Blackwell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackwell clear sky chart from 2005-11-06 to 2026-02-01.

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	7% 4 2 4 3 3 3 3 5% 7% 59%	127440
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	124891
Transparency	Day	25% 4 9% 21% 26% 15%	131285
	Night	23% 4 8% 22% 26% 17%	129110
Seeing	Day	9% 7% 24% 31% 23% 6%	16627
	Night	9% 7% 31% 38% 15% 1	26675

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	9% 5% 3 5% 3 3 2 2 4 6% 58%	4167
	11th to 20th	Day	4 3 1 3 2 2 3 2 4 6% 70%	4235
	21st to end	Day	13% 7% 2 4 3 2 3 3 4 6% 54%	5104
February	1st to 10th	Day	9% 4 1 4 2 3 4 2 4 7% 59%	2000
	11th to 20th	Day	8% 7% 2 6% 4 3 4 3 3 5% 56%	1949
	21st to end	Day	9% 7% 2 4 3 1 3 2 4 5% 60%	1650
March	1st to 10th	Day	7% 4 1 4 3 3 3 3 4 6% 61%	4517
	11th to 20th	Day	10% 5% 1 4 3 3 3 3 4 5% 59%	4980
	21st to end	Day	4 4 2 4 3 4 4 4 5% 7% 59%	5462
April	1st to 10th	Day	7% 4 2 4 3 3 4 4 4 7% 59%	2317
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 5% 5% 55%	2374
	21st to end	Day	8% 5% 1 4 4 3 3 3 4 6% 60%	2431
May	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 6% 54%	5862
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 6% 54%	5862
	21st to end	Day	10% 5% 2 5% 4 4 4 3 6% 8% 51%	6434
June	1st to 10th	Day	4 3 1 4 3 3 4 4 5% 8% 62%	2600
	11th to 20th	Day	4 4 1 3 2 3 3 3 6% 8% 61%	2600
	21st to end	Day	6% 4 1 3 3 3 4 3 6% 7% 60%	2600
July	1st to 10th	Day	10% 5% 2 5% 3 4 5% 4 7% 9% 46%	5900
	11th to 20th	Day	3 3 1 4 3 4 3 3 6% 9% 61%	5900
	21st to end	Day	3 2 1 3 3 3 3 3 5% 9% 66%	6422
August	1st to 10th	Day	1 1 1 2 2 3 3 4 5% 7% 71%	3943
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 8% 65%	3846
	21st to end	Day	3 3 1 5% 4 4 5% 4 6% 9% 57%	4312
September	1st to 10th	Day	10% 5% 2 5% 3 4 5% 4 5% 6% 51%	2267
	11th to 20th	Day	9% 5% 2 5% 4 5% 4 3 5% 8% 51%	2200
	21st to end	Day	9% 3 2 4 3 4 3 3 5% 5% 58%	2200
October	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 7% 65%	2200
	11th to 20th	Day	9% 4 2 4 2 2 2 2 3 5% 67%	2190
	21st to end	Day	11% 3 1 3 2 2 2 2 3 4 67%	2185
November	1st to 10th	Day	8% 4 1 4 2 3 2 2 4 5% 64%	1853
	11th to 20th	Day	4 4 1 3 2 2 3 3 4 6% 66%	1890
	21st to end	Day	9% 5% 2 4 3 3 3 2 4 6% 58%	1890
December	1st to 10th	Day	12% 7% 2 6% 3 3 3 2 3 5% 53%	3030
	11th to 20th	Day	7% 4 2 5% 3 4 4 3 4 6% 59%	3481
	21st to end	Day	7% 5% 1 4 2 2 2 2 2 4 68%	4587
January	1st to 10th	Night	8% 6% 2 5% 2 3 2 2 4 6% 60%	6018
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 5% 71%	6006
	21st to end	Night	12% 6% 2 4 3 3 3 3 5% 6% 53%	6116
February	1st to 10th	Night	9% 5% 2 4 3 3 2 3 3 6% 61%	2400
	11th to 20th	Night	11% 6% 2 4 3 4 3 3 4 6% 55%	2338
	21st to end	Night	8% 6% 2 4 2 3 2 2 3 7% 62%	1980
