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 2026-07-31.

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	15% 8% 3 7% 5% 4 4 4 5% 7% 37%	142218
	Night	18% 9% 3 8% 5% 4 4 3 5% 6% 34%	139494
Transparency	Day	46% 8% 17% 20% 8% 1	141928
	Night	51% 8% 14% 19% 8% 1	139366
Seeing	Day	19% 5% 14% 25% 26% 12%	17565
	Night	23% 6% 17% 26% 22% 6%	25720

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	31% 15% 5% 11% 6% 5% 4 3 4 4 12%	4125
	11th to 20th	Day	16% 11% 4 8% 5% 4 4 4 6% 7% 31%	4219
	21st to end	Day	23% 13% 4 11% 6% 5% 4 3 4 5% 20%	5343
February	1st to 10th	Day	23% 14% 4 9% 7% 6% 5% 4 5% 5% 17%	1890
	11th to 20th	Day	23% 13% 4 9% 5% 5% 5% 4 4 5% 22%	1845
	21st to end	Day	20% 14% 4 9% 7% 5% 6% 4 5% 5% 20%	1583
March	1st to 10th	Day	17% 13% 4 10% 7% 6% 6% 5% 6% 7% 18%	4793
	11th to 20th	Day	18% 12% 4 9% 5% 4 5% 4 5% 6% 28%	5110
	21st to end	Day	20% 12% 4 10% 7% 7% 6% 4 6% 6% 18%	5737
April	1st to 10th	Day	19% 11% 3 8% 6% 6% 5% 4 7% 8% 23%	2520
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 5% 6% 8% 30%	2581
	21st to end	Day	18% 9% 3 8% 6% 5% 5% 3 6% 7% 32%	2769
May	1st to 10th	Day	10% 6% 2 6% 5% 4 4 4 5% 6% 47%	8195
	11th to 20th	Day	10% 7% 3 7% 5% 5% 5% 4 7% 10% 36%	8160
	21st to end	Day	14% 7% 2 6% 4 5% 5% 4 6% 9% 38%	9009
June	1st to 10th	Day	15% 8% 2 6% 6% 4 4 4 6% 7% 38%	3150
	11th to 20th	Day	12% 6% 2 8% 6% 5% 5% 4 6% 7% 39%	3150
	21st to end	Day	9% 6% 3 6% 5% 5% 5% 4 7% 7% 44%	3150
July	1st to 10th	Day	5% 3 1 3 3 2 3 2 4 5% 68%	6930
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 5% 75%	6930
	21st to end	Day	7% 2 1 3 2 2 3 3 4 6% 66%	7028
August	1st to 10th	Day	4 2 1 3 2 2 3 2 4 6% 70%	4455
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 5% 8% 48%	4156
	21st to end	Day	12% 5% 2 7% 4 4 4 3 5% 7% 47%	4542
September	1st to 10th	Day	8% 4 2 6% 4 3 4 3 4 8% 53%	2400
	11th to 20th	Day	11% 7% 3 7% 6% 4 5% 4 6% 7% 42%	2395
	21st to end	Day	16% 7% 3 8% 5% 4 5% 3 4 7% 38%	2105
October	1st to 10th	Day	17% 6% 3 6% 5% 5% 5% 5% 7% 8% 33%	3280
	11th to 20th	Day	28% 9% 3 8% 5% 4 4 3 5% 6% 25%	3261
	21st to end	Day	21% 10% 4 11% 6% 6% 5% 4 5% 6% 21%	3341
November	1st to 10th	Day	28% 11% 3 9% 6% 6% 6% 4 4 5% 18%	1648
	11th to 20th	Day	27% 11% 4 7% 6% 5% 5% 4 3 5% 22%	1680
	21st to end	Day	28% 12% 4 9% 6% 6% 5% 3 4 5% 18%	1680
December	1st to 10th	Day	20% 11% 4 12% 8% 5% 5% 4 5% 6% 20%	2700
	11th to 20th	Day	30% 14% 6% 11% 6% 5% 4 3 4 5% 11%	2783
	21st to end	Day	26% 16% 6% 12% 7% 5% 6% 4 4 5% 11%	3575
January	1st to 10th	Night	28% 16% 5% 11% 6% 5% 5% 3 4 4 12%	8770
	11th to 20th	Night	20% 10% 3 7% 4 3 3 3 4 6% 36%	8677
	21st to end	Night	23% 12% 3 9% 5% 5% 4 3 4 5% 26%	8840
February	1st to 10th	Night	27% 13% 4 8% 5% 4 4 3 4 5% 22%	2730
	11th to 20th	Night	23% 11% 4 8% 5% 4 4 3 4 6% 27%	2662
	21st to end	Night	23% 13% 4 8% 5% 5% 5% 3 4 6% 24%	2223
