Cooke Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cooke Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	127286
	Night	9% 6% 2 6% 4 5% 5% 4 7% 9% 43%	124416
Transparency	Day	35% 10% 17% 20% 14% 5%	131046
	Night	35% 9% 16% 19% 15% 6%	128725
Seeing	Day	9% 6% 21% 27% 23% 14%	17006
	Night	10% 5% 20% 32% 25% 7%	27074

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	9% 5% 2 5% 4 3 4 3 5% 8% 52%	4146
	11th to 20th	Day	20% 9% 3 8% 4 4 4 3 5% 7% 32%	4114
	21st to end	Day	12% 5% 2 5% 3 4 4 3 6% 9% 47%	4811
February	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 8% 44%	2000
	11th to 20th	Day	10% 4 2 5% 4 4 4 3 5% 6% 54%	1948
	21st to end	Day	8% 6% 2 6% 5% 4 4 3 6% 7% 50%	1685
March	1st to 10th	Day	8% 5% 2 7% 4 5% 5% 4 6% 9% 46%	4935
	11th to 20th	Day	6% 4 2 4 2 3 3 3 6% 9% 59%	4980
	21st to end	Day	9% 4 2 5% 4 3 5% 4 6% 9% 51%	5465
April	1st to 10th	Day	7% 3 2 4 4 4 5% 4 7% 9% 52%	2200
	11th to 20th	Day	9% 5% 2 5% 5% 5% 6% 4 6% 7% 48%	2176
	21st to end	Day	5% 2 2 5% 4 5% 5% 4 7% 9% 55%	2442
May	1st to 10th	Day	3 2 1 4 4 4 5% 4 8% 11% 53%	5862
	11th to 20th	Day	9% 4 1 5% 4 4 5% 4 6% 9% 47%	5861
	21st to end	Day	4 3 1 4 4 4 5% 5% 7% 11% 52%	6433
June	1st to 10th	Day	9% 8% 2 8% 6% 5% 6% 5% 7% 11% 31%	2600
	11th to 20th	Day	6% 5% 2 6% 6% 7% 8% 7% 11% 12% 30%	2600
	21st to end	Day	7% 6% 2 8% 6% 7% 7% 6% 10% 12% 29%	2600
July	1st to 10th	Day	5% 5% 3 8% 8% 8% 9% 8% 11% 15% 20%	5900
	11th to 20th	Day	3 4 2 7% 6% 8% 8% 7% 10% 15% 28%	5900
	21st to end	Day	4 4 2 7% 6% 7% 8% 8% 11% 14% 30%	6424
August	1st to 10th	Day	8% 5% 2 8% 6% 7% 7% 6% 8% 12% 31%	4234
	11th to 20th	Day	4 4 2 6% 5% 6% 6% 6% 10% 14% 39%	3853
	21st to end	Day	7% 6% 3 8% 6% 7% 8% 6% 10% 13% 27%	3982
September	1st to 10th	Day	7% 6% 3 9% 8% 8% 7% 7% 11% 12% 22%	2200
	11th to 20th	Day	9% 7% 3 9% 7% 6% 7% 6% 8% 9% 28%	2200
	21st to end	Day	7% 5% 3 8% 6% 7% 7% 4 9% 9% 35%	2200
October	1st to 10th	Day	10% 6% 4 8% 6% 6% 6% 4 6% 9% 37%	2200
	11th to 20th	Day	6% 4 2 5% 3 4 4 3 6% 7% 55%	2045
	21st to end	Day	6% 3 2 6% 4 4 5% 4 5% 8% 52%	2200
November	1st to 10th	Day	7% 5% 2 6% 4 4 3 4 5% 7% 52%	2057
	11th to 20th	Day	7% 6% 2 6% 5% 4 4 3 5% 7% 51%	2095
	21st to end	Day	7% 6% 3 6% 4 5% 5% 4 7% 9% 45%	1890
December	1st to 10th	Day	5% 3 2 4 3 4 4 4 6% 7% 57%	3020
	11th to 20th	Day	9% 6% 2 6% 5% 3 4 4 5% 9% 48%	3463
	21st to end	Day	12% 7% 2 7% 5% 4 5% 4 5% 8% 41%	4565
January	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 4 9% 53%	6019
	11th to 20th	Night	21% 9% 3 8% 4 4 5% 4 6% 7% 29%	5979
	21st to end	Night	15% 6% 2 7% 4 4 4 3 6% 9% 40%	6141
February	1st to 10th	Night	12% 8% 2 6% 4 4 4 4 6% 7% 44%	2400
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 6% 48%	2338
	21st to end	Night	8% 7% 2 7% 4 4 3 3 5% 7% 49%	1944
March	1st to 10th	Night	13% 7% 2 7% 4 4 5% 3 5% 6% 44%	4918
