Bellevue
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bellevue clear sky chart from 2005-11-06 to 2026-03-18.

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% 3 8% 5% 4 4 3 5% 6% 33%	135550
	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	134601
Transparency	Day	51% 6% 14% 19% 8% 1	135267
	Night	55% 7% 13% 17% 7% 1	134463
Seeing	Day	24% 9% 17% 26% 18% 5%	16697
	Night	28% 9% 12% 27% 21% 3	25215

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	35% 14% 5% 12% 6% 5% 4 3 4 4 8%	4284
	11th to 20th	Day	18% 11% 4 10% 6% 5% 4 3 5% 6% 27%	4716
	21st to end	Day	28% 11% 3 7% 5% 3 3 3 4 7% 25%	5338
February	1st to 10th	Day	28% 13% 4 9% 7% 4 5% 4 6% 5% 15%	1890
	11th to 20th	Day	26% 13% 5% 10% 5% 5% 3 3 4 6% 20%	1845
	21st to end	Day	26% 16% 4 8% 6% 4 5% 3 4 4 20%	1631
March	1st to 10th	Day	20% 13% 4 10% 7% 6% 6% 4 5% 6% 18%	4708
	11th to 20th	Day	20% 14% 5% 9% 5% 5% 4 3 4 5% 26%	5078
	21st to end	Day	23% 12% 4 11% 7% 7% 6% 4 5% 6% 14%	5678
April	1st to 10th	Day	23% 13% 3 8% 6% 5% 5% 5% 7% 6% 18%	2400
	11th to 20th	Day	20% 11% 3 7% 6% 5% 5% 5% 5% 7% 28%	2390
	21st to end	Day	18% 10% 4 10% 5% 6% 4 5% 6% 7% 25%	2650
May	1st to 10th	Day	14% 8% 3 7% 4 4 5% 4 5% 7% 40%	6320
	11th to 20th	Day	21% 10% 3 7% 4 4 4 4 6% 8% 29%	6277
	21st to end	Day	19% 8% 3 7% 5% 5% 4 4 6% 8% 31%	6903
June	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 7% 31%	2831
	11th to 20th	Day	19% 10% 4 8% 6% 6% 6% 5% 6% 7% 26%	3000
	21st to end	Day	11% 7% 2 8% 5% 5% 5% 5% 6% 7% 40%	3000
July	1st to 10th	Day	8% 4 2 4 3 3 3 2 3 6% 63%	6780
	11th to 20th	Day	5% 3 2 3 2 2 2 2 3 5% 70%	6458
	21st to end	Day	8% 5% 2 4 3 3 3 3 5% 7% 58%	6874
August	1st to 10th	Day	5% 3 1 5% 3 3 3 3 5% 7% 62%	4478
	11th to 20th	Day	9% 6% 2 7% 4 5% 5% 4 6% 9% 43%	4156
	21st to end	Day	15% 9% 3 7% 4 4 4 3 4 5% 42%	4456
September	1st to 10th	Day	9% 6% 2 6% 4 5% 4 3 4 7% 50%	2400
	11th to 20th	Day	13% 8% 3 8% 6% 4 4 3 6% 7% 39%	2400
	21st to end	Day	16% 8% 2 8% 4 5% 4 4 6% 7% 37%	2115
October	1st to 10th	Day	14% 7% 2 6% 4 5% 5% 4 6% 11% 36%	3280
	11th to 20th	Day	26% 9% 4 8% 5% 5% 4 3 5% 7% 24%	3261
	21st to end	Day	26% 11% 4 9% 6% 5% 4 4 5% 6% 19%	3345
November	1st to 10th	Day	33% 10% 3 9% 5% 5% 3 4 5% 7% 18%	1698
	11th to 20th	Day	32% 11% 4 8% 5% 4 3 3 4 5% 21%	1680
	21st to end	Day	32% 11% 4 7% 4 4 4 3 5% 6% 19%	1680
December	1st to 10th	Day	22% 9% 3 9% 6% 7% 5% 4 6% 8% 21%	2700
	11th to 20th	Day	32% 13% 4 9% 6% 5% 5% 4 5% 5% 12%	3105
	21st to end	Day	30% 13% 5% 10% 6% 6% 5% 3 5% 4 13%	3745
January	1st to 10th	Night	33% 15% 5% 12% 6% 5% 4 3 4 3 10%	8577
	11th to 20th	Night	19% 11% 4 7% 4 4 5% 3 4 7% 32%	8182
	21st to end	Night	25% 9% 3 8% 5% 4 4 3 5% 6% 27%	8845
February	1st to 10th	Night	28% 13% 4 9% 6% 4 4 4 4 6% 19%	2730
	11th to 20th	Night	24% 13% 3 9% 5% 5% 4 3 4 6% 23%	2662
