The Winter-Hawk Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Winter-Hawk Memorial Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	13% 9% 3 7% 5% 5% 5% 4 6% 8% 37%	127143
	Night	14% 9% 3 7% 5% 4 4 4 6% 7% 37%	124204
Transparency	Day	42% 10% 13% 19% 14% 3	130867
	Night	42% 9% 12% 19% 14% 3	128549
Seeing	Day	18% 13% 24% 24% 16% 6%	16491
	Night	17% 11% 22% 31% 17% 2	26566

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	12% 19% 5% 11% 4 3 3 2 4 6% 30%	4065
	11th to 20th	Day	11% 15% 8% 7% 5% 5% 4 3 4 4 35%	4060
	21st to end	Day	15% 14% 4 8% 4 3 3 2 4 5% 37%	4466
February	1st to 10th	Day	16% 13% 3 8% 5% 4 4 4 4 5% 33%	1905
	11th to 20th	Day	20% 13% 5% 10% 4 3 4 3 4 5% 30%	1948
	21st to end	Day	19% 12% 2 7% 3 4 4 3 5% 5% 36%	1650
March	1st to 10th	Day	16% 14% 3 7% 4 4 4 3 4 5% 36%	4753
	11th to 20th	Day	11% 9% 2 5% 3 3 3 3 5% 7% 47%	4980
	21st to end	Day	15% 8% 2 6% 4 4 4 4 5% 6% 41%	5465
April	1st to 10th	Day	13% 8% 2 6% 4 5% 3 4 7% 8% 41%	2200
	11th to 20th	Day	14% 7% 2 5% 5% 4 4 4 5% 7% 43%	2427
	21st to end	Day	15% 7% 3 6% 5% 5% 4 4 6% 7% 39%	2600
May	1st to 10th	Day	15% 8% 3 9% 6% 5% 6% 5% 7% 8% 28%	5862
	11th to 20th	Day	15% 8% 3 9% 6% 6% 6% 5% 7% 7% 28%	5861
	21st to end	Day	14% 7% 3 7% 5% 5% 6% 4 7% 9% 33%	6433
June	1st to 10th	Day	10% 6% 2 6% 6% 6% 7% 5% 7% 10% 35%	2600
	11th to 20th	Day	6% 5% 3 7% 7% 6% 8% 6% 9% 10% 34%	2600
	21st to end	Day	9% 5% 2 6% 5% 6% 7% 6% 8% 11% 36%	2600
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 40%	5900
	11th to 20th	Day	11% 5% 2 7% 6% 6% 7% 5% 8% 10% 32%	5900
	21st to end	Day	12% 8% 3 10% 7% 6% 8% 5% 8% 9% 24%	6424
August	1st to 10th	Day	11% 6% 2 7% 6% 6% 7% 5% 7% 11% 33%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 10% 35%	4174
	21st to end	Day	6% 4 2 4 4 4 5% 4 7% 8% 52%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	2200
	11th to 20th	Day	14% 5% 2 7% 4 5% 4 4 6% 8% 42%	2200
	21st to end	Day	13% 6% 3 6% 5% 4 4 5% 6% 7% 42%	2200
October	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 6% 51%	2186
	11th to 20th	Day	13% 6% 2 5% 3 3 3 2 5% 6% 53%	2000
	21st to end	Day	15% 7% 2 4 3 4 3 3 4 5% 50%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 4 2 4 6% 52%	1910
	11th to 20th	Day	11% 8% 3 5% 3 4 4 4 5% 8% 46%	1890
	21st to end	Day	13% 11% 4 6% 3 3 3 3 5% 6% 43%	1890
December	1st to 10th	Day	14% 10% 3 8% 5% 4 4 4 5% 5% 39%	3020
	11th to 20th	Day	16% 13% 3 8% 4 4 5% 4 7% 6% 29%	3463
	21st to end	Day	12% 14% 4 7% 5% 5% 4 4 5% 8% 33%	4565
January	1st to 10th	Night	15% 14% 6% 11% 5% 5% 4 3 4 5% 30%	5903
	11th to 20th	Night	15% 12% 4 10% 7% 4 3 2 4 4 34%	5900
	21st to end	Night	16% 10% 4 8% 4 3 3 3 5% 5% 38%	6490
February	1st to 10th	Night	17% 11% 3 7% 4 4 4 4 5% 6% 37%	2490
	11th to 20th	Night	21% 12% 3 8% 4 4 4 3 5% 4 32%	2338
	21st to end	Night	19% 11% 3 6% 4 4 4 4 6% 6% 33%	1980
