Oak Harbor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Harbor clear sky chart from 2005-11-06 to 2024-01-15.

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	18% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	74801
	Night	21% 10% 4 7% 6% 3 4 4 5% 6% 30%	74970
Transparency	Day	51% 7% 17% 18% 7% 1	74492
	Night	55% 6% 14% 16% 7% 1	74797
Seeing	Day	22% 6% 18% 26% 20% 7%	14100
	Night	28% 8% 16% 25% 20% 5%	22563

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	28% 15% 5% 8% 6% 4 5% 4 4 4 16%	1329
	11th to 20th	Day	25% 11% 5% 9% 8% 4 5% 4 6% 6% 16%	1370
	21st to end	Day	26% 14% 6% 10% 8% 5% 4 5% 4 5% 14%	1621
February	1st to 10th	Day	26% 16% 4 7% 7% 4 5% 5% 4 6% 16%	1620
	11th to 20th	Day	23% 13% 5% 9% 6% 3 6% 4 4 5% 21%	1575
	21st to end	Day	22% 14% 6% 8% 6% 4 5% 4 5% 5% 22%	1370
March	1st to 10th	Day	22% 12% 4 8% 7% 4 6% 5% 6% 7% 20%	1851
	11th to 20th	Day	22% 12% 4 7% 7% 4 4 4 5% 6% 25%	1980
	21st to end	Day	19% 12% 4 9% 7% 4 6% 6% 5% 6% 20%	2240
April	1st to 10th	Day	20% 11% 5% 8% 7% 4 6% 6% 7% 7% 22%	2160
	11th to 20th	Day	18% 11% 4 6% 7% 4 6% 5% 6% 7% 26%	2183
	21st to end	Day	19% 9% 4 8% 6% 3 5% 5% 6% 7% 27%	2383
May	1st to 10th	Day	14% 7% 3 6% 6% 4 5% 5% 6% 7% 35%	2520
	11th to 20th	Day	17% 7% 3 5% 6% 3 6% 5% 6% 9% 34%	2477
	21st to end	Day	20% 9% 4 7% 6% 2 4 4 5% 7% 32%	2723
June	1st to 10th	Day	20% 6% 3 5% 7% 3 5% 5% 5% 7% 34%	2654
	11th to 20th	Day	18% 8% 3 7% 6% 4 5% 5% 5% 7% 32%	2700
	21st to end	Day	10% 6% 3 6% 5% 3 5% 6% 6% 8% 43%	2700
July	1st to 10th	Day	10% 5% 2 4 4 2 4 4 5% 6% 54%	2700
	11th to 20th	Day	9% 6% 2 4 4 3 3 3 5% 7% 55%	2669
	21st to end	Day	7% 3 1 3 3 2 2 3 4 6% 65%	2741
August	1st to 10th	Day	10% 4 2 4 3 2 3 4 4 7% 57%	2472
	11th to 20th	Day	7% 4 2 4 4 2 4 4 5% 8% 56%	2298
	21st to end	Day	10% 5% 3 5% 4 3 5% 5% 5% 8% 48%	2491
September	1st to 10th	Day	10% 6% 2 4 5% 3 3 4 4 7% 51%	2160
	11th to 20th	Day	13% 7% 3 6% 6% 3 5% 5% 5% 7% 41%	2160
	21st to end	Day	16% 9% 3 7% 5% 3 4 5% 4 7% 36%	1897
October	1st to 10th	Day	18% 9% 4 7% 6% 3 5% 6% 5% 7% 31%	1800
	11th to 20th	Day	19% 10% 5% 9% 7% 3 5% 5% 6% 7% 24%	1800
	21st to end	Day	25% 12% 4 7% 5% 4 5% 5% 6% 7% 21%	1872
November	1st to 10th	Day	28% 10% 4 8% 7% 4 5% 5% 5% 6% 19%	1493
	11th to 20th	Day	26% 10% 5% 7% 6% 3 6% 4 5% 6% 22%	1520
	21st to end	Day	28% 10% 5% 8% 7% 3 5% 6% 5% 7% 17%	1520
December	1st to 10th	Day	23% 13% 4 7% 7% 3 4 5% 5% 6% 22%	1520
	11th to 20th	Day	24% 14% 5% 10% 9% 4 5% 5% 5% 4 13%	1790
	21st to end	Day	22% 16% 6% 11% 8% 3 5% 6% 4 5% 12%	2442
January	1st to 10th	Night	28% 14% 4 7% 7% 4 4 4 4 4 18%	2836
	11th to 20th	Night	27% 13% 5% 7% 5% 3 5% 4 4 5% 21%	2674
	21st to end	Night	25% 11% 4 8% 6% 5% 4 5% 6% 7% 19%	2732
February	1st to 10th	Night	26% 13% 5% 7% 6% 3 5% 4 5% 5% 21%	2340
	11th to 20th	Night	22% 15% 4 7% 6% 3 4 4 4 5% 24%	2272
