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 2024-11-28.

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	22% 15% 4 7% 4 3 3 3 4 5% 30%	105713
	Night	24% 18% 4 7% 4 3 3 3 4 5% 27%	102423
Transparency	Day	58% 6% 10% 15% 9% 2	105460
	Night	61% 6% 10% 14% 8% 2	102322
Seeing	Day	31% 14% 25% 20% 8% 1	15468
	Night	34% 8% 23% 23% 11% 1	23609

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	29% 29% 4 5% 3 2 2 2 3 4 16%	2219
	11th to 20th	Day	24% 18% 4 9% 4 3 2 2 2 5% 26%	2199
	21st to end	Day	46% 22% 3 6% 2 2 2 1 2 2 12%	2703
February	1st to 10th	Day	25% 24% 5% 7% 4 3 4 3 4 5% 18%	1710
	11th to 20th	Day	20% 25% 4 7% 4 3 3 2 4 5% 23%	1665
	21st to end	Day	15% 30% 5% 10% 5% 4 3 3 4 3 18%	1418
March	1st to 10th	Day	18% 27% 7% 9% 5% 3 3 3 3 4 19%	3248
	11th to 20th	Day	17% 26% 4 6% 3 3 2 2 3 5% 29%	3490
	21st to end	Day	20% 26% 6% 10% 6% 4 3 2 3 3 17%	3930
April	1st to 10th	Day	24% 26% 5% 9% 5% 4 4 3 4 4 13%	2280
	11th to 20th	Day	23% 25% 5% 8% 5% 4 4 2 3 3 19%	2360
	21st to end	Day	25% 23% 5% 9% 5% 4 4 3 3 3 16%	2536
May	1st to 10th	Day	16% 16% 5% 9% 5% 4 4 3 4 4 29%	4420
	11th to 20th	Day	15% 13% 3 7% 5% 5% 6% 5% 7% 9% 25%	4377
	21st to end	Day	29% 18% 4 8% 5% 4 4 3 4 5% 15%	4988
June	1st to 10th	Day	32% 13% 4 7% 4 4 4 3 4 5% 20%	2850
	11th to 20th	Day	31% 15% 5% 9% 5% 4 4 3 4 5% 15%	2850
	21st to end	Day	24% 11% 3 8% 5% 4 3 3 4 6% 30%	2850
July	1st to 10th	Day	10% 6% 2 4 3 3 2 2 3 4 60%	4740
	11th to 20th	Day	8% 4 1 3 3 2 2 2 4 5% 65%	4740
	21st to end	Day	13% 5% 2 5% 4 3 4 4 6% 7% 47%	4931
August	1st to 10th	Day	8% 4 1 3 2 2 3 2 4 7% 64%	4384
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 5% 6% 40%	4026
	21st to end	Day	20% 8% 2 6% 4 4 4 3 6% 6% 38%	4437
September	1st to 10th	Day	16% 6% 3 6% 4 3 4 3 5% 6% 44%	2280
	11th to 20th	Day	23% 9% 3 6% 4 3 4 2 4 5% 38%	2275
	21st to end	Day	28% 10% 4 5% 3 3 3 3 5% 5% 32%	2000
October	1st to 10th	Day	26% 11% 4 8% 6% 4 3 3 5% 5% 25%	3180
	11th to 20th	Day	37% 11% 3 7% 4 3 4 3 4 4 21%	3161
	21st to end	Day	28% 19% 5% 11% 6% 4 3 3 4 4 14%	3141
November	1st to 10th	Day	38% 16% 3 8% 3 4 3 3 3 3 17%	1568
	11th to 20th	Day	33% 24% 4 6% 2 2 2 2 2 3 20%	1600
	21st to end	Day	32% 20% 5% 5% 4 4 3 2 3 4 19%	1576
December	1st to 10th	Day	26% 21% 4 7% 3 2 2 2 2 3 28%	1454
	11th to 20th	Day	33% 23% 4 9% 4 2 2 2 2 2 16%	1685
	21st to end	Day	31% 24% 5% 8% 5% 3 3 2 2 3 13%	2442
January	1st to 10th	Night	29% 29% 4 8% 4 3 2 2 3 3 15%	4726
	11th to 20th	Night	26% 16% 3 6% 3 3 3 4 5% 7% 23%	4699
	21st to end	Night	41% 24% 4 6% 3 2 1 1 2 3 12%	4947
February	1st to 10th	Night	25% 22% 5% 7% 5% 3 3 3 4 5% 17%	2470
	11th to 20th	Night	20% 25% 5% 7% 4 3 3 2 4 4 23%	2402
	21st to end	Night	17% 27% 4 9% 4 3 3 3 3 4 22%	2035
