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 2025-05-11.

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	9% 5% 2 5% 3 3 4 3 5% 7% 55%	114403
	Night	8% 5% 2 4 3 3 3 3 4 6% 59%	114760
Transparency	Day	28% 5% 9% 24% 24% 9%	118174
	Night	25% 5% 8% 23% 26% 12%	119118
Seeing	Day	11% 8% 25% 30% 20% 6%	15842
	Night	10% 8% 33% 35% 13% 1	25853

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	10% 8% 2 5% 3 3 3 3 4 5% 54%	4077
	11th to 20th	Day	7% 4 1 3 2 2 2 2 3 6% 67%	4080
	21st to end	Day	17% 9% 2 4 2 2 2 2 4 5% 51%	4488
February	1st to 10th	Day	12% 6% 2 6% 3 3 3 3 5% 6% 51%	1950
	11th to 20th	Day	12% 7% 2 6% 4 3 3 3 5% 5% 51%	1949
	21st to end	Day	17% 8% 2 4 2 2 1 2 3 4 54%	1650
March	1st to 10th	Day	13% 5% 1 4 3 3 3 3 5% 6% 55%	4867
	11th to 20th	Day	8% 5% 2 4 3 3 4 3 4 5% 59%	4980
	21st to end	Day	9% 7% 2 6% 4 4 4 4 6% 7% 49%	5462
April	1st to 10th	Day	11% 6% 2 4 3 3 4 2 4 6% 55%	2200
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 5% 6% 51%	2296
	21st to end	Day	10% 5% 3 5% 4 4 4 4 5% 6% 49%	2600
May	1st to 10th	Day	13% 8% 2 7% 4 4 4 3 5% 6% 43%	4212
	11th to 20th	Day	12% 6% 2 6% 4 3 3 3 5% 6% 49%	4083
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 4 5% 8% 42%	4477
June	1st to 10th	Day	6% 4 2 5% 4 5% 5% 4 6% 8% 51%	2470
	11th to 20th	Day	5% 3 2 4 4 4 4 4 6% 8% 56%	2470
	21st to end	Day	3 4 1 4 3 3 3 3 6% 8% 61%	2470
July	1st to 10th	Day	7% 4 2 5% 3 4 4 4 6% 8% 53%	4120
	11th to 20th	Day	3 3 1 4 4 3 4 4 6% 10% 57%	4120
	21st to end	Day	4 3 1 4 4 4 5% 4 6% 9% 57%	4465
August	1st to 10th	Day	2 3 1 4 3 3 3 3 6% 8% 65%	4104
	11th to 20th	Day	4 2 1 4 3 3 4 4 6% 10% 59%	4041
	21st to end	Day	3 2 1 4 4 4 4 4 6% 8% 60%	4089
September	1st to 10th	Day	11% 4 2 4 3 3 3 2 5% 7% 57%	2090
	11th to 20th	Day	11% 4 2 6% 4 5% 5% 3 5% 7% 47%	2090
	21st to end	Day	8% 2 2 4 3 3 3 3 4 6% 61%	2090
October	1st to 10th	Day	5% 2 1 4 3 4 4 3 4 7% 62%	2076
	11th to 20th	Day	9% 3 2 3 2 2 2 2 4 5% 66%	1900
	21st to end	Day	12% 4 2 4 2 3 2 2 3 4 62%	2090
November	1st to 10th	Day	10% 4 2 5% 3 3 3 3 4 6% 60%	1928
	11th to 20th	Day	6% 3 1 4 2 2 3 2 4 5% 67%	1800
	21st to end	Day	11% 6% 1 4 3 3 3 2 3 5% 59%	1800
December	1st to 10th	Day	11% 7% 3 6% 3 2 3 3 4 5% 53%	2940
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 4 4 53%	3391
	21st to end	Day	13% 8% 2 6% 4 3 3 2 4 5% 50%	4488
January	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	5888
	11th to 20th	Night	5% 4 1 3 2 2 2 2 4 5% 70%	5900
	21st to end	Night	17% 8% 2 4 2 2 2 3 3 4 52%	6490
February	1st to 10th	Night	11% 5% 2 6% 4 3 2 4 4 5% 55%	2466
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 5% 6% 52%	2338
	21st to end	Night	10% 7% 2 5% 3 3 3 2 3 5% 56%	1980
March	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 53%	4976
