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 2025-07-06.

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	23% 10% 3 7% 5% 4 4 3 5% 6% 31%	124174
	Night	26% 11% 3 7% 4 4 4 3 4 5% 29%	123310
Transparency	Day	55% 6% 13% 17% 8% 1	123931
	Night	58% 6% 12% 15% 7% 1	123196
Seeing	Day	28% 9% 20% 25% 14% 4	15878
	Night	35% 6% 14% 26% 17% 2	24260

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	41% 16% 3 8% 5% 5% 4 3 3 3 9%	3240
	11th to 20th	Day	20% 10% 4 8% 5% 5% 5% 4 6% 8% 26%	3616
	21st to end	Day	33% 11% 3 7% 3 3 3 3 4 4 27%	4068
February	1st to 10th	Day	34% 12% 4 8% 5% 4 4 3 4 5% 17%	1800
	11th to 20th	Day	31% 14% 4 8% 5% 4 3 3 4 4 21%	1755
	21st to end	Day	33% 14% 4 8% 5% 4 4 2 3 4 20%	1560
March	1st to 10th	Day	24% 14% 4 9% 6% 5% 5% 4 5% 6% 19%	4594
	11th to 20th	Day	22% 14% 5% 8% 5% 4 4 3 3 5% 25%	5000
	21st to end	Day	29% 12% 3 10% 6% 6% 5% 4 6% 6% 13%	5698
April	1st to 10th	Day	27% 13% 4 9% 5% 5% 5% 4 6% 6% 16%	2400
	11th to 20th	Day	26% 10% 3 8% 5% 5% 4 4 5% 5% 24%	2388
	21st to end	Day	24% 11% 3 10% 6% 5% 5% 4 6% 5% 21%	2668
May	1st to 10th	Day	18% 8% 2 7% 4 5% 4 3 5% 6% 39%	6320
	11th to 20th	Day	22% 10% 3 8% 5% 5% 5% 4 6% 7% 25%	6277
	21st to end	Day	25% 9% 3 7% 4 4 4 3 5% 8% 28%	6903
June	1st to 10th	Day	24% 8% 3 6% 5% 5% 4 3 5% 7% 29%	2851
	11th to 20th	Day	26% 10% 3 8% 6% 5% 5% 4 5% 6% 22%	3000
	21st to end	Day	15% 8% 3 8% 4 4 4 4 6% 7% 36%	3000
July	1st to 10th	Day	8% 4 2 4 3 2 3 2 4 5% 62%	4816
	11th to 20th	Day	7% 3 1 3 3 2 3 2 3 4 68%	4530
	21st to end	Day	12% 4 2 5% 3 3 3 3 4 5% 55%	4790
August	1st to 10th	Day	7% 3 1 3 3 3 4 3 5% 8% 61%	4376
	11th to 20th	Day	10% 7% 3 8% 5% 5% 4 4 6% 8% 40%	4026
	21st to end	Day	16% 8% 3 8% 4 4 4 4 5% 6% 40%	4318
September	1st to 10th	Day	13% 5% 3 7% 4 4 3 3 4 6% 48%	2280
	11th to 20th	Day	16% 8% 3 8% 4 5% 4 3 5% 6% 38%	2280
	21st to end	Day	21% 7% 2 6% 5% 4 4 4 4 6% 38%	2015
October	1st to 10th	Day	19% 7% 2 6% 4 4 5% 4 6% 10% 33%	3180
	11th to 20th	Day	37% 9% 2 6% 4 3 3 2 3 6% 23%	3161
	21st to end	Day	30% 11% 4 9% 6% 5% 4 3 4 5% 18%	3213
November	1st to 10th	Day	39% 11% 3 6% 4 3 5% 4 5% 5% 16%	1616
	11th to 20th	Day	37% 10% 3 7% 4 4 4 2 5% 4 21%	1600
	21st to end	Day	36% 12% 3 7% 4 4 2 3 5% 5% 19%	1600
December	1st to 10th	Day	26% 9% 3 6% 4 5% 5% 4 6% 7% 26%	2620
	11th to 20th	Day	36% 15% 4 9% 5% 3 4 3 4 5% 12%	3025
	21st to end	Day	35% 17% 5% 10% 5% 4 4 3 4 4 11%	3590
January	1st to 10th	Night	38% 15% 3 10% 5% 4 4 3 3 4 10%	6700
	11th to 20th	Night	22% 10% 4 7% 4 4 4 3 5% 7% 29%	6282
	21st to end	Night	30% 10% 3 7% 4 4 3 3 3 4 30%	6890
February	1st to 10th	Night	34% 13% 3 7% 5% 4 4 3 4 5% 19%	2600
	11th to 20th	Night	32% 15% 3 7% 4 3 4 3 3 4 22%	2532
	21st to end	Night	31% 14% 3 7% 4 3 4 3 5% 5% 22%	2070
