Wilson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilson Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	11% 6% 2 6% 4 4 4 3 5% 7% 48%	143690
	Night	11% 6% 2 5% 3 3 3 3 5% 7% 52%	138556
Transparency	Day	34% 5% 11% 26% 19% 4	147328
	Night	31% 5% 10% 26% 22% 6%	142985
Seeing	Day	15% 12% 29% 26% 14% 3	17933
	Night	14% 12% 32% 29% 11% 1	27684

By Month

Percentage Cloud Cover SE (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	13% 10% 3 6% 4 3 3 2 5% 6% 44%	5289
	11th to 20th	Day	6% 7% 2 6% 4 3 4 2 3 5% 58%	5310
	21st to end	Day	16% 10% 2 6% 3 2 2 2 4 5% 48%	5812
February	1st to 10th	Day	16% 11% 3 6% 4 3 3 3 5% 6% 42%	1890
	11th to 20th	Day	15% 8% 2 7% 4 3 3 3 4 6% 46%	1954
	21st to end	Day	14% 7% 2 5% 4 4 4 3 5% 5% 48%	1794
March	1st to 10th	Day	12% 7% 2 5% 3 3 3 3 4 5% 52%	5046
	11th to 20th	Day	9% 6% 2 4 3 3 3 3 4 7% 55%	5090
	21st to end	Day	13% 8% 2 8% 5% 4 5% 4 6% 7% 37%	5583
April	1st to 10th	Day	13% 6% 3 5% 4 3 4 3 6% 6% 46%	2347
	11th to 20th	Day	16% 6% 2 6% 4 3 4 4 5% 6% 44%	2494
	21st to end	Day	20% 8% 3 7% 3 3 4 3 4 6% 38%	2658
May	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 5% 6% 42%	7617
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 5% 7% 44%	7627
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 7% 32%	8392
June	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 4 7% 8% 48%	2730
	11th to 20th	Day	9% 5% 1 5% 4 4 5% 5% 6% 8% 48%	2730
	21st to end	Day	6% 3 1 4 3 3 4 4 6% 8% 57%	2730
July	1st to 10th	Day	5% 4 1 5% 4 5% 5% 4 8% 11% 47%	7680
	11th to 20th	Day	5% 4 2 6% 4 4 5% 4 7% 10% 47%	7680
	21st to end	Day	4 3 1 5% 4 4 5% 4 8% 11% 51%	8337
August	1st to 10th	Day	5% 3 1 4 3 3 4 4 6% 9% 58%	4364
	11th to 20th	Day	8% 4 2 5% 4 5% 4 4 7% 10% 50%	4224
	21st to end	Day	5% 4 2 5% 4 4 4 4 5% 8% 56%	4397
September	1st to 10th	Day	10% 4 2 6% 3 4 4 3 4 6% 54%	2297
	11th to 20th	Day	11% 5% 2 5% 3 4 5% 4 6% 9% 47%	2200
	21st to end	Day	9% 4 2 5% 3 4 4 3 5% 7% 55%	2200
October	1st to 10th	Day	9% 4 1 4 3 3 4 2 5% 7% 57%	2045
	11th to 20th	Day	11% 4 1 4 3 2 3 2 4 5% 61%	2000
	21st to end	Day	19% 5% 2 4 3 3 2 2 3 5% 52%	2200
November	1st to 10th	Day	11% 4 2 4 2 3 3 3 3 6% 59%	2058
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 4 6% 57%	1927
	21st to end	Day	15% 7% 2 4 3 2 2 3 3 6% 52%	1890
December	1st to 10th	Day	10% 9% 3 6% 4 3 2 2 4 5% 52%	3030
	11th to 20th	Day	12% 7% 3 7% 3 3 3 3 4 5% 52%	3481
	21st to end	Day	20% 16% 6% 7% 4 3 3 2 3 4 32%	4587
January	1st to 10th	Night	13% 8% 2 6% 4 3 3 3 5% 6% 47%	7634
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 4 5% 59%	7680
	21st to end	Night	16% 8% 3 5% 4 3 3 3 4 5% 46%	8402
February	1st to 10th	Night	16% 8% 2 6% 4 3 3 3 4 5% 46%	2730
	11th to 20th	Night	14% 8% 3 7% 4 2 3 2 4 6% 48%	2569
	21st to end	Night	14% 7% 2 5% 3 3 3 3 3 4 52%	2010
