Double AA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Double AA Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	8% 5% 2 5% 3 3 4 3 5% 7% 55%	148798
	Night	8% 4 2 4 3 3 3 3 5% 7% 59%	142674
Transparency	Day	28% 5% 9% 24% 25% 8%	152528
	Night	25% 5% 9% 24% 27% 11%	147011
Seeing	Day	11% 8% 25% 31% 20% 6%	17785
	Night	10% 8% 33% 35% 13% 1	27554

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	8% 6% 2 5% 3 3 3 3 4 6% 56%	5289
	11th to 20th	Day	6% 4 1 3 2 2 2 2 3 6% 68%	5310
	21st to end	Day	18% 8% 2 5% 3 2 3 2 4 5% 48%	5812
February	1st to 10th	Day	11% 6% 2 6% 2 3 3 2 4 7% 52%	2048
	11th to 20th	Day	12% 7% 2 6% 4 3 3 2 5% 5% 51%	2049
	21st to end	Day	16% 8% 2 4 2 2 1 2 3 4 56%	1730
March	1st to 10th	Day	13% 5% 1 4 3 3 3 3 5% 6% 55%	4975
	11th to 20th	Day	8% 5% 2 4 3 3 4 3 4 5% 60%	5090
	21st to end	Day	9% 7% 2 6% 4 4 4 4 6% 6% 49%	5583
April	1st to 10th	Day	11% 5% 2 4 3 3 4 3 4 6% 55%	2310
	11th to 20th	Day	11% 6% 3 6% 3 4 4 3 5% 6% 50%	2413
	21st to end	Day	11% 5% 3 6% 4 4 4 4 5% 6% 48%	2730
May	1st to 10th	Day	14% 8% 2 7% 4 4 4 3 5% 6% 43%	7617
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 6% 49%	7627
	21st to end	Day	12% 7% 2 8% 5% 5% 5% 4 6% 8% 39%	8392
June	1st to 10th	Day	7% 4 2 5% 4 5% 5% 5% 6% 8% 50%	2730
	11th to 20th	Day	6% 3 2 4 4 4 5% 4 6% 8% 55%	2730
	21st to end	Day	3 4 1 3 3 3 3 3 6% 8% 61%	2730
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 6% 9% 48%	7680
	11th to 20th	Day	4 3 2 6% 5% 5% 5% 4 7% 10% 51%	7680
	21st to end	Day	2 2 1 3 3 3 4 3 5% 8% 66%	8337
August	1st to 10th	Day	1 2 1 3 2 3 3 3 5% 8% 69%	6014
	11th to 20th	Day	3 2 1 3 2 2 3 3 5% 8% 70%	5948
	21st to end	Day	3 2 1 4 3 4 4 4 6% 8% 61%	5942
September	1st to 10th	Day	10% 4 2 4 3 3 3 2 4 7% 58%	2310
	11th to 20th	Day	11% 4 2 6% 4 5% 5% 3 5% 7% 48%	2233
	21st to end	Day	8% 2 2 4 3 3 3 3 5% 6% 61%	2200
October	1st to 10th	Day	5% 2 1 4 3 4 4 3 4 6% 64%	2185
	11th to 20th	Day	9% 3 2 3 2 2 2 2 4 6% 66%	2000
	21st to end	Day	12% 5% 2 4 2 3 2 2 3 4 62%	2200
November	1st to 10th	Day	10% 4 2 4 3 3 2 3 3 6% 61%	2026
	11th to 20th	Day	6% 3 1 3 2 2 3 2 4 5% 68%	1890
	21st to end	Day	11% 5% 1 4 3 3 3 2 4 6% 58%	1890
December	1st to 10th	Day	11% 7% 3 6% 3 3 3 3 4 5% 53%	3030
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 4 5% 54%	3481
	21st to end	Day	13% 7% 2 6% 4 3 3 2 4 5% 50%	4587
January	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 56%	7634
	11th to 20th	Night	4 3 1 3 2 2 2 2 3 5% 74%	7680
	21st to end	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	8402
February	1st to 10th	Night	10% 5% 2 6% 4 3 2 4 4 5% 55%	2589
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 5% 6% 53%	2458
	21st to end	Night	10% 7% 2 5% 3 3 3 2 3 5% 57%	2076
March	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 53%	5088
