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 2025-03-12.

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% 9% 3 9% 6% 5% 5% 4 5% 6% 32%	112731
	Night	16% 11% 4 8% 5% 5% 5% 4 5% 6% 31%	114947
Transparency	Day	51% 8% 14% 17% 8% 1	112486
	Night	52% 8% 13% 17% 8% 1	114838
Seeing	Day	19% 14% 25% 25% 14% 3	15542
	Night	18% 10% 22% 28% 19% 3	24069

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	22% 14% 4 11% 8% 7% 6% 5% 6% 6% 12%	3179
	11th to 20th	Day	15% 8% 3 10% 7% 6% 6% 4 7% 8% 24%	3494
	21st to end	Day	22% 12% 4 8% 5% 5% 4 4 4 5% 25%	4185
February	1st to 10th	Day	15% 13% 4 9% 8% 8% 6% 6% 6% 7% 18%	1800
	11th to 20th	Day	16% 13% 5% 10% 5% 5% 6% 4 6% 6% 23%	1755
	21st to end	Day	12% 12% 5% 10% 6% 6% 6% 5% 7% 5% 26%	1530
March	1st to 10th	Day	13% 14% 4 10% 8% 8% 6% 6% 6% 6% 19%	3361
	11th to 20th	Day	12% 9% 3 7% 4 5% 5% 4 5% 6% 38%	3503
	21st to end	Day	17% 11% 4 11% 7% 6% 6% 5% 6% 7% 20%	3917
April	1st to 10th	Day	17% 11% 4 9% 6% 6% 7% 5% 7% 7% 20%	2280
	11th to 20th	Day	16% 13% 4 9% 8% 5% 6% 4 5% 6% 26%	2301
	21st to end	Day	14% 10% 4 10% 6% 6% 6% 5% 6% 7% 25%	2489
May	1st to 10th	Day	14% 9% 3 7% 5% 5% 5% 3 5% 7% 37%	4420
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	4377
	21st to end	Day	19% 12% 4 11% 7% 7% 5% 4 5% 5% 20%	4813
June	1st to 10th	Day	16% 10% 4 10% 6% 6% 5% 5% 6% 6% 26%	2757
	11th to 20th	Day	17% 13% 4 10% 8% 7% 6% 5% 6% 6% 18%	2850
	21st to end	Day	11% 8% 3 8% 7% 5% 5% 4 6% 8% 35%	2850
July	1st to 10th	Day	8% 6% 2 6% 4 4 3 2 3 5% 58%	4740
	11th to 20th	Day	5% 4 1 4 4 3 3 4 5% 6% 61%	4617
	21st to end	Day	9% 7% 4 8% 5% 4 4 3 5% 5% 47%	4790
August	1st to 10th	Day	5% 5% 2 5% 4 4 4 4 5% 7% 55%	4280
	11th to 20th	Day	10% 10% 3 7% 4 5% 4 4 6% 8% 38%	4026
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 5% 41%	4370
September	1st to 10th	Day	8% 8% 2 6% 4 4 4 3 5% 6% 50%	2280
	11th to 20th	Day	11% 8% 3 8% 5% 5% 5% 4 6% 6% 40%	2280
	21st to end	Day	11% 7% 2 8% 5% 5% 6% 4 5% 7% 41%	2000
October	1st to 10th	Day	10% 7% 3 6% 4 4 5% 5% 7% 10% 38%	3180
	11th to 20th	Day	19% 9% 4 9% 6% 5% 5% 4 6% 8% 26%	3161
	21st to end	Day	17% 10% 5% 11% 6% 6% 6% 4 6% 7% 23%	3294
November	1st to 10th	Day	20% 11% 3 10% 6% 6% 7% 5% 6% 6% 20%	1583
	11th to 20th	Day	20% 11% 3 11% 7% 5% 5% 3 4 6% 25%	1600
	21st to end	Day	20% 10% 3 10% 7% 5% 5% 5% 6% 7% 23%	1600
December	1st to 10th	Day	13% 8% 4 9% 9% 8% 7% 5% 6% 8% 22%	2620
	11th to 20th	Day	26% 11% 3 11% 8% 6% 7% 4 5% 6% 13%	2951
	21st to end	Day	22% 13% 6% 12% 8% 6% 5% 4 5% 5% 14%	3498
January	1st to 10th	Night	23% 15% 5% 10% 7% 6% 6% 4 5% 5% 14%	6766
	11th to 20th	Night	14% 10% 4 9% 6% 5% 5% 3 5% 7% 32%	6401
	21st to end	Night	21% 14% 4 8% 4 4 5% 3 4 5% 27%	6776
February	1st to 10th	Night	15% 15% 4 10% 6% 6% 5% 4 6% 7% 22%	2600
	11th to 20th	Night	15% 14% 3 8% 6% 5% 5% 5% 6% 6% 27%	2532
