Martens Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Martens Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 8% 3 8% 5% 5% 5% 4 6% 7% 39%	147310
	Night	9% 8% 3 9% 6% 5% 5% 4 6% 7% 39%	143127
Transparency	Day	43% 6% 15% 25% 10% 1	141925
	Night	44% 6% 14% 24% 10% 1	139522
Seeing	Day	15% 9% 31% 29% 13% 3	18614
	Night	15% 8% 33% 32% 11% 1	28485

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% 10% 5% 15% 11% 8% 8% 5% 6% 7% 20%	4123
	11th to 20th	Day	3 8% 3 12% 10% 10% 14% 10% 8% 6% 17%	4479
	21st to end	Day	4 9% 5% 12% 7% 7% 5% 5% 7% 7% 33%	5188
February	1st to 10th	Day	9% 14% 6% 11% 7% 6% 6% 5% 5% 6% 27%	1838
	11th to 20th	Day	6% 11% 5% 11% 7% 6% 7% 7% 7% 5% 27%	1844
	21st to end	Day	7% 9% 5% 13% 9% 6% 7% 4 4 7% 30%	1701
March	1st to 10th	Day	11% 9% 4 8% 5% 5% 5% 4 6% 9% 34%	4687
	11th to 20th	Day	8% 10% 5% 14% 8% 5% 5% 4 7% 8% 26%	5090
	21st to end	Day	15% 14% 5% 11% 5% 4 4 4 5% 7% 27%	5585
April	1st to 10th	Day	11% 13% 4 9% 5% 5% 5% 4 5% 6% 35%	2525
	11th to 20th	Day	15% 12% 4 8% 5% 5% 5% 4 5% 5% 32%	2699
	21st to end	Day	20% 10% 3 7% 5% 5% 4 4 5% 7% 32%	2730
May	1st to 10th	Day	12% 7% 2 5% 4 4 4 4 5% 7% 46%	7655
	11th to 20th	Day	18% 9% 2 6% 4 4 4 4 6% 7% 37%	8416
	21st to end	Day	13% 7% 2 6% 4 4 4 3 5% 7% 46%	9682
June	1st to 10th	Day	13% 6% 3 5% 4 5% 5% 4 7% 9% 40%	3150
	11th to 20th	Day	14% 5% 2 6% 4 5% 5% 4 7% 8% 40%	3150
	21st to end	Day	13% 6% 2 6% 5% 5% 6% 4 6% 8% 39%	3150
July	1st to 10th	Day	7% 3 2 6% 4 5% 5% 4 7% 9% 48%	6949
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 8% 50%	6950
	21st to end	Day	9% 4 2 5% 4 4 5% 5% 7% 10% 46%	7184
August	1st to 10th	Day	12% 4 2 6% 4 4 5% 4 6% 8% 45%	5938
	11th to 20th	Day	14% 4 2 5% 4 4 4 3 6% 9% 46%	5821
	21st to end	Day	8% 4 1 5% 3 4 4 3 5% 8% 56%	6490
September	1st to 10th	Day	12% 4 2 5% 4 4 4 3 5% 7% 51%	2434
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 3 6% 7% 43%	2200
	21st to end	Day	12% 6% 2 5% 4 3 4 3 6% 7% 48%	2200
October	1st to 10th	Day	12% 5% 2 4 3 4 4 3 4 5% 54%	3369
	11th to 20th	Day	13% 8% 2 5% 4 4 3 3 5% 7% 46%	3188
	21st to end	Day	12% 9% 3 9% 5% 5% 4 4 5% 7% 38%	3190
November	1st to 10th	Day	13% 12% 5% 9% 4 5% 4 3 4 5% 35%	1760
	11th to 20th	Day	10% 15% 5% 11% 6% 5% 5% 3 5% 5% 30%	1680
	21st to end	Day	10% 14% 5% 13% 7% 6% 4 3 4 5% 31%	1617
December	1st to 10th	Day	10% 11% 5% 15% 14% 8% 6% 4 4 5% 18%	2360
	11th to 20th	Day	8% 15% 6% 13% 7% 5% 5% 5% 5% 7% 25%	2713
	21st to end	Day	11% 14% 5% 16% 7% 6% 4 3 4 4 26%	3575
January	1st to 10th	Night	5% 10% 6% 14% 9% 8% 7% 5% 7% 6% 23%	8799
	11th to 20th	Night	5% 11% 5% 11% 8% 8% 10% 7% 7% 6% 23%	8508
	21st to end	Night	5% 8% 6% 12% 6% 5% 5% 4 6% 8% 34%	9026
February	1st to 10th	Night	7% 13% 4 13% 6% 5% 6% 4 4 6% 30%	2781
	11th to 20th	Night	7% 9% 4 9% 7% 6% 7% 5% 6% 6% 35%	2663