March	1st to 10th	Night	14% 9% 2 5% 3 3 3 2 3 5% 50%	5029
	11th to 20th	Night	8% 4 2 4 2 2 3 3 4 7% 61%	5090
	21st to end	Night	12% 6% 2 6% 3 3 4 3 5% 8% 48%	5588
April	1st to 10th	Night	13% 6% 2 5% 2 2 3 3 5% 5% 53%	2272
	11th to 20th	Night	15% 6% 2 5% 3 2 3 3 5% 6% 49%	2075
	21st to end	Night	16% 7% 3 6% 3 3 3 3 4 6% 46%	1958
May	1st to 10th	Night	15% 6% 2 6% 3 3 4 3 5% 6% 45%	5266
	11th to 20th	Night	13% 6% 2 6% 3 4 4 3 5% 8% 47%	5265
	21st to end	Night	17% 6% 2 6% 4 3 3 3 6% 8% 41%	5793
June	1st to 10th	Night	8% 4 2 5% 3 4 5% 4 5% 8% 53%	1890
	11th to 20th	Night	5% 3 1 4 4 4 3 5% 6% 11% 54%	1890
	21st to end	Night	4 3 1 3 2 3 3 3 5% 8% 63%	1890
July	1st to 10th	Night	5% 3 1 4 3 4 4 4 8% 12% 51%	5308
	11th to 20th	Night	4 3 2 5% 4 3 4 3 7% 9% 56%	5307
	21st to end	Night	3 2 1 3 3 3 4 3 6% 12% 60%	5770
August	1st to 10th	Night	2 2 1 3 2 3 3 3 6% 10% 64%	3016
	11th to 20th	Night	5% 3 2 5% 4 5% 5% 4 6% 8% 53%	2924
	21st to end	Night	3 3 1 3 3 3 4 3 5% 8% 64%	3490
September	1st to 10th	Night	8% 4 1 5% 3 2 3 2 4 6% 59%	2098
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 6% 7% 52%	2200
	21st to end	Night	8% 4 1 4 4 3 3 3 5% 6% 59%	2200
October	1st to 10th	Night	7% 2 1 4 3 3 3 3 4 7% 64%	2335
	11th to 20th	Night	11% 5% 1 4 3 3 3 2 4 5% 60%	2400
	21st to end	Night	15% 5% 2 4 3 3 3 2 4 5% 55%	2640
November	1st to 10th	Night	11% 5% 2 3 2 2 2 3 3 4 63%	2460
	11th to 20th	Night	10% 6% 2 5% 3 2 3 3 3 5% 59%	2688
	21st to end	Night	15% 6% 2 4 3 2 3 3 4 6% 53%	2730
December	1st to 10th	Night	9% 8% 2 5% 3 3 3 3 4 6% 56%	4380
	11th to 20th	Night	15% 6% 2 5% 3 3 3 2 3 4 53%	4946
	21st to end	Night	18% 11% 4 6% 3 3 3 2 4 5% 40%	6633

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	40% 3 8% 29% 17% 2	5290
	11th to 20th	Day	28% 4 10% 29% 23% 6%	5310
	21st to end	Day	40% 2 7% 24% 24% 4	5812
February	1st to 10th	Day	44% 4 10% 24% 14% 3	1890
	11th to 20th	Day	41% 4 9% 26% 17% 4	1963
	21st to end	Day	40% 3 7% 23% 19% 8%	1794
March	1st to 10th	Day	35% 4 5% 21% 18% 16%	5046
	11th to 20th	Day	29% 5% 10% 20% 22% 14%	5090
	21st to end	Day	43% 6% 10% 30% 10% 1	5585
April	1st to 10th	Day	38% 6% 11% 23% 18% 5%	2347
	11th to 20th	Day	40% 5% 13% 21% 17% 5%	2494
	21st to end	Day	48% 5% 9% 22% 15% 1	2658
May	1st to 10th	Day	43% 5% 9% 23% 18% 2	7655
	11th to 20th	Day	38% 6% 11% 22% 20% 3	7631
	21st to end	Day	49% 9% 11% 15% 14% 2	8405
June	1st to 10th	Day	33% 8% 15% 26% 17% 2	2730
	11th to 20th	Day	34% 7% 13% 29% 16% 2	2730
	21st to end	Day	25% 6% 16% 29% 19% 4	2730
July	1st to 10th	Day	28% 7% 18% 31% 14% 2	7616
	11th to 20th	Day	29% 8% 16% 27% 16% 4	7611
	21st to end	Day	23% 8% 19% 36% 13%	8155
August	1st to 10th	Day	21% 6% 14% 29% 25% 5%	4350
	11th to 20th	Day	29% 7% 20% 26% 16% 2	4263
	21st to end	Day	25% 7% 12% 39% 13% 4	4396