	21st to end	Night	13% 12% 4 10% 7% 6% 6% 4 7% 6% 26%	2100
March	1st to 10th	Night	13% 17% 5% 11% 6% 6% 6% 4 6% 7% 19%	3732
	11th to 20th	Night	11% 10% 2 6% 4 5% 5% 4 6% 8% 39%	3504
	21st to end	Night	20% 15% 4 11% 6% 6% 5% 4 5% 5% 19%	3736
April	1st to 10th	Night	16% 10% 3 9% 8% 6% 6% 6% 6% 6% 24%	1900
	11th to 20th	Night	15% 10% 3 9% 5% 5% 6% 5% 7% 6% 29%	1825
	21st to end	Night	14% 10% 3 9% 5% 5% 6% 5% 5% 7% 30%	1691
May	1st to 10th	Night	15% 9% 3 6% 4 4 4 3 4 5% 43%	2540
	11th to 20th	Night	11% 8% 3 6% 5% 5% 6% 4 6% 7% 39%	2516
	21st to end	Night	11% 10% 3 10% 7% 7% 8% 5% 7% 7% 26%	2762
June	1st to 10th	Night	15% 7% 3 8% 5% 6% 6% 5% 6% 7% 32%	1419
	11th to 20th	Night	15% 11% 4 9% 8% 5% 5% 3 5% 8% 27%	1330
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 6% 7% 45%	1330
July	1st to 10th	Night	11% 5% 2 4 3 3 2 1 3 4 62%	2220
	11th to 20th	Night	3 2 1 3 2 3 3 3 4 6% 70%	2340
	21st to end	Night	8% 6% 3 5% 4 4 4 3 6% 6% 52%	2762
August	1st to 10th	Night	4 2 1 4 3 3 3 3 4 6% 68%	2651
	11th to 20th	Night	8% 7% 3 6% 4 3 4 3 5% 8% 50%	2812
	21st to end	Night	14% 6% 2 6% 4 4 3 3 4 6% 47%	3301
September	1st to 10th	Night	5% 4 2 5% 4 5% 3 3 6% 6% 57%	1900
	11th to 20th	Night	11% 8% 3 8% 5% 4 4 4 6% 6% 42%	1900
	21st to end	Night	12% 7% 3 7% 4 6% 5% 3 5% 7% 41%	2178
October	1st to 10th	Night	11% 7% 3 7% 5% 4 4 4 6% 9% 40%	3800
	11th to 20th	Night	18% 8% 3 8% 5% 5% 5% 4 5% 7% 30%	3777
	21st to end	Night	18% 13% 4 10% 6% 5% 4 4 5% 6% 26%	4269
November	1st to 10th	Night	21% 9% 3 8% 6% 6% 5% 4 6% 5% 25%	2711
	11th to 20th	Night	20% 11% 3 10% 6% 6% 5% 4 6% 6% 25%	2800
	21st to end	Night	16% 11% 4 9% 6% 7% 6% 5% 7% 6% 24%	2800
December	1st to 10th	Night	11% 9% 4 9% 7% 8% 7% 4 6% 8% 27%	4560
	11th to 20th	Night	28% 13% 5% 10% 6% 5% 6% 4 5% 5% 13%	5248
	21st to end	Night	24% 16% 6% 13% 6% 5% 4 3 4 4 13%	7458

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	71% 7% 9% 9% 3	3179
	11th to 20th	Day	58% 8% 14% 16% 4 1	3518
	21st to end	Day	64% 5% 10% 14% 7%	4185
February	1st to 10th	Day	67% 8% 13% 7% 5% 1	1800
	11th to 20th	Day	62% 9% 12% 9% 7% 1	1764
	21st to end	Day	60% 8% 12% 14% 5% 1	1530
March	1st to 10th	Day	58% 13% 13% 11% 4	3361
	11th to 20th	Day	47% 7% 12% 29% 4 1	3503
	21st to end	Day	61% 10% 15% 13% 2	3917
April	1st to 10th	Day	60% 9% 17% 11% 2	2280
	11th to 20th	Day	55% 9% 12% 15% 7% 2	2301
	21st to end	Day	56% 9% 15% 12% 6% 1	2489
May	1st to 10th	Day	44% 10% 16% 20% 10% 2	4420
	11th to 20th	Day	43% 12% 22% 18% 5%	4380
	21st to end	Day	61% 11% 16% 10% 2	4814
June	1st to 10th	Day	54% 9% 19% 12% 5% 1	2757
	11th to 20th	Day	61% 9% 16% 11% 3	2850
	21st to end	Day	44% 10% 21% 17% 7% 2	2850
July	1st to 10th	Day	30% 5% 9% 21% 23% 11%	4666
	11th to 20th	Day	21% 7% 10% 29% 31% 1	4536
	21st to end	Day	35% 7% 15% 30% 11% 2	4639
August	1st to 10th	Day	26% 6% 13% 36% 17% 1	4263
	11th to 20th	Day	42% 8% 19% 21% 8% 2	4065
