W & B Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the W & B Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	3 4 2 5% 4 4 4 4 6% 7% 57%	145442
	Night	2 4 2 4 3 3 4 3 5% 8% 60%	145060
Transparency	Day	22% 9% 8% 16% 27% 19%	140531
	Night	18% 8% 8% 17% 28% 21%	140979
Seeing	Day	12% 21% 28% 23% 13% 3	17023
	Night	8% 17% 26% 29% 17% 3	27224

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 7% 5% 11% 4 3 4 4 4 5% 52%	5318
	11th to 20th	Day	1 4 2 3 2 2 2 2 4 5% 74%	5340
	21st to end	Day	4 9% 3 5% 3 3 4 4 5% 5% 54%	5844
February	1st to 10th	Day	3 8% 2 4 3 2 3 3 4 6% 64%	2053
	11th to 20th	Day	4 8% 3 5% 3 3 3 3 4 7% 59%	2049
	21st to end	Day	2 6% 2 5% 4 3 3 2 4 5% 64%	1730
March	1st to 10th	Day	2 6% 3 5% 3 3 3 3 4 5% 63%	4853
	11th to 20th	Day	2 5% 3 5% 3 3 3 3 5% 8% 61%	5110
	21st to end	Day	1 7% 3 5% 3 4 4 3 5% 7% 56%	5818
April	1st to 10th	Day	2 5% 2 4 3 3 3 4 5% 6% 63%	2520
	11th to 20th	Day	2 5% 2 4 3 2 2 2 5% 5% 68%	2491
	21st to end	Day	2 4 1 3 3 2 2 3 4 5% 71%	2520
May	1st to 10th	Day	3 3 2 4 3 3 4 3 4 5% 65%	7057
	11th to 20th	Day	1 2 1 2 3 3 4 3 4 6% 70%	7032
	21st to end	Day	1 1 1 3 2 3 3 2 5% 6% 72%	8233
June	1st to 10th	Day	2 3 1 3 3 4 4 4 6% 8% 60%	2940
	11th to 20th	Day	2 2 1 3 2 2 3 3 6% 7% 68%	2940
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 5% 8% 8% 46%	2870
July	1st to 10th	Day	5% 4 2 7% 5% 6% 7% 6% 8% 10% 40%	6009
	11th to 20th	Day	2 3 2 6% 5% 7% 8% 7% 10% 11% 39%	6010
	21st to end	Day	6% 4 3 7% 7% 8% 9% 8% 10% 11% 27%	6516
August	1st to 10th	Day	4 3 2 6% 5% 6% 7% 6% 9% 12% 40%	5348
	11th to 20th	Day	4 5% 2 8% 6% 6% 8% 7% 9% 10% 35%	5366
	21st to end	Day	4 4 2 7% 6% 6% 8% 6% 9% 10% 37%	5984
September	1st to 10th	Day	10% 7% 2 8% 5% 5% 5% 5% 7% 7% 40%	2400
	11th to 20th	Day	7% 6% 2 6% 4 4 4 4 6% 7% 51%	2400
	21st to end	Day	4 4 1 4 3 3 4 3 5% 6% 63%	2200
October	1st to 10th	Day	3 3 2 5% 3 3 3 2 4 6% 65%	3276
	11th to 20th	Day	2 3 2 5% 2 3 3 3 5% 8% 65%	3250
	21st to end	Day	1 2 1 2 1 2 2 1 2 3 83%	3548
November	1st to 10th	Day	1 3 2 4 3 2 3 2 4 6% 69%	2059
	11th to 20th	Day	2 4 2 4 2 3 3 3 4 6% 66%	1924
	21st to end	Day	3 6% 2 4 3 3 3 3 6% 7% 58%	1890
December	1st to 10th	Day	3 5% 2 4 2 3 3 2 5% 6% 65%	3030
	11th to 20th	Day	3 5% 2 5% 3 3 3 3 4 6% 62%	3158
	21st to end	Day	2 5% 2 4 2 2 2 2 4 8% 66%	4356
January	1st to 10th	Night	2 8% 4 9% 3 4 4 3 5% 6% 52%	7605
	11th to 20th	Night	1 3 1 2 3 3 2 2 3 5% 75%	7650
	21st to end	Night	3 9% 3 4 3 4 5% 4 6% 7% 52%	8370
February	1st to 10th	Night	1 8% 2 4 3 2 3 3 5% 7% 63%	2563
	11th to 20th	Night	4 8% 2 4 2 3 4 3 5% 6% 60%	2458
	21st to end	Night	3 7% 2 4 3 3 3 3 5% 6% 61%	2076
March	1st to 10th	Night	2 7% 3 4 3 3 3 3 5% 5% 63%	5235
	11th to 20th	Night	2 7% 3 5% 3 3 3 3 4 8% 59%	5110
	21st to end	Night	1 7% 3 5% 3 3 3 2 5% 6% 61%	5387
