Nylund Forest Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nylund Forest Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	12% 9% 4 9% 5% 5% 5% 4 6% 7% 36%	132067
	Night	11% 10% 4 10% 6% 5% 5% 4 6% 7% 34%	130715
Transparency	Day	45% 7% 13% 23% 10% 1	135597
	Night	47% 7% 13% 22% 11% 1	135252
Seeing	Day	16% 11% 31% 26% 13% 3	18098
	Night	15% 9% 33% 31% 11% 1	28127

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	6% 14% 6% 16% 9% 7% 5% 4 5% 6% 23%	4124
	11th to 20th	Day	5% 10% 7% 15% 9% 7% 9% 8% 9% 6% 15%	4381
	21st to end	Day	3 10% 8% 14% 7% 6% 6% 6% 7% 8% 24%	5752
February	1st to 10th	Day	8% 13% 5% 12% 6% 6% 5% 4 6% 6% 27%	1890
	11th to 20th	Day	8% 10% 3 8% 7% 7% 6% 4 6% 6% 35%	1844
	21st to end	Day	9% 11% 4 11% 7% 6% 5% 3 6% 7% 31%	1557
March	1st to 10th	Day	10% 9% 3 7% 5% 4 5% 4 5% 9% 39%	4992
	11th to 20th	Day	8% 9% 3 9% 6% 4 5% 5% 6% 9% 35%	5090
	21st to end	Day	20% 12% 4 9% 4 4 4 3 4 5% 31%	5596
April	1st to 10th	Day	16% 12% 4 10% 5% 4 4 3 4 6% 32%	2520
	11th to 20th	Day	17% 12% 3 9% 5% 4 4 3 5% 6% 32%	2604
	21st to end	Day	22% 9% 4 6% 5% 5% 4 3 4 6% 33%	2730
May	1st to 10th	Day	15% 7% 2 5% 3 3 4 3 4 6% 48%	6169
	11th to 20th	Day	16% 9% 3 6% 4 4 3 3 5% 6% 41%	6319
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 6% 7% 39%	6935
June	1st to 10th	Day	11% 5% 3 6% 5% 5% 5% 4 7% 9% 41%	2940
	11th to 20th	Day	15% 8% 3 7% 6% 4 5% 4 6% 8% 35%	3000
	21st to end	Day	11% 5% 2 7% 5% 5% 6% 4 8% 9% 37%	3000
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 10% 43%	6800
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 9% 47%	6722
	21st to end	Day	8% 5% 2 6% 4 4 4 5% 8% 10% 45%	6924
August	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 43%	4237
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 5% 6% 8% 42%	4171
	21st to end	Day	15% 7% 3 6% 4 4 5% 4 5% 7% 42%	4581
September	1st to 10th	Day	10% 6% 2 7% 4 4 5% 3 6% 9% 44%	2400
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 3 6% 7% 40%	2325
	21st to end	Day	15% 8% 2 5% 4 4 4 3 5% 6% 44%	2133
October	1st to 10th	Day	17% 9% 3 7% 4 3 4 4 5% 7% 39%	2000
	11th to 20th	Day	17% 13% 4 7% 5% 4 4 3 4 5% 35%	2000
	21st to end	Day	18% 13% 4 8% 5% 4 4 3 5% 6% 29%	2144
November	1st to 10th	Day	12% 14% 5% 11% 6% 5% 5% 4 4 6% 30%	1647
	11th to 20th	Day	12% 17% 6% 14% 6% 6% 3 3 4 6% 25%	1680
	21st to end	Day	11% 12% 6% 13% 7% 5% 6% 4 5% 7% 23%	1680
December	1st to 10th	Day	10% 12% 5% 15% 15% 8% 5% 4 4 5% 17%	2690
	11th to 20th	Day	8% 17% 7% 12% 6% 4 5% 3 6% 5% 26%	2915
	21st to end	Day	17% 18% 6% 20% 9% 5% 3 3 3 4 13%	3575
January	1st to 10th	Night	6% 10% 5% 17% 10% 7% 6% 4 6% 6% 24%	8799
	11th to 20th	Night	4 11% 5% 13% 8% 7% 8% 9% 9% 6% 19%	8606
	21st to end	Night	4 10% 8% 12% 5% 6% 7% 5% 7% 8% 28%	8446
February	1st to 10th	Night	6% 11% 5% 9% 6% 6% 5% 5% 7% 7% 33%	2730
	11th to 20th	Night	6% 10% 3 9% 7% 4 6% 5% 6% 8% 36%	2663