September	1st to 10th	Day	31% 5% 10% 24% 21% 9%	2297
	11th to 20th	Day	32% 7% 13% 22% 21% 5%	2200
	21st to end	Day	28% 5% 10% 24% 27% 6%	2200
October	1st to 10th	Day	18% 3 8% 16% 42% 13%	3331
	11th to 20th	Day	16% 3 4 21% 42% 14%	3240
	21st to end	Day	29% 3 7% 27% 28% 5%	3538
November	1st to 10th	Day	27% 4 7% 28% 26% 7%	2058
	11th to 20th	Day	31% 3 8% 27% 23% 8%	1927
	21st to end	Day	35% 3 9% 26% 23% 4	1890
December	1st to 10th	Day	35% 4 7% 34% 20% 1	3030
	11th to 20th	Day	35% 3 6% 29% 24% 3	3479
	21st to end	Day	56% 5% 7% 21% 9% 2	4587
January	1st to 10th	Night	37% 4 10% 29% 18% 2	7635
	11th to 20th	Night	25% 5% 10% 21% 34% 6%	7680
	21st to end	Night	41% 3 9% 19% 26% 3	8402
February	1st to 10th	Night	41% 3 11% 24% 17% 4	2730
	11th to 20th	Night	39% 3 9% 26% 18% 5%	2570
	21st to end	Night	36% 4 7% 20% 24% 10%	2010
March	1st to 10th	Night	39% 4 7% 20% 20% 11%	5029
	11th to 20th	Night	25% 3 8% 24% 26% 13%	5090
	21st to end	Night	34% 5% 12% 32% 15% 2	5588
April	1st to 10th	Night	33% 5% 10% 23% 21% 8%	2272
	11th to 20th	Night	35% 5% 11% 22% 20% 7%	2075
	21st to end	Night	40% 4 11% 23% 19% 3	1958
May	1st to 10th	Night	39% 6% 11% 21% 20% 3	5293
	11th to 20th	Night	36% 6% 11% 24% 19% 5%	5266
	21st to end	Night	40% 7% 16% 21% 13% 2	5805
June	1st to 10th	Night	27% 9% 15% 28% 18% 3	1890
	11th to 20th	Night	23% 7% 16% 31% 21% 3	1890
	21st to end	Night	20% 5% 14% 30% 25% 6%	1890
July	1st to 10th	Night	23% 7% 19% 31% 18% 3	5265
	11th to 20th	Night	23% 7% 15% 31% 19% 4	5256
	21st to end	Night	18% 5% 15% 43% 18% 1	5641
August	1st to 10th	Night	16% 5% 15% 32% 27% 6%	3006
	11th to 20th	Night	28% 7% 16% 27% 19% 3	2951
	21st to end	Night	19% 6% 11% 39% 20% 5%	3490
September	1st to 10th	Night	27% 5% 9% 27% 23% 10%	2098
	11th to 20th	Night	30% 6% 10% 24% 23% 6%	2200
	21st to end	Night	26% 4 8% 25% 30% 7%	2200
October	1st to 10th	Night	14% 3 6% 14% 39% 24%	3816
	11th to 20th	Night	20% 3 5% 18% 35% 18%	3899
	21st to end	Night	29% 4 7% 26% 27% 8%	4252
November	1st to 10th	Night	26% 3 8% 25% 30% 8%	2460
	11th to 20th	Night	28% 3 7% 27% 26% 9%	2688
	21st to end	Night	34% 3 8% 24% 26% 5%	2730
December	1st to 10th	Night	31% 4 9% 37% 17% 2	4380
	11th to 20th	Night	37% 3 6% 23% 26% 5%	4947
	21st to end	Night	47% 4 8% 25% 12% 4	6633

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	23% 19% 37% 17% 3	330
	11th to 20th	Day	24% 25% 36% 12% 3	330
	21st to end	Day	21% 23% 38% 13% 4	363
February	1st to 10th	Day	26% 19% 38% 14% 3	330
	11th to 20th	Day	24% 24% 38% 13% 1	330
	21st to end	Day	26% 25% 36% 12% 1	273
March	1st to 10th	Day	26% 25% 32% 15% 2	330
	11th to 20th	Day	25% 23% 37% 13% 3	330
	21st to end	Day	25% 28% 33% 12% 2	363
April	1st to 10th	Day	20% 27% 34% 15% 4	367
	11th to 20th	Day	23% 25% 34% 14% 3	532
	21st to end	Day	24% 19% 35% 15% 6% 1	678
