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 2024-02-25.

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	13% 6% 3 5% 4 3 4 4 5% 7% 47%	78883
	Night	12% 6% 2 4 4 2 4 4 4 7% 51%	80475
Transparency	Day	37% 6% 11% 25% 18% 4	78607
	Night	33% 5% 11% 24% 22% 5%	80271
Seeing	Day	15% 12% 29% 26% 15% 4	14443
	Night	14% 13% 32% 29% 11% 1	24499

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% 12% 4 5% 4 3 3 3 4 5% 45%	2847
	11th to 20th	Day	9% 7% 2 6% 6% 3 5% 4 4 5% 49%	2850
	21st to end	Day	22% 12% 2 4 3 1 2 2 4 5% 44%	3135
February	1st to 10th	Day	16% 10% 4 6% 5% 2 3 3 5% 6% 41%	1710
	11th to 20th	Day	15% 6% 3 6% 5% 2 3 3 4 6% 46%	1762
	21st to end	Day	16% 8% 3 5% 4 3 4 3 4 5% 45%	1574
March	1st to 10th	Day	14% 9% 3 5% 4 2 3 4 4 6% 47%	1980
	11th to 20th	Day	20% 7% 2 4 4 2 4 3 4 7% 42%	1980
	21st to end	Day	20% 9% 3 7% 5% 2 4 4 4 6% 35%	2178
April	1st to 10th	Day	12% 6% 3 5% 5% 3 4 4 6% 7% 46%	2011
	11th to 20th	Day	16% 7% 2 5% 4 2 4 5% 5% 6% 43%	2134
	21st to end	Day	20% 7% 3 6% 4 2 4 4 4 6% 40%	2277
May	1st to 10th	Day	20% 6% 3 4 5% 3 5% 4 4 6% 41%	2340
	11th to 20th	Day	14% 7% 2 6% 5% 4 5% 5% 5% 7% 41%	2302
	21st to end	Day	13% 7% 3 6% 6% 4 5% 5% 7% 9% 35%	2531
June	1st to 10th	Day	7% 4 2 5% 5% 3 5% 5% 6% 9% 49%	2340
	11th to 20th	Day	8% 4 2 4 5% 3 6% 6% 6% 7% 49%	2340
	21st to end	Day	6% 2 1 3 4 2 5% 5% 6% 8% 58%	2340
July	1st to 10th	Day	8% 4 2 4 5% 4 6% 6% 8% 10% 43%	2340
	11th to 20th	Day	4 3 1 4 5% 2 4 5% 6% 10% 57%	2340
	21st to end	Day	7% 4 2 4 5% 3 6% 5% 7% 10% 47%	2546
August	1st to 10th	Day	6% 3 2 4 5% 4 5% 5% 7% 10% 49%	2328
	11th to 20th	Day	9% 4 2 4 5% 4 4 5% 6% 9% 47%	2301
	21st to end	Day	5% 4 2 4 5% 3 4 5% 6% 8% 54%	2430
September	1st to 10th	Day	11% 4 2 5% 4 3 4 4 4 6% 53%	2068
	11th to 20th	Day	12% 5% 2 4 4 4 4 5% 6% 8% 46%	1980
	21st to end	Day	9% 3 1 4 4 2 4 3 5% 6% 57%	1980
October	1st to 10th	Day	10% 4 1 3 3 2 4 4 5% 7% 55%	1841
	11th to 20th	Day	12% 5% 2 4 3 2 2 3 4 5% 59%	1800
	21st to end	Day	19% 5% 2 3 3 2 2 3 3 5% 52%	1980
November	1st to 10th	Day	11% 3 2 4 3 2 4 4 3 6% 59%	1858
	11th to 20th	Day	12% 5% 2 4 3 2 3 3 3 6% 57%	1744
	21st to end	Day	16% 7% 2 3 3 1 3 3 3 7% 52%	1710
December	1st to 10th	Day	14% 7% 3 5% 4 3 3 3 4 6% 48%	1710
	11th to 20th	Day	15% 8% 3 6% 4 3 3 3 4 6% 44%	2161
	21st to end	Day	17% 15% 7% 6% 5% 3 4 3 4 5% 31%	3135
January	1st to 10th	Night	11% 9% 4 5% 5% 2 3 4 4 6% 47%	4108
	11th to 20th	Night	9% 6% 2 6% 5% 3 5% 6% 4 5% 49%	4120
	21st to end	Night	21% 8% 3 4 3 2 2 3 3 4 45%	4532
February	1st to 10th	Night	16% 9% 2 6% 5% 3 3 3 3 6% 45%	2470
	11th to 20th	Night	14% 7% 3 6% 4 2 3 3 4 6% 50%	2319
	21st to end	Night	16% 7% 2 4 3 2 4 4 3 3 50%	1774
March	1st to 10th	Night	12% 6% 3 5% 4 2 4 4 4 6% 49%	1980