April	1st to 10th	Night	2 4 2 4 3 2 4 4 6% 6% 65%	2100
	11th to 20th	Night	3 5% 2 3 3 3 3 3 4 7% 64%	2077
	21st to end	Night	3 4 1 3 2 2 3 2 4 6% 69%	2100
May	1st to 10th	Night	3 4 1 3 3 3 4 3 4 6% 66%	5890
	11th to 20th	Night	1 2 1 2 2 2 3 3 5% 8% 70%	5860
	21st to end	Night	1 1 1 2 2 3 3 2 4 5% 76%	5939
June	1st to 10th	Night	3 2 2 4 5% 3 6% 5% 7% 8% 56%	1680
	11th to 20th	Night	3 2 1 2 2 3 3 3 5% 8% 66%	1680
	21st to end	Night	4 5% 2 6% 6% 5% 6% 4 7% 9% 46%	1740
July	1st to 10th	Night	3 3 2 6% 6% 6% 5% 4 7% 9% 48%	4190
	11th to 20th	Night	2 2 1 5% 4 5% 6% 6% 9% 12% 48%	4190
	21st to end	Night	4 4 1 6% 5% 5% 6% 7% 12% 15% 35%	4576
August	1st to 10th	Night	2 2 1 4 3 4 5% 5% 10% 14% 50%	4464
	11th to 20th	Night	3 3 1 5% 4 5% 6% 5% 9% 15% 44%	4483
	21st to end	Night	2 3 2 6% 4 5% 6% 5% 8% 12% 47%	4993
September	1st to 10th	Night	4 5% 2 6% 5% 5% 5% 5% 7% 8% 48%	2000
	11th to 20th	Night	5% 3 1 4 4 4 5% 4 5% 8% 57%	2000
	21st to end	Night	3 3 1 3 3 3 5% 4 4 7% 65%	2198
October	1st to 10th	Night	2 2 1 3 2 2 2 3 4 6% 74%	3910
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 6% 70%	3898
	21st to end	Night	3 1 1 3 2 2 2 2 3 5% 76%	4252
November	1st to 10th	Night	3 3 1 3 2 3 2 2 5% 6% 69%	2460
	11th to 20th	Night	2 3 1 3 3 3 3 3 5% 7% 67%	2672
	21st to end	Night	3 6% 2 5% 3 3 4 3 5% 6% 59%	2730
December	1st to 10th	Night	3 4 1 3 2 3 3 3 6% 8% 65%	4380
	11th to 20th	Night	1 4 2 5% 3 3 4 3 5% 6% 62%	5251
	21st to end	Night	2 7% 2 4 3 3 2 2 5% 7% 63%	6893

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	19% 15% 4 10% 36% 15%	5321
	11th to 20th	Day	11% 7% 3 14% 35% 31%	5340
	21st to end	Day	22% 11% 3 10% 29% 26%	5844
February	1st to 10th	Day	20% 7% 4 15% 29% 25%	2053
	11th to 20th	Day	22% 8% 4 19% 26% 21%	2059
	21st to end	Day	18% 9% 2 11% 28% 32%	1730
March	1st to 10th	Day	20% 5% 2 8% 29% 36%	4853
	11th to 20th	Day	16% 8% 6% 13% 33% 23%	5110
	21st to end	Day	17% 14% 3 16% 35% 15%	5818
April	1st to 10th	Day	18% 6% 5% 17% 35% 19%	2520
	11th to 20th	Day	18% 5% 3 13% 33% 29%	2494
	21st to end	Day	15% 6% 3 16% 36% 24%	2520
May	1st to 10th	Day	17% 7% 6% 14% 34% 22%	7057
	11th to 20th	Day	12% 5% 6% 16% 35% 26%	7035
	21st to end	Day	11% 5% 6% 14% 24% 39%	8234
June	1st to 10th	Day	20% 7% 11% 20% 24% 18%	2940
	11th to 20th	Day	15% 5% 5% 17% 26% 33%	2940
	21st to end	Day	32% 9% 10% 18% 19% 12%	2870
July	1st to 10th	Day	35% 12% 16% 18% 13% 6%	5946
	11th to 20th	Day	30% 15% 19% 19% 15% 3	5941
	21st to end	Day	40% 18% 19% 15% 7% 1	6332
August	1st to 10th	Day	33% 12% 16% 28% 10% 2	3892
	11th to 20th	Day	33% 14% 14% 23% 15% 1	3882
	21st to end	Day	36% 13% 18% 24% 8% 1	4311
September	1st to 10th	Day	40% 8% 10% 17% 18% 7%	2400
	11th to 20th	Day	32% 7% 9% 20% 26% 6%	2400
	21st to end	Day	22% 6% 9% 22% 31% 11%	2200
