Land-o-Clouds Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Land-o-Clouds Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	12% 9% 3 8% 5% 4 4 4 6% 7% 37%	130876
	Night	12% 10% 4 9% 5% 4 4 3 5% 6% 37%	130262
Transparency	Day	44% 6% 12% 24% 13% 1	134363
	Night	46% 6% 11% 22% 14% 2	134850
Seeing	Day	16% 12% 30% 26% 13% 4	17642
	Night	17% 10% 32% 29% 11% 1	27646

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% 12% 5% 15% 7% 5% 5% 4 5% 6% 25%	4691
	11th to 20th	Day	7% 12% 7% 12% 5% 4 7% 6% 6% 7% 28%	4710
	21st to end	Day	10% 11% 5% 12% 5% 5% 5% 4 4 6% 34%	5156
February	1st to 10th	Day	10% 14% 6% 10% 5% 5% 5% 4 5% 6% 30%	1884
	11th to 20th	Day	11% 12% 4 7% 4 5% 5% 3 5% 6% 37%	1967
	21st to end	Day	12% 15% 4 9% 5% 4 5% 4 5% 6% 31%	1650
March	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 45%	4480
	11th to 20th	Day	11% 7% 2 8% 5% 4 4 3 5% 7% 43%	4689
	21st to end	Day	22% 13% 4 9% 4 4 3 3 4 5% 29%	5967
April	1st to 10th	Day	15% 14% 4 8% 5% 4 5% 4 5% 6% 31%	2400
	11th to 20th	Day	18% 11% 3 6% 5% 4 5% 4 5% 5% 35%	2374
	21st to end	Day	18% 10% 3 8% 6% 4 4 4 5% 6% 33%	2543
May	1st to 10th	Day	12% 8% 3 6% 3 4 4 4 5% 7% 43%	5972
	11th to 20th	Day	11% 5% 2 7% 4 4 4 4 6% 8% 44%	6319
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 40%	6935
June	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 43%	2800
	11th to 20th	Day	11% 7% 2 7% 5% 4 5% 4 6% 8% 41%	2800
	21st to end	Day	10% 6% 2 6% 4 5% 4 4 6% 10% 43%	2800
July	1st to 10th	Day	11% 7% 2 6% 4 4 5% 4 6% 10% 39%	6360
	11th to 20th	Day	7% 4 2 4 4 4 5% 4 7% 11% 47%	6360
	21st to end	Day	8% 6% 2 6% 4 5% 5% 4 7% 9% 45%	6924
August	1st to 10th	Day	9% 5% 3 7% 4 5% 5% 5% 7% 9% 42%	4563
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 7% 9% 43%	3988
	21st to end	Day	10% 7% 3 7% 4 5% 5% 4 6% 8% 42%	4312
September	1st to 10th	Day	11% 6% 3 6% 5% 4 4 4 6% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 3 4 5% 8% 49%	2290
	21st to end	Day	13% 8% 3 6% 4 4 4 3 6% 8% 41%	2200
October	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 5% 7% 44%	2106
	11th to 20th	Day	20% 11% 3 8% 5% 5% 5% 4 5% 6% 29%	1965
	21st to end	Day	16% 12% 4 8% 4 4 5% 3 4 6% 35%	1980
November	1st to 10th	Day	13% 9% 3 8% 4 4 3 4 5% 7% 40%	1852
	11th to 20th	Day	11% 13% 5% 12% 6% 5% 5% 4 5% 6% 27%	1890
	21st to end	Day	15% 13% 5% 10% 5% 4 3 3 4 6% 31%	1701
December	1st to 10th	Day	7% 10% 6% 14% 9% 7% 6% 4 6% 6% 25%	2690
	11th to 20th	Day	10% 15% 5% 15% 7% 6% 5% 3 4 6% 24%	3088
	21st to end	Day	18% 20% 7% 14% 6% 4 4 3 4 5% 16%	4070
January	1st to 10th	Night	11% 12% 6% 13% 7% 4 4 3 4 4 33%	8232
	11th to 20th	Night	9% 14% 6% 15% 7% 5% 6% 5% 5% 4 24%	8280
	21st to end	Night	8% 13% 6% 12% 7% 6% 5% 3 4 5% 31%	9058
February	1st to 10th	Night	11% 14% 5% 10% 6% 5% 4 3 4 5% 34%	2735
	11th to 20th	Night	9% 10% 4 10% 5% 5% 5% 4 4 5% 40%	2429
