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 2024-01-17.

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	13% 9% 4 7% 6% 4 5% 5% 5% 7% 35%	74801
	Night	12% 9% 4 8% 6% 3 5% 5% 5% 7% 36%	75032
Transparency	Day	47% 7% 13% 22% 9% 1	74490
	Night	47% 7% 13% 22% 10% 1	74863
Seeing	Day	15% 10% 31% 26% 14% 3	14931
	Night	16% 9% 33% 31% 11% 1	24980

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	9% 10% 6% 10% 9% 5% 6% 4 6% 8% 26%	1328
	11th to 20th	Day	9% 14% 6% 14% 10% 4 6% 6% 6% 7% 17%	1374
	21st to end	Day	8% 13% 7% 14% 10% 6% 6% 6% 6% 7% 17%	1764
February	1st to 10th	Day	8% 12% 6% 10% 7% 4 6% 5% 6% 7% 29%	1620
	11th to 20th	Day	8% 11% 3 7% 8% 6% 6% 5% 5% 6% 34%	1574
	21st to end	Day	10% 11% 5% 10% 9% 5% 5% 4 6% 6% 29%	1332
March	1st to 10th	Day	12% 11% 5% 9% 7% 4 6% 5% 5% 8% 30%	1953
	11th to 20th	Day	13% 10% 4 8% 7% 3 5% 5% 5% 7% 34%	1980
	21st to end	Day	18% 11% 4 8% 5% 3 5% 4 5% 6% 31%	2178
April	1st to 10th	Day	16% 11% 5% 9% 7% 3 4 4 4 6% 32%	2160
	11th to 20th	Day	18% 12% 4 7% 6% 3 5% 4 5% 7% 31%	2227
	21st to end	Day	22% 9% 4 6% 5% 4 4 4 5% 6% 32%	2340
May	1st to 10th	Day	19% 9% 3 7% 5% 3 5% 5% 4 6% 35%	2452
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 4 5% 8% 38%	2479
	21st to end	Day	15% 6% 3 6% 5% 3 5% 5% 6% 8% 37%	2725
June	1st to 10th	Day	12% 5% 3 6% 5% 3 5% 5% 6% 9% 41%	2646
	11th to 20th	Day	14% 6% 3 6% 6% 3 5% 5% 6% 8% 37%	2700
	21st to end	Day	11% 4 2 6% 6% 4 6% 6% 8% 10% 37%	2700
July	1st to 10th	Day	7% 4 2 4 4 3 4 5% 7% 9% 51%	2700
	11th to 20th	Day	10% 5% 3 4 4 4 5% 6% 6% 9% 44%	2674
	21st to end	Day	5% 4 2 4 5% 3 5% 6% 7% 9% 50%	2743
August	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 5% 7% 8% 46%	2331
	11th to 20th	Day	12% 6% 2 6% 5% 3 4 5% 6% 7% 44%	2301
	21st to end	Day	11% 5% 2 5% 5% 2 4 6% 5% 7% 48%	2525
September	1st to 10th	Day	11% 6% 2 5% 6% 3 5% 4 6% 9% 44%	2160
	11th to 20th	Day	13% 6% 3 6% 7% 4 5% 4 6% 7% 40%	2093
	21st to end	Day	17% 8% 3 5% 5% 3 4 4 5% 6% 41%	1921
October	1st to 10th	Day	18% 9% 3 6% 5% 3 4 4 5% 6% 37%	1800
	11th to 20th	Day	18% 13% 4 6% 5% 4 4 4 4 5% 34%	1800
	21st to end	Day	18% 12% 5% 7% 6% 3 4 5% 5% 6% 29%	1931
November	1st to 10th	Day	12% 12% 5% 10% 6% 4 5% 4 4 6% 31%	1487
	11th to 20th	Day	12% 16% 7% 13% 7% 5% 4 4 4 6% 23%	1520
	21st to end	Day	11% 12% 6% 10% 9% 4 6% 6% 5% 7% 24%	1520
December	1st to 10th	Day	12% 13% 5% 12% 10% 5% 7% 4 5% 6% 22%	1520
	11th to 20th	Day	10% 14% 6% 11% 9% 4 7% 5% 5% 5% 22%	1801
	21st to end	Day	21% 16% 7% 11% 10% 4 4 4 3 4 17%	2442
January	1st to 10th	Night	8% 8% 5% 10% 10% 5% 6% 6% 7% 9% 27%	2837
	11th to 20th	Night	9% 13% 7% 11% 9% 4 6% 6% 5% 7% 24%	2728
	21st to end	Night	10% 13% 6% 11% 9% 4 6% 6% 6% 8% 21%	2590
February	1st to 10th	Night	6% 10% 6% 9% 7% 4 6% 7% 6% 7% 34%	2340
	11th to 20th	Night	6% 10% 4 8% 7% 3 6% 6% 7% 8% 35%	2273
