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 2024-11-04.

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 5% 4 6% 8% 41%	101217
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	97759
Transparency	Day	39% 6% 11% 24% 16% 3	104854
	Night	39% 5% 11% 24% 17% 4	102262
Seeing	Day	19% 12% 28% 24% 12% 4	15012
	Night	18% 11% 30% 29% 12% 1	24495

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	17% 5% 2 4 3 3 4 3 4 7% 48%	2581
	11th to 20th	Day	16% 11% 3 6% 4 3 4 3 5% 5% 39%	2840
	21st to end	Day	17% 9% 2 7% 5% 5% 4 3 4 6% 37%	3124
February	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 5% 7% 39%	1710
	11th to 20th	Day	18% 8% 2 5% 3 4 4 4 5% 7% 41%	1727
	21st to end	Day	19% 8% 3 7% 4 4 4 3 4 6% 39%	1570
March	1st to 10th	Day	22% 7% 2 5% 4 3 4 4 5% 8% 37%	3364
	11th to 20th	Day	10% 4 2 5% 4 4 5% 4 5% 7% 50%	3480
	21st to end	Day	21% 6% 2 5% 5% 3 3 3 5% 6% 40%	3896
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 6% 7% 43%	2280
	11th to 20th	Day	13% 5% 2 5% 5% 4 4 4 5% 6% 48%	2254
	21st to end	Day	19% 5% 2 5% 4 3 4 3 6% 6% 42%	2280
May	1st to 10th	Day	22% 8% 3 7% 5% 4 5% 4 6% 8% 29%	3983
	11th to 20th	Day	21% 7% 3 6% 4 4 5% 4 6% 9% 30%	4297
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 7% 9% 44%	4823
June	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 7% 9% 42%	2660
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 42%	2660
	21st to end	Day	10% 4 1 4 5% 5% 6% 5% 7% 11% 42%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 6% 9% 10% 42%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 8% 12% 37%	4320
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	4466
August	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 5% 7% 10% 33%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 9% 38%	3855
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 7% 9% 46%	4180
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 4 5% 7% 45%	2280
	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% 5% 2 4 4 4 4 3 5% 7% 51%	1781
	11th to 20th	Day	14% 5% 2 4 2 4 3 4 6% 7% 49%	1706
	21st to end	Day	14% 6% 2 4 4 3 4 3 5% 6% 48%	1520
December	1st to 10th	Day	15% 6% 2 5% 4 4 5% 4 6% 6% 42%	1520
	11th to 20th	Day	16% 6% 2 5% 4 3 3 3 5% 7% 47%	1910
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 5% 7% 36%	2772
January	1st to 10th	Night	15% 7% 2 6% 3 4 4 3 5% 7% 45%	4362
	11th to 20th	Night	18% 7% 2 4 3 3 3 3 4 6% 46%	4120
	21st to end	Night	19% 9% 3 6% 4 3 3 2 4 5% 41%	4532
February	1st to 10th	Night	15% 7% 2 5% 4 4 5% 4 6% 6% 42%	2470
	11th to 20th	Night	19% 7% 3 5% 4 4 3 3 4 6% 43%	2337
	21st to end	Night	18% 7% 3 5% 3 3 3 3 5% 6% 43%	1884
March	1st to 10th	Night	21% 7% 3 6% 4 3 4 4 4 6% 38%	3488
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 9% 49%	3480
	21st to end	Night	15% 8% 2 6% 4 3 2 3 4 5% 46%	3736
April	1st to 10th	Night	15% 7% 3 6% 5% 4 4 5% 5% 6% 41%	1900
	11th to 20th	Night	13% 6% 2 6% 4 3 4 4 4 7% 47%	1875
	21st to end	Night	18% 4 2 4 3 4 5% 4 6% 8% 42%	1900
May	1st to 10th	Night	21% 9% 3 7% 4 4 4 4 6% 7% 32%	2938
	11th to 20th	Night	22% 9% 3 6% 5% 4 4 4 6% 6% 32%	2594
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 7% 11% 42%	2732
June	1st to 10th	Night	10% 4 2 5% 3 5% 5% 4 8% 9% 44%	1520
	11th to 20th	Night	11% 6% 2 6% 4 4 4 5% 7% 10% 41%	1520
	21st to end	Night	9% 5% 2 6% 6% 5% 4 4 8% 10% 42%	1520
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 6% 5% 7% 10% 41%	2520
	11th to 20th	Night	8% 5% 3 8% 5% 5% 6% 5% 7% 11% 38%	2636
	21st to end	Night	11% 8% 3 7% 5% 5% 5% 5% 7% 8% 37%	3087
August	1st to 10th	Night	17% 6% 3 6% 5% 5% 5% 4 7% 9% 32%	2825
	11th to 20th	Night	11% 7% 3 9% 5% 5% 6% 4 7% 8% 35%	2940
	21st to end	Night	10% 6% 2 5% 4 3 4 3 5% 8% 51%	3476
September	1st to 10th	Night	13% 6% 2 5% 4 4 5% 3 5% 7% 45%	1900
	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 4 3 3 4 5% 54%	2368
	11th to 20th	Night	14% 7% 2 4 4 3 3 3 4 6% 50%	2470
	21st to end	Night	13% 5% 2 4 5% 3 3 5% 5% 7% 48%	2660
December	1st to 10th	Night	16% 8% 2 5% 4 4 4 3 5% 6% 42%	2660
	11th to 20th	Night	17% 5% 2 6% 4 4 3 4 5% 7% 43%	3285
	21st to end	Night	16% 7% 2 6% 4 5% 5% 4 6% 7% 37%	4884

