LIGO Hanford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the LIGO Hanford Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	6% 6% 3 6% 5% 3 5% 5% 5% 7% 48%	74789
	Night	7% 8% 3 6% 6% 3 5% 5% 6% 7% 44%	74959
Transparency	Day	36% 6% 11% 21% 18% 7%	74492
	Night	40% 7% 12% 19% 16% 6%	74774
Seeing	Day	9% 7% 20% 28% 24% 12%	13274
	Night	10% 6% 22% 35% 22% 4	22452

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	13% 17% 7% 10% 9% 5% 6% 5% 6% 6% 18%	1519
	11th to 20th	Day	12% 15% 8% 13% 10% 4 5% 5% 5% 5% 18%	1456
	21st to end	Day	10% 16% 6% 10% 9% 5% 9% 6% 5% 6% 18%	1584
February	1st to 10th	Day	11% 12% 5% 10% 9% 4 7% 5% 6% 7% 26%	1570
	11th to 20th	Day	8% 10% 4 7% 7% 4 6% 5% 5% 8% 36%	1624
	21st to end	Day	8% 7% 3 7% 6% 4 6% 6% 7% 9% 36%	1480
March	1st to 10th	Day	8% 8% 4 6% 7% 4 7% 6% 7% 9% 36%	1800
	11th to 20th	Day	4 7% 4 7% 8% 5% 7% 7% 6% 8% 36%	1880
	21st to end	Day	8% 8% 4 7% 7% 4 7% 7% 7% 9% 33%	2350
April	1st to 10th	Day	6% 7% 3 6% 7% 4 6% 5% 6% 8% 40%	2160
	11th to 20th	Day	5% 6% 2 5% 6% 3 6% 5% 6% 8% 48%	2132
	21st to end	Day	5% 5% 2 5% 5% 3 5% 5% 6% 9% 50%	2313
May	1st to 10th	Day	4 3 1 4 5% 3 5% 4 6% 9% 55%	2520
	11th to 20th	Day	6% 5% 2 7% 5% 4 5% 6% 7% 8% 45%	2477
	21st to end	Day	7% 5% 3 5% 5% 4 5% 5% 6% 8% 47%	2723
June	1st to 10th	Day	9% 4 3 5% 4 3 4 5% 5% 8% 50%	2520
	11th to 20th	Day	6% 4 2 4 4 3 4 5% 6% 8% 54%	2520
	21st to end	Day	4 2 1 3 3 2 3 3 5% 7% 66%	2520
July	1st to 10th	Day	1 2 1 2 3 1 2 3 5% 6% 75%	2520
	11th to 20th	Day	1 1 1 2 1 2 3 3 3 6% 76%	2520
	21st to end	Day	1 1 1 2 1 2 2 3 4 84%	2741
August	1st to 10th	Day	2 2 1 2 2 1 2 3 3 5% 76%	2507
	11th to 20th	Day	2 1 1 2 2 1 2 2 3 6% 78%	2332
	21st to end	Day	3 2 1 2 3 2 3 4 5% 5% 70%	2376
September	1st to 10th	Day	2 2 1 2 3 2 2 3 4 5% 74%	2160
	11th to 20th	Day	4 3 1 4 4 2 4 5% 5% 7% 59%	2128
	21st to end	Day	5% 5% 2 4 4 3 4 4 6% 7% 57%	1980
October	1st to 10th	Day	8% 4 2 4 5% 2 3 4 5% 8% 55%	1812
	11th to 20th	Day	5% 5% 2 5% 5% 3 6% 6% 7% 8% 49%	1792
	21st to end	Day	10% 7% 2 6% 6% 5% 5% 5% 5% 7% 42%	1782
November	1st to 10th	Day	9% 7% 4 8% 7% 4 7% 6% 5% 8% 36%	1674
	11th to 20th	Day	8% 13% 5% 9% 7% 6% 6% 5% 6% 7% 29%	1558
	21st to end	Day	9% 11% 6% 10% 9% 6% 7% 6% 6% 6% 24%	1520
December	1st to 10th	Day	8% 11% 6% 13% 9% 4 7% 6% 6% 6% 23%	1520
	11th to 20th	Day	12% 17% 8% 13% 9% 5% 6% 5% 5% 5% 15%	1925
	21st to end	Day	11% 15% 6% 15% 12% 5% 6% 6% 4 4 15%	2794
January	1st to 10th	Night	13% 14% 5% 9% 7% 4 6% 6% 6% 7% 21%	2646
