Nederland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nederland Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	4 6% 2 6% 4 4 5% 4 6% 8% 52%	158217
	Night	2 5% 2 5% 3 3 4 3 5% 8% 60%	151126
Transparency	Day	28% 7% 12% 22% 21% 10%	153158
	Night	22% 6% 13% 25% 25% 9%	147206
Seeing	Day	11% 18% 37% 23% 9% 2	18275
	Night	9% 17% 39% 24% 10% 2	28214

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	1 5% 4 9% 4 3 2 2 4 5% 61%	4720
	11th to 20th	Day	2 1 5% 3 4 3 2 4 7% 68%	4954
	21st to end	Day	4 2 5% 3 4 4 3 5% 7% 63%	5844
February	1st to 10th	Day	4 3 8% 4 4 3 3 6% 9% 56%	1890
	11th to 20th	Day	4 2 5% 4 3 4 3 7% 10% 58%	1990
	21st to end	Day	1 4 4 6% 3 3 3 3 5% 9% 58%	1730
March	1st to 10th	Day	1 7% 4 8% 4 3 3 3 4 7% 55%	4600
	11th to 20th	Day	1 10% 6% 9% 5% 4 3 3 4 8% 46%	5110
	21st to end	Day	2 9% 3 6% 3 4 5% 4 7% 11% 48%	5910
April	1st to 10th	Day	3 7% 3 5% 4 4 5% 3 7% 9% 50%	2520
	11th to 20th	Day	6% 12% 4 8% 3 4 4 3 5% 8% 43%	2491
	21st to end	Day	6% 10% 4 6% 4 4 5% 6% 7% 7% 40%	2520
May	1st to 10th	Day	7% 13% 4 7% 4 4 4 4 6% 8% 40%	7481
	11th to 20th	Day	8% 9% 2 5% 4 5% 5% 4 7% 8% 44%	8191
	21st to end	Day	6% 6% 2 6% 4 4 6% 5% 7% 9% 45%	9026
June	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 6% 8% 9% 45%	2940
	11th to 20th	Day	6% 5% 1 4 3 4 6% 4 7% 8% 50%	2940
	21st to end	Day	5% 5% 1 5% 5% 5% 6% 5% 8% 9% 48%	2940
July	1st to 10th	Day	4 3 1 4 4 5% 6% 5% 7% 9% 53%	8249
	11th to 20th	Day	3 2 1 4 4 5% 6% 5% 8% 10% 51%	8250
	21st to end	Day	5% 4 2 7% 6% 5% 7% 6% 8% 9% 41%	8724
August	1st to 10th	Day	4 4 2 5% 3 4 5% 5% 7% 10% 49%	7549
	11th to 20th	Day	4 3 1 5% 4 4 6% 5% 7% 10% 49%	7084
	21st to end	Day	6% 5% 2 7% 5% 6% 6% 5% 8% 9% 39%	7751
September	1st to 10th	Day	5% 5% 1 5% 4 3 4 4 6% 7% 54%	2520
	11th to 20th	Day	6% 5% 2 5% 4 4 4 4 6% 7% 55%	2517
	21st to end	Day	7% 6% 2 4 3 3 3 2 4 6% 59%	2290
October	1st to 10th	Day	3 4 1 3 3 2 3 2 3 5% 71%	3370
	11th to 20th	Day	4 5% 2 6% 3 3 4 4 5% 7% 58%	3280
	21st to end	Day	3 5% 2 4 3 3 3 3 4 6% 63%	3457
November	1st to 10th	Day	1 4 1 4 2 3 2 3 5% 8% 66%	1853
	11th to 20th	Day	1 4 2 3 3 3 2 2 6% 11% 63%	1890
	21st to end	Day	1 4 3 7% 4 3 3 3 5% 8% 59%	1738
December	1st to 10th	Day	2 2 5% 3 2 3 2 3 6% 72%	2700
	11th to 20th	Day	1 3 1 4 2 2 3 3 5% 10% 65%	3106
	21st to end	Day	1 6% 2 6% 3 3 4 4 7% 9% 56%	4092
January	1st to 10th	Night	4 3 8% 4 4 3 3 4 7% 60%	8201
	11th to 20th	Night	1 1 5% 4 3 4 3 4 7% 68%	8093
	21st to end	Night	4 1 4 3 3 3 2 4 7% 66%	8370
February	1st to 10th	Night	4 2 5% 4 3 3 3 5% 9% 62%	2730
	11th to 20th	Night	4 2 5% 4 2 3 4 5% 8% 62%	2515
	21st to end	Night	5% 3 6% 3 3 3 3 5% 8% 62%	2076
March	1st to 10th	Night	7% 3 6% 4 3 3 3 5% 7% 58%	5478
	11th to 20th	Night	1 9% 4 6% 3 3 4 2 5% 7% 56%	5110
