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 2024-01-14.

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 4 4 3 4 4 5% 7% 52%	74924
	Night	9% 5% 2 4 4 2 4 4 5% 7% 55%	74864
Transparency	Day	32% 4 12% 28% 20% 5%	74649
	Night	30% 4 11% 27% 22% 6%	74659
Seeing	Day	11% 9% 28% 30% 18% 4	14399
	Night	11% 8% 33% 35% 12% 1	24421

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	7% 7% 2 4 4 2 3 4 4 9% 54%	1707
	11th to 20th	Day	10% 7% 2 4 5% 3 3 5% 3 7% 51%	1647
	21st to end	Day	7% 6% 3 6% 4 3 4 3 5% 7% 51%	1782
February	1st to 10th	Day	9% 10% 3 5% 5% 3 3 4 3 7% 47%	1638
	11th to 20th	Day	7% 6% 2 6% 6% 3 4 4 6% 5% 51%	1749
	21st to end	Day	8% 6% 3 4 4 2 4 5% 6% 7% 51%	1480
March	1st to 10th	Day	9% 6% 2 5% 5% 2 5% 6% 4 7% 51%	1912
	11th to 20th	Day	17% 7% 2 4 4 2 3 3 5% 5% 49%	1980
	21st to end	Day	14% 7% 3 5% 5% 4 4 4 5% 7% 42%	2178
April	1st to 10th	Day	11% 4 2 4 4 3 3 5% 4 8% 52%	1980
	11th to 20th	Day	12% 7% 3 5% 6% 2 5% 4 4 7% 46%	2212
	21st to end	Day	16% 6% 2 4 5% 3 4 4 5% 6% 45%	2340
May	1st to 10th	Day	15% 7% 3 6% 5% 3 4 5% 4 6% 43%	2340
	11th to 20th	Day	12% 5% 2 5% 4 2 4 4 4 7% 50%	2302
	21st to end	Day	14% 6% 3 6% 5% 3 6% 5% 5% 8% 39%	2531
June	1st to 10th	Day	5% 3 2 3 5% 3 4 4 6% 10% 55%	2340
	11th to 20th	Day	7% 3 2 3 4 2 4 5% 6% 10% 54%	2340
	21st to end	Day	6% 3 1 3 4 2 4 3 6% 8% 59%	2340
July	1st to 10th	Day	7% 4 2 4 5% 3 5% 4 6% 9% 51%	2340
	11th to 20th	Day	3 2 1 3 4 2 5% 5% 7% 9% 59%	2340
	21st to end	Day	7% 3 2 4 5% 4 5% 6% 7% 9% 49%	2546
August	1st to 10th	Day	6% 4 2 5% 5% 3 5% 6% 7% 9% 47%	2328
	11th to 20th	Day	7% 4 2 5% 4 3 4 5% 5% 8% 54%	2301
	21st to end	Day	4 3 1 4 3 3 4 5% 7% 9% 57%	2490
September	1st to 10th	Day	9% 5% 2 4 3 2 4 5% 4 7% 55%	1980
	11th to 20th	Day	10% 5% 2 4 4 3 4 4 5% 7% 53%	1980
	21st to end	Day	7% 3 2 4 4 3 4 4 4 7% 57%	1980
October	1st to 10th	Day	12% 4 1 4 3 2 3 5% 4 5% 56%	1931
	11th to 20th	Day	11% 3 1 3 3 2 3 3 3 5% 64%	1800
	21st to end	Day	12% 5% 3 4 3 2 3 3 3 5% 57%	1980
November	1st to 10th	Day	7% 4 2 3 3 2 3 3 4 7% 63%	1704
	11th to 20th	Day	7% 4 2 4 3 2 3 4 4 6% 60%	1710
	21st to end	Day	13% 4 2 3 3 2 2 3 3 6% 59%	1710
December	1st to 10th	Day	9% 5% 2 5% 4 2 3 3 4 8% 55%	1710
	11th to 20th	Day	9% 7% 3 5% 4 3 4 4 4 6% 51%	2161
	21st to end	Day	11% 10% 4 8% 6% 3 5% 4 5% 6% 38%	3135
January	1st to 10th	Night	6% 6% 2 5% 5% 2 3 4 5% 6% 55%	2458
	11th to 20th	Night	8% 6% 2 5% 4 2 3 4 4 5% 56%	2386
	21st to end	Night	8% 5% 3 4 4 2 4 3 4 5% 59%	2574
February	1st to 10th	Night	9% 8% 3 6% 5% 2 4 4 5% 6% 47%	2336
	11th to 20th	Night	10% 8% 3 4 4 2 4 4 4 7% 49%	2098
	21st to end	Night	9% 6% 3 4 4 2 3 3 5% 5% 56%	1776
March	1st to 10th	Night	8% 6% 2 4 4 2 3 5% 5% 8% 53%	2046
	11th to 20th	Night	14% 7% 2 4 4 2 4 4 5% 5% 51%	1980
