Great Basin College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Great Basin College clear sky chart from 2005-11-06 to 2026-10-14.

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 6% 4 4 4 4 6% 7% 56%	157549
	Night	4 6% 2 6% 5% 5% 5% 4 7% 8% 49%	151651
Transparency	Day	24% 7% 10% 18% 21% 20%	152407
	Night	28% 8% 12% 18% 19% 14%	147787
Seeing	Day	6% 9% 23% 30% 23% 10%	17237
	Night	7% 4 19% 42% 25% 3	26652

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% 13% 7% 7% 6% 5% 7% 9% 29%	4722
	11th to 20th	Day	2 7% 4 7% 5% 5% 5% 5% 8% 8% 44%	4740
	21st to end	Day	3 6% 3 10% 10% 8% 7% 5% 7% 8% 33%	5382
February	1st to 10th	Day	4 7% 5% 9% 6% 6% 7% 6% 8% 11% 32%	2016
	11th to 20th	Day	3 7% 4 9% 7% 6% 7% 7% 9% 8% 33%	2050
	21st to end	Day	1 6% 3 9% 6% 6% 7% 5% 7% 10% 40%	1730
March	1st to 10th	Day	3 13% 5% 10% 7% 6% 6% 5% 9% 10% 28%	4618
	11th to 20th	Day	2 7% 3 6% 4 5% 5% 4 9% 10% 45%	4975
	21st to end	Day	3 9% 4 11% 8% 7% 6% 5% 6% 8% 34%	5755
April	1st to 10th	Day	4 8% 2 7% 5% 5% 5% 5% 7% 9% 41%	2520
	11th to 20th	Day	4 6% 2 6% 5% 5% 6% 5% 7% 8% 46%	2505
	21st to end	Day	6% 6% 2 6% 4 5% 4 4 6% 8% 49%	2730
May	1st to 10th	Day	3 5% 2 5% 4 4 4 3 5% 6% 58%	7626
	11th to 20th	Day	3 5% 2 5% 4 3 4 3 4 7% 60%	7596
	21st to end	Day	2 3 2 4 4 4 4 4 6% 7% 61%	8649
June	1st to 10th	Day	3 3 2 5% 4 4 5% 4 6% 7% 57%	2940
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 6% 68%	2940
	21st to end	Day	2 2 1 2 2 2 3 2 4 5% 75%	2940
July	1st to 10th	Day	1 1 1 3 2 2 3 3 4 5% 76%	8219
	11th to 20th	Day	2 2 1 3 3 3 4 3 5% 7% 67%	8220
	21st to end	Day	1 1 1 3 2 3 4 3 5% 7% 70%	8737
August	1st to 10th	Day	2 1 1 2 2 2 3 2 4 6% 76%	7549
	11th to 20th	Day	2 2 1 4 2 3 3 3 5% 6% 69%	7451
	21st to end	Day	2 3 1 4 3 4 5% 3 6% 7% 63%	7750
September	1st to 10th	Day	1 2 1 3 2 3 3 2 4 5% 73%	2494
	11th to 20th	Day	2 2 1 4 3 3 4 2 4 6% 68%	2310
	21st to end	Day	3 3 1 4 3 3 3 2 4 4 71%	2310
October	1st to 10th	Day	3 3 1 4 3 3 3 3 4 5% 69%	3710
	11th to 20th	Day	3 5% 1 3 3 3 2 3 4 6% 67%	3320
	21st to end	Day	3 3 1 4 3 4 4 3 6% 9% 60%	3200
November	1st to 10th	Day	4 5% 4 8% 5% 5% 4 4 7% 9% 47%	1854
	11th to 20th	Day	5% 7% 3 5% 4 4 5% 5% 8% 8% 45%	1890
	21st to end	Day	4 7% 2 9% 5% 5% 5% 4 5% 9% 44%	1890
December	1st to 10th	Day	3 4 1 5% 5% 4 3 3 5% 9% 58%	3011
	11th to 20th	Day	4 9% 4 8% 5% 5% 6% 5% 9% 11% 33%	3108
	21st to end	Day	4 8% 4 10% 6% 7% 7% 5% 8% 9% 31%	4092
January	1st to 10th	Night	4 10% 3 11% 7% 7% 8% 6% 9% 11% 25%	8172
	11th to 20th	Night	2 9% 4 8% 5% 6% 5% 5% 12% 10% 34%	8220
	21st to end	Night	3 8% 3 9% 7% 5% 6% 4 6% 8% 42%	8806
February	1st to 10th	Night	4 8% 3 9% 6% 5% 6% 5% 8% 10% 35%	2599
	11th to 20th	Night	3 8% 4 8% 6% 5% 6% 6% 8% 10% 37%	2458
	21st to end	Night	2 8% 4 8% 7% 8% 7% 6% 7% 9% 35%	2076
March	1st to 10th	Night	4 13% 4 10% 6% 6% 5% 5% 9% 10% 30%	5496
