Wilson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilson clear sky chart from 2005-11-06 to 2024-04-25.

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% 10% 4 8% 6% 4 5% 5% 5% 6% 40%	83783
	Night	5% 11% 5% 9% 6% 3 5% 5% 5% 7% 41%	85391
Transparency	Day	45% 7% 11% 18% 14% 5%	83489
	Night	45% 7% 12% 19% 13% 4	85205
Seeing	Day	12% 19% 32% 25% 11% 2	14775
	Night	10% 11% 25% 35% 16% 1	24576

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	2 14% 7% 20% 11% 4 6% 4 5% 4 24%	2441
	11th to 20th	Day	3 14% 8% 15% 8% 4 5% 5% 5% 6% 29%	2860
	21st to end	Day	2 22% 9% 13% 7% 3 4 4 5% 4 27%	3146
February	1st to 10th	Day	3 17% 7% 15% 9% 5% 5% 4 6% 6% 23%	1710
	11th to 20th	Day	3 18% 8% 11% 9% 3 5% 5% 5% 6% 27%	1665
	21st to end	Day	4 17% 9% 15% 9% 5% 5% 6% 4 5% 22%	1517
March	1st to 10th	Day	2 15% 8% 15% 8% 4 4 4 4 5% 32%	3381
	11th to 20th	Day	3 11% 5% 8% 6% 3 4 4 5% 5% 47%	3490
	21st to end	Day	5% 20% 7% 12% 8% 4 7% 7% 6% 7% 16%	3825
April	1st to 10th	Day	8% 15% 6% 10% 8% 4 5% 5% 5% 7% 28%	2182
	11th to 20th	Day	7% 13% 6% 8% 9% 4 6% 6% 5% 7% 30%	2390
	21st to end	Day	7% 13% 4 9% 7% 4 6% 6% 6% 9% 30%	2394
May	1st to 10th	Day	9% 10% 4 8% 7% 4 5% 6% 6% 8% 33%	2340
	11th to 20th	Day	9% 8% 2 6% 7% 4 6% 7% 7% 9% 35%	2358
	21st to end	Day	14% 11% 5% 9% 8% 4 6% 6% 6% 7% 25%	2724
June	1st to 10th	Day	10% 6% 2 6% 5% 3 6% 6% 7% 8% 40%	2520
	11th to 20th	Day	9% 6% 2 6% 6% 3 5% 6% 6% 7% 45%	2689
	21st to end	Day	3 2 1 2 4 3 4 4 5% 7% 65%	2700
July	1st to 10th	Day	2 2 1 3 3 2 4 5% 6% 9% 63%	2700
	11th to 20th	Day	2 2 2 3 4 3 4 5% 5% 6% 64%	2467
	21st to end	Day	4 3 1 4 5% 3 5% 5% 5% 7% 58%	2545
August	1st to 10th	Day	5% 4 2 4 5% 4 4 5% 6% 6% 55%	2328
	11th to 20th	Day	2 3 1 3 4 3 3 5% 6% 6% 64%	2299
	21st to end	Day	5% 3 1 3 4 3 4 5% 6% 7% 60%	2552
September	1st to 10th	Day	5% 3 2 4 6% 3 4 4 5% 5% 60%	2139
	11th to 20th	Day	7% 5% 1 4 4 3 4 4 5% 7% 55%	1980
	21st to end	Day	8% 5% 2 4 5% 2 3 4 4 6% 58%	1980
October	1st to 10th	Day	10% 9% 3 6% 6% 3 5% 4 6% 6% 44%	1836
	11th to 20th	Day	8% 8% 3 5% 6% 4 4 5% 4 5% 49%	1800
	21st to end	Day	7% 10% 3 5% 6% 4 4 4 5% 7% 45%	1954
November	1st to 10th	Day	10% 13% 5% 7% 5% 3 4 4 5% 6% 38%	1661
	11th to 20th	Day	5% 17% 5% 7% 5% 3 5% 4 4 7% 37%	1520
	21st to end	Day	2 13% 6% 11% 7% 3 5% 5% 4 6% 38%	1520
December	1st to 10th	Day	3 15% 7% 13% 9% 5% 5% 5% 4 5% 29%	1520
	11th to 20th	Day	5% 18% 8% 13% 8% 4 5% 5% 5% 7% 23%	1926
	21st to end	Day	3 13% 5% 12% 8% 4 6% 4 5% 7% 33%	2724
January	1st to 10th	Night	2 10% 7% 17% 9% 5% 6% 6% 5% 6% 26%	4519
	11th to 20th	Night	3 14% 9% 18% 10% 4 4 4 4 5% 26%	4110
	21st to end	Night	2 24% 8% 13% 7% 3 4 4 4 4 26%	4521
February	1st to 10th	Night	2 16% 8% 13% 8% 5% 6% 6% 5% 7% 25%	2470
	11th to 20th	Night	3 14% 9% 11% 8% 4 5% 4 5% 5% 31%	2402
	21st to end	Night	3 13% 9% 15% 9% 4 5% 5% 5% 6% 26%	1932
