Bothell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bothell clear sky chart from 2005-11-06 to 2024-04-10.

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	21% 10% 4 7% 6% 3 5% 4 5% 6% 29%	83105
	Night	24% 10% 4 7% 6% 3 4 4 4 5% 28%	85648
Transparency	Day	54% 10% 13% 16% 6% 1	82823
	Night	57% 10% 12% 15% 6% 1	85510
Seeing	Day	24% 9% 19% 26% 17% 5%	14238
	Night	29% 8% 15% 25% 20% 3	22841

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	39% 14% 5% 8% 6% 3 3 3 3 4 11%	2251
	11th to 20th	Day	26% 11% 4 8% 6% 3 4 4 5% 6% 22%	2516
	21st to end	Day	40% 13% 5% 8% 5% 3 3 3 4 5% 11%	2858
February	1st to 10th	Day	31% 12% 4 7% 6% 4 5% 5% 4 6% 15%	1710
	11th to 20th	Day	26% 15% 5% 7% 6% 4 5% 3 5% 5% 19%	1665
	21st to end	Day	27% 15% 4 8% 5% 4 5% 5% 3 5% 20%	1479
March	1st to 10th	Day	20% 13% 4 8% 8% 4 5% 5% 6% 7% 20%	3200
	11th to 20th	Day	19% 10% 4 6% 6% 3 4 5% 4 6% 34%	3490
	21st to end	Day	24% 13% 5% 10% 7% 4 6% 5% 5% 6% 16%	3965
April	1st to 10th	Day	26% 12% 5% 7% 7% 3 5% 4 6% 7% 18%	2270
	11th to 20th	Day	23% 10% 4 7% 7% 4 6% 5% 5% 7% 23%	2151
	21st to end	Day	19% 9% 4 9% 8% 4 6% 5% 5% 7% 23%	2388
May	1st to 10th	Day	16% 9% 4 7% 6% 4 6% 5% 5% 7% 30%	2520
	11th to 20th	Day	18% 7% 3 7% 6% 4 5% 5% 6% 8% 30%	2477
	21st to end	Day	23% 9% 4 7% 6% 4 5% 4 5% 6% 25%	2723
June	1st to 10th	Day	22% 8% 3 6% 6% 4 6% 5% 5% 8% 28%	2564
	11th to 20th	Day	22% 10% 4 8% 7% 4 6% 5% 5% 7% 23%	2700
	21st to end	Day	12% 7% 3 7% 5% 4 5% 5% 6% 8% 39%	2700
July	1st to 10th	Day	10% 6% 3 6% 4 2 4 5% 5% 6% 50%	2700
	11th to 20th	Day	10% 5% 2 4 4 2 4 4 5% 8% 52%	2588
	21st to end	Day	8% 4 2 3 3 2 3 3 4 5% 63%	2741
August	1st to 10th	Day	10% 5% 2 4 3 3 4 4 5% 6% 55%	2477
	11th to 20th	Day	8% 4 2 4 3 3 3 4 4 9% 56%	2298
	21st to end	Day	12% 6% 3 6% 5% 3 4 4 5% 6% 45%	2466
September	1st to 10th	Day	11% 6% 2 6% 5% 3 4 3 4 8% 49%	2160
	11th to 20th	Day	14% 8% 3 7% 6% 3 4 4 5% 7% 38%	2160
	21st to end	Day	19% 8% 3 5% 6% 4 5% 4 5% 6% 37%	1904
October	1st to 10th	Day	19% 8% 4 7% 7% 3 5% 4 6% 8% 29%	1800
	11th to 20th	Day	24% 10% 4 7% 6% 2 4 5% 6% 7% 26%	1800
	21st to end	Day	30% 9% 4 7% 5% 3 5% 5% 6% 6% 22%	1864
November	1st to 10th	Day	36% 12% 3 6% 5% 3 4 4 4 5% 17%	1529
	11th to 20th	Day	33% 12% 4 6% 5% 3 4 4 3 5% 22%	1520
	21st to end	Day	34% 11% 3 6% 5% 2 4 4 5% 6% 20%	1520
December	1st to 10th	Day	30% 10% 4 8% 6% 3 5% 3 3 5% 23%	1520
	11th to 20th	Day	32% 13% 5% 8% 7% 4 5% 5% 4 4 14%	1925
	21st to end	Day	27% 13% 5% 11% 7% 4 5% 5% 4 6% 13%	2506
January	1st to 10th	Night	38% 13% 4 8% 6% 3 5% 4 4 4 11%	4689
	11th to 20th	Night	28% 13% 5% 5% 5% 2 3 3 4 7% 23%	4382
	21st to end	Night	36% 13% 4 8% 7% 3 5% 4 4 4 12%	4796
February	1st to 10th	Night	32% 11% 4 7% 7% 4 4 4 4 6% 18%	2470
	11th to 20th	Night	26% 13% 4 7% 5% 4 4 4 4 6% 22%	2402
	21st to end	Night	28% 14% 4 6% 5% 3 4 4 4 6% 21%	1975
