Sawmill Trailhead
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sawmill Trailhead clear sky chart from 2005-11-06 to 2025-03-27.

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 3 2 2 3 3 4 6% 73%	113675
	Night	1 2 1 2 2 2 3 3 4 6% 74%	114608
Transparency	Day	10% 3 5% 12% 26% 44%	113441
	Night	9% 2 4 13% 27% 43%	114432
Seeing	Day	7% 13% 30% 33% 14% 3	16121
	Night	4 8% 25% 34% 23% 6%	26515

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	2 2 1 3 2 3 3 2 4 5% 73%	4098
	11th to 20th	Day	2 2 1 2 2 3 3 3 5% 6% 72%	4174
	21st to end	Day	4 5% 1 5% 3 3 4 3 6% 9% 56%	5016
February	1st to 10th	Day	4 4 2 4 3 4 4 3 5% 7% 62%	2000
	11th to 20th	Day	3 2 1 4 3 3 4 3 6% 8% 63%	1950
	21st to end	Day	2 4 2 4 4 4 4 3 6% 8% 59%	1650
March	1st to 10th	Day	2 3 1 4 3 3 4 3 6% 11% 62%	3238
	11th to 20th	Day	3 4 2 5% 4 3 4 3 5% 8% 59%	3494
	21st to end	Day	3 4 1 2 2 3 3 3 5% 6% 67%	4001
April	1st to 10th	Day	2 3 2 4 2 4 3 3 6% 7% 66%	2280
	11th to 20th	Day	1 2 1 2 2 2 2 4 6% 77%	2249
	21st to end	Day	1 1 2 1 2 3 3 4 7% 77%	2280
May	1st to 10th	Day	1 2 1 2 1 1 2 1 3 4 83%	4098
	11th to 20th	Day	1 1 1 1 1 1 2 3 4 84%	4071
	21st to end	Day	1 1 1 1 2 1 2 3 87%	4477
June	1st to 10th	Day	1 1 2 2 1 1 2 3 6% 81%	2470
	11th to 20th	Day	1 1 1 2 1 3 3 87%	2470
	21st to end	Day	2 2 1 2 2 1 2 1 2 3 83%	2470
July	1st to 10th	Day	1 1 1 1 1 1 2 2 3 86%	4110
	11th to 20th	Day	1 1 1 3 2 3 4 4 5% 9% 67%	4110
	21st to end	Day	1 1 1 2 2 3 4 4 6% 6% 69%	4408
August	1st to 10th	Day	1 2 1 2 2 3 4 4 6% 8% 68%	3783
	11th to 20th	Day	1 1 2 1 2 2 3 3 5% 79%	3724
	21st to end	Day	1 1 1 2 2 2 2 3 4 82%	4180
September	1st to 10th	Day	3 2 1 3 2 3 3 3 4 5% 73%	2233
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	2090
	21st to end	Day	1 2 1 2 1 2 1 2 3 4 81%	2090
October	1st to 10th	Day	1 1 1 3 2 2 3 2 4 5% 77%	3490
	11th to 20th	Day	1 1 1 1 2 1 2 4 88%	3469
	21st to end	Day	1 1 2 2 3 3 3 5% 6% 75%	3269
November	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 76%	1764
	11th to 20th	Day	1 2 1 3 2 3 2 3 5% 6% 72%	1800
	21st to end	Day	3 3 2 4 3 3 4 3 5% 7% 65%	1800
December	1st to 10th	Day	3 2 1 3 2 3 3 3 6% 7% 68%	2950
	11th to 20th	Day	2 4 1 4 3 4 6% 5% 7% 7% 57%	3409
	21st to end	Day	4 4 1 4 4 5% 6% 4 7% 10% 51%	4510
January	1st to 10th	Night	1 2 1 3 2 3 3 3 5% 7% 71%	5867
	11th to 20th	Night	1 2 1 2 2 2 3 2 4 6% 73%	5792
	21st to end	Night	3 3 1 4 4 4 5% 5% 6% 6% 59%	5984
February	1st to 10th	Night	2 4 2 5% 3 3 3 4 5% 7% 63%	2400
	11th to 20th	Night	2 3 1 3 3 4 4 3 5% 7% 64%	2338
	21st to end	Night	1 3 2 4 3 4 4 4 7% 8% 59%	1980
March	1st to 10th	Night	1 3 1 3 3 3 4 4 7% 8% 63%	3856
	11th to 20th	Night	2 3 2 3 3 4 5% 4 7% 8% 60%	3706
