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 2025-04-11.

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	12% 7% 2 5% 4 4 4 3 5% 7% 48%	114155
	Night	11% 6% 2 5% 3 3 3 3 4 6% 52%	114338
Transparency	Day	34% 5% 11% 25% 19% 5%	117834
	Night	31% 5% 10% 25% 22% 6%	118788
Seeing	Day	15% 13% 29% 26% 14% 3	15948
	Night	14% 13% 32% 29% 11% 1	25973

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	15% 12% 4 6% 4 3 3 2 6% 5% 41%	4077
	11th to 20th	Day	7% 6% 2 7% 5% 4 4 3 4 5% 53%	4080
	21st to end	Day	18% 10% 2 5% 3 2 2 2 4 4 48%	4488
February	1st to 10th	Day	17% 11% 3 6% 4 3 3 2 5% 6% 41%	1800
	11th to 20th	Day	15% 8% 2 7% 3 3 3 3 4 6% 45%	1858
	21st to end	Day	14% 8% 2 6% 4 4 4 3 4 5% 46%	1711
March	1st to 10th	Day	12% 7% 2 5% 3 2 3 3 4 5% 53%	4936
	11th to 20th	Day	9% 6% 2 4 3 3 3 3 5% 7% 55%	4980
	21st to end	Day	14% 8% 2 8% 5% 4 5% 4 6% 8% 36%	5462
April	1st to 10th	Day	13% 6% 3 5% 4 3 4 3 6% 6% 45%	2235
	11th to 20th	Day	16% 7% 2 6% 4 3 4 4 5% 6% 44%	2254
	21st to end	Day	20% 7% 3 7% 4 3 4 3 5% 5% 40%	2404
May	1st to 10th	Day	23% 8% 3 6% 4 3 3 2 4 5% 39%	4082
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 7% 43%	4082
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 7% 7% 36%	4477
June	1st to 10th	Day	7% 4 2 5% 5% 5% 5% 4 7% 9% 49%	2470
	11th to 20th	Day	8% 5% 1 5% 4 4 5% 4 6% 8% 50%	2470
	21st to end	Day	6% 3 1 3 3 3 4 4 6% 8% 58%	2470
July	1st to 10th	Day	8% 6% 2 5% 5% 4 5% 5% 7% 10% 44%	4120
	11th to 20th	Day	5% 4 1 5% 4 3 4 4 6% 10% 53%	4120
	21st to end	Day	6% 4 1 5% 4 4 5% 5% 8% 11% 49%	4465
August	1st to 10th	Day	5% 3 1 4 3 4 4 4 6% 9% 58%	4104
	11th to 20th	Day	8% 4 2 5% 4 5% 4 4 7% 10% 49%	4041
	21st to end	Day	4 4 1 5% 4 4 4 4 5% 8% 57%	4262
September	1st to 10th	Day	11% 4 2 6% 3 4 4 3 4 6% 54%	2182
	11th to 20th	Day	11% 5% 2 5% 3 4 5% 4 6% 8% 47%	2090
	21st to end	Day	9% 3 1 5% 3 4 4 3 5% 7% 56%	2090
October	1st to 10th	Day	10% 4 1 4 3 3 4 3 5% 7% 56%	1943
	11th to 20th	Day	11% 5% 1 4 3 2 3 2 4 5% 61%	1900
	21st to end	Day	18% 5% 2 4 3 3 2 2 3 5% 53%	2090
November	1st to 10th	Day	12% 4 2 4 2 3 4 3 3 6% 57%	1958
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 4 6% 56%	1835
	21st to end	Day	15% 7% 2 4 3 2 2 3 3 6% 53%	1800
December	1st to 10th	Day	10% 9% 3 6% 4 3 2 2 4 5% 52%	2940
	11th to 20th	Day	12% 7% 3 7% 3 3 3 3 4 5% 52%	3391
	21st to end	Day	21% 17% 6% 7% 4 3 3 2 3 4 31%	4488
January	1st to 10th	Night	14% 8% 3 7% 4 3 3 4 5% 6% 44%	5888
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 4 6% 53%	5900
	21st to end	Night	17% 8% 3 6% 3 2 2 2 3 5% 48%	6490
February	1st to 10th	Night	16% 9% 2 6% 4 3 3 3 4 6% 45%	2600
	11th to 20th	Night	14% 8% 3 7% 4 2 3 2 4 6% 48%	2444
	21st to end	Night	15% 7% 2 5% 3 3 3 3 3 3 51%	1917
