Washington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Washington clear sky chart from 2005-11-06 to 2025-09-04.

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	14% 6% 2 6% 4 4 4 4 6% 8% 41%	126053
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	122672
Transparency	Day	39% 6% 11% 24% 17% 3	129646
	Night	39% 5% 11% 23% 18% 4	127149
Seeing	Day	19% 13% 28% 24% 12% 4	16074
	Night	18% 11% 30% 29% 12% 1	25542

By Month

Percentage Cloud Cover SE (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	15% 4 1 4 3 3 4 3 4 6% 52%	3689
	11th to 20th	Day	15% 9% 3 5% 4 3 4 3 4 5% 47%	4060
	21st to end	Day	15% 7% 2 7% 5% 4 4 3 4 6% 43%	4466
February	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 5% 7% 39%	1800
	11th to 20th	Day	19% 8% 2 5% 3 4 4 4 5% 6% 40%	1821
	21st to end	Day	18% 7% 3 6% 4 4 4 3 4 6% 41%	1650
March	1st to 10th	Day	18% 6% 2 5% 4 3 4 4 5% 7% 43%	4836
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 5% 6% 50%	4980
	21st to end	Day	17% 6% 2 6% 5% 4 4 3 5% 7% 41%	5574
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 43%	2400
	11th to 20th	Day	13% 5% 2 5% 5% 4 4 4 5% 6% 47%	2374
	21st to end	Day	18% 5% 2 5% 4 3 4 3 6% 7% 42%	2400
May	1st to 10th	Day	19% 8% 3 6% 5% 4 5% 4 7% 9% 31%	5721
	11th to 20th	Day	21% 6% 2 6% 4 4 5% 4 7% 10% 31%	6175
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 9% 39%	6934
June	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 43%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 5% 7% 8% 41%	2800
	21st to end	Day	9% 4 1 4 5% 4 6% 5% 7% 11% 43%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 8% 10% 44%	6360
	11th to 20th	Day	9% 7% 3 7% 5% 6% 6% 5% 9% 12% 29%	6184
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	6424
August	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 5% 7% 10% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 9% 38%	3979
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 7% 9% 46%	4312
September	1st to 10th	Day	14% 4 2 6% 4 4 4 4 5% 7% 46%	2316
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 10% 46%	2280
	21st to end	Day	19% 7% 2 7% 3 4 4 5% 5% 6% 38%	2004
October	1st to 10th	Day	16% 6% 2 5% 4 4 4 3 5% 6% 46%	1900
	11th to 20th	Day	16% 4 2 5% 3 3 3 3 5% 6% 50%	1900
	21st to end	Day	14% 5% 2 6% 3 3 4 3 5% 6% 50%	2090
November	1st to 10th	Day	12% 4 2 4 4 4 4 3 5% 7% 51%	1845