	11th to 20th	Night	18% 5% 2 4 3 3 3 3 4 5% 49%	1980
	21st to end	Night	17% 6% 3 5% 4 2 3 4 4 7% 46%	2178
April	1st to 10th	Night	12% 5% 2 4 3 1 3 5% 5% 6% 54%	1948
	11th to 20th	Night	15% 6% 2 4 4 2 3 4 5% 7% 49%	1775
	21st to end	Night	16% 7% 3 5% 4 2 3 3 4 6% 47%	1679
May	1st to 10th	Night	15% 4 2 5% 5% 2 5% 4 6% 6% 45%	1620
	11th to 20th	Night	14% 6% 2 4 3 3 4 4 5% 7% 49%	1593
	21st to end	Night	11% 4 3 5% 5% 3 5% 4 8% 11% 42%	1745
June	1st to 10th	Night	6% 3 2 4 3 3 5% 5% 5% 8% 56%	1620
	11th to 20th	Night	5% 2 2 3 3 3 4 5% 6% 12% 56%	1620
	21st to end	Night	3 2 2 2 3 2 4 3 5% 8% 65%	1620
July	1st to 10th	Night	6% 3 1 4 4 2 4 5% 8% 12% 51%	1620
	11th to 20th	Night	2 2 1 4 3 2 4 4 6% 11% 62%	1620
	21st to end	Night	5% 3 2 4 4 3 5% 3 6% 11% 54%	1772
August	1st to 10th	Night	4 3 1 3 3 2 4 5% 7% 12% 56%	1611
	11th to 20th	Night	7% 3 2 4 4 3 6% 6% 6% 7% 53%	1600
	21st to end	Night	3 3 2 3 4 3 5% 5% 5% 8% 60%	1926
September	1st to 10th	Night	8% 4 2 4 4 2 3 3 4 7% 59%	1888
	11th to 20th	Night	10% 5% 2 4 4 2 4 4 5% 7% 52%	1980
	21st to end	Night	8% 4 1 3 4 2 3 3 4 6% 61%	1980
October	1st to 10th	Night	8% 3 1 4 4 2 3 4 4 7% 61%	2101
	11th to 20th	Night	12% 4 1 3 3 2 3 3 3 5% 59%	2160
	21st to end	Night	15% 5% 2 4 3 2 3 3 3 5% 55%	2376
November	1st to 10th	Night	11% 4 2 3 2 2 2 3 3 4 64%	2220
	11th to 20th	Night	10% 5% 2 4 3 2 3 3 3 6% 59%	2432
	21st to end	Night	15% 6% 2 3 3 2 3 3 4 6% 54%	2470
December	1st to 10th	Night	13% 6% 3 5% 4 2 3 4 4 7% 50%	2470
	11th to 20th	Night	13% 7% 3 5% 5% 3 4 4 3 6% 48%	3036
	21st to end	Night	17% 10% 5% 6% 4 3 3 3 4 5% 39%	4532

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	43% 3 8% 25% 19% 2	2848
	11th to 20th	Day	33% 6% 16% 28% 13% 4	2850
	21st to end	Day	46% 3 9% 21% 20% 2	3135
February	1st to 10th	Day	45% 5% 10% 23% 14% 3	1710
	11th to 20th	Day	41% 4 9% 26% 17% 3	1771
	21st to end	Day	43% 3 7% 22% 16% 9%	1574
March	1st to 10th	Day	39% 4 7% 22% 19% 9%	1980
	11th to 20th	Day	44% 5% 10% 22% 15% 4	1980
	21st to end	Day	50% 5% 10% 21% 12% 2	2178
April	1st to 10th	Day	37% 6% 12% 23% 18% 4	2011
	11th to 20th	Day	41% 5% 13% 22% 15% 4	2134
	21st to end	Day	46% 5% 9% 23% 16% 1	2277
May	1st to 10th	Day	45% 5% 10% 21% 17% 1	2340
	11th to 20th	Day	42% 6% 14% 24% 13% 1	2306
	21st to end	Day	43% 8% 15% 22% 10% 1	2531
June	1st to 10th	Day	32% 8% 15% 27% 16% 2	2340
	11th to 20th	Day	33% 7% 14% 30% 15% 2	2340
	21st to end	Day	24% 6% 15% 30% 21% 5%	2340
July	1st to 10th	Day	33% 9% 16% 26% 14% 2	2276
	11th to 20th	Day	22% 8% 14% 33% 21% 3	2271
	21st to end	Day	30% 7% 16% 31% 16% 1	2405
August	1st to 10th	Day	27% 8% 16% 28% 17% 3	2314
	11th to 20th	Day	32% 8% 13% 25% 20% 3	2301
	21st to end	Day	27% 7% 13% 28% 19% 6%	2430
September	1st to 10th	Day	32% 5% 10% 24% 21% 8%	2068
