White Mills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the White Mills Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 8% 3 7% 5% 4 5% 4 6% 7% 38%	126712
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	123712
Transparency	Day	43% 5% 11% 23% 16% 2	130378
	Night	43% 4 10% 22% 18% 3	128117
Seeing	Day	16% 10% 23% 25% 19% 9%	16684
	Night	17% 7% 25% 31% 18% 2	26376

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	14% 12% 8% 9% 5% 4 4 4 5% 6% 29%	3617
	11th to 20th	Day	14% 10% 4 14% 5% 4 4 2 4 4 35%	3734
	21st to end	Day	20% 9% 6% 6% 3 4 4 2 5% 6% 35%	4632
February	1st to 10th	Day	17% 12% 5% 9% 4 3 3 4 4 5% 35%	2000
	11th to 20th	Day	20% 9% 4 8% 5% 4 4 3 5% 6% 33%	1948
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 4 5% 35%	1650
March	1st to 10th	Day	18% 12% 4 7% 4 4 4 3 4 5% 36%	4480
	11th to 20th	Day	13% 10% 3 6% 4 3 3 3 4 7% 45%	4839
	21st to end	Day	12% 9% 3 8% 5% 5% 4 3 5% 7% 40%	5640
April	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 8% 39%	2400
	11th to 20th	Day	14% 8% 3 7% 5% 4 3 4 5% 6% 41%	2374
	21st to end	Day	16% 7% 2 7% 5% 4 4 3 5% 7% 38%	2525
May	1st to 10th	Day	19% 9% 3 8% 6% 6% 6% 4 6% 8% 26%	5862
	11th to 20th	Day	16% 8% 3 8% 5% 5% 5% 5% 6% 7% 33%	5861
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 5% 7% 8% 36%	6433
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	2645
	11th to 20th	Day	6% 4 2 5% 6% 5% 6% 5% 7% 10% 44%	2800
	21st to end	Day	7% 5% 2 5% 5% 5% 6% 6% 7% 11% 42%	2800
July	1st to 10th	Day	9% 5% 3 7% 5% 5% 6% 5% 7% 10% 40%	6360
	11th to 20th	Day	8% 5% 2 7% 6% 5% 7% 5% 8% 11% 35%	6273
	21st to end	Day	10% 9% 3 8% 6% 6% 7% 5% 8% 10% 27%	6424
August	1st to 10th	Day	5% 4 2 5% 4 4 5% 4 7% 11% 48%	4234
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 39%	3862
	21st to end	Day	5% 4 1 4 4 4 4 4 6% 9% 55%	4312
September	1st to 10th	Day	13% 5% 2 7% 5% 5% 5% 4 5% 7% 42%	2342
	11th to 20th	Day	11% 7% 2 5% 4 4 3 4 5% 9% 46%	2200
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 7% 47%	2200
October	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 4 5% 57%	2200
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 5% 6% 53%	2131
	21st to end	Day	19% 6% 2 6% 4 3 4 3 4 5% 44%	1980
November	1st to 10th	Day	12% 6% 2 5% 4 3 4 2 4 6% 52%	1852
	11th to 20th	Day	14% 8% 2 6% 4 4 3 3 4 5% 46%	1863
	21st to end	Day	17% 9% 4 6% 3 3 4 3 5% 6% 41%	1800
December	1st to 10th	Day	17% 9% 3 7% 4 3 3 3 4 5% 41%	2930
	11th to 20th	Day	18% 11% 5% 9% 6% 5% 4 3 4 5% 31%	3373
	21st to end	Day	18% 15% 5% 10% 5% 4 4 3 4 6% 25%	4136
January	1st to 10th	Night	15% 15% 5% 11% 5% 4 4 3 4 6% 28%	6348
	11th to 20th	Night	17% 12% 4 10% 5% 5% 4 2 4 4 33%	6208
	21st to end	Night	20% 11% 3 7% 4 3 3 2 4 5% 39%	6306
February	1st to 10th	Night	21% 11% 3 8% 5% 4 4 4 5% 6% 29%	2400
	11th to 20th	Night	20% 9% 4 9% 4 3 4 3 4 4 35%	2338
