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 2025-06-10.

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% 54%	123115
	Night	4 6% 2 7% 5% 5% 5% 4 6% 8% 48%	123951
Transparency	Day	26% 7% 10% 18% 20% 17%	122864
	Night	29% 9% 12% 19% 19% 13%	123790
Seeing	Day	6% 9% 23% 30% 23% 10%	15435
	Night	7% 4 19% 41% 25% 3	24992

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 10% 6% 15% 7% 6% 6% 5% 7% 8% 26%	3639
	11th to 20th	Day	2 9% 5% 9% 7% 7% 7% 5% 7% 7% 35%	3640
	21st to end	Day	3 7% 3 11% 11% 9% 6% 4 6% 7% 32%	4157
February	1st to 10th	Day	4 7% 5% 10% 6% 6% 7% 5% 8% 11% 31%	1920
	11th to 20th	Day	3 8% 4 9% 7% 6% 7% 6% 9% 8% 33%	1950
	21st to end	Day	1 6% 2 9% 6% 6% 7% 5% 8% 10% 40%	1650
March	1st to 10th	Day	3 13% 5% 10% 6% 6% 6% 5% 9% 10% 27%	4518
	11th to 20th	Day	2 7% 3 6% 4 5% 5% 4 10% 10% 44%	4868
	21st to end	Day	4 9% 4 11% 8% 7% 6% 5% 6% 8% 34%	5631
April	1st to 10th	Day	4 8% 3 8% 6% 5% 5% 5% 7% 9% 40%	2400
	11th to 20th	Day	4 6% 2 6% 5% 5% 6% 5% 6% 8% 46%	2384
	21st to end	Day	6% 6% 2 6% 4 5% 4 4 6% 8% 49%	2600
May	1st to 10th	Day	4 6% 2 5% 4 4 4 3 5% 6% 57%	5880
	11th to 20th	Day	4 5% 2 6% 4 4 4 4 5% 7% 57%	5841
	21st to end	Day	2 3 1 3 3 3 4 3 5% 7% 65%	6637
June	1st to 10th	Day	3 4 2 5% 4 4 5% 4 6% 7% 56%	2789
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 6% 66%	2660
	21st to end	Day	2 2 1 2 2 2 3 2 4 5% 75%	2660
July	1st to 10th	Day	1 1 2 1 2 2 2 3 4 81%	4420
	11th to 20th	Day	2 2 1 3 3 3 4 3 5% 6% 67%	4420
	21st to end	Day	2 2 1 4 3 3 4 4 5% 7% 65%	4683
August	1st to 10th	Day	3 2 1 4 2 3 3 3 4 7% 68%	4092
	11th to 20th	Day	1 1 1 3 2 2 3 2 3 4 78%	3966
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 78%	4180
September	1st to 10th	Day	2 1 1 3 2 2 3 2 4 5% 76%	2256
	11th to 20th	Day	2 2 1 4 3 3 4 2 4 6% 69%	2090
	21st to end	Day	3 3 1 4 3 3 2 2 3 4 72%	2090
October	1st to 10th	Day	3 3 1 4 3 3 3 3 4 5% 69%	3490
	11th to 20th	Day	3 5% 1 3 3 3 2 3 4 6% 68%	3185
	21st to end	Day	3 3 1 3 3 4 4 4 7% 9% 60%	3101
November	1st to 10th	Day	4 5% 4 8% 5% 5% 4 4 7% 8% 46%	1764
	11th to 20th	Day	5% 7% 3 5% 4 4 5% 5% 8% 8% 46%	1800
	21st to end	Day	5% 7% 3 9% 5% 5% 5% 4 5% 9% 43%	1800
December	1st to 10th	Day	3 5% 1 5% 5% 4 3 3 5% 9% 58%	2922
	11th to 20th	Day	4 10% 4 8% 5% 5% 6% 5% 9% 11% 33%	3028
	21st to end	Day	4 8% 4 10% 6% 7% 7% 5% 8% 9% 30%	4004
January	1st to 10th	Night	3 10% 3 12% 7% 7% 8% 6% 8% 11% 26%	6306
	11th to 20th	Night	3 11% 5% 10% 7% 7% 6% 4 7% 9% 32%	6320
	21st to end	Night	4 9% 4 11% 8% 6% 6% 4 5% 6% 37%	6803
February	1st to 10th	Night	4 8% 3 9% 7% 5% 6% 5% 8% 10% 34%	2475
	11th to 20th	Night	3 8% 4 7% 6% 5% 6% 6% 8% 10% 38%	2338
	21st to end	Night	2 8% 4 8% 7% 8% 7% 6% 7% 9% 35%	1980
March	1st to 10th	Night	4 13% 4 10% 6% 6% 5% 5% 9% 10% 29%	5376