	11th to 20th	Day	34% 7% 13% 20% 22% 5%	1980
	21st to end	Day	27% 5% 11% 24% 27% 6%	1980
October	1st to 10th	Day	29% 5% 13% 23% 25% 5%	1841
	11th to 20th	Day	27% 4 7% 24% 30% 8%	1800
	21st to end	Day	36% 3 8% 26% 18% 8%	1980
November	1st to 10th	Day	28% 5% 8% 28% 25% 7%	1858
	11th to 20th	Day	31% 3 8% 27% 24% 6%	1744
	21st to end	Day	36% 3 9% 25% 24% 4	1710
December	1st to 10th	Day	38% 5% 8% 28% 19% 2	1710
	11th to 20th	Day	41% 4 7% 22% 22% 3	2159
	21st to end	Day	55% 6% 8% 19% 10% 2	3135
January	1st to 10th	Night	38% 5% 9% 26% 19% 3	4108
	11th to 20th	Night	32% 8% 15% 21% 22% 2	4120
	21st to end	Night	43% 4 10% 19% 20% 3	4532
February	1st to 10th	Night	42% 4 12% 22% 17% 4	2470
	11th to 20th	Night	38% 3 8% 27% 19% 5%	2320
	21st to end	Night	39% 4 7% 20% 21% 9%	1774
March	1st to 10th	Night	38% 3 9% 19% 20% 10%	1980
	11th to 20th	Night	38% 4 12% 22% 19% 5%	1980
	21st to end	Night	39% 5% 11% 26% 16% 3	2178
April	1st to 10th	Night	32% 6% 10% 23% 22% 8%	1948
	11th to 20th	Night	35% 5% 11% 23% 19% 7%	1775
	21st to end	Night	40% 4 9% 24% 21% 3	1679
May	1st to 10th	Night	39% 7% 11% 21% 19% 3	1620
	11th to 20th	Night	34% 6% 14% 25% 18% 2	1594
	21st to end	Night	35% 7% 18% 25% 13% 1	1746
June	1st to 10th	Night	24% 9% 15% 29% 19% 3	1620
	11th to 20th	Night	21% 7% 16% 32% 21% 3	1620
	21st to end	Night	18% 5% 13% 31% 26% 7%	1620
July	1st to 10th	Night	24% 7% 15% 31% 18% 3	1575
	11th to 20th	Night	17% 6% 13% 30% 28% 5%	1566
	21st to end	Night	24% 6% 13% 34% 22% 2	1673
August	1st to 10th	Night	19% 7% 17% 35% 20% 2	1601
	11th to 20th	Night	27% 8% 11% 21% 28% 5%	1600
	21st to end	Night	22% 6% 13% 28% 22% 8%	1926
September	1st to 10th	Night	26% 5% 9% 27% 23% 9%	1888
	11th to 20th	Night	31% 6% 10% 21% 25% 7%	1980
	21st to end	Night	25% 4 8% 26% 30% 7%	1980
October	1st to 10th	Night	25% 4 11% 24% 28% 9%	2101
	11th to 20th	Night	29% 4 6% 21% 30% 10%	2160
	21st to end	Night	33% 4 8% 22% 25% 9%	2376
November	1st to 10th	Night	26% 3 8% 25% 29% 8%	2220
	11th to 20th	Night	29% 4 7% 26% 26% 8%	2432
	21st to end	Night	34% 3 8% 22% 28% 6%	2470
December	1st to 10th	Night	35% 5% 11% 26% 19% 4	2470
	11th to 20th	Night	39% 4 7% 21% 24% 4	3037
	21st to end	Night	47% 5% 9% 22% 14% 3	4532

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% 34% 19% 3	270
	11th to 20th	Day	26% 27% 34% 11% 3	270
	21st to end	Day	22% 25% 33% 14% 5%	297
February	1st to 10th	Day	26% 20% 38% 13% 3	270
	11th to 20th	Day	23% 23% 38% 14% 1	270
	21st to end	Day	30% 24% 36% 9%	210
March	1st to 10th	Day	25% 25% 33% 15% 3	240
	11th to 20th	Day	26% 21% 34% 16% 3	240
	21st to end	Day	27% 29% 30% 13% 2	264
April	1st to 10th	Day	14% 28% 38% 15% 4	271
	11th to 20th	Day	21% 25% 33% 16% 4	412
	21st to end	Day	23% 19% 36% 14% 7% 1	537
