CFAS
Clear Sky Chart
History

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

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	12% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	113210
	Night	12% 6% 2 6% 4 4 4 4 6% 7% 44%	115092
Transparency	Day	40% 7% 12% 18% 17% 6%	116855
	Night	37% 6% 12% 18% 19% 7%	119586
Seeing	Day	16% 12% 27% 25% 14% 6%	16076
	Night	15% 9% 26% 32% 16% 2	26496

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	13% 6% 2 5% 3 3 3 2 4 5% 55%	4056
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 5% 55%	4060
	21st to end	Day	12% 10% 3 7% 4 4 4 3 4 5% 43%	4466
February	1st to 10th	Day	16% 7% 2 6% 4 5% 4 4 5% 7% 41%	1800
	11th to 20th	Day	19% 7% 3 6% 4 3 3 2 4 5% 44%	1857
	21st to end	Day	14% 6% 3 5% 6% 5% 4 3 6% 6% 42%	1753
March	1st to 10th	Day	22% 8% 2 6% 4 4 4 3 5% 6% 37%	4935
	11th to 20th	Day	10% 4 1 4 4 4 5% 3 5% 6% 53%	4980
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 5% 49%	5465
April	1st to 10th	Day	10% 5% 2 5% 3 3 4 4 4 8% 51%	2365
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 3 5% 7% 49%	2254
	21st to end	Day	11% 5% 1 5% 4 4 4 5% 5% 8% 49%	2280
May	1st to 10th	Day	8% 6% 3 8% 5% 6% 6% 5% 7% 9% 37%	3815
	11th to 20th	Day	17% 8% 3 8% 5% 5% 6% 4 5% 8% 31%	4081
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 8% 9% 41%	4476
June	1st to 10th	Day	11% 7% 2 8% 5% 6% 7% 5% 7% 9% 32%	2470
	11th to 20th	Day	9% 6% 2 6% 6% 6% 6% 5% 8% 11% 35%	2470
	21st to end	Day	8% 7% 2 8% 7% 6% 7% 6% 9% 10% 30%	2470
July	1st to 10th	Day	9% 6% 3 9% 6% 7% 8% 7% 9% 12% 25%	4120
	11th to 20th	Day	9% 7% 3 11% 8% 8% 8% 7% 8% 9% 22%	4120
	21st to end	Day	9% 8% 4 10% 8% 8% 8% 6% 9% 9% 21%	4466
August	1st to 10th	Day	23% 12% 4 9% 6% 6% 6% 4 6% 9% 17%	4104
	11th to 20th	Day	10% 7% 3 8% 7% 6% 7% 5% 6% 11% 30%	3765
	21st to end	Day	7% 5% 3 8% 5% 6% 6% 5% 7% 10% 39%	4180
September	1st to 10th	Day	10% 7% 3 8% 5% 6% 6% 4 8% 10% 35%	2280
	11th to 20th	Day	17% 8% 3 7% 6% 4 5% 4 5% 7% 33%	2234
	21st to end	Day	18% 8% 3 7% 5% 5% 4 4 5% 6% 35%	2005
October	1st to 10th	Day	16% 7% 2 6% 4 5% 3 4 5% 6% 43%	1900
	11th to 20th	Day	11% 5% 1 5% 4 3 3 3 5% 6% 54%	1900
	21st to end	Day	10% 6% 2 6% 3 4 4 3 4 5% 54%	2090
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 2 5% 6% 52%	1957
	11th to 20th	Day	15% 6% 3 5% 4 3 2 3 4 5% 50%	2000
	21st to end	Day	11% 5% 2 7% 4 4 3 3 4 7% 49%	1783
December	1st to 10th	Day	10% 5% 2 5% 5% 4 4 3 4 5% 53%	2600
	11th to 20th	Day	15% 7% 3 5% 3 3 4 3 5% 6% 45%	3187
	21st to end	Day	14% 6% 2 6% 4 3 4 3 5% 8% 45%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 55%	5889
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 50%	5900
	21st to end	Night	14% 8% 3 7% 4 4 4 3 4 6% 45%	6490
February	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 5% 6% 44%	2600
	11th to 20th	Night	19% 8% 2 6% 4 4 3 4 4 6% 40%	2428
