Friday Harbor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Friday Harbor clear sky chart from 2005-11-06 to 2025-03-27.

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	16% 8% 3 7% 5% 4 4 4 5% 6% 36%	112589
	Night	19% 10% 3 8% 5% 4 4 3 5% 6% 33%	115412
Transparency	Day	47% 9% 15% 19% 8% 1	112340
	Night	51% 8% 14% 18% 7% 1	115307
Seeing	Day	18% 4 13% 25% 27% 13%	15715
	Night	23% 6% 17% 25% 22% 6%	24154

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	30% 14% 4 11% 6% 5% 4 4 4 4 13%	3179
	11th to 20th	Day	16% 13% 4 9% 5% 4 4 4 6% 7% 28%	3231
	21st to end	Day	25% 14% 4 11% 6% 5% 4 4 4 5% 18%	4119
February	1st to 10th	Day	23% 15% 4 9% 7% 6% 5% 4 5% 5% 17%	1800
	11th to 20th	Day	24% 14% 4 8% 5% 5% 5% 4 4 5% 22%	1755
	21st to end	Day	21% 14% 4 9% 7% 5% 6% 4 5% 5% 20%	1510
March	1st to 10th	Day	16% 13% 4 10% 7% 6% 6% 5% 7% 8% 18%	3361
	11th to 20th	Day	16% 9% 3 7% 4 4 5% 4 5% 7% 35%	3600
	21st to end	Day	19% 12% 4 9% 6% 6% 5% 4 6% 6% 22%	3981
April	1st to 10th	Day	19% 12% 3 8% 6% 6% 5% 4 7% 7% 23%	2280
	11th to 20th	Day	18% 10% 3 7% 5% 5% 5% 5% 6% 7% 29%	2332
	21st to end	Day	19% 10% 3 8% 6% 5% 5% 3 6% 7% 28%	2505
May	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 5% 7% 44%	4420
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 10% 44%	4377
	21st to end	Day	20% 10% 3 7% 5% 5% 5% 4 5% 7% 30%	4829
June	1st to 10th	Day	16% 8% 2 6% 6% 4 5% 4 6% 7% 37%	2850
	11th to 20th	Day	13% 6% 2 8% 6% 5% 5% 5% 6% 8% 37%	2850
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 7% 7% 45%	2850
July	1st to 10th	Day	4 3 1 4 3 2 3 2 3 4 71%	4740
	11th to 20th	Day	4 2 1 2 3 2 2 3 3 5% 73%	4740
	21st to end	Day	10% 3 1 3 2 2 2 2 4 5% 64%	4802
August	1st to 10th	Day	4 2 1 3 2 2 3 2 4 6% 71%	4318
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 5% 8% 49%	4026
	21st to end	Day	12% 5% 2 7% 4 4 4 3 5% 7% 47%	4403
September	1st to 10th	Day	9% 4 2 6% 4 3 4 3 4 7% 53%	2280
	11th to 20th	Day	12% 7% 3 7% 6% 4 5% 4 6% 7% 41%	2275
	21st to end	Day	16% 7% 3 8% 5% 4 5% 4 4 7% 38%	2000
October	1st to 10th	Day	17% 6% 3 6% 5% 5% 6% 5% 7% 8% 33%	3180
	11th to 20th	Day	28% 9% 3 8% 5% 4 4 3 5% 6% 24%	3161
	21st to end	Day	21% 10% 4 11% 6% 6% 5% 4 5% 6% 21%	3238
November	1st to 10th	Day	29% 11% 3 9% 6% 6% 6% 4 4 5% 18%	1568
	11th to 20th	Day	26% 11% 4 8% 6% 5% 5% 4 3 5% 23%	1600
	21st to end	Day	27% 12% 4 9% 6% 6% 6% 3 4 5% 19%	1600
December	1st to 10th	Day	19% 11% 4 12% 8% 5% 5% 4 5% 6% 21%	2620
	11th to 20th	Day	30% 14% 6% 11% 6% 5% 4 3 4 5% 11%	2711
	21st to end	Day	26% 16% 6% 12% 7% 5% 6% 4 4 5% 11%	3498
January	1st to 10th	Night	28% 16% 5% 11% 7% 5% 5% 3 4 4 13%	6766
	11th to 20th	Night	18% 10% 3 7% 4 4 4 4 5% 7% 33%	6665
	21st to end	Night	26% 12% 3 9% 5% 5% 4 3 4 5% 25%	6837
February	1st to 10th	Night	27% 13% 4 8% 5% 4 4 3 5% 5% 22%	2600
	11th to 20th	Night	23% 11% 4 8% 5% 4 4 3 4 5% 28%	2532
