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 2024-03-18.

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% 3 5% 5% 3 5% 5% 6% 9% 50%	78730
	Night	3 5% 2 4 4 2 4 4 5% 8% 59%	81121
Transparency	Day	30% 8% 14% 22% 20% 6%	78438
	Night	23% 7% 15% 25% 22% 7%	80935
Seeing	Day	8% 18% 38% 24% 11% 2	14632
	Night	7% 16% 38% 25% 12% 2	24864

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	3 3 8% 6% 3 3 4 5% 8% 57%	2538
	11th to 20th	Day	4 3 7% 4 2 4 3 5% 9% 60%	2642
	21st to end	Day	7% 4 6% 4 2 4 4 6% 8% 55%	3146
February	1st to 10th	Day	5% 3 8% 5% 3 3 3 5% 10% 54%	1710
	11th to 20th	Day	4 2 5% 4 2 4 4 6% 11% 59%	1795
	21st to end	Day	1 4 5% 6% 4 2 3 4 5% 9% 57%	1570
March	1st to 10th	Day	2 6% 3 6% 5% 3 4 6% 4 9% 52%	1900
	11th to 20th	Day	3 10% 6% 6% 5% 2 4 4 5% 10% 45%	2058
	21st to end	Day	4 10% 5% 5% 4 2 4 5% 7% 11% 44%	2294
April	1st to 10th	Day	3 7% 3 4 4 3 6% 4 7% 9% 49%	2160
	11th to 20th	Day	7% 11% 4 6% 4 3 4 5% 5% 8% 41%	2131
	21st to end	Day	7% 10% 4 6% 5% 3 6% 6% 7% 8% 39%	2160
May	1st to 10th	Day	9% 12% 3 5% 6% 3 6% 5% 5% 9% 37%	2285
	11th to 20th	Day	13% 13% 2 4 5% 3 5% 5% 7% 9% 35%	2478
	21st to end	Day	9% 7% 3 5% 6% 4 6% 6% 7% 9% 38%	2724
June	1st to 10th	Day	6% 5% 2 5% 5% 3 6% 7% 8% 9% 45%	2520
	11th to 20th	Day	7% 5% 1 4 4 3 5% 6% 7% 8% 49%	2520
	21st to end	Day	5% 5% 2 4 5% 4 6% 7% 8% 9% 47%	2520
July	1st to 10th	Day	8% 5% 2 4 5% 3 6% 7% 8% 10% 42%	2520
	11th to 20th	Day	6% 4 2 5% 5% 3 6% 5% 7% 9% 47%	2520
	21st to end	Day	7% 5% 3 6% 6% 3 7% 7% 8% 9% 39%	2674
August	1st to 10th	Day	7% 6% 3 6% 5% 4 6% 7% 6% 11% 39%	2328
	11th to 20th	Day	7% 4 2 4 5% 4 5% 6% 7% 8% 49%	2153
	21st to end	Day	7% 5% 2 4 5% 4 6% 6% 7% 8% 45%	2376
September	1st to 10th	Day	5% 5% 2 4 5% 3 3 5% 6% 8% 55%	2160
	11th to 20th	Day	6% 5% 2 3 5% 3 4 4 5% 8% 56%	2158
	21st to end	Day	7% 5% 1 3 3 2 3 3 4 6% 61%	1963
October	1st to 10th	Day	5% 6% 3 4 5% 2 5% 5% 5% 6% 54%	1800
	11th to 20th	Day	3 4 2 4 3 2 4 3 4 6% 65%	1800
	21st to end	Day	4 6% 2 4 3 2 3 4 4 8% 60%	1931
November	1st to 10th	Day	1 4 1 3 2 2 3 3 4 9% 67%	1673
	11th to 20th	Day	1 4 3 3 2 2 2 3 5% 11% 64%	1710
	21st to end	Day	1 4 3 7% 4 2 3 3 5% 9% 59%	1573
December	1st to 10th	Day	4 2 6% 5% 3 4 3 4 9% 60%	1520
	11th to 20th	Day	2 4 2 4 3 2 3 4 6% 12% 57%	1926
	21st to end	Day	1 5% 3 7% 5% 2 4 4 6% 9% 54%	2794
January	1st to 10th	Night	3 2 8% 5% 3 4 4 4 9% 57%	4417
	11th to 20th	Night	2 1 3 4 2 3 4 5% 9% 67%	4357
	21st to end	Night	1 7% 3 6% 4 2 3 3 4 8% 58%	4521
February	1st to 10th	Night	4 3 5% 4 2 3 4 5% 9% 60%	2470
	11th to 20th	Night	3 2 5% 5% 2 3 4 5% 9% 62%	2270
	21st to end	Night	1 5% 3 6% 4 2 3 3 5% 8% 61%	1884
March	1st to 10th	Night	1 5% 2 4 4 2 4 4 6% 10% 58%	2276
	11th to 20th	Night	3 7% 3 5% 3 2 4 4 5% 7% 58%	2068
	21st to end	Night	3 7% 3 5% 4 2 4 3 5% 9% 56%	2062
April	1st to 10th	Night	2 6% 2 4 3 2 4 5% 6% 7% 59%	1800
	11th to 20th	Night	5% 8% 4 5% 4 3 3 4 6% 7% 53%	1777
	21st to end	Night	7% 11% 3 4 4 2 4 4 5% 9% 49%	1800
May	1st to 10th	Night	8% 8% 3 5% 4 2 5% 5% 6% 9% 44%	1671