	21st to end	Night	24% 13% 3 8% 7% 3 4 4 5% 6% 23%	1885
March	1st to 10th	Night	22% 12% 4 8% 6% 4 5% 5% 5% 6% 24%	2106
	11th to 20th	Night	24% 13% 5% 7% 6% 4 4 4 4 6% 23%	1979
	21st to end	Night	26% 14% 4 6% 7% 3 5% 5% 6% 6% 19%	2116
April	1st to 10th	Night	22% 12% 3 7% 7% 3 6% 6% 6% 8% 22%	1800
	11th to 20th	Night	19% 10% 4 7% 6% 3 5% 6% 6% 6% 27%	1723
	21st to end	Night	22% 10% 5% 7% 7% 5% 3 3 5% 6% 27%	1576
May	1st to 10th	Night	16% 7% 3 6% 5% 3 5% 4 4 7% 40%	1440
	11th to 20th	Night	17% 6% 2 5% 6% 4 5% 5% 7% 8% 34%	1416
	21st to end	Night	20% 8% 3 7% 6% 3 5% 5% 6% 8% 30%	1552
June	1st to 10th	Night	19% 7% 4 7% 6% 3 4 5% 5% 5% 33%	1303
	11th to 20th	Night	19% 9% 3 7% 6% 4 6% 4 6% 7% 30%	1260
	21st to end	Night	12% 6% 2 5% 5% 3 5% 5% 5% 6% 45%	1260
July	1st to 10th	Night	11% 5% 2 4 4 2 3 3 4 6% 56%	1260
	11th to 20th	Night	9% 4 2 3 4 2 2 3 4 7% 59%	1290
	21st to end	Night	7% 2 1 2 4 2 3 3 4 5% 67%	1576
August	1st to 10th	Night	9% 3 2 4 3 2 3 3 5% 6% 60%	1463
	11th to 20th	Night	7% 3 1 2 3 2 3 4 4 6% 64%	1602
	21st to end	Night	10% 5% 1 4 4 3 4 4 6% 8% 51%	1863
September	1st to 10th	Night	10% 3 2 4 4 2 4 4 5% 7% 54%	1800
	11th to 20th	Night	14% 8% 2 6% 5% 3 4 5% 5% 6% 42%	1800
	21st to end	Night	15% 7% 3 6% 5% 3 4 3 4 7% 43%	2058
October	1st to 10th	Night	15% 8% 4 7% 6% 4 3 4 4 5% 39%	2160
	11th to 20th	Night	21% 9% 2 6% 5% 4 4 4 5% 6% 34%	2160
	21st to end	Night	22% 9% 3 6% 7% 3 4 4 5% 6% 30%	2484
November	1st to 10th	Night	29% 11% 4 6% 6% 4 5% 4 4 6% 21%	2580
	11th to 20th	Night	25% 12% 4 7% 7% 3 5% 4 5% 5% 22%	2660
	21st to end	Night	27% 10% 5% 7% 6% 4 4 5% 5% 6% 21%	2660
December	1st to 10th	Night	24% 13% 4 7% 6% 3 4 4 4 6% 25%	2660
	11th to 20th	Night	24% 15% 5% 11% 8% 4 5% 4 4 5% 15%	3410
	21st to end	Night	26% 17% 5% 8% 7% 3 5% 4 4 5% 15%	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	71% 7% 8% 10% 4 1	1329
	11th to 20th	Day	66% 8% 13% 8% 5% 1	1370
	21st to end	Day	72% 7% 9% 9% 3	1621
February	1st to 10th	Day	69% 8% 9% 8% 3 2	1620
	11th to 20th	Day	65% 5% 12% 11% 6% 1	1584
	21st to end	Day	64% 7% 12% 11% 5% 1	1370
March	1st to 10th	Day	63% 6% 12% 13% 4 1	1851
	11th to 20th	Day	63% 5% 14% 14% 4 1	1980
	21st to end	Day	64% 7% 14% 13% 1	2240
April	1st to 10th	Day	60% 8% 18% 11% 2	2160
	11th to 20th	Day	56% 6% 14% 15% 7% 1	2183
	21st to end	Day	55% 7% 15% 15% 6% 1	2383
May	1st to 10th	Day	47% 8% 20% 19% 7% 1	2520
	11th to 20th	Day	47% 8% 22% 17% 6%	2480
	21st to end	Day	52% 6% 21% 17% 3	2724
June	1st to 10th	Day	49% 6% 22% 16% 6% 1	2654
	11th to 20th	Day	51% 8% 22% 15% 4	2700
	21st to end	Day	38% 7% 24% 21% 7% 2	2700
July	1st to 10th	Day	31% 7% 20% 27% 12% 3	2626
	11th to 20th	Day	29% 5% 20% 32% 12% 1	2588
	21st to end	Day	21% 4 19% 39% 14% 3	2589
August	1st to 10th	Day	27% 7% 23% 29% 12% 2	2455
	11th to 20th	Day	26% 7% 22% 30% 13% 3	2298
