FAAC Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the FAAC Observing Site clear sky chart from 2005-11-06 to 2026-02-01.

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% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	126926
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	125190
Transparency	Day	48% 5% 11% 21% 12% 1	130482
	Night	49% 5% 11% 21% 13% 1	129692
Seeing	Day	20% 10% 28% 23% 14% 5%	17232
	Night	20% 9% 29% 29% 12% 1	27282

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	12% 18% 7% 16% 6% 5% 4 4 4 5% 21%	3697
	11th to 20th	Day	14% 16% 6% 18% 9% 8% 7% 3 5% 5% 10%	3700
	21st to end	Day	22% 18% 5% 12% 5% 4 5% 3 4 4 17%	4359
February	1st to 10th	Day	14% 15% 6% 12% 8% 4 6% 3 5% 6% 21%	1800
	11th to 20th	Day	14% 13% 5% 14% 8% 5% 4 3 5% 4 24%	1933
	21st to end	Day	14% 17% 5% 11% 5% 4 4 3 5% 6% 25%	1650
March	1st to 10th	Day	14% 12% 5% 11% 6% 4 4 3 4 5% 30%	4518
	11th to 20th	Day	11% 12% 4 8% 5% 4 4 3 5% 7% 35%	4980
	21st to end	Day	17% 15% 4 9% 5% 5% 4 3 5% 6% 27%	5465
April	1st to 10th	Day	17% 13% 4 7% 5% 5% 4 4 5% 6% 30%	2370
	11th to 20th	Day	16% 10% 3 8% 5% 4 4 4 5% 7% 35%	2502
	21st to end	Day	16% 10% 3 8% 5% 4 4 4 5% 7% 35%	2600
May	1st to 10th	Day	19% 10% 3 6% 4 4 4 3 5% 6% 34%	5862
	11th to 20th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	14% 7% 3 6% 4 4 4 3 5% 8% 41%	6558
June	1st to 10th	Day	10% 7% 2 5% 5% 5% 4 5% 6% 9% 43%	2800
	11th to 20th	Day	10% 7% 1 6% 5% 5% 5% 4 7% 9% 40%	2800
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 39%	2800
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 11% 38%	6360
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 4 7% 10% 45%	6360
	21st to end	Day	7% 6% 2 5% 5% 5% 5% 4 7% 9% 45%	6655
August	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 7% 8% 43%	4234
	11th to 20th	Day	10% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	4174
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 5% 8% 10% 41%	4545
September	1st to 10th	Day	10% 6% 2 6% 6% 5% 5% 4 5% 8% 43%	2290
	11th to 20th	Day	9% 5% 2 5% 4 4 4 5% 6% 8% 48%	2200
	21st to end	Day	13% 7% 3 7% 5% 4 6% 4 6% 7% 37%	2200
October	1st to 10th	Day	13% 6% 3 6% 5% 4 4 3 5% 7% 43%	2176
	11th to 20th	Day	16% 10% 3 9% 5% 5% 4 4 5% 6% 33%	2000
	21st to end	Day	19% 11% 3 8% 5% 4 4 4 5% 7% 30%	1995
November	1st to 10th	Day	12% 10% 3 7% 4 5% 4 4 5% 7% 38%	1852
	11th to 20th	Day	15% 15% 4 9% 5% 4 4 3 5% 5% 30%	1890
	21st to end	Day	19% 12% 5% 9% 6% 5% 4 3 4 5% 27%	1890
December	1st to 10th	Day	12% 15% 8% 16% 7% 6% 5% 3 4 6% 19%	2761
	11th to 20th	Day	18% 21% 8% 10% 6% 5% 4 4 5% 4 16%	3019
	21st to end	Day	24% 21% 8% 12% 4 4 4 2 3 4 13%	4070
January	1st to 10th	Night	14% 15% 7% 15% 6% 5% 4 3 4 5% 22%	6488
	11th to 20th	Night	12% 15% 7% 15% 8% 7% 6% 4 5% 6% 14%	6500
	21st to end	Night	18% 16% 8% 11% 5% 5% 5% 3 4 5% 20%	6845
February	1st to 10th	Night	16% 16% 4 11% 7% 5% 5% 3 4 5% 23%	2601
