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 2025-10-04.

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 9% 3 7% 4 4 4 4 5% 7% 48%	136019
	Night	4 9% 4 8% 4 4 4 4 5% 7% 48%	130733
Transparency	Day	36% 7% 12% 20% 17% 8%	130746
	Night	36% 8% 13% 22% 16% 6%	127020
Seeing	Day	12% 19% 32% 25% 10% 2	16846
	Night	10% 11% 26% 36% 16% 1	26379

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 13% 6% 18% 6% 5% 5% 4 6% 6% 28%	3513
	11th to 20th	Day	2 10% 6% 13% 5% 5% 5% 4 6% 7% 38%	4100
	21st to end	Day	1 16% 6% 10% 4 3 3 3 5% 5% 42%	4510
February	1st to 10th	Day	3 18% 7% 17% 7% 6% 5% 4 6% 6% 21%	1800
	11th to 20th	Day	2 19% 7% 12% 8% 5% 5% 3 5% 7% 26%	1755
	21st to end	Day	4 18% 7% 17% 7% 5% 5% 4 5% 5% 23%	1595
March	1st to 10th	Day	2 16% 6% 13% 6% 5% 4 3 5% 5% 36%	4865
	11th to 20th	Day	3 15% 4 8% 5% 5% 5% 4 6% 6% 40%	5000
	21st to end	Day	3 20% 5% 11% 6% 6% 7% 5% 6% 7% 23%	5486
April	1st to 10th	Day	7% 16% 5% 10% 7% 5% 5% 4 5% 7% 30%	2297
	11th to 20th	Day	6% 13% 5% 10% 7% 5% 6% 4 6% 7% 31%	2518
	21st to end	Day	6% 14% 4 10% 6% 5% 6% 5% 6% 8% 30%	2600
May	1st to 10th	Day	7% 11% 3 6% 5% 4 5% 4 6% 7% 43%	5880
	11th to 20th	Day	6% 6% 2 6% 5% 5% 6% 5% 7% 8% 43%	5996
	21st to end	Day	6% 8% 3 6% 5% 4 4 4 5% 8% 46%	6925
June	1st to 10th	Day	9% 7% 2 6% 5% 5% 5% 4 7% 8% 42%	2800
	11th to 20th	Day	8% 6% 2 6% 5% 4 5% 4 6% 7% 47%	2989
	21st to end	Day	3 2 1 3 3 4 4 3 5% 7% 66%	3000
July	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 6% 69%	6779
	11th to 20th	Day	1 1 1 2 2 3 4 3 5% 6% 70%	6171
	21st to end	Day	2 2 1 3 4 4 4 4 6% 7% 64%	6401
August	1st to 10th	Day	2 2 1 3 3 4 5% 4 7% 8% 62%	5779
	11th to 20th	Day	2 1 1 3 3 3 4 4 6% 6% 67%	5798
	21st to end	Day	5% 3 2 4 3 4 3 3 4 6% 63%	6394
September	1st to 10th	Day	4 3 2 4 5% 3 3 3 5% 6% 62%	2376
	11th to 20th	Day	6% 5% 1 5% 4 5% 4 3 6% 6% 54%	2200
	21st to end	Day	7% 5% 2 5% 4 3 3 3 4 6% 60%	2200
October	1st to 10th	Day	5% 6% 2 4 3 3 3 2 5% 4 63%	3266
	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% 13% 5% 8% 3 4 4 3 5% 6% 39%	1750
	11th to 20th	Day	5% 17% 4 8% 4 5% 5% 3 5% 7% 36%	1600
	21st to end	Day	2 14% 5% 11% 5% 4 5% 4 5% 6% 38%	1600
December	1st to 10th	Day	2 12% 4 9% 4 4 3 3 3 4 52%	2620
	11th to 20th	Day	3 16% 7% 12% 6% 5% 5% 4 5% 7% 29%	3026
	21st to end	Day	3 14% 5% 13% 6% 6% 6% 4 5% 8% 29%	3892
January	1st to 10th	Night	1 11% 6% 15% 6% 6% 5% 4 5% 6% 34%	6452
	11th to 20th	Night	2 12% 6% 15% 6% 5% 4 4 5% 6% 35%	5880
	21st to end	Night	2 17% 5% 11% 4 3 3 3 3 5% 44%	6468
February	1st to 10th	Night	3 17% 7% 14% 6% 7% 5% 5% 5% 7% 24%	2600
	11th to 20th	Night	3 15% 8% 13% 7% 6% 4 4 5% 5% 31%	2532
	21st to end	Night	3 15% 7% 16% 7% 5% 5% 4 5% 6% 27%	2030
