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 2026-08-15.

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% 39%	142437
	Night	12% 7% 2 6% 4 4 4 4 6% 8% 43%	139472
Transparency	Day	41% 7% 13% 19% 16% 5%	146038
	Night	38% 6% 13% 19% 18% 6%	143943
Seeing	Day	16% 12% 27% 25% 14% 5%	17935
	Night	15% 9% 26% 32% 16% 2	28269

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% 5% 2 5% 3 3 3 3 4 6% 53%	5258
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 5% 56%	5280
	21st to end	Day	14% 11% 3 7% 4 4 4 3 4 5% 40%	5780
February	1st to 10th	Day	16% 7% 2 6% 4 5% 4 3 5% 7% 42%	1890
	11th to 20th	Day	19% 7% 3 6% 4 3 3 3 4 5% 44%	1952
	21st to end	Day	15% 6% 2 6% 6% 5% 4 3 5% 6% 42%	1838
March	1st to 10th	Day	21% 8% 2 6% 4 4 4 3 5% 6% 37%	5045
	11th to 20th	Day	10% 4 1 4 4 4 5% 3 5% 6% 53%	5090
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 5% 49%	5586
April	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 4 8% 52%	2483
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 8% 51%	2494
	21st to end	Day	10% 5% 1 5% 3 4 4 4 5% 8% 49%	2520
May	1st to 10th	Day	11% 7% 3 8% 5% 5% 5% 4 6% 9% 36%	7098
	11th to 20th	Day	14% 6% 2 7% 4 4 5% 4 6% 9% 38%	7626
	21st to end	Day	10% 8% 3 8% 6% 6% 7% 5% 7% 8% 33%	8391
June	1st to 10th	Day	11% 7% 2 8% 5% 6% 7% 5% 7% 9% 33%	2730
	11th to 20th	Day	9% 5% 2 6% 6% 6% 7% 5% 8% 11% 34%	2730
	21st to end	Day	7% 6% 2 7% 7% 6% 7% 6% 9% 10% 32%	2730
July	1st to 10th	Day	7% 6% 2 8% 6% 6% 7% 6% 9% 11% 31%	7680
	11th to 20th	Day	7% 7% 3 10% 7% 7% 8% 7% 8% 11% 24%	7680
	21st to end	Day	10% 8% 3 9% 7% 7% 7% 6% 8% 10% 23%	8340
August	1st to 10th	Day	22% 12% 4 9% 6% 6% 6% 4 6% 9% 17%	4364
	11th to 20th	Day	10% 7% 3 8% 7% 6% 7% 5% 6% 11% 31%	3935
	21st to end	Day	7% 5% 3 8% 5% 6% 6% 5% 7% 10% 39%	4312
September	1st to 10th	Day	10% 7% 3 8% 5% 6% 6% 4 7% 10% 36%	2400
	11th to 20th	Day	17% 8% 3 7% 6% 4 5% 4 5% 7% 33%	2351
	21st to end	Day	19% 8% 3 7% 5% 5% 4 4 5% 6% 35%	2110
October	1st to 10th	Day	15% 7% 2 6% 4 5% 3 3 5% 7% 44%	2000
	11th to 20th	Day	12% 5% 1 5% 4 3 3 3 5% 6% 53%	2000
	21st to end	Day	10% 6% 2 6% 3 3 4 3 4 5% 55%	2200
November	1st to 10th	Day	11% 6% 2 5% 4 3 3 2 5% 6% 54%	2057
	11th to 20th	Day	14% 5% 2 5% 4 3 2 3 4 5% 52%	2100
	21st to end	Day	11% 5% 2 7% 4 4 3 3 4 7% 49%	1871
December	1st to 10th	Day	11% 5% 2 5% 5% 4 4 3 4 5% 53%	2680
	11th to 20th	Day	15% 7% 3 5% 4 3 4 3 5% 6% 45%	3271
	21st to end	Day	14% 6% 2 6% 4 3 4 3 5% 8% 45%	4565
January	1st to 10th	Night	10% 7% 2 5% 3 3 3 3 4 5% 54%	7635
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 5% 6% 53%	7680
	21st to end	Night	17% 8% 3 7% 4 4 4 3 4 5% 42%	8401
February	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 4 6% 45%	2730
	11th to 20th	Night	18% 8% 3 6% 3 4 3 4 4 7% 40%	2552
	21st to end	Night	16% 6% 3 6% 4 4 4 4 6% 6% 42%	1964
March	1st to 10th	Night	19% 10% 3 7% 4 4 4 2 4 5% 39%	5028
