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 2026-05-17.

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 5% 4 4 4 3 5% 7% 49%	132288
	Night	12% 6% 2 5% 3 3 3 3 5% 6% 53%	130886
Transparency	Day	33% 5% 10% 21% 20% 10%	136044
	Night	32% 4 10% 20% 21% 13%	135181
Seeing	Day	13% 7% 23% 30% 21% 6%	16877
	Night	14% 7% 28% 35% 15% 1	26867

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	11% 8% 2 5% 3 3 3 3 4 5% 52%	5289
	11th to 20th	Day	12% 5% 2 4 3 2 3 2 5% 6% 56%	5310
	21st to end	Day	22% 8% 3 8% 4 3 2 2 3 5% 41%	6088
February	1st to 10th	Day	18% 6% 3 5% 3 3 3 3 4 6% 47%	2100
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 5% 6% 46%	2049
	21st to end	Day	18% 6% 2 4 2 3 3 3 3 4 53%	1730
March	1st to 10th	Day	12% 6% 2 6% 4 3 3 3 4 6% 50%	5016
	11th to 20th	Day	13% 5% 1 5% 3 3 4 3 4 6% 52%	5090
	21st to end	Day	11% 7% 3 6% 4 4 4 3 5% 7% 47%	5583
April	1st to 10th	Day	13% 8% 3 7% 5% 3 4 3 5% 5% 43%	2310
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 5% 6% 43%	2286
	21st to end	Day	14% 7% 2 5% 4 4 4 3 5% 6% 46%	2703
May	1st to 10th	Day	18% 7% 3 6% 5% 4 5% 4 6% 7% 36%	5992
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 46%	5941
	21st to end	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 35%	6434
June	1st to 10th	Day	5% 4 2 5% 4 5% 5% 5% 7% 8% 51%	2600
	11th to 20th	Day	7% 3 2 5% 3 4 4 4 6% 10% 51%	2600
	21st to end	Day	6% 5% 2 4 5% 5% 6% 5% 8% 10% 44%	2600
July	1st to 10th	Day	11% 6% 2 6% 5% 6% 5% 5% 6% 9% 41%	5900
	11th to 20th	Day	4 3 1 4 4 4 5% 4 7% 11% 52%	5900
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 7% 10% 47%	6422
August	1st to 10th	Day	1 1 1 2 3 3 3 3 4 8% 72%	4234
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 10% 61%	3945
	21st to end	Day	4 3 2 5% 4 4 6% 4 7% 11% 51%	4119
September	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 6% 7% 49%	2200
	11th to 20th	Day	10% 5% 2 6% 4 6% 5% 4 5% 9% 45%	2200
	21st to end	Day	6% 4 2 6% 4 4 4 3 6% 8% 55%	2200
October	1st to 10th	Day	7% 3 2 4 3 3 3 4 4 6% 62%	2200
	11th to 20th	Day	11% 4 2 4 3 3 3 2 4 5% 59%	2039
	21st to end	Day	10% 4 2 5% 3 3 2 2 4 5% 61%	2200
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 5% 52%	2058
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 4 6% 55%	1962
	21st to end	Day	15% 6% 3 6% 4 3 3 3 4 5% 48%	1890
December	1st to 10th	Day	20% 6% 2 6% 3 3 3 3 4 4 45%	3030
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 6% 46%	3481
	21st to end	Day	13% 6% 2 4 2 2 2 2 3 4 59%	4587
January	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 3 4 57%	7634
	11th to 20th	Night	8% 3 1 3 2 2 2 2 5% 6% 65%	7680
	21st to end	Night	26% 7% 3 6% 3 3 3 2 3 5% 41%	8181
February	1st to 10th	Night	15% 7% 2 5% 3 2 2 3 4 5% 51%	2520
	11th to 20th	Night	17% 6% 2 5% 3 2 3 2 4 6% 50%	2458
	21st to end	Night	17% 6% 2 4 4 3 4 1 3 5% 50%	2076
