Ancient Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ancient Sky Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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	11% 5% 2 4 4 3 4 4 5% 7% 49%	75023
	Night	11% 5% 2 4 4 2 3 4 5% 7% 54%	75052
Transparency	Day	34% 5% 10% 21% 20% 9%	74748
	Night	32% 4 11% 21% 20% 12%	74847
Seeing	Day	13% 7% 22% 30% 22% 6%	13820
	Night	14% 7% 28% 35% 15% 1	23885

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	12% 6% 2 4 3 1 3 3 4 6% 55%	1707
	11th to 20th	Day	17% 5% 2 4 3 2 3 3 4 5% 51%	1710
	21st to end	Day	14% 7% 2 4 4 2 3 3 4 5% 52%	1917
February	1st to 10th	Day	18% 6% 3 4 3 3 2 4 4 6% 48%	1800
	11th to 20th	Day	16% 6% 2 5% 3 3 3 4 5% 6% 47%	1749
	21st to end	Day	19% 6% 2 4 3 2 2 4 3 4 50%	1480
March	1st to 10th	Day	19% 6% 3 6% 5% 3 3 4 4 6% 42%	1964
	11th to 20th	Day	15% 6% 3 5% 4 3 4 4 4 6% 46%	1980
	21st to end	Day	12% 6% 2 4 4 2 3 4 5% 6% 50%	2178
April	1st to 10th	Day	13% 8% 3 6% 5% 3 4 4 5% 6% 44%	1980
	11th to 20th	Day	16% 6% 2 5% 4 3 4 3 4 6% 45%	1956
	21st to end	Day	14% 6% 2 4 4 3 4 4 5% 6% 49%	2316
May	1st to 10th	Day	12% 6% 4 6% 5% 3 6% 5% 5% 6% 43%	2340
	11th to 20th	Day	13% 7% 2 5% 5% 3 6% 4 5% 7% 41%	2302
	21st to end	Day	14% 5% 3 6% 6% 4 6% 6% 7% 8% 34%	2531
June	1st to 10th	Day	5% 3 2 4 4 4 5% 5% 7% 8% 52%	2340
	11th to 20th	Day	7% 3 1 4 4 3 4 5% 6% 10% 52%	2340
	21st to end	Day	7% 4 3 4 6% 4 7% 6% 8% 10% 42%	2340
July	1st to 10th	Day	9% 4 2 5% 6% 5% 6% 6% 6% 10% 41%	2340
	11th to 20th	Day	5% 2 2 3 3 3 6% 5% 7% 10% 54%	2340
	21st to end	Day	4 2 2 3 4 3 5% 5% 6% 10% 55%	2546
August	1st to 10th	Day	1 1 1 2 3 2 4 4 5% 10% 66%	2328
	11th to 20th	Day	6% 4 2 4 4 3 5% 5% 6% 10% 51%	2180
	21st to end	Day	5% 3 2 5% 5% 3 7% 5% 7% 11% 48%	2273
September	1st to 10th	Day	8% 4 2 5% 6% 3 5% 5% 6% 7% 49%	1980
	11th to 20th	Day	11% 5% 2 5% 6% 4 6% 5% 5% 9% 42%	1980
	21st to end	Day	6% 4 2 4 5% 2 5% 5% 6% 8% 54%	1980
October	1st to 10th	Day	7% 3 2 4 4 3 4 4 4 6% 59%	1980
	11th to 20th	Day	12% 4 2 4 3 3 3 3 4 5% 56%	1836
	21st to end	Day	11% 4 2 4 3 2 2 3 4 5% 60%	1980
November	1st to 10th	Day	13% 6% 2 4 4 2 4 3 4 6% 52%	1858
	11th to 20th	Day	9% 5% 2 4 5% 3 4 4 4 6% 54%	1776
	21st to end	Day	16% 6% 3 5% 5% 2 3 4 4 5% 48%	1710
December	1st to 10th	Day	13% 6% 2 5% 3 2 4 4 4 5% 51%	1710
	11th to 20th	Day	14% 6% 3 5% 5% 2 4 5% 4 5% 46%	2161
	21st to end	Day	15% 6% 2 4 3 1 2 3 3 4 55%	3135
January	1st to 10th	Night	15% 5% 1 3 3 2 3 3 4 6% 56%	2458
	11th to 20th	Night	14% 6% 2 5% 3 2 3 2 3 6% 53%	2470
	21st to end	Night	15% 6% 3 5% 4 2 3 3 3 6% 51%	2597
February	1st to 10th	Night	16% 7% 3 5% 3 2 2 3 4 6% 51%	2160
	11th to 20th	Night	18% 5% 2 4 3 2 3 3 3 5% 51%	2098
	21st to end	Night	18% 6% 2 4 5% 3 4 2 3 6% 48%	1776
March	1st to 10th	Night	15% 7% 3 3 4 2 3 4 6% 7% 45%	1994
