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 2026-08-30.

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% 6% 5% 7% 9% 40%	143636
	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	138571
Transparency	Day	37% 8% 14% 18% 15% 7%	147435
	Night	35% 7% 14% 18% 17% 8%	142844
Seeing	Day	11% 9% 24% 26% 20% 10%	17504
	Night	12% 8% 24% 30% 22% 5%	27453

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	10% 6% 2 6% 4 4 4 3 5% 7% 50%	5258
	11th to 20th	Day	16% 7% 2 5% 4 4 4 4 6% 7% 42%	5280
	21st to end	Day	10% 6% 3 6% 4 4 5% 4 6% 8% 44%	6353
February	1st to 10th	Day	15% 8% 3 6% 4 4 4 4 4 6% 43%	2100
	11th to 20th	Day	15% 6% 2 5% 4 3 4 2 5% 7% 47%	2048
	21st to end	Day	14% 7% 3 5% 3 4 3 3 5% 7% 47%	1730
March	1st to 10th	Day	19% 7% 2 5% 4 3 3 3 6% 7% 40%	4851
	11th to 20th	Day	8% 4 1 5% 4 4 4 4 6% 8% 53%	5090
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 7% 49%	5586
April	1st to 10th	Day	9% 5% 2 6% 6% 4 5% 4 6% 7% 46%	2310
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 4 7% 49%	2420
	21st to end	Day	5% 4 1 5% 4 5% 5% 3 5% 9% 53%	2625
May	1st to 10th	Day	12% 6% 3 7% 5% 4 5% 4 5% 8% 39%	7616
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 4 6% 9% 43%	7626
	21st to end	Day	4 4 2 6% 6% 6% 7% 6% 8% 12% 38%	8391
June	1st to 10th	Day	8% 7% 2 10% 7% 6% 7% 6% 8% 10% 27%	2730
	11th to 20th	Day	5% 5% 2 8% 7% 8% 9% 7% 10% 10% 29%	2730
	21st to end	Day	7% 5% 3 8% 7% 6% 8% 7% 9% 10% 29%	2730
July	1st to 10th	Day	6% 6% 3 8% 7% 8% 9% 7% 10% 13% 24%	7680
	11th to 20th	Day	4 4 2 7% 7% 7% 8% 7% 10% 13% 32%	7680
	21st to end	Day	5% 4 2 7% 7% 8% 8% 7% 11% 15% 27%	8340
August	1st to 10th	Day	10% 6% 2 9% 7% 7% 7% 6% 8% 12% 26%	4214
	11th to 20th	Day	6% 6% 2 6% 6% 6% 6% 6% 8% 11% 38%	3969
	21st to end	Day	9% 6% 3 9% 8% 7% 6% 6% 9% 10% 28%	4302
September	1st to 10th	Day	9% 8% 3 8% 7% 6% 8% 5% 8% 11% 26%	2200
	11th to 20th	Day	10% 6% 3 10% 6% 6% 6% 6% 7% 8% 32%	2200
	21st to end	Day	12% 6% 2 5% 4 4 5% 5% 6% 7% 44%	2200
October	1st to 10th	Day	11% 6% 2 7% 4 5% 4 4 4 7% 46%	2200
	11th to 20th	Day	6% 4 1 4 4 4 3 3 4 6% 61%	2116
	21st to end	Day	8% 5% 2 5% 4 3 4 3 6% 6% 54%	2200
November	1st to 10th	Day	9% 4 2 4 3 4 4 3 5% 6% 56%	2033
	11th to 20th	Day	10% 5% 2 5% 3 3 2 3 5% 5% 57%	1890
	21st to end	Day	11% 4 2 6% 5% 4 4 3 5% 7% 49%	1890
December	1st to 10th	Day	9% 6% 2 7% 5% 4 3 3 4 6% 52%	3020
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 4 6% 47%	3463
	21st to end	Day	16% 8% 3 6% 4 3 4 3 5% 5% 43%	4565
January	1st to 10th	Night	10% 7% 2 5% 3 4 4 4 5% 7% 50%	7635
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 7% 42%	7680
	21st to end	Night	15% 8% 3 6% 4 3 4 3 5% 7% 41%	7823
February	1st to 10th	Night	13% 8% 2 7% 3 4 4 3 5% 7% 45%	2520
	11th to 20th	Night	16% 7% 2 6% 4 4 3 3 4 5% 47%	2458
	21st to end	Night	13% 7% 3 6% 4 3 3 3 5% 6% 47%	2076
