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 2024-11-28.

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	14% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	101229
	Night	14% 11% 4 8% 5% 4 4 3 5% 7% 34%	98303
Transparency	Day	47% 6% 12% 22% 12% 1	104830
	Night	48% 5% 11% 22% 13% 1	102830
Seeing	Day	19% 10% 28% 23% 14% 6%	15680
	Night	20% 9% 29% 29% 12% 1	25630

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	16% 21% 6% 10% 6% 5% 4 4 4 5% 21%	2527
	11th to 20th	Day	16% 18% 6% 12% 7% 7% 7% 4 5% 5% 12%	2530
	21st to end	Day	29% 21% 6% 11% 5% 3 2 2 3 4 14%	2974
February	1st to 10th	Day	14% 15% 6% 12% 8% 4 6% 3 5% 6% 21%	1710
	11th to 20th	Day	14% 13% 5% 14% 8% 5% 4 3 5% 4 25%	1833
	21st to end	Day	15% 17% 5% 11% 5% 4 4 3 5% 6% 25%	1570
March	1st to 10th	Day	17% 16% 5% 9% 6% 4 4 3 5% 5% 27%	3144
	11th to 20th	Day	12% 13% 4 10% 6% 5% 5% 3 5% 6% 32%	3480
	21st to end	Day	18% 13% 4 10% 6% 5% 4 3 5% 6% 27%	3815
April	1st to 10th	Day	17% 13% 3 7% 5% 4 4 4 5% 6% 30%	2251
	11th to 20th	Day	17% 10% 3 7% 5% 3 4 4 5% 7% 35%	2375
	21st to end	Day	17% 10% 3 8% 5% 4 4 4 5% 6% 35%	2470
May	1st to 10th	Day	17% 9% 3 6% 5% 5% 4 4 5% 6% 35%	4082
	11th to 20th	Day	16% 7% 2 6% 5% 5% 5% 4 6% 9% 35%	4081
	21st to end	Day	10% 7% 3 6% 4 4 4 3 5% 9% 45%	4559
June	1st to 10th	Day	10% 7% 2 5% 5% 5% 4 5% 6% 9% 42%	2660
	11th to 20th	Day	10% 7% 1 6% 5% 5% 5% 4 7% 9% 40%	2660
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 38%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 9% 37%	4440
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 7% 10% 47%	4440
	21st to end	Day	6% 6% 2 6% 5% 6% 6% 5% 7% 8% 42%	4627
August	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 7% 8% 42%	4104
	11th to 20th	Day	10% 7% 2 7% 5% 5% 5% 4 6% 8% 41%	4044
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 5% 8% 10% 41%	4406
September	1st to 10th	Day	11% 6% 2 7% 6% 5% 5% 5% 5% 8% 42%	2176
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 8% 47%	2090
	21st to end	Day	13% 7% 3 7% 5% 4 6% 5% 6% 8% 37%	2090
October	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 3 5% 7% 42%	2067
	11th to 20th	Day	17% 10% 3 9% 5% 5% 4 4 4 6% 33%	1900
	21st to end	Day	20% 11% 3 8% 5% 4 4 4 5% 7% 29%	1895
November	1st to 10th	Day	12% 10% 3 7% 5% 5% 4 4 5% 7% 38%	1762
	11th to 20th	Day	15% 15% 4 9% 5% 5% 5% 4 5% 5% 29%	1800
	21st to end	Day	20% 13% 5% 8% 6% 5% 4 3 4 5% 28%	1773
December	1st to 10th	Day	14% 17% 6% 12% 7% 5% 5% 3 4 5% 21%	1571
	11th to 20th	Day	14% 17% 7% 11% 5% 5% 4 4 7% 5% 20%	1880
	21st to end	Day	23% 22% 8% 12% 5% 5% 4 2 3 4 11%	2783
January	1st to 10th	Night	16% 16% 7% 12% 6% 4 4 3 4 5% 22%	4428
	11th to 20th	Night	15% 17% 7% 15% 6% 5% 7% 4 5% 6% 14%	4440
	21st to end	Night	24% 17% 9% 11% 5% 4 4 3 3 4 15%	4684
February	1st to 10th	Night	17% 16% 4 11% 7% 5% 4 4 4 5% 23%	2470
