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 2025-03-27.

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% 37%	112413
	Night	9% 9% 4 9% 6% 5% 5% 4 5% 7% 37%	115701
Transparency	Day	44% 7% 16% 24% 9% 1	112141
	Night	45% 7% 14% 24% 10% 1	115563
Seeing	Day	15% 8% 31% 29% 14% 3	16600
	Night	15% 8% 33% 32% 12% 1	26732

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	4 10% 5% 14% 11% 8% 8% 6% 7% 8% 20%	3178
	11th to 20th	Day	4 7% 4 12% 11% 9% 12% 10% 9% 6% 18%	3435
	21st to end	Day	5% 10% 6% 15% 7% 7% 5% 4 6% 6% 31%	4004
February	1st to 10th	Day	9% 13% 6% 10% 7% 6% 6% 5% 5% 5% 28%	1750
	11th to 20th	Day	6% 11% 5% 11% 7% 6% 8% 7% 8% 5% 27%	1754
	21st to end	Day	7% 9% 5% 13% 8% 6% 6% 4 5% 7% 30%	1622
March	1st to 10th	Day	12% 11% 4 8% 6% 5% 4 3 4 7% 35%	3291
	11th to 20th	Day	9% 10% 5% 13% 8% 4 5% 4 7% 8% 27%	3590
	21st to end	Day	15% 12% 4 12% 5% 4 4 3 5% 6% 28%	3880
April	1st to 10th	Day	10% 12% 4 10% 6% 5% 5% 4 6% 6% 35%	2284
	11th to 20th	Day	16% 12% 4 7% 5% 5% 5% 4 5% 5% 32%	2439
	21st to end	Day	20% 10% 3 7% 4 5% 4 4 5% 7% 32%	2470
May	1st to 10th	Day	19% 7% 2 6% 4 4 4 5% 6% 8% 33%	4120
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 4 7% 8% 35%	4516
	21st to end	Day	18% 9% 3 7% 4 5% 4 4 5% 7% 35%	5173
June	1st to 10th	Day	13% 6% 3 5% 4 4 5% 4 7% 9% 39%	2850
	11th to 20th	Day	15% 5% 2 6% 4 4 4 4 7% 8% 41%	2850
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 4 7% 8% 40%	2850
July	1st to 10th	Day	8% 3 1 5% 4 5% 5% 4 7% 10% 46%	4750
	11th to 20th	Day	6% 3 1 4 4 4 5% 4 7% 9% 54%	4750
	21st to end	Day	6% 4 1 4 3 4 4 5% 7% 10% 51%	4897
August	1st to 10th	Day	10% 4 2 5% 4 5% 5% 4 5% 8% 48%	4201
	11th to 20th	Day	14% 4 2 5% 4 4 4 3 6% 9% 46%	4041
	21st to end	Day	10% 5% 1 5% 3 4 4 3 5% 7% 52%	4532
September	1st to 10th	Day	12% 4 2 5% 4 3 4 3 5% 7% 51%	2313
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 3 6% 7% 45%	2090
	21st to end	Day	13% 6% 2 5% 4 3 4 3 6% 7% 46%	2090
October	1st to 10th	Day	12% 5% 2 5% 3 4 4 3 4 5% 54%	3266
	11th to 20th	Day	13% 8% 2 5% 4 4 3 3 5% 7% 47%	3090
	21st to end	Day	11% 9% 3 9% 5% 5% 4 4 5% 7% 37%	3091
November	1st to 10th	Day	13% 12% 5% 9% 4 5% 4 3 4 5% 35%	1675
	11th to 20th	Day	10% 15% 6% 11% 6% 5% 5% 4 5% 5% 29%	1600
	21st to end	Day	10% 14% 5% 13% 7% 6% 4 3 4 5% 30%	1540
December	1st to 10th	Day	9% 11% 5% 15% 14% 8% 6% 4 4 5% 18%	2290
	11th to 20th	Day	8% 15% 6% 13% 7% 5% 5% 5% 5% 7% 25%	2643
	21st to end	Day	10% 14% 6% 16% 7% 6% 4 3 3 4 26%	3498
January	1st to 10th	Night	5% 10% 6% 14% 9% 8% 7% 5% 7% 6% 24%	6787
	11th to 20th	Night	5% 11% 4 11% 8% 8% 9% 8% 8% 6% 22%	6542
	21st to end	Night	6% 11% 7% 15% 7% 5% 5% 4 5% 6% 29%	6974
February	1st to 10th	Night	7% 12% 4 13% 6% 5% 6% 4 4 6% 31%	2649
	11th to 20th	Night	6% 9% 4 9% 7% 6% 7% 5% 6% 7% 35%	2533
	21st to end	Night	6% 11% 5% 10% 6% 5% 5% 3 4 7% 38%	2025