	11th to 20th	Night	3 6% 3 6% 5% 5% 6% 5% 9% 10% 43%	5232
	21st to end	Night	4 8% 3 7% 5% 5% 5% 4 7% 10% 42%	5459
April	1st to 10th	Night	4 6% 3 7% 5% 5% 6% 6% 8% 9% 41%	2100
	11th to 20th	Night	4 7% 2 5% 4 4 5% 5% 6% 7% 51%	2062
	21st to end	Night	7% 8% 2 5% 4 3 4 4 6% 5% 52%	1890
May	1st to 10th	Night	5% 5% 1 4 4 3 4 3 5% 7% 59%	5321
	11th to 20th	Night	3 4 2 5% 3 3 4 4 5% 8% 60%	5296
	21st to end	Night	4 5% 2 6% 4 4 5% 4 5% 7% 55%	5537
June	1st to 10th	Night	3 4 1 4 4 4 4 3 5% 8% 61%	1680
	11th to 20th	Night	4 4 3 3 2 3 3 5% 6% 66%	1680
	21st to end	Night	3 2 1 3 3 3 3 3 5% 6% 70%	1680
July	1st to 10th	Night	2 2 1 4 3 3 3 3 4 5% 71%	4740
	11th to 20th	Night	3 3 2 5% 4 3 4 4 6% 7% 58%	4740
	21st to end	Night	3 3 2 6% 4 4 5% 4 6% 6% 56%	5348
August	1st to 10th	Night	2 2 1 3 2 3 4 3 5% 5% 71%	5273
	11th to 20th	Night	3 4 2 6% 3 4 4 3 4 6% 59%	5407
	21st to end	Night	5% 4 1 6% 4 4 4 3 5% 6% 57%	6499
September	1st to 10th	Night	3 2 1 3 3 3 3 3 5% 6% 69%	2126
	11th to 20th	Night	3 3 1 3 2 3 3 3 4 6% 68%	2310
	21st to end	Night	4 3 1 3 2 2 3 2 3 4 73%	2310
October	1st to 10th	Night	3 3 1 3 2 3 3 2 4 4 73%	3710
	11th to 20th	Night	4 4 2 5% 3 3 3 3 4 6% 63%	3882
	21st to end	Night	3 3 1 3 3 3 4 3 5% 8% 64%	4600
November	1st to 10th	Night	6% 6% 2 6% 4 4 4 4 6% 7% 50%	2665
	11th to 20th	Night	4 6% 2 6% 4 4 4 4 7% 8% 51%	2730
	21st to end	Night	3 6% 2 6% 6% 5% 5% 5% 8% 11% 43%	2730
December	1st to 10th	Night	3 5% 2 5% 3 3 4 3 5% 10% 56%	4395
	11th to 20th	Night	5% 8% 3 8% 6% 7% 7% 6% 9% 13% 30%	5316
	21st to end	Night	5% 10% 3 12% 8% 8% 7% 6% 9% 9% 23%	7106

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	47% 12% 20% 14% 7%	4723
	11th to 20th	Day	32% 12% 13% 19% 22% 2	4740
	21st to end	Day	37% 13% 13% 16% 18% 3	5382
February	1st to 10th	Day	41% 11% 17% 21% 7% 3	2016
	11th to 20th	Day	39% 12% 16% 20% 11% 2	2060
	21st to end	Day	34% 10% 20% 23% 9% 4	1730
March	1st to 10th	Day	42% 12% 14% 16% 11% 4	4618
	11th to 20th	Day	28% 9% 17% 30% 13% 2	4975
	21st to end	Day	43% 12% 17% 18% 8% 3	5755
April	1st to 10th	Day	36% 9% 16% 22% 15% 3	2520
	11th to 20th	Day	31% 9% 14% 22% 20% 4	2508
	21st to end	Day	32% 7% 13% 25% 19% 4	2730
May	1st to 10th	Day	22% 7% 11% 25% 25% 10%	7626
	11th to 20th	Day	22% 6% 6% 21% 34% 11%	7599
	21st to end	Day	20% 6% 8% 23% 33% 10%	8650
June	1st to 10th	Day	24% 6% 11% 19% 30% 10%	2940
	11th to 20th	Day	18% 5% 7% 16% 29% 25%	2940
	21st to end	Day	12% 3 5% 14% 31% 36%	2940
July	1st to 10th	Day	11% 3 3 7% 16% 59%	8151
	11th to 20th	Day	14% 4 5% 8% 19% 50%	8145
	21st to end	Day	12% 4 5% 8% 23% 48%	8541
August	1st to 10th	Day	12% 3 4 7% 21% 53%	5978
	11th to 20th	Day	17% 4 7% 13% 21% 38%	5874
	21st to end	Day	15% 4 6% 17% 25% 34%	6077
September	1st to 10th	Day	14% 3 6% 11% 28% 39%	2494
