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 2025-09-04.

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	19% 9% 3 7% 4 4 4 3 5% 6% 35%	130281
	Night	23% 10% 3 7% 5% 4 4 3 5% 6% 31%	127152
Transparency	Day	30% 40% 11% 13% 6% 1	129991
	Night	34% 40% 9% 11% 5% 1	127019
Seeing	Day	22% 9% 21% 26% 17% 4	16322
	Night	29% 8% 15% 24% 19% 4	24504

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	36% 15% 4 10% 5% 5% 4 3 4 4 11%	3179
	11th to 20th	Day	19% 12% 3 8% 4 4 4 4 5% 7% 29%	3313
	21st to end	Day	34% 13% 3 7% 4 3 3 2 3 4 24%	4046
February	1st to 10th	Day	30% 13% 4 8% 6% 4 4 3 5% 6% 17%	1800
	11th to 20th	Day	28% 13% 4 9% 5% 4 4 3 5% 5% 21%	1755
	21st to end	Day	25% 15% 4 8% 6% 4 5% 3 6% 5% 19%	1528
March	1st to 10th	Day	22% 14% 4 9% 6% 5% 5% 3 5% 6% 20%	4623
	11th to 20th	Day	19% 13% 4 10% 6% 5% 4 4 5% 7% 25%	5000
	21st to end	Day	24% 12% 4 10% 6% 6% 5% 4 6% 6% 18%	5660
April	1st to 10th	Day	24% 12% 4 7% 6% 5% 5% 4 6% 7% 20%	2400
	11th to 20th	Day	19% 10% 4 8% 5% 5% 5% 4 6% 7% 28%	2434
	21st to end	Day	22% 10% 3 9% 4 4 4 4 7% 6% 27%	2669
May	1st to 10th	Day	14% 9% 3 7% 4 3 4 3 4 5% 43%	6320
	11th to 20th	Day	17% 10% 4 7% 5% 5% 5% 4 6% 7% 31%	6277
	21st to end	Day	20% 8% 3 6% 4 4 5% 5% 6% 9% 31%	6903
June	1st to 10th	Day	19% 6% 2 7% 5% 5% 5% 5% 5% 8% 33%	2979
	11th to 20th	Day	19% 8% 3 8% 6% 4 4 5% 7% 7% 29%	3000
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	3000
July	1st to 10th	Day	7% 3 1 4 3 3 3 3 4 6% 64%	6780
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 5% 72%	6744
	21st to end	Day	7% 3 1 3 2 3 3 3 5% 6% 64%	6874
August	1st to 10th	Day	4 3 1 3 2 3 2 2 5% 7% 67%	4518
	11th to 20th	Day	8% 6% 3 5% 4 4 5% 4 6% 8% 48%	4156
	21st to end	Day	15% 6% 3 7% 4 4 4 3 5% 7% 42%	4514
September	1st to 10th	Day	10% 6% 2 7% 4 4 3 3 4 7% 51%	2317
	11th to 20th	Day	16% 6% 3 7% 5% 4 4 3 5% 7% 40%	2280
	21st to end	Day	22% 7% 3 8% 4 4 3 3 4 8% 36%	2005
October	1st to 10th	Day	19% 6% 2 6% 4 5% 5% 4 7% 8% 33%	3180
	11th to 20th	Day	32% 9% 2 7% 4 4 3 3 5% 7% 25%	3161
	21st to end	Day	25% 11% 5% 10% 6% 5% 5% 4 6% 6% 17%	3229
November	1st to 10th	Day	35% 9% 5% 9% 5% 4 3 3 4 5% 17%	1568
	11th to 20th	Day	34% 12% 3 8% 4 4 3 3 4 4 21%	1600
	21st to end	Day	33% 12% 4 9% 6% 4 4 4 4 4 17%	1600
December	1st to 10th	Day	23% 11% 3 9% 6% 5% 5% 4 6% 8% 23%	2620
	11th to 20th	Day	34% 15% 4 9% 5% 5% 4 3 4 4 13%	2751
	21st to end	Day	33% 17% 4 9% 5% 5% 4 3 4 5% 10%	3498
January	1st to 10th	Night	34% 15% 4 10% 5% 5% 4 3 4 4 12%	6766
	11th to 20th	Night	22% 9% 3 6% 4 4 4 4 6% 9% 30%	6583
	21st to end	Night	32% 11% 3 6% 4 4 3 2 3 4 28%	6908
February	1st to 10th	Night	33% 13% 3 7% 6% 3 3 3 4 5% 20%	2600
	11th to 20th	Night	27% 15% 3 7% 5% 4 4 3 4 5% 23%	2532
	21st to end	Night	29% 13% 3 8% 5% 4 4 3 4 5% 23%	2101