March	1st to 10th	Night	8% 10% 3 8% 6% 6% 5% 3 5% 7% 40%	3789
	11th to 20th	Night	8% 9% 3 9% 7% 4 4 4 5% 7% 38%	3590
	21st to end	Night	12% 13% 5% 11% 5% 4 5% 4 5% 6% 30%	3889
April	1st to 10th	Night	9% 9% 2 7% 6% 4 5% 4 6% 7% 41%	1891
	11th to 20th	Night	15% 12% 4 7% 5% 4 5% 3 5% 5% 37%	1689
	21st to end	Night	16% 9% 4 6% 4 4 4 4 5% 6% 37%	1710
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 5% 8% 39%	2850
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 6% 7% 42%	2384
	21st to end	Night	15% 7% 2 5% 5% 3 3 3 5% 7% 45%	2414
June	1st to 10th	Night	10% 4 1 4 4 3 4 5% 7% 7% 49%	1330
	11th to 20th	Night	11% 6% 1 5% 4 4 4 4 6% 9% 46%	1330
	21st to end	Night	7% 3 1 4 5% 4 5% 5% 6% 9% 50%	1330
July	1st to 10th	Night	7% 3 1 5% 4 3 4 4 6% 8% 55%	2220
	11th to 20th	Night	5% 3 1 4 3 3 3 3 4 7% 65%	2220
	21st to end	Night	5% 3 2 5% 4 4 5% 5% 7% 11% 48%	2664
August	1st to 10th	Night	6% 3 1 6% 4 4 4 4 6% 7% 55%	2736
	11th to 20th	Night	11% 4 1 4 3 4 4 3 7% 9% 49%	2798
	21st to end	Night	11% 4 2 5% 3 2 3 3 4 6% 55%	3135
September	1st to 10th	Night	8% 5% 2 4 3 3 4 3 6% 8% 55%	1865
	11th to 20th	Night	10% 4 1 4 3 3 4 4 5% 7% 53%	2090
	21st to end	Night	14% 5% 2 4 4 3 4 3 4 7% 50%	2090
October	1st to 10th	Night	11% 5% 1 5% 3 3 4 3 5% 7% 53%	3662
	11th to 20th	Night	10% 8% 2 6% 4 4 4 3 5% 7% 47%	3829
	21st to end	Night	13% 10% 3 7% 4 4 4 3 5% 6% 42%	4467
November	1st to 10th	Night	11% 10% 3 8% 6% 5% 5% 4 7% 7% 34%	2620
	11th to 20th	Night	8% 14% 5% 12% 6% 5% 5% 4 5% 5% 32%	2800
	21st to end	Night	11% 11% 4 11% 5% 5% 4 3 5% 6% 34%	2855
December	1st to 10th	Night	7% 12% 4 14% 10% 7% 6% 4 6% 6% 23%	4900
	11th to 20th	Night	8% 13% 5% 14% 8% 6% 6% 4 5% 6% 25%	5564
	21st to end	Night	11% 13% 5% 14% 8% 6% 5% 3 4 5% 26%	7480

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	59% 8% 12% 17% 4	3179
	11th to 20th	Day	59% 7% 14% 14% 6%	3435
	21st to end	Day	54% 6% 16% 20% 4	4004
February	1st to 10th	Day	56% 5% 14% 15% 10% 1	1750
	11th to 20th	Day	52% 7% 13% 18% 9% 1	1763
	21st to end	Day	54% 6% 14% 19% 7% 1	1622
March	1st to 10th	Day	49% 7% 16% 21% 8%	3291
	11th to 20th	Day	52% 7% 15% 20% 5%	3590
	21st to end	Day	54% 8% 14% 19% 5%	3880
April	1st to 10th	Day	48% 7% 13% 21% 10% 1	2284
	11th to 20th	Day	52% 6% 14% 17% 11% 1	2442
	21st to end	Day	51% 6% 15% 20% 6% 1	2470
May	1st to 10th	Day	43% 9% 16% 22% 10% 1	4120
	11th to 20th	Day	42% 9% 14% 24% 10% 1	4520
	21st to end	Day	45% 9% 15% 23% 8% 1	5173
June	1st to 10th	Day	38% 8% 19% 25% 9% 1	2850
	11th to 20th	Day	39% 9% 17% 25% 10% 1	2850
	21st to end	Day	39% 8% 17% 28% 8%	2850
July	1st to 10th	Day	30% 9% 23% 29% 8%	4676
	11th to 20th	Day	25% 7% 20% 38% 10%	4669
	21st to end	Day	26% 8% 22% 34% 10% 1	4743
August	1st to 10th	Day	33% 7% 17% 28% 15% 1	4186
	11th to 20th	Day	35% 6% 16% 33% 10%	4080
