Astronomy Adventures Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astronomy Adventures Observing Site clear sky chart from 2005-11-06 to 2025-11-03.

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 3 3 3 4 4 6% 8% 63%	134773
	Night	2 3 1 4 3 4 4 4 6% 8% 60%	132722
Transparency	Day	18% 5% 10% 22% 28% 18%	129786
	Night	20% 5% 9% 21% 28% 17%	128712
Seeing	Day	5% 12% 27% 30% 21% 6%	16432
	Night	4 7% 29% 41% 18% 1	26517

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	1 5% 3 5% 3 3 3 3 5% 7% 62%	4034
	11th to 20th	Day	1 4 1 3 2 2 3 3 4 6% 70%	4100
	21st to end	Day	2 4 2 4 4 4 4 4 5% 7% 61%	4510
February	1st to 10th	Day	1 4 1 5% 4 4 3 3 5% 7% 62%	1827
	11th to 20th	Day	2 3 1 4 3 3 3 3 5% 8% 62%	1949
	21st to end	Day	1 3 2 3 3 3 3 3 4 7% 66%	1650
March	1st to 10th	Day	1 4 1 2 2 2 3 3 5% 7% 68%	4748
	11th to 20th	Day	2 3 2 5% 4 4 4 4 6% 9% 58%	5000
	21st to end	Day	1 3 1 4 3 3 4 4 5% 8% 64%	5659
April	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 8% 67%	2400
	11th to 20th	Day	2 3 1 4 3 3 3 3 5% 7% 65%	2371
	21st to end	Day	2 3 1 3 2 3 3 3 5% 6% 70%	2400
May	1st to 10th	Day	3 2 1 3 4 3 3 4 6% 8% 62%	5440
	11th to 20th	Day	2 2 1 2 2 3 3 3 5% 7% 69%	5610
	21st to end	Day	1 1 2 2 2 3 3 5% 7% 75%	6925
June	1st to 10th	Day	1 2 1 3 2 3 4 4 7% 9% 65%	2800
	11th to 20th	Day	1 1 1 3 2 2 3 3 5% 7% 72%	2800
	21st to end	Day	4 3 1 3 4 3 4 4 6% 8% 59%	2800
July	1st to 10th	Day	1 2 1 3 4 4 5% 5% 7% 7% 61%	6212
	11th to 20th	Day	1 1 1 3 3 4 6% 6% 10% 13% 52%	5880
	21st to end	Day	3 3 1 4 5% 5% 7% 6% 9% 11% 44%	6401
August	1st to 10th	Day	2 2 1 3 4 4 4 5% 8% 12% 56%	5618
	11th to 20th	Day	2 2 1 3 3 3 5% 4 6% 10% 59%	5366
	21st to end	Day	2 3 1 5% 4 6% 6% 6% 8% 11% 48%	5984
September	1st to 10th	Day	5% 4 1 3 3 4 5% 4 7% 9% 55%	2400
	11th to 20th	Day	4 3 1 3 4 3 5% 4 6% 7% 61%	2400
	21st to end	Day	2 2 1 3 2 2 4 2 4 7% 70%	2175
October	1st to 10th	Day	3 2 1 2 2 2 3 2 4 5% 74%	3270
	11th to 20th	Day	3 3 1 3 3 3 3 3 6% 6% 67%	3250
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 79%	3548
November	1st to 10th	Day	2 2 1 2 2 2 3 3 5% 7% 72%	1927
	11th to 20th	Day	1 3 1 2 2 2 3 3 4 7% 73%	1800
	21st to end	Day	2 3 1 3 3 3 3 2 6% 7% 66%	1800
December	1st to 10th	Day	1 2 2 2 2 2 2 3 5% 78%	2689
	11th to 20th	Day	1 2 1 4 3 3 4 2 5% 7% 68%	3026
	21st to end	Day	1 3 1 3 3 3 3 3 4 6% 70%	4004
January	1st to 10th	Night	1 5% 3 6% 4 4 4 3 6% 7% 58%	5959
	11th to 20th	Night	1 3 1 3 3 3 4 3 4 5% 69%	5880
	21st to end	Night	2 5% 2 4 2 3 5% 4 5% 8% 60%	6468
February	1st to 10th	Night	1 5% 2 4 2 3 3 3 5% 6% 65%	2570
	11th to 20th	Night	3 5% 2 4 4 4 3 3 5% 8% 60%	2338
	21st to end	Night	2 4 2 4 3 3 4 2 4 5% 67%	1980
March	1st to 10th	Night	2 4 1 3 3 3 4 3 5% 5% 66%	5121