	11th to 20th	Night	3 6% 3 6% 5% 5% 6% 5% 9% 11% 42%	5119
	21st to end	Night	4 8% 3 7% 5% 5% 5% 4 7% 10% 42%	5341
April	1st to 10th	Night	5% 6% 3 7% 6% 5% 6% 6% 8% 9% 40%	2000
	11th to 20th	Night	5% 7% 2 5% 4 4 6% 5% 6% 7% 50%	1963
	21st to end	Night	7% 8% 2 5% 4 4 4 4 6% 5% 52%	1800
May	1st to 10th	Night	6% 6% 2 5% 4 3 3 3 4 7% 58%	4100
	11th to 20th	Night	4 5% 2 5% 4 4 4 4 5% 7% 55%	4074
	21st to end	Night	4 4 2 5% 3 4 4 4 5% 7% 59%	4244
June	1st to 10th	Night	3 4 1 4 3 4 5% 3 5% 8% 61%	1595
	11th to 20th	Night	5% 4 1 3 3 3 4 3 5% 6% 65%	1520
	21st to end	Night	3 2 1 3 3 3 3 3 5% 5% 70%	1520
July	1st to 10th	Night	2 1 1 3 2 2 3 2 4 5% 75%	2540
	11th to 20th	Night	3 3 2 6% 4 4 5% 4 6% 8% 55%	2540
	21st to end	Night	3 3 2 5% 4 4 6% 4 7% 6% 55%	2872
August	1st to 10th	Night	2 3 1 4 3 4 5% 4 6% 6% 61%	2847
	11th to 20th	Night	2 2 1 4 2 3 3 3 4 4 72%	2871
	21st to end	Night	3 2 1 3 2 2 3 2 3 4 75%	3498
September	1st to 10th	Night	4 1 1 3 2 2 3 3 5% 6% 71%	1924
	11th to 20th	Night	4 3 2 3 2 3 3 3 4 6% 68%	2090
	21st to end	Night	4 3 1 3 2 2 2 2 3 4 74%	2090
October	1st to 10th	Night	2 3 1 3 2 3 3 2 4 4 74%	3490
	11th to 20th	Night	3 4 2 5% 3 3 3 3 4 6% 64%	3731
	21st to end	Night	3 3 1 3 3 3 4 3 5% 8% 64%	4457
November	1st to 10th	Night	6% 6% 2 6% 4 4 5% 4 6% 7% 50%	2535
	11th to 20th	Night	4 6% 2 6% 4 4 4 4 7% 8% 52%	2600
	21st to end	Night	3 6% 2 7% 6% 5% 5% 5% 7% 10% 42%	2600
December	1st to 10th	Night	3 5% 2 5% 3 3 4 3 5% 10% 56%	4264
	11th to 20th	Night	5% 8% 3 8% 6% 7% 7% 6% 9% 13% 29%	5176
	21st to end	Night	5% 10% 3 12% 8% 8% 7% 6% 9% 9% 22%	6952

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	49% 12% 16% 15% 7%	3639
	11th to 20th	Day	41% 13% 14% 17% 14% 1	3640
	21st to end	Day	39% 15% 12% 14% 16% 4	4157
February	1st to 10th	Day	42% 12% 18% 19% 7% 3	1920
	11th to 20th	Day	40% 11% 17% 20% 11% 2	1960
	21st to end	Day	33% 10% 20% 23% 9% 4	1650
March	1st to 10th	Day	43% 12% 14% 16% 11% 4	4518
	11th to 20th	Day	29% 9% 17% 30% 13% 1	4868
	21st to end	Day	44% 12% 17% 17% 8% 2	5631
April	1st to 10th	Day	37% 10% 16% 22% 13% 3	2400
	11th to 20th	Day	31% 9% 14% 22% 20% 4	2387
	21st to end	Day	32% 7% 14% 26% 18% 3	2600
May	1st to 10th	Day	23% 8% 12% 26% 24% 6%	5880
	11th to 20th	Day	23% 7% 7% 23% 33% 7%	5844
	21st to end	Day	17% 6% 8% 24% 35% 9%	6638
June	1st to 10th	Day	24% 6% 12% 20% 29% 9%	2789
	11th to 20th	Day	19% 5% 8% 17% 30% 22%	2660
	21st to end	Day	12% 3 5% 14% 29% 37%	2660
July	1st to 10th	Day	9% 2 3 7% 13% 66%	4351
	11th to 20th	Day	14% 4 4 7% 17% 53%	4345
	21st to end	Day	15% 5% 6% 9% 25% 41%	4535
August	1st to 10th	Day	16% 3 5% 8% 19% 49%	4078
	11th to 20th	Day	11% 3 4 7% 21% 55%	4005
	21st to end	Day	11% 2 4 10% 26% 47%	4179
September	1st to 10th	Day	12% 3 6% 10% 28% 41%	2256
	11th to 20th	Day	16% 5% 7% 16% 29% 27%	2090
