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 2025-06-10.

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 6% 4 4 4 3 5% 6% 58%	123954
	Night	3 5% 2 5% 4 4 4 3 5% 7% 58%	123270
Transparency	Day	23% 5% 9% 19% 25% 18%	123722
	Night	24% 5% 10% 21% 25% 16%	123090
Seeing	Day	7% 13% 27% 28% 18% 6%	15741
	Night	6% 8% 23% 38% 22% 3	25960

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	2 7% 4 10% 6% 5% 4 4 6% 8% 42%	4098
	11th to 20th	Day	3 6% 2 6% 6% 6% 5% 3 4 6% 52%	4100
	21st to end	Day	3 10% 4 8% 5% 4 3 3 4 5% 51%	4510
February	1st to 10th	Day	2 7% 3 8% 5% 5% 4 4 7% 8% 48%	1830
	11th to 20th	Day	4 9% 4 6% 5% 4 4 3 5% 5% 51%	1950
	21st to end	Day	3 6% 3 8% 5% 4 4 4 5% 6% 51%	1650
March	1st to 10th	Day	2 12% 4 8% 4 3 4 3 4 5% 49%	4707
	11th to 20th	Day	3 10% 5% 10% 6% 5% 4 4 5% 6% 42%	5000
	21st to end	Day	3 9% 4 8% 5% 5% 6% 4 7% 8% 42%	5486
April	1st to 10th	Day	3 5% 3 6% 4 4 4 4 5% 7% 54%	2232
	11th to 20th	Day	3 7% 2 7% 3 3 4 3 6% 6% 54%	2477
	21st to end	Day	4 5% 2 5% 4 3 4 3 5% 7% 57%	2600
May	1st to 10th	Day	5% 4 2 4 4 3 4 3 4 6% 61%	5880
	11th to 20th	Day	3 5% 2 5% 4 4 4 3 6% 7% 57%	5841
	21st to end	Day	3 3 2 4 4 3 4 3 5% 6% 62%	6423
June	1st to 10th	Day	1 2 1 4 3 4 4 3 6% 7% 64%	2589
	11th to 20th	Day	2 4 1 3 2 3 3 3 4 4 72%	2470
	21st to end	Day	2 2 1 2 2 2 3 3 4 6% 73%	2470
July	1st to 10th	Day	2 2 1 2 2 2 2 3 3 4 78%	4110
	11th to 20th	Day	3 3 2 6% 6% 6% 7% 6% 7% 9% 47%	4110
	21st to end	Day	2 3 1 5% 4 6% 6% 5% 6% 7% 54%	4454
August	1st to 10th	Day	2 3 2 5% 4 4 5% 6% 7% 8% 54%	4092
	11th to 20th	Day	2 1 1 3 3 3 3 3 5% 6% 69%	4028
	21st to end	Day	2 2 1 2 2 2 4 3 4 5% 73%	4336
September	1st to 10th	Day	3 2 1 3 3 3 4 2 4 6% 68%	2115
	11th to 20th	Day	3 2 1 4 3 3 3 2 3 4 72%	2090
	21st to end	Day	2 2 1 3 2 2 3 2 4 4 76%	2090
October	1st to 10th	Day	3 3 1 4 2 3 2 3 3 4 72%	3472
	11th to 20th	Day	2 3 1 4 2 2 4 3 5% 7% 67%	3150
	21st to end	Day	4 5% 2 4 2 1 1 2 3 4 72%	3361
November	1st to 10th	Day	4 5% 2 5% 3 3 4 2 5% 7% 62%	1764
	11th to 20th	Day	3 4 1 5% 3 3 3 3 5% 8% 63%	1800
	21st to end	Day	4 7% 2 6% 4 3 3 3 5% 6% 57%	1800
December	1st to 10th	Day	2 4 2 5% 4 3 3 3 4 7% 64%	2950
	11th to 20th	Day	4 6% 3 8% 4 4 4 4 6% 10% 49%	3409
	21st to end	Day	3 8% 3 9% 6% 6% 5% 5% 7% 9% 39%	4510
January	1st to 10th	Night	2 8% 4 8% 5% 4 4 4 6% 8% 50%	5867
	11th to 20th	Night	2 6% 2 6% 4 5% 4 3 5% 7% 55%	5880
	21st to end	Night	3 8% 3 8% 5% 4 4 3 5% 7% 51%	6468
February	1st to 10th	Night	3 9% 2 6% 4 3 4 5% 5% 8% 51%	2567
	11th to 20th	Night	4 8% 3 7% 5% 4 4 4 5% 7% 50%	2338
	21st to end	Night	3 8% 2 7% 5% 5% 4 4 6% 7% 50%	1980
March	1st to 10th	Night	4 8% 3 6% 5% 4 4 3 5% 6% 50%	5186
	11th to 20th	Night	5% 11% 3 9% 6% 5% 5% 3 5% 6% 41%	5000
	21st to end	Night	3 7% 3 6% 5% 4 4 4 6% 6% 51%	5489