March	1st to 10th	Night	2 15% 7% 15% 7% 3 4 4 4 5% 33%	3492
	11th to 20th	Night	2 9% 4 6% 4 3 4 4 6% 7% 51%	3490
	21st to end	Night	4 15% 7% 9% 7% 4 6% 6% 7% 7% 27%	3828
April	1st to 10th	Night	5% 12% 4 8% 7% 3 4 6% 6% 8% 36%	1996
	11th to 20th	Night	6% 9% 5% 7% 7% 4 6% 5% 6% 8% 38%	1736
	21st to end	Night	6% 10% 3 4 4 3 5% 5% 6% 8% 46%	1665
May	1st to 10th	Night	8% 8% 3 5% 5% 3 5% 4 6% 9% 44%	1620
	11th to 20th	Night	7% 6% 2 5% 4 3 4 6% 7% 9% 47%	1537
	21st to end	Night	11% 8% 3 6% 7% 4 6% 6% 6% 9% 34%	1552
June	1st to 10th	Night	10% 5% 2 5% 4 3 3 5% 7% 9% 47%	1440
	11th to 20th	Night	9% 4 2 4 4 2 4 4 5% 9% 53%	1266
	21st to end	Night	3 1 1 2 2 1 4 3 4 7% 71%	1260
July	1st to 10th	Night	1 1 1 2 3 3 4 4 7% 8% 66%	1260
	11th to 20th	Night	2 1 2 2 1 3 4 5% 7% 73%	1485
	21st to end	Night	2 2 1 3 3 2 4 5% 6% 7% 66%	1773
August	1st to 10th	Night	4 3 2 4 4 3 4 5% 6% 8% 58%	1611
	11th to 20th	Night	2 2 1 2 3 2 3 3 5% 9% 67%	1602
	21st to end	Night	4 3 2 2 3 2 3 4 5% 7% 65%	1804
September	1st to 10th	Night	5% 3 1 2 3 2 4 4 4 7% 65%	1818
	11th to 20th	Night	4 3 2 4 4 3 4 5% 5% 7% 58%	1980
	21st to end	Night	8% 5% 1 4 2 2 3 3 3 6% 63%	1980
October	1st to 10th	Night	11% 8% 3 5% 4 2 4 4 4 7% 47%	2124
	11th to 20th	Night	8% 8% 2 4 4 3 3 3 4 6% 54%	2160
	21st to end	Night	5% 10% 3 4 4 3 4 5% 5% 7% 51%	2400
November	1st to 10th	Night	10% 12% 4 8% 5% 2 4 4 5% 6% 40%	2405
	11th to 20th	Night	6% 13% 4 7% 5% 4 5% 6% 6% 6% 39%	2651
	21st to end	Night	3 11% 5% 8% 6% 3 5% 5% 6% 7% 41%	2660
December	1st to 10th	Night	4 17% 7% 12% 8% 5% 5% 4 5% 5% 29%	2660
	11th to 20th	Night	5% 14% 7% 12% 8% 4 4 5% 6% 6% 28%	3271
	21st to end	Night	4 15% 5% 11% 8% 4 5% 6% 5% 6% 32%	4911

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	63% 7% 10% 16% 3 1	2441
	11th to 20th	Day	59% 6% 13% 12% 8% 2	2860
	21st to end	Day	61% 7% 10% 13% 8% 1	3146
February	1st to 10th	Day	65% 8% 10% 12% 4 1	1710
	11th to 20th	Day	62% 7% 11% 12% 5% 2	1674
	21st to end	Day	66% 7% 7% 13% 5% 2	1517
March	1st to 10th	Day	58% 5% 9% 15% 12% 1	3381
	11th to 20th	Day	42% 7% 15% 25% 10% 1	3490
	21st to end	Day	72% 8% 8% 10% 2	3825
April	1st to 10th	Day	57% 8% 12% 14% 8% 1	2182
	11th to 20th	Day	56% 7% 11% 18% 8% 1	2393
	21st to end	Day	52% 8% 11% 23% 6%	2394
May	1st to 10th	Day	50% 8% 11% 22% 8% 1	2340
	11th to 20th	Day	45% 10% 11% 25% 8%	2361
	21st to end	Day	58% 9% 12% 16% 5%	2724
June	1st to 10th	Day	39% 10% 15% 25% 11% 1	2520
	11th to 20th	Day	38% 7% 10% 19% 17% 8%	2689
	21st to end	Day	19% 6% 9% 22% 32% 11%	2700
July	1st to 10th	Day	19% 6% 10% 19% 32% 14%	2626
	11th to 20th	Day	21% 4 8% 14% 27% 25%	2390
	21st to end	Day	25% 6% 8% 13% 26% 22%	2404
August	1st to 10th	Day	28% 7% 10% 17% 26% 13%	2313
	11th to 20th	Day	20% 5% 8% 20% 31% 15%	2299
	21st to end	Day	24% 5% 8% 18% 32% 13%	2552
