Area 52 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Area 52 Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	2 3 1 4 3 3 3 3 4 6% 68%	139429
	Night	2 3 1 4 3 3 3 3 5% 6% 67%	139186
Transparency	Day	17% 3 7% 17% 27% 29%	134399
	Night	18% 4 8% 19% 28% 23%	135198
Seeing	Day	4 6% 19% 30% 26% 14%	16697
	Night	4 3 18% 43% 28% 4	26408

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	3 6% 2 5% 3 4 4 4 5% 7% 57%	4722
	11th to 20th	Day	1 3 1 3 2 2 2 2 3 5% 75%	4740
	21st to end	Day	3 6% 2 4 3 3 4 4 6% 8% 58%	5822
February	1st to 10th	Day	3 4 1 4 3 2 3 4 5% 9% 62%	2100
	11th to 20th	Day	2 5% 2 5% 4 4 4 4 5% 8% 58%	2050
	21st to end	Day	2 5% 2 4 4 4 4 3 5% 6% 61%	1730
March	1st to 10th	Day	2 6% 3 7% 4 4 4 3 5% 9% 53%	4539
	11th to 20th	Day	1 5% 2 5% 4 4 4 4 6% 8% 56%	4811
	21st to end	Day	1 7% 2 6% 4 5% 4 4 6% 9% 52%	5739
April	1st to 10th	Day	2 4 2 5% 4 4 3 3 4 8% 60%	2400
	11th to 20th	Day	3 4 1 4 3 4 3 2 5% 6% 65%	2369
	21st to end	Day	3 4 2 4 4 3 3 4 5% 6% 64%	2507
May	1st to 10th	Day	2 4 1 4 3 3 4 3 4 5% 67%	5880
	11th to 20th	Day	2 4 2 4 3 3 3 3 4 5% 67%	5841
	21st to end	Day	2 3 1 4 3 3 3 3 4 6% 68%	6423
June	1st to 10th	Day	2 2 1 4 3 4 3 4 6% 7% 63%	2778
	11th to 20th	Day	1 2 1 3 2 3 3 2 3 5% 75%	2800
	21st to end	Day	1 2 1 3 2 2 2 2 3 5% 79%	2800
July	1st to 10th	Day	1 1 1 2 2 2 2 2 2 3 83%	6319
	11th to 20th	Day	2 2 1 3 2 2 3 3 5% 7% 72%	6320
	21st to end	Day	1 2 1 3 2 3 3 3 4 6% 72%	6558
August	1st to 10th	Day	2 2 1 2 2 1 2 2 2 3 82%	5742
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	5364
	21st to end	Day	1 2 1 3 2 3 4 3 4 6% 71%	5984
September	1st to 10th	Day	1 2 1 3 3 2 3 3 4 6% 73%	2395
	11th to 20th	Day	1 2 1 2 2 2 3 2 3 5% 76%	2200
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 79%	2200
October	1st to 10th	Day	1 1 1 2 2 2 2 2 3 5% 77%	3600
	11th to 20th	Day	1 1 1 2 2 3 3 2 3 4 78%	3431
	21st to end	Day	1 2 1 2 2 2 2 2 3 6% 77%	3200
November	1st to 10th	Day	2 4 2 3 2 2 2 3 5% 8% 67%	1854
	11th to 20th	Day	1 3 1 3 2 2 2 3 4 7% 71%	1890
	21st to end	Day	2 3 1 3 3 3 3 3 5% 7% 66%	1890
December	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 6% 71%	3040
	11th to 20th	Day	4 7% 2 5% 4 3 3 3 6% 9% 53%	3299
	21st to end	Day	2 4 2 5% 4 4 4 4 6% 8% 57%	4092
January	1st to 10th	Night	2 4 2 5% 4 5% 5% 3 4 7% 59%	8172
	11th to 20th	Night	1 2 1 3 3 2 3 3 4 6% 73%	8220
	21st to end	Night	3 7% 2 5% 3 3 3 3 5% 6% 59%	8373
February	1st to 10th	Night	2 5% 2 5% 3 3 3 3 5% 7% 62%	2520
	11th to 20th	Night	3 5% 1 4 3 4 4 3 6% 8% 60%	2458
	21st to end	Night	1 5% 2 4 3 4 3 3 5% 7% 61%	2076
March	1st to 10th	Night	2 5% 2 6% 4 3 4 3 5% 6% 61%	5400
	11th to 20th	Night	1 4 2 5% 4 4 6% 5% 6% 8% 56%	5187
	21st to end	Night	1 4 2 5% 3 3 4 4 6% 7% 58%	5231
