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 2024-10-20.

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% 38%	101546
	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 34%	97218
Transparency	Day	46% 9% 16% 19% 9% 1	101297
	Night	51% 9% 13% 18% 8% 1	97113
Seeing	Day	18% 4 13% 25% 28% 13%	15298
	Night	23% 6% 17% 26% 22% 6%	23485

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 11% 6% 5% 4 4 4 5% 13%	2219
	11th to 20th	Day	22% 15% 4 9% 5% 5% 4 3 6% 5% 21%	2243
	21st to end	Day	33% 17% 5% 12% 6% 5% 4 4 4 3 9%	2867
February	1st to 10th	Day	24% 14% 4 8% 7% 6% 5% 4 5% 5% 17%	1710
	11th to 20th	Day	23% 13% 5% 8% 5% 5% 5% 3 4 5% 22%	1665
	21st to end	Day	21% 14% 4 9% 7% 5% 5% 4 5% 5% 21%	1437
March	1st to 10th	Day	16% 12% 4 9% 7% 6% 6% 5% 7% 8% 19%	3257
	11th to 20th	Day	16% 9% 3 7% 4 4 5% 4 5% 7% 36%	3490
	21st to end	Day	19% 12% 4 9% 6% 6% 6% 4 6% 6% 23%	3913
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	18% 7% 3 7% 4 5% 4 4 6% 8% 33%	1900
	11th to 20th	Day	22% 11% 4 9% 6% 4 5% 4 5% 6% 24%	1891
	21st to end	Day	25% 9% 4 8% 5% 5% 4 3 5% 7% 24%	1864
November	1st to 10th	Day	29% 11% 3 9% 6% 6% 6% 4 3 5% 18%	1488
	11th to 20th	Day	27% 12% 4 7% 6% 5% 4 4 3 5% 23%	1520
	21st to end	Day	28% 12% 4 9% 6% 6% 6% 3 4 5% 19%	1520
December	1st to 10th	Day	24% 15% 3 8% 7% 5% 4 3 5% 4 22%	1520
	11th to 20th	Day	27% 15% 6% 13% 6% 6% 5% 3 4 5% 11%	1723
	21st to end	Day	23% 15% 6% 13% 7% 5% 6% 4 5% 5% 11%	2442
January	1st to 10th	Night	26% 15% 5% 11% 7% 6% 5% 4 5% 5% 13%	4726
	11th to 20th	Night	26% 12% 4 8% 4 3 3 3 5% 7% 25%	4653
	21st to end	Night	33% 16% 4 10% 5% 4 4 3 4 5% 13%	4786
February	1st to 10th	Night	28% 12% 4 8% 5% 4 4 3 5% 5% 22%	2470
	11th to 20th	Night	23% 11% 4 8% 6% 5% 4 3 4 5% 27%	2402
	21st to end	Night	24% 13% 4 7% 5% 4 4 3 4 6% 24%	2017
March	1st to 10th	Night	17% 11% 3 9% 6% 6% 6% 5% 6% 7% 23%	3615
	11th to 20th	Night	15% 9% 2 5% 5% 4 5% 3 6% 7% 39%	3489
	21st to end	Night	22% 14% 5% 9% 6% 5% 4 4 5% 4 22%	3739
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	16% 7% 3 7% 4 4 5% 3 5% 5% 41%	2280
	11th to 20th	Night	21% 8% 2 6% 5% 4 4 4 6% 6% 34%	2273
	21st to end	Night	24% 8% 3 6% 5% 4 3 3 5% 5% 33%	2489
November	1st to 10th	Night	29% 11% 3 9% 6% 5% 4 4 4 5% 21%	2590
	11th to 20th	Night	27% 11% 3 7% 5% 4 4 4 5% 6% 25%	2660
	21st to end	Night	27% 11% 3 8% 6% 5% 5% 3 4 5% 22%	2660
December	1st to 10th	Night	23% 12% 3 8% 5% 4 5% 4 4 6% 25%	2660
	11th to 20th	Night	27% 16% 6% 12% 6% 4 4 4 4 4 13%	3477
	21st to end	Night	25% 15% 5% 10% 6% 5% 4 3 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	70% 11% 7% 9% 2	2219
	11th to 20th	Day	63% 8% 12% 11% 6%	2264
	21st to end	Day	78% 9% 6% 5% 2	2867
February	1st to 10th	Day	67% 11% 9% 8% 3 1	1710
	11th to 20th	Day	64% 9% 10% 11% 5% 1	1674
	21st to end	Day	63% 9% 11% 10% 5% 1	1437
March	1st to 10th	Day	54% 12% 16% 13% 5%	3257
	11th to 20th	Day	46% 9% 17% 24% 4 1	3490
	21st to end	Day	58% 12% 15% 13% 3	3913
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	48% 10% 16% 20% 6%	1900
	11th to 20th	Day	59% 9% 13% 15% 4	1891
	21st to end	Day	59% 10% 11% 12% 7%	1864
November	1st to 10th	Day	65% 9% 12% 10% 3 1	1488
	11th to 20th	Day	64% 8% 10% 12% 4 2	1520
	21st to end	Day	67% 10% 8% 9% 6%	1520
December	1st to 10th	Day	63% 8% 9% 13% 6%	1520
	11th to 20th	Day	74% 9% 8% 5% 3 1	1726
	21st to end	Day	70% 12% 10% 6% 2	2442
January	1st to 10th	Night	70% 11% 8% 8% 3	4726
	11th to 20th	Night	60% 6% 13% 15% 4 1	4694
	21st to end	Night	72% 9% 9% 8% 2	4786
February	1st to 10th	Night	65% 7% 12% 10% 5% 1	2470
	11th to 20th	Night	60% 9% 11% 13% 6% 1	2404
	21st to end	Night	63% 6% 10% 13% 7% 1	2017
March	1st to 10th	Night	55% 11% 13% 15% 5%	3615
	11th to 20th	Night	43% 10% 18% 26% 3 1	3490
	21st to end	Night	63% 12% 11% 13% 1	3739
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	46% 7% 15% 23% 10%	2280
	11th to 20th	Night	51% 7% 16% 19% 7%	2273
	21st to end	Night	53% 8% 13% 18% 9%	2489
November	1st to 10th	Night	65% 9% 11% 11% 4 1	2590
	11th to 20th	Night	61% 9% 12% 10% 5% 2	2660
	21st to end	Night	64% 9% 10% 10% 6% 1	2660
December	1st to 10th	Night	60% 9% 12% 13% 6%	2660
	11th to 20th	Night	73% 11% 8% 5% 3 1	3473
	21st to end	Night	68% 9% 10% 10% 3	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	31% 4 18% 27% 16% 4	365
	21st to end	Day	23% 10% 21% 23% 19% 5%	319
March	1st to 10th	Day	23% 7% 18% 27% 19% 6%	460
	11th to 20th	Day	25% 4 17% 23% 21% 9%	460
	21st to end	Day	24% 7% 14% 28% 21% 5%	506
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	29% 5% 13% 22% 20% 11%	487
	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	26% 7% 20% 27% 17% 3	744
	21st to end	Night	29% 11% 21% 21% 14% 4	628
March	1st to 10th	Night	26% 8% 19% 25% 17% 5%	760
	11th to 20th	Night	25% 8% 19% 24% 19% 5%	760
	21st to end	Night	27% 8% 18% 27% 17% 3	836
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%	758
	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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