SETEC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SETEC Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	9% 5% 2 5% 4 3 4 3 5% 7% 53%	127503
	Night	9% 5% 2 5% 3 3 4 3 5% 7% 55%	124471
Transparency	Day	31% 4 11% 27% 22% 5%	131207
	Night	29% 4 11% 27% 24% 6%	128830
Seeing	Day	12% 9% 29% 30% 16% 4	17152
	Night	11% 9% 33% 35% 11% 1	27120

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	11% 7% 2 8% 4 4 3 3 5% 7% 45%	4167
	11th to 20th	Day	6% 7% 3 8% 5% 5% 4 3 5% 6% 49%	4134
	21st to end	Day	8% 9% 3 7% 4 3 3 2 5% 5% 51%	4488
February	1st to 10th	Day	9% 11% 3 6% 4 4 3 3 4 7% 45%	1821
	11th to 20th	Day	7% 7% 3 7% 5% 4 4 3 6% 5% 51%	1949
	21st to end	Day	7% 6% 2 4 3 4 4 4 6% 7% 52%	1650
March	1st to 10th	Day	7% 6% 2 5% 3 3 3 3 4 6% 58%	4794
	11th to 20th	Day	8% 5% 1 4 3 3 3 3 6% 7% 59%	4980
	21st to end	Day	9% 5% 2 6% 4 4 4 4 7% 9% 47%	5462
April	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 8% 52%	2200
	11th to 20th	Day	12% 7% 2 6% 4 4 4 3 4 7% 47%	2462
	21st to end	Day	16% 6% 2 5% 4 4 4 4 5% 7% 44%	2600
May	1st to 10th	Day	18% 7% 2 6% 4 3 3 3 4 5% 45%	5862
	11th to 20th	Day	10% 4 2 4 3 3 3 3 4 6% 57%	5862
	21st to end	Day	19% 7% 2 6% 4 4 4 3 4 6% 42%	6434
June	1st to 10th	Day	6% 4 2 4 4 4 4 3 6% 9% 54%	2600
	11th to 20th	Day	6% 4 1 4 5% 3 4 4 6% 9% 54%	2600
	21st to end	Day	5% 3 1 4 3 3 4 3 6% 8% 58%	2600
July	1st to 10th	Day	7% 4 1 5% 4 4 4 4 6% 8% 52%	5900
	11th to 20th	Day	8% 4 2 5% 3 4 4 4 6% 8% 53%	5900
	21st to end	Day	5% 3 1 5% 4 4 4 4 7% 10% 53%	6422
August	1st to 10th	Day	6% 4 2 6% 4 4 5% 4 6% 8% 51%	4234
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 9% 54%	4171
	21st to end	Day	4 3 1 4 3 3 3 3 6% 9% 61%	4502
September	1st to 10th	Day	9% 5% 2 5% 3 3 4 4 4 7% 56%	2200
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 6% 54%	2200
	21st to end	Day	8% 4 2 5% 3 4 4 3 4 7% 56%	2200
October	1st to 10th	Day	11% 3 1 4 3 3 3 3 4 5% 58%	2145
	11th to 20th	Day	10% 3 1 3 2 3 3 2 4 4 64%	2000
	21st to end	Day	12% 6% 2 4 2 3 3 2 3 5% 57%	2200
November	1st to 10th	Day	8% 4 1 3 3 3 3 2 3 7% 63%	1886
	11th to 20th	Day	7% 5% 2 5% 3 3 3 3 4 7% 59%	1890
	21st to end	Day	12% 5% 2 3 2 2 2 2 4 6% 60%	1890
December	1st to 10th	Day	5% 4 2 4 3 2 2 3 4 6% 65%	3030
	11th to 20th	Day	6% 5% 2 5% 3 3 3 2 4 6% 61%	3481
	21st to end	Day	13% 10% 4 8% 4 4 4 3 4 6% 39%	4587
January	1st to 10th	Night	12% 7% 2 7% 4 3 4 3 5% 6% 47%	6018
	11th to 20th	Night	4 5% 2 6% 4 4 4 3 5% 6% 57%	5979
	21st to end	Night	12% 8% 3 5% 3 3 3 2 3 4 54%	6490
February	1st to 10th	Night	9% 9% 3 6% 5% 3 3 3 6% 6% 47%	2595
	11th to 20th	Night	10% 8% 3 5% 3 4 3 4 4 7% 48%	2338
	21st to end	Night	8% 6% 2 5% 3 3 3 3 5% 4 58%	1980
March	1st to 10th	Night	6% 5% 2 5% 3 2 3 3 4 6% 61%	5064
	11th to 20th	Night	6% 4 1 4 3 4 4 3 5% 6% 61%	4980