	21st to end	Night	6% 11% 5% 10% 6% 5% 5% 3 5% 7% 37%	2123
March	1st to 10th	Night	7% 9% 3 7% 5% 5% 5% 3 6% 7% 43%	5401
	11th to 20th	Night	9% 9% 3 10% 7% 5% 5% 5% 5% 7% 35%	5090
	21st to end	Night	13% 14% 5% 10% 5% 4 5% 4 5% 6% 31%	5588
April	1st to 10th	Night	10% 9% 3 7% 6% 4 5% 4 6% 7% 40%	2090
	11th to 20th	Night	14% 12% 4 8% 5% 3 5% 3 5% 5% 37%	1869
	21st to end	Night	16% 9% 3 6% 4 4 4 3 5% 6% 38%	1890
May	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 46%	5292
	11th to 20th	Night	16% 7% 2 5% 4 4 4 3 5% 7% 44%	4471
	21st to end	Night	12% 5% 2 4 4 3 3 3 5% 7% 50%	4526
June	1st to 10th	Night	10% 4 1 4 4 3 4 5% 7% 7% 49%	1470
	11th to 20th	Night	11% 6% 1 5% 4 4 4 4 6% 10% 46%	1470
	21st to end	Night	8% 4 1 5% 5% 4 5% 5% 6% 9% 48%	1470
July	1st to 10th	Night	6% 4 1 5% 3 3 4 4 6% 8% 56%	3250
	11th to 20th	Night	6% 3 1 4 3 3 4 3 5% 8% 59%	3250
	21st to end	Night	7% 4 2 6% 4 4 5% 4 7% 10% 46%	3908
August	1st to 10th	Night	11% 3 1 5% 4 4 4 3 6% 7% 51%	3872
	11th to 20th	Night	12% 4 1 4 3 3 4 3 6% 9% 51%	4028
	21st to end	Night	8% 3 1 5% 3 2 3 3 5% 7% 58%	4487
September	1st to 10th	Night	7% 5% 2 4 3 3 4 3 6% 8% 55%	1964
	11th to 20th	Night	11% 4 2 4 3 3 4 4 5% 7% 52%	2200
	21st to end	Night	13% 5% 2 4 4 3 4 3 4 7% 51%	2200
October	1st to 10th	Night	11% 5% 1 5% 3 3 4 3 5% 7% 53%	3778
	11th to 20th	Night	11% 8% 2 7% 4 4 4 3 5% 7% 46%	3951
	21st to end	Night	13% 10% 3 7% 4 4 4 3 5% 6% 41%	4610
November	1st to 10th	Night	11% 10% 3 8% 6% 5% 5% 4 6% 7% 34%	2755
	11th to 20th	Night	8% 13% 5% 12% 6% 5% 4 4 5% 6% 33%	2940
	21st to end	Night	11% 12% 4 12% 5% 5% 4 3 5% 6% 33%	2998
December	1st to 10th	Night	7% 12% 4 15% 10% 7% 6% 4 6% 6% 23%	5050
	11th to 20th	Night	8% 13% 5% 14% 8% 6% 6% 4 5% 6% 26%	5714
	21st to end	Night	11% 13% 5% 14% 8% 6% 5% 3 4 5% 25%	7645

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	61% 6% 12% 16% 4	4125
	11th to 20th	Day	62% 7% 13% 14% 5%	4479
	21st to end	Day	48% 6% 14% 23% 8%	5188
February	1st to 10th	Day	56% 5% 14% 15% 10%	1838
	11th to 20th	Day	53% 7% 13% 19% 8% 1	1853
	21st to end	Day	55% 6% 13% 19% 7% 1	1701
March	1st to 10th	Day	45% 6% 20% 20% 9%	4687
	11th to 20th	Day	54% 6% 15% 20% 5%	5090
	21st to end	Day	55% 7% 13% 20% 4	5585
April	1st to 10th	Day	49% 7% 12% 21% 9% 1	2525
	11th to 20th	Day	52% 6% 15% 17% 11% 1	2702
	21st to end	Day	51% 6% 15% 20% 7% 1	2730
May	1st to 10th	Day	35% 6% 12% 32% 14%	7655
	11th to 20th	Day	45% 5% 11% 24% 12% 3	8420
	21st to end	Day	36% 6% 11% 28% 18% 1	9683
June	1st to 10th	Day	39% 8% 18% 26% 9% 1	3150
	11th to 20th	Day	39% 8% 17% 25% 10% 1	3150
	21st to end	Day	40% 8% 17% 27% 8%	3150
July	1st to 10th	Day	30% 9% 24% 31% 6%	6876
	11th to 20th	Day	31% 6% 18% 36% 8%	6869
	21st to end	Day	32% 8% 22% 28% 10%	6983
August	1st to 10th	Day	33% 7% 16% 28% 16% 1	4319
	11th to 20th	Day	35% 6% 16% 33% 10%	4210
