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 2024-02-15.

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	17% 9% 4 7% 6% 3 5% 5% 5% 6% 34%	77952
	Night	21% 10% 4 7% 6% 3 4 4 5% 6% 31%	81071
Transparency	Day	34% 39% 11% 11% 4 1	77664
	Night	38% 38% 9% 10% 4 1	80939
Seeing	Day	18% 4 12% 24% 28% 13%	14268
	Night	23% 5% 17% 25% 22% 6%	22728

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% 14% 4 10% 7% 3 5% 5% 4 5% 13%	2219
	11th to 20th	Day	23% 15% 5% 8% 6% 3 4 4 5% 5% 21%	2243
	21st to end	Day	33% 16% 6% 10% 7% 4 4 4 3 3 9%	2867
February	1st to 10th	Day	25% 13% 4 8% 8% 5% 5% 5% 5% 6% 17%	1710
	11th to 20th	Day	24% 12% 5% 7% 6% 4 5% 5% 4 6% 23%	1612
	21st to end	Day	21% 12% 5% 7% 8% 4 6% 5% 5% 5% 22%	1354
March	1st to 10th	Day	20% 12% 6% 7% 7% 4 5% 6% 6% 6% 22%	1867
	11th to 20th	Day	24% 11% 5% 8% 6% 3 5% 5% 4 6% 23%	1980
	21st to end	Day	20% 12% 5% 7% 9% 5% 6% 5% 5% 7% 21%	2228
April	1st to 10th	Day	20% 11% 4 6% 6% 5% 5% 5% 6% 8% 23%	2160
	11th to 20th	Day	19% 9% 4 6% 6% 4 5% 6% 6% 8% 28%	2207
	21st to end	Day	18% 9% 4 7% 8% 4 5% 4 6% 7% 29%	2373
May	1st to 10th	Day	12% 6% 3 6% 7% 4 6% 5% 6% 7% 37%	2520
	11th to 20th	Day	13% 7% 2 6% 6% 3 5% 5% 6% 8% 37%	2477
	21st to end	Day	15% 8% 4 7% 6% 4 5% 4 5% 7% 36%	2725
June	1st to 10th	Day	16% 7% 3 5% 6% 3 5% 4 6% 7% 37%	2700
	11th to 20th	Day	13% 5% 3 7% 7% 4 6% 5% 6% 8% 37%	2700
	21st to end	Day	10% 5% 3 5% 6% 3 4 5% 6% 7% 46%	2700
July	1st to 10th	Day	7% 4 2 5% 5% 2 4 4 4 6% 57%	2700
	11th to 20th	Day	7% 3 2 3 4 2 3 5% 5% 7% 58%	2700
	21st to end	Day	5% 2 1 2 2 1 2 3 4 5% 72%	2753
August	1st to 10th	Day	6% 4 2 3 3 2 3 4 4 7% 61%	2463
	11th to 20th	Day	5% 4 2 3 3 2 3 4 5% 8% 60%	2298
	21st to end	Day	8% 4 1 4 5% 3 4 5% 6% 9% 52%	2508
September	1st to 10th	Day	9% 4 2 5% 4 2 3 4 4 7% 53%	2160
	11th to 20th	Day	12% 6% 3 6% 7% 3 5% 5% 5% 7% 42%	2156
	21st to end	Day	16% 7% 3 7% 5% 3 4 5% 4 7% 38%	1895
October	1st to 10th	Day	19% 7% 3 6% 5% 3 4 5% 6% 8% 34%	1800
	11th to 20th	Day	22% 10% 5% 8% 8% 3 4 5% 5% 6% 24%	1800
	21st to end	Day	25% 9% 4 7% 6% 3 4 4 5% 7% 25%	1864
November	1st to 10th	Day	29% 10% 3 7% 7% 5% 6% 5% 3 5% 18%	1488
	11th to 20th	Day	27% 10% 5% 7% 6% 4 5% 5% 3 5% 23%	1520
	21st to end	Day	28% 10% 5% 8% 6% 4 6% 4 4 5% 19%	1520
December	1st to 10th	Day	25% 13% 4 7% 7% 3 5% 4 5% 4 22%	1520
	11th to 20th	Day	28% 14% 6% 10% 8% 4 6% 4 4 5% 11%	1723
	21st to end	Day	24% 14% 6% 11% 8% 4 6% 5% 5% 5% 11%	2442
January	1st to 10th	Night	26% 14% 5% 9% 8% 4 5% 5% 5% 5% 13%	4726
	11th to 20th	Night	26% 12% 4 7% 5% 2 4 4 5% 7% 25%	4653
	21st to end	Night	33% 15% 4 9% 6% 3 4 4 4 5% 13%	4786
February	1st to 10th	Night	29% 11% 4 8% 6% 3 5% 4 5% 5% 22%	2470
	11th to 20th	Night	23% 11% 4 7% 6% 3 5% 4 4 5% 28%	2329
	21st to end	Night	23% 12% 5% 7% 6% 3 4 5% 5% 6% 25%	1902
