NF/ Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NF/ Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	3 3 1 4 3 4 4 4 5% 7% 62%	146369
	Night	3 3 1 4 3 3 4 3 5% 7% 63%	143887
Transparency	Day	19% 5% 7% 14% 28% 27%	141344
	Night	19% 4 7% 14% 30% 26%	139919
Seeing	Day	8% 14% 28% 29% 17% 4	16716
	Night	5% 7% 24% 38% 22% 4	26720

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	2 7% 2 6% 4 4 3 3 5% 5% 58%	5318
	11th to 20th	Day	2 3 2 1 2 2 2 3 5% 79%	5340
	21st to end	Day	7% 4 2 4 3 3 3 3 6% 7% 59%	5844
February	1st to 10th	Day	2 5% 2 4 3 3 3 3 5% 5% 65%	1890
	11th to 20th	Day	5% 4 2 4 3 3 3 4 4 6% 63%	1982
	21st to end	Day	3 6% 1 3 2 2 3 3 4 4 68%	1882
March	1st to 10th	Day	2 5% 1 4 3 3 3 3 3 6% 67%	5064
	11th to 20th	Day	5% 5% 1 4 3 3 3 4 5% 6% 62%	5110
	21st to end	Day	1 4 2 5% 4 4 4 4 6% 7% 59%	5607
April	1st to 10th	Day	2 2 2 3 3 3 3 3 4 6% 69%	2456
	11th to 20th	Day	2 3 1 2 2 2 2 2 3 5% 74%	2491
	21st to end	Day	2 2 1 2 2 1 2 2 4 5% 76%	2520
May	1st to 10th	Day	1 3 1 3 3 3 3 3 4 6% 70%	7057
	11th to 20th	Day	1 1 1 2 2 2 3 3 4 5% 77%	7426
	21st to end	Day	1 1 1 2 2 2 2 2 3 4 79%	8370
June	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 8% 66%	2730
	11th to 20th	Day	2 2 1 2 2 2 3 2 4 6% 73%	2730
	21st to end	Day	4 3 1 5% 4 5% 5% 5% 6% 7% 55%	2730
July	1st to 10th	Day	3 3 1 5% 5% 5% 6% 5% 6% 9% 52%	6009
	11th to 20th	Day	2 2 1 5% 6% 7% 8% 6% 9% 11% 42%	5946
	21st to end	Day	4 5% 3 8% 6% 8% 8% 6% 9% 11% 32%	6401
August	1st to 10th	Day	3 2 1 5% 4 5% 6% 6% 9% 10% 49%	5779
	11th to 20th	Day	3 3 1 5% 5% 6% 6% 6% 9% 11% 45%	5371
	21st to end	Day	3 3 2 6% 5% 6% 7% 6% 8% 10% 45%	5984
September	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 7% 8% 45%	2400
	11th to 20th	Day	6% 4 1 4 3 4 5% 4 6% 7% 56%	2312
	21st to end	Day	5% 2 1 3 2 3 3 3 4 7% 67%	2145
October	1st to 10th	Day	3 2 1 3 3 2 3 3 4 6% 70%	3270
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 4 73%	3250
	21st to end	Day	2 1 1 1 1 1 1 1 2 4 84%	3548
November	1st to 10th	Day	2 2 1 2 3 3 3 2 3 6% 73%	2059
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 5% 71%	2100
	21st to end	Day	5% 5% 2 4 3 4 4 3 5% 6% 61%	2100
December	1st to 10th	Day	4 3 1 3 2 2 4 3 4 5% 68%	3040
	11th to 20th	Day	4 4 2 5% 3 3 3 2 4 7% 64%	3499
	21st to end	Day	2 3 1 2 2 3 3 3 4 6% 69%	4609
January	1st to 10th	Night	3 6% 2 5% 4 4 4 4 6% 8% 54%	7605
	11th to 20th	Night	2 2 1 1 2 2 2 1 2 4 82%	7650
	21st to end	Night	6% 5% 2 5% 3 3 4 4 6% 7% 57%	8370
February	1st to 10th	Night	2 3 1 4 3 3 4 4 6% 5% 65%	2730
	11th to 20th	Night	5% 4 2 4 2 3 3 3 4 6% 63%	2524
	21st to end	Night	3 4 1 3 2 3 3 3 5% 5% 68%	1920
March	1st to 10th	Night	2 3 1 3 3 2 3 3 5% 8% 68%	5050
	11th to 20th	Night	5% 4 1 3 2 2 3 3 5% 7% 65%	5110