	11th to 20th	Night	7% 4 1 3 2 3 3 2 4 7% 64%	4980
	21st to end	Night	8% 6% 2 6% 4 4 5% 3 5% 6% 52%	5467
April	1st to 10th	Night	8% 5% 2 4 3 3 3 3 5% 8% 55%	2200
	11th to 20th	Night	9% 4 2 4 3 3 4 3 5% 6% 55%	2047
	21st to end	Night	10% 4 1 5% 3 3 4 3 6% 7% 55%	1800
May	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 5% 48%	2913
	11th to 20th	Night	11% 6% 2 6% 4 3 4 3 5% 7% 49%	2828
	21st to end	Night	7% 4 1 5% 3 3 4 4 6% 8% 53%	3090
June	1st to 10th	Night	4 2 1 3 4 3 4 3 7% 11% 59%	1710
	11th to 20th	Night	3 3 1 4 2 3 3 3 7% 11% 61%	1710
	21st to end	Night	2 2 1 3 3 3 3 3 4 9% 67%	1710
July	1st to 10th	Night	5% 3 1 5% 3 3 4 3 6% 9% 58%	2850
	11th to 20th	Night	2 1 1 2 2 3 2 3 5% 9% 70%	2850
	21st to end	Night	3 2 1 4 3 3 3 3 6% 8% 65%	3098
August	1st to 10th	Night	1 2 1 3 2 2 3 3 5% 9% 71%	2836
	11th to 20th	Night	3 2 1 3 2 2 3 3 5% 10% 68%	2798
	21st to end	Night	3 2 1 3 2 2 3 3 5% 9% 67%	3569
September	1st to 10th	Night	8% 4 1 5% 4 3 3 3 5% 6% 59%	2090
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 5% 6% 52%	2090
	21st to end	Night	7% 2 1 3 2 2 4 2 4 7% 65%	2090
October	1st to 10th	Night	5% 2 1 3 3 3 3 3 5% 7% 65%	2090
	11th to 20th	Night	9% 3 1 3 2 2 2 3 3 4 66%	2275
	21st to end	Night	12% 3 2 3 2 2 2 2 3 5% 64%	2508
November	1st to 10th	Night	9% 3 2 4 3 3 2 2 4 4 65%	2367
	11th to 20th	Night	5% 3 1 3 2 3 3 3 4 7% 66%	2600
	21st to end	Night	12% 4 1 4 3 3 3 3 5% 6% 57%	2600
December	1st to 10th	Night	9% 6% 2 4 3 3 2 2 3 5% 60%	4250
	11th to 20th	Night	10% 6% 2 7% 3 3 3 3 3 4 56%	4816
	21st to end	Night	12% 7% 2 5% 3 3 3 2 4 5% 54%	6490

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	32% 4 7% 27% 25% 5%	4078
	11th to 20th	Day	21% 3 7% 21% 41% 8%	4080
	21st to end	Day	38% 3 5% 18% 24% 12%	4488
February	1st to 10th	Day	35% 5% 7% 21% 23% 9%	1950
	11th to 20th	Day	36% 5% 8% 21% 21% 9%	1959
	21st to end	Day	36% 3 5% 14% 30% 14%	1650
March	1st to 10th	Day	30% 4 7% 15% 22% 22%	4867
	11th to 20th	Day	27% 6% 8% 17% 20% 21%	4980
	21st to end	Day	32% 8% 10% 22% 22% 6%	5464
April	1st to 10th	Day	31% 5% 10% 24% 19% 11%	2200
	11th to 20th	Day	34% 6% 9% 20% 20% 11%	2296
	21st to end	Day	34% 6% 10% 23% 21% 5%	2600
May	1st to 10th	Day	39% 8% 11% 19% 13% 10%	4250
	11th to 20th	Day	34% 8% 12% 24% 21% 2	4087
	21st to end	Day	38% 10% 13% 18% 14% 7%	4489
June	1st to 10th	Day	29% 8% 16% 27% 17% 3	2470
	11th to 20th	Day	25% 7% 13% 27% 23% 5%	2470
	21st to end	Day	22% 7% 13% 28% 25% 5%	2470
July	1st to 10th	Day	27% 7% 12% 36% 16% 2	4056
	11th to 20th	Day	20% 6% 11% 36% 25% 3	4051
	21st to end	Day	22% 7% 13% 33% 25% 1	4325
August	1st to 10th	Day	18% 5% 11% 25% 31% 10%	4090
	11th to 20th	Day	19% 7% 13% 37% 22% 3	4080
	21st to end	Day	20% 6% 16% 37% 17% 3	4088
September	1st to 10th	Day	27% 5% 10% 22% 24% 12%	2090
