The Dutch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Dutch Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	127086
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 45%	124289
Transparency	Day	39% 5% 11% 21% 18% 6%	130862
	Night	37% 5% 10% 20% 21% 7%	128582
Seeing	Day	16% 9% 21% 24% 20% 10%	16461
	Night	15% 8% 22% 30% 21% 4	26506

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	17% 8% 2 4 2 3 3 3 4 5% 49%	4065
	11th to 20th	Day	11% 7% 2 4 3 2 3 2 5% 6% 54%	4060
	21st to end	Day	22% 8% 3 6% 4 3 2 2 3 5% 41%	4618
February	1st to 10th	Day	15% 7% 3 8% 4 3 5% 4 6% 6% 40%	2000
	11th to 20th	Day	22% 6% 1 5% 3 3 4 3 4 5% 42%	1948
	21st to end	Day	18% 7% 3 6% 4 4 4 4 5% 6% 41%	1650
March	1st to 10th	Day	22% 9% 2 6% 4 4 4 3 4 5% 37%	4681
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 5% 7% 52%	4980
	21st to end	Day	16% 5% 2 7% 4 4 4 3 4 6% 46%	5465
April	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 5% 7% 48%	2210
	11th to 20th	Day	11% 4 2 7% 4 4 4 3 5% 7% 49%	2374
	21st to end	Day	9% 4 2 5% 3 4 4 4 6% 7% 51%	2578
May	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 4 6% 9% 39%	5862
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 5% 6% 9% 31%	5861
	21st to end	Day	12% 6% 2 6% 5% 4 5% 4 6% 8% 43%	6433
June	1st to 10th	Day	9% 6% 3 7% 6% 6% 5% 5% 8% 11% 36%	2600
	11th to 20th	Day	5% 4 1 4 4 6% 5% 6% 8% 12% 44%	2600
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 9% 10% 43%	2600
July	1st to 10th	Day	7% 6% 3 7% 6% 6% 7% 6% 9% 11% 31%	5900
	11th to 20th	Day	5% 4 2 7% 5% 6% 7% 6% 8% 12% 38%	5900
	21st to end	Day	8% 7% 3 8% 7% 7% 7% 7% 8% 11% 27%	6424
August	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 11% 34%	4234
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 5% 6% 11% 46%	3953
	21st to end	Day	8% 5% 1 5% 4 5% 5% 4 6% 9% 49%	4279
September	1st to 10th	Day	11% 6% 3 6% 5% 5% 4 5% 7% 10% 40%	2200
	11th to 20th	Day	15% 6% 2 5% 4 4 5% 3 6% 7% 43%	2200
	21st to end	Day	13% 6% 2 6% 3 4 4 3 5% 7% 47%	2200
October	1st to 10th	Day	11% 6% 2 5% 3 4 4 3 4 6% 55%	2200
	11th to 20th	Day	11% 4 2 4 3 3 3 3 4 7% 58%	2070
	21st to end	Day	12% 5% 2 4 3 3 3 3 4 5% 56%	2200
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 4 5% 55%	1913
	11th to 20th	Day	13% 5% 1 4 3 3 3 3 4 6% 54%	1890
	21st to end	Day	12% 6% 2 5% 4 3 4 4 5% 7% 48%	1890
December	1st to 10th	Day	17% 7% 2 5% 4 3 4 3 5% 6% 46%	3020
	11th to 20th	Day	17% 8% 2 6% 4 4 4 3 4 5% 44%	3463
	21st to end	Day	22% 7% 2 4 3 3 4 3 4 7% 41%	4565
January	1st to 10th	Night	14% 5% 2 4 3 4 3 3 5% 6% 51%	5903
	11th to 20th	Night	12% 7% 2 5% 4 3 4 3 5% 6% 48%	5900
	21st to end	Night	18% 9% 2 5% 3 4 3 3 4 6% 43%	6334
February	1st to 10th	Night	15% 6% 2 6% 5% 4 4 3 4 6% 44%	2400
	11th to 20th	Night	20% 7% 3 6% 3 3 3 3 4 5% 43%	2338
	21st to end	Night	18% 6% 2 6% 4 4 4 3 4 6% 43%	1980
