Issaquah
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Issaquah clear sky chart from 2005-11-06 to 2025-05-11.

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	22% 10% 3 7% 5% 4 4 3 5% 6% 31%	118314
	Night	25% 11% 3 7% 4 4 4 3 4 5% 29%	119226
Transparency	Day	55% 6% 12% 17% 8% 1	118073
	Night	58% 6% 11% 16% 8% 1	119113
Seeing	Day	28% 9% 18% 26% 15% 4	15571
	Night	33% 6% 12% 26% 19% 3	24066

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% 14% 4 8% 5% 4 4 3 5% 4 9%	3477
	11th to 20th	Day	19% 10% 4 9% 6% 5% 5% 4 5% 7% 26%	3616
	21st to end	Day	30% 11% 3 8% 4 3 3 2 4 4 29%	4096
February	1st to 10th	Day	33% 13% 4 8% 6% 4 4 3 4 7% 15%	1800
	11th to 20th	Day	32% 14% 4 9% 5% 4 3 3 4 5% 19%	1755
	21st to end	Day	30% 15% 4 8% 5% 5% 4 3 3 4 19%	1565
March	1st to 10th	Day	21% 14% 5% 10% 6% 6% 5% 4 5% 7% 17%	4598
	11th to 20th	Day	21% 16% 5% 10% 6% 4 4 3 4 4 25%	5000
	21st to end	Day	27% 13% 4 10% 7% 6% 6% 5% 5% 5% 12%	5695
April	1st to 10th	Day	27% 14% 3 8% 6% 4 5% 4 6% 6% 16%	2400
	11th to 20th	Day	23% 12% 3 7% 6% 5% 5% 4 5% 6% 25%	2379
	21st to end	Day	23% 11% 4 10% 5% 5% 4 4 6% 7% 21%	2655
May	1st to 10th	Day	19% 9% 2 7% 5% 4 5% 3 5% 7% 34%	4560
	11th to 20th	Day	20% 7% 2 6% 4 4 3 4 5% 9% 36%	4380
	21st to end	Day	30% 11% 3 10% 5% 5% 4 3 5% 6% 18%	4813
June	1st to 10th	Day	23% 9% 3 7% 6% 5% 4 4 6% 6% 26%	2661
	11th to 20th	Day	25% 11% 4 8% 6% 5% 5% 4 6% 6% 20%	2850
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 6% 6% 37%	2850
July	1st to 10th	Day	8% 5% 2 4 3 3 3 3 3 4 62%	4740
	11th to 20th	Day	7% 3 1 3 3 2 3 3 4 5% 66%	4462
	21st to end	Day	12% 5% 2 5% 3 3 3 3 4 6% 55%	4790
August	1st to 10th	Day	7% 3 2 4 3 3 3 3 5% 7% 61%	4347
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 4 7% 8% 38%	4026
	21st to end	Day	18% 9% 3 6% 4 4 3 3 4 6% 40%	4313
September	1st to 10th	Day	12% 6% 2 6% 3 4 3 3 5% 6% 49%	2280
	11th to 20th	Day	16% 9% 3 7% 5% 4 5% 4 5% 7% 37%	2280
	21st to end	Day	19% 9% 2 8% 4 4 4 3 5% 5% 37%	2015
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 10% 36%	3180
	11th to 20th	Day	31% 8% 3 7% 4 4 4 4 5% 6% 25%	3161
	21st to end	Day	31% 10% 4 9% 4 5% 5% 3 5% 5% 18%	3236
November	1st to 10th	Day	37% 10% 3 8% 4 4 3 4 4 6% 17%	1626
	11th to 20th	Day	34% 12% 3 8% 5% 4 3 3 4 4 21%	1600
	21st to end	Day	34% 11% 3 8% 5% 4 3 2 5% 6% 21%	1600
December	1st to 10th	Day	25% 8% 3 6% 4 4 5% 4 4 6% 31%	2620
	11th to 20th	Day	35% 13% 4 9% 5% 5% 4 3 4 5% 13%	3025
	21st to end	Day	34% 14% 4 9% 6% 5% 4 3 3 4 13%	3863
January	1st to 10th	Night	38% 15% 4 10% 5% 4 4 3 3 4 10%	6407
	11th to 20th	Night	20% 11% 4 7% 4 4 4 4 5% 7% 30%	6282
	21st to end	Night	27% 10% 3 7% 5% 4 3 3 4 5% 29%	6847
February	1st to 10th	Night	32% 13% 4 8% 6% 4 4 4 3 5% 17%	2600
	11th to 20th	Night	30% 13% 4 8% 4 3 3 3 5% 5% 22%	2532
