Everett
Clear Sky Chart
History

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

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	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	105636
	Night	23% 11% 3 8% 5% 4 4 3 4 5% 30%	102015
Transparency	Day	54% 3 13% 19% 9% 1	105392
	Night	59% 3 13% 18% 7% 1	101905
Seeing	Day	23% 9% 20% 25% 17% 5%	15122
	Night	28% 9% 15% 24% 20% 4	23463

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% 19% 5% 9% 5% 4 3 3 3 3 11%	2219
	11th to 20th	Day	26% 11% 3 9% 5% 5% 4 3 5% 6% 22%	2488
	21st to end	Day	36% 15% 5% 10% 6% 5% 4 2 4 4 10%	2837
February	1st to 10th	Day	32% 13% 4 9% 5% 5% 4 3 5% 5% 15%	1710
	11th to 20th	Day	27% 14% 4 10% 6% 4 3 4 5% 4 20%	1665
	21st to end	Day	27% 15% 4 8% 6% 5% 4 3 4 4 20%	1475
March	1st to 10th	Day	18% 14% 4 9% 6% 6% 5% 4 6% 7% 19%	3197
	11th to 20th	Day	19% 11% 3 7% 4 5% 4 3 5% 5% 34%	3490
	21st to end	Day	25% 13% 4 10% 6% 5% 6% 4 5% 6% 16%	3965
April	1st to 10th	Day	23% 14% 4 8% 5% 6% 4 5% 6% 7% 17%	2280
	11th to 20th	Day	23% 10% 4 8% 7% 5% 5% 3 5% 6% 24%	2278
	21st to end	Day	20% 11% 3 10% 7% 6% 5% 4 6% 6% 22%	2527
May	1st to 10th	Day	18% 8% 2 6% 4 4 3 3 5% 6% 39%	4420
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 6% 9% 36%	4377
	21st to end	Day	28% 11% 3 9% 6% 4 4 3 5% 6% 21%	4813
June	1st to 10th	Day	21% 8% 3 8% 6% 4 4 4 5% 8% 29%	2731
	11th to 20th	Day	22% 10% 3 8% 5% 5% 5% 4 6% 7% 24%	2850
	21st to end	Day	13% 7% 4 7% 5% 4 4 4 6% 8% 38%	2850
July	1st to 10th	Day	7% 4 2 4 3 3 3 3 4 5% 63%	4740
	11th to 20th	Day	6% 3 1 3 3 2 2 2 3 5% 69%	4563
	21st to end	Day	10% 4 2 4 3 4 4 3 5% 5% 57%	4790
August	1st to 10th	Day	5% 3 1 3 2 3 3 3 5% 8% 63%	4354
	11th to 20th	Day	9% 6% 3 8% 6% 4 4 4 6% 9% 41%	4026
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 6% 41%	4339
September	1st to 10th	Day	9% 7% 3 7% 3 4 4 3 5% 7% 48%	2280
	11th to 20th	Day	14% 8% 4 8% 5% 4 4 3 6% 6% 38%	2280
	21st to end	Day	18% 8% 3 7% 4 5% 4 3 5% 6% 36%	2009
October	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 7% 10% 32%	3180
	11th to 20th	Day	32% 11% 3 8% 4 3 3 3 4 6% 22%	3161
	21st to end	Day	25% 12% 5% 10% 6% 6% 4 4 4 6% 18%	3233
November	1st to 10th	Day	34% 11% 3 8% 4 4 5% 3 4 5% 16%	1564
	11th to 20th	Day	34% 11% 3 7% 5% 3 4 3 5% 4 22%	1520
