Wilsall
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilsall clear sky chart from 2005-11-06 to 2026-07-16.

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	5% 7% 3 7% 5% 5% 5% 4 6% 8% 45%	147007
	Night	4 7% 3 8% 5% 5% 5% 4 6% 8% 44%	143093
Transparency	Day	33% 7% 13% 25% 16% 5%	141702
	Night	34% 8% 16% 27% 13% 3	139406
Seeing	Day	11% 11% 28% 28% 16% 5%	17800
	Night	9% 6% 27% 42% 15% 1	27231

By Month

Percentage Cloud Cover SW (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	3 11% 6% 11% 7% 6% 5% 5% 8% 10% 29%	4124
	11th to 20th	Day	1 6% 4 10% 8% 7% 6% 4 6% 8% 40%	4477
	21st to end	Day	2 8% 3 9% 5% 5% 5% 4 6% 7% 47%	5819
February	1st to 10th	Day	3 11% 4 15% 9% 8% 7% 5% 7% 6% 26%	1890
	11th to 20th	Day	1 10% 6% 16% 9% 6% 6% 4 6% 7% 29%	1845
	21st to end	Day	2 8% 4 14% 9% 7% 7% 4 7% 8% 30%	1611
March	1st to 10th	Day	2 7% 4 10% 5% 5% 5% 4 6% 6% 46%	4933
	11th to 20th	Day	3 10% 4 8% 6% 6% 7% 5% 7% 9% 35%	5110
	21st to end	Day	4 13% 5% 11% 6% 7% 8% 6% 8% 8% 24%	5607
April	1st to 10th	Day	6% 12% 4 9% 6% 6% 7% 4 7% 9% 29%	2447
	11th to 20th	Day	5% 11% 5% 11% 7% 6% 6% 5% 6% 7% 30%	2681
	21st to end	Day	9% 12% 4 9% 6% 6% 6% 5% 6% 8% 29%	2730
May	1st to 10th	Day	8% 10% 3 6% 4 5% 5% 5% 6% 10% 38%	7644
	11th to 20th	Day	10% 8% 3 7% 5% 5% 6% 5% 7% 9% 34%	8191
	21st to end	Day	10% 7% 2 6% 4 4 4 4 7% 9% 42%	9077
June	1st to 10th	Day	9% 8% 3 7% 5% 5% 5% 5% 8% 8% 36%	3150
	11th to 20th	Day	10% 7% 2 7% 6% 5% 6% 5% 7% 8% 36%	3150
	21st to end	Day	5% 3 2 5% 4 5% 5% 5% 7% 8% 52%	3150
July	1st to 10th	Day	3 1 1 4 4 5% 5% 5% 7% 7% 57%	6929
	11th to 20th	Day	4 2 1 3 3 3 4 4 6% 7% 65%	6856
	21st to end	Day	3 2 2 3 3 4 5% 4 7% 8% 58%	6639
August	1st to 10th	Day	4 3 1 4 3 4 5% 4 7% 10% 54%	5779
	11th to 20th	Day	3 2 1 4 3 4 4 4 5% 7% 63%	5798
	21st to end	Day	7% 4 1 4 3 3 4 3 5% 6% 59%	6468
September	1st to 10th	Day	6% 3 1 4 3 3 4 3 5% 7% 61%	2400
	11th to 20th	Day	8% 5% 2 6% 3 4 5% 4 5% 8% 50%	2220
	21st to end	Day	7% 5% 2 5% 3 4 4 4 5% 8% 54%	2200
October	1st to 10th	Day	7% 5% 2 5% 3 3 4 2 4 5% 60%	3289
	11th to 20th	Day	6% 6% 2 6% 4 3 4 3 4 6% 57%	3250
	21st to end	Day	3 7% 3 8% 5% 6% 6% 4 6% 8% 43%	3402
November	1st to 10th	Day	6% 9% 4 9% 6% 6% 5% 4 6% 8% 38%	1723
	11th to 20th	Day	3 9% 4 9% 6% 6% 5% 4 7% 8% 38%	1680
	21st to end	Day	2 9% 4 10% 5% 5% 6% 5% 7% 7% 40%	1680
