Shadowlands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shadowlands Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	12% 6% 2 6% 4 4 4 3 5% 7% 47%	132455
	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	130972
Transparency	Day	36% 5% 9% 25% 20% 5%	136066
	Night	33% 4 9% 24% 22% 7%	135438
Seeing	Day	15% 9% 22% 26% 22% 7%	17464
	Night	15% 7% 23% 36% 18% 2	27338

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	15% 9% 3 6% 3 4 4 2 3 4 47%	5289
	11th to 20th	Day	7% 6% 2 6% 3 3 3 2 4 4 58%	5310
	21st to end	Day	19% 8% 2 4 3 3 3 3 4 5% 47%	5812
February	1st to 10th	Day	21% 9% 3 6% 3 2 2 2 3 3 45%	1890
	11th to 20th	Day	17% 9% 2 6% 4 3 3 3 5% 4 44%	1870
	21st to end	Day	17% 8% 2 6% 4 3 3 3 4 5% 43%	1903
March	1st to 10th	Day	18% 8% 3 5% 4 3 3 2 4 5% 45%	5046
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 5% 6% 54%	5090
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 7% 42%	5583
April	1st to 10th	Day	14% 6% 3 7% 4 4 4 3 4 6% 46%	2426
	11th to 20th	Day	15% 7% 3 6% 5% 3 4 3 4 5% 44%	2494
	21st to end	Day	17% 8% 3 8% 5% 4 4 3 5% 5% 38%	2520
May	1st to 10th	Day	17% 8% 3 7% 4 4 4 3 5% 7% 38%	5870
	11th to 20th	Day	12% 6% 2 8% 5% 5% 5% 4 6% 8% 39%	5992
	21st to end	Day	17% 7% 3 7% 5% 4 5% 3 6% 7% 35%	6577
June	1st to 10th	Day	8% 6% 3 6% 4 5% 4 4 7% 9% 43%	2601
	11th to 20th	Day	6% 4 2 7% 5% 5% 6% 5% 7% 8% 45%	2600
	21st to end	Day	4 3 2 4 4 5% 6% 5% 8% 9% 53%	2600
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 39%	5900
	11th to 20th	Day	4 4 2 5% 4 4 5% 4 6% 8% 55%	5900
	21st to end	Day	5% 4 2 6% 5% 5% 5% 5% 7% 10% 44%	6422
August	1st to 10th	Day	4 2 1 4 4 3 4 4 6% 8% 59%	4234
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 9% 46%	4057
	21st to end	Day	4 3 1 5% 4 4 5% 4 6% 8% 54%	4312
September	1st to 10th	Day	10% 5% 2 4 4 4 4 3 5% 7% 51%	2400
	11th to 20th	Day	11% 4 2 4 3 4 5% 4 5% 9% 47%	2241
	21st to end	Day	8% 4 1 5% 4 3 5% 4 5% 6% 55%	2165
October	1st to 10th	Day	7% 4 1 4 4 3 4 3 4 6% 58%	2000
	11th to 20th	Day	13% 4 2 4 3 2 3 3 3 5% 59%	2000
	21st to end	Day	18% 5% 2 5% 3 2 3 2 4 5% 49%	2200
November	1st to 10th	Day	12% 5% 1 4 3 3 3 3 4 5% 56%	2058
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 6% 56%	2100
	21st to end	Day	16% 7% 3 5% 3 2 3 2 4 5% 49%	1895
December	1st to 10th	Day	16% 7% 2 5% 3 2 3 2 5% 5% 50%	3030
	11th to 20th	Day	17% 8% 3 6% 3 3 3 3 4 6% 44%	3481
	21st to end	Day	19% 13% 4 7% 3 3 3 3 4 5% 37%	4587
January	1st to 10th	Night	14% 7% 2 6% 4 3 3 3 5% 6% 47%	7634
	11th to 20th	Night	7% 5% 1 3 2 2 2 2 4 5% 66%	7680
	21st to end	Night	22% 8% 2 5% 3 3 3 2 3 5% 44%	8402
February	1st to 10th	Night	18% 8% 2 6% 4 3 2 2 5% 5% 45%	2730
	11th to 20th	Night	17% 7% 3 5% 3 2 3 3 5% 5% 46%	2647
	21st to end	Night	16% 6% 2 6% 4 3 3 3 4 6% 49%	1908
March	1st to 10th	Night	20% 7% 2 6% 4 3 3 2 4 5% 45%	5029
