Diamond Trailhead
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Diamond Trailhead clear sky chart from 2005-11-06 to 2026-02-01.

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	21% 15% 4 7% 4 4 4 3 4 5% 30%	131564
	Night	23% 18% 4 7% 4 3 3 3 4 5% 28%	129164
Transparency	Day	57% 6% 12% 16% 8% 2	131268
	Night	60% 5% 11% 14% 8% 2	129037
Seeing	Day	31% 14% 26% 20% 8% 2	16900
	Night	34% 8% 24% 23% 11% 1	25182

By Month

Percentage Cloud Cover NW (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	27% 28% 4 5% 3 3 3 3 4 4 16%	3249
	11th to 20th	Day	17% 13% 4 8% 5% 4 3 3 3 6% 35%	3229
	21st to end	Day	35% 18% 3 5% 2 2 2 1 2 2 28%	3985
February	1st to 10th	Day	24% 23% 6% 7% 4 3 4 3 4 5% 19%	1801
	11th to 20th	Day	20% 25% 4 7% 4 3 3 2 4 5% 24%	1755
	21st to end	Day	15% 30% 5% 10% 5% 4 4 3 4 3 18%	1491
March	1st to 10th	Day	20% 23% 6% 10% 5% 4 4 3 3 4 19%	4667
	11th to 20th	Day	15% 29% 6% 9% 5% 4 3 2 3 4 22%	5000
	21st to end	Day	20% 27% 6% 12% 6% 5% 3 2 3 3 13%	5630
April	1st to 10th	Day	23% 25% 5% 9% 5% 4 4 4 4 4 13%	2400
	11th to 20th	Day	22% 24% 5% 8% 5% 4 4 2 3 3 20%	2486
	21st to end	Day	24% 22% 5% 9% 5% 4 4 3 3 3 18%	2670
May	1st to 10th	Day	14% 14% 5% 9% 6% 4 4 3 4 4 33%	6320
	11th to 20th	Day	18% 16% 4 8% 5% 4 5% 4 6% 8% 21%	6277
	21st to end	Day	25% 15% 4 8% 5% 5% 5% 4 5% 6% 19%	7148
June	1st to 10th	Day	31% 12% 4 7% 4 4 4 3 4 5% 22%	3000
	11th to 20th	Day	30% 15% 5% 10% 6% 4 4 3 4 5% 15%	3000
	21st to end	Day	24% 12% 3 8% 5% 4 4 3 4 6% 29%	3000
July	1st to 10th	Day	11% 5% 2 4 3 3 3 3 4 5% 56%	6780
	11th to 20th	Day	6% 4 2 3 2 2 2 3 4 6% 66%	6780
	21st to end	Day	11% 5% 1 4 3 3 4 3 5% 7% 54%	7085
August	1st to 10th	Day	8% 4 1 3 2 2 3 2 4 7% 63%	4524
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 5% 6% 40%	4156
	21st to end	Day	19% 7% 2 6% 4 4 4 3 6% 6% 39%	4577
September	1st to 10th	Day	15% 6% 3 6% 4 3 4 3 5% 6% 44%	2400
	11th to 20th	Day	22% 9% 3 6% 4 3 4 3 4 5% 38%	2395
	21st to end	Day	27% 10% 4 5% 3 3 3 3 5% 5% 32%	2105
October	1st to 10th	Day	26% 11% 4 8% 6% 4 3 3 5% 5% 25%	3280
	11th to 20th	Day	36% 12% 3 7% 4 3 4 3 4 4 21%	3261
	21st to end	Day	28% 19% 6% 11% 6% 4 3 3 3 4 14%	3241
November	1st to 10th	Day	36% 17% 3 8% 3 4 3 3 3 3 17%	1648
	11th to 20th	Day	34% 24% 4 6% 2 2 2 2 2 3 19%	1680
