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 2025-04-26.

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 6% 2 6% 4 4 5% 4 6% 8% 51%	117869
	Night	2 6% 2 6% 4 4 4 4 6% 8% 53%	119705
Transparency	Day	27% 8% 13% 22% 20% 11%	117630
	Night	26% 7% 14% 25% 20% 8%	119533
Seeing	Day	7% 12% 27% 30% 18% 6%	16034
	Night	5% 8% 28% 38% 19% 2	26341

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 12% 7% 5% 6% 5% 7% 10% 37%	4024
	11th to 20th	Day	1 5% 2 9% 6% 6% 6% 5% 7% 9% 43%	4100
	21st to end	Day	1 7% 4 9% 4 4 5% 4 6% 11% 45%	4510
February	1st to 10th	Day	1 6% 3 8% 7% 6% 6% 4 8% 9% 41%	1800
	11th to 20th	Day	1 8% 3 9% 7% 6% 5% 4 7% 9% 41%	1755
	21st to end	Day	1 7% 3 8% 5% 4 5% 5% 8% 8% 46%	1615
March	1st to 10th	Day	1 8% 3 8% 5% 5% 5% 4 6% 9% 47%	4954
	11th to 20th	Day	2 10% 3 7% 5% 5% 5% 4 6% 9% 44%	5000
	21st to end	Day	3 11% 4 10% 7% 6% 7% 5% 7% 8% 32%	5486
April	1st to 10th	Day	2 7% 3 7% 5% 6% 5% 5% 7% 9% 45%	2303
	11th to 20th	Day	5% 8% 3 8% 5% 5% 5% 5% 7% 8% 41%	2424
	21st to end	Day	5% 7% 3 6% 6% 5% 5% 5% 7% 8% 43%	2537
May	1st to 10th	Day	5% 10% 4 9% 6% 6% 6% 5% 6% 8% 35%	4110
	11th to 20th	Day	5% 6% 2 5% 3 4 5% 4 7% 9% 51%	4071
	21st to end	Day	4 6% 2 6% 5% 5% 6% 4 7% 8% 49%	4790
June	1st to 10th	Day	3 5% 1 5% 5% 4 5% 4 7% 10% 51%	2660
	11th to 20th	Day	4 5% 1 4 3 3 4 3 6% 7% 58%	2660
	21st to end	Day	2 2 1 4 3 4 4 4 6% 6% 65%	2660
July	1st to 10th	Day	2 2 1 3 3 3 3 3 4 5% 72%	4369
	11th to 20th	Day	2 3 1 4 5% 5% 6% 6% 7% 7% 54%	4110
	21st to end	Day	3 3 1 3 3 4 4 4 6% 7% 62%	4454
August	1st to 10th	Day	3 3 1 5% 4 5% 5% 5% 7% 7% 56%	4092
	11th to 20th	Day	2 2 1 5% 4 5% 5% 4 7% 8% 56%	4028
	21st to end	Day	2 3 1 4 3 4 5% 4 5% 6% 64%	4377
September	1st to 10th	Day	3 3 2 3 3 4 4 3 5% 7% 64%	2275
	11th to 20th	Day	3 4 1 5% 3 3 3 3 5% 7% 63%	2090
	21st to end	Day	5% 3 1 4 3 3 3 2 4 5% 66%	2090
October	1st to 10th	Day	3 3 1 3 3 2 3 2 4 6% 70%	3207
	11th to 20th	Day	7% 6% 1 4 3 3 3 3 4 7% 58%	3150
	21st to end	Day	3 6% 2 5% 4 3 4 3 6% 7% 57%	3438
November	1st to 10th	Day	3 5% 3 5% 4 4 4 5% 5% 9% 54%	1835
	11th to 20th	Day	2 5% 2 6% 3 4 4 4 6% 8% 55%	1645
	21st to end	Day	2 5% 3 6% 5% 3 4 5% 7% 8% 51%	1600
December	1st to 10th	Day	1 4 1 4 2 3 3 3 6% 13% 60%	2620
	11th to 20th	Day	1 5% 2 7% 4 5% 6% 6% 9% 13% 42%	3026
	21st to end	Day	2 9% 3 9% 6% 6% 6% 5% 8% 10% 36%	4004
January	1st to 10th	Night	1 7% 4 11% 5% 5% 6% 6% 8% 9% 39%	5985
	11th to 20th	Night	1 6% 4 11% 7% 6% 5% 4 6% 8% 43%	5880
	21st to end	Night	2 10% 4 10% 6% 6% 4 4 6% 8% 41%	6468
February	1st to 10th	Night	1 6% 3 8% 5% 5% 5% 4 7% 10% 47%	2600
	11th to 20th	Night	1 6% 2 7% 5% 5% 6% 5% 7% 8% 47%	2532
	21st to end	Night	1 6% 3 7% 5% 4 5% 5% 7% 10% 47%	2014
March	1st to 10th	Night	2 8% 2 6% 5% 5% 5% 5% 6% 7% 48%	4940