	21st to end	Night	27% 15% 3 7% 5% 4 4 4 5% 6% 20%	2174
March	1st to 10th	Night	17% 12% 4 9% 6% 6% 7% 5% 7% 7% 20%	5389
	11th to 20th	Night	22% 12% 3 7% 4 4 4 3 5% 6% 30%	5076
	21st to end	Night	28% 13% 5% 10% 5% 5% 5% 4 5% 5% 15%	5297
April	1st to 10th	Night	22% 11% 3 9% 6% 6% 5% 5% 7% 6% 21%	2000
	11th to 20th	Night	18% 9% 3 9% 6% 5% 4 4 6% 6% 31%	1957
	21st to end	Night	21% 11% 3 8% 6% 5% 4 3 6% 6% 27%	1750
May	1st to 10th	Night	14% 8% 3 7% 4 5% 3 3 4 5% 43%	3640
	11th to 20th	Night	17% 10% 5% 9% 5% 5% 5% 4 4 7% 30%	3616
	21st to end	Night	17% 11% 4 9% 6% 6% 6% 3 6% 7% 25%	3972
June	1st to 10th	Night	16% 8% 3 9% 6% 4 6% 3 4 7% 34%	1563
	11th to 20th	Night	18% 10% 4 10% 6% 5% 5% 4 4 7% 28%	1400
	21st to end	Night	11% 6% 2 9% 6% 5% 4 4 5% 7% 43%	1400
July	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 3 4 59%	3180
	11th to 20th	Night	3 3 1 3 2 3 2 2 4 4 74%	3500
	21st to end	Night	7% 4 2 4 3 3 3 3 5% 6% 60%	3969
August	1st to 10th	Night	4 3 1 4 3 3 3 2 3 6% 68%	2671
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 5% 7% 50%	2902
	21st to end	Night	16% 7% 2 7% 3 4 3 2 4 5% 48%	3459
September	1st to 10th	Night	9% 4 2 5% 3 4 3 3 5% 6% 55%	2000
	11th to 20th	Night	14% 8% 3 7% 4 4 4 3 5% 5% 42%	2000
	21st to end	Night	16% 8% 3 6% 4 5% 4 3 5% 5% 40%	2280
October	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 6% 8% 38%	3920
	11th to 20th	Night	25% 9% 3 7% 4 4 5% 4 5% 5% 30%	3897
	21st to end	Night	27% 12% 4 8% 4 4 4 3 5% 6% 23%	4459
November	1st to 10th	Night	32% 10% 3 6% 5% 5% 6% 3 5% 5% 21%	2817
	11th to 20th	Night	32% 11% 4 8% 4 4 4 4 4 5% 20%	2940
	21st to end	Night	30% 11% 3 8% 5% 4 5% 3 5% 6% 20%	2940
December	1st to 10th	Night	23% 9% 3 9% 6% 5% 5% 4 5% 6% 26%	4700
	11th to 20th	Night	32% 14% 5% 10% 6% 4 4 3 4 4 12%	5320
	21st to end	Night	28% 16% 5% 11% 6% 5% 5% 3 4 4 13%	7417

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	81% 5% 7% 6% 1	4284
	11th to 20th	Day	56% 5% 10% 18% 11% 1	4740
	21st to end	Day	60% 3 9% 16% 12%	5339
February	1st to 10th	Day	69% 7% 11% 9% 4 1	1890
	11th to 20th	Day	67% 6% 8% 11% 7%	1854
	21st to end	Day	68% 8% 9% 12% 4	1631
March	1st to 10th	Day	62% 7% 13% 12% 5%	4708
	11th to 20th	Day	60% 6% 12% 20% 2	5078
	21st to end	Day	69% 8% 13% 10% 1	5678
April	1st to 10th	Day	66% 9% 15% 10% 1	2400
	11th to 20th	Day	57% 9% 12% 15% 6% 1	2390
	21st to end	Day	61% 9% 13% 13% 4 1	2650
May	1st to 10th	Day	44% 8% 15% 24% 9%	6320
	11th to 20th	Day	53% 9% 18% 18% 2	6280
	21st to end	Day	52% 7% 24% 15% 2	6904
June	1st to 10th	Day	53% 10% 17% 15% 5%	2831
	11th to 20th	Day	60% 9% 15% 13% 3	3000
	21st to end	Day	44% 9% 19% 19% 8% 1	3000
July	1st to 10th	Day	25% 5% 18% 23% 20% 9%	6706
	11th to 20th	Day	19% 4 14% 31% 29% 2	6380
	21st to end	Day	27% 4 24% 32% 11% 1	6681
August	1st to 10th	Day	23% 6% 13% 40% 17% 2	4461
	11th to 20th	Day	39% 7% 22% 23% 7% 2	4195