	11th to 20th	Day	13% 6% 1 5% 3 4 3 4 6% 7% 49%	1795
	21st to end	Day	14% 6% 2 5% 4 3 4 3 5% 6% 48%	1600
December	1st to 10th	Day	11% 5% 2 6% 4 3 4 4 5% 6% 49%	2600
	11th to 20th	Day	18% 8% 2 5% 3 3 3 3 5% 6% 44%	2990
	21st to end	Day	16% 8% 3 6% 5% 4 4 4 5% 7% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 6% 49%	6240
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 4 6% 52%	5900
	21st to end	Night	15% 8% 2 6% 4 3 3 2 4 5% 47%	6490
February	1st to 10th	Night	16% 7% 2 5% 4 4 5% 4 6% 6% 41%	2600
	11th to 20th	Night	19% 7% 3 5% 4 4 3 3 4 6% 43%	2463
	21st to end	Night	17% 7% 3 5% 3 3 4 3 5% 7% 43%	1980
March	1st to 10th	Night	17% 6% 2 6% 3 3 3 3 4 5% 48%	5016
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 8% 50%	4980
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 6% 45%	5344
April	1st to 10th	Night	15% 7% 2 6% 5% 4 4 5% 5% 6% 41%	2000
	11th to 20th	Night	14% 6% 2 6% 4 3 4 4 5% 7% 46%	1975
	21st to end	Night	17% 4 2 5% 3 4 5% 4 6% 8% 42%	2000
May	1st to 10th	Night	20% 10% 3 7% 5% 4 5% 4 6% 7% 30%	4214
	11th to 20th	Night	22% 8% 2 5% 4 4 5% 4 7% 8% 31%	3716
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 7% 9% 35%	3920
June	1st to 10th	Night	10% 5% 2 5% 3 5% 5% 4 8% 9% 44%	1600
	11th to 20th	Night	12% 6% 2 6% 4 4 4 5% 7% 10% 41%	1600
	21st to end	Night	9% 5% 2 6% 6% 5% 4 4 8% 10% 42%	1600
July	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 7% 10% 40%	3598
	11th to 20th	Night	9% 7% 3 9% 5% 6% 7% 6% 8% 11% 29%	3769
	21st to end	Night	9% 6% 2 6% 5% 4 5% 5% 7% 9% 41%	4427
August	1st to 10th	Night	17% 6% 3 6% 5% 5% 5% 4 7% 9% 33%	2915
	11th to 20th	Night	11% 7% 3 9% 5% 5% 6% 4 7% 8% 35%	3035
	21st to end	Night	10% 6% 2 5% 4 3 4 3 5% 8% 51%	3586
September	1st to 10th	Night	13% 6% 2 5% 5% 4 5% 3 5% 7% 45%	1931
	11th to 20th	Night	10% 7% 2 6% 3 3 4 3 6% 7% 50%	1900
	21st to end	Night	19% 7% 2 6% 4 4 3 3 6% 7% 37%	2172
October	1st to 10th	Night	19% 6% 2 5% 3 3 3 3 4 6% 46%	2280
	11th to 20th	Night	15% 5% 2 5% 4 3 3 2 5% 5% 52%	2280
	21st to end	Night	14% 6% 2 4 3 3 4 4 4 7% 50%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 5% 54%	2451
	11th to 20th	Night	14% 7% 2 4 4 3 3 3 4 6% 50%	2601
	21st to end	Night	13% 5% 2 4 5% 3 3 5% 6% 7% 49%	2800
December	1st to 10th	Night	10% 6% 2 5% 4 4 3 3 5% 7% 52%	4580
	11th to 20th	Night	21% 5% 2 5% 3 4 4 4 5% 6% 40%	5205
	21st to end	Night	17% 7% 2 6% 4 4 4 4 6% 7% 38%	6996