	21st to end	Night	9% 11% 4 10% 7% 6% 5% 5% 5% 6% 31%	2249
March	1st to 10th	Night	10% 8% 2 7% 4 4 4 3 5% 7% 46%	5124
	11th to 20th	Night	10% 10% 3 11% 6% 6% 5% 4 5% 6% 32%	5090
	21st to end	Night	19% 14% 5% 9% 5% 4 3 3 4 5% 28%	5574
April	1st to 10th	Night	15% 10% 4 9% 4 5% 4 3 5% 6% 35%	2100
	11th to 20th	Night	16% 12% 3 8% 5% 4 4 2 5% 6% 36%	1963
	21st to end	Night	19% 9% 4 8% 4 3 4 3 5% 6% 36%	1890
May	1st to 10th	Night	12% 8% 2 5% 4 3 3 3 4 6% 49%	3754
	11th to 20th	Night	13% 7% 3 7% 3 3 3 3 5% 6% 47%	3596
	21st to end	Night	15% 8% 3 6% 5% 4 5% 4 5% 7% 38%	3942
June	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 7% 9% 45%	1456
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 5% 7% 44%	1400
	21st to end	Night	10% 6% 2 7% 4 4 4 4 7% 8% 45%	1400
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 11% 47%	3178
	11th to 20th	Night	8% 3 1 4 3 3 3 3 6% 9% 56%	3253
	21st to end	Night	8% 4 2 5% 4 4 4 4 7% 10% 47%	3947
August	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 5% 8% 50%	2920
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 5% 8% 48%	2888
	21st to end	Night	13% 6% 2 6% 4 3 4 3 5% 9% 44%	3325
September	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 7% 6% 51%	2000
	11th to 20th	Night	9% 5% 2 6% 4 3 5% 3 5% 7% 50%	2070
	21st to end	Night	13% 6% 2 6% 4 3 3 3 5% 6% 49%	2246
October	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 6% 44%	2400
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 3 4 6% 36%	2400
	21st to end	Night	18% 11% 3 9% 5% 4 4 4 5% 6% 32%	2690
November	1st to 10th	Night	13% 11% 3 9% 6% 5% 4 4 5% 6% 33%	2871
	11th to 20th	Night	14% 13% 5% 10% 7% 4 4 4 4 6% 29%	2940
	21st to end	Night	12% 9% 4 12% 8% 6% 4 4 6% 7% 27%	2940
December	1st to 10th	Night	6% 11% 6% 17% 11% 11% 6% 4 5% 6% 17%	4720
	11th to 20th	Night	9% 15% 6% 15% 7% 6% 5% 5% 6% 6% 21%	5500
	21st to end	Night	19% 19% 6% 17% 6% 4 3 2 4 4 16%	7645

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	62% 5% 9% 13% 10% 1	4125
	11th to 20th	Day	62% 6% 8% 17% 7%	4381
	21st to end	Day	54% 6% 9% 20% 10%	5752
February	1st to 10th	Day	56% 4 11% 20% 9%	1890
	11th to 20th	Day	47% 6% 13% 21% 10% 1	1853
	21st to end	Day	52% 7% 13% 17% 11% 1	1557
March	1st to 10th	Day	41% 6% 15% 23% 11% 4	4992
	11th to 20th	Day	43% 6% 15% 25% 10% 1	5090
	21st to end	Day	55% 6% 11% 20% 8% 1	5598
April	1st to 10th	Day	53% 5% 10% 19% 12% 2	2520
	11th to 20th	Day	52% 6% 13% 17% 11% 1	2604
	21st to end	Day	52% 6% 10% 21% 11%	2730
May	1st to 10th	Day	36% 6% 13% 26% 17% 1	6208
	11th to 20th	Day	43% 7% 11% 21% 14% 4	6323
	21st to end	Day	40% 8% 11% 28% 13% 1	6949
June	1st to 10th	Day	37% 9% 18% 26% 9% 1	2940
	11th to 20th	Day	46% 7% 14% 21% 12% 1	3000
	21st to end	Day	40% 9% 15% 27% 9% 1	3000
July	1st to 10th	Day	31% 10% 20% 29% 9%	6726
	11th to 20th	Day	32% 7% 18% 33% 9% 1	6641
	21st to end	Day	32% 8% 20% 30% 11%	6728