	21st to end	Night	1 3 1 2 2 3 4 4 7% 8% 65%	3786
April	1st to 10th	Night	2 3 1 2 4 3 2 3 3 7% 69%	1900
	11th to 20th	Night	1 2 1 2 1 2 2 2 3 5% 79%	1879
	21st to end	Night	1 1 1 1 2 2 2 4 6% 80%	1900
May	1st to 10th	Night	1 1 1 1 1 1 2 6% 85%	2870
	11th to 20th	Night	1 1 1 2 2 2 4 87%	2834
	21st to end	Night	1 1 1 1 1 3 91%	3110
June	1st to 10th	Night	1 1 2 1 1 2 2 3 4 84%	1710
	11th to 20th	Night	1 1 1 1 2 2 2 90%	1710
	21st to end	Night	1 1 1 1 1 2 1 2 2 86%	1710
July	1st to 10th	Night	1 1 1 1 1 1 2 2 91%	2860
	11th to 20th	Night	1 1 1 1 2 2 2 3 5% 82%	2860
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 80%	3140
August	1st to 10th	Night	1 1 1 1 1 2 2 3 5% 83%	3166
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 84%	3126
	21st to end	Night	1 1 1 1 1 1 1 1 2 90%	3498
September	1st to 10th	Night	1 2 1 2 2 2 2 2 3 4 80%	1947
	11th to 20th	Night	1 1 1 1 1 1 1 2 91%	2090
	21st to end	Night	1 1 2 1 1 1 1 2 3 88%	2090
October	1st to 10th	Night	1 1 1 1 1 2 2 2 3 85%	3490
	11th to 20th	Night	1 1 1 1 1 2 4 89%	3469
	21st to end	Night	1 2 1 1 1 2 2 4 6% 79%	4286
November	1st to 10th	Night	1 2 2 2 3 3 5% 5% 77%	2535
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 6% 73%	2600
	21st to end	Night	3 3 2 3 3 3 4 3 7% 8% 63%	2600
December	1st to 10th	Night	2 2 1 2 2 2 3 3 5% 6% 70%	4240
	11th to 20th	Night	2 2 1 3 3 3 5% 5% 7% 10% 58%	4811
	21st to end	Night	3 2 1 3 3 4 4 4 7% 10% 58%	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	11% 2 7% 15% 42% 23%	4099
	11th to 20th	Day	12% 3 6% 20% 21% 38%	4174
	21st to end	Day	18% 3 9% 16% 27% 27%	5016
February	1st to 10th	Day	19% 2 6% 12% 30% 31%	2000
	11th to 20th	Day	15% 3 4 16% 33% 29%	1960
	21st to end	Day	19% 3 4 19% 27% 29%	1650
March	1st to 10th	Day	12% 2 8% 17% 35% 25%	3238
	11th to 20th	Day	18% 4 6% 22% 30% 19%	3494
	21st to end	Day	15% 2 4 22% 40% 17%	4001
April	1st to 10th	Day	15% 2 4 13% 37% 28%	2280
	11th to 20th	Day	8% 1 3 11% 33% 44%	2252
	21st to end	Day	7% 2 2 13% 36% 40%	2280
May	1st to 10th	Day	6% 1 2 9% 31% 50%	4098
	11th to 20th	Day	4 1 3 6% 22% 64%	4074
	21st to end	Day	4 1 1 6% 25% 62%	4477
June	1st to 10th	Day	6% 1 2 8% 30% 52%	2470
	11th to 20th	Day	4 1 1 5% 16% 73%	2470
	21st to end	Day	9% 1 2 4 10% 75%	2470
July	1st to 10th	Day	6% 1 2 4 8% 80%	4046
	11th to 20th	Day	11% 5% 5% 12% 28% 39%	4041
	21st to end	Day	11% 5% 7% 16% 18% 43%	4269
August	1st to 10th	Day	12% 5% 8% 14% 29% 32%	3770
	11th to 20th	Day	8% 3 5% 8% 18% 58%	3760
	21st to end	Day	7% 2 3 5% 14% 69%	4179
September	1st to 10th	Day	13% 3 4 9% 17% 54%	2233
	11th to 20th	Day	6% 1 1 4 22% 66%	2090
	21st to end	Day	8% 2 2 7% 23% 58%	2090
October	1st to 10th	Day	6% 2 4 7% 16% 65%	3490