March	1st to 10th	Night	5% 4 2 4 3 3 4 3 5% 7% 61%	5340
	11th to 20th	Night	9% 4 1 3 2 3 3 3 5% 6% 60%	4980
	21st to end	Night	5% 4 1 4 3 3 4 3 5% 6% 62%	5467
April	1st to 10th	Night	5% 4 2 3 4 3 3 3 5% 7% 61%	2077
	11th to 20th	Night	9% 4 2 3 2 3 4 3 5% 8% 57%	1977
	21st to end	Night	10% 3 2 3 3 2 2 2 4 6% 63%	1965
May	1st to 10th	Night	12% 5% 2 4 3 3 2 3 4 6% 56%	4053
	11th to 20th	Night	5% 4 2 3 3 4 4 3 6% 7% 60%	4053
	21st to end	Night	10% 5% 2 4 3 3 4 4 7% 9% 51%	4443
June	1st to 10th	Night	2 2 1 3 3 3 4 3 8% 11% 59%	1800
	11th to 20th	Night	3 2 1 3 2 2 4 4 6% 9% 64%	1800
	21st to end	Night	4 3 1 2 2 3 3 4 6% 7% 65%	1800
July	1st to 10th	Night	8% 6% 2 5% 3 3 4 4 5% 10% 50%	4078
	11th to 20th	Night	2 2 3 2 3 2 3 5% 9% 69%	4077
	21st to end	Night	2 2 1 3 2 3 4 4 6% 8% 67%	4449
August	1st to 10th	Night	1 1 1 3 2 3 3 3 4 8% 73%	3212
	11th to 20th	Night	2 3 1 2 3 3 4 3 5% 8% 68%	3213
	21st to end	Night	3 2 1 4 3 4 4 4 6% 8% 62%	3597
September	1st to 10th	Night	6% 4 2 4 3 4 3 3 6% 8% 56%	2131
	11th to 20th	Night	7% 5% 2 5% 3 4 3 4 6% 8% 53%	2200
	21st to end	Night	7% 4 1 3 3 3 3 3 5% 8% 60%	2200
October	1st to 10th	Night	4 2 1 3 2 3 3 3 5% 7% 67%	2200
	11th to 20th	Night	8% 3 1 3 3 2 2 2 3 5% 68%	2200
	21st to end	Night	8% 3 1 3 2 2 3 2 3 5% 68%	2637
November	1st to 10th	Night	9% 4 1 4 2 2 2 2 3 5% 66%	2665
	11th to 20th	Night	5% 2 1 3 2 3 3 3 4 7% 67%	2730
	21st to end	Night	12% 4 2 4 4 3 3 2 5% 6% 56%	2730
December	1st to 10th	Night	9% 8% 3 5% 3 2 2 2 3 6% 57%	4380
	11th to 20th	Night	6% 4 2 5% 3 3 3 3 4 6% 61%	4946
	21st to end	Night	7% 6% 2 5% 3 2 3 3 4 5% 62%	6633

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	28% 3 7% 20% 29% 14%	4168
	11th to 20th	Day	17% 3 6% 22% 37% 15%	4235
	21st to end	Day	32% 3 5% 15% 26% 19%	5104
February	1st to 10th	Day	26% 3 7% 16% 27% 20%	2000
	11th to 20th	Day	33% 3 5% 21% 22% 16%	1959
	21st to end	Day	28% 2 5% 12% 30% 23%	1650
March	1st to 10th	Day	25% 3 5% 12% 23% 32%	4517
	11th to 20th	Day	28% 4 7% 9% 22% 30%	4980
	21st to end	Day	24% 4 8% 15% 29% 20%	5464
April	1st to 10th	Day	24% 5% 9% 17% 23% 21%	2317
	11th to 20th	Day	31% 3 8% 14% 24% 19%	2374
	21st to end	Day	27% 4 8% 18% 26% 16%	2431
May	1st to 10th	Day	32% 5% 7% 15% 23% 19%	5900
	11th to 20th	Day	32% 4 7% 14% 27% 16%	5866
	21st to end	Day	31% 6% 11% 26% 14% 12%	6447
June	1st to 10th	Day	19% 7% 16% 30% 22% 6%	2600
	11th to 20th	Day	21% 5% 11% 28% 26% 9%	2600
	21st to end	Day	22% 6% 13% 26% 24% 10%	2600
July	1st to 10th	Day	33% 8% 14% 28% 15% 3	5836
	11th to 20th	Day	20% 5% 17% 33% 22% 3	5831
	21st to end	Day	15% 6% 15% 35% 25% 4	6240
August	1st to 10th	Day	13% 4 11% 31% 36% 6%	3929
	11th to 20th	Day	17% 6% 14% 34% 26% 3	3882
	21st to end	Day	22% 8% 16% 25% 23% 5%	4311
September	1st to 10th	Day	32% 6% 9% 21% 22% 10%	2267