September	1st to 10th	Day	26% 4 7% 15% 33% 14%	2139
	11th to 20th	Day	30% 6% 9% 22% 25% 9%	1980
	21st to end	Day	30% 5% 11% 27% 24% 4	1980
October	1st to 10th	Day	44% 6% 10% 24% 13% 3	1836
	11th to 20th	Day	39% 7% 13% 28% 10% 2	1800
	21st to end	Day	42% 7% 15% 23% 12% 1	1954
November	1st to 10th	Day	50% 7% 14% 20% 9% 2	1661
	11th to 20th	Day	50% 6% 17% 18% 8% 1	1520
	21st to end	Day	50% 8% 14% 18% 9% 1	1520
December	1st to 10th	Day	60% 8% 9% 11% 8% 4	1520
	11th to 20th	Day	65% 7% 10% 14% 5%	1924
	21st to end	Day	51% 7% 10% 14% 10% 7%	2724
January	1st to 10th	Night	60% 7% 9% 16% 7% 1	4519
	11th to 20th	Night	65% 6% 10% 12% 6% 1	4110
	21st to end	Night	62% 10% 10% 11% 6% 1	4521
February	1st to 10th	Night	62% 9% 11% 12% 6%	2470
	11th to 20th	Night	59% 7% 10% 15% 8% 1	2403
	21st to end	Night	62% 8% 10% 13% 5% 2	1932
March	1st to 10th	Night	56% 6% 9% 16% 12%	3492
	11th to 20th	Night	34% 9% 17% 26% 14% 1	3490
	21st to end	Night	57% 11% 13% 17% 2 1	3828
April	1st to 10th	Night	47% 10% 16% 15% 10% 2	1996
	11th to 20th	Night	46% 9% 16% 19% 8% 1	1736
	21st to end	Night	38% 9% 15% 29% 9%	1665
May	1st to 10th	Night	39% 8% 12% 29% 12%	1620
	11th to 20th	Night	34% 10% 16% 27% 13%	1538
	21st to end	Night	45% 12% 15% 20% 7% 1	1552
June	1st to 10th	Night	35% 9% 17% 26% 12% 1	1440
	11th to 20th	Night	30% 6% 14% 24% 19% 7%	1266
	21st to end	Night	15% 5% 9% 24% 33% 14%	1260
July	1st to 10th	Night	16% 6% 10% 22% 33% 13%	1225
	11th to 20th	Night	13% 3 7% 19% 31% 28%	1439
	21st to end	Night	17% 5% 9% 17% 29% 24%	1674
August	1st to 10th	Night	24% 6% 10% 18% 25% 18%	1602
	11th to 20th	Night	16% 5% 7% 22% 33% 15%	1602
	21st to end	Night	20% 4 8% 19% 32% 17%	1804
September	1st to 10th	Night	21% 5% 7% 17% 30% 20%	1818
	11th to 20th	Night	24% 7% 10% 22% 26% 11%	1980
	21st to end	Night	26% 4 10% 27% 28% 5%	1980
October	1st to 10th	Night	38% 5% 13% 24% 16% 4	2124
	11th to 20th	Night	34% 5% 13% 32% 14% 2	2160
	21st to end	Night	35% 8% 16% 27% 13% 1	2400
November	1st to 10th	Night	46% 9% 13% 22% 9% 2	2405
	11th to 20th	Night	46% 9% 16% 19% 9% 1	2651
	21st to end	Night	45% 8% 15% 20% 10% 2	2660
December	1st to 10th	Night	59% 7% 14% 11% 7% 3	2660
	11th to 20th	Night	60% 8% 12% 14% 6%	3272
	21st to end	Night	53% 8% 9% 15% 8% 6%	4911

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	14% 22% 29% 27% 8% 1	214
	11th to 20th	Day	7% 18% 39% 24% 11% 1	270
	21st to end	Day	9% 18% 34% 29% 9%	297
February	1st to 10th	Day	13% 37% 31% 17% 2	270
	11th to 20th	Day	15% 30% 29% 21% 6%	270
	21st to end	Day	16% 28% 34% 16% 5%	329
March	1st to 10th	Day	16% 35% 32% 16% 2 1	460
	11th to 20th	Day	12% 25% 33% 23% 6%	460
	21st to end	Day	19% 28% 34% 17% 2	506
April	1st to 10th	Day	20% 28% 30% 16% 5%	460
	11th to 20th	Day	16% 31% 33% 14% 5% 1	452
	21st to end	Day	18% 24% 26% 21% 10% 1	437
