NFCC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NFCC Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	10% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	114022
	Night	10% 6% 2 6% 4 4 4 4 6% 8% 45%	114588
Transparency	Day	37% 8% 13% 17% 17% 8%	117865
	Night	35% 7% 13% 17% 18% 9%	118884
Seeing	Day	11% 9% 24% 25% 20% 10%	15656
	Night	12% 8% 24% 30% 22% 5%	25753

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	11% 5% 2 5% 4 3 3 2 4 6% 55%	4056
	11th to 20th	Day	18% 7% 2 5% 3 4 4 3 5% 6% 43%	4060
	21st to end	Day	11% 6% 2 6% 4 4 4 3 6% 9% 45%	4903
February	1st to 10th	Day	15% 8% 3 6% 4 4 4 4 4 7% 42%	2000
	11th to 20th	Day	16% 6% 1 5% 3 3 4 2 5% 7% 47%	1948
	21st to end	Day	14% 7% 3 5% 3 3 3 3 5% 7% 46%	1650
March	1st to 10th	Day	20% 7% 2 5% 4 3 3 3 6% 7% 40%	4746
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 8% 52%	4980
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 7% 49%	5465
April	1st to 10th	Day	9% 5% 2 6% 6% 4 5% 4 6% 7% 47%	2200
	11th to 20th	Day	11% 6% 2 7% 4 4 4 4 4 7% 48%	2304
	21st to end	Day	5% 4 1 6% 4 5% 5% 4 5% 8% 53%	2435
May	1st to 10th	Day	7% 4 2 5% 4 5% 5% 5% 6% 8% 49%	4082
	11th to 20th	Day	11% 7% 3 8% 6% 5% 6% 4 6% 7% 37%	4081
	21st to end	Day	4 4 2 5% 5% 5% 6% 5% 7% 11% 47%	4476
June	1st to 10th	Day	8% 7% 3 10% 7% 6% 7% 6% 8% 10% 28%	2470
	11th to 20th	Day	5% 5% 2 8% 7% 8% 9% 7% 10% 11% 30%	2470
	21st to end	Day	7% 6% 3 9% 8% 7% 8% 7% 9% 10% 28%	2470
July	1st to 10th	Day	8% 7% 3 9% 8% 8% 9% 7% 9% 12% 19%	4120
	11th to 20th	Day	5% 4 2 7% 7% 6% 8% 6% 9% 12% 34%	4120
	21st to end	Day	5% 6% 2 8% 9% 9% 9% 8% 11% 12% 20%	4466
August	1st to 10th	Day	10% 6% 2 9% 6% 7% 7% 6% 8% 12% 26%	3962
	11th to 20th	Day	6% 6% 2 6% 6% 6% 6% 6% 8% 11% 38%	3729
	21st to end	Day	8% 6% 2 9% 8% 7% 6% 6% 9% 10% 29%	4066
September	1st to 10th	Day	10% 8% 3 8% 7% 6% 8% 6% 8% 11% 24%	2090
	11th to 20th	Day	11% 7% 3 10% 6% 6% 6% 6% 7% 9% 29%	2090
	21st to end	Day	12% 6% 2 5% 4 5% 5% 5% 5% 7% 44%	2090
October	1st to 10th	Day	11% 6% 2 6% 4 5% 4 4 4 7% 47%	2090
	11th to 20th	Day	6% 4 1 4 4 4 3 3 4 6% 61%	2010
	21st to end	Day	8% 4 2 5% 4 3 4 3 6% 6% 54%	2090
November	1st to 10th	Day	9% 4 2 4 4 4 3 3 5% 6% 56%	1934
	11th to 20th	Day	10% 5% 2 5% 3 4 2 3 5% 6% 54%	1800
	21st to end	Day	12% 4 2 6% 4 3 4 3 5% 7% 50%	1800
December	1st to 10th	Day	8% 5% 2 7% 5% 4 3 3 4 6% 53%	2930
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 4 6% 46%	3373
	21st to end	Day	17% 8% 3 6% 4 3 4 3 5% 5% 42%	4466
January	1st to 10th	Night	11% 6% 2 5% 3 4 3 3 5% 6% 53%	5889
	11th to 20th	Night	13% 7% 3 7% 5% 5% 4 4 5% 7% 40%	5900
	21st to end	Night	16% 8% 2 5% 4 3 4 3 5% 7% 42%	6048
February	1st to 10th	Night	13% 8% 2 7% 3 4 4 3 5% 6% 43%	2400
	11th to 20th	Night	16% 7% 2 6% 4 4 3 3 4 5% 47%	2338
	21st to end	Night	14% 8% 3 5% 4 3 3 3 5% 6% 47%	1980