	11th to 20th	Day	32% 7% 10% 22% 23% 7%	2200
	21st to end	Day	27% 4 8% 20% 27% 14%	2200
October	1st to 10th	Day	12% 3 7% 14% 32% 31%	3590
	11th to 20th	Day	22% 3 5% 21% 24% 26%	3546
	21st to end	Day	17% 2 4 24% 39% 14%	3512
November	1st to 10th	Day	24% 4 6% 20% 28% 18%	1853
	11th to 20th	Day	20% 3 9% 21% 28% 19%	1890
	21st to end	Day	29% 3 6% 19% 28% 16%	1890
December	1st to 10th	Day	35% 3 5% 18% 27% 12%	3030
	11th to 20th	Day	28% 4 7% 14% 29% 18%	3479
	21st to end	Day	23% 2 4 21% 32% 18%	4587
January	1st to 10th	Night	27% 2 5% 21% 31% 13%	6018
	11th to 20th	Night	17% 2 5% 20% 33% 22%	6006
	21st to end	Night	33% 4 6% 15% 25% 17%	6116
February	1st to 10th	Night	28% 2 6% 15% 26% 22%	2400
	11th to 20th	Night	32% 4 7% 18% 21% 19%	2338
	21st to end	Night	26% 2 5% 14% 27% 26%	1980
March	1st to 10th	Night	23% 3 5% 13% 28% 28%	5340
	11th to 20th	Night	25% 4 8% 12% 16% 35%	4980
	21st to end	Night	22% 4 8% 18% 29% 19%	5467
April	1st to 10th	Night	23% 4 10% 21% 17% 24%	2077
	11th to 20th	Night	27% 5% 8% 17% 23% 20%	1977
	21st to end	Night	24% 4 9% 22% 28% 14%	1965
May	1st to 10th	Night	30% 6% 8% 19% 20% 18%	4080
	11th to 20th	Night	22% 7% 9% 17% 25% 20%	4054
	21st to end	Night	29% 7% 11% 22% 16% 15%	4452
June	1st to 10th	Night	18% 7% 17% 31% 21% 6%	1800
	11th to 20th	Night	16% 6% 12% 30% 27% 9%	1800
	21st to end	Night	18% 6% 12% 30% 23% 11%	1800
July	1st to 10th	Night	30% 6% 14% 31% 16% 3	4035
	11th to 20th	Night	13% 4 13% 37% 29% 5%	4026
	21st to end	Night	14% 6% 14% 36% 27% 3	4320
August	1st to 10th	Night	11% 4 9% 29% 40% 7%	3202
	11th to 20th	Night	15% 5% 14% 36% 25% 5%	3243
	21st to end	Night	19% 7% 19% 27% 23% 5%	3597
September	1st to 10th	Night	26% 7% 10% 24% 21% 12%	2131
	11th to 20th	Night	28% 5% 12% 24% 23% 7%	2200
	21st to end	Night	23% 4 8% 23% 25% 17%	2200
October	1st to 10th	Night	10% 3 7% 14% 31% 34%	3590
	11th to 20th	Night	18% 3 7% 17% 28% 27%	3570
	21st to end	Night	17% 2 4 23% 39% 16%	4261
November	1st to 10th	Night	24% 3 7% 20% 26% 21%	2665
	11th to 20th	Night	18% 4 8% 20% 31% 20%	2730
	21st to end	Night	31% 3 6% 15% 29% 17%	2730
December	1st to 10th	Night	31% 2 5% 20% 27% 14%	4380
	11th to 20th	Night	26% 3 6% 17% 29% 19%	4947
	21st to end	Night	26% 2 4 23% 26% 18%	6633

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	13% 12% 39% 29% 7% 1	330
	11th to 20th	Day	13% 15% 32% 28% 11% 2	330
	21st to end	Day	12% 15% 39% 25% 8% 1	363
February	1st to 10th	Day	17% 11% 36% 28% 8% 1	300
	11th to 20th	Day	18% 15% 40% 21% 5% 1	300
	21st to end	Day	14% 18% 42% 21% 5%	249
March	1st to 10th	Day	14% 15% 39% 28% 4	300
	11th to 20th	Day	19% 13% 37% 24% 6% 1	300
	21st to end	Day	8% 17% 43% 26% 5%	330
April	1st to 10th	Day	9% 17% 45% 22% 6%	417
	11th to 20th	Day	11% 18% 47% 21% 3 1	492
