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Salt Lake Astronomical Society

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Stansbury Park Observatory Complex (SPOC)
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stansbury Park Observatory Complex (SPOC) clear sky chart from 2005-11-06 to 2024-11-06.

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	5% 7% 2 6% 4 4 4 4 5% 7% 52%	105111
	Night	6% 9% 3 6% 4 4 4 4 5% 7% 49%	102683
Transparency	Day	30% 6% 9% 19% 22% 14%	104877
	Night	32% 6% 9% 20% 20% 12%	102506
Seeing	Day	8% 10% 25% 34% 19% 4	14786
	Night	10% 6% 20% 36% 24% 5%	25066

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	7% 20% 6% 11% 5% 6% 6% 5% 6% 6% 23%	2858
	11th to 20th	Day	8% 17% 6% 15% 7% 4 4 5% 5% 7% 24%	2860
	21st to end	Day	9% 15% 5% 8% 6% 5% 6% 5% 4 8% 29%	3146
February	1st to 10th	Day	9% 13% 4 10% 5% 5% 4 4 5% 8% 33%	1710
	11th to 20th	Day	8% 14% 4 9% 5% 4 5% 4 6% 7% 35%	1759
	21st to end	Day	5% 13% 5% 9% 6% 6% 5% 4 5% 7% 35%	1570
March	1st to 10th	Day	4 11% 4 10% 6% 5% 5% 4 5% 7% 39%	3319
	11th to 20th	Day	4 11% 3 5% 4 4 4 4 6% 7% 48%	3490
	21st to end	Day	8% 16% 4 9% 6% 6% 6% 6% 6% 8% 26%	3825
April	1st to 10th	Day	9% 10% 2 6% 4 5% 6% 4 6% 7% 42%	2090
	11th to 20th	Day	8% 9% 3 6% 5% 5% 5% 3 6% 6% 45%	2426
	21st to end	Day	7% 8% 2 6% 4 3 4 4 6% 7% 48%	2470
May	1st to 10th	Day	6% 9% 2 7% 4 4 5% 4 7% 7% 44%	4110
	11th to 20th	Day	5% 4 1 4 3 3 3 3 6% 8% 61%	4071
	21st to end	Day	5% 5% 2 5% 4 4 5% 4 6% 7% 54%	4477
June	1st to 10th	Day	3 3 1 4 3 4 4 5% 5% 10% 57%	2470
	11th to 20th	Day	4 3 1 4 3 3 3 4 4 7% 64%	2470
	21st to end	Day	1 2 1 3 2 2 3 3 4 6% 73%	2624
July	1st to 10th	Day	1 1 1 2 2 2 3 2 4 5% 77%	4203
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 8% 63%	4110
	21st to end	Day	2 2 1 3 3 3 3 3 6% 7% 67%	4454
August	1st to 10th	Day	2 2 1 3 3 3 3 4 5% 7% 66%	4092
	11th to 20th	Day	2 1 1 4 3 4 4 4 5% 6% 66%	4028
	21st to end	Day	2 2 1 3 2 3 3 3 4 5% 73%	4521
September	1st to 10th	Day	2 2 1 3 2 2 2 4 5% 5% 72%	2111
	11th to 20th	Day	3 2 1 3 3 3 4 3 5% 6% 67%	2090
	21st to end	Day	6% 3 1 3 3 2 3 2 3 5% 69%	2090
October	1st to 10th	Day	4 3 1 3 3 3 3 3 4 4 68%	3367
	11th to 20th	Day	8% 5% 2 3 2 3 3 3 4 6% 60%	3150
	21st to end	Day	7% 6% 2 5% 3 3 3 3 5% 6% 57%	3378
November	1st to 10th	Day	8% 8% 3 5% 4 5% 5% 5% 5% 7% 46%	1719
	11th to 20th	Day	11% 8% 3 6% 4 3 4 3 5% 7% 47%	1710
	21st to end	Day	7% 13% 4 7% 6% 5% 4 3 5% 6% 41%	1656
December	1st to 10th	Day	9% 12% 3 9% 5% 5% 5% 4 5% 7% 36%	1520
	11th to 20th	Day	8% 14% 4 9% 6% 6% 7% 5% 7% 9% 25%	2021
	21st to end	Day	7% 15% 5% 10% 5% 5% 7% 4 6% 7% 29%	3146
January	1st to 10th	Night	7% 19% 6% 11% 4 4 5% 3 4 7% 31%	4097
	11th to 20th	Night	12% 18% 4 10% 6% 4 5% 4 5% 6% 26%	4110
	21st to end	Night	8% 17% 3 7% 5% 4 5% 5% 6% 7% 32%	4521
February	1st to 10th	Night	8% 12% 3 9% 5% 5% 5% 4 7% 6% 35%	2470