	21st to end	Night	2 2 1 3 3 3 3 3 5% 8% 68%	5610
April	1st to 10th	Night	2 2 1 2 2 2 4 3 5% 6% 70%	2163
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 5% 75%	2077
	21st to end	Night	1 2 2 2 3 2 2 4 4 76%	2100
May	1st to 10th	Night	3 3 1 3 2 3 3 3 4 5% 70%	5890
	11th to 20th	Night	1 1 2 2 2 2 2 4 5% 77%	5449
	21st to end	Night	1 3 1 3 2 2 2 2 3 4 78%	5837
June	1st to 10th	Night	3 3 1 4 3 3 3 4 6% 6% 64%	1890
	11th to 20th	Night	2 2 1 2 2 3 3 3 4 6% 72%	1890
	21st to end	Night	4 4 2 5% 5% 4 5% 5% 7% 8% 50%	1890
July	1st to 10th	Night	4 3 1 5% 5% 5% 5% 5% 8% 10% 49%	4190
	11th to 20th	Night	2 3 2 6% 5% 6% 7% 6% 10% 14% 38%	4145
	21st to end	Night	3 5% 3 9% 6% 7% 7% 7% 9% 12% 32%	4472
August	1st to 10th	Night	2 3 2 6% 4 6% 6% 6% 10% 14% 41%	4033
	11th to 20th	Night	2 3 2 6% 6% 6% 8% 7% 11% 15% 33%	4475
	21st to end	Night	3 5% 2 7% 6% 6% 7% 6% 9% 10% 38%	4993
September	1st to 10th	Night	6% 5% 2 5% 5% 5% 5% 5% 7% 9% 47%	2000
	11th to 20th	Night	6% 3 1 4 4 4 4 3 5% 7% 59%	2082
	21st to end	Night	5% 3 1 3 2 3 3 3 4 5% 68%	2255
October	1st to 10th	Night	3 2 1 3 2 3 3 3 5% 5% 71%	3920
	11th to 20th	Night	4 3 1 3 2 2 2 2 3 4 74%	3898
	21st to end	Night	2 1 1 1 2 2 2 3 5% 80%	4252
November	1st to 10th	Night	2 2 1 2 2 2 3 2 4 7% 74%	2460
	11th to 20th	Night	3 3 1 3 2 2 2 2 5% 5% 72%	2520
	21st to end	Night	6% 4 2 4 2 3 3 2 4 7% 63%	2520
December	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 74%	4365
	11th to 20th	Night	2 3 1 4 3 3 4 3 4 6% 65%	4941
	21st to end	Night	4 4 1 3 2 3 2 3 5% 6% 68%	6611

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	25% 3 4 14% 37% 15%	5321
	11th to 20th	Day	9% 2 3 13% 36% 37%	5340
	21st to end	Day	23% 3 4 12% 28% 28%	5844
February	1st to 10th	Day	19% 4 5% 14% 27% 32%	1890
	11th to 20th	Day	21% 4 6% 15% 29% 25%	1992
	21st to end	Day	19% 2 4 11% 31% 33%	1882
March	1st to 10th	Day	18% 2 4 7% 34% 36%	5064
	11th to 20th	Day	20% 5% 5% 14% 33% 24%	5110
	21st to end	Day	20% 4 5% 13% 36% 22%	5607
April	1st to 10th	Day	15% 2 5% 11% 36% 30%	2456
	11th to 20th	Day	13% 2 4 9% 32% 41%	2494
	21st to end	Day	11% 2 5% 11% 33% 39%	2520
May	1st to 10th	Day	13% 5% 5% 12% 27% 38%	7057
	11th to 20th	Day	9% 3 4 11% 31% 43%	7429
	21st to end	Day	9% 2 3 7% 20% 58%	8371
June	1st to 10th	Day	16% 3 5% 11% 30% 34%	2730
	11th to 20th	Day	11% 3 4 8% 22% 52%	2730
	21st to end	Day	23% 7% 10% 13% 19% 27%	2730
July	1st to 10th	Day	23% 9% 12% 16% 23% 18%	5946
	11th to 20th	Day	25% 11% 20% 22% 19% 3	5877
	21st to end	Day	37% 12% 21% 17% 10% 4	6217
August	1st to 10th	Day	27% 11% 16% 28% 14% 3	4208
	11th to 20th	Day	23% 8% 13% 20% 31% 4	3887
	21st to end	Day	33% 9% 17% 17% 18% 5%	4311
September	1st to 10th	Day	33% 8% 10% 17% 20% 13%	2400
	11th to 20th	Day	23% 6% 8% 17% 29% 16%	2312