	21st to end	Day	34% 6% 21% 28% 9% 1	2491
September	1st to 10th	Day	32% 7% 21% 24% 13% 4	2160
	11th to 20th	Day	43% 6% 17% 17% 16% 1	2160
	21st to end	Day	47% 7% 17% 18% 10% 1	1897
October	1st to 10th	Day	51% 6% 17% 19% 7%	1800
	11th to 20th	Day	58% 8% 15% 15% 4 1	1800
	21st to end	Day	62% 7% 10% 13% 7%	1872
November	1st to 10th	Day	66% 7% 13% 10% 3	1493
	11th to 20th	Day	66% 5% 10% 13% 5% 2	1520
	21st to end	Day	67% 7% 9% 10% 7% 1	1520
December	1st to 10th	Day	63% 7% 10% 13% 7% 1	1520
	11th to 20th	Day	75% 6% 9% 7% 3 1	1792
	21st to end	Day	71% 9% 10% 7% 2	2442
January	1st to 10th	Night	71% 5% 9% 11% 3 1	2836
	11th to 20th	Night	65% 5% 13% 10% 5% 1	2674
	21st to end	Night	64% 6% 13% 13% 3 1	2732
February	1st to 10th	Night	67% 6% 12% 9% 5% 2	2340
	11th to 20th	Night	62% 7% 12% 12% 6% 1	2274
	21st to end	Night	62% 6% 10% 16% 6% 1	1885
March	1st to 10th	Night	62% 6% 12% 14% 5%	2106
	11th to 20th	Night	63% 6% 11% 15% 3 1	1980
	21st to end	Night	66% 7% 16% 11% 1	2116
April	1st to 10th	Night	61% 9% 14% 13% 3	1800
	11th to 20th	Night	57% 7% 14% 14% 6% 2	1723
	21st to end	Night	60% 5% 12% 15% 7% 1	1576
May	1st to 10th	Night	43% 7% 20% 21% 9% 1	1440
	11th to 20th	Night	45% 7% 21% 19% 7% 1	1416
	21st to end	Night	51% 7% 19% 18% 4	1552
June	1st to 10th	Night	52% 6% 17% 17% 5% 2	1303
	11th to 20th	Night	52% 7% 22% 15% 3	1260
	21st to end	Night	39% 6% 24% 21% 7% 3	1260
July	1st to 10th	Night	32% 5% 18% 28% 14% 4	1225
	11th to 20th	Night	28% 4 21% 30% 15% 3	1248
	21st to end	Night	18% 5% 16% 33% 23% 4	1488
August	1st to 10th	Night	25% 6% 18% 31% 17% 4	1455
	11th to 20th	Night	21% 5% 21% 30% 19% 4	1602
	21st to end	Night	31% 6% 19% 31% 12% 1	1863
September	1st to 10th	Night	28% 7% 18% 29% 14% 5%	1800
	11th to 20th	Night	42% 6% 13% 18% 17% 4	1800
	21st to end	Night	43% 7% 15% 25% 9% 1	2058
October	1st to 10th	Night	48% 5% 14% 23% 9%	2160
	11th to 20th	Night	51% 6% 17% 19% 6%	2160
	21st to end	Night	55% 6% 14% 18% 7%	2484
November	1st to 10th	Night	67% 6% 11% 12% 3 1	2580
	11th to 20th	Night	64% 6% 13% 10% 5% 2	2660
	21st to end	Night	65% 7% 11% 10% 6% 1	2660
December	1st to 10th	Night	60% 7% 12% 14% 7%	2660
	11th to 20th	Night	73% 7% 11% 6% 3 1	3407
	21st to end	Night	70% 7% 11% 9% 2	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	37% 12% 22% 11% 12% 6%	180
	11th to 20th	Day	28% 16% 21% 20% 8% 7%	168
	21st to end	Day	37% 8% 22% 17% 13% 4	213
February	1st to 10th	Day	39% 10% 19% 19% 12% 1	340
	11th to 20th	Day	35% 11% 18% 22% 13% 2	335
	21st to end	Day	33% 10% 20% 21% 14% 2	298
March	1st to 10th	Day	33% 8% 21% 21% 14% 3	420
	11th to 20th	Day	33% 8% 17% 21% 16% 6%	420
	21st to end	Day	30% 8% 20% 24% 14% 3	462
April	1st to 10th	Day	29% 10% 20% 24% 14% 4	420
	11th to 20th	Day	23% 10% 21% 24% 15% 7%	412
	21st to end	Day	26% 8% 20% 25% 17% 4	420