	21st to end	Night	9% 6% 2 5% 4 4 4 3 5% 6% 51%	5467
April	1st to 10th	Night	11% 5% 2 4 3 3 4 3 6% 7% 53%	2200
	11th to 20th	Night	13% 5% 2 6% 3 3 4 3 6% 7% 49%	1882
	21st to end	Night	14% 6% 2 5% 4 5% 4 3 5% 6% 47%	1800
May	1st to 10th	Night	17% 6% 1 4 3 3 3 3 5% 6% 48%	4053
	11th to 20th	Night	10% 4 1 4 3 3 3 2 4 8% 57%	4053
	21st to end	Night	16% 5% 2 4 3 2 3 3 5% 7% 52%	4443
June	1st to 10th	Night	6% 4 1 3 4 3 4 4 7% 11% 54%	1800
	11th to 20th	Night	5% 3 1 4 3 4 4 5% 7% 10% 55%	1800
	21st to end	Night	5% 2 1 4 3 4 4 3 5% 8% 61%	1800
July	1st to 10th	Night	5% 4 1 4 3 3 4 5% 7% 10% 54%	4078
	11th to 20th	Night	4 3 2 5% 4 3 5% 4 6% 11% 53%	4077
	21st to end	Night	5% 4 2 5% 4 4 5% 4 7% 10% 50%	4449
August	1st to 10th	Night	4 3 1 5% 4 4 4 3 5% 9% 57%	2926
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 8% 52%	2888
	21st to end	Night	4 3 1 4 3 3 4 3 5% 8% 64%	3388
September	1st to 10th	Night	8% 5% 2 4 3 3 3 3 5% 6% 60%	2200
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 5% 7% 57%	2200
	21st to end	Night	6% 4 1 5% 3 3 3 3 5% 7% 60%	2200
October	1st to 10th	Night	9% 5% 2 5% 3 3 3 3 4 7% 58%	2234
	11th to 20th	Night	10% 3 1 3 3 2 2 2 4 5% 65%	2400
	21st to end	Night	12% 4 1 4 2 2 2 2 3 5% 61%	2640
November	1st to 10th	Night	8% 4 1 4 2 3 2 2 3 6% 65%	2630
	11th to 20th	Night	7% 5% 2 3 3 2 4 3 5% 5% 62%	2730
	21st to end	Night	12% 5% 1 4 2 3 3 3 4 5% 57%	2730
December	1st to 10th	Night	5% 4 2 4 3 3 3 2 4 5% 66%	4380
	11th to 20th	Night	7% 5% 2 5% 3 3 3 2 4 6% 61%	4946
	21st to end	Night	14% 7% 3 6% 4 4 3 3 4 6% 45%	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	39% 3 11% 24% 19% 4	4168
	11th to 20th	Day	36% 4 11% 22% 22% 5%	4134
	21st to end	Day	38% 2 9% 24% 24% 3	4488
February	1st to 10th	Day	40% 3 11% 26% 15% 5%	1821
	11th to 20th	Day	35% 3 13% 27% 16% 5%	1959
	21st to end	Day	31% 3 8% 25% 25% 7%	1650
March	1st to 10th	Day	30% 2 6% 19% 27% 16%	4794
	11th to 20th	Day	25% 3 11% 18% 28% 15%	4980
	21st to end	Day	34% 5% 12% 34% 13% 2	5464
April	1st to 10th	Day	32% 4 11% 26% 21% 6%	2200
	11th to 20th	Day	39% 4 10% 25% 18% 5%	2462
	21st to end	Day	41% 3 12% 29% 14% 2	2600
May	1st to 10th	Day	43% 4 10% 20% 19% 4	5900
	11th to 20th	Day	29% 4 10% 22% 28% 8%	5866
	21st to end	Day	44% 4 10% 21% 18% 4	6447
June	1st to 10th	Day	26% 5% 18% 30% 16% 4	2600
	11th to 20th	Day	27% 5% 14% 28% 21% 5%	2600
	21st to end	Day	22% 5% 15% 27% 23% 7%	2600
July	1st to 10th	Day	28% 5% 13% 25% 23% 5%	5836
	11th to 20th	Day	28% 5% 11% 31% 22% 3	5831
	21st to end	Day	24% 6% 16% 36% 17% 1	6240
August	1st to 10th	Day	29% 5% 13% 27% 22% 3	4220
	11th to 20th	Day	28% 5% 17% 28% 18% 3	4210
	21st to end	Day	20% 5% 15% 36% 19% 5%	4501
September	1st to 10th	Day	29% 4 10% 26% 23% 8%	2200