March	1st to 10th	Night	23% 11% 4 7% 6% 3 5% 5% 5% 7% 26%	2091
	11th to 20th	Night	23% 12% 4 6% 6% 3 5% 5% 5% 6% 24%	1979
	21st to end	Night	24% 12% 5% 7% 7% 3 5% 5% 5% 5% 23%	2127
April	1st to 10th	Night	20% 10% 4 7% 7% 3 5% 5% 6% 6% 26%	1800
	11th to 20th	Night	18% 8% 4 7% 6% 3 6% 5% 6% 7% 30%	1701
	21st to end	Night	18% 10% 4 8% 7% 3 5% 4 5% 7% 30%	1585
May	1st to 10th	Night	13% 5% 2 6% 5% 3 5% 4 6% 8% 43%	1440
	11th to 20th	Night	12% 7% 3 6% 5% 3 5% 5% 7% 8% 38%	1416
	21st to end	Night	12% 8% 3 7% 7% 4 5% 5% 6% 7% 35%	1547
June	1st to 10th	Night	16% 7% 3 7% 5% 3 5% 5% 7% 6% 36%	1260
	11th to 20th	Night	13% 6% 3 6% 5% 4 6% 5% 6% 9% 37%	1260
	21st to end	Night	11% 3 2 5% 5% 3 4 5% 7% 8% 47%	1260
July	1st to 10th	Night	8% 4 1 3 4 3 4 4 3 5% 60%	1260
	11th to 20th	Night	7% 4 2 3 3 2 4 3 4 6% 62%	1260
	21st to end	Night	4 2 1 1 2 1 2 3 3 5% 75%	1562
August	1st to 10th	Night	5% 4 1 4 4 2 3 4 4 5% 64%	1474
	11th to 20th	Night	4 3 1 3 2 2 2 4 5% 7% 67%	1602
	21st to end	Night	7% 3 2 5% 4 3 4 4 5% 9% 55%	1846
September	1st to 10th	Night	7% 2 2 4 4 2 3 4 5% 8% 60%	1800
	11th to 20th	Night	14% 5% 2 6% 5% 2 5% 4 4 5% 48%	1801
	21st to end	Night	15% 5% 2 7% 6% 3 4 5% 4 6% 44%	2064
October	1st to 10th	Night	16% 7% 3 6% 5% 3 5% 4 5% 5% 42%	2160
	11th to 20th	Night	21% 8% 2 5% 6% 3 4 4 6% 6% 34%	2160
	21st to end	Night	24% 8% 3 6% 6% 3 4 3 5% 6% 33%	2489
November	1st to 10th	Night	29% 11% 3 8% 7% 4 4 5% 4 5% 22%	2590
	11th to 20th	Night	27% 10% 4 6% 6% 3 4 4 5% 6% 25%	2660
	21st to end	Night	27% 10% 4 7% 7% 4 5% 5% 4 5% 22%	2660
December	1st to 10th	Night	24% 11% 4 7% 6% 3 5% 5% 4 6% 26%	2660
	11th to 20th	Night	27% 15% 6% 10% 7% 3 5% 4 4 4 13%	3477
	21st to end	Night	25% 14% 6% 9% 7% 4 5% 4 4 5% 18%	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	57% 32% 5% 5%	2219
	11th to 20th	Day	47% 28% 12% 9% 4	2264
	21st to end	Day	64% 27% 5% 3 1	2867
February	1st to 10th	Day	44% 43% 6% 5% 2	1710
	11th to 20th	Day	40% 41% 7% 8% 3	1621
	21st to end	Day	43% 40% 8% 6% 2	1354
March	1st to 10th	Day	41% 40% 8% 8% 2 1	1867
	11th to 20th	Day	42% 39% 9% 8% 2 1	1980
	21st to end	Day	42% 39% 9% 8% 1	2228
April	1st to 10th	Day	38% 42% 11% 7% 2	2160
	11th to 20th	Day	35% 40% 7% 10% 6% 1	2207
	21st to end	Day	35% 41% 11% 9% 4 1	2373
May	1st to 10th	Day	32% 40% 12% 13% 4	2520
	11th to 20th	Day	28% 41% 13% 14% 5% 1	2480
	21st to end	Day	34% 41% 12% 11% 2	2726
June	1st to 10th	Day	32% 40% 13% 10% 4 1	2700
	11th to 20th	Day	32% 41% 16% 9% 2	2700
	21st to end	Day	25% 40% 15% 13% 6% 1	2700
July	1st to 10th	Day	18% 40% 15% 19% 7% 2	2626
	11th to 20th	Day	17% 39% 14% 22% 7% 1	2619
	21st to end	Day	11% 40% 12% 25% 9% 3	2600
August	1st to 10th	Day	18% 39% 16% 21% 5% 1	2446
	11th to 20th	Day	17% 38% 14% 19% 10% 2	2298
