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 2024-11-05.

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	4 10% 3 8% 5% 4 5% 4 5% 6% 45%	105767
	Night	4 10% 4 9% 5% 4 4 4 5% 6% 45%	101962
Transparency	Day	40% 6% 10% 19% 17% 8%	105513
	Night	40% 7% 11% 20% 16% 6%	101804
Seeing	Day	12% 19% 32% 25% 11% 2	15576
	Night	10% 11% 26% 36% 16% 1	25183

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% 23% 7% 6% 5% 3 5% 4 24%	2441
	11th to 20th	Day	2 15% 7% 17% 6% 5% 5% 3 5% 6% 29%	2860
	21st to end	Day	2 23% 8% 14% 5% 4 4 3 4 4 27%	3146
February	1st to 10th	Day	3 18% 7% 16% 7% 6% 5% 4 6% 6% 22%	1710
	11th to 20th	Day	2 20% 8% 12% 8% 5% 5% 3 5% 6% 27%	1665
	21st to end	Day	4 19% 8% 17% 7% 6% 5% 4 5% 5% 22%	1517
March	1st to 10th	Day	2 16% 7% 16% 6% 5% 4 3 5% 5% 32%	3381
	11th to 20th	Day	3 12% 4 9% 5% 4 3 3 5% 5% 47%	3490
	21st to end	Day	5% 21% 7% 14% 7% 6% 7% 5% 6% 7% 16%	3825
April	1st to 10th	Day	7% 16% 5% 11% 7% 5% 5% 4 5% 7% 28%	2182
	11th to 20th	Day	7% 14% 5% 10% 8% 5% 6% 4 5% 7% 29%	2390
	21st to end	Day	6% 14% 4 10% 6% 5% 6% 4 6% 8% 29%	2470
May	1st to 10th	Day	8% 15% 4 8% 7% 5% 5% 5% 6% 7% 30%	4110
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 5% 6% 8% 48%	4184
	21st to end	Day	9% 11% 4 9% 6% 5% 5% 4 5% 6% 36%	4825
June	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 4 7% 8% 41%	2660
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 6% 6% 45%	2839
	21st to end	Day	3 2 1 3 3 4 4 3 5% 7% 65%	2850
July	1st to 10th	Day	2 1 1 2 2 3 3 3 5% 7% 71%	4740
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 7% 66%	4319
	21st to end	Day	3 2 1 4 4 4 5% 4 6% 7% 61%	4454
August	1st to 10th	Day	3 3 1 3 3 4 5% 4 7% 8% 59%	4092
	11th to 20th	Day	2 2 1 4 4 4 4 4 5% 6% 64%	4028
	21st to end	Day	3 2 1 3 3 3 3 3 5% 6% 68%	4473
September	1st to 10th	Day	5% 4 2 4 5% 3 3 3 5% 5% 61%	2257
	11th to 20th	Day	7% 6% 1 5% 4 5% 4 3 5% 6% 54%	2090
	21st to end	Day	7% 5% 1 5% 4 3 3 3 4 6% 59%	2090
October	1st to 10th	Day	6% 6% 2 4 3 3 3 2 4 4 64%	3234
	11th to 20th	Day	5% 6% 2 5% 4 4 4 3 3 5% 59%	3150
	21st to end	Day	5% 10% 2 6% 4 5% 5% 4 7% 8% 45%	3388
November	1st to 10th	Day	9% 14% 5% 8% 4 4 4 3 5% 6% 37%	1697
	11th to 20th	Day	5% 18% 4 8% 5% 4 5% 3 5% 6% 37%	1520
	21st to end	Day	2 14% 5% 11% 6% 4 5% 4 5% 6% 38%	1520
December	1st to 10th	Day	3 16% 6% 15% 7% 7% 4 4 4 5% 29%	1520
	11th to 20th	Day	5% 18% 7% 14% 6% 5% 5% 4 5% 7% 23%	1926
	21st to end	Day	3 14% 5% 13% 6% 6% 6% 3 5% 7% 33%	2724
January	1st to 10th	Night	2 11% 7% 19% 7% 6% 5% 5% 5% 6% 26%	4519
	11th to 20th	Night	3 15% 8% 20% 8% 5% 4 3 4 5% 26%	4110
	21st to end	Night	2 25% 7% 15% 6% 4 4 3 4 4 26%	4521
February	1st to 10th	Night	2 17% 8% 14% 6% 7% 5% 4 5% 7% 25%	2470
	11th to 20th	Night	3 15% 8% 13% 7% 5% 4 4 5% 5% 31%	2402
	21st to end	Night	3 16% 8% 16% 7% 5% 5% 4 5% 6% 25%	1932