March	1st to 10th	Night	14% 9% 2 5% 3 3 3 2 3 5% 50%	4919
	11th to 20th	Night	8% 4 2 4 2 2 3 3 4 7% 61%	4980
	21st to end	Night	12% 6% 2 6% 3 3 4 3 5% 8% 48%	5467
April	1st to 10th	Night	13% 6% 2 5% 3 2 4 3 5% 5% 53%	2164
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	1881
	21st to end	Night	16% 7% 3 6% 3 3 3 3 4 6% 47%	1772
May	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 5% 7% 43%	2823
	11th to 20th	Night	15% 6% 2 6% 3 4 4 3 5% 6% 45%	2823
	21st to end	Night	12% 5% 2 5% 4 3 3 4 7% 10% 45%	3090
June	1st to 10th	Night	6% 4 2 4 3 4 5% 4 5% 9% 55%	1710
	11th to 20th	Night	4 3 1 3 3 4 3 5% 6% 12% 56%	1710
	21st to end	Night	4 3 1 3 2 3 3 3 6% 8% 64%	1710
July	1st to 10th	Night	9% 4 1 5% 3 3 3 3 7% 9% 51%	2850
	11th to 20th	Night	3 2 2 4 3 3 4 3 6% 9% 63%	2850
	21st to end	Night	6% 3 1 4 3 3 4 3 6% 10% 56%	3098
August	1st to 10th	Night	3 2 1 3 2 3 3 3 6% 10% 63%	2836
	11th to 20th	Night	6% 3 2 5% 5% 5% 6% 4 6% 7% 52%	2798
	21st to end	Night	3 3 1 3 3 3 4 3 5% 8% 64%	3383
September	1st to 10th	Night	8% 4 1 5% 3 2 3 2 4 6% 60%	1993
	11th to 20th	Night	9% 6% 2 5% 3 4 3 4 5% 7% 52%	2090
	21st to end	Night	8% 4 1 4 4 3 3 2 5% 6% 59%	2090
October	1st to 10th	Night	7% 3 1 4 3 3 3 3 4 7% 63%	2218
	11th to 20th	Night	12% 4 1 4 3 3 3 2 3 5% 60%	2280
	21st to end	Night	14% 5% 2 4 3 3 3 2 4 5% 55%	2508
November	1st to 10th	Night	12% 5% 2 3 2 2 2 3 3 4 62%	2340
	11th to 20th	Night	10% 6% 2 4 3 2 3 2 3 5% 59%	2560
	21st to end	Night	14% 6% 2 3 3 2 3 3 4 6% 55%	2600
December	1st to 10th	Night	9% 8% 2 5% 3 2 3 3 4 6% 56%	4250
	11th to 20th	Night	16% 6% 2 5% 3 3 3 2 3 4 52%	4816
	21st to end	Night	19% 12% 4 6% 3 3 3 2 4 5% 39%	6490

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	45% 3 9% 26% 16% 1	4078
	11th to 20th	Day	30% 5% 12% 27% 21% 5%	4080
	21st to end	Day	42% 2 7% 21% 24% 4	4488
February	1st to 10th	Day	46% 4 10% 23% 14% 3	1800
	11th to 20th	Day	42% 4 9% 25% 17% 3	1867
	21st to end	Day	41% 3 7% 23% 18% 8%	1711
March	1st to 10th	Day	35% 4 5% 22% 19% 16%	4936
	11th to 20th	Day	29% 5% 10% 20% 22% 14%	4980
	21st to end	Day	44% 6% 10% 29% 10% 1	5464
April	1st to 10th	Day	38% 6% 11% 22% 18% 5%	2235
	11th to 20th	Day	40% 5% 13% 21% 16% 4	2254
	21st to end	Day	46% 5% 9% 22% 16% 1	2404
May	1st to 10th	Day	48% 5% 9% 21% 15% 2	4120
	11th to 20th	Day	39% 8% 13% 23% 16% 1	4086
	21st to end	Day	43% 10% 14% 19% 12% 2	4489
June	1st to 10th	Day	32% 8% 15% 27% 16% 2	2470
	11th to 20th	Day	32% 7% 13% 30% 16% 2	2470
	21st to end	Day	24% 6% 15% 30% 20% 5%	2470
July	1st to 10th	Day	33% 8% 17% 26% 14% 3	4056
	11th to 20th	Day	25% 6% 11% 36% 20% 2	4051
	21st to end	Day	26% 8% 19% 33% 14%	4325
August	1st to 10th	Day	22% 6% 14% 28% 25% 5%	4090
	11th to 20th	Day	30% 7% 20% 24% 16% 2	4080
	21st to end	Day	24% 7% 12% 40% 14% 3	4261
