Lyman
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lyman clear sky chart from 2005-11-06 to 2026-08-30.

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	3 5% 2 6% 4 5% 5% 4 6% 8% 52%	152743
	Night	2 5% 2 6% 4 4 4 4 6% 8% 54%	147098
Transparency	Day	27% 7% 12% 21% 21% 12%	147523
	Night	26% 7% 13% 25% 21% 9%	143326
Seeing	Day	7% 12% 27% 29% 18% 6%	17950
	Night	5% 8% 28% 39% 18% 2	28069

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 6% 4 11% 7% 5% 6% 5% 7% 9% 39%	5218
	11th to 20th	Day	1 4 2 7% 5% 5% 5% 4 6% 8% 53%	5340
	21st to end	Day	1 6% 3 8% 4 4 5% 4 5% 11% 48%	5844
February	1st to 10th	Day	1 6% 3 8% 6% 6% 6% 4 8% 9% 43%	1890
	11th to 20th	Day	1 8% 3 9% 7% 6% 5% 4 7% 9% 40%	1845
	21st to end	Day	1 7% 3 8% 4 4 5% 5% 8% 9% 47%	1693
March	1st to 10th	Day	2 8% 3 8% 5% 5% 5% 4 5% 8% 47%	5064
	11th to 20th	Day	2 10% 3 7% 5% 5% 5% 4 6% 9% 45%	5110
	21st to end	Day	3 10% 4 10% 7% 6% 7% 5% 7% 8% 33%	5607
April	1st to 10th	Day	2 6% 3 7% 5% 6% 5% 4 7% 9% 46%	2418
	11th to 20th	Day	5% 8% 3 8% 6% 5% 5% 5% 7% 8% 41%	2547
	21st to end	Day	5% 7% 3 6% 6% 5% 5% 5% 7% 8% 43%	2730
May	1st to 10th	Day	4 7% 2 6% 5% 5% 5% 4 6% 8% 48%	7626
	11th to 20th	Day	5% 6% 2 5% 4 4 5% 4 7% 8% 49%	7596
	21st to end	Day	4 4 2 5% 5% 5% 5% 4 6% 8% 52%	8963
June	1st to 10th	Day	3 5% 1 5% 4 4 4 4 7% 10% 52%	2940
	11th to 20th	Day	4 4 1 4 3 3 4 3 6% 7% 60%	2940
	21st to end	Day	3 2 1 4 3 4 4 4 6% 6% 64%	2940
July	1st to 10th	Day	2 2 1 4 4 4 5% 4 6% 6% 61%	8132
	11th to 20th	Day	2 3 1 5% 5% 6% 6% 5% 7% 7% 54%	7650
	21st to end	Day	3 3 1 4 4 5% 5% 5% 7% 8% 54%	8304
August	1st to 10th	Day	3 2 1 4 4 4 5% 4 6% 6% 61%	5909
	11th to 20th	Day	2 2 1 5% 4 5% 5% 4 6% 8% 58%	5928
	21st to end	Day	3 4 2 6% 4 5% 5% 4 5% 6% 56%	6375
September	1st to 10th	Day	3 3 2 3 3 4 4 3 5% 7% 64%	2394
	11th to 20th	Day	3 4 1 5% 3 3 3 3 5% 7% 62%	2200
	21st to end	Day	5% 3 1 4 3 3 3 3 4 5% 65%	2200
October	1st to 10th	Day	3 3 1 3 3 3 3 2 4 5% 69%	3308
	11th to 20th	Day	7% 6% 1 4 3 3 3 3 4 7% 59%	3250
	21st to end	Day	3 6% 2 5% 3 3 4 3 6% 7% 58%	3548
November	1st to 10th	Day	3 5% 3 5% 4 4 4 5% 5% 9% 55%	1929
	11th to 20th	Day	2 5% 2 6% 3 4 4 4 7% 8% 54%	1727
	21st to end	Day	2 5% 3 6% 5% 3 4 5% 7% 8% 52%	1680
December	1st to 10th	Day	1 4 2 4 2 3 3 3 6% 13% 60%	2700
	11th to 20th	Day	1 5% 2 7% 4 5% 6% 6% 9% 13% 43%	3106
	21st to end	Day	2 8% 3 9% 6% 6% 6% 5% 8% 10% 37%	4092
January	1st to 10th	Night	1 7% 4 10% 6% 5% 5% 5% 8% 9% 41%	7748
	11th to 20th	Night	1 4 3 8% 5% 4 4 3 5% 7% 55%	7650
	21st to end	Night	1 8% 3 9% 5% 5% 5% 4 6% 8% 45%	8370
February	1st to 10th	Night	1 6% 3 7% 5% 5% 5% 4 7% 10% 47%	2730
	11th to 20th	Night	1 6% 3 7% 5% 5% 6% 5% 8% 9% 46%	2662
	21st to end	Night	1 5% 3 7% 5% 4 5% 5% 7% 10% 48%	2111
March	1st to 10th	Night	2 8% 3 6% 5% 5% 5% 5% 6% 7% 48%	5050