	11th to 20th	Night	7% 3 1 3 2 3 3 2 4 7% 64%	5090
	21st to end	Night	8% 5% 2 6% 4 4 4 3 5% 6% 52%	5588
April	1st to 10th	Night	9% 5% 2 4 3 4 3 3 5% 8% 54%	2310
	11th to 20th	Night	10% 4 2 4 3 3 4 3 5% 7% 55%	2150
	21st to end	Night	10% 4 1 5% 3 3 4 4 6% 7% 53%	1890
May	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 6% 7% 46%	5266
	11th to 20th	Night	9% 4 2 5% 3 3 4 4 6% 8% 52%	5265
	21st to end	Night	11% 5% 2 6% 4 3 4 4 6% 9% 47%	5793
June	1st to 10th	Night	4 3 1 3 4 3 4 3 7% 11% 57%	1890
	11th to 20th	Night	3 3 1 4 2 3 3 3 7% 10% 60%	1890
	21st to end	Night	2 2 1 3 2 3 3 3 4 9% 68%	1890
July	1st to 10th	Night	5% 4 2 6% 4 4 5% 3 6% 10% 52%	5308
	11th to 20th	Night	2 2 1 3 2 3 4 3 6% 10% 65%	5307
	21st to end	Night	2 1 1 2 2 2 2 3 4 7% 75%	5770
August	1st to 10th	Night	1 1 1 2 1 2 2 2 4 8% 75%	4156
	11th to 20th	Night	2 1 2 2 2 2 2 4 7% 75%	4118
	21st to end	Night	3 2 1 3 2 2 3 3 5% 9% 68%	5180
September	1st to 10th	Night	7% 4 1 4 4 3 3 2 4 6% 61%	2310
	11th to 20th	Night	8% 5% 2 5% 4 4 4 3 5% 6% 53%	2233
	21st to end	Night	7% 2 1 3 2 2 4 2 5% 7% 64%	2200
October	1st to 10th	Night	5% 2 1 3 3 3 3 3 5% 6% 66%	2200
	11th to 20th	Night	8% 4 1 3 2 2 2 3 4 5% 67%	2395
	21st to end	Night	12% 4 2 3 2 2 2 2 3 4 64%	2640
November	1st to 10th	Night	9% 3 2 3 3 3 2 2 4 4 65%	2489
	11th to 20th	Night	4 3 1 3 2 3 3 3 4 6% 66%	2730
	21st to end	Night	12% 4 1 4 3 3 2 3 5% 5% 57%	2730
December	1st to 10th	Night	9% 6% 2 4 3 3 2 2 3 5% 60%	4380
	11th to 20th	Night	10% 5% 2 6% 3 3 3 3 3 4 57%	4946
	21st to end	Night	12% 7% 2 5% 3 3 3 2 4 5% 55%	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	29% 4 7% 27% 29% 5%	5290
	11th to 20th	Day	20% 2 6% 19% 45% 8%	5310
	21st to end	Day	40% 3 4 21% 22% 11%	5812
February	1st to 10th	Day	34% 5% 6% 21% 24% 9%	2048
	11th to 20th	Day	36% 5% 8% 21% 21% 9%	2059
	21st to end	Day	34% 3 4 14% 31% 14%	1730
March	1st to 10th	Day	30% 4 7% 15% 22% 21%	4975
	11th to 20th	Day	27% 5% 8% 18% 21% 21%	5090
	21st to end	Day	32% 8% 10% 23% 22% 6%	5585
April	1st to 10th	Day	31% 5% 9% 24% 19% 11%	2310
	11th to 20th	Day	35% 6% 9% 19% 20% 11%	2413
	21st to end	Day	35% 6% 10% 23% 21% 5%	2730
May	1st to 10th	Day	40% 7% 11% 20% 16% 6%	7655
	11th to 20th	Day	35% 7% 9% 22% 24% 4	7631
	21st to end	Day	41% 8% 14% 19% 12% 6%	8405
June	1st to 10th	Day	30% 7% 16% 27% 17% 3	2730
	11th to 20th	Day	26% 7% 13% 27% 22% 4	2730
	21st to end	Day	21% 7% 14% 28% 25% 5%	2730
July	1st to 10th	Day	32% 6% 12% 32% 17% 1	7616
	11th to 20th	Day	25% 8% 15% 29% 21% 2	7611
	21st to end	Day	15% 5% 12% 37% 30% 1	8155
August	1st to 10th	Day	14% 4 9% 27% 32% 14%	6000
	11th to 20th	Day	13% 5% 11% 34% 33% 3	5987
	21st to end	Day	20% 6% 13% 34% 24% 4	5941
September	1st to 10th	Day	27% 4 10% 23% 26% 11%	2310