May	1st to 10th	Day	19% 18% 37% 20% 5%	603
	11th to 20th	Day	19% 13% 33% 24% 9% 3	625
	21st to end	Day	16% 12% 32% 21% 13% 6%	679
June	1st to 10th	Day	7% 6% 23% 36% 23% 5%	630
	11th to 20th	Day	8% 5% 30% 37% 16% 3	630
	21st to end	Day	5% 3 33% 37% 18% 4	630
July	1st to 10th	Day	8% 3 17% 35% 28% 10%	621
	11th to 20th	Day	3 2 19% 43% 26% 6%	618
	21st to end	Day	8% 1 12% 38% 30% 10%	684
August	1st to 10th	Day	8% 3 18% 41% 25% 6%	626
	11th to 20th	Day	10% 3 18% 35% 29% 6%	615
	21st to end	Day	7% 1 15% 36% 32% 9%	549
September	1st to 10th	Day	10% 2 15% 35% 30% 8%	358
	11th to 20th	Day	6% 3 24% 37% 26% 5%	270
	21st to end	Day	13% 6% 23% 34% 18% 5%	270
October	1st to 10th	Day	12% 9% 36% 24% 14% 4	270
	11th to 20th	Day	11% 19% 43% 21% 6%	270
	21st to end	Day	27% 16% 33% 16% 6% 2	297
November	1st to 10th	Day	14% 15% 41% 22% 7% 1	270
	11th to 20th	Day	16% 19% 39% 16% 7% 2	270
	21st to end	Day	23% 19% 38% 17% 3	270
December	1st to 10th	Day	18% 21% 37% 17% 5% 2	270
	11th to 20th	Day	24% 20% 38% 12% 6%	270
	21st to end	Day	20% 27% 38% 13% 2	297
January	1st to 10th	Night	16% 20% 35% 23% 5%	950
	11th to 20th	Night	18% 20% 37% 22% 4	950
	21st to end	Night	15% 20% 36% 25% 4	1045
February	1st to 10th	Night	21% 19% 37% 20% 3	950
	11th to 20th	Night	20% 20% 36% 21% 2	845
	21st to end	Night	21% 21% 36% 19% 3	554
March	1st to 10th	Night	17% 17% 39% 24% 4	540
	11th to 20th	Night	20% 16% 39% 22% 3	540
	21st to end	Night	20% 17% 39% 20% 4	594
April	1st to 10th	Night	16% 18% 41% 21% 4	540
	11th to 20th	Night	18% 15% 36% 25% 6%	533
	21st to end	Night	19% 14% 41% 19% 7% 1	540
May	1st to 10th	Night	15% 12% 40% 28% 6%	540
	11th to 20th	Night	15% 10% 33% 29% 12% 1	531
	21st to end	Night	12% 7% 35% 28% 17% 1	584
June	1st to 10th	Night	6% 4 22% 40% 26% 1	540
	11th to 20th	Night	5% 3 33% 41% 17% 1	540
	21st to end	Night	3 2 30% 46% 17% 1	540
July	1st to 10th	Night	5% 2 18% 47% 25% 4	525
	11th to 20th	Night	2 1 18% 45% 29% 4	522
	21st to end	Night	5% 2 16% 44% 31% 3	591
August	1st to 10th	Night	4 2 20% 45% 27% 2	537
	11th to 20th	Night	6% 2 19% 44% 26% 3	534
	21st to end	Night	4 2 16% 39% 35% 4	594
September	1st to 10th	Night	10% 1 17% 47% 24%	540
	11th to 20th	Night	11% 2 21% 40% 22% 4	540
	21st to end	Night	9% 6% 23% 36% 22% 4	540
October	1st to 10th	Night	8% 5% 33% 31% 20% 2	661
	11th to 20th	Night	14% 11% 38% 27% 8%	720
	21st to end	Night	18% 14% 33% 26% 9% 1	792
November	1st to 10th	Night	13% 13% 43% 21% 10% 1	740
	11th to 20th	Night	13% 17% 37% 26% 7%	912
	21st to end	Night	18% 17% 35% 23% 7%	950
December	1st to 10th	Night	17% 17% 33% 28% 5%	950
	11th to 20th	Night	20% 16% 35% 25% 4	950
	21st to end	Night	19% 21% 34% 24% 3	1045

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