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 2025-03-27.

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 5% 2 5% 3 4 4 4 6% 7% 57%	112644
	Night	3 5% 2 4 3 3 4 3 5% 7% 61%	115518
Transparency	Day	22% 9% 7% 17% 27% 18%	112409
	Night	19% 9% 7% 17% 28% 20%	115343
Seeing	Day	11% 21% 27% 23% 14% 3	15164
	Night	8% 17% 25% 29% 17% 3	25532

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 8% 6% 12% 3 3 3 3 4 4 53%	4098
	11th to 20th	Day	1 5% 3 4 2 2 2 2 4 5% 71%	4100
	21st to end	Day	3 10% 4 5% 2 3 4 3 5% 5% 55%	4510
February	1st to 10th	Day	3 8% 2 4 3 2 3 3 4 6% 63%	1955
	11th to 20th	Day	4 8% 3 5% 3 2 3 3 4 7% 60%	1949
	21st to end	Day	2 7% 2 5% 4 3 3 2 4 5% 63%	1650
March	1st to 10th	Day	2 8% 3 6% 4 3 3 3 4 5% 60%	3413
	11th to 20th	Day	3 6% 2 6% 4 3 3 3 4 9% 58%	3600
	21st to end	Day	1 9% 4 5% 4 4 4 4 6% 8% 52%	4034
April	1st to 10th	Day	2 5% 2 3 3 3 3 4 5% 6% 65%	2280
	11th to 20th	Day	2 5% 2 4 3 2 2 2 5% 5% 68%	2251
	21st to end	Day	2 4 1 4 3 2 2 3 4 5% 70%	2280
May	1st to 10th	Day	2 2 1 2 2 2 2 2 2 3 78%	3800
	11th to 20th	Day	2 3 1 3 3 4 4 3 5% 7% 66%	3765
	21st to end	Day	1 2 1 2 1 2 2 2 4 5% 79%	4410
June	1st to 10th	Day	2 3 1 3 3 4 4 4 6% 8% 60%	2660
	11th to 20th	Day	3 3 1 3 2 2 3 3 6% 7% 68%	2660
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 6% 9% 8% 45%	2597
July	1st to 10th	Day	5% 4 1 6% 5% 6% 6% 6% 9% 11% 40%	4110
	11th to 20th	Day	3 3 2 6% 6% 7% 7% 7% 9% 10% 40%	4110
	21st to end	Day	9% 5% 3 7% 6% 8% 8% 8% 11% 11% 26%	4438
August	1st to 10th	Day	5% 4 2 6% 5% 6% 7% 6% 10% 12% 37%	3784
	11th to 20th	Day	5% 4 2 6% 5% 6% 8% 7% 9% 9% 40%	3726
	21st to end	Day	5% 5% 2 7% 6% 7% 8% 6% 10% 10% 34%	4180
September	1st to 10th	Day	9% 7% 2 8% 5% 5% 5% 4 7% 7% 40%	2280
	11th to 20th	Day	7% 5% 2 6% 4 4 4 4 6% 6% 52%	2280
	21st to end	Day	4 4 1 4 2 3 4 3 5% 6% 63%	2090
October	1st to 10th	Day	3 3 2 5% 3 3 3 2 5% 6% 66%	3176
	11th to 20th	Day	2 3 1 5% 2 3 3 3 5% 8% 65%	3150
	21st to end	Day	1 2 1 2 1 2 2 1 2 3 83%	3438
November	1st to 10th	Day	1 3 2 4 3 2 3 3 4 7% 68%	1959
	11th to 20th	Day	2 4 2 4 2 3 3 2 4 6% 66%	1833
	21st to end	Day	4 6% 2 4 3 3 4 3 6% 7% 57%	1800
December	1st to 10th	Day	3 5% 2 4 2 3 3 2 5% 6% 64%	2940
	11th to 20th	Day	3 5% 2 5% 3 3 3 3 4 6% 62%	3076
	21st to end	Day	2 5% 2 4 2 2 2 2 4 8% 66%	4262
January	1st to 10th	Night	1 7% 4 9% 4 4 4 3 4 5% 55%	5867
	11th to 20th	Night	1 3 1 3 4 3 3 2 3 5% 72%	5880
	21st to end	Night	3 9% 3 5% 3 4 5% 4 5% 6% 54%	6468
February	1st to 10th	Night	1 8% 2 4 3 2 3 3 5% 7% 62%	2441
	11th to 20th	Night	4 8% 2 3 2 2 4 3 5% 7% 60%	2338
	21st to end	Night	3 7% 2 4 3 3 4 3 6% 5% 61%	1980
March	1st to 10th	Night	2 8% 2 4 4 3 3 3 4 5% 62%	3672
	11th to 20th	Night	3 9% 2 5% 4 4 4 3 5% 8% 53%	3600
	21st to end	Night	2 9% 3 5% 4 4 3 3 5% 7% 54%	3750