April	1st to 10th	Night	2 4 1 4 3 3 4 3 6% 6% 63%	2000
	11th to 20th	Night	3 4 1 4 3 2 3 2 4 5% 68%	1979
	21st to end	Night	3 4 1 4 3 3 3 3 5% 7% 62%	1890
May	1st to 10th	Night	3 3 2 4 3 3 4 3 4 6% 65%	4100
	11th to 20th	Night	2 3 1 3 3 4 3 4 5% 6% 67%	4074
	21st to end	Night	2 3 1 5% 4 4 4 3 5% 6% 63%	4474
June	1st to 10th	Night	2 1 1 3 3 3 3 4 6% 7% 65%	1616
	11th to 20th	Night	2 2 1 3 2 3 3 3 4 5% 72%	1600
	21st to end	Night	1 2 1 3 3 2 2 3 4 4 75%	1600
July	1st to 10th	Night	1 1 1 2 1 2 2 2 3 3 82%	3640
	11th to 20th	Night	1 2 1 3 3 3 3 3 5% 7% 69%	3640
	21st to end	Night	1 2 1 4 3 3 3 2 4 5% 70%	4305
August	1st to 10th	Night	2 1 1 2 2 2 3 2 3 4 77%	4069
	11th to 20th	Night	1 1 1 1 1 2 1 3 4 84%	4506
	21st to end	Night	2 2 1 3 3 3 3 3 5% 6% 71%	5015
September	1st to 10th	Night	1 1 3 3 2 3 4 3 4 76%	2005
	11th to 20th	Night	2 1 1 3 3 2 2 2 3 4 76%	2200
	21st to end	Night	2 1 1 2 2 2 2 2 2 3 81%	2200
October	1st to 10th	Night	2 1 1 2 2 2 3 3 3 4 77%	3600
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 77%	3720
	21st to end	Night	1 3 1 4 2 2 2 2 3 4 76%	4600
November	1st to 10th	Night	2 3 1 3 2 2 3 2 4 7% 71%	2665
	11th to 20th	Night	2 3 1 3 2 3 3 3 4 7% 68%	2730
	21st to end	Night	1 3 1 3 3 3 3 3 5% 6% 68%	2730
December	1st to 10th	Night	1 3 1 4 3 3 3 3 4 6% 68%	4370
	11th to 20th	Night	4 5% 1 4 3 3 3 3 6% 11% 57%	5115
	21st to end	Night	2 5% 2 6% 5% 6% 6% 5% 7% 10% 47%	7106

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	24% 6% 18% 27% 21% 4	4723
	11th to 20th	Day	14% 3 10% 19% 28% 27%	4740
	21st to end	Day	23% 4 11% 23% 27% 12%	5822
February	1st to 10th	Day	21% 5% 11% 28% 25% 11%	2100
	11th to 20th	Day	24% 4 8% 26% 30% 8%	2060
	21st to end	Day	23% 4 11% 29% 24% 8%	1730
March	1st to 10th	Day	27% 3 7% 21% 30% 11%	4539
	11th to 20th	Day	23% 6% 9% 30% 26% 7%	4811
	21st to end	Day	26% 6% 11% 24% 24% 9%	5739
April	1st to 10th	Day	23% 4 8% 24% 25% 16%	2400
	11th to 20th	Day	20% 4 7% 22% 30% 17%	2372
	21st to end	Day	20% 4 7% 24% 34% 11%	2507
May	1st to 10th	Day	19% 4 6% 20% 38% 14%	5880
	11th to 20th	Day	18% 3 4 12% 40% 23%	5844
	21st to end	Day	17% 3 5% 12% 37% 26%	6424
June	1st to 10th	Day	18% 4 6% 15% 29% 29%	2778
	11th to 20th	Day	13% 2 3 8% 27% 47%	2800
	21st to end	Day	10% 3 2 5% 23% 57%	2800
July	1st to 10th	Day	9% 1 2 3 10% 75%	6251
	11th to 20th	Day	12% 2 6% 5% 14% 61%	6245
	21st to end	Day	13% 3 5% 9% 17% 53%	6370
August	1st to 10th	Day	13% 2 2 7% 26% 50%	4183
	11th to 20th	Day	6% 2 3 5% 16% 68%	3880
	21st to end	Day	11% 2 2 7% 21% 57%	4311
September	1st to 10th	Day	13% 3 4 9% 23% 48%	2395
	11th to 20th	Day	12% 3 3 11% 29% 42%	2200
	21st to end	Day	10% 2 4 11% 34% 39%	2200
October	1st to 10th	Day	11% 2 5% 8% 32% 42%	3600