	11th to 20th	Day	31% 4 10% 28% 23% 4	2200
	21st to end	Day	27% 5% 8% 27% 25% 8%	2200
October	1st to 10th	Day	20% 3 6% 19% 44% 8%	3496
	11th to 20th	Day	18% 2 7% 29% 35% 9%	3240
	21st to end	Day	26% 3 7% 26% 29% 9%	3538
November	1st to 10th	Day	22% 3 9% 31% 27% 7%	1886
	11th to 20th	Day	27% 2 8% 29% 28% 6%	1890
	21st to end	Day	28% 2 11% 30% 24% 5%	1890
December	1st to 10th	Day	22% 2 8% 43% 23% 2	3030
	11th to 20th	Day	27% 3 7% 36% 25% 3	3479
	21st to end	Day	45% 4 7% 29% 13% 2	4587
January	1st to 10th	Night	37% 3 11% 29% 17% 3	6018
	11th to 20th	Night	27% 4 12% 25% 26% 6%	5979
	21st to end	Night	36% 2 8% 22% 27% 5%	6490
February	1st to 10th	Night	37% 3 12% 25% 17% 5%	2595
	11th to 20th	Night	37% 3 12% 25% 19% 5%	2338
	21st to end	Night	30% 2 8% 23% 25% 11%	1980
March	1st to 10th	Night	26% 2 6% 23% 30% 13%	5064
	11th to 20th	Night	25% 3 7% 18% 30% 18%	4980
	21st to end	Night	33% 4 14% 34% 13% 2	5467
April	1st to 10th	Night	31% 4 10% 24% 24% 8%	2200
	11th to 20th	Night	33% 5% 12% 26% 20% 5%	1882
	21st to end	Night	38% 4 11% 23% 20% 4	1800
May	1st to 10th	Night	38% 4 13% 19% 21% 5%	4080
	11th to 20th	Night	27% 5% 13% 26% 19% 10%	4054
	21st to end	Night	34% 4 15% 24% 20% 3	4452
June	1st to 10th	Night	26% 5% 21% 29% 17% 3	1800
	11th to 20th	Night	23% 5% 16% 30% 21% 5%	1800
	21st to end	Night	22% 4 13% 31% 23% 8%	1800
July	1st to 10th	Night	24% 6% 17% 27% 22% 4	4035
	11th to 20th	Night	25% 4 11% 33% 24% 3	4026
	21st to end	Night	28% 6% 14% 36% 16% 1	4320
August	1st to 10th	Night	24% 4 13% 28% 23% 7%	2916
	11th to 20th	Night	29% 6% 14% 26% 21% 4	2915
	21st to end	Night	19% 5% 15% 37% 19% 5%	3388
September	1st to 10th	Night	27% 3 10% 25% 24% 11%	2200
	11th to 20th	Night	28% 4 10% 26% 26% 6%	2200
	21st to end	Night	24% 4 9% 25% 30% 8%	2200
October	1st to 10th	Night	20% 2 6% 15% 46% 12%	3650
	11th to 20th	Night	17% 2 6% 26% 34% 15%	3899
	21st to end	Night	22% 3 7% 24% 38% 7%	4252
November	1st to 10th	Night	23% 2 10% 28% 28% 9%	2630
	11th to 20th	Night	25% 3 7% 30% 29% 7%	2730
	21st to end	Night	30% 3 8% 28% 23% 8%	2730
December	1st to 10th	Night	21% 3 10% 42% 21% 2	4380
	11th to 20th	Night	27% 2 7% 34% 25% 6%	4947
	21st to end	Night	40% 4 9% 30% 14% 4	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	14% 16% 38% 22% 10% 1	330
	11th to 20th	Day	17% 16% 39% 25% 3	318
	21st to end	Day	8% 12% 48% 26% 5%	330
February	1st to 10th	Day	18% 19% 39% 18% 4 1	300
	11th to 20th	Day	17% 16% 45% 16% 6%	300
	21st to end	Day	11% 18% 49% 18% 4	249
March	1st to 10th	Day	12% 19% 41% 23% 5%	300
	11th to 20th	Day	20% 15% 39% 21% 6%	300
	21st to end	Day	18% 22% 39% 18% 2 1	330
April	1st to 10th	Day	19% 17% 37% 22% 5%	300
	11th to 20th	Day	16% 17% 42% 20% 5%	580
	21st to end	Day	21% 15% 35% 19% 8% 2	700
May	1st to 10th	Day	16% 12% 35% 28% 8% 1	703
