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 2024-11-02.

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 2 1 2 2 2 2 2 4 5% 75%	101552
	Night	3 2 1 2 2 2 2 2 4 5% 73%	97688
Transparency	Day	12% 2 5% 14% 26% 40%	101310
	Night	14% 3 6% 15% 26% 37%	97521
Seeing	Day	5% 11% 25% 33% 20% 7%	15406
	Night	4 6% 19% 36% 29% 7%	25192

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 1 3 2 2 3 3 5% 7% 66%	2858
	11th to 20th	Day	4 3 2 4 3 3 3 3 5% 8% 63%	2860
	21st to end	Day	8% 5% 2 5% 3 3 4 4 5% 7% 56%	3315
February	1st to 10th	Day	8% 3 1 3 2 2 2 2 4 6% 66%	1900
	11th to 20th	Day	5% 3 2 1 3 4 3 4 7% 67%	1850
	21st to end	Day	6% 4 1 3 3 4 3 4 4 6% 63%	1570
March	1st to 10th	Day	6% 2 1 4 2 2 3 4 5% 7% 63%	3138
	11th to 20th	Day	4 3 1 3 3 2 3 3 5% 7% 66%	3413
	21st to end	Day	6% 2 1 3 2 3 4 4 6% 9% 60%	3903
April	1st to 10th	Day	2 3 1 3 2 3 3 3 5% 6% 70%	2280
	11th to 20th	Day	2 2 1 2 2 1 3 2 4 6% 75%	2249
	21st to end	Day	2 2 2 2 2 3 3 3 5% 77%	2370
May	1st to 10th	Day	1 1 1 2 2 1 2 2 3 5% 80%	4110
	11th to 20th	Day	1 1 1 1 2 2 2 3 5% 82%	4071
	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%	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	2 1 1 2 2 2 2 2 4 4 78%	2090
	11th to 20th	Day	2 1 1 2 1 2 2 1 3 3 82%	2076
	21st to end	Day	2 1 1 1 1 1 1 2 3 5% 82%	1896
November	1st to 10th	Day	2 3 1 1 2 2 2 2 4 6% 77%	1683
	11th to 20th	Day	2 2 1 2 1 2 2 1 3 5% 78%	1710
	21st to end	Day	4 2 1 3 2 2 2 3 4 6% 72%	1710
December	1st to 10th	Day	4 2 1 3 3 2 4 3 5% 6% 67%	1710
	11th to 20th	Day	7% 4 1 4 3 3 4 3 6% 10% 54%	2169
	21st to end	Day	4 3 2 4 3 3 5% 5% 8% 9% 53%	3146
January	1st to 10th	Night	4 4 2 4 3 3 3 3 4 5% 66%	4097
	11th to 20th	Night	3 2 1 4 3 3 3 3 5% 8% 65%	4110
	21st to end	Night	11% 4 1 3 3 2 2 3 5% 7% 58%	4355
February	1st to 10th	Night	6% 5% 1 3 3 2 2 3 4 5% 64%	2280
	11th to 20th	Night	4 4 1 3 3 3 3 2 4 6% 67%	2218
	21st to end	Night	5% 4 1 3 3 2 3 2 5% 6% 66%	1884
March	1st to 10th	Night	6% 3 1 3 2 2 3 3 6% 7% 63%	3736
	11th to 20th	Night	4 3 1 3 3 3 4 3 5% 8% 62%	3566
	21st to end	Night	6% 3 1 3 3 3 3 3 5% 6% 66%	3746
April	1st to 10th	Night	2 2 1 3 3 2 3 2 3 6% 73%	1900
	11th to 20th	Night	2 2 1 2 1 2 3 3 5% 5% 74%	1879
	21st to end	Night	2 1 1 1 2 2 2 4 6% 79%	1810
May	1st to 10th	Night	3 1 2 1 1 2 2 3 5% 81%	2860
	11th to 20th	Night	1 1 1 1 2 2 3 3 85%	2834
	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%	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	2 2 2 2 2 2 1 3 5% 79%	2090
	11th to 20th	Night	1 1 1 1 2 2 2 3 3 83%	2104
	21st to end	Night	1 1 2 2 1 2 2 4 4 81%	2698
November	1st to 10th	Night	2 2 1 2 2 2 2 1 3 5% 78%	2430
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 6% 77%	2470
	21st to end	Night	3 3 1 3 2 2 3 2 4 5% 72%	2470
December	1st to 10th	Night	5% 4 1 4 3 3 3 3 4 6% 66%	2470
	11th to 20th	Night	8% 3 1 5% 3 4 3 3 5% 9% 56%	3041
	21st to end	Night	6% 3 1 5% 5% 5% 5% 4 6% 7% 52%	4521

