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 2024-02-10.

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% 4 7% 7% 4 6% 6% 6% 8% 40%	78039
	Night	4 7% 3 8% 7% 4 6% 6% 6% 8% 40%	80950
Transparency	Day	37% 9% 14% 23% 14% 4	77736
	Night	36% 9% 16% 24% 11% 3	80775
Seeing	Day	11% 11% 28% 29% 17% 5%	14426
	Night	9% 6% 26% 42% 15% 1	24119

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	1 10% 5% 9% 10% 5% 7% 7% 8% 10% 29%	2219
	11th to 20th	Day	2 8% 5% 12% 11% 7% 7% 5% 6% 7% 29%	2389
	21st to end	Day	1 7% 4 8% 8% 4 6% 6% 6% 8% 42%	3121
February	1st to 10th	Day	2 10% 4 15% 11% 5% 8% 6% 6% 7% 26%	1701
	11th to 20th	Day	1 10% 6% 14% 11% 5% 6% 5% 6% 7% 27%	1575
	21st to end	Day	2 8% 6% 13% 12% 5% 7% 5% 6% 8% 27%	1377
March	1st to 10th	Day	3 8% 4 12% 9% 5% 7% 7% 7% 8% 31%	1926
	11th to 20th	Day	3 8% 4 7% 8% 6% 7% 6% 7% 9% 33%	1980
	21st to end	Day	4 12% 6% 11% 10% 5% 8% 6% 6% 8% 24%	2178
April	1st to 10th	Day	7% 11% 5% 8% 8% 5% 7% 6% 6% 9% 27%	2096
	11th to 20th	Day	6% 11% 6% 10% 8% 5% 7% 7% 6% 7% 28%	2293
	21st to end	Day	10% 12% 4 9% 7% 4 6% 6% 6% 8% 28%	2340
May	1st to 10th	Day	9% 9% 3 7% 7% 4 7% 8% 6% 8% 32%	2344
	11th to 20th	Day	11% 8% 3 7% 7% 4 6% 6% 8% 9% 30%	2478
	21st to end	Day	14% 10% 4 9% 7% 4 5% 6% 7% 9% 25%	2736
June	1st to 10th	Day	10% 8% 4 6% 6% 4 6% 7% 8% 9% 33%	2700
	11th to 20th	Day	10% 7% 3 7% 7% 4 6% 7% 7% 8% 34%	2700
	21st to end	Day	4 3 2 4 5% 4 5% 6% 7% 8% 52%	2700
July	1st to 10th	Day	3 2 1 3 5% 3 5% 8% 7% 9% 54%	2700
	11th to 20th	Day	2 2 1 3 4 3 4 6% 6% 7% 61%	2700
	21st to end	Day	3 3 1 3 4 2 5% 5% 6% 8% 60%	2647
August	1st to 10th	Day	5% 4 2 3 4 3 4 6% 7% 9% 53%	2328
	11th to 20th	Day	5% 3 1 3 4 3 5% 5% 6% 8% 59%	2299
	21st to end	Day	5% 3 1 4 4 2 4 5% 7% 7% 57%	2574
September	1st to 10th	Day	7% 3 2 3 4 2 4 4 5% 7% 60%	2160
	11th to 20th	Day	8% 4 2 5% 4 3 5% 5% 5% 7% 52%	1998
	21st to end	Day	8% 5% 2 4 4 3 4 5% 6% 8% 51%	1980
October	1st to 10th	Day	11% 8% 4 8% 6% 3 4 4 6% 7% 40%	1818
	11th to 20th	Day	8% 7% 3 8% 6% 3 5% 5% 5% 6% 45%	1800
	21st to end	Day	5% 7% 3 8% 8% 4 7% 6% 6% 9% 38%	1901
November	1st to 10th	Day	6% 8% 5% 8% 7% 4 6% 5% 6% 7% 37%	1556
	11th to 20th	Day	3 8% 5% 8% 7% 4 7% 6% 7% 8% 37%	1520
	21st to end	Day	3 7% 5% 9% 7% 4 7% 5% 6% 7% 40%	1520
December	1st to 10th	Day	2 6% 4 7% 9% 6% 8% 8% 7% 8% 36%	1520
	11th to 20th	Day	2 7% 4 9% 9% 6% 10% 7% 7% 8% 31%	1723
	21st to end	Day	3 8% 6% 10% 8% 4 7% 6% 6% 8% 36%	2442
January	1st to 10th	Night	1 7% 4 9% 7% 4 6% 7% 8% 9% 36%	4726
	11th to 20th	Night	1 8% 4 13% 11% 6% 7% 6% 5% 8% 30%	4602
	21st to end	Night	1 7% 4 8% 8% 4 8% 7% 7% 8% 38%	4543
February	1st to 10th	Night	3 12% 8% 13% 10% 6% 7% 5% 5% 6% 24%	2463
	11th to 20th	Night	2 11% 5% 12% 10% 5% 6% 6% 5% 7% 32%	2272
	21st to end	Night	1 9% 5% 11% 10% 6% 8% 6% 6% 6% 31%	1875