April	1st to 10th	Night	2 4 1 3 3 2 4 4 6% 6% 65%	1900
	11th to 20th	Night	3 5% 2 3 3 3 3 3 4 7% 64%	1877
	21st to end	Night	3 5% 1 3 3 2 3 2 4 6% 68%	1900
May	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 79%	3170
	11th to 20th	Night	2 4 1 3 3 2 3 3 4 6% 69%	3140
	21st to end	Night	1 1 1 1 2 2 2 2 3 4 81%	3155
June	1st to 10th	Night	3 3 2 5% 5% 3 6% 4 7% 8% 55%	1520
	11th to 20th	Night	3 2 1 2 2 3 3 3 5% 8% 68%	1520
	21st to end	Night	4 5% 2 7% 6% 5% 6% 4 7% 8% 46%	1574
July	1st to 10th	Night	4 3 2 6% 5% 6% 6% 5% 9% 11% 43%	2860
	11th to 20th	Night	3 2 1 5% 4 5% 6% 5% 7% 10% 51%	2860
	21st to end	Night	6% 5% 1 6% 5% 5% 6% 7% 11% 14% 35%	3123
August	1st to 10th	Night	3 3 1 4 4 4 5% 5% 10% 14% 47%	3155
	11th to 20th	Night	4 2 1 4 3 4 5% 4 7% 12% 55%	3113
	21st to end	Night	2 3 2 7% 5% 5% 7% 6% 9% 13% 41%	3487
September	1st to 10th	Night	4 5% 2 6% 5% 5% 5% 5% 7% 8% 48%	1900
	11th to 20th	Night	5% 3 1 4 4 4 5% 4 5% 8% 57%	1900
	21st to end	Night	3 3 1 3 3 2 5% 3 5% 7% 66%	2088
October	1st to 10th	Night	2 2 1 3 2 2 2 3 4 6% 74%	3790
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 6% 70%	3778
	21st to end	Night	3 1 1 3 2 2 2 2 3 5% 76%	4120
November	1st to 10th	Night	3 4 1 4 2 3 2 3 5% 7% 68%	2340
	11th to 20th	Night	2 3 1 2 3 3 3 3 5% 6% 68%	2545
	21st to end	Night	4 6% 2 5% 4 3 4 3 5% 6% 58%	2600
December	1st to 10th	Night	3 4 1 3 2 3 3 3 6% 8% 64%	4250
	11th to 20th	Night	1 4 2 5% 3 4 4 3 5% 6% 62%	5113
	21st to end	Night	2 7% 2 4 3 3 3 2 5% 7% 63%	6744

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	20% 17% 3 10% 33% 17%	4099
	11th to 20th	Day	13% 8% 4 15% 38% 22%	4100
	21st to end	Day	20% 13% 3 11% 28% 25%	4510
February	1st to 10th	Day	20% 7% 4 16% 28% 25%	1955
	11th to 20th	Day	22% 8% 4 19% 26% 22%	1959
	21st to end	Day	19% 9% 2 10% 27% 33%	1650
March	1st to 10th	Day	25% 6% 3 10% 32% 24%	3413
	11th to 20th	Day	22% 9% 9% 18% 30% 13%	3600
	21st to end	Day	18% 18% 3 17% 31% 12%	4034
April	1st to 10th	Day	17% 6% 4 16% 35% 21%	2280
	11th to 20th	Day	18% 5% 3 12% 32% 30%	2254
	21st to end	Day	16% 6% 3 17% 36% 22%	2280
May	1st to 10th	Day	11% 5% 2 9% 38% 35%	3800
	11th to 20th	Day	17% 7% 6% 14% 35% 21%	3768
	21st to end	Day	10% 3 3 12% 22% 50%	4410
June	1st to 10th	Day	20% 7% 11% 20% 24% 18%	2660
	11th to 20th	Day	16% 4 5% 17% 25% 33%	2660
	21st to end	Day	32% 10% 10% 18% 19% 12%	2597
July	1st to 10th	Day	34% 12% 19% 14% 13% 7%	4046
	11th to 20th	Day	32% 12% 14% 21% 18% 3	4041
	21st to end	Day	39% 16% 19% 18% 7%	4298
August	1st to 10th	Day	33% 12% 16% 29% 10% 1	3772
	11th to 20th	Day	33% 14% 14% 24% 15% 1	3762
	21st to end	Day	37% 13% 18% 24% 7% 1	4179
September	1st to 10th	Day	41% 8% 9% 16% 18% 8%	2280
	11th to 20th	Day	32% 7% 9% 20% 26% 6%	2280
	21st to end	Day	22% 6% 9% 21% 31% 11%	2090