	21st to end	Night	15% 13% 3 7% 5% 4 5% 3 5% 5% 35%	1980
March	1st to 10th	Night	14% 8% 2 6% 4 4 5% 3 5% 6% 44%	5373
	11th to 20th	Night	9% 8% 3 7% 5% 5% 5% 3 5% 8% 42%	5314
	21st to end	Night	19% 12% 4 9% 4 4 4 3 4 5% 31%	4962
April	1st to 10th	Night	18% 12% 3 6% 4 3 4 3 4 7% 35%	2000
	11th to 20th	Night	18% 11% 3 7% 4 4 3 3 5% 5% 37%	1975
	21st to end	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 37%	1855
May	1st to 10th	Night	14% 7% 2 7% 4 4 4 3 4 7% 42%	3913
	11th to 20th	Night	9% 5% 2 7% 5% 6% 5% 4 7% 8% 42%	3596
	21st to end	Night	17% 9% 3 7% 4 5% 4 3 4 6% 38%	3942
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	1600
	11th to 20th	Night	10% 7% 3 6% 4 4 4 4 6% 9% 43%	1600
	21st to end	Night	9% 6% 2 7% 4 4 4 4 6% 8% 48%	1600
July	1st to 10th	Night	11% 6% 3 5% 4 4 4 4 5% 9% 46%	3618
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 5% 9% 52%	3617
	21st to end	Night	5% 5% 2 6% 3 4 5% 4 7% 8% 52%	3947
August	1st to 10th	Night	10% 5% 2 5% 3 4 4 4 7% 9% 47%	2597
	11th to 20th	Night	11% 7% 3 6% 4 4 5% 3 5% 8% 45%	3057
	21st to end	Night	7% 6% 2 5% 4 4 5% 4 6% 8% 49%	3586
September	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 7% 51%	2000
	11th to 20th	Night	11% 4 2 5% 4 3 4 4 5% 6% 52%	2089
	21st to end	Night	12% 7% 3 7% 4 3 3 3 5% 6% 49%	2200
October	1st to 10th	Night	11% 7% 3 7% 4 4 3 3 5% 6% 48%	2290
	11th to 20th	Night	16% 10% 3 6% 4 4 4 4 5% 7% 38%	2435
	21st to end	Night	17% 12% 3 6% 3 4 4 4 6% 7% 34%	2860
November	1st to 10th	Night	12% 9% 3 7% 5% 3 4 3 5% 6% 44%	2666
	11th to 20th	Night	12% 13% 4 10% 7% 5% 4 3 5% 6% 31%	2730
	21st to end	Night	14% 13% 4 9% 6% 4 4 3 5% 6% 32%	2917
December	1st to 10th	Night	8% 11% 3 10% 7% 6% 6% 4 6% 7% 31%	4720
	11th to 20th	Night	12% 16% 7% 12% 6% 5% 5% 3 4 5% 26%	5339
	21st to end	Night	17% 17% 6% 13% 6% 5% 4 3 4 5% 20%	7150

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	60% 5% 15% 14% 5%	4692
	11th to 20th	Day	54% 6% 8% 20% 12% 1	4710
	21st to end	Day	52% 4 8% 21% 15%	5156
February	1st to 10th	Day	54% 5% 10% 20% 10% 1	1884
	11th to 20th	Day	46% 5% 11% 25% 11% 2	1977
	21st to end	Day	53% 6% 10% 19% 10% 3	1650
March	1st to 10th	Day	39% 4 12% 24% 15% 5%	4480
	11th to 20th	Day	40% 4 15% 24% 15% 1	4689
	21st to end	Day	59% 4 9% 17% 11% 1	5969
April	1st to 10th	Day	52% 6% 12% 19% 10%	2400
	11th to 20th	Day	50% 6% 9% 20% 13% 2	2374
	21st to end	Day	51% 7% 11% 20% 11% 1	2543
May	1st to 10th	Day	39% 6% 12% 25% 16% 3	6007
	11th to 20th	Day	37% 8% 12% 27% 16% 1	6323
	21st to end	Day	40% 6% 10% 27% 15% 2	6949
June	1st to 10th	Day	36% 7% 12% 28% 16% 1	2800
	11th to 20th	Day	40% 7% 13% 25% 15% 1	2800
	21st to end	Day	36% 8% 14% 26% 15% 1	2800
July	1st to 10th	Day	38% 7% 13% 31% 10% 1	6291
	11th to 20th	Day	29% 7% 15% 33% 15% 1	6285
	21st to end	Day	33% 9% 14% 23% 19% 2	6728
August	1st to 10th	Day	35% 8% 15% 28% 13% 1	4548
