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 2024-10-20.

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	4 5% 2 5% 4 4 4 4 6% 7% 55%	100334
	Night	3 5% 2 5% 3 3 4 4 5% 7% 58%	98557
Transparency	Day	24% 10% 8% 17% 25% 17%	100099
	Night	20% 10% 8% 18% 26% 18%	98382
Seeing	Day	10% 21% 28% 24% 14% 3	14668
	Night	8% 16% 25% 29% 18% 3	24856

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 8% 7% 13% 3 3 3 3 4 4 52%	2858
	11th to 20th	Day	2 5% 3 5% 2 2 3 2 4 5% 67%	2860
	21st to end	Day	4 14% 6% 7% 3 4 4 4 6% 4 44%	3146
February	1st to 10th	Day	3 9% 2 4 3 2 3 3 4 6% 62%	1857
	11th to 20th	Day	4 8% 3 5% 2 2 3 2 3 7% 61%	1849
	21st to end	Day	2 7% 2 5% 4 3 3 2 4 5% 63%	1570
March	1st to 10th	Day	2 8% 3 6% 4 3 3 3 4 5% 60%	3308
	11th to 20th	Day	3 6% 2 6% 4 3 3 3 4 9% 57%	3490
	21st to end	Day	1 9% 4 5% 4 4 5% 4 6% 8% 52%	3967
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	5% 5% 2 6% 3 3 4 2 5% 6% 60%	1906
	11th to 20th	Day	3 3 1 4 2 3 3 3 4 6% 68%	1890
	21st to end	Day	2 3 1 3 2 2 3 2 3 4 74%	1980
November	1st to 10th	Day	1 3 2 4 3 2 3 3 4 7% 68%	1859
	11th to 20th	Day	2 4 2 4 2 3 3 2 4 7% 65%	1742
	21st to end	Day	4 7% 2 5% 3 3 4 3 6% 7% 57%	1710
December	1st to 10th	Day	5% 7% 2 3 2 3 4 3 5% 7% 59%	1710
	11th to 20th	Day	4 8% 3 6% 4 3 3 4 5% 6% 53%	1963
	21st to end	Day	2 7% 3 5% 3 3 3 2 4 6% 62%	2968
January	1st to 10th	Night	2 5% 5% 10% 4 4 4 3 4 4 55%	4097
	11th to 20th	Night	2 5% 2 4 4 4 3 2 3 5% 67%	4110
	21st to end	Night	4 13% 3 5% 4 5% 7% 5% 6% 6% 44%	4521
February	1st to 10th	Night	2 9% 2 4 3 2 3 3 5% 7% 61%	2319
	11th to 20th	Night	5% 8% 2 4 2 2 3 3 5% 6% 60%	2218
	21st to end	Night	3 7% 2 4 3 3 4 3 6% 5% 60%	1884
March	1st to 10th	Night	2 8% 2 4 4 3 3 3 4 5% 62%	3558
	11th to 20th	Night	3 9% 2 5% 4 4 4 3 5% 8% 53%	3490
	21st to end	Night	2 9% 3 5% 4 4 3 3 5% 7% 54%	3679
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	3 4 1 4 3 3 3 5% 6% 8% 61%	2270
	11th to 20th	Night	2 3 1 3 3 2 3 2 3 6% 73%	2274
	21st to end	Night	4 2 1 4 2 2 3 2 3 4 72%	2376
November	1st to 10th	Night	2 3 1 3 2 3 2 3 5% 7% 68%	2220
	11th to 20th	Night	2 3 2 2 3 3 3 3 5% 6% 68%	2417
	21st to end	Night	4 6% 2 5% 4 3 4 3 5% 6% 58%	2470
December	1st to 10th	Night	5% 7% 2 4 3 3 4 3 5% 7% 58%	2470
	11th to 20th	Night	2 7% 3 7% 4 4 4 3 5% 7% 54%	3229
	21st to end	Night	3 9% 3 5% 3 3 3 2 4 5% 60%	4713

