Great Basin National Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Great Basin National Park clear sky chart from 2005-11-06 to 2025-07-06.

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	2 4 2 4 4 4 5% 4 6% 8% 58%	124858
	Night	2 4 2 4 3 4 4 4 6% 8% 60%	122892
Transparency	Day	21% 6% 9% 19% 27% 18%	124618
	Night	20% 6% 10% 21% 27% 16%	122720
Seeing	Day	8% 16% 30% 28% 15% 4	15939
	Night	6% 9% 25% 34% 21% 5%	25574

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	4 3 5% 4 4 5% 4 8% 14% 49%	4098
	11th to 20th	Day	3 1 4 4 4 4 4 7% 8% 61%	4100
	21st to end	Day	2 6% 3 7% 3 4 4 4 5% 8% 53%	4510
February	1st to 10th	Day	1 5% 2 6% 4 4 4 5% 7% 10% 50%	1880
	11th to 20th	Day	2 7% 1 6% 4 4 5% 4 7% 9% 50%	1950
	21st to end	Day	1 7% 3 4 4 4 4 4 6% 9% 53%	1650
March	1st to 10th	Day	1 9% 3 7% 6% 5% 5% 4 6% 7% 48%	4579
	11th to 20th	Day	1 9% 3 5% 4 5% 5% 5% 6% 8% 49%	5000
	21st to end	Day	1 6% 2 5% 4 5% 7% 6% 9% 11% 44%	5486
April	1st to 10th	Day	2 6% 2 6% 5% 5% 4 6% 7% 7% 52%	2321
	11th to 20th	Day	2 6% 2 4 5% 5% 4 4 6% 8% 53%	2432
	21st to end	Day	3 6% 2 5% 4 4 5% 4 6% 8% 53%	2600
May	1st to 10th	Day	4 5% 1 4 5% 4 5% 4 6% 6% 55%	5880
	11th to 20th	Day	2 4 2 5% 4 4 5% 5% 6% 8% 55%	5841
	21st to end	Day	3 4 1 4 4 5% 6% 4 6% 8% 54%	6423
June	1st to 10th	Day	1 3 1 4 5% 4 5% 5% 7% 8% 56%	2600
	11th to 20th	Day	2 3 1 3 3 3 3 3 4 6% 68%	2731
	21st to end	Day	2 2 1 3 4 3 4 3 6% 5% 67%	2800
July	1st to 10th	Day	2 2 1 3 2 3 2 2 3 3 76%	4491
	11th to 20th	Day	2 3 1 4 4 5% 6% 6% 8% 9% 52%	4174
	21st to end	Day	3 3 1 4 5% 6% 6% 5% 7% 8% 52%	4454
August	1st to 10th	Day	1 2 1 3 3 4 4 4 5% 7% 65%	4092
	11th to 20th	Day	2 1 1 3 3 3 3 3 3 4 74%	4028
	21st to end	Day	2 2 1 3 2 3 3 2 4 5% 73%	4221
September	1st to 10th	Day	3 3 1 4 3 3 3 3 4 5% 68%	2168
	11th to 20th	Day	1 3 1 4 2 3 3 2 4 6% 70%	2090
	21st to end	Day	2 3 3 3 2 2 2 3 5% 74%	2090
October	1st to 10th	Day	2 2 1 3 3 3 4 3 4 6% 69%	3490
	11th to 20th	Day	2 3 1 2 2 3 3 3 5% 6% 71%	3215
	21st to end	Day	1 2 1 2 2 2 4 4 4 5% 72%	3231
November	1st to 10th	Day	1 5% 1 3 3 4 4 3 6% 9% 60%	1764
	11th to 20th	Day	1 2 1 3 3 3 4 4 7% 8% 64%	1800
	21st to end	Day	2 4 2 5% 4 4 5% 4 5% 7% 58%	1800
December	1st to 10th	Day	1 3 1 3 3 3 3 3 6% 9% 66%	2950
	11th to 20th	Day	1 5% 3 5% 4 3 4 4 8% 12% 51%	3409
	21st to end	Day	3 2 6% 6% 7% 7% 6% 8% 11% 44%	4510
January	1st to 10th	Night	4 2 4 3 4 4 5% 7% 11% 56%	5867
	11th to 20th	Night	2 1 5% 3 3 4 4 8% 11% 58%	5880
	21st to end	Night	1 6% 3 6% 5% 3 3 3 5% 8% 58%	6468
February	1st to 10th	Night	1 4 2 5% 4 4 4 4 7% 10% 56%	2515
	11th to 20th	Night	1 5% 2 5% 3 5% 5% 5% 7% 9% 53%	2338
	21st to end	Night	1 4 2 5% 4 4 5% 4 6% 9% 55%	1980
March	1st to 10th	Night	1 7% 3 6% 4 5% 6% 5% 5% 6% 52%	5300
	11th to 20th	Night	3 9% 3 6% 4 4 4 7% 7% 9% 45%	5000
