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 2024-01-16.

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	2 3 1 3 4 2 4 5% 6% 8% 62%	74718
	Night	3 3 2 3 4 2 4 4 5% 8% 61%	75088
Transparency	Day	20% 6% 10% 22% 27% 16%	74441
	Night	20% 6% 10% 21% 27% 15%	74886
Seeing	Day	5% 12% 26% 30% 20% 6%	13961
	Night	4 7% 28% 41% 18% 1	24202

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 2 1 2 3 3 4 4 5% 10% 64%	1684
	11th to 20th	Day	3 4 2 3 2 2 3 4 4 5% 69%	1665
	21st to end	Day	3 4 2 3 3 2 3 4 4 7% 64%	1782
February	1st to 10th	Day	2 4 1 5% 4 3 3 4 5% 7% 63%	1644
	11th to 20th	Day	2 3 2 4 4 2 4 4 5% 9% 61%	1749
	21st to end	Day	1 3 2 3 3 3 3 4 4 8% 65%	1480
March	1st to 10th	Day	2 3 1 3 3 3 4 5% 7% 8% 61%	1894
	11th to 20th	Day	2 5% 3 4 4 3 4 3 4 7% 61%	1980
	21st to end	Day	2 3 1 3 3 2 4 5% 5% 9% 62%	2248
April	1st to 10th	Day	2 2 1 2 3 2 3 4 5% 8% 68%	2160
	11th to 20th	Day	2 3 2 4 4 2 3 4 5% 7% 64%	2131
	21st to end	Day	3 3 1 3 2 2 3 3 5% 7% 68%	2160
May	1st to 10th	Day	2 2 1 3 3 2 3 4 5% 9% 68%	2160
	11th to 20th	Day	3 3 2 3 4 3 4 5% 6% 7% 60%	2197
	21st to end	Day	2 2 1 2 3 2 3 4 5% 8% 67%	2724
June	1st to 10th	Day	2 1 3 2 2 3 5% 7% 10% 65%	2520
	11th to 20th	Day	2 1 1 2 3 1 3 4 4 7% 72%	2520
	21st to end	Day	4 3 1 2 4 2 4 5% 6% 8% 60%	2520
July	1st to 10th	Day	3 3 2 3 6% 3 6% 7% 8% 10% 48%	2477
	11th to 20th	Day	2 3 1 3 5% 3 5% 7% 8% 12% 50%	2340
	21st to end	Day	3 3 2 4 6% 4 7% 9% 9% 11% 40%	2545
August	1st to 10th	Day	3 3 2 4 5% 4 5% 7% 9% 11% 48%	2267
	11th to 20th	Day	2 3 2 4 5% 3 6% 7% 8% 11% 49%	2123
	21st to end	Day	3 3 2 3 5% 4 5% 6% 7% 11% 52%	2376
September	1st to 10th	Day	5% 4 2 3 4 3 5% 5% 7% 9% 53%	2160
	11th to 20th	Day	4 3 1 3 4 2 5% 5% 6% 7% 61%	2160
	21st to end	Day	2 2 1 2 3 2 4 3 4 7% 70%	1959
October	1st to 10th	Day	6% 3 2 4 3 3 4 4 5% 6% 61%	1800
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 4 77%	1800
	21st to end	Day	3 3 1 3 3 2 2 2 3 4 74%	1980
November	1st to 10th	Day	2 1 1 1 2 1 3 3 5% 7% 74%	1810
	11th to 20th	Day	1 2 1 2 2 1 3 4 4 7% 72%	1710
	21st to end	Day	2 3 2 3 4 3 3 3 5% 7% 66%	1710
December	1st to 10th	Day	2 4 1 3 3 2 4 4 5% 7% 66%	1563
	11th to 20th	Day	2 3 2 5% 4 3 4 4 6% 7% 60%	1926
	21st to end	Day	2 4 1 3 4 2 3 4 4 7% 65%	2794
January	1st to 10th	Night	2 3 2 3 3 1 3 3 4 7% 68%	2495
	11th to 20th	Night	3 3 1 3 5% 2 4 3 4 6% 66%	2405
	21st to end	Night	3 5% 1 3 3 2 3 4 5% 8% 64%	2574
February	1st to 10th	Night	1 5% 2 3 3 2 3 4 4 6% 66%	2313
	11th to 20th	Night	3 5% 2 3 4 2 4 4 4 8% 60%	2098
	21st to end	Night	2 4 2 3 4 2 4 3 4 5% 67%	1776
March	1st to 10th	Night	3 5% 2 3 3 3 4 4 5% 7% 61%	2063
	11th to 20th	Night	4 4 2 4 4 2 5% 4 5% 6% 61%	1980
	21st to end	Night	3 3 2 4 3 2 4 4 4 6% 64%	2106
