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 2024-11-12.

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% 42%	101066
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 44%	98075
Transparency	Day	38% 6% 11% 21% 19% 6%	104886
	Night	37% 5% 10% 20% 20% 8%	102394
Seeing	Day	15% 9% 21% 24% 20% 11%	14971
	Night	15% 8% 21% 30% 22% 4	24985

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	18% 8% 2 5% 3 3 4 4 5% 6% 39%	2836
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 6% 5% 53%	2840
	21st to end	Day	26% 8% 2 5% 3 2 2 2 4 5% 41%	3220
February	1st to 10th	Day	16% 7% 3 8% 4 3 5% 4 5% 6% 39%	1900
	11th to 20th	Day	21% 7% 1 5% 3 3 4 3 4 5% 42%	1848
	21st to end	Day	18% 7% 3 6% 4 4 4 4 4 6% 40%	1570
March	1st to 10th	Day	24% 11% 3 6% 4 3 3 2 4 5% 36%	3265
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 6% 7% 48%	3480
	21st to end	Day	19% 5% 2 6% 4 4 4 3 4 7% 42%	3815
April	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 5% 6% 48%	2099
	11th to 20th	Day	11% 4 2 7% 4 4 4 3 5% 8% 48%	2254
	21st to end	Day	9% 4 1 5% 3 4 4 4 6% 8% 52%	2449
May	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 5% 10% 44%	4082
	11th to 20th	Day	17% 7% 2 7% 4 4 4 4 5% 8% 38%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 9% 47%	4476
June	1st to 10th	Day	9% 6% 2 7% 6% 6% 5% 5% 8% 10% 36%	2470
	11th to 20th	Day	5% 4 1 4 4 5% 5% 6% 8% 12% 45%	2470
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 9% 10% 42%	2470
July	1st to 10th	Day	9% 6% 3 8% 6% 7% 8% 7% 9% 12% 26%	4120
	11th to 20th	Day	7% 5% 2 8% 6% 6% 7% 6% 8% 12% 35%	4120
	21st to end	Day	9% 9% 3 9% 7% 7% 8% 7% 8% 11% 21%	4466
August	1st to 10th	Day	9% 6% 2 6% 5% 6% 6% 5% 9% 12% 35%	4104
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 5% 6% 11% 46%	3830
	21st to end	Day	7% 4 1 5% 4 5% 5% 4 6% 8% 50%	4148
September	1st to 10th	Day	11% 6% 3 6% 5% 5% 4 5% 6% 9% 39%	2090
	11th to 20th	Day	15% 7% 2 5% 4 4 5% 3 6% 8% 41%	2090
	21st to end	Day	14% 5% 2 6% 3 4 4 3 5% 7% 48%	2090
October	1st to 10th	Day	11% 6% 2 4 3 3 3 3 4 6% 56%	2090
	11th to 20th	Day	11% 4 2 4 3 3 3 3 4 7% 57%	1967
	21st to end	Day	11% 5% 2 4 3 3 3 3 4 5% 56%	2090
November	1st to 10th	Day	13% 5% 2 5% 4 3 3 3 4 5% 54%	1820
	11th to 20th	Day	13% 5% 1 5% 3 3 3 3 5% 6% 52%	1719
	21st to end	Day	13% 6% 3 5% 4 3 4 4 5% 7% 47%	1710
December	1st to 10th	Day	18% 9% 2 6% 5% 4 4 3 5% 6% 38%	1710
	11th to 20th	Day	19% 6% 2 6% 4 4 3 3 4 5% 44%	2153
	21st to end	Day	22% 8% 2 5% 3 3 4 3 5% 8% 37%	3124
January	1st to 10th	Night	18% 6% 3 5% 3 4 4 3 5% 7% 42%	4109
	11th to 20th	Night	11% 7% 3 5% 4 4 4 4 5% 6% 47%	4120
	21st to end	Night	23% 9% 3 6% 3 3 3 2 3 4 40%	4432
February	1st to 10th	Night	15% 7% 2 6% 5% 4 4 3 5% 6% 43%	2280
	11th to 20th	Night	19% 7% 3 6% 3 3 3 3 4 5% 44%	2218
	21st to end	Night	19% 6% 2 6% 4 4 4 3 4 6% 42%	1884