March	1st to 10th	Night	15% 11% 4 9% 6% 6% 6% 5% 6% 7% 25%	5319
	11th to 20th	Night	17% 11% 3 7% 5% 5% 5% 4 6% 7% 31%	5109
	21st to end	Night	25% 13% 4 9% 5% 5% 5% 4 5% 5% 19%	5479
April	1st to 10th	Night	20% 10% 3 8% 6% 5% 4 4 6% 6% 27%	2100
	11th to 20th	Night	16% 8% 3 8% 6% 5% 5% 4 6% 7% 32%	1987
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 5% 7% 33%	1849
May	1st to 10th	Night	11% 5% 2 7% 4 4 3 3 4 5% 52%	4724
	11th to 20th	Night	10% 6% 2 8% 5% 5% 4 4 6% 8% 42%	4700
	21st to end	Night	12% 7% 3 8% 5% 5% 5% 4 7% 8% 36%	5145
June	1st to 10th	Night	15% 8% 2 8% 4 4 5% 5% 7% 6% 36%	1470
	11th to 20th	Night	12% 7% 2 7% 5% 5% 5% 3 6% 9% 39%	1470
	21st to end	Night	11% 4 2 6% 5% 3 5% 4 7% 8% 44%	1470
July	1st to 10th	Night	6% 3 1 2 2 3 3 2 4 5% 69%	3250
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 4 77%	3250
	21st to end	Night	6% 3 1 3 3 2 3 2 4 5% 67%	4020
August	1st to 10th	Night	3 2 1 3 2 2 3 2 4 5% 73%	2696
	11th to 20th	Night	10% 5% 2 6% 3 3 4 3 5% 7% 53%	2902
	21st to end	Night	11% 3 2 5% 4 4 4 3 5% 8% 51%	3369
September	1st to 10th	Night	6% 2 1 5% 3 3 3 3 5% 8% 60%	2000
	11th to 20th	Night	14% 6% 2 6% 4 3 4 3 4 5% 48%	2002
	21st to end	Night	16% 5% 2 8% 5% 3 3 3 4 6% 44%	2294
October	1st to 10th	Night	14% 6% 2 7% 4 4 5% 3 5% 6% 42%	3920
	11th to 20th	Night	23% 8% 2 7% 5% 4 4 4 5% 6% 32%	3897
	21st to end	Night	22% 10% 3 7% 5% 4 4 3 5% 5% 30%	4460
November	1st to 10th	Night	29% 11% 3 9% 6% 5% 4 4 4 5% 20%	2870
	11th to 20th	Night	27% 12% 3 8% 5% 4 4 4 5% 5% 23%	2940
	21st to end	Night	27% 11% 4 9% 6% 5% 5% 3 4 5% 21%	2940
December	1st to 10th	Night	20% 10% 3 9% 6% 5% 6% 4 5% 7% 26%	4700
	11th to 20th	Night	29% 17% 6% 11% 6% 4 4 3 4 4 12%	5637
	21st to end	Night	23% 14% 5% 10% 6% 5% 5% 4 5% 5% 17%	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	75% 8% 7% 9% 2	4125
	11th to 20th	Day	51% 8% 12% 21% 9%	4240
	21st to end	Day	64% 7% 9% 16% 4	5344
February	1st to 10th	Day	66% 11% 9% 8% 4 2	1890
	11th to 20th	Day	63% 9% 10% 12% 6% 1	1854
	21st to end	Day	63% 9% 11% 10% 5% 1	1583
March	1st to 10th	Day	58% 10% 13% 14% 4	4793
	11th to 20th	Day	55% 8% 14% 20% 3 1	5110
	21st to end	Day	62% 11% 13% 11% 2	5737
April	1st to 10th	Day	56% 11% 17% 14% 3	2520
	11th to 20th	Day	49% 9% 14% 17% 9% 2	2581
	21st to end	Day	49% 10% 15% 19% 7% 1	2769
May	1st to 10th	Day	37% 7% 18% 27% 10%	8195
	11th to 20th	Day	40% 11% 22% 23% 4	8163
	21st to end	Day	41% 9% 25% 19% 5%	9010
June	1st to 10th	Day	44% 8% 19% 19% 7% 1	3150
	11th to 20th	Day	43% 10% 19% 21% 6%	3150
	21st to end	Day	38% 10% 21% 22% 8% 2	3150
July	1st to 10th	Day	20% 5% 23% 24% 21% 8%	6856
	11th to 20th	Day	16% 4 20% 33% 25% 2	6849
	21st to end	Day	20% 7% 30% 31% 11% 1	6834
August	1st to 10th	Day	18% 7% 19% 38% 17% 1	4438
	11th to 20th	Day	35% 8% 26% 22% 7% 1	4195