March	1st to 10th	Night	2 17% 6% 13% 5% 4 3 3 4 5% 39%	5028
	11th to 20th	Night	2 12% 5% 7% 5% 4 4 4 6% 6% 46%	5000
	21st to end	Night	4 13% 5% 9% 5% 5% 6% 5% 7% 7% 34%	5489
April	1st to 10th	Night	5% 13% 3 8% 6% 4 4 5% 6% 7% 38%	2101
	11th to 20th	Night	6% 10% 4 8% 6% 5% 5% 4 6% 7% 39%	1828
	21st to end	Night	6% 11% 3 6% 4 5% 4 4 6% 8% 43%	1800
May	1st to 10th	Night	6% 11% 2 5% 4 4 4 3 5% 6% 49%	4100
	11th to 20th	Night	7% 5% 2 5% 3 4 5% 4 7% 8% 50%	3905
	21st to end	Night	5% 6% 2 4 3 3 4 4 5% 9% 55%	3972
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 7% 8% 48%	1600
	11th to 20th	Night	8% 4 2 4 3 3 4 3 6% 8% 55%	1406
	21st to end	Night	3 1 1 3 2 2 3 3 4 7% 71%	1400
July	1st to 10th	Night	1 1 1 2 2 3 4 3 6% 6% 71%	3180
	11th to 20th	Night	1 1 1 1 2 3 3 6% 8% 75%	3783
	21st to end	Night	1 1 1 2 2 3 3 4 6% 7% 71%	4472
August	1st to 10th	Night	2 2 1 2 2 2 4 4 6% 8% 67%	4033
	11th to 20th	Night	1 1 1 3 3 2 4 3 5% 8% 68%	4051
	21st to end	Night	5% 3 2 4 2 2 3 2 4 6% 66%	4570
September	1st to 10th	Night	4 3 1 3 2 2 4 3 5% 6% 66%	2020
	11th to 20th	Night	4 4 1 4 4 4 5% 4 6% 7% 57%	2200
	21st to end	Night	7% 5% 1 4 2 2 2 2 3 6% 65%	2200
October	1st to 10th	Night	6% 5% 1 4 2 2 2 2 3 5% 68%	3772
	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 4 4 3 5% 6% 41%	2535
	11th to 20th	Night	5% 13% 4 8% 4 5% 5% 5% 6% 6% 39%	2791
	21st to end	Night	3 12% 4 8% 6% 5% 4 4 6% 7% 41%	2800
December	1st to 10th	Night	2 12% 3 7% 4 4 4 3 4 5% 50%	4570
	11th to 20th	Night	3 16% 8% 12% 6% 5% 5% 4 5% 6% 31%	5181
	21st to end	Night	3 16% 5% 11% 6% 5% 5% 4 6% 6% 30%	7038

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	55% 9% 13% 14% 9% 1	3513
	11th to 20th	Day	45% 8% 17% 17% 10% 3	4100
	21st to end	Day	45% 6% 11% 21% 14% 3	4510
February	1st to 10th	Day	65% 8% 10% 12% 4 1	1800
	11th to 20th	Day	61% 8% 12% 12% 5% 2	1764
	21st to end	Day	64% 8% 8% 14% 5% 2	1595
March	1st to 10th	Day	51% 5% 11% 18% 11% 3	4865
	11th to 20th	Day	42% 9% 14% 25% 10% 1	5000
	21st to end	Day	59% 13% 13% 14% 2	5486
April	1st to 10th	Day	55% 8% 13% 15% 8% 1	2297
	11th to 20th	Day	54% 7% 11% 18% 8% 1	2521
	21st to end	Day	52% 9% 11% 22% 6%	2600
May	1st to 10th	Day	40% 6% 14% 27% 12% 1	5880
	11th to 20th	Day	33% 10% 13% 28% 12% 3	5999
	21st to end	Day	36% 7% 13% 30% 14% 1	6926
June	1st to 10th	Day	37% 9% 15% 26% 12% 1	2800
	11th to 20th	Day	36% 7% 10% 20% 19% 8%	2989
	21st to end	Day	18% 6% 9% 23% 33% 12%	3000
July	1st to 10th	Day	14% 5% 6% 13% 33% 29%	6706
	11th to 20th	Day	13% 4 6% 16% 29% 32%	6094
	21st to end	Day	17% 5% 7% 18% 36% 18%	6217
August	1st to 10th	Day	20% 6% 9% 17% 31% 18%	4208
	11th to 20th	Day	17% 7% 8% 22% 32% 14%	4197
