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 2024-11-04.

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	20% 9% 3 7% 4 4 4 3 5% 6% 35%	105514
	Night	24% 10% 3 7% 5% 4 4 3 4 5% 31%	102106
Transparency	Day	26% 48% 8% 11% 5% 1	105266
	Night	30% 48% 7% 9% 5% 1	102000
Seeing	Day	22% 9% 21% 26% 17% 4	15260
	Night	29% 8% 15% 24% 19% 4	23493

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 10% 5% 4 4 3 3 3 10%	2219
	11th to 20th	Day	26% 15% 3 9% 5% 4 3 3 4 6% 23%	2297
	21st to end	Day	43% 16% 4 8% 5% 3 3 3 3 3 10%	2822
February	1st to 10th	Day	31% 14% 4 8% 6% 4 4 2 5% 6% 16%	1710
	11th to 20th	Day	28% 14% 4 9% 5% 4 4 3 4 5% 21%	1665
	21st to end	Day	25% 15% 4 8% 6% 4 4 3 6% 5% 20%	1454
March	1st to 10th	Day	22% 15% 4 9% 5% 5% 4 3 4 7% 21%	3217
	11th to 20th	Day	18% 10% 3 8% 5% 4 4 3 5% 7% 33%	3490
	21st to end	Day	24% 12% 3 9% 5% 5% 5% 4 5% 6% 23%	3947
April	1st to 10th	Day	24% 12% 4 7% 6% 5% 5% 4 6% 7% 20%	2280
	11th to 20th	Day	20% 10% 4 8% 5% 6% 4 4 6% 7% 27%	2311
	21st to end	Day	22% 11% 3 9% 5% 5% 4 4 7% 6% 25%	2535
May	1st to 10th	Day	16% 9% 3 6% 4 3 4 4 4 5% 43%	4420
	11th to 20th	Day	13% 6% 3 6% 4 5% 4 4 7% 9% 40%	4377
	21st to end	Day	25% 10% 3 7% 5% 5% 4 4 5% 6% 26%	4813
June	1st to 10th	Day	20% 7% 2 7% 5% 5% 5% 5% 5% 7% 31%	2830
	11th to 20th	Day	20% 8% 3 8% 6% 4 4 5% 6% 7% 29%	2850
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	2850
July	1st to 10th	Day	7% 4 1 3 2 2 3 2 4 5% 67%	4740
	11th to 20th	Day	5% 3 1 2 2 3 2 2 4 6% 71%	4718
	21st to end	Day	11% 4 1 3 3 3 3 3 4 6% 60%	4790
August	1st to 10th	Day	4 3 1 3 2 3 2 2 5% 7% 68%	4379
	11th to 20th	Day	8% 6% 3 5% 4 4 5% 4 6% 8% 48%	4026
	21st to end	Day	15% 6% 3 7% 4 4 4 3 5% 7% 41%	4376
September	1st to 10th	Day	10% 6% 2 7% 4 4 3 3 4 6% 51%	2280
	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%	1512
	11th to 20th	Day	35% 12% 3 7% 4 4 3 3 4 4 22%	1520
	21st to end	Day	33% 13% 4 9% 6% 3 4 4 4 4 17%	1520
December	1st to 10th	Day	30% 15% 3 7% 4 3 3 3 4 4 24%	1520
	11th to 20th	Day	33% 15% 4 8% 5% 5% 4 3 3 5% 16%	1749
	21st to end	Day	32% 16% 4 9% 5% 5% 4 3 4 5% 12%	2442
January	1st to 10th	Night	36% 16% 4 10% 5% 4 4 3 3 4 12%	4726
	11th to 20th	Night	30% 9% 3 6% 4 3 4 3 5% 7% 25%	4599
	21st to end	Night	42% 15% 4 7% 4 4 3 3 3 4 13%	4831
February	1st to 10th	Night	34% 13% 3 7% 5% 3 3 3 4 5% 20%	2470
	11th to 20th	Night	27% 15% 3 7% 5% 4 4 3 4 5% 23%	2402
	21st to end	Night	29% 13% 4 8% 5% 3 4 3 4 5% 24%	1999
March	1st to 10th	Night	22% 15% 4 8% 6% 5% 5% 4 6% 6% 21%	3655
	11th to 20th	Night	21% 8% 2 6% 4 4 5% 4 4 6% 35%	3489
	21st to end	Night	29% 15% 4 9% 6% 4 4 3 4 4 19%	3705
April	1st to 10th	Night	26% 14% 3 6% 5% 4 4 3 6% 6% 22%	1900
	11th to 20th	Night	23% 11% 3 7% 6% 5% 5% 3 5% 5% 28%	1818
	21st to end	Night	25% 11% 3 8% 5% 4 5% 3 4 6% 24%	1644
May	1st to 10th	Night	16% 10% 3 6% 4 3 3 2 4 5% 45%	2540
