SFA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SFA Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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% 6% 2 6% 4 4 4 4 6% 7% 45%	149016
	Night	13% 6% 2 5% 3 3 4 3 5% 7% 48%	143400
Transparency	Day	37% 5% 11% 22% 18% 6%	152626
	Night	35% 5% 11% 20% 20% 9%	147844
Seeing	Day	14% 9% 24% 27% 20% 6%	17726
	Night	16% 9% 25% 32% 17% 2	27457

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% 9% 3 5% 3 3 3 3 4 5% 46%	5289
	11th to 20th	Day	12% 5% 2 4 3 2 3 3 5% 7% 54%	5310
	21st to end	Day	26% 10% 2 6% 3 2 2 2 3 4 40%	5812
February	1st to 10th	Day	21% 9% 3 6% 3 3 3 3 4 6% 39%	1890
	11th to 20th	Day	21% 10% 3 6% 4 3 3 4 4 4 38%	2054
	21st to end	Day	20% 8% 3 6% 4 3 3 2 3 5% 42%	1903
March	1st to 10th	Day	19% 12% 3 8% 4 3 3 2 3 5% 38%	5046
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 4 6% 50%	5090
	21st to end	Day	16% 9% 2 6% 4 4 4 3 5% 6% 41%	5583
April	1st to 10th	Day	14% 7% 3 7% 6% 4 4 3 5% 6% 41%	2344
	11th to 20th	Day	14% 7% 3 6% 4 5% 5% 5% 6% 7% 38%	2494
	21st to end	Day	12% 6% 3 7% 5% 4 4 4 5% 6% 42%	2520
May	1st to 10th	Day	21% 8% 3 7% 5% 4 5% 3 5% 6% 34%	7176
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 6% 39%	7627
	21st to end	Day	11% 7% 3 8% 6% 6% 5% 5% 7% 10% 33%	8392
June	1st to 10th	Day	7% 6% 2 6% 6% 5% 6% 5% 8% 9% 38%	2730
	11th to 20th	Day	6% 5% 2 6% 6% 5% 5% 5% 8% 10% 41%	2730
	21st to end	Day	5% 5% 2 6% 5% 6% 6% 6% 8% 10% 41%	2730
July	1st to 10th	Day	6% 5% 2 6% 6% 6% 6% 5% 8% 12% 37%	7680
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	7680
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 9% 48%	8337
August	1st to 10th	Day	2 1 1 2 2 3 4 3 6% 9% 67%	6014
	11th to 20th	Day	2 2 1 3 2 3 3 3 6% 9% 66%	5692
	21st to end	Day	6% 4 2 5% 5% 6% 6% 5% 8% 10% 43%	6080
September	1st to 10th	Day	7% 6% 2 6% 5% 4 6% 5% 7% 9% 44%	2424
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 4 6% 7% 44%	2310
	21st to end	Day	7% 4 1 5% 4 4 4 4 6% 7% 53%	2310
October	1st to 10th	Day	8% 4 1 4 2 3 3 3 5% 6% 61%	2120
	11th to 20th	Day	11% 6% 2 4 3 2 4 3 4 6% 55%	2100
	21st to end	Day	14% 5% 2 5% 3 3 3 2 3 5% 55%	2280
November	1st to 10th	Day	12% 6% 2 6% 3 4 4 3 5% 5% 50%	2058
	11th to 20th	Day	11% 6% 2 6% 4 3 2 3 5% 5% 53%	2100
	21st to end	Day	17% 7% 2 6% 4 3 3 2 4 5% 45%	2013
December	1st to 10th	Day	21% 5% 1 5% 4 3 3 2 4 5% 46%	3030
	11th to 20th	Day	17% 8% 2 6% 4 4 4 3 5% 5% 40%	3481
	21st to end	Day	17% 8% 3 7% 4 3 3 3 4 4 44%	4587
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 2 4 4 51%	7634
	11th to 20th	Night	11% 5% 2 4 2 2 3 3 5% 5% 57%	7680
	21st to end	Night	27% 11% 2 6% 3 3 3 2 3 4 36%	8402
February	1st to 10th	Night	17% 7% 2 5% 4 3 3 2 5% 6% 45%	2730
	11th to 20th	Night	21% 9% 3 6% 4 3 2 2 3 5% 43%	2468
	21st to end	Night	21% 7% 2 5% 3 2 4 3 3 5% 46%	1903