	21st to end	Day	32% 20% 4 6% 4 4 3 2 3 4 18%	1680
December	1st to 10th	Day	22% 16% 4 8% 4 4 4 2 3 4 29%	2574
	11th to 20th	Day	34% 25% 5% 10% 5% 3 2 2 2 2 11%	2715
	21st to end	Day	31% 25% 5% 9% 5% 4 3 2 3 3 10%	3575
January	1st to 10th	Night	26% 26% 5% 9% 4 3 3 2 3 3 17%	6916
	11th to 20th	Night	19% 12% 3 6% 3 4 4 3 5% 8% 33%	6889
	21st to end	Night	30% 18% 3 6% 3 2 2 2 3 4 28%	7208
February	1st to 10th	Night	23% 21% 6% 8% 5% 3 3 3 4 5% 18%	2600
	11th to 20th	Night	19% 25% 5% 8% 4 3 3 2 4 4 24%	2532
	21st to end	Night	18% 27% 4 9% 4 3 3 3 3 3 21%	2138
March	1st to 10th	Night	21% 22% 6% 9% 4 3 4 3 4 4 20%	5214
	11th to 20th	Night	14% 30% 5% 9% 4 3 3 3 3 4 23%	4999
	21st to end	Night	23% 25% 6% 10% 4 4 3 2 3 4 16%	5344
April	1st to 10th	Night	23% 23% 4 8% 6% 4 4 3 5% 4 16%	2000
	11th to 20th	Night	19% 22% 4 7% 4 3 3 3 4 6% 25%	1861
	21st to end	Night	24% 18% 4 8% 4 4 2 2 4 6% 23%	1730
May	1st to 10th	Night	16% 15% 4 6% 3 3 3 2 3 4 42%	3640
	11th to 20th	Night	18% 16% 4 8% 4 4 4 3 5% 7% 26%	3616
	21st to end	Night	22% 17% 4 7% 4 5% 4 3 4 6% 24%	3724
June	1st to 10th	Night	26% 11% 3 7% 5% 4 4 4 4 5% 28%	1400
	11th to 20th	Night	25% 15% 4 8% 6% 4 4 4 5% 5% 22%	1400
	21st to end	Night	21% 11% 2 7% 6% 4 3 3 4 5% 34%	1400
July	1st to 10th	Night	11% 6% 2 3 2 3 2 2 3 5% 61%	3180
	11th to 20th	Night	5% 3 1 2 2 2 1 2 3 5% 73%	3180
	21st to end	Night	10% 7% 2 4 2 3 3 3 4 7% 54%	3757
August	1st to 10th	Night	8% 3 1 2 2 1 3 2 4 6% 69%	2624
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 4 6% 41%	2902
	21st to end	Night	16% 10% 3 6% 4 3 3 3 5% 6% 40%	3334
September	1st to 10th	Night	13% 7% 2 6% 2 3 3 3 5% 8% 48%	2000
	11th to 20th	Night	22% 9% 3 5% 4 3 4 2 5% 5% 39%	2002
	21st to end	Night	27% 12% 3 5% 3 3 3 3 3 6% 33%	2294
October	1st to 10th	Night	22% 15% 5% 9% 5% 4 4 3 4 5% 25%	3920
	11th to 20th	Night	31% 14% 3 7% 4 4 3 2 4 4 25%	3897
	21st to end	Night	29% 19% 5% 8% 4 3 3 3 4 4 18%	4556
November	1st to 10th	Night	38% 18% 4 6% 4 3 3 2 3 4 14%	2870
	11th to 20th	Night	34% 22% 4 5% 4 3 3 3 2 3 18%	2940
	21st to end	Night	30% 22% 3 7% 3 3 3 2 3 3 20%	2940
December	1st to 10th	Night	22% 17% 5% 7% 4 4 4 3 4 6% 25%	4824
	11th to 20th	Night	34% 23% 5% 8% 4 3 3 2 3 3 11%	5710
	21st to end	Night	32% 27% 5% 9% 4 3 3 2 2 3 11%	7623