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 52%	5090
	21st to end	Night	14% 6% 3 5% 4 3 4 3 5% 8% 46%	5588
April	1st to 10th	Night	13% 5% 2 6% 3 3 3 3 4 7% 49%	2192
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 5% 6% 49%	2075
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 7% 45%	2100
May	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	4243
	11th to 20th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	4143
	21st to end	Night	15% 7% 2 7% 4 4 4 3 6% 8% 39%	4542
June	1st to 10th	Night	4 4 2 4 4 3 4 4 7% 9% 55%	1800
	11th to 20th	Night	4 4 1 3 3 3 3 4 6% 10% 58%	1800
	21st to end	Night	3 2 1 3 3 3 4 3 5% 9% 64%	1800
July	1st to 10th	Night	7% 4 2 6% 5% 5% 5% 5% 7% 11% 43%	4078
	11th to 20th	Night	5% 3 1 4 3 3 3 3 6% 9% 60%	4077
	21st to end	Night	3 3 2 6% 4 4 5% 4 7% 10% 51%	4449
August	1st to 10th	Night	3 2 2 4 3 4 4 4 5% 8% 60%	2926
	11th to 20th	Night	5% 4 2 5% 3 4 4 4 5% 9% 56%	2984
	21st to end	Night	4 3 1 5% 4 4 4 3 6% 8% 58%	3586
September	1st to 10th	Night	8% 4 1 4 4 3 4 3 5% 8% 57%	2000
	11th to 20th	Night	9% 4 1 5% 4 3 4 4 7% 8% 50%	2154
	21st to end	Night	8% 4 1 5% 3 4 4 3 5% 7% 56%	2214
October	1st to 10th	Night	6% 3 1 4 2 3 4 3 5% 7% 62%	2400
	11th to 20th	Night	12% 4 2 3 3 3 3 2 4 5% 61%	2400
	21st to end	Night	18% 5% 2 5% 2 2 2 2 3 4 53%	2640
November	1st to 10th	Night	11% 5% 2 4 3 3 3 3 4 5% 58%	2460
	11th to 20th	Night	12% 5% 1 3 3 3 2 3 5% 6% 58%	2520
	21st to end	Night	17% 5% 2 4 3 3 3 2 3 6% 51%	2722
December	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 3 5% 51%	4380
	11th to 20th	Night	19% 6% 2 5% 3 3 3 2 4 4 49%	4946
	21st to end	Night	21% 9% 3 6% 4 3 3 2 3 4 40%	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	43% 2 8% 26% 20% 2	5290
	11th to 20th	Day	31% 3 7% 23% 31% 6%	5310
	21st to end	Day	42% 2 4 21% 24% 7%	5812
February	1st to 10th	Day	45% 3 10% 21% 17% 4	1890
	11th to 20th	Day	44% 3 8% 22% 20% 4	1879
	21st to end	Day	44% 4 6% 17% 21% 8%	1903
March	1st to 10th	Day	43% 4 5% 18% 16% 14%	5046
	11th to 20th	Day	31% 4 7% 19% 24% 14%	5090
	21st to end	Day	41% 4 9% 29% 16% 1	5585
April	1st to 10th	Day	41% 4 9% 20% 20% 5%	2426
	11th to 20th	Day	42% 4 9% 21% 18% 5%	2494
	21st to end	Day	47% 6% 7% 21% 17% 2	2520
May	1st to 10th	Day	46% 5% 9% 23% 16% 1	5907
	11th to 20th	Day	41% 7% 11% 22% 18% 2	5996
	21st to end	Day	47% 6% 10% 18% 17% 1	6590
June	1st to 10th	Day	36% 8% 13% 25% 17% 2	2601
	11th to 20th	Day	34% 7% 14% 24% 19% 2	2600
	21st to end	Day	26% 8% 13% 29% 21% 4	2600
July	1st to 10th	Day	34% 8% 17% 29% 10% 1	5836
	11th to 20th	Day	25% 6% 12% 37% 18% 1	5831
	21st to end	Day	32% 8% 14% 33% 12% 1	6240
August	1st to 10th	Day	22% 6% 13% 31% 23% 6%	4220
	11th to 20th	Day	33% 7% 15% 31% 13% 1	4096
	21st to end	Day	25% 8% 13% 36% 15% 3	4311