May	1st to 10th	Day	12% 23% 32% 24% 8% 1	423
	11th to 20th	Day	16% 15% 32% 24% 12% 2	505
	21st to end	Day	20% 13% 24% 26% 15% 3	679
June	1st to 10th	Day	11% 16% 25% 30% 16% 3	630
	11th to 20th	Day	13% 21% 34% 22% 10%	630
	21st to end	Day	5% 11% 35% 31% 16% 3	630
July	1st to 10th	Day	4 7% 32% 38% 18% 2	621
	11th to 20th	Day	3 13% 39% 32% 11% 1	516
	21st to end	Day	4 9% 33% 37% 14% 2	487
August	1st to 10th	Day	6% 10% 38% 33% 12% 1	447
	11th to 20th	Day	4 10% 32% 32% 19% 3	437
	21st to end	Day	6% 14% 31% 33% 13% 3	495
September	1st to 10th	Day	6% 15% 34% 27% 16% 2	450
	11th to 20th	Day	12% 16% 31% 26% 11% 4	450
	21st to end	Day	10% 15% 31% 28% 14% 2	450
October	1st to 10th	Day	11% 17% 27% 25% 17% 2	306
	11th to 20th	Day	12% 23% 22% 25% 15% 4	270
	21st to end	Day	17% 21% 29% 26% 6% 1	297
November	1st to 10th	Day	20% 22% 30% 17% 8% 3	270
	11th to 20th	Day	12% 23% 33% 22% 8% 2	270
	21st to end	Day	7% 21% 34% 29% 10%	270
December	1st to 10th	Day	15% 21% 29% 24% 10% 1	270
	11th to 20th	Day	17% 21% 33% 23% 5%	270
	21st to end	Day	16% 22% 32% 18% 10% 1	277
January	1st to 10th	Night	9% 16% 28% 36% 10% 1	903
	11th to 20th	Night	10% 15% 31% 31% 12% 1	760
	21st to end	Night	9% 9% 36% 36% 10% 1	836
February	1st to 10th	Night	9% 20% 33% 31% 6% 1	760
	11th to 20th	Night	11% 17% 32% 32% 8% 1	744
	21st to end	Night	9% 18% 34% 29% 8%	628
March	1st to 10th	Night	9% 20% 29% 33% 9% 1	760
	11th to 20th	Night	9% 13% 28% 35% 14% 1	760
	21st to end	Night	13% 13% 31% 32% 10%	836
April	1st to 10th	Night	11% 17% 29% 31% 11% 2	666
	11th to 20th	Night	12% 13% 33% 32% 10% 1	563
	21st to end	Night	12% 9% 26% 32% 20% 1	555
May	1st to 10th	Night	12% 11% 21% 37% 19% 1	540
	11th to 20th	Night	12% 6% 23% 33% 23% 2	531
	21st to end	Night	15% 9% 21% 35% 19% 1	584
June	1st to 10th	Night	12% 9% 21% 33% 23% 2	540
	11th to 20th	Night	11% 8% 22% 43% 16%	366
	21st to end	Night	3 1 15% 47% 31% 3	360
July	1st to 10th	Night	1 1 15% 48% 31% 3	350
	11th to 20th	Night	3 3 16% 51% 28%	472
	21st to end	Night	3 1 14% 48% 32% 2	591
August	1st to 10th	Night	5% 3 16% 44% 31% 2	537
	11th to 20th	Night	2 2 11% 48% 34% 3	534
	21st to end	Night	6% 5% 16% 38% 33% 3	594
September	1st to 10th	Night	7% 3 18% 39% 31% 3	558
	11th to 20th	Night	6% 4 19% 37% 30% 4	720
	21st to end	Night	12% 5% 18% 36% 25% 5%	720
October	1st to 10th	Night	13% 8% 22% 35% 19% 3	720
	11th to 20th	Night	14% 8% 22% 33% 21% 2	720
	21st to end	Night	11% 9% 23% 41% 13% 3	792
November	1st to 10th	Night	15% 14% 24% 34% 11% 1	740
	11th to 20th	Night	14% 15% 30% 33% 6% 1	941
	21st to end	Night	11% 14% 27% 34% 13% 1	950
December	1st to 10th	Night	15% 13% 31% 33% 7% 1	950
	11th to 20th	Night	15% 15% 29% 34% 7%	950
	21st to end	Night	14% 16% 32% 28% 8% 1	1045

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