	21st to end	Night	16% 7% 2 5% 4 3 4 3 5% 7% 44%	2178
April	1st to 10th	Night	10% 4 2 3 4 2 4 4 6% 8% 54%	1980
	11th to 20th	Night	13% 5% 2 5% 4 3 4 5% 5% 7% 47%	1692
	21st to end	Night	13% 6% 2 4 4 3 5% 3 5% 6% 49%	1620
May	1st to 10th	Night	16% 6% 2 4 5% 2 4 5% 5% 7% 45%	1620
	11th to 20th	Night	12% 4 1 3 3 2 3 5% 5% 10% 51%	1593
	21st to end	Night	10% 5% 1 4 4 2 5% 5% 7% 10% 47%	1745
June	1st to 10th	Night	5% 4 2 2 4 2 4 5% 7% 10% 55%	1620
	11th to 20th	Night	5% 3 1 3 3 2 5% 5% 7% 10% 55%	1620
	21st to end	Night	6% 2 1 3 3 3 3 4 4 9% 62%	1620
July	1st to 10th	Night	7% 3 2 3 4 3 4 5% 6% 10% 53%	1620
	11th to 20th	Night	3 2 1 3 3 2 5% 5% 6% 11% 59%	1620
	21st to end	Night	5% 4 1 3 5% 3 4 5% 7% 11% 50%	1772
August	1st to 10th	Night	4 2 2 4 4 3 5% 6% 6% 10% 55%	1611
	11th to 20th	Night	6% 4 2 5% 5% 3 4 4 5% 9% 54%	1600
	21st to end	Night	3 3 1 3 4 3 5% 4 5% 7% 62%	1862
September	1st to 10th	Night	9% 4 2 3 4 2 3 3 4 5% 60%	1980
	11th to 20th	Night	8% 4 2 4 3 2 4 4 5% 7% 56%	1980
	21st to end	Night	6% 3 1 4 3 2 3 4 5% 8% 61%	1980
October	1st to 10th	Night	10% 4 2 4 4 2 3 4 4 7% 56%	2010
	11th to 20th	Night	10% 3 1 2 3 1 2 3 4 5% 65%	2160
	21st to end	Night	11% 4 1 4 3 2 2 3 3 6% 61%	2376
November	1st to 10th	Night	7% 3 1 3 3 2 2 3 3 6% 66%	2373
	11th to 20th	Night	8% 4 2 3 2 1 4 4 4 6% 62%	2470
	21st to end	Night	12% 4 2 3 3 2 3 3 3 6% 58%	2470
December	1st to 10th	Night	7% 5% 3 4 3 2 3 4 5% 7% 56%	2470
	11th to 20th	Night	9% 6% 2 5% 4 2 3 3 4 7% 55%	3036
	21st to end	Night	14% 8% 3 6% 5% 3 4 4 4 7% 43%	4532

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	29% 3 11% 30% 24% 3	1708
	11th to 20th	Day	35% 3 13% 29% 18% 3	1647
	21st to end	Day	33% 3 13% 31% 18% 3	1782
February	1st to 10th	Day	39% 3 12% 27% 15% 5%	1638
	11th to 20th	Day	34% 3 14% 27% 16% 5%	1759
	21st to end	Day	32% 3 9% 27% 23% 6%	1480
March	1st to 10th	Day	35% 3 11% 23% 21% 8%	1912
	11th to 20th	Day	38% 3 9% 22% 22% 5%	1980
	21st to end	Day	42% 5% 11% 27% 12% 4	2178
April	1st to 10th	Day	31% 4 12% 27% 20% 6%	1980
	11th to 20th	Day	40% 3 10% 26% 16% 4	2212
	21st to end	Day	40% 3 11% 29% 14% 2	2340
May	1st to 10th	Day	42% 5% 11% 20% 19% 3	2340
	11th to 20th	Day	34% 3 15% 28% 18% 1	2306
	21st to end	Day	43% 5% 15% 26% 9% 2	2531
June	1st to 10th	Day	26% 5% 20% 31% 15% 4	2340
	11th to 20th	Day	26% 5% 14% 28% 21% 6%	2340
	21st to end	Day	22% 5% 14% 29% 24% 6%	2340
July	1st to 10th	Day	29% 5% 16% 28% 18% 4	2276
	11th to 20th	Day	20% 6% 14% 30% 25% 6%	2271
	21st to end	Day	28% 6% 15% 30% 20% 2	2405
August	1st to 10th	Day	30% 6% 16% 28% 17% 3	2314
	11th to 20th	Day	29% 5% 13% 31% 18% 5%	2301
	21st to end	Day	23% 5% 13% 28% 22% 9%	2490
September	1st to 10th	Day	29% 4 9% 27% 21% 9%	1980
