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 2025-01-07.

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% 6% 5% 5% 4 6% 7% 35%	105225
	Night	11% 10% 4 10% 6% 5% 5% 4 5% 7% 34%	103935
Transparency	Day	46% 7% 13% 23% 9% 1	108797
	Night	48% 7% 12% 22% 10% 1	108493
Seeing	Day	15% 10% 31% 26% 14% 4	16305
	Night	15% 9% 33% 31% 11% 1	26303

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	7% 15% 6% 15% 7% 6% 5% 3 5% 6% 23%	2260
	11th to 20th	Day	5% 10% 5% 16% 11% 9% 10% 8% 7% 5% 15%	2344
	21st to end	Day	6% 14% 12% 17% 8% 6% 5% 3 5% 6% 18%	3103
February	1st to 10th	Day	8% 13% 6% 11% 6% 5% 5% 4 6% 7% 29%	1710
	11th to 20th	Day	8% 11% 3 9% 7% 8% 6% 4 6% 6% 35%	1664
	21st to end	Day	9% 12% 4 11% 7% 6% 4 3 6% 7% 30%	1413
March	1st to 10th	Day	10% 11% 4 9% 6% 5% 5% 4 5% 7% 34%	3397
	11th to 20th	Day	9% 9% 4 9% 6% 4 5% 4 6% 8% 37%	3480
	21st to end	Day	19% 14% 4 9% 5% 4 3 3 5% 6% 28%	3827
April	1st to 10th	Day	16% 11% 4 10% 6% 4 4 3 4 6% 33%	2280
	11th to 20th	Day	18% 12% 3 8% 5% 4 5% 3 5% 6% 31%	2353
	21st to end	Day	22% 9% 4 7% 5% 5% 4 4 4 5% 32%	2470
May	1st to 10th	Day	19% 8% 2 7% 4 4 5% 4 5% 7% 36%	4291
	11th to 20th	Day	15% 7% 3 6% 4 5% 4 4 6% 7% 41%	4399
	21st to end	Day	17% 7% 3 8% 5% 4 5% 4 6% 8% 33%	4824
June	1st to 10th	Day	12% 5% 3 6% 5% 5% 4 4 7% 9% 41%	2793
	11th to 20th	Day	15% 7% 2 7% 5% 4 5% 4 6% 8% 37%	2850
	21st to end	Day	11% 5% 2 7% 5% 6% 6% 4 8% 9% 37%	2850
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 9% 40%	4750
	11th to 20th	Day	6% 4 2 4 3 4 5% 4 6% 10% 51%	4698
	21st to end	Day	6% 5% 2 6% 4 4 4 5% 8% 9% 47%	4813
August	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 8% 9% 43%	4107
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 5% 6% 8% 42%	4041
	21st to end	Day	15% 7% 3 6% 4 4 5% 4 4 6% 42%	4441
September	1st to 10th	Day	10% 6% 2 6% 4 4 5% 3 6% 9% 45%	2280
	11th to 20th	Day	12% 6% 2 7% 6% 5% 5% 3 6% 7% 41%	2209
	21st to end	Day	16% 8% 2 5% 4 4 4 3 5% 6% 43%	2027
October	1st to 10th	Day	17% 9% 3 7% 5% 3 4 3 5% 6% 38%	1900
	11th to 20th	Day	17% 13% 4 7% 4 4 4 3 4 5% 35%	1900
	21st to end	Day	18% 13% 4 8% 5% 4 4 3 5% 6% 29%	2037
November	1st to 10th	Day	12% 14% 5% 11% 5% 5% 5% 4 4 6% 30%	1567
	11th to 20th	Day	12% 17% 6% 14% 6% 6% 3 3 4 6% 23%	1600
	21st to end	Day	11% 13% 6% 12% 8% 5% 6% 5% 5% 7% 23%	1600
December	1st to 10th	Day	10% 12% 5% 15% 15% 8% 5% 4 5% 5% 17%	2610
	11th to 20th	Day	8% 17% 7% 12% 6% 4 5% 3 6% 5% 26%	2839
	21st to end	Day	17% 18% 6% 20% 9% 5% 3 2 2 4 13%	3498
January	1st to 10th	Night	8% 11% 5% 15% 9% 6% 5% 4 6% 7% 26%	4828
	11th to 20th	Night	5% 12% 5% 13% 9% 8% 9% 5% 6% 6% 20%	4623
	21st to end	Night	7% 16% 12% 15% 6% 5% 5% 3 5% 5% 20%	4562
February	1st to 10th	Night	6% 11% 5% 10% 6% 6% 5% 5% 7% 7% 34%	2470
	11th to 20th	Night	6% 11% 4 9% 7% 4 6% 5% 7% 8% 35%	2403
