Willard L. Eccles Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willard L. Eccles Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	3 5% 2 5% 4 4 4 3 5% 6% 59%	158119
	Night	3 5% 2 5% 4 3 4 3 5% 6% 59%	151311
Transparency	Day	23% 5% 8% 19% 25% 20%	152917
	Night	23% 5% 9% 20% 25% 18%	147515
Seeing	Day	8% 14% 28% 28% 17% 6%	17580
	Night	7% 8% 24% 38% 21% 3	27712

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	3 9% 5% 11% 6% 5% 4 4 5% 7% 42%	5318
	11th to 20th	Day	2 5% 2 5% 5% 5% 4 3 4 6% 61%	5340
	21st to end	Day	3 9% 4 7% 4 3 3 3 4 6% 54%	5844
February	1st to 10th	Day	2 7% 3 8% 5% 5% 3 4 7% 8% 49%	1922
	11th to 20th	Day	4 9% 4 6% 5% 4 5% 3 5% 5% 50%	2050
	21st to end	Day	3 6% 3 8% 5% 4 4 4 5% 7% 51%	1730
March	1st to 10th	Day	2 12% 4 8% 4 3 4 3 4 5% 50%	4811
	11th to 20th	Day	3 10% 4 10% 6% 5% 4 4 5% 6% 43%	5110
	21st to end	Day	3 9% 4 8% 5% 5% 6% 4 7% 8% 42%	5607
April	1st to 10th	Day	3 5% 3 6% 4 4 4 4 5% 7% 55%	2344
	11th to 20th	Day	3 7% 2 6% 3 3 4 3 5% 6% 55%	2602
	21st to end	Day	4 5% 2 5% 4 3 4 3 5% 7% 57%	2730
May	1st to 10th	Day	4 4 1 3 3 3 4 3 4 6% 65%	7626
	11th to 20th	Day	3 5% 2 4 3 4 3 3 5% 6% 61%	7596
	21st to end	Day	3 3 2 5% 3 3 4 3 5% 6% 62%	8370
June	1st to 10th	Day	1 2 1 4 3 4 4 3 6% 7% 64%	2730
	11th to 20th	Day	2 4 1 3 2 3 3 3 4 4 72%	2730
	21st to end	Day	1 2 1 2 2 3 3 3 4 6% 74%	2730
July	1st to 10th	Day	1 2 1 3 2 2 2 3 3 4 77%	7649
	11th to 20th	Day	2 3 1 6% 5% 6% 6% 5% 7% 9% 50%	7650
	21st to end	Day	2 2 1 4 4 4 4 4 5% 6% 64%	8304
August	1st to 10th	Day	1 2 1 4 3 4 4 5% 6% 7% 63%	7549
	11th to 20th	Day	2 2 1 4 3 3 4 3 4 6% 67%	7565
	21st to end	Day	2 3 2 5% 4 4 6% 4 6% 7% 57%	8033
September	1st to 10th	Day	3 2 1 4 3 3 4 3 4 5% 68%	2337
	11th to 20th	Day	2 2 1 3 3 3 3 2 3 4 73%	2310
	21st to end	Day	2 2 1 3 2 2 3 2 4 5% 75%	2310
October	1st to 10th	Day	3 2 1 4 2 3 2 2 3 4 73%	3692
	11th to 20th	Day	2 4 1 4 2 2 4 3 5% 7% 67%	3280
	21st to end	Day	3 5% 2 4 2 2 1 2 3 4 72%	3468
November	1st to 10th	Day	4 5% 2 4 3 3 3 2 4 7% 63%	1854
	11th to 20th	Day	4 4 1 5% 3 3 3 3 5% 8% 61%	1890
	21st to end	Day	4 7% 2 6% 4 3 3 3 5% 6% 57%	1890
December	1st to 10th	Day	2 4 2 5% 4 3 3 3 4 7% 65%	3040
	11th to 20th	Day	3 6% 3 8% 4 4 4 4 6% 10% 49%	3499
	21st to end	Day	3 8% 3 9% 6% 6% 5% 5% 7% 9% 40%	4609
January	1st to 10th	Night	4 10% 4 8% 4 4 4 3 5% 7% 47%	7605
	11th to 20th	Night	2 4 2 5% 4 4 4 3 5% 6% 63%	7650
	21st to end	Night	3 8% 3 7% 4 4 3 3 4 7% 55%	8370
February	1st to 10th	Night	3 8% 3 6% 4 3 4 5% 5% 8% 51%	2695
	11th to 20th	Night	4 8% 3 7% 5% 4 4 4 5% 7% 49%	2458
	21st to end	Night	2 7% 2 7% 5% 5% 4 4 6% 8% 50%	2076
March	1st to 10th	Night	4 8% 3 6% 5% 4 4 3 5% 6% 51%	5302
	11th to 20th	Night	5% 11% 3 9% 6% 5% 4 3 5% 6% 42%	5110