March	1st to 10th	Night	2 16% 6% 16% 5% 5% 4 3 4 6% 33%	3492
	11th to 20th	Night	2 9% 4 6% 4 4 4 3 6% 6% 51%	3490
	21st to end	Night	4 15% 6% 11% 6% 6% 6% 5% 7% 7% 27%	3828
April	1st to 10th	Night	5% 14% 4 9% 6% 4 4 4 6% 7% 36%	1996
	11th to 20th	Night	6% 10% 4 8% 6% 5% 5% 4 6% 7% 38%	1736
	21st to end	Night	5% 11% 3 6% 4 5% 4 4 6% 7% 44%	1710
May	1st to 10th	Night	7% 15% 3 6% 4 5% 5% 3 6% 8% 37%	2860
	11th to 20th	Night	5% 4 1 5% 3 4 4 4 6% 8% 55%	2721
	21st to end	Night	7% 9% 3 6% 5% 4 5% 4 6% 8% 44%	2762
June	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 7% 8% 47%	1520
	11th to 20th	Night	8% 5% 2 4 3 3 4 3 6% 8% 54%	1336
	21st to end	Night	2 1 1 3 2 2 3 3 4 8% 71%	1330
July	1st to 10th	Night	1 1 2 2 3 3 3 6% 6% 71%	2220
	11th to 20th	Night	1 1 2 1 2 3 3 6% 9% 72%	2641
	21st to end	Night	1 1 1 3 2 3 3 4 6% 8% 68%	3109
August	1st to 10th	Night	2 2 1 3 2 2 4 4 7% 8% 63%	2847
	11th to 20th	Night	1 2 2 4 3 3 5% 4 6% 8% 63%	2811
	21st to end	Night	3 2 1 3 2 2 2 3 4 5% 73%	3194
September	1st to 10th	Night	5% 3 1 3 2 2 4 3 5% 6% 66%	1919
	11th to 20th	Night	5% 4 1 5% 4 4 5% 4 5% 7% 57%	2090
	21st to end	Night	8% 5% 1 4 2 2 2 2 3 5% 65%	2090
October	1st to 10th	Night	6% 5% 1 4 2 2 2 2 3 5% 68%	3738
	11th to 20th	Night	7% 6% 2 4 3 3 4 3 5% 6% 57%	3778
	21st to end	Night	4 9% 3 5% 3 3 4 4 6% 8% 51%	4168
November	1st to 10th	Night	9% 13% 4 8% 4 3 4 3 5% 6% 40%	2469
	11th to 20th	Night	5% 14% 4 8% 4 5% 5% 5% 6% 6% 39%	2651
	21st to end	Night	3 12% 4 8% 6% 5% 4 3 6% 6% 41%	2660
December	1st to 10th	Night	4 18% 5% 12% 7% 6% 5% 3 5% 5% 29%	2660
	11th to 20th	Night	5% 15% 7% 13% 7% 5% 4 4 6% 6% 28%	3271
	21st to end	Night	3 16% 5% 12% 7% 5% 5% 4 5% 6% 31%	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	53% 9% 11% 22% 5%	2470
May	1st to 10th	Day	52% 6% 11% 21% 8% 1	4110
	11th to 20th	Day	32% 8% 12% 31% 13% 4	4187
	21st to end	Day	48% 7% 11% 26% 8%	4825
June	1st to 10th	Day	38% 9% 15% 25% 11% 1	2660
	11th to 20th	Day	37% 7% 10% 20% 18% 8%	2839
	21st to end	Day	19% 6% 9% 22% 32% 12%	2850
July	1st to 10th	Day	12% 4 7% 14% 34% 30%	4666
	11th to 20th	Day	15% 4 7% 13% 24% 36%	4242
	21st to end	Day	20% 5% 7% 17% 33% 18%	4314
August	1st to 10th	Day	20% 6% 9% 17% 30% 18%	4078
	11th to 20th	Day	18% 7% 8% 22% 32% 14%	4067
	21st to end	Day	17% 5% 8% 19% 31% 20%	4472
September	1st to 10th	Day	25% 4 7% 17% 33% 14%	2257
	11th to 20th	Day	29% 7% 10% 22% 25% 8%	2090
	21st to end	Day	28% 4 11% 27% 24% 5%	2090
October	1st to 10th	Day	27% 3 6% 21% 33% 9%	3234
	11th to 20th	Day	30% 5% 11% 35% 16% 5%	3150
	21st to end	Day	37% 8% 18% 25% 12%	3388
November	1st to 10th	Day	49% 7% 14% 20% 8% 2	1697
	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	39% 9% 15% 28% 8%	1710
May	1st to 10th	Night	47% 6% 11% 23% 11% 1	2860
	11th to 20th	Night	25% 10% 14% 35% 14% 3	2722
