Merk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Merk Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	7% 7% 3 7% 4 4 4 3 5% 7% 51%	131108
	Night	8% 8% 3 7% 4 4 4 3 5% 7% 45%	128320
Transparency	Day	33% 5% 10% 22% 20% 9%	130835
	Night	38% 6% 11% 22% 17% 7%	128170
Seeing	Day	10% 7% 20% 27% 24% 13%	15645
	Night	12% 5% 22% 36% 22% 3	24692

By Month

Percentage Cloud Cover NW (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	15% 19% 10% 16% 5% 5% 3 3 4 4 16%	3639
	11th to 20th	Day	13% 12% 3 9% 5% 5% 4 3 6% 7% 34%	3616
	21st to end	Day	17% 15% 5% 10% 4 4 3 3 4 5% 32%	4004
February	1st to 10th	Day	13% 14% 4 10% 6% 5% 4 4 6% 7% 27%	1710
	11th to 20th	Day	10% 12% 5% 10% 5% 4 4 4 5% 5% 36%	1808
	21st to end	Day	9% 12% 3 9% 6% 5% 5% 4 6% 6% 36%	1650
March	1st to 10th	Day	5% 7% 3 8% 6% 5% 5% 5% 8% 9% 40%	4518
	11th to 20th	Day	5% 7% 3 9% 5% 4 4 4 5% 7% 47%	4731
	21st to end	Day	8% 9% 3 9% 6% 6% 6% 5% 8% 9% 32%	5942
April	1st to 10th	Day	8% 9% 3 8% 5% 5% 5% 4 6% 8% 39%	2400
	11th to 20th	Day	5% 6% 2 6% 5% 5% 4 4 7% 9% 47%	2372
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 4 6% 8% 49%	2603
May	1st to 10th	Day	7% 5% 2 4 3 3 3 3 5% 8% 58%	6320
	11th to 20th	Day	6% 6% 2 6% 4 4 4 4 6% 8% 51%	6277
	21st to end	Day	5% 6% 2 6% 4 4 4 3 6% 7% 53%	6903
June	1st to 10th	Day	8% 6% 2 6% 5% 4 4 4 6% 8% 47%	2800
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 5% 8% 53%	2800
	21st to end	Day	3 3 2 4 3 3 3 3 5% 6% 64%	2800
July	1st to 10th	Day	2 1 1 2 2 2 2 2 3 4 79%	6320
	11th to 20th	Day	1 1 1 2 2 2 2 2 2 4 82%	6320
	21st to end	Day	2 2 1 2 1 2 2 2 3 6% 77%	6874
August	1st to 10th	Day	2 2 1 3 2 2 3 2 4 5% 75%	4541
	11th to 20th	Day	2 2 1 2 2 2 3 3 5% 6% 72%	4263
	21st to end	Day	4 4 2 5% 3 3 3 3 4 6% 65%	4312
September	1st to 10th	Day	3 2 1 2 2 2 3 3 5% 7% 70%	2400
	11th to 20th	Day	3 4 2 5% 4 4 4 3 6% 7% 57%	2355
	21st to end	Day	5% 5% 1 5% 4 4 4 3 6% 7% 58%	2200
October	1st to 10th	Day	5% 3 1 3 3 3 3 2 6% 9% 62%	3316
	11th to 20th	Day	5% 3 2 6% 6% 6% 6% 4 6% 8% 48%	3204
	21st to end	Day	9% 7% 3 8% 3 4 5% 5% 6% 7% 44%	3200
November	1st to 10th	Day	12% 10% 3 9% 6% 6% 5% 4 5% 5% 35%	1854
	11th to 20th	Day	12% 14% 5% 11% 6% 5% 4 3 6% 6% 29%	1711
	21st to end	Day	12% 15% 5% 12% 6% 6% 5% 4 5% 6% 25%	1680
December	1st to 10th	Day	10% 11% 7% 13% 8% 7% 6% 4 5% 6% 23%	2636
	11th to 20th	Day	17% 20% 9% 15% 6% 5% 3 3 4 4 14%	3025
	21st to end	Day	15% 18% 7% 15% 8% 6% 4 3 4 4 16%	4004
January	1st to 10th	Night	19% 19% 8% 13% 5% 5% 4 3 4 5% 14%	6306
	11th to 20th	Night	11% 9% 4 8% 4 4 4 4 5% 8% 38%	6282
	21st to end	Night	13% 16% 7% 11% 4 3 3 3 4 5% 32%	6952
February	1st to 10th	Night	11% 15% 4 9% 6% 5% 4 4 5% 6% 32%	2688
	11th to 20th	Night	10% 9% 4 8% 5% 5% 5% 4 6% 7% 36%	2478
	21st to end	Night	7% 11% 4 7% 6% 6% 5% 5% 7% 9% 34%	1980
