LEAR Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the LEAR Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	4 6% 2 6% 4 4 4 4 5% 7% 53%	135454
	Night	4 7% 3 7% 4 4 4 4 5% 7% 51%	133097
Transparency	Day	27% 8% 11% 19% 20% 14%	130188
	Night	29% 8% 13% 21% 19% 11%	129366
Seeing	Day	8% 15% 31% 31% 13% 2	16205
	Night	9% 9% 24% 36% 20% 3	26884

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	5% 13% 5% 14% 7% 5% 5% 4 6% 7% 30%	4098
	11th to 20th	Day	5% 15% 5% 13% 7% 5% 5% 3 5% 8% 29%	4100
	21st to end	Day	5% 11% 4 8% 4 5% 5% 4 5% 6% 44%	4510
February	1st to 10th	Day	8% 12% 4 10% 6% 5% 6% 5% 5% 6% 33%	1800
	11th to 20th	Day	7% 13% 4 11% 7% 4 5% 5% 6% 6% 33%	1834
	21st to end	Day	5% 13% 5% 10% 5% 5% 4 4 6% 8% 35%	1650
March	1st to 10th	Day	2 12% 6% 11% 6% 5% 5% 4 5% 6% 38%	4821
	11th to 20th	Day	4 10% 4 8% 5% 6% 6% 4 6% 7% 41%	5000
	21st to end	Day	5% 13% 4 11% 7% 7% 6% 4 5% 6% 31%	5486
April	1st to 10th	Day	7% 10% 3 6% 5% 4 6% 4 6% 6% 43%	2211
	11th to 20th	Day	6% 9% 3 7% 6% 5% 4 4 6% 6% 44%	2531
	21st to end	Day	6% 8% 2 6% 5% 5% 4 4 5% 8% 47%	2600
May	1st to 10th	Day	7% 8% 2 6% 4 4 4 4 6% 7% 50%	5880
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 7% 8% 50%	5841
	21st to end	Day	4 4 2 5% 3 3 4 4 6% 7% 58%	6423
June	1st to 10th	Day	3 3 2 5% 3 4 4 4 7% 8% 57%	2600
	11th to 20th	Day	4 4 1 4 3 3 3 3 4 7% 64%	2600
	21st to end	Day	2 2 1 2 3 3 3 2 4 5% 73%	2600
July	1st to 10th	Day	1 1 1 2 3 3 3 3 4 5% 74%	5879
	11th to 20th	Day	2 2 1 3 3 4 5% 4 6% 7% 62%	5880
	21st to end	Day	2 2 1 3 3 3 3 3 4 6% 71%	6401
August	1st to 10th	Day	1 1 1 3 2 3 3 3 5% 6% 71%	5779
	11th to 20th	Day	1 1 1 3 3 4 3 3 4 6% 70%	5798
	21st to end	Day	3 2 1 4 4 4 5% 4 6% 7% 61%	6427
September	1st to 10th	Day	2 2 1 3 2 2 4 3 5% 6% 70%	2247
	11th to 20th	Day	3 2 1 3 3 3 4 4 5% 7% 65%	2200
	21st to end	Day	5% 3 1 3 2 2 2 2 4 7% 70%	2200
October	1st to 10th	Day	4 4 1 3 2 3 3 2 3 4 70%	3445
	11th to 20th	Day	6% 4 1 4 3 3 3 2 5% 7% 61%	3250
	21st to end	Day	6% 6% 2 5% 3 3 3 3 4 6% 60%	3527
November	1st to 10th	Day	7% 7% 3 6% 3 3 4 4 6% 7% 50%	1854
	11th to 20th	Day	8% 8% 2 6% 4 4 4 4 5% 7% 46%	1890
	21st to end	Day	4 12% 3 8% 5% 5% 4 3 6% 8% 43%	1777
December	1st to 10th	Day	5% 6% 2 6% 3 2 4 4 7% 11% 50%	2700
	11th to 20th	Day	5% 9% 3 9% 5% 5% 6% 4 6% 9% 39%	3105
	21st to end	Day	6% 12% 5% 13% 7% 5% 5% 4 6% 7% 31%	4510
January	1st to 10th	Night	5% 12% 5% 15% 6% 5% 4 3 4 8% 33%	5867
	11th to 20th	Night	7% 14% 5% 11% 5% 6% 6% 5% 6% 8% 27%	5880
	21st to end	Night	4 9% 3 8% 5% 5% 5% 3 5% 9% 43%	6468
February	1st to 10th	Night	6% 11% 4 9% 6% 5% 5% 4 6% 7% 36%	2600
	11th to 20th	Night	6% 12% 3 9% 5% 6% 5% 3 5% 7% 40%	2448
	21st to end	Night	5% 12% 4 9% 5% 4 5% 4 5% 7% 40%	1980
March	1st to 10th	Night	3 12% 4 11% 6% 5% 5% 4 5% 7% 38%	5073
