SRAF
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SRAF clear sky chart from 2005-11-06 to 2026-03-03.

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	7% 7% 2 6% 4 4 4 3 5% 6% 53%	139579
	Night	10% 8% 2 6% 4 4 4 3 5% 6% 48%	138914
Transparency	Day	32% 5% 10% 20% 19% 13%	134351
	Night	37% 5% 12% 19% 17% 10%	135152
Seeing	Day	11% 8% 22% 31% 23% 5%	16035
	Night	14% 4 18% 39% 23% 3	25455

By Month

Percentage Cloud Cover SW (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	15% 18% 6% 10% 7% 6% 6% 4 6% 5% 17%	4722
	11th to 20th	Day	9% 10% 3 7% 4 5% 4 3 5% 7% 41%	4740
	21st to end	Day	12% 12% 4 10% 6% 5% 5% 3 4 5% 35%	5187
February	1st to 10th	Day	15% 14% 5% 9% 5% 5% 4 3 5% 7% 29%	1835
	11th to 20th	Day	12% 14% 3 8% 5% 4 6% 3 6% 8% 31%	2050
	21st to end	Day	12% 12% 4 7% 6% 5% 6% 4 5% 7% 33%	1730
March	1st to 10th	Day	8% 10% 4 9% 6% 6% 6% 5% 8% 8% 30%	4539
	11th to 20th	Day	9% 10% 3 7% 4 4 4 4 5% 6% 46%	4780
	21st to end	Day	9% 11% 3 9% 6% 5% 6% 5% 7% 8% 30%	5779
April	1st to 10th	Day	11% 10% 3 7% 5% 4 4 4 6% 8% 39%	2400
	11th to 20th	Day	7% 8% 3 7% 4 4 4 4 6% 8% 45%	2409
	21st to end	Day	9% 7% 2 7% 4 4 5% 3 6% 8% 45%	2600
May	1st to 10th	Day	7% 6% 2 5% 4 4 4 3 5% 6% 56%	5880
	11th to 20th	Day	6% 5% 2 5% 3 3 4 3 5% 6% 57%	6093
	21st to end	Day	5% 4 1 4 3 3 4 3 5% 8% 61%	6902
June	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 44%	2800
	11th to 20th	Day	7% 4 2 4 3 3 4 4 6% 7% 55%	2800
	21st to end	Day	3 3 2 3 2 2 3 3 5% 5% 70%	2800
July	1st to 10th	Day	2 1 2 2 2 2 2 3 4 80%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 82%	6320
	21st to end	Day	3 3 1 3 2 2 3 3 4 7% 71%	6880
August	1st to 10th	Day	3 3 1 4 3 3 3 3 5% 8% 64%	6010
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 3 81%	5798
	21st to end	Day	4 4 1 4 3 3 3 2 4 4 68%	5992
September	1st to 10th	Day	3 2 1 2 2 2 3 2 4 5% 73%	2400
	11th to 20th	Day	5% 3 1 4 3 2 4 3 5% 7% 63%	2227
	21st to end	Day	7% 3 2 4 2 3 3 2 4 5% 66%	2200
October	1st to 10th	Day	5% 4 1 4 3 3 3 2 3 4 67%	3600
	11th to 20th	Day	8% 4 2 5% 4 3 4 3 5% 6% 55%	3116
	21st to end	Day	8% 8% 2 6% 4 4 5% 3 5% 7% 48%	3200
November	1st to 10th	Day	15% 8% 3 6% 4 4 5% 4 5% 6% 41%	1854
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 6% 8% 33%	1890
	21st to end	Day	12% 8% 3 9% 5% 5% 5% 5% 7% 8% 33%	1827
