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 2025-08-05.

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% 7% 33%	130239
	Night	23% 10% 3 8% 5% 4 4 3 5% 5% 29%	126497
Transparency	Day	50% 8% 15% 17% 8% 1	129954
	Night	55% 9% 13% 16% 7% 1	126359
Seeing	Day	24% 9% 19% 26% 17% 5%	16028
	Night	28% 8% 15% 25% 20% 3	24343

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	37% 15% 5% 9% 6% 6% 4 3 4 4 9%	3239
	11th to 20th	Day	20% 10% 4 10% 7% 5% 5% 3 6% 7% 24%	3616
	21st to end	Day	31% 11% 3 7% 3 3 3 3 4 5% 25%	4096
February	1st to 10th	Day	29% 13% 4 9% 5% 6% 5% 4 4 6% 15%	1800
	11th to 20th	Day	26% 15% 4 8% 5% 5% 4 2 5% 5% 19%	1755
	21st to end	Day	27% 15% 4 9% 4 5% 5% 3 3 5% 20%	1555
March	1st to 10th	Day	21% 15% 4 10% 6% 5% 5% 4 5% 6% 17%	4606
	11th to 20th	Day	19% 14% 5% 9% 5% 5% 4 4 4 5% 26%	5000
	21st to end	Day	24% 12% 4 11% 6% 6% 7% 4 6% 6% 13%	5678
April	1st to 10th	Day	25% 13% 4 8% 6% 5% 5% 3 6% 7% 17%	2400
	11th to 20th	Day	21% 11% 3 7% 6% 5% 5% 4 5% 7% 26%	2394
	21st to end	Day	19% 10% 3 10% 6% 6% 6% 4 5% 7% 24%	2654
May	1st to 10th	Day	15% 8% 3 7% 4 4 4 3 5% 7% 40%	6320
	11th to 20th	Day	22% 9% 3 8% 5% 4 5% 4 6% 8% 27%	6277
	21st to end	Day	22% 9% 3 7% 4 4 4 3 6% 8% 29%	6903
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 6% 8% 30%	2850
	11th to 20th	Day	21% 10% 3 9% 6% 5% 6% 4 6% 7% 23%	3000
	21st to end	Day	12% 8% 3 8% 5% 5% 4 4 6% 7% 38%	3000
July	1st to 10th	Day	8% 4 2 4 2 3 3 3 4 5% 61%	6780
	11th to 20th	Day	4 3 1 3 3 2 2 2 4 5% 70%	6486
	21st to end	Day	8% 4 2 5% 3 3 3 3 5% 7% 57%	6874
August	1st to 10th	Day	6% 3 1 4 2 3 3 3 5% 7% 62%	4402
	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	35% 12% 3 7% 4 4 5% 4 5% 5% 17%	1611
	11th to 20th	Day	32% 13% 3 7% 5% 4 4 3 4 5% 20%	1600
	21st to end	Day	33% 12% 3 8% 4 3 4 3 5% 5% 19%	1600
December	1st to 10th	Day	22% 9% 3 9% 7% 5% 6% 3 6% 7% 22%	2620
	11th to 20th	Day	34% 14% 4 10% 6% 5% 5% 3 4 4 11%	3025
	21st to end	Day	31% 14% 4 11% 6% 5% 5% 3 4 5% 11%	3590
January	1st to 10th	Night	35% 14% 5% 11% 6% 5% 5% 3 4 4 10%	6701
	11th to 20th	Night	20% 10% 4 6% 4 4 5% 3 6% 8% 29%	6282
	21st to end	Night	28% 11% 3 7% 4 4 4 3 4 5% 28%	6847
February	1st to 10th	Night	30% 13% 4 8% 6% 5% 4 4 4 6% 18%	2600
	11th to 20th	Night	26% 13% 4 8% 5% 5% 3 4 4 6% 22%	2532
	21st to end	Night	28% 15% 3 7% 4 4 4 3 4 6% 21%	2075
March	1st to 10th	Night	18% 14% 4 9% 6% 6% 7% 5% 6% 7% 18%	5275
	11th to 20th	Night	23% 12% 4 8% 4 4 4 3 5% 5% 29%	4999
	21st to end	Night	30% 14% 4 10% 5% 5% 5% 4 5% 4 14%	5297
April	1st to 10th	Night	24% 11% 3 8% 6% 5% 6% 5% 6% 6% 19%	2000
	11th to 20th	Night	19% 10% 3 8% 5% 5% 5% 4 5% 5% 31%	1953
	21st to end	Night	23% 10% 4 9% 6% 4 4 4 5% 6% 26%	1745
May	1st to 10th	Night	15% 9% 3 7% 4 4 4 3 4 4 42%	3640
	11th to 20th	Night	22% 10% 3 8% 5% 4 4 3 4 6% 31%	3616
	21st to end	Night	22% 11% 3 8% 5% 5% 5% 4 6% 7% 24%	3972
June	1st to 10th	Night	19% 8% 4 7% 6% 5% 5% 3 6% 5% 34%	1545
	11th to 20th	Night	21% 13% 3 9% 5% 5% 4 3 5% 5% 26%	1400
	21st to end	Night	13% 6% 3 7% 5% 5% 3 4 5% 7% 42%	1400
July	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 3 4 58%	3180
	11th to 20th	Night	3 2 1 3 2 2 2 3 3 4 74%	3470
	21st to end	Night	8% 4 1 4 3 3 4 3 5% 7% 58%	3969
August	1st to 10th	Night	4 3 1 4 3 3 3 2 4 5% 68%	2615
	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% 5% 4 3 4 5% 21%	2684
	11th to 20th	Night	33% 10% 4 8% 5% 4 4 3 4 4 21%	2800
	21st to end	Night	31% 10% 3 8% 6% 5% 4 3 4 5% 20%	2800
December	1st to 10th	Night	23% 10% 3 8% 6% 5% 5% 5% 6% 6% 25%	4560
	11th to 20th	Night	33% 14% 5% 10% 6% 4 4 3 4 4 12%	5180
	21st to end	Night	31% 18% 5% 11% 5% 5% 4 3 4 4 12%	7343