April	1st to 10th	Night	4 6% 3 6% 4 4 5% 4 6% 6% 53%	2164
	11th to 20th	Night	5% 6% 2 4 4 4 4 3 6% 7% 56%	1870
	21st to end	Night	4 5% 2 4 3 3 4 4 6% 8% 57%	1800
May	1st to 10th	Night	5% 5% 2 6% 4 4 4 3 5% 6% 55%	4100
	11th to 20th	Night	6% 5% 2 5% 3 3 4 4 5% 6% 58%	4074
	21st to end	Night	4 5% 1 5% 3 3 4 3 6% 6% 60%	4474
June	1st to 10th	Night	2 4 2 4 4 3 3 3 6% 8% 61%	1795
	11th to 20th	Night	3 3 1 4 3 3 3 3 4 5% 69%	1710
	21st to end	Night	2 2 1 3 2 3 3 3 6% 6% 70%	1710
July	1st to 10th	Night	1 2 1 2 2 2 3 2 4 4 78%	2860
	11th to 20th	Night	3 5% 2 7% 6% 6% 6% 5% 8% 9% 42%	2860
	21st to end	Night	3 4 2 5% 4 5% 6% 5% 7% 7% 53%	3109
August	1st to 10th	Night	4 6% 2 6% 4 4 3 4 6% 8% 53%	2847
	11th to 20th	Night	1 2 1 3 2 2 3 3 5% 6% 73%	2811
	21st to end	Night	3 2 1 2 1 2 3 2 3 4 77%	3333
September	1st to 10th	Night	3 3 1 3 3 2 3 3 4 7% 67%	2064
	11th to 20th	Night	2 3 1 4 3 3 3 2 4 5% 71%	2090
	21st to end	Night	2 2 1 3 2 2 2 3 4 4 77%	2090
October	1st to 10th	Night	3 3 1 3 2 2 2 2 3 4 74%	3507
	11th to 20th	Night	5% 4 1 4 2 3 3 3 4 5% 66%	3778
	21st to end	Night	4 4 1 4 2 3 3 2 4 5% 67%	4195
November	1st to 10th	Night	3 4 2 4 3 2 3 2 5% 6% 66%	2535
	11th to 20th	Night	2 3 1 3 3 3 3 3 5% 7% 67%	2600
	21st to end	Night	4 6% 2 6% 4 3 4 2 5% 7% 57%	2600
December	1st to 10th	Night	3 4 2 4 2 3 3 4 5% 6% 65%	4240
	11th to 20th	Night	2 5% 2 6% 4 4 3 3 5% 9% 57%	4811
	21st to end	Night	3 8% 3 9% 5% 4 4 4 6% 9% 46%	6468

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	35% 8% 20% 23% 14% 1	4099
	11th to 20th	Day	30% 8% 13% 22% 21% 6%	4100
	21st to end	Day	33% 6% 12% 22% 22% 5%	4510
February	1st to 10th	Day	31% 7% 16% 25% 17% 5%	1830
	11th to 20th	Day	33% 4 13% 25% 19% 5%	1960
	21st to end	Day	29% 5% 13% 27% 20% 6%	1650
March	1st to 10th	Day	31% 7% 8% 23% 22% 8%	4707
	11th to 20th	Day	37% 7% 9% 24% 19% 4	5000
	21st to end	Day	33% 8% 12% 26% 18% 3	5486
April	1st to 10th	Day	27% 5% 8% 26% 25% 10%	2232
	11th to 20th	Day	27% 4 8% 23% 25% 13%	2480
	21st to end	Day	24% 5% 8% 22% 32% 9%	2600
May	1st to 10th	Day	22% 5% 7% 21% 36% 9%	5880
	11th to 20th	Day	21% 5% 7% 20% 32% 15%	5844
	21st to end	Day	18% 5% 6% 15% 38% 18%	6424
June	1st to 10th	Day	16% 4 8% 15% 30% 28%	2589
	11th to 20th	Day	16% 2 4 9% 28% 41%	2470
	21st to end	Day	11% 2 4 7% 25% 51%	2470
July	1st to 10th	Day	11% 2 3 5% 8% 70%	4046
	11th to 20th	Day	25% 7% 8% 10% 22% 28%	4041
	21st to end	Day	23% 6% 8% 13% 17% 33%	4314
August	1st to 10th	Day	22% 6% 7% 17% 22% 26%	4078
	11th to 20th	Day	14% 4 7% 14% 21% 40%	4067
	21st to end	Day	12% 3 4 12% 25% 44%	4335
September	1st to 10th	Day	17% 4 6% 10% 24% 40%	2115
	11th to 20th	Day	15% 2 5% 16% 27% 34%	2090
	21st to end	Day	12% 2 4 16% 36% 30%	2090
October	1st to 10th	Day	15% 3 5% 11% 30% 36%	3472