March	1st to 10th	Night	19% 15% 4 8% 6% 5% 5% 4 6% 6% 22%	5256
	11th to 20th	Night	22% 12% 4 8% 5% 4 5% 4 4 6% 27%	4999
	21st to end	Night	32% 14% 4 10% 6% 4 4 3 4 4 16%	5314
April	1st to 10th	Night	26% 13% 4 7% 5% 4 4 3 6% 6% 22%	2000
	11th to 20th	Night	22% 11% 3 7% 5% 5% 5% 3 5% 5% 29%	1914
	21st to end	Night	25% 11% 3 8% 5% 4 5% 3 4 6% 26%	1730
May	1st to 10th	Night	15% 10% 3 6% 4 3 3 2 4 4 46%	3640
	11th to 20th	Night	18% 8% 3 8% 5% 5% 4 4 5% 6% 34%	3616
	21st to end	Night	13% 9% 3 10% 7% 6% 6% 5% 6% 6% 28%	3972
June	1st to 10th	Night	18% 6% 2 9% 6% 5% 5% 4 6% 5% 35%	1417
	11th to 20th	Night	17% 10% 4 7% 4 6% 5% 5% 6% 8% 29%	1400
	21st to end	Night	15% 6% 3 6% 5% 5% 4 3 6% 8% 41%	1400
July	1st to 10th	Night	8% 3 1 4 3 3 3 2 3 5% 63%	3180
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 4 77%	3216
	21st to end	Night	7% 3 1 4 2 3 3 3 3 6% 64%	3969
August	1st to 10th	Night	5% 2 1 2 2 2 3 3 5% 6% 69%	2631
	11th to 20th	Night	9% 4 2 5% 4 4 4 3 5% 8% 52%	2902
	21st to end	Night	12% 5% 2 7% 5% 5% 5% 4 6% 9% 42%	3400
September	1st to 10th	Night	9% 3 2 5% 4 4 3 3 5% 6% 56%	1930
	11th to 20th	Night	17% 7% 2 6% 4 4 3 2 4 6% 44%	1900
	21st to end	Night	18% 7% 2 6% 4 3 4 4 5% 5% 41%	2170
October	1st to 10th	Night	18% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	3800
	11th to 20th	Night	28% 7% 3 6% 4 3 5% 3 5% 6% 29%	3777
	21st to end	Night	26% 10% 4 8% 5% 4 4 3 5% 6% 24%	4332
November	1st to 10th	Night	37% 11% 3 8% 4 5% 4 3 4 4 18%	2728
	11th to 20th	Night	34% 11% 3 8% 4 4 4 3 4 4 22%	2800
	21st to end	Night	33% 11% 3 8% 6% 5% 4 3 4 4 19%	2800
December	1st to 10th	Night	24% 11% 3 8% 4 4 5% 4 6% 8% 23%	4560
	11th to 20th	Night	36% 14% 4 9% 5% 4 4 3 4 5% 13%	5451
	21st to end	Night	30% 14% 5% 10% 6% 5% 5% 3 4 4 13%	7458

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% 30% 5% 7% 1	3179
	11th to 20th	Day	36% 29% 9% 17% 7% 2	3334
	21st to end	Day	48% 28% 5% 13% 6%	4046
February	1st to 10th	Day	27% 61% 3 3 4 2	1800
	11th to 20th	Day	28% 62% 3 4 3	1764
	21st to end	Day	26% 62% 4 4 2 1	1528
March	1st to 10th	Day	44% 32% 9% 12% 3	4623
	11th to 20th	Day	41% 32% 10% 15% 2	5000
	21st to end	Day	47% 33% 9% 8% 2	5660
April	1st to 10th	Day	24% 66% 5% 5%	2400
	11th to 20th	Day	20% 64% 6% 6% 2 1	2434
	21st to end	Day	21% 65% 5% 7% 3	2669
May	1st to 10th	Day	28% 32% 12% 19% 8%	6320
	11th to 20th	Day	37% 36% 14% 12% 1	6280
	21st to end	Day	34% 34% 18% 13% 1	6904
June	1st to 10th	Day	16% 64% 8% 8% 3	2979
	11th to 20th	Day	20% 64% 5% 8% 3	3000
	21st to end	Day	16% 64% 8% 10% 2	3000
July	1st to 10th	Day	15% 28% 16% 17% 16% 8%	6706
	11th to 20th	Day	9% 26% 15% 26% 20% 3	6663
	21st to end	Day	17% 27% 27% 22% 7%	6681
August	1st to 10th	Day	9% 38% 8% 30% 15% 1	4501
	11th to 20th	Day	24% 40% 20% 12% 3 1	4195
	21st to end	Day	29% 37% 10% 14% 9%	4513