	21st to end	Night	3 7% 3 6% 5% 4 4 4 6% 7% 52%	5610
April	1st to 10th	Night	4 6% 3 6% 4 4 4 4 6% 6% 54%	2272
	11th to 20th	Night	5% 6% 2 4 4 4 4 3 6% 7% 56%	1964
	21st to end	Night	4 6% 1 4 3 3 4 4 6% 8% 56%	1890
May	1st to 10th	Night	5% 5% 2 5% 3 3 4 3 4 5% 60%	5321
	11th to 20th	Night	7% 4 1 4 3 3 3 3 4 6% 61%	5296
	21st to end	Night	3 5% 1 5% 3 3 4 3 6% 6% 61%	5837
June	1st to 10th	Night	2 4 2 4 4 3 3 3 6% 8% 61%	1890
	11th to 20th	Night	3 3 1 3 3 3 3 3 4 6% 69%	1890
	21st to end	Night	2 2 1 3 2 3 3 3 5% 6% 71%	1890
July	1st to 10th	Night	2 2 1 3 2 2 3 2 3 3 78%	5340
	11th to 20th	Night	3 5% 3 7% 5% 6% 6% 5% 8% 9% 43%	5340
	21st to end	Night	2 3 1 4 3 3 5% 3 5% 7% 63%	5805
August	1st to 10th	Night	3 4 1 5% 3 3 3 3 4 7% 64%	5273
	11th to 20th	Night	1 2 1 3 2 3 4 3 5% 5% 70%	5291
	21st to end	Night	3 4 2 5% 3 4 4 4 5% 7% 60%	6197
September	1st to 10th	Night	3 3 1 4 3 2 3 3 4 7% 67%	2282
	11th to 20th	Night	2 3 1 4 3 3 3 2 4 5% 72%	2310
	21st to end	Night	2 2 1 3 2 2 2 2 4 4 75%	2310
October	1st to 10th	Night	3 3 1 3 2 2 2 2 3 4 75%	3727
	11th to 20th	Night	5% 4 1 4 2 3 3 3 4 5% 65%	3934
	21st to end	Night	4 4 1 4 2 3 3 2 4 5% 67%	4329
November	1st to 10th	Night	2 4 2 4 3 2 3 2 5% 6% 67%	2665
	11th to 20th	Night	2 4 1 4 3 3 3 3 5% 7% 65%	2730
	21st to end	Night	4 6% 2 6% 4 3 4 2 5% 7% 58%	2730
December	1st to 10th	Night	3 4 2 4 2 3 3 4 5% 6% 66%	4370
	11th to 20th	Night	2 5% 2 6% 4 4 3 3 5% 9% 57%	4941
	21st to end	Night	3 8% 3 9% 5% 4 4 4 6% 9% 46%	6611

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	38% 7% 19% 24% 11% 1	5321
	11th to 20th	Day	24% 6% 10% 23% 29% 8%	5340
	21st to end	Day	30% 5% 12% 23% 24% 6%	5844
February	1st to 10th	Day	30% 6% 15% 25% 18% 4	1922
	11th to 20th	Day	34% 4 13% 25% 19% 5%	2060
	21st to end	Day	28% 5% 14% 28% 20% 6%	1730
March	1st to 10th	Day	31% 7% 8% 23% 23% 8%	4811
	11th to 20th	Day	36% 7% 9% 25% 19% 5%	5110
	21st to end	Day	33% 8% 12% 26% 18% 3	5607
April	1st to 10th	Day	27% 5% 8% 25% 25% 10%	2344
	11th to 20th	Day	26% 4 8% 24% 25% 13%	2605
	21st to end	Day	24% 5% 8% 21% 33% 9%	2730
May	1st to 10th	Day	19% 4 6% 19% 39% 14%	7626
	11th to 20th	Day	20% 5% 6% 17% 34% 19%	7599
	21st to end	Day	19% 4 5% 16% 40% 15%	8371
June	1st to 10th	Day	16% 4 7% 14% 30% 30%	2730
	11th to 20th	Day	15% 2 4 8% 28% 43%	2730
	21st to end	Day	11% 2 4 7% 26% 50%	2730
July	1st to 10th	Day	11% 2 3 4 13% 66%	7586
	11th to 20th	Day	24% 7% 9% 12% 17% 31%	7581
	21st to end	Day	18% 5% 6% 14% 17% 40%	8120
August	1st to 10th	Day	19% 5% 6% 16% 20% 35%	5978
	11th to 20th	Day	20% 5% 7% 21% 18% 29%	5964
	21st to end	Day	18% 6% 6% 19% 21% 31%	6300
September	1st to 10th	Day	17% 4 6% 10% 25% 38%	2337
	11th to 20th	Day	15% 3 5% 16% 29% 33%	2310
	21st to end	Day	13% 2 4 17% 36% 28%	2310
October	1st to 10th	Day	15% 3 5% 11% 31% 35%	3692
