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 2024-12-06.

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% 5% 4 4 3 5% 6% 34%	105995
	Night	22% 10% 3 8% 5% 4 4 3 5% 5% 30%	102274
Transparency	Day	51% 7% 13% 18% 8% 1	105753
	Night	55% 7% 13% 16% 7% 1	102163
Seeing	Day	24% 9% 17% 26% 18% 5%	15137
	Night	28% 9% 12% 27% 21% 4	23553

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% 14% 4 11% 6% 4 3 3 4 4 11%	2294
	11th to 20th	Day	25% 12% 4 9% 5% 4 5% 3 5% 5% 22%	2516
	21st to end	Day	37% 15% 4 9% 6% 3 4 3 4 5% 11%	2862
February	1st to 10th	Day	29% 13% 4 8% 7% 4 5% 4 6% 5% 15%	1710
	11th to 20th	Day	26% 13% 5% 9% 6% 5% 3 3 4 5% 20%	1665
	21st to end	Day	25% 16% 4 8% 6% 5% 5% 3 4 3 21%	1479
March	1st to 10th	Day	17% 12% 4 10% 7% 7% 6% 4 5% 6% 21%	3200
	11th to 20th	Day	19% 9% 4 7% 5% 5% 4 3 4 5% 35%	3490
	21st to end	Day	22% 13% 4 11% 7% 6% 6% 4 5% 6% 17%	3965
April	1st to 10th	Day	23% 13% 3 8% 7% 5% 5% 5% 7% 6% 19%	2280
	11th to 20th	Day	20% 12% 3 7% 6% 5% 5% 4 5% 6% 27%	2269
	21st to end	Day	19% 10% 4 10% 5% 6% 4 5% 6% 7% 24%	2517
May	1st to 10th	Day	17% 8% 3 6% 4 5% 5% 4 5% 6% 38%	4420
	11th to 20th	Day	18% 7% 3 6% 4 3 4 3 6% 8% 39%	4377
	21st to end	Day	25% 11% 3 9% 6% 6% 5% 4 5% 6% 22%	4813
June	1st to 10th	Day	20% 8% 3 7% 6% 5% 5% 4 7% 7% 29%	2689
	11th to 20th	Day	19% 10% 4 8% 6% 6% 6% 5% 6% 7% 25%	2850
	21st to end	Day	11% 7% 2 8% 5% 5% 5% 5% 6% 7% 40%	2850
July	1st to 10th	Day	6% 4 2 4 3 3 3 3 3 6% 64%	4740
	11th to 20th	Day	5% 3 2 3 3 2 3 2 4 5% 69%	4516
	21st to end	Day	10% 5% 2 5% 3 3 3 3 4 6% 57%	4790
August	1st to 10th	Day	5% 3 1 5% 3 3 3 3 5% 7% 63%	4340
	11th to 20th	Day	9% 7% 2 7% 4 5% 5% 4 6% 9% 43%	4026
	21st to end	Day	16% 9% 3 7% 4 3 4 3 4 5% 42%	4319
September	1st to 10th	Day	9% 6% 2 6% 4 4 4 3 4 7% 51%	2280
	11th to 20th	Day	13% 8% 3 8% 6% 5% 5% 3 6% 7% 38%	2280
	21st to end	Day	16% 8% 2 8% 5% 4 4 4 6% 7% 37%	2009
October	1st to 10th	Day	14% 7% 2 6% 4 5% 5% 4 6% 11% 36%	3180
	11th to 20th	Day	26% 9% 4 8% 5% 5% 4 3 5% 7% 24%	3161
	21st to end	Day	26% 12% 4 9% 6% 5% 4 4 5% 6% 19%	3241
November	1st to 10th	Day	33% 10% 3 9% 5% 5% 3 4 5% 6% 17%	1616
	11th to 20th	Day	31% 11% 4 8% 5% 4 3 3 4 5% 21%	1600
	21st to end	Day	31% 11% 4 7% 4 4 4 3 5% 6% 19%	1600
December	1st to 10th	Day	27% 11% 3 8% 5% 6% 4 3 4 6% 22%	1560
	11th to 20th	Day	30% 13% 5% 9% 6% 5% 5% 4 5% 4 15%	1925
	21st to end	Day	25% 13% 5% 11% 7% 6% 6% 3 5% 4 13%	2566
January	1st to 10th	Night	35% 13% 4 10% 5% 6% 4 3 4 4 11%	4631
	11th to 20th	Night	27% 13% 4 7% 4 3 4 3 4 7% 23%	4382
	21st to end	Night	34% 13% 4 10% 5% 4 5% 3 4 5% 13%	4791
February	1st to 10th	Night	30% 13% 3 9% 6% 4 4 4 4 5% 18%	2470
	11th to 20th	Night	25% 13% 3 8% 5% 5% 4 3 5% 6% 23%	2402
	21st to end	Night	27% 15% 3 7% 5% 4 4 3 6% 6% 20%	1974
March	1st to 10th	Night	17% 13% 4 10% 6% 7% 7% 5% 6% 7% 18%	3657