	11th to 20th	Night	7% 13% 4 8% 5% 4 4 3 5% 5% 42%	2305
	21st to end	Night	5% 15% 5% 7% 5% 4 4 5% 6% 7% 39%	1884
March	1st to 10th	Night	5% 10% 4 11% 5% 5% 5% 4 6% 8% 36%	3555
	11th to 20th	Night	3 10% 3 6% 4 4 5% 4 7% 7% 47%	3490
	21st to end	Night	7% 14% 4 9% 5% 5% 5% 5% 6% 6% 35%	3828
April	1st to 10th	Night	10% 10% 3 4 4 3 3 3 4 6% 48%	2090
	11th to 20th	Night	8% 9% 2 6% 4 3 4 4 7% 7% 46%	1699
	21st to end	Night	9% 8% 2 6% 4 3 4 4 5% 6% 49%	1710
May	1st to 10th	Night	6% 8% 2 6% 5% 5% 6% 5% 7% 7% 42%	2860
	11th to 20th	Night	5% 3 2 4 3 4 4 3 5% 8% 60%	2834
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 8% 52%	3110
June	1st to 10th	Night	5% 4 2 4 4 3 4 5% 6% 8% 56%	1710
	11th to 20th	Night	4 3 1 4 3 3 3 4 6% 6% 63%	1710
	21st to end	Night	2 2 1 3 3 3 3 3 6% 8% 65%	1542
July	1st to 10th	Night	2 2 1 3 2 2 3 2 4 5% 75%	2750
	11th to 20th	Night	2 3 2 5% 5% 5% 6% 5% 7% 9% 52%	2860
	21st to end	Night	3 4 2 5% 4 4 4 4 5% 6% 60%	3109
August	1st to 10th	Night	2 3 2 3 4 4 4 4 7% 8% 59%	2847
	11th to 20th	Night	2 3 1 3 3 4 5% 4 6% 6% 62%	2811
	21st to end	Night	4 2 1 3 2 2 2 3 4 5% 72%	3146
September	1st to 10th	Night	4 2 1 3 3 2 3 3 4 6% 67%	2063
	11th to 20th	Night	4 3 1 4 2 3 3 3 3 6% 67%	2090
	21st to end	Night	6% 2 1 3 3 2 3 2 3 4 71%	2090
October	1st to 10th	Night	4 3 1 3 3 3 3 3 3 4 70%	3608
	11th to 20th	Night	9% 5% 2 4 3 3 4 2 4 5% 60%	3778
	21st to end	Night	7% 8% 2 5% 3 3 4 4 5% 7% 53%	4175
November	1st to 10th	Night	9% 9% 2 6% 3 3 4 3 6% 7% 48%	2482
	11th to 20th	Night	7% 10% 3 6% 4 3 3 4 5% 7% 47%	2470
	21st to end	Night	7% 11% 4 6% 5% 4 3 3 4 7% 45%	2498
December	1st to 10th	Night	9% 11% 3 7% 5% 4 4 4 7% 7% 40%	2660
	11th to 20th	Night	10% 13% 3 7% 5% 5% 5% 4 5% 8% 35%	3200
	21st to end	Night	9% 14% 5% 10% 5% 4 5% 4 5% 6% 33%	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	54% 11% 12% 16% 6%	2859
	11th to 20th	Day	55% 9% 17% 15% 4 1	2860
	21st to end	Day	49% 10% 19% 13% 7% 2	3146
February	1st to 10th	Day	50% 6% 13% 20% 9% 2	1710
	11th to 20th	Day	48% 7% 12% 19% 13% 1	1768
	21st to end	Day	49% 6% 12% 21% 11% 1	1570
March	1st to 10th	Day	40% 9% 10% 24% 15% 2	3319
	11th to 20th	Day	34% 7% 12% 35% 12% 1	3490
	21st to end	Day	50% 10% 14% 17% 8% 1	3825
April	1st to 10th	Day	42% 7% 11% 20% 16% 5%	2090
	11th to 20th	Day	40% 6% 10% 21% 19% 4	2429
	21st to end	Day	33% 7% 11% 27% 21% 2	2470
May	1st to 10th	Day	32% 12% 7% 23% 21% 5%	4110
	11th to 20th	Day	21% 5% 5% 28% 35% 5%	4074
	21st to end	Day	27% 6% 9% 27% 27% 4	4477
June	1st to 10th	Day	23% 6% 8% 23% 31% 9%	2470
	11th to 20th	Day	21% 5% 5% 19% 30% 19%	2470
	21st to end	Day	13% 4 3 12% 31% 36%	2624
July	1st to 10th	Day	11% 3 4 7% 15% 60%	4139
	11th to 20th	Day	15% 6% 5% 11% 27% 37%	4041
	21st to end	Day	15% 4 5% 11% 30% 34%	4314
August	1st to 10th	Day	17% 5% 4 18% 28% 28%	4078
