Coopers Hawk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coopers Hawk Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 2 7% 5% 5% 5% 5% 7% 8% 40%	126538
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	123169
Transparency	Day	38% 8% 14% 18% 16% 6%	130197
	Night	36% 7% 13% 18% 18% 8%	127579
Seeing	Day	13% 8% 23% 28% 20% 8%	16730
	Night	13% 7% 23% 32% 21% 4	26774

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% 4 2 5% 3 3 4 3 4 5% 54%	4056
	11th to 20th	Day	15% 6% 2 5% 3 2 3 3 4 7% 50%	4060
	21st to end	Day	14% 7% 2 5% 4 3 4 3 5% 7% 45%	4466
February	1st to 10th	Day	14% 7% 2 5% 4 4 5% 4 6% 7% 42%	1859
	11th to 20th	Day	19% 6% 2 5% 3 2 2 3 5% 7% 45%	1948
	21st to end	Day	13% 7% 3 6% 4 3 4 3 6% 7% 47%	1742
March	1st to 10th	Day	22% 8% 2 6% 4 4 3 4 5% 5% 38%	4935
	11th to 20th	Day	8% 4 2 4 3 3 3 3 6% 7% 56%	4980
	21st to end	Day	13% 6% 2 6% 5% 4 4 3 4 5% 48%	5465
April	1st to 10th	Day	9% 4 1 4 5% 4 4 5% 5% 8% 50%	2200
	11th to 20th	Day	11% 4 2 5% 4 4 3 3 6% 7% 50%	2329
	21st to end	Day	5% 4 2 4 4 4 4 5% 7% 8% 53%	2450
May	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 10% 39%	5862
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 7% 9% 37%	5861
	21st to end	Day	7% 4 2 7% 5% 5% 6% 5% 7% 10% 43%	6433
June	1st to 10th	Day	8% 6% 3 8% 7% 6% 7% 7% 8% 9% 30%	2600
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 6% 8% 11% 35%	2600
	21st to end	Day	6% 6% 3 8% 7% 7% 7% 6% 9% 11% 31%	2600
July	1st to 10th	Day	11% 8% 3 10% 8% 8% 8% 6% 10% 10% 18%	5900
	11th to 20th	Day	4 3 2 9% 7% 8% 9% 7% 10% 15% 26%	5900
	21st to end	Day	5% 5% 2 9% 7% 7% 8% 7% 9% 13% 27%	6424
August	1st to 10th	Day	19% 7% 3 9% 6% 6% 7% 6% 8% 10% 18%	4234
	11th to 20th	Day	5% 5% 2 8% 6% 6% 8% 7% 9% 12% 31%	4148
	21st to end	Day	10% 7% 3 8% 6% 6% 5% 5% 8% 10% 32%	4229
September	1st to 10th	Day	10% 7% 3 8% 7% 6% 6% 6% 7% 9% 31%	2200
	11th to 20th	Day	13% 7% 3 7% 5% 5% 6% 5% 7% 7% 35%	2200
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 3 6% 8% 37%	2200
October	1st to 10th	Day	12% 7% 3 7% 4 4 5% 3 4 6% 45%	2096
	11th to 20th	Day	9% 5% 2 4 3 3 3 2 4 6% 60%	1980
	21st to end	Day	11% 5% 2 5% 3 4 4 2 4 6% 55%	2090
November	1st to 10th	Day	11% 5% 2 5% 4 4 3 4 5% 6% 51%	1957
	11th to 20th	Day	12% 7% 2 5% 4 3 3 3 4 7% 50%	1965
	21st to end	Day	14% 7% 2 5% 4 3 3 3 5% 6% 50%	1800
December	1st to 10th	Day	7% 4 2 6% 4 3 5% 4 5% 6% 53%	2930
	11th to 20th	Day	14% 6% 3 5% 3 3 3 3 5% 6% 50%	3373
	21st to end	Day	18% 9% 3 6% 3 3 3 3 4 6% 41%	4466
January	1st to 10th	Night	10% 5% 2 4 3 4 4 2 4 5% 57%	5889
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 44%	5900
	21st to end	Night	13% 8% 3 6% 4 4 4 3 4 7% 45%	6490
February	1st to 10th	Night	16% 6% 2 5% 4 4 5% 3 5% 6% 44%	2538
	11th to 20th	Night	17% 7% 3 5% 4 3 3 3 5% 6% 45%	2338
	21st to end	Night	11% 6% 3 6% 5% 5% 4 4 5% 6% 46%	1886