	11th to 20th	Day	13% 2 2 11% 36% 35%	3431
	21st to end	Day	11% 1 2 21% 48% 16%	3200
November	1st to 10th	Day	16% 3 8% 26% 32% 15%	1854
	11th to 20th	Day	14% 3 8% 29% 35% 10%	1890
	21st to end	Day	18% 3 9% 27% 30% 13%	1890
December	1st to 10th	Day	15% 3 8% 23% 32% 19%	3040
	11th to 20th	Day	27% 5% 11% 27% 26% 4	3298
	21st to end	Day	23% 7% 17% 28% 21% 4	4092
January	1st to 10th	Night	21% 8% 17% 26% 25% 3	8172
	11th to 20th	Night	14% 3 9% 21% 28% 25%	8220
	21st to end	Night	26% 4 11% 22% 25% 13%	8373
February	1st to 10th	Night	22% 4 11% 27% 26% 10%	2520
	11th to 20th	Night	21% 5% 9% 23% 32% 10%	2459
	21st to end	Night	21% 5% 13% 28% 25% 9%	2076
March	1st to 10th	Night	22% 4 7% 23% 28% 15%	5400
	11th to 20th	Night	22% 6% 10% 31% 24% 7%	5187
	21st to end	Night	22% 6% 13% 26% 27% 5%	5231
April	1st to 10th	Night	20% 5% 9% 29% 25% 12%	2000
	11th to 20th	Night	19% 3 7% 26% 29% 15%	1979
	21st to end	Night	21% 4 6% 25% 36% 8%	1890
May	1st to 10th	Night	19% 5% 6% 17% 41% 12%	4097
	11th to 20th	Night	14% 5% 6% 16% 42% 17%	4075
	21st to end	Night	19% 4 7% 11% 37% 21%	4474
June	1st to 10th	Night	15% 6% 7% 16% 32% 25%	1616
	11th to 20th	Night	13% 5% 3 9% 28% 42%	1600
	21st to end	Night	12% 2 3 5% 25% 52%	1600
July	1st to 10th	Night	9% 1 3 4 9% 74%	3600
	11th to 20th	Night	14% 2 5% 7% 17% 55%	3592
	21st to end	Night	16% 3 5% 9% 13% 54%	4181
August	1st to 10th	Night	16% 3 5% 9% 26% 42%	2951
	11th to 20th	Night	7% 2 3 5% 14% 68%	3256
	21st to end	Night	9% 2 2 6% 21% 59%	3608
September	1st to 10th	Night	12% 3 5% 8% 23% 48%	2005
	11th to 20th	Night	13% 3 4 11% 28% 42%	2200
	21st to end	Night	11% 2 3 12% 29% 43%	2200
October	1st to 10th	Night	11% 2 5% 10% 30% 43%	3600
	11th to 20th	Night	12% 2 3 13% 36% 34%	3720
	21st to end	Night	14% 1 3 16% 46% 19%	4600
November	1st to 10th	Night	14% 3 8% 27% 32% 15%	2665
	11th to 20th	Night	15% 4 7% 26% 37% 11%	2730
	21st to end	Night	18% 3 7% 25% 36% 12%	2730
December	1st to 10th	Night	18% 4 7% 23% 30% 20%	4370
	11th to 20th	Night	22% 5% 14% 23% 30% 5%	5115
	21st to end	Night	30% 7% 15% 24% 20% 4	7106

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	9% 7% 18% 38% 25% 3	220
	11th to 20th	Day	5% 4 22% 30% 30% 10%	220
	21st to end	Day	5% 3 16% 34% 30% 12%	396
February	1st to 10th	Day	6% 11% 28% 29% 19% 7%	540
	11th to 20th	Day	7% 10% 27% 33% 19% 4	535
	21st to end	Day	5% 8% 29% 40% 14% 5%	446
March	1st to 10th	Day	9% 12% 26% 29% 19% 5%	509
	11th to 20th	Day	6% 11% 28% 30% 18% 6%	500
	21st to end	Day	8% 10% 31% 29% 16% 6%	550
April	1st to 10th	Day	6% 12% 25% 26% 21% 10%	500
	11th to 20th	Day	5% 14% 25% 31% 17% 7%	492
	21st to end	Day	7% 9% 23% 30% 20% 11%	500
May	1st to 10th	Day	4 11% 26% 28% 22% 10%	503