	11th to 20th	Night	12% 9% 2 4 4 2 3 5% 7% 9% 43%	1417
	21st to end	Night	6% 5% 3 4 4 2 4 4 5% 7% 55%	1552
June	1st to 10th	Night	4 4 1 3 3 2 4 4 6% 10% 60%	1440
	11th to 20th	Night	5% 3 1 2 3 2 4 4 5% 8% 61%	1440
	21st to end	Night	4 3 1 3 3 2 3 4 7% 8% 61%	1440
July	1st to 10th	Night	6% 4 2 4 4 3 4 6% 7% 9% 52%	1440
	11th to 20th	Night	2 2 1 3 4 2 5% 6% 6% 9% 60%	1440
	21st to end	Night	6% 3 2 4 4 3 5% 5% 8% 10% 49%	1639
August	1st to 10th	Night	4 5% 2 4 4 3 4 6% 6% 9% 53%	1611
	11th to 20th	Night	6% 3 1 3 3 3 5% 4 6% 8% 57%	1748
	21st to end	Night	4 3 2 4 5% 3 4 5% 6% 8% 55%	1980
September	1st to 10th	Night	6% 3 2 4 4 2 4 3 6% 7% 60%	1800
	11th to 20th	Night	6% 3 1 3 2 2 3 4 4 7% 65%	1801
	21st to end	Night	7% 4 2 3 3 2 3 4 5% 5% 64%	1993
October	1st to 10th	Night	5% 5% 2 4 3 3 3 4 4 7% 61%	2160
	11th to 20th	Night	2 4 1 3 2 1 3 2 4 6% 71%	2160
	21st to end	Night	4 7% 3 5% 4 2 3 3 4 8% 58%	2405
November	1st to 10th	Night	1 3 1 3 3 2 3 3 4 7% 70%	2405
	11th to 20th	Night	1 4 2 4 3 2 3 4 5% 9% 65%	2470
	21st to end	Night	1 4 2 6% 3 2 4 3 6% 7% 61%	2603
December	1st to 10th	Night	1 4 2 4 3 2 3 3 5% 9% 64%	2660
	11th to 20th	Night	1 5% 3 6% 4 2 3 4 6% 10% 55%	3271
	21st to end	Night	1 5% 3 5% 3 2 5% 6% 7% 9% 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	28% 8% 13% 26% 22% 4	1900
	11th to 20th	Day	34% 6% 14% 24% 18% 3	2058
	21st to end	Day	35% 7% 17% 24% 14% 4	2294
April	1st to 10th	Day	30% 8% 13% 24% 18% 7%	2160
	11th to 20th	Day	40% 8% 14% 18% 16% 4	2134
	21st to end	Day	39% 10% 13% 22% 14% 1	2160
May	1st to 10th	Day	43% 10% 12% 20% 13% 2	2285
	11th to 20th	Day	44% 12% 12% 21% 9% 1	2482
	21st to end	Day	41% 10% 13% 20% 13% 5%	2724
June	1st to 10th	Day	30% 12% 16% 20% 16% 7%	2520
	11th to 20th	Day	30% 10% 11% 14% 22% 13%	2520
	21st to end	Day	29% 9% 13% 17% 22% 9%	2520
July	1st to 10th	Day	33% 11% 13% 16% 19% 8%	2451
	11th to 20th	Day	30% 10% 13% 20% 17% 9%	2445
	21st to end	Day	36% 13% 17% 18% 12% 4	2525
August	1st to 10th	Day	36% 13% 15% 17% 13% 5%	2313
	11th to 20th	Day	30% 8% 15% 16% 21% 9%	2153
	21st to end	Day	33% 8% 13% 18% 22% 7%	2376
September	1st to 10th	Day	25% 7% 12% 18% 22% 15%	2160
	11th to 20th	Day	25% 8% 13% 20% 25% 10%	2158
	21st to end	Day	25% 5% 8% 21% 32% 8%	1963
October	1st to 10th	Day	29% 8% 10% 19% 27% 7%	1800
	11th to 20th	Day	20% 5% 10% 26% 29% 9%	1800
	21st to end	Day	24% 6% 13% 25% 23% 9%	1931
November	1st to 10th	Day	17% 8% 13% 22% 31% 9%	1673
	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	21% 8% 12% 28% 26% 5%	2276
	11th to 20th	Night	26% 7% 13% 27% 23% 5%	2068
	21st to end	Night	28% 7% 15% 28% 19% 4	2062
April	1st to 10th	Night	23% 9% 13% 26% 22% 7%	1800
	11th to 20th	Night	30% 7% 14% 24% 20% 5%	1777
	21st to end	Night	32% 8% 13% 27% 19% 3	1800
May	1st to 10th	Night	34% 10% 14% 23% 15% 3	1671
	11th to 20th	Night	36% 10% 18% 24% 12% 1	1418
	21st to end	Night	27% 8% 18% 24% 17% 6%	1552
June	1st to 10th	Night	21% 7% 20% 28% 16% 8%	1440
	11th to 20th	Night	19% 7% 13% 18% 24% 18%	1440