	11th to 20th	Day	32% 3 10% 28% 23% 5%	1980
	21st to end	Day	26% 5% 9% 28% 26% 7%	1980
October	1st to 10th	Day	30% 4 9% 25% 28% 5%	1931
	11th to 20th	Day	25% 2 6% 27% 30% 9%	1800
	21st to end	Day	32% 3 8% 26% 23% 8%	1980
November	1st to 10th	Day	23% 3 9% 30% 28% 8%	1704
	11th to 20th	Day	27% 2 9% 28% 29% 5%	1710
	21st to end	Day	29% 2 9% 29% 26% 5%	1710
December	1st to 10th	Day	30% 3 12% 34% 19% 3	1710
	11th to 20th	Day	35% 3 9% 29% 21% 4	2159
	21st to end	Day	46% 4 8% 27% 14% 2	3135
January	1st to 10th	Night	30% 3 10% 31% 23% 4	2458
	11th to 20th	Night	31% 4 9% 29% 24% 4	2386
	21st to end	Night	29% 2 11% 32% 22% 4	2574
February	1st to 10th	Night	37% 3 12% 25% 17% 5%	2336
	11th to 20th	Night	36% 3 12% 26% 19% 5%	2098
	21st to end	Night	32% 2 9% 24% 23% 10%	1776
March	1st to 10th	Night	32% 2 9% 24% 24% 9%	2046
	11th to 20th	Night	37% 3 9% 23% 21% 6%	1980
	21st to end	Night	41% 4 11% 26% 15% 3	2178
April	1st to 10th	Night	30% 4 10% 25% 24% 7%	1980
	11th to 20th	Night	35% 4 12% 26% 18% 4	1692
	21st to end	Night	36% 4 11% 24% 20% 5%	1620
May	1st to 10th	Night	39% 4 13% 22% 19% 3	1620
	11th to 20th	Night	29% 5% 17% 30% 16% 2	1594
	21st to end	Night	32% 6% 21% 27% 12% 2	1746
June	1st to 10th	Night	25% 5% 21% 30% 16% 3	1620
	11th to 20th	Night	22% 5% 16% 31% 21% 5%	1620
	21st to end	Night	21% 4 13% 31% 23% 8%	1620
July	1st to 10th	Night	25% 6% 15% 26% 24% 4	1575
	11th to 20th	Night	19% 4 13% 32% 25% 6%	1566
	21st to end	Night	27% 5% 13% 31% 22% 2	1673
August	1st to 10th	Night	24% 5% 16% 33% 17% 5%	1601
	11th to 20th	Night	27% 4 12% 31% 22% 4	1600
	21st to end	Night	21% 6% 13% 27% 26% 9%	1862
September	1st to 10th	Night	28% 3 9% 26% 23% 11%	1980
	11th to 20th	Night	29% 4 10% 24% 27% 5%	1980
	21st to end	Night	22% 4 9% 25% 30% 9%	1980
October	1st to 10th	Night	30% 3 9% 21% 29% 8%	2010
	11th to 20th	Night	23% 2 6% 27% 31% 11%	2160
	21st to end	Night	27% 3 8% 24% 28% 10%	2376
November	1st to 10th	Night	22% 2 10% 28% 28% 9%	2373
	11th to 20th	Night	25% 3 7% 30% 29% 6%	2470
	21st to end	Night	30% 3 8% 26% 25% 9%	2470
December	1st to 10th	Night	28% 4 13% 30% 22% 4	2470
	11th to 20th	Night	31% 3 8% 31% 22% 5%	3037
	21st to end	Night	41% 5% 10% 28% 13% 4	4532

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	13% 13% 37% 24% 11% 1	270
	11th to 20th	Day	18% 14% 39% 25% 3	249
	21st to end	Day	9% 11% 50% 27% 3	264
February	1st to 10th	Day	17% 18% 41% 19% 4 1	240
	11th to 20th	Day	15% 14% 48% 17% 6%	240
	21st to end	Day	12% 20% 45% 18% 5% 1	198
March	1st to 10th	Day	11% 19% 41% 24% 5%	240
	11th to 20th	Day	21% 15% 35% 22% 6%	240
	21st to end	Day	19% 20% 39% 19% 2 1	264
April	1st to 10th	Day	15% 15% 40% 24% 7%	240
	11th to 20th	Day	14% 16% 43% 22% 5%	490
	21st to end	Day	20% 16% 34% 20% 9% 2	600
