Bellingham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bellingham clear sky chart from 2005-11-06 to 2024-03-28.

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	22% 9% 3 6% 5% 3 4 4 5% 6% 32%	78414
	Night	27% 10% 3 6% 5% 3 4 4 4 6% 28%	81534
Transparency	Day	54% 9% 13% 16% 7% 1	78127
	Night	58% 8% 12% 14% 7% 1	81401
Seeing	Day	22% 9% 21% 26% 17% 4	14342
	Night	30% 8% 15% 23% 19% 4	22844

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	38% 16% 3 8% 6% 3 4 4 3 3 10%	2219
	11th to 20th	Day	27% 14% 4 8% 6% 3 3 3 4 6% 23%	2297
	21st to end	Day	43% 15% 4 7% 6% 2 3 4 3 3 10%	2822
February	1st to 10th	Day	32% 12% 4 7% 7% 4 4 3 5% 6% 17%	1710
	11th to 20th	Day	28% 13% 4 7% 6% 3 5% 4 4 5% 21%	1665
	21st to end	Day	26% 14% 4 7% 6% 3 5% 4 6% 5% 20%	1454
March	1st to 10th	Day	27% 13% 5% 7% 7% 3 4 4 4 6% 21%	1943
	11th to 20th	Day	28% 11% 4 9% 6% 3 3 4 4 6% 22%	2090
	21st to end	Day	26% 12% 5% 8% 6% 4 5% 4 5% 6% 20%	2327
April	1st to 10th	Day	25% 11% 4 6% 7% 4 5% 5% 6% 7% 20%	2160
	11th to 20th	Day	21% 10% 4 7% 6% 4 5% 5% 6% 7% 26%	2187
	21st to end	Day	21% 10% 4 8% 6% 3 4 5% 6% 7% 26%	2401
May	1st to 10th	Day	17% 7% 3 6% 6% 3 5% 5% 5% 6% 35%	2520
	11th to 20th	Day	15% 8% 3 6% 6% 4 5% 5% 5% 8% 34%	2477
	21st to end	Day	18% 9% 4 7% 6% 3 4 5% 5% 6% 33%	2723
June	1st to 10th	Day	20% 6% 3 6% 6% 3 6% 6% 5% 8% 32%	2681
	11th to 20th	Day	20% 7% 4 7% 7% 3 4 5% 6% 7% 29%	2700
	21st to end	Day	12% 6% 3 5% 5% 4 5% 5% 5% 8% 41%	2700
July	1st to 10th	Day	10% 5% 2 5% 4 2 4 4 5% 6% 53%	2700
	11th to 20th	Day	8% 4 2 3 3 3 4 4 5% 8% 55%	2692
	21st to end	Day	7% 3 1 3 2 1 3 4 4 6% 67%	2741
August	1st to 10th	Day	8% 5% 2 4 3 2 3 3 5% 7% 59%	2498
	11th to 20th	Day	7% 4 2 3 3 2 4 5% 5% 7% 58%	2298
	21st to end	Day	13% 5% 2 5% 4 3 4 4 5% 7% 48%	2494
September	1st to 10th	Day	11% 6% 2 6% 5% 3 3 3 4 6% 51%	2160
	11th to 20th	Day	16% 6% 3 5% 6% 3 4 4 4 7% 41%	2160
	21st to end	Day	22% 6% 3 6% 5% 3 3 4 4 7% 37%	1900
October	1st to 10th	Day	22% 9% 2 6% 5% 3 5% 3 5% 8% 32%	1800
	11th to 20th	Day	28% 8% 3 7% 5% 3 3 4 6% 8% 25%	1800
	21st to end	Day	29% 9% 4 7% 6% 3 4 3 6% 5% 23%	1856
November	1st to 10th	Day	36% 9% 5% 8% 6% 3 3 3 4 5% 17%	1488
	11th to 20th	Day	35% 11% 3 6% 5% 3 4 3 3 5% 22%	1520
	21st to end	Day	33% 11% 4 8% 7% 2 4 5% 4 5% 17%	1520
December	1st to 10th	Day	31% 13% 4 6% 5% 2 3 4 4 4 24%	1520
	11th to 20th	Day	33% 14% 4 8% 5% 4 4 4 3 5% 16%	1749
	21st to end	Day	33% 15% 4 8% 6% 3 5% 4 4 5% 12%	2442
January	1st to 10th	Night	36% 15% 5% 9% 6% 3 4 3 3 4 12%	4726
	11th to 20th	Night	31% 9% 3 5% 4 2 4 4 5% 8% 25%	4599
	21st to end	Night	42% 14% 4 6% 5% 3 3 3 3 4 13%	4831
February	1st to 10th	Night	34% 12% 4 6% 6% 3 3 3 4 5% 20%	2470
	11th to 20th	Night	27% 15% 4 6% 7% 3 4 3 4 5% 23%	2402
	21st to end	Night	29% 12% 4 7% 6% 3 3 4 4 5% 24%	1999