March	1st to 10th	Night	20% 12% 3 6% 4 4 3 3 4 5% 37%	5098
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 5% 8% 50%	4980
	21st to end	Night	12% 7% 2 6% 5% 4 4 4 6% 7% 42%	5467
April	1st to 10th	Night	13% 8% 2 5% 4 3 4 4 7% 8% 41%	2200
	11th to 20th	Night	13% 9% 2 6% 5% 4 3 3 5% 7% 43%	1937
	21st to end	Night	15% 6% 2 6% 4 4 4 4 6% 7% 41%	1800
May	1st to 10th	Night	15% 8% 3 7% 5% 5% 6% 5% 8% 10% 29%	4033
	11th to 20th	Night	16% 8% 3 8% 7% 6% 5% 4 7% 8% 27%	4032
	21st to end	Night	16% 8% 3 8% 5% 5% 5% 4 6% 8% 32%	4421
June	1st to 10th	Night	11% 6% 3 5% 6% 5% 6% 5% 8% 11% 35%	1800
	11th to 20th	Night	5% 5% 3 6% 5% 5% 6% 4 8% 12% 40%	1800
	21st to end	Night	8% 6% 2 6% 6% 6% 6% 4 6% 11% 40%	1800
July	1st to 10th	Night	13% 5% 2 7% 5% 5% 6% 6% 8% 10% 32%	4058
	11th to 20th	Night	9% 6% 3 8% 5% 5% 6% 5% 8% 11% 32%	4057
	21st to end	Night	12% 9% 4 11% 6% 7% 6% 5% 8% 10% 22%	4427
August	1st to 10th	Night	10% 5% 2 8% 5% 5% 5% 6% 7% 10% 36%	2915
	11th to 20th	Night	13% 6% 2 7% 5% 5% 5% 4 7% 8% 38%	2875
	21st to end	Night	6% 4 1 5% 4 4 6% 4 7% 8% 49%	3520
September	1st to 10th	Night	11% 6% 2 7% 4 4 5% 4 6% 7% 43%	2200
	11th to 20th	Night	14% 6% 2 7% 4 4 4 4 7% 8% 40%	2200
	21st to end	Night	13% 6% 2 6% 3 5% 4 3 6% 7% 46%	2200
October	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 51%	2212
	11th to 20th	Night	14% 6% 1 5% 4 3 3 3 4 6% 52%	2400
	21st to end	Night	14% 6% 2 4 3 3 3 3 4 6% 50%	2640
November	1st to 10th	Night	15% 7% 2 4 3 3 3 3 4 5% 50%	2606
	11th to 20th	Night	14% 8% 2 5% 3 4 3 3 5% 7% 46%	2730
	21st to end	Night	15% 9% 2 5% 3 4 3 3 5% 6% 46%	2730
December	1st to 10th	Night	14% 13% 5% 8% 4 4 3 3 5% 5% 37%	4380
	11th to 20th	Night	19% 11% 4 9% 4 4 4 3 4 6% 31%	4952
	21st to end	Night	12% 15% 4 8% 4 4 5% 4 6% 8% 32%	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	51% 8% 8% 20% 11% 2	4066
	11th to 20th	Day	49% 7% 7% 15% 21% 1	4060
	21st to end	Day	50% 5% 6% 21% 17% 1	4466
February	1st to 10th	Day	52% 6% 9% 16% 16% 2	1905
	11th to 20th	Day	56% 6% 8% 15% 14% 2	1958
	21st to end	Day	48% 6% 9% 15% 17% 5%	1650
March	1st to 10th	Day	48% 6% 8% 14% 16% 8%	4753
	11th to 20th	Day	35% 7% 7% 18% 23% 9%	4980
	21st to end	Day	42% 6% 9% 28% 13% 2	5465
April	1st to 10th	Day	39% 8% 12% 22% 17% 3	2200
	11th to 20th	Day	39% 7% 10% 19% 20% 5%	2427
	21st to end	Day	43% 8% 11% 23% 12% 2	2600
May	1st to 10th	Day	47% 13% 14% 18% 7% 1	5897
	11th to 20th	Day	51% 12% 15% 17% 5% 1	5865
	21st to end	Day	42% 12% 14% 17% 12% 2	6447
June	1st to 10th	Day	41% 13% 17% 18% 10% 1	2600
	11th to 20th	Day	39% 14% 17% 16% 12% 1	2600
	21st to end	Day	39% 14% 15% 19% 12% 2	2600
July	1st to 10th	Day	34% 15% 20% 19% 12%	5835
	11th to 20th	Day	42% 15% 21% 16% 4 1	5831
	21st to end	Day	52% 14% 15% 12% 7%	6240
August	1st to 10th	Day	42% 13% 18% 20% 7%	4221
	11th to 20th	Day	37% 14% 21% 19% 8% 1	4213
