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 2025-01-22.

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%	105271
	Night	11% 5% 2 4 3 3 3 3 5% 7% 55%	104229
Transparency	Day	31% 5% 10% 22% 23% 10%	109090
	Night	29% 4 10% 22% 22% 13%	108537
Seeing	Day	12% 8% 24% 30% 21% 6%	15177
	Night	12% 7% 30% 36% 14% 1	25212

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	11% 9% 2 4 2 2 3 3 4 6% 55%	2937
	11th to 20th	Day	14% 5% 1 3 2 2 3 3 5% 5% 57%	2940
	21st to end	Day	16% 6% 3 6% 4 3 3 2 3 5% 49%	3313
February	1st to 10th	Day	16% 7% 2 5% 3 2 3 3 4 5% 51%	1900
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 5% 5% 49%	1849
	21st to end	Day	17% 6% 1 4 3 2 2 2 4 5% 53%	1570
March	1st to 10th	Day	14% 7% 2 7% 4 3 3 3 4 5% 46%	3368
	11th to 20th	Day	19% 6% 2 6% 4 5% 4 4 5% 7% 37%	3480
	21st to end	Day	10% 6% 3 6% 4 4 4 4 6% 7% 46%	3814
April	1st to 10th	Day	12% 7% 3 5% 5% 4 4 3 5% 6% 47%	2090
	11th to 20th	Day	15% 6% 3 6% 4 4 3 3 5% 6% 46%	2107
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 5% 50%	2439
May	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 6% 7% 37%	4082
	11th to 20th	Day	14% 7% 2 6% 4 4 3 3 5% 6% 47%	4082
	21st to end	Day	12% 5% 2 6% 5% 5% 6% 5% 8% 9% 37%	4477
June	1st to 10th	Day	5% 4 1 4 4 4 4 4 7% 8% 55%	2470
	11th to 20th	Day	5% 4 1 4 4 5% 5% 4 6% 9% 53%	2470
	21st to end	Day	7% 3 1 4 5% 4 5% 4 7% 9% 48%	2470
July	1st to 10th	Day	5% 4 1 5% 5% 5% 6% 4 7% 8% 52%	4120
	11th to 20th	Day	3 3 1 3 3 3 4 4 6% 9% 62%	4120
	21st to end	Day	6% 5% 2 5% 5% 5% 6% 5% 8% 11% 44%	4465
August	1st to 10th	Day	1 1 1 3 2 3 3 3 4 8% 71%	4104
	11th to 20th	Day	4 2 1 3 3 3 3 3 5% 8% 65%	3765
	21st to end	Day	4 3 1 4 4 4 5% 4 7% 10% 55%	4037
September	1st to 10th	Day	11% 4 1 5% 4 4 5% 4 5% 7% 49%	2090
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 4 7% 48%	2090
	21st to end	Day	8% 3 1 4 4 5% 4 3 5% 7% 57%	2090
October	1st to 10th	Day	5% 3 1 4 3 4 4 3 4 7% 61%	2090
	11th to 20th	Day	10% 5% 2 4 3 2 3 3 4 4 60%	1957
	21st to end	Day	12% 4 1 4 3 3 3 2 3 4 62%	2090
November	1st to 10th	Day	14% 5% 2 4 4 3 3 3 4 5% 53%	1958
	11th to 20th	Day	8% 4 2 5% 3 3 3 3 5% 7% 57%	1818
	21st to end	Day	16% 6% 2 5% 4 3 3 3 3 5% 52%	1800
December	1st to 10th	Day	17% 7% 2 6% 3 3 4 3 3 4 46%	2940
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 3 5% 5% 45%	3391
	21st to end	Day	11% 5% 2 4 3 3 3 2 3 5% 60%	4488
January	1st to 10th	Night	15% 5% 2 3 2 2 2 2 3 4 59%	4238
	11th to 20th	Night	11% 4 1 3 3 1 2 1 3 4 66%	4250
	21st to end	Night	17% 7% 3 5% 4 3 3 2 3 6% 46%	4404
February	1st to 10th	Night	13% 5% 3 6% 4 2 2 3 4 5% 52%	2280
	11th to 20th	Night	15% 6% 2 5% 3 2 3 2 4 6% 52%	2218
	21st to end	Night	15% 5% 2 5% 3 2 4 3 5% 5% 52%	1884
March	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 5% 7% 49%	3486