March	1st to 10th	Night	20% 16% 4 10% 6% 5% 5% 4 6% 5% 19%	5298
	11th to 20th	Night	26% 14% 3 8% 4 4 3 2 4 5% 27%	4999
	21st to end	Night	35% 15% 4 8% 5% 4 4 3 4 4 14%	5276
April	1st to 10th	Night	29% 13% 3 9% 5% 5% 6% 3 5% 6% 17%	2000
	11th to 20th	Night	25% 9% 3 7% 5% 4 4 3 5% 5% 29%	1959
	21st to end	Night	27% 10% 4 8% 6% 5% 4 4 5% 6% 22%	1730
May	1st to 10th	Night	19% 9% 3 7% 4 3 3 2 4 4 42%	3640
	11th to 20th	Night	26% 10% 4 7% 4 3 3 3 4 5% 31%	3616
	21st to end	Night	26% 9% 3 8% 4 5% 4 4 5% 7% 25%	3972
June	1st to 10th	Night	23% 8% 3 7% 4 5% 4 3 5% 6% 32%	1544
	11th to 20th	Night	28% 10% 3 7% 6% 5% 3 2 5% 5% 25%	1400
	21st to end	Night	17% 7% 2 5% 4 4 5% 4 5% 6% 41%	1400
July	1st to 10th	Night	10% 5% 1 4 3 3 2 2 3 3 63%	2255
	11th to 20th	Night	6% 2 1 2 2 2 2 2 4 5% 72%	2427
	21st to end	Night	13% 5% 2 4 3 3 3 2 4 5% 56%	2762
August	1st to 10th	Night	5% 2 1 3 3 3 2 2 5% 5% 69%	2552
	11th to 20th	Night	15% 7% 3 5% 3 3 3 3 4 5% 50%	2812
	21st to end	Night	19% 6% 3 6% 3 3 3 2 3 5% 47%	3356
September	1st to 10th	Night	12% 6% 2 6% 3 3 3 3 4 5% 54%	1900
	11th to 20th	Night	19% 7% 3 6% 4 4 3 3 4 5% 40%	1900
	21st to end	Night	21% 9% 3 7% 4 3 3 3 4 5% 40%	2161
October	1st to 10th	Night	17% 7% 2 7% 5% 5% 4 4 5% 7% 35%	3800
	11th to 20th	Night	31% 9% 2 6% 3 3 3 3 5% 6% 29%	3777
	21st to end	Night	35% 12% 3 7% 5% 4 3 2 3 5% 20%	4335
November	1st to 10th	Night	38% 10% 2 7% 4 4 4 3 4 5% 19%	2682
	11th to 20th	Night	40% 9% 3 6% 4 3 3 2 4 4 22%	2800
	21st to end	Night	37% 10% 3 6% 4 4 4 4 4 4 20%	2800
December	1st to 10th	Night	26% 10% 3 6% 4 3 5% 4 6% 7% 26%	4560
	11th to 20th	Night	35% 15% 4 9% 5% 4 3 3 4 5% 13%	5180
	21st to end	Night	33% 18% 5% 10% 5% 5% 4 3 4 4 10%	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	79% 4 6% 8% 2	3240
	11th to 20th	Day	54% 6% 11% 20% 7% 1	3640
	21st to end	Day	63% 4 7% 15% 11%	4068
February	1st to 10th	Day	71% 7% 9% 9% 3 1	1800
	11th to 20th	Day	70% 4 9% 8% 8%	1764
	21st to end	Day	70% 5% 10% 11% 5%	1560
March	1st to 10th	Day	64% 6% 13% 12% 5%	4594
	11th to 20th	Day	64% 6% 11% 17% 2	5000
	21st to end	Day	71% 8% 12% 9%	5698
April	1st to 10th	Day	70% 7% 15% 8% 1	2400
	11th to 20th	Day	63% 5% 13% 13% 6% 1	2388
	21st to end	Day	66% 7% 12% 10% 4	2668
May	1st to 10th	Day	47% 5% 18% 23% 8%	6320
	11th to 20th	Day	57% 9% 19% 13% 2	6280
	21st to end	Day	56% 9% 21% 13% 1	6904
June	1st to 10th	Day	57% 7% 18% 13% 5% 1	2851
	11th to 20th	Day	64% 8% 15% 11% 2	3000
	21st to end	Day	47% 9% 19% 18% 6% 1	3000
July	1st to 10th	Day	27% 5% 10% 22% 25% 11%	4742
	11th to 20th	Day	22% 5% 9% 30% 32% 2	4451
	21st to end	Day	31% 5% 17% 32% 13% 2	4639
August	1st to 10th	Day	24% 7% 14% 39% 16% 1	4359
	11th to 20th	Day	42% 10% 19% 21% 7% 2	4065
	21st to end	Day	44% 5% 14% 25% 11% 1	4317
September	1st to 10th	Day	39% 5% 16% 23% 12% 5%	2280