	11th to 20th	Night	13% 14% 5% 10% 6% 5% 5% 3 6% 5% 27%	2231
	21st to end	Night	18% 11% 4 11% 5% 4 4 3 6% 6% 29%	1884
March	1st to 10th	Night	16% 13% 3 8% 5% 4 3 2 5% 7% 35%	3706
	11th to 20th	Night	9% 9% 2 6% 4 5% 5% 4 5% 8% 42%	3480
	21st to end	Night	16% 10% 3 9% 5% 4 4 3 5% 6% 34%	3817
April	1st to 10th	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 35%	1943
	11th to 20th	Night	19% 9% 3 5% 4 4 5% 4 6% 7% 36%	1753
	21st to end	Night	17% 10% 2 6% 5% 4 4 3 4 6% 38%	1710
May	1st to 10th	Night	16% 9% 2 7% 5% 4 5% 3 6% 8% 34%	2813
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 41%	2812
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 9% 46%	2995
June	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 6% 8% 47%	1520
	11th to 20th	Night	10% 5% 2 7% 5% 4 4 3 7% 10% 43%	1520
	21st to end	Night	9% 5% 3 6% 4 3 3 3 7% 10% 47%	1520
July	1st to 10th	Night	8% 4 2 5% 5% 5% 5% 5% 6% 10% 47%	2530
	11th to 20th	Night	8% 4 2 4 3 3 4 4 5% 8% 56%	2530
	21st to end	Night	5% 5% 2 7% 5% 5% 5% 4 8% 9% 47%	2929
August	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 9% 47%	2825
	11th to 20th	Night	10% 6% 3 5% 3 4 5% 3 6% 8% 49%	2785
	21st to end	Night	8% 6% 3 7% 4 5% 5% 3 5% 7% 47%	3247
September	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 6% 48%	1985
	11th to 20th	Night	9% 5% 2 5% 3 3 4 2 4 6% 56%	2090
	21st to end	Night	13% 7% 2 6% 4 5% 5% 4 5% 7% 42%	2090
October	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 4 6% 46%	2111
	11th to 20th	Night	19% 10% 4 7% 4 4 5% 4 4 6% 35%	2280
	21st to end	Night	21% 10% 3 6% 4 4 4 4 5% 6% 34%	2698
November	1st to 10th	Night	13% 10% 2 8% 4 3 3 3 4 6% 42%	2536
	11th to 20th	Night	15% 12% 4 8% 5% 5% 5% 4 6% 6% 31%	2600
	21st to end	Night	18% 15% 4 9% 5% 4 3 4 5% 5% 28%	2562
December	1st to 10th	Night	15% 17% 6% 11% 7% 5% 5% 3 4 6% 20%	2596
	11th to 20th	Night	14% 15% 5% 12% 7% 5% 5% 4 6% 6% 22%	3299
	21st to end	Night	26% 23% 6% 10% 5% 4 4 3 4 4 11%	4884

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	66% 5% 10% 14% 5%	2528
	11th to 20th	Day	69% 9% 10% 7% 3	2530
	21st to end	Day	76% 3 9% 8% 3 1	2974
February	1st to 10th	Day	63% 6% 10% 13% 8% 1	1710
	11th to 20th	Day	61% 5% 8% 16% 9% 1	1843
	21st to end	Day	61% 3 12% 16% 7% 1	1570
March	1st to 10th	Day	59% 5% 11% 17% 7% 1	3144
	11th to 20th	Day	50% 6% 13% 20% 8% 2	3480
	21st to end	Day	58% 4 12% 15% 9% 2	3815
April	1st to 10th	Day	53% 5% 9% 20% 12%	2251
	11th to 20th	Day	49% 5% 11% 21% 13% 2	2375
	21st to end	Day	49% 5% 13% 17% 14% 1	2470
May	1st to 10th	Day	46% 6% 11% 23% 12% 1	4120
	11th to 20th	Day	45% 6% 15% 24% 9% 1	4085
	21st to end	Day	36% 5% 12% 20% 23% 4	4572
June	1st to 10th	Day	38% 6% 12% 28% 14% 3	2660
	11th to 20th	Day	39% 6% 13% 24% 17% 2	2660
	21st to end	Day	39% 7% 15% 22% 16% 2	2660
July	1st to 10th	Day	41% 6% 14% 21% 16% 2	4371
	11th to 20th	Day	30% 6% 13% 33% 17% 1	4365
