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 2024-11-02.

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 4 5% 7% 57%	101205
	Night	3 6% 2 6% 4 4 4 3 5% 7% 56%	98052
Transparency	Day	23% 5% 9% 19% 23% 20%	100972
	Night	25% 5% 11% 21% 22% 17%	97875
Seeing	Day	7% 13% 27% 29% 19% 6%	14959
	Night	6% 7% 23% 39% 22% 3	25138

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% 12% 6% 5% 5% 4 6% 7% 39%	2858
	11th to 20th	Day	4 9% 3 9% 6% 4 5% 4 6% 8% 43%	2860
	21st to end	Day	4 11% 4 8% 5% 4 4 3 5% 6% 46%	3146
February	1st to 10th	Day	2 7% 3 8% 5% 5% 3 3 7% 8% 48%	1738
	11th to 20th	Day	4 9% 4 6% 4 4 4 3 5% 5% 51%	1850
	21st to end	Day	3 6% 3 9% 6% 4 4 4 5% 6% 49%	1570
March	1st to 10th	Day	2 10% 3 8% 4 4 4 4 5% 6% 49%	3275
	11th to 20th	Day	4 11% 5% 8% 5% 5% 4 4 5% 6% 43%	3490
	21st to end	Day	3 9% 3 8% 5% 5% 6% 4 8% 9% 40%	3825
April	1st to 10th	Day	3 5% 3 6% 4 4 4 3 5% 7% 55%	2120
	11th to 20th	Day	3 7% 2 6% 3 3 4 3 6% 6% 55%	2351
	21st to end	Day	4 5% 2 5% 3 3 4 3 5% 7% 57%	2470
May	1st to 10th	Day	4 3 1 4 4 3 4 4 5% 8% 61%	4110
	11th to 20th	Day	2 4 1 4 3 4 3 3 6% 8% 61%	4071
	21st to end	Day	4 4 2 4 4 4 4 3 5% 6% 61%	4477
June	1st to 10th	Day	1 2 1 4 3 4 4 3 6% 7% 64%	2470
	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	4 3 2 5% 3 3 3 3 4 6% 64%	2085
	11th to 20th	Day	2 3 1 4 2 2 3 2 3 6% 72%	1900
	21st to end	Day	3 4 2 4 2 2 2 3 4 5% 68%	2046
November	1st to 10th	Day	4 5% 2 5% 3 3 4 2 5% 7% 62%	1683
	11th to 20th	Day	4 4 1 5% 3 3 3 3 5% 8% 62%	1710
	21st to end	Day	4 7% 2 6% 5% 3 3 3 5% 5% 57%	1710
December	1st to 10th	Day	4 7% 2 7% 5% 4 4 4 4 7% 52%	1710
	11th to 20th	Day	3 7% 4 10% 5% 5% 5% 4 6% 9% 42%	2169
	21st to end	Day	3 9% 4 9% 6% 6% 5% 5% 7% 8% 38%	3146
January	1st to 10th	Night	2 8% 4 9% 5% 5% 4 4 6% 7% 46%	4097
	11th to 20th	Night	3 8% 3 8% 5% 4 5% 4 6% 8% 45%	4110
	21st to end	Night	4 9% 3 8% 5% 4 4 3 6% 10% 44%	4521
February	1st to 10th	Night	3 8% 2 6% 4 3 4 5% 5% 8% 51%	2439
	11th to 20th	Night	4 8% 3 6% 5% 4 4 4 5% 6% 51%	2218
	21st to end	Night	3 8% 2 8% 5% 5% 5% 5% 6% 7% 49%	1884
March	1st to 10th	Night	4 8% 3 7% 5% 5% 5% 4 5% 7% 48%	3598
	11th to 20th	Night	7% 10% 4 8% 5% 4 4 3 5% 6% 44%	3490
	21st to end	Night	5% 8% 3 8% 5% 5% 5% 4 6% 7% 45%	3828
April	1st to 10th	Night	4 6% 3 6% 4 4 5% 4 6% 6% 53%	2056
	11th to 20th	Night	5% 6% 2 4 4 4 4 3 5% 7% 56%	1776
	21st to end	Night	4 5% 2 4 3 3 3 4 6% 8% 57%	1710
May	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 5% 7% 56%	2860
	11th to 20th	Night	6% 4 1 5% 3 3 4 4 5% 6% 59%	2834
	21st to end	Night	5% 5% 1 5% 3 3 4 3 6% 6% 59%	3110
June	1st to 10th	Night	3 4 2 3 4 3 3 3 6% 8% 62%	1710
	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	4 4 2 4 2 3 3 2 4 5% 66%	2095
	11th to 20th	Night	3 3 1 3 3 3 3 3 4 5% 70%	2280
	21st to end	Night	3 4 1 3 2 3 3 3 4 6% 68%	2550
November	1st to 10th	Night	3 4 2 4 3 2 3 2 5% 6% 67%	2430
	11th to 20th	Night	2 3 1 3 3 3 3 3 5% 7% 67%	2470
	21st to end	Night	4 6% 2 6% 4 3 4 3 5% 7% 57%	2470
December	1st to 10th	Night	4 6% 3 6% 3 4 4 5% 6% 7% 53%	2470
	11th to 20th	Night	4 7% 3 7% 5% 5% 4 4 5% 9% 48%	3041
	21st to end	Night	3 9% 4 10% 5% 5% 5% 4 6% 8% 42%	4521