August	1st to 10th	Day	34% 8% 16% 28% 13% 1	4222
	11th to 20th	Day	37% 9% 17% 30% 8%	4210
	21st to end	Day	41% 7% 16% 26% 9% 1	4580
September	1st to 10th	Day	35% 7% 17% 31% 10%	2400
	11th to 20th	Day	42% 8% 19% 24% 7%	2325
	21st to end	Day	41% 7% 13% 31% 8% 1	2133
October	1st to 10th	Day	35% 6% 14% 31% 13% 1	3260
	11th to 20th	Day	43% 5% 10% 23% 18% 1	3240
	21st to end	Day	53% 5% 14% 22% 5%	3436
November	1st to 10th	Day	55% 7% 10% 18% 10% 1	1647
	11th to 20th	Day	60% 6% 12% 15% 6% 1	1680
	21st to end	Day	59% 7% 14% 15% 4 2	1680
December	1st to 10th	Day	68% 7% 10% 12% 3 1	2690
	11th to 20th	Day	58% 5% 8% 18% 10%	2912
	21st to end	Day	76% 5% 5% 10% 4	3575
January	1st to 10th	Night	58% 6% 13% 13% 10%	8800
	11th to 20th	Night	56% 7% 11% 16% 10%	8606
	21st to end	Night	51% 6% 11% 23% 9%	8446
February	1st to 10th	Night	49% 6% 12% 22% 11% 1	2730
	11th to 20th	Night	43% 7% 13% 24% 12% 1	2664
	21st to end	Night	50% 7% 11% 20% 10% 1	2249
March	1st to 10th	Night	37% 6% 17% 21% 16% 3	5124
	11th to 20th	Night	49% 7% 11% 20% 12%	5090
	21st to end	Night	58% 5% 9% 17% 10% 1	5574
April	1st to 10th	Night	50% 5% 10% 19% 15% 1	2100
	11th to 20th	Night	48% 6% 10% 20% 15% 1	1963
	21st to end	Night	47% 6% 10% 23% 13% 1	1890
May	1st to 10th	Night	36% 6% 13% 27% 17% 1	3780
	11th to 20th	Night	37% 6% 12% 22% 18% 4	3597
	21st to end	Night	42% 8% 11% 25% 14% 1	3950
June	1st to 10th	Night	33% 10% 19% 27% 9% 2	1456
	11th to 20th	Night	37% 9% 14% 23% 16% 1	1400
	21st to end	Night	35% 7% 17% 29% 11% 1	1400
July	1st to 10th	Night	29% 10% 20% 29% 11%	3145
	11th to 20th	Night	23% 7% 20% 37% 11% 2	3214
	21st to end	Night	31% 8% 19% 30% 12% 1	3832
August	1st to 10th	Night	30% 8% 19% 26% 15% 2	2911
	11th to 20th	Night	33% 7% 15% 34% 10% 1	2915
	21st to end	Night	36% 8% 18% 27% 11% 1	3325
September	1st to 10th	Night	30% 9% 16% 34% 11% 1	2000
	11th to 20th	Night	33% 7% 19% 30% 11%	2070
	21st to end	Night	36% 5% 12% 35% 11%	2246
October	1st to 10th	Night	28% 4 9% 36% 18% 4	3920
	11th to 20th	Night	42% 5% 10% 24% 18% 1	3899
	21st to end	Night	48% 6% 13% 22% 10%	4338
November	1st to 10th	Night	49% 7% 12% 23% 8% 1	2871
	11th to 20th	Night	55% 7% 13% 17% 7% 1	2940
	21st to end	Night	54% 9% 15% 17% 5% 1	2940
December	1st to 10th	Night	66% 8% 12% 11% 3	4720
	11th to 20th	Night	61% 6% 11% 14% 9% 1	5502
	21st to end	Night	72% 4 7% 12% 4	7645

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	11% 12% 38% 29% 9% 1	330
	11th to 20th	Day	16% 10% 32% 33% 9%	330
	21st to end	Day	11% 9% 40% 24% 12% 3	363
February	1st to 10th	Day	16% 16% 40% 21% 6% 1	330
	11th to 20th	Day	13% 16% 37% 23% 8% 2	330
	21st to end	Day	14% 11% 43% 27% 4	273
March	1st to 10th	Day	15% 10% 46% 21% 7% 1	330
	11th to 20th	Day	16% 13% 34% 25% 10% 2	330
	21st to end	Day	21% 16% 34% 15% 12% 2	364
April	1st to 10th	Day	23% 15% 34% 23% 5% 1	540
	11th to 20th	Day	23% 14% 31% 22% 10% 1	642
	21st to end	Day	23% 13% 29% 23% 11% 1	750