	21st to end	Day	31% 6% 20% 33% 10%	4531
September	1st to 10th	Day	33% 5% 16% 33% 12% 1	2313
	11th to 20th	Day	38% 7% 19% 27% 9% 1	2090
	21st to end	Day	36% 6% 17% 32% 9%	2090
October	1st to 10th	Day	33% 4 12% 34% 16% 1	3266
	11th to 20th	Day	38% 4 11% 31% 16%	3090
	21st to end	Day	46% 6% 13% 25% 10%	3091
November	1st to 10th	Day	50% 7% 15% 18% 9% 1	1675
	11th to 20th	Day	57% 6% 14% 16% 7% 1	1600
	21st to end	Day	56% 6% 14% 16% 7%	1540
December	1st to 10th	Day	68% 5% 13% 11% 3	2290
	11th to 20th	Day	58% 6% 14% 14% 8%	2640
	21st to end	Day	62% 5% 8% 19% 6%	3498
January	1st to 10th	Night	57% 7% 13% 19% 4	6787
	11th to 20th	Night	57% 7% 13% 18% 5%	6542
	21st to end	Night	55% 6% 13% 20% 5%	6974
February	1st to 10th	Night	53% 5% 13% 18% 10%	2649
	11th to 20th	Night	46% 6% 14% 20% 13%	2534
	21st to end	Night	47% 5% 13% 24% 10% 1	2025
March	1st to 10th	Night	42% 7% 16% 25% 10% 1	3789
	11th to 20th	Night	42% 8% 15% 23% 11% 1	3590
	21st to end	Night	52% 7% 13% 19% 8% 1	3889
April	1st to 10th	Night	40% 7% 14% 25% 12% 2	1891
	11th to 20th	Night	49% 5% 11% 19% 14% 2	1689
	21st to end	Night	46% 6% 14% 23% 9% 2	1710
May	1st to 10th	Night	42% 8% 13% 26% 9% 1	2850
	11th to 20th	Night	39% 8% 12% 26% 12% 3	2385
	21st to end	Night	37% 7% 14% 29% 12%	2414
June	1st to 10th	Night	29% 9% 20% 29% 10% 3	1330
	11th to 20th	Night	34% 8% 15% 28% 14% 2	1330
	21st to end	Night	29% 8% 21% 33% 7% 2	1330
July	1st to 10th	Night	25% 8% 26% 31% 10% 1	2185
	11th to 20th	Night	20% 6% 15% 48% 10% 2	2178
	21st to end	Night	27% 8% 23% 29% 11% 1	2580
August	1st to 10th	Night	27% 6% 15% 35% 16% 2	2728
	11th to 20th	Night	30% 6% 20% 31% 12% 1	2825
	21st to end	Night	31% 5% 16% 32% 16%	3135
September	1st to 10th	Night	27% 7% 15% 34% 17% 1	1865
	11th to 20th	Night	29% 6% 20% 31% 13% 1	2090
	21st to end	Night	34% 6% 13% 31% 16% 1	2090
October	1st to 10th	Night	32% 5% 12% 32% 17% 3	3662
	11th to 20th	Night	37% 5% 10% 31% 17% 1	3829
	21st to end	Night	43% 5% 13% 27% 12%	4467
November	1st to 10th	Night	47% 7% 15% 18% 11% 2	2620
	11th to 20th	Night	53% 6% 15% 17% 8% 1	2800
	21st to end	Night	51% 7% 17% 18% 7%	2855
December	1st to 10th	Night	61% 8% 15% 13% 3	4900
	11th to 20th	Night	59% 7% 13% 16% 5%	5566
	21st to end	Night	61% 6% 11% 16% 6%	7480

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% 9% 35% 32% 11% 3	300
	11th to 20th	Day	12% 10% 33% 34% 10% 1	300
	21st to end	Day	16% 7% 38% 28% 9% 2	330
February	1st to 10th	Day	17% 14% 35% 27% 6% 1	300
	11th to 20th	Day	9% 12% 42% 25% 10% 2	300
	21st to end	Day	9% 8% 45% 31% 8% 1	249
March	1st to 10th	Day	16% 12% 28% 33% 9% 2	300
	11th to 20th	Day	16% 11% 29% 30% 10% 3	300
	21st to end	Day	17% 8% 30% 31% 10% 3	315
April	1st to 10th	Day	18% 13% 33% 24% 11% 1	464
	11th to 20th	Day	24% 10% 30% 23% 10% 2	640
	21st to end	Day	24% 10% 25% 27% 12% 3	650