March	1st to 10th	Night	21% 7% 2 6% 4 4 4 3 5% 6% 37%	5171
	11th to 20th	Night	10% 5% 1 4 3 3 4 4 5% 8% 53%	4980
	21st to end	Night	15% 5% 2 6% 4 4 4 3 5% 6% 46%	5467
April	1st to 10th	Night	11% 5% 1 5% 4 4 4 4 6% 8% 48%	2200
	11th to 20th	Night	13% 5% 2 5% 3 4 2 4 5% 7% 51%	1980
	21st to end	Night	8% 5% 2 5% 5% 4 4 4 5% 7% 51%	1820
May	1st to 10th	Night	10% 6% 2 8% 6% 5% 5% 4 7% 9% 38%	4033
	11th to 20th	Night	18% 6% 2 7% 5% 5% 5% 4 6% 7% 34%	4032
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	4421
June	1st to 10th	Night	7% 5% 2 8% 5% 5% 5% 5% 8% 11% 39%	1800
	11th to 20th	Night	5% 5% 1 5% 5% 4 5% 5% 7% 13% 44%	1800
	21st to end	Night	5% 3 2 6% 5% 6% 6% 5% 8% 12% 41%	1800
July	1st to 10th	Night	9% 6% 3 8% 6% 6% 7% 5% 8% 11% 32%	4058
	11th to 20th	Night	5% 5% 2 7% 5% 5% 7% 6% 9% 15% 34%	4057
	21st to end	Night	10% 7% 3 8% 6% 5% 6% 5% 8% 11% 29%	4427
August	1st to 10th	Night	9% 5% 2 7% 5% 6% 6% 5% 9% 13% 32%	2915
	11th to 20th	Night	8% 4 2 5% 4 5% 5% 4 6% 10% 46%	3092
	21st to end	Night	8% 5% 2 5% 4 4 4 4 5% 8% 52%	3586
September	1st to 10th	Night	8% 6% 2 6% 5% 5% 7% 5% 7% 9% 41%	2200
	11th to 20th	Night	12% 5% 3 6% 3 5% 5% 3 6% 8% 44%	2200
	21st to end	Night	11% 6% 2 6% 4 3 5% 4 6% 8% 46%	2200
October	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 52%	2200
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 4 60%	2329
	21st to end	Night	12% 4 1 4 3 3 4 3 4 5% 57%	2640
November	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 5% 55%	2601
	11th to 20th	Night	14% 5% 2 4 3 2 3 2 4 5% 55%	2730
	21st to end	Night	14% 6% 2 4 4 4 4 3 5% 6% 49%	2730
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 2 4 5% 52%	4380
	11th to 20th	Night	17% 5% 2 5% 3 3 3 3 5% 6% 47%	4952
	21st to end	Night	21% 7% 2 6% 3 4 4 3 5% 5% 42%	6633

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	39% 3 4 22% 24% 8%	4066
	11th to 20th	Day	33% 3 4 22% 29% 9%	4060
	21st to end	Day	48% 2 4 16% 21% 9%	4618
February	1st to 10th	Day	44% 4 9% 19% 19% 5%	2000
	11th to 20th	Day	46% 4 5% 19% 21% 6%	1958
	21st to end	Day	44% 4 7% 17% 20% 8%	1650
March	1st to 10th	Day	51% 3 6% 11% 22% 7%	4681
	11th to 20th	Day	32% 4 6% 15% 24% 18%	4980
	21st to end	Day	43% 3 6% 21% 21% 7%	5465
April	1st to 10th	Day	36% 5% 9% 23% 20% 7%	2210
	11th to 20th	Day	34% 4 8% 21% 24% 9%	2374
	21st to end	Day	31% 6% 10% 26% 24% 3	2578
May	1st to 10th	Day	42% 6% 14% 22% 15% 1	5897
	11th to 20th	Day	49% 5% 16% 20% 9% 1	5865
	21st to end	Day	39% 6% 12% 23% 19% 2	6447
June	1st to 10th	Day	41% 8% 16% 23% 11% 2	2600
	11th to 20th	Day	31% 9% 20% 24% 15% 1	2600
	21st to end	Day	34% 9% 19% 22% 13% 3	2600
July	1st to 10th	Day	45% 7% 18% 23% 6% 1	5835
	11th to 20th	Day	38% 7% 28% 23% 4 1	5831
	21st to end	Day	50% 9% 17% 20% 4	6240
August	1st to 10th	Day	42% 6% 16% 30% 6%	4221
	11th to 20th	Day	33% 6% 16% 28% 14% 2	3992