	21st to end	Night	31% 15% 3 7% 4 4 4 2 5% 5% 19%	2065
March	1st to 10th	Night	19% 15% 4 9% 6% 5% 5% 5% 6% 7% 20%	5293
	11th to 20th	Night	24% 14% 3 7% 4 4 4 3 4 5% 28%	4999
	21st to end	Night	33% 14% 3 9% 5% 5% 5% 3 5% 5% 13%	5280
April	1st to 10th	Night	26% 12% 3 9% 6% 5% 5% 5% 5% 6% 18%	2000
	11th to 20th	Night	20% 11% 3 8% 5% 5% 5% 3 5% 6% 28%	1969
	21st to end	Night	24% 12% 3 8% 5% 5% 4 3 5% 6% 24%	1745
May	1st to 10th	Night	20% 9% 3 7% 5% 3 3 3 4 4 39%	2620
	11th to 20th	Night	17% 9% 3 7% 5% 4 5% 4 5% 8% 34%	2519
	21st to end	Night	26% 14% 4 10% 5% 5% 5% 3 4 4 20%	2762
June	1st to 10th	Night	19% 9% 4 9% 6% 5% 4 3 4 6% 30%	1514
	11th to 20th	Night	21% 12% 3 10% 6% 5% 4 3 5% 6% 25%	1330
	21st to end	Night	14% 6% 3 8% 4 4 4 3 6% 6% 43%	1330
July	1st to 10th	Night	8% 5% 2 5% 3 3 2 3 3 4 61%	2220
	11th to 20th	Night	5% 3 1 3 2 2 2 2 3 4 72%	2494
	21st to end	Night	12% 5% 2 4 3 3 3 2 4 5% 58%	2762
August	1st to 10th	Night	5% 3 2 3 2 2 2 2 5% 6% 68%	2582
	11th to 20th	Night	12% 6% 3 6% 3 4 4 3 4 7% 49%	2812
	21st to end	Night	20% 7% 2 5% 3 3 2 2 3 5% 48%	3360
September	1st to 10th	Night	10% 5% 2 6% 4 3 4 2 4 6% 53%	1900
	11th to 20th	Night	18% 8% 3 6% 3 3 4 4 4 5% 41%	1900
	21st to end	Night	19% 8% 3 7% 4 4 3 3 5% 5% 40%	2161
October	1st to 10th	Night	18% 7% 2 7% 5% 4 4 3 6% 9% 37%	3800
	11th to 20th	Night	29% 8% 3 6% 3 4 4 3 5% 5% 29%	3777
	21st to end	Night	32% 13% 3 7% 4 4 4 3 4 5% 21%	4325
November	1st to 10th	Night	34% 10% 3 8% 4 4 4 3 4 5% 20%	2668
	11th to 20th	Night	35% 10% 3 8% 5% 3 3 2 4 4 21%	2800
	21st to end	Night	32% 11% 3 9% 5% 4 4 3 4 5% 21%	2800
December	1st to 10th	Night	24% 10% 2 6% 5% 4 4 3 4 6% 31%	4560
	11th to 20th	Night	35% 15% 4 10% 4 5% 4 3 3 4 13%	5180
	21st to end	Night	32% 17% 4 10% 5% 5% 4 3 4 4 12%	7031

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% 5% 6% 9% 3	3477
	11th to 20th	Day	55% 7% 12% 18% 8% 1	3640
	21st to end	Day	60% 4 7% 17% 12%	4096
February	1st to 10th	Day	72% 4 10% 9% 4 1	1800
	11th to 20th	Day	71% 5% 7% 10% 7% 1	1764
	21st to end	Day	71% 4 9% 12% 4	1565
March	1st to 10th	Day	64% 6% 13% 11% 5%	4598
	11th to 20th	Day	63% 6% 12% 18% 1	5000
	21st to end	Day	70% 9% 11% 9% 1	5695
April	1st to 10th	Day	70% 7% 14% 8% 1	2400
	11th to 20th	Day	62% 6% 12% 14% 6% 1	2379
	21st to end	Day	66% 6% 12% 11% 4 1	2655
May	1st to 10th	Day	51% 5% 15% 19% 10%	4560
	11th to 20th	Day	48% 7% 24% 18% 3	4383
	21st to end	Day	68% 7% 16% 8% 1	4814
June	1st to 10th	Day	61% 7% 15% 12% 5%	2661
	11th to 20th	Day	66% 6% 14% 11% 3	2850
	21st to end	Day	47% 8% 17% 19% 8% 1	2850
July	1st to 10th	Day	27% 6% 8% 21% 22% 15%	4666
	11th to 20th	Day	24% 4 7% 28% 35% 3	4385
	21st to end	Day	30% 6% 14% 33% 14% 2	4639
August	1st to 10th	Day	23% 6% 11% 40% 18% 2	4330
	11th to 20th	Day	41% 10% 19% 20% 7% 2	4065
	21st to end	Day	45% 5% 12% 26% 11% 1	4312