	11th to 20th	Night	14% 5% 3 5% 5% 3 4 5% 5% 6% 46%	1980
	21st to end	Night	12% 5% 3 4 4 2 4 4 4 7% 52%	2178
April	1st to 10th	Night	11% 6% 2 4 5% 3 4 4 4 8% 49%	1980
	11th to 20th	Night	15% 5% 2 4 5% 2 4 4 5% 6% 46%	1953
	21st to end	Night	13% 6% 3 5% 3 2 3 4 5% 7% 50%	1640
May	1st to 10th	Night	10% 5% 3 5% 4 2 4 5% 6% 9% 48%	1620
	11th to 20th	Night	11% 6% 2 5% 5% 2 4 4 5% 8% 48%	1593
	21st to end	Night	11% 5% 2 4 3 3 5% 6% 7% 8% 46%	1745
June	1st to 10th	Night	4 3 2 3 3 2 3 4 7% 9% 60%	1620
	11th to 20th	Night	4 2 1 3 3 3 4 4 6% 8% 61%	1620
	21st to end	Night	6% 3 2 3 4 3 5% 4 7% 9% 54%	1620
July	1st to 10th	Night	6% 4 1 4 4 3 4 7% 6% 11% 51%	1620
	11th to 20th	Night	2 1 1 2 3 2 3 4 5% 10% 66%	1620
	21st to end	Night	2 2 1 2 3 2 3 5% 6% 12% 61%	1772
August	1st to 10th	Night	1 1 1 1 1 1 2 4 5% 10% 73%	1611
	11th to 20th	Night	6% 3 2 4 4 2 3 4 6% 9% 56%	1716
	21st to end	Night	4 4 1 4 5% 3 5% 5% 6% 11% 53%	2081
September	1st to 10th	Night	8% 5% 2 4 4 3 4 5% 7% 7% 52%	1980
	11th to 20th	Night	10% 5% 2 5% 6% 3 3 4 6% 8% 47%	1980
	21st to end	Night	8% 4 1 3 4 2 3 4 5% 7% 58%	1980
October	1st to 10th	Night	6% 3 1 3 3 2 4 3 4 7% 64%	1980
	11th to 20th	Night	11% 4 2 4 3 2 2 4 4 6% 57%	2106
	21st to end	Night	13% 5% 2 3 3 1 3 3 3 5% 59%	2376
November	1st to 10th	Night	12% 5% 2 4 3 2 3 3 4 6% 56%	2220
	11th to 20th	Night	7% 6% 2 4 3 2 4 3 4 6% 58%	2400
	21st to end	Night	16% 6% 2 4 4 2 4 4 4 5% 48%	2470
December	1st to 10th	Night	12% 5% 2 4 4 3 3 4 4 7% 53%	2470
	11th to 20th	Night	13% 8% 3 6% 4 2 3 4 4 5% 48%	3036
	21st to end	Night	16% 7% 2 3 3 1 2 2 3 4 56%	4532

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% 3 6% 21% 24% 14%	1708
	11th to 20th	Day	36% 4 6% 16% 25% 13%	1710
	21st to end	Day	37% 3 6% 14% 26% 15%	1917
February	1st to 10th	Day	41% 3 6% 12% 22% 15%	1800
	11th to 20th	Day	40% 3 7% 18% 19% 13%	1759
	21st to end	Day	39% 4 5% 16% 20% 16%	1480
March	1st to 10th	Day	45% 4 8% 13% 20% 11%	1964
	11th to 20th	Day	41% 4 7% 13% 21% 14%	1980
	21st to end	Day	35% 4 10% 17% 22% 12%	2178
April	1st to 10th	Day	42% 4 8% 19% 15% 12%	1980
	11th to 20th	Day	41% 4 8% 17% 19% 11%	1956
	21st to end	Day	38% 4 6% 21% 22% 9%	2316
May	1st to 10th	Day	41% 6% 9% 17% 19% 8%	2340
	11th to 20th	Day	43% 6% 10% 19% 18% 4	2306
	21st to end	Day	45% 8% 12% 20% 14% 2	2531
June	1st to 10th	Day	28% 7% 13% 29% 20% 4	2340
	11th to 20th	Day	27% 6% 14% 29% 18% 5%	2340
	21st to end	Day	34% 8% 13% 24% 18% 3	2340
July	1st to 10th	Day	37% 7% 17% 26% 11% 1	2276
	11th to 20th	Day	23% 7% 17% 27% 22% 3	2271
	21st to end	Day	21% 7% 21% 34% 16% 1	2405
August	1st to 10th	Day	15% 5% 16% 39% 23% 2	2314
	11th to 20th	Day	26% 8% 16% 30% 14% 5%	2180
	21st to end	Day	29% 9% 15% 27% 16% 4	2273
