Landolt Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Landolt Astronomical Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	11% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	125898
	Night	12% 6% 2 6% 4 4 4 4 5% 8% 44%	123614
Transparency	Day	40% 7% 12% 18% 16% 7%	129512
	Night	37% 6% 13% 19% 17% 8%	128076
Seeing	Day	13% 7% 18% 24% 24% 15%	16075
	Night	14% 6% 20% 31% 24% 6%	25701

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	16% 7% 5% 8% 4 4 4 3 4 6% 40%	4077
	11th to 20th	Day	13% 8% 3 6% 4 3 3 3 4 5% 49%	4080
	21st to end	Day	21% 10% 3 7% 4 4 4 3 4 5% 34%	4576
February	1st to 10th	Day	16% 9% 3 7% 5% 4 4 3 5% 6% 38%	2000
	11th to 20th	Day	17% 8% 2 6% 4 3 5% 3 5% 6% 40%	1948
	21st to end	Day	19% 7% 3 7% 4 4 4 4 5% 7% 36%	1660
March	1st to 10th	Day	19% 10% 3 8% 5% 4 4 3 5% 6% 33%	4936
	11th to 20th	Day	11% 5% 2 7% 5% 4 5% 3 6% 8% 43%	4980
	21st to end	Day	14% 8% 3 7% 5% 5% 4 3 5% 7% 40%	5588
April	1st to 10th	Day	12% 7% 3 7% 4 5% 5% 5% 6% 7% 40%	2400
	11th to 20th	Day	13% 7% 3 6% 5% 5% 4 4 7% 8% 39%	2374
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 6% 9% 43%	2400
May	1st to 10th	Day	16% 6% 3 8% 5% 6% 6% 5% 6% 9% 31%	5405
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 5% 6% 9% 45%	5404
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	5932
June	1st to 10th	Day	7% 5% 2 7% 6% 7% 6% 6% 9% 10% 34%	2535
	11th to 20th	Day	5% 5% 2 8% 8% 7% 8% 7% 8% 9% 32%	2600
	21st to end	Day	6% 6% 3 8% 8% 8% 9% 7% 9% 9% 27%	2600
July	1st to 10th	Day	5% 6% 3 9% 7% 8% 8% 7% 9% 11% 28%	5900
	11th to 20th	Day	6% 4 3 8% 7% 8% 9% 7% 10% 12% 28%	5900
	21st to end	Day	8% 7% 3 10% 7% 7% 7% 6% 8% 12% 26%	6196
August	1st to 10th	Day	3 3 1 5% 4 5% 6% 4 6% 10% 53%	3905
	11th to 20th	Day	6% 4 2 6% 6% 6% 7% 6% 9% 12% 37%	3849
	21st to end	Day	10% 6% 3 9% 6% 6% 6% 5% 7% 7% 34%	4312
September	1st to 10th	Day	11% 6% 3 7% 6% 6% 7% 5% 8% 9% 34%	2400
	11th to 20th	Day	11% 6% 2 7% 5% 6% 5% 4 6% 9% 39%	2400
	21st to end	Day	9% 5% 3 6% 4 5% 4 4 6% 7% 47%	2181
October	1st to 10th	Day	6% 4 3 5% 4 4 5% 3 5% 7% 55%	1930
	11th to 20th	Day	7% 5% 2 6% 2 4 3 4 6% 7% 55%	1900
	21st to end	Day	11% 4 1 4 3 3 4 3 5% 7% 54%	2090
November	1st to 10th	Day	8% 5% 2 6% 5% 3 4 4 6% 6% 50%	1958
	11th to 20th	Day	11% 5% 1 6% 4 3 3 4 4 4 53%	2000
	21st to end	Day	13% 7% 3 6% 4 4 5% 4 5% 6% 45%	2000
December	1st to 10th	Day	18% 8% 3 7% 4 4 5% 3 4 5% 40%	3260
	11th to 20th	Day	11% 7% 3 7% 4 5% 5% 3 5% 6% 44%	3734
	21st to end	Day	15% 8% 2 7% 4 4 4 3 5% 6% 43%	4488
January	1st to 10th	Night	19% 6% 2 5% 3 3 3 2 4 5% 48%	5888
	11th to 20th	Night	18% 7% 3 5% 3 3 3 2 4 5% 48%	5900
	21st to end	Night	23% 8% 2 6% 3 4 4 3 4 6% 36%	6380
February	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 4 5% 42%	2400
	11th to 20th	Night	20% 7% 2 6% 3 4 3 3 5% 6% 41%	2338
	21st to end	Night	19% 9% 2 7% 4 4 3 3 5% 7% 38%	1980
