Logan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Logan Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	2 4 2 5% 4 4 4 4 5% 7% 59%	140665
	Night	1 3 1 4 3 3 4 3 5% 7% 65%	139040
Transparency	Day	23% 5% 8% 14% 27% 22%	135570
	Night	17% 5% 9% 17% 30% 21%	135140
Seeing	Day	10% 18% 27% 24% 16% 6%	16490
	Night	5% 14% 26% 30% 20% 5%	26633

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 2 6% 3 3 4 4 6% 7% 61%	5318
	11th to 20th	Day	2 1 3 2 1 2 2 3 6% 76%	5340
	21st to end	Day	2 6% 3 5% 3 3 4 3 6% 6% 60%	5844
February	1st to 10th	Day	1 4 1 5% 4 2 3 3 5% 6% 66%	1890
	11th to 20th	Day	2 3 2 4 3 3 4 4 5% 7% 62%	1915
	21st to end	Day	4 2 4 3 3 3 3 6% 7% 66%	1903
March	1st to 10th	Day	3 2 4 3 3 3 3 4 7% 67%	5064
	11th to 20th	Day	1 3 2 6% 4 4 4 4 7% 8% 57%	5110
	21st to end	Day	1 5% 3 6% 4 3 3 3 5% 6% 61%	5607
April	1st to 10th	Day	1 3 1 4 3 3 3 3 5% 7% 67%	2395
	11th to 20th	Day	1 5% 2 4 3 3 3 3 4 4 68%	2444
	21st to end	Day	2 4 1 3 2 3 3 3 4 7% 68%	2400
May	1st to 10th	Day	2 3 2 4 3 4 4 3 4 6% 65%	5721
	11th to 20th	Day	1 3 1 4 3 3 3 3 4 5% 69%	5841
	21st to end	Day	3 3 2 4 3 3 3 2 3 5% 69%	6423
June	1st to 10th	Day	2 3 1 3 4 3 4 3 5% 6% 66%	2600
	11th to 20th	Day	3 2 1 3 3 3 3 2 4 5% 71%	2600
	21st to end	Day	4 4 2 6% 6% 5% 5% 5% 6% 7% 51%	2600
July	1st to 10th	Day	3 3 2 6% 5% 5% 5% 5% 6% 6% 53%	5879
	11th to 20th	Day	3 3 2 8% 7% 9% 9% 7% 8% 9% 34%	5880
	21st to end	Day	6% 5% 3 9% 8% 7% 8% 6% 7% 8% 33%	6401
August	1st to 10th	Day	4 5% 3 9% 7% 8% 8% 6% 7% 7% 35%	5779
	11th to 20th	Day	5% 4 2 8% 8% 8% 8% 6% 9% 8% 35%	5630
	21st to end	Day	4 4 2 8% 6% 8% 8% 6% 8% 10% 34%	5984
September	1st to 10th	Day	6% 6% 2 8% 5% 5% 5% 5% 6% 7% 43%	2390
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 3 4 6% 54%	2200
	21st to end	Day	4 4 1 5% 3 4 4 3 4 6% 63%	2200
October	1st to 10th	Day	4 4 2 5% 3 3 3 3 3 5% 65%	3276
	11th to 20th	Day	2 3 1 4 3 2 3 2 5% 8% 68%	3250
	21st to end	Day	1 1 1 2 1 2 1 1 3 5% 82%	3548
November	1st to 10th	Day	1 2 1 2 2 2 3 2 4 6% 75%	2059
	11th to 20th	Day	1 3 1 3 2 2 3 3 5% 6% 71%	2100
	21st to end	Day	1 5% 1 4 3 2 3 3 4 7% 66%	1926
December	1st to 10th	Day	1 2 1 3 2 2 3 3 4 5% 75%	3040
	11th to 20th	Day	1 4 2 4 3 3 3 2 4 8% 65%	3499
	21st to end	Day	2 1 5% 3 4 3 3 5% 7% 67%	4609
January	1st to 10th	Night	1 4 3 8% 5% 4 4 4 5% 6% 55%	7605
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 86%	7650
	21st to end	Night	1 5% 3 5% 4 4 5% 4 6% 6% 59%	8370
February	1st to 10th	Night	3 1 3 3 2 4 4 6% 7% 67%	2730
	11th to 20th	Night	1 4 1 5% 3 4 4 3 5% 6% 64%	2586
	21st to end	Night	1 1 3 2 3 3 3 5% 8% 70%	1903
March	1st to 10th	Night	3 1 3 2 2 3 2 3 6% 74%	5050
	11th to 20th	Night	1 3 2 3 2 3 3 3 5% 7% 68%	5110
	21st to end	Night	1 2 1 4 3 3 3 3 5% 7% 67%	5610
