Pahrump Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pahrump Valley Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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 2 1 2 2 2 2 2 4 5% 75%	118888
	Night	3 2 1 2 2 2 2 2 4 5% 74%	118995
Transparency	Day	7% 40% 3 8% 16% 25%	118646
	Night	8% 39% 3 9% 17% 24%	118827
Seeing	Day	4 11% 25% 33% 20% 7%	16078
	Night	4 6% 19% 36% 29% 7%	25941

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 2 1 2 2 2 3 2 4 7% 72%	4098
	11th to 20th	Day	3 2 1 3 2 2 2 2 4 6% 74%	4100
	21st to end	Day	6% 5% 2 5% 3 3 3 3 5% 6% 58%	4760
February	1st to 10th	Day	7% 3 1 3 2 2 2 2 5% 6% 67%	2000
	11th to 20th	Day	6% 3 2 2 3 4 3 4 7% 67%	1950
	21st to end	Day	6% 3 1 3 3 3 2 4 4 6% 65%	1650
March	1st to 10th	Day	7% 4 1 4 3 2 3 3 5% 6% 62%	4518
	11th to 20th	Day	4 3 1 3 3 3 3 3 6% 7% 63%	4885
	21st to end	Day	4 2 1 3 2 3 3 3 6% 8% 65%	5590
April	1st to 10th	Day	2 3 1 3 2 3 3 3 5% 6% 70%	2400
	11th to 20th	Day	2 2 1 2 2 2 3 2 4 6% 75%	2369
	21st to end	Day	2 2 2 2 2 3 3 3 5% 77%	2495
May	1st to 10th	Day	1 1 1 2 2 1 2 2 3 5% 80%	4240
	11th to 20th	Day	1 1 1 1 2 2 2 3 5% 82%	4073
	21st to end	Day	1 1 1 1 1 1 1 2 3 4 84%	4477
June	1st to 10th	Day	1 1 1 2 2 1 2 2 3 5% 81%	2470
	11th to 20th	Day	1 1 1 1 1 1 3 3 87%	2533
	21st to end	Day	1 1 1 1 2 2 2 2 4 85%	2660
July	1st to 10th	Day	1 1 1 1 1 2 1 2 3 86%	4420
	11th to 20th	Day	2 1 1 2 2 2 2 2 4 5% 76%	4120
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 78%	4454
August	1st to 10th	Day	2 1 2 2 2 2 2 4 5% 79%	4014
	11th to 20th	Day	1 1 1 1 1 1 2 4 87%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4180
September	1st to 10th	Day	3 1 1 2 2 2 2 2 3 4 78%	2232
	11th to 20th	Day	1 1 1 1 2 1 2 3 86%	2090
	21st to end	Day	1 1 2 1 1 1 2 3 4 85%	2090
October	1st to 10th	Day	1 1 1 1 1 1 1 3 3 85%	3490
	11th to 20th	Day	1 1 1 2 1 1 2 1 2 4 85%	3442
	21st to end	Day	1 1 2 1 1 2 2 3 5% 81%	3131
November	1st to 10th	Day	2 3 1 1 2 2 1 2 4 6% 77%	1764
	11th to 20th	Day	2 2 1 2 1 2 2 1 3 5% 79%	1800
	21st to end	Day	4 2 1 3 2 2 3 3 4 6% 72%	1800
December	1st to 10th	Day	2 1 1 2 2 2 3 3 6% 6% 69%	2950
	11th to 20th	Day	5% 3 1 4 3 3 4 3 6% 10% 58%	3409
	21st to end	Day	3 3 1 4 3 3 5% 5% 7% 9% 58%	4510
January	1st to 10th	Night	3 3 1 3 2 2 2 2 4 6% 72%	5867
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 6% 75%	5880
	21st to end	Night	9% 4 1 3 2 2 3 3 5% 6% 62%	6226
February	1st to 10th	Night	6% 5% 1 3 3 2 2 3 4 5% 64%	2400
	11th to 20th	Night	5% 4 1 3 3 3 3 2 4 6% 67%	2338
	21st to end	Night	5% 4 1 3 3 2 2 2 5% 6% 67%	1980
March	1st to 10th	Night	6% 4 1 3 2 2 3 3 5% 7% 65%	5376
	11th to 20th	Night	4 3 1 3 3 4 4 4 6% 8% 60%	5114