	21st to end	Day	33% 13% 3 7% 5% 4 3 4 5% 5% 19%	1520
December	1st to 10th	Day	27% 14% 4 9% 4 4 4 3 4 4 23%	1520
	11th to 20th	Day	32% 13% 5% 8% 6% 5% 5% 3 4 5% 15%	1925
	21st to end	Day	25% 16% 5% 11% 7% 5% 4 3 5% 5% 14%	2460
January	1st to 10th	Night	36% 15% 4 10% 6% 4 4 3 4 3 11%	4726
	11th to 20th	Night	28% 12% 4 7% 4 4 3 3 5% 7% 23%	4406
	21st to end	Night	34% 14% 4 10% 6% 4 4 3 4 4 12%	4817
February	1st to 10th	Night	31% 13% 3 9% 5% 4 4 3 4 5% 19%	2470
	11th to 20th	Night	28% 15% 4 8% 5% 4 4 3 4 4 21%	2402
	21st to end	Night	28% 13% 3 8% 4 4 5% 4 4 5% 21%	1978
March	1st to 10th	Night	18% 15% 4 11% 7% 6% 5% 5% 6% 5% 17%	3674
	11th to 20th	Night	20% 10% 3 6% 3 4 4 2 4 6% 37%	3489
	21st to end	Night	28% 16% 4 10% 5% 4 4 4 4 5% 16%	3688
April	1st to 10th	Night	25% 12% 3 9% 6% 5% 5% 4 6% 6% 19%	1900
	11th to 20th	Night	22% 12% 3 7% 6% 4 5% 4 6% 5% 28%	1851
	21st to end	Night	23% 12% 3 9% 6% 6% 5% 3 6% 5% 23%	1653
May	1st to 10th	Night	18% 9% 3 6% 4 3 3 3 4 4 42%	2540
	11th to 20th	Night	19% 7% 3 6% 4 4 3 3 5% 5% 41%	2516
	21st to end	Night	29% 10% 3 9% 5% 4 4 3 5% 5% 23%	2762
June	1st to 10th	Night	20% 9% 3 7% 5% 5% 5% 4 5% 6% 31%	1444
	11th to 20th	Night	25% 11% 3 9% 5% 4 4 3 6% 5% 25%	1330
	21st to end	Night	15% 5% 2 6% 3 5% 4 4 7% 9% 40%	1330
July	1st to 10th	Night	9% 4 2 4 3 3 3 2 3 4 62%	2220
	11th to 20th	Night	5% 3 1 2 2 2 2 3 4 6% 72%	2380
	21st to end	Night	12% 6% 1 5% 2 3 3 3 4 5% 56%	2762
August	1st to 10th	Night	4 3 1 3 3 2 3 2 4 6% 69%	2576
	11th to 20th	Night	13% 7% 2 6% 3 3 3 3 4 6% 50%	2812
	21st to end	Night	16% 6% 3 7% 4 3 3 3 3 5% 48%	3333
September	1st to 10th	Night	10% 5% 2 6% 3 3 4 3 5% 5% 54%	1900
	11th to 20th	Night	16% 9% 2 7% 6% 3 4 3 4 5% 40%	1900
	21st to end	Night	17% 8% 3 8% 4 4 3 3 4 6% 39%	2166
October	1st to 10th	Night	15% 7% 2 8% 5% 5% 5% 4 6% 8% 35%	3800
	11th to 20th	Night	27% 10% 2 6% 4 4 4 3 5% 6% 29%	3777
	21st to end	Night	29% 14% 4 9% 5% 4 3 3 4 5% 20%	4328
November	1st to 10th	Night	34% 11% 4 7% 5% 4 4 3 4 5% 20%	2634
	11th to 20th	Night	33% 11% 3 7% 4 4 4 3 4 4 22%	2660
	21st to end	Night	33% 12% 3 8% 5% 4 4 3 4 5% 20%	2660
December	1st to 10th	Night	30% 12% 3 7% 5% 4 3 3 5% 6% 23%	2660
	11th to 20th	Night	29% 15% 5% 9% 6% 5% 5% 3 4 5% 16%	3280
	21st to end	Night	27% 15% 5% 11% 6% 5% 5% 3 5% 4 14%	5191