March	1st to 10th	Night	19% 9% 2 6% 4 3 4 2 4 6% 41%	4918
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 6% 8% 52%	4980
	21st to end	Night	13% 5% 2 5% 4 4 3 3 5% 6% 51%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 5% 3 3 5% 7% 50%	2200
	11th to 20th	Night	11% 3 1 5% 4 3 4 3 5% 7% 54%	2020
	21st to end	Night	6% 4 1 4 4 4 4 4 7% 8% 53%	1965
May	1st to 10th	Night	6% 6% 2 6% 5% 4 6% 4 7% 10% 44%	4033
	11th to 20th	Night	14% 5% 2 6% 3 4 5% 4 7% 10% 38%	4032
	21st to end	Night	9% 5% 2 5% 4 4 5% 5% 7% 9% 45%	4421
June	1st to 10th	Night	9% 6% 2 7% 6% 5% 5% 4 8% 13% 35%	1800
	11th to 20th	Night	6% 5% 3 7% 5% 6% 5% 6% 9% 12% 36%	1800
	21st to end	Night	5% 5% 2 6% 5% 6% 6% 6% 10% 14% 35%	1800
July	1st to 10th	Night	9% 8% 3 9% 7% 6% 7% 6% 8% 10% 27%	4058
	11th to 20th	Night	4 4 2 7% 6% 6% 7% 5% 10% 15% 33%	4057
	21st to end	Night	7% 6% 3 8% 6% 6% 7% 6% 8% 10% 31%	4427
August	1st to 10th	Night	18% 7% 2 7% 5% 5% 7% 5% 8% 12% 23%	2915
	11th to 20th	Night	7% 5% 2 7% 5% 6% 6% 5% 8% 12% 36%	2875
	21st to end	Night	8% 6% 2 6% 5% 5% 4 5% 7% 10% 44%	3640
September	1st to 10th	Night	10% 7% 3 6% 5% 5% 6% 5% 8% 9% 36%	2200
	11th to 20th	Night	9% 6% 3 8% 5% 5% 5% 5% 6% 8% 39%	2200
	21st to end	Night	12% 7% 3 7% 5% 4 5% 4 6% 7% 37%	2200
October	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 5% 6% 46%	2302
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 4 5% 58%	2377
	21st to end	Night	8% 4 2 6% 4 4 4 3 4 5% 56%	2508
November	1st to 10th	Night	12% 4 2 5% 3 3 4 3 5% 7% 52%	2341
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 50%	2430
	21st to end	Night	11% 6% 2 6% 4 4 4 3 5% 6% 48%	2600
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 4 6% 56%	4250
	11th to 20th	Night	16% 8% 3 7% 4 3 3 3 4 5% 43%	4822
	21st to end	Night	17% 7% 2 6% 4 4 4 3 4 5% 43%	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	30% 5% 7% 16% 31% 11%	4057
	11th to 20th	Day	35% 4 7% 17% 27% 10%	4060
	21st to end	Day	38% 5% 7% 18% 20% 11%	4466
February	1st to 10th	Day	40% 6% 10% 19% 18% 7%	1859
	11th to 20th	Day	40% 4 9% 16% 23% 8%	1958
	21st to end	Day	37% 5% 9% 18% 20% 11%	1742
March	1st to 10th	Day	47% 5% 7% 13% 18% 10%	4935
	11th to 20th	Day	27% 5% 11% 17% 21% 19%	4980
	21st to end	Day	39% 4 8% 19% 18% 11%	5465
April	1st to 10th	Day	30% 6% 13% 22% 18% 10%	2200
	11th to 20th	Day	32% 5% 10% 21% 21% 11%	2329
	21st to end	Day	25% 7% 13% 29% 20% 6%	2450
May	1st to 10th	Day	35% 9% 18% 24% 11% 3	5897
	11th to 20th	Day	41% 7% 18% 22% 11% 1	5865
	21st to end	Day	33% 9% 16% 20% 21% 1	6447
June	1st to 10th	Day	42% 12% 18% 16% 11% 1	2600
	11th to 20th	Day	35% 11% 22% 23% 8%	2600
	21st to end	Day	39% 14% 22% 16% 8% 1	2600
July	1st to 10th	Day	51% 10% 25% 11% 2 1	5835
	11th to 20th	Day	39% 13% 34% 11% 2	5831
	21st to end	Day	40% 13% 25% 19% 3	6240
August	1st to 10th	Day	56% 11% 22% 9% 2	4221