September	1st to 10th	Day	18% 61% 8% 8% 4 1	2317
	11th to 20th	Day	20% 62% 5% 6% 6% 1	2280
	21st to end	Day	20% 61% 7% 6% 5%	2005
October	1st to 10th	Day	29% 39% 13% 16% 2	3180
	11th to 20th	Day	40% 38% 7% 8% 6%	3161
	21st to end	Day	42% 40% 7% 8% 3	3229
November	1st to 10th	Day	31% 57% 6% 5% 1	1568
	11th to 20th	Day	27% 58% 3 7% 3 2	1600
	21st to end	Day	27% 58% 3 6% 5% 1	1600
December	1st to 10th	Day	35% 39% 8% 12% 6%	2620
	11th to 20th	Day	52% 34% 6% 6% 1	2754
	21st to end	Day	57% 31% 8% 3 1	3498
January	1st to 10th	Night	56% 30% 6% 6% 2	6766
	11th to 20th	Night	31% 32% 13% 15% 8% 1	6624
	21st to end	Night	44% 29% 8% 14% 5%	6908
February	1st to 10th	Night	25% 62% 4 3 4 2	2600
	11th to 20th	Night	28% 61% 3 4 2 1	2534
	21st to end	Night	23% 63% 4 6% 3 1	2101
March	1st to 10th	Night	40% 33% 10% 10% 6%	5256
	11th to 20th	Night	38% 33% 10% 17% 2	5000
	21st to end	Night	52% 33% 9% 6% 1	5314
April	1st to 10th	Night	25% 63% 4 7% 1	2000
	11th to 20th	Night	22% 63% 5% 5% 3 1	1914
	21st to end	Night	22% 63% 5% 8% 2 1	1730
May	1st to 10th	Night	30% 30% 6% 27% 7%	3637
	11th to 20th	Night	36% 33% 18% 11% 2	3616
	21st to end	Night	39% 35% 13% 13% 1	3972
June	1st to 10th	Night	16% 63% 7% 10% 3 1	1417
	11th to 20th	Night	18% 63% 6% 11% 2	1400
	21st to end	Night	17% 61% 9% 10% 3	1400
July	1st to 10th	Night	17% 26% 14% 20% 15% 8%	3145
	11th to 20th	Night	7% 25% 14% 27% 24% 3	3174
	21st to end	Night	19% 26% 17% 24% 12% 1	3857
August	1st to 10th	Night	7% 35% 9% 28% 20% 1	2623
	11th to 20th	Night	23% 38% 20% 14% 4 1	2929
	21st to end	Night	26% 39% 14% 13% 7%	3400
September	1st to 10th	Night	15% 61% 8% 10% 6% 1	1930
	11th to 20th	Night	21% 60% 6% 6% 6% 1	1900
	21st to end	Night	19% 61% 6% 10% 4	2170
October	1st to 10th	Night	32% 40% 14% 12% 2	3800
	11th to 20th	Night	37% 39% 8% 11% 4	3777
	21st to end	Night	38% 40% 9% 9% 3	4332
November	1st to 10th	Night	32% 56% 6% 4 1	2728
	11th to 20th	Night	29% 57% 4 5% 4 2	2800
	21st to end	Night	28% 58% 4 4 5% 1	2800
December	1st to 10th	Night	35% 39% 12% 10% 4	4560
	11th to 20th	Night	52% 33% 6% 7% 1	5447
	21st to end	Night	53% 33% 6% 6% 1	7458

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	39% 11% 20% 17% 9% 6%	200
	11th to 20th	Day	37% 14% 15% 17% 14% 4	200
	21st to end	Day	39% 11% 19% 13% 13% 4	236
February	1st to 10th	Day	40% 10% 23% 17% 8% 3	400
	11th to 20th	Day	34% 13% 22% 21% 9% 1	395
	21st to end	Day	34% 14% 23% 19% 9% 1	354
March	1st to 10th	Day	30% 13% 26% 18% 12% 1	500
	11th to 20th	Day	30% 15% 17% 19% 16% 4	500
	21st to end	Day	32% 13% 25% 20% 8% 1	550
April	1st to 10th	Day	27% 11% 26% 21% 11% 2	500
	11th to 20th	Day	20% 14% 28% 22% 13% 3	492
	21st to end	Day	25% 10% 24% 25% 13% 3	500
May	1st to 10th	Day	15% 9% 26% 31% 17% 2	503
	11th to 20th	Day	19% 7% 20% 30% 21% 3	528