May	1st to 10th	Day	15% 12% 34% 28% 9% 1	603
	11th to 20th	Day	18% 9% 28% 28% 15% 3	625
	21st to end	Day	17% 9% 28% 27% 16% 4	679
June	1st to 10th	Day	6% 3 22% 38% 25% 7%	630
	11th to 20th	Day	8% 3 28% 37% 21% 2	630
	21st to end	Day	5% 5% 25% 44% 18% 2	630
July	1st to 10th	Day	8% 2 12% 35% 33% 11%	621
	11th to 20th	Day	4 2 13% 40% 32% 9%	618
	21st to end	Day	7% 2 11% 36% 34% 10%	684
August	1st to 10th	Day	7% 2 15% 41% 29% 4	626
	11th to 20th	Day	8% 1 13% 39% 34% 5%	615
	21st to end	Day	5% 2 14% 37% 34% 8%	609
September	1st to 10th	Day	13% 1 9% 35% 33% 9%	270
	11th to 20th	Day	6% 2 18% 37% 30% 6%	270
	21st to end	Day	14% 5% 27% 29% 18% 6%	270
October	1st to 10th	Day	14% 8% 28% 24% 19% 8%	270
	11th to 20th	Day	9% 11% 43% 26% 11% 1	270
	21st to end	Day	19% 14% 31% 23% 11% 3	297
November	1st to 10th	Day	9% 13% 31% 31% 13% 4	270
	11th to 20th	Day	13% 17% 36% 20% 13% 1	270
	21st to end	Day	19% 12% 32% 27% 9% 1	270
December	1st to 10th	Day	10% 11% 36% 30% 11% 3	270
	11th to 20th	Day	11% 15% 35% 29% 10% 1	270
	21st to end	Day	15% 21% 40% 22% 3	297
January	1st to 10th	Night	9% 12% 37% 34% 8%	950
	11th to 20th	Night	13% 11% 38% 31% 7% 1	918
	21st to end	Night	11% 11% 39% 33% 6%	990
February	1st to 10th	Night	14% 12% 40% 30% 4	896
	11th to 20th	Night	15% 12% 38% 29% 5%	704
	21st to end	Night	12% 15% 37% 31% 5%	592
March	1st to 10th	Night	10% 13% 42% 30% 4	606
	11th to 20th	Night	16% 14% 37% 27% 7%	540
	21st to end	Night	20% 11% 40% 24% 5%	594
April	1st to 10th	Night	15% 13% 42% 22% 8%	540
	11th to 20th	Night	17% 13% 39% 26% 5%	533
	21st to end	Night	16% 14% 35% 29% 6% 1	540
May	1st to 10th	Night	17% 10% 35% 30% 7%	540
	11th to 20th	Night	14% 7% 29% 37% 13% 1	531
	21st to end	Night	12% 6% 31% 34% 15% 2	584
June	1st to 10th	Night	6% 4 26% 37% 24% 2	540
	11th to 20th	Night	5% 5% 34% 42% 14%	540
	21st to end	Night	6% 3 30% 47% 13% 1	540
July	1st to 10th	Night	7% 3 14% 45% 29% 2	525
	11th to 20th	Night	3 1 15% 55% 24% 2	522
	21st to end	Night	6% 2 13% 47% 31% 1	591
August	1st to 10th	Night	4 1 18% 49% 27% 1	537
	11th to 20th	Night	8% 2 12% 52% 25% 1	534
	21st to end	Night	4 1 14% 45% 32% 3	594
September	1st to 10th	Night	10% 3 23% 42% 21% 2	540
	11th to 20th	Night	9% 2 27% 40% 20% 2	540
	21st to end	Night	7% 5% 29% 39% 19% 1	540
October	1st to 10th	Night	11% 6% 28% 34% 19% 2	570
	11th to 20th	Night	13% 6% 40% 34% 8%	720
	21st to end	Night	13% 10% 35% 30% 11% 1	792
November	1st to 10th	Night	10% 6% 38% 34% 12%	893
	11th to 20th	Night	10% 12% 39% 31% 8% 1	950
	21st to end	Night	16% 10% 29% 35% 10%	950
December	1st to 10th	Night	11% 9% 36% 35% 8% 1	950
	11th to 20th	Night	15% 10% 38% 32% 5%	950
	21st to end	Night	13% 12% 38% 31% 5%	1045

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