Everett Astro SP
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Everett Astro SP clear sky chart from 2005-11-06 to 2026-10-29.

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% 9% 3 7% 4 4 4 3 5% 6% 33%	154368
	Night	25% 10% 3 7% 4 4 4 3 4 5% 31%	146403
Transparency	Day	51% 6% 15% 18% 8% 1	154084
	Night	56% 6% 13% 17% 7% 1	146269
Seeing	Day	28% 9% 20% 25% 14% 4	17627
	Night	34% 6% 14% 26% 17% 2	25879

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	42% 17% 3 8% 5% 4 4 3 3 3 8%	4200
	11th to 20th	Day	20% 9% 3 8% 5% 4 4 4 6% 8% 29%	4716
	21st to end	Day	31% 10% 3 7% 4 4 4 2 3 4 28%	5264
February	1st to 10th	Day	34% 12% 4 7% 5% 4 4 3 4 5% 17%	1890
	11th to 20th	Day	30% 14% 4 8% 5% 4 3 3 4 4 21%	1845
	21st to end	Day	32% 14% 4 8% 5% 4 4 3 3 4 20%	1636
March	1st to 10th	Day	24% 14% 4 9% 6% 5% 5% 4 5% 6% 19%	4695
	11th to 20th	Day	24% 14% 5% 8% 5% 4 4 3 3 5% 25%	5110
	21st to end	Day	29% 12% 3 10% 6% 6% 5% 4 6% 6% 13%	5824
April	1st to 10th	Day	26% 12% 4 9% 5% 5% 5% 4 6% 6% 18%	2520
	11th to 20th	Day	26% 11% 3 8% 5% 5% 5% 4 5% 5% 24%	2509
	21st to end	Day	23% 11% 3 10% 6% 5% 5% 4 6% 5% 22%	2801
May	1st to 10th	Day	14% 7% 2 7% 4 5% 4 4 5% 6% 40%	8195
	11th to 20th	Day	21% 10% 3 8% 5% 5% 5% 4 7% 8% 24%	8160
	21st to end	Day	23% 8% 3 7% 4 4 4 3 6% 9% 30%	8993
June	1st to 10th	Day	24% 8% 3 6% 5% 5% 4 3 5% 7% 29%	2994
	11th to 20th	Day	25% 9% 3 7% 6% 5% 5% 4 5% 6% 25%	3150
	21st to end	Day	15% 8% 3 8% 4 5% 4 4 6% 7% 36%	3150
July	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 7% 55%	8820
	11th to 20th	Day	5% 4 1 4 3 3 3 2 4 5% 66%	8442
	21st to end	Day	8% 5% 2 5% 4 4 4 3 5% 7% 53%	8916
August	1st to 10th	Day	7% 3 1 3 2 3 3 2 4 7% 63%	6402
	11th to 20th	Day	8% 6% 3 7% 5% 5% 4 3 6% 8% 45%	5923
	21st to end	Day	14% 7% 3 8% 4 4 5% 4 6% 7% 38%	6282
September	1st to 10th	Day	12% 5% 3 7% 4 4 3 3 4 7% 46%	2520
	11th to 20th	Day	16% 8% 3 8% 4 5% 4 3 5% 6% 39%	2520
	21st to end	Day	21% 7% 2 6% 5% 4 4 4 4 6% 38%	2226
October	1st to 10th	Day	18% 7% 2 6% 4 4 5% 4 6% 10% 33%	3380
	11th to 20th	Day	35% 9% 3 6% 4 3 3 2 3 6% 25%	3361
	21st to end	Day	30% 11% 4 9% 6% 5% 4 3 4 5% 18%	3392
November	1st to 10th	Day	38% 10% 3 6% 4 3 4 4 5% 6% 16%	1698
	11th to 20th	Day	38% 10% 3 7% 4 4 3 2 5% 4 21%	1680
	21st to end	Day	36% 13% 3 7% 4 4 2 3 5% 5% 19%	1680
December	1st to 10th	Day	27% 9% 3 6% 4 5% 5% 4 6% 7% 25%	2700
	11th to 20th	Day	37% 15% 4 9% 5% 3 4 3 3 5% 12%	3105
	21st to end	Day	34% 16% 5% 10% 5% 4 4 3 4 4 11%	3669
January	1st to 10th	Night	38% 16% 4 10% 5% 4 4 3 3 4 9%	8676
	11th to 20th	Night	22% 9% 5% 7% 4 4 4 3 5% 6% 30%	8182
	21st to end	Night	29% 9% 3 7% 5% 4 3 3 3 4 30%	8906
February	1st to 10th	Night	33% 13% 3 7% 5% 4 4 3 4 5% 19%	2730
	11th to 20th	Night	30% 14% 3 7% 5% 3 4 3 3 5% 23%	2662
	21st to end	Night	31% 14% 3 7% 4 4 4 3 4 5% 21%	2170