March	1st to 10th	Night	21% 25% 5% 8% 4 3 3 3 4 4 20%	3626
	11th to 20th	Night	16% 25% 3 6% 3 3 3 3 3 4 29%	3489
	21st to end	Night	21% 23% 5% 9% 5% 4 3 3 3 3 21%	3722
April	1st to 10th	Night	23% 23% 4 8% 6% 4 4 3 5% 4 17%	1900
	11th to 20th	Night	20% 22% 4 7% 4 4 4 3 3 5% 25%	1767
	21st to end	Night	25% 19% 4 8% 4 4 2 2 4 6% 22%	1644
May	1st to 10th	Night	15% 16% 4 5% 4 3 4 2 3 4 40%	2540
	11th to 20th	Night	16% 11% 4 8% 3 4 4 4 6% 8% 33%	2516
	21st to end	Night	28% 19% 4 6% 3 4 3 3 4 5% 21%	2584
June	1st to 10th	Night	27% 12% 4 8% 5% 4 4 4 4 4 25%	1330
	11th to 20th	Night	25% 15% 4 8% 5% 4 4 4 5% 5% 22%	1330
	21st to end	Night	21% 10% 2 7% 5% 4 3 3 4 5% 34%	1330
July	1st to 10th	Night	9% 5% 2 4 2 3 2 2 2 5% 64%	2220
	11th to 20th	Night	7% 4 1 2 3 2 1 2 4 5% 69%	2220
	21st to end	Night	14% 8% 2 4 2 3 3 3 4 6% 49%	2620
August	1st to 10th	Night	7% 3 1 2 2 1 2 2 4 6% 70%	2544
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 4 6% 41%	2812
	21st to end	Night	17% 10% 3 6% 4 3 3 3 5% 6% 39%	3232
September	1st to 10th	Night	13% 6% 2 5% 2 3 4 3 5% 7% 48%	1900
	11th to 20th	Night	23% 9% 3 5% 4 3 3 2 5% 5% 39%	1902
	21st to end	Night	26% 12% 3 5% 3 3 3 3 3 6% 32%	2179
October	1st to 10th	Night	22% 15% 5% 9% 5% 4 4 3 4 5% 24%	3800
	11th to 20th	Night	31% 14% 3 7% 4 4 3 2 4 4 24%	3777
	21st to end	Night	29% 18% 5% 8% 4 3 3 3 4 4 18%	4414
November	1st to 10th	Night	38% 18% 4 6% 4 3 3 2 3 4 15%	2730
	11th to 20th	Night	34% 22% 4 5% 4 3 3 2 2 3 18%	2800
	21st to end	Night	30% 22% 3 7% 3 3 3 2 3 3 20%	2758
December	1st to 10th	Night	25% 18% 4 6% 5% 3 3 2 4 4 25%	2724
	11th to 20th	Night	34% 23% 5% 8% 4 3 2 2 3 3 13%	3520
	21st to end	Night	33% 25% 4 7% 4 3 3 2 2 3 14%	5214

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	73% 8% 6% 9% 3 1	2219
	11th to 20th	Day	63% 3 9% 12% 7% 6%	2220
	21st to end	Day	83% 3 4 7% 3 1	2703
February	1st to 10th	Day	70% 5% 8% 8% 7% 2	1710
	11th to 20th	Day	66% 5% 7% 12% 7% 2	1674
	21st to end	Day	71% 6% 7% 9% 4 2	1418
March	1st to 10th	Day	71% 6% 8% 9% 6%	3248
	11th to 20th	Day	61% 3 8% 19% 8%	3490
	21st to end	Day	73% 6% 9% 8% 3 1	3930
April	1st to 10th	Day	75% 7% 10% 7% 1	2280
	11th to 20th	Day	71% 4 7% 8% 8% 1	2360
	21st to end	Day	75% 6% 6% 9% 5%	2536
May	1st to 10th	Day	57% 7% 12% 17% 7%	4420
	11th to 20th	Day	50% 11% 20% 15% 4	4380
	21st to end	Day	70% 9% 10% 8% 3	4989
June	1st to 10th	Day	67% 8% 10% 9% 4 2	2850
	11th to 20th	Day	73% 6% 11% 8% 3	2850
	21st to end	Day	58% 5% 13% 16% 7% 2	2850
July	1st to 10th	Day	29% 4 9% 26% 21% 12%	4666
	11th to 20th	Day	23% 5% 10% 36% 23% 3	4659
	21st to end	Day	34% 7% 19% 22% 15% 3	4775
August	1st to 10th	Day	22% 6% 10% 37% 24% 1	4367
	11th to 20th	Day	45% 7% 20% 18% 9% 2	4065
	21st to end	Day	46% 7% 17% 20% 10% 1	4436