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	40% 7% 19% 23% 10%	2859
	11th to 20th	Day	37% 9% 18% 25% 11%	2860
	21st to end	Day	36% 7% 17% 27% 11% 2	3146
February	1st to 10th	Day	31% 7% 16% 26% 16% 4	1738
	11th to 20th	Day	33% 4 13% 26% 19% 5%	1860
	21st to end	Day	30% 5% 14% 27% 19% 5%	1570
March	1st to 10th	Day	30% 7% 11% 28% 19% 6%	3275
	11th to 20th	Day	36% 7% 11% 27% 16% 4	3490
	21st to end	Day	31% 9% 14% 26% 17% 3	3825
April	1st to 10th	Day	27% 4 8% 27% 24% 10%	2120
	11th to 20th	Day	26% 4 9% 24% 25% 12%	2354
	21st to end	Day	24% 5% 8% 22% 31% 10%	2470
May	1st to 10th	Day	18% 5% 7% 21% 37% 12%	4110
	11th to 20th	Day	16% 5% 8% 22% 32% 17%	4074
	21st to end	Day	19% 5% 7% 18% 31% 20%	4477
June	1st to 10th	Day	15% 3 8% 14% 31% 29%	2470
	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	21% 3 7% 18% 30% 21%	2085
	11th to 20th	Day	15% 3 5% 21% 38% 17%	1900
	21st to end	Day	19% 2 6% 30% 33% 10%	2046
November	1st to 10th	Day	21% 3 11% 26% 31% 7%	1683
	11th to 20th	Day	20% 4 9% 34% 28% 5%	1710
	21st to end	Day	27% 4 12% 27% 26% 5%	1710
December	1st to 10th	Day	31% 7% 12% 29% 17% 4	1710
	11th to 20th	Day	36% 7% 17% 22% 17%	2168
	21st to end	Day	37% 7% 18% 20% 17% 2	3146
January	1st to 10th	Night	32% 7% 19% 28% 13% 1	4097
	11th to 20th	Night	33% 8% 21% 26% 11% 1	4110
	21st to end	Night	33% 9% 20% 24% 12% 2	4521
February	1st to 10th	Night	30% 6% 14% 30% 16% 4	2439
	11th to 20th	Night	31% 4 13% 24% 23% 5%	2219
	21st to end	Night	31% 6% 13% 24% 23% 3	1884
March	1st to 10th	Night	30% 6% 14% 26% 20% 4	3598
	11th to 20th	Night	35% 6% 10% 27% 19% 3	3490
	21st to end	Night	31% 9% 13% 23% 20% 4	3828
April	1st to 10th	Night	28% 4 10% 26% 21% 10%	2056
	11th to 20th	Night	25% 4 9% 22% 28% 12%	1776
	21st to end	Night	23% 5% 10% 22% 33% 7%	1710
May	1st to 10th	Night	22% 7% 7% 20% 30% 15%	2860
	11th to 20th	Night	20% 6% 8% 18% 34% 14%	2835
	21st to end	Night	20% 7% 8% 18% 31% 17%	3110
June	1st to 10th	Night	18% 3 6% 15% 33% 26%	1710
	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	21% 3 5% 16% 33% 22%	2095
	11th to 20th	Night	16% 3 6% 17% 38% 19%	2280
	21st to end	Night	17% 4 7% 26% 37% 10%	2550
November	1st to 10th	Night	18% 3 11% 26% 32% 10%	2430
	11th to 20th	Night	17% 3 11% 30% 32% 7%	2470
	21st to end	Night	26% 4 7% 28% 29% 5%	2470
December	1st to 10th	Night	27% 7% 13% 31% 17% 5%	2470
	11th to 20th	Night	31% 6% 14% 31% 14% 3	3041
	21st to end	Night	37% 7% 15% 19% 18% 4	4521