	21st to end	Day	37% 8% 17% 26% 11% 1	4541
September	1st to 10th	Day	32% 9% 21% 23% 12% 3	2400
	11th to 20th	Day	43% 8% 16% 19% 13% 1	2395
	21st to end	Day	45% 9% 18% 18% 9% 1	2105
October	1st to 10th	Day	46% 10% 18% 22% 4	3280
	11th to 20th	Day	59% 7% 12% 15% 6%	3261
	21st to end	Day	61% 9% 12% 11% 6%	3341
November	1st to 10th	Day	65% 9% 12% 10% 3	1648
	11th to 20th	Day	65% 8% 10% 12% 4 2	1680
	21st to end	Day	67% 10% 8% 9% 5%	1680
December	1st to 10th	Day	61% 7% 11% 16% 5%	2700
	11th to 20th	Day	75% 8% 8% 6% 2 1	2786
	21st to end	Day	72% 10% 9% 7% 2	3575
January	1st to 10th	Night	74% 8% 8% 8% 2	8770
	11th to 20th	Night	49% 7% 14% 20% 10%	8718
	21st to end	Night	58% 8% 11% 17% 6%	8840
February	1st to 10th	Night	64% 7% 12% 10% 6% 1	2730
	11th to 20th	Night	59% 9% 10% 14% 6% 1	2664
	21st to end	Night	62% 6% 10% 13% 7% 1	2223
March	1st to 10th	Night	54% 9% 14% 16% 6%	5319
	11th to 20th	Night	51% 9% 14% 22% 3 1	5110
	21st to end	Night	64% 10% 12% 12% 1	5479
April	1st to 10th	Night	57% 9% 15% 15% 4	2100
	11th to 20th	Night	51% 10% 14% 16% 7% 2	1987
	21st to end	Night	53% 8% 13% 16% 9%	1849
May	1st to 10th	Night	36% 5% 12% 34% 13%	4721
	11th to 20th	Night	39% 10% 22% 22% 7%	4700
	21st to end	Night	43% 10% 22% 21% 4	5149
June	1st to 10th	Night	48% 9% 15% 20% 7% 2	1470
	11th to 20th	Night	44% 9% 18% 23% 5%	1470
	21st to end	Night	37% 8% 21% 22% 8% 3	1470
July	1st to 10th	Night	19% 5% 18% 31% 17% 10%	3215
	11th to 20th	Night	12% 4 18% 32% 29% 4	3208
	21st to end	Night	21% 5% 21% 33% 18% 2	3909
August	1st to 10th	Night	15% 5% 15% 37% 26% 2	2688
	11th to 20th	Night	32% 7% 25% 23% 10% 3	2929
	21st to end	Night	33% 8% 16% 31% 12% 1	3369
September	1st to 10th	Night	25% 9% 17% 30% 14% 5%	2000
	11th to 20th	Night	40% 8% 13% 20% 16% 3	2002
	21st to end	Night	43% 8% 15% 24% 9% 1	2294
October	1st to 10th	Night	43% 7% 21% 24% 6%	3920
	11th to 20th	Night	53% 6% 15% 21% 5%	3897
	21st to end	Night	55% 8% 14% 15% 7%	4460
November	1st to 10th	Night	66% 9% 11% 11% 3 1	2870
	11th to 20th	Night	63% 8% 11% 10% 5% 2	2940
	21st to end	Night	65% 9% 10% 10% 5% 1	2940
December	1st to 10th	Night	56% 9% 13% 18% 5%	4700
	11th to 20th	Night	74% 8% 8% 7% 3	5633
	21st to end	Night	67% 9% 11% 11% 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% 9% 28% 16% 11% 3	220
	11th to 20th	Day	27% 11% 22% 23% 15% 2	220
	21st to end	Day	33% 6% 22% 21% 15% 2	295
February	1st to 10th	Day	31% 6% 21% 24% 14% 3	430
	11th to 20th	Day	31% 4 19% 27% 16% 3	425
	21st to end	Day	26% 11% 21% 20% 17% 4	369
March	1st to 10th	Day	26% 7% 18% 26% 17% 5%	540
	11th to 20th	Day	30% 5% 18% 21% 18% 8%	540
	21st to end	Day	26% 8% 16% 27% 19% 4	594
April	1st to 10th	Day	21% 8% 17% 29% 17% 8%	540
	11th to 20th	Day	19% 5% 16% 31% 18% 10%	532
	21st to end	Day	19% 4 17% 27% 22% 10%	540
May	1st to 10th	Day	12% 4 13% 24% 36% 10%	543
