High Desert Astronomy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the High Desert Astronomy clear sky chart from 2005-11-06 to 2025-05-26.

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%	119095
	Night	3 2 1 2 2 2 2 2 4 5% 74%	119139
Transparency	Day	12% 2 5% 14% 27% 40%	118853
	Night	13% 2 5% 15% 27% 37%	118972
Seeing	Day	4 11% 25% 33% 20% 7%	16142
	Night	4 6% 19% 36% 29% 7%	25980

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%	4763
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%	4884
	21st to end	Day	4 2 1 3 2 3 3 3 6% 8% 66%	5603
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%	4201
	21st to end	Day	1 1 1 1 1 1 1 2 3 4 85%	4544
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%	2539
	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%	4124
	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%	2233
	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%	3443
	21st to end	Day	1 1 2 1 1 2 2 3 5% 81%	3116
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%	6224
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%	5115
	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%	1905
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 4 85%	2924
	21st to end	Night	1 1 1 1 1 2 1 3 4 85%	3159
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%	1631
	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%	2823
	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%	2925
	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%	4427
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	13% 3 6% 31% 33% 14%	4099
	11th to 20th	Day	14% 3 10% 20% 25% 28%	4100
	21st to end	Day	25% 3 7% 21% 26% 19%	4763
February	1st to 10th	Day	20% 3 7% 17% 32% 21%	2000
	11th to 20th	Day	18% 3 6% 18% 34% 21%	1960
	21st to end	Day	20% 3 8% 18% 28% 23%	1650
March	1st to 10th	Day	22% 2 5% 20% 27% 24%	4518
	11th to 20th	Day	20% 4 8% 22% 29% 17%	4884
	21st to end	Day	15% 3 11% 20% 33% 18%	5603
April	1st to 10th	Day	16% 2 6% 18% 31% 27%	2400
	11th to 20th	Day	12% 3 5% 16% 30% 35%	2372
	21st to end	Day	11% 2 5% 14% 37% 31%	2495
May	1st to 10th	Day	8% 2 5% 12% 33% 41%	4240
	11th to 20th	Day	6% 1 4 8% 35% 45%	4204
	21st to end	Day	7% 1 3 9% 34% 46%	4544
June	1st to 10th	Day	9% 2 3 9% 25% 52%	2470
	11th to 20th	Day	5% 1 2 6% 19% 67%	2539
	21st to end	Day	7% 1 2 4 13% 73%	2660
July	1st to 10th	Day	5% 1 2 3 8% 80%	4351
	11th to 20th	Day	11% 1 4 6% 15% 63%	4054
	21st to end	Day	10% 3 3 9% 20% 55%	4314
August	1st to 10th	Day	10% 1 3 7% 28% 51%	4000
	11th to 20th	Day	5% 1 2 4 16% 71%	3760
	21st to end	Day	5% 1 2 5% 15% 73%	4179
September	1st to 10th	Day	11% 2 3 7% 18% 58%	2233
	11th to 20th	Day	6% 1 3 6% 26% 57%	2090
	21st to end	Day	6% 1 2 7% 27% 56%	2090
October	1st to 10th	Day	7% 2 3 6% 20% 63%	3490
	11th to 20th	Day	7% 1 2 8% 33% 49%	3443
	21st to end	Day	9% 1 2 8% 35% 44%	3116
November	1st to 10th	Day	10% 1 5% 13% 42% 29%	1764
	11th to 20th	Day	11% 2 6% 15% 35% 30%	1800
	21st to end	Day	15% 2 6% 19% 34% 24%	1800
December	1st to 10th	Day	14% 3 7% 15% 36% 25%	2950
	11th to 20th	Day	21% 3 10% 24% 33% 8%	3408
	21st to end	Day	20% 6% 13% 31% 22% 7%	4510
January	1st to 10th	Night	16% 2 5% 27% 35% 15%	5867
	11th to 20th	Night	12% 3 8% 23% 28% 26%	5880
	21st to end	Night	23% 3 7% 19% 26% 23%	6224
February	1st to 10th	Night	23% 2 8% 16% 29% 22%	2400
	11th to 20th	Night	19% 3 6% 19% 28% 25%	2339
	21st to end	Night	18% 3 7% 18% 31% 23%	1980
March	1st to 10th	Night	18% 3 7% 17% 29% 25%	5376
	11th to 20th	Night	20% 5% 8% 25% 27% 15%	5115
	21st to end	Night	15% 2 7% 21% 36% 19%	5368
April	1st to 10th	Night	14% 3 6% 20% 30% 28%	2000
	11th to 20th	Night	12% 3 5% 16% 32% 32%	1979
	21st to end	Night	8% 2 5% 16% 34% 34%	1905
May	1st to 10th	Night	9% 2 4 10% 30% 45%	2950
	11th to 20th	Night	6% 1 4 8% 31% 50%	2925
	21st to end	Night	6% 2 3 9% 30% 51%	3159
June	1st to 10th	Night	6% 2 3 9% 23% 57%	1710
	11th to 20th	Night	7% 2 3 6% 18% 65%	1631
	21st to end	Night	5% 1 1 5% 11% 77%	1520
July	1st to 10th	Night	4 1 1 3 8% 83%	2500
	11th to 20th	Night	9% 1 3 6% 18% 62%	2771
	21st to end	Night	12% 3 3 11% 18% 53%	3011
August	1st to 10th	Night	11% 2 2 8% 23% 55%	2916
	11th to 20th	Night	5% 1 3 5% 18% 68%	3156
	21st to end	Night	5% 1 2 3 14% 75%	3498
September	1st to 10th	Night	10% 3 4 7% 16% 60%	1947
	11th to 20th	Night	6% 2 3 7% 23% 59%	2090
	21st to end	Night	6% 1 2 7% 24% 60%	2090
October	1st to 10th	Night	6% 1 3 6% 15% 68%	3490
	11th to 20th	Night	6% 1 2 6% 25% 59%	3494
	21st to end	Night	8% 1 3 8% 30% 49%	4427
November	1st to 10th	Night	11% 1 4 15% 36% 32%	2535
	11th to 20th	Night	11% 2 5% 15% 34% 34%	2600
	21st to end	Night	15% 3 5% 16% 30% 31%	2600
December	1st to 10th	Night	15% 2 6% 15% 37% 25%	4240
	11th to 20th	Night	16% 3 9% 24% 37% 11%	4811
	21st to end	Night	23% 4 14% 30% 22% 7%	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% 10% 19% 31% 24% 7%	440
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% 28% 13% 6%	503
	11th to 20th	Day	2 14% 31% 31% 17% 5%	528
	21st to end	Day	2 13% 28% 36% 16% 6%	550
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%	476
	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% 6%	705
May	1st to 10th	Night	4 6% 27% 40% 21% 3	600
	11th to 20th	Night	2 8% 20% 36% 30% 4	591
	21st to end	Night	2 9% 20% 38% 27% 3	634
June	1st to 10th	Night	1 5% 20% 38% 32% 4	570
	11th to 20th	Night	1 5% 22% 36% 33% 4	491
	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	2 1 7% 42% 43% 5%	530
	21st to end	Night	3 1 6% 36% 50% 4	624
August	1st to 10th	Night	2 1 11% 43% 40% 3	606
	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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