	11th to 20th	Day	34% 6% 8% 22% 21% 8%	2233
	21st to end	Day	23% 6% 7% 23% 29% 11%	2200
October	1st to 10th	Day	13% 4 6% 13% 39% 24%	3562
	11th to 20th	Day	16% 3 6% 20% 38% 17%	3240
	21st to end	Day	22% 3 5% 31% 28% 10%	3538
November	1st to 10th	Day	26% 4 7% 22% 30% 11%	2026
	11th to 20th	Day	20% 3 9% 23% 32% 13%	1890
	21st to end	Day	30% 3 6% 21% 29% 11%	1890
December	1st to 10th	Day	35% 3 5% 31% 23% 4	3030
	11th to 20th	Day	34% 5% 6% 13% 32% 10%	3479
	21st to end	Day	36% 4 5% 26% 21% 8%	4587
January	1st to 10th	Night	28% 5% 7% 26% 29% 6%	7635
	11th to 20th	Night	15% 2 7% 21% 39% 16%	7680
	21st to end	Night	41% 4 6% 15% 24% 11%	8402
February	1st to 10th	Night	32% 4 8% 20% 25% 11%	2589
	11th to 20th	Night	34% 4 8% 20% 24% 11%	2458
	21st to end	Night	31% 3 6% 17% 24% 19%	2076
March	1st to 10th	Night	32% 5% 6% 14% 22% 21%	5088
	11th to 20th	Night	22% 4 8% 20% 19% 28%	5090
	21st to end	Night	32% 7% 8% 19% 30% 5%	5588
April	1st to 10th	Night	28% 6% 10% 21% 21% 14%	2310
	11th to 20th	Night	29% 7% 10% 18% 23% 13%	2150
	21st to end	Night	29% 7% 12% 23% 23% 5%	1890
May	1st to 10th	Night	36% 7% 13% 21% 18% 6%	5293
	11th to 20th	Night	28% 6% 12% 22% 24% 7%	5266
	21st to end	Night	33% 8% 17% 22% 13% 8%	5805
June	1st to 10th	Night	21% 8% 16% 33% 18% 4	1890
	11th to 20th	Night	18% 6% 15% 30% 24% 7%	1890
	21st to end	Night	15% 5% 14% 29% 29% 8%	1890
July	1st to 10th	Night	27% 7% 12% 34% 20% 2	5265
	11th to 20th	Night	16% 7% 17% 33% 25% 3	5256
	21st to end	Night	12% 4 10% 40% 30% 3	5641
August	1st to 10th	Night	9% 3 7% 31% 36% 14%	4146
	11th to 20th	Night	11% 3 8% 34% 37% 6%	4145
	21st to end	Night	14% 4 14% 34% 29% 5%	5180
September	1st to 10th	Night	25% 4 10% 22% 29% 10%	2310
	11th to 20th	Night	30% 7% 8% 22% 22% 10%	2233
	21st to end	Night	20% 5% 7% 22% 30% 16%	2200
October	1st to 10th	Night	12% 3 6% 14% 35% 30%	3590
	11th to 20th	Night	15% 3 6% 19% 34% 23%	3894
	21st to end	Night	22% 3 5% 29% 28% 14%	4252
November	1st to 10th	Night	24% 4 7% 21% 29% 16%	2489
	11th to 20th	Night	20% 4 9% 22% 30% 15%	2730
	21st to end	Night	30% 4 7% 18% 29% 12%	2730
December	1st to 10th	Night	29% 3 5% 27% 28% 9%	4380
	11th to 20th	Night	32% 5% 5% 15% 28% 16%	4947
	21st to end	Night	33% 4 6% 21% 27% 9%	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	14% 15% 37% 27% 7%	330
	11th to 20th	Day	16% 16% 37% 23% 6% 2	330
	21st to end	Day	18% 9% 39% 25% 7% 1	363
February	1st to 10th	Day	16% 11% 44% 22% 7%	330
	11th to 20th	Day	18% 14% 48% 17% 2	330
	21st to end	Day	21% 16% 41% 18% 4	273
March	1st to 10th	Day	20% 19% 34% 21% 7%	330
	11th to 20th	Day	19% 19% 42% 15% 5% 1	330
	21st to end	Day	14% 15% 46% 21% 4 1	363
April	1st to 10th	Day	17% 18% 33% 25% 7%	330
	11th to 20th	Day	16% 18% 42% 20% 3	451
	21st to end	Day	15% 16% 39% 22% 6% 1	750