	21st to end	Day	30% 6% 20% 33% 10%	4674
September	1st to 10th	Day	33% 6% 16% 33% 12% 1	2434
	11th to 20th	Day	39% 7% 19% 25% 9% 1	2200
	21st to end	Day	35% 6% 17% 33% 9%	2200
October	1st to 10th	Day	32% 4 12% 34% 16% 1	3369
	11th to 20th	Day	39% 4 11% 30% 16%	3188
	21st to end	Day	46% 6% 13% 25% 9%	3190
November	1st to 10th	Day	51% 7% 15% 18% 9% 1	1760
	11th to 20th	Day	56% 6% 14% 16% 7% 1	1680
	21st to end	Day	56% 6% 14% 17% 7%	1617
December	1st to 10th	Day	68% 5% 13% 11% 3	2360
	11th to 20th	Day	57% 6% 14% 14% 8%	2710
	21st to end	Day	62% 5% 8% 18% 6%	3575
January	1st to 10th	Night	58% 7% 14% 17% 5%	8800
	11th to 20th	Night	57% 7% 13% 19% 5%	8508
	21st to end	Night	47% 5% 13% 25% 10%	9026
February	1st to 10th	Night	54% 5% 14% 18% 9%	2781
	11th to 20th	Night	46% 6% 14% 21% 13%	2664
	21st to end	Night	47% 5% 13% 24% 10% 1	2123
March	1st to 10th	Night	38% 6% 19% 24% 10% 2	5401
	11th to 20th	Night	47% 7% 13% 21% 12% 1	5090
	21st to end	Night	54% 7% 12% 20% 8%	5588
April	1st to 10th	Night	41% 7% 13% 25% 12% 2	2090
	11th to 20th	Night	49% 4 12% 20% 14% 2	1869
	21st to end	Night	45% 5% 13% 24% 10% 2	1890
May	1st to 10th	Night	36% 6% 11% 30% 16% 1	5293
	11th to 20th	Night	38% 6% 10% 25% 19% 3	4472
	21st to end	Night	33% 5% 13% 27% 22% 1	4526
June	1st to 10th	Night	29% 8% 20% 30% 10% 2	1470
	11th to 20th	Night	34% 8% 16% 27% 13% 2	1470
	21st to end	Night	32% 8% 20% 32% 7% 1	1470
July	1st to 10th	Night	25% 7% 24% 34% 9% 1	3215
	11th to 20th	Night	24% 7% 17% 42% 9% 1	3208
	21st to end	Night	31% 8% 25% 25% 10% 1	3797
August	1st to 10th	Night	27% 6% 15% 34% 16% 2	2815
	11th to 20th	Night	30% 6% 20% 31% 12% 1	2915
	21st to end	Night	30% 5% 17% 32% 15%	3234
September	1st to 10th	Night	26% 7% 16% 34% 16% 1	1964
	11th to 20th	Night	30% 6% 20% 30% 13% 1	2200
	21st to end	Night	33% 6% 13% 31% 16% 1	2200
October	1st to 10th	Night	32% 5% 12% 32% 17% 3	3778
	11th to 20th	Night	38% 4 10% 30% 17% 1	3951
	21st to end	Night	44% 5% 13% 26% 11%	4610
November	1st to 10th	Night	48% 7% 14% 18% 11% 2	2755
	11th to 20th	Night	52% 6% 15% 17% 8% 1	2940
	21st to end	Night	51% 6% 16% 18% 7%	2998
December	1st to 10th	Night	61% 8% 15% 13% 4	5050
	11th to 20th	Night	59% 7% 13% 16% 5%	5716
	21st to end	Night	61% 6% 11% 16% 6%	7645

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	10% 10% 35% 31% 11% 3	330
	11th to 20th	Day	12% 12% 34% 32% 9% 1	330
	21st to end	Day	16% 7% 40% 27% 9% 2	363
February	1st to 10th	Day	16% 15% 35% 26% 6% 2	330
	11th to 20th	Day	9% 12% 40% 26% 10% 2	330
	21st to end	Day	9% 8% 45% 29% 8% 1	273
March	1st to 10th	Day	16% 11% 30% 32% 8% 2	330
	11th to 20th	Day	16% 13% 30% 29% 10% 3	330
	21st to end	Day	15% 11% 33% 28% 10% 3	363
April	1st to 10th	Day	19% 14% 32% 24% 10% 1	545
	11th to 20th	Day	23% 12% 31% 23% 9% 1	740
	21st to end	Day	24% 11% 25% 27% 11% 2	750