September	1st to 10th	Day	34% 6% 11% 24% 17% 9%	1980
	11th to 20th	Day	38% 8% 11% 20% 17% 5%	1980
	21st to end	Day	28% 6% 11% 22% 22% 12%	1980
October	1st to 10th	Day	27% 5% 10% 22% 23% 13%	1980
	11th to 20th	Day	32% 3 7% 17% 22% 19%	1836
	21st to end	Day	29% 3 5% 16% 27% 20%	1980
November	1st to 10th	Day	36% 3 9% 19% 22% 10%	1858
	11th to 20th	Day	33% 4 9% 19% 21% 15%	1776
	21st to end	Day	40% 4 6% 15% 20% 14%	1710
December	1st to 10th	Day	36% 4 9% 19% 23% 9%	1710
	11th to 20th	Day	40% 4 8% 12% 23% 13%	2159
	21st to end	Day	33% 3 5% 14% 27% 16%	3135
January	1st to 10th	Night	32% 2 5% 18% 24% 18%	2458
	11th to 20th	Night	36% 2 6% 16% 25% 15%	2470
	21st to end	Night	37% 3 5% 15% 21% 19%	2597
February	1st to 10th	Night	38% 3 7% 13% 21% 19%	2160
	11th to 20th	Night	39% 3 6% 17% 20% 16%	2098
	21st to end	Night	41% 2 6% 14% 18% 18%	1776
March	1st to 10th	Night	39% 5% 9% 15% 23% 10%	1994
	11th to 20th	Night	41% 5% 9% 12% 20% 14%	1980
	21st to end	Night	34% 5% 11% 18% 20% 13%	2178
April	1st to 10th	Night	36% 5% 9% 19% 17% 14%	1980
	11th to 20th	Night	39% 5% 10% 14% 18% 14%	1953
	21st to end	Night	36% 4 10% 21% 20% 10%	1640
May	1st to 10th	Night	34% 6% 12% 21% 19% 9%	1620
	11th to 20th	Night	35% 6% 12% 27% 17% 4	1594
	21st to end	Night	34% 6% 16% 29% 14% 2	1746
June	1st to 10th	Night	20% 8% 16% 30% 23% 4	1620
	11th to 20th	Night	21% 5% 16% 31% 20% 6%	1620
	21st to end	Night	27% 6% 16% 29% 19% 4	1620
July	1st to 10th	Night	26% 10% 20% 30% 12% 1	1575
	11th to 20th	Night	15% 7% 22% 31% 24% 1	1566
	21st to end	Night	14% 7% 25% 40% 13%	1673
August	1st to 10th	Night	9% 5% 21% 40% 22% 3	1601
	11th to 20th	Night	25% 7% 20% 30% 14% 4	1716
	21st to end	Night	26% 7% 20% 29% 14% 5%	2081
September	1st to 10th	Night	29% 8% 13% 25% 17% 8%	1980
	11th to 20th	Night	35% 6% 16% 22% 15% 6%	1980
	21st to end	Night	26% 5% 13% 24% 19% 13%	1980
October	1st to 10th	Night	22% 4 11% 26% 22% 15%	1980
	11th to 20th	Night	29% 4 8% 16% 22% 20%	2106
	21st to end	Night	30% 3 5% 17% 22% 23%	2376
November	1st to 10th	Night	32% 3 8% 20% 23% 14%	2220
	11th to 20th	Night	29% 4 9% 20% 23% 16%	2400
	21st to end	Night	40% 3 7% 15% 20% 15%	2470
December	1st to 10th	Night	33% 4 9% 21% 21% 12%	2470
	11th to 20th	Night	40% 2 6% 12% 22% 18%	3037
	21st to end	Night	35% 2 5% 15% 28% 16%	4532

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	20% 14% 35% 26% 4	270
	11th to 20th	Day	24% 13% 33% 23% 7% 1	270
	21st to end	Day	18% 18% 40% 18% 5%	282
February	1st to 10th	Day	25% 15% 36% 19% 5%	240
	11th to 20th	Day	26% 18% 35% 17% 4	240
	21st to end	Day	31% 16% 35% 18%	198
March	1st to 10th	Day	24% 14% 38% 20% 4	240
	11th to 20th	Day	17% 21% 35% 20% 8%	240
	21st to end	Day	17% 16% 42% 19% 7%	264
April	1st to 10th	Day	22% 15% 40% 20% 3	240
	11th to 20th	Day	18% 17% 35% 23% 7%	234
