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 2025-06-10.

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 4 4 6% 8% 52%	123593
	Night	2 5% 2 5% 3 3 3 3 5% 8% 60%	123556
Transparency	Day	28% 7% 13% 22% 22% 9%	123339
	Night	22% 6% 13% 25% 25% 9%	123398
Seeing	Day	9% 18% 38% 23% 10% 2	16375
	Night	8% 17% 38% 25% 11% 2	26472

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 4 9% 4 3 2 3 5% 6% 60%	3638
	11th to 20th	Day	3 2 6% 4 5% 3 3 4 7% 63%	3798
	21st to end	Day	5% 3 6% 4 3 3 3 5% 7% 61%	4510
February	1st to 10th	Day	5% 3 9% 4 4 3 3 6% 9% 55%	1800
	11th to 20th	Day	4 2 5% 4 3 4 3 7% 10% 58%	1893
	21st to end	Day	1 5% 4 7% 3 3 3 3 5% 9% 57%	1650
March	1st to 10th	Day	1 7% 4 8% 4 3 3 4 4 7% 55%	4500
	11th to 20th	Day	1 10% 6% 9% 5% 4 3 3 4 8% 46%	5000
	21st to end	Day	2 9% 3 6% 3 4 5% 4 7% 11% 48%	5783
April	1st to 10th	Day	3 7% 3 5% 4 4 5% 3 7% 9% 49%	2400
	11th to 20th	Day	6% 12% 4 8% 3 4 4 3 5% 8% 42%	2371
	21st to end	Day	7% 10% 3 6% 4 4 5% 6% 7% 7% 40%	2400
May	1st to 10th	Day	9% 13% 2 6% 4 4 5% 4 7% 9% 38%	5771
	11th to 20th	Day	7% 8% 2 5% 4 5% 5% 4 8% 9% 44%	6298
	21st to end	Day	6% 6% 2 5% 4 4 5% 5% 7% 9% 47%	6925
June	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 6% 8% 9% 45%	2788
	11th to 20th	Day	7% 5% 1 5% 3 5% 6% 4 7% 8% 49%	2660
	21st to end	Day	5% 5% 1 5% 5% 5% 6% 5% 8% 9% 47%	2660
July	1st to 10th	Day	6% 4 1 4 4 4 5% 4 7% 8% 53%	4430
	11th to 20th	Day	4 3 2 5% 5% 6% 7% 6% 9% 10% 43%	4430
	21st to end	Day	6% 4 2 6% 5% 4 6% 5% 7% 8% 47%	4681
August	1st to 10th	Day	7% 6% 2 7% 4 5% 6% 5% 7% 11% 38%	4092
	11th to 20th	Day	5% 4 1 5% 4 5% 6% 5% 7% 8% 50%	3768
	21st to end	Day	5% 4 2 6% 5% 6% 5% 4 7% 7% 48%	4180
September	1st to 10th	Day	5% 5% 1 5% 4 3 4 4 6% 7% 55%	2280
	11th to 20th	Day	6% 5% 2 4 4 4 4 4 5% 7% 56%	2277
	21st to end	Day	7% 6% 1 4 3 3 3 2 4 6% 62%	2072
October	1st to 10th	Day	3 4 1 3 3 2 3 2 3 4 72%	3170
	11th to 20th	Day	4 5% 2 6% 3 3 4 4 5% 7% 57%	3150
	21st to end	Day	3 5% 2 4 3 3 3 3 4 6% 63%	3350
November	1st to 10th	Day	1 4 1 4 2 3 3 3 5% 8% 65%	1763
	11th to 20th	Day	1 4 2 3 2 3 2 2 6% 10% 64%	1800
	21st to end	Day	1 4 3 7% 4 4 3 3 5% 9% 59%	1655
December	1st to 10th	Day	2 2 4 3 2 3 2 3 6% 72%	2620
	11th to 20th	Day	1 3 1 4 2 2 3 3 5% 10% 65%	3026
	21st to end	Day	1 6% 2 6% 3 3 4 4 7% 9% 55%	4004
January	1st to 10th	Night	3 2 9% 4 4 3 3 4 8% 60%	6327
	11th to 20th	Night	2 1 5% 4 3 4 3 5% 7% 67%	6225
	21st to end	Night	1 5% 2 6% 4 3 3 2 4 7% 64%	6468
February	1st to 10th	Night	4 3 6% 4 3 3 3 5% 9% 61%	2600
	11th to 20th	Night	3 2 5% 5% 2 3 4 5% 8% 61%	2392
	21st to end	Night	5% 3 6% 3 3 3 3 5% 8% 61%	1980
March	1st to 10th	Night	7% 3 6% 4 3 3 3 5% 7% 58%	5358
	11th to 20th	Night	1 9% 4 6% 3 3 4 2 5% 7% 56%	5000