	21st to end	Night	2 4 2 4 3 4 5% 5% 6% 10% 56%	5489
April	1st to 10th	Night	3 3 2 4 5% 5% 5% 5% 6% 8% 55%	2075
	11th to 20th	Night	3 6% 2 3 3 4 5% 5% 6% 8% 57%	1915
	21st to end	Night	4 3 2 3 3 5% 5% 5% 6% 8% 57%	1800
May	1st to 10th	Night	5% 6% 1 4 4 4 4 4 6% 8% 54%	4100
	11th to 20th	Night	4 4 1 5% 3 3 4 4 5% 7% 60%	4074
	21st to end	Night	3 4 1 5% 4 4 5% 4 6% 7% 57%	4474
June	1st to 10th	Night	3 4 1 3 4 4 4 4 5% 9% 59%	1800
	11th to 20th	Night	2 3 1 4 3 3 3 2 3 6% 69%	1660
	21st to end	Night	2 2 1 4 3 2 3 3 6% 7% 67%	1600
July	1st to 10th	Night	1 1 1 3 2 2 2 3 3 4 78%	2580
	11th to 20th	Night	4 4 2 5% 5% 6% 6% 6% 7% 8% 46%	2786
	21st to end	Night	3 3 1 5% 5% 7% 8% 5% 7% 8% 49%	3109
August	1st to 10th	Night	3 3 1 5% 3 4 4 3 5% 7% 60%	2847
	11th to 20th	Night	1 2 2 2 3 3 2 4 5% 76%	2811
	21st to end	Night	2 1 2 2 2 3 2 3 5% 78%	3453
September	1st to 10th	Night	3 3 1 3 3 2 4 3 5% 4 70%	2010
	11th to 20th	Night	2 3 1 3 2 3 4 3 5% 6% 69%	2090
	21st to end	Night	2 2 1 2 2 2 2 2 3 5% 78%	2090
October	1st to 10th	Night	3 2 1 3 2 2 2 2 4 5% 74%	3490
	11th to 20th	Night	3 3 1 3 3 3 3 3 4 6% 69%	3713
	21st to end	Night	2 3 1 3 2 4 3 3 5% 5% 69%	4324
November	1st to 10th	Night	1 4 2 3 2 3 3 3 6% 8% 65%	2535
	11th to 20th	Night	2 3 1 3 2 3 3 3 6% 7% 67%	2600
	21st to end	Night	2 4 1 4 3 3 5% 4 7% 9% 58%	2600
December	1st to 10th	Night	1 2 1 3 2 3 4 3 4 7% 69%	4240
	11th to 20th	Night	1 3 2 4 3 4 4 4 7% 11% 59%	4811
	21st to end	Night	1 4 2 6% 5% 5% 5% 5% 8% 11% 47%	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	21% 8% 20% 31% 18% 2	4099
	11th to 20th	Day	22% 8% 13% 23% 25% 9%	4100
	21st to end	Day	28% 6% 14% 21% 25% 6%	4510
February	1st to 10th	Day	27% 10% 14% 24% 17% 8%	1880
	11th to 20th	Day	28% 8% 14% 22% 22% 7%	1960
	21st to end	Day	26% 8% 14% 24% 22% 6%	1650
March	1st to 10th	Day	32% 9% 6% 20% 19% 14%	4579
	11th to 20th	Day	25% 6% 8% 30% 22% 8%	5000
	21st to end	Day	26% 10% 15% 27% 20% 3	5486
April	1st to 10th	Day	26% 9% 11% 24% 21% 8%	2321
	11th to 20th	Day	26% 8% 9% 24% 25% 8%	2435
	21st to end	Day	25% 8% 9% 25% 28% 5%	2600
May	1st to 10th	Day	23% 6% 9% 27% 29% 7%	5880
	11th to 20th	Day	20% 7% 7% 18% 38% 9%	5844
	21st to end	Day	20% 8% 6% 16% 39% 10%	6424
June	1st to 10th	Day	20% 7% 9% 16% 33% 14%	2600
	11th to 20th	Day	17% 5% 5% 12% 29% 33%	2731
	21st to end	Day	15% 5% 5% 10% 25% 40%	2800
July	1st to 10th	Day	14% 3 3 4 10% 66%	4422
	11th to 20th	Day	21% 7% 5% 9% 23% 36%	4102
	21st to end	Day	21% 6% 7% 13% 18% 35%	4314
August	1st to 10th	Day	15% 4 5% 10% 32% 33%	4078
	11th to 20th	Day	13% 4 5% 8% 23% 47%	4067
	21st to end	Day	12% 4 4 8% 27% 45%	4220
September	1st to 10th	Day	18% 5% 5% 9% 28% 34%	2168
	11th to 20th	Day	15% 5% 5% 17% 29% 29%	2090
	21st to end	Day	14% 4 4 17% 33% 28%	2090