	11th to 20th	Day	15% 4 14% 20% 31% 16%	568
	21st to end	Day	14% 4 13% 29% 26% 14%	620
June	1st to 10th	Day	17% 4 11% 31% 28% 10%	780
	11th to 20th	Day	12% 4 14% 36% 27% 7%	780
	21st to end	Day	10% 2 12% 29% 33% 15%	780
July	1st to 10th	Day	9% 2 12% 26% 35% 17%	771
	11th to 20th	Day	6% 1 12% 28% 35% 18%	768
	21st to end	Day	5% 2 7% 23% 37% 27%	629
August	1st to 10th	Day	6% 2 6% 20% 34% 32%	527
	11th to 20th	Day	6% 1 5% 20% 41% 27%	517
	21st to end	Day	10% 1 6% 24% 41% 17%	583
September	1st to 10th	Day	11% 1 6% 26% 41% 15%	530
	11th to 20th	Day	14% 2 11% 25% 33% 15%	530
	21st to end	Day	17% 5% 10% 26% 29% 12%	530
October	1st to 10th	Day	22% 3 14% 23% 25% 12%	530
	11th to 20th	Day	30% 5% 14% 22% 19% 11%	530
	21st to end	Day	27% 9% 21% 24% 13% 6%	462
November	1st to 10th	Day	31% 7% 20% 26% 15% 1	220
	11th to 20th	Day	32% 8% 17% 21% 15% 7%	220
	21st to end	Day	30% 10% 23% 18% 11% 7%	220
December	1st to 10th	Day	35% 6% 24% 12% 20% 3	220
	11th to 20th	Day	38% 12% 23% 17% 10%	220
	21st to end	Day	40% 10% 26% 19% 6%	242
January	1st to 10th	Night	34% 10% 25% 19% 10% 2	950
	11th to 20th	Night	31% 8% 17% 23% 19% 2	931
	21st to end	Night	31% 6% 17% 23% 19% 4	924
February	1st to 10th	Night	33% 7% 20% 24% 13% 4	840
	11th to 20th	Night	27% 8% 20% 26% 17% 3	824
	21st to end	Night	29% 12% 21% 21% 14% 3	692
March	1st to 10th	Night	27% 9% 19% 25% 17% 4	840
	11th to 20th	Night	28% 8% 19% 23% 17% 5%	840
	21st to end	Night	28% 9% 18% 27% 16% 3	924
April	1st to 10th	Night	22% 7% 18% 28% 19% 5%	840
	11th to 20th	Night	19% 4 23% 29% 19% 6%	739
	21st to end	Night	19% 4 23% 33% 15% 6%	589
May	1st to 10th	Night	13% 2 19% 33% 28% 5%	420
	11th to 20th	Night	15% 5% 14% 27% 31% 8%	414
	21st to end	Night	13% 5% 16% 31% 29% 6%	455
June	1st to 10th	Night	19% 4 18% 26% 31% 3	420
	11th to 20th	Night	14% 4 20% 37% 22% 4	420
	21st to end	Night	12% 2 13% 34% 30% 8%	420
July	1st to 10th	Night	9% 1 15% 31% 36% 7%	410
	11th to 20th	Night	7% 2 14% 34% 37% 7%	408
	21st to end	Night	6% 2 8% 31% 42% 10%	458
August	1st to 10th	Night	6% 2 9% 26% 42% 16%	446
	11th to 20th	Night	7% 1 6% 23% 47% 16%	594
	21st to end	Night	10% 1 9% 29% 41% 10%	732
September	1st to 10th	Night	8% 1 6% 33% 37% 14%	800
	11th to 20th	Night	15% 2 9% 24% 33% 18%	800
	21st to end	Night	18% 3 12% 28% 31% 9%	800
October	1st to 10th	Night	19% 3 13% 26% 27% 13%	800
	11th to 20th	Night	25% 4 14% 26% 23% 8%	800
	21st to end	Night	28% 5% 20% 25% 18% 5%	880
November	1st to 10th	Night	32% 7% 22% 22% 16% 1	820
	11th to 20th	Night	32% 8% 22% 22% 14% 3	840
	21st to end	Night	31% 7% 22% 22% 13% 5%	840
December	1st to 10th	Night	30% 7% 17% 24% 19% 3	840
	11th to 20th	Night	37% 9% 24% 20% 9% 2	925
	21st to end	Night	34% 11% 26% 19% 9% 1	1045

Page generated at 2026-07-28 00:25:19. Version ac07

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