	11th to 20th	Day	7% 6% 22% 29% 21% 15%	528
	21st to end	Day	8% 5% 16% 27% 27% 17%	573
June	1st to 10th	Day	3 7% 17% 25% 26% 22%	530
	11th to 20th	Day	3 8% 20% 29% 25% 15%	530
	21st to end	Day	3 4 14% 28% 35% 17%	530
July	1st to 10th	Day	2 2 13% 35% 32% 17%	526
	11th to 20th	Day	3 2 7% 33% 32% 23%	524
	21st to end	Day	4 1 7% 32% 33% 23%	575
August	1st to 10th	Day	5% 1 12% 36% 27% 19%	527
	11th to 20th	Day	3 1 14% 28% 30% 25%	517
	21st to end	Day	2 2 13% 28% 35% 20%	583
September	1st to 10th	Day	3 3 12% 28% 29% 25%	530
	11th to 20th	Day	2 5% 18% 30% 28% 18%	530
	21st to end	Day	2 4 13% 30% 37% 13%	530
October	1st to 10th	Day	3 7% 18% 27% 28% 17%	530
	11th to 20th	Day	4 4 20% 24% 35% 14%	515
	21st to end	Day	3 3 17% 29% 33% 16%	363
November	1st to 10th	Day	4 3 18% 31% 27% 17%	330
	11th to 20th	Day	2 6% 15% 40% 22% 14%	330
	21st to end	Day	3 4 18% 32% 32% 12%	330
December	1st to 10th	Day	5% 3 17% 35% 30% 10%	330
	11th to 20th	Day	4 2 18% 37% 27% 12%	283
	21st to end	Day	5% 5% 25% 34% 25% 6%	242
January	1st to 10th	Night	5% 4 23% 39% 23% 6%	840
	11th to 20th	Night	3 2 20% 47% 24% 5%	840
	21st to end	Night	6% 1 19% 45% 22% 6%	924
February	1st to 10th	Night	5% 4 23% 42% 23% 3	840
	11th to 20th	Night	4 4 26% 42% 20% 4	824
	21st to end	Night	5% 3 24% 49% 16% 2	692
March	1st to 10th	Night	4 5% 23% 48% 17% 3	808
	11th to 20th	Night	4 4 26% 42% 21% 4	800
	21st to end	Night	5% 7% 28% 40% 18% 2	880
April	1st to 10th	Night	4 8% 22% 45% 20% 1	800
	11th to 20th	Night	5% 6% 29% 41% 18% 3	791
	21st to end	Night	5% 6% 25% 40% 22% 3	690
May	1st to 10th	Night	5% 6% 31% 39% 18% 2	600
	11th to 20th	Night	6% 4 21% 41% 26% 2	591
	21st to end	Night	6% 6% 18% 38% 29% 3	650
June	1st to 10th	Night	2 3 13% 45% 34% 3	416
	11th to 20th	Night	2 3 19% 49% 26% 1	400
	21st to end	Night	1 1 11% 51% 34% 2	400
July	1st to 10th	Night	4 1 8% 48% 34% 5%	390
	11th to 20th	Night	3 5% 45% 41% 4	388
	21st to end	Night	3 1 6% 45% 41% 4	581
August	1st to 10th	Night	4 2 13% 49% 31% 2	607
	11th to 20th	Night	2 8% 41% 44% 6%	792
	21st to end	Night	3 1 6% 40% 46% 5%	880
September	1st to 10th	Night	3 1 10% 33% 47% 7%	800
	11th to 20th	Night	3 2 12% 38% 40% 6%	800
	21st to end	Night	3 2 12% 40% 34% 10%	800
October	1st to 10th	Night	4 1 16% 41% 32% 6%	800
	11th to 20th	Night	2 3 15% 40% 34% 6%	800
	21st to end	Night	1 2 17% 41% 30% 9%	880
November	1st to 10th	Night	3 3 12% 42% 31% 8%	820
	11th to 20th	Night	3 2 17% 46% 26% 5%	840
	21st to end	Night	2 2 19% 43% 28% 6%	840
December	1st to 10th	Night	5% 2 18% 48% 20% 6%	840
	11th to 20th	Night	8% 1 18% 47% 21% 4	840
	21st to end	Night	4 3 25% 48% 18% 2	924

Page generated at 2026-02-27 22:40:19. Version ac07

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