October	1st to 10th	Day	20% 4 6% 13% 33% 24%	3176
	11th to 20th	Day	19% 4 7% 22% 30% 18%	3150
	21st to end	Day	9% 3 2 18% 38% 30%	3438
November	1st to 10th	Day	17% 5% 4 22% 33% 20%	1959
	11th to 20th	Day	17% 6% 5% 18% 28% 25%	1833
	21st to end	Day	22% 8% 6% 15% 29% 21%	1800
December	1st to 10th	Day	19% 6% 4 18% 33% 20%	2940
	11th to 20th	Day	20% 7% 4 10% 38% 21%	3074
	21st to end	Day	13% 8% 4 16% 34% 25%	4262
January	1st to 10th	Night	22% 13% 2 11% 37% 15%	5867
	11th to 20th	Night	13% 7% 3 16% 35% 26%	5880
	21st to end	Night	20% 14% 2 11% 26% 26%	6468
February	1st to 10th	Night	18% 8% 5% 16% 24% 28%	2441
	11th to 20th	Night	19% 10% 4 16% 28% 23%	2338
	21st to end	Night	18% 10% 4 11% 27% 31%	1980
March	1st to 10th	Night	18% 10% 3 13% 36% 20%	3672
	11th to 20th	Night	26% 10% 9% 14% 28% 13%	3600
	21st to end	Night	13% 21% 4 19% 30% 13%	3750
April	1st to 10th	Night	16% 7% 5% 15% 37% 20%	1900
	11th to 20th	Night	16% 9% 5% 14% 30% 26%	1877
	21st to end	Night	16% 6% 4 19% 36% 18%	1900
May	1st to 10th	Night	10% 4 3 8% 39% 37%	3170
	11th to 20th	Night	14% 8% 6% 15% 36% 21%	3141
	21st to end	Night	9% 3 4 11% 21% 52%	3156
June	1st to 10th	Night	24% 8% 8% 19% 21% 20%	1520
	11th to 20th	Night	16% 4 4 17% 22% 37%	1520
	21st to end	Night	34% 7% 11% 18% 15% 15%	1574
July	1st to 10th	Night	30% 9% 21% 19% 16% 6%	2815
	11th to 20th	Night	24% 10% 15% 29% 17% 5%	2806
	21st to end	Night	32% 10% 26% 22% 9% 1	3025
August	1st to 10th	Night	22% 8% 19% 33% 15% 2	3144
	11th to 20th	Night	20% 9% 15% 38% 17% 1	3143
	21st to end	Night	27% 12% 22% 27% 12% 1	3487
September	1st to 10th	Night	30% 10% 12% 17% 20% 10%	1900
	11th to 20th	Night	25% 9% 9% 20% 26% 11%	1900
	21st to end	Night	19% 7% 9% 22% 33% 11%	2088
October	1st to 10th	Night	13% 5% 5% 15% 35% 27%	3790
	11th to 20th	Night	14% 6% 7% 20% 35% 18%	3778
	21st to end	Night	10% 5% 2 19% 33% 32%	4120
November	1st to 10th	Night	16% 6% 4 19% 33% 22%	2340
	11th to 20th	Night	14% 7% 5% 17% 31% 25%	2545
	21st to end	Night	23% 7% 5% 17% 28% 20%	2600
December	1st to 10th	Night	18% 6% 4 16% 37% 18%	4250
	11th to 20th	Night	18% 7% 6% 11% 30% 28%	5114
	21st to end	Night	16% 8% 3 16% 33% 24%	6744

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	19% 33% 24% 22% 3	300
	11th to 20th	Day	19% 28% 25% 21% 5% 1	300
	21st to end	Day	15% 32% 29% 15% 9%	330
February	1st to 10th	Day	20% 29% 25% 19% 6% 1	300
	11th to 20th	Day	22% 34% 28% 13% 3	300
	21st to end	Day	26% 37% 22% 11% 4	249
March	1st to 10th	Day	22% 35% 29% 10% 3	405
	11th to 20th	Day	26% 29% 27% 12% 5% 1	500
	21st to end	Day	19% 42% 29% 8% 2 1	531
April	1st to 10th	Day	12% 40% 33% 12% 2 1	460
	11th to 20th	Day	15% 40% 32% 10% 2 1	452
	21st to end	Day	13% 38% 33% 12% 3	460
May	1st to 10th	Day	7% 34% 43% 12% 3	463
