LINEAR
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the LINEAR 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	2 2 1 3 3 3 3 3 5% 7% 66%	134725
	Night	3 3 1 4 3 3 4 3 6% 7% 62%	133783
Transparency	Day	17% 4 7% 18% 29% 25%	129741
	Night	20% 4 6% 16% 29% 24%	129770
Seeing	Day	5% 7% 18% 29% 27% 15%	16120
	Night	5% 4 23% 44% 23% 2	26486

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	2 7% 2 6% 4 3 3 2 4 5% 62%	4098
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 6% 76%	4100
	21st to end	Day	4 6% 2 4 2 2 3 3 5% 6% 64%	4510
February	1st to 10th	Day	2 4 1 4 2 3 2 2 4 7% 69%	1835
	11th to 20th	Day	5% 4 1 3 3 3 3 3 4 6% 64%	1949
	21st to end	Day	3 4 1 4 2 2 2 2 4 6% 71%	1650
March	1st to 10th	Day	2 3 1 3 2 3 3 3 5% 6% 69%	4908
	11th to 20th	Day	3 3 2 4 3 4 4 3 5% 7% 63%	5000
	21st to end	Day	1 4 2 4 3 4 4 3 6% 6% 63%	5572
April	1st to 10th	Day	2 3 1 3 2 3 2 3 5% 6% 71%	2400
	11th to 20th	Day	3 3 1 2 3 2 2 2 3 6% 72%	2371
	21st to end	Day	2 2 1 3 2 2 3 2 4 5% 75%	2400
May	1st to 10th	Day	1 1 2 1 2 3 2 5% 6% 77%	5440
	11th to 20th	Day	1 2 1 3 3 3 3 3 4 5% 72%	5479
	21st to end	Day	1 1 1 1 2 2 2 3 6% 80%	6423
June	1st to 10th	Day	1 2 1 2 3 3 4 4 7% 10% 64%	2790
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 6% 75%	2800
	21st to end	Day	4 3 1 5% 3 4 5% 4 7% 7% 56%	2648
July	1st to 10th	Day	2 2 1 3 3 4 5% 4 7% 9% 59%	5879
	11th to 20th	Day	1 1 1 2 2 4 5% 4 9% 12% 58%	5880
	21st to end	Day	3 3 2 5% 5% 5% 7% 6% 9% 12% 43%	6401
August	1st to 10th	Day	1 1 1 3 3 3 4 5% 8% 12% 59%	5591
	11th to 20th	Day	2 2 1 3 3 4 4 4 7% 11% 59%	5366
	21st to end	Day	2 2 1 4 4 5% 6% 5% 9% 11% 52%	5984
September	1st to 10th	Day	5% 3 1 4 4 4 5% 5% 6% 8% 54%	2400
	11th to 20th	Day	5% 3 1 4 3 4 4 4 5% 7% 61%	2400
	21st to end	Day	3 2 1 3 2 2 3 2 4 6% 73%	2152
October	1st to 10th	Day	2 2 1 3 2 2 2 2 3 5% 78%	3270
	11th to 20th	Day	3 2 1 3 2 2 3 2 4 7% 71%	3250
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	3548
November	1st to 10th	Day	2 2 1 2 2 2 2 3 4 6% 76%	2059
	11th to 20th	Day	2 2 1 2 1 2 3 3 4 5% 74%	1989
	21st to end	Day	5% 3 1 4 3 3 3 3 5% 5% 64%	1890
December	1st to 10th	Day	2 3 1 2 2 2 2 2 4 7% 72%	2839
	11th to 20th	Day	3 2 1 4 4 3 3 2 3 5% 69%	3082
	21st to end	Day	3 3 1 3 2 2 3 2 4 6% 72%	4372
January	1st to 10th	Night	3 8% 3 5% 3 3 3 2 4 6% 60%	5867
	11th to 20th	Night	2 3 1 2 2 2 2 2 4 4 76%	5880
	21st to end	Night	5% 5% 2 5% 3 3 3 3 5% 6% 60%	6468
February	1st to 10th	Night	3 4 2 3 2 3 3 2 4 6% 69%	2563
	11th to 20th	Night	5% 5% 2 4 3 2 3 2 4 7% 63%	2338
	21st to end	Night	3 4 2 3 3 2 3 3 6% 6% 65%	1980
March	1st to 10th	Night	4 4 1 3 2 2 3 3 4 5% 69%	4970
	11th to 20th	Night	7% 4 2 4 3 3 3 2 4 5% 63%	5000