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 6% 7% 49%	5090
	21st to end	Night	14% 5% 2 4 3 4 3 3 5% 6% 51%	5588
April	1st to 10th	Night	11% 5% 2 5% 3 4 4 4 6% 7% 49%	2137
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 5% 7% 51%	2075
	21st to end	Night	8% 5% 1 5% 4 5% 4 3 6% 7% 52%	2100
May	1st to 10th	Night	10% 7% 2 8% 5% 5% 5% 4 7% 10% 38%	5798
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	5234
	21st to end	Night	12% 7% 2 8% 5% 5% 6% 5% 7% 9% 34%	5761
June	1st to 10th	Night	9% 6% 3 8% 6% 4 5% 5% 8% 11% 36%	1890
	11th to 20th	Night	9% 6% 2 7% 4 5% 5% 4 8% 10% 40%	1890
	21st to end	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 11% 38%	1890
July	1st to 10th	Night	6% 5% 2 7% 5% 6% 6% 6% 9% 12% 36%	5278
	11th to 20th	Night	10% 7% 3 8% 5% 6% 5% 5% 9% 14% 28%	5277
	21st to end	Night	12% 8% 3 8% 6% 6% 7% 6% 9% 11% 25%	5736
August	1st to 10th	Night	20% 10% 4 8% 5% 5% 5% 4 7% 9% 24%	3005
	11th to 20th	Night	7% 6% 3 8% 5% 6% 6% 5% 8% 11% 35%	3169
	21st to end	Night	6% 5% 2 7% 5% 4 5% 5% 8% 10% 42%	3586
September	1st to 10th	Night	9% 6% 2 6% 4 5% 5% 5% 7% 10% 43%	2000
	11th to 20th	Night	12% 7% 2 7% 5% 4 5% 3 7% 9% 39%	2044
	21st to end	Night	18% 7% 3 6% 5% 4 4 3 5% 7% 38%	2290
October	1st to 10th	Night	13% 8% 2 7% 4 3 4 3 5% 6% 46%	2400
	11th to 20th	Night	11% 5% 1 6% 3 3 3 2 5% 5% 56%	2400
	21st to end	Night	11% 5% 1 5% 3 3 3 2 5% 6% 55%	2640
November	1st to 10th	Night	13% 5% 2 5% 4 2 3 3 5% 5% 54%	2461
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 6% 50%	2520
	21st to end	Night	12% 6% 2 5% 3 4 4 3 5% 7% 50%	2736
December	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 6% 53%	4720
	11th to 20th	Night	14% 8% 3 7% 3 4 4 4 4 7% 43%	5134
	21st to end	Night	13% 6% 2 5% 3 3 4 4 5% 7% 48%	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	33% 3 11% 20% 26% 7%	5259
	11th to 20th	Day	32% 3 4 16% 31% 13%	5280
	21st to end	Day	47% 4 6% 15% 18% 10%	5780
February	1st to 10th	Day	42% 4 9% 20% 19% 5%	1890
	11th to 20th	Day	44% 4 7% 16% 22% 7%	1961
	21st to end	Day	42% 5% 10% 18% 18% 8%	1838
March	1st to 10th	Day	48% 4 9% 13% 18% 8%	5045
	11th to 20th	Day	30% 5% 9% 15% 24% 17%	5090
	21st to end	Day	36% 4 10% 20% 20% 9%	5586
April	1st to 10th	Day	31% 5% 11% 23% 23% 7%	2483
	11th to 20th	Day	30% 6% 12% 25% 19% 9%	2494
	21st to end	Day	32% 6% 14% 21% 22% 5%	2520
May	1st to 10th	Day	44% 7% 17% 22% 8% 2	7130
	11th to 20th	Day	41% 7% 14% 23% 14% 2	7630
	21st to end	Day	44% 9% 14% 18% 13% 2	8405
June	1st to 10th	Day	44% 9% 14% 21% 11% 1	2730
	11th to 20th	Day	38% 11% 19% 21% 10% 1	2730
	21st to end	Day	40% 12% 18% 20% 8% 1	2730
July	1st to 10th	Day	41% 11% 21% 22% 5%	7615
	11th to 20th	Day	48% 13% 23% 12% 4	7611
	21st to end	Day	50% 11% 16% 15% 8% 1	8156
August	1st to 10th	Day	62% 9% 17% 9% 3	4351
	11th to 20th	Day	46% 10% 16% 19% 10%	3972