	21st to end	Day	18% 5% 8% 20% 31% 19%	4613
September	1st to 10th	Day	24% 4 8% 18% 33% 13%	2376
	11th to 20th	Day	29% 7% 10% 22% 25% 8%	2200
	21st to end	Day	28% 5% 11% 26% 25% 5%	2200
October	1st to 10th	Day	27% 4 6% 22% 32% 9%	3266
	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	48% 7% 14% 20% 9% 2	1750
	11th to 20th	Day	49% 7% 17% 18% 8% 1	1600
	21st to end	Day	48% 9% 15% 19% 9% 1	1600
December	1st to 10th	Day	37% 7% 25% 23% 6% 2	2620
	11th to 20th	Day	54% 10% 20% 13% 3	3024
	21st to end	Day	47% 11% 16% 13% 8% 5%	3892
January	1st to 10th	Night	51% 8% 11% 20% 10%	6452
	11th to 20th	Night	49% 9% 14% 17% 9% 3	5880
	21st to end	Night	44% 7% 11% 20% 16% 2	6468
February	1st to 10th	Night	62% 10% 11% 12% 6%	2600
	11th to 20th	Night	57% 7% 11% 15% 8% 1	2533
	21st to end	Night	59% 8% 10% 14% 6% 2	2030
March	1st to 10th	Night	51% 5% 11% 18% 12% 3	5028
	11th to 20th	Night	34% 11% 15% 27% 12% 1	5000
	21st to end	Night	45% 14% 17% 21% 3	5489
April	1st to 10th	Night	46% 10% 16% 17% 11% 2	2101
	11th to 20th	Night	45% 9% 16% 20% 9% 1	1828
	21st to end	Night	39% 10% 15% 28% 9%	1800
May	1st to 10th	Night	37% 6% 11% 30% 15% 1	4097
	11th to 20th	Night	27% 10% 14% 34% 13% 2	3906
	21st to end	Night	27% 6% 15% 31% 19% 2	3972
June	1st to 10th	Night	34% 9% 16% 27% 13% 1	1600
	11th to 20th	Night	28% 6% 14% 24% 21% 7%	1406
	21st to end	Night	15% 5% 9% 25% 32% 15%	1400
July	1st to 10th	Night	12% 5% 7% 15% 34% 28%	3145
	11th to 20th	Night	7% 3 6% 18% 35% 31%	3737
	21st to end	Night	10% 4 7% 20% 37% 21%	4344
August	1st to 10th	Night	18% 5% 7% 16% 37% 17%	2928
	11th to 20th	Night	18% 7% 9% 26% 26% 14%	2928
	21st to end	Night	16% 3 7% 21% 35% 18%	3294
September	1st to 10th	Night	20% 5% 7% 19% 31% 18%	2020
	11th to 20th	Night	24% 8% 10% 22% 26% 10%	2200
	21st to end	Night	24% 4 11% 27% 28% 6%	2200
October	1st to 10th	Night	22% 3 8% 26% 35% 7%	3772
	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	44% 9% 13% 22% 10% 2	2535
	11th to 20th	Night	45% 9% 17% 20% 9% 1	2791
	21st to end	Night	44% 8% 16% 20% 10% 2	2800
December	1st to 10th	Night	37% 8% 27% 21% 6% 2	4570
	11th to 20th	Night	52% 12% 20% 13% 4	5182
	21st to end	Night	48% 12% 14% 16% 7% 4	7038

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	13% 23% 30% 26% 7% 1	238
	11th to 20th	Day	6% 19% 39% 24% 10% 1	300
	21st to end	Day	8% 18% 35% 30% 8%	330
February	1st to 10th	Day	18% 36% 29% 15% 2	300
	11th to 20th	Day	15% 29% 31% 20% 5%	300
	21st to end	Day	17% 28% 33% 17% 5%	359
March	1st to 10th	Day	15% 34% 32% 16% 3 1	500
	11th to 20th	Day	14% 26% 32% 22% 6%	500
	21st to end	Day	19% 28% 33% 17% 3	550
April	1st to 10th	Day	20% 26% 31% 16% 6%	500
	11th to 20th	Day	16% 30% 34% 15% 4 1	492