	21st to end	Night	2 3 1 3 3 3 3 3 5% 7% 66%	5398
April	1st to 10th	Night	2 2 2 3 3 3 3 3 5% 6% 69%	2000
	11th to 20th	Night	3 3 1 3 3 2 3 3 5% 6% 67%	1977
	21st to end	Night	3 2 1 4 2 3 3 3 5% 4 71%	2000
May	1st to 10th	Night	4 3 2 4 3 3 3 3 5% 5% 65%	4540
	11th to 20th	Night	3 2 1 3 2 3 3 3 5% 7% 67%	4432
	21st to end	Night	1 1 1 2 2 2 3 3 4 5% 77%	4474
June	1st to 10th	Night	3 3 1 5% 3 4 5% 4 8% 6% 58%	1600
	11th to 20th	Night	2 2 1 3 3 3 3 3 6% 6% 69%	1600
	21st to end	Night	5% 4 2 6% 5% 4 4 6% 7% 9% 49%	1740
July	1st to 10th	Night	3 3 1 6% 4 5% 6% 4 7% 10% 50%	4100
	11th to 20th	Night	2 3 2 5% 5% 5% 6% 5% 10% 13% 44%	4100
	21st to end	Night	4 4 2 7% 7% 7% 8% 7% 9% 12% 33%	4472
August	1st to 10th	Night	3 3 2 6% 5% 5% 7% 5% 9% 12% 42%	4220
	11th to 20th	Night	2 3 2 6% 5% 6% 7% 6% 10% 13% 39%	4483
	21st to end	Night	3 4 2 8% 6% 7% 8% 7% 9% 12% 34%	4993
September	1st to 10th	Night	5% 3 2 5% 5% 5% 6% 5% 6% 10% 49%	2000
	11th to 20th	Night	6% 3 2 5% 3 4 5% 4 6% 8% 55%	2000
	21st to end	Night	4 3 1 4 3 4 3 3 5% 5% 64%	2241
October	1st to 10th	Night	2 2 1 2 3 2 3 3 5% 6% 72%	3920
	11th to 20th	Night	4 3 1 3 3 3 3 3 4 6% 69%	3898
	21st to end	Night	2 1 1 1 2 1 2 2 4 5% 80%	4252
November	1st to 10th	Night	3 1 1 2 1 2 2 3 5% 7% 74%	2460
	11th to 20th	Night	2 3 1 3 2 2 2 2 3 5% 75%	2606
	21st to end	Night	5% 4 1 3 3 2 2 2 3 6% 68%	2730
December	1st to 10th	Night	3 2 1 2 2 2 2 2 3 6% 74%	4567
	11th to 20th	Night	3 2 1 3 2 3 4 3 5% 8% 66%	5280
	21st to end	Night	3 4 1 3 2 2 2 2 4 5% 70%	6634

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	25% 4 3 18% 43% 7%	4099
	11th to 20th	Day	11% 2 6% 21% 40% 19%	4100
	21st to end	Day	22% 3 5% 18% 32% 20%	4510
February	1st to 10th	Day	17% 2 5% 18% 33% 24%	1835
	11th to 20th	Day	21% 3 5% 19% 31% 21%	1959
	21st to end	Day	16% 2 3 12% 36% 32%	1650
March	1st to 10th	Day	16% 3 2 8% 33% 37%	4908
	11th to 20th	Day	20% 5% 4 17% 27% 27%	5000
	21st to end	Day	19% 4 3 14% 36% 24%	5572
April	1st to 10th	Day	14% 2 4 12% 34% 34%	2400
	11th to 20th	Day	16% 3 3 10% 28% 42%	2374
	21st to end	Day	13% 2 4 13% 33% 36%	2400
May	1st to 10th	Day	9% 3 4 16% 31% 38%	5440
	11th to 20th	Day	13% 4 5% 11% 28% 41%	5482
	21st to end	Day	8% 3 4 9% 17% 60%	6424
June	1st to 10th	Day	14% 3 8% 17% 24% 34%	2790
	11th to 20th	Day	11% 2 3 9% 21% 53%	2800
	21st to end	Day	24% 5% 6% 16% 21% 27%	2648
July	1st to 10th	Day	19% 6% 12% 15% 24% 24%	5816
	11th to 20th	Day	15% 5% 17% 31% 23% 8%	5811
	21st to end	Day	28% 9% 16% 26% 16% 5%	6217
August	1st to 10th	Day	17% 7% 13% 29% 27% 7%	4064
	11th to 20th	Day	17% 5% 10% 21% 32% 15%	3882
	21st to end	Day	25% 8% 15% 27% 20% 6%	4311
September	1st to 10th	Day	25% 6% 10% 20% 24% 16%	2400
	11th to 20th	Day	21% 6% 7% 19% 25% 22%	2400