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	72% 7% 7% 10% 3 1	3249
	11th to 20th	Day	52% 5% 12% 16% 10% 4	3250
	21st to end	Day	68% 2 4 17% 9% 1	3985
February	1st to 10th	Day	69% 5% 7% 8% 8% 2	1801
	11th to 20th	Day	65% 5% 7% 13% 7% 2	1764
	21st to end	Day	72% 6% 7% 9% 4 2	1491
March	1st to 10th	Day	70% 6% 6% 11% 6%	4667
	11th to 20th	Day	68% 4 6% 15% 6%	5000
	21st to end	Day	77% 5% 7% 7% 2 1	5630
April	1st to 10th	Day	75% 7% 10% 7% 1	2400
	11th to 20th	Day	70% 5% 7% 10% 8% 1	2486
	21st to end	Day	72% 6% 7% 10% 5%	2670
May	1st to 10th	Day	53% 6% 13% 18% 10% 1	6320
	11th to 20th	Day	59% 9% 18% 12% 3	6280
	21st to end	Day	64% 9% 13% 11% 3	7149
June	1st to 10th	Day	64% 8% 11% 9% 5% 2	3000
	11th to 20th	Day	72% 6% 11% 9% 3	3000
	21st to end	Day	58% 5% 13% 16% 7% 2	3000
July	1st to 10th	Day	31% 5% 16% 23% 16% 8%	6706
	11th to 20th	Day	21% 6% 16% 32% 21% 2	6699
	21st to end	Day	29% 6% 26% 23% 14% 2	6886
August	1st to 10th	Day	23% 6% 10% 36% 23% 1	4507
	11th to 20th	Day	45% 7% 20% 18% 9% 2	4195
	21st to end	Day	45% 7% 17% 20% 10% 1	4576
September	1st to 10th	Day	40% 8% 12% 25% 12% 4	2400
	11th to 20th	Day	50% 5% 10% 18% 16% 3	2395
	21st to end	Day	56% 6% 12% 15% 9% 2	2105
October	1st to 10th	Day	61% 6% 11% 16% 7%	3280
	11th to 20th	Day	68% 5% 8% 13% 6%	3261
	21st to end	Day	74% 5% 7% 9% 4 1	3241
November	1st to 10th	Day	73% 6% 6% 8% 5% 1	1648
	11th to 20th	Day	74% 4 6% 7% 6% 3	1680
	21st to end	Day	72% 5% 5% 9% 7% 2	1680
December	1st to 10th	Day	61% 4 10% 14% 9% 1	2574
	11th to 20th	Day	83% 3 5% 5% 2 1	2718
	21st to end	Day	81% 6% 4 7% 3 1	3575
January	1st to 10th	Night	75% 4 5% 9% 7% 1	6916
	11th to 20th	Night	49% 6% 12% 20% 10% 3	6930
	21st to end	Night	63% 4 8% 15% 9% 1	7208
February	1st to 10th	Night	69% 6% 7% 8% 8% 2	2600
	11th to 20th	Night	66% 5% 8% 10% 7% 3	2534
	21st to end	Night	68% 4 9% 10% 6% 2	2138
March	1st to 10th	Night	68% 5% 8% 11% 7% 1	5214
	11th to 20th	Night	67% 6% 7% 13% 8%	5000
	21st to end	Night	73% 7% 9% 7% 2 1	5344
April	1st to 10th	Night	71% 7% 11% 10% 1	2000
	11th to 20th	Night	61% 7% 10% 13% 7% 1	1861
	21st to end	Night	64% 6% 12% 12% 6%	1730
May	1st to 10th	Night	47% 5% 16% 22% 10%	3637
	11th to 20th	Night	54% 8% 22% 13% 3	3616
	21st to end	Night	63% 7% 14% 13% 3	3724
June	1st to 10th	Night	59% 6% 18% 9% 5% 3	1400
	11th to 20th	Night	64% 7% 13% 11% 4	1400
	21st to end	Night	53% 9% 14% 16% 6% 2	1400
July	1st to 10th	Night	28% 6% 19% 24% 17% 6%	3145
	11th to 20th	Night	16% 6% 20% 31% 25% 1	3138
	21st to end	Night	30% 5% 23% 23% 14% 4	3651
August	1st to 10th	Night	19% 4 12% 35% 28% 2	2616
	11th to 20th	Night	44% 7% 21% 15% 9% 3	2929
	21st to end	Night	45% 6% 18% 21% 9% 2	3334
September	1st to 10th	Night	35% 9% 16% 24% 13% 4	2000
	11th to 20th	Night	48% 6% 11% 17% 13% 5%	2002
	21st to end	Night	55% 5% 14% 16% 8% 2	2294
October	1st to 10th	Night	62% 6% 13% 13% 6%	3920
	11th to 20th	Night	65% 5% 7% 16% 8%	3897
	21st to end	Night	70% 5% 6% 13% 5% 1	4556
November	1st to 10th	Night	75% 4 7% 7% 4 1	2870
	11th to 20th	Night	73% 4 7% 7% 5% 3	2940
	21st to end	Night	71% 4 6% 9% 9% 2	2940
December	1st to 10th	Night	61% 5% 10% 15% 7% 2	4824
	11th to 20th	Night	80% 5% 6% 6% 3 1	5706
	21st to end	Night	81% 4 4 7% 4 1	7623