	21st to end	Day	43% 7% 13% 24% 12% 1	4369
September	1st to 10th	Day	34% 6% 17% 23% 14% 5%	2280
	11th to 20th	Day	43% 10% 14% 16% 15% 2	2280
	21st to end	Day	44% 8% 17% 18% 10% 2	2000
October	1st to 10th	Day	39% 8% 20% 26% 7%	3180
	11th to 20th	Day	56% 8% 15% 13% 7%	3161
	21st to end	Day	60% 8% 13% 11% 7% 1	3294
November	1st to 10th	Day	64% 9% 13% 10% 5% 1	1583
	11th to 20th	Day	63% 9% 9% 12% 6% 1	1600
	21st to end	Day	64% 7% 11% 11% 6% 1	1600
December	1st to 10th	Day	60% 7% 12% 14% 7%	2620
	11th to 20th	Day	73% 9% 8% 8% 2	2954
	21st to end	Day	72% 8% 9% 8% 2	3498
January	1st to 10th	Night	73% 7% 9% 8% 2	6766
	11th to 20th	Night	52% 7% 13% 21% 6% 1	6439
	21st to end	Night	61% 6% 10% 15% 7%	6776
February	1st to 10th	Night	62% 9% 12% 10% 7%	2600
	11th to 20th	Night	58% 9% 13% 12% 7% 1	2534
	21st to end	Night	58% 8% 12% 15% 6% 1	2100
March	1st to 10th	Night	62% 10% 12% 12% 4	3732
	11th to 20th	Night	41% 8% 17% 29% 4 1	3505
	21st to end	Night	65% 10% 13% 10% 2	3736
April	1st to 10th	Night	55% 12% 16% 14% 3	1900
	11th to 20th	Night	51% 9% 15% 14% 9% 2	1825
	21st to end	Night	53% 9% 14% 16% 7% 1	1691
May	1st to 10th	Night	44% 8% 13% 24% 11%	2540
	11th to 20th	Night	40% 10% 23% 19% 8% 1	2516
	21st to end	Night	48% 16% 19% 14% 3	2762
June	1st to 10th	Night	46% 7% 23% 18% 5% 1	1419
	11th to 20th	Night	52% 8% 22% 15% 3	1330
	21st to end	Night	37% 7% 26% 18% 8% 3	1330
July	1st to 10th	Night	28% 5% 10% 22% 22% 14%	2185
	11th to 20th	Night	15% 5% 12% 32% 34% 1	2298
	21st to end	Night	29% 7% 15% 28% 17% 3	2674
August	1st to 10th	Night	17% 5% 14% 39% 23% 2	2643
	11th to 20th	Night	31% 9% 26% 21% 10% 3	2839
	21st to end	Night	38% 7% 14% 26% 14% 1	3301
September	1st to 10th	Night	26% 7% 19% 26% 17% 6%	1900
	11th to 20th	Night	42% 9% 13% 16% 15% 5%	1900
	21st to end	Night	42% 8% 13% 22% 12% 2	2178
October	1st to 10th	Night	40% 7% 15% 31% 6%	3800
	11th to 20th	Night	52% 7% 13% 21% 6%	3777
	21st to end	Night	58% 8% 11% 14% 8% 1	4269
November	1st to 10th	Night	61% 8% 13% 13% 4 1	2711
	11th to 20th	Night	62% 7% 12% 10% 7% 2	2800
	21st to end	Night	60% 9% 13% 9% 7% 1	2800
December	1st to 10th	Night	56% 9% 13% 15% 7%	4560
	11th to 20th	Night	74% 8% 10% 6% 2	5244
	21st to end	Night	71% 10% 8% 9% 2	7458

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% 14% 23% 18% 9% 4	200
	11th to 20th	Day	33% 14% 19% 18% 16% 2	200
	21st to end	Day	27% 19% 24% 18% 10% 2	297
February	1st to 10th	Day	25% 22% 28% 16% 8% 1	400
	11th to 20th	Day	25% 23% 29% 16% 6%	395
	21st to end	Day	21% 25% 29% 19% 5% 1	354
March	1st to 10th	Day	21% 22% 32% 17% 7% 1	500
	11th to 20th	Day	22% 21% 23% 20% 11% 3	465
	21st to end	Day	25% 21% 28% 18% 7%	506
April	1st to 10th	Day	23% 23% 25% 18% 8% 3	460
	11th to 20th	Day	19% 24% 28% 17% 9% 3	452
	21st to end	Day	21% 20% 32% 20% 6% 1	460
May	1st to 10th	Day	14% 13% 30% 29% 11% 2	463