March	1st to 10th	Night	3 6% 4 9% 8% 5% 7% 7% 6% 8% 36%	2034
	11th to 20th	Night	4 7% 3 6% 6% 3 6% 7% 7% 11% 38%	1980
	21st to end	Night	4 12% 4 10% 8% 5% 5% 7% 7% 8% 30%	2178
April	1st to 10th	Night	7% 9% 4 9% 7% 4 6% 6% 7% 7% 35%	1863
	11th to 20th	Night	6% 11% 4 7% 7% 5% 7% 5% 7% 8% 33%	1612
	21st to end	Night	11% 10% 3 7% 5% 3 5% 6% 7% 8% 35%	1620
May	1st to 10th	Night	8% 9% 3 6% 8% 3 5% 6% 7% 9% 37%	1615
	11th to 20th	Night	9% 8% 2 6% 6% 4 6% 7% 7% 9% 36%	1417
	21st to end	Night	12% 9% 4 7% 5% 4 7% 6% 6% 11% 30%	1536
June	1st to 10th	Night	11% 6% 3 6% 6% 3 7% 8% 6% 8% 35%	1260
	11th to 20th	Night	12% 7% 3 7% 6% 4 6% 5% 6% 7% 38%	1260
	21st to end	Night	5% 3 2 4 4 3 5% 5% 6% 9% 55%	1260
July	1st to 10th	Night	6% 2 1 3 4 3 4 4 7% 10% 56%	1260
	11th to 20th	Night	3 2 2 2 3 2 4 5% 6% 10% 61%	1260
	21st to end	Night	3 3 1 4 4 3 4 4 6% 8% 61%	1666
August	1st to 10th	Night	4 4 2 4 4 3 5% 6% 7% 9% 52%	1611
	11th to 20th	Night	4 2 1 3 4 3 5% 5% 7% 7% 57%	1602
	21st to end	Night	5% 3 2 5% 5% 4 5% 5% 6% 8% 52%	1782
September	1st to 10th	Night	6% 4 1 5% 5% 3 4 4 5% 7% 57%	1800
	11th to 20th	Night	8% 4 2 5% 4 2 4 5% 7% 8% 51%	1959
	21st to end	Night	9% 5% 2 4 4 3 4 4 5% 7% 52%	1980
October	1st to 10th	Night	10% 8% 3 6% 6% 3 5% 4 5% 6% 43%	2142
	11th to 20th	Night	7% 7% 3 6% 6% 3 4 4 4 7% 48%	2160
	21st to end	Night	6% 6% 3 6% 6% 4 6% 7% 6% 7% 44%	2452
November	1st to 10th	Night	6% 9% 4 6% 5% 4 6% 5% 6% 9% 40%	2503
	11th to 20th	Night	3 8% 4 8% 8% 4 6% 6% 6% 8% 39%	2660
	21st to end	Night	2 6% 3 7% 7% 4 6% 6% 7% 10% 42%	2660
December	1st to 10th	Night	3 7% 4 9% 9% 5% 7% 5% 6% 8% 37%	2660
	11th to 20th	Night	2 6% 4 8% 8% 5% 7% 7% 6% 9% 38%	3463
	21st to end	Night	2 6% 4 8% 7% 4 5% 6% 6% 9% 42%	5214

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	39% 13% 15% 29% 4	2219
	11th to 20th	Day	46% 10% 17% 14% 12% 1	2389
	21st to end	Day	32% 12% 23% 23% 7% 1	3121
February	1st to 10th	Day	48% 12% 14% 19% 5% 1	1701
	11th to 20th	Day	47% 12% 15% 19% 7% 1	1584
	21st to end	Day	47% 11% 9% 18% 12% 2	1377
March	1st to 10th	Day	42% 12% 16% 22% 8% 1	1926
	11th to 20th	Day	40% 10% 17% 21% 11% 1	1980
	21st to end	Day	51% 10% 16% 18% 4 1	2178
April	1st to 10th	Day	47% 9% 15% 20% 8% 1	2096
	11th to 20th	Day	48% 9% 14% 21% 8%	2296
	21st to end	Day	50% 7% 14% 21% 7%	2340
May	1st to 10th	Day	44% 8% 13% 26% 8% 1	2344
	11th to 20th	Day	44% 10% 16% 22% 7% 1	2482
	21st to end	Day	52% 8% 20% 15% 5% 1	2736
June	1st to 10th	Day	42% 9% 18% 22% 7% 1	2700
	11th to 20th	Day	42% 9% 13% 20% 13% 4	2700
	21st to end	Day	24% 7% 13% 28% 23% 4	2700
July	1st to 10th	Day	20% 9% 12% 29% 24% 7%	2626
	11th to 20th	Day	19% 5% 8% 24% 25% 19%	2619
	21st to end	Day	19% 5% 7% 21% 30% 19%	2499
August	1st to 10th	Day	25% 6% 9% 24% 26% 10%	2313
	11th to 20th	Day	21% 5% 7% 28% 28% 11%	2299