March	1st to 10th	Night	23% 8% 2 6% 4 4 3 3 4 5% 37%	3587
	11th to 20th	Night	11% 5% 2 4 3 3 4 4 5% 8% 50%	3480
	21st to end	Night	17% 6% 2 6% 4 4 4 3 5% 6% 43%	3817
April	1st to 10th	Night	11% 5% 1 5% 4 4 4 4 6% 8% 48%	2090
	11th to 20th	Night	13% 5% 2 5% 4 4 2 4 5% 7% 50%	1880
	21st to end	Night	8% 4 2 4 4 4 4 4 5% 8% 53%	1729
May	1st to 10th	Night	9% 5% 2 8% 6% 5% 5% 4 6% 9% 42%	2813
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 3 6% 6% 42%	2812
	21st to end	Night	7% 4 2 4 3 4 5% 4 7% 11% 50%	3079
June	1st to 10th	Night	7% 5% 2 8% 5% 5% 6% 5% 8% 11% 38%	1710
	11th to 20th	Night	5% 4 1 5% 5% 4 5% 5% 7% 14% 45%	1710
	21st to end	Night	5% 4 2 6% 6% 6% 6% 5% 8% 11% 40%	1710
July	1st to 10th	Night	10% 6% 2 8% 7% 6% 6% 5% 7% 11% 31%	2840
	11th to 20th	Night	6% 5% 2 7% 6% 5% 8% 6% 9% 12% 33%	2840
	21st to end	Night	12% 7% 3 8% 7% 6% 7% 6% 9% 12% 23%	3087
August	1st to 10th	Night	8% 5% 2 7% 5% 6% 6% 5% 9% 13% 33%	2825
	11th to 20th	Night	8% 4 2 5% 4 5% 5% 4 6% 10% 46%	2995
	21st to end	Night	7% 4 2 5% 4 4 4 4 5% 8% 53%	3476
September	1st to 10th	Night	8% 6% 2 6% 5% 5% 6% 5% 7% 9% 41%	2090
	11th to 20th	Night	13% 6% 3 6% 4 4 5% 3 7% 7% 43%	2090
	21st to end	Night	11% 5% 2 6% 4 3 4 4 6% 7% 47%	2090
October	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2090
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 4 4 59%	2212
	21st to end	Night	11% 4 1 4 3 3 4 3 4 5% 58%	2508
November	1st to 10th	Night	13% 6% 2 4 3 3 3 2 4 6% 55%	2474
	11th to 20th	Night	15% 5% 2 4 3 2 3 2 4 5% 54%	2484
	21st to end	Night	14% 6% 1 4 4 4 4 3 5% 6% 48%	2470
December	1st to 10th	Night	17% 7% 2 5% 3 4 4 3 5% 6% 43%	2470
	11th to 20th	Night	17% 5% 2 5% 3 3 3 2 4 6% 49%	3042
	21st to end	Night	19% 9% 2 7% 4 4 4 4 6% 6% 35%	4532

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	44% 5% 6% 18% 21% 6%	2837
	11th to 20th	Day	32% 4 6% 26% 25% 8%	2840
	21st to end	Day	48% 3 4 16% 22% 8%	3220
February	1st to 10th	Day	45% 4 9% 18% 19% 5%	1900
	11th to 20th	Day	45% 4 5% 18% 22% 6%	1858
	21st to end	Day	46% 4 7% 16% 19% 8%	1570
March	1st to 10th	Day	51% 4 8% 13% 15% 8%	3265
	11th to 20th	Day	35% 6% 8% 18% 25% 9%	3480
	21st to end	Day	44% 3 8% 21% 18% 6%	3815
April	1st to 10th	Day	36% 5% 9% 22% 21% 7%	2099
	11th to 20th	Day	36% 5% 8% 19% 23% 9%	2254
	21st to end	Day	30% 6% 10% 27% 25% 3	2449
May	1st to 10th	Day	36% 7% 13% 24% 19% 2	4120
	11th to 20th	Day	44% 6% 11% 24% 13% 1	4085
	21st to end	Day	31% 7% 14% 25% 21% 2	4489
June	1st to 10th	Day	41% 9% 16% 22% 10% 2	2470
	11th to 20th	Day	29% 9% 20% 25% 15% 1	2470
	21st to end	Day	34% 9% 19% 22% 13% 3	2470
July	1st to 10th	Day	48% 8% 20% 18% 5% 1	4055
	11th to 20th	Day	41% 9% 21% 23% 5% 1	4051
	21st to end	Day	54% 13% 15% 14% 4	4324
August	1st to 10th	Day	41% 7% 16% 31% 6%	4091
	11th to 20th	Day	33% 6% 16% 28% 15% 2	3869