	11th to 20th	Night	2 8% 4 7% 4 4 5% 3 6% 9% 48%	5110
	21st to end	Night	2 9% 4 8% 5% 5% 5% 4 6% 8% 45%	5610
April	1st to 10th	Night	2 7% 2 6% 5% 4 5% 5% 5% 7% 52%	2200
	11th to 20th	Night	4 8% 2 6% 4 4 5% 5% 7% 8% 47%	2017
	21st to end	Night	3 6% 2 6% 5% 4 6% 4 7% 8% 48%	1890
May	1st to 10th	Night	5% 7% 2 5% 4 5% 4 4 6% 7% 52%	5321
	11th to 20th	Night	5% 5% 2 5% 3 3 3 3 5% 7% 58%	5296
	21st to end	Night	4 5% 2 5% 4 4 5% 4 6% 9% 53%	5241
June	1st to 10th	Night	3 4 1 3 3 3 4 4 7% 8% 60%	1680
	11th to 20th	Night	4 3 1 3 3 2 4 3 5% 7% 66%	1680
	21st to end	Night	3 2 1 3 2 2 3 3 5% 6% 69%	1680
July	1st to 10th	Night	1 3 1 4 3 3 4 4 5% 7% 66%	4851
	11th to 20th	Night	1 2 1 4 4 4 5% 5% 7% 8% 57%	5340
	21st to end	Night	2 2 1 5% 4 4 5% 4 7% 8% 57%	5805
August	1st to 10th	Night	2 1 1 3 3 3 4 4 6% 7% 66%	4123
	11th to 20th	Night	3 3 1 5% 3 4 4 3 5% 6% 62%	4141
	21st to end	Night	2 3 1 4 4 4 5% 4 6% 7% 61%	4799
September	1st to 10th	Night	3 2 1 4 2 2 3 3 4 7% 68%	2003
	11th to 20th	Night	3 2 1 4 3 3 4 3 6% 8% 64%	2200
	21st to end	Night	5% 3 1 4 2 3 2 3 4 5% 69%	2200
October	1st to 10th	Night	3 3 1 3 2 3 3 2 3 5% 74%	3883
	11th to 20th	Night	5% 6% 2 5% 3 3 3 3 4 6% 61%	3898
	21st to end	Night	4 7% 2 4 3 3 3 3 7% 8% 56%	4252
November	1st to 10th	Night	2 5% 2 6% 3 3 4 4 6% 9% 58%	2588
	11th to 20th	Night	3 5% 2 6% 4 4 4 4 6% 9% 54%	2870
	21st to end	Night	2 6% 3 6% 4 4 4 4 6% 6% 55%	2940
December	1st to 10th	Night	1 4 2 5% 3 4 4 4 7% 10% 57%	4710
	11th to 20th	Night	1 6% 3 7% 4 4 6% 5% 7% 8% 49%	5321
	21st to end	Night	2 7% 3 9% 6% 5% 6% 5% 9% 11% 37%	7128

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% 12% 20% 22% 11%	5221
	11th to 20th	Day	23% 10% 15% 30% 16% 5%	5340
	21st to end	Day	28% 9% 18% 26% 16% 3	5844
February	1st to 10th	Day	33% 11% 19% 21% 14% 2	1890
	11th to 20th	Day	36% 10% 14% 26% 13% 2	1854
	21st to end	Day	30% 9% 17% 25% 16% 3	1693
March	1st to 10th	Day	31% 7% 12% 31% 13% 6%	5064
	11th to 20th	Day	33% 8% 17% 25% 16% 1	5110
	21st to end	Day	42% 11% 18% 21% 8% 1	5607
April	1st to 10th	Day	32% 7% 17% 23% 18% 3	2418
	11th to 20th	Day	37% 8% 13% 23% 18% 1	2550
	21st to end	Day	34% 8% 13% 25% 19% 1	2730
May	1st to 10th	Day	28% 7% 11% 22% 24% 7%	7626
	11th to 20th	Day	29% 7% 8% 22% 29% 6%	7599
	21st to end	Day	26% 6% 10% 23% 30% 5%	8964
June	1st to 10th	Day	24% 8% 13% 23% 26% 6%	2940
	11th to 20th	Day	21% 5% 9% 18% 30% 16%	2940
	21st to end	Day	18% 5% 8% 16% 30% 24%	2940
July	1st to 10th	Day	21% 5% 4 8% 21% 41%	8065
	11th to 20th	Day	25% 6% 8% 13% 22% 27%	7581
	21st to end	Day	24% 7% 7% 15% 22% 26%	8120
August	1st to 10th	Day	22% 6% 7% 15% 26% 24%	4338
	11th to 20th	Day	23% 6% 8% 18% 27% 18%	4327
	21st to end	Day	21% 5% 7% 12% 28% 27%	4630
September	1st to 10th	Day	19% 5% 8% 18% 27% 23%	2394
	11th to 20th	Day	20% 6% 9% 20% 29% 16%	2200