	21st to end	Day	19% 10% 24% 27% 15% 4	573
June	1st to 10th	Day	18% 8% 28% 28% 15% 2	709
	11th to 20th	Day	18% 10% 26% 33% 11% 2	730
	21st to end	Day	14% 6% 22% 33% 20% 4	730
July	1st to 10th	Day	12% 5% 21% 37% 23% 2	721
	11th to 20th	Day	7% 5% 19% 41% 23% 5%	708
	21st to end	Day	7% 3 16% 34% 32% 7%	575
August	1st to 10th	Day	7% 4 16% 33% 30% 10%	527
	11th to 20th	Day	6% 4 15% 37% 31% 7%	517
	21st to end	Day	12% 7% 19% 32% 21% 9%	583
September	1st to 10th	Day	11% 5% 25% 34% 20% 6%	503
	11th to 20th	Day	17% 8% 22% 27% 21% 5%	490
	21st to end	Day	24% 9% 19% 26% 18% 5%	490
October	1st to 10th	Day	27% 7% 20% 24% 16% 6%	490
	11th to 20th	Day	33% 8% 18% 21% 14% 6%	490
	21st to end	Day	31% 14% 18% 21% 11% 5%	408
November	1st to 10th	Day	36% 16% 19% 18% 12% 1	200
	11th to 20th	Day	38% 13% 18% 13% 11% 10%	200
	21st to end	Day	36% 10% 24% 13% 10% 9%	200
December	1st to 10th	Day	38% 14% 8% 11% 19% 11%	200
	11th to 20th	Day	42% 17% 14% 16% 11% 1	200
	21st to end	Day	39% 18% 16% 16% 9% 1	220
January	1st to 10th	Night	44% 13% 17% 14% 10% 3	900
	11th to 20th	Night	39% 9% 14% 20% 17% 2	867
	21st to end	Night	38% 9% 15% 23% 13% 2	880
February	1st to 10th	Night	40% 8% 20% 23% 7% 2	800
	11th to 20th	Night	36% 10% 19% 23% 11% 2	784
	21st to end	Night	38% 10% 20% 20% 11% 2	660
March	1st to 10th	Night	32% 12% 18% 20% 16% 2	800
	11th to 20th	Night	34% 13% 15% 22% 14% 3	800
	21st to end	Night	35% 10% 19% 22% 12% 2	880
April	1st to 10th	Night	31% 8% 19% 27% 13% 3	800
	11th to 20th	Night	27% 8% 19% 28% 16% 2	726
	21st to end	Night	30% 6% 16% 29% 15% 4	530
May	1st to 10th	Night	18% 7% 16% 30% 26% 4	400
	11th to 20th	Night	18% 6% 11% 29% 30% 5%	394
	21st to end	Night	17% 4 15% 32% 28% 5%	433
June	1st to 10th	Night	19% 7% 16% 31% 24% 4	400
	11th to 20th	Night	19% 8% 18% 33% 19% 4	400
	21st to end	Night	15% 4 15% 29% 32% 6%	400
July	1st to 10th	Night	10% 3 14% 30% 39% 4	390
	11th to 20th	Night	7% 3 13% 35% 35% 5%	388
	21st to end	Night	5% 3 8% 31% 44% 8%	438
August	1st to 10th	Night	9% 2 8% 27% 41% 14%	407
	11th to 20th	Night	8% 2 7% 25% 46% 12%	594
	21st to end	Night	10% 4 11% 28% 39% 8%	750
September	1st to 10th	Night	10% 2 11% 35% 33% 8%	772
	11th to 20th	Night	19% 5% 12% 22% 31% 11%	760
	21st to end	Night	23% 6% 13% 26% 27% 6%	760
October	1st to 10th	Night	22% 5% 13% 28% 25% 8%	760
	11th to 20th	Night	32% 7% 13% 21% 20% 6%	760
	21st to end	Night	36% 9% 16% 23% 13% 4	836
November	1st to 10th	Night	42% 11% 16% 20% 10% 1	780
	11th to 20th	Night	41% 10% 19% 16% 11% 3	800
	21st to end	Night	39% 10% 18% 17% 12% 4	800
December	1st to 10th	Night	34% 8% 15% 24% 15% 4	800
	11th to 20th	Night	43% 15% 19% 17% 6% 1	865
	21st to end	Night	43% 14% 19% 18% 6%	990

Page generated at 2025-08-31 22:51:09. Version ac07

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