	21st to end	Day	32% 5% 16% 25% 19% 4	4148
September	1st to 10th	Day	40% 8% 14% 16% 18% 4	2090
	11th to 20th	Day	41% 6% 10% 19% 21% 2	2090
	21st to end	Day	38% 4 9% 22% 22% 6%	2090
October	1st to 10th	Day	27% 3 6% 14% 37% 13%	3480
	11th to 20th	Day	18% 3 4 17% 29% 29%	3246
	21st to end	Day	25% 2 4 22% 34% 12%	3428
November	1st to 10th	Day	34% 4 7% 16% 30% 10%	1820
	11th to 20th	Day	34% 3 6% 20% 26% 11%	1719
	21st to end	Day	36% 5% 8% 19% 25% 7%	1710
December	1st to 10th	Day	47% 4 6% 19% 17% 6%	1710
	11th to 20th	Day	43% 3 5% 16% 25% 8%	2150
	21st to end	Day	44% 6% 9% 18% 20% 3	3124
January	1st to 10th	Night	42% 4 5% 17% 25% 7%	4109
	11th to 20th	Night	37% 5% 5% 24% 21% 9%	4120
	21st to end	Night	50% 3 3 12% 25% 7%	4432
February	1st to 10th	Night	42% 4 7% 18% 19% 10%	2280
	11th to 20th	Night	44% 4 5% 18% 21% 9%	2219
	21st to end	Night	44% 4 7% 16% 18% 11%	1884
March	1st to 10th	Night	49% 4 9% 15% 15% 8%	3587
	11th to 20th	Night	31% 4 9% 16% 27% 12%	3480
	21st to end	Night	42% 4 6% 19% 19% 10%	3817
April	1st to 10th	Night	33% 4 10% 20% 23% 9%	2090
	11th to 20th	Night	34% 4 6% 22% 23% 11%	1880
	21st to end	Night	29% 5% 11% 26% 23% 6%	1729
May	1st to 10th	Night	36% 8% 14% 22% 18% 3	2840
	11th to 20th	Night	41% 6% 12% 24% 15% 1	2813
	21st to end	Night	26% 7% 16% 27% 21% 4	3087
June	1st to 10th	Night	37% 11% 18% 21% 12% 2	1710
	11th to 20th	Night	29% 8% 19% 29% 13% 3	1710
	21st to end	Night	33% 10% 20% 19% 15% 4	1710
July	1st to 10th	Night	46% 10% 21% 17% 5% 1	2795
	11th to 20th	Night	39% 10% 22% 20% 7% 1	2786
	21st to end	Night	51% 12% 18% 14% 4 1	2988
August	1st to 10th	Night	42% 7% 17% 29% 5%	2814
	11th to 20th	Night	33% 6% 17% 28% 14% 1	3022
	21st to end	Night	31% 6% 14% 23% 24% 2	3476
September	1st to 10th	Night	36% 10% 15% 18% 17% 4	2090
	11th to 20th	Night	38% 8% 12% 20% 19% 4	2090
	21st to end	Night	34% 6% 13% 18% 21% 8%	2090
October	1st to 10th	Night	30% 3 7% 13% 34% 13%	3480
	11th to 20th	Night	18% 2 4 16% 29% 31%	3672
	21st to end	Night	23% 2 3 22% 31% 19%	4120
November	1st to 10th	Night	32% 4 8% 17% 26% 13%	2474
	11th to 20th	Night	33% 3 6% 18% 26% 14%	2484
	21st to end	Night	37% 4 7% 19% 25% 8%	2470
December	1st to 10th	Night	41% 5% 8% 20% 20% 6%	2470
	11th to 20th	Night	37% 4 7% 17% 28% 7%	3044
	21st to end	Night	49% 5% 7% 20% 17% 3	4532

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	23% 21% 39% 15% 2	270
	11th to 20th	Day	23% 21% 31% 17% 8% 1	270
	21st to end	Day	21% 24% 37% 13% 5% 1	297
February	1st to 10th	Day	22% 20% 34% 19% 5%	270
	11th to 20th	Day	36% 16% 30% 15% 3	270
	21st to end	Day	28% 17% 38% 15% 2	225
March	1st to 10th	Day	20% 16% 32% 23% 8% 1	350
	11th to 20th	Day	20% 15% 33% 23% 8%	460
	21st to end	Day	19% 16% 31% 26% 8%	506
April	1st to 10th	Day	18% 17% 26% 26% 13% 1	460
	11th to 20th	Day	13% 15% 30% 21% 18% 3	452