	11th to 20th	Day	35% 7% 13% 35% 10% 1	4027
	21st to end	Day	38% 7% 12% 25% 17% 1	4311
September	1st to 10th	Day	37% 7% 12% 28% 16% 1	2400
	11th to 20th	Day	34% 5% 15% 32% 14% 1	2290
	21st to end	Day	41% 5% 11% 30% 12% 1	2200
October	1st to 10th	Day	24% 4 7% 38% 26% 1	3431
	11th to 20th	Day	42% 6% 9% 24% 17% 1	3181
	21st to end	Day	44% 5% 11% 27% 12% 1	3180
November	1st to 10th	Day	44% 5% 13% 24% 13% 2	1852
	11th to 20th	Day	55% 7% 12% 18% 7% 1	1890
	21st to end	Day	54% 5% 13% 20% 6%	1701
December	1st to 10th	Day	56% 10% 13% 17% 4	2690
	11th to 20th	Day	60% 5% 10% 16% 7% 1	3085
	21st to end	Day	71% 5% 6% 11% 7%	4070
January	1st to 10th	Night	55% 4 9% 21% 10%	8233
	11th to 20th	Night	61% 6% 8% 11% 14% 1	8280
	21st to end	Night	55% 4 9% 19% 12%	9058
February	1st to 10th	Night	53% 4 9% 20% 12% 2	2735
	11th to 20th	Night	47% 5% 10% 25% 13% 1	2430
	21st to end	Night	51% 4 12% 19% 11% 2	1980
March	1st to 10th	Night	40% 5% 14% 24% 13% 3	5373
	11th to 20th	Night	39% 5% 17% 23% 13% 3	5314
	21st to end	Night	55% 5% 9% 17% 12% 1	4962
April	1st to 10th	Night	49% 5% 10% 18% 18% 1	2000
	11th to 20th	Night	49% 4 9% 20% 15% 3	1975
	21st to end	Night	46% 5% 12% 22% 14% 1	1855
May	1st to 10th	Night	41% 5% 12% 19% 20% 3	3940
	11th to 20th	Night	36% 10% 13% 22% 18% 2	3597
	21st to end	Night	46% 6% 9% 24% 12% 4	3950
June	1st to 10th	Night	33% 8% 12% 30% 17% 1	1600
	11th to 20th	Night	36% 7% 10% 27% 17% 2	1600
	21st to end	Night	34% 7% 15% 24% 16% 3	1600
July	1st to 10th	Night	35% 7% 14% 31% 10% 3	3580
	11th to 20th	Night	28% 7% 15% 30% 17% 2	3572
	21st to end	Night	27% 7% 15% 26% 22% 2	3832
August	1st to 10th	Night	32% 7% 14% 27% 18% 2	2588
	11th to 20th	Night	36% 7% 11% 30% 15%	3084
	21st to end	Night	30% 8% 14% 25% 20% 3	3586
September	1st to 10th	Night	31% 7% 12% 26% 23% 2	2000
	11th to 20th	Night	32% 6% 11% 33% 17% 1	2089
	21st to end	Night	36% 6% 10% 30% 17% 1	2200
October	1st to 10th	Night	27% 4 8% 38% 22% 2	3745
	11th to 20th	Night	35% 5% 11% 29% 17% 3	3958
	21st to end	Night	44% 5% 10% 25% 17%	4610
November	1st to 10th	Night	41% 5% 11% 24% 16% 2	2666
	11th to 20th	Night	54% 5% 11% 18% 10% 2	2730
	21st to end	Night	53% 5% 14% 21% 7% 1	2917
December	1st to 10th	Night	47% 10% 16% 19% 6% 1	4720
	11th to 20th	Night	62% 5% 9% 13% 11% 1	5341
	21st to end	Night	66% 5% 9% 13% 7%	7150

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% 15% 36% 25% 6% 1	330
	11th to 20th	Day	16% 18% 37% 24% 6%	330
	21st to end	Day	19% 15% 37% 23% 5% 1	363
February	1st to 10th	Day	13% 18% 43% 18% 7% 1	330
	11th to 20th	Day	17% 21% 36% 21% 5%	315
	21st to end	Day	20% 21% 43% 14% 1	249
March	1st to 10th	Day	18% 18% 44% 17% 4	300
	11th to 20th	Day	21% 20% 31% 21% 4 3	315
	21st to end	Day	27% 13% 30% 23% 6% 1	550
April	1st to 10th	Day	24% 13% 36% 21% 5%	500
	11th to 20th	Day	25% 17% 28% 18% 9% 2	492
	21st to end	Day	21% 14% 30% 22% 11% 1	500