March	1st to 10th	Night	19% 14% 5% 8% 7% 4 7% 6% 5% 6% 17%	3661
	11th to 20th	Night	19% 9% 3 6% 4 3 4 3 4 6% 39%	3489
	21st to end	Night	29% 14% 4 8% 7% 4 5% 4 4 4 17%	3688
April	1st to 10th	Night	24% 11% 4 7% 7% 4 5% 6% 6% 6% 19%	1900
	11th to 20th	Night	20% 9% 4 8% 6% 3 6% 5% 5% 5% 28%	1757
	21st to end	Night	24% 9% 4 8% 7% 3 5% 5% 5% 6% 25%	1571
May	1st to 10th	Night	17% 9% 4 6% 5% 3 5% 4 6% 5% 36%	1440
	11th to 20th	Night	18% 8% 2 6% 6% 4 5% 5% 6% 7% 34%	1416
	21st to end	Night	19% 10% 3 8% 6% 4 6% 4 5% 6% 30%	1552
June	1st to 10th	Night	20% 7% 4 7% 7% 3 5% 4 5% 5% 32%	1391
	11th to 20th	Night	23% 12% 4 8% 6% 3 5% 4 5% 6% 25%	1260
	21st to end	Night	12% 5% 3 6% 6% 4 3 5% 5% 8% 44%	1260
July	1st to 10th	Night	10% 7% 2 4 5% 3 4 4 4 6% 51%	1260
	11th to 20th	Night	7% 5% 3 5% 4 3 3 4 4 5% 58%	1368
	21st to end	Night	7% 4 1 4 3 2 3 3 3 6% 64%	1576
August	1st to 10th	Night	8% 5% 2 4 4 2 3 3 4 7% 59%	1458
	11th to 20th	Night	7% 4 1 3 4 2 2 4 4 7% 63%	1602
	21st to end	Night	12% 5% 2 5% 4 2 4 3 4 7% 51%	1890
September	1st to 10th	Night	9% 5% 2 5% 4 2 4 4 4 6% 53%	1800
	11th to 20th	Night	16% 8% 3 6% 5% 3 4 5% 4 5% 42%	1800
	21st to end	Night	17% 7% 3 7% 6% 3 4 4 4 5% 39%	2051
October	1st to 10th	Night	19% 8% 4 7% 6% 4 4 4 4 7% 35%	2160
	11th to 20th	Night	24% 8% 3 5% 5% 3 4 5% 5% 6% 32%	2160
	21st to end	Night	27% 8% 3 6% 5% 3 3 4 5% 7% 28%	2491
November	1st to 10th	Night	33% 9% 4 6% 6% 3 5% 4 4 4 21%	2546
	11th to 20th	Night	32% 9% 4 7% 6% 3 3 4 4 4 22%	2660
	21st to end	Night	32% 9% 4 7% 6% 4 4 4 4 5% 21%	2660
December	1st to 10th	Night	29% 10% 5% 7% 6% 2 3 4 4 5% 24%	2660
	11th to 20th	Night	29% 13% 6% 9% 7% 3 5% 4 5% 4 15%	3280
	21st to end	Night	29% 15% 5% 9% 7% 4 4 4 4 4 15%	5127

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% 8% 6% 7% 2	2251
	11th to 20th	Day	60% 9% 11% 14% 4 1	2540
	21st to end	Day	76% 8% 9% 6% 2	2858
February	1st to 10th	Day	70% 9% 10% 7% 3 1	1710
	11th to 20th	Day	67% 8% 9% 9% 6%	1674
	21st to end	Day	66% 10% 9% 10% 4	1479
March	1st to 10th	Day	58% 12% 13% 12% 4	3200
	11th to 20th	Day	48% 11% 15% 24% 2	3490
	21st to end	Day	64% 13% 12% 10% 1	3965
April	1st to 10th	Day	64% 13% 14% 9% 1	2270
	11th to 20th	Day	61% 9% 11% 12% 5% 1	2151
	21st to end	Day	61% 11% 11% 11% 5% 1	2388
May	1st to 10th	Day	52% 11% 16% 16% 5%	2520
	11th to 20th	Day	51% 13% 17% 15% 5%	2480
	21st to end	Day	59% 10% 15% 14% 2	2724
June	1st to 10th	Day	55% 10% 17% 12% 5% 1	2564
	11th to 20th	Day	60% 11% 14% 11% 3	2700
	21st to end	Day	43% 11% 19% 19% 7% 1	2700
July	1st to 10th	Day	35% 11% 14% 25% 13% 2	2626
	11th to 20th	Day	32% 9% 14% 30% 14% 1	2509
	21st to end	Day	23% 7% 15% 36% 17% 3	2589
August	1st to 10th	Day	30% 9% 16% 29% 13% 2	2460
	11th to 20th	Day	26% 9% 21% 29% 12% 3	2298
	21st to end	Day	39% 8% 15% 29% 9% 1	2466
