The Lair
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Lair 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	10% 5% 2 5% 4 4 4 3 5% 7% 51%	132274
	Night	11% 5% 2 4 3 3 3 3 5% 7% 55%	130889
Transparency	Day	32% 4 9% 21% 22% 11%	136054
	Night	30% 3 9% 21% 22% 14%	135173
Seeing	Day	12% 8% 25% 30% 21% 5%	16913
	Night	12% 7% 31% 36% 13% 1	26911

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	10% 7% 2 5% 3 3 3 3 4 5% 55%	5289
	11th to 20th	Day	11% 5% 1 4 3 2 3 3 5% 5% 59%	5310
	21st to end	Day	20% 7% 2 5% 4 4 3 3 3 5% 45%	6147
February	1st to 10th	Day	15% 7% 2 5% 3 2 3 3 4 5% 52%	2100
	11th to 20th	Day	15% 7% 2 5% 4 3 3 3 4 5% 49%	2049
	21st to end	Day	15% 6% 1 4 3 2 2 2 4 5% 55%	1730
March	1st to 10th	Day	11% 6% 2 7% 4 3 3 3 4 5% 51%	4950
	11th to 20th	Day	13% 4 2 4 3 4 3 3 5% 6% 53%	5090
	21st to end	Day	10% 6% 2 5% 3 4 4 4 5% 7% 49%	5583
April	1st to 10th	Day	12% 8% 3 6% 5% 4 4 3 5% 6% 45%	2310
	11th to 20th	Day	15% 6% 3 7% 4 4 3 3 5% 6% 44%	2331
	21st to end	Day	13% 5% 2 6% 4 4 4 4 5% 6% 48%	2696
May	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 6% 7% 40%	5992
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 7% 46%	5941
	21st to end	Day	12% 6% 3 6% 5% 5% 5% 4 7% 9% 37%	6434
June	1st to 10th	Day	5% 4 1 4 4 4 4 4 7% 8% 54%	2600
	11th to 20th	Day	5% 4 1 4 4 5% 5% 4 6% 9% 53%	2600
	21st to end	Day	7% 3 1 4 5% 4 5% 5% 7% 9% 49%	2600
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 4 7% 8% 41%	5900
	11th to 20th	Day	3 3 1 4 3 4 5% 4 7% 10% 56%	5900
	21st to end	Day	4 4 2 5% 4 4 5% 5% 7% 11% 49%	6422
August	1st to 10th	Day	1 1 1 3 2 3 3 3 4 8% 71%	4234
	11th to 20th	Day	4 2 1 3 3 3 3 3 5% 8% 64%	3886
	21st to end	Day	4 3 1 4 4 4 5% 4 7% 9% 55%	4165
September	1st to 10th	Day	11% 4 1 5% 4 4 5% 4 6% 7% 50%	2200
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 4 7% 49%	2200
	21st to end	Day	8% 3 1 4 4 5% 4 3 5% 7% 57%	2200
October	1st to 10th	Day	5% 3 1 4 3 3 4 3 4 7% 62%	2200
	11th to 20th	Day	9% 5% 2 4 3 3 3 3 4 5% 61%	2060
	21st to end	Day	11% 4 1 5% 3 3 3 2 3 4 62%	2200
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 5% 54%	2058
	11th to 20th	Day	8% 4 2 5% 3 3 3 3 5% 7% 58%	1909
	21st to end	Day	15% 6% 2 5% 4 4 3 3 3 5% 51%	1890
December	1st to 10th	Day	17% 7% 2 6% 3 3 4 3 3 5% 47%	3030
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 3 5% 5% 46%	3481
	21st to end	Day	11% 5% 2 4 3 3 3 2 3 5% 60%	4587
January	1st to 10th	Night	13% 5% 2 4 3 2 2 2 3 4 60%	7634
	11th to 20th	Night	8% 3 1 3 2 2 2 2 3 6% 67%	7680
	21st to end	Night	23% 7% 3 5% 4 3 3 2 3 5% 42%	8122
February	1st to 10th	Night	13% 5% 3 5% 4 2 2 3 4 5% 54%	2520
	11th to 20th	Night	15% 5% 2 5% 3 2 3 2 4 6% 52%	2458
	21st to end	Night	15% 4 2 4 3 2 4 3 4 5% 53%	2076
March	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 6% 7% 50%	5126