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	37% 4 8% 32% 15% 4	2582
	11th to 20th	Day	46% 4 10% 21% 15% 4	2840
	21st to end	Day	48% 5% 10% 23% 11% 2	3124
February	1st to 10th	Day	43% 5% 13% 24% 14% 2	1710
	11th to 20th	Day	42% 5% 8% 26% 16% 3	1736
	21st to end	Day	46% 5% 11% 19% 16% 3	1570
March	1st to 10th	Day	45% 5% 10% 24% 13% 3	3364
	11th to 20th	Day	33% 4 8% 27% 23% 4	3480
	21st to end	Day	45% 5% 10% 21% 16% 3	3896
April	1st to 10th	Day	39% 6% 11% 21% 18% 5%	2280
	11th to 20th	Day	37% 4 10% 22% 21% 6%	2254
	21st to end	Day	41% 5% 11% 22% 18% 4	2280
May	1st to 10th	Day	51% 8% 10% 19% 11% 2	4019
	11th to 20th	Day	48% 9% 12% 21% 8% 1	4301
	21st to end	Day	33% 8% 14% 24% 17% 3	4837
June	1st to 10th	Day	36% 8% 13% 26% 16% 2	2660
	11th to 20th	Day	37% 10% 11% 24% 16% 2	2660
	21st to end	Day	33% 9% 15% 24% 17% 2	2660
July	1st to 10th	Day	34% 10% 19% 20% 12% 5%	4370
	11th to 20th	Day	37% 7% 17% 27% 10% 2	4245
	21st to end	Day	42% 8% 11% 21% 17% 1	4324
August	1st to 10th	Day	44% 8% 15% 24% 8%	4091
	11th to 20th	Day	39% 8% 12% 26% 14% 1	3894
	21st to end	Day	34% 7% 12% 28% 17% 2	4180
September	1st to 10th	Day	38% 5% 13% 19% 20% 4	2280
	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% 21% 4	1781
	11th to 20th	Day	34% 4 10% 24% 23% 5%	1706
	21st to end	Day	37% 4 9% 28% 18% 3	1520
December	1st to 10th	Day	41% 5% 14% 26% 13% 2	1520
	11th to 20th	Day	38% 4 10% 32% 14% 3	1906
	21st to end	Day	45% 7% 13% 24% 11% 1	2772
January	1st to 10th	Night	39% 4 9% 27% 17% 4	4362
	11th to 20th	Night	40% 4 9% 25% 19% 3	4120
	21st to end	Night	46% 4 7% 27% 15% 1	4532
February	1st to 10th	Night	41% 5% 12% 25% 15% 2	2470
	11th to 20th	Night	43% 3 8% 22% 18% 5%	2339
	21st to end	Night	42% 4 10% 22% 17% 5%	1884
March	1st to 10th	Night	47% 5% 8% 26% 13% 2	3488
	11th to 20th	Night	32% 4 11% 30% 19% 4	3480
	21st to end	Night	41% 5% 8% 22% 19% 6%	3736
April	1st to 10th	Night	41% 6% 9% 19% 20% 4	1900
	11th to 20th	Night	37% 5% 11% 21% 20% 6%	1875
	21st to end	Night	38% 6% 11% 25% 15% 6%	1900
May	1st to 10th	Night	50% 8% 11% 19% 10% 3	2968
	11th to 20th	Night	50% 8% 10% 20% 10% 2	2595
	21st to end	Night	33% 8% 18% 23% 14% 3	2739
June	1st to 10th	Night	35% 7% 13% 27% 17% 2	1520
	11th to 20th	Night	36% 8% 14% 25% 14% 3	1520
	21st to end	Night	35% 7% 15% 22% 19% 3	1520
July	1st to 10th	Night	37% 9% 20% 18% 12% 3	2480
	11th to 20th	Night	40% 8% 18% 23% 10% 1	2588
	21st to end	Night	42% 8% 13% 22% 15% 1	2988
August	1st to 10th	Night	45% 8% 17% 21% 8%	2814
	11th to 20th	Night	45% 7% 11% 25% 11% 1	2967
	21st to end	Night	33% 5% 11% 31% 18% 2	3476
September	1st to 10th	Night	38% 7% 10% 22% 18% 5%	1900
	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% 3 9% 26% 23% 6%	2368
	11th to 20th	Night	36% 4 10% 22% 23% 6%	2470
	21st to end	Night	33% 6% 11% 26% 21% 3	2660
December	1st to 10th	Night	42% 5% 11% 24% 15% 2	2660
	11th to 20th	Night	40% 4 11% 23% 19% 3	3288
	21st to end	Night	44% 7% 14% 23% 11% 1	4884

