Sandstone Trailhead
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandstone Trailhead clear sky chart from 2005-11-06 to 2026-04-02.

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	4 3 1 3 2 2 3 3 4 7% 67%	139813
	Night	5% 4 1 3 3 2 3 2 4 6% 67%	139559
Transparency	Day	11% 37% 5% 11% 18% 18%	134900
	Night	12% 36% 5% 11% 18% 18%	135445
Seeing	Day	7% 8% 25% 34% 19% 7%	17486
	Night	6% 4 13% 32% 35% 10%	27358

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	9% 3 1 3 2 2 3 2 4 5% 66%	4800
	11th to 20th	Day	5% 3 1 2 2 2 2 2 4 7% 70%	5340
	21st to end	Day	9% 5% 2 4 3 4 4 3 6% 9% 53%	6198
February	1st to 10th	Day	10% 5% 2 3 3 2 2 3 4 5% 60%	2100
	11th to 20th	Day	10% 4 1 2 4 2 3 3 5% 8% 59%	2050
	21st to end	Day	8% 3 2 4 3 3 3 3 4 7% 59%	1730
March	1st to 10th	Day	11% 4 2 5% 3 3 3 3 5% 6% 55%	4618
	11th to 20th	Day	9% 4 1 4 3 3 3 3 4 7% 60%	4902
	21st to end	Day	7% 4 1 4 4 4 5% 4 7% 9% 51%	6069
April	1st to 10th	Day	6% 5% 1 3 3 3 3 3 4 9% 60%	2413
	11th to 20th	Day	4 4 1 4 2 2 3 3 5% 7% 65%	2369
	21st to end	Day	4 5% 1 4 2 2 2 2 4 7% 66%	2400
May	1st to 10th	Day	4 5% 1 4 2 2 3 3 4 7% 66%	5440
	11th to 20th	Day	3 6% 2 5% 3 3 4 3 5% 8% 60%	5831
	21st to end	Day	2 3 1 4 3 3 4 3 5% 9% 63%	6423
June	1st to 10th	Day	4 7% 2 5% 3 3 3 3 5% 8% 57%	2733
	11th to 20th	Day	3 4 1 4 2 2 3 3 5% 8% 65%	2800
	21st to end	Day	2 3 1 3 2 2 2 2 4 6% 73%	2800
July	1st to 10th	Day	1 1 1 1 1 2 2 2 5% 84%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 6% 76%	6171
	21st to end	Day	1 1 1 2 1 1 2 2 4 6% 79%	6080
August	1st to 10th	Day	1 1 1 2 1 2 2 2 3 5% 82%	5346
	11th to 20th	Day	1 1 1 1 1 1 3 6% 84%	5364
	21st to end	Day	1 1 1 1 2 2 1 3 5% 83%	5984
September	1st to 10th	Day	1 2 1 3 1 2 2 2 4 6% 77%	2400
	11th to 20th	Day	3 2 1 3 1 2 2 2 4 6% 74%	2297
	21st to end	Day	2 2 1 2 2 2 3 2 4 5% 75%	2200
October	1st to 10th	Day	2 2 1 1 2 2 2 3 4 81%	3579
	11th to 20th	Day	2 5% 1 4 3 3 3 2 4 6% 68%	3261
	21st to end	Day	3 2 1 4 2 2 3 3 5% 5% 69%	3558
November	1st to 10th	Day	3 2 1 3 2 2 3 3 4 6% 71%	1874
	11th to 20th	Day	6% 3 1 3 3 3 2 2 4 6% 67%	1890
	21st to end	Day	5% 3 2 4 3 3 3 3 5% 7% 62%	1890
December	1st to 10th	Day	4 2 1 3 3 4 4 3 5% 6% 64%	3040
	11th to 20th	Day	10% 4 1 4 3 4 5% 4 5% 7% 53%	3452
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 9% 46%	4092
January	1st to 10th	Night	10% 5% 2 4 3 3 2 2 3 5% 62%	8082
	11th to 20th	Night	5% 3 1 3 2 2 2 2 4 5% 70%	7650
	21st to end	Night	10% 7% 2 4 3 3 4 3 4 5% 55%	8017
February	1st to 10th	Night	8% 6% 1 5% 3 2 3 2 4 5% 61%	2520
	11th to 20th	Night	9% 4 1 4 3 3 3 2 4 6% 59%	2458
	21st to end	Night	10% 4 1 4 3 3 3 3 4 5% 59%	2076
March	1st to 10th	Night	7% 7% 2 4 3 3 4 4 6% 6% 55%	5496
	11th to 20th	Night	6% 7% 2 5% 4 3 3 3 4 5% 60%	5301