December	1st to 10th	Day	11% 8% 2 8% 4 3 4 3 5% 6% 46%	2700
	11th to 20th	Day	18% 15% 4 12% 7% 6% 5% 4 6% 6% 17%	3108
	21st to end	Day	18% 14% 4 10% 6% 5% 5% 4 6% 6% 23%	4092
January	1st to 10th	Night	19% 15% 4 11% 6% 5% 4 3 5% 5% 21%	8172
	11th to 20th	Night	13% 10% 2 5% 3 4 3 3 4 5% 47%	8220
	21st to end	Night	12% 9% 3 8% 4 4 4 3 5% 6% 41%	8994
February	1st to 10th	Night	17% 12% 3 8% 5% 4 4 4 5% 7% 32%	2783
	11th to 20th	Night	14% 12% 2 6% 5% 5% 4 3 5% 6% 36%	2458
	21st to end	Night	13% 14% 3 7% 5% 4 4 5% 6% 7% 32%	2076
March	1st to 10th	Night	9% 13% 3 7% 5% 4 4 3 6% 7% 39%	5400
	11th to 20th	Night	13% 13% 3 6% 4 3 3 3 5% 5% 43%	5207
	21st to end	Night	10% 10% 3 8% 5% 6% 6% 5% 7% 9% 31%	5192
April	1st to 10th	Night	12% 8% 2 6% 4 4 3 4 6% 7% 43%	2000
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 4 4 5% 47%	1937
	21st to end	Night	7% 6% 2 6% 5% 4 4 3 5% 6% 51%	1800
May	1st to 10th	Night	11% 4 1 3 2 3 3 3 5% 7% 57%	4100
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 5% 7% 54%	3805
	21st to end	Night	5% 5% 2 4 4 3 3 2 3 5% 63%	3972
June	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 47%	1600
	11th to 20th	Night	7% 5% 2 4 3 4 4 3 4 6% 58%	1600
	21st to end	Night	3 3 1 3 4 3 4 3 5% 6% 67%	1600
July	1st to 10th	Night	2 2 1 3 2 2 3 2 4 4 76%	3640
	11th to 20th	Night	1 1 1 1 1 2 1 3 5% 83%	3640
	21st to end	Night	2 2 1 4 3 3 4 4 6% 9% 62%	3970
August	1st to 10th	Night	2 3 1 5% 3 3 4 4 6% 7% 61%	3779
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 5% 80%	4051
	21st to end	Night	5% 3 1 4 2 3 3 2 4 5% 68%	5005
September	1st to 10th	Night	3 2 1 2 2 2 3 3 4 5% 74%	2000
	11th to 20th	Night	5% 4 2 4 3 2 3 3 4 6% 63%	2172
	21st to end	Night	7% 3 2 4 2 2 3 2 3 5% 67%	2200
October	1st to 10th	Night	4 3 1 3 3 3 3 3 3 4 69%	3600
	11th to 20th	Night	7% 4 2 4 3 4 4 3 4 6% 59%	4033
	21st to end	Night	11% 6% 2 5% 4 4 4 4 6% 7% 47%	4600
November	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	2665
	11th to 20th	Night	18% 9% 2 6% 5% 4 4 4 6% 6% 36%	2730
	21st to end	Night	14% 9% 3 6% 4 4 5% 3 5% 6% 41%	2791
December	1st to 10th	Night	12% 8% 2 5% 3 3 3 3 5% 7% 50%	4700
	11th to 20th	Night	19% 14% 4 10% 6% 5% 5% 4 6% 6% 21%	5316
	21st to end	Night	17% 12% 3 9% 5% 5% 5% 4 6% 7% 25%	7106