	11th to 20th	Day	16% 5% 5% 24% 32% 19%	3150
	21st to end	Day	18% 2 4 26% 40% 11%	3361
November	1st to 10th	Day	22% 3 11% 25% 31% 7%	1764
	11th to 20th	Day	19% 4 9% 33% 30% 5%	1800
	21st to end	Day	27% 4 11% 26% 27% 6%	1800
December	1st to 10th	Day	20% 5% 7% 33% 30% 5%	2950
	11th to 20th	Day	28% 6% 14% 24% 27%	3408
	21st to end	Day	37% 7% 14% 24% 16% 1	4510
January	1st to 10th	Night	29% 7% 20% 26% 18% 1	5867
	11th to 20th	Night	25% 8% 15% 22% 23% 7%	5880
	21st to end	Night	31% 7% 14% 19% 22% 7%	6468
February	1st to 10th	Night	30% 7% 14% 29% 17% 4	2567
	11th to 20th	Night	31% 5% 13% 24% 22% 5%	2339
	21st to end	Night	30% 6% 12% 23% 23% 5%	1980
March	1st to 10th	Night	29% 7% 10% 22% 22% 10%	5186
	11th to 20th	Night	39% 6% 8% 24% 18% 4	5000
	21st to end	Night	27% 7% 12% 27% 23% 3	5489
April	1st to 10th	Night	28% 5% 10% 25% 22% 10%	2164
	11th to 20th	Night	25% 5% 9% 21% 28% 13%	1870
	21st to end	Night	23% 5% 10% 23% 33% 7%	1800
May	1st to 10th	Night	27% 6% 7% 18% 30% 12%	4097
	11th to 20th	Night	23% 5% 6% 18% 37% 10%	4075
	21st to end	Night	20% 7% 6% 16% 38% 14%	4474
June	1st to 10th	Night	19% 3 6% 15% 32% 25%	1795
	11th to 20th	Night	16% 3 5% 8% 26% 42%	1710
	21st to end	Night	12% 3 5% 8% 20% 53%	1710
July	1st to 10th	Night	9% 3 4 6% 7% 71%	2815
	11th to 20th	Night	29% 8% 7% 12% 20% 25%	2806
	21st to end	Night	26% 6% 8% 16% 15% 30%	3011
August	1st to 10th	Night	26% 5% 8% 15% 21% 24%	2838
	11th to 20th	Night	12% 4 8% 15% 20% 41%	2838
	21st to end	Night	12% 2 6% 12% 18% 51%	3333
September	1st to 10th	Night	17% 3 8% 10% 24% 38%	2064
	11th to 20th	Night	16% 3 5% 18% 21% 36%	2090
	21st to end	Night	12% 2 5% 16% 33% 32%	2090
October	1st to 10th	Night	15% 3 4 10% 28% 40%	3507
	11th to 20th	Night	20% 4 5% 16% 34% 21%	3778
	21st to end	Night	19% 4 4 21% 43% 9%	4195
November	1st to 10th	Night	19% 3 10% 25% 32% 11%	2535
	11th to 20th	Night	17% 3 10% 30% 33% 7%	2600
	21st to end	Night	26% 4 7% 28% 30% 6%	2600
December	1st to 10th	Night	19% 5% 8% 31% 30% 8%	4240
	11th to 20th	Night	24% 5% 15% 32% 21% 3	4811
	21st to end	Night	33% 7% 14% 26% 17% 3	6468

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% 29% 24% 17% 3	300
	11th to 20th	Day	8% 14% 31% 28% 15% 3	300
	21st to end	Day	5% 15% 21% 30% 23% 5%	330
February	1st to 10th	Day	10% 22% 25% 23% 15% 5%	330
	11th to 20th	Day	10% 18% 29% 26% 14% 3	495
	21st to end	Day	9% 19% 33% 26% 11% 2	414
March	1st to 10th	Day	13% 20% 26% 26% 12% 2	500
	11th to 20th	Day	11% 21% 29% 24% 11% 4	500
	21st to end	Day	15% 24% 29% 22% 9% 1	550
April	1st to 10th	Day	8% 21% 29% 25% 11% 5%	500
	11th to 20th	Day	12% 22% 37% 19% 9% 1	492
	21st to end	Day	9% 21% 30% 24% 13% 3	500
May	1st to 10th	Day	10% 17% 33% 23% 13% 3	503
	11th to 20th	Day	8% 15% 28% 28% 17% 4	528