	21st to end	Night	24% 13% 4 7% 5% 5% 5% 3 4 6% 24%	2120
March	1st to 10th	Night	17% 11% 3 9% 6% 6% 6% 5% 6% 7% 23%	3731
	11th to 20th	Night	15% 9% 2 6% 5% 5% 5% 3 6% 7% 38%	3599
	21st to end	Night	22% 14% 4 9% 6% 5% 4 4 5% 4 22%	3809
April	1st to 10th	Night	21% 11% 3 8% 6% 5% 4 4 7% 6% 26%	1900
	11th to 20th	Night	17% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	1796
	21st to end	Night	18% 11% 4 8% 6% 4 5% 3 5% 7% 29%	1673
May	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 5% 6% 49%	2540
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 50%	2516
	21st to end	Night	15% 9% 3 9% 6% 6% 6% 4 6% 6% 30%	2743
June	1st to 10th	Night	16% 8% 2 8% 4 4 5% 5% 6% 6% 36%	1330
	11th to 20th	Night	13% 7% 2 7% 5% 5% 5% 4 6% 9% 37%	1330
	21st to end	Night	11% 4 2 6% 5% 3 5% 4 7% 8% 45%	1330
July	1st to 10th	Night	5% 3 1 3 2 3 3 2 4 5% 70%	2220
	11th to 20th	Night	4 2 1 2 2 2 2 2 2 4 76%	2220
	21st to end	Night	8% 4 2 3 3 2 3 2 4 5% 64%	2748
August	1st to 10th	Night	3 2 1 3 2 2 3 2 4 5% 73%	2613
	11th to 20th	Night	9% 5% 2 6% 3 3 4 3 5% 7% 53%	2812
	21st to end	Night	11% 3 2 5% 4 4 4 3 5% 9% 50%	3266
September	1st to 10th	Night	7% 2 1 5% 3 3 3 3 5% 7% 60%	1900
	11th to 20th	Night	14% 6% 2 7% 4 3 4 3 4 5% 47%	1902
	21st to end	Night	15% 5% 2 8% 5% 4 4 3 5% 6% 44%	2179
October	1st to 10th	Night	15% 6% 2 6% 4 4 5% 3 5% 6% 42%	3800
	11th to 20th	Night	24% 8% 2 7% 5% 4 4 4 5% 6% 32%	3777
	21st to end	Night	22% 10% 4 7% 5% 4 4 3 5% 5% 30%	4321
November	1st to 10th	Night	29% 12% 3 9% 6% 5% 4 4 4 5% 21%	2730
	11th to 20th	Night	27% 11% 3 8% 5% 4 4 4 5% 6% 24%	2800
	21st to end	Night	26% 11% 4 9% 6% 5% 5% 3 5% 5% 21%	2800
December	1st to 10th	Night	19% 10% 3 9% 6% 5% 6% 4 5% 7% 26%	4560
	11th to 20th	Night	29% 17% 6% 11% 6% 4 4 3 4 4 12%	5489
	21st to end	Night	23% 14% 5% 10% 6% 5% 5% 4 4 5% 17%	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	72% 9% 7% 9% 2	3179
	11th to 20th	Day	54% 8% 13% 17% 7%	3252
	21st to end	Day	65% 8% 10% 15% 3	4119
February	1st to 10th	Day	66% 11% 9% 8% 4 2	1800
	11th to 20th	Day	64% 9% 10% 11% 6% 1	1764
	21st to end	Day	64% 9% 10% 11% 5% 1	1510
March	1st to 10th	Day	55% 12% 15% 13% 5%	3361
	11th to 20th	Day	47% 9% 17% 24% 4 1	3600
	21st to end	Day	58% 12% 14% 12% 3	3981
April	1st to 10th	Day	56% 11% 16% 13% 3	2280
	11th to 20th	Day	51% 9% 13% 16% 9% 2	2332
	21st to end	Day	52% 10% 13% 17% 7% 1	2505
May	1st to 10th	Day	39% 9% 16% 24% 11% 1	4420
	11th to 20th	Day	31% 13% 25% 25% 6%	4380
	21st to end	Day	51% 11% 20% 14% 3	4830
June	1st to 10th	Day	46% 9% 19% 17% 7% 1	2850
	11th to 20th	Day	45% 11% 20% 19% 5%	2850
	21st to end	Day	37% 10% 21% 22% 8% 2	2850
July	1st to 10th	Day	18% 6% 11% 28% 26% 11%	4666
	11th to 20th	Day	17% 5% 12% 36% 28% 2	4659
	21st to end	Day	23% 7% 23% 31% 14% 2	4650
August	1st to 10th	Day	17% 7% 19% 39% 17% 1	4301