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	78% 7% 7% 6% 2	3239
	11th to 20th	Day	55% 10% 12% 16% 6% 1	3640
	21st to end	Day	60% 6% 10% 14% 9%	4096
February	1st to 10th	Day	69% 9% 9% 8% 4 1	1800
	11th to 20th	Day	67% 8% 9% 9% 7%	1764
	21st to end	Day	66% 10% 9% 11% 4	1555
March	1st to 10th	Day	61% 10% 12% 12% 4	4606
	11th to 20th	Day	58% 9% 13% 18% 2	5000
	21st to end	Day	66% 11% 14% 8% 1	5678
April	1st to 10th	Day	63% 13% 15% 9% 1	2400
	11th to 20th	Day	58% 9% 12% 13% 6% 1	2394
	21st to end	Day	59% 11% 12% 12% 5% 1	2654
May	1st to 10th	Day	42% 9% 16% 23% 10%	6320
	11th to 20th	Day	52% 11% 19% 16% 3	6280
	21st to end	Day	51% 10% 22% 15% 1	6904
June	1st to 10th	Day	52% 10% 18% 14% 5% 1	2850
	11th to 20th	Day	59% 11% 14% 12% 4	3000
	21st to end	Day	44% 10% 19% 19% 7% 1	3000
July	1st to 10th	Day	25% 7% 18% 21% 19% 10%	6706
	11th to 20th	Day	19% 5% 15% 31% 29% 2	6407
	21st to end	Day	27% 6% 24% 31% 11% 1	6681
August	1st to 10th	Day	21% 7% 13% 40% 18% 1	4385
	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	67% 9% 12% 9% 3	1611
	11th to 20th	Day	66% 9% 9% 9% 6% 2	1600
	21st to end	Day	66% 9% 9% 10% 7% 1	1600
December	1st to 10th	Day	58% 7% 10% 14% 10%	2620
	11th to 20th	Day	73% 8% 10% 7% 1	3028
	21st to end	Day	74% 9% 11% 5% 2	3590
January	1st to 10th	Night	77% 8% 7% 6% 2	6701
	11th to 20th	Night	49% 9% 15% 21% 6% 1	6320
	21st to end	Night	58% 7% 10% 16% 8%	6847
February	1st to 10th	Night	67% 9% 10% 9% 5% 1	2600
	11th to 20th	Night	63% 8% 11% 10% 7% 1	2534
	21st to end	Night	63% 8% 12% 12% 4	2075
March	1st to 10th	Night	58% 12% 14% 12% 4	5275
	11th to 20th	Night	55% 10% 15% 18% 2	5000
	21st to end	Night	71% 10% 12% 8%	5297
April	1st to 10th	Night	61% 13% 14% 11% 2	2000
	11th to 20th	Night	53% 11% 14% 13% 8% 1	1953
	21st to end	Night	58% 10% 12% 14% 5% 1	1745
May	1st to 10th	Night	43% 9% 12% 28% 8%	3637
	11th to 20th	Night	52% 11% 19% 15% 3	3616
	21st to end	Night	58% 12% 18% 12% 1	3972
June	1st to 10th	Night	50% 9% 17% 17% 6%	1545
	11th to 20th	Night	58% 11% 13% 15% 3	1400
	21st to end	Night	42% 9% 22% 19% 6% 3	1400
July	1st to 10th	Night	30% 6% 15% 19% 17% 12%	3145
	11th to 20th	Night	15% 4 16% 28% 35% 2	3426
	21st to end	Night	25% 6% 22% 31% 14% 2	3857
August	1st to 10th	Night	19% 6% 12% 40% 21% 2	2607
	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	2684
	11th to 20th	Night	67% 7% 11% 9% 4 2	2800
	21st to end	Night	65% 9% 9% 9% 7%	2800
December	1st to 10th	Night	55% 10% 11% 16% 7%	4560
	11th to 20th	Night	73% 7% 11% 7% 2	5176
	21st to end	Night	74% 10% 9% 6% 2	7343