May	1st to 10th	Day	20% 13% 29% 24% 11% 3	709
	11th to 20th	Day	17% 12% 31% 26% 11% 3	725
	21st to end	Day	16% 10% 28% 30% 14% 2	789
June	1st to 10th	Day	10% 10% 30% 30% 16% 3	730
	11th to 20th	Day	16% 10% 35% 26% 11% 2	730
	21st to end	Day	13% 9% 30% 28% 14% 5%	730
July	1st to 10th	Day	9% 6% 24% 28% 25% 9%	721
	11th to 20th	Day	10% 7% 28% 26% 21% 8%	718
	21st to end	Day	7% 5% 27% 31% 23% 8%	794
August	1st to 10th	Day	12% 6% 23% 31% 23% 6%	726
	11th to 20th	Day	13% 6% 23% 30% 22% 7%	715
	21st to end	Day	14% 8% 24% 31% 19% 5%	748
September	1st to 10th	Day	10% 8% 30% 31% 17% 4	530
	11th to 20th	Day	14% 7% 27% 32% 16% 4	455
	21st to end	Day	18% 10% 32% 20% 13% 6%	330
October	1st to 10th	Day	18% 12% 33% 22% 10% 5%	330
	11th to 20th	Day	20% 17% 34% 22% 6%	330
	21st to end	Day	23% 15% 39% 15% 8% 1	363
November	1st to 10th	Day	18% 15% 35% 23% 7% 1	330
	11th to 20th	Day	19% 16% 41% 18% 4 1	330
	21st to end	Day	18% 15% 39% 23% 5%	330
December	1st to 10th	Day	15% 13% 39% 25% 8% 1	330
	11th to 20th	Day	17% 20% 33% 25% 6%	330
	21st to end	Day	25% 10% 34% 23% 7% 2	363
January	1st to 10th	Night	12% 11% 35% 32% 8% 2	1050
	11th to 20th	Night	14% 11% 37% 28% 9% 1	1050
	21st to end	Night	14% 11% 38% 28% 8% 1	1155
February	1st to 10th	Night	11% 11% 38% 31% 8% 1	1050
	11th to 20th	Night	11% 11% 42% 26% 8% 1	1029
	21st to end	Night	13% 10% 38% 28% 9% 2	865
March	1st to 10th	Night	13% 13% 36% 28% 8% 1	677
	11th to 20th	Night	16% 11% 41% 26% 6% 1	630
	21st to end	Night	21% 10% 30% 27% 11% 2	693
April	1st to 10th	Night	21% 9% 36% 29% 5%	630
	11th to 20th	Night	21% 9% 34% 28% 7% 1	623
	21st to end	Night	23% 7% 30% 30% 10% 1	630
May	1st to 10th	Night	18% 11% 33% 30% 8% 1	603
	11th to 20th	Night	17% 7% 32% 34% 9% 1	591
	21st to end	Night	14% 8% 28% 39% 12%	650
June	1st to 10th	Night	11% 7% 31% 36% 13% 2	600
	11th to 20th	Night	14% 4 34% 40% 8%	600
	21st to end	Night	12% 7% 27% 37% 17% 1	600
July	1st to 10th	Night	6% 2 24% 43% 23% 2	585
	11th to 20th	Night	10% 3 25% 40% 21% 1	582
	21st to end	Night	6% 3 23% 43% 23% 2	657
August	1st to 10th	Night	8% 4 19% 48% 20% 2	597
	11th to 20th	Night	13% 3 19% 38% 25% 2	594
	21st to end	Night	11% 6% 23% 37% 21% 2	660
September	1st to 10th	Night	11% 5% 25% 40% 18% 2	600
	11th to 20th	Night	12% 6% 29% 34% 18% 1	600
	21st to end	Night	16% 7% 33% 27% 14% 2	646
October	1st to 10th	Night	19% 10% 32% 28% 11% 1	800
	11th to 20th	Night	19% 12% 36% 28% 5% 1	800
	21st to end	Night	24% 11% 32% 28% 4	900
November	1st to 10th	Night	17% 12% 33% 30% 8%	1025
	11th to 20th	Night	19% 12% 36% 25% 8% 1	1050
	21st to end	Night	17% 13% 38% 26% 6%	1050
December	1st to 10th	Night	14% 11% 35% 30% 8% 1	1050
	11th to 20th	Night	19% 13% 36% 25% 6% 1	1050
	21st to end	Night	24% 11% 34% 24% 6% 1	1155

Page generated at 2026-04-29 01:21:40. Version ac07

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