	21st to end	Night	18% 7% 13% 22% 28% 12%	1440
July	1st to 10th	Night	27% 9% 14% 21% 21% 8%	1400
	11th to 20th	Night	19% 8% 15% 26% 21% 12%	1392
	21st to end	Night	26% 12% 18% 26% 13% 5%	1548
August	1st to 10th	Night	27% 10% 18% 25% 14% 6%	1602
	11th to 20th	Night	22% 7% 14% 22% 25% 10%	1748
	21st to end	Night	25% 7% 13% 23% 25% 8%	1980
September	1st to 10th	Night	24% 6% 12% 19% 23% 16%	1800
	11th to 20th	Night	19% 6% 11% 25% 25% 14%	1801
	21st to end	Night	22% 5% 10% 24% 30% 10%	1993
October	1st to 10th	Night	24% 6% 11% 21% 28% 9%	2160
	11th to 20th	Night	16% 4 13% 24% 30% 13%	2160
	21st to end	Night	26% 4 13% 24% 24% 8%	2405
November	1st to 10th	Night	16% 6% 13% 25% 29% 12%	2405
	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% 41% 18% 2	449
	21st to end	Day	13% 32% 41% 11% 2	462
April	1st to 10th	Day	12% 31% 41% 13% 2	420
	11th to 20th	Day	13% 33% 39% 12% 3	412
	21st to end	Day	14% 24% 41% 17% 4 1	420
May	1st to 10th	Day	14% 21% 40% 18% 6% 1	517
	11th to 20th	Day	18% 11% 36% 24% 9% 2	625
	21st to end	Day	11% 14% 32% 29% 13% 2	679
June	1st to 10th	Day	6% 8% 31% 33% 18% 5%	630
	11th to 20th	Day	7% 9% 42% 26% 13% 2	630
	21st to end	Day	5% 9% 38% 26% 17% 4	630
July	1st to 10th	Day	7% 3 25% 36% 23% 5%	621
	11th to 20th	Day	5% 3 28% 37% 21% 6%	618
	21st to end	Day	5% 4 25% 33% 26% 6%	616
August	1st to 10th	Day	6% 6% 32% 39% 15% 2	447
	11th to 20th	Day	4 6% 36% 32% 19% 3	437
	21st to end	Day	7% 8% 30% 37% 15% 3	495
September	1st to 10th	Day	7% 9% 34% 32% 15% 4	450
	11th to 20th	Day	3 10% 40% 33% 13% 1	448
	21st to end	Day	7% 13% 33% 27% 16% 3	270
October	1st to 10th	Day	6% 17% 33% 24% 15% 4	270
	11th to 20th	Day	4 21% 45% 22% 5% 3	270
	21st to end	Day	9% 24% 44% 18% 4 1	297
November	1st to 10th	Day	5% 23% 47% 16% 9%	270
	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%	752
	21st to end	Night	7% 23% 47% 19% 3 1	676
April	1st to 10th	Night	9% 16% 50% 22% 3	540
	11th to 20th	Night	10% 20% 42% 20% 8%	533
	21st to end	Night	13% 17% 38% 22% 9% 1	540
May	1st to 10th	Night	11% 10% 41% 25% 11% 1	540
	11th to 20th	Night	18% 5% 28% 35% 13% 2	531
	21st to end	Night	9% 9% 30% 33% 16% 2	584
June	1st to 10th	Night	6% 3 26% 39% 22% 4	540
	11th to 20th	Night	7% 5% 34% 39% 14% 1	540
	21st to end	Night	4 3 22% 42% 27% 2	540
July	1st to 10th	Night	6% 2 13% 39% 34% 6%	525
	11th to 20th	Night	1 1 14% 41% 38% 4	522
	21st to end	Night	6% 2 9% 39% 38% 6%	591
August	1st to 10th	Night	5% 3 12% 44% 32% 3	537
	11th to 20th	Night	5% 1 14% 46% 29% 5%	534
	21st to end	Night	5% 1 16% 40% 33% 6%	594
September	1st to 10th	Night	5% 2 22% 39% 26% 6%	540
	11th to 20th	Night	8% 3 26% 38% 19% 5%	540
	21st to end	Night	7% 8% 29% 37% 16% 3	553
October	1st to 10th	Night	8% 11% 35% 30% 13% 3	720
	11th to 20th	Night	6% 14% 43% 26% 9% 2	720
	21st to end	Night	8% 18% 46% 19% 7% 2	821
November	1st to 10th	Night	5% 19% 50% 19% 6% 1	925
	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 2024-03-15 12:01:39. Version ac07

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