	11th to 20th	Day	16% 5% 5% 23% 32% 18%	3280
	21st to end	Day	18% 2 4 26% 40% 11%	3468
November	1st to 10th	Day	21% 3 11% 26% 31% 8%	1854
	11th to 20th	Day	21% 4 8% 33% 29% 5%	1890
	21st to end	Day	26% 4 11% 27% 26% 6%	1890
December	1st to 10th	Day	20% 5% 7% 34% 29% 5%	3040
	11th to 20th	Day	28% 6% 14% 24% 26% 1	3498
	21st to end	Day	37% 7% 14% 24% 16% 1	4609
January	1st to 10th	Night	34% 6% 17% 26% 16% 1	7605
	11th to 20th	Night	20% 6% 12% 23% 28% 10%	7650
	21st to end	Night	28% 6% 13% 21% 26% 7%	8370
February	1st to 10th	Night	29% 6% 13% 29% 18% 4	2695
	11th to 20th	Night	33% 5% 12% 24% 22% 5%	2459
	21st to end	Night	29% 6% 12% 25% 23% 5%	2076
March	1st to 10th	Night	28% 7% 10% 22% 23% 10%	5302
	11th to 20th	Night	38% 6% 8% 24% 18% 5%	5110
	21st to end	Night	27% 7% 12% 27% 23% 4	5610
April	1st to 10th	Night	28% 4 10% 25% 24% 10%	2272
	11th to 20th	Night	25% 4 9% 22% 27% 13%	1964
	21st to end	Night	23% 5% 9% 23% 33% 7%	1890
May	1st to 10th	Night	23% 5% 6% 16% 34% 16%	5319
	11th to 20th	Night	22% 4 5% 15% 36% 18%	5297
	21st to end	Night	19% 6% 5% 17% 41% 12%	5838
June	1st to 10th	Night	19% 3 6% 14% 32% 27%	1890
	11th to 20th	Night	16% 3 5% 8% 25% 43%	1890
	21st to end	Night	12% 3 4 8% 21% 52%	1890
July	1st to 10th	Night	12% 3 3 4 14% 65%	5295
	11th to 20th	Night	31% 7% 9% 9% 15% 29%	5286
	21st to end	Night	20% 5% 7% 15% 15% 39%	5677
August	1st to 10th	Night	23% 4 6% 14% 17% 36%	4168
	11th to 20th	Night	19% 4 9% 20% 19% 29%	4168
	21st to end	Night	16% 3 6% 20% 20% 35%	4855
September	1st to 10th	Night	17% 3 8% 11% 24% 37%	2282
	11th to 20th	Night	16% 3 5% 18% 23% 35%	2310
	21st to end	Night	14% 2 6% 17% 32% 30%	2310
October	1st to 10th	Night	15% 3 3 10% 29% 39%	3727
	11th to 20th	Night	21% 4 4 16% 35% 20%	3934
	21st to end	Night	19% 4 4 21% 43% 9%	4329
November	1st to 10th	Night	18% 3 10% 26% 33% 10%	2665
	11th to 20th	Night	19% 3 10% 29% 32% 6%	2730
	21st to end	Night	26% 4 8% 28% 29% 5%	2730
December	1st to 10th	Night	19% 5% 8% 31% 29% 8%	4370
	11th to 20th	Night	24% 5% 14% 32% 21% 4	4941
	21st to end	Night	33% 7% 14% 26% 17% 3	6611

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% 15% 30% 23% 15% 3	330
	11th to 20th	Day	7% 15% 30% 29% 16% 3	330
	21st to end	Day	6% 15% 21% 31% 22% 5%	363
February	1st to 10th	Day	10% 21% 25% 24% 15% 4	362
	11th to 20th	Day	13% 19% 29% 24% 13% 2	535
	21st to end	Day	9% 18% 34% 27% 11% 2	446
March	1st to 10th	Day	13% 21% 26% 25% 12% 2	540
	11th to 20th	Day	11% 21% 29% 24% 11% 4	540
	21st to end	Day	14% 25% 30% 22% 8% 1	594
April	1st to 10th	Day	8% 21% 29% 26% 11% 5%	540
	11th to 20th	Day	13% 22% 36% 19% 9% 1	532
	21st to end	Day	10% 22% 30% 22% 12% 3	540
May	1st to 10th	Day	10% 18% 34% 23% 13% 3	543
	11th to 20th	Day	9% 16% 28% 28% 16% 4	568