September	1st to 10th	Day	41% 8% 11% 24% 11% 4	2280
	11th to 20th	Day	51% 4 9% 17% 16% 3	2275
	21st to end	Day	56% 6% 11% 16% 9% 2	2000
October	1st to 10th	Day	61% 7% 10% 16% 7%	3180
	11th to 20th	Day	68% 5% 8% 13% 6%	3161
	21st to end	Day	73% 5% 7% 9% 4 1	3141
November	1st to 10th	Day	73% 7% 6% 8% 5% 1	1568
	11th to 20th	Day	73% 4 7% 7% 6% 3	1600
	21st to end	Day	71% 5% 5% 9% 7% 2	1576
December	1st to 10th	Day	65% 5% 8% 11% 10% 2	1454
	11th to 20th	Day	76% 4 6% 8% 3 2	1688
	21st to end	Day	78% 6% 4 8% 3 1	2442
January	1st to 10th	Night	77% 4 4 8% 5% 1	4726
	11th to 20th	Night	59% 6% 10% 13% 8% 4	4740
	21st to end	Night	80% 4 4 6% 4 1	4947
February	1st to 10th	Night	70% 5% 7% 8% 7% 2	2470
	11th to 20th	Night	66% 5% 9% 10% 7% 3	2404
	21st to end	Night	67% 4 9% 11% 6% 2	2035
March	1st to 10th	Night	68% 5% 9% 11% 7% 1	3626
	11th to 20th	Night	59% 6% 9% 16% 10%	3490
	21st to end	Night	68% 8% 11% 8% 3 2	3722
April	1st to 10th	Night	70% 7% 10% 10% 1	1900
	11th to 20th	Night	62% 7% 9% 13% 8% 1	1767
	21st to end	Night	65% 6% 12% 11% 6%	1644
May	1st to 10th	Night	48% 6% 15% 23% 8%	2540
	11th to 20th	Night	45% 9% 25% 17% 4	2516
	21st to end	Night	66% 9% 12% 10% 3	2584
June	1st to 10th	Night	62% 6% 17% 9% 4 3	1330
	11th to 20th	Night	65% 7% 14% 10% 4	1330
	21st to end	Night	52% 9% 14% 16% 6% 2	1330
July	1st to 10th	Night	26% 5% 11% 26% 24% 9%	2185
	11th to 20th	Night	19% 5% 12% 35% 27% 2	2178
	21st to end	Night	35% 6% 14% 25% 13% 6%	2537
August	1st to 10th	Night	18% 4 12% 36% 29% 2	2536
	11th to 20th	Night	44% 7% 21% 16% 9% 3	2839
	21st to end	Night	46% 6% 18% 21% 8% 1	3232
September	1st to 10th	Night	35% 9% 15% 24% 13% 4	1900
	11th to 20th	Night	48% 6% 11% 16% 14% 5%	1902
	21st to end	Night	55% 5% 13% 16% 8% 3	2179
October	1st to 10th	Night	62% 6% 12% 12% 7%	3800
	11th to 20th	Night	65% 5% 6% 16% 8%	3777
	21st to end	Night	69% 5% 6% 13% 5% 1	4414
November	1st to 10th	Night	75% 4 7% 7% 4 2	2730
	11th to 20th	Night	73% 4 7% 7% 6% 3	2800
	21st to end	Night	71% 4 6% 8% 9% 2	2758
December	1st to 10th	Night	64% 6% 9% 11% 9% 2	2724
	11th to 20th	Night	78% 6% 5% 5% 4 2	3516
	21st to end	Night	79% 3 3 8% 5% 1	5214

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	42% 15% 19% 14% 8% 2	180
	11th to 20th	Day	43% 9% 21% 21% 7%	180
	21st to end	Day	43% 8% 24% 18% 6%	203
February	1st to 10th	Day	40% 15% 28% 12% 5%	370
	11th to 20th	Day	36% 14% 28% 18% 3 1	365
	21st to end	Day	31% 22% 27% 16% 4	310
March	1st to 10th	Day	34% 17% 25% 18% 5%	460
	11th to 20th	Day	35% 18% 19% 19% 9% 1	460
	21st to end	Day	40% 16% 23% 13% 7% 1	506
April	1st to 10th	Day	43% 17% 20% 15% 5% 1	460
	11th to 20th	Day	37% 18% 23% 15% 6% 1	452
	21st to end	Day	40% 18% 20% 15% 7% 1	460
May	1st to 10th	Day	28% 16% 26% 24% 6% 1	463