	11th to 20th	Day	4 1 2 8% 28% 58%	3469
	21st to end	Day	5% 2 6% 6% 27% 54%	3269
November	1st to 10th	Day	10% 1 3 10% 32% 43%	1764
	11th to 20th	Day	10% 3 4 12% 28% 44%	1800
	21st to end	Day	16% 3 5% 13% 28% 35%	1800
December	1st to 10th	Day	13% 2 5% 12% 32% 36%	2950
	11th to 20th	Day	15% 5% 9% 19% 30% 22%	3408
	21st to end	Day	18% 7% 10% 24% 26% 15%	4510
January	1st to 10th	Night	9% 2 7% 17% 36% 28%	5867
	11th to 20th	Night	11% 3 5% 21% 26% 34%	5792
	21st to end	Night	17% 4 9% 14% 30% 27%	5984
February	1st to 10th	Night	17% 3 5% 14% 26% 36%	2400
	11th to 20th	Night	16% 3 5% 16% 29% 31%	2339
	21st to end	Night	16% 4 5% 16% 29% 30%	1980
March	1st to 10th	Night	13% 2 5% 20% 38% 22%	3856
	11th to 20th	Night	12% 5% 9% 23% 34% 17%	3706
	21st to end	Night	12% 2 5% 19% 38% 23%	3786
April	1st to 10th	Night	14% 2 4 18% 35% 27%	1900
	11th to 20th	Night	10% 1 2 11% 34% 42%	1879
	21st to end	Night	6% 2 3 12% 37% 40%	1900
May	1st to 10th	Night	5% 1 3 10% 29% 52%	2870
	11th to 20th	Night	4 1 3 6% 26% 60%	2835
	21st to end	Night	3 1 1 8% 23% 65%	3110
June	1st to 10th	Night	5% 1 2 9% 25% 57%	1710
	11th to 20th	Night	4 1 6% 15% 73%	1710
	21st to end	Night	7% 1 4 10% 77%	1710
July	1st to 10th	Night	3 1 1 2 10% 82%	2815
	11th to 20th	Night	6% 2 3 9% 27% 53%	2806
	21st to end	Night	8% 2 4 17% 25% 44%	3041
August	1st to 10th	Night	5% 2 3 18% 34% 38%	3156
	11th to 20th	Night	5% 2 5% 10% 19% 59%	3156
	21st to end	Night	5% 1 1 4 14% 75%	3498
September	1st to 10th	Night	9% 2 3 9% 20% 57%	1947
	11th to 20th	Night	5% 1 1 4 19% 70%	2090
	21st to end	Night	6% 1 1 6% 26% 61%	2090
October	1st to 10th	Night	6% 1 2 6% 19% 66%	3490
	11th to 20th	Night	3 1 1 5% 22% 68%	3469
	21st to end	Night	5% 2 3 7% 26% 58%	4286
November	1st to 10th	Night	7% 1 4 10% 33% 45%	2535
	11th to 20th	Night	11% 2 3 11% 30% 43%	2600
	21st to end	Night	16% 3 5% 14% 29% 34%	2600
December	1st to 10th	Night	12% 2 5% 11% 25% 44%	4240
	11th to 20th	Night	14% 4 8% 18% 32% 24%	4811
	21st to end	Night	15% 5% 8% 27% 27% 18%	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	12% 16% 30% 31% 9% 1	300
	11th to 20th	Day	14% 12% 29% 31% 13% 1	335
	21st to end	Day	11% 21% 34% 26% 8% 1	550
February	1st to 10th	Day	12% 19% 35% 23% 7% 3	500
	11th to 20th	Day	14% 20% 36% 24% 5%	495
	21st to end	Day	15% 26% 34% 20% 5% 1	414
March	1st to 10th	Day	15% 25% 36% 19% 5% 1	500
	11th to 20th	Day	16% 18% 38% 19% 8% 1	500
	21st to end	Day	14% 22% 40% 20% 4 1	531
April	1st to 10th	Day	10% 19% 32% 28% 10% 2	460
	11th to 20th	Day	6% 24% 38% 23% 8% 1	452
	21st to end	Day	7% 19% 30% 28% 13% 3	460
May	1st to 10th	Day	5% 18% 33% 32% 9% 3	463
	11th to 20th	Day	6% 14% 36% 31% 10% 3	488