March	1st to 10th	Night	19% 9% 3 6% 4 4 3 3 4 6% 38%	5106
	11th to 20th	Night	9% 4 1 4 3 3 4 4 7% 9% 53%	4980
	21st to end	Night	14% 6% 2 5% 3 4 4 3 4 6% 49%	5467
April	1st to 10th	Night	10% 6% 3 5% 4 4 3 4 5% 7% 48%	2200
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 6% 7% 51%	2060
	21st to end	Night	6% 3 2 5% 3 4 4 3 6% 8% 57%	1857
May	1st to 10th	Night	6% 5% 1 6% 4 5% 5% 4 6% 10% 50%	2813
	11th to 20th	Night	10% 6% 3 6% 4 5% 4 4 6% 9% 44%	2812
	21st to end	Night	4 3 1 4 3 3 4 3 8% 10% 55%	3079
June	1st to 10th	Night	8% 5% 3 6% 4 5% 6% 6% 7% 13% 38%	1710
	11th to 20th	Night	4 5% 2 6% 6% 6% 6% 6% 8% 12% 39%	1710
	21st to end	Night	6% 6% 2 6% 5% 4 6% 5% 8% 13% 38%	1710
July	1st to 10th	Night	7% 9% 3 10% 7% 8% 8% 7% 10% 9% 22%	2840
	11th to 20th	Night	3 4 2 5% 4 4 6% 6% 10% 14% 42%	2840
	21st to end	Night	5% 5% 3 6% 6% 6% 7% 8% 11% 15% 28%	3087
August	1st to 10th	Night	9% 4 2 5% 5% 6% 6% 6% 9% 13% 36%	2948
	11th to 20th	Night	4 3 2 6% 4 5% 6% 6% 10% 14% 40%	3100
	21st to end	Night	9% 5% 3 8% 6% 6% 6% 5% 9% 12% 30%	3557
September	1st to 10th	Night	9% 8% 2 8% 6% 6% 7% 6% 9% 11% 28%	2090
	11th to 20th	Night	10% 8% 3 9% 6% 6% 6% 5% 8% 9% 31%	2090
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 5% 6% 8% 44%	2090
October	1st to 10th	Night	8% 6% 3 7% 4 4 4 3 4 8% 50%	2090
	11th to 20th	Night	7% 4 2 4 3 3 3 3 4 7% 60%	2167
	21st to end	Night	7% 6% 2 5% 4 4 4 4 5% 6% 54%	2508
November	1st to 10th	Night	9% 5% 2 5% 4 5% 3 3 5% 7% 52%	2360
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 4 6% 52%	2600
	21st to end	Night	11% 7% 2 5% 3 3 3 3 6% 6% 50%	2600
December	1st to 10th	Night	9% 5% 2 6% 4 4 3 3 5% 5% 55%	4250
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 5% 50%	4822
	21st to end	Night	16% 8% 2 6% 3 3 4 3 4 6% 43%	6490

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	32% 4 6% 15% 32% 12%	4057
	11th to 20th	Day	41% 5% 6% 13% 21% 14%	4060
	21st to end	Day	36% 4 9% 20% 20% 11%	4903
February	1st to 10th	Day	42% 5% 9% 15% 19% 10%	2000
	11th to 20th	Day	37% 5% 8% 15% 23% 12%	1958
	21st to end	Day	36% 5% 10% 17% 19% 13%	1650
March	1st to 10th	Day	43% 5% 9% 15% 18% 10%	4746
	11th to 20th	Day	29% 6% 10% 14% 21% 20%	4980
	21st to end	Day	34% 5% 9% 24% 16% 12%	5465
April	1st to 10th	Day	34% 7% 11% 18% 17% 13%	2200
	11th to 20th	Day	36% 5% 8% 19% 21% 12%	2304
	21st to end	Day	28% 7% 11% 22% 22% 10%	2435
May	1st to 10th	Day	28% 9% 15% 23% 18% 6%	4120
	11th to 20th	Day	42% 8% 12% 20% 15% 2	4085
	21st to end	Day	27% 8% 18% 24% 21% 2	4489
June	1st to 10th	Day	46% 12% 16% 18% 8% 1	2470
	11th to 20th	Day	41% 13% 20% 17% 8% 1	2470
	21st to end	Day	44% 14% 20% 13% 7% 1	2470
July	1st to 10th	Day	50% 16% 24% 7% 2	4055
	11th to 20th	Day	36% 14% 34% 13% 2	4051
	21st to end	Day	48% 19% 24% 9% 2	4324
August	1st to 10th	Day	45% 12% 21% 15% 6%	3949
	11th to 20th	Day	36% 12% 18% 23% 11%	3765