	21st to end	Night	9% 6% 2 4 3 3 2 3 4 6% 57%	5146
April	1st to 10th	Night	6% 4 1 4 3 3 3 2 5% 7% 62%	2010
	11th to 20th	Night	3 5% 2 4 2 3 3 2 4 6% 67%	1979
	21st to end	Night	3 5% 1 3 3 3 2 2 5% 7% 66%	2000
May	1st to 10th	Night	2 5% 2 4 2 3 3 3 4 5% 67%	4560
	11th to 20th	Night	1 5% 2 6% 3 3 2 2 4 5% 66%	4083
	21st to end	Night	1 2 1 3 2 2 2 2 3 5% 77%	4474
June	1st to 10th	Night	2 5% 1 5% 3 2 4 4 5% 7% 61%	1661
	11th to 20th	Night	2 3 1 2 3 2 2 2 5% 8% 70%	1600
	21st to end	Night	1 3 2 3 2 2 2 1 4 8% 73%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 3 4 84%	3640
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 5% 77%	3777
	21st to end	Night	1 1 1 2 2 2 2 2 3 5% 79%	4806
August	1st to 10th	Night	1 1 2 1 2 2 2 4 6% 79%	4486
	11th to 20th	Night	1 1 1 2 1 1 2 2 3 5% 82%	4506
	21st to end	Night	1 1 1 2 1 1 2 2 3 5% 82%	5015
September	1st to 10th	Night	2 3 1 3 2 2 2 2 4 6% 73%	2000
	11th to 20th	Night	2 2 1 3 2 1 2 2 4 5% 75%	2101
	21st to end	Night	2 2 1 2 2 2 2 2 4 6% 77%	2200
October	1st to 10th	Night	2 1 1 2 1 1 2 2 3 3 83%	3617
	11th to 20th	Night	3 2 2 4 3 2 3 3 5% 6% 67%	3897
	21st to end	Night	3 3 1 3 2 2 2 2 3 5% 77%	4253
November	1st to 10th	Night	2 3 1 3 2 2 3 2 5% 6% 70%	2643
	11th to 20th	Night	5% 4 2 3 2 2 2 3 4 5% 68%	2730
	21st to end	Night	6% 3 1 4 4 3 4 3 4 7% 62%	2730
December	1st to 10th	Night	5% 3 1 4 3 3 3 3 4 6% 67%	4370
	11th to 20th	Night	11% 5% 1 4 3 3 4 4 6% 7% 52%	4969
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	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	17% 21% 6% 16% 22% 18%	4801
	11th to 20th	Day	12% 22% 7% 10% 14% 35%	5340
	21st to end	Day	21% 23% 5% 13% 20% 19%	6198
February	1st to 10th	Day	13% 58% 3 7% 10% 9%	2100
	11th to 20th	Day	12% 58% 2 7% 12% 10%	2060
	21st to end	Day	9% 59% 3 10% 10% 9%	1730
March	1st to 10th	Day	21% 30% 7% 13% 16% 13%	4618
	11th to 20th	Day	17% 29% 10% 21% 14% 9%	4902
	21st to end	Day	19% 32% 12% 22% 13% 3	6069
April	1st to 10th	Day	7% 62% 4 10% 12% 4	2413
	11th to 20th	Day	7% 62% 4 8% 12% 7%	2372
	21st to end	Day	8% 63% 4 12% 10% 4	2400
May	1st to 10th	Day	15% 31% 8% 14% 22% 11%	5440
	11th to 20th	Day	17% 31% 10% 17% 20% 5%	5834
	21st to end	Day	12% 31% 11% 23% 17% 6%	6424
June	1st to 10th	Day	10% 63% 3 9% 11% 4	2733
	11th to 20th	Day	7% 62% 2 7% 12% 11%	2800
	21st to end	Day	6% 60% 1 5% 13% 15%	2800
July	1st to 10th	Day	3 25% 2 5% 14% 52%	6251
	11th to 20th	Day	7% 25% 4 6% 27% 31%	6096
	21st to end	Day	5% 24% 3 12% 32% 24%	5906
August	1st to 10th	Day	4 35% 1 6% 32% 21%	3890
	11th to 20th	Day	2 35% 1 4 22% 36%	3880
	21st to end	Day	2 35% 2 6% 23% 32%	4311
September	1st to 10th	Day	5% 57% 2 5% 15% 15%	2400
	11th to 20th	Day	8% 57% 3 6% 15% 11%	2297