	11th to 20th	Day	35% 9% 26% 22% 7% 1	4065
	21st to end	Day	38% 8% 17% 26% 10%	4402
September	1st to 10th	Day	31% 9% 20% 23% 13% 4	2280
	11th to 20th	Day	44% 8% 16% 18% 13% 1	2275
	21st to end	Day	46% 9% 17% 18% 9% 1	2000
October	1st to 10th	Day	47% 10% 18% 22% 4	3180
	11th to 20th	Day	59% 7% 12% 15% 6%	3161
	21st to end	Day	61% 9% 12% 11% 6%	3238
November	1st to 10th	Day	66% 9% 12% 10% 3 1	1568
	11th to 20th	Day	64% 8% 10% 13% 4 2	1600
	21st to end	Day	67% 10% 8% 9% 6%	1600
December	1st to 10th	Day	61% 7% 11% 16% 5%	2620
	11th to 20th	Day	75% 8% 8% 6% 2 1	2714
	21st to end	Day	72% 10% 9% 7% 2	3498
January	1st to 10th	Night	72% 9% 8% 8% 2	6766
	11th to 20th	Night	49% 8% 17% 19% 5%	6706
	21st to end	Night	59% 8% 12% 15% 5%	6837
February	1st to 10th	Night	65% 7% 12% 9% 6% 1	2600
	11th to 20th	Night	60% 9% 10% 14% 6% 1	2534
	21st to end	Night	63% 6% 10% 13% 7% 1	2120
March	1st to 10th	Night	55% 11% 14% 15% 5%	3731
	11th to 20th	Night	43% 10% 18% 25% 3 1	3600
	21st to end	Night	63% 12% 11% 13% 1	3809
April	1st to 10th	Night	58% 10% 14% 14% 4	1900
	11th to 20th	Night	52% 10% 12% 16% 7% 3	1796
	21st to end	Night	56% 9% 11% 15% 9%	1673
May	1st to 10th	Night	37% 7% 14% 30% 11%	2540
	11th to 20th	Night	29% 11% 27% 24% 9% 1	2516
	21st to end	Night	51% 13% 18% 14% 4	2743
June	1st to 10th	Night	48% 9% 15% 20% 6% 2	1330
	11th to 20th	Night	45% 10% 19% 20% 5%	1330
	21st to end	Night	37% 8% 22% 22% 8% 3	1330
July	1st to 10th	Night	19% 6% 9% 30% 22% 14%	2185
	11th to 20th	Night	15% 4 12% 31% 35% 4	2178
	21st to end	Night	25% 5% 14% 30% 23% 3	2661
August	1st to 10th	Night	15% 5% 14% 37% 27% 2	2605
	11th to 20th	Night	32% 7% 25% 23% 10% 3	2839
	21st to end	Night	33% 8% 16% 31% 11%	3266
September	1st to 10th	Night	25% 9% 17% 30% 15% 5%	1900
	11th to 20th	Night	40% 8% 12% 19% 17% 3	1902
	21st to end	Night	42% 8% 15% 24% 10% 1	2179
October	1st to 10th	Night	43% 7% 21% 23% 6%	3800
	11th to 20th	Night	54% 6% 15% 21% 5%	3777
	21st to end	Night	55% 8% 14% 15% 7%	4321
November	1st to 10th	Night	66% 9% 11% 11% 3 1	2730
	11th to 20th	Night	62% 9% 12% 10% 5% 2	2800
	21st to end	Night	65% 9% 10% 10% 6% 1	2800
December	1st to 10th	Night	55% 9% 13% 18% 5%	4560
	11th to 20th	Night	74% 8% 8% 7% 3	5485
	21st to end	Night	67% 9% 11% 10% 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	32% 9% 28% 17% 12% 3	200
	11th to 20th	Day	28% 12% 22% 24% 14% 2	200
	21st to end	Day	33% 7% 22% 21% 16% 2	270
February	1st to 10th	Day	31% 6% 22% 23% 15% 4	400
	11th to 20th	Day	32% 4 19% 26% 16% 4	395
	21st to end	Day	25% 10% 20% 22% 19% 5%	344
March	1st to 10th	Day	24% 7% 18% 27% 18% 5%	500
	11th to 20th	Day	26% 5% 18% 23% 20% 9%	500
	21st to end	Day	26% 7% 15% 27% 20% 5%	531
April	1st to 10th	Day	21% 8% 17% 27% 18% 8%	460
	11th to 20th	Day	21% 4 13% 31% 19% 11%	452
	21st to end	Day	21% 4 16% 26% 23% 11%	460