March	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 6% 7% 47%	5058
	11th to 20th	Night	12% 6% 2 4 3 4 4 3 5% 5% 51%	5090
	21st to end	Night	13% 10% 3 6% 4 3 3 2 4 6% 47%	5588
April	1st to 10th	Night	12% 7% 2 5% 4 3 3 4 5% 7% 48%	2310
	11th to 20th	Night	16% 6% 2 5% 5% 3 3 3 6% 6% 44%	2283
	21st to end	Night	15% 8% 2 5% 3 3 3 3 5% 6% 47%	1911
May	1st to 10th	Night	21% 8% 2 5% 4 3 4 3 4 7% 38%	4143
	11th to 20th	Night	8% 6% 2 7% 4 4 5% 3 6% 7% 49%	4107
	21st to end	Night	13% 7% 2 6% 4 4 5% 4 7% 8% 40%	4443
June	1st to 10th	Night	4 3 1 4 3 3 3 4 7% 8% 59%	1800
	11th to 20th	Night	5% 3 1 4 3 4 4 3 7% 8% 59%	1800
	21st to end	Night	5% 4 2 3 4 3 5% 3 6% 8% 56%	1800
July	1st to 10th	Night	9% 5% 3 5% 4 3 4 4 6% 9% 46%	4078
	11th to 20th	Night	3 2 1 4 3 3 4 3 7% 11% 60%	4077
	21st to end	Night	5% 5% 1 5% 4 4 4 5% 7% 10% 51%	4449
August	1st to 10th	Night	1 1 1 1 1 1 2 2 4 7% 78%	2926
	11th to 20th	Night	3 2 1 3 3 2 3 3 6% 10% 63%	3109
	21st to end	Night	4 3 1 4 3 3 3 4 6% 10% 58%	3782
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 7% 7% 51%	2200
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 7% 49%	2200
	21st to end	Night	7% 3 1 4 3 3 3 3 6% 7% 59%	2200
October	1st to 10th	Night	6% 3 1 3 2 3 3 2 4 6% 66%	2200
	11th to 20th	Night	10% 4 2 4 3 3 2 3 4 6% 60%	2340
	21st to end	Night	12% 5% 2 3 3 2 3 2 3 4 61%	2640
November	1st to 10th	Night	12% 5% 2 4 3 3 3 2 4 5% 57%	2460
	11th to 20th	Night	7% 6% 2 4 3 3 4 2 4 5% 60%	2654
	21st to end	Night	16% 6% 2 5% 3 3 4 3 4 5% 49%	2730
December	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 5% 6% 45%	4380
	11th to 20th	Night	18% 9% 2 6% 3 3 3 2 3 4 48%	4946
	21st to end	Night	14% 7% 2 4 2 2 2 2 3 4 58%	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	35% 3 6% 24% 23% 9%	5290
	11th to 20th	Day	30% 2 5% 15% 29% 19%	5310
	21st to end	Day	48% 2 5% 12% 22% 11%	6088
February	1st to 10th	Day	41% 3 6% 12% 22% 16%	2100
	11th to 20th	Day	39% 3 8% 19% 19% 12%	2059
	21st to end	Day	37% 4 5% 17% 22% 16%	1730
March	1st to 10th	Day	36% 3 5% 17% 18% 21%	5016
	11th to 20th	Day	35% 5% 6% 9% 16% 30%	5090
	21st to end	Day	37% 5% 9% 19% 23% 8%	5585
April	1st to 10th	Day	42% 4 8% 17% 15% 14%	2310
	11th to 20th	Day	43% 4 8% 16% 18% 11%	2286
	21st to end	Day	40% 4 8% 20% 20% 8%	2703
May	1st to 10th	Day	46% 6% 11% 17% 16% 4	6030
	11th to 20th	Day	36% 6% 11% 16% 24% 7%	5945
	21st to end	Day	41% 8% 12% 22% 13% 4	6447
June	1st to 10th	Day	28% 7% 13% 28% 20% 3	2600
	11th to 20th	Day	28% 6% 14% 28% 19% 5%	2600
	21st to end	Day	32% 8% 13% 25% 19% 3	2600
July	1st to 10th	Day	39% 7% 19% 28% 6% 1	5836
	11th to 20th	Day	25% 7% 16% 34% 16% 2	5831
	21st to end	Day	30% 7% 17% 27% 16% 3	6240
August	1st to 10th	Day	12% 4 11% 38% 34% 1	4220
	11th to 20th	Day	17% 6% 20% 39% 14% 2	3984
