Salt Lake City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Salt Lake City clear sky chart from 2005-11-06 to 2025-11-03.

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	6% 8% 3 6% 3 3 4 3 5% 6% 53%	135224
	Night	8% 9% 3 5% 3 3 4 3 5% 6% 50%	132314
Transparency	Day	32% 6% 9% 19% 21% 14%	129958
	Night	34% 5% 10% 19% 20% 11%	128583
Seeing	Day	12% 15% 32% 29% 10% 2	16335
	Night	13% 8% 24% 35% 18% 3	26739

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	11% 20% 5% 10% 3 3 4 3 5% 7% 29%	4098
	11th to 20th	Day	8% 19% 6% 15% 6% 4 4 2 4 5% 27%	4100
	21st to end	Day	8% 12% 4 9% 3 3 3 3 4 7% 43%	4510
February	1st to 10th	Day	12% 16% 5% 9% 5% 4 4 4 4 6% 32%	1800
	11th to 20th	Day	11% 17% 4 8% 5% 4 3 3 5% 7% 32%	1827
	21st to end	Day	7% 16% 6% 10% 5% 4 4 5% 6% 6% 31%	1650
March	1st to 10th	Day	5% 18% 5% 11% 4 4 4 2 5% 6% 36%	4821
	11th to 20th	Day	6% 13% 4 7% 4 4 4 4 5% 7% 42%	5000
	21st to end	Day	10% 17% 4 10% 6% 5% 4 4 4 5% 30%	5486
April	1st to 10th	Day	10% 10% 4 6% 4 4 4 5% 6% 7% 41%	2217
	11th to 20th	Day	10% 13% 2 6% 5% 4 4 4 5% 6% 42%	2533
	21st to end	Day	10% 9% 3 6% 4 4 4 4 6% 8% 45%	2600
May	1st to 10th	Day	8% 9% 2 5% 4 4 4 4 5% 7% 49%	5880
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 6% 8% 51%	5841
	21st to end	Day	6% 4 1 5% 3 3 4 3 6% 7% 58%	6423
June	1st to 10th	Day	4 4 1 5% 3 3 4 4 7% 8% 57%	2670
	11th to 20th	Day	5% 4 1 3 3 3 3 3 4 6% 65%	2800
	21st to end	Day	3 2 1 2 3 3 3 3 4 4 73%	2607
July	1st to 10th	Day	1 1 1 2 2 3 3 3 4 4 74%	5879
	11th to 20th	Day	3 2 1 3 2 3 4 4 6% 6% 67%	5880
	21st to end	Day	2 2 1 3 2 2 3 3 5% 6% 72%	6401
August	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 5% 71%	5779
	11th to 20th	Day	2 2 1 3 3 3 3 3 4 6% 71%	5798
	21st to end	Day	3 3 1 4 3 4 4 3 6% 6% 63%	6429
September	1st to 10th	Day	3 3 1 2 2 3 3 3 5% 5% 71%	2255
	11th to 20th	Day	5% 3 1 3 3 3 3 4 5% 6% 65%	2200
	21st to end	Day	7% 3 1 3 2 2 3 2 4 5% 68%	2200
October	1st to 10th	Day	6% 3 1 3 3 3 3 2 3 4 69%	3440
	11th to 20th	Day	8% 6% 1 3 2 3 3 3 4 7% 61%	3250
	21st to end	Day	7% 7% 2 5% 3 4 3 3 4 5% 57%	3522
November	1st to 10th	Day	9% 10% 2 7% 5% 4 3 3 5% 6% 47%	1782
	11th to 20th	Day	13% 10% 3 5% 3 3 3 2 5% 7% 45%	1800
	21st to end	Day	9% 12% 3 7% 5% 4 4 2 4 5% 44%	1659
December	1st to 10th	Day	6% 9% 2 5% 3 3 3 3 5% 7% 53%	2620
	11th to 20th	Day	9% 14% 5% 8% 4 5% 4 4 5% 9% 33%	3026
	21st to end	Day	13% 17% 5% 9% 5% 4 4 4 5% 6% 29%	4441
January	1st to 10th	Night	10% 20% 7% 8% 3 2 3 3 5% 6% 34%	5867
	11th to 20th	Night	12% 18% 5% 10% 4 3 3 3 7% 7% 29%	5880
	21st to end	Night	7% 13% 3 7% 4 3 3 3 4 5% 48%	6468
February	1st to 10th	Night	12% 15% 3 7% 4 4 5% 3 6% 7% 34%	2600
	11th to 20th	Night	10% 15% 4 7% 4 4 3 3 4 5% 41%	2458
	21st to end	Night	6% 16% 5% 8% 4 3 4 3 5% 6% 39%	1980