	11th to 20th	Night	13% 6% 2 6% 3 4 4 4 6% 8% 45%	2516
	21st to end	Night	18% 11% 3 10% 7% 6% 4 4 6% 6% 26%	2762
June	1st to 10th	Night	19% 6% 3 9% 6% 5% 4 4 5% 5% 33%	1346
	11th to 20th	Night	18% 10% 4 7% 4 5% 5% 5% 6% 8% 28%	1330
	21st to end	Night	14% 5% 3 6% 5% 5% 4 3 6% 8% 42%	1330
July	1st to 10th	Night	7% 3 1 5% 4 3 3 2 3 5% 65%	2220
	11th to 20th	Night	4 2 2 3 2 2 2 2 2 4 76%	2242
	21st to end	Night	10% 4 1 4 2 3 3 3 4 5% 60%	2762
August	1st to 10th	Night	4 1 1 2 2 2 3 3 5% 6% 70%	2550
	11th to 20th	Night	8% 4 2 5% 4 4 4 3 5% 8% 53%	2812
	21st to end	Night	12% 5% 2 7% 5% 5% 5% 4 6% 9% 41%	3296
September	1st to 10th	Night	9% 3 2 5% 4 4 3 3 5% 6% 56%	1900
	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 3 4 18%	2636
	11th to 20th	Night	34% 11% 3 7% 4 4 4 3 4 4 23%	2660
	21st to end	Night	34% 11% 3 7% 6% 5% 4 2 4 4 20%	2660
December	1st to 10th	Night	30% 11% 3 7% 4 4 3 3 4 5% 25%	2660
	11th to 20th	Night	37% 14% 4 9% 5% 3 3 3 3 4 14%	3453
	21st to end	Night	33% 15% 5% 10% 5% 4 4 3 3 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	49% 41% 4 4 2	2219
	11th to 20th	Day	38% 37% 7% 9% 5% 3	2318
	21st to end	Day	54% 40% 3 2 1	2822
February	1st to 10th	Day	26% 64% 2 2 3 2	1710
	11th to 20th	Day	26% 65% 3 3 3	1674
	21st to end	Day	24% 65% 4 3 3 1	1454
March	1st to 10th	Day	35% 45% 9% 9% 3	3217
	11th to 20th	Day	26% 43% 10% 19% 2	3490
	21st to end	Day	37% 44% 9% 8% 2	3947
April	1st to 10th	Day	22% 69% 4 5%	2280
	11th to 20th	Day	20% 67% 5% 5% 2 1	2311
	21st to end	Day	21% 68% 4 5% 3	2535
May	1st to 10th	Day	23% 44% 9% 15% 8%	4420
	11th to 20th	Day	20% 49% 16% 14% 1	4380
	21st to end	Day	35% 45% 12% 6% 1	4814
June	1st to 10th	Day	17% 67% 7% 7% 2	2830
	11th to 20th	Day	18% 67% 5% 7% 3	2850
	21st to end	Day	14% 67% 7% 9% 2	2850
July	1st to 10th	Day	11% 37% 5% 16% 20% 11%	4666
	11th to 20th	Day	7% 36% 6% 27% 22% 2	4637
	21st to end	Day	19% 37% 18% 19% 7% 1	4639
August	1st to 10th	Day	8% 39% 7% 31% 15% 1	4362
	11th to 20th	Day	23% 41% 20% 12% 3 1	4065
	21st to end	Day	29% 38% 10% 14% 8%	4375
September	1st to 10th	Day	18% 62% 7% 8% 4 1	2280
	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	30% 59% 5% 5% 1	1512
	11th to 20th	Day	25% 61% 3 7% 3 2	1520
	21st to end	Day	25% 60% 3 6% 5% 1	1520
December	1st to 10th	Day	26% 61% 2 5% 5% 1	1520
	11th to 20th	Day	37% 50% 5% 5% 2	1752
	21st to end	Day	48% 40% 7% 3 1 1	2442
January	1st to 10th	Night	48% 41% 4 4 1 1	4726
	11th to 20th	Night	32% 41% 7% 11% 7% 1	4640
	21st to end	Night	53% 40% 4 4	4831
February	1st to 10th	Night	24% 65% 3 3 3 2	2470
	11th to 20th	Night	27% 64% 3 3 2 1	2404
	21st to end	Night	21% 65% 3 6% 4 1	1999
March	1st to 10th	Night	35% 44% 7% 7% 6%	3655
	11th to 20th	Night	23% 44% 12% 20% 2	3490
	21st to end	Night	42% 44% 8% 6% 1	3705
April	1st to 10th	Night	22% 66% 4 7% 1	1900
	11th to 20th	Night	21% 67% 4 5% 2 1	1818
	21st to end	Night	22% 66% 3 6% 3 1	1644