	11th to 20th	Night	17% 7% 2 5% 4 4 4 4 6% 6% 40%	3480
	21st to end	Night	9% 7% 2 6% 4 4 4 3 5% 6% 49%	3817
April	1st to 10th	Night	9% 5% 1 5% 4 3 4 3 5% 7% 53%	2090
	11th to 20th	Night	12% 6% 3 6% 4 4 3 4 4 7% 48%	2018
	21st to end	Night	10% 6% 2 4 3 3 4 3 6% 7% 52%	1739
May	1st to 10th	Night	22% 8% 1 4 3 2 3 2 4 6% 43%	2823
	11th to 20th	Night	9% 7% 2 5% 3 3 4 3 5% 7% 51%	2823
	21st to end	Night	10% 5% 2 6% 3 4 5% 4 7% 8% 47%	3090
June	1st to 10th	Night	2 2 1 3 3 3 4 4 7% 9% 61%	1710
	11th to 20th	Night	4 3 2 3 3 3 3 4 6% 10% 63%	1710
	21st to end	Night	5% 3 1 2 3 4 4 3 6% 8% 59%	1710
July	1st to 10th	Night	5% 2 1 4 3 3 3 2 6% 9% 62%	2850
	11th to 20th	Night	2 2 1 2 2 2 3 3 6% 9% 67%	2850
	21st to end	Night	2 5% 2 5% 5% 4 5% 4 7% 10% 52%	3098
August	1st to 10th	Night	1 1 2 1 2 2 2 4 6% 78%	2836
	11th to 20th	Night	4 2 1 3 2 2 4 3 6% 9% 65%	3069
	21st to end	Night	3 3 1 4 3 3 4 3 6% 10% 61%	3611
September	1st to 10th	Night	8% 4 1 4 3 4 5% 3 6% 7% 54%	2090
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 6% 8% 50%	2090
	21st to end	Night	8% 3 1 4 2 3 4 3 6% 6% 61%	2090
October	1st to 10th	Night	5% 2 1 3 2 3 3 2 4 6% 68%	2090
	11th to 20th	Night	11% 3 1 3 2 3 3 3 4 6% 60%	2202
	21st to end	Night	12% 5% 2 3 2 2 3 2 3 4 62%	2508
November	1st to 10th	Night	11% 5% 1 4 3 3 3 2 3 5% 58%	2340
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 4 6% 61%	2579
	21st to end	Night	14% 5% 2 4 3 3 4 4 4 6% 51%	2600
December	1st to 10th	Night	17% 5% 2 5% 3 3 3 3 4 7% 47%	4250
	11th to 20th	Night	16% 8% 2 6% 3 3 3 2 4 5% 49%	4816
	21st to end	Night	13% 6% 2 4 2 2 2 2 3 4 60%	6490

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% 4 4 18% 27% 14%	2938
	11th to 20th	Day	29% 3 6% 16% 30% 15%	2940
	21st to end	Day	39% 4 4 13% 27% 13%	3313
February	1st to 10th	Day	37% 4 6% 15% 21% 17%	1900
	11th to 20th	Day	37% 3 6% 19% 21% 14%	1859
	21st to end	Day	35% 3 4 15% 23% 18%	1570
March	1st to 10th	Day	39% 6% 6% 13% 22% 14%	3368
	11th to 20th	Day	46% 6% 12% 11% 16% 9%	3480
	21st to end	Day	35% 7% 11% 15% 24% 9%	3814
April	1st to 10th	Day	38% 4 8% 18% 16% 15%	2090
	11th to 20th	Day	39% 5% 8% 17% 19% 12%	2107
	21st to end	Day	34% 5% 7% 21% 23% 9%	2439
May	1st to 10th	Day	45% 7% 11% 16% 16% 5%	4120
	11th to 20th	Day	38% 6% 8% 18% 26% 5%	4086
	21st to end	Day	39% 9% 13% 24% 10% 4	4489
June	1st to 10th	Day	25% 6% 13% 29% 22% 4	2470
	11th to 20th	Day	26% 5% 14% 28% 20% 7%	2470
	21st to end	Day	30% 8% 12% 22% 23% 5%	2470
July	1st to 10th	Day	30% 5% 16% 38% 11% 1	4056
	11th to 20th	Day	20% 4 13% 38% 20% 5%	4051
	21st to end	Day	31% 5% 15% 35% 14% 1	4325
August	1st to 10th	Day	13% 3 12% 33% 37% 1	4090
	11th to 20th	Day	18% 4 18% 40% 18% 3	3802
	21st to end	Day	24% 6% 20% 28% 20% 3	4036
September	1st to 10th	Day	33% 5% 10% 22% 19% 10%	2090