March	1st to 10th	Night	8% 17% 4 9% 4 4 4 4 5% 6% 36%	5073
	11th to 20th	Night	7% 14% 3 6% 4 4 5% 4 6% 7% 40%	5000
	21st to end	Night	11% 17% 3 6% 3 4 4 3 5% 5% 39%	5489
April	1st to 10th	Night	12% 12% 2 5% 4 3 3 3 4 5% 47%	2181
	11th to 20th	Night	8% 10% 3 5% 4 3 4 4 5% 8% 46%	1811
	21st to end	Night	11% 8% 2 6% 4 4 4 4 5% 6% 45%	1800
May	1st to 10th	Night	9% 11% 2 4 3 3 3 3 6% 6% 50%	4100
	11th to 20th	Night	8% 4 1 4 3 3 4 3 6% 7% 56%	4074
	21st to end	Night	7% 4 2 5% 4 4 4 3 5% 7% 57%	4474
June	1st to 10th	Night	6% 5% 2 3 3 3 4 3 5% 8% 58%	1720
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 8% 64%	1600
	21st to end	Night	4 1 1 4 2 3 2 3 5% 7% 68%	1780
July	1st to 10th	Night	2 2 1 4 3 3 3 3 4 4 71%	4100
	11th to 20th	Night	2 4 1 4 4 4 4 4 7% 8% 57%	4100
	21st to end	Night	3 3 1 3 3 4 4 3 5% 6% 66%	4472
August	1st to 10th	Night	2 2 1 3 3 3 3 4 5% 7% 68%	4033
	11th to 20th	Night	2 2 1 4 4 4 4 3 4 6% 65%	4051
	21st to end	Night	4 3 2 5% 4 4 4 4 5% 5% 59%	4548
September	1st to 10th	Night	3 3 1 4 3 3 3 2 5% 5% 68%	2141
	11th to 20th	Night	6% 3 1 3 2 3 3 3 4 6% 66%	2200
	21st to end	Night	6% 3 1 3 2 2 2 2 3 5% 70%	2200
October	1st to 10th	Night	5% 3 1 3 2 3 3 2 3 4 71%	3755
	11th to 20th	Night	9% 5% 1 4 2 3 3 2 4 6% 60%	3898
	21st to end	Night	9% 6% 2 4 3 3 3 3 5% 7% 55%	4273
November	1st to 10th	Night	9% 9% 2 4 3 3 4 3 6% 7% 50%	2561
	11th to 20th	Night	12% 9% 2 5% 3 3 3 3 6% 7% 47%	2600
	21st to end	Night	9% 11% 3 4 4 3 4 3 4 6% 48%	2716
December	1st to 10th	Night	6% 8% 3 5% 3 3 3 2 9% 9% 50%	4570
	11th to 20th	Night	12% 13% 3 6% 3 3 4 3 5% 7% 42%	5181
	21st to end	Night	12% 15% 5% 9% 5% 4 4 4 5% 6% 33%	6560

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	56% 5% 12% 16% 8% 2	4099
	11th to 20th	Day	58% 7% 11% 17% 6% 1	4100
	21st to end	Day	41% 6% 14% 22% 16% 1	4510
February	1st to 10th	Day	54% 7% 13% 15% 10% 1	1800
	11th to 20th	Day	53% 8% 12% 16% 10% 1	1836
	21st to end	Day	52% 8% 12% 17% 10% 1	1650
March	1st to 10th	Day	48% 8% 8% 20% 12% 4	4821
	11th to 20th	Day	39% 8% 9% 31% 12% 1	5000
	21st to end	Day	57% 6% 15% 15% 6% 1	5486
April	1st to 10th	Day	42% 8% 13% 19% 13% 4	2217
	11th to 20th	Day	43% 6% 11% 20% 17% 2	2536
	21st to end	Day	37% 8% 10% 25% 19% 1	2600
May	1st to 10th	Day	34% 7% 10% 23% 21% 5%	5880
	11th to 20th	Day	30% 6% 8% 28% 25% 4	5844
	21st to end	Day	24% 6% 7% 25% 33% 3	6424
June	1st to 10th	Day	23% 7% 9% 23% 29% 8%	2670
	11th to 20th	Day	22% 5% 7% 18% 29% 20%	2800
	21st to end	Day	15% 4 5% 11% 33% 33%	2607
July	1st to 10th	Day	13% 4 4 6% 16% 57%	5816
	11th to 20th	Day	16% 4 4 9% 27% 40%	5811
	21st to end	Day	13% 4 4 7% 24% 47%	6217
August	1st to 10th	Day	18% 5% 5% 16% 28% 28%	4208
	11th to 20th	Day	18% 5% 6% 13% 33% 25%	4197
	21st to end	Day	15% 3 5% 15% 32% 30%	4636
September	1st to 10th	Day	16% 5% 6% 17% 29% 27%	2255