	21st to end	Day	44% 6% 12% 27% 10% 1	4455
September	1st to 10th	Day	36% 6% 16% 26% 12% 5%	2400
	11th to 20th	Day	45% 7% 14% 18% 14% 2	2400
	21st to end	Day	46% 8% 17% 18% 9% 2	2115
October	1st to 10th	Day	42% 6% 17% 30% 5%	3280
	11th to 20th	Day	60% 5% 12% 14% 7%	3261
	21st to end	Day	64% 6% 12% 10% 7% 1	3345
November	1st to 10th	Day	66% 8% 12% 11% 3	1698
	11th to 20th	Day	67% 8% 8% 10% 6% 2	1680
	21st to end	Day	65% 8% 8% 10% 7% 1	1680
December	1st to 10th	Day	57% 6% 9% 17% 11%	2700
	11th to 20th	Day	73% 6% 11% 8% 1	3108
	21st to end	Day	73% 6% 11% 8% 2	3745
January	1st to 10th	Night	79% 6% 6% 7% 2	8577
	11th to 20th	Night	50% 7% 12% 20% 10% 1	8220
	21st to end	Night	58% 4 9% 19% 10%	8845
February	1st to 10th	Night	68% 6% 10% 9% 6%	2730
	11th to 20th	Night	62% 8% 10% 11% 7% 1	2664
	21st to end	Night	65% 7% 10% 13% 4	2174
March	1st to 10th	Night	59% 8% 15% 14% 4	5389
	11th to 20th	Night	56% 8% 15% 20% 2	5077
	21st to end	Night	71% 8% 12% 9%	5297
April	1st to 10th	Night	62% 11% 13% 12% 2	2000
	11th to 20th	Night	54% 9% 14% 14% 8% 1	1957
	21st to end	Night	59% 7% 14% 14% 5% 1	1750
May	1st to 10th	Night	43% 7% 13% 27% 9%	3637
	11th to 20th	Night	54% 9% 18% 16% 3	3616
	21st to end	Night	59% 10% 19% 12% 1	3972
June	1st to 10th	Night	50% 8% 18% 18% 5%	1563
	11th to 20th	Night	58% 9% 15% 16% 3	1400
	21st to end	Night	42% 7% 22% 20% 6% 2	1400
July	1st to 10th	Night	31% 5% 16% 20% 17% 12%	3145
	11th to 20th	Night	17% 3 15% 30% 34% 1	3456
	21st to end	Night	26% 5% 21% 33% 13% 2	3857
August	1st to 10th	Night	20% 4 13% 40% 21% 2	2663
	11th to 20th	Night	35% 6% 24% 24% 9% 2	2929
	21st to end	Night	41% 5% 15% 29% 9% 1	3459
September	1st to 10th	Night	31% 7% 17% 28% 13% 4	2000
	11th to 20th	Night	44% 7% 11% 22% 13% 4	2000
	21st to end	Night	47% 6% 13% 21% 10% 2	2280
October	1st to 10th	Night	43% 6% 18% 28% 4	3920
	11th to 20th	Night	57% 6% 13% 18% 6%	3897
	21st to end	Night	63% 6% 10% 13% 7%	4459
November	1st to 10th	Night	65% 7% 13% 11% 3	2817
	11th to 20th	Night	67% 7% 10% 10% 4 2	2940
	21st to end	Night	66% 7% 9% 10% 8%	2940
December	1st to 10th	Night	56% 6% 9% 20% 9% 1	4700
	11th to 20th	Night	74% 5% 11% 8% 2	5316
	21st to end	Night	73% 8% 9% 8% 2	7417

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	42% 8% 14% 18% 11% 6%	220
	11th to 20th	Day	28% 11% 15% 21% 19% 7%	220
	21st to end	Day	38% 12% 12% 18% 13% 7%	290
February	1st to 10th	Day	40% 8% 18% 19% 12% 3	430
	11th to 20th	Day	39% 12% 21% 18% 8% 2	425
	21st to end	Day	36% 14% 20% 19% 9% 2	395
March	1st to 10th	Day	35% 13% 21% 19% 9% 3	540
	11th to 20th	Day	32% 14% 18% 18% 14% 4	529
	21st to end	Day	33% 13% 23% 19% 11% 1	550
April	1st to 10th	Day	31% 13% 21% 19% 13% 3	500
	11th to 20th	Day	23% 15% 25% 21% 10% 5%	492
	21st to end	Day	24% 11% 24% 25% 13% 3	500
May	1st to 10th	Day	17% 11% 22% 31% 16% 3	503