March	1st to 10th	Night	18% 11% 3 7% 4 4 4 3 4 6% 38%	4924
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 6% 46%	4980
	21st to end	Night	16% 7% 3 6% 3 4 4 4 5% 6% 43%	5340
April	1st to 10th	Night	13% 6% 2 6% 4 3 3 4 6% 7% 45%	2000
	11th to 20th	Night	12% 7% 2 6% 4 4 3 3 5% 6% 48%	1975
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 7% 52%	2000
May	1st to 10th	Night	15% 7% 3 7% 5% 4 4 4 6% 8% 36%	4490
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 5% 8% 42%	4489
	21st to end	Night	8% 6% 2 6% 5% 4 5% 4 7% 11% 41%	4922
June	1st to 10th	Night	5% 5% 2 5% 5% 5% 6% 6% 7% 10% 44%	1858
	11th to 20th	Night	3 4 2 6% 5% 6% 5% 4 8% 13% 45%	1800
	21st to end	Night	6% 5% 2 5% 7% 5% 6% 6% 8% 11% 38%	1800
July	1st to 10th	Night	3 5% 2 8% 6% 6% 6% 6% 9% 12% 36%	4078
	11th to 20th	Night	4 4 1 5% 4 5% 6% 5% 10% 14% 43%	4077
	21st to end	Night	4 5% 2 7% 5% 6% 6% 5% 9% 13% 36%	4662
August	1st to 10th	Night	2 3 1 3 3 3 4 4 6% 11% 60%	3244
	11th to 20th	Night	3 3 2 6% 4 5% 6% 5% 9% 14% 44%	3200
	21st to end	Night	6% 5% 3 8% 6% 6% 5% 6% 7% 9% 40%	3586
September	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 4 8% 10% 42%	2000
	11th to 20th	Night	9% 6% 3 7% 6% 5% 5% 5% 6% 8% 41%	2000
	21st to end	Night	7% 4 2 5% 4 4 5% 4 5% 8% 51%	2191
October	1st to 10th	Night	6% 3 2 4 4 3 4 3 5% 6% 59%	2317
	11th to 20th	Night	6% 5% 2 4 4 3 3 4 5% 6% 59%	2280
	21st to end	Night	11% 4 2 5% 3 3 3 3 4 6% 57%	2508
November	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 5% 6% 56%	2340
	11th to 20th	Night	12% 4 2 4 4 3 3 3 5% 5% 55%	2400
	21st to end	Night	13% 8% 3 6% 3 3 4 3 4 6% 48%	2400
December	1st to 10th	Night	18% 7% 3 7% 4 3 4 3 3 5% 42%	3920
	11th to 20th	Night	10% 9% 3 6% 5% 4 4 3 5% 6% 45%	4457
	21st to end	Night	17% 9% 2 6% 4 3 3 2 3 5% 44%	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	45% 4 5% 17% 23% 5%	4078
	11th to 20th	Day	39% 3 4 16% 28% 9%	4080
	21st to end	Day	53% 3 6% 13% 15% 11%	4576
February	1st to 10th	Day	48% 4 8% 13% 18% 10%	2000
	11th to 20th	Day	44% 5% 7% 15% 20% 9%	1958
	21st to end	Day	48% 4 7% 16% 17% 8%	1660
March	1st to 10th	Day	51% 4 8% 13% 14% 11%	4936
	11th to 20th	Day	39% 5% 8% 15% 17% 17%	4980
	21st to end	Day	43% 5% 9% 17% 18% 7%	5590
April	1st to 10th	Day	41% 6% 10% 16% 18% 9%	2400
	11th to 20th	Day	42% 5% 9% 19% 16% 10%	2374
	21st to end	Day	36% 6% 12% 20% 22% 5%	2400
May	1st to 10th	Day	48% 6% 13% 21% 9% 3	5440
	11th to 20th	Day	34% 6% 12% 26% 19% 2	5408
	21st to end	Day	34% 8% 16% 29% 12% 1	5945
June	1st to 10th	Day	41% 10% 14% 22% 13% 1	2535
	11th to 20th	Day	43% 11% 14% 19% 12% 1	2600
	21st to end	Day	45% 13% 17% 19% 6% 1	2600
July	1st to 10th	Day	44% 12% 23% 17% 3	5836
	11th to 20th	Day	43% 15% 31% 10% 2	5831
	21st to end	Day	47% 10% 18% 22% 2	6019
August	1st to 10th	Day	25% 7% 15% 30% 23%	3893
