Seabeck
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seabeck clear sky chart from 2005-11-06 to 2026-07-16.

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	14% 10% 3 9% 6% 5% 5% 4 6% 7% 32%	142329
	Night	15% 11% 4 8% 5% 5% 5% 4 5% 6% 32%	139074
Transparency	Day	50% 8% 15% 18% 8% 1	142043
	Night	52% 8% 14% 17% 8% 1	138942
Seeing	Day	19% 14% 24% 25% 14% 3	17377
	Night	18% 10% 21% 28% 19% 3	25660

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	23% 15% 5% 11% 7% 6% 6% 5% 6% 5% 11%	4125
	11th to 20th	Day	12% 9% 4 10% 7% 6% 6% 4 7% 8% 28%	4552
	21st to end	Day	23% 12% 4 8% 5% 5% 4 4 4 5% 25%	5422
February	1st to 10th	Day	15% 13% 4 9% 8% 8% 6% 6% 6% 7% 18%	1890
	11th to 20th	Day	16% 13% 5% 10% 5% 5% 6% 4 6% 6% 24%	1845
	21st to end	Day	13% 12% 5% 9% 6% 6% 6% 5% 6% 5% 26%	1604
March	1st to 10th	Day	13% 15% 4 10% 7% 7% 7% 5% 6% 6% 19%	4793
	11th to 20th	Day	12% 13% 4 10% 5% 6% 5% 5% 5% 6% 29%	5110
	21st to end	Day	14% 11% 4 12% 8% 7% 7% 5% 7% 7% 17%	5741
April	1st to 10th	Day	17% 10% 4 9% 6% 6% 7% 5% 7% 7% 21%	2520
	11th to 20th	Day	15% 12% 4 9% 8% 5% 6% 4 5% 6% 26%	2547
	21st to end	Day	14% 10% 4 9% 6% 6% 6% 5% 6% 7% 26%	2751
May	1st to 10th	Day	11% 7% 3 8% 5% 5% 5% 4 5% 7% 40%	8195
	11th to 20th	Day	15% 10% 4 9% 6% 5% 5% 4 7% 8% 26%	8160
	21st to end	Day	14% 8% 3 8% 5% 6% 5% 4 6% 9% 30%	8993
June	1st to 10th	Day	15% 10% 4 9% 6% 6% 5% 5% 6% 7% 28%	3047
	11th to 20th	Day	16% 12% 4 9% 8% 7% 6% 5% 6% 6% 21%	3150
	21st to end	Day	11% 8% 3 9% 7% 5% 6% 4 6% 7% 34%	3150
July	1st to 10th	Day	9% 7% 2 6% 3 4 3 2 4 5% 56%	6930
	11th to 20th	Day	5% 4 1 4 3 3 3 3 4 5% 63%	6677
	21st to end	Day	8% 7% 3 7% 4 4 4 3 5% 6% 49%	6874
August	1st to 10th	Day	5% 5% 2 5% 4 4 4 4 5% 7% 54%	4416
	11th to 20th	Day	11% 10% 3 7% 4 5% 4 4 6% 8% 38%	4156
	21st to end	Day	13% 8% 3 8% 5% 4 4 3 5% 6% 41%	4508
September	1st to 10th	Day	8% 8% 2 6% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	10% 8% 3 8% 5% 5% 5% 4 6% 6% 41%	2400
	21st to end	Day	11% 7% 3 8% 4 5% 6% 4 5% 7% 41%	2105
October	1st to 10th	Day	10% 8% 3 6% 4 4 5% 5% 7% 10% 38%	3280
	11th to 20th	Day	19% 9% 4 9% 6% 5% 5% 4 7% 8% 26%	3261
	21st to end	Day	17% 10% 5% 11% 6% 6% 6% 4 6% 7% 22%	3399
November	1st to 10th	Day	21% 11% 3 10% 6% 5% 6% 5% 6% 7% 20%	1664
	11th to 20th	Day	21% 11% 3 10% 7% 5% 5% 3 4 6% 24%	1680
	21st to end	Day	21% 11% 3 9% 7% 5% 5% 5% 5% 7% 22%	1680
December	1st to 10th	Day	13% 9% 4 9% 9% 8% 7% 5% 6% 8% 22%	2700
	11th to 20th	Day	26% 12% 3 12% 8% 6% 7% 4 5% 6% 13%	3029
	21st to end	Day	22% 13% 6% 13% 8% 6% 5% 4 5% 5% 14%	3575
January	1st to 10th	Night	24% 15% 5% 10% 6% 6% 6% 4 5% 5% 15%	8770
	11th to 20th	Night	13% 12% 5% 9% 5% 4 4 3 4 6% 36%	8329
	21st to end	Night	20% 13% 4 9% 5% 5% 5% 3 4 5% 28%	8767
February	1st to 10th	Night	14% 15% 4 10% 6% 6% 5% 4 6% 7% 22%	2730
	11th to 20th	Night	14% 14% 3 8% 6% 5% 5% 5% 6% 6% 27%	2662