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% 1 6% 9% 2 1	2219
	11th to 20th	Day	64% 2 9% 17% 7% 1	2512
	21st to end	Day	79% 1 8% 10% 2	2837
February	1st to 10th	Day	72% 3 11% 9% 3 1	1710
	11th to 20th	Day	70% 2 10% 10% 8% 1	1674
	21st to end	Day	71% 1 9% 13% 6%	1475
March	1st to 10th	Day	66% 1 11% 17% 6%	3197
	11th to 20th	Day	55% 2 16% 25% 2 1	3490
	21st to end	Day	72% 2 13% 12% 1	3965
April	1st to 10th	Day	69% 4 15% 11% 1	2280
	11th to 20th	Day	64% 2 11% 14% 7% 1	2278
	21st to end	Day	66% 3 11% 14% 6% 1	2527
May	1st to 10th	Day	50% 2 12% 24% 12%	4420
	11th to 20th	Day	47% 4 23% 22% 4	4380
	21st to end	Day	69% 2 18% 10% 1	4814
June	1st to 10th	Day	58% 3 17% 16% 5% 1	2731
	11th to 20th	Day	63% 4 16% 14% 4	2850
	21st to end	Day	46% 4 21% 21% 7% 1	2850
July	1st to 10th	Day	27% 3 9% 24% 25% 12%	4666
	11th to 20th	Day	21% 2 9% 32% 32% 3	4483
	21st to end	Day	30% 5% 17% 33% 14% 2	4639
August	1st to 10th	Day	22% 4 15% 41% 17% 1	4337
	11th to 20th	Day	40% 8% 20% 24% 7% 2	4065
	21st to end	Day	44% 5% 13% 27% 11% 1	4338
September	1st to 10th	Day	39% 2 15% 27% 13% 5%	2280
	11th to 20th	Day	48% 3 14% 19% 14% 1	2280
	21st to end	Day	48% 4 18% 19% 10% 2	2009
October	1st to 10th	Day	46% 5% 17% 26% 5%	3180
	11th to 20th	Day	64% 2 11% 15% 7%	3161
	21st to end	Day	66% 5% 11% 10% 8%	3233
November	1st to 10th	Day	71% 3 12% 10% 4	1564
	11th to 20th	Day	68% 3 10% 11% 7% 2	1520
	21st to end	Day	69% 3 6% 13% 9% 1	1520
December	1st to 10th	Day	67% 3 8% 12% 9% 1	1520
	11th to 20th	Day	72% 2 11% 12% 2 1	1928
	21st to end	Day	75% 1 11% 10% 2	2460
January	1st to 10th	Night	81% 2 7% 8% 2 1	4726
	11th to 20th	Night	64% 2 7% 19% 7% 1	4444
	21st to end	Night	78% 1 8% 9% 3	4817
February	1st to 10th	Night	70% 2 10% 12% 5% 1	2470
	11th to 20th	Night	68% 3 11% 11% 7% 1	2404
	21st to end	Night	66% 2 11% 15% 5% 1	1978
March	1st to 10th	Night	69% 1 9% 15% 6%	3674
	11th to 20th	Night	51% 2 20% 24% 2 1	3490
	21st to end	Night	74% 2 13% 10% 1	3688
April	1st to 10th	Night	67% 4 14% 13% 2	1900
	11th to 20th	Night	60% 3 14% 14% 8% 2	1851
	21st to end	Night	65% 3 12% 15% 6% 1	1653
May	1st to 10th	Night	49% 2 11% 29% 9% 1	2540
	11th to 20th	Night	48% 2 22% 23% 5%	2516
	21st to end	Night	67% 4 18% 10% 1	2762
June	1st to 10th	Night	56% 3 16% 19% 5% 1	1444
	11th to 20th	Night	63% 3 15% 16% 2	1330
	21st to end	Night	44% 3 22% 23% 5% 3	1330
July	1st to 10th	Night	28% 3 9% 22% 22% 16%	2185
	11th to 20th	Night	18% 2 9% 29% 40% 2	2337
	21st to end	Night	31% 4 13% 29% 20% 3	2674
August	1st to 10th	Night	19% 3 15% 41% 20% 2	2568
	11th to 20th	Night	36% 6% 23% 25% 9% 2	2839
	21st to end	Night	42% 4 15% 30% 9%	3333
September	1st to 10th	Night	35% 3 15% 30% 14% 4	1900
	11th to 20th	Night	48% 3 11% 21% 14% 3	1900
	21st to end	Night	48% 3 14% 23% 10% 2	2166
October	1st to 10th	Night	46% 6% 18% 26% 4	3800
	11th to 20th	Night	57% 3 14% 19% 7%	3777
	21st to end	Night	67% 4 11% 12% 6%	4328
November	1st to 10th	Night	68% 3 12% 13% 3 1	2634
	11th to 20th	Night	68% 2 11% 13% 5% 2	2660
	21st to end	Night	68% 2 8% 12% 9% 1	2660
December	1st to 10th	Night	64% 3 11% 14% 8% 1	2660
	11th to 20th	Night	73% 3 11% 10% 3 1	3276
	21st to end	Night	76% 1 10% 10% 3	5191

