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 2026-06-16.

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	14% 6% 2 6% 4 4 4 3 5% 7% 45%	137568
	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	134763
Transparency	Day	39% 4 10% 23% 19% 6%	141177
	Night	35% 3 9% 22% 22% 9%	139234
Seeing	Day	16% 9% 23% 26% 20% 6%	17418
	Night	16% 7% 24% 32% 18% 2	27253

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	17% 8% 2 5% 3 3 3 3 4 6% 47%	5289
	11th to 20th	Day	9% 7% 2 5% 3 3 3 2 4 5% 57%	5310
	21st to end	Day	23% 6% 2 5% 3 3 3 2 3 4 45%	5812
February	1st to 10th	Day	25% 7% 2 5% 4 4 3 3 3 4 40%	1890
	11th to 20th	Day	22% 8% 2 5% 4 3 3 2 4 5% 41%	1932
	21st to end	Day	25% 7% 2 5% 3 2 3 2 3 4 42%	1903
March	1st to 10th	Day	23% 9% 3 6% 2 3 3 3 4 5% 41%	5046
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 5% 6% 49%	5090
	21st to end	Day	18% 8% 3 7% 4 4 3 3 5% 5% 41%	5583
April	1st to 10th	Day	17% 6% 2 6% 4 4 3 3 5% 6% 43%	2415
	11th to 20th	Day	18% 6% 3 5% 3 4 4 3 5% 7% 43%	2494
	21st to end	Day	19% 9% 2 5% 4 3 3 3 5% 6% 40%	2520
May	1st to 10th	Day	22% 9% 3 6% 3 3 3 3 5% 6% 36%	7278
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	7627
	21st to end	Day	16% 9% 3 9% 5% 5% 5% 4 6% 7% 30%	8392
June	1st to 10th	Day	9% 5% 2 6% 6% 5% 5% 4 6% 9% 42%	2730
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 7% 9% 42%	2666
	21st to end	Day	5% 4 2 5% 4 5% 5% 5% 7% 10% 49%	2600
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 6% 5% 8% 12% 36%	5900
	11th to 20th	Day	5% 3 1 4 3 3 4 3 6% 9% 57%	5900
	21st to end	Day	8% 7% 3 7% 5% 5% 5% 4 7% 9% 42%	6422
August	1st to 10th	Day	3 2 1 4 3 4 4 4 5% 9% 62%	4234
	11th to 20th	Day	6% 3 2 5% 4 4 4 4 7% 10% 50%	4010
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 6% 9% 47%	4312
September	1st to 10th	Day	10% 6% 2 5% 4 4 4 3 5% 6% 50%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 5% 6% 7% 46%	2228
	21st to end	Day	9% 5% 2 5% 3 4 4 3 5% 6% 53%	2170
October	1st to 10th	Day	8% 3 2 5% 3 3 3 3 4 6% 58%	2000
	11th to 20th	Day	14% 4 2 4 3 2 3 2 4 5% 58%	2000
	21st to end	Day	19% 6% 2 5% 3 2 3 2 3 5% 51%	2200
November	1st to 10th	Day	15% 6% 2 5% 3 3 2 2 4 6% 52%	2058
	11th to 20th	Day	15% 5% 2 4 3 3 3 3 4 6% 53%	2100
	21st to end	Day	21% 7% 2 5% 3 2 3 2 2 5% 48%	1959
December	1st to 10th	Day	20% 6% 2 5% 3 3 3 2 4 4 49%	3030
	11th to 20th	Day	25% 8% 2 5% 3 2 3 2 3 5% 43%	3481
	21st to end	Day	23% 8% 3 6% 3 4 3 2 4 5% 39%	4587
January	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 6% 49%	7634
	11th to 20th	Night	10% 4 1 3 2 2 2 1 3 4 69%	7680
	21st to end	Night	26% 7% 2 5% 3 3 3 2 3 4 43%	8402
February	1st to 10th	Night	21% 6% 2 5% 3 3 3 3 4 4 45%	2730
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 5% 4 44%	2591
	21st to end	Night	19% 6% 2 5% 3 2 3 2 3 4 51%	1903