March	1st to 10th	Night	19% 8% 3 6% 4 4 3 3 5% 6% 39%	5220
	11th to 20th	Night	9% 4 1 4 3 3 4 4 7% 9% 53%	5090
	21st to end	Night	13% 6% 2 5% 3 4 4 3 4 6% 49%	5588
April	1st to 10th	Night	11% 6% 2 5% 4 4 3 4 5% 7% 47%	2310
	11th to 20th	Night	8% 5% 2 5% 3 4 5% 4 5% 7% 53%	2164
	21st to end	Night	6% 3 2 5% 3 5% 4 3 6% 8% 56%	1991
May	1st to 10th	Night	10% 7% 2 7% 4 4 4 4 5% 9% 43%	5237
	11th to 20th	Night	12% 6% 3 5% 3 4 4 4 6% 10% 45%	5234
	21st to end	Night	5% 4 2 6% 4 5% 5% 4 8% 12% 45%	5761
June	1st to 10th	Night	8% 6% 3 6% 4 5% 6% 6% 8% 13% 36%	1890
	11th to 20th	Night	5% 5% 2 6% 6% 6% 6% 6% 8% 12% 38%	1890
	21st to end	Night	6% 5% 2 7% 5% 4 7% 5% 8% 13% 38%	1890
July	1st to 10th	Night	5% 6% 3 8% 6% 8% 8% 6% 10% 11% 30%	5278
	11th to 20th	Night	4 4 2 6% 5% 6% 7% 6% 11% 15% 35%	5277
	21st to end	Night	5% 4 3 7% 5% 5% 7% 7% 10% 15% 31%	5736
August	1st to 10th	Night	9% 5% 2 5% 5% 5% 6% 6% 9% 13% 35%	3136
	11th to 20th	Night	4 3 2 5% 4 5% 6% 6% 10% 14% 40%	3300
	21st to end	Night	9% 6% 3 8% 6% 6% 6% 5% 9% 12% 30%	3760
September	1st to 10th	Night	8% 8% 2 8% 6% 6% 6% 6% 9% 11% 30%	2200
	11th to 20th	Night	10% 8% 2 8% 6% 6% 6% 5% 8% 9% 33%	2200
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 8% 45%	2200
October	1st to 10th	Night	8% 6% 3 7% 4 4 4 3 4 8% 49%	2200
	11th to 20th	Night	7% 4 2 4 3 3 3 3 4 6% 61%	2281
	21st to end	Night	7% 6% 2 5% 4 4 4 4 5% 5% 54%	2640
November	1st to 10th	Night	9% 5% 2 5% 4 5% 3 3 5% 7% 53%	2481
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 4 6% 54%	2730
	21st to end	Night	11% 7% 2 5% 4 4 3 3 6% 6% 50%	2730
December	1st to 10th	Night	10% 5% 2 6% 4 4 3 3 5% 5% 54%	4380
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 5% 51%	4952
	21st to end	Night	16% 8% 2 6% 3 3 4 3 4 6% 44%	6633

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	35% 4 8% 17% 28% 9%	5259
	11th to 20th	Day	40% 5% 5% 14% 20% 16%	5280
	21st to end	Day	37% 4 9% 20% 18% 12%	6353
February	1st to 10th	Day	41% 5% 8% 15% 20% 11%	2100
	11th to 20th	Day	37% 5% 8% 16% 23% 11%	2058
	21st to end	Day	36% 5% 9% 17% 19% 13%	1730
March	1st to 10th	Day	43% 5% 9% 15% 18% 10%	4851
	11th to 20th	Day	29% 6% 10% 15% 21% 20%	5090
	21st to end	Day	34% 5% 9% 24% 16% 12%	5586
April	1st to 10th	Day	34% 7% 11% 18% 17% 13%	2310
	11th to 20th	Day	34% 5% 8% 19% 22% 12%	2420
	21st to end	Day	27% 6% 11% 24% 21% 10%	2625
May	1st to 10th	Day	41% 7% 14% 20% 14% 4	7651
	11th to 20th	Day	35% 7% 16% 25% 14% 2	7630
	21st to end	Day	35% 11% 20% 18% 15% 2	8405
June	1st to 10th	Day	46% 11% 15% 17% 9% 1	2730
	11th to 20th	Day	42% 13% 21% 16% 7% 1	2730
	21st to end	Day	43% 15% 20% 14% 7% 1	2730
July	1st to 10th	Day	45% 15% 27% 11% 2	7615
	11th to 20th	Day	37% 15% 34% 13% 1	7611
	21st to end	Day	38% 15% 25% 20% 1	8156
August	1st to 10th	Day	46% 12% 21% 15% 6%	4201
	11th to 20th	Day	36% 11% 20% 22% 10%	4005