	21st to end	Day	16% 4 6% 19% 32% 22%	2145
October	1st to 10th	Day	13% 4 7% 11% 29% 36%	3270
	11th to 20th	Day	16% 3 4 11% 38% 28%	3250
	21st to end	Day	8% 1 2 10% 34% 45%	3548
November	1st to 10th	Day	14% 2 5% 17% 34% 28%	2059
	11th to 20th	Day	17% 3 4 16% 32% 28%	2100
	21st to end	Day	23% 3 6% 16% 31% 20%	2100
December	1st to 10th	Day	18% 4 5% 19% 36% 20%	3040
	11th to 20th	Day	22% 3 4 13% 37% 21%	3498
	21st to end	Day	15% 3 6% 18% 36% 22%	4609
January	1st to 10th	Night	25% 5% 6% 16% 36% 13%	7605
	11th to 20th	Night	9% 1 3 11% 41% 34%	7650
	21st to end	Night	26% 3 4 12% 26% 28%	8370
February	1st to 10th	Night	17% 4 6% 16% 26% 30%	2730
	11th to 20th	Night	20% 3 6% 17% 30% 23%	2524
	21st to end	Night	17% 3 4 13% 31% 31%	1920
March	1st to 10th	Night	13% 3 4 9% 39% 31%	5050
	11th to 20th	Night	17% 4 5% 15% 36% 23%	5110
	21st to end	Night	14% 3 4 21% 38% 20%	5610
April	1st to 10th	Night	13% 2 6% 14% 40% 25%	2163
	11th to 20th	Night	12% 2 4 11% 32% 39%	2077
	21st to end	Night	10% 3 4 14% 36% 33%	2100
May	1st to 10th	Night	16% 4 5% 12% 30% 33%	5888
	11th to 20th	Night	9% 3 4 11% 35% 38%	5450
	21st to end	Night	12% 2 3 5% 21% 57%	5838
June	1st to 10th	Night	18% 4 6% 11% 27% 34%	1890
	11th to 20th	Night	13% 3 5% 8% 23% 49%	1890
	21st to end	Night	28% 7% 10% 12% 16% 27%	1890
July	1st to 10th	Night	25% 8% 14% 19% 20% 14%	4145
	11th to 20th	Night	29% 10% 21% 19% 18% 3	4091
	21st to end	Night	39% 9% 23% 16% 7% 5%	4344
August	1st to 10th	Night	28% 11% 20% 27% 9% 4	2928
	11th to 20th	Night	29% 9% 15% 17% 25% 4	3235
	21st to end	Night	40% 10% 17% 16% 14% 2	3597
September	1st to 10th	Night	29% 9% 13% 18% 18% 14%	2000
	11th to 20th	Night	23% 7% 10% 18% 25% 17%	2082
	21st to end	Night	18% 4 7% 17% 33% 21%	2255
October	1st to 10th	Night	14% 5% 6% 10% 32% 34%	3920
	11th to 20th	Night	14% 3 4 13% 36% 30%	3898
	21st to end	Night	9% 2 2 11% 31% 46%	4252
November	1st to 10th	Night	12% 2 5% 14% 37% 29%	2460
	11th to 20th	Night	15% 2 4 18% 31% 30%	2520
	21st to end	Night	21% 3 5% 17% 31% 22%	2520
December	1st to 10th	Night	13% 3 5% 21% 40% 17%	4365
	11th to 20th	Night	19% 3 6% 12% 33% 27%	4941
	21st to end	Night	17% 3 5% 18% 34% 24%	6611

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	15% 17% 34% 24% 10%	330
	11th to 20th	Day	12% 15% 32% 25% 12% 4	330
	21st to end	Day	9% 16% 35% 26% 12% 2	363
February	1st to 10th	Day	6% 20% 31% 27% 12% 4	330
	11th to 20th	Day	18% 18% 32% 26% 6%	330
	21st to end	Day	16% 34% 30% 15% 4	425
March	1st to 10th	Day	15% 27% 35% 18% 4 1	540
	11th to 20th	Day	16% 22% 31% 21% 8% 2	540
	21st to end	Day	10% 31% 38% 14% 6% 1	594
April	1st to 10th	Day	10% 24% 38% 21% 6% 2	540
	11th to 20th	Day	11% 29% 36% 18% 5% 1	532
	21st to end	Day	11% 25% 39% 18% 6% 1	540
May	1st to 10th	Day	8% 21% 41% 22% 8%	543