March	1st to 10th	Night	28% 11% 4 6% 6% 3 4 4 5% 5% 24%	2235
	11th to 20th	Night	29% 9% 3 6% 6% 3 5% 5% 3 5% 24%	2089
	21st to end	Night	31% 13% 4 8% 7% 2 5% 4 4 5% 19%	2183
April	1st to 10th	Night	27% 12% 4 6% 6% 3 5% 4 6% 6% 22%	1800
	11th to 20th	Night	24% 9% 4 7% 6% 4 5% 4 5% 5% 26%	1722
	21st to end	Night	26% 10% 4 7% 6% 3 4 5% 4 6% 25%	1558
May	1st to 10th	Night	17% 7% 2 6% 5% 3 4 4 7% 7% 39%	1440
	11th to 20th	Night	15% 6% 3 6% 5% 3 6% 6% 7% 9% 35%	1416
	21st to end	Night	15% 8% 3 7% 7% 4 4 5% 7% 7% 34%	1552
June	1st to 10th	Night	18% 6% 3 8% 8% 3 5% 5% 5% 6% 33%	1276
	11th to 20th	Night	18% 9% 4 7% 5% 3 5% 5% 6% 9% 29%	1260
	21st to end	Night	14% 4 3 5% 6% 3 5% 4 6% 9% 43%	1260
July	1st to 10th	Night	11% 4 2 5% 5% 3 4 3 3 6% 55%	1260
	11th to 20th	Night	8% 4 3 4 3 2 3 3 4 5% 60%	1268
	21st to end	Night	6% 2 1 3 2 2 3 3 3 6% 69%	1576
August	1st to 10th	Night	8% 2 1 3 4 2 4 4 5% 7% 61%	1439
	11th to 20th	Night	5% 2 2 3 3 2 3 3 5% 8% 63%	1602
	21st to end	Night	8% 4 2 6% 5% 4 4 5% 6% 8% 48%	1862
September	1st to 10th	Night	10% 3 2 4 4 3 4 4 4 6% 56%	1800
	11th to 20th	Night	17% 6% 3 6% 5% 3 4 3 4 6% 45%	1800
	21st to end	Night	18% 7% 2 6% 5% 2 4 4 5% 5% 41%	2055
October	1st to 10th	Night	19% 7% 3 5% 5% 3 6% 4 4 6% 39%	2160
	11th to 20th	Night	27% 7% 3 6% 5% 3 4 4 5% 6% 30%	2160
	21st to end	Night	30% 8% 3 6% 4 3 4 4 4 6% 29%	2499
November	1st to 10th	Night	37% 10% 3 7% 5% 4 4 4 3 4 18%	2588
	11th to 20th	Night	35% 10% 4 6% 5% 3 3 4 4 4 23%	2660
	21st to end	Night	34% 9% 4 7% 7% 3 5% 3 4 4 20%	2660
December	1st to 10th	Night	30% 10% 4 6% 5% 3 4 4 4 6% 25%	2660
	11th to 20th	Night	37% 13% 5% 8% 6% 3 4 4 3 4 14%	3453
	21st to end	Night	34% 14% 5% 9% 6% 3 4 3 4 4 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	78% 8% 6% 6% 2	2219
	11th to 20th	Day	63% 7% 9% 12% 6% 3	2318
	21st to end	Day	80% 5% 7% 6% 2	2822
February	1st to 10th	Day	70% 7% 9% 7% 5% 2	1710
	11th to 20th	Day	66% 9% 8% 9% 6% 1	1674
	21st to end	Day	66% 9% 9% 10% 5% 1	1454
March	1st to 10th	Day	65% 9% 11% 11% 4 1	1943
	11th to 20th	Day	67% 6% 10% 13% 3 1	2090
	21st to end	Day	67% 10% 11% 10% 2	2327
April	1st to 10th	Day	64% 12% 13% 9% 2	2160
	11th to 20th	Day	60% 11% 9% 11% 7% 1	2187
	21st to end	Day	60% 10% 12% 11% 6% 1	2401
May	1st to 10th	Day	50% 12% 14% 17% 7% 1	2520
	11th to 20th	Day	48% 13% 18% 15% 6%	2480
	21st to end	Day	54% 12% 15% 17% 3	2724
June	1st to 10th	Day	53% 10% 16% 15% 6% 1	2681
	11th to 20th	Day	55% 12% 17% 12% 4	2700
	21st to end	Day	43% 10% 18% 20% 7% 1	2700
July	1st to 10th	Day	34% 9% 17% 26% 12% 3	2626
	11th to 20th	Day	29% 10% 17% 31% 11% 1	2611
	21st to end	Day	20% 7% 17% 37% 15% 3	2589
August	1st to 10th	Day	27% 9% 20% 29% 12% 2	2481
	11th to 20th	Day	25% 9% 20% 29% 13% 3	2298
	21st to end	Day	36% 9% 19% 28% 8% 1	2494