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	40% 15% 20% 16% 7% 1	220
	11th to 20th	Day	38% 8% 22% 24% 7% 1	220
	21st to end	Day	41% 7% 24% 20% 6% 1	249
February	1st to 10th	Day	38% 17% 27% 12% 6% 1	401
	11th to 20th	Day	36% 13% 28% 18% 4 1	395
	21st to end	Day	33% 22% 26% 15% 4	335
March	1st to 10th	Day	35% 17% 25% 18% 5%	500
	11th to 20th	Day	36% 17% 20% 18% 8% 1	500
	21st to end	Day	40% 17% 23% 12% 7% 1	550
April	1st to 10th	Day	42% 16% 20% 15% 5% 1	500
	11th to 20th	Day	35% 18% 25% 15% 6% 1	492
	21st to end	Day	38% 18% 20% 15% 8% 2	500
May	1st to 10th	Day	28% 16% 27% 22% 6% 1	503
	11th to 20th	Day	33% 9% 25% 21% 11% 1	528
	21st to end	Day	31% 13% 28% 18% 8% 1	675
June	1st to 10th	Day	36% 15% 26% 18% 4 1	730
	11th to 20th	Day	36% 18% 24% 16% 5% 1	730
	21st to end	Day	27% 15% 25% 25% 6% 1	730
July	1st to 10th	Day	21% 13% 31% 26% 8% 2	721
	11th to 20th	Day	15% 13% 35% 27% 8% 2	718
	21st to end	Day	13% 10% 31% 31% 12% 3	669
August	1st to 10th	Day	14% 10% 28% 30% 13% 6%	527
	11th to 20th	Day	15% 9% 29% 31% 13% 3	517
	21st to end	Day	19% 14% 30% 25% 11% 2	583
September	1st to 10th	Day	20% 15% 29% 25% 9% 2	530
	11th to 20th	Day	29% 11% 26% 24% 8% 2	530
	21st to end	Day	32% 16% 25% 19% 8% 1	530
October	1st to 10th	Day	35% 14% 23% 19% 8% 1	530
	11th to 20th	Day	42% 10% 21% 16% 8% 2	530
	21st to end	Day	44% 13% 22% 14% 6% 1	415
November	1st to 10th	Day	43% 14% 22% 16% 5%	220
	11th to 20th	Day	50% 9% 16% 14% 11% 1	220
	21st to end	Day	43% 13% 21% 10% 9% 4	220
December	1st to 10th	Day	42% 8% 17% 16% 13% 3	220
	11th to 20th	Day	46% 13% 20% 15% 5%	220
	21st to end	Day	37% 11% 31% 17% 4	242
January	1st to 10th	Night	45% 11% 23% 16% 4 1	950
	11th to 20th	Night	38% 9% 29% 20% 4	950
	21st to end	Night	37% 11% 27% 20% 4	954
February	1st to 10th	Night	35% 10% 32% 19% 4	800
	11th to 20th	Night	32% 11% 32% 22% 4	784
	21st to end	Night	34% 13% 27% 17% 8%	660
March	1st to 10th	Night	36% 11% 24% 20% 9% 1	800
	11th to 20th	Night	41% 10% 21% 21% 7%	800
	21st to end	Night	40% 9% 25% 17% 9% 1	880
April	1st to 10th	Night	35% 9% 23% 23% 8% 1	800
	11th to 20th	Night	31% 9% 26% 27% 7% 1	673
	21st to end	Night	34% 6% 28% 21% 11%	530
May	1st to 10th	Night	23% 6% 24% 31% 16% 1	400
	11th to 20th	Night	27% 5% 19% 31% 16% 1	394
	21st to end	Night	30% 5% 23% 30% 12%	433
June	1st to 10th	Night	31% 6% 27% 22% 14% 1	400
	11th to 20th	Night	32% 7% 23% 24% 13% 1	400
	21st to end	Night	27% 6% 19% 30% 18% 1	400
July	1st to 10th	Night	15% 4 16% 36% 28% 1	390
	11th to 20th	Night	14% 3 13% 38% 30% 2	388
	21st to end	Night	11% 3 12% 35% 36% 3	438
August	1st to 10th	Night	17% 1 10% 34% 36% 3	400
	11th to 20th	Night	15% 2 10% 37% 33% 3	594
	21st to end	Night	20% 4 18% 33% 25% 2	710
September	1st to 10th	Night	17% 4 23% 35% 21% 1	800
	11th to 20th	Night	26% 4 19% 26% 21% 3	800
	21st to end	Night	34% 6% 20% 27% 12% 1	800
October	1st to 10th	Night	30% 7% 24% 25% 14% 1	800
	11th to 20th	Night	39% 7% 21% 22% 10% 1	800
	21st to end	Night	44% 8% 26% 16% 5% 1	880
November	1st to 10th	Night	50% 10% 22% 16% 1	820
	11th to 20th	Night	47% 9% 24% 15% 4 1	840
	21st to end	Night	40% 13% 23% 18% 5% 1	840
December	1st to 10th	Night	36% 9% 30% 18% 6%	879
	11th to 20th	Night	48% 11% 28% 12% 2	950
	21st to end	Night	44% 14% 28% 14% 1	1045

Page generated at 2026-01-28 22:21:45. Version ac07

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