May	1st to 10th	Day	18% 11% 28% 29% 11% 3	753
	11th to 20th	Day	22% 7% 30% 28% 11% 2	775
	21st to end	Day	17% 6% 24% 32% 17% 4	844
June	1st to 10th	Day	15% 10% 29% 27% 14% 5%	780
	11th to 20th	Day	16% 9% 36% 26% 12% 2	780
	21st to end	Day	16% 7% 29% 29% 14% 5%	780
July	1st to 10th	Day	7% 3 30% 30% 20% 9%	771
	11th to 20th	Day	10% 6% 32% 29% 18% 5%	768
	21st to end	Day	5% 4 29% 37% 19% 6%	845
August	1st to 10th	Day	8% 4 26% 34% 23% 6%	726
	11th to 20th	Day	14% 4 20% 33% 21% 8%	715
	21st to end	Day	11% 8% 28% 28% 21% 5%	803
September	1st to 10th	Day	12% 8% 31% 30% 15% 4	564
	11th to 20th	Day	16% 7% 32% 30% 15% 2	330
	21st to end	Day	18% 8% 35% 26% 11% 3	330
October	1st to 10th	Day	19% 10% 31% 26% 12% 2	330
	11th to 20th	Day	17% 12% 38% 27% 6% 1	330
	21st to end	Day	20% 12% 36% 21% 9% 2	363
November	1st to 10th	Day	22% 9% 31% 26% 10% 1	330
	11th to 20th	Day	16% 14% 36% 25% 9% 1	330
	21st to end	Day	15% 13% 39% 24% 8%	330
December	1st to 10th	Day	14% 11% 36% 28% 9% 2	330
	11th to 20th	Day	20% 15% 33% 27% 5%	330
	21st to end	Day	18% 8% 35% 28% 8% 3	363
January	1st to 10th	Night	13% 10% 34% 30% 11% 1	1050
	11th to 20th	Night	14% 9% 37% 32% 8%	1050
	21st to end	Night	14% 10% 38% 30% 8% 1	1155
February	1st to 10th	Night	13% 12% 35% 30% 9% 1	1050
	11th to 20th	Night	13% 9% 36% 30% 10% 3	1029
	21st to end	Night	14% 9% 36% 30% 10% 2	739
March	1st to 10th	Night	16% 9% 35% 30% 9% 2	799
	11th to 20th	Night	13% 9% 37% 30% 9% 1	630
	21st to end	Night	17% 6% 34% 28% 12% 2	693
April	1st to 10th	Night	15% 9% 34% 33% 9%	630
	11th to 20th	Night	20% 8% 32% 31% 10%	623
	21st to end	Night	21% 7% 26% 34% 12% 1	630
May	1st to 10th	Night	18% 7% 30% 34% 10% 1	630
	11th to 20th	Night	19% 6% 31% 32% 12%	621
	21st to end	Night	17% 6% 28% 35% 13%	683
June	1st to 10th	Night	12% 7% 27% 40% 13% 1	630
	11th to 20th	Night	13% 6% 35% 33% 13%	630
	21st to end	Night	9% 4 27% 43% 16% 1	630
July	1st to 10th	Night	6% 2 26% 42% 22% 2	615
	11th to 20th	Night	6% 2 32% 42% 16% 1	612
	21st to end	Night	6% 2 25% 46% 19% 2	687
August	1st to 10th	Night	6% 4 24% 45% 20% 1	597
	11th to 20th	Night	10% 2 26% 40% 20% 2	594
	21st to end	Night	9% 4 27% 40% 20%	660
September	1st to 10th	Night	10% 4 29% 37% 19% 2	600
	11th to 20th	Night	11% 6% 30% 38% 15% 1	600
	21st to end	Night	14% 7% 31% 33% 15% 1	600
October	1st to 10th	Night	18% 6% 36% 31% 8%	710
	11th to 20th	Night	20% 9% 35% 28% 8%	828
	21st to end	Night	20% 11% 34% 26% 9%	1100
November	1st to 10th	Night	16% 10% 36% 26% 11% 1	1025
	11th to 20th	Night	15% 10% 35% 28% 10% 1	1050
	21st to end	Night	17% 13% 39% 23% 7% 1	1050
December	1st to 10th	Night	14% 10% 36% 30% 9% 1	1050
	11th to 20th	Night	18% 11% 33% 28% 8% 1	1050
	21st to end	Night	23% 8% 33% 27% 9% 1	1155

Page generated at 2026-07-28 02:25:20. Version ac07

Copyright © 2026 Attilla Danko.