	11th to 20th	Day	37% 5% 11% 19% 22% 6%	2090
	21st to end	Day	27% 5% 10% 22% 22% 15%	2090
October	1st to 10th	Day	14% 3 6% 12% 27% 37%	3480
	11th to 20th	Day	19% 2 7% 18% 29% 24%	3236
	21st to end	Day	23% 2 2 19% 40% 13%	3428
November	1st to 10th	Day	34% 3 9% 19% 24% 11%	1958
	11th to 20th	Day	28% 4 9% 18% 23% 18%	1818
	21st to end	Day	37% 3 4 19% 21% 15%	1800
December	1st to 10th	Day	43% 2 5% 17% 22% 10%	2940
	11th to 20th	Day	41% 3 7% 11% 24% 14%	3389
	21st to end	Day	30% 2 4 18% 29% 18%	4488
January	1st to 10th	Night	31% 2 4 17% 28% 17%	4238
	11th to 20th	Night	25% 2 4 18% 29% 23%	4250
	21st to end	Night	42% 4 4 13% 24% 13%	4404
February	1st to 10th	Night	34% 4 7% 13% 22% 19%	2280
	11th to 20th	Night	36% 2 6% 18% 22% 16%	2218
	21st to end	Night	35% 4 6% 14% 21% 20%	1884
March	1st to 10th	Night	34% 6% 7% 15% 25% 13%	3486
	11th to 20th	Night	42% 7% 10% 16% 15% 9%	3480
	21st to end	Night	35% 6% 10% 18% 22% 9%	3817
April	1st to 10th	Night	31% 6% 9% 20% 19% 16%	2090
	11th to 20th	Night	36% 5% 10% 14% 19% 15%	2018
	21st to end	Night	32% 5% 11% 21% 22% 10%	1739
May	1st to 10th	Night	42% 5% 10% 20% 15% 7%	2850
	11th to 20th	Night	31% 7% 13% 22% 20% 8%	2824
	21st to end	Night	34% 7% 14% 30% 10% 5%	3099
June	1st to 10th	Night	18% 7% 17% 31% 22% 4	1710
	11th to 20th	Night	18% 5% 16% 32% 22% 7%	1710
	21st to end	Night	23% 7% 13% 30% 23% 5%	1710
July	1st to 10th	Night	21% 5% 20% 44% 9% 1	2805
	11th to 20th	Night	15% 3 19% 41% 20% 2	2796
	21st to end	Night	27% 4 20% 37% 11% 1	2999
August	1st to 10th	Night	9% 3 12% 37% 37% 2	2826
	11th to 20th	Night	17% 4 25% 36% 16% 3	3098
	21st to end	Night	20% 8% 22% 29% 19% 3	3611
September	1st to 10th	Night	29% 7% 12% 22% 19% 10%	2090
	11th to 20th	Night	33% 6% 14% 23% 18% 6%	2090
	21st to end	Night	24% 4 11% 25% 20% 15%	2090
October	1st to 10th	Night	11% 3 7% 15% 22% 43%	3480
	11th to 20th	Night	19% 2 8% 17% 30% 23%	3649
	21st to end	Night	21% 2 3 22% 36% 16%	4120
November	1st to 10th	Night	29% 4 8% 20% 25% 14%	2340
	11th to 20th	Night	26% 3 8% 20% 23% 19%	2579
	21st to end	Night	35% 4 8% 17% 22% 16%	2600
December	1st to 10th	Night	39% 2 6% 19% 22% 11%	4250
	11th to 20th	Night	39% 2 6% 11% 25% 17%	4817
	21st to end	Night	32% 2 3 18% 26% 19%	6490

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	17% 18% 36% 23% 5%	300
	11th to 20th	Day	19% 14% 39% 20% 6% 1	300
	21st to end	Day	16% 19% 39% 19% 7%	300
February	1st to 10th	Day	22% 17% 34% 22% 4	270
	11th to 20th	Day	20% 20% 39% 19% 3	270
	21st to end	Day	25% 19% 39% 16% 1	225
March	1st to 10th	Day	19% 16% 46% 16% 3	270
	11th to 20th	Day	21% 18% 39% 18% 5%	270
	21st to end	Day	16% 17% 43% 19% 5%	297
April	1st to 10th	Day	19% 20% 46% 12% 3	270
	11th to 20th	Day	18% 20% 39% 20% 4	305