	21st to end	Day	23% 5% 9% 26% 25% 12%	2574
September	1st to 10th	Day	23% 4 8% 25% 27% 12%	2160
	11th to 20th	Day	29% 5% 10% 25% 24% 7%	1998
	21st to end	Day	28% 6% 12% 31% 19% 5%	1980
October	1st to 10th	Day	41% 5% 12% 25% 15% 2	1818
	11th to 20th	Day	36% 8% 15% 25% 13% 3	1800
	21st to end	Day	36% 11% 16% 27% 9% 1	1901
November	1st to 10th	Day	41% 9% 13% 24% 11% 2	1556
	11th to 20th	Day	37% 9% 20% 23% 9% 1	1520
	21st to end	Day	37% 9% 16% 25% 13%	1520
December	1st to 10th	Day	37% 13% 18% 23% 8% 2	1520
	11th to 20th	Day	39% 14% 25% 19% 3 1	1722
	21st to end	Day	40% 9% 8% 24% 11% 7%	2442
January	1st to 10th	Night	34% 11% 16% 31% 7%	4726
	11th to 20th	Night	46% 10% 15% 19% 8% 2	4602
	21st to end	Night	34% 14% 22% 24% 6% 1	4543
February	1st to 10th	Night	52% 12% 13% 16% 6%	2463
	11th to 20th	Night	45% 11% 16% 19% 8%	2273
	21st to end	Night	47% 9% 12% 19% 12% 2	1875
March	1st to 10th	Night	39% 11% 17% 23% 10%	2034
	11th to 20th	Night	34% 10% 21% 25% 10% 1	1980
	21st to end	Night	46% 10% 19% 21% 5%	2178
April	1st to 10th	Night	45% 8% 16% 23% 8% 1	1863
	11th to 20th	Night	44% 8% 15% 25% 7% 1	1612
	21st to end	Night	42% 9% 20% 23% 6%	1620
May	1st to 10th	Night	40% 10% 17% 27% 6% 1	1615
	11th to 20th	Night	42% 9% 18% 21% 8% 1	1418
	21st to end	Night	47% 9% 23% 15% 4 1	1536
June	1st to 10th	Night	43% 10% 19% 21% 6%	1260
	11th to 20th	Night	43% 8% 13% 23% 10% 3	1260
	21st to end	Night	24% 7% 16% 30% 18% 4	1260
July	1st to 10th	Night	22% 7% 16% 31% 21% 5%	1225
	11th to 20th	Night	17% 6% 12% 28% 25% 14%	1218
	21st to end	Night	20% 5% 10% 24% 29% 12%	1574
August	1st to 10th	Night	25% 7% 15% 29% 18% 7%	1602
	11th to 20th	Night	22% 6% 12% 28% 22% 9%	1602
	21st to end	Night	27% 6% 11% 26% 22% 9%	1782
September	1st to 10th	Night	26% 6% 9% 24% 25% 11%	1800
	11th to 20th	Night	28% 6% 13% 28% 21% 4	1959
	21st to end	Night	31% 5% 12% 30% 20% 3	1980
October	1st to 10th	Night	39% 7% 12% 25% 14% 2	2142
	11th to 20th	Night	35% 7% 15% 26% 15% 3	2160
	21st to end	Night	33% 8% 18% 26% 13% 1	2452
November	1st to 10th	Night	36% 8% 14% 27% 12% 2	2503
	11th to 20th	Night	36% 10% 19% 28% 7% 1	2660
	21st to end	Night	31% 9% 19% 27% 12% 2	2660
December	1st to 10th	Night	38% 12% 18% 20% 10% 2	2660
	11th to 20th	Night	35% 10% 24% 25% 5% 1	3464
	21st to end	Night	34% 9% 12% 27% 9% 9%	5214

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% 14% 20% 32% 22% 3	180
	11th to 20th	Day	12% 8% 33% 28% 14% 4	180
	21st to end	Day	8% 10% 28% 41% 10% 3	292
February	1st to 10th	Day	10% 18% 40% 25% 6%	267
	11th to 20th	Day	10% 13% 38% 28% 9% 1	240
	21st to end	Day	9% 17% 35% 25% 12% 2	243
March	1st to 10th	Day	10% 19% 38% 27% 5% 1	420
	11th to 20th	Day	10% 19% 32% 25% 12% 2	420
	21st to end	Day	12% 19% 31% 25% 10% 3	462
April	1st to 10th	Day	17% 18% 35% 21% 9% 1	420
	11th to 20th	Day	12% 23% 33% 24% 8% 1	412