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	18% 21% 4 10% 33% 14%	2859
	11th to 20th	Day	14% 11% 5% 20% 35% 16%	2860
	21st to end	Day	27% 18% 3 13% 25% 13%	3146
February	1st to 10th	Day	21% 7% 5% 16% 29% 23%	1857
	11th to 20th	Day	23% 8% 4 17% 26% 23%	1859
	21st to end	Day	19% 9% 2 10% 28% 31%	1570
March	1st to 10th	Day	26% 7% 3 10% 32% 23%	3308
	11th to 20th	Day	23% 8% 9% 19% 29% 12%	3490
	21st to end	Day	18% 18% 3 17% 31% 12%	3967
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	25% 6% 9% 18% 25% 18%	1906
	11th to 20th	Day	20% 5% 4 15% 29% 27%	1890
	21st to end	Day	15% 5% 4 22% 35% 20%	1980
November	1st to 10th	Day	17% 5% 4 21% 34% 19%	1859
	11th to 20th	Day	18% 6% 5% 19% 29% 24%	1742
	21st to end	Day	22% 8% 5% 15% 30% 20%	1710
December	1st to 10th	Day	21% 8% 5% 23% 28% 15%	1710
	11th to 20th	Day	27% 10% 6% 14% 27% 16%	1961
	21st to end	Day	17% 11% 4 15% 26% 27%	2968
January	1st to 10th	Night	18% 18% 3 14% 30% 18%	4097
	11th to 20th	Night	15% 10% 4 21% 37% 14%	4110
	21st to end	Night	26% 19% 3 13% 26% 13%	4521
February	1st to 10th	Night	19% 8% 5% 16% 25% 26%	2319
	11th to 20th	Night	19% 10% 4 15% 28% 24%	2218
	21st to end	Night	19% 10% 3 11% 28% 29%	1884
March	1st to 10th	Night	19% 10% 3 13% 36% 19%	3558
	11th to 20th	Night	26% 10% 9% 14% 28% 12%	3490
	21st to end	Night	13% 21% 4 20% 30% 13%	3679
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	21% 7% 8% 19% 27% 17%	2270
	11th to 20th	Night	16% 5% 4 17% 30% 28%	2274
	21st to end	Night	16% 5% 4 18% 33% 24%	2376
November	1st to 10th	Night	16% 5% 4 20% 33% 22%	2220
	11th to 20th	Night	14% 7% 5% 18% 31% 24%	2417
	21st to end	Night	24% 7% 4 16% 29% 19%	2470
December	1st to 10th	Night	21% 10% 5% 18% 26% 19%	2470
	11th to 20th	Night	25% 9% 8% 15% 22% 20%	3230
	21st to end	Night	21% 10% 4 13% 27% 24%	4713

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% 31% 24% 23% 3	270
	11th to 20th	Day	17% 28% 24% 24% 6% 1	270
	21st to end	Day	10% 33% 30% 16% 9%	297
February	1st to 10th	Day	18% 29% 27% 21% 6% 1	270
	11th to 20th	Day	18% 36% 28% 14% 3	270
	21st to end	Day	26% 39% 21% 11% 3	225
March	1st to 10th	Day	19% 37% 29% 11% 4 1	370
	11th to 20th	Day	22% 30% 29% 12% 5% 1	460
	21st to end	Day	19% 43% 28% 7% 2 1	506
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%	297
	21st to end	Day	10% 24% 26% 22% 14% 4	297
November	1st to 10th	Day	7% 35% 30% 17% 8% 3	270
	11th to 20th	Day	7% 21% 30% 25% 14% 3	270
	21st to end	Day	14% 33% 26% 19% 7% 1	270
December	1st to 10th	Day	16% 30% 24% 22% 5% 1	270
	11th to 20th	Day	11% 36% 29% 19% 6%	270
	21st to end	Day	19% 34% 27% 14% 5%	297
January	1st to 10th	Night	8% 19% 27% 29% 13% 4	760
	11th to 20th	Night	10% 20% 28% 27% 12% 3	760
	21st to end	Night	10% 22% 26% 26% 14% 1	836
February	1st to 10th	Night	9% 22% 30% 25% 11% 2	760
	11th to 20th	Night	12% 23% 35% 23% 6% 1	744
	21st to end	Night	12% 33% 31% 19% 5% 1	628
March	1st to 10th	Night	13% 28% 26% 24% 8% 1	760
	11th to 20th	Night	13% 21% 26% 28% 12%	760
	21st to end	Night	10% 28% 32% 22% 7% 1	751
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	758
	21st to end	Night	7% 16% 21% 27% 22% 8%	792
November	1st to 10th	Night	6% 18% 21% 31% 20% 5%	740
	11th to 20th	Night	5% 13% 24% 34% 19% 4	897
	21st to end	Night	10% 20% 24% 25% 17% 4	950
December	1st to 10th	Night	12% 21% 22% 29% 14% 2	950
	11th to 20th	Night	9% 25% 29% 26% 10% 1	950
	21st to end	Night	15% 20% 26% 27% 10% 1	950

Page generated at 2024-10-17 11:09:42. Version ac07

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