May	1st to 10th	Day	20% 11% 31% 27% 9% 2	548
	11th to 20th	Day	15% 11% 31% 26% 12% 4	725
	21st to end	Day	15% 9% 30% 26% 15% 5%	789
June	1st to 10th	Day	11% 8% 31% 32% 14% 4	730
	11th to 20th	Day	13% 9% 32% 28% 14% 4	730
	21st to end	Day	13% 10% 28% 30% 15% 3	730
July	1st to 10th	Day	7% 5% 21% 33% 25% 10%	721
	11th to 20th	Day	10% 6% 24% 33% 19% 8%	718
	21st to end	Day	7% 4 27% 34% 20% 8%	794
August	1st to 10th	Day	8% 5% 23% 34% 22% 10%	726
	11th to 20th	Day	14% 4 17% 28% 25% 12%	542
	21st to end	Day	12% 7% 21% 27% 22% 10%	583
September	1st to 10th	Day	14% 7% 22% 28% 21% 9%	530
	11th to 20th	Day	10% 4 20% 34% 22% 10%	530
	21st to end	Day	15% 11% 24% 25% 20% 5%	530
October	1st to 10th	Day	16% 12% 24% 26% 17% 5%	436
	11th to 20th	Day	19% 18% 32% 22% 7% 1	330
	21st to end	Day	24% 12% 38% 19% 6% 1	363
November	1st to 10th	Day	16% 16% 37% 21% 8% 1	330
	11th to 20th	Day	16% 19% 38% 21% 5%	330
	21st to end	Day	23% 18% 37% 18% 4	330
December	1st to 10th	Day	18% 21% 40% 16% 5% 1	330
	11th to 20th	Day	20% 20% 39% 16% 5% 1	330
	21st to end	Day	24% 21% 35% 15% 4 1	363
January	1st to 10th	Night	17% 14% 38% 24% 7%	1050
	11th to 20th	Night	18% 14% 40% 22% 5% 1	1050
	21st to end	Night	17% 12% 39% 26% 6%	1155
February	1st to 10th	Night	20% 13% 37% 23% 7%	1050
	11th to 20th	Night	14% 16% 38% 25% 6%	834
	21st to end	Night	22% 13% 37% 22% 5%	660
March	1st to 10th	Night	21% 11% 35% 24% 8% 1	800
	11th to 20th	Night	16% 13% 36% 28% 6% 1	767
	21st to end	Night	23% 8% 28% 32% 8% 1	660
April	1st to 10th	Night	23% 11% 37% 24% 5%	600
	11th to 20th	Night	23% 12% 31% 27% 6% 1	593
	21st to end	Night	19% 10% 34% 30% 8% 1	600
May	1st to 10th	Night	19% 8% 33% 31% 8% 1	600
	11th to 20th	Night	16% 8% 35% 31% 9% 1	591
	21st to end	Night	13% 6% 27% 39% 14% 1	650
June	1st to 10th	Night	12% 5% 28% 38% 17% 1	600
	11th to 20th	Night	13% 5% 34% 35% 12% 1	600
	21st to end	Night	11% 4 28% 39% 17% 1	600
July	1st to 10th	Night	9% 3 17% 42% 27% 2	585
	11th to 20th	Night	7% 3 23% 42% 23% 2	582
	21st to end	Night	7% 2 21% 45% 24% 2	657
August	1st to 10th	Night	9% 1 16% 46% 25% 3	597
	11th to 20th	Night	10% 3 16% 41% 27% 3	594
	21st to end	Night	10% 3 19% 43% 22% 2	660
September	1st to 10th	Night	11% 5% 23% 34% 22% 5%	600
	11th to 20th	Night	13% 5% 20% 34% 24% 5%	689
	21st to end	Night	15% 7% 25% 31% 20% 3	800
October	1st to 10th	Night	14% 8% 30% 30% 17% 2	800
	11th to 20th	Night	21% 14% 32% 25% 8%	800
	21st to end	Night	23% 11% 28% 27% 9% 1	880
November	1st to 10th	Night	17% 13% 35% 25% 8% 1	820
	11th to 20th	Night	19% 18% 35% 22% 6%	840
	21st to end	Night	21% 15% 37% 21% 6%	1027
December	1st to 10th	Night	19% 16% 38% 21% 6%	1050
	11th to 20th	Night	21% 16% 37% 20% 5% 1	1050
	21st to end	Night	25% 14% 37% 22% 3	1155

Page generated at 2026-02-13 00:42:17. Version ac07

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