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 2026-09-14.

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% 34%	154309
	Night	23% 10% 3 7% 4 4 4 3 4 5% 31%	145374
Transparency	Day	51% 7% 14% 18% 9% 1	154027
	Night	55% 6% 12% 17% 8% 1	145238
Seeing	Day	27% 9% 19% 26% 15% 4	17388
	Night	32% 7% 13% 26% 19% 3	25607

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% 16% 4 9% 5% 4 4 3 5% 4 7%	4521
	11th to 20th	Day	19% 8% 3 9% 6% 5% 5% 4 5% 6% 30%	4716
	21st to end	Day	29% 11% 3 8% 4 3 3 2 4 5% 29%	5306
February	1st to 10th	Day	32% 12% 4 8% 6% 4 4 4 5% 7% 15%	1890
	11th to 20th	Day	31% 14% 4 9% 5% 4 3 3 4 5% 19%	1845
	21st to end	Day	30% 15% 4 9% 5% 5% 4 3 3 4 18%	1641
March	1st to 10th	Day	21% 14% 5% 10% 6% 6% 5% 4 5% 7% 17%	4700
	11th to 20th	Day	23% 15% 4 10% 6% 4 4 3 4 4 25%	5110
	21st to end	Day	27% 13% 4 10% 7% 6% 6% 5% 5% 5% 12%	5820
April	1st to 10th	Day	26% 14% 3 8% 6% 4 5% 4 6% 6% 17%	2520
	11th to 20th	Day	23% 12% 3 8% 6% 5% 5% 5% 5% 6% 24%	2499
	21st to end	Day	23% 11% 4 10% 5% 5% 4 4 6% 7% 21%	2788
May	1st to 10th	Day	13% 8% 2 7% 4 4 5% 4 6% 8% 40%	8195
	11th to 20th	Day	22% 10% 3 8% 5% 5% 5% 4 6% 8% 24%	8160
	21st to end	Day	22% 8% 2 7% 4 4 4 4 6% 8% 30%	8993
June	1st to 10th	Day	22% 9% 3 7% 6% 5% 5% 4 6% 6% 27%	2941
	11th to 20th	Day	23% 10% 3 8% 6% 5% 5% 4 6% 6% 23%	3150
	21st to end	Day	14% 8% 3 8% 6% 4 5% 4 6% 7% 36%	3150
July	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 4 6% 56%	8820
	11th to 20th	Day	5% 4 2 4 3 2 3 3 4 5% 65%	8304
	21st to end	Day	8% 5% 2 5% 3 3 4 3 5% 7% 55%	8916
August	1st to 10th	Day	6% 3 2 4 3 2 3 3 4 6% 64%	6359
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	5923
	21st to end	Day	15% 8% 3 6% 4 4 4 3 5% 8% 39%	6263
September	1st to 10th	Day	12% 6% 3 6% 4 4 4 3 5% 6% 47%	2520
	11th to 20th	Day	15% 9% 2 7% 5% 4 4 4 5% 6% 38%	2437
	21st to end	Day	18% 9% 2 8% 3 4 4 3 5% 5% 38%	2120
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 10% 36%	3280
	11th to 20th	Day	31% 8% 3 7% 4 4 4 4 5% 6% 25%	3261
	21st to end	Day	30% 10% 4 9% 4 5% 5% 3 5% 5% 18%	3339
November	1st to 10th	Day	36% 10% 3 8% 4 4 3 4 5% 6% 17%	1709
	11th to 20th	Day	35% 12% 3 8% 5% 4 3 3 4 4 20%	1680
	21st to end	Day	34% 11% 3 8% 5% 4 3 2 5% 6% 20%	1680
December	1st to 10th	Day	26% 8% 3 6% 4 4 5% 3 4 6% 30%	2700
	11th to 20th	Day	36% 13% 4 9% 5% 5% 4 3 4 5% 13%	3105
	21st to end	Day	34% 14% 4 9% 6% 5% 4 3 4 4 13%	3948
January	1st to 10th	Night	38% 16% 4 10% 6% 4 4 3 3 3 9%	8299
	11th to 20th	Night	20% 9% 3 7% 4 4 4 4 5% 7% 33%	8182
	21st to end	Night	26% 10% 3 7% 5% 4 4 3 4 5% 30%	8863
February	1st to 10th	Night	32% 13% 4 8% 6% 4 4 4 4 5% 17%	2730
	11th to 20th	Night	29% 13% 4 8% 4 3 3 3 5% 5% 22%	2662
	21st to end	Night	31% 15% 3 8% 4 4 4 2 4 5% 19%	2165