	21st to end	Night	2 6% 2 5% 3 3 4 3 5% 9% 55%	5293
April	1st to 10th	Night	2 6% 2 4 3 3 4 4 6% 7% 59%	2100
	11th to 20th	Night	5% 8% 3 6% 4 3 3 3 5% 8% 52%	2077
	21st to end	Night	6% 11% 2 5% 3 3 4 3 5% 9% 49%	2100
May	1st to 10th	Night	8% 12% 3 6% 3 3 3 3 5% 8% 46%	5462
	11th to 20th	Night	7% 7% 2 6% 4 4 3 4 5% 7% 50%	4701
	21st to end	Night	4 7% 2 4 4 3 4 3 4 7% 57%	5181
June	1st to 10th	Night	4 4 1 3 3 3 3 4 6% 9% 59%	1680
	11th to 20th	Night	5% 4 1 3 3 2 4 3 5% 8% 63%	1680
	21st to end	Night	4 3 1 4 3 3 3 4 7% 8% 61%	1680
July	1st to 10th	Night	3 3 1 3 2 2 4 4 6% 7% 64%	4740
	11th to 20th	Night	3 3 1 3 3 2 3 4 6% 10% 62%	4740
	21st to end	Night	3 2 1 4 3 4 6% 6% 9% 11% 50%	5380
August	1st to 10th	Night	3 4 1 3 3 4 5% 5% 7% 9% 56%	5273
	11th to 20th	Night	3 3 1 3 3 3 4 4 7% 10% 59%	5759
	21st to end	Night	3 4 2 5% 4 5% 5% 5% 9% 11% 48%	6465
September	1st to 10th	Night	6% 4 1 4 4 3 4 3 6% 7% 60%	2100
	11th to 20th	Night	5% 4 1 3 3 2 3 3 5% 8% 63%	2102
	21st to end	Night	7% 4 2 4 3 3 4 3 5% 5% 62%	2326
October	1st to 10th	Night	3 3 1 3 2 2 2 2 3 5% 75%	4040
	11th to 20th	Night	2 5% 2 3 2 3 3 3 5% 7% 66%	3934
	21st to end	Night	3 5% 2 5% 2 3 2 2 3 6% 67%	4309
November	1st to 10th	Night	1 4 2 4 3 2 3 3 5% 6% 69%	2665
	11th to 20th	Night	1 4 2 4 2 3 3 3 5% 9% 65%	2730
	21st to end	Night	1 4 2 6% 3 3 3 3 6% 7% 61%	2877
December	1st to 10th	Night	2 1 3 2 2 2 1 3 6% 77%	4710
	11th to 20th	Night	3 2 5% 3 2 2 3 5% 8% 66%	5321
	21st to end	Night	1 6% 2 6% 3 3 4 4 7% 8% 55%	7128

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	24% 5% 12% 28% 26% 5%	4723
	11th to 20th	Day	18% 5% 16% 25% 24% 12%	4954
	21st to end	Day	21% 5% 10% 25% 27% 12%	5844
February	1st to 10th	Day	27% 6% 19% 23% 18% 7%	1890
	11th to 20th	Day	24% 7% 15% 25% 23% 5%	2000
	21st to end	Day	27% 4 15% 22% 23% 9%	1730
March	1st to 10th	Day	28% 5% 10% 17% 30% 11%	4600
	11th to 20th	Day	32% 7% 10% 24% 21% 5%	5110
	21st to end	Day	29% 7% 16% 33% 14% 2	5910
April	1st to 10th	Day	30% 7% 12% 25% 19% 7%	2520
	11th to 20th	Day	39% 7% 13% 18% 17% 6%	2494
	21st to end	Day	38% 10% 13% 23% 15% 2	2520
May	1st to 10th	Day	39% 6% 13% 25% 14% 2	7481
	11th to 20th	Day	34% 8% 11% 19% 20% 7%	8195
	21st to end	Day	32% 8% 13% 23% 19% 5%	9027
June	1st to 10th	Day	29% 12% 16% 20% 17% 6%	2940
	11th to 20th	Day	28% 9% 11% 15% 22% 14%	2940
	21st to end	Day	28% 9% 13% 17% 22% 11%	2940
July	1st to 10th	Day	23% 6% 9% 16% 22% 24%	8181
	11th to 20th	Day	23% 8% 11% 23% 18% 17%	8175
	21st to end	Day	34% 10% 15% 18% 12% 10%	8529
August	1st to 10th	Day	29% 9% 12% 17% 17% 15%	5978
	11th to 20th	Day	29% 10% 18% 17% 16% 9%	5588
	21st to end	Day	32% 9% 16% 19% 16% 7%	6078
September	1st to 10th	Day	25% 7% 12% 20% 22% 14%	2520
	11th to 20th	Day	26% 7% 12% 21% 25% 9%	2517
	21st to end	Day	26% 4 8% 21% 31% 9%	2290