	11th to 20th	Night	3 4 2 5% 4 4 5% 4 5% 6% 58%	5000
	21st to end	Night	2 2 1 4 3 3 4 3 5% 7% 67%	5306
April	1st to 10th	Night	3 3 1 3 3 4 3 3 5% 7% 64%	2000
	11th to 20th	Night	3 4 2 4 3 3 4 3 5% 6% 63%	1977
	21st to end	Night	3 2 1 3 3 2 3 3 5% 6% 68%	2000
May	1st to 10th	Night	3 4 1 4 3 4 4 4 7% 8% 56%	4540
	11th to 20th	Night	3 3 1 4 3 3 4 4 5% 7% 62%	4286
	21st to end	Night	2 2 1 3 2 3 4 3 5% 8% 67%	3972
June	1st to 10th	Night	2 3 1 4 4 5% 4 5% 8% 10% 55%	1600
	11th to 20th	Night	2 2 1 3 2 3 4 4 5% 7% 67%	1600
	21st to end	Night	4 4 2 5% 5% 5% 4 5% 8% 8% 51%	1600
July	1st to 10th	Night	2 2 1 5% 5% 4 5% 5% 7% 8% 55%	3748
	11th to 20th	Night	3 2 1 4 5% 6% 7% 7% 11% 14% 39%	4100
	21st to end	Night	5% 4 2 7% 6% 6% 7% 6% 9% 12% 36%	4472
August	1st to 10th	Night	2 3 2 8% 6% 7% 8% 7% 8% 10% 39%	4193
	11th to 20th	Night	2 3 1 5% 5% 6% 7% 6% 9% 13% 42%	4483
	21st to end	Night	4 3 3 7% 6% 7% 7% 6% 10% 13% 33%	4993
September	1st to 10th	Night	4 3 1 5% 5% 5% 5% 5% 8% 11% 48%	2000
	11th to 20th	Night	4 3 2 5% 3 4 4 4 7% 7% 58%	2000
	21st to end	Night	3 3 1 3 3 3 3 3 4 7% 67%	2221
October	1st to 10th	Night	3 2 1 3 2 2 2 2 4 5% 74%	3920
	11th to 20th	Night	4 3 1 4 3 2 4 3 5% 7% 64%	3898
	21st to end	Night	2 1 1 1 2 1 2 2 3 6% 79%	4252
November	1st to 10th	Night	1 2 1 2 2 2 2 3 4 5% 76%	2394
	11th to 20th	Night	1 2 1 2 2 2 2 2 5% 6% 74%	2600
	21st to end	Night	4 4 2 4 4 3 3 3 4 6% 65%	2600
December	1st to 10th	Night	2 2 1 2 1 2 2 2 4 7% 75%	4496
	11th to 20th	Night	2 2 1 2 2 3 3 2 5% 9% 69%	5181
	21st to end	Night	2 3 1 4 3 3 2 3 4 7% 68%	6974

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	21% 3 8% 32% 31% 5%	4035
	11th to 20th	Day	15% 3 9% 28% 28% 17%	4100
	21st to end	Day	20% 5% 7% 18% 35% 14%	4510
February	1st to 10th	Day	20% 4 11% 24% 28% 12%	1827
	11th to 20th	Day	20% 5% 7% 26% 30% 12%	1959
	21st to end	Day	17% 4 6% 20% 32% 22%	1650
March	1st to 10th	Day	15% 4 4 13% 38% 27%	4748
	11th to 20th	Day	18% 8% 7% 22% 31% 14%	5000
	21st to end	Day	16% 4 6% 29% 36% 10%	5659
April	1st to 10th	Day	15% 4 7% 20% 34% 20%	2400
	11th to 20th	Day	19% 4 6% 18% 30% 23%	2374
	21st to end	Day	14% 4 8% 21% 36% 18%	2400
May	1st to 10th	Day	18% 6% 11% 19% 27% 19%	5440
	11th to 20th	Day	14% 5% 7% 14% 32% 29%	5613
	21st to end	Day	9% 4 9% 15% 19% 44%	6926
June	1st to 10th	Day	15% 5% 10% 22% 25% 23%	2800
	11th to 20th	Day	13% 3 5% 14% 23% 42%	2800
	21st to end	Day	22% 6% 9% 19% 21% 24%	2800
July	1st to 10th	Day	19% 9% 11% 15% 20% 26%	6146
	11th to 20th	Day	19% 9% 22% 29% 14% 7%	5811
	21st to end	Day	28% 9% 19% 26% 14% 3	6217
August	1st to 10th	Day	23% 11% 20% 24% 15% 7%	4091
	11th to 20th	Day	19% 6% 12% 26% 23% 14%	3882
	21st to end	Day	28% 11% 18% 23% 18% 3	4311
September	1st to 10th	Day	24% 7% 12% 22% 24% 11%	2400