	21st to end	Night	20% 11% 4 8% 4 4 4 3 4 6% 33%	1980
March	1st to 10th	Night	19% 14% 3 8% 4 4 4 3 4 4 34%	5373
	11th to 20th	Night	12% 7% 2 6% 4 3 4 3 5% 6% 48%	5120
	21st to end	Night	12% 9% 3 9% 5% 5% 5% 4 5% 6% 37%	5330
April	1st to 10th	Night	12% 6% 2 6% 5% 4 5% 4 6% 7% 43%	2000
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 5% 6% 43%	1975
	21st to end	Night	14% 8% 2 8% 6% 3 4 4 6% 7% 38%	1872
May	1st to 10th	Night	18% 11% 3 8% 5% 5% 5% 4 6% 8% 27%	4033
	11th to 20th	Night	14% 8% 3 9% 6% 5% 5% 4 6% 8% 31%	4032
	21st to end	Night	10% 8% 2 8% 5% 4 5% 4 6% 9% 37%	4421
June	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 7% 8% 47%	1749
	11th to 20th	Night	5% 4 2 6% 4 5% 4 4 7% 9% 51%	1600
	21st to end	Night	3 4 1 5% 5% 5% 4 5% 9% 13% 48%	1600
July	1st to 10th	Night	6% 4 2 6% 5% 5% 4 4 7% 10% 47%	3618
	11th to 20th	Night	6% 5% 2 5% 4 5% 5% 4 8% 11% 45%	3695
	21st to end	Night	7% 7% 3 10% 7% 6% 6% 5% 8% 10% 31%	4427
August	1st to 10th	Night	6% 4 2 5% 3 3 4 3 6% 8% 57%	2915
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 4 8% 10% 48%	3183
	21st to end	Night	4 2 1 3 2 3 3 4 5% 9% 63%	3586
September	1st to 10th	Night	11% 5% 2 4 4 4 3 4 6% 7% 50%	2036
	11th to 20th	Night	10% 6% 2 5% 4 4 3 4 5% 8% 50%	2200
	21st to end	Night	11% 5% 2 5% 3 3 4 4 4 7% 52%	2200
October	1st to 10th	Night	9% 4 1 4 2 3 3 3 4 6% 61%	2200
	11th to 20th	Night	14% 5% 1 5% 4 3 3 3 3 6% 52%	2265
	21st to end	Night	17% 7% 2 5% 3 3 4 3 4 6% 47%	2860
November	1st to 10th	Night	11% 7% 2 5% 4 4 4 3 4 5% 52%	2666
	11th to 20th	Night	13% 7% 2 6% 4 4 4 2 4 7% 46%	2692
	21st to end	Night	17% 8% 4 6% 4 3 3 3 5% 5% 42%	2600
December	1st to 10th	Night	16% 12% 4 8% 4 4 3 3 4 4 39%	4250
	11th to 20th	Night	21% 9% 3 7% 4 3 3 3 4 6% 36%	4822
	21st to end	Night	19% 16% 5% 10% 6% 5% 4 3 4 4 23%	6812

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	56% 4 9% 21% 9% 1	3618
	11th to 20th	Day	54% 3 6% 20% 15% 2	3734
	21st to end	Day	51% 3 8% 21% 13% 3	4632
February	1st to 10th	Day	52% 4 8% 21% 13% 1	2000
	11th to 20th	Day	53% 4 8% 20% 12% 3	1958
	21st to end	Day	51% 5% 8% 18% 14% 4	1650
March	1st to 10th	Day	52% 3 4 17% 19% 5%	4480
	11th to 20th	Day	41% 3 8% 25% 20% 3	4839
	21st to end	Day	44% 4 10% 28% 13% 1	5642
April	1st to 10th	Day	41% 6% 11% 23% 17% 2	2400
	11th to 20th	Day	43% 4 10% 22% 18% 3	2374
	21st to end	Day	45% 5% 10% 25% 12% 2	2525
May	1st to 10th	Day	54% 7% 12% 17% 10%	5897
	11th to 20th	Day	50% 6% 12% 21% 11%	5865
	21st to end	Day	43% 6% 13% 19% 17% 2	6447
June	1st to 10th	Day	39% 7% 10% 23% 18% 2	2645
	11th to 20th	Day	32% 10% 14% 24% 19% 2	2800
	21st to end	Day	34% 8% 16% 21% 19% 4	2800
July	1st to 10th	Day	39% 5% 14% 24% 17% 1	6291
	11th to 20th	Day	40% 7% 17% 26% 8% 2	6198
	21st to end	Day	50% 6% 18% 19% 7% 1	6240
August	1st to 10th	Day	28% 6% 15% 34% 16% 1	4221