	21st to end	Day	17% 25% 29% 20% 8% 1	500
May	1st to 10th	Day	13% 22% 32% 25% 8% 1	503
	11th to 20th	Day	17% 16% 33% 22% 10% 2	592
	21st to end	Day	19% 13% 25% 27% 15% 2	789
June	1st to 10th	Day	10% 17% 26% 29% 15% 3	730
	11th to 20th	Day	14% 22% 34% 21% 9%	730
	21st to end	Day	6% 13% 35% 29% 14% 2	730
July	1st to 10th	Day	3 10% 32% 37% 16% 2	721
	11th to 20th	Day	3 13% 40% 32% 11% 1	604
	21st to end	Day	4 11% 34% 36% 13% 2	575
August	1st to 10th	Day	5% 13% 39% 31% 11% 1	527
	11th to 20th	Day	4 12% 34% 32% 16% 3	517
	21st to end	Day	7% 16% 31% 30% 13% 3	583
September	1st to 10th	Day	5% 13% 32% 29% 17% 3	530
	11th to 20th	Day	11% 17% 32% 25% 12% 4	530
	21st to end	Day	10% 15% 29% 28% 14% 3	530
October	1st to 10th	Day	10% 17% 30% 25% 15% 3	349
	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	18% 21% 31% 19% 8% 3	300
	11th to 20th	Day	13% 24% 33% 21% 7% 2	300
	21st to end	Day	7% 20% 37% 27% 9%	300
December	1st to 10th	Day	14% 20% 27% 27% 10% 1	300
	11th to 20th	Day	16% 22% 35% 21% 5%	300
	21st to end	Day	17% 22% 31% 19% 9% 1	307
January	1st to 10th	Night	10% 16% 30% 35% 10% 1	950
	11th to 20th	Night	10% 15% 33% 30% 12% 1	800
	21st to end	Night	8% 9% 36% 36% 9% 1	880
February	1st to 10th	Night	11% 21% 32% 29% 6% 1	800
	11th to 20th	Night	10% 17% 32% 32% 8% 1	784
	21st to end	Night	9% 18% 34% 30% 8%	660
March	1st to 10th	Night	9% 20% 30% 33% 9% 1	800
	11th to 20th	Night	10% 13% 28% 35% 13% 1	800
	21st to end	Night	12% 14% 31% 32% 10%	880
April	1st to 10th	Night	10% 17% 29% 31% 11% 2	701
	11th to 20th	Night	12% 13% 34% 32% 9% 1	593
	21st to end	Night	12% 11% 27% 31% 18% 1	600
May	1st to 10th	Night	12% 12% 21% 36% 18% 1	600
	11th to 20th	Night	12% 8% 26% 33% 21% 2	591
	21st to end	Night	14% 10% 22% 35% 19% 1	650
June	1st to 10th	Night	11% 10% 22% 33% 22% 2	600
	11th to 20th	Night	10% 9% 23% 42% 15%	406
	21st to end	Night	3 2 17% 47% 29% 3	400
July	1st to 10th	Night	1 2 16% 49% 29% 2	390
	11th to 20th	Night	2 3 16% 50% 28%	527
	21st to end	Night	2 1 15% 49% 30% 2	657
August	1st to 10th	Night	4 3 18% 44% 28% 2	597
	11th to 20th	Night	2 2 14% 48% 31% 3	594
	21st to end	Night	6% 5% 19% 37% 31% 3	660
September	1st to 10th	Night	6% 2 17% 41% 31% 3	620
	11th to 20th	Night	6% 4 20% 38% 29% 4	800
	21st to end	Night	11% 5% 17% 38% 25% 4	800
October	1st to 10th	Night	13% 8% 22% 35% 20% 2	772
	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% 34% 11% 1	780
	11th to 20th	Night	13% 16% 32% 32% 6% 1	991
	21st to end	Night	11% 14% 29% 33% 12% 1	1000
December	1st to 10th	Night	14% 13% 32% 34% 7% 1	1000
	11th to 20th	Night	15% 16% 30% 33% 7%	1000
	21st to end	Night	14% 17% 32% 28% 8% 1	1100

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