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	40% 10% 16% 14% 14% 7%	200
	11th to 20th	Day	35% 9% 16% 20% 16% 6%	200
	21st to end	Day	39% 12% 15% 17% 12% 5%	260
February	1st to 10th	Day	41% 10% 21% 17% 9% 3	400
	11th to 20th	Day	39% 13% 22% 16% 8% 2	395
	21st to end	Day	35% 15% 19% 20% 8% 2	367
March	1st to 10th	Day	31% 15% 24% 16% 12% 2	500
	11th to 20th	Day	32% 13% 19% 18% 15% 4	500
	21st to end	Day	35% 14% 21% 20% 9% 1	550
April	1st to 10th	Day	31% 12% 23% 20% 11% 2	500
	11th to 20th	Day	25% 15% 27% 20% 9% 5%	492
	21st to end	Day	23% 13% 24% 25% 13% 2	500
May	1st to 10th	Day	17% 9% 24% 34% 14% 3	503
	11th to 20th	Day	24% 9% 20% 24% 16% 7%	528
	21st to end	Day	24% 9% 19% 28% 14% 7%	573
June	1st to 10th	Day	22% 8% 21% 29% 16% 4	580
	11th to 20th	Day	22% 9% 22% 29% 15% 3	730
	21st to end	Day	15% 5% 21% 35% 21% 5%	730
July	1st to 10th	Day	11% 4 18% 35% 27% 4	721
	11th to 20th	Day	8% 4 16% 36% 29% 6%	595
	21st to end	Day	8% 3 14% 34% 32% 8%	575
August	1st to 10th	Day	11% 3 11% 35% 27% 12%	504
	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	40% 12% 18% 17% 9% 3	223
	11th to 20th	Day	41% 13% 14% 14% 10% 9%	200
	21st to end	Day	34% 14% 19% 15% 10% 10%	200
December	1st to 10th	Day	41% 6% 15% 11% 15% 14%	200
	11th to 20th	Day	42% 12% 22% 15% 10% 1	200
	21st to end	Day	41% 10% 18% 19% 10% 3	220
January	1st to 10th	Night	44% 11% 14% 16% 13% 2	886
	11th to 20th	Night	35% 10% 12% 23% 17% 2	800
	21st to end	Night	36% 9% 14% 24% 15% 3	880
February	1st to 10th	Night	36% 9% 17% 24% 11% 2	800
	11th to 20th	Night	34% 9% 15% 26% 14% 2	784
	21st to end	Night	35% 10% 18% 24% 11% 2	660
March	1st to 10th	Night	31% 11% 17% 26% 14% 2	800
	11th to 20th	Night	31% 12% 17% 24% 14% 2	800
	21st to end	Night	30% 13% 22% 24% 10% 1	880
April	1st to 10th	Night	30% 10% 18% 26% 16% 2	800
	11th to 20th	Night	25% 8% 21% 28% 15% 2	765
	21st to end	Night	28% 8% 17% 31% 14% 2	545
May	1st to 10th	Night	21% 8% 18% 31% 20% 2	400
	11th to 20th	Night	21% 7% 13% 30% 26% 3	394
	21st to end	Night	24% 6% 16% 26% 24% 4	433
June	1st to 10th	Night	20% 8% 18% 32% 20% 3	400
	11th to 20th	Night	24% 10% 17% 29% 18% 2	400
	21st to end	Night	15% 4 19% 31% 29% 4	400
July	1st to 10th	Night	13% 4 12% 29% 37% 4	390
	11th to 20th	Night	9% 2 13% 37% 35% 4	388
	21st to end	Night	9% 3 10% 30% 45% 4	438
August	1st to 10th	Night	10% 2 6% 28% 45% 9%	414
	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	36% 10% 17% 24% 12% 1	780
	11th to 20th	Night	39% 9% 18% 18% 14% 2	800
	21st to end	Night	36% 9% 17% 21% 14% 4	800
December	1st to 10th	Night	33% 7% 15% 24% 18% 3	800
	11th to 20th	Night	40% 12% 16% 21% 10% 2	800
	21st to end	Night	45% 12% 17% 17% 9% 1	965

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