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 2024-03-01.

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	24% 9% 4 7% 5% 3 4 4 5% 6% 29%	78689
	Night	26% 10% 4 7% 5% 3 4 4 4 5% 27%	80573
Transparency	Day	59% 6% 12% 16% 7% 1	78409
	Night	60% 6% 11% 15% 7% 1	80433
Seeing	Day	29% 8% 18% 26% 16% 4	14005
	Night	33% 6% 12% 26% 20% 3	22583

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% 5% 8% 6% 2 3 3 4 3 11%	2419
	11th to 20th	Day	26% 12% 4 8% 6% 3 4 4 4 6% 22%	2516
	21st to end	Day	38% 14% 4 8% 6% 3 3 3 4 5% 12%	2858
February	1st to 10th	Day	34% 11% 5% 7% 6% 3 4 4 4 6% 15%	1710
	11th to 20th	Day	32% 13% 4 8% 6% 2 4 4 4 5% 19%	1665
	21st to end	Day	30% 14% 5% 8% 6% 3 5% 4 3 4 19%	1489
March	1st to 10th	Day	29% 11% 5% 8% 6% 3 5% 4 4 6% 17%	1829
	11th to 20th	Day	33% 12% 4 7% 6% 2 5% 4 4 5% 18%	1980
	21st to end	Day	30% 13% 4 8% 7% 4 5% 6% 4 5% 14%	2258
April	1st to 10th	Day	28% 13% 4 8% 6% 3 5% 5% 6% 5% 16%	2160
	11th to 20th	Day	25% 10% 4 7% 6% 3 6% 6% 5% 6% 22%	2137
	21st to end	Day	24% 10% 5% 9% 7% 4 4 5% 5% 7% 20%	2389
May	1st to 10th	Day	19% 11% 3 6% 6% 3 5% 5% 5% 6% 29%	2520
	11th to 20th	Day	21% 8% 4 7% 6% 4 4 6% 5% 8% 27%	2477
	21st to end	Day	28% 10% 4 7% 5% 3 5% 5% 5% 6% 23%	2723
June	1st to 10th	Day	24% 8% 4 6% 7% 4 5% 5% 6% 6% 25%	2520
	11th to 20th	Day	26% 10% 4 8% 7% 4 6% 5% 5% 6% 20%	2700
	21st to end	Day	14% 6% 3 7% 6% 3 5% 5% 6% 7% 38%	2700
July	1st to 10th	Day	12% 7% 3 5% 5% 3 4 5% 5% 6% 48%	2700
	11th to 20th	Day	12% 5% 2 5% 4 2 4 4 5% 7% 49%	2548
	21st to end	Day	10% 4 2 3 3 2 3 4 4 6% 60%	2741
August	1st to 10th	Day	12% 6% 2 4 4 2 3 4 5% 6% 52%	2479
	11th to 20th	Day	10% 4 1 4 4 2 4 4 5% 8% 55%	2298
	21st to end	Day	14% 7% 3 5% 5% 3 4 4 5% 7% 44%	2457
September	1st to 10th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 49%	2160
	11th to 20th	Day	16% 8% 3 6% 6% 3 5% 4 5% 6% 38%	2160
	21st to end	Day	19% 9% 2 6% 5% 3 4 4 5% 5% 37%	1910
October	1st to 10th	Day	21% 9% 3 6% 5% 3 6% 5% 6% 7% 29%	1800
	11th to 20th	Day	27% 9% 4 6% 5% 3 4 5% 5% 6% 27%	1800
	21st to end	Day	33% 8% 3 7% 4 3 5% 3 5% 5% 23%	1863
November	1st to 10th	Day	38% 9% 3 6% 5% 3 4 5% 4 6% 17%	1543
	11th to 20th	Day	35% 11% 3 6% 5% 3 3 3 4 4 22%	1520
	21st to end	Day	35% 10% 4 6% 5% 3 3 2 4 5% 21%	1520
December	1st to 10th	Day	31% 10% 4 6% 5% 4 5% 4 3 5% 23%	1520
	11th to 20th	Day	35% 12% 4 7% 6% 3 4 3 5% 5% 18%	1925
	21st to end	Day	28% 13% 5% 8% 8% 4 5% 4 4 5% 15%	2695
January	1st to 10th	Night	39% 15% 4 8% 6% 3 4 3 3 3 11%	4479
	11th to 20th	Night	28% 12% 5% 7% 5% 3 4 4 5% 6% 22%	4382
	21st to end	Night	36% 12% 4 7% 7% 3 4 4 4 5% 13%	4796
February	1st to 10th	Night	34% 11% 4 7% 7% 3 4 4 3 5% 17%	2470
	11th to 20th	Night	30% 12% 4 7% 5% 3 3 4 5% 5% 22%	2402
	21st to end	Night	31% 14% 4 6% 5% 2 5% 3 5% 5% 19%	1965