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% 11% 29% 26% 18% 3	270
	11th to 20th	Day	7% 12% 31% 30% 16% 3	270
	21st to end	Day	5% 13% 19% 32% 25% 6%	297
February	1st to 10th	Day	8% 19% 26% 25% 16% 5%	298
	11th to 20th	Day	9% 17% 28% 27% 15% 3	455
	21st to end	Day	9% 19% 32% 27% 11% 2	382
March	1st to 10th	Day	12% 20% 26% 27% 13% 2	460
	11th to 20th	Day	9% 20% 30% 25% 12% 4	460
	21st to end	Day	15% 24% 29% 23% 9% 1	506
April	1st to 10th	Day	8% 22% 28% 25% 12% 5%	460
	11th to 20th	Day	11% 22% 37% 20% 9% 2	452
	21st to end	Day	9% 20% 29% 25% 14% 3	460
May	1st to 10th	Day	9% 18% 35% 23% 13% 2	463
	11th to 20th	Day	7% 13% 28% 30% 18% 4	488
	21st to end	Day	8% 14% 29% 27% 15% 7%	529
June	1st to 10th	Day	3 11% 30% 28% 23% 5%	490
	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% 7% 27% 30% 21% 5%	330
November	1st to 10th	Day	9% 12% 22% 25% 22% 9%	273
	11th to 20th	Day	3 12% 23% 30% 21% 10%	270
	21st to end	Day	9% 10% 22% 25% 26% 8%	270
December	1st to 10th	Day	6% 11% 26% 33% 19% 5%	270
	11th to 20th	Day	9% 8% 22% 37% 20% 5%	270
	21st to end	Day	14% 15% 22% 32% 14% 4	297
January	1st to 10th	Night	8% 6% 23% 35% 23% 6%	760
	11th to 20th	Night	6% 8% 20% 40% 22% 4	760
	21st to end	Night	8% 6% 21% 41% 19% 4	836
February	1st to 10th	Night	6% 7% 25% 39% 18% 3	760
	11th to 20th	Night	6% 10% 26% 41% 16% 2	744
	21st to end	Night	9% 7% 30% 39% 13% 1	628
March	1st to 10th	Night	9% 13% 24% 38% 15% 1	760
	11th to 20th	Night	7% 10% 25% 36% 21% 1	760
	21st to end	Night	8% 15% 28% 37% 11% 2	836
April	1st to 10th	Night	7% 14% 27% 35% 16% 1	760
	11th to 20th	Night	8% 14% 30% 34% 13% 1	648
	21st to end	Night	6% 14% 28% 35% 15% 2	570
May	1st to 10th	Night	9% 10% 32% 35% 12% 1	570
	11th to 20th	Night	6% 8% 23% 39% 21% 1	561
	21st to end	Night	9% 10% 27% 33% 19% 2	617
June	1st to 10th	Night	4 7% 30% 39% 20% 1	570
	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	5% 8% 21% 34% 28% 4	748
	11th to 20th	Night	3 6% 17% 46% 22% 5%	760
	21st to end	Night	6% 7% 23% 37% 24% 5%	760
December	1st to 10th	Night	8% 5% 21% 43% 21% 3	760
	11th to 20th	Night	8% 6% 23% 41% 20% 2	760
	21st to end	Night	10% 8% 24% 39% 18% 2	836

Page generated at 2024-10-30 11:19:24. Version ac07

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