March	1st to 10th	Night	19% 15% 4 9% 6% 5% 5% 5% 6% 7% 20%	5411
	11th to 20th	Night	25% 14% 3 7% 4 4 4 2 4 5% 28%	5109
	21st to end	Night	32% 14% 3 9% 5% 5% 5% 3 5% 5% 13%	5396
April	1st to 10th	Night	25% 11% 3 9% 6% 5% 5% 4 5% 6% 19%	2100
	11th to 20th	Night	21% 11% 3 8% 5% 5% 5% 3 5% 7% 27%	2069
	21st to end	Night	24% 12% 3 8% 5% 5% 4 3 5% 6% 25%	1832
May	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 4 5% 44%	4724
	11th to 20th	Night	20% 14% 4 10% 6% 5% 4 4 5% 7% 23%	4700
	21st to end	Night	19% 12% 4 9% 5% 5% 5% 4 6% 7% 27%	5178
June	1st to 10th	Night	19% 8% 4 9% 6% 5% 5% 3 5% 6% 31%	1673
	11th to 20th	Night	20% 11% 3 9% 6% 4 4 4 5% 7% 27%	1470
	21st to end	Night	14% 7% 3 8% 4 4 4 3 6% 6% 42%	1470
July	1st to 10th	Night	9% 5% 2 6% 3 3 3 3 4 5% 56%	4140
	11th to 20th	Night	4 3 2 4 3 3 3 3 4 5% 68%	4652
	21st to end	Night	8% 4 2 5% 3 3 4 3 5% 6% 58%	5152
August	1st to 10th	Night	4 3 1 3 2 2 2 2 4 5% 72%	3778
	11th to 20th	Night	9% 5% 2 5% 3 4 4 3 5% 8% 51%	4142
	21st to end	Night	18% 7% 2 6% 3 3 3 2 4 6% 47%	4898
September	1st to 10th	Night	10% 5% 2 6% 4 3 4 2 5% 6% 52%	2100
	11th to 20th	Night	18% 8% 3 6% 3 3 4 4 4 5% 41%	2030
	21st to end	Night	19% 8% 3 7% 4 4 3 3 5% 5% 40%	2276
October	1st to 10th	Night	17% 7% 2 7% 5% 4 4 3 6% 8% 37%	3920
	11th to 20th	Night	29% 8% 3 6% 4 4 4 3 5% 5% 29%	3897
	21st to end	Night	32% 13% 3 7% 4 4 4 3 4 5% 22%	4464
November	1st to 10th	Night	34% 10% 3 8% 4 4 4 3 4 5% 19%	2805
	11th to 20th	Night	36% 11% 3 8% 5% 3 3 2 4 4 21%	2940
	21st to end	Night	33% 11% 3 9% 4 4 4 3 4 5% 21%	2940
December	1st to 10th	Night	25% 10% 2 6% 5% 4 4 3 4 6% 31%	4700
	11th to 20th	Night	36% 15% 4 9% 4 4 4 3 3 4 13%	5320
	21st to end	Night	32% 17% 4 10% 5% 5% 4 3 4 4 12%	7187

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% 6% 6% 8% 2	4521
	11th to 20th	Day	50% 7% 11% 16% 15% 1	4740
	21st to end	Day	60% 3 6% 16% 14%	5307
February	1st to 10th	Day	71% 5% 9% 9% 4 1	1890
	11th to 20th	Day	70% 4 7% 10% 7% 1	1854
	21st to end	Day	72% 4 9% 11% 4	1641
March	1st to 10th	Day	64% 6% 13% 11% 5%	4700
	11th to 20th	Day	63% 6% 11% 18% 1	5110
	21st to end	Day	70% 9% 11% 9% 1	5820
April	1st to 10th	Day	69% 7% 15% 9% 1	2520
	11th to 20th	Day	62% 6% 12% 14% 6% 1	2499
	21st to end	Day	65% 6% 12% 12% 4 1	2788
May	1st to 10th	Day	43% 6% 19% 24% 8%	8195
	11th to 20th	Day	56% 10% 19% 13% 2	8163
	21st to end	Day	51% 8% 25% 14% 2	8994
June	1st to 10th	Day	58% 7% 16% 14% 5%	2941
	11th to 20th	Day	63% 6% 13% 14% 4	3150
	21st to end	Day	48% 8% 17% 19% 8% 1	3150
July	1st to 10th	Day	28% 8% 17% 21% 18% 8%	8746
	11th to 20th	Day	22% 5% 12% 29% 30% 3	8227
	21st to end	Day	27% 7% 21% 33% 12% 1	8723
August	1st to 10th	Day	22% 5% 13% 33% 23% 4	6342
	11th to 20th	Day	36% 11% 22% 24% 5% 1	5962
	21st to end	Day	42% 7% 15% 27% 8%	6262
September	1st to 10th	Day	37% 7% 14% 26% 11% 5%	2520