	21st to end	Night	4 2 1 2 2 3 3 3 4 6% 70%	5368
April	1st to 10th	Night	2 2 1 3 3 3 3 3 3 6% 73%	2000
	11th to 20th	Night	2 2 1 2 2 2 3 3 5% 5% 74%	1979
	21st to end	Night	2 1 1 1 2 2 2 4 5% 78%	1903
May	1st to 10th	Night	3 1 1 2 1 1 2 2 3 5% 80%	2950
	11th to 20th	Night	1 1 1 1 2 2 3 3 85%	2838
	21st to end	Night	1 1 1 1 1 2 1 3 4 85%	3110
June	1st to 10th	Night	1 1 1 2 1 1 1 3 5% 83%	1710
	11th to 20th	Night	1 1 1 1 1 1 1 3 3 85%	1637
	21st to end	Night	1 1 1 1 1 2 2 3 88%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 2 90%	2540
	11th to 20th	Night	1 1 1 2 2 2 2 3 3 5% 79%	2828
	21st to end	Night	1 2 1 2 2 2 3 2 5% 6% 74%	3109
August	1st to 10th	Night	1 2 2 2 2 2 2 3 4 79%	2910
	11th to 20th	Night	1 1 1 1 1 1 3 4 86%	3126
	21st to end	Night	1 1 1 1 1 1 1 2 2 89%	3498
September	1st to 10th	Night	1 2 1 2 2 2 2 2 3 4 78%	1947
	11th to 20th	Night	1 2 1 2 2 2 3 3 86%	2090
	21st to end	Night	1 1 1 1 1 1 1 2 2 88%	2090
October	1st to 10th	Night	1 1 1 1 1 1 1 2 4 86%	3490
	11th to 20th	Night	1 1 1 1 2 2 3 4 86%	3494
	21st to end	Night	1 1 2 1 1 2 2 4 4 82%	4423
November	1st to 10th	Night	2 2 1 2 2 2 2 1 3 5% 78%	2535
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 5% 77%	2600
	21st to end	Night	3 3 1 3 2 2 3 2 4 5% 72%	2600
December	1st to 10th	Night	3 2 2 2 2 3 3 5% 7% 70%	4240
	11th to 20th	Night	5% 2 1 3 2 3 3 3 5% 9% 65%	4811
	21st to end	Night	4 3 1 4 4 4 4 3 6% 10% 57%	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	9% 27% 4 24% 25% 11%	4099
	11th to 20th	Day	11% 28% 7% 14% 16% 24%	4100
	21st to end	Day	20% 28% 5% 14% 17% 15%	4760
February	1st to 10th	Day	9% 62% 4 6% 14% 6%	2000
	11th to 20th	Day	8% 60% 2 5% 15% 9%	1960
	21st to end	Day	7% 61% 1 6% 13% 11%	1650
March	1st to 10th	Day	17% 28% 3 15% 18% 19%	4518
	11th to 20th	Day	15% 30% 6% 17% 21% 12%	4885
	21st to end	Day	11% 29% 8% 13% 25% 13%	5590
April	1st to 10th	Day	6% 61% 3 7% 12% 11%	2400
	11th to 20th	Day	4 62% 2 4 14% 14%	2372
	21st to end	Day	5% 61% 3 5% 16% 10%	2495
May	1st to 10th	Day	6% 38% 3 7% 22% 25%	4240
	11th to 20th	Day	3 38% 3 4 24% 29%	4076
	21st to end	Day	4 38% 2 5% 21% 31%	4477
June	1st to 10th	Day	2 64% 1 1 10% 22%	2470
	11th to 20th	Day	1 64% 1 2 7% 26%	2533
	21st to end	Day	2 62% 1 1 6% 28%	2660
July	1st to 10th	Day	2 34% 1 2 4 57%	4351
	11th to 20th	Day	6% 34% 3 3 7% 46%	4050
	21st to end	Day	4 34% 1 5% 13% 44%	4314
August	1st to 10th	Day	7% 36% 2 4 22% 29%	4000
	11th to 20th	Day	2 35% 1 2 8% 52%	3760
	21st to end	Day	2 35% 1 1 9% 52%	4179
September	1st to 10th	Day	4 59% 2 2 10% 24%	2232
	11th to 20th	Day	2 59% 1 2 14% 23%	2090
	21st to end	Day	3 59% 1 4 12% 21%	2090
October	1st to 10th	Day	2 35% 1 2 10% 50%	3490