	21st to end	Night	15% 6% 3 6% 4 4 3 4 6% 6% 43%	1873
March	1st to 10th	Night	19% 10% 3 7% 4 4 4 2 4 4 39%	4918
	11th to 20th	Night	13% 5% 2 4 4 3 4 4 6% 7% 49%	4980
	21st to end	Night	14% 5% 2 5% 3 4 3 3 5% 6% 51%	5467
April	1st to 10th	Night	12% 5% 2 5% 3 3 4 4 5% 7% 49%	2035
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 5% 6% 49%	1882
	21st to end	Night	8% 5% 1 5% 4 5% 3 3 6% 7% 53%	1900
May	1st to 10th	Night	7% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	3109
	11th to 20th	Night	13% 8% 3 7% 5% 4 5% 4 6% 7% 37%	2812
	21st to end	Night	8% 4 2 7% 5% 5% 4 4 7% 10% 45%	3079
June	1st to 10th	Night	9% 6% 3 8% 6% 4 5% 5% 8% 11% 36%	1710
	11th to 20th	Night	9% 6% 2 7% 4 5% 5% 5% 8% 10% 41%	1710
	21st to end	Night	8% 6% 2 7% 6% 6% 5% 5% 8% 11% 36%	1710
July	1st to 10th	Night	7% 6% 3 8% 6% 6% 7% 5% 8% 12% 32%	2840
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 5% 9% 12% 30%	2840
	21st to end	Night	11% 7% 3 8% 6% 6% 7% 6% 8% 10% 27%	3087
August	1st to 10th	Night	20% 10% 4 8% 5% 4 5% 4 7% 9% 24%	2825
	11th to 20th	Night	7% 6% 3 8% 5% 6% 6% 5% 8% 10% 35%	3032
	21st to end	Night	6% 5% 2 7% 5% 4 5% 5% 8% 10% 43%	3476
September	1st to 10th	Night	9% 5% 2 5% 4 5% 5% 5% 7% 10% 42%	1900
	11th to 20th	Night	12% 7% 2 7% 5% 4 5% 3 7% 9% 39%	1942
	21st to end	Night	17% 7% 3 6% 5% 4 5% 3 5% 7% 38%	2175
October	1st to 10th	Night	14% 8% 2 7% 4 3 4 3 4 6% 46%	2280
	11th to 20th	Night	10% 5% 1 6% 3 3 3 2 5% 5% 56%	2280
	21st to end	Night	10% 5% 1 5% 3 3 3 2 5% 6% 55%	2508
November	1st to 10th	Night	13% 5% 2 5% 4 2 3 3 5% 5% 53%	2341
	11th to 20th	Night	14% 7% 3 6% 4 3 3 3 4 6% 48%	2400
	21st to end	Night	12% 6% 1 5% 3 4 4 3 5% 7% 50%	2605
December	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 6% 54%	4580
	11th to 20th	Night	15% 8% 3 7% 3 4 4 4 4 7% 43%	4999
	21st to end	Night	14% 6% 2 5% 4 3 4 4 5% 6% 47%	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	33% 3 5% 17% 32% 9%	4057
	11th to 20th	Day	32% 3 5% 17% 32% 11%	4060
	21st to end	Day	44% 5% 6% 14% 21% 10%	4466
February	1st to 10th	Day	43% 4 10% 20% 19% 5%	1800
	11th to 20th	Day	44% 4 7% 16% 22% 7%	1866
	21st to end	Day	41% 5% 10% 19% 18% 8%	1753
March	1st to 10th	Day	49% 4 8% 12% 19% 8%	4935
	11th to 20th	Day	30% 5% 9% 15% 24% 17%	4980
	21st to end	Day	37% 4 9% 20% 21% 9%	5465
April	1st to 10th	Day	31% 5% 11% 22% 22% 8%	2365
	11th to 20th	Day	32% 6% 12% 21% 19% 10%	2254
	21st to end	Day	31% 7% 14% 21% 22% 5%	2280
May	1st to 10th	Day	40% 10% 19% 19% 10% 2	3850
	11th to 20th	Day	48% 8% 11% 19% 13% 1	4085
	21st to end	Day	34% 10% 17% 24% 14% 2	4489
June	1st to 10th	Day	45% 10% 14% 21% 10% 1	2470
	11th to 20th	Day	38% 11% 18% 21% 11% 1	2470
	21st to end	Day	42% 13% 18% 18% 8% 1	2470
July	1st to 10th	Day	45% 12% 18% 19% 5% 1	4055
	11th to 20th	Day	52% 13% 21% 10% 4	4051
	21st to end	Day	52% 13% 15% 13% 7%	4324
August	1st to 10th	Day	62% 9% 17% 9% 3	4091