	21st to end	Day	19% 41% 15% 19% 5% 1	2508
September	1st to 10th	Day	18% 38% 15% 17% 9% 3	2160
	11th to 20th	Day	25% 39% 12% 14% 9% 1	2156
	21st to end	Day	31% 41% 10% 13% 5% 1	1895
October	1st to 10th	Day	31% 41% 11% 13% 5%	1800
	11th to 20th	Day	39% 41% 10% 9% 1 1	1800
	21st to end	Day	38% 41% 10% 6% 5%	1864
November	1st to 10th	Day	42% 43% 6% 5% 2 1	1488
	11th to 20th	Day	42% 42% 7% 6% 2	1520
	21st to end	Day	44% 44% 5% 5% 2	1520
December	1st to 10th	Day	42% 42% 6% 6% 4	1520
	11th to 20th	Day	51% 38% 7% 3 1	1726
	21st to end	Day	54% 34% 7% 4 1	2442
January	1st to 10th	Night	56% 32% 7% 4 1	4726
	11th to 20th	Night	44% 29% 11% 13% 2 1	4694
	21st to end	Night	60% 27% 6% 5% 1	4786
February	1st to 10th	Night	44% 38% 9% 7% 2	2470
	11th to 20th	Night	37% 40% 8% 9% 5%	2331
	21st to end	Night	43% 39% 7% 8% 3	1902
March	1st to 10th	Night	40% 41% 10% 8% 2	2091
	11th to 20th	Night	41% 41% 8% 8% 2 1	1980
	21st to end	Night	42% 41% 9% 7% 1	2127
April	1st to 10th	Night	39% 41% 10% 8% 2	1800
	11th to 20th	Night	34% 40% 10% 10% 5% 2	1701
	21st to end	Night	39% 40% 8% 9% 5%	1585
May	1st to 10th	Night	28% 40% 14% 13% 5%	1440
	11th to 20th	Night	27% 40% 14% 13% 6% 1	1416
	21st to end	Night	34% 41% 11% 10% 3	1547
June	1st to 10th	Night	34% 39% 12% 10% 4 2	1260
	11th to 20th	Night	31% 41% 16% 11% 2	1260
	21st to end	Night	24% 39% 15% 14% 5% 3	1260
July	1st to 10th	Night	17% 41% 12% 19% 9% 3	1225
	11th to 20th	Night	15% 39% 13% 23% 9% 2	1218
	21st to end	Night	9% 40% 10% 21% 17% 4	1475
August	1st to 10th	Night	17% 38% 15% 20% 9% 1	1466
	11th to 20th	Night	13% 37% 14% 19% 13% 4	1602
	21st to end	Night	19% 39% 12% 21% 8%	1846
September	1st to 10th	Night	13% 39% 10% 22% 11% 4	1800
	11th to 20th	Night	23% 40% 9% 13% 12% 3	1801
	21st to end	Night	27% 40% 11% 14% 6% 1	2064
October	1st to 10th	Night	29% 39% 10% 13% 8%	2160
	11th to 20th	Night	32% 39% 13% 13% 3	2160
	21st to end	Night	34% 40% 10% 11% 5%	2489
November	1st to 10th	Night	41% 44% 6% 6% 3 1	2590
	11th to 20th	Night	40% 43% 9% 6% 2	2660
	21st to end	Night	41% 44% 7% 6% 2	2660
December	1st to 10th	Night	39% 42% 10% 6% 3	2660
	11th to 20th	Night	51% 40% 5% 3 1	3473
	21st to end	Night	52% 32% 8% 8% 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	28% 10% 28% 17% 13% 3	180
	11th to 20th	Day	29% 12% 19% 24% 13% 2	180
	21st to end	Day	34% 7% 22% 20% 15% 1	245
February	1st to 10th	Day	32% 5% 22% 24% 15% 3	370
	11th to 20th	Day	32% 3 18% 26% 16% 5%	348
	21st to end	Day	23% 9% 21% 23% 19% 5%	290
March	1st to 10th	Day	25% 6% 17% 27% 19% 6%	420
	11th to 20th	Day	27% 4 18% 24% 20% 7%	420
	21st to end	Day	25% 6% 15% 27% 21% 5%	462
April	1st to 10th	Day	22% 8% 17% 26% 18% 9%	420
	11th to 20th	Day	22% 4 13% 29% 20% 12%	412
	21st to end	Day	20% 3 16% 26% 24% 11%	420
May	1st to 10th	Day	13% 4 10% 24% 39% 11%	423