	11th to 20th	Night	17% 9% 3 6% 3 4 4 4 5% 6% 40%	3489
	21st to end	Night	25% 15% 5% 9% 5% 5% 5% 4 5% 5% 17%	3688
April	1st to 10th	Night	22% 10% 3 9% 6% 5% 5% 5% 7% 6% 21%	1900
	11th to 20th	Night	18% 9% 3 9% 6% 5% 4 4 6% 6% 30%	1858
	21st to end	Night	21% 11% 3 8% 7% 5% 4 3 6% 6% 25%	1663
May	1st to 10th	Night	16% 9% 3 8% 4 4 3 3 4 5% 40%	2540
	11th to 20th	Night	15% 8% 3 8% 4 4 5% 3 4 7% 38%	2516
	21st to end	Night	20% 12% 4 10% 7% 6% 6% 3 4 5% 22%	2762
June	1st to 10th	Night	17% 8% 3 9% 6% 5% 6% 3 5% 7% 33%	1485
	11th to 20th	Night	18% 10% 4 10% 6% 5% 5% 5% 4 6% 28%	1330
	21st to end	Night	11% 6% 2 8% 6% 5% 4 4 5% 7% 43%	1330
July	1st to 10th	Night	7% 5% 2 5% 3 3 3 2 4 4 61%	2220
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 4 73%	2442
	21st to end	Night	10% 5% 2 5% 3 3 3 3 4 5% 58%	2762
August	1st to 10th	Night	4 3 1 4 3 3 3 2 3 5% 69%	2589
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 5% 7% 51%	2812
	21st to end	Night	17% 7% 2 6% 3 3 3 2 3 5% 48%	3354
September	1st to 10th	Night	9% 4 2 5% 3 3 3 3 4 6% 55%	1900
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 5% 5% 42%	1900
	21st to end	Night	17% 8% 3 6% 4 5% 4 3 5% 5% 40%	2166
October	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 6% 9% 38%	3800
	11th to 20th	Night	25% 8% 3 7% 4 4 5% 4 5% 5% 29%	3777
	21st to end	Night	27% 12% 4 8% 4 4 4 3 5% 6% 23%	4321
November	1st to 10th	Night	32% 10% 3 6% 5% 5% 6% 3 5% 5% 21%	2679
	11th to 20th	Night	31% 11% 4 8% 5% 4 4 4 4 5% 21%	2800
	21st to end	Night	29% 11% 3 8% 5% 4 5% 4 5% 6% 20%	2800
December	1st to 10th	Night	27% 12% 3 9% 6% 4 4 3 4 5% 25%	2736
	11th to 20th	Night	28% 13% 5% 10% 7% 5% 5% 3 5% 4 16%	3280
	21st to end	Night	27% 14% 5% 12% 6% 5% 5% 4 4 4 15%	5068

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	77% 6% 6% 7% 2 1	2294
	11th to 20th	Day	63% 7% 10% 14% 5% 1	2540
	21st to end	Day	76% 5% 10% 7% 2	2862
February	1st to 10th	Day	70% 7% 12% 7% 4 1	1710
	11th to 20th	Day	67% 7% 9% 11% 6% 1	1674
	21st to end	Day	67% 8% 9% 11% 5%	1479
March	1st to 10th	Day	60% 9% 14% 13% 4	3200
	11th to 20th	Day	50% 8% 14% 26% 2	3490
	21st to end	Day	66% 10% 12% 11% 1	3965
April	1st to 10th	Day	66% 10% 14% 9% 1	2280
	11th to 20th	Day	58% 9% 12% 14% 6% 1	2269
	21st to end	Day	62% 9% 12% 12% 5% 1	2517
May	1st to 10th	Day	45% 10% 13% 21% 10%	4420
	11th to 20th	Day	43% 10% 23% 21% 4	4380
	21st to end	Day	65% 8% 18% 8% 1	4814
June	1st to 10th	Day	55% 10% 17% 13% 5%	2689
	11th to 20th	Day	61% 9% 15% 12% 3	2850
	21st to end	Day	44% 9% 19% 20% 8% 1	2850
July	1st to 10th	Day	25% 6% 9% 23% 25% 13%	4666
	11th to 20th	Day	20% 5% 8% 29% 34% 3	4438
	21st to end	Day	29% 5% 16% 33% 15% 2	4639
August	1st to 10th	Day	22% 6% 12% 41% 17% 2	4323
	11th to 20th	Day	39% 7% 23% 23% 7% 2	4065
	21st to end	Day	44% 6% 12% 27% 10% 1	4318