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	28% 21% 33% 14% 4 1	198
	11th to 20th	Day	29% 25% 30% 12% 4	270
	21st to end	Day	22% 26% 32% 14% 5%	297
February	1st to 10th	Day	25% 23% 35% 13% 4	270
	11th to 20th	Day	33% 20% 37% 8% 1	334
	21st to end	Day	31% 18% 31% 18% 2	382
March	1st to 10th	Day	25% 22% 31% 18% 3 1	460
	11th to 20th	Day	28% 19% 32% 15% 5% 1	460
	21st to end	Day	25% 18% 32% 18% 6% 1	506
April	1st to 10th	Day	22% 22% 33% 16% 6% 1	460
	11th to 20th	Day	18% 19% 34% 19% 7% 3	452
	21st to end	Day	26% 15% 31% 20% 7% 1	460
May	1st to 10th	Day	24% 17% 28% 20% 8% 3	463
	11th to 20th	Day	21% 13% 28% 25% 9% 3	487
	21st to end	Day	17% 8% 25% 29% 18% 3	529
June	1st to 10th	Day	14% 9% 27% 29% 15% 6%	490
	11th to 20th	Day	16% 7% 27% 29% 17% 5%	490
	21st to end	Day	11% 5% 30% 33% 17% 4	490
July	1st to 10th	Day	13% 2 16% 39% 19% 11%	485
	11th to 20th	Day	6% 3 21% 36% 27% 7%	484
	21st to end	Day	12% 4 23% 34% 21% 6%	532
August	1st to 10th	Day	13% 4 20% 31% 23% 8%	486
	11th to 20th	Day	12% 4 18% 33% 24% 9%	479
	21st to end	Day	10% 3 22% 33% 20% 12%	539
September	1st to 10th	Day	16% 7% 22% 26% 17% 13%	490
	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	16% 14% 32% 25% 10% 3	416
	11th to 20th	Day	18% 16% 34% 22% 8% 2	366
	21st to end	Day	23% 22% 29% 17% 9% 1	180
December	1st to 10th	Day	18% 24% 37% 15% 4 2	180
	11th to 20th	Day	26% 21% 29% 18% 7%	180
	21st to end	Day	26% 17% 33% 19% 5% 1	198
January	1st to 10th	Night	20% 17% 39% 20% 3	760
	11th to 20th	Night	23% 14% 33% 25% 5%	760
	21st to end	Night	19% 17% 37% 25% 2	836
February	1st to 10th	Night	19% 16% 33% 28% 3	760
	11th to 20th	Night	26% 20% 34% 19% 1	743
	21st to end	Night	25% 17% 36% 19% 3	628
March	1st to 10th	Night	20% 16% 39% 23% 2	760
	11th to 20th	Night	22% 17% 36% 19% 5% 1	760
	21st to end	Night	19% 16% 38% 22% 4 1	836
April	1st to 10th	Night	19% 20% 35% 21% 4	760
	11th to 20th	Night	17% 15% 33% 29% 6%	751
	21st to end	Night	22% 11% 34% 25% 9%	760
May	1st to 10th	Night	20% 10% 31% 27% 11%	646
	11th to 20th	Night	19% 8% 27% 36% 9%	434
	21st to end	Night	14% 6% 23% 38% 17% 1	411
June	1st to 10th	Night	11% 3 31% 35% 19% 1	380
	11th to 20th	Night	14% 4 27% 39% 16% 1	380
	21st to end	Night	11% 3 26% 44% 16% 1	380
July	1st to 10th	Night	8% 3 21% 42% 24% 2	370
	11th to 20th	Night	6% 3 16% 43% 29% 3	432
	21st to end	Night	9% 2 21% 40% 25% 3	624
August	1st to 10th	Night	11% 2 13% 43% 29% 2	567
	11th to 20th	Night	13% 4 15% 40% 25% 2	654
	21st to end	Night	11% 3 19% 36% 27% 4	836
September	1st to 10th	Night	15% 3 20% 31% 26% 4	760
	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% 9% 34% 29% 12% 2	755
	11th to 20th	Night	18% 13% 32% 28% 8% 1	760
	21st to end	Night	17% 14% 32% 27% 9% 1	760
December	1st to 10th	Night	22% 13% 33% 23% 8% 1	760
	11th to 20th	Night	18% 11% 39% 26% 5%	760
	21st to end	Night	21% 12% 32% 31% 3	836

Page generated at 2024-11-01 22:47:40. Version ac07

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