	21st to end	Night	10% 12% 4 9% 7% 6% 5% 5% 5% 6% 31%	2041
March	1st to 10th	Night	8% 9% 3 8% 5% 4 3 3 6% 7% 43%	3484
	11th to 20th	Night	9% 9% 3 11% 7% 5% 5% 4 5% 5% 36%	3480
	21st to end	Night	19% 16% 6% 10% 5% 3 3 3 3 4 28%	3803
April	1st to 10th	Night	15% 10% 4 9% 4 5% 3 3 5% 6% 36%	1900
	11th to 20th	Night	17% 12% 3 7% 5% 4 3 2 5% 5% 36%	1774
	21st to end	Night	18% 10% 4 7% 4 3 4 3 5% 6% 35%	1710
May	1st to 10th	Night	16% 10% 3 7% 5% 4 4 3 5% 7% 39%	2613
	11th to 20th	Night	14% 7% 2 6% 4 3 4 4 5% 7% 45%	2506
	21st to end	Night	18% 8% 2 7% 5% 4 5% 4 4 6% 36%	2743
June	1st to 10th	Night	9% 6% 2 5% 4 4 5% 4 7% 9% 45%	1383
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 5% 6% 7% 45%	1330
	21st to end	Night	9% 6% 2 6% 4 4 5% 3 7% 8% 46%	1330
July	1st to 10th	Night	9% 3 2 5% 4 4 6% 5% 8% 11% 43%	2220
	11th to 20th	Night	5% 3 1 4 4 3 3 3 6% 8% 61%	2270
	21st to end	Night	5% 3 1 4 4 4 4 4 7% 11% 54%	2750
August	1st to 10th	Night	8% 5% 2 5% 4 3 4 4 6% 8% 50%	2830
	11th to 20th	Night	12% 5% 2 4 3 4 4 3 6% 8% 48%	2798
	21st to end	Night	13% 6% 2 6% 4 3 4 3 5% 9% 44%	3223
September	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 7% 6% 51%	1900
	11th to 20th	Night	9% 5% 2 6% 4 3 5% 3 5% 7% 50%	1966
	21st to end	Night	14% 6% 3 6% 4 3 3 3 5% 6% 49%	2133
October	1st to 10th	Night	15% 7% 2 6% 5% 4 4 3 5% 6% 43%	2280
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 3 4 6% 37%	2280
	21st to end	Night	18% 11% 3 9% 5% 4 4 4 5% 6% 31%	2555
November	1st to 10th	Night	13% 11% 3 9% 6% 5% 4 4 5% 6% 33%	2731
	11th to 20th	Night	14% 13% 5% 10% 7% 5% 4 4 4 6% 27%	2800
	21st to end	Night	12% 9% 4 11% 8% 6% 4 4 6% 7% 28%	2800
December	1st to 10th	Night	6% 11% 6% 17% 11% 11% 7% 4 5% 6% 17%	4580
	11th to 20th	Night	9% 15% 6% 15% 7% 6% 4 4 6% 6% 21%	5356
	21st to end	Night	19% 19% 6% 17% 6% 5% 3 2 4 4 16%	7480

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	61% 6% 10% 17% 6%	2261
	11th to 20th	Day	64% 8% 8% 15% 4	2344
	21st to end	Day	66% 7% 8% 14% 6%	3103
February	1st to 10th	Day	54% 4 11% 21% 10% 1	1710
	11th to 20th	Day	48% 7% 13% 21% 9% 2	1673
	21st to end	Day	54% 6% 11% 16% 12% 1	1413
March	1st to 10th	Day	48% 7% 11% 23% 10%	3397
	11th to 20th	Day	43% 6% 14% 27% 9%	3480
	21st to end	Day	57% 7% 11% 20% 5%	3827
April	1st to 10th	Day	52% 5% 10% 19% 12% 2	2280
	11th to 20th	Day	52% 6% 13% 17% 12% 1	2353
	21st to end	Day	53% 6% 10% 20% 11%	2470
May	1st to 10th	Day	45% 7% 13% 22% 12% 2	4330
	11th to 20th	Day	40% 8% 14% 26% 9% 3	4403
	21st to end	Day	45% 9% 13% 25% 8% 1	4837
June	1st to 10th	Day	37% 9% 18% 26% 9% 1	2793
	11th to 20th	Day	44% 7% 13% 22% 12% 1	2850
	21st to end	Day	39% 9% 15% 27% 9%	2850
July	1st to 10th	Day	35% 11% 17% 24% 12% 1	4676
	11th to 20th	Day	25% 8% 18% 36% 12% 1	4617
	21st to end	Day	30% 9% 19% 32% 11% 1	4662
August	1st to 10th	Day	34% 8% 16% 29% 13% 1	4092