	11th to 20th	Day	47% 7% 11% 18% 14% 2	2437
	21st to end	Day	49% 5% 16% 19% 9% 2	2120
October	1st to 10th	Day	43% 6% 14% 30% 7% 1	3280
	11th to 20th	Day	60% 5% 10% 15% 8% 1	3261
	21st to end	Day	65% 8% 10% 9% 7% 1	3339
November	1st to 10th	Day	69% 6% 11% 11% 4	1709
	11th to 20th	Day	70% 5% 8% 9% 6% 2	1680
	21st to end	Day	69% 5% 6% 10% 8% 2	1680
December	1st to 10th	Day	56% 5% 7% 17% 14% 1	2700
	11th to 20th	Day	74% 6% 9% 9% 2	3108
	21st to end	Day	74% 6% 9% 8% 2	3948
January	1st to 10th	Night	79% 6% 5% 7% 3	8299
	11th to 20th	Night	48% 7% 11% 18% 14% 2	8220
	21st to end	Night	58% 5% 8% 17% 12% 1	8863
February	1st to 10th	Night	70% 5% 10% 9% 6% 1	2730
	11th to 20th	Night	65% 7% 9% 10% 7% 2	2664
	21st to end	Night	68% 5% 10% 11% 4	2165
March	1st to 10th	Night	61% 7% 14% 13% 5%	5411
	11th to 20th	Night	60% 6% 15% 17% 2	5110
	21st to end	Night	71% 8% 12% 8%	5396
April	1st to 10th	Night	66% 8% 13% 11% 2	2100
	11th to 20th	Night	59% 7% 13% 13% 8% 1	2069
	21st to end	Night	61% 8% 13% 12% 6% 1	1832
May	1st to 10th	Night	42% 7% 14% 29% 7%	4721
	11th to 20th	Night	60% 9% 15% 13% 2	4700
	21st to end	Night	57% 8% 21% 12% 2	5182
June	1st to 10th	Night	58% 5% 16% 16% 5%	1673
	11th to 20th	Night	60% 6% 15% 16% 3	1470
	21st to end	Night	45% 7% 19% 20% 7% 2	1470
July	1st to 10th	Night	30% 8% 18% 20% 13% 11%	4105
	11th to 20th	Night	20% 4 12% 26% 36% 1	4606
	21st to end	Night	28% 6% 19% 33% 12% 2	5040
August	1st to 10th	Night	17% 4 10% 43% 22% 4	3770
	11th to 20th	Night	31% 9% 26% 25% 6% 1	4169
	21st to end	Night	40% 7% 17% 29% 7% 1	4898
September	1st to 10th	Night	34% 6% 17% 26% 13% 4	2100
	11th to 20th	Night	47% 7% 11% 19% 13% 4	2030
	21st to end	Night	48% 7% 13% 20% 10% 2	2276
October	1st to 10th	Night	44% 6% 15% 29% 6% 1	3920
	11th to 20th	Night	58% 5% 10% 17% 9% 1	3897
	21st to end	Night	64% 6% 10% 12% 8%	4464
November	1st to 10th	Night	68% 6% 11% 11% 3 1	2805
	11th to 20th	Night	70% 5% 8% 10% 5% 2	2940
	21st to end	Night	68% 7% 6% 10% 9% 1	2940
December	1st to 10th	Night	56% 5% 8% 17% 14% 1	4700
	11th to 20th	Night	74% 5% 8% 10% 2	5316
	21st to end	Night	73% 8% 9% 7% 2	7187

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	43% 9% 15% 18% 9% 5%	220
	11th to 20th	Day	30% 13% 15% 21% 16% 5%	220
	21st to end	Day	43% 10% 13% 16% 14% 4	284
February	1st to 10th	Day	42% 10% 20% 17% 9% 2	430
	11th to 20th	Day	44% 11% 22% 15% 8% 1	425
	21st to end	Day	41% 12% 20% 20% 6% 1	399
March	1st to 10th	Day	37% 15% 20% 18% 9% 1	540
	11th to 20th	Day	38% 12% 18% 18% 11% 3	540
	21st to end	Day	39% 11% 22% 19% 8% 1	594
April	1st to 10th	Day	35% 12% 21% 17% 11% 2	540
	11th to 20th	Day	29% 14% 26% 19% 9% 4	532
	21st to end	Day	31% 11% 24% 23% 10% 2	540
May	1st to 10th	Day	21% 11% 23% 30% 12% 2	543
	11th to 20th	Day	31% 10% 18% 23% 13% 5%	568
