18% 11%	1838
May	1st to 10th	Night	48% 4 8% 18% 12% 10%	2850
	11th to 20th	Night	34% 5% 11% 22% 18% 9%	2824
	21st to end	Night	36% 6% 13% 28% 12% 5%	3099
June	1st to 10th	Night	21% 6% 18% 33% 19% 3	1710
	11th to 20th	Night	24% 6% 15% 29% 22% 5%	1710
	21st to end	Night	28% 6% 17% 28% 17% 4	1710
July	1st to 10th	Night	25% 8% 23% 37% 6% 1	2805
	11th to 20th	Night	24% 6% 19% 32% 17% 2	2796
	21st to end	Night	36% 6% 21% 28% 8% 1	2999
August	1st to 10th	Night	14% 3 16% 46% 21% 1	3067
	11th to 20th	Night	18% 5% 30% 35% 10% 2	3143
	21st to end	Night	26% 10% 20% 25% 17% 2	3542
September	1st to 10th	Night	34% 6% 13% 21% 18% 8%	2090
	11th to 20th	Night	35% 8% 15% 21% 15% 6%	2090
	21st to end	Night	31% 7% 12% 20% 18% 13%	2090
October	1st to 10th	Night	17% 3 7% 14% 23% 36%	3480
	11th to 20th	Night	25% 3 9% 14% 30% 19%	3514
	21st to end	Night	27% 2 5% 18% 34% 15%	4120
November	1st to 10th	Night	34% 4 7% 19% 22% 14%	2365
	11th to 20th	Night	29% 4 8% 21% 20% 18%	2600
	21st to end	Night	36% 4 8% 16% 21% 16%	2600
December	1st to 10th	Night	46% 3 8% 15% 15% 13%	4250
	11th to 20th	Night	42% 2 5% 11% 21% 18%	4817
	21st to end	Night	32% 2 3 15% 28% 18%	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% 14% 37% 19% 5% 1	300
	11th to 20th	Day	27% 13% 33% 21% 5% 1	300
	21st to end	Day	21% 17% 36% 19% 6%	300
February	1st to 10th	Day	27% 13% 31% 21% 6%	270
	11th to 20th	Day	26% 20% 34% 16% 3 1	270
	21st to end	Day	24% 20% 40% 14% 2	225
March	1st to 10th	Day	22% 19% 40% 17% 2	270
	11th to 20th	Day	19% 25% 37% 15% 4	270
	21st to end	Day	20% 20% 42% 16% 2 1	297
April	1st to 10th	Day	24% 20% 40% 14% 2	270
	11th to 20th	Day	19% 21% 38% 19% 3	385
	21st to end	Day	20% 15% 42% 19% 4	520
May	1st to 10th	Day	18% 13% 40% 23% 5%	653
	11th to 20th	Day	22% 9% 31% 29% 8% 1	675
	21st to end	Day	20% 7% 33% 29% 10%	734
June	1st to 10th	Day	8% 4 20% 44% 21% 3	680
	11th to 20th	Day	11% 2 21% 47% 17% 2	680
	21st to end	Day	10% 2 15% 39% 27% 7%	680
July	1st to 10th	Day	12% 3 12% 36% 30% 8%	671
	11th to 20th	Day	6% 1 7% 43% 36% 7%	668
	21st to end	Day	6% 1 7% 38% 39% 8%	739
August	1st to 10th	Day	3 6% 40% 39% 12%	534
	11th to 20th	Day	8% 6% 36% 39% 10%	477
	21st to end	Day	7% 1 7% 31% 38% 16%	506
September	1st to 10th	Day	9% 1 11% 30% 37% 12%	300
	11th to 20th	Day	6% 1 5% 34% 43% 11%	300
	21st to end	Day	10% 1 9% 35% 35% 10%	300
October	1st to 10th	Day	5% 2 18% 38% 29% 7%	300
	11th to 20th	Day	13% 5% 21% 41% 18% 3	300
	21st to end	Day	19% 13% 27% 27% 12% 2	330
November	1st to 10th	Day	21% 7% 32% 26% 12% 2	300
	11th to 20th	Day	20% 12% 27% 25% 11% 5%	300
	21st to end	Day	22% 19% 34% 15% 8% 2	300
December	1st to 10th	Day	20% 10% 31% 28% 10% 2	300
	11th to 20th	Day	21% 15% 34% 20% 9% 1	300
	21st to end	Day	19% 14% 36% 23% 7% 1	330
January	1st to 10th	Night	18% 10% 31% 33% 9%	1000
	11th to 20th	Night	21% 11% 32% 30% 6%	995
	21st to end	Night	23% 10% 30% 30% 7%	840
February	1st to 10th	Night	23% 12% 31% 28% 6%	760
	11th to 20th	Night	24% 13% 34% 26% 4	744
	21st to end	Night	23% 11% 44% 21% 1	628
March	1st to 10th	Night	23% 12% 35% 26% 3	667
	11th to 20th	Night	21% 11% 36% 28% 5%	570
	21st to end	Night	18% 12% 42% 25% 3	627
April	1st to 10th	Night	15% 14% 42% 26% 3	570
	11th to 20th	Night	20% 13% 40% 24% 2	563
	21st to end	Night	20% 11% 43% 21% 5%	570
May	1st to 10th	Night	18% 12% 40% 25% 5%	570
	11th to 20th	Night	16% 7% 36% 33% 7% 1	561
	21st to end	Night	17% 7% 34% 33% 9%	617
June	1st to 10th	Night	5% 2 27% 47% 18% 1	570
	11th to 20th	Night	8% 2 32% 47% 11%	570
	21st to end	Night	8% 3 23% 44% 21% 2	570
July	1st to 10th	Night	10% 2 11% 50% 25% 3	555
	11th to 20th	Night	5% 1 12% 52% 28% 2	552
	21st to end	Night	6% 1 9% 53% 28% 3	624
August	1st to 10th	Night	2 1 9% 51% 34% 2	567
	11th to 20th	Night	5% 1 7% 52% 34% 2	564
	21st to end	Night	7% 2 7% 41% 38% 5%	627
September	1st to 10th	Night	11% 1 9% 42% 34% 4	570
	11th to 20th	Night	12% 1 8% 39% 38% 2	570
	21st to end	Night	9% 2 13% 39% 32% 4	570
October	1st to 10th	Night	9% 1 14% 45% 29% 2	570
	11th to 20th	Night	14% 5% 20% 40% 18% 2	599
	21st to end	Night	17% 8% 25% 28% 21% 2	836
November	1st to 10th	Night	18% 6% 22% 36% 17% 1	805
	11th to 20th	Night	14% 7% 26% 37% 14% 1	1000
	21st to end	Night	22% 10% 27% 30% 11% 1	1000
December	1st to 10th	Night	20% 8% 27% 36% 9% 1	1000
	11th to 20th	Night	23% 10% 29% 30% 7%	1000
	21st to end	Night	18% 11% 29% 34% 8%	1100

Page generated at 2025-01-19 02:06:48. Version ac07

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