October	1st to 10th	Day	17% 5% 6% 15% 38% 20%	3370
	11th to 20th	Day	25% 4 11% 28% 25% 7%	3280
	21st to end	Day	22% 5% 8% 24% 34% 7%	3457
November	1st to 10th	Day	18% 7% 12% 23% 32% 8%	1853
	11th to 20th	Day	18% 6% 12% 29% 27% 8%	1890
	21st to end	Day	23% 5% 14% 26% 22% 10%	1738
December	1st to 10th	Day	16% 5% 10% 32% 28% 8%	2700
	11th to 20th	Day	15% 5% 12% 37% 25% 6%	3104
	21st to end	Day	24% 5% 11% 26% 25% 8%	4092
January	1st to 10th	Night	25% 4 9% 30% 26% 6%	8202
	11th to 20th	Night	17% 6% 15% 24% 25% 13%	8093
	21st to end	Night	19% 5% 12% 23% 26% 14%	8370
February	1st to 10th	Night	21% 7% 18% 26% 22% 6%	2730
	11th to 20th	Night	21% 7% 15% 28% 23% 6%	2515
	21st to end	Night	22% 5% 13% 25% 24% 12%	2076
March	1st to 10th	Night	25% 5% 9% 18% 31% 11%	5478
	11th to 20th	Night	27% 6% 10% 19% 28% 10%	5110
	21st to end	Night	25% 7% 15% 36% 16% 2	5293
April	1st to 10th	Night	23% 9% 13% 27% 22% 7%	2100
	11th to 20th	Night	31% 7% 12% 23% 22% 5%	2077
	21st to end	Night	32% 8% 12% 26% 19% 3	2100
May	1st to 10th	Night	36% 6% 13% 26% 15% 3	5460
	11th to 20th	Night	30% 9% 12% 24% 20% 5%	4702
	21st to end	Night	26% 7% 16% 26% 22% 4	5182
June	1st to 10th	Night	21% 7% 19% 29% 17% 8%	1680
	11th to 20th	Night	17% 7% 13% 20% 24% 19%	1680
	21st to end	Night	19% 7% 12% 21% 27% 14%	1680
July	1st to 10th	Night	18% 4 8% 15% 34% 21%	4700
	11th to 20th	Night	16% 5% 16% 27% 23% 14%	4692
	21st to end	Night	23% 9% 23% 21% 13% 11%	5263
August	1st to 10th	Night	23% 7% 14% 20% 26% 10%	4168
	11th to 20th	Night	20% 9% 27% 22% 15% 7%	4543
	21st to end	Night	23% 8% 24% 21% 19% 4	5069
September	1st to 10th	Night	23% 5% 11% 21% 25% 14%	2100
	11th to 20th	Night	20% 6% 10% 25% 25% 13%	2102
	21st to end	Night	24% 5% 10% 23% 29% 10%	2326
October	1st to 10th	Night	15% 4 7% 21% 34% 20%	4040
	11th to 20th	Night	19% 4 11% 29% 29% 8%	3934
	21st to end	Night	20% 3 8% 25% 32% 11%	4309
November	1st to 10th	Night	16% 6% 12% 25% 29% 11%	2665
	11th to 20th	Night	18% 5% 13% 29% 28% 8%	2730
	21st to end	Night	22% 5% 12% 25% 27% 8%	2877
December	1st to 10th	Night	12% 5% 10% 32% 33% 7%	4710
	11th to 20th	Night	15% 5% 12% 30% 33% 4	5322
	21st to end	Night	25% 6% 13% 23% 28% 5%	7128

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	16% 20% 43% 14% 6% 2	330
	11th to 20th	Day	14% 33% 38% 12% 2 1	330
	21st to end	Day	10% 31% 41% 13% 4 1	363
February	1st to 10th	Day	16% 35% 40% 6% 2 1	330
	11th to 20th	Day	14% 32% 42% 10% 2	330
	21st to end	Day	19% 34% 39% 8%	273
March	1st to 10th	Day	10% 30% 43% 13% 3	330
	11th to 20th	Day	19% 26% 36% 16% 3	540
	21st to end	Day	15% 32% 39% 12% 2	594
April	1st to 10th	Day	14% 31% 41% 12% 2 1	540
	11th to 20th	Day	13% 35% 39% 10% 2	532
	21st to end	Day	16% 25% 39% 17% 3 1	540
May	1st to 10th	Day	15% 22% 39% 17% 6% 1	653