	21st to end	Day	13% 11% 24% 28% 19% 4	460
May	1st to 10th	Day	12% 11% 27% 26% 18% 7%	463
	11th to 20th	Day	14% 5% 21% 24% 23% 12%	487
	21st to end	Day	13% 4 16% 27% 25% 15%	529
June	1st to 10th	Day	11% 2 16% 26% 30% 14%	490
	11th to 20th	Day	8% 3 14% 26% 24% 25%	490
	21st to end	Day	9% 4 12% 29% 27% 18%	490
July	1st to 10th	Day	8% 3 8% 20% 34% 28%	485
	11th to 20th	Day	8% 2 7% 20% 31% 33%	484
	21st to end	Day	6% 2 5% 21% 39% 26%	532
August	1st to 10th	Day	12% 4 15% 21% 24% 24%	486
	11th to 20th	Day	10% 3 9% 20% 35% 23%	479
	21st to end	Day	13% 3 7% 25% 31% 20%	539
September	1st to 10th	Day	10% 4 13% 28% 27% 18%	490
	11th to 20th	Day	17% 4 12% 21% 32% 14%	490
	21st to end	Day	15% 4 14% 26% 32% 9%	490
October	1st to 10th	Day	15% 4 19% 32% 21% 9%	490
	11th to 20th	Day	12% 9% 22% 27% 22% 8%	490
	21st to end	Day	15% 8% 22% 28% 23% 5%	539
November	1st to 10th	Day	20% 6% 26% 26% 19% 3	358
	11th to 20th	Day	17% 14% 35% 26% 6% 2	273
	21st to end	Day	13% 14% 32% 25% 13% 3	270
December	1st to 10th	Day	29% 17% 28% 18% 7% 1	270
	11th to 20th	Day	26% 16% 28% 24% 6% 1	270
	21st to end	Day	29% 17% 30% 18% 5% 1	297
January	1st to 10th	Night	23% 16% 30% 25% 5% 1	760
	11th to 20th	Night	20% 16% 30% 26% 9%	760
	21st to end	Night	22% 14% 36% 25% 4	836
February	1st to 10th	Night	19% 12% 30% 34% 6%	760
	11th to 20th	Night	23% 16% 32% 26% 3	743
	21st to end	Night	21% 16% 36% 25% 3	628
March	1st to 10th	Night	17% 12% 30% 34% 6%	760
	11th to 20th	Night	18% 10% 36% 26% 9%	760
	21st to end	Night	16% 13% 29% 31% 10%	836
April	1st to 10th	Night	14% 12% 31% 31% 12%	760
	11th to 20th	Night	16% 10% 28% 28% 17% 1	568
	21st to end	Night	10% 8% 22% 38% 21% 1	570
May	1st to 10th	Night	11% 5% 27% 34% 22% 1	570
	11th to 20th	Night	11% 4 20% 35% 25% 5%	561
	21st to end	Night	12% 2 12% 33% 35% 7%	617
June	1st to 10th	Night	8% 3 11% 37% 36% 6%	570
	11th to 20th	Night	5% 2 11% 30% 37% 14%	570
	21st to end	Night	7% 1 8% 35% 40% 9%	570
July	1st to 10th	Night	10% 2 5% 23% 47% 13%	555
	11th to 20th	Night	7% 3 5% 21% 48% 16%	552
	21st to end	Night	7% 1 6% 29% 46% 11%	624
August	1st to 10th	Night	8% 2 5% 31% 43% 11%	567
	11th to 20th	Night	9% 1 9% 26% 46% 9%	564
	21st to end	Night	13% 4 7% 25% 43% 9%	627
September	1st to 10th	Night	9% 3 11% 26% 41% 11%	760
	11th to 20th	Night	14% 3 10% 28% 39% 6%	760
	21st to end	Night	13% 3 13% 33% 35% 3	760
October	1st to 10th	Night	14% 3 16% 35% 25% 7%	760
	11th to 20th	Night	12% 6% 23% 32% 23% 3	760
	21st to end	Night	14% 7% 19% 35% 22% 3	836
November	1st to 10th	Night	15% 6% 22% 33% 22% 2	780
	11th to 20th	Night	18% 10% 26% 32% 14% 1	765
	21st to end	Night	16% 10% 26% 32% 14% 1	760
December	1st to 10th	Night	19% 12% 27% 30% 11% 1	760
	11th to 20th	Night	20% 11% 29% 30% 10% 1	760
	21st to end	Night	26% 12% 26% 28% 7%	836

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