	11th to 20th	Day	23% 6% 9% 18% 26% 18%	2400
	21st to end	Day	15% 5% 6% 22% 32% 20%	2175
October	1st to 10th	Day	14% 4 4 13% 33% 32%	3270
	11th to 20th	Day	18% 4 7% 16% 31% 24%	3250
	21st to end	Day	10% 2 4 20% 40% 24%	3548
November	1st to 10th	Day	12% 2 6% 22% 40% 17%	1927
	11th to 20th	Day	13% 3 8% 24% 40% 13%	1800
	21st to end	Day	18% 4 7% 22% 33% 16%	1800
December	1st to 10th	Day	11% 3 7% 30% 39% 11%	2689
	11th to 20th	Day	18% 3 8% 31% 34% 6%	3024
	21st to end	Day	16% 3 10% 27% 33% 12%	4004
January	1st to 10th	Night	23% 3 10% 25% 30% 9%	5959
	11th to 20th	Night	15% 4 8% 27% 31% 16%	5880
	21st to end	Night	19% 5% 9% 19% 38% 10%	6468
February	1st to 10th	Night	18% 4 11% 25% 28% 14%	2570
	11th to 20th	Night	21% 5% 9% 25% 27% 14%	2338
	21st to end	Night	18% 3 6% 16% 34% 23%	1980
March	1st to 10th	Night	17% 4 4 12% 39% 24%	5121
	11th to 20th	Night	22% 7% 6% 17% 32% 16%	5000
	21st to end	Night	14% 4 7% 26% 38% 11%	5306
April	1st to 10th	Night	18% 4 6% 19% 31% 22%	2000
	11th to 20th	Night	20% 3 6% 19% 26% 25%	1977
	21st to end	Night	17% 4 8% 20% 34% 17%	2000
May	1st to 10th	Night	21% 7% 11% 19% 21% 20%	4537
	11th to 20th	Night	18% 7% 8% 16% 26% 26%	4287
	21st to end	Night	14% 6% 10% 11% 14% 45%	3972
June	1st to 10th	Night	22% 7% 13% 17% 19% 23%	1600
	11th to 20th	Night	16% 4 6% 14% 21% 40%	1600
	21st to end	Night	26% 8% 12% 15% 16% 22%	1600
July	1st to 10th	Night	26% 8% 12% 19% 14% 20%	3706
	11th to 20th	Night	27% 11% 26% 21% 9% 6%	4046
	21st to end	Night	36% 10% 19% 22% 11% 2	4344
August	1st to 10th	Night	37% 12% 16% 17% 12% 6%	3044
	11th to 20th	Night	28% 11% 16% 17% 17% 11%	3243
	21st to end	Night	37% 10% 17% 22% 9% 4	3597
September	1st to 10th	Night	26% 10% 15% 20% 20% 10%	2000
	11th to 20th	Night	22% 8% 11% 17% 22% 19%	2000
	21st to end	Night	18% 5% 9% 18% 29% 21%	2221
October	1st to 10th	Night	14% 3 6% 15% 30% 32%	3920
	11th to 20th	Night	19% 5% 7% 14% 28% 27%	3898
	21st to end	Night	10% 2 4 22% 40% 23%	4252
November	1st to 10th	Night	11% 3 6% 21% 41% 18%	2394
	11th to 20th	Night	12% 3 6% 23% 40% 16%	2600
	21st to end	Night	21% 4 6% 19% 32% 18%	2600
December	1st to 10th	Night	11% 3 6% 34% 32% 14%	4496
	11th to 20th	Night	14% 3 7% 30% 36% 10%	5182
	21st to end	Night	17% 3 9% 27% 36% 9%	6974

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	5% 15% 38% 31% 11%	300
	11th to 20th	Day	8% 15% 37% 27% 12% 1	300
	21st to end	Day	3 10% 37% 35% 13% 2	330
February	1st to 10th	Day	4 19% 38% 28% 11%	300
	11th to 20th	Day	6% 20% 42% 25% 6%	300
	21st to end	Day	10% 22% 41% 21% 5%	249
March	1st to 10th	Day	6% 23% 33% 28% 10%	405
	11th to 20th	Day	10% 26% 31% 23% 8% 1	500
	21st to end	Day	10% 31% 31% 19% 8%	550
April	1st to 10th	Day	9% 22% 37% 23% 8% 2	500
	11th to 20th	Day	9% 27% 39% 17% 8% 1	492