	11th to 20th	Day	17% 7% 5% 13% 32% 26%	4067
	21st to end	Day	15% 4 4 13% 33% 32%	4520
September	1st to 10th	Day	14% 5% 5% 17% 30% 28%	2111
	11th to 20th	Day	19% 5% 7% 20% 28% 22%	2090
	21st to end	Day	19% 5% 5% 19% 35% 18%	2090
October	1st to 10th	Day	21% 4 5% 14% 38% 19%	3367
	11th to 20th	Day	24% 5% 5% 27% 30% 8%	3150
	21st to end	Day	26% 6% 10% 32% 23% 3	3378
November	1st to 10th	Day	37% 6% 13% 22% 20% 3	1719
	11th to 20th	Day	36% 6% 11% 31% 15% 1	1710
	21st to end	Day	43% 4 13% 24% 14% 1	1656
December	1st to 10th	Day	44% 8% 17% 21% 8% 2	1520
	11th to 20th	Day	49% 6% 14% 22% 10%	2020
	21st to end	Day	47% 8% 15% 16% 13% 1	3146
January	1st to 10th	Night	49% 8% 13% 21% 9% 1	4097
	11th to 20th	Night	54% 9% 14% 16% 6% 1	4110
	21st to end	Night	43% 10% 21% 17% 7% 2	4521
February	1st to 10th	Night	45% 7% 15% 23% 8% 1	2470
	11th to 20th	Night	42% 5% 11% 23% 17% 2	2306
	21st to end	Night	42% 7% 10% 24% 15% 3	1884
March	1st to 10th	Night	40% 7% 11% 25% 16% 1	3555
	11th to 20th	Night	31% 7% 13% 32% 16% 2	3490
	21st to end	Night	42% 10% 10% 23% 13% 2	3828
April	1st to 10th	Night	36% 6% 10% 25% 17% 7%	2090
	11th to 20th	Night	35% 7% 9% 23% 23% 4	1699
	21st to end	Night	34% 6% 11% 24% 23% 2	1710
May	1st to 10th	Night	31% 9% 8% 25% 23% 4	2860
	11th to 20th	Night	20% 6% 6% 29% 32% 7%	2835
	21st to end	Night	25% 7% 10% 25% 29% 4	3110
June	1st to 10th	Night	25% 7% 7% 23% 27% 11%	1710
	11th to 20th	Night	20% 5% 7% 18% 30% 20%	1710
	21st to end	Night	17% 4 3 12% 26% 37%	1542
July	1st to 10th	Night	13% 3 3 8% 12% 60%	2705
	11th to 20th	Night	25% 5% 5% 8% 23% 33%	2806
	21st to end	Night	24% 5% 6% 9% 23% 34%	3011
August	1st to 10th	Night	19% 6% 6% 12% 28% 29%	2838
	11th to 20th	Night	20% 6% 5% 11% 30% 28%	2838
	21st to end	Night	16% 4 4 10% 24% 42%	3146
September	1st to 10th	Night	19% 3 4 15% 25% 34%	2063
	11th to 20th	Night	19% 4 5% 17% 30% 25%	2090
	21st to end	Night	19% 4 3 16% 32% 26%	2090
October	1st to 10th	Night	18% 3 5% 13% 34% 26%	3608
	11th to 20th	Night	26% 5% 5% 25% 27% 12%	3778
	21st to end	Night	28% 6% 6% 28% 27% 5%	4175
November	1st to 10th	Night	33% 7% 10% 24% 20% 6%	2482
	11th to 20th	Night	34% 5% 12% 29% 17% 3	2470
	21st to end	Night	37% 5% 10% 29% 16% 3	2498
December	1st to 10th	Night	40% 7% 16% 24% 11% 3	2660
	11th to 20th	Night	44% 7% 14% 23% 12%	3200
	21st to end	Night	48% 7% 15% 17% 13% 1	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	15% 13% 28% 26% 14% 4	270
	11th to 20th	Day	13% 12% 27% 29% 16% 3	270
	21st to end	Day	16% 10% 22% 35% 14% 4	297
February	1st to 10th	Day	15% 15% 30% 27% 10% 3	270
	11th to 20th	Day	13% 16% 32% 25% 13% 1	364
	21st to end	Day	8% 17% 36% 30% 8% 1	382
March	1st to 10th	Day	11% 15% 31% 33% 10% 1	460
	11th to 20th	Day	8% 13% 35% 32% 11% 2	460
	21st to end	Day	16% 16% 34% 28% 6%	506
April	1st to 10th	Day	13% 13% 31% 25% 16% 2	460
	11th to 20th	Day	13% 22% 25% 28% 10% 2	452
