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 2024-10-20.

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% 72%	100808
	Night	1 2 1 2 2 2 3 3 4 6% 74%	98167
Transparency	Day	11% 2 4 12% 26% 43%	100574
	Night	11% 2 4 14% 27% 42%	97991
Seeing	Day	6% 12% 30% 34% 14% 3	15576
	Night	4 8% 25% 34% 23% 6%	25883

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 3 1 3 2 3 3 2 5% 6% 68%	2858
	11th to 20th	Day	4 3 1 3 3 3 4 4 6% 8% 61%	2906
	21st to end	Day	6% 4 1 4 3 3 4 3 7% 8% 56%	3498
February	1st to 10th	Day	4 4 2 4 3 3 3 3 5% 7% 62%	1900
	11th to 20th	Day	3 2 1 4 3 3 4 3 6% 8% 64%	1850
	21st to end	Day	2 4 2 4 4 4 4 3 6% 9% 58%	1570
March	1st to 10th	Day	1 3 1 4 3 3 4 2 6% 11% 62%	3138
	11th to 20th	Day	3 4 2 5% 3 3 4 3 5% 8% 60%	3387
	21st to end	Day	3 4 1 2 2 3 3 3 5% 6% 67%	3934
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 3 3 3 4 6% 74%	2090
	11th to 20th	Day	1 1 1 1 2 3 2 2 4 83%	2080
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 79%	1885
November	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 75%	1674
	11th to 20th	Day	1 2 1 3 3 3 3 3 5% 6% 71%	1710
	21st to end	Day	3 3 2 3 3 2 4 3 5% 7% 65%	1710
December	1st to 10th	Day	4 4 1 5% 3 3 4 3 6% 7% 59%	1710
	11th to 20th	Day	3 6% 2 5% 5% 4 5% 5% 7% 8% 50%	2169
	21st to end	Day	5% 5% 2 4 4 6% 5% 4 7% 8% 52%	3146
January	1st to 10th	Night	2 3 1 3 2 2 3 3 6% 6% 69%	4097
	11th to 20th	Night	2 3 1 3 3 3 4 3 6% 9% 63%	4048
	21st to end	Night	5% 4 1 4 3 3 4 4 6% 6% 59%	4180
February	1st to 10th	Night	2 4 2 5% 3 3 3 4 5% 7% 63%	2280
	11th to 20th	Night	2 3 1 3 3 3 4 3 5% 7% 65%	2218
	21st to end	Night	1 3 2 5% 3 5% 4 4 7% 8% 57%	1884
March	1st to 10th	Night	1 3 1 3 3 3 4 4 7% 8% 63%	3736
	11th to 20th	Night	2 3 2 3 3 3 4 4 7% 8% 61%	3593
	21st to end	Night	1 3 1 2 2 3 4 4 7% 8% 65%	3715
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 2 2 2 3 2 3 4 79%	2090
	11th to 20th	Night	2 1 1 1 1 2 2 2 4 85%	2084
	21st to end	Night	1 1 2 2 2 3 2 3 4 80%	2465
November	1st to 10th	Night	1 2 2 2 3 3 4 5% 77%	2405
	11th to 20th	Night	2 2 1 3 2 3 3 2 4 6% 73%	2470
	21st to end	Night	3 3 1 3 3 3 4 3 7% 7% 64%	2470
December	1st to 10th	Night	3 4 2 3 4 3 4 3 6% 8% 60%	2470
	11th to 20th	Night	3 4 2 5% 4 3 5% 5% 6% 10% 54%	3041
	21st to end	Night	5% 3 2 4 4 4 5% 4 7% 7% 56%	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	14% 3 6% 15% 42% 21%	2859
	11th to 20th	Day	17% 4 8% 28% 27% 16%	2906
	21st to end	Day	22% 3 9% 19% 32% 16%	3498
February	1st to 10th	Day	20% 2 6% 12% 30% 31%	1900
	11th to 20th	Day	15% 3 3 16% 32% 30%	1860
	21st to end	Day	19% 3 4 19% 27% 28%	1570
March	1st to 10th	Day	12% 2 8% 18% 36% 25%	3138
	11th to 20th	Day	18% 4 6% 22% 31% 20%	3387
	21st to end	Day	15% 2 4 22% 40% 17%	3934
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	10% 2 3 11% 24% 50%	2090
	11th to 20th	Day	6% 2 2 9% 25% 56%	2080