	21st to end	Day	36% 8% 14% 20% 21% 1	4480
August	1st to 10th	Day	37% 7% 12% 26% 17% 1	4091
	11th to 20th	Day	39% 6% 10% 32% 13% 1	4083
	21st to end	Day	36% 7% 14% 30% 12% 1	4405
September	1st to 10th	Day	40% 6% 13% 24% 14% 2	2176
	11th to 20th	Day	32% 6% 10% 33% 17% 1	2090
	21st to end	Day	43% 6% 13% 24% 14%	2090
October	1st to 10th	Day	31% 5% 9% 33% 21% 1	3431
	11th to 20th	Day	41% 4 10% 25% 19%	3140
	21st to end	Day	46% 4 13% 25% 11% 2	3095
November	1st to 10th	Day	45% 6% 11% 23% 14% 1	1762
	11th to 20th	Day	56% 4 12% 19% 8% 2	1800
	21st to end	Day	58% 5% 13% 17% 7% 1	1773
December	1st to 10th	Day	66% 4 10% 15% 4	1571
	11th to 20th	Day	62% 6% 13% 14% 4	1877
	21st to end	Day	78% 4 8% 6% 3	2783
January	1st to 10th	Night	63% 5% 10% 15% 6% 1	4428
	11th to 20th	Night	70% 6% 11% 10% 4	4440
	21st to end	Night	72% 4 9% 11% 3 1	4684
February	1st to 10th	Night	63% 5% 10% 14% 6% 1	2470
	11th to 20th	Night	57% 6% 9% 19% 9% 1	2232
	21st to end	Night	56% 4 12% 17% 9% 2	1884
March	1st to 10th	Night	50% 4 13% 25% 7% 1	3706
	11th to 20th	Night	37% 7% 14% 22% 19% 1	3480
	21st to end	Night	50% 5% 13% 21% 11% 1	3817
April	1st to 10th	Night	48% 5% 10% 22% 14% 1	1943
	11th to 20th	Night	46% 5% 11% 19% 18% 2	1753
	21st to end	Night	47% 5% 11% 19% 15% 3	1710
May	1st to 10th	Night	47% 5% 12% 22% 12% 2	2840
	11th to 20th	Night	42% 5% 12% 26% 14% 2	2813
	21st to end	Night	33% 6% 14% 22% 20% 4	3003
June	1st to 10th	Night	33% 7% 11% 29% 18% 3	1520
	11th to 20th	Night	36% 5% 15% 25% 17% 2	1520
	21st to end	Night	32% 6% 14% 28% 19% 1	1520
July	1st to 10th	Night	32% 6% 12% 29% 14% 7%	2490
	11th to 20th	Night	27% 5% 12% 33% 22% 2	2482
	21st to end	Night	30% 8% 18% 19% 23% 2	2836
August	1st to 10th	Night	35% 5% 14% 28% 17% 1	2814
	11th to 20th	Night	32% 6% 10% 37% 13% 2	2812
	21st to end	Night	36% 6% 11% 31% 15% 1	3247
September	1st to 10th	Night	34% 6% 12% 27% 19% 2	1985
	11th to 20th	Night	30% 5% 10% 31% 22% 2	2090
	21st to end	Night	40% 5% 12% 27% 14% 1	2090
October	1st to 10th	Night	26% 4 10% 34% 26% 1	3527
	11th to 20th	Night	43% 5% 10% 23% 19%	3779
	21st to end	Night	40% 4 10% 32% 14% 1	4436
November	1st to 10th	Night	43% 5% 11% 25% 16% 1	2536
	11th to 20th	Night	51% 5% 14% 19% 9% 1	2600
	21st to end	Night	57% 4 13% 19% 7%	2562
December	1st to 10th	Night	65% 5% 10% 14% 6%	2596
	11th to 20th	Night	61% 6% 12% 15% 6% 1	3301
	21st to end	Night	76% 5% 10% 7% 2	4884

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	26% 14% 37% 17% 6% 1	270
	11th to 20th	Day	26% 13% 43% 14% 3 1	270
	21st to end	Day	31% 13% 35% 15% 5%	297
February	1st to 10th	Day	25% 21% 35% 13% 4 1	270
	11th to 20th	Day	29% 20% 35% 13% 2 1	270
	21st to end	Day	30% 25% 32% 11% 2	225
March	1st to 10th	Day	27% 21% 32% 17% 2	281
	11th to 20th	Day	25% 11% 33% 22% 7% 1	460
	21st to end	Day	30% 12% 32% 20% 5% 1	506
April	1st to 10th	Day	23% 14% 34% 21% 7% 1	460