	21st to end	Day	12% 14% 29% 25% 18% 2	460
May	1st to 10th	Day	6% 14% 38% 28% 11% 3	463
	11th to 20th	Day	7% 11% 28% 30% 19% 6%	488
	21st to end	Day	6% 11% 26% 31% 22% 5%	529
June	1st to 10th	Day	3 7% 28% 35% 22% 5%	490
	11th to 20th	Day	5% 14% 30% 31% 17% 3	490
	21st to end	Day	2 8% 27% 40% 19% 4	490
July	1st to 10th	Day	2 3 20% 48% 23% 4	486
	11th to 20th	Day	2 2 21% 46% 23% 5%	484
	21st to end	Day	3 2 12% 49% 27% 7%	531
August	1st to 10th	Day	3 2 21% 46% 24% 4	487
	11th to 20th	Day	3 4 17% 43% 26% 7%	477
	21st to end	Day	4 5% 17% 38% 30% 6%	539
September	1st to 10th	Day	3 5% 18% 37% 27% 12%	490
	11th to 20th	Day	5% 10% 21% 32% 26% 6%	490
	21st to end	Day	6% 6% 20% 33% 29% 6%	490
October	1st to 10th	Day	7% 8% 26% 31% 22% 7%	419
	11th to 20th	Day	7% 11% 22% 27% 24% 9%	300
	21st to end	Day	11% 8% 24% 33% 21% 2	330
November	1st to 10th	Day	16% 12% 21% 25% 19% 6%	285
	11th to 20th	Day	11% 10% 21% 34% 16% 8%	270
	21st to end	Day	16% 8% 19% 26% 27% 5%	270
December	1st to 10th	Day	13% 10% 24% 33% 16% 4	270
	11th to 20th	Day	15% 7% 22% 39% 14% 1	270
	21st to end	Day	19% 11% 26% 30% 14% 1	297
January	1st to 10th	Night	14% 9% 19% 32% 20% 6%	760
	11th to 20th	Night	15% 7% 20% 33% 20% 5%	760
	21st to end	Night	17% 7% 16% 36% 20% 5%	836
February	1st to 10th	Night	16% 5% 23% 38% 15% 4	760
	11th to 20th	Night	14% 5% 22% 38% 18% 3	744
	21st to end	Night	12% 8% 25% 39% 15% 2	628
March	1st to 10th	Night	13% 8% 24% 38% 15% 3	760
	11th to 20th	Night	8% 7% 21% 39% 21% 3	760
	21st to end	Night	13% 7% 29% 37% 12% 2	836
April	1st to 10th	Night	16% 8% 23% 35% 16% 2	760
	11th to 20th	Night	12% 8% 29% 34% 15% 2	568
	21st to end	Night	12% 7% 20% 38% 19% 3	570
May	1st to 10th	Night	9% 6% 25% 39% 18% 2	570
	11th to 20th	Night	8% 4 19% 35% 29% 5%	561
	21st to end	Night	9% 6% 18% 34% 28% 5%	617
June	1st to 10th	Night	6% 6% 16% 39% 30% 4	570
	11th to 20th	Night	4 9% 21% 39% 25% 2	570
	21st to end	Night	2 3 20% 38% 30% 6%	402
July	1st to 10th	Night	4 2 17% 35% 37% 5%	484
	11th to 20th	Night	4 2 14% 45% 32% 3	552
	21st to end	Night	5% 2 12% 31% 46% 4	624
August	1st to 10th	Night	4 2 18% 38% 36% 3	567
	11th to 20th	Night	2 2 17% 41% 34% 4	564
	21st to end	Night	4 4 15% 36% 36% 6%	627
September	1st to 10th	Night	4 4 13% 35% 36% 8%	738
	11th to 20th	Night	5% 4 16% 34% 32% 8%	760
	21st to end	Night	6% 4 17% 31% 30% 11%	760
October	1st to 10th	Night	8% 4 19% 37% 26% 6%	760
	11th to 20th	Night	10% 4 17% 35% 27% 8%	760
	21st to end	Night	10% 5% 22% 31% 25% 6%	836
November	1st to 10th	Night	13% 8% 18% 31% 25% 6%	764
	11th to 20th	Night	13% 4 21% 38% 17% 7%	760
	21st to end	Night	11% 5% 18% 38% 21% 6%	788
December	1st to 10th	Night	15% 5% 19% 39% 20% 2	950
	11th to 20th	Night	19% 5% 20% 36% 17% 3	904
	21st to end	Night	19% 9% 23% 33% 15% 1	836

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