	11th to 20th	Day	21% 6% 7% 18% 29% 19%	2200
	21st to end	Day	19% 4 6% 21% 34% 15%	2200
October	1st to 10th	Day	21% 4 4 13% 37% 21%	3440
	11th to 20th	Day	25% 4 6% 29% 28% 7%	3250
	21st to end	Day	30% 5% 10% 31% 21% 4	3522
November	1st to 10th	Day	39% 7% 12% 22% 18% 3	1782
	11th to 20th	Day	41% 5% 10% 28% 15% 1	1800
	21st to end	Day	44% 6% 11% 23% 15% 1	1659
December	1st to 10th	Day	29% 8% 16% 38% 8% 1	2620
	11th to 20th	Day	49% 6% 14% 21% 9%	3024
	21st to end	Day	55% 7% 10% 17% 11%	4441
January	1st to 10th	Night	51% 5% 12% 18% 11% 1	5867
	11th to 20th	Night	53% 5% 13% 21% 6% 1	5880
	21st to end	Night	38% 6% 13% 23% 18% 1	6468
February	1st to 10th	Night	49% 7% 14% 19% 9% 2	2600
	11th to 20th	Night	46% 6% 11% 20% 16% 2	2459
	21st to end	Night	46% 6% 11% 21% 13% 3	1980
March	1st to 10th	Night	47% 7% 7% 19% 16% 4	5073
	11th to 20th	Night	42% 7% 10% 25% 16% 1	5000
	21st to end	Night	47% 7% 14% 19% 13% 1	5489
April	1st to 10th	Night	40% 5% 13% 20% 16% 6%	2181
	11th to 20th	Night	36% 7% 11% 21% 21% 4	1811
	21st to end	Night	39% 6% 13% 22% 20% 2	1800
May	1st to 10th	Night	33% 5% 12% 19% 27% 4	4097
	11th to 20th	Night	25% 6% 7% 27% 31% 4	4075
	21st to end	Night	25% 6% 8% 21% 37% 3	4474
June	1st to 10th	Night	26% 6% 8% 21% 29% 10%	1720
	11th to 20th	Night	20% 5% 8% 18% 28% 21%	1600
	21st to end	Night	17% 3 5% 10% 29% 37%	1780
July	1st to 10th	Night	17% 4 3 7% 13% 56%	4055
	11th to 20th	Night	23% 5% 3 7% 23% 39%	4046
	21st to end	Night	20% 4 4 7% 22% 44%	4344
August	1st to 10th	Night	19% 6% 6% 11% 28% 30%	2928
	11th to 20th	Night	24% 5% 6% 11% 28% 27%	2928
	21st to end	Night	17% 5% 5% 10% 29% 34%	3272
September	1st to 10th	Night	20% 4 6% 15% 24% 31%	2141
	11th to 20th	Night	20% 5% 7% 17% 28% 23%	2200
	21st to end	Night	19% 4 5% 17% 33% 22%	2200
October	1st to 10th	Night	20% 4 5% 13% 34% 25%	3755
	11th to 20th	Night	26% 4 4 28% 27% 11%	3898
	21st to end	Night	29% 5% 6% 29% 26% 4	4273
November	1st to 10th	Night	34% 5% 11% 23% 21% 6%	2561
	11th to 20th	Night	36% 6% 12% 26% 17% 3	2600
	21st to end	Night	38% 5% 11% 26% 16% 4	2716
December	1st to 10th	Night	29% 5% 17% 35% 12% 2	4570
	11th to 20th	Night	43% 5% 18% 24% 10%	5182
	21st to end	Night	52% 6% 14% 17% 10% 1	6560

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	26% 14% 27% 21% 10% 2	300
	11th to 20th	Day	19% 13% 30% 24% 12% 2	300
	21st to end	Day	17% 11% 31% 32% 8% 2	330
February	1st to 10th	Day	24% 21% 29% 19% 6%	300
	11th to 20th	Day	19% 18% 34% 23% 6% 1	372
	21st to end	Day	13% 20% 39% 21% 6% 1	414
March	1st to 10th	Day	17% 22% 32% 19% 8% 1	500
	11th to 20th	Day	14% 20% 34% 24% 7%	500
	21st to end	Day	20% 23% 33% 21% 3	550
April	1st to 10th	Day	18% 20% 30% 24% 6% 1	500
	11th to 20th	Day	20% 23% 32% 20% 5% 1	492
	21st to end	Day	19% 22% 28% 25% 7% 1	500
May	1st to 10th	Day	12% 23% 34% 22% 7% 2	503