	21st to end	Day	32% 5% 16% 24% 19% 4	4279
September	1st to 10th	Day	39% 8% 14% 17% 18% 4	2200
	11th to 20th	Day	40% 6% 10% 21% 21% 2	2200
	21st to end	Day	38% 4 10% 21% 22% 6%	2200
October	1st to 10th	Day	27% 3 6% 14% 37% 13%	3590
	11th to 20th	Day	18% 3 4 17% 30% 28%	3349
	21st to end	Day	25% 2 4 22% 34% 12%	3538
November	1st to 10th	Day	33% 4 7% 16% 30% 10%	1913
	11th to 20th	Day	33% 3 6% 21% 26% 11%	1890
	21st to end	Day	35% 5% 8% 19% 26% 6%	1890
December	1st to 10th	Day	42% 2 4 18% 26% 7%	3020
	11th to 20th	Day	43% 3 8% 19% 20% 7%	3460
	21st to end	Day	43% 4 8% 23% 20% 3	4565
January	1st to 10th	Night	35% 3 4 22% 29% 7%	5903
	11th to 20th	Night	37% 3 4 21% 27% 9%	5900
	21st to end	Night	45% 2 4 15% 28% 6%	6334
February	1st to 10th	Night	42% 4 7% 19% 19% 10%	2400
	11th to 20th	Night	45% 3 5% 18% 21% 9%	2339
	21st to end	Night	42% 4 6% 16% 19% 12%	1980
March	1st to 10th	Night	49% 3 7% 14% 18% 9%	5171
	11th to 20th	Night	30% 3 7% 15% 26% 19%	4980
	21st to end	Night	39% 3 5% 20% 21% 11%	5467
April	1st to 10th	Night	33% 4 10% 21% 22% 9%	2200
	11th to 20th	Night	33% 4 6% 23% 24% 11%	1980
	21st to end	Night	31% 5% 11% 25% 22% 6%	1820
May	1st to 10th	Night	40% 6% 17% 21% 14% 2	4060
	11th to 20th	Night	49% 5% 14% 21% 10% 1	4033
	21st to end	Night	34% 6% 15% 24% 19% 3	4429
June	1st to 10th	Night	37% 11% 17% 21% 13% 2	1800
	11th to 20th	Night	30% 8% 19% 28% 12% 2	1800
	21st to end	Night	33% 10% 20% 19% 15% 4	1800
July	1st to 10th	Night	45% 8% 20% 21% 5% 1	4015
	11th to 20th	Night	38% 9% 28% 19% 5% 1	4006
	21st to end	Night	48% 9% 21% 17% 4 1	4297
August	1st to 10th	Night	43% 7% 17% 28% 5%	2904
	11th to 20th	Night	34% 6% 18% 27% 14% 1	3119
	21st to end	Night	31% 6% 14% 23% 23% 2	3586
September	1st to 10th	Night	37% 9% 15% 18% 16% 4	2200
	11th to 20th	Night	38% 8% 12% 21% 19% 4	2200
	21st to end	Night	35% 6% 13% 18% 21% 7%	2200
October	1st to 10th	Night	31% 3 8% 13% 33% 13%	3590
	11th to 20th	Night	18% 2 4 16% 29% 30%	3789
	21st to end	Night	24% 2 3 22% 31% 18%	4252
November	1st to 10th	Night	32% 3 8% 17% 27% 13%	2601
	11th to 20th	Night	32% 3 6% 18% 28% 13%	2730
	21st to end	Night	36% 4 7% 19% 26% 9%	2730
December	1st to 10th	Night	36% 3 5% 19% 29% 8%	4380
	11th to 20th	Night	38% 3 11% 17% 23% 7%	4954
	21st to end	Night	45% 3 5% 26% 19% 2	6633

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% 22% 38% 13% 2	303
	11th to 20th	Day	23% 22% 30% 16% 8% 1	300
	21st to end	Day	23% 23% 36% 13% 5% 1	330
February	1st to 10th	Day	20% 19% 36% 20% 5%	300
	11th to 20th	Day	38% 16% 29% 14% 3	300
	21st to end	Day	26% 16% 38% 18% 2	249
March	1st to 10th	Day	22% 17% 31% 22% 8% 1	384
	11th to 20th	Day	21% 15% 33% 24% 7%	500
	21st to end	Day	20% 16% 30% 26% 7%	550
April	1st to 10th	Day	17% 17% 26% 27% 12% 1	500
	11th to 20th	Day	13% 14% 31% 21% 17% 3	492
	21st to end	Day	14% 10% 24% 28% 19% 4	500