	11th to 20th	Night	11% 14% 6% 10% 7% 5% 7% 6% 5% 7% 23%	2548
	21st to end	Night	9% 12% 6% 9% 10% 5% 5% 5% 6% 7% 25%	2772
February	1st to 10th	Night	10% 8% 4 7% 8% 4 6% 5% 6% 8% 34%	2390
	11th to 20th	Night	7% 8% 3 7% 7% 4 6% 5% 6% 8% 38%	2222
	21st to end	Night	6% 9% 3 8% 6% 4 6% 6% 7% 8% 37%	1776
March	1st to 10th	Night	8% 8% 3 6% 7% 3 6% 6% 6% 8% 39%	2160
	11th to 20th	Night	7% 6% 4 6% 6% 3 6% 6% 7% 8% 40%	2074
	21st to end	Night	7% 9% 4 6% 6% 5% 7% 6% 7% 8% 35%	2005
April	1st to 10th	Night	10% 8% 3 6% 6% 4 5% 5% 6% 9% 38%	1800
	11th to 20th	Night	6% 6% 3 6% 5% 3 5% 5% 5% 6% 50%	1779
	21st to end	Night	5% 6% 3 5% 6% 4 6% 5% 6% 9% 47%	1644
May	1st to 10th	Night	4 4 2 4 4 3 5% 5% 7% 8% 55%	1440
	11th to 20th	Night	6% 5% 2 5% 5% 3 5% 5% 7% 9% 47%	1416
	21st to end	Night	5% 5% 1 3 5% 3 6% 6% 9% 10% 46%	1552
June	1st to 10th	Night	8% 3 2 4 5% 2 4 5% 6% 8% 53%	1440
	11th to 20th	Night	8% 4 2 5% 4 2 3 4 4 6% 56%	1440
	21st to end	Night	3 2 1 3 3 2 4 5% 5% 6% 64%	1440
July	1st to 10th	Night	2 1 1 2 2 1 3 2 5% 6% 73%	1440
	11th to 20th	Night	2 1 1 2 2 1 3 3 4 4 78%	1440
	21st to end	Night	1 1 1 1 1 1 2 3 2 4 83%	1576
August	1st to 10th	Night	2 1 1 2 2 3 2 3 3 6% 75%	1432
	11th to 20th	Night	2 2 1 2 2 2 2 3 3 5% 77%	1564
	21st to end	Night	1 2 1 2 3 2 3 4 5% 6% 71%	1980
September	1st to 10th	Night	2 2 1 2 2 2 2 3 3 5% 76%	1800
	11th to 20th	Night	3 3 2 4 3 2 4 5% 4 7% 64%	1831
	21st to end	Night	4 3 2 4 4 3 3 4 5% 9% 61%	1980
October	1st to 10th	Night	8% 4 2 5% 4 3 4 4 4 6% 55%	2146
	11th to 20th	Night	6% 5% 2 5% 5% 3 4 5% 6% 7% 52%	2168
	21st to end	Night	8% 5% 2 6% 6% 4 5% 4 5% 7% 48%	2574
November	1st to 10th	Night	11% 7% 4 7% 6% 4 6% 5% 5% 7% 38%	2405
	11th to 20th	Night	10% 8% 4 7% 7% 5% 6% 6% 7% 6% 34%	2617
	21st to end	Night	9% 9% 4 7% 7% 4 6% 6% 7% 8% 34%	2660
December	1st to 10th	Night	9% 11% 5% 10% 9% 5% 6% 5% 6% 6% 28%	2660
	11th to 20th	Night	13% 16% 8% 12% 9% 4 6% 5% 5% 5% 17%	3280
	21st to end	Night	11% 17% 5% 9% 8% 5% 5% 6% 6% 7% 22%	4862

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	67% 10% 11% 8% 4 1	1519
	11th to 20th	Day	71% 8% 11% 9% 2	1456
	21st to end	Day	68% 9% 10% 10% 3 1	1584
February	1st to 10th	Day	59% 9% 16% 11% 5%	1570
	11th to 20th	Day	48% 9% 14% 21% 8% 1	1633
	21st to end	Day	45% 9% 15% 21% 9% 1	1480
March	1st to 10th	Day	43% 10% 14% 24% 9% 1	1800
	11th to 20th	Day	46% 7% 12% 25% 10%	1880
	21st to end	Day	45% 10% 17% 20% 8% 1	2350