	21st to end	Night	38% 8% 13% 27% 12% 1	2762
June	1st to 10th	Night	35% 9% 17% 26% 12% 1	1520
	11th to 20th	Night	29% 6% 14% 24% 20% 8%	1336
	21st to end	Night	15% 5% 9% 23% 33% 15%	1330
July	1st to 10th	Night	12% 5% 7% 14% 38% 24%	2185
	11th to 20th	Night	9% 3 6% 18% 32% 32%	2595
	21st to end	Night	12% 4 9% 24% 34% 18%	3011
August	1st to 10th	Night	18% 5% 7% 15% 36% 18%	2838
	11th to 20th	Night	18% 7% 9% 26% 26% 14%	2838
	21st to end	Night	15% 3 7% 21% 35% 19%	3194
September	1st to 10th	Night	20% 5% 7% 18% 31% 19%	1919
	11th to 20th	Night	24% 8% 10% 21% 26% 10%	2090
	21st to end	Night	25% 4 10% 27% 28% 6%	2090
October	1st to 10th	Night	22% 3 8% 25% 35% 7%	3738
	11th to 20th	Night	30% 5% 9% 35% 16% 5%	3778
	21st to end	Night	30% 8% 17% 30% 14% 1	4168
November	1st to 10th	Night	45% 9% 13% 22% 9% 2	2469
	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% 27% 21% 8% 1	460
May	1st to 10th	Day	14% 23% 32% 23% 7% 1	463
	11th to 20th	Day	16% 15% 32% 23% 11% 2	549
	21st to end	Day	19% 13% 24% 26% 15% 2	734
June	1st to 10th	Day	11% 16% 26% 29% 15% 3	680
	11th to 20th	Day	14% 22% 34% 21% 9%	680
	21st to end	Day	5% 13% 36% 29% 15% 3	680
July	1st to 10th	Day	4 9% 32% 37% 17% 2	671
	11th to 20th	Day	4 13% 39% 32% 11% 1	560
	21st to end	Day	4 10% 34% 37% 14% 2	531
August	1st to 10th	Day	6% 11% 38% 32% 11% 1	487
	11th to 20th	Day	4 12% 32% 31% 17% 3	477
	21st to end	Day	7% 16% 29% 31% 13% 3	539
September	1st to 10th	Day	6% 14% 33% 28% 17% 3	490
	11th to 20th	Day	12% 18% 31% 25% 11% 3	490
	21st to end	Day	10% 16% 30% 28% 14% 2	490
October	1st to 10th	Day	10% 17% 29% 25% 16% 3	338
	11th to 20th	Day	11% 22% 21% 24% 18% 4	300
	21st to end	Day	17% 21% 29% 25% 6% 1	330
November	1st to 10th	Day	19% 22% 30% 17% 8% 3	282
	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% 10% 26% 31% 19% 1	570
May	1st to 10th	Night	12% 12% 22% 36% 18% 1	570
	11th to 20th	Night	11% 7% 24% 34% 22% 2	561
	21st to end	Night	15% 10% 21% 35% 19% 1	617
June	1st to 10th	Night	12% 9% 22% 33% 23% 2	570
	11th to 20th	Night	11% 9% 22% 42% 15%	386
	21st to end	Night	3 1 16% 47% 30% 3	380
July	1st to 10th	Night	1 2 17% 47% 31% 2	370
	11th to 20th	Night	3 3 16% 51% 28%	500
	21st to end	Night	3 1 14% 49% 31% 2	624
August	1st to 10th	Night	5% 3 17% 44% 30% 2	567
	11th to 20th	Night	2 2 14% 48% 32% 3	564
	21st to end	Night	6% 5% 18% 37% 32% 3	627
September	1st to 10th	Night	7% 2 17% 40% 31% 3	589
	11th to 20th	Night	6% 4 19% 37% 29% 4	760
	21st to end	Night	11% 5% 18% 37% 25% 4	760
October	1st to 10th	Night	13% 8% 22% 35% 20% 3	760
	11th to 20th	Night	13% 8% 21% 34% 22% 2	760
	21st to end	Night	11% 10% 24% 40% 12% 3	836
November	1st to 10th	Night	15% 14% 25% 33% 11% 1	760
	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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