	21st to end	Day	15% 17% 41% 19% 7%	619
May	1st to 10th	Day	13% 13% 38% 30% 6%	653
	11th to 20th	Day	16% 8% 32% 32% 11% 2	675
	21st to end	Day	14% 6% 31% 35% 13% 2	734
June	1st to 10th	Day	6% 3 21% 41% 23% 7%	680
	11th to 20th	Day	5% 1 21% 50% 20% 3	680
	21st to end	Day	5% 1 15% 38% 33% 8%	680
July	1st to 10th	Day	7% 1 11% 37% 33% 11%	671
	11th to 20th	Day	2 6% 36% 46% 8%	668
	21st to end	Day	4 1 5% 36% 43% 11%	739
August	1st to 10th	Day	2 4 37% 45% 12%	676
	11th to 20th	Day	6% 5% 35% 42% 11%	504
	21st to end	Day	5% 7% 27% 40% 21%	461
September	1st to 10th	Day	13% 7% 18% 42% 20%	300
	11th to 20th	Day	5% 1 8% 24% 45% 17%	300
	21st to end	Day	13% 2 7% 33% 31% 15%	300
October	1st to 10th	Day	4 3 15% 34% 35% 10%	300
	11th to 20th	Day	9% 3 28% 35% 22% 2	300
	21st to end	Day	20% 9% 25% 23% 20% 2	330
November	1st to 10th	Day	20% 5% 35% 27% 9% 3	300
	11th to 20th	Day	12% 12% 30% 29% 12% 5%	300
	21st to end	Day	21% 15% 37% 15% 12% 1	300
December	1st to 10th	Day	17% 9% 33% 29% 9% 3	300
	11th to 20th	Day	15% 14% 39% 22% 9%	300
	21st to end	Day	15% 14% 40% 23% 6% 2	330
January	1st to 10th	Night	15% 9% 39% 28% 8%	1000
	11th to 20th	Night	16% 12% 35% 31% 6%	1000
	21st to end	Night	17% 11% 38% 29% 6%	961
February	1st to 10th	Night	16% 13% 39% 28% 4	760
	11th to 20th	Night	18% 14% 42% 23% 3	744
	21st to end	Night	19% 14% 46% 18% 2	628
March	1st to 10th	Night	17% 15% 38% 28% 2	615
	11th to 20th	Night	15% 13% 39% 28% 4	570
	21st to end	Night	11% 13% 48% 23% 4	627
April	1st to 10th	Night	9% 12% 50% 26% 2	570
	11th to 20th	Night	13% 14% 46% 23% 3	563
	21st to end	Night	11% 13% 45% 25% 7%	570
May	1st to 10th	Night	14% 11% 45% 28% 3	570
	11th to 20th	Night	10% 6% 40% 33% 10% 1	561
	21st to end	Night	9% 6% 39% 35% 11%	617
June	1st to 10th	Night	2 2 29% 46% 18% 2	570
	11th to 20th	Night	4 2 32% 51% 11% 1	570
	21st to end	Night	4 2 19% 50% 22% 2	570
July	1st to 10th	Night	6% 1 11% 53% 27% 2	555
	11th to 20th	Night	3 1 11% 54% 30% 1	552
	21st to end	Night	2 11% 53% 31% 3	624
August	1st to 10th	Night	2 7% 56% 32% 3	567
	11th to 20th	Night	7% 1 6% 50% 33% 4	564
	21st to end	Night	4 1 6% 47% 36% 6%	627
September	1st to 10th	Night	9% 1 13% 41% 32% 4	570
	11th to 20th	Night	11% 1 8% 44% 34% 2	570
	21st to end	Night	9% 13% 43% 32% 3	570
October	1st to 10th	Night	5% 1 17% 48% 26% 3	570
	11th to 20th	Night	10% 5% 28% 40% 16% 2	682
	21st to end	Night	14% 9% 31% 30% 16% 1	836
November	1st to 10th	Night	13% 3 30% 39% 13% 1	780
	11th to 20th	Night	8% 8% 34% 36% 13% 1	979
	21st to end	Night	16% 11% 32% 30% 10% 1	1000
December	1st to 10th	Night	14% 9% 32% 37% 9%	1000
	11th to 20th	Night	19% 9% 33% 33% 6%	1000
	21st to end	Night	17% 10% 34% 31% 7%	1100

Page generated at 2025-01-19 02:06:44. Version ac07

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