	11th to 20th	Night	5% 5% 1 4 3 3 4 3 6% 9% 57%	4980
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 8% 50%	5467
April	1st to 10th	Night	7% 5% 2 5% 3 3 4 4 7% 10% 50%	2200
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 5% 9% 53%	2173
	21st to end	Night	7% 3 1 5% 3 4 4 3 5% 8% 57%	1971
May	1st to 10th	Night	4 3 1 4 3 4 4 4 6% 10% 55%	4033
	11th to 20th	Night	9% 4 2 5% 3 3 4 4 6% 9% 52%	4032
	21st to end	Night	3 2 1 4 4 4 4 4 6% 10% 58%	4421
June	1st to 10th	Night	9% 7% 2 7% 4 5% 5% 5% 7% 10% 40%	1800
	11th to 20th	Night	5% 5% 2 6% 6% 6% 7% 6% 9% 13% 36%	1800
	21st to end	Night	6% 5% 3 7% 6% 7% 7% 6% 8% 13% 33%	1800
July	1st to 10th	Night	5% 7% 3 10% 7% 8% 8% 6% 10% 15% 21%	4058
	11th to 20th	Night	3 4 2 7% 6% 7% 8% 8% 12% 15% 27%	4057
	21st to end	Night	4 6% 3 10% 7% 8% 9% 7% 11% 12% 23%	4427
August	1st to 10th	Night	7% 4 2 7% 6% 6% 6% 6% 10% 13% 33%	2915
	11th to 20th	Night	3 3 1 6% 4 5% 6% 5% 10% 16% 40%	3191
	21st to end	Night	7% 6% 3 9% 6% 6% 8% 6% 9% 12% 28%	3881
September	1st to 10th	Night	6% 6% 3 9% 7% 6% 8% 6% 10% 11% 28%	2200
	11th to 20th	Night	10% 6% 3 9% 6% 7% 6% 6% 8% 11% 29%	2200
	21st to end	Night	8% 6% 2 8% 6% 6% 6% 5% 7% 10% 37%	2200
October	1st to 10th	Night	10% 6% 2 8% 5% 5% 6% 4 7% 9% 40%	2200
	11th to 20th	Night	5% 5% 1 4 5% 3 4 3 6% 9% 56%	2355
	21st to end	Night	8% 5% 2 7% 4 4 3 3 5% 7% 51%	2640
November	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 6% 8% 53%	2461
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 5% 7% 51%	2520
	21st to end	Night	7% 6% 3 6% 4 4 4 3 6% 9% 47%	2730
December	1st to 10th	Night	6% 4 2 5% 4 3 3 4 5% 7% 57%	4380
	11th to 20th	Night	9% 7% 2 5% 4 4 4 3 5% 7% 51%	4952
	21st to end	Night	15% 8% 3 7% 4 4 4 4 6% 7% 38%	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	30% 5% 9% 16% 27% 13%	4147
	11th to 20th	Day	52% 4 5% 16% 15% 8%	4114
	21st to end	Day	32% 5% 10% 26% 20% 8%	4811
February	1st to 10th	Day	37% 7% 9% 15% 19% 12%	2000
	11th to 20th	Day	30% 6% 10% 19% 24% 11%	1958
	21st to end	Day	32% 7% 11% 18% 21% 11%	1685
March	1st to 10th	Day	32% 8% 13% 20% 20% 7%	4935
	11th to 20th	Day	23% 5% 9% 23% 23% 17%	4980
	21st to end	Day	30% 6% 10% 28% 19% 8%	5465
April	1st to 10th	Day	27% 8% 15% 21% 20% 9%	2200
	11th to 20th	Day	34% 8% 10% 16% 23% 9%	2176
	21st to end	Day	24% 7% 13% 24% 24% 7%	2442
May	1st to 10th	Day	22% 9% 25% 27% 14% 4	5897
	11th to 20th	Day	30% 10% 17% 29% 12% 1	5865
	21st to end	Day	23% 9% 20% 32% 15% 2	6447
June	1st to 10th	Day	45% 13% 19% 17% 6%	2600
	11th to 20th	Day	41% 17% 21% 18% 4	2600
	21st to end	Day	43% 16% 21% 15% 4	2600
July	1st to 10th	Day	47% 16% 32% 5%	5835
	11th to 20th	Day	40% 16% 35% 9%	5831
	21st to end	Day	38% 15% 27% 19% 1	6240
August	1st to 10th	Day	42% 14% 29% 13% 3	4221
	11th to 20th	Day	32% 15% 27% 24% 3	3889
	21st to end	Day	43% 16% 24% 16% 2	3982