	21st to end	Day	13% 4 5% 19% 33% 27%	2152
October	1st to 10th	Day	12% 2 5% 12% 28% 42%	3270
	11th to 20th	Day	14% 3 5% 12% 34% 32%	3250
	21st to end	Day	7% 1 2 16% 39% 36%	3548
November	1st to 10th	Day	11% 3 3 19% 39% 25%	2059
	11th to 20th	Day	13% 3 5% 20% 36% 23%	1989
	21st to end	Day	22% 3 4 20% 35% 17%	1890
December	1st to 10th	Day	14% 2 4 28% 36% 15%	2839
	11th to 20th	Day	20% 2 6% 22% 39% 12%	3080
	21st to end	Day	15% 3 5% 27% 36% 14%	4372
January	1st to 10th	Night	27% 3 4 15% 41% 10%	5867
	11th to 20th	Night	13% 2 5% 19% 42% 19%	5880
	21st to end	Night	24% 4 5% 16% 29% 22%	6468
February	1st to 10th	Night	18% 2 4 18% 32% 25%	2563
	11th to 20th	Night	23% 2 4 19% 29% 23%	2338
	21st to end	Night	19% 3 3 10% 32% 32%	1980
March	1st to 10th	Night	18% 3 2 10% 36% 31%	4970
	11th to 20th	Night	24% 4 3 13% 32% 24%	5000
	21st to end	Night	16% 5% 3 15% 35% 26%	5398
April	1st to 10th	Night	16% 3 4 10% 35% 32%	2000
	11th to 20th	Night	18% 3 4 10% 26% 39%	1977
	21st to end	Night	16% 3 4 11% 30% 35%	2000
May	1st to 10th	Night	19% 4 5% 12% 23% 37%	4537
	11th to 20th	Night	15% 4 7% 10% 24% 40%	4433
	21st to end	Night	10% 3 4 8% 13% 62%	4474
June	1st to 10th	Night	22% 5% 9% 14% 17% 33%	1600
	11th to 20th	Night	15% 3 5% 10% 19% 49%	1600
	21st to end	Night	29% 7% 10% 15% 16% 23%	1740
July	1st to 10th	Night	26% 7% 13% 16% 19% 18%	4055
	11th to 20th	Night	26% 8% 17% 26% 17% 6%	4046
	21st to end	Night	37% 9% 18% 22% 11% 3	4344
August	1st to 10th	Night	26% 10% 13% 26% 17% 7%	3071
	11th to 20th	Night	27% 8% 12% 18% 28% 7%	3243
	21st to end	Night	35% 9% 15% 22% 15% 5%	3597
September	1st to 10th	Night	28% 7% 13% 18% 20% 15%	2000
	11th to 20th	Night	25% 5% 10% 18% 19% 24%	2000
	21st to end	Night	21% 4 5% 15% 29% 26%	2241
October	1st to 10th	Night	14% 3 6% 10% 28% 39%	3920
	11th to 20th	Night	17% 4 5% 12% 26% 35%	3898
	21st to end	Night	10% 2 3 13% 35% 37%	4252
November	1st to 10th	Night	11% 3 3 18% 37% 28%	2460
	11th to 20th	Night	13% 3 3 18% 35% 28%	2606
	21st to end	Night	20% 3 4 19% 35% 19%	2730
December	1st to 10th	Night	13% 3 4 26% 35% 18%	4567
	11th to 20th	Night	17% 2 5% 18% 43% 15%	5281
	21st to end	Night	17% 2 4 25% 39% 13%	6634

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	7% 7% 19% 43% 21% 3	300
	11th to 20th	Day	6% 6% 19% 42% 21% 6%	300
	21st to end	Day	5% 5% 28% 36% 22% 5%	330
February	1st to 10th	Day	5% 8% 21% 38% 22% 6%	300
	11th to 20th	Day	9% 9% 25% 38% 20%	300
	21st to end	Day	11% 10% 30% 32% 12% 4	249
March	1st to 10th	Day	7% 17% 28% 31% 14% 2	478
	11th to 20th	Day	8% 15% 26% 31% 18% 3	500
	21st to end	Day	7% 21% 31% 23% 16% 1	550
April	1st to 10th	Day	6% 16% 26% 33% 13% 6%	500
	11th to 20th	Day	8% 16% 33% 26% 14% 3	492