March	1st to 10th	Night	29% 11% 4 5% 5% 3 4 4 4 7% 23%	2134
	11th to 20th	Night	29% 10% 3 7% 5% 3 5% 4 5% 6% 24%	1979
	21st to end	Night	29% 12% 4 8% 6% 4 5% 5% 5% 5% 16%	2098
April	1st to 10th	Night	27% 11% 4 8% 7% 4 5% 5% 6% 7% 18%	1800
	11th to 20th	Night	21% 10% 4 7% 6% 3 6% 5% 5% 7% 26%	1771
	21st to end	Night	25% 11% 4 7% 6% 4 5% 4 5% 6% 23%	1571
May	1st to 10th	Night	21% 8% 3 6% 5% 3 4 5% 5% 6% 34%	1440
	11th to 20th	Night	20% 8% 2 6% 6% 4 5% 7% 5% 8% 29%	1416
	21st to end	Night	22% 9% 4 8% 6% 3 5% 4 5% 6% 28%	1552
June	1st to 10th	Night	20% 7% 4 9% 6% 4 5% 4 5% 7% 30%	1435
	11th to 20th	Night	22% 11% 4 9% 7% 3 4 5% 4 7% 25%	1260
	21st to end	Night	13% 5% 3 6% 4 3 4 3 6% 6% 44%	1260
July	1st to 10th	Night	10% 6% 3 6% 5% 3 4 4 4 6% 51%	1260
	11th to 20th	Night	10% 4 3 5% 3 2 4 4 4 6% 56%	1408
	21st to end	Night	7% 4 1 3 3 2 3 3 3 5% 66%	1576
August	1st to 10th	Night	8% 4 2 3 3 2 3 3 5% 8% 58%	1457
	11th to 20th	Night	7% 3 2 3 2 2 3 4 4 7% 62%	1602
	21st to end	Night	13% 6% 2 5% 4 3 3 3 4 6% 51%	1898
September	1st to 10th	Night	11% 5% 2 5% 5% 2 4 3 5% 6% 53%	1800
	11th to 20th	Night	18% 8% 3 5% 4 3 4 4 4 5% 42%	1800
	21st to end	Night	18% 8% 3 6% 5% 2 4 3 5% 5% 40%	2046
October	1st to 10th	Night	21% 7% 3 7% 6% 3 4 3 5% 6% 35%	2160
	11th to 20th	Night	26% 8% 3 6% 5% 3 4 4 4 5% 31%	2160
	21st to end	Night	30% 9% 3 6% 5% 3 3 4 4 6% 28%	2492
November	1st to 10th	Night	34% 9% 4 7% 5% 3 4 3 4 5% 20%	2531
	11th to 20th	Night	34% 9% 3 7% 6% 3 4 3 4 5% 22%	2660
	21st to end	Night	33% 10% 3 8% 5% 2 4 4 4 5% 22%	2660
December	1st to 10th	Night	31% 10% 3 7% 6% 2 4 3 4 5% 24%	2660
	11th to 20th	Night	33% 12% 4 8% 5% 4 4 4 4 5% 17%	3280
	21st to end	Night	29% 13% 4 9% 7% 4 5% 5% 4 4 15%	4913

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% 5% 5% 8% 3 1	2419
	11th to 20th	Day	64% 5% 11% 13% 6% 1	2540
	21st to end	Day	78% 4 6% 8% 3	2858
February	1st to 10th	Day	72% 4 10% 9% 4 1	1710
	11th to 20th	Day	70% 5% 8% 10% 6% 1	1674
	21st to end	Day	72% 3 10% 11% 4	1489
March	1st to 10th	Day	69% 5% 11% 12% 3	1829
	11th to 20th	Day	71% 4 9% 12% 3 1	1980
	21st to end	Day	75% 6% 9% 9% 1	2258
April	1st to 10th	Day	71% 6% 14% 8% 1	2160
	11th to 20th	Day	66% 6% 10% 13% 5% 1	2137
	21st to end	Day	68% 6% 11% 10% 5% 1	2389
May	1st to 10th	Day	58% 7% 15% 15% 5%	2520
	11th to 20th	Day	57% 7% 18% 14% 4	2480
	21st to end	Day	65% 7% 14% 13% 1	2724
June	1st to 10th	Day	61% 7% 16% 11% 5%	2520
	11th to 20th	Day	68% 6% 14% 10% 3	2700
	21st to end	Day	47% 8% 17% 19% 8% 1	2700
July	1st to 10th	Day	41% 8% 12% 25% 12% 2	2626
	11th to 20th	Day	38% 6% 11% 29% 15% 1	2471
	21st to end	Day	26% 6% 12% 35% 17% 3	2589
August	1st to 10th	Day	35% 6% 14% 29% 14% 2	2462
	11th to 20th	Day	30% 6% 20% 29% 13% 3	2298
	21st to end	Day	42% 5% 14% 29% 9% 1	2457