September	1st to 10th	Day	31% 5% 10% 23% 22% 9%	2182
	11th to 20th	Day	33% 7% 13% 21% 22% 5%	2090
	21st to end	Day	28% 5% 11% 24% 27% 6%	2090
October	1st to 10th	Day	18% 3 8% 16% 42% 14%	3229
	11th to 20th	Day	16% 2 4 21% 42% 14%	3140
	21st to end	Day	29% 3 7% 28% 28% 6%	3428
November	1st to 10th	Day	28% 4 7% 27% 25% 7%	1958
	11th to 20th	Day	31% 3 8% 26% 23% 8%	1835
	21st to end	Day	34% 3 9% 26% 24% 4	1800
December	1st to 10th	Day	35% 4 7% 34% 20% 1	2940
	11th to 20th	Day	36% 3 6% 29% 24% 3	3389
	21st to end	Day	57% 5% 6% 20% 9% 2	4488
January	1st to 10th	Night	38% 4 11% 29% 15% 3	5888
	11th to 20th	Night	27% 7% 12% 20% 30% 4	5900
	21st to end	Night	41% 3 8% 18% 26% 4	6490
February	1st to 10th	Night	42% 3 11% 23% 17% 4	2600
	11th to 20th	Night	39% 3 9% 26% 19% 4	2445
	21st to end	Night	38% 4 7% 20% 23% 9%	1917
March	1st to 10th	Night	38% 3 7% 20% 20% 12%	4919
	11th to 20th	Night	26% 3 8% 24% 26% 13%	4980
	21st to end	Night	34% 5% 12% 31% 15% 2	5467
April	1st to 10th	Night	33% 5% 10% 22% 21% 8%	2164
	11th to 20th	Night	35% 5% 11% 23% 19% 8%	1881
	21st to end	Night	40% 4 10% 23% 20% 3	1772
May	1st to 10th	Night	41% 7% 11% 20% 18% 4	2850
	11th to 20th	Night	38% 7% 12% 25% 16% 1	2824
	21st to end	Night	32% 8% 17% 25% 17% 1	3099
June	1st to 10th	Night	25% 9% 15% 29% 19% 3	1710
	11th to 20th	Night	20% 7% 16% 32% 21% 3	1710
	21st to end	Night	19% 5% 14% 31% 25% 6%	1710
July	1st to 10th	Night	28% 6% 18% 26% 17% 5%	2805
	11th to 20th	Night	17% 6% 10% 39% 25% 3	2796
	21st to end	Night	23% 6% 16% 37% 18% 1	2999
August	1st to 10th	Night	16% 5% 15% 31% 26% 6%	2826
	11th to 20th	Night	29% 7% 16% 26% 20% 3	2825
	21st to end	Night	18% 6% 11% 40% 21% 5%	3383
September	1st to 10th	Night	26% 5% 9% 26% 23% 10%	1993
	11th to 20th	Night	30% 6% 10% 22% 24% 7%	2090
	21st to end	Night	26% 5% 8% 25% 30% 7%	2090
October	1st to 10th	Night	14% 2 6% 14% 39% 24%	3699
	11th to 20th	Night	20% 3 5% 18% 36% 19%	3779
	21st to end	Night	28% 4 7% 26% 26% 8%	4120
November	1st to 10th	Night	27% 3 8% 25% 29% 8%	2340
	11th to 20th	Night	29% 4 7% 25% 26% 9%	2560
	21st to end	Night	32% 3 8% 24% 27% 6%	2600
December	1st to 10th	Night	31% 4 9% 37% 17% 2	4250
	11th to 20th	Night	37% 3 5% 23% 26% 5%	4817
	21st to end	Night	48% 4 8% 24% 12% 4	6490

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	24% 19% 35% 18% 3	300
	11th to 20th	Day	24% 25% 35% 13% 3	300
	21st to end	Day	22% 23% 36% 14% 5%	330
February	1st to 10th	Day	28% 20% 36% 12% 3	300
	11th to 20th	Day	25% 25% 37% 13% 1	300
	21st to end	Day	27% 25% 35% 12% 1	249
March	1st to 10th	Day	24% 24% 33% 16% 2	300
	11th to 20th	Day	24% 23% 36% 13% 3	300
	21st to end	Day	26% 28% 32% 12% 2	330
April	1st to 10th	Day	20% 28% 34% 14% 4	335
	11th to 20th	Day	22% 25% 34% 15% 4	452