September	1st to 10th	Day	49% 19% 17% 11% 4	2200
	11th to 20th	Day	48% 15% 19% 13% 5%	2200
	21st to end	Day	42% 12% 17% 16% 10% 3	2200
October	1st to 10th	Day	55% 10% 12% 10% 7% 5%	3590
	11th to 20th	Day	23% 6% 7% 22% 34% 9%	3311
	21st to end	Day	27% 5% 8% 22% 30% 7%	3538
November	1st to 10th	Day	31% 7% 10% 22% 18% 12%	2057
	11th to 20th	Day	32% 6% 11% 19% 20% 12%	2095
	21st to end	Day	34% 6% 9% 21% 20% 9%	1890
December	1st to 10th	Day	25% 6% 17% 23% 21% 10%	3020
	11th to 20th	Day	33% 5% 11% 21% 18% 12%	3460
	21st to end	Day	39% 8% 10% 21% 17% 5%	4565
January	1st to 10th	Night	29% 5% 8% 16% 31% 11%	6019
	11th to 20th	Night	54% 5% 9% 14% 14% 4	5979
	21st to end	Night	40% 7% 10% 19% 17% 8%	6141
February	1st to 10th	Night	38% 6% 11% 16% 18% 11%	2400
	11th to 20th	Night	33% 7% 12% 19% 21% 9%	2339
	21st to end	Night	34% 6% 13% 18% 22% 7%	1944
March	1st to 10th	Night	39% 8% 10% 16% 17% 10%	4918
	11th to 20th	Night	22% 7% 15% 21% 18% 18%	4980
	21st to end	Night	32% 6% 10% 27% 20% 6%	5467
April	1st to 10th	Night	27% 8% 17% 23% 19% 6%	2200
	11th to 20th	Night	27% 6% 13% 24% 21% 7%	2173
	21st to end	Night	25% 6% 14% 26% 24% 6%	1971
May	1st to 10th	Night	21% 10% 24% 29% 12% 4	4060
	11th to 20th	Night	27% 11% 16% 28% 16% 2	4033
	21st to end	Night	20% 8% 19% 37% 15% 1	4429
June	1st to 10th	Night	36% 12% 24% 22% 6%	1800
	11th to 20th	Night	35% 14% 28% 19% 3	1800
	21st to end	Night	39% 17% 24% 16% 4	1800
July	1st to 10th	Night	49% 16% 30% 4 1	4015
	11th to 20th	Night	38% 17% 38% 7%	4006
	21st to end	Night	47% 15% 24% 13% 1	4297
August	1st to 10th	Night	37% 15% 33% 12% 2	2904
	11th to 20th	Night	28% 15% 33% 23% 2	3221
	21st to end	Night	44% 16% 24% 13% 3	3881
September	1st to 10th	Night	44% 19% 22% 11% 4	2200
	11th to 20th	Night	45% 14% 20% 17% 4 1	2200
	21st to end	Night	38% 13% 18% 19% 10% 2	2200
October	1st to 10th	Night	56% 10% 11% 12% 7% 4	3590
	11th to 20th	Night	22% 5% 8% 24% 36% 6%	3828
	21st to end	Night	30% 6% 7% 19% 31% 6%	4252
November	1st to 10th	Night	29% 7% 12% 21% 20% 11%	2461
	11th to 20th	Night	32% 6% 10% 21% 21% 11%	2520
	21st to end	Night	33% 6% 10% 24% 18% 9%	2730
December	1st to 10th	Night	26% 5% 14% 25% 17% 13%	4380
	11th to 20th	Night	33% 6% 12% 21% 19% 10%	4954
	21st to end	Night	42% 8% 11% 19% 15% 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	10% 13% 48% 21% 7% 1	330
	11th to 20th	Day	13% 14% 39% 23% 9% 2	318
	21st to end	Day	15% 18% 46% 18% 3	330
February	1st to 10th	Day	13% 13% 35% 25% 12% 2	300
	11th to 20th	Day	13% 11% 39% 25% 9% 2	300
	21st to end	Day	8% 13% 40% 26% 13% 1	284
March	1st to 10th	Day	7% 15% 40% 28% 8% 1	500
	11th to 20th	Day	12% 14% 32% 27% 14% 2	500
	21st to end	Day	9% 11% 35% 29% 14% 2	550
April	1st to 10th	Day	10% 12% 33% 29% 14% 2	500
	11th to 20th	Day	10% 13% 27% 30% 17% 3	492
	21st to end	Day	6% 6% 26% 35% 21% 6%	500