	11th to 20th	Day	16% 5% 7% 15% 31% 26%	2310
	21st to end	Day	17% 3 5% 17% 32% 25%	2310
October	1st to 10th	Day	18% 4 6% 13% 30% 29%	3710
	11th to 20th	Day	20% 3 6% 21% 36% 14%	3320
	21st to end	Day	18% 6% 11% 33% 24% 8%	3200
November	1st to 10th	Day	32% 9% 11% 24% 19% 5%	1854
	11th to 20th	Day	31% 10% 13% 31% 14% 2	1890
	21st to end	Day	37% 6% 15% 24% 16% 3	1890
December	1st to 10th	Day	24% 7% 13% 33% 21% 2	3011
	11th to 20th	Day	39% 13% 18% 21% 9% 1	3108
	21st to end	Day	41% 13% 16% 20% 9%	4092
January	1st to 10th	Night	44% 16% 18% 16% 6%	8172
	11th to 20th	Night	34% 15% 17% 16% 15% 3	8220
	21st to end	Night	30% 14% 13% 18% 23% 2	8806
February	1st to 10th	Night	38% 11% 19% 22% 7% 3	2599
	11th to 20th	Night	37% 11% 17% 22% 13% 2	2459
	21st to end	Night	36% 10% 19% 23% 9% 3	2076
March	1st to 10th	Night	41% 9% 15% 19% 10% 5%	5496
	11th to 20th	Night	27% 10% 16% 28% 17% 2	5232
	21st to end	Night	31% 11% 20% 25% 11% 3	5459
April	1st to 10th	Night	34% 10% 16% 23% 15% 3	2100
	11th to 20th	Night	29% 8% 13% 27% 19% 5%	2062
	21st to end	Night	30% 8% 13% 23% 24% 3	1890
May	1st to 10th	Night	22% 6% 10% 22% 32% 8%	5319
	11th to 20th	Night	20% 6% 7% 20% 35% 13%	5297
	21st to end	Night	24% 7% 8% 19% 30% 11%	5538
June	1st to 10th	Night	21% 6% 13% 20% 31% 9%	1680
	11th to 20th	Night	19% 4 6% 17% 29% 25%	1680
	21st to end	Night	14% 4 6% 11% 32% 33%	1680
July	1st to 10th	Night	16% 3 4 6% 16% 57%	4700
	11th to 20th	Night	20% 5% 7% 9% 17% 42%	4692
	21st to end	Night	23% 5% 5% 9% 17% 41%	5232
August	1st to 10th	Night	16% 4 5% 8% 16% 50%	4168
	11th to 20th	Night	24% 4 7% 12% 19% 34%	4258
	21st to end	Night	22% 4 6% 12% 20% 36%	5092
September	1st to 10th	Night	15% 4 6% 11% 27% 38%	2126
	11th to 20th	Night	17% 4 7% 14% 28% 30%	2310
	21st to end	Night	16% 3 5% 15% 32% 29%	2310
October	1st to 10th	Night	15% 4 5% 12% 33% 30%	3710
	11th to 20th	Night	22% 5% 5% 17% 35% 16%	3882
	21st to end	Night	18% 4 8% 30% 34% 7%	4600
November	1st to 10th	Night	31% 7% 12% 21% 20% 7%	2665
	11th to 20th	Night	27% 8% 15% 25% 22% 3	2730
	21st to end	Night	31% 8% 15% 27% 15% 2	2730
December	1st to 10th	Night	24% 7% 12% 34% 21% 2	4395
	11th to 20th	Night	38% 13% 19% 20% 9% 1	5316
	21st to end	Night	45% 14% 18% 16% 6%	7106

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	11% 10% 24% 31% 20% 4	220
	11th to 20th	Day	8% 6% 15% 34% 24% 14%	220
	21st to end	Day	5% 7% 18% 30% 26% 13%	277
February	1st to 10th	Day	7% 9% 30% 28% 19% 7%	456
	11th to 20th	Day	7% 11% 29% 33% 16% 5%	535
	21st to end	Day	3 12% 30% 34% 17% 4	446
March	1st to 10th	Day	7% 10% 25% 33% 22% 4	540
	11th to 20th	Day	4 12% 25% 30% 22% 6%	540
	21st to end	Day	7% 12% 27% 34% 16% 4	594
April	1st to 10th	Day	9% 12% 22% 27% 22% 8%	540
	11th to 20th	Day	5% 14% 29% 33% 13% 5%	532
	21st to end	Day	10% 11% 23% 30% 21% 5%	540
