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 2025-03-12.

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 8% 5% 5% 5% 4 6% 7% 43%	112526
	Night	4 8% 3 8% 5% 5% 5% 4 6% 8% 43%	115245
Transparency	Day	34% 8% 13% 24% 16% 6%	112263
	Night	35% 8% 16% 26% 13% 3	115098
Seeing	Day	11% 11% 29% 29% 16% 5%	15882
	Night	9% 6% 27% 42% 15% 1	25561

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 14% 7% 12% 8% 7% 6% 4 6% 8% 26%	3179
	11th to 20th	Day	1 7% 4 12% 10% 9% 6% 5% 6% 7% 33%	3433
	21st to end	Day	1 6% 3 8% 6% 5% 5% 4 6% 7% 47%	4485
February	1st to 10th	Day	3 11% 4 16% 9% 8% 7% 5% 7% 6% 25%	1800
	11th to 20th	Day	1 10% 6% 16% 9% 6% 6% 4 6% 7% 28%	1755
	21st to end	Day	2 9% 5% 14% 10% 7% 7% 5% 7% 8% 28%	1536
March	1st to 10th	Day	2 7% 4 11% 6% 5% 6% 5% 6% 6% 43%	3462
	11th to 20th	Day	2 10% 3 8% 6% 6% 6% 4 6% 8% 41%	3502
	21st to end	Day	4 16% 6% 13% 8% 7% 8% 5% 7% 8% 20%	3825
April	1st to 10th	Day	6% 13% 4 10% 6% 6% 7% 4 6% 9% 27%	2213
	11th to 20th	Day	5% 11% 5% 11% 7% 6% 6% 6% 6% 7% 29%	2423
	21st to end	Day	10% 13% 4 10% 6% 6% 6% 5% 6% 7% 28%	2470
May	1st to 10th	Day	9% 17% 4 10% 6% 6% 6% 5% 5% 7% 25%	4128
	11th to 20th	Day	8% 8% 3 9% 6% 6% 6% 5% 8% 10% 33%	4388
	21st to end	Day	11% 10% 3 8% 5% 5% 5% 4 7% 9% 33%	4850
June	1st to 10th	Day	9% 8% 3 7% 6% 5% 5% 5% 8% 8% 35%	2850
	11th to 20th	Day	10% 8% 2 8% 6% 6% 6% 5% 7% 7% 35%	2850
	21st to end	Day	4 3 2 5% 4 5% 5% 5% 8% 8% 52%	2850
July	1st to 10th	Day	3 2 1 3 4 4 5% 5% 8% 8% 58%	4740
	11th to 20th	Day	2 2 1 3 3 3 4 5% 6% 8% 64%	4740
	21st to end	Day	3 2 1 3 3 3 4 4 6% 7% 63%	4624
August	1st to 10th	Day	5% 3 1 4 4 5% 6% 4 7% 9% 51%	4092
	11th to 20th	Day	4 3 1 4 4 5% 5% 4 6% 7% 58%	4028
	21st to end	Day	4 3 1 4 3 4 4 4 6% 6% 61%	4521
September	1st to 10th	Day	6% 3 1 4 3 3 4 3 5% 7% 61%	2280
	11th to 20th	Day	8% 5% 2 6% 3 4 5% 4 5% 8% 50%	2109
	21st to end	Day	8% 5% 2 5% 3 4 4 4 6% 8% 53%	2090
October	1st to 10th	Day	6% 5% 2 5% 3 3 4 2 4 5% 60%	3188
	11th to 20th	Day	6% 5% 2 6% 4 3 4 3 4 6% 58%	3150
	21st to end	Day	3 7% 3 8% 6% 6% 6% 5% 6% 8% 42%	3297
November	1st to 10th	Day	6% 9% 4 9% 6% 6% 5% 4 6% 8% 37%	1639
	11th to 20th	Day	3 9% 4 9% 6% 6% 5% 4 7% 8% 38%	1600
	21st to end	Day	2 9% 4 10% 5% 6% 6% 5% 7% 7% 40%	1600
December	1st to 10th	Day	1 5% 3 6% 6% 5% 6% 5% 6% 7% 48%	2620
	11th to 20th	Day	4 8% 3 9% 7% 8% 7% 4 7% 8% 34%	2711
	21st to end	Day	4 11% 6% 10% 6% 5% 6% 4 6% 7% 34%	3498
January	1st to 10th	Night	2 12% 5% 9% 6% 5% 6% 5% 7% 8% 36%	6766
	11th to 20th	Night	1 8% 4 13% 9% 7% 6% 4 6% 7% 34%	6572
	21st to end	Night	1 7% 3 8% 5% 6% 6% 5% 6% 7% 46%	6490
February	1st to 10th	Night	3 13% 6% 14% 9% 8% 7% 4 6% 6% 24%	2600
	11th to 20th	Night	1 12% 5% 13% 8% 6% 6% 4 6% 7% 30%	2532
	21st to end	Night	1 10% 4 12% 9% 7% 7% 5% 6% 6% 34%	2090
March	1st to 10th	Night	2 7% 4 9% 6% 6% 6% 5% 7% 8% 40%	3620