	11th to 20th	Day	36% 9% 17% 30% 8% 1	4080
	21st to end	Day	42% 7% 16% 26% 9% 1	4440
September	1st to 10th	Day	35% 6% 17% 32% 10%	2280
	11th to 20th	Day	41% 8% 18% 25% 8%	2209
	21st to end	Day	42% 6% 13% 30% 7% 1	2027
October	1st to 10th	Day	36% 6% 14% 31% 13% 1	3160
	11th to 20th	Day	42% 6% 10% 23% 18% 1	3140
	21st to end	Day	53% 5% 14% 22% 5%	3329
November	1st to 10th	Day	54% 7% 11% 19% 10% 1	1567
	11th to 20th	Day	62% 6% 12% 13% 7% 1	1600
	21st to end	Day	59% 7% 14% 14% 4 2	1600
December	1st to 10th	Day	68% 7% 10% 11% 3 1	2610
	11th to 20th	Day	58% 5% 8% 18% 10%	2836
	21st to end	Day	76% 5% 5% 10% 4	3498
January	1st to 10th	Night	55% 7% 12% 19% 6%	4828
	11th to 20th	Night	62% 8% 12% 13% 5% 1	4623
	21st to end	Night	64% 6% 8% 17% 5%	4562
February	1st to 10th	Night	47% 6% 12% 23% 11% 1	2470
	11th to 20th	Night	44% 7% 13% 23% 11% 1	2404
	21st to end	Night	51% 7% 10% 20% 11% 1	2041
March	1st to 10th	Night	39% 7% 16% 23% 14% 2	3484
	11th to 20th	Night	45% 7% 12% 23% 12%	3480
	21st to end	Night	60% 5% 9% 17% 8% 1	3803
April	1st to 10th	Night	49% 5% 10% 19% 15% 1	1900
	11th to 20th	Night	49% 5% 8% 21% 15% 1	1774
	21st to end	Night	48% 7% 10% 22% 13% 1	1710
May	1st to 10th	Night	43% 7% 12% 25% 11% 1	2639
	11th to 20th	Night	37% 7% 12% 24% 16% 3	2507
	21st to end	Night	45% 9% 12% 21% 11% 1	2751
June	1st to 10th	Night	32% 11% 19% 27% 9% 2	1383
	11th to 20th	Night	35% 9% 14% 23% 16% 1	1330
	21st to end	Night	34% 7% 17% 29% 11% 1	1330
July	1st to 10th	Night	31% 11% 20% 25% 13%	2185
	11th to 20th	Night	20% 7% 15% 41% 14% 2	2228
	21st to end	Night	23% 9% 19% 35% 13% 1	2662
August	1st to 10th	Night	30% 8% 19% 26% 15% 2	2821
	11th to 20th	Night	33% 7% 15% 34% 10% 1	2825
	21st to end	Night	36% 8% 18% 27% 11% 1	3223
September	1st to 10th	Night	30% 8% 16% 33% 12% 1	1900
	11th to 20th	Night	32% 7% 20% 30% 11%	1966
	21st to end	Night	37% 5% 12% 34% 11%	2133
October	1st to 10th	Night	29% 4 9% 37% 18% 4	3800
	11th to 20th	Night	42% 5% 10% 24% 18% 1	3779
	21st to end	Night	48% 6% 13% 22% 10%	4203
November	1st to 10th	Night	49% 7% 12% 23% 8% 1	2731
	11th to 20th	Night	57% 7% 13% 15% 7% 1	2800
	21st to end	Night	53% 9% 16% 17% 5% 1	2800
December	1st to 10th	Night	66% 8% 13% 11% 2	4580
	11th to 20th	Night	61% 6% 10% 14% 9% 1	5358
	21st to end	Night	73% 4 7% 12% 4	7480

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% 36% 30% 10% 1	288
	11th to 20th	Day	16% 9% 31% 33% 10%	270
	21st to end	Day	12% 6% 37% 26% 14% 4	297
February	1st to 10th	Day	16% 14% 40% 22% 7%	270
	11th to 20th	Day	14% 17% 37% 23% 7% 2	270
	21st to end	Day	15% 11% 42% 28% 4	225
March	1st to 10th	Day	14% 11% 43% 24% 8% 1	270
	11th to 20th	Day	16% 11% 33% 26% 12% 2	270
	21st to end	Day	22% 13% 33% 15% 14% 3	298
April	1st to 10th	Day	22% 15% 32% 25% 6% 1	460
	11th to 20th	Day	24% 14% 27% 23% 11% 1	551