	11th to 20th	Night	2 8% 4 7% 4 4 5% 3 6% 9% 48%	5000
	21st to end	Night	2 9% 4 8% 5% 5% 5% 4 6% 7% 44%	5489
April	1st to 10th	Night	3 7% 2 6% 5% 4 5% 5% 5% 7% 51%	2095
	11th to 20th	Night	5% 8% 2 6% 4 4 5% 5% 7% 8% 47%	1920
	21st to end	Night	3 7% 2 5% 5% 4 6% 3 7% 8% 49%	1759
May	1st to 10th	Night	5% 9% 3 7% 5% 6% 5% 4 6% 7% 43%	2860
	11th to 20th	Night	5% 5% 2 5% 3 4 4 4 5% 8% 55%	2834
	21st to end	Night	4 4 2 6% 4 4 5% 3 6% 9% 54%	2794
June	1st to 10th	Night	3 4 2 3 3 3 4 4 7% 8% 58%	1520
	11th to 20th	Night	4 3 1 3 2 2 4 3 5% 7% 65%	1520
	21st to end	Night	3 2 1 3 2 2 3 3 5% 6% 69%	1520
July	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 6% 72%	2595
	11th to 20th	Night	1 2 1 3 4 4 5% 5% 8% 8% 58%	2860
	21st to end	Night	1 2 1 4 3 4 5% 5% 7% 8% 60%	3109
August	1st to 10th	Night	2 2 1 3 3 4 5% 4 7% 7% 62%	2847
	11th to 20th	Night	3 2 1 6% 4 4 4 3 5% 6% 61%	2811
	21st to end	Night	1 1 1 2 2 2 2 3 4 6% 75%	3288
September	1st to 10th	Night	3 2 1 3 2 2 3 3 4 7% 70%	1903
	11th to 20th	Night	3 2 1 4 3 3 3 3 6% 8% 65%	2090
	21st to end	Night	4 3 1 3 2 3 2 2 4 4 70%	2090
October	1st to 10th	Night	3 3 1 3 2 2 3 2 3 5% 74%	3764
	11th to 20th	Night	5% 6% 2 5% 3 3 3 3 4 6% 61%	3778
	21st to end	Night	4 7% 2 4 3 3 3 3 7% 8% 56%	4120
November	1st to 10th	Night	2 5% 2 6% 4 3 4 4 6% 9% 57%	2462
	11th to 20th	Night	2 5% 2 6% 4 4 4 5% 6% 9% 54%	2733
	21st to end	Night	2 6% 3 6% 4 4 4 4 6% 6% 55%	2800
December	1st to 10th	Night	1 4 2 5% 3 4 4 3 7% 10% 57%	4570
	11th to 20th	Night	1 6% 3 7% 4 4 6% 5% 7% 8% 49%	5181
	21st to end	Night	2 7% 3 9% 6% 5% 6% 5% 9% 11% 36%	6974

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	34% 12% 20% 21% 13%	4025
	11th to 20th	Day	29% 13% 16% 23% 16% 3	4100
	21st to end	Day	29% 10% 19% 26% 13% 3	4510
February	1st to 10th	Day	33% 12% 19% 19% 14% 2	1800
	11th to 20th	Day	35% 10% 14% 26% 13% 2	1764
	21st to end	Day	30% 9% 18% 24% 15% 4	1615
March	1st to 10th	Day	31% 7% 12% 31% 13% 6%	4954
	11th to 20th	Day	34% 8% 17% 25% 16% 1	5000
	21st to end	Day	42% 11% 18% 21% 7% 1	5486
April	1st to 10th	Day	32% 8% 17% 22% 17% 4	2303
	11th to 20th	Day	37% 8% 13% 23% 17% 1	2427
	21st to end	Day	34% 8% 14% 25% 18% 1	2537
May	1st to 10th	Day	37% 9% 15% 18% 17% 5%	4110
	11th to 20th	Day	25% 9% 12% 27% 23% 5%	4074
	21st to end	Day	26% 8% 13% 24% 26% 2	4790
June	1st to 10th	Day	25% 8% 14% 24% 23% 6%	2660
	11th to 20th	Day	23% 6% 10% 19% 29% 14%	2660
	21st to end	Day	17% 5% 8% 16% 29% 25%	2660
July	1st to 10th	Day	14% 4 6% 9% 17% 50%	4301
	11th to 20th	Day	23% 6% 6% 12% 23% 30%	4041
	21st to end	Day	20% 5% 7% 11% 24% 33%	4314
August	1st to 10th	Day	22% 7% 7% 15% 26% 22%	4078
	11th to 20th	Day	23% 6% 8% 18% 27% 18%	4067
	21st to end	Day	20% 5% 7% 12% 29% 28%	4376
September	1st to 10th	Day	19% 5% 8% 17% 27% 24%	2275