	11th to 20th	Day	17% 10% 28% 26% 15% 3	725
	21st to end	Day	17% 10% 28% 27% 15% 3	789
June	1st to 10th	Day	6% 5% 24% 36% 23% 6%	730
	11th to 20th	Day	7% 4 30% 36% 20% 3	730
	21st to end	Day	5% 5% 29% 43% 16% 2	730
July	1st to 10th	Day	7% 2 15% 35% 31% 10%	721
	11th to 20th	Day	5% 2 16% 40% 29% 8%	718
	21st to end	Day	7% 3 13% 36% 32% 9%	794
August	1st to 10th	Day	7% 2 18% 41% 27% 4	726
	11th to 20th	Day	8% 1 15% 41% 30% 5%	715
	21st to end	Day	6% 2 15% 37% 33% 7%	708
September	1st to 10th	Day	12% 2 9% 40% 28% 8%	330
	11th to 20th	Day	6% 4 19% 39% 27% 5%	330
	21st to end	Day	15% 5% 27% 30% 18% 6%	330
October	1st to 10th	Day	12% 7% 31% 26% 17% 7%	330
	11th to 20th	Day	9% 12% 42% 26% 10% 1	330
	21st to end	Day	20% 14% 31% 22% 10% 2	363
November	1st to 10th	Day	12% 14% 34% 28% 10% 3	330
	11th to 20th	Day	14% 18% 35% 20% 13% 1	330
	21st to end	Day	20% 13% 34% 25% 7% 1	330
December	1st to 10th	Day	10% 13% 40% 26% 9% 2	330
	11th to 20th	Day	11% 15% 38% 27% 8% 1	330
	21st to end	Day	15% 19% 39% 22% 6%	363
January	1st to 10th	Night	10% 12% 39% 33% 7%	1050
	11th to 20th	Night	13% 12% 39% 29% 6% 1	1031
	21st to end	Night	12% 11% 39% 32% 5%	1100
February	1st to 10th	Night	14% 13% 39% 30% 4	995
	11th to 20th	Night	15% 15% 37% 29% 4	784
	21st to end	Night	11% 14% 38% 32% 5%	660
March	1st to 10th	Night	11% 13% 42% 30% 4	672
	11th to 20th	Night	15% 14% 38% 27% 7%	600
	21st to end	Night	18% 12% 40% 25% 5%	660
April	1st to 10th	Night	16% 14% 42% 22% 7%	600
	11th to 20th	Night	17% 13% 40% 24% 4	593
	21st to end	Night	17% 14% 36% 27% 6% 1	600
May	1st to 10th	Night	17% 9% 35% 32% 7%	600
	11th to 20th	Night	13% 8% 29% 36% 13% 1	591
	21st to end	Night	13% 7% 30% 34% 14% 2	650
June	1st to 10th	Night	7% 4 27% 38% 23% 2	600
	11th to 20th	Night	5% 5% 35% 41% 14%	600
	21st to end	Night	6% 4 33% 46% 11% 1	600
July	1st to 10th	Night	6% 2 17% 45% 27% 2	585
	11th to 20th	Night	3 1 18% 53% 23% 2	582
	21st to end	Night	6% 2 16% 46% 29% 1	657
August	1st to 10th	Night	4 1 21% 48% 26% 1	597
	11th to 20th	Night	8% 2 13% 52% 24% 1	594
	21st to end	Night	5% 1 15% 46% 30% 2	660
September	1st to 10th	Night	10% 3 23% 42% 21% 2	600
	11th to 20th	Night	9% 2 29% 40% 18% 2	600
	21st to end	Night	9% 4 27% 40% 18% 2	600
October	1st to 10th	Night	11% 6% 29% 34% 19% 2	634
	11th to 20th	Night	12% 6% 41% 34% 7%	800
	21st to end	Night	14% 11% 35% 29% 10%	880
November	1st to 10th	Night	11% 6% 38% 33% 11%	990
	11th to 20th	Night	10% 13% 38% 32% 8%	1050
	21st to end	Night	16% 11% 29% 35% 9%	1050
December	1st to 10th	Night	11% 10% 36% 34% 8%	1050
	11th to 20th	Night	14% 11% 39% 31% 4	1050
	21st to end	Night	13% 12% 38% 32% 5%	1155

Page generated at 2026-01-14 00:14:29. Version ac07

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