	21st to end	Day	25% 9% 18% 26% 19% 2	4118
September	1st to 10th	Day	34% 6% 10% 22% 18% 10%	2200
	11th to 20th	Day	35% 8% 11% 21% 19% 5%	2200
	21st to end	Day	27% 5% 10% 22% 21% 14%	2200
October	1st to 10th	Day	15% 3 6% 13% 25% 38%	3590
	11th to 20th	Day	19% 2 8% 19% 30% 22%	3292
	21st to end	Day	23% 3 5% 20% 36% 14%	3538
November	1st to 10th	Day	36% 3 8% 17% 24% 11%	2058
	11th to 20th	Day	31% 4 8% 19% 21% 16%	1962
	21st to end	Day	40% 3 6% 16% 20% 14%	1890
December	1st to 10th	Day	44% 3 7% 15% 22% 9%	3030
	11th to 20th	Day	39% 5% 7% 12% 24% 13%	3479
	21st to end	Day	30% 3 5% 16% 28% 17%	4587
January	1st to 10th	Night	33% 2 5% 20% 27% 13%	7635
	11th to 20th	Night	23% 2 4 18% 31% 24%	7680
	21st to end	Night	48% 2 3 11% 21% 13%	8181
February	1st to 10th	Night	37% 3 6% 13% 22% 19%	2520
	11th to 20th	Night	39% 3 6% 18% 19% 15%	2458
	21st to end	Night	39% 2 6% 14% 21% 17%	2076
March	1st to 10th	Night	36% 4 7% 16% 21% 15%	5058
	11th to 20th	Night	35% 5% 8% 11% 15% 26%	5090
	21st to end	Night	40% 5% 7% 17% 21% 10%	5588
April	1st to 10th	Night	37% 5% 9% 18% 16% 15%	2310
	11th to 20th	Night	40% 5% 10% 14% 17% 14%	2283
	21st to end	Night	39% 4 11% 19% 19% 9%	1911
May	1st to 10th	Night	45% 5% 10% 19% 14% 6%	4170
	11th to 20th	Night	34% 7% 11% 20% 18% 11%	4108
	21st to end	Night	39% 7% 14% 25% 10% 4	4452
June	1st to 10th	Night	20% 7% 16% 30% 23% 3	1800
	11th to 20th	Night	22% 5% 15% 31% 20% 6%	1800
	21st to end	Night	26% 6% 16% 29% 19% 4	1800
July	1st to 10th	Night	34% 8% 23% 29% 5% 1	4035
	11th to 20th	Night	19% 7% 23% 36% 15% 1	4026
	21st to end	Night	27% 8% 21% 26% 14% 4	4320
August	1st to 10th	Night	7% 3 13% 41% 33% 2	2916
	11th to 20th	Night	17% 5% 29% 34% 13% 2	3136
	21st to end	Night	21% 8% 22% 28% 18% 3	3782
September	1st to 10th	Night	30% 7% 12% 24% 17% 10%	2200
	11th to 20th	Night	34% 6% 15% 24% 16% 7%	2200
	21st to end	Night	24% 5% 13% 25% 19% 14%	2200
October	1st to 10th	Night	13% 2 7% 15% 22% 41%	3590
	11th to 20th	Night	18% 3 8% 17% 32% 21%	3800
	21st to end	Night	24% 2 5% 19% 32% 17%	4252
November	1st to 10th	Night	32% 3 8% 19% 24% 14%	2460
	11th to 20th	Night	28% 3 9% 20% 23% 17%	2654
	21st to end	Night	38% 3 7% 16% 20% 15%	2730
December	1st to 10th	Night	42% 3 7% 18% 20% 11%	4380
	11th to 20th	Night	42% 2 6% 10% 23% 17%	4947
	21st to end	Night	33% 2 4 16% 27% 17%	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	20% 14% 37% 24% 5%	330
	11th to 20th	Day	21% 15% 33% 24% 6% 1	330
	21st to end	Day	19% 18% 39% 18% 5%	363
February	1st to 10th	Day	25% 14% 36% 20% 5%	330
	11th to 20th	Day	24% 19% 35% 19% 4	330
	21st to end	Day	28% 14% 40% 16% 1	273
March	1st to 10th	Day	23% 15% 38% 20% 3	330
	11th to 20th	Day	17% 20% 38% 18% 6%	330
	21st to end	Day	16% 14% 40% 23% 7% 1	363
April	1st to 10th	Day	23% 17% 37% 18% 5%	330
	11th to 20th	Day	19% 18% 38% 19% 5%	324