April	1st to 10th	Night	3 2 2 3 3 2 4 3 6% 7% 65%	1800
	11th to 20th	Night	3 4 2 3 4 3 4 4 5% 6% 63%	1777
	21st to end	Night	3 2 1 3 3 2 3 4 5% 6% 68%	1800
May	1st to 10th	Night	3 3 1 3 4 2 3 4 6% 7% 64%	1800
	11th to 20th	Night	4 4 2 4 5% 1 5% 5% 5% 8% 56%	1692
	21st to end	Night	3 2 1 3 3 2 4 4 4 8% 67%	1552
June	1st to 10th	Night	1 2 1 3 4 3 4 7% 7% 11% 55%	1440
	11th to 20th	Night	2 1 1 3 3 2 4 4 5% 7% 69%	1440
	21st to end	Night	4 3 2 3 5% 3 5% 6% 8% 8% 53%	1440
July	1st to 10th	Night	3 3 1 5% 6% 3 6% 8% 9% 11% 44%	1478
	11th to 20th	Night	3 3 2 3 5% 3 7% 9% 9% 14% 43%	1620
	21st to end	Night	4 4 2 6% 7% 5% 6% 7% 9% 16% 35%	1773
August	1st to 10th	Night	2 3 2 5% 6% 3 7% 9% 10% 11% 42%	1671
	11th to 20th	Night	3 5% 2 5% 6% 5% 6% 7% 8% 13% 39%	1778
	21st to end	Night	3 2 2 4 5% 4 6% 6% 9% 12% 47%	1980
September	1st to 10th	Night	4 3 1 4 6% 4 5% 6% 9% 11% 47%	1800
	11th to 20th	Night	4 3 2 4 4 3 4 5% 7% 8% 58%	1800
	21st to end	Night	3 3 1 3 3 2 4 4 4 6% 67%	1997
October	1st to 10th	Night	6% 3 2 4 4 2 4 3 5% 6% 62%	2160
	11th to 20th	Night	3 2 1 2 2 1 2 3 4 5% 76%	2160
	21st to end	Night	4 2 1 2 3 1 2 3 3 6% 72%	2376
November	1st to 10th	Night	1 2 1 1 2 1 2 3 4 5% 78%	2248
	11th to 20th	Night	1 2 1 2 3 2 2 3 5% 6% 74%	2470
	21st to end	Night	4 3 2 3 4 2 3 3 4 6% 65%	2470
December	1st to 10th	Night	3 4 1 2 3 1 3 4 5% 7% 67%	2612
	11th to 20th	Night	3 3 1 3 4 3 4 4 5% 9% 60%	3271
	21st to end	Night	3 5% 2 4 4 3 3 3 4 6% 63%	4873

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	15% 5% 11% 32% 31% 6%	1685
	11th to 20th	Day	18% 4 8% 29% 33% 7%	1665
	21st to end	Day	20% 5% 7% 25% 32% 11%	1782
February	1st to 10th	Day	20% 4 12% 26% 27% 11%	1644
	11th to 20th	Day	22% 5% 7% 26% 28% 12%	1759
	21st to end	Day	18% 4 6% 21% 33% 19%	1480
March	1st to 10th	Day	19% 6% 7% 23% 30% 16%	1894
	11th to 20th	Day	23% 3 8% 19% 32% 15%	1980
	21st to end	Day	18% 5% 9% 22% 32% 14%	2248
April	1st to 10th	Day	15% 4 7% 20% 34% 20%	2160
	11th to 20th	Day	20% 4 6% 19% 30% 21%	2134
	21st to end	Day	16% 4 8% 22% 34% 17%	2160
May	1st to 10th	Day	15% 4 8% 18% 32% 23%	2160
	11th to 20th	Day	23% 6% 10% 18% 25% 17%	2200
	21st to end	Day	16% 6% 9% 18% 24% 28%	2724
June	1st to 10th	Day	14% 5% 10% 22% 25% 23%	2520
	11th to 20th	Day	13% 3 6% 13% 23% 42%	2520
	21st to end	Day	21% 6% 9% 19% 22% 24%	2520
July	1st to 10th	Day	27% 11% 14% 16% 17% 15%	2410
	11th to 20th	Day	23% 9% 18% 25% 15% 9%	2271
	21st to end	Day	31% 12% 21% 24% 9% 2	2404
August	1st to 10th	Day	26% 12% 18% 21% 14% 9%	2252
	11th to 20th	Day	26% 10% 15% 23% 18% 8%	2123
	21st to end	Day	24% 9% 14% 25% 22% 5%	2376
September	1st to 10th	Day	25% 7% 12% 22% 22% 11%	2160
	11th to 20th	Day	23% 6% 9% 18% 25% 19%	2160