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% 10% 16% 16% 12% 8%	180
	11th to 20th	Day	35% 14% 17% 14% 14% 6%	180
	21st to end	Day	41% 11% 16% 16% 11% 4	228
February	1st to 10th	Day	40% 11% 21% 17% 9% 2	370
	11th to 20th	Day	36% 13% 24% 17% 8% 1	365
	21st to end	Day	32% 17% 19% 23% 8% 2	337
March	1st to 10th	Day	29% 17% 25% 16% 12% 2	460
	11th to 20th	Day	27% 15% 18% 20% 15% 5%	460
	21st to end	Day	32% 14% 25% 19% 9% 1	506
April	1st to 10th	Day	27% 17% 22% 20% 12% 3	460
	11th to 20th	Day	23% 15% 29% 17% 10% 6%	452
	21st to end	Day	21% 15% 27% 23% 12% 2	460
May	1st to 10th	Day	13% 12% 25% 33% 13% 3	463
	11th to 20th	Day	20% 8% 20% 26% 19% 7%	488
	21st to end	Day	22% 12% 18% 26% 16% 6%	529
June	1st to 10th	Day	21% 9% 23% 27% 16% 3	561
	11th to 20th	Day	21% 9% 25% 28% 14% 3	680
	21st to end	Day	14% 4 21% 34% 21% 5%	680
July	1st to 10th	Day	11% 3 19% 36% 26% 4	671
	11th to 20th	Day	8% 4 18% 34% 28% 8%	570
	21st to end	Day	8% 2 16% 38% 29% 8%	531
August	1st to 10th	Day	9% 3 11% 37% 28% 11%	487
	11th to 20th	Day	7% 3 13% 36% 29% 12%	477
	21st to end	Day	13% 5% 23% 30% 21% 9%	539
September	1st to 10th	Day	15% 5% 22% 34% 18% 7%	490
	11th to 20th	Day	17% 8% 19% 27% 25% 4	490
	21st to end	Day	22% 9% 21% 27% 16% 6%	490
October	1st to 10th	Day	23% 9% 20% 23% 18% 7%	490
	11th to 20th	Day	31% 10% 17% 21% 16% 6%	490
	21st to end	Day	32% 13% 18% 19% 13% 5%	412
November	1st to 10th	Day	44% 11% 17% 17% 10% 2	208
	11th to 20th	Day	41% 11% 12% 14% 12% 11%	180
	21st to end	Day	35% 14% 17% 13% 9% 12%	180
December	1st to 10th	Day	42% 11% 12% 11% 13% 11%	180
	11th to 20th	Day	40% 13% 19% 15% 12% 1	180
	21st to end	Day	41% 9% 17% 24% 7% 3	198
January	1st to 10th	Night	41% 13% 14% 16% 14% 2	850
	11th to 20th	Night	36% 12% 13% 18% 18% 3	765
	21st to end	Night	36% 10% 15% 22% 14% 3	836
February	1st to 10th	Night	36% 10% 15% 25% 11% 2	760
	11th to 20th	Night	32% 11% 15% 23% 16% 2	744
	21st to end	Night	32% 10% 19% 24% 12% 3	628
March	1st to 10th	Night	31% 12% 17% 24% 14% 2	760
	11th to 20th	Night	31% 15% 15% 21% 16% 2	760
	21st to end	Night	30% 15% 21% 24% 9% 1	836
April	1st to 10th	Night	28% 12% 18% 25% 14% 2	760
	11th to 20th	Night	25% 10% 21% 26% 15% 3	723
	21st to end	Night	27% 10% 21% 27% 13% 2	513
May	1st to 10th	Night	19% 9% 16% 32% 22% 2	380
	11th to 20th	Night	20% 6% 13% 29% 28% 3	374
	21st to end	Night	22% 7% 17% 26% 23% 5%	411
June	1st to 10th	Night	23% 8% 17% 30% 18% 4	380
	11th to 20th	Night	27% 8% 15% 30% 18% 2	380
	21st to end	Night	18% 3 12% 35% 28% 3	380
July	1st to 10th	Night	13% 3 11% 32% 36% 4	370
	11th to 20th	Night	8% 4 12% 36% 36% 4	368
	21st to end	Night	9% 4 10% 30% 42% 6%	416
August	1st to 10th	Night	9% 2 7% 28% 45% 10%	399
	11th to 20th	Night	9% 1 7% 25% 43% 14%	564
	21st to end	Night	13% 5% 12% 29% 34% 6%	736
September	1st to 10th	Night	13% 2 11% 33% 34% 7%	760
	11th to 20th	Night	19% 5% 12% 22% 31% 11%	760
	21st to end	Night	22% 6% 15% 26% 26% 6%	760
October	1st to 10th	Night	21% 7% 12% 27% 26% 7%	760
	11th to 20th	Night	29% 8% 14% 22% 22% 5%	760
	21st to end	Night	33% 10% 16% 22% 15% 4	836
November	1st to 10th	Night	39% 12% 15% 22% 12% 1	764
	11th to 20th	Night	38% 8% 19% 19% 13% 3	760
	21st to end	Night	37% 9% 15% 19% 15% 5%	760
December	1st to 10th	Night	33% 8% 15% 25% 16% 3	760
	11th to 20th	Night	40% 12% 16% 19% 11% 1	760
	21st to end	Night	40% 13% 17% 19% 10% 1	930

Page generated at 2024-11-03 10:19:00. Version ac07

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