	11th to 20th	Day	32% 9% 25% 22% 11% 1	488
	21st to end	Day	31% 13% 28% 19% 8% 1	626
June	1st to 10th	Day	38% 15% 25% 18% 4 1	680
	11th to 20th	Day	38% 17% 24% 16% 5% 1	680
	21st to end	Day	26% 14% 26% 26% 6% 1	680
July	1st to 10th	Day	21% 13% 31% 26% 8% 2	671
	11th to 20th	Day	15% 13% 34% 28% 8% 2	668
	21st to end	Day	14% 10% 31% 30% 12% 3	620
August	1st to 10th	Day	14% 10% 27% 30% 13% 5%	487
	11th to 20th	Day	13% 8% 29% 33% 14% 3	477
	21st to end	Day	21% 14% 30% 24% 10% 2	539
September	1st to 10th	Day	21% 16% 30% 24% 7% 2	490
	11th to 20th	Day	31% 11% 26% 23% 8% 1	490
	21st to end	Day	32% 14% 25% 19% 8% 1	490
October	1st to 10th	Day	35% 13% 23% 19% 9% 1	490
	11th to 20th	Day	43% 8% 20% 17% 8% 2	490
	21st to end	Day	44% 12% 23% 14% 6% 1	381
November	1st to 10th	Day	41% 14% 22% 18% 6%	200
	11th to 20th	Day	48% 10% 15% 15% 12% 2	200
	21st to end	Day	44% 12% 20% 10% 10% 4	194
December	1st to 10th	Day	43% 9% 18% 15% 13% 2	180
	11th to 20th	Day	42% 12% 23% 16% 6% 1	180
	21st to end	Day	35% 11% 33% 16% 4	198
January	1st to 10th	Night	47% 10% 22% 16% 4 1	850
	11th to 20th	Night	41% 9% 25% 20% 5%	850
	21st to end	Night	39% 12% 25% 18% 5%	861
February	1st to 10th	Night	37% 9% 32% 18% 4	760
	11th to 20th	Night	32% 11% 32% 22% 3	744
	21st to end	Night	33% 12% 28% 18% 9%	628
March	1st to 10th	Night	36% 11% 23% 20% 9% 1	760
	11th to 20th	Night	41% 10% 22% 21% 7%	760
	21st to end	Night	39% 9% 25% 17% 9% 1	836
April	1st to 10th	Night	35% 9% 24% 23% 9% 1	760
	11th to 20th	Night	32% 8% 26% 26% 7% 1	639
	21st to end	Night	35% 6% 28% 20% 10%	504
May	1st to 10th	Night	23% 6% 24% 31% 16% 1	380
	11th to 20th	Night	26% 4 19% 32% 17% 1	374
	21st to end	Night	31% 5% 23% 29% 12%	411
June	1st to 10th	Night	32% 6% 27% 21% 14% 1	380
	11th to 20th	Night	33% 7% 23% 24% 12% 1	380
	21st to end	Night	27% 5% 20% 30% 17% 1	380
July	1st to 10th	Night	15% 4 17% 36% 27% 1	370
	11th to 20th	Night	15% 3 13% 37% 29% 2	368
	21st to end	Night	11% 3 12% 36% 35% 3	416
August	1st to 10th	Night	15% 10% 35% 36% 3	380
	11th to 20th	Night	13% 2 10% 38% 34% 3	564
	21st to end	Night	21% 4 18% 31% 24% 2	674
September	1st to 10th	Night	17% 4 23% 35% 19% 1	760
	11th to 20th	Night	27% 4 19% 24% 22% 4	760
	21st to end	Night	33% 6% 20% 27% 13% 1	760
October	1st to 10th	Night	30% 7% 23% 25% 14% 1	760
	11th to 20th	Night	39% 7% 20% 22% 10% 1	760
	21st to end	Night	44% 8% 26% 17% 5% 1	836
November	1st to 10th	Night	50% 10% 22% 17% 2	780
	11th to 20th	Night	48% 9% 24% 15% 4 1	800
	21st to end	Night	40% 14% 22% 18% 5% 1	788
December	1st to 10th	Night	35% 9% 31% 18% 6% 1	791
	11th to 20th	Night	47% 10% 29% 12% 2	850
	21st to end	Night	45% 12% 28% 14% 1	935

Page generated at 2024-11-24 21:22:21. Version ac07

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