	21st to end	Day	15% 5% 6% 23% 33% 19%	1959
October	1st to 10th	Day	24% 6% 7% 19% 25% 19%	1800
	11th to 20th	Day	14% 3 4 14% 34% 32%	1800
	21st to end	Day	16% 3 6% 21% 36% 18%	1980
November	1st to 10th	Day	11% 2 7% 22% 39% 18%	1810
	11th to 20th	Day	13% 3 8% 25% 39% 12%	1710
	21st to end	Day	18% 4 6% 23% 33% 15%	1710
December	1st to 10th	Day	17% 4 11% 33% 26% 8%	1563
	11th to 20th	Day	23% 4 13% 23% 29% 8%	1924
	21st to end	Day	19% 3 9% 21% 32% 16%	2794
January	1st to 10th	Night	17% 4 11% 29% 32% 7%	2495
	11th to 20th	Night	19% 5% 7% 27% 32% 9%	2405
	21st to end	Night	18% 5% 10% 23% 36% 9%	2574
February	1st to 10th	Night	18% 4 11% 26% 28% 13%	2313
	11th to 20th	Night	23% 5% 9% 24% 26% 14%	2098
	21st to end	Night	18% 3 7% 17% 34% 21%	1776
March	1st to 10th	Night	20% 5% 6% 24% 29% 16%	2063
	11th to 20th	Night	22% 5% 8% 19% 31% 15%	1980
	21st to end	Night	20% 5% 7% 19% 34% 15%	2106
April	1st to 10th	Night	17% 4 7% 20% 32% 21%	1800
	11th to 20th	Night	21% 3 6% 20% 27% 23%	1777
	21st to end	Night	18% 4 8% 19% 35% 16%	1800
May	1st to 10th	Night	18% 4 10% 14% 31% 22%	1800
	11th to 20th	Night	27% 7% 10% 18% 20% 17%	1693
	21st to end	Night	17% 5% 10% 17% 23% 28%	1552
June	1st to 10th	Night	21% 8% 14% 17% 19% 22%	1440
	11th to 20th	Night	15% 4 6% 14% 20% 40%	1440
	21st to end	Night	24% 9% 12% 15% 17% 24%	1440
July	1st to 10th	Night	31% 11% 14% 19% 12% 13%	1436
	11th to 20th	Night	25% 13% 21% 19% 13% 8%	1566
	21st to end	Night	34% 14% 24% 20% 7% 1	1674
August	1st to 10th	Night	28% 14% 20% 19% 11% 8%	1662
	11th to 20th	Night	31% 13% 19% 17% 12% 7%	1778
	21st to end	Night	24% 10% 17% 27% 14% 7%	1980
September	1st to 10th	Night	26% 10% 16% 19% 19% 10%	1800
	11th to 20th	Night	22% 8% 12% 17% 21% 20%	1800
	21st to end	Night	19% 5% 8% 18% 29% 21%	1997
October	1st to 10th	Night	23% 5% 9% 20% 20% 23%	2160
	11th to 20th	Night	12% 4 5% 14% 34% 31%	2160
	21st to end	Night	15% 3 6% 19% 38% 19%	2376
November	1st to 10th	Night	10% 3 6% 22% 41% 18%	2248
	11th to 20th	Night	12% 3 6% 23% 40% 15%	2470
	21st to end	Night	21% 4 6% 20% 32% 17%	2470
December	1st to 10th	Night	17% 4 10% 31% 29% 8%	2612
	11th to 20th	Night	21% 4 11% 26% 30% 9%	3272
	21st to end	Night	21% 3 10% 22% 32% 12%	4873

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% 30% 11%	270
	11th to 20th	Day	9% 15% 36% 27% 12%	255
	21st to end	Day	3 10% 38% 38% 10% 1	264
February	1st to 10th	Day	3 18% 39% 28% 12%	240
	11th to 20th	Day	8% 22% 41% 22% 8%	240
	21st to end	Day	9% 25% 40% 22% 4	198
March	1st to 10th	Day	5% 23% 33% 28% 10%	334
	11th to 20th	Day	10% 26% 30% 23% 10% 1	420
	21st to end	Day	10% 34% 30% 18% 8%	462
April	1st to 10th	Day	8% 22% 37% 23% 8% 2	420
	11th to 20th	Day	10% 28% 36% 16% 8% 1	412