May	1st to 10th	Day	16% 6% 20% 30% 24% 3	423
	11th to 20th	Day	22% 3 14% 30% 25% 6%	448
	21st to end	Day	18% 4 22% 31% 18% 9%	485
June	1st to 10th	Day	20% 5% 25% 30% 15% 4	584
	11th to 20th	Day	20% 5% 24% 34% 13% 4	630
	21st to end	Day	11% 5% 22% 31% 23% 8%	630
July	1st to 10th	Day	11% 3 19% 35% 25% 8%	621
	11th to 20th	Day	8% 5% 18% 34% 27% 7%	587
	21st to end	Day	5% 2 13% 30% 33% 16%	487
August	1st to 10th	Day	11% 3 7% 28% 35% 15%	447
	11th to 20th	Day	6% 1 10% 30% 37% 15%	437
	21st to end	Day	11% 4 14% 32% 29% 10%	495
September	1st to 10th	Day	17% 3 14% 31% 28% 8%	450
	11th to 20th	Day	14% 4 16% 25% 34% 6%	450
	21st to end	Day	22% 6% 15% 26% 23% 8%	450
October	1st to 10th	Day	24% 6% 14% 25% 22% 10%	450
	11th to 20th	Day	31% 9% 17% 18% 17% 8%	450
	21st to end	Day	34% 9% 19% 19% 14% 6%	387
November	1st to 10th	Day	36% 10% 23% 16% 13% 2	183
	11th to 20th	Day	34% 9% 21% 14% 14% 8%	180
	21st to end	Day	33% 13% 19% 16% 8% 11%	180
December	1st to 10th	Day	39% 12% 14% 10% 14% 10%	180
	11th to 20th	Day	39% 12% 22% 17% 8% 2	180
	21st to end	Day	39% 8% 23% 22% 6% 2	198
January	1st to 10th	Night	40% 12% 18% 16% 12% 3	850
	11th to 20th	Night	39% 9% 12% 18% 19% 2	782
	21st to end	Night	34% 9% 15% 22% 16% 4	792
February	1st to 10th	Night	38% 10% 13% 25% 11% 3	720
	11th to 20th	Night	32% 9% 16% 25% 16% 2	704
	21st to end	Night	34% 9% 17% 22% 14% 3	592
March	1st to 10th	Night	32% 11% 18% 23% 14% 3	720
	11th to 20th	Night	32% 9% 17% 23% 15% 4	720
	21st to end	Night	34% 10% 14% 28% 11% 2	792
April	1st to 10th	Night	32% 8% 16% 26% 16% 3	720
	11th to 20th	Night	25% 9% 18% 27% 16% 5%	655
	21st to end	Night	29% 6% 19% 29% 14% 3	496
May	1st to 10th	Night	18% 8% 15% 32% 24% 3	360
	11th to 20th	Night	21% 3 14% 27% 27% 7%	354
	21st to end	Night	20% 5% 16% 31% 23% 5%	389
June	1st to 10th	Night	21% 5% 20% 31% 21% 2	360
	11th to 20th	Night	21% 7% 22% 31% 16% 3	360
	21st to end	Night	16% 4 17% 34% 24% 5%	360
July	1st to 10th	Night	11% 3 18% 34% 30% 4	350
	11th to 20th	Night	11% 4 16% 34% 30% 5%	348
	21st to end	Night	6% 3 17% 28% 38% 7%	394
August	1st to 10th	Night	10% 3 10% 26% 42% 10%	389
	11th to 20th	Night	8% 1 7% 25% 43% 15%	534
	21st to end	Night	12% 3 15% 29% 32% 9%	675
September	1st to 10th	Night	11% 2 11% 35% 32% 9%	720
	11th to 20th	Night	18% 4 9% 24% 31% 14%	720
	21st to end	Night	18% 4 16% 27% 27% 8%	720
October	1st to 10th	Night	21% 6% 11% 24% 28% 10%	720
	11th to 20th	Night	29% 6% 13% 22% 22% 8%	720
	21st to end	Night	30% 10% 15% 24% 17% 4	792
November	1st to 10th	Night	35% 10% 18% 24% 12% 1	740
	11th to 20th	Night	34% 11% 17% 21% 15% 2	760
	21st to end	Night	36% 9% 19% 18% 13% 5%	760
December	1st to 10th	Night	31% 10% 14% 24% 18% 4	760
	11th to 20th	Night	39% 10% 20% 21% 9% 1	800
	21st to end	Night	38% 12% 21% 19% 10% 1	935

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