March	1st to 10th	Night	21% 16% 4 10% 6% 5% 5% 4 6% 5% 19%	5417
	11th to 20th	Night	28% 14% 3 8% 4 4 3 2 4 5% 27%	5109
	21st to end	Night	34% 15% 4 9% 5% 4 4 3 4 4 14%	5392
April	1st to 10th	Night	29% 12% 3 8% 4 5% 6% 3 5% 6% 19%	2100
	11th to 20th	Night	25% 10% 3 7% 5% 4 4 3 5% 5% 28%	2058
	21st to end	Night	26% 10% 3 8% 6% 5% 4 4 5% 6% 24%	1817
May	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 5% 4 44%	4724
	11th to 20th	Night	25% 10% 4 8% 5% 4 4 3 4 6% 27%	4700
	21st to end	Night	22% 8% 3 7% 4 5% 4 4 6% 7% 29%	5178
June	1st to 10th	Night	23% 8% 3 7% 5% 5% 5% 3 5% 6% 31%	1621
	11th to 20th	Night	26% 10% 3 7% 6% 5% 3 3 5% 5% 27%	1470
	21st to end	Night	17% 7% 2 5% 4 4 5% 4 6% 6% 40%	1470
July	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 4 55%	4140
	11th to 20th	Night	6% 3 1 3 2 2 2 2 4 5% 69%	4515
	21st to end	Night	9% 5% 2 5% 3 3 3 3 5% 6% 56%	5152
August	1st to 10th	Night	6% 2 1 2 2 2 2 2 4 5% 73%	3747
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 4 7% 53%	4142
	21st to end	Night	19% 7% 3 6% 3 3 3 2 3 5% 46%	4879
September	1st to 10th	Night	11% 6% 2 6% 3 3 3 3 5% 5% 53%	2100
	11th to 20th	Night	18% 8% 3 6% 4 4 3 3 4 5% 41%	2100
	21st to end	Night	20% 9% 3 7% 4 3 3 3 4 5% 40%	2389
October	1st to 10th	Night	17% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	4040
	11th to 20th	Night	30% 9% 2 6% 3 3 3 3 5% 6% 30%	4017
	21st to end	Night	34% 12% 3 8% 5% 4 3 2 4 5% 21%	4574
November	1st to 10th	Night	38% 10% 2 7% 4 4 4 3 4 5% 19%	2820
	11th to 20th	Night	40% 10% 3 6% 4 3 3 2 4 4 21%	2940
	21st to end	Night	37% 11% 3 6% 4 4 4 3 4 4 20%	2940
December	1st to 10th	Night	27% 10% 3 6% 4 3 4 4 6% 7% 25%	4700
	11th to 20th	Night	36% 15% 4 9% 5% 4 3 3 4 5% 13%	5320
	21st to end	Night	33% 18% 5% 10% 5% 5% 4 3 4 4 10%	7506

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	82% 4 6% 6% 2	4200
	11th to 20th	Day	50% 6% 11% 22% 11% 1	4740
	21st to end	Day	62% 4 6% 16% 12%	5265
February	1st to 10th	Day	70% 7% 9% 9% 4 1	1890
	11th to 20th	Day	70% 4 8% 9% 8%	1854
	21st to end	Day	70% 5% 10% 11% 5%	1636
March	1st to 10th	Day	64% 5% 13% 12% 5%	4695
	11th to 20th	Day	64% 6% 11% 17% 2	5110
	21st to end	Day	70% 8% 12% 9%	5824
April	1st to 10th	Day	68% 7% 16% 9% 1	2520
	11th to 20th	Day	63% 5% 13% 12% 6% 1	2509
	21st to end	Day	65% 7% 12% 11% 4	2801
May	1st to 10th	Day	44% 5% 20% 24% 7%	8195
	11th to 20th	Day	56% 9% 20% 12% 2	8163
	21st to end	Day	52% 10% 24% 13% 1	8994
June	1st to 10th	Day	57% 7% 17% 13% 5% 1	2994
	11th to 20th	Day	61% 8% 15% 13% 3	3150
	21st to end	Day	48% 9% 18% 18% 6% 1	3150
July	1st to 10th	Day	30% 6% 19% 22% 17% 6%	8746
	11th to 20th	Day	22% 5% 16% 30% 27% 2	8363
	21st to end	Day	30% 6% 23% 30% 10% 1	8723
August	1st to 10th	Day	23% 5% 15% 33% 21% 3	6385
	11th to 20th	Day	37% 9% 25% 22% 5% 1	5962