	11th to 20th	Night	12% 5% 2 4 3 3 4 3 5% 6% 53%	5090
	21st to end	Night	10% 7% 2 5% 3 4 3 3 4 7% 52%	5588
April	1st to 10th	Night	10% 5% 1 5% 4 4 4 3 5% 7% 51%	2310
	11th to 20th	Night	12% 6% 3 6% 4 4 4 4 4 7% 47%	2233
	21st to end	Night	12% 6% 2 4 3 3 4 2 5% 7% 51%	1922
May	1st to 10th	Night	20% 7% 1 4 3 3 3 3 5% 6% 44%	4143
	11th to 20th	Night	7% 6% 2 6% 4 5% 5% 4 6% 7% 49%	4107
	21st to end	Night	11% 6% 2 6% 3 4 5% 4 6% 9% 43%	4443
June	1st to 10th	Night	2 2 1 4 3 3 4 4 7% 9% 60%	1800
	11th to 20th	Night	4 3 1 3 3 3 3 4 6% 10% 62%	1800
	21st to end	Night	5% 3 1 2 3 4 4 3 6% 8% 60%	1800
July	1st to 10th	Night	8% 5% 2 6% 4 3 4 3 6% 11% 48%	4078
	11th to 20th	Night	2 2 1 3 2 3 4 3 6% 11% 64%	4077
	21st to end	Night	2 4 2 4 4 4 5% 4 6% 10% 56%	4449
August	1st to 10th	Night	1 1 2 2 2 2 2 4 6% 78%	2926
	11th to 20th	Night	3 2 1 3 2 2 4 3 6% 10% 65%	3167
	21st to end	Night	3 3 1 4 3 3 4 3 6% 10% 60%	3725
September	1st to 10th	Night	8% 4 1 4 4 4 5% 3 6% 7% 53%	2200
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 6% 8% 50%	2200
	21st to end	Night	8% 3 1 4 2 3 4 3 6% 7% 60%	2200
October	1st to 10th	Night	5% 2 1 3 2 3 3 2 4 6% 69%	2200
	11th to 20th	Night	10% 3 1 3 2 3 3 3 4 5% 62%	2318
	21st to end	Night	11% 5% 2 3 2 2 3 2 4 4 63%	2640
November	1st to 10th	Night	11% 5% 1 3 3 3 3 2 3 5% 60%	2460
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 4 6% 61%	2708
	21st to end	Night	15% 5% 2 4 3 3 3 4 4 6% 51%	2730
December	1st to 10th	Night	17% 5% 2 5% 3 3 3 3 4 6% 48%	4380
	11th to 20th	Night	16% 8% 2 6% 3 3 3 2 3 5% 49%	4946
	21st to end	Night	13% 6% 2 4 2 2 2 2 3 4 60%	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	33% 2 4 24% 27% 9%	5290
	11th to 20th	Day	29% 2 4 18% 30% 18%	5310
	21st to end	Day	45% 2 3 14% 24% 12%	6147
February	1st to 10th	Day	37% 3 6% 15% 22% 18%	2100
	11th to 20th	Day	38% 3 6% 20% 20% 13%	2059
	21st to end	Day	34% 3 4 16% 25% 18%	1730
March	1st to 10th	Day	35% 4 4 14% 21% 21%	4950
	11th to 20th	Day	33% 4 8% 9% 14% 32%	5090
	21st to end	Day	34% 5% 9% 20% 23% 10%	5585
April	1st to 10th	Day	39% 4 8% 17% 16% 15%	2310
	11th to 20th	Day	41% 4 8% 17% 19% 12%	2331
	21st to end	Day	36% 5% 8% 21% 21% 9%	2696
May	1st to 10th	Day	43% 5% 10% 18% 19% 5%	6030
	11th to 20th	Day	37% 4 9% 15% 25% 9%	5945
	21st to end	Day	41% 6% 10% 23% 13% 6%	6447
June	1st to 10th	Day	26% 6% 13% 29% 23% 4	2600
	11th to 20th	Day	26% 5% 13% 28% 21% 7%	2600
	21st to end	Day	29% 7% 13% 23% 23% 5%	2600
July	1st to 10th	Day	41% 4 17% 29% 9% 1	5836
	11th to 20th	Day	23% 3 16% 35% 19% 3	5831
	21st to end	Day	27% 4 16% 31% 19% 3	6240
August	1st to 10th	Day	13% 3 12% 34% 37% 1	4220
	11th to 20th	Day	18% 4 18% 39% 18% 3	3923
	21st to end	Day	25% 6% 19% 28% 20% 3	4164