	21st to end	Day	5% 15% 32% 27% 18% 4	500
May	1st to 10th	Day	3 11% 30% 29% 21% 5%	503
	11th to 20th	Day	3 9% 28% 30% 20% 9%	528
	21st to end	Day	3 9% 30% 28% 24% 6%	573
June	1st to 10th	Day	3 3 15% 37% 32% 10%	720
	11th to 20th	Day	2 4 15% 35% 32% 12%	730
	21st to end	Day	6% 1 14% 28% 31% 20%	578
July	1st to 10th	Day	4 1 6% 23% 36% 31%	526
	11th to 20th	Day	3 3 18% 31% 45%	524
	21st to end	Day	3 2 4 14% 34% 43%	575
August	1st to 10th	Day	2 2 4 15% 40% 37%	527
	11th to 20th	Day	3 1 4 21% 40% 31%	517
	21st to end	Day	5% 1 4 19% 39% 33%	583
September	1st to 10th	Day	6% 1 8% 20% 38% 26%	530
	11th to 20th	Day	4 2 9% 31% 38% 18%	530
	21st to end	Day	3 4 10% 26% 35% 22%	383
October	1st to 10th	Day	5% 2 14% 21% 36% 21%	330
	11th to 20th	Day	6% 4 13% 27% 33% 17%	330
	21st to end	Day	6% 7% 12% 26% 33% 17%	363
November	1st to 10th	Day	6% 4 18% 31% 32% 9%	330
	11th to 20th	Day	4 5% 12% 31% 34% 14%	330
	21st to end	Day	7% 7% 21% 36% 22% 7%	330
December	1st to 10th	Day	7% 3 16% 41% 26% 7%	330
	11th to 20th	Day	4 4 21% 40% 21% 10%	321
	21st to end	Day	8% 6% 18% 45% 19% 4	330
January	1st to 10th	Night	6% 4 23% 51% 16% 2	800
	11th to 20th	Night	6% 3 30% 43% 17% 1	800
	21st to end	Night	6% 5% 29% 45% 15%	880
February	1st to 10th	Night	4 4 32% 46% 14% 1	800
	11th to 20th	Night	8% 5% 33% 46% 8%	784
	21st to end	Night	5% 8% 40% 42% 5%	660
March	1st to 10th	Night	5% 8% 35% 42% 10%	800
	11th to 20th	Night	6% 7% 30% 43% 14% 1	800
	21st to end	Night	4 8% 35% 43% 10%	840
April	1st to 10th	Night	3 5% 41% 43% 9%	600
	11th to 20th	Night	5% 8% 39% 39% 8%	593
	21st to end	Night	4 7% 39% 42% 9%	600
May	1st to 10th	Night	4 4 33% 50% 9%	600
	11th to 20th	Night	5% 5% 26% 47% 17% 1	591
	21st to end	Night	2 4 28% 54% 12%	650
June	1st to 10th	Night	4 4 21% 51% 21% 1	600
	11th to 20th	Night	2 2 19% 57% 20% 1	600
	21st to end	Night	6% 2 11% 45% 35% 1	600
July	1st to 10th	Night	4 1 7% 39% 44% 5%	585
	11th to 20th	Night	3 2 4 30% 55% 5%	582
	21st to end	Night	4 1 5% 27% 55% 8%	657
August	1st to 10th	Night	3 1 4 27% 55% 10%	597
	11th to 20th	Night	4 1 7% 28% 55% 5%	594
	21st to end	Night	4 1 5% 32% 53% 5%	660
September	1st to 10th	Night	6% 2 6% 40% 45% 3	600
	11th to 20th	Night	6% 1 12% 46% 34% 2	600
	21st to end	Night	4 2 14% 44% 34% 2	696
October	1st to 10th	Night	4 2 16% 49% 27% 1	800
	11th to 20th	Night	4 3 21% 49% 22% 1	800
	21st to end	Night	4 3 20% 41% 28% 3	880
November	1st to 10th	Night	3 2 19% 46% 29% 1	820
	11th to 20th	Night	4 2 20% 47% 25% 2	926
	21st to end	Night	7% 4 23% 41% 23% 2	1050
December	1st to 10th	Night	6% 4 24% 49% 17% 1	1050
	11th to 20th	Night	7% 4 24% 46% 19%	1036
	21st to end	Night	7% 3 26% 51% 12% 1	955

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