	21st to end	Day	28% 10% 15% 35% 11% 2	4312
September	1st to 10th	Day	39% 11% 14% 17% 15% 4	2200
	11th to 20th	Day	40% 10% 13% 20% 15% 2	2200
	21st to end	Day	40% 8% 15% 19% 14% 4	2200
October	1st to 10th	Day	27% 7% 10% 18% 28% 10%	3563
	11th to 20th	Day	25% 5% 7% 25% 21% 18%	3240
	21st to end	Day	27% 5% 7% 28% 23% 9%	3538
November	1st to 10th	Day	33% 6% 10% 21% 26% 5%	1910
	11th to 20th	Day	36% 6% 9% 22% 23% 4	1890
	21st to end	Day	41% 6% 9% 22% 18% 4	1890
December	1st to 10th	Day	45% 7% 8% 17% 21% 2	3020
	11th to 20th	Day	50% 8% 11% 17% 12% 2	3460
	21st to end	Day	46% 9% 14% 21% 9% 1	4565
January	1st to 10th	Night	53% 8% 9% 17% 11% 1	5903
	11th to 20th	Night	49% 7% 8% 15% 17% 4	5900
	21st to end	Night	46% 6% 7% 19% 19% 1	6490
February	1st to 10th	Night	48% 6% 10% 18% 16% 2	2490
	11th to 20th	Night	55% 5% 9% 15% 12% 3	2339
	21st to end	Night	50% 6% 9% 17% 14% 4	1980
March	1st to 10th	Night	48% 7% 8% 17% 16% 4	5098
	11th to 20th	Night	31% 7% 10% 19% 23% 10%	4980
	21st to end	Night	39% 8% 13% 26% 13% 2	5467
April	1st to 10th	Night	39% 8% 13% 21% 15% 3	2200
	11th to 20th	Night	40% 7% 9% 21% 19% 4	1937
	21st to end	Night	40% 8% 12% 25% 13% 1	1800
May	1st to 10th	Night	45% 12% 18% 18% 6% 1	4060
	11th to 20th	Night	51% 11% 15% 18% 4	4033
	21st to end	Night	43% 13% 14% 20% 9% 1	4429
June	1st to 10th	Night	39% 13% 20% 18% 11% 1	1800
	11th to 20th	Night	33% 13% 21% 21% 11% 1	1800
	21st to end	Night	36% 12% 17% 21% 11% 3	1800
July	1st to 10th	Night	40% 14% 19% 19% 8%	4015
	11th to 20th	Night	41% 14% 24% 16% 4 1	4006
	21st to end	Night	51% 15% 15% 12% 6%	4297
August	1st to 10th	Night	38% 13% 21% 21% 6%	2904
	11th to 20th	Night	39% 12% 20% 21% 8%	2902
	21st to end	Night	28% 11% 16% 32% 12% 1	3520
September	1st to 10th	Night	39% 9% 15% 19% 14% 4	2200
	11th to 20th	Night	39% 11% 15% 18% 16% 1	2200
	21st to end	Night	37% 9% 11% 22% 17% 3	2200
October	1st to 10th	Night	31% 7% 10% 18% 28% 7%	3615
	11th to 20th	Night	26% 5% 8% 21% 23% 16%	3899
	21st to end	Night	25% 5% 6% 28% 23% 13%	4252
November	1st to 10th	Night	36% 6% 10% 20% 21% 6%	2606
	11th to 20th	Night	38% 6% 8% 20% 23% 6%	2730
	21st to end	Night	40% 5% 7% 26% 18% 4	2730
December	1st to 10th	Night	48% 7% 9% 16% 19% 1	4380
	11th to 20th	Night	52% 8% 10% 16% 13% 2	4954
	21st to end	Night	46% 9% 15% 18% 11% 1	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	25% 29% 30% 13% 2	303
	11th to 20th	Day	37% 23% 25% 12% 2	300
	21st to end	Day	26% 27% 33% 11% 3	330
February	1st to 10th	Day	21% 28% 36% 14% 2	300
	11th to 20th	Day	40% 28% 24% 7% 1	300
	21st to end	Day	37% 23% 31% 8% 1	249
March	1st to 10th	Day	28% 19% 28% 17% 7%	417
	11th to 20th	Day	25% 20% 32% 18% 4 1	500
	21st to end	Day	26% 21% 27% 20% 6%	550
April	1st to 10th	Day	22% 21% 25% 22% 9% 1	500
	11th to 20th	Day	19% 20% 26% 24% 10% 2	492