	21st to end	Day	46% 11% 19% 20% 4	4302
September	1st to 10th	Day	48% 12% 16% 16% 7% 2	2200
	11th to 20th	Day	46% 11% 14% 15% 12% 2	2200
	21st to end	Day	37% 7% 13% 21% 16% 6%	2200
October	1st to 10th	Day	45% 4 7% 10% 17% 16%	3590
	11th to 20th	Day	16% 2 5% 18% 29% 29%	3421
	21st to end	Day	21% 3 5% 16% 38% 17%	3538
November	1st to 10th	Day	28% 4 9% 19% 24% 16%	2033
	11th to 20th	Day	30% 4 8% 16% 28% 15%	1890
	21st to end	Day	34% 6% 7% 19% 22% 11%	1890
December	1st to 10th	Day	35% 4 7% 16% 26% 12%	3020
	11th to 20th	Day	39% 4 7% 20% 22% 9%	3460
	21st to end	Day	43% 4 6% 18% 24% 5%	4565
January	1st to 10th	Night	33% 4 10% 16% 27% 11%	7636
	11th to 20th	Night	41% 4 5% 16% 24% 11%	7680
	21st to end	Night	43% 4 7% 19% 19% 8%	7823
February	1st to 10th	Night	39% 6% 7% 15% 21% 13%	2520
	11th to 20th	Night	40% 5% 9% 14% 19% 14%	2459
	21st to end	Night	38% 5% 8% 18% 17% 15%	2076
March	1st to 10th	Night	46% 5% 8% 14% 17% 9%	5220
	11th to 20th	Night	27% 5% 10% 19% 22% 18%	5090
	21st to end	Night	36% 4 8% 20% 20% 12%	5588
April	1st to 10th	Night	35% 6% 11% 17% 18% 13%	2310
	11th to 20th	Night	30% 6% 9% 18% 25% 12%	2164
	21st to end	Night	26% 5% 11% 26% 23% 9%	1991
May	1st to 10th	Night	36% 8% 14% 24% 12% 6%	5264
	11th to 20th	Night	33% 7% 17% 26% 13% 3	5235
	21st to end	Night	29% 9% 22% 26% 12% 1	5771
June	1st to 10th	Night	36% 10% 22% 20% 10% 1	1890
	11th to 20th	Night	33% 14% 25% 19% 9% 1	1890
	21st to end	Night	34% 14% 24% 20% 7% 1	1890
July	1st to 10th	Night	44% 14% 28% 11% 2	5235
	11th to 20th	Night	33% 16% 37% 13% 1	5226
	21st to end	Night	37% 14% 31% 17% 1	5606
August	1st to 10th	Night	35% 14% 28% 17% 6%	3125
	11th to 20th	Night	29% 14% 29% 23% 7%	3330
	21st to end	Night	42% 13% 25% 15% 4 1	3760
September	1st to 10th	Night	43% 13% 20% 15% 8% 2	2200
	11th to 20th	Night	44% 12% 14% 17% 12% 1	2200
	21st to end	Night	34% 9% 15% 18% 17% 7%	2200
October	1st to 10th	Night	43% 5% 9% 12% 18% 13%	3590
	11th to 20th	Night	17% 2 4 17% 33% 26%	3715
	21st to end	Night	21% 4 4 16% 37% 17%	4252
November	1st to 10th	Night	31% 5% 8% 20% 20% 16%	2481
	11th to 20th	Night	32% 4 8% 16% 25% 15%	2730
	21st to end	Night	33% 5% 11% 19% 20% 12%	2730
December	1st to 10th	Night	33% 4 8% 16% 24% 16%	4380
	11th to 20th	Night	37% 4 8% 20% 22% 10%	4954
	21st to end	Night	41% 5% 8% 17% 25% 5%	6633

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% 16% 2	330
	11th to 20th	Day	17% 19% 37% 20% 5% 2	330
	21st to end	Day	25% 22% 39% 10% 3	363
February	1st to 10th	Day	20% 17% 38% 18% 8%	330
	11th to 20th	Day	23% 20% 38% 15% 3	330
	21st to end	Day	17% 24% 38% 18% 3	273
March	1st to 10th	Day	15% 19% 31% 27% 8%	445
	11th to 20th	Day	16% 17% 31% 25% 10% 1	540
	21st to end	Day	15% 15% 33% 25% 10% 1	594
April	1st to 10th	Day	14% 17% 30% 24% 14% 1	540
	11th to 20th	Day	10% 14% 32% 28% 14% 2	532
	21st to end	Day	5% 12% 30% 31% 18% 4	540