	21st to end	Day	17% 3 5% 16% 31% 26%	2090
October	1st to 10th	Day	19% 4 6% 13% 30% 29%	3490
	11th to 20th	Day	19% 3 6% 21% 36% 15%	3185
	21st to end	Day	18% 6% 11% 32% 24% 8%	3101
November	1st to 10th	Day	33% 9% 11% 24% 19% 5%	1764
	11th to 20th	Day	31% 9% 13% 31% 14% 2	1800
	21st to end	Day	38% 6% 15% 23% 16% 3	1800
December	1st to 10th	Day	24% 7% 12% 33% 22% 2	2922
	11th to 20th	Day	39% 13% 18% 21% 9%	3028
	21st to end	Day	41% 13% 16% 20% 9%	4004
January	1st to 10th	Night	45% 13% 18% 17% 8%	6306
	11th to 20th	Night	42% 14% 14% 15% 13% 3	6320
	21st to end	Night	35% 16% 11% 16% 20% 2	6803
February	1st to 10th	Night	38% 11% 19% 21% 7% 3	2475
	11th to 20th	Night	35% 11% 17% 22% 13% 2	2339
	21st to end	Night	37% 10% 19% 22% 9% 3	1980
March	1st to 10th	Night	41% 9% 15% 19% 10% 6%	5376
	11th to 20th	Night	28% 10% 17% 28% 17% 1	5119
	21st to end	Night	31% 11% 20% 25% 10% 2	5341
April	1st to 10th	Night	34% 11% 16% 23% 14% 3	2000
	11th to 20th	Night	29% 8% 13% 27% 18% 5%	1963
	21st to end	Night	30% 8% 14% 23% 23% 3	1800
May	1st to 10th	Night	24% 7% 11% 25% 28% 5%	4097
	11th to 20th	Night	24% 7% 8% 24% 30% 7%	4075
	21st to end	Night	22% 7% 9% 20% 31% 12%	4244
June	1st to 10th	Night	21% 6% 14% 21% 31% 7%	1595
	11th to 20th	Night	20% 5% 7% 18% 29% 21%	1520
	21st to end	Night	14% 4 7% 11% 29% 35%	1520
July	1st to 10th	Night	12% 3 5% 6% 12% 62%	2500
	11th to 20th	Night	23% 5% 6% 8% 17% 41%	2492
	21st to end	Night	24% 5% 5% 10% 17% 39%	2782
August	1st to 10th	Night	19% 5% 6% 11% 18% 42%	2838
	11th to 20th	Night	15% 3 5% 7% 20% 49%	2898
	21st to end	Night	14% 2 4 9% 20% 51%	3498
September	1st to 10th	Night	14% 3 6% 10% 26% 40%	1924
	11th to 20th	Night	18% 4 7% 14% 27% 31%	2090
	21st to end	Night	15% 3 5% 14% 32% 30%	2090
October	1st to 10th	Night	15% 4 5% 12% 33% 31%	3490
	11th to 20th	Night	22% 5% 5% 17% 35% 17%	3731
	21st to end	Night	18% 4 8% 30% 34% 7%	4457
November	1st to 10th	Night	32% 8% 12% 20% 20% 8%	2535
	11th to 20th	Night	27% 8% 14% 25% 23% 3	2600
	21st to end	Night	32% 8% 15% 26% 16% 3	2600
December	1st to 10th	Night	23% 7% 11% 34% 22% 2	4264
	11th to 20th	Night	38% 13% 20% 20% 9% 1	5176
	21st to end	Night	45% 14% 18% 16% 6%	6952

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% 11% 24% 30% 21% 5%	200
	11th to 20th	Day	9% 7% 16% 34% 23% 12%	200
	21st to end	Day	6% 8% 18% 31% 24% 13%	252
February	1st to 10th	Day	7% 10% 31% 28% 18% 6%	420
	11th to 20th	Day	6% 10% 29% 32% 17% 5%	495
	21st to end	Day	3 11% 29% 34% 18% 4	414
March	1st to 10th	Day	7% 9% 23% 34% 22% 4	500
	11th to 20th	Day	4 12% 25% 30% 23% 6%	500
	21st to end	Day	7% 12% 26% 35% 16% 4	550
April	1st to 10th	Day	9% 12% 23% 28% 22% 6%	500
	11th to 20th	Day	5% 13% 28% 34% 13% 6%	492
	21st to end	Day	10% 10% 23% 31% 21% 5%	500
May	1st to 10th	Day	5% 13% 25% 30% 17% 9%	503