September	1st to 10th	Day	37% 8% 16% 22% 12% 4	2160
	11th to 20th	Day	44% 9% 13% 18% 15% 2	2160
	21st to end	Day	48% 9% 14% 17% 10% 2	1900
October	1st to 10th	Day	50% 8% 15% 17% 9%	1800
	11th to 20th	Day	57% 9% 16% 13% 4 1	1800
	21st to end	Day	61% 9% 11% 11% 7%	1856
November	1st to 10th	Day	70% 6% 10% 10% 3 1	1488
	11th to 20th	Day	68% 7% 8% 10% 5% 2	1520
	21st to end	Day	70% 5% 9% 9% 6% 1	1520
December	1st to 10th	Day	65% 8% 6% 11% 9% 1	1520
	11th to 20th	Day	69% 9% 8% 9% 3 1	1752
	21st to end	Day	74% 9% 9% 5% 2 1	2442
January	1st to 10th	Night	76% 8% 7% 7% 2 1	4726
	11th to 20th	Night	57% 9% 10% 15% 8% 2	4640
	21st to end	Night	77% 5% 6% 8% 2 1	4831
February	1st to 10th	Night	69% 6% 9% 8% 5% 3	2470
	11th to 20th	Night	66% 8% 9% 10% 6% 2	2404
	21st to end	Night	63% 7% 10% 14% 6% 1	1999
March	1st to 10th	Night	61% 8% 13% 11% 6%	2235
	11th to 20th	Night	63% 7% 11% 14% 5% 1	2090
	21st to end	Night	68% 8% 13% 10% 1	2183
April	1st to 10th	Night	61% 11% 11% 14% 2	1800
	11th to 20th	Night	59% 7% 11% 14% 6% 2	1722
	21st to end	Night	62% 6% 12% 11% 7% 1	1558
May	1st to 10th	Night	46% 9% 18% 17% 8% 1	1440
	11th to 20th	Night	44% 10% 21% 16% 8%	1416
	21st to end	Night	47% 11% 18% 19% 4	1552
June	1st to 10th	Night	51% 8% 17% 16% 6% 2	1276
	11th to 20th	Night	52% 10% 21% 14% 3	1260
	21st to end	Night	38% 9% 22% 22% 6% 4	1260
July	1st to 10th	Night	34% 6% 14% 27% 15% 4	1225
	11th to 20th	Night	29% 7% 17% 31% 14% 2	1226
	21st to end	Night	17% 6% 14% 35% 22% 6%	1488
August	1st to 10th	Night	23% 7% 19% 28% 19% 3	1431
	11th to 20th	Night	20% 6% 22% 31% 16% 5%	1602
	21st to end	Night	33% 7% 18% 29% 12%	1862
September	1st to 10th	Night	30% 8% 17% 27% 14% 4	1800
	11th to 20th	Night	42% 8% 13% 17% 16% 4	1800
	21st to end	Night	45% 8% 15% 21% 10% 2	2055
October	1st to 10th	Night	47% 8% 14% 22% 9%	2160
	11th to 20th	Night	56% 7% 15% 15% 6% 1	2160
	21st to end	Night	58% 8% 11% 14% 9%	2499
November	1st to 10th	Night	70% 7% 10% 9% 4 1	2588
	11th to 20th	Night	67% 7% 10% 8% 6% 2	2660
	21st to end	Night	68% 7% 9% 9% 6% 1	2660
December	1st to 10th	Night	62% 8% 10% 11% 8% 1	2660
	11th to 20th	Night	75% 6% 9% 6% 4 1	3449
	21st to end	Night	73% 9% 7% 7% 3 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	38% 11% 21% 14% 10% 6%	180
	11th to 20th	Day	39% 14% 14% 16% 12% 4	180
	21st to end	Day	42% 12% 19% 12% 13% 2	213
February	1st to 10th	Day	40% 10% 24% 17% 8% 2	370
	11th to 20th	Day	35% 13% 22% 20% 9% 1	365
	21st to end	Day	33% 14% 23% 20% 9% 1	328
March	1st to 10th	Day	30% 13% 26% 18% 12% 1	460
	11th to 20th	Day	28% 14% 17% 19% 17% 4	460
	21st to end	Day	33% 13% 24% 20% 9% 1	491
April	1st to 10th	Day	29% 11% 25% 21% 12% 2	420
	11th to 20th	Day	22% 13% 25% 22% 14% 4	412
	21st to end	Day	24% 10% 25% 26% 13% 2	420
May	1st to 10th	Day	15% 8% 27% 31% 16% 2	423