May	1st to 10th	Day	19% 17% 38% 21% 5%	753
	11th to 20th	Day	19% 15% 33% 22% 9% 3	775
	21st to end	Day	18% 13% 31% 22% 12% 5%	844
June	1st to 10th	Day	8% 8% 25% 34% 21% 4	780
	11th to 20th	Day	9% 6% 29% 37% 16% 3	780
	21st to end	Day	6% 4 33% 37% 16% 3	780
July	1st to 10th	Day	8% 3 18% 36% 27% 8%	771
	11th to 20th	Day	3 2 20% 43% 25% 5%	768
	21st to end	Day	7% 1 13% 39% 29% 10%	849
August	1st to 10th	Day	7% 3 19% 41% 25% 5%	776
	11th to 20th	Day	9% 3 17% 38% 28% 6%	736
	21st to end	Day	8% 2 15% 37% 30% 8%	642
September	1st to 10th	Day	10% 2 16% 35% 28% 8%	427
	11th to 20th	Day	5% 3 24% 38% 25% 5%	330
	21st to end	Day	13% 6% 23% 34% 18% 6%	330
October	1st to 10th	Day	10% 7% 33% 30% 16% 4	330
	11th to 20th	Day	10% 17% 38% 26% 8%	330
	21st to end	Day	26% 16% 32% 19% 6% 1	363
November	1st to 10th	Day	15% 15% 42% 22% 5% 1	330
	11th to 20th	Day	17% 19% 38% 17% 8% 2	330
	21st to end	Day	22% 19% 38% 17% 4	330
December	1st to 10th	Day	18% 21% 38% 16% 5% 2	330
	11th to 20th	Day	22% 22% 38% 12% 5%	330
	21st to end	Day	21% 25% 37% 14% 2	363
January	1st to 10th	Night	16% 20% 36% 23% 5%	1050
	11th to 20th	Night	17% 20% 38% 21% 4	1050
	21st to end	Night	15% 19% 37% 25% 4	1155
February	1st to 10th	Night	20% 19% 38% 20% 3	1050
	11th to 20th	Night	22% 21% 35% 20% 2	935
	21st to end	Night	19% 21% 37% 21% 3	626
March	1st to 10th	Night	19% 17% 38% 23% 3	630
	11th to 20th	Night	19% 17% 39% 22% 3	630
	21st to end	Night	19% 17% 41% 19% 4	693
April	1st to 10th	Night	17% 17% 40% 22% 4	630
	11th to 20th	Night	19% 16% 38% 23% 5%	623
	21st to end	Night	20% 15% 40% 19% 6%	630
May	1st to 10th	Night	14% 12% 39% 29% 6%	630
	11th to 20th	Night	15% 11% 32% 29% 12% 1	621
	21st to end	Night	13% 8% 33% 29% 16% 1	683
June	1st to 10th	Night	8% 5% 23% 40% 23% 1	630
	11th to 20th	Night	6% 3 31% 41% 17% 1	630
	21st to end	Night	4 2 31% 46% 16% 1	630
July	1st to 10th	Night	5% 2 18% 47% 25% 4	615
	11th to 20th	Night	3 1 18% 46% 28% 3	612
	21st to end	Night	4 2 16% 45% 31% 3	690
August	1st to 10th	Night	3 2 21% 45% 27% 2	627
	11th to 20th	Night	6% 2 18% 44% 27% 3	606
	21st to end	Night	5% 2 15% 40% 35% 3	660
September	1st to 10th	Night	10% 1 17% 47% 25% 1	600
	11th to 20th	Night	10% 2 20% 41% 22% 4	600
	21st to end	Night	10% 6% 22% 36% 23% 4	600
October	1st to 10th	Night	7% 5% 32% 33% 20% 2	735
	11th to 20th	Night	13% 11% 38% 29% 9%	800
	21st to end	Night	18% 14% 33% 26% 9% 1	880
November	1st to 10th	Night	14% 13% 42% 21% 9% 1	820
	11th to 20th	Night	13% 17% 37% 26% 7%	1008
	21st to end	Night	19% 16% 36% 22% 7%	1050
December	1st to 10th	Night	16% 17% 35% 27% 4	1050
	11th to 20th	Night	20% 15% 36% 24% 4	1050
	21st to end	Night	19% 20% 34% 25% 3	1155

Page generated at 2026-08-12 02:37:15. Version ac07

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