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	34% 3 6% 28% 23% 6%	3690
	11th to 20th	Day	41% 3 8% 22% 20% 7%	4060
	21st to end	Day	43% 4 9% 28% 13% 3	4466
February	1st to 10th	Day	43% 5% 12% 25% 13% 2	1800
	11th to 20th	Day	43% 5% 8% 25% 16% 3	1830
	21st to end	Day	45% 4 11% 20% 16% 3	1650
March	1st to 10th	Day	40% 5% 8% 23% 17% 7%	4836
	11th to 20th	Day	35% 3 9% 25% 22% 5%	4980
	21st to end	Day	42% 5% 10% 19% 18% 6%	5574
April	1st to 10th	Day	40% 6% 11% 21% 18% 5%	2400
	11th to 20th	Day	37% 4 10% 22% 20% 6%	2374
	21st to end	Day	40% 5% 10% 23% 18% 4	2400
May	1st to 10th	Day	47% 8% 14% 18% 11% 2	5754
	11th to 20th	Day	47% 8% 11% 24% 9% 1	6179
	21st to end	Day	40% 7% 13% 22% 16% 2	6949
June	1st to 10th	Day	36% 8% 13% 26% 16% 2	2800
	11th to 20th	Day	38% 10% 11% 23% 16% 2	2800
	21st to end	Day	33% 9% 15% 24% 18% 2	2800
July	1st to 10th	Day	34% 10% 18% 20% 14% 4	6290
	11th to 20th	Day	44% 9% 17% 21% 7% 1	6109
	21st to end	Day	37% 8% 10% 23% 22% 1	6240
August	1st to 10th	Day	44% 8% 15% 24% 8%	4221
	11th to 20th	Day	40% 8% 12% 26% 14% 1	4018
	21st to end	Day	34% 7% 12% 28% 18% 2	4312
September	1st to 10th	Day	37% 5% 13% 20% 20% 4	2316
	11th to 20th	Day	34% 6% 9% 26% 21% 4	2280
	21st to end	Day	46% 6% 9% 20% 17% 3	2004
October	1st to 10th	Day	38% 4 6% 18% 29% 5%	3160
	11th to 20th	Day	25% 4 7% 29% 27% 9%	3140
	21st to end	Day	23% 3 9% 30% 28% 8%	3428
November	1st to 10th	Day	33% 4 11% 27% 20% 5%	1845
	11th to 20th	Day	34% 4 10% 24% 23% 5%	1795
	21st to end	Day	37% 4 9% 28% 18% 3	1600
December	1st to 10th	Day	36% 4 13% 28% 15% 4	2600
	11th to 20th	Day	41% 4 13% 29% 11% 2	2986
	21st to end	Day	43% 6% 14% 21% 15%	3960
January	1st to 10th	Night	36% 4 7% 25% 24% 5%	6240
	11th to 20th	Night	35% 3 8% 25% 22% 7%	5900
	21st to end	Night	40% 3 7% 28% 19% 3	6490
February	1st to 10th	Night	42% 5% 13% 24% 15% 2	2600
	11th to 20th	Night	44% 3 8% 22% 18% 5%	2465
	21st to end	Night	42% 4 10% 22% 17% 5%	1980
March	1st to 10th	Night	40% 4 7% 26% 15% 9%	5016
	11th to 20th	Night	33% 4 11% 26% 19% 7%	4980
	21st to end	Night	40% 4 8% 22% 20% 6%	5344
April	1st to 10th	Night	42% 6% 9% 19% 20% 4	2000
	11th to 20th	Night	38% 5% 11% 21% 20% 5%	1975
	21st to end	Night	38% 6% 11% 25% 16% 5%	2000
May	1st to 10th	Night	52% 7% 12% 17% 9% 3	4244
	11th to 20th	Night	49% 7% 10% 21% 12% 1	3717
	21st to end	Night	44% 7% 14% 22% 11% 2	3927
June	1st to 10th	Night	35% 6% 12% 26% 17% 2	1600
	11th to 20th	Night	37% 8% 14% 25% 13% 3	1600
	21st to end	Night	35% 7% 14% 22% 19% 3	1600
July	1st to 10th	Night	39% 10% 17% 18% 14% 2	3560
	11th to 20th	Night	47% 9% 18% 17% 7% 1	3724
	21st to end	Night	36% 8% 13% 24% 19% 1	4297
August	1st to 10th	Night	45% 7% 17% 21% 9%	2904
	11th to 20th	Night	44% 7% 11% 25% 11% 1	3062
	21st to end	Night	33% 5% 11% 31% 18% 2	3586
September	1st to 10th	Night	38% 6% 10% 22% 18% 5%	1931
	11th to 20th	Night	34% 5% 9% 27% 22% 4	1900
	21st to end	Night	45% 6% 10% 19% 17% 3	2172
October	1st to 10th	Night	40% 5% 9% 21% 20% 6%	3800
	11th to 20th	Night	24% 3 10% 22% 31% 8%	3779
	21st to end	Night	23% 3 9% 32% 25% 8%	4120
November	1st to 10th	Night	33% 4 9% 25% 23% 6%	2451
	11th to 20th	Night	36% 4 10% 23% 22% 6%	2601
	21st to end	Night	34% 6% 11% 25% 21% 3	2800
December	1st to 10th	Night	33% 4 12% 27% 21% 3	4580
	11th to 20th	Night	44% 4 12% 19% 16% 4	5208
	21st to end	Night	43% 6% 14% 21% 15% 1	6996