	11th to 20th	Day	37% 13% 21% 20% 8%	4187
	21st to end	Day	44% 9% 16% 27% 4	4229
September	1st to 10th	Day	44% 11% 15% 14% 13% 3	2200
	11th to 20th	Day	44% 11% 13% 18% 12% 3	2200
	21st to end	Day	43% 8% 13% 18% 15% 3	2200
October	1st to 10th	Day	42% 5% 7% 13% 23% 10%	3408
	11th to 20th	Day	16% 3 6% 14% 30% 32%	3220
	21st to end	Day	23% 4 7% 19% 34% 12%	3428
November	1st to 10th	Day	32% 6% 10% 16% 23% 14%	1957
	11th to 20th	Day	35% 5% 7% 17% 23% 12%	1965
	21st to end	Day	35% 5% 9% 21% 21% 9%	1800
December	1st to 10th	Day	31% 5% 10% 17% 23% 14%	2930
	11th to 20th	Day	35% 5% 9% 23% 23% 5%	3370
	21st to end	Day	45% 4 6% 20% 22% 3	4466
January	1st to 10th	Night	29% 5% 7% 14% 35% 10%	5889
	11th to 20th	Night	39% 5% 5% 17% 26% 8%	5900
	21st to end	Night	40% 5% 8% 13% 24% 11%	6490
February	1st to 10th	Night	41% 5% 9% 19% 19% 8%	2538
	11th to 20th	Night	40% 6% 8% 15% 21% 10%	2339
	21st to end	Night	38% 6% 9% 18% 16% 13%	1886
March	1st to 10th	Night	45% 5% 8% 13% 17% 12%	4918
	11th to 20th	Night	28% 6% 11% 13% 24% 18%	4980
	21st to end	Night	34% 4 11% 17% 21% 12%	5467
April	1st to 10th	Night	33% 6% 12% 20% 19% 9%	2200
	11th to 20th	Night	29% 6% 9% 24% 20% 12%	2020
	21st to end	Night	27% 6% 14% 26% 22% 6%	1965
May	1st to 10th	Night	33% 8% 20% 26% 11% 3	4060
	11th to 20th	Night	38% 9% 22% 21% 10% 1	4033
	21st to end	Night	32% 8% 16% 26% 16% 2	4429
June	1st to 10th	Night	38% 11% 22% 19% 9% 1	1800
	11th to 20th	Night	36% 12% 21% 24% 7%	1800
	21st to end	Night	32% 17% 26% 19% 6% 1	1800
July	1st to 10th	Night	47% 11% 24% 15% 2 1	4015
	11th to 20th	Night	37% 13% 37% 10% 2	4006
	21st to end	Night	44% 12% 23% 18% 3	4297
August	1st to 10th	Night	50% 12% 24% 11% 2	2904
	11th to 20th	Night	38% 11% 24% 21% 6%	2902
	21st to end	Night	35% 9% 21% 31% 4	3640
September	1st to 10th	Night	40% 13% 17% 17% 10% 2	2200
	11th to 20th	Night	40% 11% 17% 18% 12% 2	2200
	21st to end	Night	43% 9% 14% 18% 14% 2	2200
October	1st to 10th	Night	38% 5% 10% 16% 24% 7%	3770
	11th to 20th	Night	17% 4 5% 12% 26% 35%	3876
	21st to end	Night	24% 5% 6% 19% 31% 16%	4120
November	1st to 10th	Night	32% 5% 10% 16% 23% 14%	2341
	11th to 20th	Night	36% 5% 6% 17% 23% 12%	2430
	21st to end	Night	36% 5% 9% 18% 22% 10%	2600
December	1st to 10th	Night	30% 4 9% 16% 29% 12%	4250
	11th to 20th	Night	44% 4 10% 15% 19% 7%	4824
	21st to end	Night	42% 5% 7% 19% 24% 3	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	13% 20% 43% 20% 3 1	300
	11th to 20th	Day	12% 19% 37% 21% 9% 2	300
	21st to end	Day	21% 22% 37% 18% 2	330
February	1st to 10th	Day	15% 17% 39% 22% 7%	300
	11th to 20th	Day	29% 16% 32% 21% 2	300
	21st to end	Day	13% 18% 40% 23% 6%	341
March	1st to 10th	Day	16% 18% 30% 24% 11% 1	500
	11th to 20th	Day	12% 13% 42% 24% 7% 3	500
	21st to end	Day	13% 15% 39% 23% 9% 1	550
April	1st to 10th	Day	13% 17% 33% 26% 10% 2	500
	11th to 20th	Day	11% 12% 32% 27% 13% 5%	492