	11th to 20th	Day	5% 17% 37% 32% 7% 2	568
	21st to end	Day	4 17% 41% 29% 9% 1	617
June	1st to 10th	Day	4 9% 36% 35% 14% 1	570
	11th to 20th	Day	3 10% 38% 33% 15% 2	570
	21st to end	Day	5% 5% 27% 39% 20% 4	570
July	1st to 10th	Day	5% 2 16% 42% 28% 8%	566
	11th to 20th	Day	4 11% 35% 38% 12%	545
	21st to end	Day	5% 1 10% 27% 41% 16%	575
August	1st to 10th	Day	6% 1 8% 33% 38% 14%	527
	11th to 20th	Day	6% 2 11% 39% 30% 12%	517
	21st to end	Day	5% 2 11% 33% 37% 12%	583
September	1st to 10th	Day	10% 3 16% 39% 27% 6%	530
	11th to 20th	Day	5% 2 24% 40% 22% 6%	530
	21st to end	Day	5% 6% 27% 37% 20% 4	475
October	1st to 10th	Day	8% 10% 26% 29% 21% 6%	330
	11th to 20th	Day	11% 12% 24% 32% 17% 4	330
	21st to end	Day	6% 12% 24% 30% 22% 5%	363
November	1st to 10th	Day	5% 15% 33% 31% 13% 4	330
	11th to 20th	Day	8% 11% 23% 36% 19% 3	330
	21st to end	Day	11% 18% 29% 31% 10% 1	330
December	1st to 10th	Day	7% 18% 29% 29% 16% 1	330
	11th to 20th	Day	7% 17% 32% 30% 12% 2	330
	21st to end	Day	14% 20% 31% 26% 7% 1	363
January	1st to 10th	Night	8% 9% 27% 36% 16% 4	840
	11th to 20th	Night	6% 9% 30% 37% 15% 3	840
	21st to end	Night	9% 10% 29% 34% 15% 2	924
February	1st to 10th	Night	3 10% 34% 35% 15% 3	840
	11th to 20th	Night	9% 12% 32% 36% 10% 1	824
	21st to end	Night	7% 16% 40% 28% 8% 1	692
March	1st to 10th	Night	6% 15% 36% 32% 10% 1	840
	11th to 20th	Night	6% 13% 30% 35% 14% 2	840
	21st to end	Night	4 14% 36% 34% 12% 1	924
April	1st to 10th	Night	4 12% 40% 36% 8% 1	693
	11th to 20th	Night	3 16% 41% 29% 11% 1	623
	21st to end	Night	4 14% 38% 34% 9% 2	630
May	1st to 10th	Night	4 10% 36% 42% 8%	630
	11th to 20th	Night	3 8% 29% 46% 14% 1	621
	21st to end	Night	2 5% 35% 45% 12%	683
June	1st to 10th	Night	3 4 24% 50% 18% 1	630
	11th to 20th	Night	3 3 25% 51% 17% 1	630
	21st to end	Night	4 3 18% 44% 29% 2	630
July	1st to 10th	Night	6% 1 10% 41% 36% 7%	615
	11th to 20th	Night	5% 6% 30% 50% 9%	597
	21st to end	Night	6% 1 7% 27% 48% 12%	657
August	1st to 10th	Night	5% 1 4 24% 53% 13%	597
	11th to 20th	Night	3 1 8% 29% 51% 9%	594
	21st to end	Night	4 1 7% 30% 50% 8%	660
September	1st to 10th	Night	7% 1 7% 38% 44% 4	600
	11th to 20th	Night	6% 1 8% 44% 35% 5%	682
	21st to end	Night	5% 2 14% 43% 32% 5%	800
October	1st to 10th	Night	5% 2 19% 47% 25% 2	800
	11th to 20th	Night	6% 4 20% 45% 22% 4	800
	21st to end	Night	4 5% 21% 40% 25% 6%	880
November	1st to 10th	Night	3 4 21% 42% 23% 6%	820
	11th to 20th	Night	4 6% 22% 44% 21% 3	840
	21st to end	Night	7% 10% 25% 35% 18% 5%	840
December	1st to 10th	Night	7% 8% 25% 39% 16% 4	840
	11th to 20th	Night	5% 9% 28% 39% 17% 3	840
	21st to end	Night	9% 12% 25% 39% 14% 2	924

Page generated at 2026-07-13 01:14:54. Version ac07

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