	11th to 20th	Day	18% 12% 38% 22% 8% 2	775
	21st to end	Day	11% 14% 34% 28% 11% 1	844
June	1st to 10th	Day	6% 10% 33% 33% 15% 4	780
	11th to 20th	Day	8% 12% 42% 24% 11% 2	780
	21st to end	Day	6% 13% 39% 24% 14% 3	780
July	1st to 10th	Day	7% 6% 28% 34% 20% 4	771
	11th to 20th	Day	5% 4 28% 38% 20% 5%	768
	21st to end	Day	5% 5% 27% 33% 23% 5%	769
August	1st to 10th	Day	6% 9% 35% 35% 13% 2	567
	11th to 20th	Day	5% 7% 37% 31% 17% 3	557
	21st to end	Day	7% 10% 31% 35% 14% 3	627
September	1st to 10th	Day	6% 9% 37% 30% 14% 3	570
	11th to 20th	Day	4 13% 40% 31% 11% 1	567
	21st to end	Day	8% 16% 33% 26% 14% 3	360
October	1st to 10th	Day	6% 16% 36% 24% 14% 4	360
	11th to 20th	Day	8% 21% 43% 21% 5% 2	339
	21st to end	Day	10% 24% 46% 16% 4 1	363
November	1st to 10th	Day	10% 23% 45% 15% 7%	330
	11th to 20th	Day	14% 25% 43% 14% 3 1	330
	21st to end	Day	11% 27% 42% 18% 2 1	330
December	1st to 10th	Day	16% 28% 38% 13% 3 1	330
	11th to 20th	Day	18% 27% 45% 9% 1	330
	21st to end	Day	15% 34% 37% 11% 3	363
January	1st to 10th	Night	11% 24% 42% 16% 6% 1	1050
	11th to 20th	Night	12% 32% 36% 15% 5% 1	997
	21st to end	Night	10% 24% 49% 13% 4	924
February	1st to 10th	Night	10% 28% 48% 11% 3	840
	11th to 20th	Night	8% 27% 47% 16% 2	824
	21st to end	Night	10% 27% 50% 11% 2	692
March	1st to 10th	Night	7% 22% 49% 17% 4	840
	11th to 20th	Night	10% 21% 39% 23% 6%	840
	21st to end	Night	8% 23% 48% 18% 3 1	788
April	1st to 10th	Night	9% 17% 49% 22% 3	630
	11th to 20th	Night	11% 21% 42% 19% 6%	623
	21st to end	Night	13% 17% 39% 22% 8% 1	630
May	1st to 10th	Night	11% 12% 41% 26% 10% 1	630
	11th to 20th	Night	17% 6% 30% 34% 11% 2	621
	21st to end	Night	10% 10% 32% 32% 14% 2	683
June	1st to 10th	Night	6% 4 29% 38% 19% 3	630
	11th to 20th	Night	7% 6% 37% 37% 12% 1	630
	21st to end	Night	4 4 27% 40% 23% 2	630
July	1st to 10th	Night	6% 3 19% 39% 29% 5%	615
	11th to 20th	Night	2 1 16% 43% 34% 4	612
	21st to end	Night	6% 2 13% 41% 33% 6%	690
August	1st to 10th	Night	5% 3 18% 44% 28% 3	627
	11th to 20th	Night	5% 1 19% 46% 25% 4	624
	21st to end	Night	5% 2 19% 41% 29% 5%	693
September	1st to 10th	Night	5% 4 25% 39% 22% 5%	630
	11th to 20th	Night	7% 4 33% 35% 17% 4	630
	21st to end	Night	7% 9% 31% 37% 14% 3	646
October	1st to 10th	Night	8% 11% 37% 30% 11% 3	840
	11th to 20th	Night	8% 15% 45% 24% 8% 2	812
	21st to end	Night	9% 18% 47% 18% 6% 2	913
November	1st to 10th	Night	7% 19% 49% 19% 5% 1	1025
	11th to 20th	Night	9% 27% 46% 15% 3 1	1050
	21st to end	Night	7% 21% 49% 20% 3	1050
December	1st to 10th	Night	12% 24% 45% 17% 2	1050
	11th to 20th	Night	12% 24% 48% 13% 3 1	1050
	21st to end	Night	9% 28% 46% 13% 4	1155

Page generated at 2026-10-11 03:06:31. Version ac07

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