	11th to 20th	Night	3 7% 3 6% 5% 5% 6% 5% 7% 9% 45%	3506
	21st to end	Night	4 14% 4 12% 8% 7% 6% 6% 8% 8% 23%	3828
April	1st to 10th	Night	6% 11% 3 9% 7% 6% 6% 5% 7% 6% 35%	1966
	11th to 20th	Night	5% 12% 3 8% 6% 7% 6% 4 8% 8% 33%	1702
	21st to end	Night	11% 10% 3 8% 4 4 5% 5% 7% 7% 35%	1710
May	1st to 10th	Night	8% 13% 5% 10% 7% 5% 6% 4 6% 7% 28%	2841
	11th to 20th	Night	7% 8% 2 7% 6% 5% 6% 5% 7% 8% 41%	2517
	21st to end	Night	10% 9% 3 10% 5% 5% 6% 4 6% 9% 32%	2732
June	1st to 10th	Night	11% 6% 3 7% 5% 5% 7% 7% 6% 8% 35%	1330
	11th to 20th	Night	12% 7% 2 8% 5% 5% 6% 4 6% 6% 39%	1330
	21st to end	Night	4 3 1 4 4 5% 4 4 6% 9% 56%	1330
July	1st to 10th	Night	5% 2 1 4 3 3 4 3 6% 7% 61%	2220
	11th to 20th	Night	3 1 1 2 2 3 3 4 6% 8% 67%	2220
	21st to end	Night	2 3 1 5% 3 4 5% 4 7% 9% 58%	2932
August	1st to 10th	Night	5% 3 2 4 4 4 6% 5% 8% 9% 50%	2847
	11th to 20th	Night	4 2 1 5% 4 5% 6% 5% 8% 8% 51%	2811
	21st to end	Night	5% 3 1 4 3 4 4 3 5% 7% 61%	3146
September	1st to 10th	Night	6% 4 1 5% 4 3 4 3 5% 7% 57%	1900
	11th to 20th	Night	8% 5% 2 6% 4 3 5% 4 7% 8% 49%	2068
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 7% 54%	2090
October	1st to 10th	Night	6% 5% 2 4 3 3 3 3 4 5% 61%	3769
	11th to 20th	Night	7% 7% 2 6% 3 3 4 3 4 7% 55%	3778
	21st to end	Night	6% 8% 3 7% 4 4 5% 4 6% 8% 45%	4258
November	1st to 10th	Night	6% 9% 3 7% 5% 5% 5% 4 6% 8% 41%	2639
	11th to 20th	Night	3 9% 4 9% 7% 5% 6% 5% 6% 8% 39%	2800
	21st to end	Night	2 7% 3 8% 6% 5% 6% 5% 7% 9% 42%	2800
December	1st to 10th	Night	2 6% 3 7% 6% 5% 5% 4 5% 7% 50%	4570
	11th to 20th	Night	4 8% 3 8% 6% 6% 6% 5% 6% 9% 38%	5477
	21st to end	Night	3 8% 4 10% 6% 5% 5% 5% 7% 9% 39%	7458

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	47% 10% 15% 24% 3	3179
	11th to 20th	Day	44% 8% 17% 17% 12% 1	3433
	21st to end	Day	30% 9% 18% 29% 13% 1	4485
February	1st to 10th	Day	48% 12% 15% 19% 6% 1	1800
	11th to 20th	Day	47% 11% 15% 19% 7% 1	1764
	21st to end	Day	44% 12% 10% 20% 12% 2	1536
March	1st to 10th	Day	34% 9% 11% 27% 18% 1	3462
	11th to 20th	Day	32% 12% 19% 29% 7%	3502
	21st to end	Day	52% 14% 14% 17% 2	3825
April	1st to 10th	Day	47% 10% 15% 20% 8% 1	2213
	11th to 20th	Day	47% 9% 14% 22% 8%	2426
	21st to end	Day	49% 7% 13% 22% 8%	2470
May	1st to 10th	Day	52% 10% 9% 21% 7% 1	4128
	11th to 20th	Day	39% 11% 15% 25% 9% 2	4392
	21st to end	Day	42% 9% 16% 24% 10%	4850
June	1st to 10th	Day	41% 9% 18% 22% 8% 1	2850
	11th to 20th	Day	42% 8% 12% 20% 14% 4	2850
	21st to end	Day	24% 7% 13% 28% 23% 5%	2850
July	1st to 10th	Day	18% 6% 10% 24% 32% 11%	4666
	11th to 20th	Day	15% 3 7% 18% 27% 28%	4659
	21st to end	Day	17% 4 7% 18% 32% 22%	4477
August	1st to 10th	Day	24% 6% 11% 22% 23% 14%	4078
	11th to 20th	Day	22% 5% 8% 29% 28% 7%	4067
	21st to end	Day	21% 4 7% 28% 27% 14%	4520