March	1st to 10th	Night	5% 7% 3 7% 5% 5% 5% 5% 7% 10% 41%	5376
	11th to 20th	Night	7% 10% 3 6% 5% 4 4 3 5% 7% 47%	5265
	21st to end	Night	6% 9% 3 8% 7% 6% 7% 5% 9% 10% 31%	5033
April	1st to 10th	Night	8% 11% 3 8% 5% 5% 5% 4 7% 7% 39%	2000
	11th to 20th	Night	7% 7% 2 6% 5% 4 4 3 6% 6% 49%	1979
	21st to end	Night	6% 5% 3 7% 5% 5% 4 5% 8% 8% 45%	1792
May	1st to 10th	Night	4 4 1 5% 3 3 4 4 5% 7% 58%	3640
	11th to 20th	Night	5% 6% 3 6% 4 4 4 4 6% 6% 51%	3616
	21st to end	Night	5% 5% 2 5% 4 4 3 3 5% 8% 57%	3972
June	1st to 10th	Night	7% 5% 2 5% 5% 3 5% 4 6% 8% 51%	1600
	11th to 20th	Night	6% 4 2 4 4 4 5% 4 5% 7% 56%	1600
	21st to end	Night	4 4 2 4 3 4 4 4 5% 8% 60%	1600
July	1st to 10th	Night	2 2 1 1 1 1 1 2 3 5% 81%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 84%	3640
	21st to end	Night	2 2 1 1 2 2 2 2 3 4 79%	3969
August	1st to 10th	Night	3 2 1 3 1 2 2 2 3 5% 76%	2609
	11th to 20th	Night	2 3 1 3 3 2 4 3 5% 5% 68%	2788
	21st to end	Night	2 3 1 4 3 3 3 3 4 6% 67%	3608
September	1st to 10th	Night	2 2 1 3 2 3 3 3 5% 5% 72%	2000
	11th to 20th	Night	3 4 1 4 3 3 5% 3 6% 6% 60%	2045
	21st to end	Night	5% 4 1 4 3 3 3 3 6% 7% 60%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 3 4 5% 8% 63%	3882
	11th to 20th	Night	6% 4 2 5% 3 4 5% 4 6% 8% 52%	3951
	21st to end	Night	10% 7% 2 6% 4 5% 5% 4 5% 7% 46%	4600
November	1st to 10th	Night	14% 8% 2 7% 5% 5% 6% 3 6% 7% 36%	2665
	11th to 20th	Night	13% 9% 3 8% 6% 5% 5% 5% 6% 6% 34%	2904
	21st to end	Night	14% 11% 4 9% 6% 5% 5% 4 5% 7% 30%	2940
December	1st to 10th	Night	8% 8% 5% 8% 5% 6% 5% 5% 7% 13% 32%	4588
	11th to 20th	Night	18% 17% 8% 14% 6% 5% 5% 4 5% 5% 12%	5180
	21st to end	Night	17% 18% 5% 11% 6% 5% 5% 3 5% 6% 20%	6952

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	70% 7% 8% 9% 5%	3639
	11th to 20th	Day	49% 6% 18% 20% 6%	3640
	21st to end	Day	56% 5% 13% 19% 7%	4004
February	1st to 10th	Day	56% 8% 15% 14% 6% 1	1710
	11th to 20th	Day	50% 7% 15% 18% 10%	1817
	21st to end	Day	46% 8% 15% 21% 9% 1	1650
March	1st to 10th	Day	37% 6% 12% 23% 20% 1	4518
	11th to 20th	Day	37% 4 10% 33% 15%	4731
	21st to end	Day	45% 8% 16% 20% 10% 2	5942
April	1st to 10th	Day	42% 6% 14% 24% 13% 2	2400
	11th to 20th	Day	34% 6% 14% 24% 18% 5%	2372
	21st to end	Day	33% 5% 12% 26% 20% 4	2603
May	1st to 10th	Day	24% 3 6% 20% 37% 9%	6320
	11th to 20th	Day	30% 5% 10% 27% 23% 5%	6280
	21st to end	Day	28% 5% 13% 31% 21% 3	6904
June	1st to 10th	Day	35% 5% 9% 24% 22% 6%	2800
	11th to 20th	Day	28% 5% 11% 28% 22% 7%	2800
	21st to end	Day	21% 4 8% 30% 27% 10%	2800
July	1st to 10th	Day	12% 2 4 18% 30% 35%	6251
	11th to 20th	Day	9% 2 3 14% 32% 40%	6245
	21st to end	Day	11% 2 4 23% 37% 22%	6681
August	1st to 10th	Day	14% 2 5% 15% 36% 28%	4524
	11th to 20th	Day	13% 4 6% 35% 32% 10%	4305
	21st to end	Day	21% 3 9% 26% 32% 9%	4311
