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 2024-11-04.

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% 5% 4 4 3 5% 6% 33%	105684
	Night	23% 10% 3 8% 5% 4 4 3 4 5% 30%	101898
Transparency	Day	51% 9% 14% 17% 8% 1	105441
	Night	55% 9% 13% 16% 7% 1	101787
Seeing	Day	24% 9% 19% 26% 17% 5%	15094
	Night	28% 8% 15% 25% 20% 3	23439

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% 15% 5% 9% 5% 4 3 3 3 4 11%	2251
	11th to 20th	Day	26% 12% 4 9% 5% 4 5% 3 5% 6% 22%	2516
	21st to end	Day	40% 14% 4 9% 4 4 3 3 4 4 11%	2858
February	1st to 10th	Day	31% 13% 4 8% 5% 6% 5% 4 4 5% 15%	1710
	11th to 20th	Day	26% 15% 4 8% 5% 5% 4 2 5% 5% 19%	1665
	21st to end	Day	26% 16% 4 9% 4 5% 5% 3 3 4 20%	1479
March	1st to 10th	Day	19% 14% 4 10% 7% 5% 5% 4 6% 7% 20%	3200
	11th to 20th	Day	19% 10% 3 7% 5% 4 4 4 4 6% 34%	3490
	21st to end	Day	24% 13% 5% 11% 6% 5% 5% 4 5% 6% 16%	3965
April	1st to 10th	Day	25% 13% 4 8% 6% 5% 5% 3 6% 7% 18%	2280
	11th to 20th	Day	22% 12% 3 8% 6% 5% 5% 4 5% 7% 25%	2273
	21st to end	Day	20% 10% 4 11% 6% 6% 6% 4 5% 7% 22%	2521
May	1st to 10th	Day	17% 8% 3 6% 4 4 4 3 5% 8% 38%	4420
	11th to 20th	Day	19% 6% 2 6% 5% 4 4 3 5% 9% 36%	4377
	21st to end	Day	28% 11% 3 8% 5% 5% 4 3 5% 6% 21%	4813
June	1st to 10th	Day	22% 8% 3 7% 5% 5% 5% 4 6% 8% 28%	2707
	11th to 20th	Day	22% 11% 3 9% 6% 5% 6% 4 5% 7% 23%	2850
	21st to end	Day	12% 8% 3 8% 5% 5% 4 4 6% 7% 38%	2850
July	1st to 10th	Day	7% 4 2 5% 2 2 3 3 4 5% 63%	4740
	11th to 20th	Day	5% 3 1 3 3 2 3 2 4 5% 68%	4530
	21st to end	Day	10% 5% 2 5% 3 3 3 3 4 5% 57%	4790
August	1st to 10th	Day	6% 3 1 4 2 3 3 2 5% 7% 63%	4345
	11th to 20th	Day	9% 7% 3 8% 4 5% 5% 4 6% 9% 41%	4026
	21st to end	Day	16% 8% 3 7% 4 4 3 3 4 6% 42%	4335
September	1st to 10th	Day	11% 6% 2 7% 4 4 4 3 5% 7% 49%	2280
	11th to 20th	Day	14% 9% 3 9% 5% 5% 4 3 6% 7% 37%	2280
	21st to end	Day	18% 8% 3 7% 5% 5% 4 3 5% 6% 36%	2009
October	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 4 7% 11% 34%	3180
	11th to 20th	Day	29% 10% 3 8% 5% 3 3 3 5% 6% 24%	3161
	21st to end	Day	27% 12% 4 10% 6% 5% 5% 3 5% 6% 17%	3237
November	1st to 10th	Day	36% 13% 3 6% 4 4 4 3 4 5% 17%	1555
	11th to 20th	Day	33% 13% 4 7% 4 3 3 3 3 5% 21%	1520
	21st to end	Day	33% 12% 3 7% 4 3 4 3 5% 5% 19%	1520
December	1st to 10th	Day	29% 11% 4 9% 5% 4 4 2 4 4 22%	1520
	11th to 20th	Day	32% 13% 4 9% 6% 5% 5% 4 4 4 14%	1925
	21st to end	Day	27% 13% 5% 12% 6% 5% 5% 4 4 6% 13%	2506
January	1st to 10th	Night	38% 14% 4 9% 5% 4 4 3 4 4 11%	4689
	11th to 20th	Night	28% 13% 4 6% 4 3 3 2 4 7% 23%	4382
	21st to end	Night	36% 14% 4 9% 5% 4 5% 3 4 4 12%	4796
February	1st to 10th	Night	31% 12% 3 8% 6% 5% 4 3 4 6% 17%	2470
	11th to 20th	Night	26% 14% 4 8% 5% 5% 3 4 4 5% 22%	2402
	21st to end	Night	28% 15% 4 7% 4 4 4 3 4 5% 21%	1975
March	1st to 10th	Night	19% 15% 5% 9% 7% 6% 7% 5% 5% 6% 17%	3661
	11th to 20th	Night	19% 9% 3 6% 3 3 4 3 4 6% 39%	3489