	21st to end	Day	45% 11% 19% 20% 4	4066
September	1st to 10th	Day	49% 12% 16% 15% 6% 2	2090
	11th to 20th	Day	48% 12% 14% 13% 11% 2	2090
	21st to end	Day	37% 7% 14% 20% 16% 6%	2090
October	1st to 10th	Day	45% 4 7% 10% 18% 17%	3480
	11th to 20th	Day	16% 3 5% 19% 29% 29%	3315
	21st to end	Day	20% 3 5% 16% 39% 17%	3428
November	1st to 10th	Day	28% 5% 9% 19% 24% 16%	1934
	11th to 20th	Day	31% 4 9% 15% 26% 15%	1800
	21st to end	Day	34% 6% 6% 20% 23% 11%	1800
December	1st to 10th	Day	34% 4 8% 16% 26% 12%	2930
	11th to 20th	Day	39% 4 7% 20% 22% 9%	3370
	21st to end	Day	44% 4 6% 18% 24% 5%	4466
January	1st to 10th	Night	32% 4 7% 12% 31% 14%	5889
	11th to 20th	Night	43% 4 6% 15% 24% 8%	5900
	21st to end	Night	41% 4 8% 19% 19% 9%	6048
February	1st to 10th	Night	40% 6% 7% 15% 20% 12%	2400
	11th to 20th	Night	40% 5% 8% 13% 19% 15%	2339
	21st to end	Night	38% 5% 7% 18% 17% 15%	1980
March	1st to 10th	Night	47% 5% 8% 14% 17% 9%	5106
	11th to 20th	Night	27% 5% 10% 19% 22% 18%	4980
	21st to end	Night	36% 4 8% 19% 20% 13%	5467
April	1st to 10th	Night	34% 6% 11% 18% 18% 13%	2200
	11th to 20th	Night	31% 6% 9% 18% 23% 12%	2060
	21st to end	Night	25% 6% 11% 26% 23% 9%	1857
May	1st to 10th	Night	25% 11% 16% 25% 16% 7%	2840
	11th to 20th	Night	35% 9% 16% 25% 13% 3	2813
	21st to end	Night	20% 9% 18% 33% 18% 2	3087
June	1st to 10th	Night	34% 11% 24% 22% 10% 1	1710
	11th to 20th	Night	32% 14% 23% 20% 9% 1	1710
	21st to end	Night	34% 14% 25% 20% 7% 1	1710
July	1st to 10th	Night	52% 16% 22% 8% 2	2795
	11th to 20th	Night	27% 17% 39% 14% 2	2786
	21st to end	Night	39% 19% 30% 10% 2	2988
August	1st to 10th	Night	34% 14% 28% 18% 6%	2937
	11th to 20th	Night	29% 14% 27% 24% 7%	3130
	21st to end	Night	41% 13% 25% 16% 4 1	3557
September	1st to 10th	Night	44% 14% 19% 15% 7% 2	2090
	11th to 20th	Night	46% 13% 14% 15% 11% 1	2090
	21st to end	Night	34% 9% 15% 17% 17% 7%	2090
October	1st to 10th	Night	43% 5% 9% 12% 18% 13%	3480
	11th to 20th	Night	17% 2 5% 17% 33% 26%	3601
	21st to end	Night	20% 4 4 17% 38% 17%	4120
November	1st to 10th	Night	32% 5% 9% 20% 19% 16%	2360
	11th to 20th	Night	34% 4 8% 15% 24% 15%	2600
	21st to end	Night	33% 5% 10% 19% 21% 12%	2600
December	1st to 10th	Night	32% 4 8% 16% 25% 16%	4250
	11th to 20th	Night	37% 4 8% 19% 21% 10%	4824
	21st to end	Night	41% 5% 8% 17% 25% 5%	6490

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% 25% 42% 15% 2	300
	11th to 20th	Day	17% 18% 36% 21% 6% 2	300
	21st to end	Day	25% 23% 38% 10% 3	330
February	1st to 10th	Day	20% 17% 37% 18% 8%	300
	11th to 20th	Day	24% 20% 38% 15% 3	300
	21st to end	Day	15% 23% 39% 20% 3	249
March	1st to 10th	Day	16% 21% 32% 24% 6%	410
	11th to 20th	Day	16% 17% 32% 24% 10% 1	500
	21st to end	Day	16% 16% 33% 24% 10%	550
April	1st to 10th	Day	14% 17% 30% 24% 14% 1	500
	11th to 20th	Day	11% 14% 33% 28% 13% 1	492