	21st to end	Day	5% 57% 2 7% 16% 12%	2200
October	1st to 10th	Day	3 35% 1 6% 12% 44%	3579
	11th to 20th	Day	13% 35% 2 8% 26% 16%	3261
	21st to end	Day	12% 35% 3 4 26% 20%	3558
November	1st to 10th	Day	5% 53% 2 6% 13% 22%	1874
	11th to 20th	Day	12% 55% 3 6% 8% 15%	1890
	21st to end	Day	12% 54% 4 7% 9% 14%	1890
December	1st to 10th	Day	12% 34% 3 8% 20% 24%	3040
	11th to 20th	Day	22% 30% 5% 10% 18% 15%	3451
	21st to end	Day	24% 29% 9% 16% 16% 6%	4092
January	1st to 10th	Night	21% 22% 4 10% 24% 18%	8082
	11th to 20th	Night	14% 21% 5% 12% 13% 36%	7650
	21st to end	Night	24% 23% 4 10% 19% 21%	8017
February	1st to 10th	Night	13% 58% 2 7% 10% 9%	2520
	11th to 20th	Night	13% 58% 2 6% 9% 11%	2459
	21st to end	Night	10% 59% 2 9% 11% 10%	2076
March	1st to 10th	Night	15% 33% 7% 13% 20% 10%	5496
	11th to 20th	Night	17% 31% 8% 20% 13% 11%	5301
	21st to end	Night	20% 30% 9% 21% 16% 3	5146
April	1st to 10th	Night	9% 61% 4 10% 11% 5%	2010
	11th to 20th	Night	8% 63% 3 8% 13% 6%	1979
	21st to end	Night	6% 62% 4 11% 11% 5%	2000
May	1st to 10th	Night	10% 33% 9% 15% 23% 9%	4557
	11th to 20th	Night	10% 33% 8% 20% 22% 6%	4084
	21st to end	Night	4 29% 8% 30% 22% 7%	4474
June	1st to 10th	Night	8% 64% 5% 9% 11% 3	1661
	11th to 20th	Night	3 62% 3 7% 13% 12%	1600
	21st to end	Night	6% 60% 1 6% 13% 14%	1600
July	1st to 10th	Night	3 25% 2 4 14% 53%	3600
	11th to 20th	Night	6% 24% 7% 8% 25% 30%	3729
	21st to end	Night	4 25% 3 13% 32% 23%	4668
August	1st to 10th	Night	5% 35% 1 9% 27% 22%	3256
	11th to 20th	Night	2 35% 1 5% 23% 33%	3256
	21st to end	Night	3 35% 2 4 25% 31%	3608
September	1st to 10th	Night	6% 58% 2 4 15% 15%	2000
	11th to 20th	Night	7% 57% 2 5% 16% 12%	2101
	21st to end	Night	4 57% 2 6% 17% 14%	2200
October	1st to 10th	Night	3 34% 1 6% 11% 45%	3617
	11th to 20th	Night	9% 36% 6% 9% 20% 19%	3897
	21st to end	Night	8% 34% 1 5% 22% 30%	4253
November	1st to 10th	Night	3 53% 2 6% 14% 22%	2643
	11th to 20th	Night	11% 54% 3 6% 10% 16%	2730
	21st to end	Night	12% 55% 3 9% 9% 12%	2730
December	1st to 10th	Night	11% 34% 3 9% 17% 27%	4370
	11th to 20th	Night	21% 31% 4 9% 14% 20%	4969
	21st to end	Night	26% 29% 7% 20% 13% 5%	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	17% 8% 38% 29% 8%	246
	11th to 20th	Day	13% 13% 35% 30% 7% 1	430
	21st to end	Day	12% 15% 37% 25% 8% 2	538
February	1st to 10th	Day	14% 14% 36% 24% 9% 3	540
	11th to 20th	Day	14% 16% 32% 30% 6% 2	535
	21st to end	Day	12% 15% 36% 27% 9% 1	446
March	1st to 10th	Day	17% 18% 35% 22% 7% 1	540
	11th to 20th	Day	14% 13% 33% 27% 11% 3	540
	21st to end	Day	11% 15% 33% 30% 9% 2	594
April	1st to 10th	Day	9% 12% 31% 31% 13% 4	505
	11th to 20th	Day	8% 15% 32% 29% 11% 4	492
	21st to end	Day	5% 12% 30% 33% 15% 5%	500
May	1st to 10th	Day	9% 11% 27% 34% 15% 4	503