	21st to end	Day	18% 17% 26% 22% 14% 3	500
May	1st to 10th	Day	20% 16% 27% 22% 10% 4	503
	11th to 20th	Day	17% 13% 23% 28% 13% 6%	527
	21st to end	Day	16% 9% 23% 24% 19% 9%	573
June	1st to 10th	Day	11% 8% 25% 28% 21% 8%	530
	11th to 20th	Day	6% 6% 21% 30% 27% 10%	530
	21st to end	Day	9% 5% 25% 29% 19% 11%	530
July	1st to 10th	Day	11% 4 14% 28% 29% 13%	525
	11th to 20th	Day	11% 5% 14% 29% 27% 15%	524
	21st to end	Day	12% 3 16% 26% 29% 13%	576
August	1st to 10th	Day	14% 6% 17% 26% 24% 13%	526
	11th to 20th	Day	11% 5% 16% 29% 28% 11%	519
	21st to end	Day	10% 4 12% 27% 31% 15%	583
September	1st to 10th	Day	11% 5% 14% 28% 30% 11%	530
	11th to 20th	Day	13% 5% 16% 26% 32% 8%	530
	21st to end	Day	16% 7% 18% 30% 24% 4	530
October	1st to 10th	Day	15% 7% 23% 30% 20% 6%	530
	11th to 20th	Day	15% 13% 23% 30% 17% 2	530
	21st to end	Day	22% 12% 22% 30% 12% 2	583
November	1st to 10th	Day	16% 15% 30% 27% 10% 2	388
	11th to 20th	Day	19% 19% 34% 22% 6%	330
	21st to end	Day	22% 17% 35% 19% 7% 1	330
December	1st to 10th	Day	28% 22% 32% 15% 2	330
	11th to 20th	Day	25% 18% 31% 23% 3	330
	21st to end	Day	22% 26% 33% 14% 4 1	363
January	1st to 10th	Night	23% 21% 29% 23% 5%	805
	11th to 20th	Night	26% 17% 30% 23% 5%	800
	21st to end	Night	22% 18% 34% 24% 2	880
February	1st to 10th	Night	23% 15% 29% 30% 3	800
	11th to 20th	Night	28% 20% 32% 19% 1	783
	21st to end	Night	26% 22% 30% 21% 1	660
March	1st to 10th	Night	24% 15% 30% 27% 5%	800
	11th to 20th	Night	22% 16% 31% 24% 6% 1	800
	21st to end	Night	20% 15% 31% 26% 8%	880
April	1st to 10th	Night	18% 18% 29% 28% 8% 1	800
	11th to 20th	Night	18% 13% 27% 29% 11% 1	638
	21st to end	Night	16% 10% 25% 38% 10% 1	600
May	1st to 10th	Night	16% 11% 25% 32% 16% 1	600
	11th to 20th	Night	14% 8% 21% 36% 18% 2	591
	21st to end	Night	11% 3 17% 34% 31% 4	650
June	1st to 10th	Night	10% 4 14% 43% 26% 4	600
	11th to 20th	Night	6% 3 16% 37% 32% 7%	600
	21st to end	Night	8% 2 14% 37% 34% 5%	600
July	1st to 10th	Night	12% 3 7% 35% 37% 6%	585
	11th to 20th	Night	9% 3 7% 30% 42% 8%	582
	21st to end	Night	8% 3 9% 31% 40% 8%	657
August	1st to 10th	Night	9% 3 9% 35% 38% 7%	597
	11th to 20th	Night	9% 5% 9% 33% 41% 3	594
	21st to end	Night	10% 2 10% 34% 37% 7%	680
September	1st to 10th	Night	13% 3 11% 33% 35% 5%	800
	11th to 20th	Night	15% 5% 12% 33% 31% 5%	800
	21st to end	Night	13% 6% 18% 36% 25% 2	800
October	1st to 10th	Night	14% 7% 17% 32% 26% 5%	800
	11th to 20th	Night	16% 7% 25% 34% 16% 2	800
	21st to end	Night	17% 11% 23% 35% 13% 2	880
November	1st to 10th	Night	17% 8% 22% 35% 15% 3	820
	11th to 20th	Night	18% 13% 23% 35% 10% 1	840
	21st to end	Night	20% 12% 25% 31% 11% 1	840
December	1st to 10th	Night	25% 14% 30% 26% 5%	840
	11th to 20th	Night	23% 13% 26% 30% 7% 1	840
	21st to end	Night	23% 15% 30% 28% 4	924

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