	11th to 20th	Day	35% 9% 22% 26% 7% 1	3885
	21st to end	Day	46% 9% 14% 15% 15% 2	4311
September	1st to 10th	Day	42% 10% 16% 16% 12% 3	2400
	11th to 20th	Day	41% 7% 14% 17% 16% 6%	2400
	21st to end	Day	35% 6% 11% 20% 18% 10%	2181
October	1st to 10th	Day	26% 4 7% 12% 22% 30%	3190
	11th to 20th	Day	18% 4 4 20% 26% 28%	3140
	21st to end	Day	24% 3 5% 11% 40% 18%	3428
November	1st to 10th	Day	32% 6% 8% 19% 25% 10%	1958
	11th to 20th	Day	34% 4 6% 16% 26% 14%	2000
	21st to end	Day	39% 4 7% 17% 23% 10%	2000
December	1st to 10th	Day	47% 3 7% 15% 21% 7%	3260
	11th to 20th	Day	41% 4 9% 14% 22% 9%	3732
	21st to end	Day	42% 4 6% 18% 24% 5%	4488
January	1st to 10th	Night	40% 3 6% 19% 25% 6%	5888
	11th to 20th	Night	40% 3 4 15% 27% 11%	5900
	21st to end	Night	50% 3 6% 13% 17% 10%	6380
February	1st to 10th	Night	44% 5% 6% 16% 19% 10%	2400
	11th to 20th	Night	45% 4 5% 16% 18% 12%	2339
	21st to end	Night	46% 4 9% 14% 15% 11%	1980
March	1st to 10th	Night	48% 4 8% 12% 17% 11%	4924
	11th to 20th	Night	39% 4 8% 15% 18% 16%	4980
	21st to end	Night	42% 5% 9% 20% 18% 7%	5340
April	1st to 10th	Night	38% 5% 12% 18% 17% 11%	2000
	11th to 20th	Night	37% 4 10% 19% 18% 12%	1975
	21st to end	Night	29% 7% 11% 22% 25% 7%	2000
May	1st to 10th	Night	44% 6% 14% 20% 12% 3	4520
	11th to 20th	Night	39% 6% 13% 25% 16% 1	4490
	21st to end	Night	35% 7% 18% 30% 10% 1	4931
June	1st to 10th	Night	31% 10% 19% 26% 13% 1	1858
	11th to 20th	Night	30% 9% 19% 27% 12% 2	1800
	21st to end	Night	34% 11% 24% 23% 8%	1800
July	1st to 10th	Night	37% 14% 31% 15% 3	4035
	11th to 20th	Night	29% 15% 43% 12% 2	4026
	21st to end	Night	37% 15% 21% 24% 3	4527
August	1st to 10th	Night	18% 8% 20% 30% 23% 1	3232
	11th to 20th	Night	28% 10% 28% 28% 5%	3230
	21st to end	Night	39% 9% 18% 17% 13% 3	3586
September	1st to 10th	Night	35% 10% 19% 20% 11% 5%	2000
	11th to 20th	Night	38% 8% 17% 17% 14% 6%	2000
	21st to end	Night	31% 6% 12% 23% 17% 11%	2191
October	1st to 10th	Night	25% 3 8% 15% 20% 29%	3837
	11th to 20th	Night	17% 3 5% 19% 28% 28%	3779
	21st to end	Night	20% 3 5% 13% 36% 24%	4120
November	1st to 10th	Night	31% 4 9% 20% 24% 13%	2340
	11th to 20th	Night	32% 3 8% 15% 26% 16%	2400
	21st to end	Night	38% 4 6% 19% 21% 12%	2400
December	1st to 10th	Night	46% 3 4 18% 23% 5%	3920
	11th to 20th	Night	39% 4 10% 17% 20% 10%	4458
	21st to end	Night	44% 3 7% 15% 23% 7%	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	20% 16% 37% 18% 7% 2	300
	11th to 20th	Day	17% 17% 37% 21% 8%	300
	21st to end	Day	25% 14% 35% 20% 5%	330
February	1st to 10th	Day	23% 13% 29% 26% 8% 1	300
	11th to 20th	Day	27% 16% 31% 18% 7% 1	300
	21st to end	Day	18% 21% 41% 17% 4	249
March	1st to 10th	Day	23% 15% 34% 22% 4	300
	11th to 20th	Day	22% 18% 33% 20% 5% 2	300
	21st to end	Day	19% 15% 32% 23% 11% 1	379
April	1st to 10th	Day	18% 14% 30% 25% 11% 1	500