	21st to end	Day	39% 9% 16% 28% 7% 1	4312
September	1st to 10th	Day	42% 10% 15% 16% 13% 4	2400
	11th to 20th	Day	48% 8% 11% 16% 15% 3	2351
	21st to end	Day	49% 6% 11% 17% 14% 3	2110
October	1st to 10th	Day	40% 4 7% 20% 24% 5%	3260
	11th to 20th	Day	23% 2 6% 17% 25% 26%	3240
	21st to end	Day	28% 4 6% 21% 30% 11%	3538
November	1st to 10th	Day	32% 4 9% 19% 26% 9%	2057
	11th to 20th	Day	36% 4 7% 18% 26% 9%	2100
	21st to end	Day	35% 5% 9% 21% 23% 8%	1871
December	1st to 10th	Day	34% 4 8% 16% 25% 13%	2680
	11th to 20th	Day	40% 4 10% 18% 21% 7%	3268
	21st to end	Day	38% 4 13% 23% 20% 2	4565
January	1st to 10th	Night	33% 3 10% 21% 28% 5%	7636
	11th to 20th	Night	32% 4 6% 15% 32% 10%	7680
	21st to end	Night	44% 4 5% 15% 22% 10%	8401
February	1st to 10th	Night	41% 4 9% 19% 20% 7%	2730
	11th to 20th	Night	44% 5% 8% 16% 20% 7%	2554
	21st to end	Night	41% 5% 9% 16% 18% 10%	1964
March	1st to 10th	Night	48% 4 7% 13% 21% 6%	5028
	11th to 20th	Night	34% 5% 9% 12% 24% 16%	5090
	21st to end	Night	35% 4 11% 23% 20% 6%	5588
April	1st to 10th	Night	34% 5% 14% 18% 21% 8%	2137
	11th to 20th	Night	34% 5% 12% 23% 18% 8%	2075
	21st to end	Night	30% 6% 15% 26% 19% 4	2100
May	1st to 10th	Night	40% 8% 19% 23% 9% 1	5828
	11th to 20th	Night	37% 7% 15% 25% 15% 1	5235
	21st to end	Night	43% 11% 17% 17% 11%	5771
June	1st to 10th	Night	40% 9% 18% 21% 10% 2	1890
	11th to 20th	Night	36% 11% 20% 24% 9% 1	1890
	21st to end	Night	37% 11% 20% 24% 7% 1	1890
July	1st to 10th	Night	37% 11% 26% 20% 5%	5235
	11th to 20th	Night	42% 13% 28% 14% 4	5226
	21st to end	Night	48% 11% 18% 16% 7%	5606
August	1st to 10th	Night	53% 11% 21% 12% 2	2994
	11th to 20th	Night	39% 11% 21% 22% 7%	3198
	21st to end	Night	33% 9% 17% 32% 7% 1	3586
September	1st to 10th	Night	35% 11% 20% 18% 13% 3	2000
	11th to 20th	Night	41% 11% 13% 18% 15% 3	2044
	21st to end	Night	46% 7% 12% 19% 13% 3	2290
October	1st to 10th	Night	40% 5% 8% 18% 24% 5%	3920
	11th to 20th	Night	21% 3 5% 17% 22% 32%	3899
	21st to end	Night	25% 3 8% 24% 29% 11%	4252
November	1st to 10th	Night	33% 5% 9% 18% 25% 10%	2461
	11th to 20th	Night	37% 4 8% 16% 26% 10%	2520
	21st to end	Night	34% 4 8% 22% 24% 7%	2736
December	1st to 10th	Night	33% 4 8% 13% 30% 13%	4720
	11th to 20th	Night	42% 4 13% 16% 18% 7%	5136
	21st to end	Night	36% 5% 12% 24% 20% 3	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	23% 26% 38% 11% 2	330
	11th to 20th	Day	22% 24% 35% 16% 2 1	330
	21st to end	Day	25% 25% 37% 10% 2	363
February	1st to 10th	Day	25% 19% 39% 14% 3	330
	11th to 20th	Day	33% 22% 32% 12% 2	439
	21st to end	Day	24% 23% 34% 16% 3	446
March	1st to 10th	Day	22% 19% 34% 19% 6%	540
	11th to 20th	Day	22% 25% 33% 14% 5% 1	540
	21st to end	Day	19% 21% 37% 16% 6% 1	594
April	1st to 10th	Day	20% 20% 29% 21% 8% 1	540
	11th to 20th	Day	13% 19% 32% 20% 11% 4	532
	21st to end	Day	13% 14% 31% 25% 13% 4	540