September	1st to 10th	Day	36% 6% 14% 27% 12% 5%	2160
	11th to 20th	Day	47% 7% 11% 18% 15% 2	2160
	21st to end	Day	50% 5% 15% 18% 10% 3	1910
October	1st to 10th	Day	54% 6% 11% 22% 7% 1	1800
	11th to 20th	Day	58% 5% 12% 18% 6% 1	1800
	21st to end	Day	63% 5% 9% 13% 10%	1863
November	1st to 10th	Day	70% 5% 10% 10% 4	1543
	11th to 20th	Day	70% 5% 8% 9% 7% 2	1520
	21st to end	Day	69% 5% 5% 11% 9% 2	1520
December	1st to 10th	Day	66% 5% 8% 13% 8% 1	1520
	11th to 20th	Day	70% 4 11% 12% 2 1	1928
	21st to end	Day	71% 6% 10% 10% 3	2695
January	1st to 10th	Night	79% 6% 5% 7% 2 1	4479
	11th to 20th	Night	63% 4 9% 16% 6% 1	4420
	21st to end	Night	75% 5% 7% 8% 4 1	4796
February	1st to 10th	Night	71% 5% 10% 9% 5% 1	2470
	11th to 20th	Night	65% 7% 10% 10% 6% 2	2404
	21st to end	Night	69% 5% 10% 11% 4	1965
March	1st to 10th	Night	63% 5% 13% 15% 5%	2134
	11th to 20th	Night	65% 5% 10% 15% 4 1	1980
	21st to end	Night	72% 6% 12% 9% 1	2098
April	1st to 10th	Night	67% 8% 13% 11% 2	1800
	11th to 20th	Night	61% 6% 12% 13% 7% 1	1771
	21st to end	Night	64% 7% 11% 11% 5% 1	1571
May	1st to 10th	Night	53% 7% 17% 16% 7% 1	1440
	11th to 20th	Night	54% 7% 19% 14% 5% 1	1416
	21st to end	Night	60% 7% 16% 14% 2	1552
June	1st to 10th	Night	59% 5% 15% 15% 5%	1435
	11th to 20th	Night	64% 6% 16% 11% 2	1260
	21st to end	Night	42% 7% 21% 20% 7% 2	1260
July	1st to 10th	Night	38% 7% 15% 23% 13% 4	1225
	11th to 20th	Night	35% 4 13% 31% 16% 1	1362
	21st to end	Night	23% 6% 12% 35% 20% 4	1488
August	1st to 10th	Night	29% 5% 13% 33% 17% 3	1449
	11th to 20th	Night	26% 6% 19% 32% 14% 4	1602
	21st to end	Night	36% 5% 15% 32% 11% 1	1898
September	1st to 10th	Night	34% 6% 15% 28% 13% 4	1800
	11th to 20th	Night	46% 6% 10% 19% 14% 4	1800
	21st to end	Night	48% 7% 11% 20% 11% 3	2046
October	1st to 10th	Night	51% 5% 13% 22% 8% 1	2160
	11th to 20th	Night	58% 5% 10% 19% 7% 1	2160
	21st to end	Night	60% 5% 10% 14% 11%	2492
November	1st to 10th	Night	68% 5% 12% 11% 4 1	2531
	11th to 20th	Night	68% 5% 9% 10% 5% 3	2660
	21st to end	Night	67% 6% 6% 10% 9% 1	2660
December	1st to 10th	Night	66% 4 9% 12% 8% 1	2660
	11th to 20th	Night	70% 5% 10% 11% 3 1	3276
	21st to end	Night	71% 7% 10% 9% 3	4913

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% 12% 18% 11% 6%	180
	11th to 20th	Day	34% 14% 14% 21% 13% 4	180
	21st to end	Day	47% 11% 11% 14% 12% 4	236
February	1st to 10th	Day	44% 8% 20% 17% 9% 2	370
	11th to 20th	Day	46% 9% 19% 16% 8% 2	365
	21st to end	Day	40% 12% 20% 21% 6% 1	343
March	1st to 10th	Day	38% 13% 19% 19% 8% 2	421
	11th to 20th	Day	36% 12% 17% 20% 12% 3	420
	21st to end	Day	41% 10% 21% 17% 10% 1	462
April	1st to 10th	Day	39% 12% 19% 15% 12% 2	420
	11th to 20th	Day	31% 13% 23% 18% 10% 5%	412
	21st to end	Day	32% 11% 23% 23% 10% 1	420