	21st to end	Night	13% 12% 4 10% 8% 6% 5% 4 7% 6% 26%	2202
March	1st to 10th	Night	12% 15% 5% 11% 6% 6% 6% 5% 6% 8% 20%	5320
	11th to 20th	Night	15% 14% 3 8% 5% 5% 5% 4 5% 7% 30%	5109
	21st to end	Night	19% 14% 4 11% 7% 6% 6% 4 6% 7% 16%	5476
April	1st to 10th	Night	16% 10% 3 9% 8% 5% 6% 6% 6% 6% 25%	2100
	11th to 20th	Night	15% 10% 3 9% 5% 5% 6% 4 6% 6% 29%	2019
	21st to end	Night	14% 9% 3 9% 5% 5% 6% 5% 5% 7% 32%	1869
May	1st to 10th	Night	11% 7% 2 6% 4 4 5% 3 4 5% 47%	4724
	11th to 20th	Night	13% 9% 4 9% 5% 5% 6% 4 6% 8% 31%	4700
	21st to end	Night	10% 8% 3 9% 6% 7% 7% 5% 7% 8% 31%	5178
June	1st to 10th	Night	15% 7% 3 7% 5% 6% 6% 5% 6% 7% 33%	1568
	11th to 20th	Night	14% 10% 4 8% 7% 5% 5% 4 5% 9% 29%	1470
	21st to end	Night	8% 6% 2 8% 4 5% 5% 4 6% 7% 44%	1470
July	1st to 10th	Night	13% 6% 2 5% 3 2 3 1 3 4 58%	3250
	11th to 20th	Night	4 2 1 3 2 2 3 2 4 5% 72%	3391
	21st to end	Night	7% 5% 2 5% 4 4 4 3 6% 7% 54%	3969
August	1st to 10th	Night	4 3 1 4 3 3 3 3 4 6% 66%	2735
	11th to 20th	Night	9% 7% 3 6% 4 3 4 3 5% 8% 50%	2902
	21st to end	Night	13% 6% 2 6% 4 4 3 3 4 6% 47%	3405
September	1st to 10th	Night	5% 4 2 5% 4 5% 4 3 6% 6% 56%	2000
	11th to 20th	Night	11% 8% 3 7% 5% 4 4 4 6% 6% 42%	2000
	21st to end	Night	12% 7% 3 7% 4 6% 5% 3 5% 7% 41%	2293
October	1st to 10th	Night	11% 8% 3 7% 5% 4 4 4 6% 9% 40%	3920
	11th to 20th	Night	18% 9% 3 8% 5% 5% 5% 4 5% 7% 31%	3897
	21st to end	Night	18% 12% 4 10% 6% 5% 4 4 5% 6% 26%	4406
November	1st to 10th	Night	21% 10% 3 8% 6% 6% 5% 4 6% 5% 25%	2850
	11th to 20th	Night	20% 11% 4 10% 6% 6% 5% 4 6% 5% 24%	2940
	21st to end	Night	16% 11% 4 9% 7% 7% 5% 5% 7% 6% 23%	2940
December	1st to 10th	Night	11% 10% 4 9% 7% 8% 7% 4 6% 8% 26%	4700
	11th to 20th	Night	27% 13% 5% 10% 6% 5% 6% 4 5% 5% 13%	5390
	21st to end	Night	24% 16% 6% 13% 7% 5% 5% 3 4 4 14%	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% 8% 9% 9% 2	4125
	11th to 20th	Day	55% 7% 11% 17% 9% 1	4576
	21st to end	Day	64% 5% 10% 14% 7%	5423
February	1st to 10th	Day	67% 8% 12% 8% 5%	1890
	11th to 20th	Day	62% 9% 12% 10% 7% 1	1854
	21st to end	Day	60% 8% 13% 14% 5% 1	1604
March	1st to 10th	Day	60% 11% 12% 12% 4	4793
	11th to 20th	Day	55% 7% 11% 22% 5%	5110
	21st to end	Day	63% 9% 14% 11% 2	5741
April	1st to 10th	Day	59% 9% 18% 12% 2	2520
	11th to 20th	Day	54% 9% 13% 15% 7% 2	2547
	21st to end	Day	55% 9% 16% 13% 6% 1	2751
May	1st to 10th	Day	40% 8% 17% 25% 9% 1	8195
	11th to 20th	Day	50% 11% 19% 17% 4	8163
	21st to end	Day	47% 10% 24% 16% 3	8994
June	1st to 10th	Day	51% 9% 19% 14% 6% 1	3047
	11th to 20th	Day	58% 9% 15% 14% 4	3150
	21st to end	Day	45% 10% 19% 17% 7% 2	3150
July	1st to 10th	Day	31% 6% 18% 20% 18% 8%	6856
	11th to 20th	Day	21% 7% 15% 27% 28% 2	6596
	21st to end	Day	32% 7% 22% 29% 8% 1	6681
August	1st to 10th	Day	27% 6% 13% 35% 17% 1	4399
	11th to 20th	Day	42% 8% 19% 21% 8% 2	4195