March	1st to 10th	Night	21% 7% 2 5% 3 3 3 3 4 5% 43%	5029
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 5% 7% 51%	5090
	21st to end	Night	17% 6% 2 5% 3 4 3 3 5% 7% 45%	5588
April	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 4 6% 48%	2205
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 7% 49%	2075
	21st to end	Night	15% 7% 3 5% 4 3 3 2 4 7% 48%	2100
May	1st to 10th	Night	21% 8% 3 6% 4 4 4 4 5% 6% 35%	5571
	11th to 20th	Night	16% 6% 3 7% 4 4 4 4 5% 7% 40%	5265
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 6% 8% 35%	5793
June	1st to 10th	Night	6% 5% 2 5% 3 4 4 3 6% 9% 53%	1890
	11th to 20th	Night	6% 4 2 4 3 4 3 3 5% 8% 57%	1845
	21st to end	Night	4 3 1 5% 3 3 2 4 5% 9% 62%	1800
July	1st to 10th	Night	7% 4 2 7% 5% 5% 5% 5% 7% 12% 42%	4078
	11th to 20th	Night	4 2 1 4 3 3 3 4 7% 11% 58%	4077
	21st to end	Night	6% 4 2 6% 5% 4 5% 4 6% 8% 48%	4449
August	1st to 10th	Night	2 2 1 3 2 2 3 2 5% 9% 69%	2926
	11th to 20th	Night	4 2 1 4 3 4 4 4 7% 11% 56%	3042
	21st to end	Night	4 4 2 5% 4 4 5% 4 7% 9% 54%	3586
September	1st to 10th	Night	5% 5% 2 6% 3 3 3 3 5% 8% 57%	2000
	11th to 20th	Night	9% 4 2 5% 4 3 5% 3 6% 8% 51%	2168
	21st to end	Night	8% 3 2 4 4 3 4 3 5% 6% 58%	2210
October	1st to 10th	Night	8% 3 2 3 2 3 2 3 5% 6% 64%	2400
	11th to 20th	Night	11% 4 1 4 2 2 2 2 4 5% 61%	2400
	21st to end	Night	17% 6% 2 4 2 3 2 2 4 3 55%	2640
November	1st to 10th	Night	14% 5% 2 4 3 2 3 2 4 5% 57%	2460
	11th to 20th	Night	11% 5% 2 4 3 2 3 3 4 4 59%	2520
	21st to end	Night	18% 6% 2 4 3 3 3 2 3 5% 51%	2657
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 5% 50%	4380
	11th to 20th	Night	22% 8% 2 5% 4 4 3 2 2 3 45%	4946
	21st to end	Night	20% 9% 2 5% 3 3 3 2 3 5% 45%	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	41% 1 9% 29% 17% 2	5290
	11th to 20th	Day	32% 1 7% 24% 31% 5%	5310
	21st to end	Day	45% 2 3 17% 23% 11%	5812
February	1st to 10th	Day	48% 4 7% 18% 19% 5%	1890
	11th to 20th	Day	47% 3 7% 20% 18% 5%	1941
	21st to end	Day	47% 3 6% 13% 22% 9%	1903
March	1st to 10th	Day	47% 3 5% 15% 18% 13%	5046
	11th to 20th	Day	36% 4 6% 15% 21% 17%	5090
	21st to end	Day	46% 3 8% 22% 17% 2	5585
April	1st to 10th	Day	42% 4 8% 21% 18% 6%	2415
	11th to 20th	Day	42% 4 9% 20% 19% 6%	2494
	21st to end	Day	45% 5% 10% 19% 18% 3	2520
May	1st to 10th	Day	49% 3 9% 24% 12% 2	7313
	11th to 20th	Day	45% 4 11% 21% 18% 2	7631
	21st to end	Day	52% 5% 13% 17% 11% 3	8405
June	1st to 10th	Day	37% 7% 13% 25% 15% 2	2730
	11th to 20th	Day	36% 8% 13% 25% 16% 2	2666
	21st to end	Day	28% 7% 14% 31% 19% 2	2600
July	1st to 10th	Day	40% 5% 20% 27% 8%	5836
	11th to 20th	Day	23% 3 17% 38% 18% 1	5831
	21st to end	Day	39% 4 15% 32% 10% 1	6240
August	1st to 10th	Day	20% 5% 15% 28% 27% 5%	4220
	11th to 20th	Day	29% 5% 22% 34% 10% 1	4049
	21st to end	Day	32% 5% 16% 30% 15% 2	4311