	11th to 20th	Day	4 35% 1 4 22% 34%	3442
	21st to end	Day	6% 35% 1 4 25% 28%	3131
November	1st to 10th	Day	1 55% 1 5% 19% 19%	1764
	11th to 20th	Day	5% 56% 2 5% 15% 17%	1800
	21st to end	Day	8% 56% 3 8% 16% 9%	1800
December	1st to 10th	Day	8% 35% 3 10% 25% 19%	2950
	11th to 20th	Day	12% 31% 8% 16% 27% 6%	3408
	21st to end	Day	15% 29% 10% 24% 17% 5%	4510
January	1st to 10th	Night	11% 26% 3 21% 27% 12%	5867
	11th to 20th	Night	9% 27% 7% 18% 19% 21%	5880
	21st to end	Night	18% 28% 5% 12% 19% 19%	6226
February	1st to 10th	Night	10% 62% 4 5% 12% 8%	2400
	11th to 20th	Night	8% 60% 2 6% 13% 10%	2339
	21st to end	Night	6% 62% 3 6% 13% 11%	1980
March	1st to 10th	Night	14% 29% 5% 11% 20% 21%	5376
	11th to 20th	Night	14% 30% 6% 20% 20% 10%	5114
	21st to end	Night	10% 28% 5% 15% 28% 15%	5368
April	1st to 10th	Night	6% 61% 2 7% 12% 11%	2000
	11th to 20th	Night	4 62% 2 5% 14% 13%	1979
	21st to end	Night	3 61% 2 8% 14% 11%	1903
May	1st to 10th	Night	6% 38% 3 5% 19% 28%	2950
	11th to 20th	Night	3 38% 3 4 19% 34%	2839
	21st to end	Night	3 38% 2 4 19% 34%	3110
June	1st to 10th	Night	1 63% 1 1 8% 25%	1710
	11th to 20th	Night	1 64% 1 1 7% 25%	1637
	21st to end	Night	2 62% 1 2 5% 29%	1520
July	1st to 10th	Night	2 34% 1 2 4 58%	2500
	11th to 20th	Night	5% 34% 2 3 11% 45%	2775
	21st to end	Night	5% 34% 1 6% 11% 42%	3011
August	1st to 10th	Night	8% 35% 2 4 18% 33%	2901
	11th to 20th	Night	3 35% 1 2 9% 49%	3156
	21st to end	Night	2 35% 1 1 8% 53%	3498
September	1st to 10th	Night	3 59% 2 3 8% 25%	1947
	11th to 20th	Night	2 59% 1 3 11% 24%	2090
	21st to end	Night	3 59% 1 3 11% 23%	2090
October	1st to 10th	Night	2 35% 1 2 6% 53%	3490
	11th to 20th	Night	4 35% 1 2 15% 43%	3494
	21st to end	Night	6% 35% 2 5% 21% 32%	4423
November	1st to 10th	Night	1 54% 1 5% 18% 20%	2535
	11th to 20th	Night	5% 56% 3 4 14% 18%	2600
	21st to end	Night	8% 56% 3 7% 13% 13%	2600
December	1st to 10th	Night	8% 35% 3 10% 28% 17%	4240
	11th to 20th	Night	8% 32% 6% 16% 29% 9%	4811
	21st to end	Night	17% 27% 11% 24% 17% 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	11% 13% 23% 32% 17% 4	300
	11th to 20th	Day	10% 8% 23% 35% 20% 5%	300
	21st to end	Day	9% 11% 19% 31% 24% 7%	437
February	1st to 10th	Day	9% 18% 26% 29% 14% 4	500
	11th to 20th	Day	9% 16% 29% 27% 16% 4	495
	21st to end	Day	9% 18% 26% 32% 12% 3	414
March	1st to 10th	Day	11% 21% 29% 23% 15% 2	500
	11th to 20th	Day	9% 16% 36% 24% 12% 3	500
	21st to end	Day	7% 20% 35% 23% 12% 3	550
April	1st to 10th	Day	5% 18% 28% 29% 14% 5%	500
	11th to 20th	Day	5% 24% 33% 23% 11% 4	492
	21st to end	Day	4 19% 33% 24% 16% 5%	500
May	1st to 10th	Day	5% 12% 35% 29% 13% 6%	503
	11th to 20th	Day	3 13% 31% 31% 18% 4	489