	21st to end	Day	9% 24% 38% 24% 6% 1	420
May	1st to 10th	Day	4 20% 42% 25% 8% 1	423
	11th to 20th	Day	5% 14% 33% 30% 15% 3	467
	21st to end	Day	4 13% 31% 37% 14% 1	679
June	1st to 10th	Day	2 6% 22% 39% 27% 4	630
	11th to 20th	Day	2 6% 24% 37% 28% 3	630
	21st to end	Day	6% 4 20% 34% 29% 6%	630
July	1st to 10th	Day	4 2 14% 35% 33% 12%	580
	11th to 20th	Day	3 7% 29% 41% 20%	444
	21st to end	Day	2 1 7% 23% 42% 25%	487
August	1st to 10th	Day	3 3 11% 29% 36% 18%	447
	11th to 20th	Day	3 1 17% 33% 34% 12%	437
	21st to end	Day	5% 1 11% 33% 35% 15%	495
September	1st to 10th	Day	5% 3 16% 33% 29% 14%	450
	11th to 20th	Day	4 2 24% 36% 26% 8%	450
	21st to end	Day	3 6% 24% 31% 27% 9%	293
October	1st to 10th	Day	6% 10% 27% 24% 26% 6%	270
	11th to 20th	Day	6% 12% 26% 28% 22% 6%	270
	21st to end	Day	10% 11% 26% 27% 20% 5%	297
November	1st to 10th	Day	4 16% 33% 32% 14% 1	270
	11th to 20th	Day	4 8% 29% 36% 21% 1	270
	21st to end	Day	9% 16% 34% 27% 14% 1	270
December	1st to 10th	Day	4 16% 32% 34% 13% 1	270
	11th to 20th	Day	8% 12% 36% 34% 10% 1	270
	21st to end	Day	9% 20% 35% 32% 4	297
January	1st to 10th	Night	5% 6% 33% 41% 15% 1	798
	11th to 20th	Night	4 6% 34% 41% 14% 1	740
	21st to end	Night	5% 8% 33% 41% 12% 2	792
February	1st to 10th	Night	3 8% 38% 41% 10% 1	720
	11th to 20th	Night	6% 10% 39% 36% 8%	704
	21st to end	Night	5% 10% 46% 35% 3	592
March	1st to 10th	Night	5% 10% 40% 36% 9%	720
	11th to 20th	Night	6% 12% 32% 39% 12%	720
	21st to end	Night	4 13% 36% 39% 8%	720
April	1st to 10th	Night	4 13% 41% 37% 4 1	540
	11th to 20th	Night	4 16% 35% 37% 8%	533
	21st to end	Night	5% 13% 41% 34% 7% 1	540
May	1st to 10th	Night	4 8% 37% 40% 11%	540
	11th to 20th	Night	6% 8% 30% 40% 15% 1	531
	21st to end	Night	4 8% 35% 41% 13%	584
June	1st to 10th	Night	2 6% 25% 46% 20% 1	540
	11th to 20th	Night	2 3 29% 49% 17% 1	540
	21st to end	Night	3 3 19% 48% 26% 1	540
July	1st to 10th	Night	4 1 13% 45% 33% 3	525
	11th to 20th	Night	4 1 9% 35% 48% 4	522
	21st to end	Night	6% 1 8% 30% 49% 7%	591
August	1st to 10th	Night	3 1 9% 31% 48% 8%	537
	11th to 20th	Night	3 2 11% 45% 36% 3	534
	21st to end	Night	3 1 8% 43% 40% 5%	594
September	1st to 10th	Night	4 3 12% 49% 31% 2	540
	11th to 20th	Night	5% 3 14% 47% 31% 1	540
	21st to end	Night	4 4 21% 45% 24% 2	580
October	1st to 10th	Night	6% 4 23% 46% 20% 1	720
	11th to 20th	Night	3 4 27% 47% 17% 2	720
	21st to end	Night	5% 6% 26% 39% 21% 3	792
November	1st to 10th	Night	2 5% 26% 46% 20% 1	768
	11th to 20th	Night	2 6% 28% 45% 18% 2	950
	21st to end	Night	5% 9% 28% 39% 17% 2	950
December	1st to 10th	Night	5% 10% 30% 43% 12%	950
	11th to 20th	Night	6% 10% 32% 41% 11%	950
	21st to end	Night	6% 9% 37% 41% 7%	1045

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