September	1st to 10th	Day	34% 6% 11% 20% 20% 9%	2400
	11th to 20th	Day	36% 6% 10% 24% 18% 6%	2228
	21st to end	Day	30% 5% 10% 22% 24% 10%	2170
October	1st to 10th	Day	17% 3 5% 12% 30% 33%	3260
	11th to 20th	Day	21% 2 3 19% 35% 21%	3240
	21st to end	Day	33% 2 4 27% 27% 7%	3538
November	1st to 10th	Day	35% 3 7% 22% 25% 8%	2058
	11th to 20th	Day	33% 3 7% 20% 27% 10%	2100
	21st to end	Day	43% 2 4 18% 26% 6%	1959
December	1st to 10th	Day	41% 2 7% 29% 18% 4	3030
	11th to 20th	Day	47% 2 4 19% 21% 7%	3479
	21st to end	Day	49% 3 5% 21% 19% 4	4587
January	1st to 10th	Night	39% 2 7% 27% 23% 3	7635
	11th to 20th	Night	23% 1 5% 20% 37% 13%	7680
	21st to end	Night	47% 2 5% 15% 21% 10%	8402
February	1st to 10th	Night	43% 3 7% 19% 20% 7%	2730
	11th to 20th	Night	45% 3 6% 18% 20% 7%	2592
	21st to end	Night	40% 3 7% 16% 23% 12%	1903
March	1st to 10th	Night	45% 3 5% 14% 19% 14%	5029
	11th to 20th	Night	34% 4 6% 19% 18% 19%	5090
	21st to end	Night	39% 4 9% 22% 22% 4	5588
April	1st to 10th	Night	38% 4 9% 21% 18% 11%	2205
	11th to 20th	Night	36% 5% 8% 20% 21% 9%	2075
	21st to end	Night	38% 4 9% 23% 22% 4	2100
May	1st to 10th	Night	49% 4 11% 20% 11% 4	5601
	11th to 20th	Night	45% 4 10% 19% 18% 4	5266
	21st to end	Night	48% 5% 16% 16% 11% 5%	5805
June	1st to 10th	Night	27% 8% 13% 30% 19% 3	1890
	11th to 20th	Night	26% 4 12% 31% 24% 3	1845
	21st to end	Night	21% 5% 13% 31% 25% 4	1800
July	1st to 10th	Night	34% 5% 20% 30% 11% 1	4035
	11th to 20th	Night	21% 3 15% 38% 21% 2	4026
	21st to end	Night	34% 4 15% 30% 15% 2	4320
August	1st to 10th	Night	14% 4 13% 28% 33% 7%	2916
	11th to 20th	Night	23% 4 20% 33% 18% 3	3069
	21st to end	Night	26% 6% 14% 25% 26% 3	3586
September	1st to 10th	Night	27% 6% 10% 23% 24% 10%	2000
	11th to 20th	Night	31% 6% 10% 24% 21% 8%	2168
	21st to end	Night	27% 4 8% 22% 25% 13%	2210
October	1st to 10th	Night	15% 3 5% 14% 25% 37%	3920
	11th to 20th	Night	18% 2 4 16% 36% 25%	3899
	21st to end	Night	30% 2 4 22% 29% 13%	4252
November	1st to 10th	Night	30% 5% 8% 19% 27% 11%	2460
	11th to 20th	Night	30% 3 7% 20% 27% 13%	2520
	21st to end	Night	38% 3 5% 19% 27% 8%	2657
December	1st to 10th	Night	39% 2 6% 28% 20% 5%	4380
	11th to 20th	Night	47% 2 4 17% 20% 10%	4947
	21st to end	Night	45% 2 7% 21% 21% 4	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	28% 14% 35% 21% 3	330
	11th to 20th	Day	22% 18% 40% 15% 4	330
	21st to end	Day	25% 20% 37% 14% 4	363
February	1st to 10th	Day	31% 14% 34% 17% 4	330
	11th to 20th	Day	32% 22% 33% 10% 2	330
	21st to end	Day	36% 19% 33% 10% 3	273
March	1st to 10th	Day	31% 15% 34% 16% 4	330
	11th to 20th	Day	31% 20% 37% 11% 2	330
	21st to end	Day	23% 21% 37% 17% 2	363
April	1st to 10th	Day	23% 17% 34% 18% 8%	435
	11th to 20th	Day	25% 15% 33% 20% 6% 1	532
	21st to end	Day	25% 13% 30% 22% 9% 1	540