	11th to 20th	Day	46% 10% 15% 19% 10%	3802
	21st to end	Day	38% 9% 17% 29% 7% 1	4180
September	1st to 10th	Day	42% 11% 16% 15% 13% 4	2280
	11th to 20th	Day	48% 8% 10% 15% 15% 4	2234
	21st to end	Day	49% 6% 10% 17% 15% 3	2005
October	1st to 10th	Day	40% 4 7% 19% 24% 5%	3160
	11th to 20th	Day	22% 2 6% 17% 25% 27%	3140
	21st to end	Day	28% 4 6% 22% 30% 11%	3428
November	1st to 10th	Day	33% 4 9% 19% 26% 10%	1957
	11th to 20th	Day	37% 4 7% 18% 25% 9%	2000
	21st to end	Day	35% 5% 9% 21% 23% 8%	1783
December	1st to 10th	Day	33% 4 8% 16% 25% 13%	2600
	11th to 20th	Day	40% 4 11% 18% 20% 7%	3184
	21st to end	Day	38% 4 13% 23% 20% 2	4466
January	1st to 10th	Night	32% 4 6% 17% 35% 7%	5889
	11th to 20th	Night	35% 4 6% 15% 33% 7%	5900
	21st to end	Night	41% 4 5% 15% 25% 9%	6490
February	1st to 10th	Night	41% 5% 9% 19% 20% 6%	2600
	11th to 20th	Night	45% 5% 8% 16% 20% 7%	2430
	21st to end	Night	40% 5% 10% 17% 19% 9%	1873
March	1st to 10th	Night	49% 4 7% 13% 21% 6%	4918
	11th to 20th	Night	34% 6% 9% 11% 24% 16%	4980
	21st to end	Night	35% 4 11% 23% 21% 6%	5467
April	1st to 10th	Night	34% 6% 14% 17% 21% 8%	2035
	11th to 20th	Night	36% 5% 12% 21% 18% 8%	1882
	21st to end	Night	29% 6% 15% 26% 19% 4	1900
May	1st to 10th	Night	35% 10% 19% 23% 11% 2	3139
	11th to 20th	Night	44% 8% 11% 22% 14%	2813
	21st to end	Night	30% 11% 21% 23% 14% 1	3087
June	1st to 10th	Night	40% 10% 18% 21% 9% 1	1710
	11th to 20th	Night	34% 11% 19% 26% 9% 1	1710
	21st to end	Night	39% 12% 20% 22% 7% 1	1710
July	1st to 10th	Night	41% 12% 22% 19% 4 1	2795
	11th to 20th	Night	42% 14% 29% 11% 4	2786
	21st to end	Night	46% 14% 17% 16% 7%	2988
August	1st to 10th	Night	53% 11% 21% 13% 2	2814
	11th to 20th	Night	39% 11% 21% 22% 7%	3061
	21st to end	Night	32% 9% 17% 33% 7% 1	3476
September	1st to 10th	Night	35% 12% 21% 16% 13% 3	1900
	11th to 20th	Night	41% 11% 13% 18% 15% 3	1942
	21st to end	Night	45% 7% 12% 19% 14% 3	2175
October	1st to 10th	Night	41% 5% 8% 17% 24% 5%	3800
	11th to 20th	Night	20% 3 6% 17% 22% 33%	3779
	21st to end	Night	25% 3 8% 24% 28% 12%	4120
November	1st to 10th	Night	33% 5% 9% 18% 25% 10%	2341
	11th to 20th	Night	38% 4 8% 16% 25% 10%	2400
	21st to end	Night	34% 4 8% 22% 25% 7%	2605
December	1st to 10th	Night	32% 4 8% 12% 30% 13%	4580
	11th to 20th	Night	42% 4 13% 16% 18% 7%	5001
	21st to end	Night	37% 5% 12% 23% 20% 3	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	24% 26% 38% 11% 1	300
	11th to 20th	Day	21% 24% 35% 16% 3 1	300
	21st to end	Day	25% 25% 37% 11% 2	330
February	1st to 10th	Day	24% 20% 39% 14% 2	300
	11th to 20th	Day	32% 22% 32% 12% 1	404
	21st to end	Day	21% 22% 36% 17% 4	414
March	1st to 10th	Day	23% 20% 33% 18% 6%	500
	11th to 20th	Day	22% 26% 33% 14% 5% 1	500
	21st to end	Day	20% 22% 36% 16% 6% 1	550
April	1st to 10th	Day	21% 21% 29% 21% 7% 1	500
	11th to 20th	Day	14% 20% 31% 19% 11% 5%	452
