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 2024-11-02.

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	4 6% 2 6% 4 4 5% 4 6% 8% 50%	101344
	Night	3 5% 2 5% 3 3 4 3 6% 8% 58%	97864
Transparency	Day	29% 8% 14% 22% 20% 8%	101089
	Night	23% 7% 15% 25% 22% 8%	97709
Seeing	Day	8% 18% 38% 24% 10% 2	15588
	Night	7% 16% 38% 25% 11% 2	25595

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 3 9% 5% 4 3 3 6% 8% 57%	2538
	11th to 20th	Day	4 2 8% 3 3 3 3 5% 9% 59%	2642
	21st to end	Day	8% 4 7% 3 3 3 3 6% 7% 55%	3146
February	1st to 10th	Day	5% 3 9% 4 4 3 3 6% 9% 54%	1710
	11th to 20th	Day	4 2 5% 4 3 4 3 7% 9% 59%	1795
	21st to end	Day	1 5% 4 7% 3 3 3 4 5% 9% 57%	1570
March	1st to 10th	Day	1 7% 5% 9% 3 4 3 4 4 7% 53%	3130
	11th to 20th	Day	2 14% 7% 9% 4 3 3 3 4 8% 43%	3490
	21st to end	Day	2 12% 4 6% 3 4 5% 4 8% 9% 43%	4031
April	1st to 10th	Day	3 7% 3 4 4 4 5% 4 7% 9% 49%	2280
	11th to 20th	Day	6% 12% 4 8% 3 4 5% 3 5% 8% 41%	2251
	21st to end	Day	7% 10% 3 6% 4 4 5% 6% 7% 7% 40%	2280
May	1st to 10th	Day	7% 14% 3 7% 4 4 4 4 7% 9% 37%	4035
	11th to 20th	Day	9% 10% 2 5% 4 5% 5% 5% 8% 10% 39%	4388
	21st to end	Day	5% 5% 2 4 3 4 5% 5% 7% 10% 50%	4825
June	1st to 10th	Day	5% 5% 2 5% 4 5% 6% 6% 8% 9% 45%	2660
	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	5% 6% 2 5% 4 3 4 4 5% 6% 56%	1900
	11th to 20th	Day	3 4 2 5% 3 3 4 3 4 6% 64%	1900
	21st to end	Day	4 6% 2 4 3 3 4 3 4 7% 60%	2038
November	1st to 10th	Day	1 4 1 3 2 3 2 3 5% 8% 67%	1682
	11th to 20th	Day	1 5% 2 3 2 3 2 2 6% 10% 64%	1710
	21st to end	Day	1 4 3 8% 4 3 3 3 5% 8% 59%	1573
December	1st to 10th	Day	4 3 7% 4 3 4 3 4 9% 59%	1520
	11th to 20th	Day	2 5% 2 4 3 3 3 4 7% 11% 57%	1926
	21st to end	Day	1 6% 2 8% 4 3 4 3 6% 8% 54%	2794
January	1st to 10th	Night	3 2 9% 4 4 4 3 4 9% 56%	4417
	11th to 20th	Night	2 1 4 3 3 4 3 5% 8% 66%	4357
	21st to end	Night	1 7% 3 7% 3 3 3 3 4 8% 58%	4521
February	1st to 10th	Night	4 3 6% 4 3 3 3 5% 9% 60%	2470
	11th to 20th	Night	4 2 6% 5% 2 3 4 5% 8% 62%	2270
	21st to end	Night	5% 3 6% 3 3 3 3 5% 8% 60%	1884
March	1st to 10th	Night	1 7% 3 6% 3 3 3 3 5% 7% 58%	3718
	11th to 20th	Night	2 12% 5% 6% 3 2 3 2 4 6% 54%	3490
	21st to end	Night	2 7% 2 6% 4 3 4 3 5% 9% 54%	3615
April	1st to 10th	Night	2 6% 2 4 3 3 4 4 6% 7% 58%	1900
	11th to 20th	Night	5% 8% 4 6% 3 3 3 3 5% 7% 52%	1877
	21st to end	Night	7% 11% 2 5% 3 3 3 3 5% 9% 49%	1900
May	1st to 10th	Night	7% 9% 2 5% 3 3 4 4 5% 8% 49%	2931
	11th to 20th	Night	9% 8% 2 6% 4 4 3 4 7% 8% 44%	2517
	21st to end	Night	4 5% 2 4 3 3 4 3 5% 7% 61%	2762
June	1st to 10th	Night	4 4 1 3 3 3 4 4 6% 9% 59%	1520
	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	4 6% 2 4 2 3 3 3 4 7% 62%	2280
	11th to 20th	Night	2 4 1 3 2 2 2 2 4 6% 70%	2280
	21st to end	Night	4 7% 3 6% 3 2 3 2 4 7% 59%	2539
November	1st to 10th	Night	1 3 1 4 3 2 3 3 5% 6% 70%	2431
	11th to 20th	Night	1 4 2 4 2 3 3 3 5% 9% 64%	2470
	21st to end	Night	1 4 2 6% 3 4 3 3 6% 7% 61%	2603
December	1st to 10th	Night	1 4 2 5% 2 3 3 2 5% 9% 64%	2660
	11th to 20th	Night	1 5% 3 7% 4 3 3 4 7% 10% 55%	3271
	21st to end	Night	1 6% 2 6% 3 3 5% 5% 7% 8% 54%	4873