	21st to end	Day	23% 11% 30% 22% 13% 1	650
May	1st to 10th	Day	22% 13% 30% 23% 10% 2	653
	11th to 20th	Day	16% 12% 31% 25% 11% 4	675
	21st to end	Day	17% 10% 28% 29% 13% 2	734
June	1st to 10th	Day	10% 9% 30% 31% 16% 4	680
	11th to 20th	Day	16% 11% 35% 25% 11% 2	680
	21st to end	Day	13% 9% 30% 28% 15% 6%	680
July	1st to 10th	Day	9% 6% 23% 27% 25% 9%	671
	11th to 20th	Day	9% 7% 28% 26% 21% 9%	668
	21st to end	Day	7% 4 26% 32% 23% 8%	739
August	1st to 10th	Day	11% 5% 23% 31% 24% 6%	676
	11th to 20th	Day	12% 6% 23% 31% 22% 7%	665
	21st to end	Day	14% 8% 23% 31% 19% 5%	696
September	1st to 10th	Day	10% 7% 30% 32% 18% 3	490
	11th to 20th	Day	13% 6% 28% 31% 17% 4	419
	21st to end	Day	20% 11% 34% 19% 12% 4	300
October	1st to 10th	Day	20% 13% 30% 21% 11% 5%	300
	11th to 20th	Day	18% 16% 35% 24% 7%	300
	21st to end	Day	23% 15% 38% 15% 7% 1	330
November	1st to 10th	Day	18% 15% 35% 24% 7% 1	300
	11th to 20th	Day	20% 15% 41% 19% 4	300
	21st to end	Day	18% 15% 41% 22% 4	300
December	1st to 10th	Day	15% 12% 39% 24% 8% 1	300
	11th to 20th	Day	17% 19% 33% 25% 6%	300
	21st to end	Day	24% 10% 33% 24% 8% 2	330
January	1st to 10th	Night	12% 11% 35% 32% 8% 2	981
	11th to 20th	Night	14% 9% 37% 29% 9% 1	950
	21st to end	Night	16% 10% 36% 28% 9% 1	1045
February	1st to 10th	Night	11% 10% 38% 32% 9% 1	950
	11th to 20th	Night	11% 12% 42% 25% 9% 2	929
	21st to end	Night	14% 9% 37% 28% 10% 2	785
March	1st to 10th	Night	13% 13% 37% 28% 9% 1	613
	11th to 20th	Night	15% 10% 41% 27% 7% 1	570
	21st to end	Night	21% 9% 28% 28% 12% 2	627
April	1st to 10th	Night	21% 7% 37% 29% 6%	570
	11th to 20th	Night	22% 9% 32% 28% 8% 1	563
	21st to end	Night	21% 7% 31% 29% 11% 1	570
May	1st to 10th	Night	18% 11% 33% 30% 7% 1	570
	11th to 20th	Night	17% 6% 32% 34% 9% 1	561
	21st to end	Night	14% 8% 28% 38% 12%	617
June	1st to 10th	Night	11% 6% 31% 36% 13% 2	570
	11th to 20th	Night	13% 4 35% 41% 8%	570
	21st to end	Night	12% 6% 26% 38% 17% 1	570
July	1st to 10th	Night	5% 2 24% 43% 24% 2	555
	11th to 20th	Night	10% 3 25% 39% 22% 1	552
	21st to end	Night	6% 3 22% 42% 24% 2	624
August	1st to 10th	Night	7% 4 19% 49% 20% 2	567
	11th to 20th	Night	13% 3 19% 38% 25% 2	564
	21st to end	Night	11% 5% 24% 37% 21% 2	627
September	1st to 10th	Night	11% 5% 24% 41% 18% 1	570
	11th to 20th	Night	12% 6% 29% 34% 18% 1	570
	21st to end	Night	16% 7% 34% 27% 13% 2	613
October	1st to 10th	Night	19% 10% 31% 28% 11% 1	760
	11th to 20th	Night	19% 11% 36% 28% 5% 1	760
	21st to end	Night	24% 11% 32% 28% 4	855
November	1st to 10th	Night	18% 11% 32% 30% 9%	975
	11th to 20th	Night	19% 12% 36% 25% 8% 1	1000
	21st to end	Night	17% 13% 38% 27% 6%	1000
December	1st to 10th	Night	14% 11% 34% 31% 8% 1	1000
	11th to 20th	Night	19% 11% 37% 25% 6% 1	1000
	21st to end	Night	24% 11% 34% 24% 7% 1	1100

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