	21st to end	Day	43% 7% 13% 24% 12% 1	4507
September	1st to 10th	Day	35% 7% 18% 22% 13% 5%	2400
	11th to 20th	Day	42% 10% 13% 18% 15% 2	2400
	21st to end	Day	44% 8% 18% 19% 10% 2	2105
October	1st to 10th	Day	39% 8% 20% 26% 7%	3280
	11th to 20th	Day	56% 8% 15% 13% 7%	3261
	21st to end	Day	60% 8% 13% 11% 7% 1	3399
November	1st to 10th	Day	63% 9% 13% 10% 4 1	1664
	11th to 20th	Day	63% 9% 9% 11% 6% 1	1680
	21st to end	Day	64% 6% 11% 11% 6% 1	1680
December	1st to 10th	Day	60% 7% 12% 14% 7%	2700
	11th to 20th	Day	73% 9% 8% 8% 2	3032
	21st to end	Day	72% 8% 9% 8% 2	3575
January	1st to 10th	Night	72% 7% 9% 9% 3	8770
	11th to 20th	Night	51% 6% 11% 19% 12% 1	8367
	21st to end	Night	59% 6% 10% 17% 8%	8767
February	1st to 10th	Night	62% 9% 12% 10% 7%	2730
	11th to 20th	Night	57% 9% 13% 12% 8% 1	2664
	21st to end	Night	59% 8% 13% 15% 5%	2202
March	1st to 10th	Night	58% 11% 13% 13% 6%	5320
	11th to 20th	Night	51% 8% 14% 23% 4 1	5110
	21st to end	Night	64% 10% 13% 10% 2	5476
April	1st to 10th	Night	54% 12% 16% 14% 3	2100
	11th to 20th	Night	50% 9% 16% 14% 10% 1	2019
	21st to end	Night	51% 9% 15% 17% 8% 1	1869
May	1st to 10th	Night	38% 6% 12% 31% 12% 1	4721
	11th to 20th	Night	47% 10% 17% 20% 6%	4700
	21st to end	Night	44% 14% 24% 17% 1	5182
June	1st to 10th	Night	45% 8% 23% 19% 5% 1	1568
	11th to 20th	Night	48% 9% 22% 18% 4	1470
	21st to end	Night	37% 7% 26% 19% 8% 3	1470
July	1st to 10th	Night	31% 6% 17% 19% 17% 9%	3215
	11th to 20th	Night	15% 5% 17% 30% 32% 2	3349
	21st to end	Night	25% 8% 24% 28% 13% 2	3857
August	1st to 10th	Night	18% 5% 14% 38% 22% 2	2727
	11th to 20th	Night	32% 9% 26% 21% 10% 3	2929
	21st to end	Night	37% 7% 14% 26% 14% 1	3405
September	1st to 10th	Night	26% 7% 20% 25% 16% 5%	2000
	11th to 20th	Night	41% 9% 14% 17% 15% 5%	2000
	21st to end	Night	43% 8% 13% 22% 12% 2	2293
October	1st to 10th	Night	41% 7% 15% 31% 6%	3920
	11th to 20th	Night	52% 7% 13% 21% 6%	3897
	21st to end	Night	57% 8% 11% 14% 8% 1	4406
November	1st to 10th	Night	61% 8% 12% 13% 4 1	2850
	11th to 20th	Night	63% 7% 12% 10% 6% 2	2940
	21st to end	Night	60% 9% 13% 9% 7% 1	2940
December	1st to 10th	Night	57% 9% 13% 14% 7%	4700
	11th to 20th	Night	73% 8% 10% 6% 2	5386
	21st to end	Night	71% 10% 8% 9% 2	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	33% 15% 24% 17% 8% 4	220
	11th to 20th	Day	32% 13% 20% 17% 17% 1	220
	21st to end	Day	28% 18% 24% 17% 11% 2	323
February	1st to 10th	Day	25% 21% 29% 16% 8% 1	430
	11th to 20th	Day	25% 23% 30% 16% 6%	425
	21st to end	Day	23% 25% 27% 19% 4 1	380
March	1st to 10th	Day	23% 22% 31% 17% 6% 1	540
	11th to 20th	Day	27% 21% 21% 18% 10% 2	540
	21st to end	Day	25% 21% 29% 19% 6%	594
April	1st to 10th	Day	22% 21% 26% 19% 9% 3	540
	11th to 20th	Day	20% 22% 26% 20% 9% 3	532
	21st to end	Day	20% 19% 30% 21% 9% 2	540
May	1st to 10th	Day	13% 13% 30% 29% 13% 3	543