	11th to 20th	Day	24% 10% 18% 23% 17% 7%	528
	21st to end	Day	23% 8% 21% 26% 16% 6%	573
June	1st to 10th	Day	22% 9% 17% 28% 18% 5%	561
	11th to 20th	Day	23% 11% 19% 29% 15% 3	730
	21st to end	Day	13% 6% 17% 37% 22% 5%	730
July	1st to 10th	Day	12% 5% 15% 35% 29% 5%	721
	11th to 20th	Day	8% 4 14% 36% 32% 7%	581
	21st to end	Day	10% 3 12% 34% 33% 8%	575
August	1st to 10th	Day	10% 6% 13% 31% 29% 11%	527
	11th to 20th	Day	9% 3 14% 32% 32% 11%	517
	21st to end	Day	16% 5% 17% 31% 23% 8%	583
September	1st to 10th	Day	12% 6% 17% 36% 23% 6%	530
	11th to 20th	Day	17% 8% 15% 31% 24% 5%	530
	21st to end	Day	20% 7% 20% 27% 20% 5%	530
October	1st to 10th	Day	21% 10% 18% 22% 22% 7%	530
	11th to 20th	Day	32% 10% 14% 23% 16% 6%	530
	21st to end	Day	35% 12% 15% 22% 13% 3	458
November	1st to 10th	Day	37% 11% 19% 17% 14% 2	247
	11th to 20th	Day	41% 9% 14% 13% 15% 7%	220
	21st to end	Day	34% 10% 12% 22% 12% 10%	220
December	1st to 10th	Day	40% 9% 10% 15% 13% 14%	220
	11th to 20th	Day	45% 13% 11% 19% 10% 1	220
	21st to end	Day	39% 11% 14% 18% 15% 3	242
January	1st to 10th	Night	45% 11% 13% 15% 13% 3	916
	11th to 20th	Night	34% 11% 10% 25% 17% 3	840
	21st to end	Night	34% 9% 11% 29% 15% 3	924
February	1st to 10th	Night	36% 9% 15% 27% 11% 2	840
	11th to 20th	Night	32% 11% 12% 27% 16% 1	824
	21st to end	Night	37% 11% 12% 27% 11% 2	692
March	1st to 10th	Night	31% 12% 14% 27% 15% 2	840
	11th to 20th	Night	30% 11% 17% 26% 13% 3	828
	21st to end	Night	33% 10% 15% 28% 11% 2	880
April	1st to 10th	Night	29% 10% 17% 28% 14% 3	800
	11th to 20th	Night	24% 10% 15% 32% 16% 3	769
	21st to end	Night	28% 10% 12% 30% 17% 3	550
May	1st to 10th	Night	19% 11% 14% 35% 20% 1	400
	11th to 20th	Night	22% 7% 9% 28% 29% 5%	394
	21st to end	Night	22% 9% 9% 26% 29% 5%	433
June	1st to 10th	Night	19% 8% 12% 32% 27% 3	400
	11th to 20th	Night	24% 11% 14% 28% 22% 2	400
	21st to end	Night	14% 7% 10% 30% 35% 4	400
July	1st to 10th	Night	13% 5% 7% 28% 42% 5%	390
	11th to 20th	Night	11% 3 7% 32% 43% 4	388
	21st to end	Night	8% 2 6% 30% 48% 6%	438
August	1st to 10th	Night	10% 3 5% 26% 47% 9%	434
	11th to 20th	Night	11% 2 5% 24% 46% 12%	594
	21st to end	Night	14% 6% 7% 28% 39% 6%	783
September	1st to 10th	Night	12% 5% 7% 35% 36% 7%	800
	11th to 20th	Night	18% 5% 6% 26% 36% 8%	800
	21st to end	Night	21% 6% 10% 30% 29% 5%	800
October	1st to 10th	Night	22% 7% 10% 28% 28% 5%	800
	11th to 20th	Night	29% 8% 12% 25% 21% 5%	800
	21st to end	Night	32% 9% 14% 26% 15% 3	880
November	1st to 10th	Night	37% 9% 15% 29% 11%	820
	11th to 20th	Night	37% 11% 13% 24% 13% 2	840
	21st to end	Night	38% 9% 13% 23% 15% 3	840
December	1st to 10th	Night	36% 6% 11% 27% 18% 2	840
	11th to 20th	Night	42% 10% 15% 21% 9% 2	840
	21st to end	Night	43% 13% 13% 21% 10% 1	998

Page generated at 2026-03-14 23:23:54. Version ac07

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