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	22% 8% 20% 26% 21% 4	2539
	11th to 20th	Day	25% 6% 24% 26% 15% 4	2642
	21st to end	Day	28% 7% 15% 25% 21% 4	3146
February	1st to 10th	Day	29% 7% 20% 22% 17% 6%	1710
	11th to 20th	Day	24% 8% 16% 25% 23% 5%	1805
	21st to end	Day	29% 4 16% 22% 21% 8%	1570
March	1st to 10th	Day	31% 6% 12% 20% 26% 6%	3130
	11th to 20th	Day	36% 8% 13% 29% 12% 2	3490
	21st to end	Day	34% 9% 17% 26% 13% 2	4031
April	1st to 10th	Day	30% 8% 13% 24% 19% 7%	2280
	11th to 20th	Day	40% 8% 14% 18% 16% 4	2254
	21st to end	Day	38% 10% 13% 23% 14% 1	2280
May	1st to 10th	Day	40% 8% 15% 19% 15% 3	4035
	11th to 20th	Day	34% 13% 14% 24% 15% 1	4392
	21st to end	Day	27% 8% 15% 25% 21% 5%	4825
June	1st to 10th	Day	29% 12% 16% 20% 17% 6%	2660
	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	28% 8% 9% 18% 28% 9%	1900
	11th to 20th	Day	21% 5% 11% 26% 28% 9%	1900
	21st to end	Day	24% 6% 12% 25% 24% 9%	2038
November	1st to 10th	Day	16% 8% 13% 23% 31% 9%	1682
	11th to 20th	Day	18% 7% 13% 29% 27% 7%	1710
	21st to end	Day	24% 5% 14% 25% 22% 11%	1573
December	1st to 10th	Day	23% 8% 17% 27% 18% 5%	1520
	11th to 20th	Day	19% 7% 17% 30% 25% 3	1924
	21st to end	Day	26% 7% 14% 23% 24% 6%	2794
January	1st to 10th	Night	26% 6% 14% 28% 22% 3	4417
	11th to 20th	Night	17% 9% 22% 26% 19% 7%	4357
	21st to end	Night	25% 8% 16% 29% 17% 5%	4521
February	1st to 10th	Night	23% 8% 19% 26% 20% 5%	2470
	11th to 20th	Night	21% 8% 15% 28% 24% 5%	2270
	21st to end	Night	23% 5% 14% 26% 22% 10%	1884
March	1st to 10th	Night	24% 6% 11% 22% 31% 6%	3718
	11th to 20th	Night	31% 8% 13% 25% 20% 3	3490
	21st to end	Night	24% 8% 15% 35% 16% 2	3615
April	1st to 10th	Night	23% 9% 13% 26% 22% 7%	1900
	11th to 20th	Night	31% 8% 13% 23% 19% 5%	1877
	21st to end	Night	32% 8% 12% 27% 19% 2	1900
May	1st to 10th	Night	29% 9% 15% 24% 17% 6%	2931
	11th to 20th	Night	30% 12% 15% 24% 17% 1	2518
	21st to end	Night	21% 7% 18% 25% 22% 6%	2762
June	1st to 10th	Night	20% 7% 20% 29% 16% 8%	1520
	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	23% 6% 10% 21% 28% 11%	2280
	11th to 20th	Night	16% 4 13% 25% 30% 12%	2280
	21st to end	Night	25% 4 13% 24% 25% 9%	2539
November	1st to 10th	Night	16% 6% 13% 25% 29% 11%	2431
	11th to 20th	Night	18% 5% 14% 29% 27% 7%	2470
	21st to end	Night	22% 6% 12% 24% 27% 9%	2603
December	1st to 10th	Night	19% 8% 17% 26% 21% 8%	2660
	11th to 20th	Night	21% 7% 18% 26% 25% 4	3272
	21st to end	Night	23% 8% 16% 23% 23% 6%	4873