	21st to end	Night	11% 11% 4 8% 8% 4 6% 6% 5% 6% 31%	1924
March	1st to 10th	Night	10% 11% 4 8% 8% 3 5% 4 5% 7% 35%	2021
	11th to 20th	Night	11% 10% 3 6% 6% 3 5% 5% 5% 5% 41%	1980
	21st to end	Night	15% 10% 5% 7% 7% 3 4 5% 5% 6% 33%	2177
April	1st to 10th	Night	15% 9% 4 8% 5% 4 4 4 4 6% 35%	1800
	11th to 20th	Night	18% 12% 3 6% 6% 3 3 3 5% 5% 36%	1680
	21st to end	Night	18% 9% 4 7% 5% 2 3 5% 5% 6% 35%	1620
May	1st to 10th	Night	16% 9% 4 7% 5% 3 3 3 5% 6% 40%	1507
	11th to 20th	Night	15% 6% 3 5% 4 2 5% 5% 6% 8% 40%	1416
	21st to end	Night	13% 5% 3 5% 6% 3 6% 6% 5% 8% 40%	1551
June	1st to 10th	Night	9% 5% 2 4 5% 3 5% 5% 7% 10% 45%	1311
	11th to 20th	Night	9% 5% 2 5% 5% 3 6% 6% 5% 8% 46%	1260
	21st to end	Night	9% 5% 2 5% 5% 2 5% 5% 6% 8% 47%	1260
July	1st to 10th	Night	4 2 2 4 3 3 4 5% 6% 10% 57%	1260
	11th to 20th	Night	8% 4 1 3 5% 3 3 4 6% 8% 55%	1284
	21st to end	Night	4 2 1 3 4 2 5% 5% 6% 9% 58%	1575
August	1st to 10th	Night	6% 4 2 4 4 3 4 5% 6% 10% 52%	1605
	11th to 20th	Night	11% 5% 2 4 4 4 4 3 4 8% 51%	1600
	21st to end	Night	11% 3 1 4 4 2 4 4 5% 8% 53%	1831
September	1st to 10th	Night	9% 5% 2 4 5% 3 3 5% 7% 6% 51%	1800
	11th to 20th	Night	9% 5% 3 5% 5% 2 4 5% 4 8% 50%	1862
	21st to end	Night	15% 5% 3 5% 5% 3 3 3 5% 6% 47%	2020
October	1st to 10th	Night	16% 7% 2 6% 6% 3 4 4 5% 6% 41%	2160
	11th to 20th	Night	15% 10% 3 8% 7% 3 4 5% 4 6% 36%	2160
	21st to end	Night	19% 10% 4 8% 6% 2 4 5% 5% 6% 32%	2420
November	1st to 10th	Night	13% 9% 4 8% 6% 4 4 5% 6% 6% 34%	2591
	11th to 20th	Night	14% 12% 5% 9% 8% 4 5% 5% 4 6% 27%	2660
	21st to end	Night	12% 9% 5% 9% 8% 4 6% 5% 6% 7% 29%	2660
December	1st to 10th	Night	8% 13% 6% 13% 9% 5% 6% 5% 5% 7% 23%	2660
	11th to 20th	Night	11% 14% 5% 12% 7% 5% 6% 6% 5% 7% 23%	3384
	21st to end	Night	21% 17% 6% 10% 7% 4 4 3 4 5% 19%	5225

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	56% 5% 11% 20% 7% 1	1329
	11th to 20th	Day	63% 7% 9% 14% 6%	1374
	21st to end	Day	65% 6% 9% 12% 7%	1764
February	1st to 10th	Day	53% 4 12% 20% 10% 1	1620
	11th to 20th	Day	50% 7% 13% 20% 9% 2	1583
	21st to end	Day	55% 7% 10% 17% 11% 1	1332
March	1st to 10th	Day	52% 7% 15% 20% 6%	1953
	11th to 20th	Day	50% 6% 14% 21% 9% 1	1980
	21st to end	Day	54% 6% 12% 20% 8% 1	2178
April	1st to 10th	Day	53% 5% 10% 19% 12% 2	2160
	11th to 20th	Day	53% 6% 12% 17% 11% 1	2227
	21st to end	Day	53% 6% 10% 20% 11%	2340
May	1st to 10th	Day	50% 5% 12% 22% 10% 1	2452
	11th to 20th	Day	44% 7% 13% 25% 9% 2	2483
	21st to end	Day	44% 8% 14% 23% 11% 1	2725
June	1st to 10th	Day	38% 9% 18% 26% 9% 1	2646
	11th to 20th	Day	44% 7% 14% 22% 12% 1	2700
	21st to end	Day	39% 9% 15% 27% 9% 1	2700
July	1st to 10th	Day	28% 8% 16% 32% 15% 1	2626
	11th to 20th	Day	34% 9% 16% 26% 13% 2	2593
	21st to end	Day	28% 9% 19% 29% 14% 1	2591
August	1st to 10th	Day	34% 9% 14% 32% 10% 2	2316