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% 10% 14% 10% 1	4723
	11th to 20th	Day	41% 6% 22% 25% 6%	4740
	21st to end	Day	51% 7% 13% 21% 8% 1	5187
February	1st to 10th	Day	55% 9% 16% 16% 3 1	1835
	11th to 20th	Day	50% 10% 15% 19% 6%	2060
	21st to end	Day	48% 9% 15% 18% 9% 1	1730
March	1st to 10th	Day	45% 7% 14% 20% 12% 1	4539
	11th to 20th	Day	39% 6% 10% 32% 13% 1	4780
	21st to end	Day	47% 10% 16% 20% 7% 1	5779
April	1st to 10th	Day	41% 7% 16% 22% 11% 2	2400
	11th to 20th	Day	36% 7% 11% 24% 18% 4	2412
	21st to end	Day	35% 8% 11% 27% 17% 2	2600
May	1st to 10th	Day	28% 5% 9% 24% 29% 5%	5880
	11th to 20th	Day	26% 4 7% 28% 26% 8%	6096
	21st to end	Day	21% 5% 9% 30% 27% 7%	6904
June	1st to 10th	Day	34% 8% 12% 25% 19% 2	2800
	11th to 20th	Day	24% 6% 9% 23% 27% 10%	2800
	21st to end	Day	16% 4 7% 19% 33% 21%	2800
July	1st to 10th	Day	11% 1 4 10% 24% 51%	6251
	11th to 20th	Day	10% 1 3 5% 22% 60%	6245
	21st to end	Day	15% 2 4 12% 34% 33%	6681
August	1st to 10th	Day	23% 2 6% 10% 26% 32%	4379
	11th to 20th	Day	11% 2 3 15% 38% 32%	4197
	21st to end	Day	15% 2 4 18% 34% 27%	4319
September	1st to 10th	Day	15% 2 5% 18% 31% 28%	2400
	11th to 20th	Day	21% 4 9% 23% 30% 13%	2227
	21st to end	Day	22% 3 7% 25% 33% 10%	2200
October	1st to 10th	Day	22% 4 7% 18% 41% 8%	3600
	11th to 20th	Day	30% 4 11% 27% 24% 3	3116
	21st to end	Day	35% 5% 12% 29% 18% 1	3200
November	1st to 10th	Day	43% 8% 15% 24% 10%	1854
	11th to 20th	Day	48% 7% 19% 20% 5%	1890
	21st to end	Day	49% 8% 20% 17% 6%	1827
December	1st to 10th	Day	40% 6% 23% 29% 3	2700
	11th to 20th	Day	64% 9% 16% 9% 2	3108
	21st to end	Day	59% 8% 16% 14% 3	4092
January	1st to 10th	Night	64% 8% 16% 11% 2	8172
	11th to 20th	Night	40% 5% 18% 27% 9%	8220
	21st to end	Night	43% 8% 13% 23% 10% 2	8994
February	1st to 10th	Night	53% 8% 16% 19% 4 1	2783
	11th to 20th	Night	47% 8% 15% 20% 8% 1	2459
	21st to end	Night	50% 8% 11% 19% 11% 1	2076
March	1st to 10th	Night	42% 5% 13% 23% 15% 2	5400
	11th to 20th	Night	44% 4 6% 26% 20% 1	5207
	21st to end	Night	45% 9% 14% 23% 9%	5192
April	1st to 10th	Night	39% 5% 12% 25% 17% 2	2000
	11th to 20th	Night	38% 5% 10% 23% 20% 3	1937
	21st to end	Night	33% 5% 11% 26% 23% 2	1800
May	1st to 10th	Night	27% 4 8% 21% 32% 8%	4097
	11th to 20th	Night	29% 4 8% 24% 30% 5%	3805
	21st to end	Night	24% 4 9% 27% 29% 8%	3972
June	1st to 10th	Night	34% 7% 12% 23% 21% 2	1600
	11th to 20th	Night	27% 5% 8% 20% 30% 9%	1600
	21st to end	Night	19% 3 6% 18% 32% 22%	1600
July	1st to 10th	Night	14% 2 3 7% 19% 55%	3600
	11th to 20th	Night	7% 1 3 5% 20% 64%	3592
	21st to end	Night	19% 3 5% 9% 31% 33%	3857
August	1st to 10th	Night	25% 2 7% 9% 22% 35%	2750
	11th to 20th	Night	11% 2 5% 9% 27% 45%	2928
	21st to end	Night	16% 2 3 12% 32% 35%	3598
September	1st to 10th	Night	14% 3 5% 13% 30% 35%	2000
	11th to 20th	Night	22% 4 7% 18% 27% 22%	2172
	21st to end	Night	21% 3 5% 20% 32% 19%	2200
October	1st to 10th	Night	21% 2 6% 18% 30% 23%	3600
	11th to 20th	Night	27% 3 10% 25% 29% 6%	4033
	21st to end	Night	36% 4 10% 28% 21% 2	4600
November	1st to 10th	Night	41% 6% 12% 22% 17% 1	2665
	11th to 20th	Night	48% 6% 14% 24% 9%	2730
	21st to end	Night	44% 6% 18% 23% 8% 1	2791
December	1st to 10th	Night	35% 7% 25% 28% 5%	4700
	11th to 20th	Night	63% 6% 17% 12% 2	5316
	21st to end	Night	54% 10% 18% 14% 4	7106