October	1st to 10th	Day	21% 4 6% 13% 34% 23%	3276
	11th to 20th	Day	20% 4 7% 21% 30% 18%	3250
	21st to end	Day	9% 3 2 18% 38% 30%	3548
November	1st to 10th	Day	16% 5% 3 21% 35% 20%	2059
	11th to 20th	Day	18% 6% 5% 19% 29% 24%	1924
	21st to end	Day	21% 8% 6% 15% 30% 21%	1890
December	1st to 10th	Day	19% 5% 4 18% 34% 21%	3030
	11th to 20th	Day	20% 6% 4 10% 38% 22%	3156
	21st to end	Day	13% 8% 4 16% 34% 25%	4356
January	1st to 10th	Night	22% 13% 3 14% 36% 13%	7605
	11th to 20th	Night	11% 6% 2 14% 34% 33%	7650
	21st to end	Night	21% 13% 3 11% 25% 28%	8370
February	1st to 10th	Night	18% 7% 5% 16% 25% 29%	2563
	11th to 20th	Night	18% 10% 4 17% 28% 22%	2458
	21st to end	Night	18% 9% 3 12% 29% 29%	2076
March	1st to 10th	Night	15% 8% 4 10% 34% 30%	5235
	11th to 20th	Night	19% 9% 7% 11% 31% 23%	5110
	21st to end	Night	12% 15% 4 18% 36% 15%	5387
April	1st to 10th	Night	16% 8% 5% 16% 36% 19%	2100
	11th to 20th	Night	15% 9% 6% 14% 31% 26%	2077
	21st to end	Night	15% 6% 4 19% 35% 21%	2100
May	1st to 10th	Night	17% 5% 7% 16% 32% 22%	5888
	11th to 20th	Night	10% 5% 7% 17% 37% 23%	5861
	21st to end	Night	12% 3 5% 13% 24% 43%	5941
June	1st to 10th	Night	23% 7% 9% 19% 22% 20%	1680
	11th to 20th	Night	16% 4 5% 17% 23% 35%	1680
	21st to end	Night	32% 7% 11% 19% 16% 15%	1740
July	1st to 10th	Night	30% 9% 17% 26% 14% 4	4145
	11th to 20th	Night	23% 10% 25% 25% 14% 3	4136
	21st to end	Night	29% 13% 27% 20% 9% 1	4448
August	1st to 10th	Night	22% 8% 19% 33% 15% 2	3244
	11th to 20th	Night	20% 9% 15% 37% 16% 1	3243
	21st to end	Night	27% 12% 22% 27% 12% 1	3597
September	1st to 10th	Night	30% 10% 12% 18% 20% 10%	2000
	11th to 20th	Night	25% 9% 10% 20% 26% 10%	2000
	21st to end	Night	19% 7% 9% 22% 33% 10%	2198
October	1st to 10th	Night	14% 4 5% 14% 35% 27%	3910
	11th to 20th	Night	14% 6% 7% 20% 34% 18%	3898
	21st to end	Night	10% 5% 2 18% 33% 32%	4252
November	1st to 10th	Night	15% 5% 4 19% 34% 23%	2460
	11th to 20th	Night	14% 7% 5% 18% 32% 24%	2672
	21st to end	Night	22% 7% 5% 17% 30% 19%	2730
December	1st to 10th	Night	18% 6% 4 16% 37% 18%	4380
	11th to 20th	Night	17% 7% 6% 11% 30% 29%	5252
	21st to end	Night	16% 8% 4 16% 33% 24%	6893

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	22% 31% 23% 20% 3	330
	11th to 20th	Day	19% 28% 26% 21% 6% 1	330
	21st to end	Day	15% 33% 28% 15% 9%	363
February	1st to 10th	Day	19% 29% 25% 19% 7% 1	330
	11th to 20th	Day	25% 33% 26% 13% 3	330
	21st to end	Day	25% 37% 24% 11% 4	273
March	1st to 10th	Day	23% 34% 29% 10% 3	440
	11th to 20th	Day	26% 29% 27% 12% 5% 1	540
	21st to end	Day	18% 39% 30% 10% 3 1	594
April	1st to 10th	Day	14% 38% 33% 12% 2 1	540
	11th to 20th	Day	19% 38% 31% 10% 2 1	532
	21st to end	Day	19% 36% 31% 11% 3	540
May	1st to 10th	Day	11% 34% 41% 11% 3	543
	11th to 20th	Day	10% 26% 39% 19% 5% 1	568