September	1st to 10th	Day	36% 7% 15% 25% 12% 4	2160
	11th to 20th	Day	46% 7% 13% 17% 15% 2	2160
	21st to end	Day	48% 8% 16% 17% 9% 2	1904
October	1st to 10th	Day	52% 9% 14% 18% 6%	1800
	11th to 20th	Day	57% 9% 14% 15% 5% 1	1800
	21st to end	Day	63% 8% 11% 11% 8%	1864
November	1st to 10th	Day	68% 9% 12% 9% 3	1529
	11th to 20th	Day	65% 9% 9% 9% 6% 2	1520
	21st to end	Day	65% 8% 9% 10% 7% 1	1520
December	1st to 10th	Day	65% 7% 9% 11% 8% 1	1520
	11th to 20th	Day	69% 10% 11% 8% 2 1	1928
	21st to end	Day	70% 11% 12% 6% 2	2506
January	1st to 10th	Night	75% 10% 7% 6% 2 1	4689
	11th to 20th	Night	60% 9% 9% 17% 5% 1	4420
	21st to end	Night	74% 8% 8% 8% 3	4796
February	1st to 10th	Night	67% 9% 11% 9% 4 1	2470
	11th to 20th	Night	64% 9% 11% 9% 6% 1	2404
	21st to end	Night	64% 8% 12% 12% 4	1975
March	1st to 10th	Night	59% 14% 13% 11% 3	3661
	11th to 20th	Night	44% 11% 18% 24% 2 1	3490
	21st to end	Night	68% 12% 12% 8%	3688
April	1st to 10th	Night	61% 13% 14% 10% 2	1900
	11th to 20th	Night	55% 12% 13% 13% 6% 1	1757
	21st to end	Night	59% 10% 12% 13% 5% 1	1571
May	1st to 10th	Night	47% 10% 17% 17% 7% 1	1440
	11th to 20th	Night	48% 10% 20% 15% 6% 1	1416
	21st to end	Night	55% 12% 16% 15% 2	1552
June	1st to 10th	Night	53% 10% 16% 16% 5%	1391
	11th to 20th	Night	60% 12% 14% 12% 2	1260
	21st to end	Night	39% 9% 22% 20% 7% 3	1260
July	1st to 10th	Night	35% 9% 15% 25% 12% 4	1225
	11th to 20th	Night	30% 7% 16% 31% 15% 1	1324
	21st to end	Night	21% 8% 12% 34% 20% 4	1488
August	1st to 10th	Night	27% 8% 14% 30% 18% 3	1450
	11th to 20th	Night	23% 10% 17% 32% 16% 3	1602
	21st to end	Night	34% 8% 16% 31% 11% 1	1890
September	1st to 10th	Night	32% 8% 16% 28% 13% 4	1800
	11th to 20th	Night	44% 9% 10% 20% 14% 4	1800
	21st to end	Night	46% 8% 13% 21% 10% 2	2051
October	1st to 10th	Night	49% 9% 13% 21% 7% 1	2160
	11th to 20th	Night	53% 9% 15% 17% 6% 1	2160
	21st to end	Night	56% 9% 12% 15% 8%	2491
November	1st to 10th	Night	64% 9% 13% 10% 3 1	2546
	11th to 20th	Night	65% 7% 11% 10% 5% 2	2660
	21st to end	Night	65% 8% 9% 9% 8%	2660
December	1st to 10th	Night	63% 8% 10% 11% 7%	2660
	11th to 20th	Night	69% 10% 12% 7% 3	3276
	21st to end	Night	68% 13% 11% 6% 2	5127

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% 15% 13% 16% 7%	180
	11th to 20th	Day	37% 10% 16% 17% 15% 5%	180
	21st to end	Day	42% 12% 14% 17% 11% 4	236
February	1st to 10th	Day	42% 11% 19% 17% 9% 3	370
	11th to 20th	Day	39% 14% 21% 16% 8% 2	365
	21st to end	Day	34% 15% 20% 21% 9% 1	339
March	1st to 10th	Day	31% 15% 23% 17% 12% 2	460
	11th to 20th	Day	31% 12% 18% 18% 16% 4	460
	21st to end	Day	35% 14% 21% 20% 9% 1	506
April	1st to 10th	Day	32% 12% 24% 19% 11% 2	457
	11th to 20th	Day	28% 13% 26% 18% 8% 6%	412
	21st to end	Day	24% 13% 25% 25% 12% 1	420
May	1st to 10th	Day	17% 9% 21% 35% 14% 3	423
	11th to 20th	Day	25% 7% 18% 26% 17% 7%	448