May	1st to 10th	Day	8% 11% 29% 28% 18% 6%	543
	11th to 20th	Day	9% 5% 27% 32% 17% 9%	567
	21st to end	Day	7% 4 19% 34% 24% 12%	617
June	1st to 10th	Day	10% 3 16% 31% 27% 13%	570
	11th to 20th	Day	6% 4 15% 26% 29% 20%	570
	21st to end	Day	7% 3 11% 26% 32% 21%	570
July	1st to 10th	Day	6% 2 10% 20% 31% 32%	565
	11th to 20th	Day	6% 2 7% 20% 33% 31%	564
	21st to end	Day	4 3 8% 22% 39% 24%	620
August	1st to 10th	Day	9% 4 9% 22% 32% 23%	566
	11th to 20th	Day	7% 2 6% 17% 40% 28%	559
	21st to end	Day	10% 3 11% 27% 31% 18%	619
September	1st to 10th	Day	12% 3 15% 24% 28% 18%	530
	11th to 20th	Day	11% 3 16% 33% 27% 11%	530
	21st to end	Day	11% 3 14% 24% 34% 14%	530
October	1st to 10th	Day	13% 7% 29% 29% 16% 5%	530
	11th to 20th	Day	6% 6% 23% 35% 23% 6%	530
	21st to end	Day	10% 9% 29% 33% 17% 2	583
November	1st to 10th	Day	13% 7% 26% 32% 18% 3	511
	11th to 20th	Day	13% 10% 42% 24% 8% 2	330
	21st to end	Day	9% 17% 39% 24% 9% 1	330
December	1st to 10th	Day	16% 20% 32% 20% 9% 2	330
	11th to 20th	Day	19% 13% 38% 24% 6%	330
	21st to end	Day	19% 11% 34% 28% 9%	363
January	1st to 10th	Night	19% 14% 35% 25% 7%	840
	11th to 20th	Night	15% 14% 33% 29% 9% 1	840
	21st to end	Night	20% 15% 36% 25% 4	924
February	1st to 10th	Night	16% 11% 34% 30% 9%	840
	11th to 20th	Night	20% 15% 31% 28% 6%	823
	21st to end	Night	17% 13% 36% 30% 4	692
March	1st to 10th	Night	15% 13% 30% 34% 8%	840
	11th to 20th	Night	14% 11% 34% 30% 12%	840
	21st to end	Night	13% 12% 34% 31% 10%	924
April	1st to 10th	Night	15% 9% 31% 33% 13%	840
	11th to 20th	Night	10% 9% 31% 33% 16% 1	712
	21st to end	Night	6% 7% 28% 39% 20% 1	630
May	1st to 10th	Night	8% 5% 26% 39% 20% 2	630
	11th to 20th	Night	7% 5% 23% 38% 24% 5%	621
	21st to end	Night	6% 2 14% 37% 36% 5%	683
June	1st to 10th	Night	9% 2 12% 38% 33% 6%	630
	11th to 20th	Night	5% 3 11% 32% 36% 13%	630
	21st to end	Night	7% 2 9% 28% 42% 14%	630
July	1st to 10th	Night	5% 2 6% 22% 43% 22%	615
	11th to 20th	Night	6% 1 4 20% 47% 22%	612
	21st to end	Night	4 1 6% 19% 54% 17%	690
August	1st to 10th	Night	7% 3 6% 22% 48% 15%	627
	11th to 20th	Night	5% 1 5% 17% 52% 19%	624
	21st to end	Night	10% 2 8% 29% 35% 16%	732
September	1st to 10th	Night	9% 3 13% 29% 36% 11%	800
	11th to 20th	Night	12% 4 14% 35% 31% 5%	800
	21st to end	Night	11% 2 18% 29% 35% 5%	800
October	1st to 10th	Night	11% 4 25% 32% 25% 3	800
	11th to 20th	Night	9% 6% 23% 34% 26% 2	800
	21st to end	Night	10% 6% 25% 38% 21% 1	880
November	1st to 10th	Night	12% 7% 27% 36% 17% 2	820
	11th to 20th	Night	12% 11% 29% 32% 16% 1	840
	21st to end	Night	13% 11% 29% 35% 12%	840
December	1st to 10th	Night	15% 9% 30% 30% 15% 1	840
	11th to 20th	Night	16% 11% 30% 31% 12% 1	840
	21st to end	Night	19% 13% 28% 27% 13%	924

Page generated at 2026-08-27 04:07:10. Version ac07

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