May	1st to 10th	Day	5% 13% 24% 31% 18% 9%	543
	11th to 20th	Day	9% 9% 28% 31% 15% 8%	568
	21st to end	Day	9% 12% 20% 26% 22% 11%	617
June	1st to 10th	Day	5% 11% 22% 26% 26% 10%	570
	11th to 20th	Day	6% 13% 25% 30% 18% 7%	570
	21st to end	Day	3 6% 24% 33% 21% 12%	570
July	1st to 10th	Day	2 6% 24% 33% 27% 8%	566
	11th to 20th	Day	2 4 20% 35% 28% 11%	564
	21st to end	Day	3 2 19% 30% 32% 14%	619
August	1st to 10th	Day	5% 4 22% 29% 28% 13%	567
	11th to 20th	Day	3 4 22% 28% 29% 15%	557
	21st to end	Day	2 4 20% 28% 28% 16%	627
September	1st to 10th	Day	2 6% 19% 28% 27% 18%	570
	11th to 20th	Day	4 10% 20% 28% 24% 14%	570
	21st to end	Day	5% 6% 19% 28% 28% 14%	570
October	1st to 10th	Day	4 9% 20% 23% 28% 16%	570
	11th to 20th	Day	7% 10% 19% 22% 28% 14%	442
	21st to end	Day	7% 9% 18% 31% 27% 9%	363
November	1st to 10th	Day	5% 12% 21% 28% 26% 8%	330
	11th to 20th	Day	7% 7% 21% 34% 24% 8%	330
	21st to end	Day	8% 8% 16% 33% 26% 8%	330
December	1st to 10th	Day	6% 9% 18% 32% 22% 13%	322
	11th to 20th	Day	5% 8% 23% 26% 27% 10%	220
	21st to end	Day	13% 11% 19% 29% 22% 5%	242
January	1st to 10th	Night	9% 7% 20% 36% 23% 5%	840
	11th to 20th	Night	5% 6% 21% 37% 27% 4	840
	21st to end	Night	9% 5% 17% 41% 23% 5%	924
February	1st to 10th	Night	7% 7% 21% 41% 21% 3	840
	11th to 20th	Night	6% 4 23% 48% 17% 2	824
	21st to end	Night	5% 6% 26% 43% 18% 2	692
March	1st to 10th	Night	7% 5% 24% 43% 18% 3	840
	11th to 20th	Night	6% 5% 23% 42% 24% 2	840
	21st to end	Night	10% 6% 23% 44% 16% 1	924
April	1st to 10th	Night	7% 5% 24% 41% 20% 2	840
	11th to 20th	Night	7% 7% 24% 41% 18% 3	814
	21st to end	Night	10% 4 23% 37% 23% 2	630
May	1st to 10th	Night	6% 7% 26% 39% 20% 2	630
	11th to 20th	Night	9% 5% 22% 35% 26% 4	621
	21st to end	Night	9% 5% 20% 37% 27% 3	578
June	1st to 10th	Night	4 3 23% 40% 28% 3	420
	11th to 20th	Night	5% 3 21% 44% 25% 1	420
	21st to end	Night	4 2 16% 52% 25% 1	420
July	1st to 10th	Night	3 1 11% 53% 30% 1	410
	11th to 20th	Night	2 1 12% 56% 28%	408
	21st to end	Night	4 1 9% 52% 34% 1	528
August	1st to 10th	Night	4 2 14% 48% 31% 1	627
	11th to 20th	Night	4 2 12% 50% 30% 2	662
	21st to end	Night	3 2 11% 44% 37% 2	924
September	1st to 10th	Night	4 2 10% 44% 36% 4	840
	11th to 20th	Night	4 2 14% 39% 34% 6%	840
	21st to end	Night	5% 2 12% 38% 36% 8%	840
October	1st to 10th	Night	6% 2 17% 38% 33% 3	840
	11th to 20th	Night	6% 6% 16% 39% 30% 3	812
	21st to end	Night	6% 3 18% 40% 27% 6%	880
November	1st to 10th	Night	9% 3 16% 40% 28% 4	820
	11th to 20th	Night	7% 4 20% 46% 20% 3	840
	21st to end	Night	5% 5% 20% 40% 25% 5%	840
December	1st to 10th	Night	8% 6% 22% 40% 21% 4	840
	11th to 20th	Night	10% 7% 18% 41% 22% 2	840
	21st to end	Night	13% 10% 23% 38% 15% 2	924

Page generated at 2026-10-11 02:06:33. Version ac07

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