September	1st to 10th	Day	14% 4 9% 28% 31% 15%	2400
	11th to 20th	Day	24% 6% 12% 25% 25% 8%	2355
	21st to end	Day	25% 5% 14% 33% 18% 4	2200
October	1st to 10th	Day	21% 4 11% 38% 25% 2	3316
	11th to 20th	Day	33% 6% 18% 27% 16%	3204
	21st to end	Day	37% 6% 14% 27% 12% 4	3200
November	1st to 10th	Day	50% 7% 13% 19% 9% 1	1854
	11th to 20th	Day	55% 7% 15% 15% 6% 1	1711
	21st to end	Day	61% 7% 14% 12% 6% 1	1680
December	1st to 10th	Day	60% 9% 12% 15% 5%	2636
	11th to 20th	Day	75% 5% 8% 9% 3	3028
	21st to end	Day	68% 9% 10% 9% 3	4004
January	1st to 10th	Night	71% 8% 10% 9% 2	6306
	11th to 20th	Night	43% 7% 20% 22% 8%	6320
	21st to end	Night	55% 7% 12% 19% 7% 1	6952
February	1st to 10th	Night	52% 6% 17% 16% 8% 1	2688
	11th to 20th	Night	45% 8% 16% 20% 11% 1	2480
	21st to end	Night	44% 8% 15% 20% 12% 1	1980
March	1st to 10th	Night	35% 6% 10% 26% 21% 2	5376
	11th to 20th	Night	37% 4 9% 32% 17% 1	5266
	21st to end	Night	44% 9% 15% 25% 7%	5033
April	1st to 10th	Night	42% 6% 12% 25% 13% 2	2000
	11th to 20th	Night	35% 4 9% 27% 19% 6%	1979
	21st to end	Night	35% 6% 11% 24% 20% 5%	1792
May	1st to 10th	Night	24% 4 6% 18% 40% 8%	3637
	11th to 20th	Night	31% 5% 9% 27% 25% 3	3616
	21st to end	Night	24% 6% 13% 34% 21% 2	3972
June	1st to 10th	Night	31% 4 9% 25% 26% 6%	1600
	11th to 20th	Night	28% 5% 8% 28% 25% 6%	1600
	21st to end	Night	22% 5% 8% 24% 29% 12%	1600
July	1st to 10th	Night	9% 1 3 18% 29% 40%	3600
	11th to 20th	Night	7% 2 3 12% 30% 47%	3592
	21st to end	Night	10% 2 4 17% 36% 31%	3857
August	1st to 10th	Night	12% 2 3 9% 39% 34%	2601
	11th to 20th	Night	18% 4 5% 26% 35% 13%	2812
	21st to end	Night	17% 4 6% 26% 30% 18%	3608
September	1st to 10th	Night	14% 4 7% 26% 28% 22%	2000
	11th to 20th	Night	22% 6% 10% 24% 27% 12%	2045
	21st to end	Night	23% 4 11% 31% 22% 9%	2200
October	1st to 10th	Night	20% 3 8% 36% 31% 1	3882
	11th to 20th	Night	29% 5% 15% 26% 22% 3	3951
	21st to end	Night	37% 5% 9% 31% 16% 2	4600
November	1st to 10th	Night	45% 8% 14% 22% 9% 2	2665
	11th to 20th	Night	48% 8% 16% 19% 8% 1	2904
	21st to end	Night	53% 8% 16% 16% 7% 1	2940
December	1st to 10th	Night	43% 10% 19% 23% 4	4588
	11th to 20th	Night	73% 7% 11% 6% 3	5176
	21st to end	Night	62% 9% 12% 12% 5%	6952

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	25% 11% 18% 26% 19% 3	200
	11th to 20th	Day	25% 6% 16% 29% 18% 8%	200
	21st to end	Day	21% 6% 20% 25% 20% 7%	220
February	1st to 10th	Day	18% 10% 24% 27% 16% 5%	310
	11th to 20th	Day	18% 9% 27% 25% 15% 6%	422
	21st to end	Day	12% 13% 27% 26% 18% 5%	414
March	1st to 10th	Day	16% 8% 29% 25% 15% 6%	500
	11th to 20th	Day	13% 11% 21% 25% 21% 9%	500
	21st to end	Day	13% 12% 27% 28% 16% 4	550
April	1st to 10th	Day	13% 10% 27% 24% 17% 8%	500
	11th to 20th	Day	9% 13% 29% 26% 16% 8%	492
	21st to end	Day	5% 10% 25% 29% 21% 10%	500