May	1st to 10th	Day	12% 12% 37% 31% 8% 1	753
	11th to 20th	Day	15% 9% 34% 26% 13% 3	775
	21st to end	Day	12% 7% 32% 30% 15% 4	844
June	1st to 10th	Day	6% 5% 19% 39% 25% 6%	780
	11th to 20th	Day	7% 4 22% 42% 20% 5%	780
	21st to end	Day	4 2 16% 41% 28% 9%	780
July	1st to 10th	Day	6% 2 10% 37% 32% 13%	771
	11th to 20th	Day	3 2 8% 36% 39% 12%	768
	21st to end	Day	3 1 6% 34% 41% 15%	849
August	1st to 10th	Day	3 1 7% 40% 38% 11%	776
	11th to 20th	Day	6% 1 10% 37% 36% 9%	765
	21st to end	Day	6% 1 6% 33% 37% 18%	569
September	1st to 10th	Day	7% 2 8% 29% 39% 16%	360
	11th to 20th	Day	5% 2 9% 29% 40% 15%	339
	21st to end	Day	11% 2 11% 33% 32% 11%	330
October	1st to 10th	Day	4 3 20% 36% 27% 10%	330
	11th to 20th	Day	10% 7% 28% 36% 15% 4	330
	21st to end	Day	21% 9% 27% 27% 13% 3	363
November	1st to 10th	Day	14% 7% 35% 31% 11% 3	330
	11th to 20th	Day	12% 9% 34% 30% 10% 5%	330
	21st to end	Day	16% 14% 41% 20% 8% 2	330
December	1st to 10th	Day	14% 11% 38% 26% 10% 1	330
	11th to 20th	Day	12% 13% 39% 28% 7% 1	330
	21st to end	Day	19% 12% 36% 27% 6%	363
January	1st to 10th	Night	11% 11% 40% 29% 8%	1050
	11th to 20th	Night	12% 15% 37% 30% 6%	1050
	21st to end	Night	13% 11% 36% 34% 6%	1155
February	1st to 10th	Night	13% 11% 43% 28% 4	909
	11th to 20th	Night	16% 16% 40% 25% 4	824
	21st to end	Night	15% 15% 46% 22% 1	692
March	1st to 10th	Night	14% 15% 40% 28% 3	665
	11th to 20th	Night	14% 14% 43% 24% 5%	630
	21st to end	Night	10% 12% 48% 26% 4	693
April	1st to 10th	Night	12% 13% 48% 23% 4	630
	11th to 20th	Night	12% 15% 45% 25% 3	623
	21st to end	Night	11% 14% 44% 23% 7% 1	630
May	1st to 10th	Night	10% 9% 46% 29% 5% 1	630
	11th to 20th	Night	11% 8% 38% 30% 12% 1	621
	21st to end	Night	7% 3 38% 37% 13% 2	683
June	1st to 10th	Night	4 3 27% 44% 21% 2	630
	11th to 20th	Night	4 2 31% 53% 10%	630
	21st to end	Night	2 1 23% 52% 20% 3	630
July	1st to 10th	Night	4 14% 47% 31% 3	615
	11th to 20th	Night	3 13% 50% 31% 2	612
	21st to end	Night	2 1 9% 54% 31% 3	690
August	1st to 10th	Night	2 1 9% 60% 26% 2	627
	11th to 20th	Night	5% 1 9% 52% 29% 3	624
	21st to end	Night	4 1 8% 48% 35% 4	693
September	1st to 10th	Night	7% 3 10% 46% 30% 3	630
	11th to 20th	Night	11% 2 13% 45% 28% 3	609
	21st to end	Night	8% 2 18% 47% 24% 2	600
October	1st to 10th	Night	6% 2 27% 41% 22% 2	600
	11th to 20th	Night	10% 6% 32% 39% 12% 1	795
	21st to end	Night	13% 10% 33% 32% 11% 1	880
November	1st to 10th	Night	9% 7% 36% 35% 11% 1	849
	11th to 20th	Night	7% 10% 37% 36% 9% 1	1050
	21st to end	Night	14% 11% 39% 26% 8%	1050
December	1st to 10th	Night	13% 10% 37% 33% 6%	1050
	11th to 20th	Night	14% 10% 40% 29% 6%	1050
	21st to end	Night	13% 12% 41% 27% 6%	1155

Page generated at 2026-09-11 03:11:53. Version ac07

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