	21st to end	Day	2 13% 28% 36% 15% 6%	529
June	1st to 10th	Day	3 9% 27% 36% 18% 7%	490
	11th to 20th	Day	2 9% 27% 33% 21% 8%	490
	21st to end	Day	1 8% 25% 38% 19% 9%	490
July	1st to 10th	Day	1 3 23% 41% 22% 10%	486
	11th to 20th	Day	2 1 14% 44% 27% 12%	484
	21st to end	Day	2 3 12% 48% 23% 13%	531
August	1st to 10th	Day	2 5% 20% 41% 22% 11%	487
	11th to 20th	Day	2 2 18% 40% 25% 12%	477
	21st to end	Day	2 3 20% 40% 25% 10%	539
September	1st to 10th	Day	2 6% 16% 37% 27% 11%	490
	11th to 20th	Day	1 9% 24% 38% 22% 6%	490
	21st to end	Day	2 9% 22% 33% 28% 6%	490
October	1st to 10th	Day	2 9% 26% 28% 23% 12%	490
	11th to 20th	Day	2 8% 23% 29% 32% 6%	475
	21st to end	Day	3 8% 20% 30% 25% 14%	330
November	1st to 10th	Day	3 15% 19% 24% 29% 9%	300
	11th to 20th	Day	4 8% 15% 37% 25% 10%	300
	21st to end	Day	5% 7% 20% 31% 28% 9%	300
December	1st to 10th	Day	5% 5% 20% 41% 21% 8%	300
	11th to 20th	Day	4 9% 21% 31% 26% 9%	300
	21st to end	Day	9% 14% 22% 34% 18% 3	330
January	1st to 10th	Night	8% 7% 21% 32% 24% 9%	800
	11th to 20th	Night	5% 5% 20% 40% 24% 7%	800
	21st to end	Night	9% 5% 23% 35% 22% 6%	880
February	1st to 10th	Night	9% 7% 25% 32% 23% 5%	800
	11th to 20th	Night	6% 8% 25% 37% 18% 4	784
	21st to end	Night	7% 9% 29% 33% 17% 4	660
March	1st to 10th	Night	5% 10% 30% 35% 15% 5%	800
	11th to 20th	Night	6% 8% 27% 31% 22% 7%	800
	21st to end	Night	5% 13% 26% 36% 17% 3	880
April	1st to 10th	Night	3 10% 25% 35% 23% 4	800
	11th to 20th	Night	3 12% 30% 33% 18% 4	791
	21st to end	Night	2 6% 27% 38% 21% 5%	703
May	1st to 10th	Night	4 6% 27% 40% 21% 3	600
	11th to 20th	Night	2 8% 20% 35% 31% 4	563
	21st to end	Night	2 9% 20% 38% 27% 4	617
June	1st to 10th	Night	1 5% 20% 38% 32% 4	570
	11th to 20th	Night	1 5% 22% 36% 32% 4	497
	21st to end	Night	1 1 15% 39% 37% 6%	380
July	1st to 10th	Night	1 1 8% 40% 44% 6%	370
	11th to 20th	Night	1 1 7% 42% 43% 5%	534
	21st to end	Night	3 1 6% 36% 50% 4	624
August	1st to 10th	Night	2 1 11% 43% 40% 3	604
	11th to 20th	Night	1 1 6% 36% 48% 8%	752
	21st to end	Night	2 1 10% 32% 43% 12%	836
September	1st to 10th	Night	1 2 9% 31% 44% 12%	760
	11th to 20th	Night	1 1 14% 38% 39% 8%	760
	21st to end	Night	1 3 15% 34% 35% 13%	760
October	1st to 10th	Night	3 3 20% 38% 29% 8%	760
	11th to 20th	Night	2 4 16% 37% 30% 12%	760
	21st to end	Night	2 7% 18% 31% 31% 11%	836
November	1st to 10th	Night	2 6% 17% 33% 31% 10%	780
	11th to 20th	Night	2 3 17% 40% 30% 8%	800
	21st to end	Night	4 5% 22% 34% 29% 7%	800
December	1st to 10th	Night	6% 5% 17% 39% 25% 7%	800
	11th to 20th	Night	8% 5% 22% 36% 24% 7%	800
	21st to end	Night	7% 11% 20% 37% 21% 4	880

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