	11th to 20th	Day	19% 5% 18% 31% 23% 4	448
	21st to end	Day	18% 9% 24% 28% 16% 5%	485
June	1st to 10th	Day	19% 8% 28% 27% 16% 2	611
	11th to 20th	Day	18% 10% 27% 33% 11% 2	630
	21st to end	Day	14% 5% 22% 34% 22% 4	630
July	1st to 10th	Day	13% 4 19% 36% 25% 2	621
	11th to 20th	Day	8% 5% 19% 39% 25% 4	610
	21st to end	Day	6% 3 15% 35% 35% 7%	487
August	1st to 10th	Day	7% 4 15% 32% 32% 10%	447
	11th to 20th	Day	5% 3 13% 37% 34% 7%	437
	21st to end	Day	13% 7% 19% 33% 21% 8%	495
September	1st to 10th	Day	12% 5% 27% 32% 20% 4	450
	11th to 20th	Day	17% 7% 21% 28% 23% 5%	450
	21st to end	Day	23% 8% 18% 27% 18% 6%	450
October	1st to 10th	Day	27% 7% 19% 24% 16% 7%	450
	11th to 20th	Day	32% 8% 18% 21% 15% 6%	450
	21st to end	Day	31% 15% 17% 19% 12% 5%	371
November	1st to 10th	Day	36% 17% 18% 17% 11% 1	180
	11th to 20th	Day	38% 11% 17% 12% 12% 11%	180
	21st to end	Day	37% 9% 23% 12% 11% 8%	180
December	1st to 10th	Day	40% 16% 9% 11% 16% 8%	180
	11th to 20th	Day	41% 16% 13% 17% 12% 1	180
	21st to end	Day	39% 17% 17% 17% 9% 2	198
January	1st to 10th	Night	44% 13% 16% 13% 10% 3	850
	11th to 20th	Night	41% 9% 14% 18% 16% 2	821
	21st to end	Night	39% 9% 15% 23% 12% 2	836
February	1st to 10th	Night	41% 8% 19% 23% 7% 2	760
	11th to 20th	Night	36% 10% 18% 23% 11% 2	744
	21st to end	Night	37% 10% 20% 20% 11% 2	628
March	1st to 10th	Night	33% 12% 18% 19% 16% 2	760
	11th to 20th	Night	34% 12% 14% 22% 14% 3	760
	21st to end	Night	35% 10% 18% 23% 13% 3	820
April	1st to 10th	Night	32% 7% 19% 26% 13% 3	720
	11th to 20th	Night	29% 7% 18% 26% 17% 3	654
	21st to end	Night	31% 5% 16% 29% 15% 3	478
May	1st to 10th	Night	19% 6% 15% 30% 26% 4	360
	11th to 20th	Night	18% 5% 10% 30% 32% 5%	354
	21st to end	Night	18% 3 14% 31% 29% 5%	389
June	1st to 10th	Night	18% 7% 16% 31% 24% 4	360
	11th to 20th	Night	20% 8% 16% 33% 19% 4	360
	21st to end	Night	13% 4 15% 29% 33% 6%	360
July	1st to 10th	Night	11% 4 11% 30% 39% 4	350
	11th to 20th	Night	8% 4 12% 34% 36% 6%	348
	21st to end	Night	5% 2 7% 32% 45% 9%	394
August	1st to 10th	Night	8% 2 8% 25% 42% 15%	365
	11th to 20th	Night	7% 1 6% 25% 49% 13%	534
	21st to end	Night	10% 4 11% 28% 38% 9%	674
September	1st to 10th	Night	11% 2 11% 36% 33% 7%	720
	11th to 20th	Night	19% 4 11% 22% 32% 12%	720
	21st to end	Night	22% 6% 13% 26% 27% 6%	720
October	1st to 10th	Night	22% 5% 12% 27% 26% 8%	720
	11th to 20th	Night	33% 7% 13% 21% 21% 6%	720
	21st to end	Night	36% 9% 16% 23% 13% 4	792
November	1st to 10th	Night	42% 11% 16% 20% 11% 1	740
	11th to 20th	Night	40% 9% 20% 16% 12% 3	760
	21st to end	Night	40% 10% 17% 17% 12% 4	760
December	1st to 10th	Night	35% 8% 16% 24% 14% 4	760
	11th to 20th	Night	43% 14% 19% 17% 6% 1	818
	21st to end	Night	44% 13% 18% 19% 6%	935

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