March	1st to 10th	Night	22% 9% 2 5% 3 3 3 3 4 6% 40%	5029
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 4 7% 49%	5090
	21st to end	Night	17% 8% 3 7% 3 4 3 3 3 5% 44%	5588
April	1st to 10th	Night	13% 5% 3 6% 4 4 5% 4 6% 7% 44%	2273
	11th to 20th	Night	14% 5% 3 6% 4 4 4 3 4 7% 44%	2075
	21st to end	Night	13% 7% 2 6% 5% 3 3 3 4 6% 48%	2100
May	1st to 10th	Night	18% 10% 3 8% 4 4 4 3 5% 8% 33%	5667
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 3 6% 7% 39%	5265
	21st to end	Night	11% 7% 2 7% 5% 4 5% 4 7% 10% 38%	5793
June	1st to 10th	Night	6% 5% 2 4 4 3 5% 5% 6% 10% 51%	1890
	11th to 20th	Night	4 3 1 3 3 3 4 4 6% 10% 58%	1890
	21st to end	Night	6% 4 2 5% 3 5% 4 4 7% 11% 50%	1890
July	1st to 10th	Night	5% 4 2 6% 5% 5% 5% 5% 8% 13% 42%	5308
	11th to 20th	Night	4 4 2 6% 5% 5% 5% 4 8% 11% 46%	5307
	21st to end	Night	8% 5% 2 4 3 3 3 3 5% 12% 52%	5770
August	1st to 10th	Night	1 1 2 2 2 3 2 6% 9% 72%	4156
	11th to 20th	Night	1 2 1 2 1 3 3 3 6% 10% 68%	4359
	21st to end	Night	4 3 1 4 4 4 5% 5% 7% 10% 52%	5046
September	1st to 10th	Night	7% 5% 2 6% 4 4 5% 3 5% 9% 51%	2190
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 5% 7% 52%	2310
	21st to end	Night	6% 4 1 5% 3 3 4 3 6% 8% 58%	2310
October	1st to 10th	Night	5% 3 1 4 3 3 4 3 4 6% 63%	2478
	11th to 20th	Night	10% 4 1 3 3 3 4 3 4 5% 60%	2520
	21st to end	Night	14% 4 1 4 3 2 3 2 3 4 59%	2737
November	1st to 10th	Night	13% 5% 1 4 3 3 3 3 4 6% 55%	2460
	11th to 20th	Night	10% 5% 2 5% 4 3 3 3 5% 6% 55%	2520
	21st to end	Night	15% 7% 2 5% 4 3 4 3 5% 5% 47%	2603
December	1st to 10th	Night	20% 8% 3 5% 3 4 3 3 3 5% 45%	4380
	11th to 20th	Night	17% 9% 2 6% 4 4 3 3 3 4 44%	4946
	21st to end	Night	15% 9% 2 5% 3 3 3 3 3 4 50%	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	41% 3 6% 25% 22% 3	5290
	11th to 20th	Day	31% 2 5% 24% 27% 11%	5310
	21st to end	Day	51% 2 3 13% 19% 12%	5812
February	1st to 10th	Day	48% 5% 5% 14% 18% 10%	1890
	11th to 20th	Day	50% 4 6% 15% 18% 8%	2064
	21st to end	Day	46% 4 6% 13% 19% 11%	1903
March	1st to 10th	Day	50% 5% 5% 10% 19% 11%	5046
	11th to 20th	Day	35% 5% 6% 15% 22% 18%	5090
	21st to end	Day	44% 4 9% 21% 17% 5%	5585
April	1st to 10th	Day	44% 6% 8% 19% 16% 7%	2344
	11th to 20th	Day	42% 7% 10% 15% 18% 8%	2494
	21st to end	Day	41% 7% 9% 17% 22% 4	2520
May	1st to 10th	Day	50% 6% 10% 21% 11% 2	7211
	11th to 20th	Day	44% 5% 12% 17% 20% 2	7631
	21st to end	Day	43% 9% 15% 20% 13% 1	8405
June	1st to 10th	Day	38% 10% 14% 24% 14% 1	2730
	11th to 20th	Day	34% 10% 15% 25% 14% 3	2730
	21st to end	Day	33% 10% 16% 27% 12% 2	2730
July	1st to 10th	Day	36% 7% 25% 26% 5%	7616
	11th to 20th	Day	35% 6% 21% 29% 9%	7611
	21st to end	Day	33% 5% 14% 35% 12%	8155
August	1st to 10th	Day	14% 6% 16% 34% 25% 6%	6000
	11th to 20th	Day	16% 5% 19% 47% 13% 1	5731