	21st to end	Night	2 6% 2 5% 3 3 4 3 5% 9% 55%	5178
April	1st to 10th	Night	2 6% 2 5% 3 3 4 4 6% 7% 58%	2000
	11th to 20th	Night	5% 8% 3 6% 4 3 3 3 5% 7% 52%	1977
	21st to end	Night	7% 11% 2 5% 3 3 4 3 5% 9% 49%	2000
May	1st to 10th	Night	9% 11% 2 5% 3 3 3 3 5% 9% 47%	4205
	11th to 20th	Night	6% 7% 2 6% 4 4 4 4 6% 8% 49%	3617
	21st to end	Night	4 7% 2 4 3 2 3 2 4 6% 61%	3972
June	1st to 10th	Night	4 4 1 4 3 3 4 4 6% 9% 59%	1596
	11th to 20th	Night	5% 4 1 3 3 3 4 4 5% 8% 62%	1520
	21st to end	Night	3 3 1 3 3 3 3 4 7% 8% 60%	1520
July	1st to 10th	Night	4 4 1 3 3 3 4 4 6% 7% 60%	2540
	11th to 20th	Night	2 3 1 4 3 3 4 4 7% 11% 57%	2540
	21st to end	Night	3 3 1 3 2 3 4 4 8% 9% 59%	2877
August	1st to 10th	Night	4 5% 2 5% 4 4 6% 5% 8% 9% 49%	2847
	11th to 20th	Night	3 3 1 3 3 4 5% 4 8% 10% 56%	3071
	21st to end	Night	3 3 2 4 4 4 5% 4 8% 9% 54%	3487
September	1st to 10th	Night	6% 4 1 4 4 3 3 2 6% 7% 60%	1900
	11th to 20th	Night	6% 3 1 3 2 2 3 3 5% 7% 65%	1902
	21st to end	Night	7% 4 1 3 2 2 4 2 5% 5% 64%	2104
October	1st to 10th	Night	3 3 1 2 2 2 2 2 3 5% 75%	3800
	11th to 20th	Night	2 5% 2 3 2 3 3 3 5% 7% 66%	3778
	21st to end	Night	3 5% 2 5% 3 3 2 2 3 6% 66%	4175
November	1st to 10th	Night	1 4 2 4 3 2 3 3 5% 6% 68%	2535
	11th to 20th	Night	1 4 2 4 2 3 3 3 5% 8% 65%	2600
	21st to end	Night	1 4 2 6% 3 4 3 3 6% 7% 61%	2740
December	1st to 10th	Night	2 1 3 2 2 2 1 3 6% 77%	4570
	11th to 20th	Night	3 2 5% 3 2 2 3 5% 8% 66%	5181
	21st to end	Night	1 6% 3 6% 3 3 4 4 7% 8% 54%	6974

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	23% 6% 14% 28% 23% 7%	3639
	11th to 20th	Day	23% 6% 18% 20% 24% 10%	3798
	21st to end	Day	24% 5% 11% 24% 25% 11%	4510
February	1st to 10th	Day	28% 7% 20% 22% 17% 7%	1800
	11th to 20th	Day	24% 8% 16% 25% 23% 5%	1903
	21st to end	Day	28% 4 15% 22% 22% 9%	1650
March	1st to 10th	Day	28% 5% 10% 17% 29% 11%	4500
	11th to 20th	Day	33% 7% 11% 24% 21% 4	5000
	21st to end	Day	30% 7% 16% 33% 13% 2	5783
April	1st to 10th	Day	30% 7% 12% 25% 19% 7%	2400
	11th to 20th	Day	39% 7% 14% 18% 17% 5%	2374
	21st to end	Day	38% 10% 13% 23% 14% 1	2400
May	1st to 10th	Day	39% 7% 14% 23% 15% 2	5771
	11th to 20th	Day	31% 10% 13% 22% 19% 6%	6302
	21st to end	Day	30% 8% 13% 23% 21% 4	6926
June	1st to 10th	Day	29% 12% 16% 20% 17% 6%	2788
	11th to 20th	Day	30% 9% 11% 15% 22% 13%	2660
	21st to end	Day	29% 9% 13% 17% 22% 10%	2660
July	1st to 10th	Day	26% 7% 9% 16% 20% 22%	4361
	11th to 20th	Day	30% 10% 12% 21% 17% 11%	4355
	21st to end	Day	30% 9% 12% 19% 15% 16%	4533
August	1st to 10th	Day	37% 11% 15% 17% 14% 5%	4078
	11th to 20th	Day	29% 9% 15% 14% 19% 13%	3804
	21st to end	Day	32% 7% 12% 15% 23% 11%	4179
September	1st to 10th	Day	25% 7% 11% 19% 23% 15%	2280
	11th to 20th	Day	25% 7% 12% 20% 25% 10%	2277
	21st to end	Day	24% 4 8% 21% 33% 9%	2072