	11th to 20th	Day	48% 7% 12% 17% 14% 2	2280
	21st to end	Day	49% 7% 17% 15% 10% 2	2015
October	1st to 10th	Day	44% 7% 17% 26% 5%	3180
	11th to 20th	Day	63% 4 9% 15% 8%	3161
	21st to end	Day	68% 5% 10% 8% 8%	3213
November	1st to 10th	Day	71% 5% 12% 8% 4	1616
	11th to 20th	Day	70% 5% 8% 8% 7% 2	1600
	21st to end	Day	69% 5% 7% 10% 8% 1	1600
December	1st to 10th	Day	56% 4 10% 16% 13%	2620
	11th to 20th	Day	77% 5% 8% 9% 1 1	3028
	21st to end	Day	77% 7% 8% 6% 2	3590
January	1st to 10th	Night	76% 6% 8% 7% 3 1	6700
	11th to 20th	Night	52% 6% 14% 19% 8% 1	6320
	21st to end	Night	60% 5% 9% 17% 10%	6890
February	1st to 10th	Night	70% 5% 9% 9% 5% 1	2600
	11th to 20th	Night	67% 6% 10% 9% 7% 1	2534
	21st to end	Night	67% 6% 10% 13% 4	2070
March	1st to 10th	Night	62% 8% 14% 11% 5%	5298
	11th to 20th	Night	62% 5% 15% 16% 2 1	5000
	21st to end	Night	73% 8% 12% 7%	5276
April	1st to 10th	Night	68% 8% 13% 11% 1	2000
	11th to 20th	Night	59% 7% 14% 11% 8% 1	1959
	21st to end	Night	63% 6% 12% 12% 5% 1	1730
May	1st to 10th	Night	48% 3 16% 25% 8%	3637
	11th to 20th	Night	57% 7% 17% 16% 3	3616
	21st to end	Night	59% 10% 19% 11% 1	3972
June	1st to 10th	Night	54% 6% 18% 16% 5%	1544
	11th to 20th	Night	61% 7% 15% 14% 3	1400
	21st to end	Night	44% 9% 20% 20% 5% 3	1400
July	1st to 10th	Night	27% 4 10% 22% 20% 17%	2220
	11th to 20th	Night	18% 3 11% 26% 39% 2	2383
	21st to end	Night	32% 5% 14% 28% 20% 2	2674
August	1st to 10th	Night	18% 5% 16% 41% 18% 2	2544
	11th to 20th	Night	37% 9% 21% 22% 9% 2	2839
	21st to end	Night	42% 5% 15% 29% 9%	3356
September	1st to 10th	Night	34% 6% 15% 27% 14% 4	1900
	11th to 20th	Night	48% 5% 11% 18% 14% 3	1900
	21st to end	Night	48% 7% 14% 20% 10% 2	2161
October	1st to 10th	Night	46% 7% 16% 27% 4	3800
	11th to 20th	Night	58% 4 12% 16% 9%	3777
	21st to end	Night	67% 5% 10% 11% 6%	4335
November	1st to 10th	Night	69% 5% 13% 9% 3 1	2682
	11th to 20th	Night	70% 4 9% 10% 5% 2	2800
	21st to end	Night	69% 5% 8% 9% 8% 1	2800
December	1st to 10th	Night	56% 5% 10% 17% 11% 1	4560
	11th to 20th	Night	75% 6% 7% 9% 2	5176
	21st to end	Night	78% 7% 7% 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	44% 7% 15% 18% 11% 6%	200
	11th to 20th	Day	37% 12% 16% 17% 18% 3	200
	21st to end	Day	46% 8% 14% 16% 12% 5%	245
February	1st to 10th	Day	44% 9% 21% 18% 7% 1	400
	11th to 20th	Day	44% 11% 21% 15% 8% 1	395
	21st to end	Day	40% 9% 20% 22% 8% 1	369
March	1st to 10th	Day	35% 11% 25% 17% 10% 1	500
	11th to 20th	Day	35% 12% 17% 22% 11% 3	500
	21st to end	Day	40% 14% 23% 14% 8% 1	550
April	1st to 10th	Day	36% 12% 24% 17% 9% 2	500
	11th to 20th	Day	29% 16% 24% 17% 10% 3	492
	21st to end	Day	29% 13% 25% 19% 11% 2	500
May	1st to 10th	Day	21% 13% 25% 27% 13% 2	503
	11th to 20th	Day	29% 10% 20% 24% 13% 4	528