	21st to end	Day	24% 8% 17% 29% 15% 7%	485
June	1st to 10th	Day	23% 8% 23% 26% 15% 5%	494
	11th to 20th	Day	23% 9% 22% 30% 14% 3	630
	21st to end	Day	13% 4 20% 36% 22% 4	630
July	1st to 10th	Day	13% 4 18% 34% 27% 4	621
	11th to 20th	Day	9% 5% 17% 33% 30% 6%	508
	21st to end	Day	8% 2 14% 35% 33% 8%	487
August	1st to 10th	Day	12% 4 11% 34% 27% 12%	447
	11th to 20th	Day	7% 2 12% 37% 31% 12%	437
	21st to end	Day	14% 6% 19% 32% 21% 8%	495
September	1st to 10th	Day	14% 8% 20% 36% 19% 4	450
	11th to 20th	Day	18% 7% 17% 28% 24% 5%	450
	21st to end	Day	20% 9% 19% 28% 17% 5%	450
October	1st to 10th	Day	22% 10% 19% 22% 19% 8%	450
	11th to 20th	Day	30% 8% 17% 22% 17% 6%	450
	21st to end	Day	34% 13% 16% 21% 12% 5%	379
November	1st to 10th	Day	41% 13% 15% 19% 9% 2	201
	11th to 20th	Day	39% 13% 13% 14% 11% 10%	180
	21st to end	Day	34% 13% 19% 13% 10% 11%	180
December	1st to 10th	Day	43% 7% 14% 11% 14% 11%	180
	11th to 20th	Day	39% 12% 23% 16% 11% 1	180
	21st to end	Day	38% 10% 18% 21% 10% 3	198
January	1st to 10th	Night	44% 11% 13% 15% 14% 3	838
	11th to 20th	Night	37% 11% 12% 21% 17% 2	760
	21st to end	Night	37% 10% 13% 23% 14% 3	836
February	1st to 10th	Night	37% 9% 17% 25% 11% 2	760
	11th to 20th	Night	34% 9% 15% 26% 14% 2	744
	21st to end	Night	35% 10% 18% 24% 11% 2	628
March	1st to 10th	Night	31% 11% 16% 26% 14% 2	760
	11th to 20th	Night	30% 13% 17% 24% 15% 2	760
	21st to end	Night	30% 12% 22% 25% 10% 1	836
April	1st to 10th	Night	30% 10% 18% 25% 16% 2	760
	11th to 20th	Night	26% 8% 19% 27% 16% 3	689
	21st to end	Night	28% 7% 17% 31% 14% 2	491
May	1st to 10th	Night	22% 8% 17% 31% 21% 2	360
	11th to 20th	Night	19% 6% 13% 31% 27% 4	354
	21st to end	Night	24% 5% 15% 26% 25% 5%	389
June	1st to 10th	Night	21% 7% 18% 31% 20% 3	360
	11th to 20th	Night	26% 10% 14% 28% 19% 2	360
	21st to end	Night	14% 3 17% 32% 30% 4	360
July	1st to 10th	Night	13% 5% 10% 28% 39% 5%	350
	11th to 20th	Night	10% 2 12% 36% 36% 4	348
	21st to end	Night	8% 3 8% 30% 46% 4	394
August	1st to 10th	Night	11% 2 6% 28% 45% 9%	384
	11th to 20th	Night	9% 1 6% 26% 44% 13%	534
	21st to end	Night	14% 4 11% 29% 36% 6%	702
September	1st to 10th	Night	13% 4 9% 36% 34% 5%	720
	11th to 20th	Night	20% 4 10% 23% 36% 8%	720
	21st to end	Night	21% 7% 12% 28% 27% 5%	720
October	1st to 10th	Night	22% 6% 12% 27% 28% 6%	720
	11th to 20th	Night	28% 7% 16% 23% 21% 5%	720
	21st to end	Night	33% 9% 16% 23% 15% 3	792
November	1st to 10th	Night	37% 10% 17% 24% 12% 1	740
	11th to 20th	Night	37% 10% 19% 19% 14% 3	760
	21st to end	Night	36% 9% 16% 20% 14% 4	760
December	1st to 10th	Night	34% 7% 15% 24% 17% 3	760
	11th to 20th	Night	39% 12% 15% 21% 10% 2	760
	21st to end	Night	44% 12% 17% 18% 9% 1	912

Page generated at 2024-04-07 06:28:09. Version ac07

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