	11th to 20th	Day	37% 6% 13% 26% 16% 1	3898
	21st to end	Day	26% 5% 10% 39% 17% 3	4311
September	1st to 10th	Day	40% 7% 11% 20% 18% 4	2342
	11th to 20th	Day	36% 5% 9% 24% 23% 3	2200
	21st to end	Day	36% 5% 9% 25% 22% 4	2200
October	1st to 10th	Day	21% 3 6% 23% 44% 4	3590
	11th to 20th	Day	23% 3 10% 26% 33% 5%	3450
	21st to end	Day	32% 3 6% 29% 22% 8%	3180
November	1st to 10th	Day	35% 3 9% 27% 22% 4	1852
	11th to 20th	Day	41% 3 7% 23% 21% 5%	1863
	21st to end	Day	45% 3 8% 23% 18% 2	1800
December	1st to 10th	Day	47% 3 7% 24% 18% 1	2930
	11th to 20th	Day	57% 3 8% 17% 13% 2	3370
	21st to end	Day	60% 4 11% 18% 7%	4136
January	1st to 10th	Night	57% 3 9% 20% 10% 1	6348
	11th to 20th	Night	57% 3 6% 18% 14% 4	6208
	21st to end	Night	49% 2 7% 19% 19% 2	6306
February	1st to 10th	Night	56% 5% 9% 18% 12% 1	2400
	11th to 20th	Night	53% 4 6% 20% 14% 2	2339
	21st to end	Night	54% 4 9% 17% 14% 3	1980
March	1st to 10th	Night	54% 3 5% 19% 15% 4	5373
	11th to 20th	Night	37% 4 8% 25% 20% 6%	5120
	21st to end	Night	47% 4 10% 23% 14% 1	5330
April	1st to 10th	Night	39% 5% 10% 23% 19% 3	2000
	11th to 20th	Night	42% 4 8% 20% 21% 5%	1975
	21st to end	Night	44% 6% 9% 24% 15% 2	1872
May	1st to 10th	Night	54% 6% 10% 21% 8% 1	4060
	11th to 20th	Night	50% 7% 11% 21% 10%	4033
	21st to end	Night	41% 7% 15% 21% 14% 2	4429
June	1st to 10th	Night	32% 7% 13% 26% 18% 3	1749
	11th to 20th	Night	29% 7% 14% 24% 23% 4	1600
	21st to end	Night	26% 10% 18% 21% 19% 6%	1600
July	1st to 10th	Night	33% 6% 13% 28% 19% 2	3580
	11th to 20th	Night	33% 7% 19% 29% 10% 2	3650
	21st to end	Night	46% 7% 16% 21% 10% 1	4297
August	1st to 10th	Night	25% 5% 13% 38% 17% 1	2904
	11th to 20th	Night	29% 6% 16% 29% 17% 1	3213
	21st to end	Night	19% 5% 12% 36% 24% 4	3586
September	1st to 10th	Night	33% 7% 10% 22% 24% 5%	2036
	11th to 20th	Night	33% 5% 9% 26% 24% 4	2200
	21st to end	Night	33% 6% 8% 23% 25% 6%	2200
October	1st to 10th	Night	20% 3 6% 19% 47% 5%	3590
	11th to 20th	Night	23% 2 6% 25% 37% 7%	3685
	21st to end	Night	31% 2 7% 21% 31% 8%	4610
November	1st to 10th	Night	36% 4 7% 25% 24% 5%	2666
	11th to 20th	Night	38% 4 7% 23% 21% 7%	2692
	21st to end	Night	45% 4 7% 24% 18% 3	2600
December	1st to 10th	Night	50% 3 8% 21% 17% 1	4250
	11th to 20th	Night	50% 3 10% 20% 16% 2	4824
	21st to end	Night	65% 3 8% 16% 6% 1	6812

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% 19% 32% 17% 8%	300
	11th to 20th	Day	25% 21% 35% 14% 5%	300
	21st to end	Day	22% 19% 40% 14% 5%	330
February	1st to 10th	Day	22% 20% 37% 17% 2 1	300
	11th to 20th	Day	28% 22% 33% 14% 3 1	300
	21st to end	Day	27% 19% 38% 14% 2	249
March	1st to 10th	Day	23% 21% 32% 20% 4 1	300
	11th to 20th	Day	22% 16% 37% 16% 7% 1	437
	21st to end	Day	21% 22% 32% 19% 5% 1	550
April	1st to 10th	Day	19% 17% 28% 25% 10% 1	500
	11th to 20th	Day	17% 16% 28% 22% 13% 3	492
	21st to end	Day	19% 11% 30% 27% 9% 3	500