	21st to end	Day	8% 1 3 8% 23% 57%	1885
November	1st to 10th	Day	10% 1 3 11% 32% 43%	1674
	11th to 20th	Day	11% 3 4 12% 28% 42%	1710
	21st to end	Day	17% 2 4 13% 29% 35%	1710
December	1st to 10th	Day	20% 2 5% 19% 22% 30%	1710
	11th to 20th	Day	23% 5% 8% 22% 24% 19%	2168
	21st to end	Day	24% 5% 8% 23% 25% 16%	3146
January	1st to 10th	Night	12% 3 5% 19% 37% 24%	4097
	11th to 20th	Night	15% 4 7% 30% 29% 16%	4048
	21st to end	Night	21% 3 8% 17% 32% 18%	4180
February	1st to 10th	Night	17% 3 4 14% 25% 36%	2280
	11th to 20th	Night	17% 2 4 16% 29% 32%	2219
	21st to end	Night	17% 4 5% 17% 29% 27%	1884
March	1st to 10th	Night	13% 2 6% 20% 38% 21%	3736
	11th to 20th	Night	12% 4 9% 22% 34% 17%	3593
	21st to end	Night	12% 2 6% 20% 38% 23%	3715
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	9% 2 2 10% 28% 49%	2090
	11th to 20th	Night	6% 1 2 8% 26% 58%	2084
	21st to end	Night	7% 1 3 10% 21% 58%	2465
November	1st to 10th	Night	8% 1 3 10% 33% 45%	2405
	11th to 20th	Night	12% 2 3 11% 30% 42%	2470
	21st to end	Night	16% 2 4 14% 30% 34%	2470
December	1st to 10th	Night	19% 2 6% 19% 23% 32%	2470
	11th to 20th	Night	20% 4 8% 19% 28% 20%	3041
	21st to end	Night	21% 5% 8% 25% 25% 16%	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	11% 15% 30% 33% 10% 1	270
	11th to 20th	Day	13% 11% 29% 32% 14% 2	303
	21st to end	Day	10% 18% 36% 26% 8% 1	506
February	1st to 10th	Day	11% 18% 35% 25% 8% 3	460
	11th to 20th	Day	12% 20% 36% 26% 6%	455
	21st to end	Day	16% 26% 34% 19% 5% 1	382
March	1st to 10th	Day	13% 25% 37% 20% 5% 1	460
	11th to 20th	Day	14% 17% 40% 20% 9% 1	460
	21st to end	Day	15% 22% 40% 20% 3	506
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% 36% 15% 1	487
	21st to end	Day	3 10% 30% 35% 20% 2	354
November	1st to 10th	Day	8% 9% 29% 37% 16% 1	270
	11th to 20th	Day	4 10% 32% 36% 15% 3	270
	21st to end	Day	10% 13% 29% 31% 16% 3	270
December	1st to 10th	Day	8% 12% 38% 30% 11% 1	270
	11th to 20th	Day	6% 13% 32% 33% 14% 2	270
	21st to end	Day	13% 16% 37% 21% 12% 1	297
January	1st to 10th	Night	6% 9% 24% 30% 21% 10%	760
	11th to 20th	Night	5% 8% 25% 34% 21% 8%	760
	21st to end	Night	8% 10% 29% 28% 20% 4	836
February	1st to 10th	Night	7% 11% 27% 28% 19% 8%	760
	11th to 20th	Night	6% 14% 30% 28% 17% 5%	744
	21st to end	Night	7% 16% 32% 25% 16% 4	628
March	1st to 10th	Night	8% 15% 34% 26% 14% 3	760
	11th to 20th	Night	6% 13% 31% 27% 17% 6%	760
	21st to end	Night	8% 18% 30% 26% 15% 4	836
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% 25% 9%	758
	21st to end	Night	2 7% 23% 29% 28% 12%	792
November	1st to 10th	Night	2 7% 23% 36% 23% 9%	740
	11th to 20th	Night	3 4 24% 32% 26% 11%	760
	21st to end	Night	4 9% 27% 32% 21% 9%	760
December	1st to 10th	Night	6% 8% 27% 28% 23% 8%	760
	11th to 20th	Night	7% 7% 30% 33% 19% 5%	760
	21st to end	Night	7% 12% 29% 27% 19% 6%	836

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