	21st to end	Day	43% 6% 17% 25% 8%	6281
September	1st to 10th	Day	39% 6% 16% 23% 11% 4	2520
	11th to 20th	Day	47% 7% 13% 19% 14% 2	2520
	21st to end	Day	49% 7% 18% 16% 9% 2	2226
October	1st to 10th	Day	44% 8% 17% 25% 5%	3380
	11th to 20th	Day	62% 4 9% 16% 8%	3361
	21st to end	Day	68% 5% 10% 8% 8%	3392
November	1st to 10th	Day	70% 5% 12% 9% 4	1698
	11th to 20th	Day	70% 5% 8% 8% 6% 2	1680
	21st to end	Day	70% 5% 7% 10% 8% 1	1680
December	1st to 10th	Day	57% 4 10% 16% 13%	2700
	11th to 20th	Day	77% 5% 8% 9% 1 1	3108
	21st to end	Day	77% 7% 8% 7% 2	3669
January	1st to 10th	Night	79% 5% 7% 6% 2	8676
	11th to 20th	Night	49% 5% 15% 18% 12% 1	8220
	21st to end	Night	59% 4 8% 20% 10%	8906
February	1st to 10th	Night	70% 5% 9% 9% 5% 1	2730
	11th to 20th	Night	66% 6% 10% 10% 8% 1	2664
	21st to end	Night	67% 6% 10% 13% 4	2170
March	1st to 10th	Night	62% 8% 13% 11% 5%	5417
	11th to 20th	Night	63% 5% 15% 16% 2	5110
	21st to end	Night	73% 8% 12% 7%	5392
April	1st to 10th	Night	66% 7% 13% 12% 1	2100
	11th to 20th	Night	59% 7% 14% 11% 8% 1	2058
	21st to end	Night	62% 6% 13% 12% 6% 1	1817
May	1st to 10th	Night	44% 4 17% 28% 7%	4721
	11th to 20th	Night	59% 8% 16% 16% 2	4700
	21st to end	Night	53% 9% 24% 12% 2	5182
June	1st to 10th	Night	55% 6% 17% 16% 5%	1621
	11th to 20th	Night	59% 7% 15% 15% 3	1470
	21st to end	Night	43% 9% 20% 20% 5% 3	1470
July	1st to 10th	Night	34% 5% 16% 23% 14% 9%	4105
	11th to 20th	Night	20% 4 14% 28% 33% 1	4471
	21st to end	Night	30% 6% 21% 30% 12% 2	5040
August	1st to 10th	Night	17% 4 15% 43% 19% 3	3739
	11th to 20th	Night	31% 10% 29% 22% 7% 1	4169
	21st to end	Night	42% 6% 18% 28% 6% 1	4879
September	1st to 10th	Night	34% 6% 17% 26% 13% 4	2100
	11th to 20th	Night	47% 5% 12% 19% 14% 3	2100
	21st to end	Night	48% 7% 13% 20% 9% 2	2389
October	1st to 10th	Night	45% 7% 16% 26% 4	4040
	11th to 20th	Night	57% 5% 12% 16% 10%	4017
	21st to end	Night	67% 5% 10% 11% 6%	4574
November	1st to 10th	Night	70% 5% 13% 9% 3 1	2820
	11th to 20th	Night	71% 4 9% 10% 4 2	2940
	21st to end	Night	69% 5% 8% 10% 8% 1	2940
December	1st to 10th	Night	57% 5% 10% 16% 11% 1	4700
	11th to 20th	Night	75% 6% 7% 9% 2	5316
	21st to end	Night	78% 7% 7% 6% 2	7506

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	44% 8% 15% 17% 10% 5%	220
	11th to 20th	Day	35% 10% 15% 17% 19% 4	220
	21st to end	Day	44% 8% 13% 17% 13% 4	268
February	1st to 10th	Day	43% 9% 20% 18% 8% 2	430
	11th to 20th	Day	43% 11% 21% 16% 8% 1	425
	21st to end	Day	41% 10% 20% 21% 8% 1	397
March	1st to 10th	Day	37% 12% 24% 16% 10% 1	540
	11th to 20th	Day	39% 11% 17% 21% 10% 2	540
	21st to end	Day	40% 13% 24% 15% 8% 1	594
April	1st to 10th	Day	34% 12% 24% 18% 10% 3	540
	11th to 20th	Day	30% 15% 23% 19% 10% 3	532
	21st to end	Day	29% 13% 26% 19% 11% 2	540
May	1st to 10th	Day	20% 13% 25% 27% 13% 2	543