September	1st to 10th	Day	32% 6% 9% 26% 19% 8%	2400
	11th to 20th	Day	33% 6% 10% 25% 20% 5%	2241
	21st to end	Day	29% 6% 9% 21% 27% 8%	2165
October	1st to 10th	Day	16% 3 5% 13% 38% 25%	3260
	11th to 20th	Day	20% 2 4 18% 39% 17%	3240
	21st to end	Day	33% 3 7% 27% 24% 6%	3538
November	1st to 10th	Day	31% 4 8% 25% 25% 7%	2058
	11th to 20th	Day	31% 3 7% 26% 24% 9%	2100
	21st to end	Day	38% 4 8% 20% 26% 5%	1895
December	1st to 10th	Day	38% 3 6% 30% 21% 2	3030
	11th to 20th	Day	43% 3 6% 22% 23% 4	3479
	21st to end	Day	51% 4 6% 23% 14% 3	4587
January	1st to 10th	Night	39% 3 9% 30% 18% 2	7635
	11th to 20th	Night	23% 2 8% 20% 38% 9%	7680
	21st to end	Night	45% 2 6% 16% 24% 7%	8402
February	1st to 10th	Night	42% 3 8% 25% 18% 4	2730
	11th to 20th	Night	40% 3 7% 23% 20% 6%	2648
	21st to end	Night	38% 3 7% 19% 22% 10%	1908
March	1st to 10th	Night	43% 4 6% 16% 19% 13%	5029
	11th to 20th	Night	33% 3 6% 23% 20% 15%	5090
	21st to end	Night	38% 4 10% 26% 18% 3	5588
April	1st to 10th	Night	35% 4 10% 21% 21% 8%	2192
	11th to 20th	Night	36% 5% 8% 22% 20% 10%	2075
	21st to end	Night	39% 5% 10% 22% 22% 2	2100
May	1st to 10th	Night	40% 7% 13% 24% 14% 3	4271
	11th to 20th	Night	41% 7% 12% 23% 15% 2	4144
	21st to end	Night	43% 7% 14% 20% 13% 4	4551
June	1st to 10th	Night	23% 8% 15% 30% 21% 3	1800
	11th to 20th	Night	21% 7% 13% 33% 24% 3	1800
	21st to end	Night	18% 5% 13% 31% 27% 6%	1800
July	1st to 10th	Night	33% 8% 16% 28% 13% 2	4035
	11th to 20th	Night	20% 6% 12% 39% 21% 1	4026
	21st to end	Night	27% 7% 16% 32% 18% 1	4320
August	1st to 10th	Night	21% 6% 12% 34% 24% 4	2916
	11th to 20th	Night	25% 5% 19% 33% 17% 2	3011
	21st to end	Night	24% 5% 12% 32% 25% 3	3586
September	1st to 10th	Night	27% 5% 8% 27% 23% 10%	2000
	11th to 20th	Night	29% 7% 11% 24% 23% 6%	2154
	21st to end	Night	26% 5% 8% 23% 29% 8%	2214
October	1st to 10th	Night	14% 3 8% 13% 33% 30%	3920
	11th to 20th	Night	18% 2 4 16% 39% 21%	3899
	21st to end	Night	29% 2 5% 26% 29% 10%	4252
November	1st to 10th	Night	30% 3 8% 22% 26% 10%	2460
	11th to 20th	Night	28% 3 7% 25% 26% 11%	2520
	21st to end	Night	36% 3 8% 21% 26% 6%	2722
December	1st to 10th	Night	37% 3 7% 31% 19% 3	4380
	11th to 20th	Night	41% 2 5% 22% 22% 8%	4947
	21st to end	Night	48% 3 8% 22% 14% 4	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	28% 12% 29% 23% 8% 1	330
	11th to 20th	Day	22% 16% 38% 18% 5%	330
	21st to end	Day	21% 19% 34% 18% 7% 1	363
February	1st to 10th	Day	27% 14% 35% 17% 5% 1	330
	11th to 20th	Day	28% 18% 36% 15% 3	330
	21st to end	Day	32% 20% 30% 14% 4	273
March	1st to 10th	Day	24% 20% 29% 21% 5% 1	330
	11th to 20th	Day	27% 18% 35% 18% 3	330
	21st to end	Day	28% 17% 36% 14% 6%	363
April	1st to 10th	Day	23% 15% 32% 19% 10% 1	446
	11th to 20th	Day	22% 16% 32% 23% 8%	532
	21st to end	Day	19% 18% 29% 23% 9% 1	540