	21st to end	Day	18% 15% 31% 22% 11% 3	420
May	1st to 10th	Day	15% 10% 30% 25% 16% 3	427
	11th to 20th	Day	18% 8% 21% 29% 18% 6%	625
	21st to end	Day	16% 8% 22% 24% 24% 7%	679
June	1st to 10th	Day	13% 9% 23% 23% 24% 7%	630
	11th to 20th	Day	16% 13% 28% 27% 13% 3	630
	21st to end	Day	7% 6% 21% 36% 28% 3	630
July	1st to 10th	Day	6% 4 25% 32% 26% 7%	621
	11th to 20th	Day	3 7% 26% 37% 24% 3	618
	21st to end	Day	3 3 26% 33% 25% 9%	550
August	1st to 10th	Day	8% 8% 23% 32% 21% 7%	447
	11th to 20th	Day	8% 4 23% 32% 20% 13%	437
	21st to end	Day	6% 7% 28% 28% 22% 9%	495
September	1st to 10th	Day	9% 7% 30% 27% 23% 5%	450
	11th to 20th	Day	12% 7% 28% 30% 16% 7%	450
	21st to end	Day	14% 9% 28% 25% 17% 7%	450
October	1st to 10th	Day	18% 5% 27% 26% 16% 9%	288
	11th to 20th	Day	15% 13% 24% 27% 14% 8%	270
	21st to end	Day	12% 14% 30% 30% 9% 4	297
November	1st to 10th	Day	15% 15% 30% 28% 10% 3	270
	11th to 20th	Day	7% 16% 37% 28% 11% 1	270
	21st to end	Day	7% 12% 30% 41% 9% 1	270
December	1st to 10th	Day	6% 17% 27% 34% 14% 2	270
	11th to 20th	Day	9% 17% 32% 31% 10% 2	198
	21st to end	Day	10% 14% 31% 29% 11% 6%	198
January	1st to 10th	Night	6% 9% 33% 41% 11%	950
	11th to 20th	Night	8% 9% 34% 38% 10% 2	864
	21st to end	Night	7% 6% 33% 43% 9% 1	836
February	1st to 10th	Night	8% 9% 36% 38% 9%	758
	11th to 20th	Night	9% 7% 34% 40% 8% 2	704
	21st to end	Night	5% 9% 29% 43% 13% 1	592
March	1st to 10th	Night	6% 6% 31% 45% 10% 1	720
	11th to 20th	Night	7% 4 29% 45% 13% 1	720
	21st to end	Night	11% 6% 27% 40% 15% 1	792
April	1st to 10th	Night	12% 6% 26% 43% 12%	603
	11th to 20th	Night	10% 7% 32% 38% 12% 1	533
	21st to end	Night	17% 6% 22% 38% 16% 1	540
May	1st to 10th	Night	12% 7% 23% 38% 19% 1	540
	11th to 20th	Night	14% 5% 15% 40% 25% 2	531
	21st to end	Night	17% 5% 18% 36% 23% 2	568
June	1st to 10th	Night	13% 5% 19% 38% 23% 3	360
	11th to 20th	Night	14% 3 26% 43% 15%	360
	21st to end	Night	6% 2 16% 51% 24% 1	360
July	1st to 10th	Night	7% 1 10% 46% 35% 2	350
	11th to 20th	Night	3 3 13% 59% 21% 1	348
	21st to end	Night	5% 1 18% 48% 27% 1	551
August	1st to 10th	Night	6% 2 14% 52% 24% 2	537
	11th to 20th	Night	5% 1 11% 52% 29% 1	534
	21st to end	Night	7% 3 16% 44% 28% 2	594
September	1st to 10th	Night	8% 2 16% 49% 21% 3	540
	11th to 20th	Night	9% 2 14% 45% 27% 2	699
	21st to end	Night	12% 3 18% 42% 22% 3	720
October	1st to 10th	Night	15% 3 18% 42% 20% 2	720
	11th to 20th	Night	11% 4 24% 42% 16% 3	720
	21st to end	Night	11% 6% 27% 42% 12% 2	792
November	1st to 10th	Night	12% 4 31% 43% 9% 1	838
	11th to 20th	Night	9% 11% 33% 40% 8%	950
	21st to end	Night	6% 4 34% 47% 9%	950
December	1st to 10th	Night	7% 8% 34% 40% 10% 1	950
	11th to 20th	Night	7% 6% 39% 40% 8% 1	950
	21st to end	Night	7% 7% 36% 38% 11% 1	1045

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