	11th to 20th	Day	18% 13% 30% 23% 14% 2	492
	21st to end	Day	12% 8% 25% 32% 19% 4	500
May	1st to 10th	Day	15% 6% 25% 30% 20% 4	503
	11th to 20th	Day	13% 4 22% 27% 25% 9%	527
	21st to end	Day	7% 3 14% 31% 28% 17%	573
June	1st to 10th	Day	9% 2 12% 27% 29% 19%	665
	11th to 20th	Day	8% 3 9% 27% 36% 17%	730
	21st to end	Day	9% 3 8% 24% 30% 26%	730
July	1st to 10th	Day	8% 2 6% 20% 37% 27%	721
	11th to 20th	Day	8% 1 4 17% 37% 32%	718
	21st to end	Day	9% 2 6% 17% 36% 30%	694
August	1st to 10th	Day	5% 1 5% 15% 35% 39%	526
	11th to 20th	Day	8% 1 4 10% 33% 44%	519
	21st to end	Day	11% 2 6% 20% 35% 27%	583
September	1st to 10th	Day	10% 3 6% 19% 41% 22%	530
	11th to 20th	Day	9% 3 8% 21% 40% 20%	530
	21st to end	Day	11% 3 11% 25% 33% 17%	507
October	1st to 10th	Day	6% 3 18% 42% 22% 9%	309
	11th to 20th	Day	7% 5% 19% 34% 21% 13%	300
	21st to end	Day	15% 8% 19% 32% 20% 5%	330
November	1st to 10th	Day	14% 3 25% 34% 19% 5%	300
	11th to 20th	Day	16% 8% 27% 30% 14% 5%	300
	21st to end	Day	16% 10% 32% 28% 11% 3	300
December	1st to 10th	Day	21% 8% 29% 25% 13% 3	300
	11th to 20th	Day	21% 12% 32% 26% 8% 1	300
	21st to end	Day	25% 11% 33% 20% 10% 1	330
January	1st to 10th	Night	22% 9% 32% 31% 7%	1000
	11th to 20th	Night	19% 10% 29% 33% 8%	1000
	21st to end	Night	22% 10% 32% 30% 6%	1055
February	1st to 10th	Night	20% 10% 31% 35% 5%	800
	11th to 20th	Night	24% 11% 26% 32% 7%	783
	21st to end	Night	22% 13% 35% 28% 3	660
March	1st to 10th	Night	17% 9% 32% 33% 9%	605
	11th to 20th	Night	16% 8% 31% 34% 11%	600
	21st to end	Night	14% 9% 32% 34% 11%	660
April	1st to 10th	Night	16% 9% 30% 33% 12% 1	600
	11th to 20th	Night	17% 9% 30% 32% 12% 1	593
	21st to end	Night	8% 5% 29% 42% 15% 1	600
May	1st to 10th	Night	12% 5% 22% 45% 16% 1	600
	11th to 20th	Night	12% 4 20% 37% 25% 2	591
	21st to end	Night	8% 3 12% 39% 33% 6%	650
June	1st to 10th	Night	7% 3 7% 37% 41% 7%	600
	11th to 20th	Night	4 2 6% 34% 47% 9%	600
	21st to end	Night	9% 1 5% 24% 45% 17%	600
July	1st to 10th	Night	6% 1 4 17% 52% 21%	585
	11th to 20th	Night	5% 1 3 13% 52% 25%	582
	21st to end	Night	5% 1 5% 17% 54% 17%	657
August	1st to 10th	Night	5% 1 3 15% 52% 25%	597
	11th to 20th	Night	5% 1 1 15% 50% 28%	594
	21st to end	Night	7% 1 5% 17% 47% 22%	660
September	1st to 10th	Night	8% 2 6% 23% 45% 16%	600
	11th to 20th	Night	12% 3 6% 23% 48% 9%	600
	21st to end	Night	9% 2 9% 30% 42% 8%	775
October	1st to 10th	Night	7% 1 14% 38% 35% 5%	773
	11th to 20th	Night	9% 4 17% 40% 28% 2	760
	21st to end	Night	13% 6% 18% 38% 23% 2	836
November	1st to 10th	Night	12% 4 19% 42% 22% 2	780
	11th to 20th	Night	14% 7% 23% 39% 17% 1	800
	21st to end	Night	17% 6% 27% 34% 16% 1	800
December	1st to 10th	Night	18% 8% 26% 33% 15%	800
	11th to 20th	Night	21% 8% 26% 37% 9%	805
	21st to end	Night	26% 8% 27% 30% 9%	1100

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