May	1st to 10th	Day	18% 12% 31% 29% 10%	695
	11th to 20th	Day	15% 9% 26% 29% 18% 3	775
	21st to end	Day	14% 6% 27% 29% 18% 5%	844
June	1st to 10th	Day	10% 5% 17% 28% 31% 10%	731
	11th to 20th	Day	6% 3 15% 35% 31% 10%	730
	21st to end	Day	4 3 13% 38% 33% 10%	730
July	1st to 10th	Day	7% 2 10% 29% 38% 14%	721
	11th to 20th	Day	4 1 8% 31% 37% 18%	718
	21st to end	Day	5% 2 7% 26% 39% 20%	794
August	1st to 10th	Day	7% 2 7% 34% 39% 11%	726
	11th to 20th	Day	11% 2 8% 30% 35% 15%	653
	21st to end	Day	7% 1 7% 23% 41% 20%	583
September	1st to 10th	Day	10% 1 8% 26% 40% 15%	530
	11th to 20th	Day	10% 2 11% 29% 34% 13%	371
	21st to end	Day	12% 4 18% 29% 22% 15%	330
October	1st to 10th	Day	4 4 20% 37% 29% 6%	330
	11th to 20th	Day	12% 10% 26% 31% 18% 2	330
	21st to end	Day	28% 13% 24% 22% 11% 2	363
November	1st to 10th	Day	19% 8% 31% 21% 18% 2	330
	11th to 20th	Day	15% 13% 35% 27% 9% 2	330
	21st to end	Day	23% 17% 23% 23% 13% 1	330
December	1st to 10th	Day	20% 15% 34% 19% 9% 2	330
	11th to 20th	Day	21% 15% 33% 21% 9%	330
	21st to end	Day	26% 15% 33% 21% 5% 1	363
January	1st to 10th	Night	19% 10% 33% 32% 6%	1050
	11th to 20th	Night	18% 12% 32% 33% 5%	1050
	21st to end	Night	18% 9% 34% 34% 4	1155
February	1st to 10th	Night	22% 11% 30% 33% 4	1050
	11th to 20th	Night	22% 10% 37% 28% 4	1013
	21st to end	Night	20% 13% 36% 29% 2	524
March	1st to 10th	Night	20% 10% 30% 35% 5%	630
	11th to 20th	Night	20% 10% 32% 31% 6%	630
	21st to end	Night	18% 10% 34% 31% 7%	693
April	1st to 10th	Night	17% 10% 33% 34% 6%	630
	11th to 20th	Night	15% 11% 34% 33% 8%	623
	21st to end	Night	19% 7% 33% 31% 10%	630
May	1st to 10th	Night	13% 5% 28% 42% 10% 1	630
	11th to 20th	Night	14% 6% 24% 37% 17% 2	621
	21st to end	Night	10% 4 23% 37% 24% 3	683
June	1st to 10th	Night	6% 3 10% 40% 39% 2	600
	11th to 20th	Night	4 1 11% 49% 33% 2	600
	21st to end	Night	4 1 11% 38% 42% 6%	600
July	1st to 10th	Night	6% 1 7% 36% 44% 6%	585
	11th to 20th	Night	4 1 6% 37% 43% 9%	582
	21st to end	Night	5% 4 35% 46% 10%	657
August	1st to 10th	Night	3 1 7% 42% 42% 5%	597
	11th to 20th	Night	8% 1 9% 34% 42% 6%	594
	21st to end	Night	6% 2 4 37% 44% 7%	660
September	1st to 10th	Night	9% 1 6% 37% 43% 4	600
	11th to 20th	Night	11% 1 9% 38% 37% 4	600
	21st to end	Night	8% 2 14% 37% 34% 5%	614
October	1st to 10th	Night	8% 3 14% 40% 32% 3	800
	11th to 20th	Night	13% 5% 23% 42% 16% 1	800
	21st to end	Night	20% 8% 22% 33% 15% 1	880
November	1st to 10th	Night	12% 6% 26% 41% 14% 1	820
	11th to 20th	Night	13% 10% 27% 38% 12%	840
	21st to end	Night	20% 10% 26% 33% 11%	1042
December	1st to 10th	Night	20% 8% 30% 35% 7%	1050
	11th to 20th	Night	22% 11% 24% 39% 5%	1050
	21st to end	Night	20% 12% 29% 34% 6%	1155

Page generated at 2026-05-29 00:53:25. Version ac07

Copyright © 2026 Attilla Danko.