	11th to 20th	Day	30% 15% 30% 13% 8% 5%	452
	21st to end	Day	23% 13% 32% 21% 9% 3	460
May	1st to 10th	Day	25% 11% 31% 20% 9% 3	463
	11th to 20th	Day	20% 9% 27% 22% 14% 7%	487
	21st to end	Day	16% 7% 23% 27% 19% 8%	573
June	1st to 10th	Day	13% 7% 29% 30% 16% 5%	680
	11th to 20th	Day	14% 6% 30% 29% 18% 4	680
	21st to end	Day	13% 8% 29% 29% 16% 6%	680
July	1st to 10th	Day	11% 3 21% 29% 25% 11%	671
	11th to 20th	Day	13% 4 26% 28% 19% 9%	668
	21st to end	Day	9% 3 24% 31% 23% 10%	628
August	1st to 10th	Day	11% 3 19% 27% 24% 16%	486
	11th to 20th	Day	15% 4 18% 25% 27% 11%	479
	21st to end	Day	13% 3 19% 29% 25% 12%	539
September	1st to 10th	Day	14% 5% 21% 25% 22% 13%	490
	11th to 20th	Day	10% 4 17% 26% 28% 15%	490
	21st to end	Day	18% 9% 23% 23% 19% 8%	490
October	1st to 10th	Day	17% 7% 28% 25% 15% 7%	490
	11th to 20th	Day	21% 15% 28% 26% 9% 1	490
	21st to end	Day	29% 16% 34% 16% 5%	344
November	1st to 10th	Day	16% 19% 33% 23% 8% 1	300
	11th to 20th	Day	24% 19% 34% 17% 5% 1	300
	21st to end	Day	27% 15% 41% 11% 5%	291
December	1st to 10th	Day	31% 18% 31% 13% 7%	203
	11th to 20th	Day	27% 20% 38% 10% 4	240
	21st to end	Day	26% 21% 35% 15% 2	297
January	1st to 10th	Night	24% 13% 36% 21% 5% 1	950
	11th to 20th	Night	26% 13% 34% 21% 5% 1	950
	21st to end	Night	22% 10% 35% 27% 6%	923
February	1st to 10th	Night	27% 12% 34% 24% 2 1	760
	11th to 20th	Night	24% 13% 35% 24% 3	743
	21st to end	Night	25% 16% 36% 21% 3	628
March	1st to 10th	Night	23% 13% 34% 25% 6%	760
	11th to 20th	Night	19% 15% 35% 24% 5% 1	760
	21st to end	Night	25% 10% 31% 26% 8%	836
April	1st to 10th	Night	20% 14% 36% 25% 5%	613
	11th to 20th	Night	23% 11% 33% 24% 9% 1	563
	21st to end	Night	22% 7% 33% 29% 8%	570
May	1st to 10th	Night	22% 6% 29% 33% 9% 1	570
	11th to 20th	Night	23% 3 27% 32% 13% 1	561
	21st to end	Night	13% 2 24% 39% 19% 2	617
June	1st to 10th	Night	13% 3 23% 44% 16% 1	570
	11th to 20th	Night	12% 3 21% 43% 19% 1	570
	21st to end	Night	11% 3 21% 44% 19% 2	570
July	1st to 10th	Night	10% 1 15% 42% 28% 5%	555
	11th to 20th	Night	11% 1 18% 40% 27% 3	552
	21st to end	Night	7% 2 18% 46% 26% 2	624
August	1st to 10th	Night	9% 1 15% 44% 28% 3	567
	11th to 20th	Night	10% 2 15% 40% 29% 4	564
	21st to end	Night	11% 2 15% 40% 29% 4	627
September	1st to 10th	Night	12% 4 18% 35% 26% 4	655
	11th to 20th	Night	13% 4 14% 34% 31% 4	760
	21st to end	Night	17% 6% 19% 32% 20% 6%	760
October	1st to 10th	Night	16% 6% 27% 31% 17% 3	760
	11th to 20th	Night	24% 10% 29% 27% 9% 1	760
	21st to end	Night	27% 10% 33% 23% 6%	836
November	1st to 10th	Night	19% 11% 32% 27% 11% 1	780
	11th to 20th	Night	22% 15% 35% 20% 8% 1	800
	21st to end	Night	26% 12% 34% 22% 5%	789
December	1st to 10th	Night	24% 14% 34% 22% 5%	760
	11th to 20th	Night	24% 12% 37% 23% 3 1	922
	21st to end	Night	33% 11% 33% 21% 2	1045

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