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	17% 5% 8% 29% 32% 9%	2859
	11th to 20th	Day	20% 5% 14% 28% 23% 10%	2860
	21st to end	Day	27% 5% 8% 28% 25% 8%	3315
February	1st to 10th	Day	20% 3 7% 17% 31% 21%	1900
	11th to 20th	Day	18% 3 6% 17% 34% 21%	1860
	21st to end	Day	21% 3 9% 18% 29% 20%	1570
March	1st to 10th	Day	18% 3 7% 26% 27% 20%	3138
	11th to 20th	Day	19% 5% 8% 22% 30% 17%	3413
	21st to end	Day	17% 4 11% 23% 31% 14%	3903
April	1st to 10th	Day	16% 2 6% 18% 30% 27%	2280
	11th to 20th	Day	12% 3 5% 16% 30% 34%	2252
	21st to end	Day	11% 2 5% 15% 35% 32%	2370
May	1st to 10th	Day	7% 2 5% 11% 33% 41%	4110
	11th to 20th	Day	6% 1 4 8% 36% 45%	4074
	21st to end	Day	7% 2 3 9% 34% 45%	4477
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	11% 3 4 11% 26% 46%	2090
	11th to 20th	Day	9% 1 4 10% 31% 46%	2076
	21st to end	Day	8% 2 2 10% 34% 43%	1896
November	1st to 10th	Day	11% 1 5% 13% 42% 28%	1683
	11th to 20th	Day	12% 2 7% 16% 34% 30%	1710
	21st to end	Day	15% 2 6% 19% 34% 25%	1710
December	1st to 10th	Day	19% 5% 10% 18% 29% 19%	1710
	11th to 20th	Day	25% 4 13% 21% 26% 10%	2168
	21st to end	Day	24% 7% 11% 27% 21% 9%	3146
January	1st to 10th	Night	21% 3 7% 22% 37% 10%	4097
	11th to 20th	Night	18% 4 12% 33% 24% 10%	4110
	21st to end	Night	26% 4 8% 27% 26% 10%	4355
February	1st to 10th	Night	23% 3 7% 16% 29% 22%	2280
	11th to 20th	Night	19% 3 7% 18% 27% 27%	2219
	21st to end	Night	19% 3 8% 19% 32% 20%	1884
March	1st to 10th	Night	17% 4 9% 21% 30% 18%	3736
	11th to 20th	Night	19% 5% 8% 25% 24% 18%	3566
	21st to end	Night	18% 3 7% 20% 35% 16%	3746
April	1st to 10th	Night	14% 3 6% 20% 30% 27%	1900
	11th to 20th	Night	13% 3 5% 17% 32% 30%	1879
	21st to end	Night	8% 2 5% 15% 34% 35%	1810
May	1st to 10th	Night	9% 2 4 10% 30% 46%	2860
	11th to 20th	Night	6% 1 4 8% 31% 51%	2835
	21st to end	Night	6% 2 3 9% 30% 50%	3110
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	10% 2 4 11% 25% 49%	2090
	11th to 20th	Night	8% 2 3 9% 28% 51%	2104
	21st to end	Night	8% 2 4 11% 27% 48%	2698
November	1st to 10th	Night	11% 1 4 16% 36% 31%	2430
	11th to 20th	Night	11% 2 5% 15% 33% 34%	2470
	21st to end	Night	15% 3 4 16% 30% 32%	2470
December	1st to 10th	Night	21% 3 8% 18% 29% 21%	2470
	11th to 20th	Night	24% 5% 11% 19% 28% 13%	3041
	21st to end	Night	29% 5% 11% 22% 23% 8%	4521

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% 13% 24% 32% 16% 3	270
	11th to 20th	Day	11% 8% 23% 33% 21% 4	270
	21st to end	Day	10% 11% 20% 29% 23% 6%	401
February	1st to 10th	Day	10% 17% 25% 28% 15% 5%	460
	11th to 20th	Day	9% 17% 29% 25% 16% 4	455
	21st to end	Day	10% 20% 27% 31% 10% 2	382
March	1st to 10th	Day	12% 21% 29% 22% 14% 2	460
	11th to 20th	Day	9% 16% 37% 24% 12% 3	460
	21st to end	Day	8% 21% 36% 24% 10% 1	506
April	1st to 10th	Day	6% 18% 29% 30% 14% 3	460
	11th to 20th	Day	6% 25% 34% 22% 10% 3	452
	21st to end	Day	4 20% 33% 24% 15% 4	460
May	1st to 10th	Day	5% 13% 37% 29% 12% 4	463
	11th to 20th	Day	3 13% 31% 31% 18% 5%	488
	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%	476
	21st to end	Day	3 8% 20% 30% 25% 14%	330
November	1st to 10th	Day	4 15% 20% 23% 30% 8%	273
	11th to 20th	Day	4 8% 15% 38% 24% 11%	270
	21st to end	Day	5% 6% 20% 30% 29% 10%	270
December	1st to 10th	Day	5% 6% 22% 43% 19% 6%	270
	11th to 20th	Day	5% 10% 22% 32% 25% 6%	270
	21st to end	Day	10% 15% 23% 33% 16% 3	297
January	1st to 10th	Night	8% 8% 20% 32% 24% 9%	760
	11th to 20th	Night	5% 5% 19% 39% 25% 7%	760
	21st to end	Night	9% 5% 24% 34% 21% 7%	836
February	1st to 10th	Night	9% 7% 24% 32% 24% 5%	760
	11th to 20th	Night	6% 9% 25% 37% 19% 5%	744
	21st to end	Night	8% 10% 30% 32% 17% 4	628
March	1st to 10th	Night	5% 11% 29% 35% 16% 5%	760
	11th to 20th	Night	6% 8% 26% 30% 23% 7%	760
	21st to end	Night	5% 13% 25% 37% 16% 3	836
April	1st to 10th	Night	3 10% 26% 35% 22% 4	760
	11th to 20th	Night	3 12% 29% 33% 18% 5%	751
	21st to end	Night	2 7% 27% 38% 20% 6%	670
May	1st to 10th	Night	4 6% 27% 40% 20% 3	570
	11th to 20th	Night	2 8% 19% 35% 31% 4	561
	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% 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 5% 17% 33% 32% 10%	748
	11th to 20th	Night	2 3 17% 40% 30% 8%	760
	21st to end	Night	4 5% 21% 34% 30% 7%	760
December	1st to 10th	Night	6% 6% 18% 39% 25% 7%	760
	11th to 20th	Night	8% 5% 22% 36% 22% 6%	760
	21st to end	Night	7% 12% 20% 36% 21% 4	836

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