May	1st to 10th	Day	19% 12% 29% 23% 12% 5%	503
	11th to 20th	Day	16% 9% 24% 29% 16% 7%	527
	21st to end	Day	14% 5% 21% 27% 20% 14%	573
June	1st to 10th	Day	11% 4 20% 28% 22% 14%	575
	11th to 20th	Day	6% 3 17% 34% 28% 11%	730
	21st to end	Day	8% 4 19% 30% 25% 15%	730
July	1st to 10th	Day	10% 4 13% 24% 29% 20%	721
	11th to 20th	Day	8% 3 14% 31% 29% 16%	683
	21st to end	Day	11% 3 10% 26% 32% 19%	576
August	1st to 10th	Day	11% 2 11% 23% 32% 20%	526
	11th to 20th	Day	12% 2 12% 29% 33% 12%	519
	21st to end	Day	10% 2 7% 25% 34% 21%	583
September	1st to 10th	Day	12% 4 13% 21% 37% 13%	530
	11th to 20th	Day	11% 2 11% 25% 34% 18%	530
	21st to end	Day	16% 5% 18% 25% 24% 12%	530
October	1st to 10th	Day	10% 3 17% 33% 28% 8%	530
	11th to 20th	Day	12% 11% 29% 26% 15% 7%	516
	21st to end	Day	23% 15% 25% 24% 12% 1	363
November	1st to 10th	Day	19% 9% 28% 30% 12% 3	330
	11th to 20th	Day	19% 17% 28% 29% 8% 1	321
	21st to end	Day	24% 11% 31% 23% 11%	300
December	1st to 10th	Day	20% 21% 29% 21% 8% 1	300
	11th to 20th	Day	22% 15% 31% 22% 10%	300
	21st to end	Day	27% 19% 30% 20% 4	330
January	1st to 10th	Night	21% 13% 35% 27% 4	1000
	11th to 20th	Night	26% 13% 35% 22% 4	939
	21st to end	Night	21% 13% 39% 26% 1	880
February	1st to 10th	Night	26% 10% 36% 25% 3	800
	11th to 20th	Night	24% 12% 40% 22% 2	783
	21st to end	Night	25% 12% 40% 22% 2	660
March	1st to 10th	Night	21% 11% 36% 27% 5%	800
	11th to 20th	Night	23% 10% 33% 28% 5% 1	800
	21st to end	Night	19% 12% 31% 30% 7%	820
April	1st to 10th	Night	16% 8% 36% 31% 9%	600
	11th to 20th	Night	19% 9% 30% 29% 12% 1	593
	21st to end	Night	17% 7% 33% 33% 10% 1	600
May	1st to 10th	Night	21% 4 28% 31% 14% 1	600
	11th to 20th	Night	14% 3 21% 39% 20% 3	591
	21st to end	Night	7% 2 17% 40% 29% 6%	650
June	1st to 10th	Night	11% 2 16% 38% 29% 4	600
	11th to 20th	Night	7% 3 12% 41% 34% 3	600
	21st to end	Night	4 1 15% 41% 34% 5%	600
July	1st to 10th	Night	9% 2 9% 27% 44% 9%	585
	11th to 20th	Night	9% 2 6% 34% 43% 6%	582
	21st to end	Night	9% 3 7% 35% 39% 7%	657
August	1st to 10th	Night	8% 1 7% 32% 45% 7%	597
	11th to 20th	Night	6% 1 8% 36% 42% 7%	594
	21st to end	Night	7% 1 6% 34% 45% 8%	660
September	1st to 10th	Night	13% 2 9% 32% 39% 5%	636
	11th to 20th	Night	10% 4 9% 33% 38% 6%	800
	21st to end	Night	12% 5% 18% 31% 31% 4	800
October	1st to 10th	Night	11% 3 16% 35% 30% 6%	800
	11th to 20th	Night	15% 6% 23% 36% 19% 2	800
	21st to end	Night	19% 6% 32% 29% 13% 1	880
November	1st to 10th	Night	13% 8% 28% 34% 15% 2	820
	11th to 20th	Night	15% 10% 32% 31% 12%	829
	21st to end	Night	22% 8% 29% 31% 10% 1	800
December	1st to 10th	Night	25% 8% 30% 30% 6% 1	800
	11th to 20th	Night	25% 8% 32% 28% 7% 1	800
	21st to end	Night	28% 12% 28% 29% 3	1020

Page generated at 2025-11-15 00:56:00. Version ac07

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