September	1st to 10th	Day	33% 5% 10% 22% 20% 10%	2200
	11th to 20th	Day	36% 5% 11% 20% 22% 6%	2200
	21st to end	Day	27% 5% 10% 22% 22% 15%	2200
October	1st to 10th	Day	14% 3 6% 13% 27% 36%	3590
	11th to 20th	Day	19% 2 7% 19% 30% 24%	3339
	21st to end	Day	23% 2 2 19% 40% 13%	3538
November	1st to 10th	Day	33% 3 8% 18% 26% 12%	2058
	11th to 20th	Day	28% 4 9% 19% 23% 18%	1909
	21st to end	Day	38% 3 4 19% 21% 14%	1890
December	1st to 10th	Day	43% 2 5% 17% 22% 11%	3030
	11th to 20th	Day	41% 3 6% 11% 25% 14%	3479
	21st to end	Day	30% 2 4 18% 29% 18%	4587
January	1st to 10th	Night	31% 1 5% 20% 29% 13%	7635
	11th to 20th	Night	22% 1 3 19% 31% 25%	7680
	21st to end	Night	47% 2 3 11% 23% 14%	8122
February	1st to 10th	Night	33% 4 7% 13% 24% 19%	2520
	11th to 20th	Night	36% 2 7% 19% 21% 15%	2458
	21st to end	Night	34% 3 6% 14% 23% 20%	2076
March	1st to 10th	Night	33% 4 6% 17% 24% 16%	5126
	11th to 20th	Night	31% 5% 7% 13% 16% 27%	5090
	21st to end	Night	34% 4 8% 19% 24% 11%	5588
April	1st to 10th	Night	32% 6% 9% 20% 17% 16%	2310
	11th to 20th	Night	37% 5% 11% 15% 19% 15%	2233
	21st to end	Night	34% 5% 12% 20% 20% 9%	1922
May	1st to 10th	Night	42% 4 8% 23% 16% 7%	4170
	11th to 20th	Night	34% 5% 12% 18% 18% 14%	4108
	21st to end	Night	37% 5% 13% 28% 11% 5%	4452
June	1st to 10th	Night	19% 7% 17% 31% 22% 4	1800
	11th to 20th	Night	19% 5% 16% 33% 21% 7%	1800
	21st to end	Night	22% 7% 14% 31% 22% 5%	1800
July	1st to 10th	Night	32% 4 21% 35% 7% 1	4035
	11th to 20th	Night	16% 3 22% 39% 18% 2	4026
	21st to end	Night	24% 5% 21% 31% 16% 4	4320
August	1st to 10th	Night	9% 3 11% 37% 37% 2	2916
	11th to 20th	Night	17% 4 25% 36% 16% 3	3196
	21st to end	Night	20% 7% 21% 29% 19% 3	3725
September	1st to 10th	Night	30% 7% 11% 22% 20% 10%	2200
	11th to 20th	Night	32% 5% 13% 25% 18% 6%	2200
	21st to end	Night	24% 4 11% 26% 20% 15%	2200
October	1st to 10th	Night	11% 3 7% 15% 23% 42%	3590
	11th to 20th	Night	18% 2 8% 18% 30% 23%	3765
	21st to end	Night	21% 2 4 21% 36% 17%	4252
November	1st to 10th	Night	28% 4 7% 20% 27% 15%	2460
	11th to 20th	Night	26% 3 8% 21% 24% 18%	2708
	21st to end	Night	35% 4 7% 17% 22% 15%	2730
December	1st to 10th	Night	39% 2 6% 19% 22% 12%	4380
	11th to 20th	Night	39% 2 6% 11% 24% 17%	4947
	21st to end	Night	31% 2 4 18% 26% 19%	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	16% 18% 37% 23% 5% 1	330
	11th to 20th	Day	19% 14% 38% 22% 6% 1	330
	21st to end	Day	18% 16% 41% 18% 6%	363
February	1st to 10th	Day	21% 14% 36% 23% 5%	330
	11th to 20th	Day	21% 18% 38% 20% 3	330
	21st to end	Day	22% 16% 40% 20% 2	273
March	1st to 10th	Day	20% 16% 44% 17% 3	330
	11th to 20th	Day	19% 18% 40% 19% 4	330
	21st to end	Day	16% 15% 41% 21% 7%	363
April	1st to 10th	Day	20% 17% 42% 17% 4	330
	11th to 20th	Day	18% 19% 40% 19% 4	369
	21st to end	Day	16% 16% 40% 20% 6%	716