	11th to 20th	Day	20% 6% 9% 20% 29% 16%	2090
	21st to end	Day	21% 4 7% 22% 35% 12%	2090
October	1st to 10th	Day	18% 3 7% 19% 31% 23%	3207
	11th to 20th	Day	26% 4 8% 30% 25% 7%	3150
	21st to end	Day	25% 4 11% 31% 24% 3	3438
November	1st to 10th	Day	26% 8% 14% 29% 18% 5%	1835
	11th to 20th	Day	24% 7% 19% 28% 20% 2	1645
	21st to end	Day	28% 7% 16% 27% 19% 2	1600
December	1st to 10th	Day	16% 8% 20% 37% 16% 3	2620
	11th to 20th	Day	26% 11% 23% 27% 11% 1	3024
	21st to end	Day	37% 10% 16% 20% 12% 6%	4004
January	1st to 10th	Night	33% 10% 20% 22% 14% 1	5985
	11th to 20th	Night	33% 11% 16% 27% 12% 1	5880
	21st to end	Night	31% 10% 18% 24% 14% 2	6468
February	1st to 10th	Night	31% 9% 21% 26% 11% 2	2600
	11th to 20th	Night	29% 9% 19% 24% 17% 2	2533
	21st to end	Night	28% 8% 20% 26% 15% 4	2014
March	1st to 10th	Night	31% 6% 11% 30% 17% 5%	4940
	11th to 20th	Night	31% 7% 17% 30% 15% 1	5000
	21st to end	Night	33% 9% 22% 25% 10% 1	5489
April	1st to 10th	Night	29% 7% 17% 25% 17% 5%	2095
	11th to 20th	Night	31% 7% 17% 25% 19% 2	1920
	21st to end	Night	29% 8% 15% 32% 16% 1	1759
May	1st to 10th	Night	33% 8% 14% 19% 22% 3	2860
	11th to 20th	Night	23% 9% 14% 29% 20% 5%	2835
	21st to end	Night	22% 7% 12% 33% 25% 1	2794
June	1st to 10th	Night	20% 7% 16% 27% 23% 7%	1520
	11th to 20th	Night	17% 4 10% 22% 31% 15%	1520
	21st to end	Night	14% 4 8% 13% 33% 28%	1520
July	1st to 10th	Night	12% 3 7% 10% 18% 50%	2554
	11th to 20th	Night	18% 5% 6% 15% 29% 27%	2806
	21st to end	Night	18% 5% 8% 15% 27% 26%	3011
August	1st to 10th	Night	18% 5% 6% 16% 30% 24%	2838
	11th to 20th	Night	24% 4 7% 16% 28% 21%	2838
	21st to end	Night	11% 4 6% 17% 32% 30%	3288
September	1st to 10th	Night	15% 3 8% 19% 29% 27%	1903
	11th to 20th	Night	17% 5% 10% 22% 29% 17%	2090
	21st to end	Night	18% 3 6% 24% 38% 11%	2090
October	1st to 10th	Night	14% 3 6% 19% 40% 18%	3764
	11th to 20th	Night	26% 3 6% 28% 27% 9%	3778
	21st to end	Night	25% 4 11% 33% 23% 4	4120
November	1st to 10th	Night	23% 9% 15% 27% 23% 4	2462
	11th to 20th	Night	24% 7% 17% 33% 16% 3	2733
	21st to end	Night	28% 6% 13% 28% 19% 5%	2800
December	1st to 10th	Night	21% 7% 16% 36% 17% 3	4570
	11th to 20th	Night	29% 8% 21% 32% 9% 1	5182
	21st to end	Night	35% 11% 19% 21% 11% 4	6974

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	9% 17% 21% 31% 19% 3	300
	11th to 20th	Day	9% 16% 29% 29% 16% 1	300
	21st to end	Day	6% 12% 31% 34% 15% 2	330
February	1st to 10th	Day	11% 24% 36% 24% 5% 1	300
	11th to 20th	Day	8% 19% 32% 30% 9% 2	300
	21st to end	Day	5% 21% 36% 28% 8% 1	369
March	1st to 10th	Day	7% 16% 37% 28% 9% 3	500
	11th to 20th	Day	10% 13% 30% 29% 16% 2	500
	21st to end	Day	9% 16% 35% 28% 9% 1	550
April	1st to 10th	Day	10% 17% 31% 26% 13% 4	500
	11th to 20th	Day	12% 20% 30% 27% 9% 2	492
	21st to end	Day	11% 18% 29% 23% 14% 4	481