December	1st to 10th	Day	2 5% 3 6% 6% 5% 6% 5% 6% 7% 48%	2700
	11th to 20th	Day	5% 8% 3 9% 6% 7% 7% 4 7% 8% 34%	2783
	21st to end	Day	4 11% 6% 10% 6% 5% 6% 4 6% 7% 34%	3575
January	1st to 10th	Night	2 11% 4 8% 5% 5% 6% 5% 8% 9% 35%	8768
	11th to 20th	Night	1 7% 3 11% 7% 6% 6% 4 5% 8% 42%	8542
	21st to end	Night	1 7% 3 9% 5% 5% 6% 5% 7% 8% 44%	8392
February	1st to 10th	Night	3 13% 6% 14% 9% 8% 6% 4 6% 6% 25%	2730
	11th to 20th	Night	1 12% 5% 13% 8% 6% 6% 4 6% 7% 31%	2662
	21st to end	Night	1 10% 4 11% 9% 7% 7% 5% 6% 6% 34%	2191
March	1st to 10th	Night	2 7% 3 8% 6% 6% 6% 5% 7% 8% 42%	5169
	11th to 20th	Night	4 9% 3 6% 5% 5% 6% 6% 7% 9% 39%	5110
	21st to end	Night	5% 13% 4 10% 7% 6% 6% 6% 8% 8% 28%	5610
April	1st to 10th	Night	6% 11% 3 9% 6% 5% 5% 4 7% 6% 36%	2172
	11th to 20th	Night	5% 11% 3 8% 6% 6% 5% 4 8% 8% 34%	1883
	21st to end	Night	11% 10% 3 8% 4 4 5% 5% 7% 7% 36%	1890
May	1st to 10th	Night	7% 9% 3 8% 5% 4 4 4 6% 7% 43%	5288
	11th to 20th	Night	9% 8% 2 7% 5% 5% 6% 5% 7% 9% 37%	4701
	21st to end	Night	9% 6% 2 7% 4 4 5% 4 6% 9% 43%	5123
June	1st to 10th	Night	10% 6% 3 7% 5% 5% 7% 6% 6% 7% 37%	1470
	11th to 20th	Night	11% 7% 2 7% 5% 6% 6% 4 6% 7% 40%	1470
	21st to end	Night	5% 3 1 5% 4 5% 4 4 6% 9% 54%	1470
July	1st to 10th	Night	5% 2 1 4 3 4 4 4 8% 9% 58%	3250
	11th to 20th	Night	6% 1 2 4 3 3 3 3 5% 7% 63%	3215
	21st to end	Night	3 3 2 5% 3 4 5% 4 9% 10% 53%	4225
August	1st to 10th	Night	5% 2 1 4 3 4 5% 5% 8% 11% 50%	4033
	11th to 20th	Night	3 2 1 5% 4 5% 5% 5% 7% 7% 56%	4051
	21st to end	Night	7% 4 1 5% 3 4 5% 3 5% 6% 56%	4509
September	1st to 10th	Night	6% 4 1 5% 4 3 4 3 5% 7% 58%	2000
	11th to 20th	Night	8% 5% 2 6% 4 3 5% 4 7% 8% 49%	2177
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 7% 55%	2200
October	1st to 10th	Night	6% 5% 2 5% 3 3 3 3 4 5% 60%	3888
	11th to 20th	Night	7% 7% 2 6% 3 3 4 3 4 7% 54%	3898
	21st to end	Night	6% 8% 3 6% 4 4 5% 4 6% 8% 46%	4394
November	1st to 10th	Night	6% 9% 3 7% 5% 6% 5% 4 6% 8% 42%	2774
	11th to 20th	Night	3 8% 4 9% 6% 5% 6% 5% 7% 8% 39%	2940
	21st to end	Night	1 7% 2 8% 5% 5% 6% 6% 7% 9% 42%	2940
December	1st to 10th	Night	2 6% 3 7% 6% 5% 5% 4 5% 7% 50%	4710
	11th to 20th	Night	5% 8% 3 8% 6% 6% 6% 5% 6% 9% 38%	5625
	21st to end	Night	3 8% 4 10% 6% 5% 5% 5% 7% 9% 40%	7623