May	1st to 10th	Day	13% 11% 28% 26% 17% 6%	503
	11th to 20th	Day	15% 5% 23% 24% 22% 11%	527
	21st to end	Day	15% 5% 17% 26% 23% 14%	573
June	1st to 10th	Day	11% 3 17% 26% 29% 14%	530
	11th to 20th	Day	8% 2 14% 28% 24% 23%	530
	21st to end	Day	8% 4 12% 29% 28% 18%	530
July	1st to 10th	Day	7% 3 8% 21% 34% 27%	525
	11th to 20th	Day	7% 2 6% 20% 34% 31%	524
	21st to end	Day	6% 2 5% 23% 40% 24%	576
August	1st to 10th	Day	14% 3 14% 22% 23% 23%	526
	11th to 20th	Day	10% 3 9% 19% 36% 23%	519
	21st to end	Day	13% 3 8% 26% 30% 20%	583
September	1st to 10th	Day	10% 4 14% 30% 26% 17%	530
	11th to 20th	Day	15% 4 11% 23% 33% 14%	530
	21st to end	Day	15% 3 14% 27% 32% 9%	530
October	1st to 10th	Day	15% 4 20% 33% 19% 8%	530
	11th to 20th	Day	12% 8% 22% 28% 23% 7%	530
	21st to end	Day	16% 8% 24% 27% 21% 4	583
November	1st to 10th	Day	19% 7% 27% 26% 18% 3	391
	11th to 20th	Day	16% 13% 38% 24% 6% 2	330
	21st to end	Day	15% 16% 32% 24% 12% 2	330
December	1st to 10th	Day	31% 16% 30% 16% 6% 1	330
	11th to 20th	Day	26% 15% 27% 26% 5% 1	330
	21st to end	Day	28% 16% 31% 19% 5% 2	363
January	1st to 10th	Night	23% 18% 30% 24% 5% 1	805
	11th to 20th	Night	20% 15% 31% 25% 9%	800
	21st to end	Night	21% 14% 36% 25% 3	880
February	1st to 10th	Night	19% 11% 31% 34% 5%	800
	11th to 20th	Night	24% 16% 32% 25% 3	783
	21st to end	Night	20% 15% 36% 26% 3	660
March	1st to 10th	Night	18% 13% 30% 34% 6%	800
	11th to 20th	Night	18% 10% 36% 27% 9%	800
	21st to end	Night	16% 13% 30% 31% 10%	880
April	1st to 10th	Night	14% 12% 32% 30% 12%	800
	11th to 20th	Night	16% 10% 28% 30% 16% 1	598
	21st to end	Night	10% 8% 21% 39% 21% 1	600
May	1st to 10th	Night	11% 5% 27% 36% 21% 1	600
	11th to 20th	Night	12% 4 21% 35% 24% 5%	591
	21st to end	Night	13% 3 13% 32% 33% 6%	650
June	1st to 10th	Night	8% 3 12% 37% 35% 6%	600
	11th to 20th	Night	6% 2 11% 31% 38% 13%	600
	21st to end	Night	7% 1 8% 34% 41% 9%	600
July	1st to 10th	Night	10% 2 5% 23% 47% 12%	585
	11th to 20th	Night	7% 3 4 21% 49% 16%	582
	21st to end	Night	7% 1 5% 28% 47% 11%	657
August	1st to 10th	Night	10% 2 5% 30% 42% 10%	597
	11th to 20th	Night	10% 1 8% 25% 46% 10%	594
	21st to end	Night	13% 4 7% 26% 41% 9%	660
September	1st to 10th	Night	9% 3 11% 28% 39% 11%	800
	11th to 20th	Night	13% 3 10% 28% 40% 6%	800
	21st to end	Night	14% 3 12% 33% 35% 3	800
October	1st to 10th	Night	14% 3 16% 36% 25% 6%	800
	11th to 20th	Night	12% 6% 23% 33% 24% 3	800
	21st to end	Night	14% 7% 19% 35% 21% 3	880
November	1st to 10th	Night	15% 6% 21% 34% 21% 1	820
	11th to 20th	Night	17% 10% 27% 32% 13% 1	840
	21st to end	Night	16% 11% 27% 31% 14% 1	840
December	1st to 10th	Night	20% 11% 28% 30% 10%	840
	11th to 20th	Night	20% 10% 29% 30% 10%	840
	21st to end	Night	26% 11% 25% 30% 7%	924

Page generated at 2025-12-30 01:14:08. Version ac07

Copyright © 2025 Attilla Danko.