	11th to 20th	Night	14% 14% 5% 11% 7% 5% 5% 3 5% 5% 27%	2351
	21st to end	Night	17% 11% 4 11% 5% 4 4 4 6% 6% 28%	1980
March	1st to 10th	Night	15% 11% 4 9% 5% 4 4 2 5% 7% 35%	5332
	11th to 20th	Night	9% 9% 2 6% 4 5% 5% 4 5% 8% 42%	4980
	21st to end	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 35%	5467
April	1st to 10th	Night	16% 10% 3 8% 5% 4 4 3 5% 7% 35%	2045
	11th to 20th	Night	18% 9% 2 6% 4 4 5% 4 6% 6% 36%	1846
	21st to end	Night	16% 10% 2 6% 5% 4 5% 3 5% 6% 39%	1800
May	1st to 10th	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 31%	4033
	11th to 20th	Night	18% 9% 3 6% 4 4 3 3 4 6% 39%	4032
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 8% 44%	4298
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 4 5% 9% 47%	1600
	11th to 20th	Night	10% 5% 2 7% 5% 4 4 3 7% 10% 43%	1600
	21st to end	Night	8% 6% 3 6% 4 3 3 4 7% 10% 47%	1600
July	1st to 10th	Night	7% 4 2 5% 5% 4 6% 5% 6% 10% 47%	3618
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 6% 9% 50%	3617
	21st to end	Night	8% 6% 2 7% 5% 4 4 3 6% 8% 48%	4204
August	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 9% 48%	2915
	11th to 20th	Night	9% 6% 3 5% 3 4 5% 4 6% 8% 48%	2875
	21st to end	Night	8% 6% 3 7% 4 5% 5% 3 5% 7% 47%	3350
September	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 6% 49%	2090
	11th to 20th	Night	9% 5% 2 5% 3 3 4 2 4 6% 57%	2200
	21st to end	Night	13% 7% 2 6% 4 5% 4 4 5% 7% 43%	2200
October	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 4 6% 47%	2222
	11th to 20th	Night	18% 10% 4 7% 5% 4 5% 4 4 6% 35%	2400
	21st to end	Night	20% 10% 3 7% 4 4 4 4 5% 6% 35%	2840
November	1st to 10th	Night	12% 11% 2 8% 4 4 4 4 4 6% 41%	2666
	11th to 20th	Night	14% 12% 4 8% 5% 5% 5% 4 6% 6% 31%	2730
	21st to end	Night	18% 15% 4 9% 5% 4 4 4 5% 5% 27%	2730
December	1st to 10th	Night	11% 15% 6% 16% 7% 5% 5% 3 4 6% 22%	4611
	11th to 20th	Night	14% 19% 6% 13% 7% 5% 4 3 5% 5% 17%	5374
	21st to end	Night	25% 23% 6% 10% 5% 3 4 3 4 4 13%	7150

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	65% 4 10% 15% 5%	3698
	11th to 20th	Day	73% 8% 8% 6% 4	3700
	21st to end	Day	71% 4 10% 10% 5%	4359
February	1st to 10th	Day	63% 6% 10% 14% 8% 1	1800
	11th to 20th	Day	63% 5% 8% 15% 8% 1	1943
	21st to end	Day	60% 3 12% 16% 8% 1	1650
March	1st to 10th	Day	56% 4 12% 16% 9% 2	4518
	11th to 20th	Day	47% 5% 16% 22% 9% 2	4980
	21st to end	Day	58% 4 10% 15% 11% 2	5465
April	1st to 10th	Day	54% 5% 9% 20% 11%	2370
	11th to 20th	Day	49% 5% 11% 22% 12% 2	2502
	21st to end	Day	48% 5% 13% 19% 14% 1	2600
May	1st to 10th	Day	49% 5% 10% 19% 14% 2	5897
	11th to 20th	Day	47% 6% 13% 21% 12% 1	5865
	21st to end	Day	41% 5% 10% 23% 19% 3	6572
June	1st to 10th	Day	37% 6% 12% 28% 14% 3	2800
	11th to 20th	Day	40% 6% 13% 24% 16% 2	2800
	21st to end	Day	38% 7% 15% 22% 16% 2	2800
July	1st to 10th	Day	38% 7% 13% 26% 15% 2	6291
	11th to 20th	Day	32% 6% 13% 29% 18% 1	6285