	11th to 20th	Day	22% 11% 27% 22% 15% 4	488
	21st to end	Day	22% 10% 25% 26% 14% 3	529
June	1st to 10th	Day	20% 14% 27% 26% 11% 2	587
	11th to 20th	Day	21% 17% 25% 26% 9% 2	680
	21st to end	Day	14% 8% 28% 31% 16% 3	680
July	1st to 10th	Day	14% 7% 21% 36% 20% 3	671
	11th to 20th	Day	7% 6% 20% 37% 23% 6%	597
	21st to end	Day	11% 4 14% 35% 29% 7%	531
August	1st to 10th	Day	10% 7% 16% 32% 29% 6%	487
	11th to 20th	Day	8% 4 16% 36% 28% 8%	477
	21st to end	Day	14% 9% 25% 30% 17% 5%	539
September	1st to 10th	Day	12% 9% 27% 34% 16% 3	490
	11th to 20th	Day	13% 12% 22% 34% 17% 2	490
	21st to end	Day	16% 14% 25% 27% 16% 2	490
October	1st to 10th	Day	17% 14% 27% 23% 17% 3	490
	11th to 20th	Day	23% 14% 25% 22% 14% 3	490
	21st to end	Day	24% 21% 24% 18% 10% 3	447
November	1st to 10th	Day	27% 18% 26% 21% 8%	207
	11th to 20th	Day	30% 18% 19% 16% 13% 6%	200
	21st to end	Day	28% 13% 26% 14% 13% 7%	200
December	1st to 10th	Day	24% 17% 21% 15% 18% 6%	200
	11th to 20th	Day	30% 19% 28% 16% 7%	200
	21st to end	Day	28% 20% 26% 17% 8% 1	220
January	1st to 10th	Night	29% 14% 25% 18% 12% 2	900
	11th to 20th	Night	24% 13% 20% 24% 17% 2	826
	21st to end	Night	23% 9% 22% 27% 16% 3	880
February	1st to 10th	Night	20% 13% 31% 24% 10% 2	800
	11th to 20th	Night	20% 11% 27% 29% 12% 2	784
	21st to end	Night	19% 11% 33% 25% 10% 1	660
March	1st to 10th	Night	17% 15% 29% 26% 12% 1	800
	11th to 20th	Night	19% 18% 22% 26% 14% 2	766
	21st to end	Night	18% 16% 26% 28% 11% 1	836
April	1st to 10th	Night	19% 13% 24% 29% 14% 3	760
	11th to 20th	Night	16% 12% 27% 28% 15% 2	697
	21st to end	Night	18% 9% 22% 34% 15% 2	551
May	1st to 10th	Night	13% 11% 19% 37% 17% 3	380
	11th to 20th	Night	14% 5% 19% 31% 28% 3	374
	21st to end	Night	16% 7% 18% 33% 23% 4	411
June	1st to 10th	Night	16% 7% 21% 32% 21% 2	380
	11th to 20th	Night	19% 8% 21% 33% 18% 2	380
	21st to end	Night	11% 6% 16% 37% 27% 3	380
July	1st to 10th	Night	11% 2 14% 28% 40% 5%	370
	11th to 20th	Night	7% 2 13% 37% 37% 5%	368
	21st to end	Night	6% 3 8% 33% 44% 5%	416
August	1st to 10th	Night	7% 2 7% 29% 42% 12%	448
	11th to 20th	Night	6% 2 8% 28% 44% 11%	564
	21st to end	Night	11% 3 14% 33% 34% 5%	717
September	1st to 10th	Night	9% 3 14% 38% 30% 6%	760
	11th to 20th	Night	13% 5% 17% 26% 32% 7%	760
	21st to end	Night	17% 7% 18% 32% 23% 4	760
October	1st to 10th	Night	14% 8% 19% 31% 24% 4	760
	11th to 20th	Night	19% 10% 18% 28% 21% 4	760
	21st to end	Night	21% 11% 23% 27% 16% 2	836
November	1st to 10th	Night	24% 14% 23% 29% 9% 1	780
	11th to 20th	Night	26% 13% 25% 21% 14% 1	800
	21st to end	Night	21% 13% 25% 25% 13% 3	800
December	1st to 10th	Night	21% 12% 19% 30% 15% 2	800
	11th to 20th	Night	24% 15% 27% 24% 9% 2	815
	21st to end	Night	27% 15% 28% 21% 8% 1	990

Page generated at 2025-03-09 05:08:41. Version ac07

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