	21st to end	Day	28% 8% 22% 25% 12% 4	617
June	1st to 10th	Day	28% 8% 20% 26% 14% 4	571
	11th to 20th	Day	27% 10% 23% 27% 11% 2	780
	21st to end	Day	15% 6% 23% 33% 19% 4	780
July	1st to 10th	Day	14% 5% 19% 36% 22% 3	771
	11th to 20th	Day	11% 5% 22% 36% 23% 4	594
	21st to end	Day	9% 5% 16% 40% 24% 5%	619
August	1st to 10th	Day	13% 6% 16% 34% 23% 8%	567
	11th to 20th	Day	9% 3 16% 37% 28% 7%	557
	21st to end	Day	17% 7% 21% 33% 15% 7%	627
September	1st to 10th	Day	14% 7% 20% 37% 17% 4	570
	11th to 20th	Day	20% 8% 18% 31% 18% 5%	543
	21st to end	Day	22% 8% 20% 29% 17% 4	530
October	1st to 10th	Day	26% 8% 20% 21% 20% 6%	530
	11th to 20th	Day	34% 9% 15% 24% 13% 5%	530
	21st to end	Day	36% 11% 20% 19% 10% 4	452
November	1st to 10th	Day	42% 13% 13% 17% 13% 2	253
	11th to 20th	Day	43% 11% 15% 9% 16% 5%	220
	21st to end	Day	36% 11% 16% 19% 10% 8%	220
December	1st to 10th	Day	45% 6% 11% 16% 9% 11%	220
	11th to 20th	Day	50% 10% 13% 18% 8% 1	220
	21st to end	Day	39% 13% 12% 21% 13% 2	242
January	1st to 10th	Night	49% 9% 15% 16% 10% 1	864
	11th to 20th	Night	39% 8% 12% 25% 14% 1	840
	21st to end	Night	38% 6% 12% 30% 13%	924
February	1st to 10th	Night	40% 7% 18% 24% 10% 1	840
	11th to 20th	Night	38% 7% 14% 26% 13% 1	824
	21st to end	Night	42% 8% 15% 23% 10% 1	692
March	1st to 10th	Night	36% 9% 15% 24% 14% 1	840
	11th to 20th	Night	38% 8% 17% 25% 11% 2	840
	21st to end	Night	38% 9% 17% 25% 9% 2	924
April	1st to 10th	Night	34% 9% 15% 25% 14% 2	840
	11th to 20th	Night	29% 8% 17% 31% 14% 2	821
	21st to end	Night	33% 8% 13% 27% 15% 3	572
May	1st to 10th	Night	23% 7% 15% 31% 23% 2	420
	11th to 20th	Night	25% 7% 8% 30% 25% 5%	414
	21st to end	Night	29% 5% 12% 24% 25% 5%	455
June	1st to 10th	Night	25% 6% 11% 27% 27% 3	420
	11th to 20th	Night	29% 8% 13% 30% 18% 3	420
	21st to end	Night	17% 5% 12% 30% 33% 4	420
July	1st to 10th	Night	15% 4 9% 30% 38% 5%	410
	11th to 20th	Night	12% 2 7% 33% 41% 5%	408
	21st to end	Night	9% 2 8% 27% 47% 7%	460
August	1st to 10th	Night	13% 3 4 25% 45% 10%	448
	11th to 20th	Night	12% 2 5% 25% 43% 13%	624
	21st to end	Night	19% 4 6% 27% 39% 6%	830
September	1st to 10th	Night	13% 3 8% 35% 35% 6%	840
	11th to 20th	Night	23% 4 8% 25% 33% 8%	812
	21st to end	Night	24% 5% 10% 29% 28% 4	800
October	1st to 10th	Night	26% 5% 9% 29% 27% 4	800
	11th to 20th	Night	33% 7% 12% 25% 18% 6%	800
	21st to end	Night	36% 8% 13% 28% 12% 3	880
November	1st to 10th	Night	41% 8% 16% 26% 9%	820
	11th to 20th	Night	42% 9% 15% 21% 11% 1	840
	21st to end	Night	40% 9% 14% 24% 12% 1	840
December	1st to 10th	Night	38% 5% 15% 25% 15% 2	840
	11th to 20th	Night	47% 8% 15% 21% 7% 1	840
	21st to end	Night	49% 9% 15% 19% 7%	945

Page generated at 2026-09-11 01:11:51. Version ac07

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