	21st to end	Day	34% 7% 15% 25% 17% 2	6079
September	1st to 10th	Day	35% 9% 15% 18% 15% 8%	2424
	11th to 20th	Day	35% 8% 14% 20% 16% 7%	2310
	21st to end	Day	29% 6% 11% 22% 20% 12%	2310
October	1st to 10th	Day	15% 4 6% 11% 25% 39%	3380
	11th to 20th	Day	20% 3 5% 20% 29% 24%	3340
	21st to end	Day	28% 3 4 16% 36% 13%	3618
November	1st to 10th	Day	36% 5% 8% 17% 25% 10%	2058
	11th to 20th	Day	34% 4 7% 19% 24% 12%	2100
	21st to end	Day	42% 5% 7% 18% 22% 7%	2013
December	1st to 10th	Day	42% 3 7% 17% 25% 6%	3030
	11th to 20th	Day	46% 3 6% 13% 21% 11%	3479
	21st to end	Day	44% 3 5% 14% 28% 6%	4587
January	1st to 10th	Night	38% 3 6% 22% 25% 5%	7635
	11th to 20th	Night	30% 3 4 18% 32% 13%	7680
	21st to end	Night	53% 2 4 13% 17% 11%	8402
February	1st to 10th	Night	41% 4 7% 15% 19% 13%	2730
	11th to 20th	Night	47% 3 6% 16% 19% 9%	2468
	21st to end	Night	43% 4 8% 13% 18% 14%	1903
March	1st to 10th	Night	46% 4 5% 12% 21% 13%	5029
	11th to 20th	Night	36% 6% 6% 15% 18% 20%	5090
	21st to end	Night	44% 5% 8% 16% 20% 7%	5588
April	1st to 10th	Night	39% 7% 10% 17% 16% 11%	2273
	11th to 20th	Night	39% 5% 11% 17% 16% 11%	2075
	21st to end	Night	38% 5% 11% 17% 23% 6%	2100
May	1st to 10th	Night	50% 6% 11% 17% 14% 3	5697
	11th to 20th	Night	44% 5% 12% 17% 19% 3	5266
	21st to end	Night	41% 7% 19% 22% 9% 2	5805
June	1st to 10th	Night	27% 9% 18% 27% 18% 1	1890
	11th to 20th	Night	20% 7% 19% 32% 18% 4	1890
	21st to end	Night	27% 8% 17% 28% 19% 2	1890
July	1st to 10th	Night	32% 7% 29% 27% 4	5265
	11th to 20th	Night	30% 7% 27% 25% 10%	5256
	21st to end	Night	27% 6% 18% 35% 13% 1	5641
August	1st to 10th	Night	10% 4 21% 32% 26% 7%	4146
	11th to 20th	Night	13% 6% 20% 48% 12% 2	4386
	21st to end	Night	26% 8% 15% 30% 18% 3	5046
September	1st to 10th	Night	30% 7% 16% 21% 17% 9%	2190
	11th to 20th	Night	31% 7% 15% 22% 18% 8%	2310
	21st to end	Night	25% 6% 12% 23% 24% 11%	2310
October	1st to 10th	Night	15% 4 5% 14% 21% 42%	3972
	11th to 20th	Night	17% 3 4 18% 32% 26%	4019
	21st to end	Night	25% 3 4 15% 37% 15%	4349
November	1st to 10th	Night	31% 5% 9% 19% 25% 12%	2460
	11th to 20th	Night	30% 5% 8% 19% 23% 14%	2520
	21st to end	Night	40% 5% 7% 16% 22% 11%	2603
December	1st to 10th	Night	44% 3 6% 13% 28% 5%	4380
	11th to 20th	Night	45% 3 7% 12% 23% 11%	4947
	21st to end	Night	39% 4 6% 14% 30% 7%	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	27% 16% 39% 14% 3	330
	11th to 20th	Day	28% 17% 35% 15% 5%	330
	21st to end	Day	30% 21% 33% 14% 2	363
February	1st to 10th	Day	26% 21% 35% 16% 2	330
	11th to 20th	Day	35% 22% 32% 11%	330
	21st to end	Day	28% 23% 39% 9% 1	273
March	1st to 10th	Day	28% 20% 32% 18% 2	330
	11th to 20th	Day	24% 25% 36% 12% 2	330
	21st to end	Day	20% 17% 40% 19% 4 1	363
April	1st to 10th	Day	25% 16% 39% 16% 3	364
	11th to 20th	Day	24% 20% 28% 21% 7% 1	532
	21st to end	Day	19% 16% 35% 21% 7% 1	540