May	1st to 10th	Night	24% 42% 6% 21% 7% 1	2540
	11th to 20th	Night	18% 44% 21% 14% 3	2516
	21st to end	Night	36% 46% 10% 7% 1	2762
June	1st to 10th	Night	16% 66% 6% 10% 2 1	1346
	11th to 20th	Night	16% 66% 6% 9% 2	1330
	21st to end	Night	15% 64% 9% 9% 3	1330
July	1st to 10th	Night	13% 36% 5% 16% 19% 12%	2185
	11th to 20th	Night	8% 35% 4 24% 28% 2	2200
	21st to end	Night	21% 36% 8% 18% 15% 1	2674
August	1st to 10th	Night	6% 36% 8% 28% 20% 1	2542
	11th to 20th	Night	22% 39% 21% 14% 4	2839
	21st to end	Night	26% 40% 14% 12% 7%	3296
September	1st to 10th	Night	15% 62% 7% 9% 6% 1	1900
	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	31% 58% 6% 4 1	2636
	11th to 20th	Night	25% 60% 4 5% 4 2	2660
	21st to end	Night	26% 60% 3 4 6% 1	2660
December	1st to 10th	Night	26% 60% 4 4 5% 1	2660
	11th to 20th	Night	40% 50% 5% 3 2 1	3449
	21st to end	Night	47% 42% 5% 5% 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	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	32% 13% 24% 21% 9% 2	506
April	1st to 10th	Day	28% 12% 26% 21% 12% 2	460
	11th to 20th	Day	21% 14% 26% 23% 13% 3	452
	21st to end	Day	26% 10% 24% 25% 13% 2	460
May	1st to 10th	Day	15% 9% 26% 31% 17% 2	463
	11th to 20th	Day	18% 6% 19% 31% 22% 3	488
	21st to end	Day	19% 10% 24% 27% 15% 5%	529
June	1st to 10th	Day	20% 8% 27% 28% 15% 2	660
	11th to 20th	Day	18% 10% 26% 33% 11% 2	680
	21st to end	Day	14% 6% 22% 34% 21% 4	680
July	1st to 10th	Day	12% 4 20% 37% 24% 2	671
	11th to 20th	Day	8% 4 18% 41% 24% 5%	659
	21st to end	Day	7% 3 15% 35% 33% 7%	531
August	1st to 10th	Day	7% 4 15% 33% 31% 10%	487
	11th to 20th	Day	6% 4 13% 37% 33% 7%	477
	21st to end	Day	13% 7% 19% 32% 21% 8%	539
September	1st to 10th	Day	11% 5% 25% 33% 20% 6%	490
	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% 17% 19% 17% 11% 1	186
	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	34% 10% 18% 23% 12% 3	836
April	1st to 10th	Night	31% 8% 19% 26% 13% 3	760
	11th to 20th	Night	27% 8% 19% 27% 17% 2	690
	21st to end	Night	30% 6% 16% 29% 15% 3	504
May	1st to 10th	Night	18% 7% 16% 30% 26% 4	380
	11th to 20th	Night	17% 5% 11% 30% 32% 5%	374
	21st to end	Night	18% 4 14% 31% 28% 5%	411
June	1st to 10th	Night	19% 7% 16% 31% 23% 4	380
	11th to 20th	Night	20% 9% 17% 32% 19% 4	380
	21st to end	Night	14% 4 15% 30% 32% 6%	380
July	1st to 10th	Night	10% 4 12% 31% 39% 4	370
	11th to 20th	Night	8% 4 13% 35% 36% 6%	368
	21st to end	Night	6% 3 8% 31% 44% 8%	416
August	1st to 10th	Night	8% 2 8% 26% 42% 14%	386
	11th to 20th	Night	7% 2 6% 25% 47% 13%	564
	21st to end	Night	11% 4 11% 28% 38% 8%	712
September	1st to 10th	Night	10% 2 11% 35% 33% 8%	760
	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	754
	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

Page generated at 2024-11-01 19:47:45. Version ac07

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