October	1st to 10th	Day	17% 5% 6% 14% 38% 21%	3170
	11th to 20th	Day	26% 4 12% 28% 24% 6%	3150
	21st to end	Day	22% 5% 8% 25% 33% 7%	3350
November	1st to 10th	Day	18% 7% 12% 22% 31% 9%	1763
	11th to 20th	Day	17% 6% 12% 28% 27% 9%	1800
	21st to end	Day	23% 5% 14% 25% 23% 11%	1655
December	1st to 10th	Day	16% 5% 11% 32% 28% 8%	2620
	11th to 20th	Day	15% 5% 12% 37% 25% 6%	3024
	21st to end	Day	25% 5% 11% 26% 25% 8%	4004
January	1st to 10th	Night	24% 5% 11% 27% 25% 7%	6327
	11th to 20th	Night	18% 7% 16% 22% 24% 13%	6225
	21st to end	Night	21% 6% 12% 25% 23% 13%	6468
February	1st to 10th	Night	22% 7% 19% 25% 21% 6%	2600
	11th to 20th	Night	21% 8% 15% 28% 23% 5%	2392
	21st to end	Night	23% 5% 13% 25% 23% 12%	1980
March	1st to 10th	Night	25% 5% 9% 18% 31% 11%	5358
	11th to 20th	Night	27% 6% 10% 19% 28% 10%	5000
	21st to end	Night	25% 7% 15% 36% 16% 2	5178
April	1st to 10th	Night	24% 9% 13% 26% 22% 7%	2000
	11th to 20th	Night	31% 7% 13% 23% 20% 5%	1977
	21st to end	Night	32% 8% 12% 27% 19% 2	2000
May	1st to 10th	Night	33% 7% 15% 26% 14% 4	4202
	11th to 20th	Night	28% 11% 14% 24% 20% 4	3618
	21st to end	Night	23% 6% 16% 26% 24% 4	3972
June	1st to 10th	Night	21% 7% 19% 29% 16% 7%	1596
	11th to 20th	Night	19% 7% 13% 19% 24% 18%	1520
	21st to end	Night	19% 7% 13% 22% 27% 12%	1520
July	1st to 10th	Night	22% 6% 10% 17% 25% 20%	2500
	11th to 20th	Night	19% 6% 14% 31% 21% 9%	2492
	21st to end	Night	18% 8% 14% 28% 15% 17%	2787
August	1st to 10th	Night	29% 10% 17% 22% 19% 4	2838
	11th to 20th	Night	21% 7% 22% 19% 20% 11%	3101
	21st to end	Night	24% 7% 16% 19% 28% 6%	3487
September	1st to 10th	Night	23% 6% 11% 20% 24% 16%	1900
	11th to 20th	Night	19% 6% 11% 24% 26% 14%	1902
	21st to end	Night	22% 5% 9% 23% 31% 10%	2104
October	1st to 10th	Night	14% 4 7% 21% 34% 20%	3800
	11th to 20th	Night	19% 4 11% 30% 28% 7%	3778
	21st to end	Night	20% 3 9% 25% 32% 11%	4175
November	1st to 10th	Night	17% 6% 13% 25% 29% 11%	2535
	11th to 20th	Night	17% 5% 14% 28% 28% 8%	2600
	21st to end	Night	22% 6% 12% 24% 27% 9%	2740
December	1st to 10th	Night	12% 5% 10% 32% 34% 7%	4570
	11th to 20th	Night	15% 5% 12% 30% 34% 4	5182
	21st to end	Night	25% 7% 13% 22% 27% 5%	6974

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	12% 19% 46% 14% 7% 2	300
	11th to 20th	Day	9% 35% 40% 13% 3 1	300
	21st to end	Day	6% 31% 44% 14% 4 1	330
February	1st to 10th	Day	13% 36% 42% 6% 2 1	300
	11th to 20th	Day	11% 34% 43% 10% 2	300
	21st to end	Day	13% 37% 41% 9%	249
March	1st to 10th	Day	7% 31% 45% 14% 3	300
	11th to 20th	Day	14% 27% 38% 17% 3	500
	21st to end	Day	15% 31% 39% 13% 2	550
April	1st to 10th	Day	12% 31% 41% 13% 2	500
	11th to 20th	Day	12% 34% 40% 11% 3	492
	21st to end	Day	15% 24% 40% 18% 3 1	500