	11th to 20th	Day	37% 9% 17% 27% 9% 1	2301
	21st to end	Day	35% 7% 14% 29% 13% 1	2525
September	1st to 10th	Day	36% 6% 16% 31% 11%	2160
	11th to 20th	Day	43% 8% 17% 24% 8%	2093
	21st to end	Day	44% 7% 13% 30% 6%	1921
October	1st to 10th	Day	47% 8% 15% 23% 7% 1	1800
	11th to 20th	Day	53% 6% 10% 21% 9% 1	1800
	21st to end	Day	56% 6% 13% 19% 6%	1931
November	1st to 10th	Day	54% 7% 11% 19% 10% 1	1487
	11th to 20th	Day	62% 7% 12% 12% 7% 1	1520
	21st to end	Day	58% 8% 14% 15% 4 2	1520
December	1st to 10th	Day	63% 7% 11% 15% 5%	1520
	11th to 20th	Day	62% 7% 10% 17% 4	1798
	21st to end	Day	70% 6% 5% 12% 6%	2442
January	1st to 10th	Night	51% 7% 13% 20% 8% 1	2837
	11th to 20th	Night	59% 7% 13% 14% 7% 1	2728
	21st to end	Night	59% 7% 9% 17% 8%	2590
February	1st to 10th	Night	48% 6% 13% 22% 11% 1	2340
	11th to 20th	Night	45% 7% 13% 23% 11% 1	2274
	21st to end	Night	51% 7% 10% 20% 11% 1	1924
March	1st to 10th	Night	49% 7% 15% 20% 10% 1	2021
	11th to 20th	Night	44% 7% 14% 23% 12% 1	1980
	21st to end	Night	51% 6% 13% 20% 8% 2	2177
April	1st to 10th	Night	50% 5% 10% 18% 15% 1	1800
	11th to 20th	Night	50% 5% 9% 20% 15% 1	1680
	21st to end	Night	49% 7% 10% 22% 12% 1	1620
May	1st to 10th	Night	45% 6% 12% 25% 11% 1	1507
	11th to 20th	Night	40% 9% 15% 22% 13% 2	1417
	21st to end	Night	41% 8% 13% 20% 16% 2	1552
June	1st to 10th	Night	33% 11% 20% 27% 8% 2	1311
	11th to 20th	Night	34% 10% 15% 24% 16% 2	1260
	21st to end	Night	34% 7% 17% 28% 12% 2	1260
July	1st to 10th	Night	22% 8% 20% 33% 16% 1	1225
	11th to 20th	Night	26% 8% 16% 31% 15% 4	1242
	21st to end	Night	22% 7% 18% 33% 17% 2	1487
August	1st to 10th	Night	26% 8% 18% 34% 10% 4	1596
	11th to 20th	Night	33% 7% 14% 32% 13% 1	1600
	21st to end	Night	29% 6% 15% 30% 18% 2	1831
September	1st to 10th	Night	31% 8% 16% 32% 12% 1	1800
	11th to 20th	Night	33% 7% 19% 30% 11%	1862
	21st to end	Night	38% 5% 13% 34% 9%	2020
October	1st to 10th	Night	43% 6% 13% 26% 11% 1	2160
	11th to 20th	Night	49% 6% 11% 24% 10%	2160
	21st to end	Night	52% 7% 12% 20% 8%	2420
November	1st to 10th	Night	49% 7% 11% 23% 8% 1	2591
	11th to 20th	Night	57% 8% 13% 15% 8% 1	2660
	21st to end	Night	52% 9% 16% 18% 5% 1	2660
December	1st to 10th	Night	60% 7% 15% 14% 4 1	2660
	11th to 20th	Night	58% 7% 13% 14% 7% 1	3386
	21st to end	Night	68% 5% 9% 13% 5%	5225

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% 11% 36% 30% 10% 1	270
	11th to 20th	Day	17% 9% 30% 33% 10%	258
	21st to end	Day	14% 6% 40% 24% 14% 3	264
February	1st to 10th	Day	16% 14% 44% 20% 6%	240
	11th to 20th	Day	16% 17% 36% 23% 6% 2	240
	21st to end	Day	15% 10% 42% 29% 4 1	198
March	1st to 10th	Day	14% 12% 41% 24% 9% 1	240
	11th to 20th	Day	17% 10% 31% 27% 13% 3	240
	21st to end	Day	21% 13% 33% 16% 15% 3	264
April	1st to 10th	Day	22% 15% 33% 23% 6% 1	420
	11th to 20th	Day	25% 12% 27% 23% 12% 1	505