September	1st to 10th	Day	22% 4 8% 26% 27% 12%	2280
	11th to 20th	Day	30% 5% 10% 25% 23% 7%	2109
	21st to end	Day	27% 6% 11% 30% 20% 6%	2090
October	1st to 10th	Day	27% 4 8% 20% 34% 8%	3188
	11th to 20th	Day	26% 5% 15% 32% 19% 2	3150
	21st to end	Day	35% 7% 12% 29% 14% 2	3297
November	1st to 10th	Day	41% 8% 14% 24% 11% 2	1639
	11th to 20th	Day	37% 9% 20% 23% 10% 1	1600
	21st to end	Day	36% 9% 16% 25% 13%	1600
December	1st to 10th	Day	28% 9% 24% 27% 11% 1	2620
	11th to 20th	Day	40% 10% 24% 21% 4 1	2710
	21st to end	Day	44% 7% 9% 25% 10% 5%	3498
January	1st to 10th	Night	40% 9% 15% 31% 5%	6766
	11th to 20th	Night	45% 8% 17% 19% 10% 1	6572
	21st to end	Night	30% 10% 18% 29% 12% 1	6490
February	1st to 10th	Night	52% 12% 13% 16% 8%	2600
	11th to 20th	Night	46% 11% 15% 18% 9% 1	2533
	21st to end	Night	44% 9% 14% 20% 12% 2	2090
March	1st to 10th	Night	32% 11% 11% 25% 19% 1	3620
	11th to 20th	Night	28% 12% 24% 29% 6%	3506
	21st to end	Night	50% 15% 17% 16% 3	3828
April	1st to 10th	Night	44% 8% 16% 23% 8% 1	1966
	11th to 20th	Night	44% 8% 14% 25% 8% 1	1702
	21st to end	Night	43% 8% 19% 23% 7%	1710
May	1st to 10th	Night	50% 11% 11% 20% 8%	2841
	11th to 20th	Night	37% 10% 15% 26% 10% 3	2518
	21st to end	Night	43% 11% 17% 24% 4	2732
June	1st to 10th	Night	42% 11% 18% 21% 7%	1330
	11th to 20th	Night	42% 8% 13% 23% 11% 3	1330
	21st to end	Night	23% 7% 16% 30% 20% 5%	1330
July	1st to 10th	Night	20% 6% 13% 29% 29% 4	2185
	11th to 20th	Night	14% 3 9% 22% 30% 21%	2178
	21st to end	Night	20% 5% 11% 25% 26% 13%	2841
August	1st to 10th	Night	24% 7% 15% 26% 19% 10%	2838
	11th to 20th	Night	26% 6% 12% 33% 17% 5%	2838
	21st to end	Night	22% 4 9% 35% 25% 6%	3146
September	1st to 10th	Night	25% 5% 9% 24% 25% 11%	1900
	11th to 20th	Night	29% 6% 13% 27% 20% 4	2068
	21st to end	Night	29% 5% 11% 30% 21% 4	2090
October	1st to 10th	Night	24% 4 9% 28% 27% 8%	3769
	11th to 20th	Night	29% 5% 16% 27% 20% 2	3778
	21st to end	Night	33% 6% 15% 27% 17% 2	4258
November	1st to 10th	Night	36% 8% 14% 28% 12% 2	2639
	11th to 20th	Night	35% 10% 18% 28% 8% 1	2800
	21st to end	Night	31% 9% 20% 27% 12% 2	2800
December	1st to 10th	Night	29% 8% 24% 27% 11% 1	4570
	11th to 20th	Night	37% 8% 26% 24% 5% 1	5478
	21st to end	Night	37% 7% 14% 27% 9% 6%	7458

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% 22% 34% 20% 3	200
	11th to 20th	Day	11% 11% 36% 26% 14% 4	200
	21st to end	Day	8% 10% 28% 43% 9% 2	325
February	1st to 10th	Day	12% 17% 40% 24% 7%	300
	11th to 20th	Day	9% 12% 39% 30% 9% 2	300
	21st to end	Day	10% 19% 36% 23% 11% 2	300
March	1st to 10th	Day	10% 17% 36% 28% 7% 1	500
	11th to 20th	Day	10% 18% 32% 25% 12% 2	465
	21st to end	Day	13% 19% 32% 24% 10% 2	506
April	1st to 10th	Day	17% 17% 35% 21% 8% 1	460
	11th to 20th	Day	11% 22% 32% 25% 8% 1	452
	21st to end	Day	18% 15% 31% 22% 11% 3	460