	11th to 20th	Day	4 26% 42% 21% 6% 1	488
	21st to end	Day	6% 26% 40% 20% 7% 1	605
June	1st to 10th	Day	5% 9% 33% 34% 17% 3	680
	11th to 20th	Day	4 10% 35% 31% 17% 3	680
	21st to end	Day	5% 6% 26% 34% 24% 5%	617
July	1st to 10th	Day	8% 3 23% 34% 26% 7%	486
	11th to 20th	Day	7% 1 11% 35% 36% 9%	484
	21st to end	Day	6% 1 10% 31% 40% 11%	531
August	1st to 10th	Day	7% 2 13% 34% 34% 9%	487
	11th to 20th	Day	8% 4 18% 36% 26% 8%	477
	21st to end	Day	8% 3 15% 34% 32% 8%	539
September	1st to 10th	Day	11% 5% 22% 36% 20% 6%	490
	11th to 20th	Day	7% 7% 32% 32% 17% 5%	490
	21st to end	Day	6% 11% 35% 29% 13% 7%	300
October	1st to 10th	Day	10% 16% 28% 24% 17% 4	300
	11th to 20th	Day	7% 19% 32% 25% 12% 5%	300
	21st to end	Day	12% 24% 26% 21% 13% 4	330
November	1st to 10th	Day	11% 34% 29% 16% 7% 3	300
	11th to 20th	Day	12% 20% 29% 24% 12% 3	300
	21st to end	Day	16% 32% 25% 19% 7% 1	300
December	1st to 10th	Day	17% 30% 24% 21% 6% 1	300
	11th to 20th	Day	13% 34% 27% 20% 7%	300
	21st to end	Day	21% 34% 26% 14% 5% 1	330
January	1st to 10th	Night	10% 19% 27% 28% 12% 3	800
	11th to 20th	Night	10% 20% 29% 27% 12% 3	800
	21st to end	Night	10% 23% 27% 25% 14% 1	880
February	1st to 10th	Night	10% 23% 29% 25% 11% 2	800
	11th to 20th	Night	14% 23% 34% 22% 6% 1	784
	21st to end	Night	12% 32% 32% 19% 5% 1	660
March	1st to 10th	Night	14% 28% 26% 23% 8% 1	800
	11th to 20th	Night	15% 21% 26% 27% 11%	800
	21st to end	Night	10% 28% 31% 23% 7% 1	777
April	1st to 10th	Night	7% 32% 34% 21% 6%	570
	11th to 20th	Night	10% 28% 33% 22% 6% 1	563
	21st to end	Night	10% 27% 31% 25% 6%	570
May	1st to 10th	Night	5% 23% 38% 27% 6%	570
	11th to 20th	Night	4 19% 28% 33% 14% 1	561
	21st to end	Night	3 17% 37% 33% 9% 1	617
June	1st to 10th	Night	2 9% 29% 41% 18% 1	570
	11th to 20th	Night	4 7% 34% 37% 17% 2	570
	21st to end	Night	6% 6% 21% 41% 24% 3	570
July	1st to 10th	Night	5% 3 20% 38% 30% 4	555
	11th to 20th	Night	3 2 13% 36% 39% 8%	552
	21st to end	Night	5% 2 13% 33% 34% 13%	624
August	1st to 10th	Night	4 1 11% 28% 43% 14%	567
	11th to 20th	Night	5% 3 13% 28% 41% 10%	564
	21st to end	Night	3 3 13% 32% 41% 7%	627
September	1st to 10th	Night	5% 5% 16% 32% 35% 6%	570
	11th to 20th	Night	7% 3 18% 38% 31% 4	570
	21st to end	Night	4 7% 22% 36% 27% 4	570
October	1st to 10th	Night	4 10% 28% 35% 20% 3	750
	11th to 20th	Night	5% 11% 28% 34% 19% 3	760
	21st to end	Night	7% 16% 22% 27% 21% 7%	836
November	1st to 10th	Night	7% 18% 21% 30% 19% 5%	780
	11th to 20th	Night	6% 14% 24% 34% 19% 4	945
	21st to end	Night	11% 21% 24% 25% 16% 4	1000
December	1st to 10th	Night	12% 21% 23% 30% 14% 2	1000
	11th to 20th	Night	9% 25% 29% 27% 10% 1	1000
	21st to end	Night	15% 20% 26% 27% 10% 1	1000

Page generated at 2025-03-24 07:04:30. Version ac07

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