	11th to 20th	Day	23% 12% 26% 22% 14% 4	568
	21st to end	Day	21% 10% 24% 25% 16% 4	617
June	1st to 10th	Day	19% 14% 26% 27% 12% 3	677
	11th to 20th	Day	19% 16% 25% 27% 11% 2	780
	21st to end	Day	14% 9% 26% 31% 17% 3	780
July	1st to 10th	Day	14% 7% 20% 35% 20% 4	771
	11th to 20th	Day	7% 6% 20% 36% 24% 6%	669
	21st to end	Day	11% 4 13% 33% 30% 8%	575
August	1st to 10th	Day	11% 7% 16% 31% 30% 6%	527
	11th to 20th	Day	9% 6% 16% 35% 26% 8%	517
	21st to end	Day	14% 9% 24% 31% 17% 6%	583
September	1st to 10th	Day	12% 8% 25% 32% 19% 4	530
	11th to 20th	Day	12% 11% 23% 32% 18% 3	530
	21st to end	Day	16% 13% 25% 27% 16% 2	530
October	1st to 10th	Day	17% 14% 26% 23% 17% 3	530
	11th to 20th	Day	23% 14% 25% 22% 13% 3	530
	21st to end	Day	26% 21% 25% 17% 9% 2	486
November	1st to 10th	Day	27% 18% 26% 20% 8%	228
	11th to 20th	Day	31% 17% 18% 18% 12% 5%	220
	21st to end	Day	30% 13% 25% 15% 12% 6%	220
December	1st to 10th	Day	26% 18% 20% 15% 16% 5%	220
	11th to 20th	Day	32% 19% 28% 15% 6%	220
	21st to end	Day	28% 19% 24% 18% 9% 2	242
January	1st to 10th	Night	29% 15% 25% 18% 11% 2	950
	11th to 20th	Night	24% 13% 20% 24% 18% 2	868
	21st to end	Night	22% 10% 22% 27% 16% 3	924
February	1st to 10th	Night	20% 14% 31% 23% 10% 2	840
	11th to 20th	Night	20% 11% 26% 28% 13% 2	824
	21st to end	Night	19% 11% 33% 25% 10% 1	692
March	1st to 10th	Night	18% 15% 29% 26% 11% 1	840
	11th to 20th	Night	22% 19% 22% 24% 12% 2	840
	21st to end	Night	19% 16% 26% 28% 10% 1	924
April	1st to 10th	Night	18% 13% 23% 28% 15% 3	840
	11th to 20th	Night	17% 12% 27% 28% 15% 1	771
	21st to end	Night	17% 9% 21% 35% 15% 2	609
May	1st to 10th	Night	13% 10% 18% 38% 19% 3	420
	11th to 20th	Night	14% 6% 19% 32% 26% 3	414
	21st to end	Night	16% 7% 16% 34% 23% 4	455
June	1st to 10th	Night	15% 7% 19% 34% 23% 2	420
	11th to 20th	Night	17% 7% 19% 35% 20% 2	420
	21st to end	Night	11% 5% 15% 36% 29% 3	420
July	1st to 10th	Night	11% 2 13% 27% 41% 5%	410
	11th to 20th	Night	7% 2 12% 38% 37% 5%	398
	21st to end	Night	6% 3 8% 33% 45% 6%	438
August	1st to 10th	Night	8% 2 7% 29% 42% 12%	472
	11th to 20th	Night	8% 3 8% 28% 43% 11%	594
	21st to end	Night	10% 3 13% 33% 35% 5%	755
September	1st to 10th	Night	8% 3 14% 38% 31% 6%	800
	11th to 20th	Night	13% 5% 17% 27% 32% 7%	800
	21st to end	Night	17% 7% 18% 32% 23% 4	800
October	1st to 10th	Night	14% 8% 20% 31% 23% 4	800
	11th to 20th	Night	19% 11% 18% 28% 21% 4	800
	21st to end	Night	21% 12% 23% 27% 16% 2	880
November	1st to 10th	Night	25% 14% 23% 29% 9% 1	820
	11th to 20th	Night	27% 13% 24% 21% 14% 1	840
	21st to end	Night	21% 13% 24% 25% 13% 3	840
December	1st to 10th	Night	22% 13% 19% 30% 15% 2	840
	11th to 20th	Night	26% 15% 26% 23% 9% 2	857
	21st to end	Night	27% 16% 27% 21% 9% 1	1045

Page generated at 2026-07-13 00:14:51. Version ac07

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