	21st to end	Night	28% 14% 4 10% 6% 5% 5% 3 4 4 17%	3688
April	1st to 10th	Night	24% 11% 3 8% 6% 5% 6% 4 6% 6% 19%	1900
	11th to 20th	Night	20% 10% 3 9% 5% 5% 5% 4 5% 5% 30%	1855
	21st to end	Night	23% 10% 4 9% 6% 4 4 4 5% 6% 25%	1658
May	1st to 10th	Night	18% 10% 3 7% 4 3 4 3 4 4 41%	2540
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 4 7% 39%	2516
	21st to end	Night	27% 12% 3 8% 5% 6% 5% 3 5% 5% 21%	2762
June	1st to 10th	Night	20% 8% 4 7% 6% 5% 4 3 6% 5% 32%	1468
	11th to 20th	Night	22% 13% 3 9% 5% 5% 4 3 5% 5% 25%	1330
	21st to end	Night	13% 6% 3 7% 5% 5% 3 4 5% 7% 42%	1330
July	1st to 10th	Night	9% 6% 2 5% 3 3 3 2 3 4 61%	2220
	11th to 20th	Night	4 3 2 3 2 2 2 3 3 4 72%	2426
	21st to end	Night	11% 5% 2 4 3 3 4 3 5% 6% 56%	2762
August	1st to 10th	Night	4 3 1 4 3 3 3 2 4 5% 69%	2583
	11th to 20th	Night	12% 6% 3 6% 4 3 3 3 4 6% 50%	2812
	21st to end	Night	17% 6% 3 6% 4 3 3 2 3 5% 48%	3338
September	1st to 10th	Night	9% 5% 2 6% 3 3 5% 3 4 6% 53%	1900
	11th to 20th	Night	16% 8% 3 7% 4 4 5% 3 4 5% 41%	1900
	21st to end	Night	17% 7% 3 8% 5% 5% 4 3 5% 5% 38%	2166
October	1st to 10th	Night	15% 7% 3 7% 4 4 5% 3 6% 9% 37%	3800
	11th to 20th	Night	27% 9% 3 6% 4 4 4 4 5% 6% 28%	3777
	21st to end	Night	30% 12% 4 9% 5% 4 3 3 4 5% 21%	4324
November	1st to 10th	Night	33% 10% 4 7% 5% 4 4 3 4 4 20%	2592
	11th to 20th	Night	32% 10% 4 8% 6% 4 4 3 4 4 22%	2660
	21st to end	Night	32% 10% 3 8% 5% 5% 4 3 4 5% 21%	2660
December	1st to 10th	Night	29% 12% 4 8% 5% 3 3 3 4 5% 24%	2660
	11th to 20th	Night	29% 14% 5% 10% 6% 4 4 3 5% 4 15%	3280
	21st to end	Night	28% 15% 5% 11% 5% 5% 4 3 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	63% 13% 14% 9% 1	2280
	11th to 20th	Day	59% 9% 12% 12% 6% 1	2273
	21st to end	Day	61% 12% 11% 11% 5% 1	2521
May	1st to 10th	Day	43% 11% 15% 21% 10%	4420
	11th to 20th	Day	41% 13% 23% 19% 4	4380
	21st to end	Day	63% 11% 16% 8% 1	4814
June	1st to 10th	Day	55% 10% 17% 12% 5% 1	2707
	11th to 20th	Day	60% 11% 14% 12% 4	2850
	21st to end	Day	44% 11% 19% 19% 7% 1	2850
July	1st to 10th	Day	24% 8% 9% 21% 24% 14%	4666
	11th to 20th	Day	20% 6% 9% 30% 33% 3	4451
	21st to end	Day	29% 5% 17% 32% 15% 2	4639
August	1st to 10th	Day	20% 7% 13% 40% 18% 1	4328
	11th to 20th	Day	39% 8% 22% 22% 7% 2	4065
	21st to end	Day	43% 6% 12% 27% 11%	4334
September	1st to 10th	Day	36% 7% 15% 25% 13% 4	2280
	11th to 20th	Day	47% 8% 14% 17% 14% 2	2280
	21st to end	Day	48% 8% 17% 17% 9% 2	2009
October	1st to 10th	Day	42% 8% 18% 26% 5%	3180
	11th to 20th	Day	61% 6% 13% 12% 7%	3161
	21st to end	Day	66% 7% 12% 8% 7% 1	3237
November	1st to 10th	Day	68% 9% 12% 8% 3	1555
	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	54% 12% 13% 13% 8% 1	1855
	21st to end	Night	59% 10% 12% 13% 5% 1	1658
May	1st to 10th	Night	44% 10% 13% 24% 8% 1	2540
	11th to 20th	Night	41% 13% 23% 19% 4	2516
	21st to end	Night	64% 13% 14% 9% 1	2762
June	1st to 10th	Night	52% 10% 16% 16% 5%	1468