	21st to end	Day	7% 23% 38% 24% 6% 1	500
May	1st to 10th	Day	7% 20% 39% 24% 8% 3	503
	11th to 20th	Day	6% 16% 31% 30% 14% 3	550
	21st to end	Day	4 12% 31% 36% 15% 2	789
June	1st to 10th	Day	2 7% 22% 39% 25% 4	730
	11th to 20th	Day	2 6% 24% 35% 28% 4	730
	21st to end	Day	6% 4 21% 34% 29% 7%	730
July	1st to 10th	Day	3 1 15% 35% 33% 12%	674
	11th to 20th	Day	3 7% 28% 40% 22%	524
	21st to end	Day	3 1 7% 23% 42% 23%	575
August	1st to 10th	Day	3 2 11% 28% 37% 19%	527
	11th to 20th	Day	3 1 15% 31% 36% 15%	517
	21st to end	Day	4 1 11% 32% 37% 15%	583
September	1st to 10th	Day	4 3 16% 32% 31% 14%	530
	11th to 20th	Day	3 2 26% 36% 26% 8%	530
	21st to end	Day	4 6% 22% 33% 27% 8%	355
October	1st to 10th	Day	5% 10% 25% 24% 28% 9%	330
	11th to 20th	Day	6% 12% 29% 26% 21% 6%	330
	21st to end	Day	8% 11% 26% 28% 21% 5%	363
November	1st to 10th	Day	5% 14% 34% 31% 14% 1	306
	11th to 20th	Day	4 9% 28% 38% 20% 1	300
	21st to end	Day	8% 15% 34% 28% 14% 1	300
December	1st to 10th	Day	4 14% 31% 33% 16% 2	300
	11th to 20th	Day	7% 11% 34% 35% 12% 1	300
	21st to end	Day	8% 19% 35% 32% 5% 1	330
January	1st to 10th	Night	4 6% 34% 40% 15% 1	840
	11th to 20th	Night	4 6% 35% 41% 13% 1	800
	21st to end	Night	5% 8% 33% 41% 13% 2	880
February	1st to 10th	Night	4 9% 39% 39% 9% 1	800
	11th to 20th	Night	5% 11% 41% 36% 7%	784
	21st to end	Night	6% 10% 47% 34% 3	660
March	1st to 10th	Night	6% 10% 40% 35% 9%	800
	11th to 20th	Night	6% 13% 33% 37% 11%	800
	21st to end	Night	4 12% 38% 39% 7%	800
April	1st to 10th	Night	5% 13% 41% 36% 5% 1	600
	11th to 20th	Night	4 17% 36% 36% 7%	593
	21st to end	Night	5% 13% 42% 34% 7% 1	600
May	1st to 10th	Night	5% 9% 36% 40% 10%	600
	11th to 20th	Night	6% 9% 31% 39% 14% 1	591
	21st to end	Night	4 8% 35% 42% 12%	650
June	1st to 10th	Night	2 6% 26% 46% 19% 1	600
	11th to 20th	Night	2 3 29% 49% 16% 1	600
	21st to end	Night	4 4 18% 47% 25% 1	600
July	1st to 10th	Night	4 2 16% 46% 31% 3	585
	11th to 20th	Night	3 1 9% 37% 46% 4	582
	21st to end	Night	6% 1 8% 32% 47% 6%	657
August	1st to 10th	Night	3 1 9% 33% 47% 7%	597
	11th to 20th	Night	3 2 10% 45% 36% 3	594
	21st to end	Night	3 9% 43% 40% 5%	660
September	1st to 10th	Night	4 3 12% 49% 30% 2	600
	11th to 20th	Night	5% 3 14% 49% 29% 1	600
	21st to end	Night	4 4 20% 48% 22% 2	646
October	1st to 10th	Night	5% 4 22% 48% 21% 1	800
	11th to 20th	Night	4 5% 27% 46% 17% 2	800
	21st to end	Night	5% 5% 26% 41% 20% 2	880
November	1st to 10th	Night	3 5% 27% 45% 19% 1	818
	11th to 20th	Night	2 6% 29% 44% 17% 2	1000
	21st to end	Night	5% 9% 29% 39% 16% 2	1000
December	1st to 10th	Night	4 9% 30% 44% 12% 1	1000
	11th to 20th	Night	5% 9% 33% 42% 11%	1000
	21st to end	Night	5% 9% 38% 40% 7%	1100

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