May	1st to 10th	Day	18% 12% 39% 25% 5%	595
	11th to 20th	Day	16% 11% 28% 30% 14% 2	775
	21st to end	Day	11% 10% 29% 31% 15% 4	844
June	1st to 10th	Day	9% 3 21% 36% 25% 6%	780
	11th to 20th	Day	7% 2 22% 40% 24% 5%	780
	21st to end	Day	7% 2 15% 37% 28% 11%	780
July	1st to 10th	Day	7% 2 12% 32% 35% 12%	771
	11th to 20th	Day	5% 1 9% 32% 43% 10%	768
	21st to end	Day	6% 1 8% 35% 38% 11%	849
August	1st to 10th	Day	3 1 7% 30% 47% 12%	776
	11th to 20th	Day	5% 1 3 31% 46% 13%	652
	21st to end	Day	11% 2 7% 25% 37% 17%	627
September	1st to 10th	Day	6% 2 12% 30% 38% 12%	474
	11th to 20th	Day	6% 2 12% 32% 36% 12%	360
	21st to end	Day	10% 3 12% 39% 28% 8%	360
October	1st to 10th	Day	8% 4 22% 44% 19% 4	360
	11th to 20th	Day	9% 7% 32% 33% 14% 5%	360
	21st to end	Day	22% 10% 28% 24% 13% 3	387
November	1st to 10th	Day	16% 8% 31% 30% 14% 2	330
	11th to 20th	Day	14% 14% 35% 31% 6% 1	330
	21st to end	Day	22% 17% 35% 20% 6% 1	330
December	1st to 10th	Day	20% 14% 39% 18% 7% 3	330
	11th to 20th	Day	26% 12% 36% 18% 7% 1	330
	21st to end	Day	25% 19% 35% 16% 4 1	363
January	1st to 10th	Night	23% 13% 33% 26% 5%	1050
	11th to 20th	Night	23% 18% 29% 25% 6%	1050
	21st to end	Night	25% 14% 32% 25% 4	1155
February	1st to 10th	Night	23% 15% 35% 24% 3	1050
	11th to 20th	Night	29% 16% 31% 21% 3	833
	21st to end	Night	25% 16% 35% 20% 3	519
March	1st to 10th	Night	26% 13% 34% 23% 4	630
	11th to 20th	Night	18% 18% 33% 26% 4 1	630
	21st to end	Night	16% 13% 40% 25% 6%	693
April	1st to 10th	Night	18% 14% 39% 23% 6%	630
	11th to 20th	Night	18% 14% 36% 21% 10%	623
	21st to end	Night	17% 11% 38% 24% 10%	630
May	1st to 10th	Night	14% 11% 39% 27% 9%	630
	11th to 20th	Night	15% 9% 29% 32% 13% 1	621
	21st to end	Night	10% 5% 30% 36% 17% 2	683
June	1st to 10th	Night	7% 3 16% 40% 30% 3	630
	11th to 20th	Night	5% 2 19% 50% 23% 2	630
	21st to end	Night	7% 1 17% 42% 28% 5%	630
July	1st to 10th	Night	6% 1 13% 38% 37% 6%	615
	11th to 20th	Night	4 1 8% 40% 42% 5%	612
	21st to end	Night	5% 1 7% 43% 37% 7%	690
August	1st to 10th	Night	3 1 4 45% 41% 6%	627
	11th to 20th	Night	4 1 6% 45% 37% 7%	624
	21st to end	Night	8% 2 7% 36% 41% 6%	693
September	1st to 10th	Night	9% 1 8% 37% 40% 5%	630
	11th to 20th	Night	11% 2 10% 31% 41% 5%	630
	21st to end	Night	7% 3 12% 39% 34% 6%	630
October	1st to 10th	Night	7% 2 16% 41% 30% 5%	798
	11th to 20th	Night	12% 6% 23% 39% 18% 3	840
	21st to end	Night	15% 7% 25% 34% 17% 2	913
November	1st to 10th	Night	13% 7% 25% 36% 17% 1	820
	11th to 20th	Night	12% 10% 29% 39% 10%	840
	21st to end	Night	19% 14% 28% 27% 12%	923
December	1st to 10th	Night	23% 13% 29% 28% 7%	1050
	11th to 20th	Night	23% 11% 33% 28% 5%	1050
	21st to end	Night	22% 13% 30% 29% 6%	1155

Page generated at 2026-10-26 03:37:47. Version ac07

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