	21st to end	Day	24% 10% 30% 21% 13% 1	600
May	1st to 10th	Day	21% 13% 30% 23% 10% 2	603
	11th to 20th	Day	16% 13% 31% 25% 11% 4	625
	21st to end	Day	16% 10% 27% 31% 14% 3	679
June	1st to 10th	Day	10% 8% 29% 32% 17% 4	630
	11th to 20th	Day	15% 9% 36% 26% 12% 2	630
	21st to end	Day	12% 9% 28% 29% 16% 6%	630
July	1st to 10th	Day	9% 6% 24% 28% 25% 8%	621
	11th to 20th	Day	10% 8% 29% 27% 21% 7%	618
	21st to end	Day	7% 4 26% 33% 23% 8%	684
August	1st to 10th	Day	9% 5% 23% 31% 25% 7%	626
	11th to 20th	Day	12% 6% 23% 30% 21% 7%	615
	21st to end	Day	13% 8% 23% 31% 20% 5%	644
September	1st to 10th	Day	11% 8% 28% 33% 18% 3	450
	11th to 20th	Day	14% 7% 28% 30% 17% 4	383
	21st to end	Day	22% 11% 35% 17% 11% 4	270
October	1st to 10th	Day	20% 11% 30% 21% 11% 6%	270
	11th to 20th	Day	18% 17% 34% 23% 7%	270
	21st to end	Day	24% 15% 39% 14% 7% 1	297
November	1st to 10th	Day	18% 14% 35% 25% 7% 1	270
	11th to 20th	Day	18% 15% 42% 20% 4	270
	21st to end	Day	18% 15% 41% 22% 4	270
December	1st to 10th	Day	14% 13% 38% 25% 9% 1	270
	11th to 20th	Day	16% 19% 33% 27% 6%	270
	21st to end	Day	24% 10% 33% 24% 7% 2	297
January	1st to 10th	Night	13% 10% 35% 32% 9% 2	950
	11th to 20th	Night	14% 10% 37% 29% 9% 1	935
	21st to end	Night	17% 10% 37% 27% 9% 1	990
February	1st to 10th	Night	10% 10% 39% 31% 8% 1	900
	11th to 20th	Night	12% 12% 42% 24% 9% 2	879
	21st to end	Night	15% 9% 36% 28% 10% 2	740
March	1st to 10th	Night	13% 13% 36% 28% 9% 1	581
	11th to 20th	Night	15% 9% 40% 27% 7% 1	540
	21st to end	Night	20% 10% 27% 28% 13% 2	594
April	1st to 10th	Night	21% 7% 36% 29% 6%	540
	11th to 20th	Night	23% 9% 30% 29% 8% 1	533
	21st to end	Night	22% 7% 30% 29% 11% 1	540
May	1st to 10th	Night	18% 10% 34% 29% 7% 1	540
	11th to 20th	Night	18% 6% 31% 34% 10% 1	531
	21st to end	Night	14% 7% 29% 38% 12% 1	584
June	1st to 10th	Night	11% 6% 31% 37% 14% 2	540
	11th to 20th	Night	12% 4 35% 41% 8%	540
	21st to end	Night	12% 6% 25% 38% 18% 1	540
July	1st to 10th	Night	5% 2 25% 43% 23% 1	525
	11th to 20th	Night	10% 3 26% 39% 20% 1	522
	21st to end	Night	6% 3 22% 43% 23% 2	591
August	1st to 10th	Night	6% 3 18% 51% 20% 2	537
	11th to 20th	Night	13% 3 19% 38% 25% 2	534
	21st to end	Night	12% 5% 23% 37% 22% 2	594
September	1st to 10th	Night	12% 4 24% 41% 17% 1	540
	11th to 20th	Night	12% 7% 28% 34% 18% 1	540
	21st to end	Night	17% 7% 34% 27% 12% 3	580
October	1st to 10th	Night	20% 10% 31% 28% 12% 1	720
	11th to 20th	Night	19% 11% 35% 28% 5% 1	720
	21st to end	Night	25% 11% 31% 28% 5%	810
November	1st to 10th	Night	17% 11% 33% 30% 9%	925
	11th to 20th	Night	18% 12% 36% 25% 8% 1	950
	21st to end	Night	17% 13% 38% 27% 6%	950
December	1st to 10th	Night	14% 12% 33% 31% 9% 1	950
	11th to 20th	Night	20% 10% 37% 26% 7% 1	950
	21st to end	Night	25% 10% 33% 25% 6% 1	1045

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