May	1st to 10th	Day	13% 4 11% 23% 38% 11%	463
	11th to 20th	Day	13% 3 10% 21% 34% 18%	488
	21st to end	Day	15% 3 10% 28% 28% 16%	532
June	1st to 10th	Day	17% 3 10% 30% 30% 11%	680
	11th to 20th	Day	13% 4 12% 35% 28% 8%	680
	21st to end	Day	9% 3 10% 27% 35% 16%	680
July	1st to 10th	Day	7% 1 8% 25% 38% 19%	671
	11th to 20th	Day	6% 1 8% 27% 38% 20%	668
	21st to end	Day	5% 2 5% 20% 39% 30%	543
August	1st to 10th	Day	5% 2 5% 19% 34% 34%	487
	11th to 20th	Day	5% 1 3 18% 43% 29%	477
	21st to end	Day	11% 1 6% 23% 41% 18%	539
September	1st to 10th	Day	11% 1 6% 26% 41% 14%	490
	11th to 20th	Day	15% 2 10% 24% 34% 16%	490
	21st to end	Day	17% 4 9% 25% 31% 13%	490
October	1st to 10th	Day	22% 3 12% 24% 26% 13%	490
	11th to 20th	Day	30% 4 13% 22% 20% 11%	490
	21st to end	Day	25% 9% 20% 25% 14% 6%	425
November	1st to 10th	Day	31% 7% 19% 27% 16% 1	200
	11th to 20th	Day	29% 9% 17% 24% 15% 8%	200
	21st to end	Day	29% 9% 25% 18% 13% 8%	200
December	1st to 10th	Day	33% 6% 25% 12% 22% 3	200
	11th to 20th	Day	36% 12% 24% 19% 11%	200
	21st to end	Day	40% 9% 26% 19% 5%	220
January	1st to 10th	Night	34% 10% 25% 19% 11% 2	900
	11th to 20th	Night	30% 8% 17% 23% 19% 2	883
	21st to end	Night	31% 7% 18% 23% 18% 4	880
February	1st to 10th	Night	33% 7% 20% 23% 13% 4	800
	11th to 20th	Night	27% 8% 20% 26% 16% 3	784
	21st to end	Night	29% 11% 21% 22% 14% 4	660
March	1st to 10th	Night	26% 8% 19% 25% 17% 4	800
	11th to 20th	Night	26% 9% 19% 24% 18% 5%	800
	21st to end	Night	28% 8% 18% 26% 17% 3	862
April	1st to 10th	Night	22% 7% 18% 28% 19% 6%	760
	11th to 20th	Night	19% 4 23% 29% 19% 6%	668
	21st to end	Night	20% 4 22% 33% 15% 5%	533
May	1st to 10th	Night	13% 2 19% 31% 29% 6%	380
	11th to 20th	Night	14% 2 13% 28% 34% 9%	374
	21st to end	Night	14% 5% 14% 31% 29% 7%	411
June	1st to 10th	Night	19% 3 18% 26% 31% 3	380
	11th to 20th	Night	14% 4 19% 36% 22% 4	380
	21st to end	Night	12% 2 13% 34% 30% 9%	380
July	1st to 10th	Night	9% 1 12% 31% 39% 8%	370
	11th to 20th	Night	7% 2 12% 34% 39% 7%	368
	21st to end	Night	6% 2 7% 30% 44% 11%	416
August	1st to 10th	Night	6% 1 8% 26% 43% 17%	423
	11th to 20th	Night	6% 1 5% 23% 49% 17%	564
	21st to end	Night	10% 1 9% 29% 40% 9%	695
September	1st to 10th	Night	8% 1 6% 34% 37% 14%	760
	11th to 20th	Night	15% 1 9% 24% 32% 19%	760
	21st to end	Night	17% 3 12% 27% 32% 9%	760
October	1st to 10th	Night	19% 3 13% 25% 27% 14%	760
	11th to 20th	Night	25% 4 13% 25% 24% 9%	760
	21st to end	Night	28% 5% 20% 24% 18% 5%	836
November	1st to 10th	Night	32% 6% 22% 22% 17% 1	780
	11th to 20th	Night	31% 8% 22% 22% 14% 3	800
	21st to end	Night	31% 7% 22% 21% 13% 5%	800
December	1st to 10th	Night	28% 7% 18% 25% 19% 3	800
	11th to 20th	Night	36% 9% 24% 21% 9% 2	877
	21st to end	Night	35% 11% 26% 19% 8% 1	990

Page generated at 2025-03-24 06:04:25. Version ac07

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