	11th to 20th	Night	5% 9% 4 9% 5% 5% 5% 4 5% 8% 41%	5000
	21st to end	Night	5% 11% 4 9% 5% 5% 5% 4 5% 5% 42%	5489
April	1st to 10th	Night	9% 9% 2 6% 5% 3 4 3 5% 6% 48%	2187
	11th to 20th	Night	5% 7% 3 6% 4 5% 6% 5% 6% 7% 48%	1814
	21st to end	Night	7% 7% 3 6% 4 4 4 4 6% 7% 48%	1800
May	1st to 10th	Night	5% 8% 2 6% 4 4 4 4 6% 7% 51%	4100
	11th to 20th	Night	4 3 2 4 3 4 4 4 6% 8% 57%	4074
	21st to end	Night	5% 4 1 4 3 3 3 3 6% 8% 59%	4474
June	1st to 10th	Night	5% 4 1 5% 3 3 4 4 6% 8% 58%	1800
	11th to 20th	Night	4 3 1 4 2 3 3 3 5% 6% 67%	1800
	21st to end	Night	2 3 1 3 2 2 3 2 6% 7% 68%	1800
July	1st to 10th	Night	1 2 1 5% 4 3 3 3 3 4 71%	4100
	11th to 20th	Night	2 3 2 5% 4 4 5% 4 7% 9% 56%	4100
	21st to end	Night	2 3 1 4 3 3 4 4 5% 6% 66%	4472
August	1st to 10th	Night	2 1 1 3 2 3 4 4 5% 8% 67%	4033
	11th to 20th	Night	1 2 1 4 3 4 4 4 5% 6% 66%	4051
	21st to end	Night	2 3 2 6% 4 4 5% 4 5% 6% 58%	4548
September	1st to 10th	Night	3 3 1 3 3 3 3 3 4 5% 68%	2150
	11th to 20th	Night	4 3 1 2 3 3 4 3 4 7% 65%	2200
	21st to end	Night	4 3 2 3 2 2 3 3 3 5% 70%	2200
October	1st to 10th	Night	4 3 1 3 3 2 3 2 3 5% 72%	3750
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 4 6% 62%	3898
	21st to end	Night	5% 6% 2 5% 3 3 3 3 5% 7% 59%	4270
November	1st to 10th	Night	6% 8% 3 6% 3 3 3 3 6% 8% 52%	2665
	11th to 20th	Night	7% 6% 2 5% 4 4 5% 3 5% 8% 51%	2730
	21st to end	Night	6% 8% 2 6% 5% 4 5% 3 5% 8% 48%	2816
December	1st to 10th	Night	4 6% 2 5% 3 3 4 5% 8% 10% 50%	4710
	11th to 20th	Night	7% 10% 3 7% 4 4 4 3 5% 8% 45%	5280
	21st to end	Night	6% 11% 4 12% 7% 5% 5% 4 6% 8% 32%	6470

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	45% 12% 21% 15% 7%	4099
	11th to 20th	Day	49% 10% 14% 19% 6% 2	4100
	21st to end	Day	38% 9% 16% 23% 14% 1	4510
February	1st to 10th	Day	48% 12% 16% 14% 10% 1	1800
	11th to 20th	Day	46% 11% 15% 17% 10%	1843
	21st to end	Day	43% 13% 15% 17% 11% 1	1650
March	1st to 10th	Day	35% 14% 12% 22% 14% 3	4821
	11th to 20th	Day	32% 9% 15% 31% 12% 1	5000
	21st to end	Day	44% 13% 17% 16% 8% 1	5486
April	1st to 10th	Day	37% 10% 14% 19% 15% 5%	2211
	11th to 20th	Day	35% 10% 14% 22% 17% 3	2534
	21st to end	Day	31% 10% 13% 24% 20% 2	2600
May	1st to 10th	Day	26% 11% 13% 22% 22% 7%	5880
	11th to 20th	Day	25% 8% 9% 26% 27% 5%	5844
	21st to end	Day	20% 8% 10% 25% 32% 6%	6424
June	1st to 10th	Day	20% 8% 10% 22% 29% 10%	2600
	11th to 20th	Day	19% 6% 7% 17% 29% 22%	2600
	21st to end	Day	13% 4 5% 12% 31% 35%	2600
July	1st to 10th	Day	13% 3 3 6% 15% 59%	5816
	11th to 20th	Day	17% 5% 5% 10% 25% 38%	5811
	21st to end	Day	13% 5% 5% 7% 21% 50%	6217
August	1st to 10th	Day	15% 5% 6% 17% 28% 29%	4208
	11th to 20th	Day	17% 5% 6% 14% 33% 26%	4197
	21st to end	Day	15% 4 6% 15% 28% 32%	4634
September	1st to 10th	Day	15% 5% 7% 16% 28% 29%	2247
	11th to 20th	Day	18% 7% 8% 18% 27% 21%	2200