	11th to 20th	Night	59% 11% 14% 13% 2	1330
	21st to end	Night	41% 9% 22% 19% 6% 3	1330
July	1st to 10th	Night	28% 6% 9% 20% 19% 17%	2185
	11th to 20th	Night	17% 4 9% 27% 40% 2	2382
	21st to end	Night	29% 7% 13% 29% 19% 3	2674
August	1st to 10th	Night	19% 6% 12% 40% 21% 2	2575
	11th to 20th	Night	35% 8% 23% 24% 9% 2	2839
	21st to end	Night	40% 6% 15% 29% 10%	3338
September	1st to 10th	Night	31% 8% 16% 27% 14% 4	1900
	11th to 20th	Night	45% 8% 11% 19% 13% 4	1900
	21st to end	Night	46% 8% 14% 21% 10% 2	2166
October	1st to 10th	Night	42% 9% 20% 26% 4	3800
	11th to 20th	Night	56% 7% 13% 17% 6%	3777
	21st to end	Night	65% 7% 11% 10% 6%	4324
November	1st to 10th	Night	65% 9% 13% 10% 3 1	2592
	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	31% 13% 24% 19% 12% 2	460
	11th to 20th	Day	26% 14% 27% 19% 9% 6%	452
	21st to end	Day	24% 13% 25% 24% 13% 1	460
May	1st to 10th	Day	18% 9% 22% 34% 14% 3	463
	11th to 20th	Day	24% 7% 19% 26% 17% 7%	488
	21st to end	Day	25% 9% 19% 27% 14% 7%	529
June	1st to 10th	Day	24% 8% 22% 27% 15% 4	537
	11th to 20th	Day	24% 9% 22% 29% 14% 3	680
	21st to end	Day	15% 5% 20% 35% 20% 5%	680
July	1st to 10th	Day	12% 4 19% 34% 27% 4	671
	11th to 20th	Day	8% 4 16% 35% 30% 6%	551
	21st to end	Day	9% 2 15% 35% 32% 7%	531
August	1st to 10th	Day	11% 3 11% 36% 27% 11%	487
	11th to 20th	Day	8% 2 13% 35% 30% 11%	477
	21st to end	Day	14% 6% 19% 31% 21% 8%	539
September	1st to 10th	Day	13% 8% 19% 36% 20% 4	490
	11th to 20th	Day	18% 7% 18% 28% 24% 5%	490
	21st to end	Day	20% 9% 20% 28% 17% 5%	490
October	1st to 10th	Day	22% 9% 20% 22% 19% 8%	490
	11th to 20th	Day	31% 8% 17% 22% 16% 6%	490
	21st to end	Day	34% 13% 16% 20% 11% 5%	416
November	1st to 10th	Day	42% 12% 16% 18% 9% 2	209
	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	31% 9% 18% 25% 16% 2	760
	11th to 20th	Night	26% 8% 20% 28% 16% 3	727
	21st to end	Night	28% 8% 17% 31% 13% 2	518
May	1st to 10th	Night	22% 8% 18% 31% 21% 2	380
	11th to 20th	Night	19% 7% 13% 31% 27% 3	374
	21st to end	Night	24% 5% 16% 26% 24% 5%	411
June	1st to 10th	Night	21% 8% 18% 31% 20% 3	380
	11th to 20th	Night	25% 11% 15% 29% 19% 2	380
	21st to end	Night	15% 3 18% 31% 29% 4	380
July	1st to 10th	Night	13% 4 12% 29% 38% 4	370
	11th to 20th	Night	10% 2 12% 36% 36% 4	368
	21st to end	Night	9% 3 10% 29% 44% 4	416
August	1st to 10th	Night	10% 2 6% 28% 46% 9%	406
	11th to 20th	Night	10% 1 7% 27% 43% 13%	564
	21st to end	Night	15% 4 11% 30% 35% 6%	741
September	1st to 10th	Night	12% 4 9% 36% 34% 5%	760
	11th to 20th	Night	20% 4 10% 24% 35% 7%	760
	21st to end	Night	21% 7% 12% 28% 27% 5%	760
October	1st to 10th	Night	22% 5% 13% 28% 27% 5%	760
	11th to 20th	Night	29% 7% 16% 24% 20% 5%	760
	21st to end	Night	33% 10% 16% 23% 15% 3	836
November	1st to 10th	Night	37% 10% 16% 24% 12% 1	754
	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

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