May	1st to 10th	Day	23% 10% 21% 32% 12% 1	423
	11th to 20th	Day	31% 7% 17% 25% 15% 5%	448
	21st to end	Day	29% 7% 21% 26% 14% 4	485
June	1st to 10th	Day	30% 6% 21% 25% 15% 4	450
	11th to 20th	Day	30% 10% 20% 26% 11% 2	630
	21st to end	Day	15% 4 22% 34% 21% 4	630
July	1st to 10th	Day	16% 5% 16% 35% 24% 3	621
	11th to 20th	Day	12% 4 18% 37% 24% 4	470
	21st to end	Day	9% 4 14% 40% 27% 6%	487
August	1st to 10th	Day	14% 5% 13% 35% 24% 9%	447
	11th to 20th	Day	9% 2 14% 38% 29% 8%	437
	21st to end	Day	18% 5% 20% 33% 17% 7%	495
September	1st to 10th	Day	16% 7% 21% 38% 15% 4	450
	11th to 20th	Day	21% 6% 16% 32% 20% 5%	450
	21st to end	Day	23% 8% 18% 29% 18% 4	450
October	1st to 10th	Day	28% 7% 17% 21% 21% 6%	450
	11th to 20th	Day	34% 7% 14% 24% 14% 6%	450
	21st to end	Day	38% 8% 17% 21% 11% 4	378
November	1st to 10th	Day	43% 9% 13% 18% 14% 2	207
	11th to 20th	Day	43% 9% 13% 9% 19% 7%	180
	21st to end	Day	39% 9% 13% 18% 12% 9%	180
December	1st to 10th	Day	46% 7% 12% 16% 9% 11%	180
	11th to 20th	Day	46% 9% 14% 21% 9% 1	180
	21st to end	Day	40% 11% 11% 23% 13% 3	198
January	1st to 10th	Night	51% 7% 14% 16% 11% 1	780
	11th to 20th	Night	42% 9% 12% 23% 13% 2	760
	21st to end	Night	40% 6% 12% 28% 12%	836
February	1st to 10th	Night	42% 6% 17% 24% 10% 1	760
	11th to 20th	Night	40% 6% 13% 26% 14% 1	744
	21st to end	Night	42% 8% 14% 24% 11% 2	628
March	1st to 10th	Night	37% 8% 15% 24% 14% 2	722
	11th to 20th	Night	36% 7% 17% 27% 11% 2	720
	21st to end	Night	39% 7% 16% 26% 10% 2	792
April	1st to 10th	Night	35% 7% 16% 25% 13% 3	720
	11th to 20th	Night	30% 8% 16% 30% 14% 3	703
	21st to end	Night	35% 8% 13% 26% 15% 3	491
May	1st to 10th	Night	24% 7% 14% 29% 23% 2	360
	11th to 20th	Night	25% 5% 8% 31% 27% 5%	354
	21st to end	Night	30% 4 12% 22% 26% 6%	389
June	1st to 10th	Night	27% 5% 11% 27% 27% 3	360
	11th to 20th	Night	32% 7% 11% 27% 19% 3	360
	21st to end	Night	16% 4 11% 29% 35% 5%	360
July	1st to 10th	Night	15% 4 8% 29% 39% 5%	350
	11th to 20th	Night	13% 3 6% 29% 43% 6%	348
	21st to end	Night	9% 1 8% 25% 48% 8%	394
August	1st to 10th	Night	14% 3 4 21% 46% 11%	383
	11th to 20th	Night	10% 2 5% 24% 44% 15%	534
	21st to end	Night	20% 3 5% 26% 40% 7%	710
September	1st to 10th	Night	14% 3 7% 36% 33% 6%	720
	11th to 20th	Night	23% 3 6% 25% 35% 8%	720
	21st to end	Night	24% 4 10% 28% 29% 5%	720
October	1st to 10th	Night	27% 4 7% 29% 29% 4	720
	11th to 20th	Night	32% 6% 11% 25% 19% 7%	720
	21st to end	Night	37% 7% 12% 28% 13% 3	792
November	1st to 10th	Night	41% 7% 16% 27% 9%	740
	11th to 20th	Night	42% 8% 14% 23% 12% 1	760
	21st to end	Night	41% 8% 13% 23% 13% 1	760
December	1st to 10th	Night	38% 5% 15% 27% 14% 2	760
	11th to 20th	Night	46% 8% 15% 23% 8% 2	760
	21st to end	Night	50% 8% 15% 20% 6%	853

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