	11th to 20th	Day	14% 3 9% 20% 34% 19%	448
	21st to end	Day	14% 3 9% 29% 29% 17%	487
June	1st to 10th	Day	17% 2 9% 31% 29% 11%	630
	11th to 20th	Day	13% 3 11% 35% 29% 8%	630
	21st to end	Day	9% 2 8% 27% 36% 17%	630
July	1st to 10th	Day	8% 1 7% 24% 39% 21%	621
	11th to 20th	Day	6% 1 8% 25% 39% 21%	618
	21st to end	Day	3 1 4 19% 42% 31%	499
August	1st to 10th	Day	6% 2 4 17% 35% 36%	447
	11th to 20th	Day	4 1 2 18% 44% 31%	437
	21st to end	Day	9% 1 6% 22% 43% 20%	495
September	1st to 10th	Day	12% 2 6% 25% 41% 14%	450
	11th to 20th	Day	15% 1 8% 23% 36% 18%	450
	21st to end	Day	16% 3 9% 26% 32% 14%	450
October	1st to 10th	Day	22% 3 10% 24% 26% 14%	450
	11th to 20th	Day	28% 5% 12% 22% 20% 12%	450
	21st to end	Day	25% 9% 20% 25% 14% 7%	388
November	1st to 10th	Day	30% 7% 18% 28% 16% 1	180
	11th to 20th	Day	28% 7% 18% 23% 16% 8%	180
	21st to end	Day	29% 7% 25% 18% 13% 8%	180
December	1st to 10th	Day	34% 7% 26% 11% 19% 3	180
	11th to 20th	Day	32% 11% 25% 20% 12%	180
	21st to end	Day	37% 8% 28% 21% 6% 1	198
January	1st to 10th	Night	33% 10% 25% 18% 11% 2	850
	11th to 20th	Night	32% 9% 17% 22% 18% 2	835
	21st to end	Night	32% 7% 18% 22% 18% 3	836
February	1st to 10th	Night	33% 6% 21% 23% 13% 4	760
	11th to 20th	Night	27% 7% 20% 26% 17% 3	722
	21st to end	Night	28% 10% 21% 21% 15% 4	592
March	1st to 10th	Night	27% 8% 19% 25% 16% 5%	720
	11th to 20th	Night	26% 8% 19% 25% 17% 5%	720
	21st to end	Night	28% 7% 18% 26% 17% 4	792
April	1st to 10th	Night	23% 7% 18% 27% 19% 6%	720
	11th to 20th	Night	21% 4 23% 27% 19% 6%	633
	21st to end	Night	20% 4 21% 34% 15% 6%	505
May	1st to 10th	Night	13% 2 19% 31% 29% 6%	360
	11th to 20th	Night	15% 2 11% 28% 35% 9%	354
	21st to end	Night	15% 4 13% 31% 30% 7%	389
June	1st to 10th	Night	19% 3 18% 27% 31% 3	360
	11th to 20th	Night	14% 3 19% 36% 23% 4	360
	21st to end	Night	11% 2 12% 34% 31% 9%	360
July	1st to 10th	Night	9% 1 11% 30% 40% 8%	350
	11th to 20th	Night	7% 2 11% 34% 39% 7%	348
	21st to end	Night	5% 1 7% 30% 45% 11%	394
August	1st to 10th	Night	6% 2 8% 26% 42% 18%	400
	11th to 20th	Night	5% 1 5% 22% 50% 17%	534
	21st to end	Night	10% 1 9% 29% 41% 10%	658
September	1st to 10th	Night	9% 1 6% 34% 37% 14%	720
	11th to 20th	Night	15% 2 8% 23% 33% 19%	720
	21st to end	Night	17% 3 11% 27% 32% 9%	720
October	1st to 10th	Night	19% 2 12% 25% 27% 14%	720
	11th to 20th	Night	25% 4 14% 25% 24% 9%	720
	21st to end	Night	28% 5% 19% 24% 19% 5%	792
November	1st to 10th	Night	31% 6% 23% 22% 17% 1	740
	11th to 20th	Night	30% 8% 22% 22% 15% 3	760
	21st to end	Night	31% 7% 22% 21% 13% 6%	760
December	1st to 10th	Night	29% 8% 18% 25% 18% 2	760
	11th to 20th	Night	35% 8% 24% 21% 9% 2	829
	21st to end	Night	34% 11% 26% 19% 9% 1	935

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