	21st to end	Day	17% 5% 8% 21% 31% 19%	2200
October	1st to 10th	Day	18% 5% 6% 13% 38% 21%	3445
	11th to 20th	Day	23% 4 8% 28% 28% 9%	3250
	21st to end	Day	25% 6% 11% 31% 23% 4	3527
November	1st to 10th	Day	32% 9% 14% 23% 20% 4	1854
	11th to 20th	Day	34% 8% 13% 29% 14% 1	1890
	21st to end	Day	38% 9% 16% 24% 12% 2	1777
December	1st to 10th	Day	28% 7% 21% 34% 8% 1	2700
	11th to 20th	Day	37% 8% 18% 28% 9%	3103
	21st to end	Day	44% 10% 16% 21% 9%	4510
January	1st to 10th	Night	43% 12% 19% 18% 8%	5867
	11th to 20th	Night	51% 11% 15% 18% 5% 1	5880
	21st to end	Night	35% 11% 18% 26% 9% 1	6468
February	1st to 10th	Night	43% 12% 18% 18% 9% 1	2600
	11th to 20th	Night	41% 8% 15% 20% 15% 1	2449
	21st to end	Night	38% 10% 14% 23% 12% 2	1980
March	1st to 10th	Night	39% 11% 11% 24% 14% 2	5073
	11th to 20th	Night	34% 10% 14% 26% 15% 1	5000
	21st to end	Night	34% 11% 19% 22% 12% 1	5489
April	1st to 10th	Night	33% 8% 14% 22% 16% 6%	2187
	11th to 20th	Night	30% 9% 12% 22% 22% 4	1814
	21st to end	Night	32% 9% 16% 20% 21% 2	1800
May	1st to 10th	Night	24% 11% 13% 23% 25% 3	4097
	11th to 20th	Night	18% 8% 10% 27% 31% 6%	4075
	21st to end	Night	20% 7% 9% 25% 37% 3	4474
June	1st to 10th	Night	22% 7% 11% 20% 28% 12%	1800
	11th to 20th	Night	17% 6% 7% 18% 30% 22%	1800
	21st to end	Night	15% 5% 5% 10% 28% 37%	1800
July	1st to 10th	Night	17% 3 3 7% 13% 56%	4055
	11th to 20th	Night	20% 4 5% 11% 23% 37%	4046
	21st to end	Night	17% 4 5% 9% 24% 42%	4344
August	1st to 10th	Night	16% 5% 8% 15% 27% 29%	2928
	11th to 20th	Night	19% 5% 8% 14% 29% 25%	2928
	21st to end	Night	16% 4 7% 11% 29% 33%	3272
September	1st to 10th	Night	17% 5% 7% 15% 25% 31%	2150
	11th to 20th	Night	18% 5% 9% 17% 27% 24%	2200
	21st to end	Night	18% 5% 6% 16% 32% 24%	2200
October	1st to 10th	Night	16% 4 6% 16% 35% 23%	3750
	11th to 20th	Night	23% 5% 6% 28% 30% 9%	3898
	21st to end	Night	23% 6% 9% 35% 24% 4	4270
November	1st to 10th	Night	30% 7% 12% 24% 22% 5%	2665
	11th to 20th	Night	29% 9% 15% 28% 16% 3	2730
	21st to end	Night	32% 9% 14% 27% 14% 3	2816
December	1st to 10th	Night	28% 8% 25% 28% 8% 2	4710
	11th to 20th	Night	35% 6% 26% 25% 8%	5281
	21st to end	Night	44% 11% 19% 19% 7%	6470

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% 19% 26% 22% 14% 2	300
	11th to 20th	Day	8% 19% 34% 24% 12% 3	300
	21st to end	Day	9% 15% 33% 30% 10% 2	330
February	1st to 10th	Day	16% 28% 29% 20% 8%	300
	11th to 20th	Day	16% 18% 37% 21% 7% 1	379
	21st to end	Day	11% 23% 36% 22% 6% 1	414
March	1st to 10th	Day	10% 24% 35% 22% 7% 1	500
	11th to 20th	Day	10% 21% 35% 27% 7%	500
	21st to end	Day	16% 23% 35% 22% 4	550
April	1st to 10th	Day	12% 21% 32% 25% 9% 2	500
	11th to 20th	Day	14% 24% 33% 22% 6% 1	492
	21st to end	Day	12% 22% 29% 26% 10% 1	500
May	1st to 10th	Day	9% 23% 33% 23% 11% 1	503