October	1st to 10th	Day	14% 4 5% 9% 35% 34%	3490
	11th to 20th	Day	12% 3 4 20% 38% 23%	3215
	21st to end	Day	13% 3 5% 27% 41% 12%	3231
November	1st to 10th	Day	20% 6% 12% 25% 27% 10%	1764
	11th to 20th	Day	15% 9% 9% 33% 29% 6%	1800
	21st to end	Day	25% 7% 10% 23% 25% 9%	1800
December	1st to 10th	Day	16% 6% 8% 31% 33% 5%	2950
	11th to 20th	Day	25% 6% 16% 28% 22% 4	3408
	21st to end	Day	26% 8% 18% 23% 21% 4	4510
January	1st to 10th	Night	19% 7% 18% 29% 25% 2	5867
	11th to 20th	Night	19% 8% 16% 25% 23% 10%	5880
	21st to end	Night	27% 6% 13% 18% 27% 8%	6468
February	1st to 10th	Night	23% 8% 16% 28% 17% 7%	2515
	11th to 20th	Night	26% 7% 13% 22% 24% 8%	2339
	21st to end	Night	25% 9% 14% 21% 22% 8%	1980
March	1st to 10th	Night	28% 9% 8% 18% 21% 15%	5300
	11th to 20th	Night	26% 7% 10% 28% 24% 5%	5000
	21st to end	Night	20% 6% 14% 26% 29% 4	5489
April	1st to 10th	Night	24% 9% 10% 27% 21% 9%	2075
	11th to 20th	Night	22% 7% 10% 26% 28% 6%	1915
	21st to end	Night	21% 8% 12% 24% 29% 6%	1800
May	1st to 10th	Night	24% 8% 9% 24% 25% 10%	4097
	11th to 20th	Night	19% 6% 7% 21% 37% 10%	4075
	21st to end	Night	20% 7% 8% 18% 36% 11%	4474
June	1st to 10th	Night	19% 8% 9% 16% 35% 13%	1800
	11th to 20th	Night	18% 4 5% 12% 28% 34%	1660
	21st to end	Night	15% 5% 5% 10% 25% 40%	1600
July	1st to 10th	Night	11% 4 3 5% 10% 67%	2540
	11th to 20th	Night	27% 7% 4 11% 21% 30%	2735
	21st to end	Night	26% 5% 9% 13% 16% 30%	3011
August	1st to 10th	Night	20% 5% 5% 10% 25% 34%	2838
	11th to 20th	Night	11% 3 5% 9% 24% 48%	2838
	21st to end	Night	10% 3 4 9% 23% 51%	3453
September	1st to 10th	Night	16% 4 7% 13% 27% 33%	2010
	11th to 20th	Night	16% 5% 8% 15% 26% 30%	2090
	21st to end	Night	11% 4 5% 15% 36% 29%	2090
October	1st to 10th	Night	13% 3 4 10% 36% 34%	3490
	11th to 20th	Night	16% 4 4 16% 36% 24%	3713
	21st to end	Night	16% 4 4 21% 41% 15%	4324
November	1st to 10th	Night	18% 7% 11% 28% 25% 11%	2535
	11th to 20th	Night	15% 7% 10% 30% 30% 8%	2600
	21st to end	Night	21% 7% 9% 26% 29% 8%	2600
December	1st to 10th	Night	16% 5% 9% 29% 33% 7%	4240
	11th to 20th	Night	20% 6% 14% 30% 25% 4	4811
	21st to end	Night	27% 8% 15% 24% 21% 4	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	12% 17% 34% 24% 11% 1	300
	11th to 20th	Day	8% 18% 31% 23% 15% 5%	300
	21st to end	Day	5% 14% 31% 25% 17% 7%	330
February	1st to 10th	Day	12% 19% 33% 19% 15% 2	380
	11th to 20th	Day	11% 19% 34% 23% 11% 1	495
	21st to end	Day	6% 21% 41% 22% 10%	414
March	1st to 10th	Day	10% 21% 30% 25% 11% 3	500
	11th to 20th	Day	11% 20% 33% 21% 12% 3	500
	21st to end	Day	12% 23% 35% 23% 7%	550
April	1st to 10th	Day	9% 26% 26% 24% 11% 3	500
	11th to 20th	Day	12% 25% 33% 21% 7% 2	492
	21st to end	Day	14% 24% 29% 20% 10% 3	500
May	1st to 10th	Day	13% 21% 33% 23% 8% 1	503
	11th to 20th	Day	10% 19% 27% 26% 14% 3	528