September	1st to 10th	Day	36% 6% 14% 26% 12% 5%	2280
	11th to 20th	Day	48% 7% 11% 18% 14% 2	2280
	21st to end	Day	50% 5% 16% 18% 10% 2	2015
October	1st to 10th	Day	43% 6% 13% 30% 7% 1	3180
	11th to 20th	Day	60% 5% 10% 16% 8% 1	3161
	21st to end	Day	65% 8% 10% 9% 8% 1	3236
November	1st to 10th	Day	70% 5% 10% 10% 4	1626
	11th to 20th	Day	70% 5% 8% 9% 6% 2	1600
	21st to end	Day	69% 5% 5% 11% 8% 2	1600
December	1st to 10th	Day	55% 5% 7% 18% 15% 1	2620
	11th to 20th	Day	73% 6% 9% 10% 2	3028
	21st to end	Day	75% 6% 9% 8% 2	3863
January	1st to 10th	Night	77% 6% 5% 8% 3 1	6407
	11th to 20th	Night	52% 6% 13% 20% 8% 1	6320
	21st to end	Night	58% 5% 9% 17% 10% 1	6847
February	1st to 10th	Night	71% 5% 10% 9% 5% 1	2600
	11th to 20th	Night	65% 7% 9% 10% 7% 2	2534
	21st to end	Night	68% 5% 10% 12% 4	2065
March	1st to 10th	Night	61% 7% 14% 13% 5%	5293
	11th to 20th	Night	59% 6% 15% 17% 2	5000
	21st to end	Night	71% 8% 12% 8%	5280
April	1st to 10th	Night	67% 8% 13% 10% 2	2000
	11th to 20th	Night	59% 6% 13% 13% 8% 1	1969
	21st to end	Night	63% 7% 12% 12% 5% 1	1745
May	1st to 10th	Night	50% 5% 12% 24% 8%	2620
	11th to 20th	Night	49% 8% 21% 18% 4 1	2519
	21st to end	Night	70% 7% 14% 8% 1	2762
June	1st to 10th	Night	59% 5% 16% 15% 5%	1514
	11th to 20th	Night	64% 6% 15% 12% 2	1330
	21st to end	Night	44% 7% 20% 20% 7% 2	1330
July	1st to 10th	Night	29% 6% 10% 19% 16% 20%	2185
	11th to 20th	Night	20% 3 8% 27% 40% 1	2448
	21st to end	Night	30% 6% 13% 30% 18% 2	2674
August	1st to 10th	Night	20% 4 11% 40% 22% 2	2574
	11th to 20th	Night	37% 10% 21% 22% 8% 2	2839
	21st to end	Night	42% 4 13% 30% 9% 1	3360
September	1st to 10th	Night	34% 6% 16% 27% 14% 4	1900
	11th to 20th	Night	47% 7% 10% 19% 13% 4	1900
	21st to end	Night	48% 7% 13% 20% 11% 2	2161
October	1st to 10th	Night	44% 6% 15% 29% 6% 1	3800
	11th to 20th	Night	58% 5% 10% 17% 9% 1	3777
	21st to end	Night	64% 7% 10% 11% 8%	4325
November	1st to 10th	Night	67% 6% 12% 11% 3 1	2668
	11th to 20th	Night	69% 5% 8% 10% 5% 2	2800
	21st to end	Night	67% 7% 6% 10% 9% 1	2800
December	1st to 10th	Night	55% 5% 8% 17% 14% 1	4560
	11th to 20th	Night	74% 5% 8% 10% 2	5176
	21st to end	Night	74% 9% 9% 7% 2	7031

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% 9% 14% 19% 10% 6%	200
	11th to 20th	Day	31% 15% 15% 20% 15% 5%	200
	21st to end	Day	44% 10% 12% 16% 13% 5%	260
February	1st to 10th	Day	43% 9% 21% 17% 9% 2	400
	11th to 20th	Day	45% 10% 20% 15% 8% 2	395
	21st to end	Day	40% 12% 20% 20% 6% 1	371
March	1st to 10th	Day	35% 15% 20% 19% 9% 2	500
	11th to 20th	Day	35% 12% 19% 19% 11% 4	500
	21st to end	Day	39% 11% 21% 19% 9% 1	550
April	1st to 10th	Day	37% 13% 21% 16% 12% 2	500
	11th to 20th	Day	29% 14% 25% 19% 9% 4	492
	21st to end	Day	31% 10% 23% 23% 10% 2	500
May	1st to 10th	Day	22% 11% 23% 30% 12% 2	503