	11th to 20th	Day	36% 6% 8% 20% 21% 9%	2090
	21st to end	Day	24% 6% 7% 23% 29% 11%	2090
October	1st to 10th	Day	14% 4 6% 13% 39% 25%	3453
	11th to 20th	Day	16% 3 6% 20% 38% 17%	3140
	21st to end	Day	21% 3 5% 32% 28% 10%	3428
November	1st to 10th	Day	27% 4 8% 22% 28% 11%	1928
	11th to 20th	Day	21% 4 9% 23% 30% 14%	1800
	21st to end	Day	30% 3 6% 20% 29% 11%	1800
December	1st to 10th	Day	35% 3 5% 31% 23% 4	2940
	11th to 20th	Day	35% 5% 6% 13% 31% 11%	3389
	21st to end	Day	37% 4 5% 26% 21% 7%	4488
January	1st to 10th	Night	30% 5% 7% 28% 25% 5%	5888
	11th to 20th	Night	18% 3 8% 19% 38% 14%	5900
	21st to end	Night	38% 4 5% 14% 26% 13%	6490
February	1st to 10th	Night	32% 4 8% 19% 24% 12%	2466
	11th to 20th	Night	34% 4 8% 20% 24% 11%	2338
	21st to end	Night	33% 3 6% 16% 23% 19%	1980
March	1st to 10th	Night	32% 5% 6% 14% 23% 21%	4976
	11th to 20th	Night	22% 4 8% 20% 18% 28%	4980
	21st to end	Night	32% 7% 8% 18% 30% 5%	5467
April	1st to 10th	Night	28% 6% 10% 20% 21% 15%	2200
	11th to 20th	Night	29% 7% 9% 18% 24% 13%	2047
	21st to end	Night	29% 6% 11% 24% 24% 6%	1800
May	1st to 10th	Night	38% 7% 10% 18% 19% 8%	2940
	11th to 20th	Night	32% 9% 12% 22% 22% 2	2829
	21st to end	Night	26% 9% 14% 25% 18% 7%	3099
June	1st to 10th	Night	19% 8% 16% 34% 18% 4	1710
	11th to 20th	Night	17% 6% 15% 30% 25% 7%	1710
	21st to end	Night	15% 6% 13% 27% 30% 9%	1710
July	1st to 10th	Night	22% 7% 12% 41% 16% 3	2805
	11th to 20th	Night	12% 4 9% 42% 29% 4	2796
	21st to end	Night	18% 5% 12% 36% 26% 2	2999
August	1st to 10th	Night	12% 4 9% 29% 38% 9%	2826
	11th to 20th	Night	13% 5% 11% 38% 27% 7%	2825
	21st to end	Night	15% 5% 18% 38% 20% 4	3569
September	1st to 10th	Night	26% 5% 10% 20% 28% 11%	2090
	11th to 20th	Night	31% 7% 9% 20% 22% 11%	2090
	21st to end	Night	20% 5% 7% 22% 30% 16%	2090
October	1st to 10th	Night	12% 3 6% 14% 34% 31%	3480
	11th to 20th	Night	15% 3 6% 19% 34% 23%	3774
	21st to end	Night	22% 3 5% 30% 28% 14%	4120
November	1st to 10th	Night	24% 4 7% 21% 27% 17%	2367
	11th to 20th	Night	20% 4 9% 22% 30% 16%	2600
	21st to end	Night	30% 4 7% 18% 29% 12%	2600
December	1st to 10th	Night	29% 3 5% 27% 28% 9%	4250
	11th to 20th	Night	32% 5% 5% 15% 28% 16%	4817
	21st to end	Night	33% 4 6% 21% 27% 9%	6490

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	16% 14% 36% 27% 6%	300
	11th to 20th	Day	16% 15% 37% 24% 6% 2	300
	21st to end	Day	18% 9% 39% 25% 8% 1	330
February	1st to 10th	Day	17% 12% 43% 21% 7%	300
	11th to 20th	Day	18% 14% 49% 16% 3	300
	21st to end	Day	22% 16% 40% 18% 4	249
March	1st to 10th	Day	19% 19% 33% 22% 6%	300
	11th to 20th	Day	19% 20% 40% 15% 4 1	300
	21st to end	Day	15% 16% 45% 19% 5% 1	330
April	1st to 10th	Day	18% 19% 32% 23% 7%	300
	11th to 20th	Day	17% 16% 43% 21% 3	414
	21st to end	Day	14% 15% 40% 23% 7% 2	700