May	1st to 10th	Day	20% 11% 27% 28% 12% 3	653
	11th to 20th	Day	20% 6% 30% 29% 13% 3	675
	21st to end	Day	19% 7% 25% 32% 15% 3	734
June	1st to 10th	Day	15% 10% 29% 26% 15% 6%	680
	11th to 20th	Day	17% 9% 36% 24% 12% 3	680
	21st to end	Day	14% 7% 29% 29% 16% 5%	680
July	1st to 10th	Day	8% 3 29% 30% 20% 9%	671
	11th to 20th	Day	10% 6% 29% 30% 20% 5%	668
	21st to end	Day	5% 4 27% 37% 20% 6%	739
August	1st to 10th	Day	8% 4 27% 33% 23% 6%	676
	11th to 20th	Day	14% 4 20% 33% 21% 8%	665
	21st to end	Day	11% 7% 28% 28% 21% 5%	748
September	1st to 10th	Day	12% 7% 31% 31% 15% 3	523
	11th to 20th	Day	14% 7% 32% 31% 14% 2	300
	21st to end	Day	19% 8% 36% 25% 9% 2	300
October	1st to 10th	Day	20% 10% 31% 25% 12% 3	300
	11th to 20th	Day	14% 10% 38% 29% 7% 1	300
	21st to end	Day	20% 11% 37% 21% 9% 2	330
November	1st to 10th	Day	22% 9% 28% 28% 11% 1	300
	11th to 20th	Day	17% 12% 37% 25% 8%	300
	21st to end	Day	16% 13% 40% 24% 7%	300
December	1st to 10th	Day	13% 11% 35% 29% 10% 2	300
	11th to 20th	Day	19% 12% 35% 29% 6%	300
	21st to end	Day	17% 7% 36% 28% 8% 4	330
January	1st to 10th	Night	13% 10% 34% 30% 12% 1	1000
	11th to 20th	Night	14% 8% 36% 33% 9%	1000
	21st to end	Night	15% 10% 37% 30% 8% 1	1100
February	1st to 10th	Night	13% 11% 35% 30% 9% 1	1000
	11th to 20th	Night	12% 9% 37% 30% 10% 3	979
	21st to end	Night	14% 9% 36% 30% 10% 2	705
March	1st to 10th	Night	16% 9% 34% 30% 10% 2	761
	11th to 20th	Night	14% 9% 37% 30% 9% 1	600
	21st to end	Night	17% 6% 32% 29% 13% 3	647
April	1st to 10th	Night	14% 9% 35% 33% 10%	570
	11th to 20th	Night	20% 7% 31% 31% 11%	563
	21st to end	Night	21% 6% 25% 34% 12% 1	570
May	1st to 10th	Night	19% 6% 31% 33% 10% 1	570
	11th to 20th	Night	19% 5% 29% 33% 13% 1	561
	21st to end	Night	18% 6% 29% 34% 12%	617
June	1st to 10th	Night	13% 6% 26% 40% 14% 1	570
	11th to 20th	Night	14% 6% 34% 32% 13% 1	570
	21st to end	Night	8% 3 27% 44% 17% 1	570
July	1st to 10th	Night	6% 3 26% 42% 22% 2	555
	11th to 20th	Night	6% 2 31% 42% 17% 1	552
	21st to end	Night	5% 2 25% 45% 20% 2	624
August	1st to 10th	Night	6% 3 25% 46% 20% 1	567
	11th to 20th	Night	10% 2 26% 41% 20% 2	564
	21st to end	Night	9% 4 27% 39% 20%	627
September	1st to 10th	Night	10% 4 29% 37% 18% 2	570
	11th to 20th	Night	10% 5% 30% 39% 16% 1	570
	21st to end	Night	15% 6% 32% 33% 14% 1	570
October	1st to 10th	Night	19% 7% 35% 31% 8%	674
	11th to 20th	Night	19% 8% 36% 28% 8% 1	786
	21st to end	Night	20% 11% 34% 26% 9%	1045
November	1st to 10th	Night	16% 10% 37% 26% 11% 1	975
	11th to 20th	Night	16% 10% 35% 28% 10% 1	1000
	21st to end	Night	17% 14% 39% 23% 7% 1	1000
December	1st to 10th	Night	14% 10% 35% 30% 9% 1	1000
	11th to 20th	Night	18% 11% 33% 29% 8% 1	1000
	21st to end	Night	23% 7% 32% 27% 9% 1	1100

Page generated at 2025-03-24 08:04:32. Version ac07

Copyright © 2025 Attilla Danko.