	21st to end	Day	33% 7% 14% 25% 20% 1	6465
August	1st to 10th	Day	37% 7% 12% 26% 17% 1	4221
	11th to 20th	Day	39% 6% 10% 32% 12% 1	4213
	21st to end	Day	36% 7% 14% 30% 12% 1	4544
September	1st to 10th	Day	39% 6% 13% 26% 14% 2	2290
	11th to 20th	Day	31% 6% 11% 33% 17% 1	2200
	21st to end	Day	43% 6% 12% 24% 15%	2200
October	1st to 10th	Day	31% 5% 9% 33% 21% 1	3540
	11th to 20th	Day	41% 4 10% 26% 19%	3240
	21st to end	Day	46% 4 13% 24% 12% 2	3195
November	1st to 10th	Day	45% 6% 11% 23% 14% 1	1852
	11th to 20th	Day	55% 4 12% 20% 7% 2	1890
	21st to end	Day	59% 5% 12% 17% 6%	1890
December	1st to 10th	Day	66% 6% 13% 11% 3	2761
	11th to 20th	Day	70% 5% 10% 12% 3	3016
	21st to end	Day	76% 4 7% 9% 5%	4070
January	1st to 10th	Night	63% 5% 8% 17% 6% 1	6488
	11th to 20th	Night	68% 6% 10% 10% 6%	6500
	21st to end	Night	65% 5% 11% 12% 7%	6845
February	1st to 10th	Night	64% 5% 10% 14% 6% 1	2601
	11th to 20th	Night	58% 5% 9% 18% 9% 1	2352
	21st to end	Night	56% 4 13% 17% 8% 2	1980
March	1st to 10th	Night	49% 4 13% 23% 10% 1	5332
	11th to 20th	Night	38% 6% 13% 27% 16% 1	4980
	21st to end	Night	49% 5% 11% 19% 14% 1	5467
April	1st to 10th	Night	49% 5% 10% 21% 14% 2	2045
	11th to 20th	Night	46% 5% 11% 19% 18% 2	1846
	21st to end	Night	46% 5% 12% 19% 15% 3	1800
May	1st to 10th	Night	53% 4 10% 16% 12% 4	4060
	11th to 20th	Night	46% 4 10% 25% 12% 3	4033
	21st to end	Night	38% 5% 12% 23% 18% 4	4306
June	1st to 10th	Night	33% 7% 11% 28% 19% 3	1600
	11th to 20th	Night	36% 5% 15% 26% 16% 2	1600
	21st to end	Night	31% 6% 14% 28% 20% 1	1600
July	1st to 10th	Night	32% 6% 13% 29% 15% 5%	3580
	11th to 20th	Night	30% 6% 13% 30% 19% 2	3572
	21st to end	Night	33% 7% 13% 21% 24% 2	4081
August	1st to 10th	Night	34% 5% 14% 28% 18% 1	2904
	11th to 20th	Night	32% 6% 10% 37% 12% 2	2902
	21st to end	Night	35% 6% 11% 31% 15% 1	3350
September	1st to 10th	Night	33% 6% 11% 29% 19% 2	2090
	11th to 20th	Night	29% 5% 10% 32% 22% 2	2200
	21st to end	Night	40% 5% 12% 27% 15% 1	2200
October	1st to 10th	Night	26% 4 10% 33% 26% 1	3638
	11th to 20th	Night	43% 5% 10% 23% 19%	3899
	21st to end	Night	40% 4 10% 32% 14% 1	4578
November	1st to 10th	Night	44% 5% 10% 25% 16% 1	2666
	11th to 20th	Night	51% 5% 14% 20% 9% 1	2730
	21st to end	Night	58% 4 12% 18% 7%	2730
December	1st to 10th	Night	63% 5% 13% 15% 4	4611
	11th to 20th	Night	67% 6% 9% 13% 5%	5376
	21st to end	Night	75% 4 8% 8% 5%	7150

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	25% 14% 39% 16% 5% 1	330
	11th to 20th	Day	27% 17% 41% 12% 3 1	330
	21st to end	Day	30% 16% 34% 15% 4	363
February	1st to 10th	Day	27% 22% 34% 13% 3 1	300
	11th to 20th	Day	30% 20% 35% 13% 2 1	300
	21st to end	Day	32% 24% 31% 11% 2	249
March	1st to 10th	Day	27% 21% 33% 17% 2	312
	11th to 20th	Day	26% 12% 33% 22% 7% 1	500
	21st to end	Day	30% 13% 33% 19% 4 1	550
April	1st to 10th	Day	25% 13% 34% 19% 7% 1	500