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	29% 21% 33% 13% 4	220
	11th to 20th	Day	28% 25% 30% 13% 4	300
	21st to end	Day	22% 26% 34% 13% 5%	330
February	1st to 10th	Day	26% 24% 34% 12% 4	300
	11th to 20th	Day	36% 20% 36% 7% 1	368
	21st to end	Day	30% 18% 32% 18% 2	414
March	1st to 10th	Day	25% 22% 31% 18% 3 1	500
	11th to 20th	Day	28% 18% 32% 15% 5% 1	500
	21st to end	Day	26% 18% 33% 17% 5% 1	550
April	1st to 10th	Day	23% 22% 32% 16% 5% 1	500
	11th to 20th	Day	20% 20% 33% 18% 6% 3	492
	21st to end	Day	25% 15% 32% 20% 7% 2	500
May	1st to 10th	Day	24% 17% 29% 20% 8% 2	503
	11th to 20th	Day	22% 14% 28% 24% 9% 2	527
	21st to end	Day	18% 9% 25% 28% 17% 3	573
June	1st to 10th	Day	13% 9% 27% 29% 15% 7%	530
	11th to 20th	Day	17% 7% 26% 29% 16% 4	530
	21st to end	Day	11% 5% 28% 32% 18% 5%	530
July	1st to 10th	Day	14% 2 17% 38% 20% 11%	525
	11th to 20th	Day	6% 3 21% 36% 27% 7%	524
	21st to end	Day	12% 3 21% 35% 22% 7%	576
August	1st to 10th	Day	14% 4 21% 31% 22% 8%	526
	11th to 20th	Day	14% 4 17% 34% 23% 9%	519
	21st to end	Day	11% 3 23% 32% 20% 11%	583
September	1st to 10th	Day	16% 7% 22% 26% 17% 13%	502
	11th to 20th	Day	12% 5% 22% 35% 17% 9%	490
	21st to end	Day	27% 10% 23% 26% 9% 5%	490
October	1st to 10th	Day	21% 8% 24% 26% 16% 6%	490
	11th to 20th	Day	17% 16% 27% 23% 14% 3	490
	21st to end	Day	17% 11% 37% 22% 10% 3	539
November	1st to 10th	Day	15% 15% 33% 25% 10% 3	438
	11th to 20th	Day	18% 16% 35% 22% 8% 2	395
	21st to end	Day	24% 23% 29% 16% 9% 1	200
December	1st to 10th	Day	19% 23% 39% 15% 4 2	200
	11th to 20th	Day	26% 20% 31% 17% 8%	200
	21st to end	Day	25% 17% 32% 20% 5%	220
January	1st to 10th	Night	20% 19% 39% 19% 3	800
	11th to 20th	Night	22% 15% 33% 25% 5%	800
	21st to end	Night	19% 17% 37% 25% 2	880
February	1st to 10th	Night	20% 16% 34% 27% 3	800
	11th to 20th	Night	27% 20% 33% 19% 1	783
	21st to end	Night	25% 17% 37% 19% 3	660
March	1st to 10th	Night	20% 16% 38% 23% 2	800
	11th to 20th	Night	22% 17% 37% 19% 5% 1	800
	21st to end	Night	19% 17% 39% 21% 4 1	880
April	1st to 10th	Night	20% 20% 36% 21% 4	800
	11th to 20th	Night	18% 15% 33% 28% 6%	791
	21st to end	Night	21% 11% 34% 26% 8%	800
May	1st to 10th	Night	21% 10% 31% 27% 11%	680
	11th to 20th	Night	20% 8% 28% 35% 9%	457
	21st to end	Night	15% 6% 24% 36% 16% 1	433
June	1st to 10th	Night	11% 3 32% 36% 18% 1	400
	11th to 20th	Night	15% 5% 27% 39% 15% 1	400
	21st to end	Night	11% 3 26% 43% 17% 1	400
July	1st to 10th	Night	8% 3 21% 42% 23% 2	390
	11th to 20th	Night	7% 3 16% 43% 29% 2	456
	21st to end	Night	9% 2 20% 42% 25% 3	657
August	1st to 10th	Night	11% 2 13% 43% 29% 2	597
	11th to 20th	Night	14% 4 15% 40% 25% 2	689
	21st to end	Night	11% 3 19% 36% 27% 4	880
September	1st to 10th	Night	15% 3 20% 31% 27% 4	773
	11th to 20th	Night	12% 4 16% 39% 25% 4	760
	21st to end	Night	23% 5% 23% 31% 17% 1	760
October	1st to 10th	Night	20% 6% 22% 28% 20% 4	760
	11th to 20th	Night	16% 10% 26% 32% 15% 1	760
	21st to end	Night	18% 10% 32% 27% 13% 1	836
November	1st to 10th	Night	14% 10% 35% 29% 12% 2	780
	11th to 20th	Night	19% 13% 32% 28% 8% 1	800
	21st to end	Night	17% 14% 33% 26% 9% 1	800
December	1st to 10th	Night	22% 13% 33% 24% 7% 1	800
	11th to 20th	Night	19% 11% 38% 27% 5%	800
	21st to end	Night	21% 12% 32% 31% 4	880

Page generated at 2025-09-01 01:51:08. Version ac07

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