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 2024-11-01.

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	3 6% 2 6% 5% 4 5% 4 6% 8% 51%	101248
	Night	2 6% 2 6% 4 4 5% 4 6% 8% 53%	97964
Transparency	Day	27% 8% 13% 20% 20% 12%	101009
	Night	26% 8% 15% 23% 19% 9%	97792
Seeing	Day	7% 11% 27% 30% 18% 7%	15443
	Night	5% 8% 28% 38% 19% 2	25607

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% 9% 39%	2798
	11th to 20th	Day	1 7% 3 10% 7% 6% 6% 5% 7% 9% 38%	2860
	21st to end	Day	1 7% 4 10% 6% 5% 6% 4 5% 8% 42%	3146
February	1st to 10th	Day	1 7% 3 8% 7% 6% 7% 4 8% 9% 41%	1710
	11th to 20th	Day	1 8% 3 8% 7% 6% 5% 4 6% 9% 42%	1665
	21st to end	Day	1 7% 3 9% 5% 5% 5% 5% 8% 9% 45%	1536
March	1st to 10th	Day	2 5% 3 8% 5% 5% 5% 5% 7% 10% 46%	3444
	11th to 20th	Day	2 10% 4 7% 5% 5% 5% 4 6% 8% 45%	3490
	21st to end	Day	3 13% 5% 11% 7% 6% 7% 5% 8% 9% 28%	3825
April	1st to 10th	Day	2 7% 3 7% 5% 6% 5% 5% 7% 9% 45%	2188
	11th to 20th	Day	5% 8% 3 8% 6% 5% 6% 4 7% 8% 41%	2301
	21st to end	Day	5% 7% 3 6% 6% 5% 5% 5% 7% 7% 43%	2470
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	5% 5% 2 5% 4 3 3 3 5% 6% 58%	1924
	11th to 20th	Day	3 4 1 4 3 3 3 3 5% 6% 63%	1900
	21st to end	Day	2 5% 2 6% 4 4 4 3 6% 8% 56%	2090
November	1st to 10th	Day	3 6% 3 5% 4 4 4 5% 5% 8% 55%	1742
	11th to 20th	Day	2 5% 2 6% 4 4 4 4 7% 8% 55%	1563
	21st to end	Day	2 5% 2 6% 6% 4 4 5% 7% 8% 51%	1520
December	1st to 10th	Day	1 6% 3 6% 3 4 5% 4 8% 10% 49%	1520
	11th to 20th	Day	1 7% 3 7% 5% 5% 6% 6% 8% 11% 41%	1926
	21st to end	Day	2 7% 3 9% 6% 5% 6% 5% 9% 9% 39%	2794
January	1st to 10th	Night	1 7% 4 11% 6% 5% 6% 5% 7% 8% 40%	4187
	11th to 20th	Night	2 7% 5% 9% 6% 5% 5% 5% 7% 9% 41%	4110
	21st to end	Night	2 11% 5% 11% 7% 7% 4 4 5% 8% 35%	4521
February	1st to 10th	Night	1 6% 3 8% 5% 5% 4 4 7% 10% 46%	2470
	11th to 20th	Night	1 6% 2 7% 5% 5% 6% 5% 7% 8% 48%	2402
	21st to end	Night	1 6% 3 7% 5% 4 5% 5% 7% 10% 46%	1917
March	1st to 10th	Night	1 6% 2 6% 5% 5% 5% 5% 7% 9% 50%	3430
	11th to 20th	Night	1 8% 3 7% 4 4 4 3 5% 8% 52%	3490
	21st to end	Night	3 10% 4 9% 6% 5% 6% 5% 6% 6% 40%	3828
April	1st to 10th	Night	3 7% 2 6% 5% 4 5% 5% 5% 6% 51%	1990
	11th to 20th	Night	4 8% 2 6% 4 4 5% 5% 7% 8% 46%	1823
	21st to end	Night	4 7% 2 5% 5% 4 6% 3 7% 8% 49%	1710
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	4 5% 1 4 2 4 4 4 4 6% 62%	2256
	11th to 20th	Night	3 5% 2 4 3 3 3 3 4 6% 65%	2280
	21st to end	Night	4 5% 2 5% 3 3 4 3 6% 7% 59%	2508
November	1st to 10th	Night	2 5% 1 5% 4 3 4 4 6% 9% 57%	2341
	11th to 20th	Night	2 5% 2 5% 4 4 3 5% 6% 9% 54%	2596
	21st to end	Night	2 6% 3 6% 4 4 4 4 6% 6% 55%	2660
December	1st to 10th	Night	1 7% 3 7% 5% 5% 5% 4 5% 8% 49%	2660
	11th to 20th	Night	2 7% 4 8% 5% 4 6% 5% 7% 8% 44%	3271
	21st to end	Night	1 7% 3 9% 6% 5% 7% 5% 8% 10% 38%	4873