	21st to end	Day	7% 14% 28% 28% 15% 8%	573
June	1st to 10th	Day	3 11% 30% 29% 23% 5%	527
	11th to 20th	Day	6% 13% 34% 23% 19% 4	490
	21st to end	Day	3 10% 29% 34% 21% 4	490
July	1st to 10th	Day	2 6% 31% 36% 20% 6%	486
	11th to 20th	Day	4 4 24% 36% 24% 9%	484
	21st to end	Day	5% 4 18% 37% 27% 9%	531
August	1st to 10th	Day	3 5% 26% 37% 23% 6%	487
	11th to 20th	Day	3 6% 25% 31% 26% 10%	477
	21st to end	Day	5% 6% 23% 32% 25% 9%	539
September	1st to 10th	Day	4 8% 20% 34% 21% 13%	490
	11th to 20th	Day	5% 14% 28% 25% 19% 9%	490
	21st to end	Day	5% 10% 26% 30% 21% 9%	490
October	1st to 10th	Day	6% 13% 26% 22% 23% 9%	485
	11th to 20th	Day	5% 13% 18% 23% 29% 12%	300
	21st to end	Day	11% 6% 27% 29% 21% 5%	330
November	1st to 10th	Day	10% 15% 20% 24% 22% 9%	300
	11th to 20th	Day	4 14% 25% 28% 20% 9%	300
	21st to end	Day	11% 12% 22% 24% 24% 7%	300
December	1st to 10th	Day	6% 11% 26% 34% 19% 5%	300
	11th to 20th	Day	9% 8% 22% 36% 20% 5%	300
	21st to end	Day	14% 17% 22% 30% 13% 3	330
January	1st to 10th	Night	9% 6% 24% 34% 22% 5%	800
	11th to 20th	Night	6% 9% 21% 39% 21% 4	800
	21st to end	Night	9% 8% 22% 40% 18% 4	880
February	1st to 10th	Night	9% 9% 25% 37% 17% 3	800
	11th to 20th	Night	6% 10% 27% 40% 15% 2	784
	21st to end	Night	9% 8% 30% 38% 13% 1	660
March	1st to 10th	Night	10% 13% 24% 37% 15% 1	800
	11th to 20th	Night	8% 11% 26% 35% 20% 1	800
	21st to end	Night	8% 14% 28% 37% 11% 1	880
April	1st to 10th	Night	7% 14% 27% 35% 15% 1	800
	11th to 20th	Night	7% 15% 30% 34% 12% 1	682
	21st to end	Night	7% 14% 29% 34% 14% 2	600
May	1st to 10th	Night	10% 10% 31% 35% 13% 1	600
	11th to 20th	Night	6% 10% 23% 38% 21% 1	591
	21st to end	Night	9% 10% 27% 33% 19% 2	650
June	1st to 10th	Night	4 7% 30% 38% 21% 1	599
	11th to 20th	Night	5% 11% 28% 38% 16% 1	570
	21st to end	Night	2 3 29% 42% 22% 1	570
July	1st to 10th	Night	2 3 22% 44% 27% 2	555
	11th to 20th	Night	5% 2 19% 45% 26% 3	552
	21st to end	Night	5% 3 16% 42% 33% 2	624
August	1st to 10th	Night	5% 2 21% 46% 26% 1	567
	11th to 20th	Night	2 3 20% 48% 25% 2	564
	21st to end	Night	4 4 19% 36% 34% 4	736
September	1st to 10th	Night	4 4 14% 36% 38% 4	760
	11th to 20th	Night	3 6% 21% 36% 29% 5%	760
	21st to end	Night	3 4 19% 35% 33% 6%	760
October	1st to 10th	Night	5% 6% 20% 39% 27% 3	760
	11th to 20th	Night	5% 5% 18% 40% 26% 6%	760
	21st to end	Night	6% 8% 21% 34% 25% 6%	836
November	1st to 10th	Night	6% 8% 20% 34% 27% 4	780
	11th to 20th	Night	4 8% 18% 44% 21% 5%	800
	21st to end	Night	7% 8% 23% 36% 23% 5%	800
December	1st to 10th	Night	8% 5% 21% 43% 21% 3	800
	11th to 20th	Night	8% 7% 23% 41% 20% 2	800
	21st to end	Night	10% 8% 25% 38% 18% 1	880

Page generated at 2025-06-07 08:28:17. Version ac07

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