April	1st to 10th	Night	1 2 1 2 3 3 3 4 6% 6% 69%	2223
	11th to 20th	Night	1 3 1 3 3 3 3 2 4 5% 72%	2037
	21st to end	Night	1 2 1 1 1 2 3 3 4 7% 76%	2000
May	1st to 10th	Night	1 2 1 4 2 3 3 2 3 5% 74%	4243
	11th to 20th	Night	1 2 1 2 2 2 2 2 5% 5% 76%	4074
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 81%	4474
June	1st to 10th	Night	1 3 1 3 2 3 4 3 4 5% 70%	1800
	11th to 20th	Night	1 2 1 2 2 1 2 2 4 4 79%	1800
	21st to end	Night	1 3 1 5% 4 4 5% 5% 5% 7% 59%	1800
July	1st to 10th	Night	2 2 1 4 4 4 5% 4 6% 8% 60%	4100
	11th to 20th	Night	3 4 2 9% 6% 7% 7% 6% 8% 11% 37%	4100
	21st to end	Night	3 3 2 7% 5% 6% 7% 5% 11% 12% 38%	4472
August	1st to 10th	Night	2 2 1 5% 4 6% 7% 6% 9% 11% 47%	4033
	11th to 20th	Night	2 4 2 6% 5% 6% 7% 7% 10% 12% 40%	4217
	21st to end	Night	4 4 2 6% 5% 6% 6% 6% 11% 11% 39%	4993
September	1st to 10th	Night	3 5% 2 6% 4 4 5% 5% 6% 9% 51%	2005
	11th to 20th	Night	3 2 1 4 3 3 3 3 5% 5% 67%	2200
	21st to end	Night	2 2 1 3 2 3 3 3 4 6% 72%	2200
October	1st to 10th	Night	2 2 1 3 2 3 3 3 4 5% 72%	3912
	11th to 20th	Night	1 1 1 2 1 2 2 2 4 6% 77%	3898
	21st to end	Night	1 2 1 2 1 1 2 2 3 4 82%	4252
November	1st to 10th	Night	2 1 2 2 2 2 2 4 6% 76%	2460
	11th to 20th	Night	1 2 1 3 1 2 2 3 4 6% 74%	2520
	21st to end	Night	1 3 1 3 3 3 3 3 6% 7% 68%	2691
December	1st to 10th	Night	1 2 1 3 2 2 2 2 4 6% 75%	4370
	11th to 20th	Night	1 2 2 4 3 3 4 3 5% 7% 67%	4941
	21st to end	Night	1 3 1 4 3 3 4 3 5% 6% 69%	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	18% 5% 9% 21% 36% 12%	5321
	11th to 20th	Day	10% 4 6% 17% 29% 35%	5340
	21st to end	Day	22% 6% 6% 10% 26% 30%	5844
February	1st to 10th	Day	19% 4 7% 18% 25% 25%	1890
	11th to 20th	Day	21% 4 8% 21% 26% 20%	1924
	21st to end	Day	17% 5% 5% 16% 31% 26%	1903
March	1st to 10th	Day	16% 4 6% 9% 32% 34%	5064
	11th to 20th	Day	21% 5% 10% 13% 34% 16%	5110
	21st to end	Day	23% 5% 7% 15% 38% 12%	5607
April	1st to 10th	Day	17% 4 7% 16% 37% 19%	2395
	11th to 20th	Day	20% 3 4 14% 33% 26%	2447
	21st to end	Day	18% 4 7% 18% 37% 18%	2400
May	1st to 10th	Day	17% 4 8% 14% 36% 21%	5721
	11th to 20th	Day	15% 5% 6% 11% 33% 30%	5844
	21st to end	Day	16% 3 5% 7% 24% 45%	6424
June	1st to 10th	Day	18% 4 7% 14% 29% 30%	2600
	11th to 20th	Day	17% 2 4 8% 23% 45%	2600
	21st to end	Day	30% 6% 9% 12% 22% 21%	2600
July	1st to 10th	Day	27% 7% 8% 9% 15% 34%	5816
	11th to 20th	Day	38% 11% 15% 19% 8% 8%	5811
	21st to end	Day	41% 10% 18% 15% 12% 4	6217
August	1st to 10th	Day	44% 11% 17% 15% 9% 4	4208
	11th to 20th	Day	38% 12% 13% 20% 14% 3	4077
	21st to end	Day	40% 9% 15% 16% 15% 4	4311
September	1st to 10th	Day	38% 7% 9% 15% 19% 12%	2390
	11th to 20th	Day	31% 5% 7% 15% 27% 16%	2200
	21st to end	Day	22% 4 7% 18% 29% 19%	2200
October	1st to 10th	Day	20% 3 7% 11% 27% 32%	3276