May	1st to 10th	Day	15% 22% 39% 18% 6% 1	608
	11th to 20th	Day	18% 11% 37% 23% 8% 2	725
	21st to end	Day	11% 14% 34% 28% 11% 1	789
June	1st to 10th	Day	6% 9% 33% 33% 16% 4	726
	11th to 20th	Day	7% 11% 43% 26% 12% 2	680
	21st to end	Day	6% 11% 38% 25% 16% 4	680
July	1st to 10th	Day	8% 5% 26% 35% 22% 4	671
	11th to 20th	Day	5% 3 28% 37% 21% 5%	668
	21st to end	Day	5% 5% 26% 33% 25% 6%	667
August	1st to 10th	Day	7% 6% 32% 38% 14% 2	487
	11th to 20th	Day	4 7% 37% 31% 17% 3	477
	21st to end	Day	6% 9% 31% 36% 15% 3	539
September	1st to 10th	Day	7% 9% 34% 32% 16% 3	490
	11th to 20th	Day	3 12% 40% 31% 12% 1	487
	21st to end	Day	8% 14% 32% 27% 15% 4	300
October	1st to 10th	Day	6% 16% 35% 25% 13% 4	300
	11th to 20th	Day	4 20% 46% 22% 6% 3	300
	21st to end	Day	10% 24% 45% 17% 4 1	330
November	1st to 10th	Day	7% 24% 47% 15% 8%	300
	11th to 20th	Day	12% 25% 44% 14% 4 1	300
	21st to end	Day	9% 27% 43% 19% 1 1	300
December	1st to 10th	Day	11% 29% 42% 15% 3 1	300
	11th to 20th	Day	13% 28% 49% 10% 1	300
	21st to end	Day	12% 33% 39% 12% 3	330
January	1st to 10th	Night	10% 24% 44% 16% 6% 1	1000
	11th to 20th	Night	11% 32% 36% 15% 5% 1	950
	21st to end	Night	8% 24% 50% 14% 4	880
February	1st to 10th	Night	10% 27% 49% 11% 3	800
	11th to 20th	Night	8% 27% 47% 16% 2	784
	21st to end	Night	8% 28% 51% 11% 2	660
March	1st to 10th	Night	6% 23% 49% 18% 5%	800
	11th to 20th	Night	8% 21% 40% 24% 7%	800
	21st to end	Night	8% 23% 47% 18% 3 1	750
April	1st to 10th	Night	9% 17% 50% 21% 3	600
	11th to 20th	Night	10% 21% 42% 20% 7%	593
	21st to end	Night	13% 18% 39% 22% 8% 1	600
May	1st to 10th	Night	11% 12% 40% 26% 10% 1	600
	11th to 20th	Night	17% 6% 30% 34% 11% 2	591
	21st to end	Night	10% 9% 31% 33% 15% 2	650
June	1st to 10th	Night	6% 3 28% 39% 20% 4	599
	11th to 20th	Night	6% 5% 36% 38% 13% 1	570
	21st to end	Night	4 3 24% 41% 26% 2	570
July	1st to 10th	Night	6% 2 15% 38% 32% 6%	555
	11th to 20th	Night	1 1 16% 41% 36% 4	552
	21st to end	Night	6% 2 10% 40% 36% 6%	624
August	1st to 10th	Night	5% 3 14% 44% 31% 3	567
	11th to 20th	Night	5% 2 17% 45% 28% 5%	564
	21st to end	Night	5% 1 18% 40% 31% 5%	627
September	1st to 10th	Night	5% 2 22% 40% 25% 5%	570
	11th to 20th	Night	8% 4 29% 37% 18% 5%	570
	21st to end	Night	7% 8% 30% 37% 15% 3	584
October	1st to 10th	Night	7% 11% 36% 30% 12% 3	760
	11th to 20th	Night	6% 14% 44% 25% 8% 2	760
	21st to end	Night	8% 18% 47% 18% 7% 2	867
November	1st to 10th	Night	5% 19% 49% 19% 6% 1	975
	11th to 20th	Night	8% 27% 46% 15% 3 1	1000
	21st to end	Night	6% 21% 49% 20% 3	1000
December	1st to 10th	Night	10% 24% 46% 17% 3	1000
	11th to 20th	Night	9% 24% 49% 14% 3 1	1000
	21st to end	Night	8% 27% 46% 14% 5%	1100

Page generated at 2025-06-07 08:28:16. Version ac07

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