	21st to end	Day	20% 13% 39% 21% 7%	723
May	1st to 10th	Day	17% 11% 37% 27% 7%	753
	11th to 20th	Day	17% 7% 32% 29% 12% 2	756
	21st to end	Day	16% 6% 28% 33% 15% 2	789
June	1st to 10th	Day	5% 3 18% 42% 25% 7%	730
	11th to 20th	Day	7% 2 18% 47% 22% 4	730
	21st to end	Day	6% 2 10% 41% 31% 10%	730
July	1st to 10th	Day	9% 3 7% 37% 33% 11%	721
	11th to 20th	Day	4 1 5% 37% 44% 9%	718
	21st to end	Day	4 2 5% 36% 44% 10%	794
August	1st to 10th	Day	1 5% 34% 46% 14%	726
	11th to 20th	Day	4 1 3 38% 41% 14%	580
	21st to end	Day	5% 1 4 25% 41% 23%	468
September	1st to 10th	Day	9% 2 6% 26% 38% 20%	330
	11th to 20th	Day	6% 1 6% 27% 46% 14%	330
	21st to end	Day	8% 2 6% 36% 32% 15%	330
October	1st to 10th	Day	4 1 14% 37% 34% 10%	330
	11th to 20th	Day	9% 5% 25% 31% 25% 5%	330
	21st to end	Day	19% 9% 24% 29% 17% 2	363
November	1st to 10th	Day	18% 8% 29% 29% 12% 4	330
	11th to 20th	Day	13% 11% 29% 32% 12% 3	330
	21st to end	Day	22% 15% 32% 19% 11% 1	330
December	1st to 10th	Day	18% 8% 35% 26% 9% 3	330
	11th to 20th	Day	19% 12% 37% 23% 7% 2	330
	21st to end	Day	18% 10% 39% 25% 6% 2	363
January	1st to 10th	Night	18% 10% 36% 29% 7%	1050
	11th to 20th	Night	18% 11% 34% 30% 7%	1050
	21st to end	Night	21% 11% 35% 27% 6%	1076
February	1st to 10th	Night	21% 12% 34% 29% 4	840
	11th to 20th	Night	22% 13% 39% 24% 2	824
	21st to end	Night	22% 14% 40% 22% 2	692
March	1st to 10th	Night	20% 15% 36% 25% 3	646
	11th to 20th	Night	17% 13% 37% 29% 4	630
	21st to end	Night	16% 10% 43% 27% 3	693
April	1st to 10th	Night	16% 11% 43% 29% 2	630
	11th to 20th	Night	20% 13% 39% 24% 5%	623
	21st to end	Night	19% 9% 41% 23% 7%	630
May	1st to 10th	Night	14% 10% 41% 27% 6%	630
	11th to 20th	Night	13% 6% 36% 33% 11% 1	609
	21st to end	Night	11% 7% 35% 34% 13%	650
June	1st to 10th	Night	5% 3 24% 42% 25% 2	600
	11th to 20th	Night	6% 2 27% 50% 14% 2	600
	21st to end	Night	6% 3 16% 47% 25% 3	600
July	1st to 10th	Night	6% 3 11% 48% 29% 3	585
	11th to 20th	Night	4 1 9% 55% 29% 3	582
	21st to end	Night	3 1 9% 51% 34% 3	657
August	1st to 10th	Night	2 7% 53% 34% 4	597
	11th to 20th	Night	7% 6% 51% 33% 4	594
	21st to end	Night	5% 1 4 45% 41% 5%	660
September	1st to 10th	Night	8% 2 10% 42% 33% 5%	600
	11th to 20th	Night	12% 2 7% 41% 36% 3	600
	21st to end	Night	9% 1 13% 39% 35% 4	600
October	1st to 10th	Night	7% 2 12% 49% 27% 5%	600
	11th to 20th	Night	12% 3 24% 39% 19% 2	740
	21st to end	Night	14% 8% 27% 32% 18% 1	880
November	1st to 10th	Night	14% 5% 25% 40% 15% 1	820
	11th to 20th	Night	10% 7% 31% 37% 14% 1	974
	21st to end	Night	19% 11% 30% 28% 12%	1050
December	1st to 10th	Night	17% 8% 33% 33% 9%	1050
	11th to 20th	Night	21% 10% 30% 33% 6%	1050
	21st to end	Night	19% 9% 33% 32% 8%	1155

Page generated at 2026-05-14 00:58:57. Version ac07

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