	21st to end	Day	21% 5% 7% 22% 34% 12%	2200
October	1st to 10th	Day	18% 3 7% 19% 31% 22%	3308
	11th to 20th	Day	26% 4 8% 30% 25% 7%	3250
	21st to end	Day	25% 4 11% 31% 25% 3	3548
November	1st to 10th	Day	25% 7% 13% 30% 19% 6%	1929
	11th to 20th	Day	25% 7% 19% 28% 19% 2	1727
	21st to end	Day	27% 7% 17% 28% 19% 2	1680
December	1st to 10th	Day	17% 8% 20% 36% 16% 3	2700
	11th to 20th	Day	26% 11% 23% 27% 11% 1	3104
	21st to end	Day	36% 10% 16% 21% 12% 5%	4092
January	1st to 10th	Night	33% 9% 22% 24% 12% 1	7749
	11th to 20th	Night	25% 8% 16% 32% 15% 3	7650
	21st to end	Night	30% 9% 17% 23% 18% 3	8370
February	1st to 10th	Night	30% 9% 21% 27% 12% 2	2730
	11th to 20th	Night	29% 9% 18% 24% 17% 2	2663
	21st to end	Night	28% 8% 19% 27% 15% 4	2111
March	1st to 10th	Night	31% 6% 11% 30% 17% 5%	5050
	11th to 20th	Night	30% 7% 17% 30% 15% 1	5110
	21st to end	Night	33% 8% 22% 26% 10% 1	5610
April	1st to 10th	Night	29% 7% 16% 26% 17% 5%	2200
	11th to 20th	Night	31% 7% 16% 25% 19% 2	2017
	21st to end	Night	29% 8% 14% 32% 16% 1	1890
May	1st to 10th	Night	28% 6% 10% 22% 29% 5%	5319
	11th to 20th	Night	24% 6% 8% 26% 28% 8%	5297
	21st to end	Night	25% 5% 10% 30% 29% 1	5242
June	1st to 10th	Night	19% 7% 14% 25% 26% 8%	1680
	11th to 20th	Night	17% 4 9% 21% 32% 17%	1680
	21st to end	Night	14% 4 7% 13% 35% 27%	1680
July	1st to 10th	Night	17% 3 5% 10% 24% 41%	4810
	11th to 20th	Night	21% 5% 7% 12% 24% 30%	5286
	21st to end	Night	22% 5% 7% 20% 26% 21%	5677
August	1st to 10th	Night	18% 5% 5% 15% 30% 26%	3018
	11th to 20th	Night	24% 4 7% 16% 27% 21%	3018
	21st to end	Night	13% 4 6% 18% 31% 28%	3475
September	1st to 10th	Night	15% 3 8% 19% 28% 26%	2003
	11th to 20th	Night	17% 5% 10% 23% 29% 17%	2200
	21st to end	Night	19% 3 6% 24% 37% 11%	2200
October	1st to 10th	Night	14% 3 6% 19% 40% 17%	3883
	11th to 20th	Night	26% 3 6% 28% 27% 9%	3898
	21st to end	Night	25% 4 11% 33% 24% 4	4252
November	1st to 10th	Night	22% 8% 14% 28% 23% 4	2588
	11th to 20th	Night	25% 7% 18% 33% 15% 2	2870
	21st to end	Night	28% 6% 14% 29% 19% 4	2940
December	1st to 10th	Night	22% 7% 17% 35% 17% 3	4710
	11th to 20th	Night	29% 7% 21% 32% 10% 1	5322
	21st to end	Night	35% 10% 19% 21% 11% 4	7128

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	10% 18% 23% 29% 17% 3	330
	11th to 20th	Day	8% 16% 30% 28% 15% 2	330
	21st to end	Day	6% 13% 32% 33% 13% 2	363
February	1st to 10th	Day	11% 22% 35% 25% 6% 2	330
	11th to 20th	Day	9% 18% 32% 30% 9% 2	330
	21st to end	Day	7% 21% 35% 28% 8% 1	399
March	1st to 10th	Day	8% 17% 37% 27% 9% 3	540
	11th to 20th	Day	11% 15% 29% 28% 16% 1	540
	21st to end	Day	9% 18% 35% 28% 10% 1	594
April	1st to 10th	Day	10% 16% 32% 25% 14% 4	540
	11th to 20th	Day	12% 20% 30% 28% 9% 2	532
	21st to end	Day	11% 19% 29% 23% 14% 3	540