	11th to 20th	Day	10% 8% 28% 30% 15% 8%	528
	21st to end	Day	10% 12% 20% 26% 22% 11%	573
June	1st to 10th	Day	5% 11% 22% 26% 26% 11%	527
	11th to 20th	Day	7% 13% 26% 30% 18% 6%	490
	21st to end	Day	3 7% 25% 34% 20% 12%	490
July	1st to 10th	Day	3 5% 24% 33% 26% 8%	486
	11th to 20th	Day	2 4 22% 35% 26% 11%	484
	21st to end	Day	3 2 19% 30% 32% 13%	531
August	1st to 10th	Day	6% 4 22% 29% 29% 11%	487
	11th to 20th	Day	3 4 23% 27% 29% 15%	477
	21st to end	Day	2 4 21% 29% 29% 15%	539
September	1st to 10th	Day	2 6% 18% 28% 27% 20%	490
	11th to 20th	Day	4 10% 19% 28% 25% 14%	490
	21st to end	Day	6% 6% 20% 29% 27% 12%	490
October	1st to 10th	Day	3 9% 22% 23% 27% 16%	490
	11th to 20th	Day	5% 10% 19% 22% 29% 15%	394
	21st to end	Day	7% 8% 18% 32% 26% 8%	330
November	1st to 10th	Day	6% 11% 21% 29% 26% 8%	300
	11th to 20th	Day	8% 6% 22% 34% 22% 8%	300
	21st to end	Day	9% 9% 15% 32% 26% 9%	300
December	1st to 10th	Day	6% 9% 17% 31% 23% 14%	293
	11th to 20th	Day	4 8% 23% 29% 28% 9%	200
	21st to end	Day	12% 10% 20% 32% 21% 5%	220
January	1st to 10th	Night	9% 7% 20% 35% 24% 5%	800
	11th to 20th	Night	5% 7% 22% 37% 25% 4	800
	21st to end	Night	9% 5% 17% 39% 24% 6%	880
February	1st to 10th	Night	7% 8% 21% 41% 20% 3	800
	11th to 20th	Night	6% 3 23% 48% 17% 2	784
	21st to end	Night	5% 6% 26% 42% 19% 2	660
March	1st to 10th	Night	7% 5% 23% 43% 18% 3	800
	11th to 20th	Night	6% 5% 22% 41% 24% 2	800
	21st to end	Night	10% 6% 23% 43% 17% 1	880
April	1st to 10th	Night	8% 5% 24% 41% 20% 2	800
	11th to 20th	Night	7% 7% 24% 40% 19% 3	775
	21st to end	Night	10% 4 24% 36% 23% 3	600
May	1st to 10th	Night	6% 7% 26% 38% 20% 3	600
	11th to 20th	Night	9% 5% 21% 34% 27% 4	591
	21st to end	Night	9% 5% 20% 36% 27% 3	550
June	1st to 10th	Night	4 3 22% 40% 29% 3	399
	11th to 20th	Night	6% 3 22% 42% 26% 1	380
	21st to end	Night	3 2 16% 52% 26% 1	380
July	1st to 10th	Night	4 1 10% 52% 32% 1	370
	11th to 20th	Night	3 1 13% 54% 29% 1	368
	21st to end	Night	3 1 9% 50% 35% 1	478
August	1st to 10th	Night	5% 2 13% 47% 32% 1	567
	11th to 20th	Night	3 2 11% 51% 31% 2	598
	21st to end	Night	2 2 11% 44% 37% 3	836
September	1st to 10th	Night	4 2 8% 43% 37% 4	760
	11th to 20th	Night	5% 2 12% 38% 36% 7%	760
	21st to end	Night	5% 2 11% 37% 36% 8%	760
October	1st to 10th	Night	6% 2 17% 39% 33% 4	760
	11th to 20th	Night	4 6% 15% 40% 32% 4	760
	21st to end	Night	6% 3 18% 40% 26% 6%	836
November	1st to 10th	Night	9% 3 16% 39% 28% 4	780
	11th to 20th	Night	7% 4 20% 46% 20% 4	800
	21st to end	Night	5% 5% 21% 39% 26% 5%	800
December	1st to 10th	Night	8% 6% 21% 39% 22% 4	800
	11th to 20th	Night	10% 7% 19% 42% 21% 2	800
	21st to end	Night	13% 9% 24% 38% 15% 2	880

Page generated at 2025-06-07 07:28:18. Version ac07

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