	21st to end	Day	31% 9% 18% 27% 12% 3	573
June	1st to 10th	Day	25% 8% 24% 27% 13% 3	581
	11th to 20th	Day	25% 9% 26% 26% 12% 2	730
	21st to end	Day	17% 6% 22% 32% 19% 4	730
July	1st to 10th	Day	14% 3 23% 38% 19% 3	697
	11th to 20th	Day	11% 6% 20% 38% 21% 4	551
	21st to end	Day	10% 4 18% 39% 23% 5%	531
August	1st to 10th	Day	12% 4 14% 38% 22% 9%	487
	11th to 20th	Day	12% 3 17% 35% 25% 8%	477
	21st to end	Day	15% 8% 23% 32% 15% 7%	539
September	1st to 10th	Day	18% 6% 22% 34% 15% 5%	490
	11th to 20th	Day	20% 8% 20% 28% 21% 3	490
	21st to end	Day	22% 8% 22% 29% 14% 4	490
October	1st to 10th	Day	28% 7% 20% 23% 18% 4	490
	11th to 20th	Day	37% 5% 16% 21% 18% 4	490
	21st to end	Day	38% 11% 17% 20% 11% 4	405
November	1st to 10th	Day	44% 10% 18% 16% 10% 1	225
	11th to 20th	Day	48% 9% 11% 14% 14% 6%	200
	21st to end	Day	41% 8% 16% 15% 11% 10%	200
December	1st to 10th	Day	49% 5% 11% 11% 16% 10%	200
	11th to 20th	Day	48% 9% 18% 15% 10% 1	200
	21st to end	Day	46% 8% 17% 19% 9% 1	220
January	1st to 10th	Night	50% 7% 14% 18% 11% 1	885
	11th to 20th	Night	41% 7% 13% 25% 12% 1	800
	21st to end	Night	42% 5% 13% 27% 13% 1	880
February	1st to 10th	Night	42% 7% 16% 26% 8% 1	800
	11th to 20th	Night	42% 6% 13% 29% 10% 1	784
	21st to end	Night	40% 7% 16% 26% 10% 1	660
March	1st to 10th	Night	36% 10% 17% 24% 12% 1	800
	11th to 20th	Night	41% 9% 14% 22% 13% 1	800
	21st to end	Night	43% 9% 17% 21% 9% 1	880
April	1st to 10th	Night	38% 7% 15% 27% 12% 1	800
	11th to 20th	Night	33% 6% 19% 29% 12% 1	771
	21st to end	Night	34% 7% 19% 24% 13% 2	530
May	1st to 10th	Night	26% 5% 15% 34% 21%	400
	11th to 20th	Night	27% 5% 12% 32% 22% 2	394
	21st to end	Night	28% 5% 13% 29% 22% 3	433
June	1st to 10th	Night	27% 5% 17% 32% 16% 4	400
	11th to 20th	Night	34% 6% 13% 29% 17% 1	400
	21st to end	Night	22% 4 12% 34% 25% 4	400
July	1st to 10th	Night	15% 4 10% 35% 33% 3	380
	11th to 20th	Night	12% 1 14% 34% 36% 3	368
	21st to end	Night	12% 3 9% 31% 41% 4	416
August	1st to 10th	Night	10% 2 6% 29% 46% 8%	388
	11th to 20th	Night	12% 2 6% 29% 40% 10%	564
	21st to end	Night	18% 3 12% 31% 33% 5%	746
September	1st to 10th	Night	17% 2 11% 35% 30% 6%	760
	11th to 20th	Night	24% 3 11% 25% 30% 6%	760
	21st to end	Night	27% 6% 10% 28% 26% 4	760
October	1st to 10th	Night	26% 4 13% 28% 25% 3	760
	11th to 20th	Night	35% 4 15% 25% 18% 3	760
	21st to end	Night	40% 6% 15% 25% 12% 2	836
November	1st to 10th	Night	45% 7% 17% 22% 8%	780
	11th to 20th	Night	47% 4 20% 19% 9% 2	800
	21st to end	Night	43% 5% 16% 20% 13% 2	800
December	1st to 10th	Night	37% 6% 16% 25% 14% 3	800
	11th to 20th	Night	49% 7% 14% 24% 6% 1	800
	21st to end	Night	51% 7% 15% 19% 7%	965

Page generated at 2025-07-02 22:30:14. Version ac07

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