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	22% 11% 21% 27% 15% 5%	220
	11th to 20th	Day	15% 8% 19% 32% 19% 7%	220
	21st to end	Day	17% 5% 18% 35% 18% 7%	242
February	1st to 10th	Day	24% 8% 27% 20% 17% 4	333
	11th to 20th	Day	19% 10% 27% 29% 13% 2	535
	21st to end	Day	18% 14% 26% 29% 12% 1	446
March	1st to 10th	Day	17% 14% 30% 27% 11% 1	509
	11th to 20th	Day	17% 11% 28% 26% 16% 2	500
	21st to end	Day	14% 17% 29% 25% 13% 2	550
April	1st to 10th	Day	15% 18% 23% 22% 17% 4	500
	11th to 20th	Day	12% 17% 30% 24% 14% 4	492
	21st to end	Day	9% 16% 30% 27% 16% 3	500
May	1st to 10th	Day	8% 16% 29% 27% 18% 3	503
	11th to 20th	Day	16% 10% 25% 27% 18% 3	528
	21st to end	Day	9% 9% 17% 33% 24% 8%	573
June	1st to 10th	Day	8% 7% 23% 32% 22% 8%	530
	11th to 20th	Day	8% 11% 27% 34% 17% 3	530
	21st to end	Day	4 6% 24% 35% 26% 6%	530
July	1st to 10th	Day	3 4 18% 42% 28% 5%	526
	11th to 20th	Day	1 3 19% 43% 29% 5%	524
	21st to end	Day	3 2 15% 40% 31% 9%	575
August	1st to 10th	Day	4 4 18% 40% 28% 6%	527
	11th to 20th	Day	3 2 19% 32% 34% 10%	517
	21st to end	Day	4 5% 16% 32% 31% 11%	583
September	1st to 10th	Day	5% 3 14% 29% 37% 12%	530
	11th to 20th	Day	7% 5% 14% 34% 32% 8%	530
	21st to end	Day	6% 4 20% 31% 32% 7%	530
October	1st to 10th	Day	7% 6% 20% 27% 33% 8%	530
	11th to 20th	Day	11% 7% 21% 26% 27% 8%	420
	21st to end	Day	12% 11% 23% 29% 21% 4	363
November	1st to 10th	Day	16% 8% 22% 30% 22% 2	330
	11th to 20th	Day	18% 6% 22% 30% 19% 5%	330
	21st to end	Day	15% 5% 19% 35% 21% 5%	297
December	1st to 10th	Day	22% 7% 17% 27% 19% 9%	220
	11th to 20th	Day	22% 7% 23% 29% 18% 1	220
	21st to end	Day	21% 9% 22% 29% 17% 2	242
January	1st to 10th	Night	23% 7% 19% 30% 19% 3	840
	11th to 20th	Night	20% 6% 17% 33% 21% 3	840
	21st to end	Night	20% 3 15% 36% 23% 3	924
February	1st to 10th	Night	23% 7% 19% 34% 16% 2	840
	11th to 20th	Night	19% 4 20% 37% 17% 2	824
	21st to end	Night	18% 8% 23% 34% 15% 2	692
March	1st to 10th	Night	17% 7% 25% 33% 15% 1	808
	11th to 20th	Night	18% 7% 20% 37% 19% 1	800
	21st to end	Night	16% 8% 24% 38% 13% 1	880
April	1st to 10th	Night	16% 6% 20% 38% 18% 2	800
	11th to 20th	Night	13% 7% 26% 36% 16% 2	749
	21st to end	Night	10% 7% 22% 43% 16% 2	600
May	1st to 10th	Night	8% 7% 22% 42% 20% 1	600
	11th to 20th	Night	10% 5% 21% 33% 28% 3	479
	21st to end	Night	11% 5% 15% 34% 31% 3	433
June	1st to 10th	Night	11% 4 18% 40% 25% 3	400
	11th to 20th	Night	8% 5% 22% 40% 23% 2	400
	21st to end	Night	3 1 17% 48% 30% 1	400
July	1st to 10th	Night	1 2 16% 47% 33% 1	390
	11th to 20th	Night	2 1 13% 58% 27%	388
	21st to end	Night	3 1 11% 50% 35% 1	438
August	1st to 10th	Night	3 1 11% 51% 31% 2	482
	11th to 20th	Night	1 1 11% 48% 37% 2	594
	21st to end	Night	4 2 11% 45% 33% 5%	870
September	1st to 10th	Night	4 1 9% 46% 38% 3	800
	11th to 20th	Night	6% 2 10% 40% 37% 6%	800
	21st to end	Night	8% 2 15% 38% 30% 7%	800
October	1st to 10th	Night	9% 2 15% 45% 27% 3	800
	11th to 20th	Night	11% 3 15% 39% 27% 5%	800
	21st to end	Night	13% 3 18% 40% 22% 4	880
November	1st to 10th	Night	18% 4 19% 36% 22% 2	820
	11th to 20th	Night	21% 5% 20% 36% 17% 2	840
	21st to end	Night	19% 3 16% 38% 19% 4	840
December	1st to 10th	Night	20% 4 18% 36% 18% 3	840
	11th to 20th	Night	25% 5% 17% 36% 15% 2	840
	21st to end	Night	29% 7% 20% 32% 11% 2	924

Page generated at 2026-02-27 23:40:21. Version ac07

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