	11th to 20th	Day	14% 2 6% 12% 42% 24%	3250
	21st to end	Day	8% 1 4 11% 39% 37%	3548
November	1st to 10th	Day	11% 3 6% 23% 36% 22%	2059
	11th to 20th	Day	15% 3 5% 16% 37% 24%	2100
	21st to end	Day	19% 5% 6% 15% 33% 21%	1926
December	1st to 10th	Day	12% 3 6% 18% 40% 21%	3040
	11th to 20th	Day	20% 3 6% 14% 37% 20%	3498
	21st to end	Day	16% 5% 7% 22% 31% 19%	4609
January	1st to 10th	Night	24% 8% 8% 22% 28% 10%	7605
	11th to 20th	Night	7% 2 6% 16% 41% 28%	7650
	21st to end	Night	22% 6% 7% 10% 28% 27%	8370
February	1st to 10th	Night	16% 5% 8% 21% 27% 23%	2730
	11th to 20th	Night	20% 5% 8% 21% 28% 19%	2587
	21st to end	Night	13% 4 7% 18% 30% 27%	1903
March	1st to 10th	Night	12% 3 6% 10% 43% 25%	5050
	11th to 20th	Night	13% 5% 10% 19% 34% 18%	5110
	21st to end	Night	13% 6% 7% 24% 38% 12%	5610
April	1st to 10th	Night	14% 4 7% 22% 37% 16%	2223
	11th to 20th	Night	15% 4 5% 18% 34% 24%	2037
	21st to end	Night	10% 4 6% 21% 41% 18%	2000
May	1st to 10th	Night	13% 3 6% 20% 44% 14%	4240
	11th to 20th	Night	10% 5% 5% 13% 41% 25%	4075
	21st to end	Night	10% 2 3 8% 27% 50%	4474
June	1st to 10th	Night	16% 4 6% 12% 32% 31%	1800
	11th to 20th	Night	10% 3 3 11% 29% 43%	1800
	21st to end	Night	22% 7% 11% 13% 23% 25%	1800
July	1st to 10th	Night	21% 6% 11% 12% 15% 34%	4055
	11th to 20th	Night	35% 10% 19% 18% 12% 6%	4046
	21st to end	Night	32% 11% 28% 11% 13% 4	4344
August	1st to 10th	Night	32% 13% 25% 15% 10% 5%	2928
	11th to 20th	Night	30% 12% 21% 17% 16% 4	3046
	21st to end	Night	31% 10% 19% 21% 15% 4	3597
September	1st to 10th	Night	28% 8% 15% 18% 20% 11%	2005
	11th to 20th	Night	18% 5% 10% 20% 31% 16%	2200
	21st to end	Night	15% 4 7% 21% 32% 22%	2200
October	1st to 10th	Night	14% 3 8% 12% 29% 34%	3912
	11th to 20th	Night	10% 2 5% 18% 37% 27%	3898
	21st to end	Night	8% 1 4 13% 41% 32%	4252
November	1st to 10th	Night	11% 3 5% 20% 39% 22%	2460
	11th to 20th	Night	13% 3 7% 18% 33% 27%	2520
	21st to end	Night	16% 4 7% 20% 31% 22%	2691
December	1st to 10th	Night	13% 3 4 24% 33% 22%	4370
	11th to 20th	Night	17% 4 8% 14% 36% 21%	4941
	21st to end	Night	16% 6% 8% 23% 25% 22%	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	9% 26% 38% 18% 7% 2	330
	11th to 20th	Day	13% 24% 29% 17% 13% 4	330
	21st to end	Day	8% 21% 30% 26% 13% 3	363
February	1st to 10th	Day	10% 24% 32% 20% 10% 3	330
	11th to 20th	Day	22% 30% 27% 17% 4 1	382
	21st to end	Day	16% 34% 31% 13% 5%	446
March	1st to 10th	Day	18% 32% 31% 14% 4 1	540
	11th to 20th	Day	19% 25% 28% 19% 8% 2	540
	21st to end	Day	16% 33% 27% 17% 5% 2	594
April	1st to 10th	Day	10% 26% 36% 17% 8% 3	540
	11th to 20th	Day	17% 29% 34% 13% 6% 1	517
	21st to end	Day	13% 29% 33% 17% 6% 2	500
May	1st to 10th	Day	8% 24% 38% 21% 7% 2	503
	11th to 20th	Day	9% 20% 35% 24% 11% 2	528