	11th to 20th	Day	4 10% 28% 34% 19% 5%	528
	21st to end	Day	7% 8% 31% 31% 18% 5%	573
June	1st to 10th	Day	7% 6% 21% 35% 23% 8%	530
	11th to 20th	Day	4 3 21% 38% 26% 8%	530
	21st to end	Day	3 3 15% 40% 29% 11%	530
July	1st to 10th	Day	2 1 9% 44% 30% 14%	526
	11th to 20th	Day	1 1 6% 40% 35% 17%	524
	21st to end	Day	1 9% 38% 34% 18%	575
August	1st to 10th	Day	2 1 10% 44% 28% 14%	527
	11th to 20th	Day	2 1 8% 42% 33% 15%	517
	21st to end	Day	1 1 12% 43% 31% 12%	583
September	1st to 10th	Day	2 2 18% 42% 25% 11%	530
	11th to 20th	Day	4 5% 24% 41% 19% 8%	530
	21st to end	Day	1 7% 23% 37% 25% 7%	530
October	1st to 10th	Day	3 7% 26% 39% 21% 4	530
	11th to 20th	Day	4 7% 31% 36% 17% 5%	530
	21st to end	Day	4 7% 29% 35% 19% 6%	583
November	1st to 10th	Day	6% 7% 22% 35% 21% 9%	350
	11th to 20th	Day	8% 11% 34% 29% 13% 5%	330
	21st to end	Day	10% 10% 33% 27% 16% 3	330
December	1st to 10th	Day	11% 10% 29% 32% 14% 3	330
	11th to 20th	Day	6% 11% 32% 31% 17% 3	319
	21st to end	Day	16% 13% 33% 25% 12% 2	242
January	1st to 10th	Night	12% 6% 21% 40% 16% 4	840
	11th to 20th	Night	10% 7% 27% 36% 18% 2	840
	21st to end	Night	13% 9% 22% 35% 17% 3	924
February	1st to 10th	Night	11% 7% 19% 36% 22% 5%	840
	11th to 20th	Night	10% 6% 20% 40% 19% 5%	824
	21st to end	Night	12% 6% 21% 39% 20% 3	692
March	1st to 10th	Night	9% 7% 20% 42% 20% 2	840
	11th to 20th	Night	8% 5% 14% 39% 28% 7%	840
	21st to end	Night	8% 5% 15% 40% 29% 4	924
April	1st to 10th	Night	7% 5% 13% 39% 32% 4	804
	11th to 20th	Night	4 4 15% 38% 34% 5%	791
	21st to end	Night	6% 4 10% 36% 35% 10%	800
May	1st to 10th	Night	6% 3 11% 36% 36% 8%	800
	11th to 20th	Night	4 4 10% 35% 39% 9%	600
	21st to end	Night	3 2 11% 32% 44% 8%	650
June	1st to 10th	Night	3 2 6% 29% 47% 13%	461
	11th to 20th	Night	2 4 26% 57% 12%	400
	21st to end	Night	3 1 2 22% 59% 14%	400
July	1st to 10th	Night	2 2 7% 68% 22%	390
	11th to 20th	Night	2 1 4 56% 36%	447
	21st to end	Night	2 1 2 9% 56% 30%	799
August	1st to 10th	Night	2 2 16% 58% 22%	796
	11th to 20th	Night	2 1 2 13% 56% 27%	792
	21st to end	Night	1 1 16% 54% 26%	880
September	1st to 10th	Night	2 1 3 21% 51% 21%	800
	11th to 20th	Night	3 1 3 24% 54% 15%	800
	21st to end	Night	2 1 5% 28% 51% 14%	800
October	1st to 10th	Night	3 1 6% 38% 44% 7%	800
	11th to 20th	Night	5% 2 10% 35% 39% 9%	800
	21st to end	Night	4 3 14% 36% 34% 10%	880
November	1st to 10th	Night	4 5% 13% 39% 30% 9%	820
	11th to 20th	Night	5% 3 18% 39% 28% 7%	840
	21st to end	Night	7% 5% 22% 38% 24% 4	840
December	1st to 10th	Night	8% 6% 23% 36% 21% 5%	840
	11th to 20th	Night	13% 5% 22% 38% 19% 3	840
	21st to end	Night	12% 7% 21% 39% 19% 3	924

Page generated at 2026-03-29 23:18:46. Version ac07

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