	21st to end	Day	6% 26% 42% 19% 7% 1	701
June	1st to 10th	Day	5% 11% 32% 33% 16% 2	780
	11th to 20th	Day	4 11% 35% 32% 15% 3	780
	21st to end	Day	5% 8% 28% 32% 23% 4	710
July	1st to 10th	Day	8% 5% 23% 34% 25% 6%	566
	11th to 20th	Day	7% 3 13% 36% 34% 8%	564
	21st to end	Day	6% 1 11% 32% 39% 11%	616
August	1st to 10th	Day	7% 2 13% 35% 33% 9%	527
	11th to 20th	Day	8% 5% 17% 36% 26% 8%	517
	21st to end	Day	8% 4 15% 34% 32% 8%	583
September	1st to 10th	Day	11% 5% 23% 36% 19% 5%	530
	11th to 20th	Day	7% 7% 32% 33% 17% 5%	530
	21st to end	Day	5% 11% 37% 28% 13% 6%	330
October	1st to 10th	Day	11% 17% 28% 24% 17% 4	330
	11th to 20th	Day	8% 21% 33% 24% 11% 4	330
	21st to end	Day	11% 25% 27% 20% 13% 4	363
November	1st to 10th	Day	10% 33% 30% 16% 7% 3	330
	11th to 20th	Day	12% 21% 29% 23% 11% 3	330
	21st to end	Day	17% 31% 26% 18% 7% 1	330
December	1st to 10th	Day	19% 31% 23% 20% 5% 1	330
	11th to 20th	Day	13% 32% 27% 20% 7%	330
	21st to end	Day	21% 34% 25% 15% 5% 1	363
January	1st to 10th	Night	10% 21% 27% 27% 12% 3	840
	11th to 20th	Night	10% 20% 29% 26% 12% 3	840
	21st to end	Night	11% 23% 27% 25% 13% 1	924
February	1st to 10th	Night	9% 22% 30% 25% 12% 2	840
	11th to 20th	Night	16% 22% 34% 21% 6% 1	824
	21st to end	Night	12% 32% 32% 19% 5% 1	692
March	1st to 10th	Night	14% 28% 26% 24% 8% 1	840
	11th to 20th	Night	14% 21% 26% 27% 11%	840
	21st to end	Night	10% 27% 30% 24% 8% 1	830
April	1st to 10th	Night	9% 31% 33% 22% 6%	630
	11th to 20th	Night	12% 28% 32% 21% 6%	623
	21st to end	Night	11% 27% 31% 25% 6%	630
May	1st to 10th	Night	6% 23% 38% 27% 6%	630
	11th to 20th	Night	7% 20% 27% 32% 13% 1	621
	21st to end	Night	4 16% 37% 34% 9% 1	683
June	1st to 10th	Night	2 10% 29% 42% 16%	630
	11th to 20th	Night	4 7% 34% 38% 16% 2	630
	21st to end	Night	6% 6% 21% 40% 23% 3	630
July	1st to 10th	Night	5% 3 20% 40% 29% 3	615
	11th to 20th	Night	3 3 14% 35% 38% 7%	612
	21st to end	Night	5% 2 13% 33% 35% 11%	687
August	1st to 10th	Night	4 1 11% 28% 43% 13%	597
	11th to 20th	Night	5% 3 13% 29% 40% 10%	594
	21st to end	Night	3 3 14% 32% 40% 7%	660
September	1st to 10th	Night	5% 5% 16% 34% 34% 6%	600
	11th to 20th	Night	7% 3 19% 38% 30% 4	600
	21st to end	Night	4 7% 22% 37% 26% 4	600
October	1st to 10th	Night	5% 10% 28% 35% 20% 3	790
	11th to 20th	Night	5% 12% 29% 33% 18% 3	800
	21st to end	Night	7% 15% 24% 27% 20% 7%	880
November	1st to 10th	Night	7% 17% 23% 29% 18% 5%	820
	11th to 20th	Night	7% 14% 24% 33% 18% 4	992
	21st to end	Night	11% 21% 24% 25% 16% 3	1050
December	1st to 10th	Night	12% 21% 23% 29% 13% 2	1050
	11th to 20th	Night	8% 25% 29% 27% 10% 1	1050
	21st to end	Night	15% 20% 26% 27% 10% 1	1050

Page generated at 2026-07-28 01:25:20. Version ac07

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