	21st to end	Day	9% 22% 38% 19% 10% 2	573
June	1st to 10th	Day	4 13% 35% 32% 13% 3	530
	11th to 20th	Day	4 16% 30% 32% 13% 5%	530
	21st to end	Day	6% 9% 27% 36% 17% 5%	530
July	1st to 10th	Day	8% 4 19% 31% 29% 10%	526
	11th to 20th	Day	8% 3 10% 30% 31% 18%	524
	21st to end	Day	11% 2 11% 26% 32% 19%	575
August	1st to 10th	Day	7% 3 12% 27% 33% 18%	527
	11th to 20th	Day	10% 2 13% 32% 29% 15%	517
	21st to end	Day	8% 3 14% 30% 31% 15%	583
September	1st to 10th	Day	9% 6% 15% 33% 28% 8%	530
	11th to 20th	Day	6% 9% 28% 31% 20% 5%	530
	21st to end	Day	8% 12% 27% 28% 20% 5%	530
October	1st to 10th	Day	6% 17% 26% 26% 18% 6%	336
	11th to 20th	Day	12% 16% 24% 27% 16% 6%	330
	21st to end	Day	8% 13% 30% 23% 21% 4	363
November	1st to 10th	Day	6% 22% 29% 26% 12% 4	330
	11th to 20th	Day	6% 15% 29% 24% 19% 6%	330
	21st to end	Day	12% 24% 27% 21% 12% 4	330
December	1st to 10th	Day	7% 23% 25% 28% 15% 2	330
	11th to 20th	Day	8% 23% 30% 24% 12% 3	330
	21st to end	Day	16% 26% 30% 20% 8% 1	363
January	1st to 10th	Night	4 17% 25% 33% 14% 6%	840
	11th to 20th	Night	5% 14% 30% 30% 17% 4	840
	21st to end	Night	5% 18% 29% 28% 15% 5%	924
February	1st to 10th	Night	5% 20% 28% 29% 14% 3	840
	11th to 20th	Night	8% 24% 33% 23% 11% 1	824
	21st to end	Night	9% 23% 33% 25% 8% 1	692
March	1st to 10th	Night	9% 27% 30% 23% 9% 2	840
	11th to 20th	Night	6% 22% 32% 24% 13% 3	840
	21st to end	Night	7% 21% 33% 25% 11% 2	924
April	1st to 10th	Night	5% 23% 34% 26% 11% 1	753
	11th to 20th	Night	7% 25% 36% 23% 8% 1	611
	21st to end	Night	3 22% 33% 28% 13% 3	600
May	1st to 10th	Night	2 16% 38% 30% 13% 2	600
	11th to 20th	Night	4 10% 37% 31% 16% 3	591
	21st to end	Night	3 15% 35% 31% 14% 2	650
June	1st to 10th	Night	2 7% 34% 36% 19% 2	600
	11th to 20th	Night	2 8% 33% 40% 15% 2	600
	21st to end	Night	2 3 23% 41% 27% 5%	600
July	1st to 10th	Night	5% 1 14% 36% 35% 7%	585
	11th to 20th	Night	4 1 10% 29% 43% 12%	582
	21st to end	Night	5% 2 9% 26% 44% 14%	657
August	1st to 10th	Night	4 1 9% 25% 46% 15%	597
	11th to 20th	Night	5% 1 9% 32% 42% 11%	594
	21st to end	Night	5% 8% 30% 45% 11%	660
September	1st to 10th	Night	4 1 17% 34% 37% 7%	605
	11th to 20th	Night	4 3 25% 33% 29% 8%	800
	21st to end	Night	2 10% 19% 36% 26% 7%	800
October	1st to 10th	Night	3 12% 31% 33% 17% 4	800
	11th to 20th	Night	4 10% 26% 36% 19% 5%	800
	21st to end	Night	4 10% 25% 31% 22% 7%	880
November	1st to 10th	Night	2 14% 25% 35% 17% 7%	820
	11th to 20th	Night	4 14% 25% 32% 21% 5%	840
	21st to end	Night	6% 20% 25% 26% 17% 6%	840
December	1st to 10th	Night	6% 18% 21% 33% 19% 4	840
	11th to 20th	Night	5% 20% 29% 28% 15% 3	840
	21st to end	Night	7% 21% 28% 30% 10% 3	924

Page generated at 2026-04-13 23:44:30. Version ac07

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