May	1st to 10th	Day	12% 15% 29% 28% 11% 5%	463
	11th to 20th	Day	11% 13% 23% 31% 17% 5%	488
	21st to end	Day	8% 10% 24% 26% 24% 8%	712
June	1st to 10th	Day	4 7% 24% 31% 25% 9%	680
	11th to 20th	Day	6% 10% 29% 33% 17% 4	680
	21st to end	Day	3 8% 25% 35% 24% 6%	680
July	1st to 10th	Day	4 5% 20% 34% 29% 9%	637
	11th to 20th	Day	5% 5% 26% 32% 21% 11%	484
	21st to end	Day	4 3 18% 34% 28% 13%	531
August	1st to 10th	Day	6% 5% 22% 30% 27% 10%	487
	11th to 20th	Day	3 4 25% 36% 23% 10%	477
	21st to end	Day	4 6% 24% 28% 24% 14%	539
September	1st to 10th	Day	4 5% 23% 30% 24% 14%	490
	11th to 20th	Day	5% 7% 30% 31% 17% 10%	490
	21st to end	Day	7% 10% 23% 27% 22% 11%	490
October	1st to 10th	Day	6% 8% 25% 22% 27% 13%	324
	11th to 20th	Day	8% 15% 17% 27% 22% 12%	300
	21st to end	Day	9% 15% 29% 31% 13% 3	330
November	1st to 10th	Day	10% 13% 28% 29% 16% 4	300
	11th to 20th	Day	10% 14% 25% 34% 14% 4	300
	21st to end	Day	8% 8% 32% 32% 15% 6%	300
December	1st to 10th	Day	5% 13% 29% 33% 15% 5%	300
	11th to 20th	Day	4 17% 29% 37% 12% 1	300
	21st to end	Day	11% 18% 34% 22% 11% 5%	330
January	1st to 10th	Night	5% 13% 29% 34% 16% 3	853
	11th to 20th	Night	5% 14% 33% 30% 15% 3	800
	21st to end	Night	4 9% 38% 35% 13% 2	880
February	1st to 10th	Night	4 15% 41% 30% 8% 2	800
	11th to 20th	Night	4 12% 37% 35% 12% 1	784
	21st to end	Night	3 13% 37% 38% 8% 1	660
March	1st to 10th	Night	5% 10% 33% 38% 13% 1	800
	11th to 20th	Night	5% 7% 28% 44% 15% 1	800
	21st to end	Night	7% 9% 36% 37% 11%	880
April	1st to 10th	Night	7% 8% 36% 36% 11% 1	695
	11th to 20th	Night	9% 11% 35% 35% 9% 1	593
	21st to end	Night	5% 10% 31% 36% 16% 1	587
May	1st to 10th	Night	9% 9% 29% 36% 15% 2	570
	11th to 20th	Night	8% 6% 20% 40% 24% 2	561
	21st to end	Night	6% 6% 26% 38% 22% 2	617
June	1st to 10th	Night	4 5% 23% 41% 25% 2	570
	11th to 20th	Night	6% 6% 27% 44% 16%	570
	21st to end	Night	3 1 19% 49% 25% 2	570
July	1st to 10th	Night	2 1 15% 40% 39% 2	555
	11th to 20th	Night	2 2 14% 47% 34% 2	552
	21st to end	Night	3 1 9% 45% 38% 4	624
August	1st to 10th	Night	4 1 12% 47% 34% 2	567
	11th to 20th	Night	3 1 12% 48% 35% 2	564
	21st to end	Night	2 2 14% 41% 37% 4	627
September	1st to 10th	Night	3 1 17% 42% 33% 3	573
	11th to 20th	Night	3 2 18% 43% 28% 6%	760
	21st to end	Night	6% 3 20% 37% 26% 8%	760
October	1st to 10th	Night	6% 4 25% 40% 23% 4	760
	11th to 20th	Night	6% 9% 23% 37% 22% 4	760
	21st to end	Night	6% 7% 30% 36% 17% 3	836
November	1st to 10th	Night	4 7% 30% 39% 17% 2	780
	11th to 20th	Night	3 14% 33% 36% 11% 3	933
	21st to end	Night	5% 9% 30% 37% 17% 2	1000
December	1st to 10th	Night	5% 11% 34% 37% 12% 1	1000
	11th to 20th	Night	5% 12% 35% 36% 11% 1	1000
	21st to end	Night	7% 15% 35% 30% 11% 2	1100

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