	21st to end	Day	14% 15% 32% 23% 12% 4	460
May	1st to 10th	Day	11% 20% 32% 23% 10% 5%	463
	11th to 20th	Day	16% 9% 28% 27% 15% 4	487
	21st to end	Day	13% 5% 23% 33% 16% 9%	529
June	1st to 10th	Day	14% 6% 26% 31% 18% 5%	490
	11th to 20th	Day	12% 5% 23% 30% 19% 11%	490
	21st to end	Day	9% 4 21% 37% 20% 9%	490
July	1st to 10th	Day	10% 3 15% 32% 29% 11%	485
	11th to 20th	Day	12% 3 12% 31% 33% 10%	484
	21st to end	Day	7% 4 18% 35% 29% 7%	532
August	1st to 10th	Day	17% 5% 17% 26% 22% 12%	486
	11th to 20th	Day	10% 3 16% 30% 25% 15%	479
	21st to end	Day	10% 3 15% 32% 23% 16%	539
September	1st to 10th	Day	12% 3 20% 32% 18% 15%	490
	11th to 20th	Day	22% 5% 16% 33% 17% 8%	490
	21st to end	Day	20% 5% 23% 27% 17% 7%	490
October	1st to 10th	Day	17% 7% 25% 26% 18% 7%	490
	11th to 20th	Day	10% 10% 30% 30% 17% 3	490
	21st to end	Day	12% 11% 28% 29% 17% 4	539
November	1st to 10th	Day	17% 11% 29% 28% 12% 4	495
	11th to 20th	Day	16% 18% 35% 22% 9% 1	500
	21st to end	Day	13% 15% 33% 23% 13% 2	355
December	1st to 10th	Day	20% 18% 36% 19% 7% 1	200
	11th to 20th	Day	20% 19% 33% 20% 5% 2	243
	21st to end	Day	23% 20% 33% 15% 6% 2	330
January	1st to 10th	Night	18% 19% 32% 26% 5%	800
	11th to 20th	Night	17% 16% 30% 32% 6%	800
	21st to end	Night	19% 15% 36% 26% 3	880
February	1st to 10th	Night	22% 13% 34% 26% 5%	800
	11th to 20th	Night	25% 16% 36% 21% 2	783
	21st to end	Night	20% 16% 32% 27% 5%	660
March	1st to 10th	Night	17% 15% 36% 26% 5%	800
	11th to 20th	Night	17% 16% 38% 23% 6% 1	800
	21st to end	Night	18% 14% 36% 25% 7%	880
April	1st to 10th	Night	15% 16% 37% 25% 7%	800
	11th to 20th	Night	15% 12% 34% 28% 11%	754
	21st to end	Night	10% 9% 40% 32% 9% 1	760
May	1st to 10th	Night	9% 11% 38% 31% 10% 1	752
	11th to 20th	Night	15% 7% 24% 39% 14%	561
	21st to end	Night	11% 2 18% 35% 29% 5%	617
June	1st to 10th	Night	10% 2 21% 42% 23% 2	570
	11th to 20th	Night	11% 3 16% 36% 28% 6%	570
	21st to end	Night	9% 4 14% 43% 27% 3	570
July	1st to 10th	Night	6% 2 12% 40% 35% 5%	555
	11th to 20th	Night	9% 1 9% 38% 38% 6%	552
	21st to end	Night	8% 2 11% 44% 31% 4	624
August	1st to 10th	Night	13% 2 9% 35% 35% 6%	567
	11th to 20th	Night	10% 2 12% 36% 34% 5%	709
	21st to end	Night	13% 1 13% 33% 34% 6%	836
September	1st to 10th	Night	12% 3 13% 34% 33% 6%	760
	11th to 20th	Night	14% 3 16% 38% 25% 4	760
	21st to end	Night	21% 5% 15% 32% 24% 4	760
October	1st to 10th	Night	17% 5% 21% 31% 22% 5%	760
	11th to 20th	Night	13% 7% 26% 33% 18% 3	760
	21st to end	Night	14% 6% 25% 32% 21% 2	836
November	1st to 10th	Night	14% 7% 26% 35% 15% 2	780
	11th to 20th	Night	19% 11% 29% 27% 13% 2	800
	21st to end	Night	14% 12% 29% 30% 13% 2	800
December	1st to 10th	Night	19% 12% 30% 32% 8% 1	800
	11th to 20th	Night	17% 12% 29% 33% 9% 1	800
	21st to end	Night	19% 15% 29% 29% 8%	880

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