	21st to end	Day	6% 11% 26% 35% 16% 6%	500
May	1st to 10th	Day	6% 13% 29% 31% 16% 7%	503
	11th to 20th	Day	12% 4 25% 30% 20% 8%	527
	21st to end	Day	12% 6% 15% 31% 24% 12%	573
June	1st to 10th	Day	10% 2 17% 34% 25% 12%	530
	11th to 20th	Day	7% 4 15% 31% 26% 18%	530
	21st to end	Day	6% 3 13% 33% 30% 15%	530
July	1st to 10th	Day	7% 1 8% 27% 36% 20%	525
	11th to 20th	Day	6% 2 5% 24% 40% 23%	524
	21st to end	Day	4 2 8% 33% 34% 18%	576
August	1st to 10th	Day	14% 3 8% 27% 32% 16%	526
	11th to 20th	Day	7% 3 6% 29% 36% 20%	519
	21st to end	Day	16% 3 10% 25% 27% 18%	583
September	1st to 10th	Day	12% 3 12% 28% 31% 15%	530
	11th to 20th	Day	17% 4 15% 28% 29% 7%	530
	21st to end	Day	16% 2 13% 31% 29% 9%	530
October	1st to 10th	Day	16% 4 22% 31% 18% 8%	530
	11th to 20th	Day	10% 6% 22% 33% 21% 8%	522
	21st to end	Day	15% 8% 19% 31% 23% 4	539
November	1st to 10th	Day	16% 6% 24% 31% 19% 4	495
	11th to 20th	Day	18% 9% 33% 26% 13% 2	465
	21st to end	Day	13% 13% 28% 32% 12% 1	300
December	1st to 10th	Day	17% 14% 32% 26% 10% 1	300
	11th to 20th	Day	18% 12% 33% 26% 8% 3	300
	21st to end	Day	25% 13% 29% 20% 12% 1	330
January	1st to 10th	Night	15% 16% 34% 29% 6%	800
	11th to 20th	Night	14% 15% 31% 30% 9% 1	800
	21st to end	Night	19% 14% 36% 26% 4	880
February	1st to 10th	Night	18% 10% 32% 33% 7%	800
	11th to 20th	Night	21% 14% 33% 28% 4	783
	21st to end	Night	14% 12% 36% 32% 5%	660
March	1st to 10th	Night	15% 12% 32% 33% 8%	800
	11th to 20th	Night	14% 12% 32% 33% 9%	800
	21st to end	Night	13% 13% 33% 31% 10% 1	880
April	1st to 10th	Night	12% 11% 34% 31% 11%	800
	11th to 20th	Night	11% 9% 30% 33% 15% 1	791
	21st to end	Night	8% 7% 28% 38% 17% 2	765
May	1st to 10th	Night	7% 6% 25% 40% 22% 2	600
	11th to 20th	Night	9% 4 23% 37% 23% 4	591
	21st to end	Night	10% 3 13% 33% 34% 7%	650
June	1st to 10th	Night	11% 2 12% 39% 31% 5%	600
	11th to 20th	Night	7% 3 10% 36% 32% 12%	600
	21st to end	Night	7% 2 10% 39% 34% 9%	600
July	1st to 10th	Night	9% 2 8% 29% 42% 11%	585
	11th to 20th	Night	9% 4 27% 46% 14%	582
	21st to end	Night	5% 1 6% 36% 44% 8%	657
August	1st to 10th	Night	10% 3 7% 29% 41% 10%	597
	11th to 20th	Night	9% 1 7% 25% 45% 12%	594
	21st to end	Night	13% 3 10% 28% 34% 12%	870
September	1st to 10th	Night	12% 3 11% 28% 38% 8%	800
	11th to 20th	Night	12% 3 12% 35% 32% 7%	800
	21st to end	Night	17% 2 13% 31% 31% 5%	800
October	1st to 10th	Night	16% 4 21% 31% 23% 6%	800
	11th to 20th	Night	12% 5% 21% 33% 24% 4	793
	21st to end	Night	10% 5% 24% 33% 24% 4	836
November	1st to 10th	Night	13% 8% 21% 34% 22% 3	780
	11th to 20th	Night	16% 9% 27% 32% 14% 1	800
	21st to end	Night	14% 10% 27% 32% 17% 1	800
December	1st to 10th	Night	16% 11% 28% 31% 14% 1	800
	11th to 20th	Night	19% 9% 28% 28% 15% 1	800
	21st to end	Night	23% 10% 29% 27% 11%	880

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