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	42% 9% 16% 27% 5%	4125
	11th to 20th	Day	38% 7% 20% 21% 13% 2	4477
	21st to end	Day	32% 8% 18% 29% 12% 1	5819
February	1st to 10th	Day	47% 12% 15% 20% 7% 1	1890
	11th to 20th	Day	46% 11% 14% 20% 8% 1	1854
	21st to end	Day	43% 11% 10% 21% 13% 2	1611
March	1st to 10th	Day	32% 6% 10% 27% 20% 5%	4933
	11th to 20th	Day	36% 9% 15% 28% 11%	5110
	21st to end	Day	46% 12% 16% 21% 4 1	5607
April	1st to 10th	Day	45% 9% 15% 21% 9% 1	2447
	11th to 20th	Day	46% 9% 14% 22% 9%	2684
	21st to end	Day	48% 7% 13% 22% 9%	2730
May	1st to 10th	Day	37% 8% 10% 29% 14% 2	7644
	11th to 20th	Day	39% 8% 14% 23% 12% 4	8195
	21st to end	Day	34% 6% 14% 29% 15% 2	9078
June	1st to 10th	Day	40% 8% 17% 23% 9% 1	3150
	11th to 20th	Day	40% 8% 13% 20% 15% 4	3150
	21st to end	Day	25% 7% 13% 28% 22% 5%	3150
July	1st to 10th	Day	20% 6% 11% 24% 28% 11%	6856
	11th to 20th	Day	16% 3 8% 21% 28% 24%	6775
	21st to end	Day	19% 6% 13% 19% 27% 16%	6448
August	1st to 10th	Day	24% 6% 11% 23% 23% 14%	4208
	11th to 20th	Day	22% 5% 8% 29% 28% 8%	4197
	21st to end	Day	21% 4 7% 27% 27% 13%	4663
September	1st to 10th	Day	22% 4 8% 27% 28% 11%	2400
	11th to 20th	Day	31% 5% 10% 26% 22% 6%	2220
	21st to end	Day	27% 5% 11% 29% 22% 6%	2200
October	1st to 10th	Day	27% 4 8% 20% 34% 8%	3289
	11th to 20th	Day	27% 5% 15% 32% 19% 2	3250
	21st to end	Day	34% 7% 12% 30% 14% 2	3402
November	1st to 10th	Day	40% 8% 14% 25% 11% 2	1723
	11th to 20th	Day	37% 9% 20% 24% 9% 1	1680
	21st to end	Day	36% 9% 17% 25% 12%	1680
December	1st to 10th	Day	28% 9% 24% 26% 11% 1	2700
	11th to 20th	Day	40% 10% 24% 21% 4 1	2782
	21st to end	Day	44% 7% 9% 26% 10% 5%	3575
January	1st to 10th	Night	38% 8% 18% 30% 7%	8769
	11th to 20th	Night	37% 7% 23% 22% 8% 3	8542
	21st to end	Night	32% 8% 18% 29% 12%	8392
February	1st to 10th	Night	50% 11% 14% 16% 8%	2730
	11th to 20th	Night	44% 11% 15% 19% 10% 1	2663
	21st to end	Night	43% 9% 13% 20% 12% 2	2191
March	1st to 10th	Night	32% 9% 10% 29% 17% 3	5169
	11th to 20th	Night	34% 10% 19% 30% 8%	5110
	21st to end	Night	45% 12% 18% 19% 6%	5610
April	1st to 10th	Night	44% 8% 16% 24% 8%	2172
	11th to 20th	Night	42% 8% 15% 27% 8% 1	1883
	21st to end	Night	42% 8% 19% 24% 7%	1890
May	1st to 10th	Night	37% 9% 16% 26% 12%	5286
	11th to 20th	Night	39% 7% 15% 26% 10% 3	4702
	21st to end	Night	34% 9% 18% 33% 6%	5124
June	1st to 10th	Night	41% 10% 17% 22% 9%	1470
	11th to 20th	Night	41% 8% 14% 23% 11% 3	1470
	21st to end	Night	25% 7% 16% 29% 19% 4	1470
July	1st to 10th	Night	21% 5% 14% 32% 24% 3	3215
	11th to 20th	Night	19% 4 9% 25% 28% 15%	3173
	21st to end	Night	21% 7% 17% 25% 20% 9%	4104
August	1st to 10th	Night	24% 7% 15% 26% 18% 10%	2928
	11th to 20th	Night	26% 6% 12% 33% 17% 6%	2928
	21st to end	Night	22% 4 9% 34% 25% 6%	3245
September	1st to 10th	Night	25% 5% 9% 25% 25% 11%	2000
	11th to 20th	Night	30% 6% 13% 28% 19% 4	2177
	21st to end	Night	29% 4 11% 29% 22% 4	2200
October	1st to 10th	Night	25% 4 9% 28% 27% 7%	3888
	11th to 20th	Night	30% 5% 16% 27% 20% 2	3898
	21st to end	Night	32% 6% 15% 28% 17% 2	4394
November	1st to 10th	Night	35% 8% 14% 29% 13% 2	2774
	11th to 20th	Night	35% 10% 18% 28% 8% 1	2940
	21st to end	Night	31% 9% 20% 27% 11% 2	2940
December	1st to 10th	Night	29% 8% 24% 26% 11% 1	4710
	11th to 20th	Night	37% 8% 25% 24% 6% 1	5626
	21st to end	Night	36% 7% 14% 27% 9% 6%	7623