	21st to end	Day	24% 19% 36% 14% 7% 1	584
May	1st to 10th	Day	19% 17% 38% 20% 5%	653
	11th to 20th	Day	19% 13% 32% 24% 9% 3	675
	21st to end	Day	16% 13% 32% 21% 13% 5%	734
June	1st to 10th	Day	7% 7% 25% 35% 22% 4	680
	11th to 20th	Day	8% 5% 29% 38% 16% 4	680
	21st to end	Day	5% 3 32% 38% 17% 4	680
July	1st to 10th	Day	8% 3 18% 35% 27% 9%	671
	11th to 20th	Day	4 2 21% 42% 25% 6%	668
	21st to end	Day	8% 1 13% 38% 29% 10%	739
August	1st to 10th	Day	7% 3 18% 40% 26% 5%	676
	11th to 20th	Day	10% 3 18% 36% 28% 5%	665
	21st to end	Day	7% 2 15% 36% 32% 8%	595
September	1st to 10th	Day	9% 2 16% 34% 30% 8%	392
	11th to 20th	Day	5% 3 25% 38% 24% 5%	300
	21st to end	Day	13% 6% 23% 35% 17% 6%	300
October	1st to 10th	Day	11% 8% 34% 26% 17% 4	300
	11th to 20th	Day	10% 18% 39% 26% 7%	300
	21st to end	Day	25% 16% 33% 18% 6% 2	330
November	1st to 10th	Day	16% 16% 40% 21% 6% 1	300
	11th to 20th	Day	18% 19% 36% 17% 7% 2	300
	21st to end	Day	22% 19% 39% 17% 4	300
December	1st to 10th	Day	18% 21% 38% 16% 5% 2	300
	11th to 20th	Day	23% 22% 38% 12% 5%	300
	21st to end	Day	23% 26% 36% 13% 2	330
January	1st to 10th	Night	16% 20% 36% 23% 5%	1000
	11th to 20th	Night	17% 19% 38% 22% 4	1000
	21st to end	Night	15% 20% 37% 25% 4	1100
February	1st to 10th	Night	21% 19% 38% 19% 3	1000
	11th to 20th	Night	21% 21% 35% 20% 2	890
	21st to end	Night	19% 21% 36% 21% 3	597
March	1st to 10th	Night	18% 18% 38% 23% 3	600
	11th to 20th	Night	20% 16% 40% 22% 3	600
	21st to end	Night	20% 17% 40% 20% 4	660
April	1st to 10th	Night	17% 18% 40% 21% 4	600
	11th to 20th	Night	18% 15% 37% 24% 5%	565
	21st to end	Night	19% 14% 41% 19% 6% 1	570
May	1st to 10th	Night	15% 12% 40% 27% 6%	570
	11th to 20th	Night	15% 10% 32% 30% 12% 1	561
	21st to end	Night	12% 8% 34% 29% 17% 1	617
June	1st to 10th	Night	6% 5% 23% 40% 24% 1	570
	11th to 20th	Night	5% 3 32% 42% 17% 1	570
	21st to end	Night	3 2 30% 47% 16% 1	570
July	1st to 10th	Night	5% 2 18% 47% 24% 4	555
	11th to 20th	Night	3 1 19% 46% 28% 3	552
	21st to end	Night	4 2 17% 43% 31% 3	624
August	1st to 10th	Night	4 2 21% 44% 28% 2	567
	11th to 20th	Night	6% 2 19% 44% 25% 3	564
	21st to end	Night	4 2 16% 40% 35% 3	627
September	1st to 10th	Night	9% 1 17% 46% 26%	570
	11th to 20th	Night	11% 2 21% 41% 21% 4	570
	21st to end	Night	10% 6% 23% 37% 22% 3	570
October	1st to 10th	Night	8% 5% 32% 32% 20% 2	698
	11th to 20th	Night	14% 11% 38% 28% 8%	760
	21st to end	Night	17% 14% 34% 26% 9% 1	836
November	1st to 10th	Night	14% 13% 42% 21% 10% 1	780
	11th to 20th	Night	13% 17% 36% 25% 8%	960
	21st to end	Night	18% 16% 37% 23% 7%	1000
December	1st to 10th	Night	17% 17% 34% 28% 5%	1000
	11th to 20th	Night	21% 15% 35% 24% 5%	1000
	21st to end	Night	19% 20% 33% 24% 3	1100

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