May	1st to 10th	Day	13% 19% 33% 22% 9% 4	543
	11th to 20th	Day	16% 10% 29% 27% 14% 4	567
	21st to end	Day	15% 6% 25% 31% 14% 8%	617
June	1st to 10th	Day	13% 7% 28% 29% 18% 5%	570
	11th to 20th	Day	11% 6% 24% 32% 18% 10%	570
	21st to end	Day	8% 4 21% 38% 20% 8%	570
July	1st to 10th	Day	9% 3 16% 33% 30% 10%	565
	11th to 20th	Day	11% 3 12% 32% 33% 10%	564
	21st to end	Day	8% 4 19% 33% 27% 8%	620
August	1st to 10th	Day	18% 6% 17% 26% 22% 11%	566
	11th to 20th	Day	11% 3 15% 31% 26% 14%	535
	21st to end	Day	11% 4 17% 33% 22% 15%	583
September	1st to 10th	Day	11% 4 21% 33% 17% 14%	530
	11th to 20th	Day	22% 5% 17% 32% 17% 7%	530
	21st to end	Day	22% 5% 22% 28% 17% 7%	530
October	1st to 10th	Day	17% 7% 25% 27% 18% 7%	530
	11th to 20th	Day	11% 11% 30% 29% 16% 3	530
	21st to end	Day	12% 13% 28% 28% 16% 3	583
November	1st to 10th	Day	16% 11% 31% 28% 11% 3	535
	11th to 20th	Day	15% 18% 35% 21% 9% 1	540
	21st to end	Day	13% 16% 32% 24% 12% 2	383
December	1st to 10th	Day	21% 17% 36% 19% 6%	220
	11th to 20th	Day	21% 18% 33% 21% 4 2	267
	21st to end	Day	23% 20% 34% 15% 6% 2	363
January	1st to 10th	Night	18% 19% 33% 26% 5%	840
	11th to 20th	Night	17% 17% 30% 31% 5%	840
	21st to end	Night	19% 15% 36% 26% 3	924
February	1st to 10th	Night	22% 13% 35% 26% 4	840
	11th to 20th	Night	24% 16% 36% 21% 2	823
	21st to end	Night	22% 16% 32% 25% 5%	692
March	1st to 10th	Night	17% 15% 36% 26% 6%	840
	11th to 20th	Night	17% 16% 38% 23% 6% 1	840
	21st to end	Night	17% 14% 37% 26% 7%	924
April	1st to 10th	Night	15% 16% 36% 27% 6%	840
	11th to 20th	Night	13% 12% 35% 29% 11%	831
	21st to end	Night	10% 9% 39% 33% 9%	840
May	1st to 10th	Night	9% 12% 37% 32% 9%	831
	11th to 20th	Night	15% 7% 24% 39% 14%	621
	21st to end	Night	12% 3 19% 34% 27% 5%	683
June	1st to 10th	Night	11% 3 20% 42% 23% 2	630
	11th to 20th	Night	11% 3 18% 37% 26% 6%	630
	21st to end	Night	8% 4 14% 43% 27% 3	630
July	1st to 10th	Night	6% 2 12% 41% 35% 4	615
	11th to 20th	Night	9% 2 8% 39% 37% 5%	612
	21st to end	Night	9% 3 12% 41% 32% 4	690
August	1st to 10th	Night	13% 2 9% 35% 34% 5%	627
	11th to 20th	Night	10% 2 12% 36% 34% 6%	762
	21st to end	Night	12% 2 13% 34% 32% 6%	880
September	1st to 10th	Night	12% 3 14% 34% 32% 6%	800
	11th to 20th	Night	14% 3 17% 37% 25% 5%	800
	21st to end	Night	21% 5% 15% 32% 24% 4	800
October	1st to 10th	Night	16% 5% 21% 32% 22% 5%	800
	11th to 20th	Night	15% 7% 26% 33% 17% 3	800
	21st to end	Night	14% 6% 25% 32% 20% 2	880
November	1st to 10th	Night	14% 8% 27% 34% 15% 2	820
	11th to 20th	Night	19% 12% 29% 27% 12% 1	840
	21st to end	Night	14% 12% 29% 30% 13% 1	840
December	1st to 10th	Night	20% 12% 29% 31% 7% 1	840
	11th to 20th	Night	17% 11% 29% 33% 9% 1	840
	21st to end	Night	19% 15% 30% 29% 8%	924

Page generated at 2026-08-12 03:37:18. Version ac07

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