	11th to 20th	Day	12% 21% 31% 24% 9% 2	528
	21st to end	Day	10% 16% 31% 28% 12% 3	573
June	1st to 10th	Day	6% 15% 34% 34% 10% 2	600
	11th to 20th	Day	8% 15% 35% 29% 12% 1	730
	21st to end	Day	4 15% 37% 35% 8% 1	537
July	1st to 10th	Day	3 9% 39% 37% 10% 2	526
	11th to 20th	Day	3 6% 39% 37% 12% 3	524
	21st to end	Day	3 5% 35% 41% 12% 3	575
August	1st to 10th	Day	5% 10% 36% 34% 12% 2	527
	11th to 20th	Day	3 7% 35% 39% 13% 4	517
	21st to end	Day	6% 8% 30% 39% 14% 3	583
September	1st to 10th	Day	4 9% 30% 36% 18% 3	530
	11th to 20th	Day	7% 18% 29% 33% 12% 2	530
	21st to end	Day	8% 12% 26% 36% 17% 1	530
October	1st to 10th	Day	12% 14% 28% 29% 17%	435
	11th to 20th	Day	12% 19% 24% 28% 16% 2	330
	21st to end	Day	13% 13% 35% 29% 9% 1	363
November	1st to 10th	Day	22% 17% 23% 28% 8% 2	306
	11th to 20th	Day	17% 13% 32% 25% 8% 4	300
	21st to end	Day	16% 13% 26% 28% 15% 2	300
December	1st to 10th	Day	17% 13% 25% 30% 13% 2	300
	11th to 20th	Day	20% 10% 32% 29% 8% 1	300
	21st to end	Day	24% 15% 28% 26% 5% 1	330
January	1st to 10th	Night	20% 7% 25% 28% 17% 3	800
	11th to 20th	Night	18% 7% 24% 32% 14% 5%	800
	21st to end	Night	16% 8% 24% 33% 16% 3	880
February	1st to 10th	Night	20% 8% 25% 34% 11% 2	800
	11th to 20th	Night	17% 8% 28% 32% 14% 2	784
	21st to end	Night	13% 10% 34% 28% 13% 1	660
March	1st to 10th	Night	16% 12% 28% 31% 10% 3	800
	11th to 20th	Night	13% 9% 26% 34% 16% 3	800
	21st to end	Night	15% 15% 25% 33% 10% 2	880
April	1st to 10th	Night	18% 12% 25% 32% 11% 2	781
	11th to 20th	Night	13% 13% 30% 31% 11% 2	593
	21st to end	Night	13% 10% 23% 34% 18% 2	600
May	1st to 10th	Night	12% 8% 30% 34% 15% 2	600
	11th to 20th	Night	10% 7% 23% 36% 20% 4	591
	21st to end	Night	9% 8% 23% 36% 20% 4	650
June	1st to 10th	Night	8% 8% 21% 40% 23% 2	600
	11th to 20th	Night	6% 10% 25% 41% 18% 1	600
	21st to end	Night	4 4 24% 37% 27% 3	600
July	1st to 10th	Night	3 5% 24% 34% 32% 2	585
	11th to 20th	Night	5% 3 20% 45% 25% 2	582
	21st to end	Night	4 3 16% 39% 36% 2	657
August	1st to 10th	Night	5% 3 21% 44% 25% 2	597
	11th to 20th	Night	3 3 21% 43% 29% 2	594
	21st to end	Night	5% 5% 18% 44% 26% 2	660
September	1st to 10th	Night	5% 6% 16% 47% 22% 4	741
	11th to 20th	Night	6% 6% 23% 37% 23% 6%	800
	21st to end	Night	8% 5% 22% 32% 26% 8%	800
October	1st to 10th	Night	10% 7% 26% 36% 19% 4	800
	11th to 20th	Night	11% 6% 23% 33% 23% 5%	800
	21st to end	Night	12% 7% 26% 32% 20% 4	880
November	1st to 10th	Night	15% 11% 20% 33% 19% 3	788
	11th to 20th	Night	16% 7% 26% 32% 15% 4	800
	21st to end	Night	15% 9% 25% 31% 16% 5%	916
December	1st to 10th	Night	17% 6% 26% 34% 16% 1	1000
	11th to 20th	Night	21% 7% 27% 31% 11% 2	1000
	21st to end	Night	25% 10% 24% 31% 10% 1	920

Page generated at 2025-10-31 00:02:40. Version ac07

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