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	33% 13% 19% 22% 12% 1	2799
	11th to 20th	Day	32% 15% 21% 20% 10% 1	2860
	21st to end	Day	32% 14% 23% 21% 7% 2	3146
February	1st to 10th	Day	34% 12% 20% 20% 12% 2	1710
	11th to 20th	Day	34% 11% 14% 25% 13% 2	1674
	21st to end	Day	31% 10% 18% 24% 15% 2	1536
March	1st to 10th	Day	27% 9% 17% 30% 14% 3	3444
	11th to 20th	Day	33% 9% 21% 27% 9% 1	3490
	21st to end	Day	44% 13% 16% 20% 6% 1	3825
April	1st to 10th	Day	33% 8% 18% 21% 17% 4	2188
	11th to 20th	Day	37% 9% 14% 23% 17% 1	2304
	21st to end	Day	34% 8% 14% 25% 18% 1	2470
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	27% 5% 10% 23% 26% 9%	1924
	11th to 20th	Day	20% 5% 13% 32% 23% 7%	1900
	21st to end	Day	24% 6% 16% 28% 23% 3	2090
November	1st to 10th	Day	25% 8% 15% 29% 18% 5%	1742
	11th to 20th	Day	25% 7% 19% 27% 20% 2	1563
	21st to end	Day	28% 7% 16% 26% 19% 3	1520
December	1st to 10th	Day	27% 12% 22% 22% 12% 5%	1520
	11th to 20th	Day	29% 12% 22% 23% 13% 1	1924
	21st to end	Day	33% 11% 17% 18% 12% 8%	2794
January	1st to 10th	Night	33% 11% 23% 20% 13% 1	4187
	11th to 20th	Night	31% 12% 21% 24% 10% 2	4110
	21st to end	Night	34% 13% 24% 20% 5% 3	4521
February	1st to 10th	Night	31% 9% 22% 26% 11% 1	2470
	11th to 20th	Night	29% 9% 18% 24% 18% 2	2403
	21st to end	Night	29% 8% 20% 26% 14% 3	1917
March	1st to 10th	Night	26% 8% 15% 32% 19% 1	3430
	11th to 20th	Night	28% 8% 23% 31% 10% 1	3490
	21st to end	Night	36% 11% 18% 26% 8% 1	3828
April	1st to 10th	Night	30% 7% 17% 24% 16% 6%	1990
	11th to 20th	Night	31% 8% 17% 26% 17% 1	1823
	21st to end	Night	29% 8% 15% 32% 16% 1	1710
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	22% 5% 10% 26% 28% 9%	2256
	11th to 20th	Night	21% 4 9% 32% 27% 6%	2280
	21st to end	Night	23% 5% 17% 29% 22% 4	2508
November	1st to 10th	Night	22% 9% 15% 27% 24% 4	2341
	11th to 20th	Night	24% 7% 18% 32% 16% 3	2596
	21st to end	Night	28% 6% 13% 28% 19% 5%	2660
December	1st to 10th	Night	32% 9% 19% 23% 12% 5%	2660
	11th to 20th	Night	32% 10% 18% 27% 11% 1	3272
	21st to end	Night	34% 11% 20% 17% 12% 6%	4873

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	8% 17% 20% 31% 21% 3	270
	11th to 20th	Day	9% 15% 28% 30% 17% 1	270
	21st to end	Day	5% 12% 31% 35% 15% 3	297
February	1st to 10th	Day	8% 24% 37% 25% 5% 1	270
	11th to 20th	Day	7% 19% 30% 32% 9% 2	270
	21st to end	Day	4 21% 37% 29% 7% 1	339
March	1st to 10th	Day	7% 16% 38% 28% 9% 2	460
	11th to 20th	Day	8% 12% 30% 31% 17% 2	460
	21st to end	Day	8% 16% 36% 30% 9% 1	506
April	1st to 10th	Day	10% 18% 31% 25% 13% 3	460
	11th to 20th	Day	12% 21% 30% 28% 7% 2	452
	21st to end	Day	11% 18% 29% 23% 15% 4	460
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% 12% 3	330
November	1st to 10th	Day	10% 14% 27% 29% 15% 5%	270
	11th to 20th	Day	9% 14% 24% 34% 15% 4	270
	21st to end	Day	8% 7% 31% 32% 15% 6%	270
December	1st to 10th	Day	6% 13% 30% 35% 13% 3	270
	11th to 20th	Day	4 17% 30% 36% 12% 1	270
	21st to end	Day	9% 17% 36% 21% 11% 5%	297
January	1st to 10th	Night	5% 13% 28% 34% 17% 3	811
	11th to 20th	Night	5% 15% 33% 30% 15% 3	760
	21st to end	Night	4 9% 38% 34% 13% 2	836
February	1st to 10th	Night	4 14% 42% 31% 8% 2	760
	11th to 20th	Night	4 12% 37% 35% 12% 1	744
	21st to end	Night	3 13% 37% 38% 8% 1	628
March	1st to 10th	Night	5% 10% 34% 38% 12% 1	760
	11th to 20th	Night	4 6% 28% 45% 15% 1	760
	21st to end	Night	8% 9% 36% 37% 10%	836
April	1st to 10th	Night	8% 9% 37% 36% 11% 1	660
	11th to 20th	Night	9% 12% 35% 36% 9% 1	563
	21st to end	Night	6% 10% 30% 36% 16% 1	570
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 6% 31% 38% 18% 2	742
	11th to 20th	Night	3 14% 32% 36% 12% 3	886
	21st to end	Night	5% 10% 30% 36% 17% 2	950
December	1st to 10th	Night	5% 11% 35% 37% 12% 1	950
	11th to 20th	Night	5% 11% 35% 37% 11% 1	950
	21st to end	Night	7% 15% 35% 31% 11% 2	1045

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