May	1st to 10th	Day	8% 8% 20% 31% 24% 9%	503
	11th to 20th	Day	11% 8% 16% 29% 24% 12%	528
	21st to end	Day	8% 4 17% 25% 27% 19%	573
June	1st to 10th	Day	9% 7% 18% 24% 25% 17%	530
	11th to 20th	Day	6% 8% 27% 30% 20% 8%	530
	21st to end	Day	3 3 17% 28% 29% 20%	530
July	1st to 10th	Day	2 2 13% 33% 33% 17%	526
	11th to 20th	Day	1 2 16% 27% 36% 18%	524
	21st to end	Day	1 2 10% 25% 37% 23%	575
August	1st to 10th	Day	4 3 10% 24% 35% 24%	527
	11th to 20th	Day	2 2 9% 28% 33% 26%	517
	21st to end	Day	4 4 16% 25% 30% 21%	583
September	1st to 10th	Day	4 3 13% 29% 30% 22%	530
	11th to 20th	Day	5% 4 17% 25% 30% 18%	530
	21st to end	Day	8% 8% 18% 26% 28% 12%	530
October	1st to 10th	Day	7% 5% 23% 28% 24% 14%	530
	11th to 20th	Day	9% 8% 21% 23% 26% 12%	498
	21st to end	Day	15% 9% 22% 32% 17% 6%	363
November	1st to 10th	Day	17% 11% 20% 29% 20% 2	330
	11th to 20th	Day	19% 13% 22% 22% 17% 7%	236
	21st to end	Day	15% 5% 24% 30% 20% 6%	220
December	1st to 10th	Day	19% 5% 20% 27% 17% 12%	204
	11th to 20th	Day	22% 5% 29% 27% 15% 4	200
	21st to end	Day	22% 8% 22% 30% 17% 2	220
January	1st to 10th	Night	25% 8% 23% 30% 13% 2	800
	11th to 20th	Night	19% 4 22% 36% 17% 3	800
	21st to end	Night	17% 3 20% 36% 21% 3	880
February	1st to 10th	Night	18% 6% 27% 33% 15% 2	800
	11th to 20th	Night	13% 5% 25% 37% 18% 1	784
	21st to end	Night	13% 10% 24% 35% 16% 2	660
March	1st to 10th	Night	12% 7% 25% 37% 16% 2	800
	11th to 20th	Night	11% 7% 25% 35% 21% 2	800
	21st to end	Night	12% 8% 28% 35% 15% 1	880
April	1st to 10th	Night	13% 6% 25% 33% 20% 2	800
	11th to 20th	Night	9% 8% 31% 32% 17% 3	791
	21st to end	Night	7% 6% 26% 37% 21% 3	592
May	1st to 10th	Night	7% 6% 29% 37% 19% 3	400
	11th to 20th	Night	8% 6% 21% 32% 29% 4	394
	21st to end	Night	6% 3 20% 38% 27% 5%	433
June	1st to 10th	Night	10% 4 20% 39% 28% 1	400
	11th to 20th	Night	7% 9% 27% 38% 18% 2	400
	21st to end	Night	6% 2 22% 41% 25% 4	400
July	1st to 10th	Night	3 2 17% 43% 33% 2	390
	11th to 20th	Night	3 4 19% 41% 32% 2	388
	21st to end	Night	1 3 11% 41% 40% 4	438
August	1st to 10th	Night	4 2 13% 36% 40% 5%	398
	11th to 20th	Night	3 1 13% 36% 39% 9%	516
	21st to end	Night	3 3 16% 35% 37% 6%	880
September	1st to 10th	Night	3 2 16% 41% 32% 6%	800
	11th to 20th	Night	5% 3 15% 33% 37% 8%	800
	21st to end	Night	6% 5% 20% 35% 31% 5%	800
October	1st to 10th	Night	10% 4 19% 39% 26% 3	800
	11th to 20th	Night	10% 6% 22% 33% 24% 4	800
	21st to end	Night	13% 5% 25% 36% 18% 3	880
November	1st to 10th	Night	18% 7% 20% 42% 12% 1	820
	11th to 20th	Night	17% 7% 25% 33% 15% 3	840
	21st to end	Night	18% 5% 20% 35% 20% 3	840
December	1st to 10th	Night	16% 5% 18% 37% 19% 4	808
	11th to 20th	Night	23% 7% 19% 36% 14% 2	800
	21st to end	Night	24% 7% 22% 35% 11% 1	880

Page generated at 2025-11-29 22:00:54. Version ac07

Copyright © 2025 Attilla Danko.