September	1st to 10th	Day	35% 6% 15% 26% 12% 5%	2280
	11th to 20th	Day	46% 7% 14% 17% 14% 2	2280
	21st to end	Day	47% 9% 16% 17% 9% 2	2009
October	1st to 10th	Day	42% 6% 17% 30% 6%	3180
	11th to 20th	Day	60% 5% 12% 14% 7%	3161
	21st to end	Day	64% 7% 12% 10% 7% 1	3241
November	1st to 10th	Day	67% 8% 11% 10% 4	1616
	11th to 20th	Day	66% 8% 8% 10% 6% 2	1600
	21st to end	Day	65% 8% 8% 10% 7% 1	1600
December	1st to 10th	Day	63% 7% 8% 13% 9% 1	1560
	11th to 20th	Day	70% 6% 12% 9% 2 1	1928
	21st to end	Day	71% 8% 12% 7% 2	2566
January	1st to 10th	Night	76% 7% 7% 6% 2 1	4631
	11th to 20th	Night	60% 7% 9% 17% 5% 1	4420
	21st to end	Night	74% 6% 8% 8% 3 1	4791
February	1st to 10th	Night	69% 6% 11% 9% 5%	2470
	11th to 20th	Night	62% 9% 11% 10% 6% 1	2404
	21st to end	Night	65% 8% 10% 13% 4	1974
March	1st to 10th	Night	60% 10% 13% 13% 4	3657
	11th to 20th	Night	44% 10% 19% 24% 3 1	3490
	21st to end	Night	69% 9% 13% 9% 1	3688
April	1st to 10th	Night	62% 11% 13% 12% 2	1900
	11th to 20th	Night	55% 9% 13% 13% 8% 1	1858
	21st to end	Night	61% 7% 13% 13% 5% 1	1663
May	1st to 10th	Night	46% 9% 13% 24% 8% 1	2540
	11th to 20th	Night	44% 11% 22% 19% 4 1	2516
	21st to end	Night	65% 11% 14% 9% 1	2762
June	1st to 10th	Night	52% 9% 17% 17% 5%	1485
	11th to 20th	Night	58% 9% 15% 14% 3	1330
	21st to end	Night	41% 7% 22% 20% 7% 2	1330
July	1st to 10th	Night	28% 6% 9% 21% 20% 17%	2185
	11th to 20th	Night	18% 4 9% 29% 40% 1	2398
	21st to end	Night	29% 6% 13% 30% 19% 3	2674
August	1st to 10th	Night	19% 4 13% 40% 22% 2	2581
	11th to 20th	Night	34% 6% 24% 25% 8% 2	2839
	21st to end	Night	41% 5% 15% 29% 9%	3354
September	1st to 10th	Night	31% 7% 16% 28% 14% 4	1900
	11th to 20th	Night	45% 7% 11% 21% 13% 4	1900
	21st to end	Night	46% 6% 13% 21% 11% 2	2166
October	1st to 10th	Night	43% 6% 18% 29% 4	3800
	11th to 20th	Night	57% 6% 13% 18% 6%	3777
	21st to end	Night	63% 6% 10% 12% 7%	4321
November	1st to 10th	Night	64% 7% 13% 11% 3 1	2679
	11th to 20th	Night	66% 7% 10% 10% 4 2	2800
	21st to end	Night	66% 7% 9% 9% 8% 1	2800
December	1st to 10th	Night	62% 7% 9% 13% 8% 1	2736
	11th to 20th	Night	69% 7% 13% 8% 3	3276
	21st to end	Night	70% 9% 9% 9% 3	5068

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	43% 8% 11% 17% 13% 8%	180
	11th to 20th	Day	31% 12% 16% 18% 19% 4	180
	21st to end	Day	41% 14% 12% 17% 10% 5%	240
February	1st to 10th	Day	41% 8% 18% 19% 11% 3	370
	11th to 20th	Day	40% 13% 19% 17% 9% 2	365
	21st to end	Day	34% 15% 20% 20% 10% 1	339
March	1st to 10th	Day	33% 13% 21% 21% 10% 3	460
	11th to 20th	Day	29% 14% 17% 20% 16% 4	460
	21st to end	Day	33% 13% 22% 19% 11% 1	506
April	1st to 10th	Day	32% 14% 20% 19% 12% 3	460
	11th to 20th	Day	24% 16% 26% 19% 10% 6%	452
	21st to end	Day	25% 12% 25% 23% 12% 2	460
May	1st to 10th	Day	18% 11% 21% 32% 15% 3	463
	11th to 20th	Day	23% 10% 17% 25% 18% 7%	488