May	1st to 10th	Day	16% 10% 30% 25% 15% 3	468
	11th to 20th	Day	18% 8% 21% 28% 18% 6%	675
	21st to end	Day	16% 9% 22% 24% 23% 7%	734
June	1st to 10th	Day	13% 10% 23% 23% 23% 6%	680
	11th to 20th	Day	16% 13% 29% 27% 13% 3	680
	21st to end	Day	6% 6% 23% 35% 26% 3	680
July	1st to 10th	Day	6% 4 25% 32% 26% 7%	671
	11th to 20th	Day	3 6% 26% 37% 24% 3	668
	21st to end	Day	4 3 27% 32% 25% 9%	597
August	1st to 10th	Day	8% 8% 23% 32% 22% 7%	487
	11th to 20th	Day	8% 4 23% 33% 20% 12%	477
	21st to end	Day	6% 8% 27% 29% 21% 9%	539
September	1st to 10th	Day	8% 6% 29% 26% 24% 7%	490
	11th to 20th	Day	12% 8% 29% 30% 15% 6%	490
	21st to end	Day	13% 10% 28% 24% 18% 7%	490
October	1st to 10th	Day	16% 6% 27% 26% 15% 10%	318
	11th to 20th	Day	13% 12% 23% 26% 16% 10%	300
	21st to end	Day	12% 13% 31% 30% 10% 4	330
November	1st to 10th	Day	14% 14% 29% 30% 10% 3	300
	11th to 20th	Day	7% 17% 37% 28% 10% 1	300
	21st to end	Day	6% 12% 33% 39% 9% 1	300
December	1st to 10th	Day	5% 17% 29% 33% 15% 2	300
	11th to 20th	Day	10% 16% 31% 31% 10% 2	220
	21st to end	Day	9% 13% 35% 27% 10% 5%	220
January	1st to 10th	Night	7% 9% 33% 41% 11%	1000
	11th to 20th	Night	8% 9% 35% 37% 9% 2	909
	21st to end	Night	7% 6% 34% 43% 9% 1	880
February	1st to 10th	Night	8% 9% 36% 37% 9%	800
	11th to 20th	Night	8% 7% 35% 40% 8% 2	784
	21st to end	Night	5% 10% 31% 42% 12% 1	660
March	1st to 10th	Night	6% 6% 32% 44% 10% 1	800
	11th to 20th	Night	7% 4 28% 46% 13% 1	766
	21st to end	Night	11% 6% 27% 40% 14% 1	836
April	1st to 10th	Night	12% 6% 27% 44% 11%	636
	11th to 20th	Night	10% 7% 31% 40% 12% 1	563
	21st to end	Night	17% 6% 22% 38% 15% 1	570
May	1st to 10th	Night	12% 7% 23% 38% 19% 1	570
	11th to 20th	Night	13% 5% 16% 40% 25% 1	561
	21st to end	Night	17% 6% 18% 36% 23% 2	600
June	1st to 10th	Night	13% 6% 19% 38% 22% 2	380
	11th to 20th	Night	13% 3 27% 42% 14%	380
	21st to end	Night	5% 2 16% 52% 23% 1	380
July	1st to 10th	Night	7% 1 10% 47% 34% 2	370
	11th to 20th	Night	3 3 13% 60% 21% 1	368
	21st to end	Night	4 2 18% 49% 26% 1	582
August	1st to 10th	Night	6% 2 14% 52% 24% 2	567
	11th to 20th	Night	4 1 12% 53% 28% 1	564
	21st to end	Night	7% 3 17% 44% 27% 2	627
September	1st to 10th	Night	8% 2 16% 49% 22% 3	570
	11th to 20th	Night	10% 3 14% 45% 26% 2	738
	21st to end	Night	12% 3 18% 42% 22% 3	760
October	1st to 10th	Night	14% 3 18% 41% 20% 2	760
	11th to 20th	Night	11% 4 24% 42% 16% 3	760
	21st to end	Night	11% 6% 28% 41% 12% 2	836
November	1st to 10th	Night	12% 4 30% 44% 10% 1	884
	11th to 20th	Night	9% 10% 35% 39% 7%	1000
	21st to end	Night	5% 4 35% 46% 9%	1000
December	1st to 10th	Night	7% 8% 35% 40% 9% 1	1000
	11th to 20th	Night	7% 6% 39% 39% 8% 1	1000
	21st to end	Night	7% 8% 37% 37% 11% 1	1100

Page generated at 2025-03-09 06:08:48. Version ac07

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