	11th to 20th	Day	29% 10% 21% 23% 13% 4	568
	21st to end	Day	30% 8% 18% 28% 13% 3	617
June	1st to 10th	Day	25% 9% 25% 26% 12% 3	624
	11th to 20th	Day	24% 9% 27% 26% 12% 2	780
	21st to end	Day	17% 7% 22% 32% 18% 4	780
July	1st to 10th	Day	15% 3 23% 38% 19% 3	771
	11th to 20th	Day	11% 6% 22% 38% 19% 4	639
	21st to end	Day	10% 5% 18% 38% 23% 6%	619
August	1st to 10th	Day	13% 5% 15% 37% 22% 9%	567
	11th to 20th	Day	12% 3 16% 35% 26% 8%	557
	21st to end	Day	15% 7% 22% 33% 15% 8%	627
September	1st to 10th	Day	17% 6% 21% 34% 17% 6%	570
	11th to 20th	Day	19% 8% 19% 27% 22% 4	570
	21st to end	Day	22% 8% 23% 29% 15% 4	570
October	1st to 10th	Day	26% 7% 21% 24% 18% 4	570
	11th to 20th	Day	35% 5% 16% 23% 17% 3	570
	21st to end	Day	37% 12% 17% 20% 11% 3	470
November	1st to 10th	Day	44% 11% 17% 16% 11% 1	247
	11th to 20th	Day	49% 8% 10% 15% 13% 5%	220
	21st to end	Day	42% 8% 15% 16% 10% 9%	220
December	1st to 10th	Day	51% 5% 10% 10% 14% 9%	220
	11th to 20th	Day	51% 8% 17% 14% 9%	220
	21st to end	Day	44% 7% 17% 19% 10% 2	242
January	1st to 10th	Night	50% 7% 14% 18% 10% 1	933
	11th to 20th	Night	40% 7% 13% 25% 14% 1	840
	21st to end	Night	42% 5% 12% 27% 13% 1	924
February	1st to 10th	Night	41% 7% 16% 26% 8% 1	840
	11th to 20th	Night	40% 6% 13% 29% 11% 1	824
	21st to end	Night	41% 7% 16% 25% 10% 1	692
March	1st to 10th	Night	37% 10% 17% 23% 12% 1	840
	11th to 20th	Night	43% 8% 14% 22% 12% 1	840
	21st to end	Night	42% 10% 17% 22% 8% 1	924
April	1st to 10th	Night	37% 7% 15% 27% 13% 1	840
	11th to 20th	Night	33% 6% 18% 28% 13% 1	810
	21st to end	Night	33% 7% 19% 26% 13% 2	557
May	1st to 10th	Night	24% 5% 15% 33% 23%	420
	11th to 20th	Night	27% 5% 13% 32% 22% 1	414
	21st to end	Night	27% 5% 13% 30% 22% 3	455
June	1st to 10th	Night	27% 6% 17% 32% 15% 3	420
	11th to 20th	Night	32% 6% 14% 29% 18% 1	420
	21st to end	Night	22% 4 11% 34% 25% 4	420
July	1st to 10th	Night	15% 4 10% 34% 34% 3	410
	11th to 20th	Night	12% 1 13% 37% 34% 3	408
	21st to end	Night	12% 2 9% 30% 41% 6%	460
August	1st to 10th	Night	10% 1 6% 30% 45% 8%	430
	11th to 20th	Night	13% 2 6% 29% 40% 10%	624
	21st to end	Night	18% 3 11% 29% 34% 5%	824
September	1st to 10th	Night	16% 2 11% 34% 31% 6%	840
	11th to 20th	Night	23% 3 11% 26% 30% 7%	840
	21st to end	Night	26% 6% 11% 29% 25% 4	840
October	1st to 10th	Night	25% 4 13% 30% 25% 4	840
	11th to 20th	Night	34% 5% 15% 27% 18% 2	840
	21st to end	Night	40% 6% 16% 25% 12% 2	912
November	1st to 10th	Night	46% 7% 17% 22% 8%	820
	11th to 20th	Night	47% 5% 19% 19% 9% 1	840
	21st to end	Night	44% 5% 17% 21% 13% 1	840
December	1st to 10th	Night	39% 5% 15% 24% 14% 2	840
	11th to 20th	Night	51% 7% 13% 23% 5% 1	840
	21st to end	Night	50% 7% 15% 20% 7%	1018

Page generated at 2026-10-26 01:37:50. Version ac07

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