May	1st to 10th	Day	18% 11% 35% 28% 8%	632
	11th to 20th	Day	16% 8% 30% 30% 14% 2	775
	21st to end	Day	13% 7% 26% 34% 17% 4	844
June	1st to 10th	Day	12% 4 18% 32% 26% 8%	780
	11th to 20th	Day	10% 3 17% 37% 27% 6%	756
	21st to end	Day	5% 3 16% 34% 30% 12%	730
July	1st to 10th	Day	6% 2 11% 32% 38% 10%	721
	11th to 20th	Day	5% 2 9% 30% 41% 14%	718
	21st to end	Day	7% 1 10% 29% 39% 15%	794
August	1st to 10th	Day	4 1 9% 32% 43% 10%	726
	11th to 20th	Day	12% 1 8% 29% 37% 13%	619
	21st to end	Day	10% 1 8% 24% 40% 17%	583
September	1st to 10th	Day	9% 2 10% 28% 38% 13%	530
	11th to 20th	Day	8% 3 14% 27% 37% 12%	358
	21st to end	Day	12% 5% 17% 31% 27% 8%	330
October	1st to 10th	Day	5% 2 26% 39% 24% 4	330
	11th to 20th	Day	12% 9% 32% 28% 17% 3	330
	21st to end	Day	28% 11% 25% 25% 9% 2	363
November	1st to 10th	Day	20% 12% 29% 25% 13% 2	330
	11th to 20th	Day	18% 13% 36% 24% 8% 1	330
	21st to end	Day	27% 15% 28% 22% 7% 2	330
December	1st to 10th	Day	25% 15% 35% 18% 5% 2	330
	11th to 20th	Day	29% 16% 30% 17% 7%	330
	21st to end	Day	25% 20% 34% 15% 5% 1	363
January	1st to 10th	Night	23% 11% 31% 29% 5%	1050
	11th to 20th	Night	21% 13% 35% 26% 5%	1050
	21st to end	Night	23% 11% 33% 28% 5%	1155
February	1st to 10th	Night	25% 13% 31% 27% 5%	1050
	11th to 20th	Night	25% 16% 33% 22% 3	957
	21st to end	Night	24% 14% 33% 26% 3	519
March	1st to 10th	Night	25% 10% 31% 29% 5%	630
	11th to 20th	Night	19% 14% 31% 30% 5%	630
	21st to end	Night	19% 10% 34% 31% 5%	693
April	1st to 10th	Night	18% 11% 35% 29% 7%	630
	11th to 20th	Night	15% 13% 36% 25% 11% 1	623
	21st to end	Night	16% 9% 34% 30% 11%	630
May	1st to 10th	Night	16% 6% 28% 39% 10%	630
	11th to 20th	Night	16% 5% 28% 33% 16% 2	621
	21st to end	Night	12% 3 24% 35% 24% 2	683
June	1st to 10th	Night	6% 2 12% 43% 34% 3	630
	11th to 20th	Night	7% 2 13% 43% 32% 2	615
	21st to end	Night	4 2 10% 36% 42% 6%	600
July	1st to 10th	Night	6% 7% 31% 49% 6%	585
	11th to 20th	Night	4 1 5% 31% 49% 9%	582
	21st to end	Night	5% 1 7% 30% 48% 9%	657
August	1st to 10th	Night	4 1 7% 36% 46% 7%	597
	11th to 20th	Night	8% 1 9% 33% 41% 8%	594
	21st to end	Night	5% 2 7% 35% 43% 7%	660
September	1st to 10th	Night	7% 2 9% 37% 40% 6%	600
	11th to 20th	Night	10% 2 11% 35% 38% 5%	600
	21st to end	Night	9% 2 14% 37% 34% 4	610
October	1st to 10th	Night	9% 2 14% 41% 30% 4	800
	11th to 20th	Night	14% 5% 25% 38% 18% 1	800
	21st to end	Night	21% 7% 24% 30% 17% 2	880
November	1st to 10th	Night	16% 5% 26% 37% 15% 1	820
	11th to 20th	Night	14% 8% 29% 37% 11%	840
	21st to end	Night	21% 11% 28% 28% 12% 1	977
December	1st to 10th	Night	22% 9% 31% 32% 7%	1050
	11th to 20th	Night	27% 9% 26% 32% 6%	1050
	21st to end	Night	24% 11% 30% 28% 7%	1155

Page generated at 2026-06-13 01:31:45. Version ac07

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