	21st to end	Day	24% 8% 20% 26% 16% 6%	529
June	1st to 10th	Day	24% 10% 18% 28% 16% 5%	519
	11th to 20th	Day	24% 11% 19% 29% 14% 3	680
	21st to end	Day	13% 6% 18% 37% 22% 4	680
July	1st to 10th	Day	12% 5% 16% 34% 28% 5%	671
	11th to 20th	Day	8% 4 14% 35% 33% 6%	538
	21st to end	Day	10% 3 13% 35% 32% 8%	531
August	1st to 10th	Day	10% 5% 12% 32% 29% 11%	487
	11th to 20th	Day	8% 3 13% 32% 34% 11%	477
	21st to end	Day	16% 5% 18% 31% 22% 7%	539
September	1st to 10th	Day	13% 7% 18% 37% 20% 5%	490
	11th to 20th	Day	18% 9% 15% 31% 23% 5%	490
	21st to end	Day	21% 8% 19% 28% 20% 5%	490
October	1st to 10th	Day	21% 11% 17% 21% 22% 7%	490
	11th to 20th	Day	32% 10% 13% 23% 16% 6%	490
	21st to end	Day	35% 11% 15% 22% 13% 4	420
November	1st to 10th	Day	38% 9% 18% 18% 14% 3	225
	11th to 20th	Day	41% 10% 16% 12% 15% 8%	200
	21st to end	Day	35% 10% 11% 22% 13% 11%	200
December	1st to 10th	Day	38% 8% 12% 13% 15% 14%	190
	11th to 20th	Day	40% 14% 12% 22% 10% 2	180
	21st to end	Day	39% 10% 15% 19% 16% 2	198
January	1st to 10th	Night	44% 12% 12% 15% 14% 3	823
	11th to 20th	Night	36% 12% 10% 23% 16% 3	760
	21st to end	Night	35% 10% 11% 27% 14% 2	836
February	1st to 10th	Night	38% 9% 14% 27% 11% 2	760
	11th to 20th	Night	32% 12% 12% 27% 16% 1	744
	21st to end	Night	37% 11% 12% 26% 12% 2	628
March	1st to 10th	Night	31% 12% 14% 26% 15% 2	760
	11th to 20th	Night	28% 12% 17% 27% 14% 3	760
	21st to end	Night	32% 11% 15% 28% 11% 2	836
April	1st to 10th	Night	29% 10% 17% 28% 14% 3	760
	11th to 20th	Night	24% 10% 15% 30% 17% 3	730
	21st to end	Night	28% 11% 13% 30% 16% 2	523
May	1st to 10th	Night	19% 11% 14% 34% 20% 1	380
	11th to 20th	Night	21% 7% 8% 29% 30% 5%	374
	21st to end	Night	22% 9% 9% 27% 28% 5%	411
June	1st to 10th	Night	20% 8% 12% 32% 26% 3	380
	11th to 20th	Night	24% 11% 13% 28% 21% 2	380
	21st to end	Night	14% 7% 10% 31% 34% 4	380
July	1st to 10th	Night	12% 5% 7% 28% 42% 6%	370
	11th to 20th	Night	11% 3 7% 32% 43% 4	368
	21st to end	Night	9% 2 6% 30% 47% 6%	416
August	1st to 10th	Night	9% 2 6% 25% 48% 10%	412
	11th to 20th	Night	9% 2 5% 24% 47% 13%	564
	21st to end	Night	15% 6% 7% 28% 38% 5%	744
September	1st to 10th	Night	12% 5% 7% 35% 35% 7%	760
	11th to 20th	Night	19% 6% 6% 24% 37% 8%	760
	21st to end	Night	21% 6% 10% 30% 28% 5%	760
October	1st to 10th	Night	22% 7% 9% 28% 29% 5%	760
	11th to 20th	Night	29% 8% 11% 24% 22% 6%	760
	21st to end	Night	33% 10% 13% 26% 15% 3	836
November	1st to 10th	Night	37% 8% 15% 29% 11%	780
	11th to 20th	Night	37% 11% 14% 23% 14% 2	800
	21st to end	Night	37% 9% 13% 22% 15% 3	800
December	1st to 10th	Night	35% 5% 12% 28% 18% 3	782
	11th to 20th	Night	41% 10% 15% 23% 10% 2	760
	21st to end	Night	43% 14% 12% 21% 10% 1	896

Page generated at 2024-12-03 03:45:14. Version ac07

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