May	1st to 10th	Day	14% 12% 38% 29% 6%	753
	11th to 20th	Day	15% 10% 33% 31% 10% 2	756
	21st to end	Day	14% 5% 32% 34% 13% 2	789
June	1st to 10th	Day	6% 3 22% 41% 22% 6%	730
	11th to 20th	Day	5% 1 21% 50% 19% 3	730
	21st to end	Day	5% 1 15% 37% 34% 8%	730
July	1st to 10th	Day	8% 1 11% 37% 33% 10%	721
	11th to 20th	Day	3 6% 37% 45% 8%	718
	21st to end	Day	4 1 5% 36% 43% 11%	794
August	1st to 10th	Day	2 4 37% 45% 12%	726
	11th to 20th	Day	6% 5% 35% 42% 12%	545
	21st to end	Day	5% 6% 27% 40% 21%	501
September	1st to 10th	Day	13% 7% 19% 42% 18%	330
	11th to 20th	Day	5% 1 7% 26% 45% 17%	330
	21st to end	Day	12% 2 7% 34% 31% 15%	330
October	1st to 10th	Day	4 2 13% 33% 36% 11%	330
	11th to 20th	Day	8% 3 28% 35% 22% 3	330
	21st to end	Day	19% 9% 25% 25% 20% 2	363
November	1st to 10th	Day	19% 6% 33% 28% 11% 4	330
	11th to 20th	Day	12% 11% 28% 33% 12% 5%	330
	21st to end	Day	20% 14% 37% 15% 12% 1	330
December	1st to 10th	Day	17% 9% 35% 28% 9% 2	330
	11th to 20th	Day	15% 15% 37% 22% 10% 1	330
	21st to end	Day	14% 13% 39% 26% 6% 2	363
January	1st to 10th	Night	15% 10% 39% 29% 8%	1050
	11th to 20th	Night	15% 12% 36% 30% 6%	1050
	21st to end	Night	18% 11% 38% 27% 5%	1055
February	1st to 10th	Night	15% 13% 39% 29% 4	840
	11th to 20th	Night	19% 14% 41% 23% 3	824
	21st to end	Night	19% 13% 45% 20% 2	692
March	1st to 10th	Night	17% 15% 39% 27% 2	679
	11th to 20th	Night	15% 14% 39% 28% 4	630
	21st to end	Night	12% 12% 48% 24% 4	693
April	1st to 10th	Night	11% 12% 48% 27% 2	630
	11th to 20th	Night	14% 15% 45% 23% 3	623
	21st to end	Night	13% 13% 44% 24% 6%	630
May	1st to 10th	Night	14% 10% 45% 28% 3	630
	11th to 20th	Night	11% 6% 40% 33% 10% 1	609
	21st to end	Night	10% 6% 39% 35% 11%	650
June	1st to 10th	Night	3 2 31% 45% 18% 2	600
	11th to 20th	Night	4 2 32% 51% 11% 1	600
	21st to end	Night	4 2 20% 51% 22% 2	600
July	1st to 10th	Night	8% 1 11% 52% 27% 2	585
	11th to 20th	Night	4 1 11% 55% 29% 1	582
	21st to end	Night	2 11% 53% 31% 2	657
August	1st to 10th	Night	2 7% 57% 32% 3	597
	11th to 20th	Night	6% 1 6% 51% 32% 4	594
	21st to end	Night	4 1 5% 47% 36% 6%	660
September	1st to 10th	Night	9% 1 13% 42% 32% 4	600
	11th to 20th	Night	10% 1 8% 45% 33% 2	600
	21st to end	Night	9% 13% 44% 31% 3	600
October	1st to 10th	Night	5% 1 16% 48% 27% 3	600
	11th to 20th	Night	10% 5% 27% 40% 17% 2	718
	21st to end	Night	13% 9% 31% 31% 15% 1	880
November	1st to 10th	Night	12% 4 29% 40% 13% 1	820
	11th to 20th	Night	8% 8% 33% 37% 13% 1	1028
	21st to end	Night	17% 11% 31% 30% 10% 1	1050
December	1st to 10th	Night	14% 8% 33% 36% 8%	1050
	11th to 20th	Night	19% 9% 33% 33% 6%	1050
	21st to end	Night	16% 10% 34% 32% 8%	1155

Page generated at 2026-05-14 00:58:55. Version ac07

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