	11th to 20th	Day	11% 16% 33% 27% 10% 3	528
	21st to end	Day	8% 12% 32% 30% 14% 4	573
June	1st to 10th	Day	4 12% 31% 35% 13% 4	530
	11th to 20th	Day	6% 14% 36% 31% 12% 1	530
	21st to end	Day	2 11% 34% 40% 10% 2	530
July	1st to 10th	Day	1 6% 31% 45% 15% 2	526
	11th to 20th	Day	3 5% 33% 40% 16% 3	524
	21st to end	Day	2 2 27% 48% 16% 3	575
August	1st to 10th	Day	3 6% 30% 43% 16% 2	527
	11th to 20th	Day	3 7% 29% 43% 15% 4	517
	21st to end	Day	3 7% 27% 40% 18% 4	583
September	1st to 10th	Day	3 7% 27% 39% 19% 5%	530
	11th to 20th	Day	4 11% 31% 35% 16% 2	530
	21st to end	Day	4 13% 22% 37% 20% 3	530
October	1st to 10th	Day	5% 16% 29% 25% 19% 5%	440
	11th to 20th	Day	7% 15% 29% 25% 21% 3	330
	21st to end	Day	9% 13% 33% 29% 15% 1	363
November	1st to 10th	Day	13% 13% 29% 28% 14% 3	330
	11th to 20th	Day	10% 16% 29% 28% 12% 5%	330
	21st to end	Day	12% 14% 25% 28% 17% 4	330
December	1st to 10th	Day	10% 15% 30% 26% 15% 4	330
	11th to 20th	Day	13% 12% 31% 34% 7% 3	321
	21st to end	Day	19% 18% 28% 28% 8%	330
January	1st to 10th	Night	13% 11% 23% 32% 16% 4	800
	11th to 20th	Night	11% 10% 23% 34% 17% 5%	800
	21st to end	Night	12% 10% 22% 35% 18% 4	880
February	1st to 10th	Night	10% 12% 29% 33% 13% 3	800
	11th to 20th	Night	11% 11% 28% 34% 14% 2	784
	21st to end	Night	10% 13% 30% 33% 13% 2	660
March	1st to 10th	Night	11% 13% 29% 33% 11% 4	800
	11th to 20th	Night	9% 10% 26% 35% 17% 3	800
	21st to end	Night	8% 15% 30% 33% 11% 3	880
April	1st to 10th	Night	14% 14% 26% 31% 13% 2	787
	11th to 20th	Night	7% 13% 30% 37% 11% 1	593
	21st to end	Night	9% 11% 26% 36% 16% 2	600
May	1st to 10th	Night	8% 9% 29% 36% 17% 2	600
	11th to 20th	Night	7% 7% 23% 35% 24% 4	591
	21st to end	Night	8% 7% 24% 35% 22% 4	650
June	1st to 10th	Night	5% 9% 18% 41% 25% 2	600
	11th to 20th	Night	4 10% 25% 40% 20% 2	600
	21st to end	Night	4 5% 22% 40% 26% 3	600
July	1st to 10th	Night	2 3 21% 38% 33% 2	585
	11th to 20th	Night	3 3 18% 47% 26% 3	582
	21st to end	Night	3 3 13% 37% 42% 1	657
August	1st to 10th	Night	4 2 19% 44% 30% 2	597
	11th to 20th	Night	3 2 18% 45% 29% 3	594
	21st to end	Night	2 4 17% 43% 31% 3	660
September	1st to 10th	Night	5% 4 15% 41% 30% 5%	750
	11th to 20th	Night	5% 6% 18% 39% 27% 6%	800
	21st to end	Night	6% 6% 17% 33% 29% 9%	800
October	1st to 10th	Night	6% 7% 23% 35% 25% 4	800
	11th to 20th	Night	7% 6% 23% 35% 23% 5%	800
	21st to end	Night	8% 9% 24% 33% 23% 4	880
November	1st to 10th	Night	11% 10% 19% 36% 20% 4	820
	11th to 20th	Night	8% 9% 29% 35% 15% 5%	840
	21st to end	Night	11% 8% 23% 33% 20% 4	926
December	1st to 10th	Night	10% 9% 27% 34% 18% 2	1050
	11th to 20th	Night	15% 9% 27% 35% 12% 1	1036
	21st to end	Night	18% 13% 26% 31% 11% 1	882

Page generated at 2025-12-14 23:13:49. Version ac07

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