	21st to end	Day	4 16% 32% 31% 12% 5%	529
June	1st to 10th	Day	2 8% 30% 37% 15% 7%	490
	11th to 20th	Day	1 11% 35% 32% 16% 5%	490
	21st to end	Day	3 6% 31% 38% 19% 4	490
July	1st to 10th	Day	1 2 20% 47% 24% 6%	486
	11th to 20th	Day	2 16% 46% 26% 10%	484
	21st to end	Day	2 4 12% 48% 24% 10%	531
August	1st to 10th	Day	2 2 21% 43% 24% 8%	487
	11th to 20th	Day	2 2 20% 43% 24% 9%	477
	21st to end	Day	1 4 17% 47% 23% 8%	539
September	1st to 10th	Day	3 2 28% 44% 19% 4	490
	11th to 20th	Day	3 7% 32% 42% 15% 2	490
	21st to end	Day	3 9% 31% 37% 19% 2	490
October	1st to 10th	Day	4 10% 34% 38% 13% 1	490
	11th to 20th	Day	3 12% 32% 37% 15% 1	490
	21st to end	Day	4 10% 30% 35% 18% 2	390
November	1st to 10th	Day	9% 12% 27% 36% 15% 1	300
	11th to 20th	Day	5% 13% 32% 33% 14% 2	300
	21st to end	Day	9% 16% 30% 29% 14% 2	300
December	1st to 10th	Day	7% 13% 39% 29% 11% 1	300
	11th to 20th	Day	6% 12% 33% 32% 15% 2	300
	21st to end	Day	13% 16% 39% 20% 11% 1	330
January	1st to 10th	Night	6% 10% 24% 30% 20% 10%	800
	11th to 20th	Night	5% 8% 26% 33% 20% 7%	800
	21st to end	Night	8% 11% 30% 28% 19% 4	880
February	1st to 10th	Night	8% 11% 28% 26% 18% 8%	800
	11th to 20th	Night	8% 15% 30% 27% 16% 4	784
	21st to end	Night	7% 15% 33% 27% 15% 4	660
March	1st to 10th	Night	9% 16% 33% 26% 13% 3	800
	11th to 20th	Night	8% 13% 32% 26% 17% 6%	800
	21st to end	Night	7% 18% 30% 26% 15% 3	862
April	1st to 10th	Night	5% 15% 33% 30% 14% 3	760
	11th to 20th	Night	5% 15% 36% 27% 13% 4	751
	21st to end	Night	4 14% 31% 31% 15% 5%	760
May	1st to 10th	Night	4 12% 32% 36% 14% 2	580
	11th to 20th	Night	2 10% 31% 38% 17% 3	561
	21st to end	Night	3 13% 33% 35% 15% 1	617
June	1st to 10th	Night	1 6% 28% 41% 22% 1	570
	11th to 20th	Night	2 5% 32% 40% 19% 1	570
	21st to end	Night	1 5% 26% 44% 23% 2	570
July	1st to 10th	Night	1 1 14% 46% 33% 5%	555
	11th to 20th	Night	2 1 10% 50% 33% 4	552
	21st to end	Night	2 2 7% 45% 39% 4	643
August	1st to 10th	Night	1 1 13% 45% 34% 7%	756
	11th to 20th	Night	1 1 7% 39% 43% 8%	752
	21st to end	Night	2 2 6% 37% 43% 10%	836
September	1st to 10th	Night	1 1 10% 37% 43% 8%	760
	11th to 20th	Night	2 3 21% 39% 27% 9%	760
	21st to end	Night	1 4 20% 36% 31% 8%	760
October	1st to 10th	Night	2 6% 26% 41% 21% 4	760
	11th to 20th	Night	3 6% 23% 35% 24% 9%	760
	21st to end	Night	2 7% 23% 29% 27% 11%	836
November	1st to 10th	Night	3 7% 22% 36% 22% 9%	780
	11th to 20th	Night	4 5% 24% 33% 24% 10%	800
	21st to end	Night	4 9% 28% 31% 20% 8%	800
December	1st to 10th	Night	6% 9% 26% 29% 23% 8%	800
	11th to 20th	Night	7% 7% 29% 33% 19% 5%	800
	21st to end	Night	8% 12% 30% 26% 18% 6%	880

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