	21st to end	Day	8% 18% 42% 24% 7% 1	531
May	1st to 10th	Day	11% 12% 37% 33% 7%	703
	11th to 20th	Day	12% 7% 34% 32% 12% 3	725
	21st to end	Day	11% 5% 33% 37% 13% 1	789
June	1st to 10th	Day	4 3 19% 48% 23% 4	730
	11th to 20th	Day	5% 2 22% 47% 22% 3	730
	21st to end	Day	4 2 13% 35% 39% 8%	730
July	1st to 10th	Day	8% 1 5% 31% 43% 12%	721
	11th to 20th	Day	2 4 35% 46% 12%	718
	21st to end	Day	2 6% 28% 48% 17%	794
August	1st to 10th	Day	2 1 3 29% 50% 16%	552
	11th to 20th	Day	5% 1 4 35% 42% 13%	517
	21st to end	Day	4 1 6% 33% 43% 14%	583
September	1st to 10th	Day	10% 2 6% 26% 38% 18%	397
	11th to 20th	Day	5% 2 8% 29% 42% 14%	330
	21st to end	Day	12% 2 10% 28% 35% 13%	330
October	1st to 10th	Day	2 2 17% 37% 32% 10%	330
	11th to 20th	Day	9% 6% 30% 32% 20% 2	330
	21st to end	Day	15% 9% 23% 31% 18% 4	363
November	1st to 10th	Day	13% 7% 29% 33% 16% 3	330
	11th to 20th	Day	11% 12% 27% 31% 15% 4	330
	21st to end	Day	13% 17% 35% 22% 12% 2	330
December	1st to 10th	Day	16% 11% 32% 29% 11% 1	330
	11th to 20th	Day	6% 14% 38% 30% 11% 1	330
	21st to end	Day	14% 12% 36% 28% 9% 1	363
January	1st to 10th	Night	10% 9% 39% 32% 9%	1050
	11th to 20th	Night	11% 11% 34% 36% 8%	1039
	21st to end	Night	11% 12% 38% 33% 6%	924
February	1st to 10th	Night	13% 10% 41% 29% 7%	800
	11th to 20th	Night	14% 12% 42% 27% 4	784
	21st to end	Night	12% 13% 48% 25% 3	660
March	1st to 10th	Night	12% 12% 43% 30% 4	792
	11th to 20th	Night	13% 12% 40% 30% 5%	600
	21st to end	Night	8% 12% 49% 28% 2	660
April	1st to 10th	Night	8% 12% 51% 25% 4	600
	11th to 20th	Night	9% 13% 52% 25% 1	593
	21st to end	Night	9% 10% 50% 26% 5%	600
May	1st to 10th	Night	8% 8% 48% 32% 5%	600
	11th to 20th	Night	8% 6% 40% 35% 9% 1	591
	21st to end	Night	9% 4 42% 37% 9%	650
June	1st to 10th	Night	3 3 30% 49% 15%	600
	11th to 20th	Night	3 1 35% 52% 9%	600
	21st to end	Night	4 2 21% 49% 23% 1	600
July	1st to 10th	Night	6% 8% 50% 32% 3	585
	11th to 20th	Night	2 7% 54% 36% 1	582
	21st to end	Night	2 5% 54% 38% 1	657
August	1st to 10th	Night	1 5% 47% 45% 2	597
	11th to 20th	Night	4 1 7% 47% 38% 2	594
	21st to end	Night	3 1 6% 44% 43% 3	660
September	1st to 10th	Night	6% 2 8% 45% 37% 3	600
	11th to 20th	Night	8% 2 9% 47% 33% 2	600
	21st to end	Night	8% 1 15% 46% 29% 1	600
October	1st to 10th	Night	4 2 21% 50% 22% 1	600
	11th to 20th	Night	9% 5% 26% 44% 15% 1	600
	21st to end	Night	9% 9% 29% 37% 15% 2	877
November	1st to 10th	Night	10% 6% 28% 41% 15% 1	1025
	11th to 20th	Night	6% 6% 32% 41% 12% 1	1050
	21st to end	Night	14% 10% 31% 33% 11% 1	1050
December	1st to 10th	Night	12% 9% 32% 39% 8% 1	1050
	11th to 20th	Night	12% 10% 37% 32% 8%	1050
	21st to end	Night	10% 9% 37% 35% 9%	1155

Page generated at 2026-01-29 00:21:43. Version ac07

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