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	8% 17% 22% 31% 19% 3	220
	11th to 20th	Day	10% 13% 36% 25% 12% 4	220
	21st to end	Day	8% 10% 28% 42% 8% 2	358
February	1st to 10th	Day	11% 17% 39% 25% 7% 1	330
	11th to 20th	Day	8% 12% 38% 30% 10% 2	330
	21st to end	Day	11% 19% 35% 22% 10% 2	327
March	1st to 10th	Day	11% 17% 36% 28% 8% 1	540
	11th to 20th	Day	13% 20% 31% 24% 11% 2	540
	21st to end	Day	12% 21% 33% 23% 9% 2	594
April	1st to 10th	Day	17% 17% 33% 22% 10% 2	540
	11th to 20th	Day	10% 22% 33% 26% 9% 1	532
	21st to end	Day	16% 15% 31% 23% 11% 3	540
May	1st to 10th	Day	15% 9% 30% 26% 16% 4	548
	11th to 20th	Day	17% 10% 22% 27% 18% 5%	775
	21st to end	Day	15% 8% 22% 26% 23% 6%	844
June	1st to 10th	Day	13% 10% 24% 24% 22% 7%	780
	11th to 20th	Day	14% 14% 30% 26% 14% 3	780
	21st to end	Day	8% 7% 24% 34% 24% 3	780
July	1st to 10th	Day	5% 4 25% 33% 26% 7%	771
	11th to 20th	Day	4 6% 27% 36% 24% 3	744
	21st to end	Day	4 4 26% 32% 25% 9%	644
August	1st to 10th	Day	7% 8% 24% 33% 21% 7%	527
	11th to 20th	Day	7% 4 23% 34% 21% 11%	517
	21st to end	Day	6% 8% 26% 30% 21% 9%	583
September	1st to 10th	Day	7% 6% 28% 26% 24% 8%	530
	11th to 20th	Day	12% 8% 28% 30% 16% 7%	530
	21st to end	Day	12% 9% 27% 24% 18% 10%	530
October	1st to 10th	Day	16% 6% 27% 27% 14% 10%	349
	11th to 20th	Day	14% 13% 23% 26% 15% 9%	330
	21st to end	Day	10% 14% 29% 31% 10% 4	363
November	1st to 10th	Day	14% 14% 30% 29% 10% 3	330
	11th to 20th	Day	7% 17% 35% 27% 12% 2	330
	21st to end	Day	7% 12% 32% 39% 9% 1	330
December	1st to 10th	Day	6% 18% 28% 32% 14% 2	330
	11th to 20th	Day	14% 17% 30% 29% 9% 2	242
	21st to end	Day	9% 13% 37% 26% 11% 5%	242
January	1st to 10th	Night	6% 9% 34% 40% 10%	1050
	11th to 20th	Night	8% 9% 36% 36% 9% 2	954
	21st to end	Night	7% 6% 34% 43% 8% 1	924
February	1st to 10th	Night	8% 8% 36% 38% 9%	840
	11th to 20th	Night	8% 7% 35% 40% 8% 2	824
	21st to end	Night	5% 10% 32% 40% 12% 1	692
March	1st to 10th	Night	6% 7% 33% 44% 10% 1	840
	11th to 20th	Night	8% 5% 30% 43% 12% 1	840
	21st to end	Night	10% 7% 29% 39% 13% 1	924
April	1st to 10th	Night	13% 6% 26% 43% 11%	702
	11th to 20th	Night	10% 7% 31% 41% 11%	623
	21st to end	Night	16% 6% 23% 40% 14% 1	630
May	1st to 10th	Night	12% 7% 23% 39% 18% 1	630
	11th to 20th	Night	13% 6% 18% 38% 24% 1	621
	21st to end	Night	16% 5% 18% 38% 22% 2	664
June	1st to 10th	Night	12% 5% 21% 39% 21% 2	420
	11th to 20th	Night	13% 3 27% 44% 14%	420
	21st to end	Night	6% 2 17% 52% 22% 1	420
July	1st to 10th	Night	6% 1 10% 48% 33% 1	410
	11th to 20th	Night	3 4 12% 61% 20% 1	398
	21st to end	Night	5% 2 18% 49% 25% 1	613
August	1st to 10th	Night	6% 2 14% 53% 23% 2	597
	11th to 20th	Night	4 1 12% 53% 28% 1	594
	21st to end	Night	7% 3 17% 44% 27% 2	660
September	1st to 10th	Night	8% 2 15% 50% 23% 3	600
	11th to 20th	Night	9% 3 14% 46% 26% 2	777
	21st to end	Night	12% 3 18% 42% 23% 3	800
October	1st to 10th	Night	14% 3 19% 42% 20% 2	800
	11th to 20th	Night	12% 4 25% 42% 16% 2	800
	21st to end	Night	11% 6% 29% 41% 12% 1	880
November	1st to 10th	Night	11% 4 31% 43% 9% 1	929
	11th to 20th	Night	9% 10% 34% 39% 8%	1050
	21st to end	Night	5% 4 36% 46% 9%	1050
December	1st to 10th	Night	7% 9% 36% 39% 9% 1	1050
	11th to 20th	Night	7% 7% 39% 38% 7% 1	1050
	21st to end	Night	7% 8% 38% 35% 10% 1	1155

Page generated at 2026-07-13 01:14:47. Version ac07

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