	21st to end	Day	5% 13% 29% 32% 18% 3	480
May	1st to 10th	Day	6% 11% 28% 29% 20% 7%	463
	11th to 20th	Day	9% 5% 25% 33% 18% 11%	487
	21st to end	Day	8% 3 19% 33% 25% 13%	529
June	1st to 10th	Day	10% 2 16% 29% 29% 14%	490
	11th to 20th	Day	5% 3 15% 26% 30% 21%	490
	21st to end	Day	7% 3 11% 24% 32% 21%	490
July	1st to 10th	Day	6% 2 10% 19% 31% 32%	485
	11th to 20th	Day	6% 2 6% 19% 33% 33%	484
	21st to end	Day	5% 3 7% 22% 38% 26%	532
August	1st to 10th	Day	9% 3 9% 21% 33% 24%	486
	11th to 20th	Day	8% 2 6% 17% 38% 29%	479
	21st to end	Day	9% 2 11% 27% 32% 18%	539
September	1st to 10th	Day	13% 3 13% 23% 29% 19%	490
	11th to 20th	Day	12% 3 15% 32% 27% 10%	490
	21st to end	Day	11% 3 13% 23% 34% 14%	490
October	1st to 10th	Day	14% 7% 28% 29% 17% 6%	490
	11th to 20th	Day	7% 6% 23% 35% 22% 7%	490
	21st to end	Day	9% 8% 28% 34% 18% 2	539
November	1st to 10th	Day	13% 8% 27% 31% 18% 3	472
	11th to 20th	Day	14% 11% 44% 22% 6% 2	300
	21st to end	Day	9% 18% 38% 24% 10% 1	300
December	1st to 10th	Day	13% 21% 33% 22% 10% 2	300
	11th to 20th	Day	20% 12% 39% 23% 5%	300
	21st to end	Day	20% 11% 34% 27% 8%	330
January	1st to 10th	Night	19% 15% 36% 24% 7%	800
	11th to 20th	Night	15% 13% 32% 29% 10% 1	800
	21st to end	Night	20% 15% 36% 25% 4	880
February	1st to 10th	Night	17% 11% 33% 30% 9%	800
	11th to 20th	Night	19% 15% 31% 28% 6%	783
	21st to end	Night	17% 12% 37% 31% 4	660
March	1st to 10th	Night	16% 13% 30% 33% 8%	800
	11th to 20th	Night	14% 11% 34% 30% 12% 1	800
	21st to end	Night	13% 13% 34% 30% 10%	880
April	1st to 10th	Night	15% 8% 31% 33% 13%	800
	11th to 20th	Night	10% 9% 32% 32% 15% 1	678
	21st to end	Night	6% 6% 29% 39% 19% 2	587
May	1st to 10th	Night	7% 4 26% 39% 22% 2	570
	11th to 20th	Night	6% 4 22% 37% 25% 5%	561
	21st to end	Night	6% 2 14% 38% 35% 5%	617
June	1st to 10th	Night	9% 1 11% 38% 35% 6%	570
	11th to 20th	Night	4 2 12% 33% 35% 14%	570
	21st to end	Night	7% 2 9% 29% 40% 14%	570
July	1st to 10th	Night	6% 2 6% 22% 43% 21%	555
	11th to 20th	Night	6% 1 3 19% 47% 24%	552
	21st to end	Night	4 5% 19% 54% 17%	624
August	1st to 10th	Night	6% 2 6% 20% 50% 17%	567
	11th to 20th	Night	5% 1 6% 17% 52% 20%	564
	21st to end	Night	10% 2 8% 29% 35% 16%	669
September	1st to 10th	Night	9% 3 12% 28% 37% 11%	760
	11th to 20th	Night	12% 4 14% 33% 31% 5%	760
	21st to end	Night	11% 2 18% 29% 36% 5%	760
October	1st to 10th	Night	11% 4 24% 31% 27% 3	760
	11th to 20th	Night	9% 6% 24% 34% 25% 2	760
	21st to end	Night	9% 6% 25% 38% 21% 1	836
November	1st to 10th	Night	12% 7% 28% 35% 17% 2	780
	11th to 20th	Night	13% 11% 30% 30% 16% 1	800
	21st to end	Night	13% 11% 28% 35% 12% 1	800
December	1st to 10th	Night	14% 9% 30% 31% 16% 1	800
	11th to 20th	Night	16% 11% 30% 31% 12% 1	800
	21st to end	Night	19% 13% 29% 26% 13%	880

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