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	10% 19% 47% 16% 7% 2	270
	11th to 20th	Day	7% 36% 40% 14% 3 1	270
	21st to end	Day	2 32% 46% 15% 4 1	297
February	1st to 10th	Day	8% 37% 45% 6% 2 1	270
	11th to 20th	Day	9% 33% 44% 11% 2	270
	21st to end	Day	10% 39% 41% 10%	225
March	1st to 10th	Day	6% 31% 44% 14% 4	270
	11th to 20th	Day	12% 26% 40% 19% 3	460
	21st to end	Day	12% 33% 40% 13% 2	506
April	1st to 10th	Day	12% 32% 41% 13% 2	460
	11th to 20th	Day	13% 33% 39% 12% 3	452
	21st to end	Day	14% 23% 41% 18% 3 1	460
May	1st to 10th	Day	14% 23% 40% 17% 6% 1	563
	11th to 20th	Day	16% 11% 37% 24% 9% 2	675
	21st to end	Day	11% 14% 33% 29% 12% 1	734
June	1st to 10th	Day	6% 8% 32% 33% 16% 4	680
	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	5% 23% 47% 15% 9%	273
	11th to 20th	Day	9% 27% 44% 16% 4 1	270
	21st to end	Day	7% 26% 44% 21% 1 1	270
December	1st to 10th	Day	9% 29% 43% 15% 3 1	270
	11th to 20th	Day	5% 30% 53% 11% 1	270
	21st to end	Day	8% 35% 40% 13% 4	297
January	1st to 10th	Night	9% 23% 44% 17% 6% 1	950
	11th to 20th	Night	9% 33% 37% 16% 5% 1	903
	21st to end	Night	7% 24% 50% 14% 4	836
February	1st to 10th	Night	9% 27% 49% 11% 3	760
	11th to 20th	Night	7% 27% 46% 17% 3	744
	21st to end	Night	7% 28% 52% 11% 2	628
March	1st to 10th	Night	6% 22% 49% 18% 5%	760
	11th to 20th	Night	7% 20% 41% 25% 7%	760
	21st to end	Night	7% 23% 47% 19% 3 1	713
April	1st to 10th	Night	9% 17% 49% 22% 3	570
	11th to 20th	Night	10% 20% 42% 20% 7%	563
	21st to end	Night	13% 17% 39% 22% 8% 1	570
May	1st to 10th	Night	11% 11% 41% 25% 11% 1	570
	11th to 20th	Night	17% 6% 29% 35% 12% 2	561
	21st to end	Night	9% 9% 31% 33% 16% 2	617
June	1st to 10th	Night	6% 3 28% 39% 21% 4	570
	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% 46% 19% 7% 2	867
November	1st to 10th	Night	5% 19% 50% 19% 6% 1	935
	11th to 20th	Night	7% 26% 47% 16% 3 1	950
	21st to end	Night	5% 21% 50% 21% 3	950
December	1st to 10th	Night	10% 24% 46% 17% 3	950
	11th to 20th	Night	7% 24% 51% 15% 3 1	950
	21st to end	Night	7% 27% 46% 15% 5%	1045

Page generated at 2024-10-30 11:19:23. Version ac07

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