	11th to 20th	Day	29% 15% 31% 13% 7% 4	492
	21st to end	Day	23% 12% 31% 21% 10% 2	500
May	1st to 10th	Day	26% 12% 31% 20% 9% 3	503
	11th to 20th	Day	20% 9% 28% 22% 14% 7%	527
	21st to end	Day	18% 8% 23% 25% 18% 7%	620
June	1st to 10th	Day	13% 7% 29% 30% 16% 4	730
	11th to 20th	Day	14% 6% 29% 30% 17% 4	730
	21st to end	Day	12% 8% 29% 30% 16% 5%	730
July	1st to 10th	Day	11% 3 21% 30% 24% 11%	721
	11th to 20th	Day	13% 5% 27% 27% 19% 9%	718
	21st to end	Day	9% 3 24% 30% 23% 10%	677
August	1st to 10th	Day	10% 2 18% 28% 25% 16%	526
	11th to 20th	Day	14% 3 18% 27% 27% 11%	519
	21st to end	Day	12% 3 20% 29% 25% 12%	583
September	1st to 10th	Day	14% 6% 21% 24% 22% 14%	530
	11th to 20th	Day	9% 4 17% 25% 29% 17%	530
	21st to end	Day	19% 8% 22% 24% 19% 8%	530
October	1st to 10th	Day	17% 7% 28% 25% 16% 7%	530
	11th to 20th	Day	20% 14% 29% 27% 9% 1	530
	21st to end	Day	28% 17% 34% 17% 5%	378
November	1st to 10th	Day	19% 18% 33% 22% 7% 1	330
	11th to 20th	Day	27% 19% 32% 16% 5% 1	330
	21st to end	Day	28% 18% 39% 12% 5%	330
December	1st to 10th	Day	28% 17% 33% 13% 9%	247
	11th to 20th	Day	30% 21% 35% 10% 3 1	294
	21st to end	Day	30% 20% 34% 13% 2	363
January	1st to 10th	Night	23% 14% 36% 21% 5% 1	1050
	11th to 20th	Night	25% 15% 36% 20% 4 1	1050
	21st to end	Night	22% 12% 35% 25% 5%	1019
February	1st to 10th	Night	27% 13% 34% 23% 2	801
	11th to 20th	Night	24% 14% 35% 24% 3	783
	21st to end	Night	25% 15% 37% 20% 3	660
March	1st to 10th	Night	23% 13% 34% 24% 6%	800
	11th to 20th	Night	19% 16% 35% 24% 5% 1	800
	21st to end	Night	25% 11% 31% 26% 7%	880
April	1st to 10th	Night	20% 14% 36% 24% 5%	645
	11th to 20th	Night	22% 12% 32% 24% 9% 1	593
	21st to end	Night	21% 8% 33% 29% 8%	600
May	1st to 10th	Night	23% 6% 29% 32% 9% 1	600
	11th to 20th	Night	23% 4 27% 31% 13% 1	591
	21st to end	Night	14% 2 25% 39% 18% 2	650
June	1st to 10th	Night	13% 3 24% 43% 16% 1	600
	11th to 20th	Night	13% 3 21% 43% 18% 1	600
	21st to end	Night	11% 3 21% 44% 20% 2	600
July	1st to 10th	Night	10% 1 15% 42% 28% 4	585
	11th to 20th	Night	11% 1 18% 40% 26% 3	582
	21st to end	Night	7% 2 17% 46% 26% 2	657
August	1st to 10th	Night	8% 1 14% 45% 29% 3	597
	11th to 20th	Night	10% 2 15% 40% 28% 4	594
	21st to end	Night	10% 2 15% 40% 28% 4	660
September	1st to 10th	Night	12% 4 19% 34% 27% 4	690
	11th to 20th	Night	12% 4 14% 34% 32% 5%	800
	21st to end	Night	17% 5% 19% 32% 20% 6%	800
October	1st to 10th	Night	16% 5% 26% 33% 18% 2	800
	11th to 20th	Night	23% 10% 29% 28% 9% 1	800
	21st to end	Night	26% 10% 33% 24% 6%	880
November	1st to 10th	Night	19% 11% 32% 27% 10% 1	820
	11th to 20th	Night	22% 15% 34% 21% 7%	840
	21st to end	Night	27% 13% 34% 22% 5%	840
December	1st to 10th	Night	23% 15% 34% 23% 5%	840
	11th to 20th	Night	24% 14% 37% 22% 3	1020
	21st to end	Night	34% 11% 34% 20% 2	1155

Page generated at 2026-01-29 01:21:41. Version ac07

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