	21st to end	Day	11% 17% 26% 26% 16% 3	573
June	1st to 10th	Day	4 12% 30% 32% 18% 4	530
	11th to 20th	Day	8% 18% 35% 25% 12% 2	530
	21st to end	Day	5% 15% 32% 30% 15% 2	530
July	1st to 10th	Day	5% 7% 29% 38% 17% 3	507
	11th to 20th	Day	5% 5% 27% 39% 18% 6%	484
	21st to end	Day	6% 5% 23% 39% 22% 6%	531
August	1st to 10th	Day	5% 5% 31% 37% 17% 5%	487
	11th to 20th	Day	5% 9% 26% 32% 21% 6%	477
	21st to end	Day	4 8% 25% 36% 20% 6%	539
September	1st to 10th	Day	4 8% 31% 28% 20% 9%	490
	11th to 20th	Day	6% 14% 30% 26% 17% 7%	490
	21st to end	Day	5% 13% 25% 31% 21% 5%	490
October	1st to 10th	Day	6% 16% 29% 26% 16% 8%	490
	11th to 20th	Day	4 15% 27% 29% 18% 8%	339
	21st to end	Day	10% 11% 29% 29% 17% 4	330
November	1st to 10th	Day	9% 16% 28% 22% 18% 7%	300
	11th to 20th	Day	7% 15% 28% 30% 14% 6%	300
	21st to end	Day	7% 15% 26% 26% 19% 7%	300
December	1st to 10th	Day	6% 13% 28% 33% 14% 5%	300
	11th to 20th	Day	6% 16% 32% 30% 13% 3	300
	21st to end	Day	10% 20% 29% 29% 9% 3	330
January	1st to 10th	Night	7% 9% 31% 29% 17% 8%	800
	11th to 20th	Night	3 11% 29% 31% 21% 5%	800
	21st to end	Night	6% 9% 30% 30% 17% 7%	880
February	1st to 10th	Night	8% 10% 32% 32% 14% 5%	800
	11th to 20th	Night	6% 10% 33% 34% 14% 3	784
	21st to end	Night	4 12% 30% 37% 14% 2	660
March	1st to 10th	Night	6% 13% 30% 34% 15% 3	800
	11th to 20th	Night	8% 11% 27% 33% 20% 3	800
	21st to end	Night	6% 14% 34% 32% 13% 2	880
April	1st to 10th	Night	6% 13% 30% 33% 15% 3	800
	11th to 20th	Night	7% 14% 31% 31% 15% 2	727
	21st to end	Night	9% 12% 29% 31% 17% 3	600
May	1st to 10th	Night	9% 11% 29% 34% 16% 2	600
	11th to 20th	Night	8% 8% 24% 36% 21% 4	591
	21st to end	Night	11% 9% 25% 29% 22% 3	650
June	1st to 10th	Night	5% 10% 25% 33% 24% 4	600
	11th to 20th	Night	4 13% 28% 35% 18% 2	460
	21st to end	Night	3 6% 21% 42% 26% 3	400
July	1st to 10th	Night	3 2 16% 42% 32% 4	380
	11th to 20th	Night	6% 3 18% 41% 27% 5%	507
	21st to end	Night	7% 2 15% 40% 33% 3	624
August	1st to 10th	Night	6% 1 21% 45% 25% 2	567
	11th to 20th	Night	4 5% 16% 41% 32% 3	564
	21st to end	Night	4 4 15% 37% 33% 7%	804
September	1st to 10th	Night	4 3 15% 35% 34% 9%	760
	11th to 20th	Night	5% 7% 16% 32% 31% 10%	760
	21st to end	Night	3 5% 18% 30% 33% 12%	760
October	1st to 10th	Night	7% 6% 19% 36% 25% 8%	760
	11th to 20th	Night	6% 6% 18% 34% 27% 9%	760
	21st to end	Night	4 7% 25% 31% 24% 8%	836
November	1st to 10th	Night	5% 10% 19% 35% 23% 8%	780
	11th to 20th	Night	5% 9% 24% 36% 19% 8%	800
	21st to end	Night	6% 9% 27% 26% 24% 8%	800
December	1st to 10th	Night	4 8% 32% 32% 18% 6%	800
	11th to 20th	Night	5% 9% 33% 33% 17% 3	800
	21st to end	Night	6% 13% 34% 34% 11% 2	880

Page generated at 2025-07-02 22:30:16. Version ac07

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