	21st to end	Day	8% 15% 29% 28% 14% 7%	617
June	1st to 10th	Day	3 11% 31% 28% 22% 5%	570
	11th to 20th	Day	6% 14% 35% 23% 17% 4	570
	21st to end	Day	5% 12% 29% 32% 19% 4	570
July	1st to 10th	Day	2 7% 34% 34% 18% 5%	566
	11th to 20th	Day	3 4 25% 36% 23% 8%	564
	21st to end	Day	5% 5% 21% 36% 24% 8%	619
August	1st to 10th	Day	3 7% 26% 36% 22% 5%	567
	11th to 20th	Day	3 7% 26% 32% 24% 9%	557
	21st to end	Day	5% 6% 24% 32% 23% 9%	627
September	1st to 10th	Day	5% 9% 23% 32% 19% 12%	570
	11th to 20th	Day	5% 16% 30% 24% 17% 8%	570
	21st to end	Day	6% 10% 27% 29% 19% 8%	570
October	1st to 10th	Day	7% 14% 27% 23% 21% 8%	565
	11th to 20th	Day	9% 15% 18% 21% 26% 11%	339
	21st to end	Day	11% 7% 27% 29% 21% 6%	363
November	1st to 10th	Day	10% 15% 21% 23% 22% 9%	330
	11th to 20th	Day	5% 15% 26% 27% 19% 9%	330
	21st to end	Day	11% 12% 23% 23% 24% 7%	330
December	1st to 10th	Day	6% 11% 26% 34% 18% 5%	330
	11th to 20th	Day	9% 9% 22% 35% 20% 5%	330
	21st to end	Day	16% 18% 22% 29% 12% 3	363
January	1st to 10th	Night	10% 7% 24% 33% 21% 5%	840
	11th to 20th	Night	6% 9% 22% 39% 21% 3	840
	21st to end	Night	9% 7% 23% 40% 18% 4	924
February	1st to 10th	Night	9% 8% 24% 38% 18% 3	840
	11th to 20th	Night	8% 11% 27% 38% 14% 2	824
	21st to end	Night	8% 8% 31% 39% 13% 1	692
March	1st to 10th	Night	10% 13% 25% 36% 15% 1	840
	11th to 20th	Night	8% 10% 26% 34% 20% 1	840
	21st to end	Night	8% 14% 29% 37% 11% 1	924
April	1st to 10th	Night	8% 14% 27% 35% 15% 1	840
	11th to 20th	Night	8% 16% 30% 34% 12% 1	716
	21st to end	Night	7% 15% 30% 33% 14% 2	630
May	1st to 10th	Night	10% 10% 30% 36% 13% 1	630
	11th to 20th	Night	7% 10% 23% 38% 20% 1	621
	21st to end	Night	9% 11% 27% 32% 19% 2	683
June	1st to 10th	Night	4 7% 29% 38% 20% 1	630
	11th to 20th	Night	5% 12% 29% 37% 16% 1	630
	21st to end	Night	3 6% 29% 40% 21% 1	630
July	1st to 10th	Night	2 4 24% 43% 25% 2	615
	11th to 20th	Night	5% 2 20% 44% 25% 3	612
	21st to end	Night	5% 3 18% 40% 32% 2	690
August	1st to 10th	Night	5% 4 23% 43% 25%	627
	11th to 20th	Night	3 3 22% 46% 25% 2	624
	21st to end	Night	4 4 19% 35% 33% 4	814
September	1st to 10th	Night	4 7% 15% 35% 35% 3	840
	11th to 20th	Night	4 7% 22% 36% 27% 4	840
	21st to end	Night	3 6% 20% 34% 32% 6%	840
October	1st to 10th	Night	5% 7% 21% 38% 27% 2	840
	11th to 20th	Night	7% 6% 18% 39% 24% 6%	812
	21st to end	Night	7% 8% 20% 34% 25% 6%	880
November	1st to 10th	Night	6% 9% 21% 35% 26% 4	820
	11th to 20th	Night	4 9% 18% 43% 21% 5%	840
	21st to end	Night	7% 8% 23% 36% 22% 4	840
December	1st to 10th	Night	7% 5% 22% 43% 20% 3	840
	11th to 20th	Night	7% 7% 23% 40% 20% 2	840
	21st to end	Night	11% 9% 24% 37% 17% 1	924

Page generated at 2026-10-11 03:06:33. Version ac07

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