	11th to 20th	Day	30% 8% 16% 25% 15% 6%	489
	21st to end	Day	30% 8% 21% 25% 13% 3	529
June	1st to 10th	Day	30% 7% 21% 25% 14% 4	491
	11th to 20th	Day	30% 10% 20% 26% 11% 2	680
	21st to end	Day	15% 5% 23% 33% 19% 4	680
July	1st to 10th	Day	15% 5% 18% 35% 23% 3	671
	11th to 20th	Day	11% 4 17% 38% 25% 4	511
	21st to end	Day	10% 5% 15% 40% 25% 5%	531
August	1st to 10th	Day	13% 5% 15% 36% 23% 9%	487
	11th to 20th	Day	9% 3 15% 38% 28% 8%	477
	21st to end	Day	18% 6% 20% 32% 17% 7%	539
September	1st to 10th	Day	15% 7% 20% 38% 16% 4	490
	11th to 20th	Day	21% 7% 17% 31% 19% 5%	490
	21st to end	Day	22% 8% 19% 30% 17% 4	490
October	1st to 10th	Day	27% 7% 18% 21% 20% 6%	490
	11th to 20th	Day	34% 8% 13% 24% 14% 6%	490
	21st to end	Day	36% 10% 18% 20% 11% 4	415
November	1st to 10th	Day	43% 10% 13% 17% 13% 3	230
	11th to 20th	Day	44% 12% 14% 9% 17% 6%	200
	21st to end	Day	37% 10% 14% 20% 11% 9%	200
December	1st to 10th	Day	44% 6% 12% 16% 10% 13%	200
	11th to 20th	Day	46% 11% 15% 20% 9% 1	200
	21st to end	Day	41% 13% 11% 21% 12% 2	220
January	1st to 10th	Night	49% 8% 15% 16% 11% 1	822
	11th to 20th	Night	40% 9% 13% 24% 13% 2	800
	21st to end	Night	39% 6% 13% 29% 13%	880
February	1st to 10th	Night	41% 7% 17% 24% 10% 1	800
	11th to 20th	Night	40% 7% 13% 26% 13% 1	784
	21st to end	Night	43% 7% 15% 24% 10% 2	660
March	1st to 10th	Night	35% 9% 16% 25% 14% 2	800
	11th to 20th	Night	35% 9% 17% 26% 11% 2	800
	21st to end	Night	39% 8% 16% 25% 10% 2	880
April	1st to 10th	Night	35% 9% 16% 25% 13% 2	800
	11th to 20th	Night	29% 8% 16% 31% 13% 2	781
	21st to end	Night	34% 9% 12% 26% 16% 3	545
May	1st to 10th	Night	24% 8% 14% 30% 22% 2	400
	11th to 20th	Night	24% 5% 8% 30% 27% 5%	375
	21st to end	Night	30% 4 11% 23% 25% 6%	411
June	1st to 10th	Night	26% 5% 12% 27% 27% 3	380
	11th to 20th	Night	31% 8% 12% 27% 19% 3	380
	21st to end	Night	17% 4 11% 29% 34% 5%	380
July	1st to 10th	Night	15% 4 8% 30% 38% 5%	370
	11th to 20th	Night	12% 2 6% 30% 44% 5%	368
	21st to end	Night	10% 1 7% 26% 47% 8%	416
August	1st to 10th	Night	13% 3 4 23% 46% 11%	405
	11th to 20th	Night	11% 2 5% 24% 43% 14%	564
	21st to end	Night	20% 4 6% 25% 39% 7%	750
September	1st to 10th	Night	14% 3 7% 35% 34% 6%	760
	11th to 20th	Night	24% 3 7% 25% 33% 8%	760
	21st to end	Night	24% 5% 10% 28% 29% 4	760
October	1st to 10th	Night	26% 4 8% 30% 28% 4	760
	11th to 20th	Night	33% 6% 11% 25% 19% 6%	760
	21st to end	Night	37% 8% 12% 28% 12% 3	836
November	1st to 10th	Night	40% 8% 16% 27% 9%	780
	11th to 20th	Night	43% 9% 14% 22% 11% 1	800
	21st to end	Night	40% 9% 14% 24% 13% 2	800
December	1st to 10th	Night	37% 5% 15% 26% 15% 2	800
	11th to 20th	Night	46% 8% 16% 22% 7% 2	800
	21st to end	Night	50% 9% 16% 19% 6%	899

Page generated at 2025-05-08 06:33:55. Version ac07

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