May	1st to 10th	Day	12% 11% 38% 31% 8% 1	703
	11th to 20th	Day	14% 8% 35% 28% 13% 3	676
	21st to end	Day	12% 8% 31% 29% 15% 4	734
June	1st to 10th	Day	6% 4 17% 40% 26% 7%	680
	11th to 20th	Day	5% 3 21% 44% 21% 6%	680
	21st to end	Day	4 2 13% 41% 30% 10%	680
July	1st to 10th	Day	6% 2 10% 36% 34% 14%	671
	11th to 20th	Day	3 2 8% 34% 40% 13%	668
	21st to end	Day	4 1 5% 34% 41% 15%	739
August	1st to 10th	Day	3 1 7% 39% 38% 12%	676
	11th to 20th	Day	7% 2 11% 36% 35% 10%	665
	21st to end	Day	5% 1 5% 32% 38% 20%	487
September	1st to 10th	Day	8% 2 7% 26% 39% 18%	300
	11th to 20th	Day	5% 2 9% 30% 38% 16%	300
	21st to end	Day	12% 3 9% 33% 33% 11%	300
October	1st to 10th	Day	4 3 21% 35% 27% 10%	300
	11th to 20th	Day	10% 7% 29% 36% 15% 3	300
	21st to end	Day	21% 9% 27% 26% 13% 3	330
November	1st to 10th	Day	15% 7% 35% 30% 11% 3	300
	11th to 20th	Day	13% 10% 34% 28% 11% 5%	300
	21st to end	Day	16% 14% 40% 19% 8% 2	300
December	1st to 10th	Day	14% 9% 38% 28% 10% 1	300
	11th to 20th	Day	13% 13% 39% 28% 7%	300
	21st to end	Day	20% 12% 37% 25% 6%	330
January	1st to 10th	Night	12% 11% 41% 29% 8%	1000
	11th to 20th	Night	13% 15% 37% 30% 6%	1000
	21st to end	Night	13% 11% 36% 34% 6%	1100
February	1st to 10th	Night	14% 12% 42% 28% 4	866
	11th to 20th	Night	16% 15% 40% 25% 3	784
	21st to end	Night	16% 16% 45% 22% 2	660
March	1st to 10th	Night	14% 15% 39% 29% 3	633
	11th to 20th	Night	14% 14% 43% 24% 6%	600
	21st to end	Night	10% 13% 47% 26% 4	660
April	1st to 10th	Night	11% 14% 49% 23% 4	600
	11th to 20th	Night	12% 15% 45% 26% 3	593
	21st to end	Night	11% 13% 44% 24% 8% 1	600
May	1st to 10th	Night	10% 9% 45% 30% 5% 1	600
	11th to 20th	Night	11% 6% 39% 31% 12% 1	563
	21st to end	Night	7% 3 38% 36% 13% 2	617
June	1st to 10th	Night	3 2 25% 46% 22% 2	570
	11th to 20th	Night	4 2 30% 54% 11%	570
	21st to end	Night	2 1 22% 52% 22% 3	570
July	1st to 10th	Night	4 14% 46% 32% 3	555
	11th to 20th	Night	3 13% 49% 32% 2	552
	21st to end	Night	2 1 9% 53% 32% 3	624
August	1st to 10th	Night	2 1 8% 59% 28% 2	567
	11th to 20th	Night	6% 1 9% 50% 31% 3	564
	21st to end	Night	4 1 7% 48% 36% 4	627
September	1st to 10th	Night	7% 3 10% 44% 32% 3	570
	11th to 20th	Night	11% 2 13% 42% 29% 3	570
	21st to end	Night	8% 1 18% 46% 25% 2	570
October	1st to 10th	Night	6% 2 28% 39% 23% 2	570
	11th to 20th	Night	10% 6% 32% 39% 11% 1	755
	21st to end	Night	13% 10% 33% 33% 11% 1	836
November	1st to 10th	Night	10% 6% 36% 36% 12% 1	807
	11th to 20th	Night	7% 11% 37% 35% 9% 1	1000
	21st to end	Night	14% 11% 40% 26% 9% 1	1000
December	1st to 10th	Night	13% 10% 36% 34% 6%	1000
	11th to 20th	Night	15% 10% 39% 30% 6%	1000
	21st to end	Night	14% 12% 40% 27% 7%	1100

Page generated at 2025-05-08 08:33:56. Version ac07

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