May	1st to 10th	Day	11% 14% 28% 26% 15% 6%	543
	11th to 20th	Day	11% 14% 26% 29% 16% 5%	568
	21st to end	Day	8% 10% 24% 27% 23% 8%	820
June	1st to 10th	Day	4 8% 25% 31% 23% 9%	780
	11th to 20th	Day	6% 11% 31% 33% 17% 4	780
	21st to end	Day	3 9% 26% 34% 22% 5%	780
July	1st to 10th	Day	5% 5% 20% 35% 28% 8%	733
	11th to 20th	Day	5% 4 25% 31% 21% 13%	564
	21st to end	Day	4 4 19% 33% 27% 13%	619
August	1st to 10th	Day	5% 6% 23% 29% 26% 10%	567
	11th to 20th	Day	3 4 23% 37% 22% 11%	557
	21st to end	Day	4 6% 24% 29% 23% 14%	620
September	1st to 10th	Day	4 4 23% 30% 25% 14%	530
	11th to 20th	Day	5% 7% 30% 31% 17% 10%	530
	21st to end	Day	7% 9% 22% 27% 23% 12%	530
October	1st to 10th	Day	5% 8% 24% 23% 27% 13%	355
	11th to 20th	Day	8% 15% 21% 25% 21% 11%	330
	21st to end	Day	9% 15% 28% 31% 13% 5%	363
November	1st to 10th	Day	9% 14% 27% 30% 16% 4	330
	11th to 20th	Day	10% 14% 25% 33% 15% 4	330
	21st to end	Day	8% 9% 31% 32% 15% 5%	330
December	1st to 10th	Day	6% 17% 28% 30% 14% 5%	330
	11th to 20th	Day	6% 18% 29% 35% 12% 1	330
	21st to end	Day	11% 18% 34% 22% 11% 4	363
January	1st to 10th	Night	5% 13% 30% 34% 15% 3	895
	11th to 20th	Night	5% 14% 33% 30% 15% 3	840
	21st to end	Night	4 9% 38% 35% 12% 2	924
February	1st to 10th	Night	4 14% 40% 32% 8% 2	840
	11th to 20th	Night	4 13% 37% 35% 11% 1	824
	21st to end	Night	3 13% 38% 38% 8% 1	692
March	1st to 10th	Night	6% 10% 34% 37% 12% 1	840
	11th to 20th	Night	5% 8% 28% 43% 15% 1	840
	21st to end	Night	7% 9% 36% 37% 10%	924
April	1st to 10th	Night	7% 9% 35% 37% 11% 1	730
	11th to 20th	Night	8% 11% 35% 36% 9% 1	623
	21st to end	Night	5% 10% 30% 38% 15% 1	630
May	1st to 10th	Night	9% 9% 28% 38% 15% 1	630
	11th to 20th	Night	8% 6% 22% 40% 22% 2	621
	21st to end	Night	6% 6% 26% 39% 21% 2	683
June	1st to 10th	Night	4 5% 24% 42% 24% 2	630
	11th to 20th	Night	6% 6% 28% 44% 15%	630
	21st to end	Night	4 3 20% 48% 23% 2	630
July	1st to 10th	Night	2 1 15% 42% 37% 2	615
	11th to 20th	Night	2 2 14% 48% 33% 2	612
	21st to end	Night	3 1 9% 46% 38% 3	690
August	1st to 10th	Night	4 1 14% 48% 31% 2	627
	11th to 20th	Night	3 2 12% 49% 33% 2	624
	21st to end	Night	2 2 14% 42% 36% 3	687
September	1st to 10th	Night	3 2 17% 43% 33% 3	603
	11th to 20th	Night	3 2 18% 44% 28% 6%	800
	21st to end	Night	6% 3 19% 38% 27% 8%	800
October	1st to 10th	Night	6% 4 25% 40% 22% 4	800
	11th to 20th	Night	6% 10% 23% 37% 21% 4	800
	21st to end	Night	6% 7% 29% 37% 17% 3	880
November	1st to 10th	Night	4 7% 30% 40% 17% 2	820
	11th to 20th	Night	4 13% 32% 37% 11% 3	980
	21st to end	Night	4 10% 30% 37% 17% 2	1050
December	1st to 10th	Night	5% 12% 34% 36% 12% 1	1050
	11th to 20th	Night	6% 12% 35% 36% 11% 1	1050
	21st to end	Night	7% 16% 35% 30% 10% 2	1155

Page generated at 2026-08-27 02:07:16. Version ac07

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