	21st to end	Day	17% 14% 39% 22% 8% 1	576
May	1st to 10th	Day	14% 10% 38% 30% 8%	603
	11th to 20th	Day	17% 5% 31% 31% 14% 2	625
	21st to end	Day	17% 5% 26% 34% 16% 3	679
June	1st to 10th	Day	5% 2 16% 42% 27% 7%	630
	11th to 20th	Day	7% 1 18% 47% 23% 4	630
	21st to end	Day	7% 3 10% 40% 31% 10%	630
July	1st to 10th	Day	8% 2 8% 38% 33% 11%	621
	11th to 20th	Day	3 1 4 38% 45% 10%	618
	21st to end	Day	3 1 3 36% 47% 10%	684
August	1st to 10th	Day	2 4 35% 46% 13%	626
	11th to 20th	Day	5% 1 3 37% 41% 13%	494
	21st to end	Day	6% 1 4 24% 40% 24%	392
September	1st to 10th	Day	8% 1 2 22% 43% 24%	270
	11th to 20th	Day	7% 1 6% 25% 46% 16%	270
	21st to end	Day	10% 2 5% 32% 35% 16%	270
October	1st to 10th	Day	5% 1 14% 37% 33% 9%	270
	11th to 20th	Day	11% 5% 24% 30% 25% 5%	270
	21st to end	Day	22% 10% 25% 26% 15% 2	297
November	1st to 10th	Day	19% 7% 29% 30% 12% 3	270
	11th to 20th	Day	16% 11% 29% 30% 11% 3	270
	21st to end	Day	24% 16% 31% 19% 9% 1	270
December	1st to 10th	Day	16% 7% 34% 28% 10% 4	270
	11th to 20th	Day	20% 13% 38% 21% 7%	270
	21st to end	Day	20% 11% 39% 24% 5% 2	297
January	1st to 10th	Night	18% 9% 36% 29% 7% 1	950
	11th to 20th	Night	19% 10% 32% 31% 7%	950
	21st to end	Night	20% 10% 36% 28% 6%	957
February	1st to 10th	Night	21% 12% 35% 28% 4	720
	11th to 20th	Night	21% 12% 39% 25% 2	704
	21st to end	Night	23% 14% 42% 19% 2	592
March	1st to 10th	Night	19% 13% 37% 27% 4	554
	11th to 20th	Night	17% 12% 36% 31% 5%	540
	21st to end	Night	15% 11% 44% 27% 4	594
April	1st to 10th	Night	14% 10% 45% 29% 2	540
	11th to 20th	Night	18% 11% 40% 24% 6%	533
	21st to end	Night	16% 9% 44% 23% 8% 1	540
May	1st to 10th	Night	12% 9% 44% 27% 7%	540
	11th to 20th	Night	13% 5% 36% 34% 12% 1	531
	21st to end	Night	11% 6% 34% 35% 14%	584
June	1st to 10th	Night	4 2 23% 43% 26% 2	540
	11th to 20th	Night	5% 1 27% 50% 14% 2	540
	21st to end	Night	6% 2 17% 45% 26% 3	540
July	1st to 10th	Night	6% 2 12% 47% 30% 4	525
	11th to 20th	Night	3 9% 56% 29% 3	522
	21st to end	Night	2 1 8% 53% 33% 3	591
August	1st to 10th	Night	1 8% 53% 33% 4	537
	11th to 20th	Night	7% 6% 48% 35% 4	534
	21st to end	Night	5% 1 4 46% 39% 5%	594
September	1st to 10th	Night	8% 2 9% 41% 35% 5%	540
	11th to 20th	Night	12% 2 6% 39% 38% 3	540
	21st to end	Night	9% 1 13% 37% 36% 4	540
October	1st to 10th	Night	7% 2 13% 48% 25% 6%	540
	11th to 20th	Night	13% 3 25% 38% 19% 2	666
	21st to end	Night	14% 8% 28% 30% 18% 2	792
November	1st to 10th	Night	14% 4 25% 39% 16% 1	740
	11th to 20th	Night	10% 7% 30% 37% 14% 1	880
	21st to end	Night	19% 11% 30% 28% 11% 1	950
December	1st to 10th	Night	16% 7% 32% 34% 10% 1	950
	11th to 20th	Night	21% 9% 31% 33% 6% 1	950
	21st to end	Night	20% 9% 33% 32% 7%	1045

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