May	1st to 10th	Day	5% 6% 28% 33% 19% 8%	503
	11th to 20th	Day	7% 3 25% 30% 23% 11%	527
	21st to end	Day	6% 3 13% 31% 29% 17%	573
June	1st to 10th	Day	11% 3 12% 30% 27% 16%	530
	11th to 20th	Day	9% 3 9% 24% 31% 24%	530
	21st to end	Day	7% 1 6% 20% 33% 33%	530
July	1st to 10th	Day	6% 1 4 18% 33% 38%	525
	11th to 20th	Day	5% 1 4 15% 36% 39%	524
	21st to end	Day	8% 1 4 23% 29% 35%	576
August	1st to 10th	Day	9% 3 4 18% 32% 34%	526
	11th to 20th	Day	7% 1 4 15% 34% 39%	519
	21st to end	Day	10% 2 6% 21% 35% 27%	583
September	1st to 10th	Day	9% 3 7% 21% 34% 26%	530
	11th to 20th	Day	7% 3 6% 30% 35% 18%	530
	21st to end	Day	9% 2 9% 28% 33% 19%	530
October	1st to 10th	Day	12% 6% 22% 33% 20% 7%	530
	11th to 20th	Day	6% 5% 22% 32% 28% 7%	530
	21st to end	Day	7% 5% 22% 40% 22% 3	583
November	1st to 10th	Day	9% 5% 23% 39% 19% 4	535
	11th to 20th	Day	9% 8% 25% 36% 19% 3	535
	21st to end	Day	7% 11% 35% 31% 13% 2	330
December	1st to 10th	Day	8% 10% 41% 26% 10% 4	330
	11th to 20th	Day	12% 9% 35% 30% 13% 1	330
	21st to end	Day	10% 10% 34% 34% 10% 2	363
January	1st to 10th	Night	12% 10% 32% 32% 13% 1	840
	11th to 20th	Night	13% 10% 25% 38% 13%	825
	21st to end	Night	16% 12% 34% 30% 8%	880
February	1st to 10th	Night	15% 9% 29% 31% 15% 1	800
	11th to 20th	Night	12% 9% 29% 36% 12% 1	783
	21st to end	Night	10% 7% 35% 39% 9%	660
March	1st to 10th	Night	9% 9% 37% 35% 9% 1	800
	11th to 20th	Night	8% 11% 27% 39% 15% 1	800
	21st to end	Night	9% 8% 31% 38% 13% 1	880
April	1st to 10th	Night	8% 9% 29% 38% 15% 1	800
	11th to 20th	Night	9% 8% 31% 38% 13%	791
	21st to end	Night	7% 4 26% 41% 22% 1	737
May	1st to 10th	Night	6% 4 15% 47% 26% 3	600
	11th to 20th	Night	7% 4 15% 39% 30% 4	591
	21st to end	Night	7% 1 11% 36% 37% 8%	650
June	1st to 10th	Night	11% 2 10% 30% 38% 9%	600
	11th to 20th	Night	8% 1 5% 22% 43% 21%	600
	21st to end	Night	9% 1 5% 16% 46% 23%	600
July	1st to 10th	Night	8% 1 3 14% 45% 29%	585
	11th to 20th	Night	5% 1 2 12% 44% 36%	582
	21st to end	Night	5% 1 4 13% 48% 29%	657
August	1st to 10th	Night	8% 1 3 12% 48% 27%	597
	11th to 20th	Night	6% 1 3 12% 45% 34%	594
	21st to end	Night	12% 3 5% 22% 39% 21%	838
September	1st to 10th	Night	10% 1 6% 25% 40% 18%	800
	11th to 20th	Night	11% 3 8% 30% 38% 11%	800
	21st to end	Night	11% 3 10% 29% 41% 8%	800
October	1st to 10th	Night	13% 3 18% 38% 24% 4	800
	11th to 20th	Night	5% 5% 17% 39% 32% 2	800
	21st to end	Night	10% 3 24% 39% 22% 2	880
November	1st to 10th	Night	10% 3 27% 37% 20% 2	820
	11th to 20th	Night	9% 6% 23% 42% 19% 1	840
	21st to end	Night	8% 6% 27% 42% 16% 1	840
December	1st to 10th	Night	10% 6% 30% 34% 19% 1	840
	11th to 20th	Night	11% 6% 29% 37% 15% 1	840
	21st to end	Night	12% 7% 28% 35% 17% 1	924

Page generated at 2026-01-14 00:14:30. Version ac07

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