April	1st to 10th	Day	41% 6% 14% 22% 15% 2	2160
	11th to 20th	Day	34% 6% 15% 24% 17% 5%	2132
	21st to end	Day	31% 6% 12% 27% 19% 4	2313
May	1st to 10th	Day	25% 5% 12% 32% 22% 4	2520
	11th to 20th	Day	35% 7% 12% 25% 17% 4	2480
	21st to end	Day	34% 6% 13% 26% 17% 3	2724
June	1st to 10th	Day	33% 6% 9% 27% 21% 5%	2520
	11th to 20th	Day	27% 6% 12% 28% 23% 5%	2520
	21st to end	Day	19% 4 7% 29% 29% 11%	2520
July	1st to 10th	Day	12% 3 7% 22% 35% 21%	2451
	11th to 20th	Day	12% 3 7% 13% 35% 30%	2445
	21st to end	Day	8% 2 4 14% 41% 32%	2589
August	1st to 10th	Day	13% 2 8% 18% 38% 22%	2490
	11th to 20th	Day	11% 3 6% 23% 40% 18%	2332
	21st to end	Day	16% 4 7% 26% 36% 12%	2376
September	1st to 10th	Day	13% 4 8% 28% 31% 16%	2160
	11th to 20th	Day	24% 6% 11% 24% 28% 8%	2128
	21st to end	Day	26% 7% 14% 29% 18% 5%	1980
October	1st to 10th	Day	30% 5% 13% 31% 18% 2	1812
	11th to 20th	Day	31% 7% 19% 29% 12% 1	1792
	21st to end	Day	43% 8% 16% 22% 11% 1	1782
November	1st to 10th	Day	47% 10% 14% 19% 8% 1	1674
	11th to 20th	Day	56% 8% 14% 15% 6% 1	1558
	21st to end	Day	61% 10% 13% 10% 6% 1	1520
December	1st to 10th	Day	64% 8% 11% 11% 5%	1520
	11th to 20th	Day	73% 8% 9% 8% 2	1928
	21st to end	Day	70% 8% 9% 9% 3	2794
January	1st to 10th	Night	62% 10% 14% 9% 4	2646
	11th to 20th	Night	61% 9% 13% 13% 4	2548
	21st to end	Night	58% 9% 15% 14% 3 1	2772
February	1st to 10th	Night	50% 8% 19% 16% 6% 1	2390
	11th to 20th	Night	43% 9% 17% 21% 9% 1	2224
	21st to end	Night	43% 10% 15% 20% 11% 2	1776
March	1st to 10th	Night	42% 7% 14% 24% 11% 1	2160
	11th to 20th	Night	40% 8% 14% 25% 12% 1	2075
	21st to end	Night	45% 8% 13% 23% 10% 1	2005
April	1st to 10th	Night	43% 6% 14% 22% 13% 2	1800
	11th to 20th	Night	33% 7% 10% 27% 18% 5%	1779
	21st to end	Night	35% 7% 13% 23% 18% 5%	1644
May	1st to 10th	Night	25% 6% 9% 29% 24% 5%	1440
	11th to 20th	Night	33% 6% 11% 22% 24% 5%	1416
	21st to end	Night	29% 9% 13% 27% 18% 4	1552
June	1st to 10th	Night	29% 6% 11% 26% 22% 6%	1440
	11th to 20th	Night	30% 5% 8% 29% 24% 5%	1440
	21st to end	Night	20% 4 8% 24% 29% 14%	1440
July	1st to 10th	Night	13% 3 6% 19% 32% 27%	1400
	11th to 20th	Night	12% 3 5% 16% 31% 34%	1392
	21st to end	Night	7% 2 5% 11% 34% 41%	1488
August	1st to 10th	Night	14% 2 5% 12% 38% 29%	1424
	11th to 20th	Night	12% 3 5% 18% 34% 28%	1564
	21st to end	Night	14% 3 7% 21% 37% 18%	1980
September	1st to 10th	Night	13% 3 7% 25% 30% 22%	1800
	11th to 20th	Night	20% 5% 10% 22% 27% 15%	1831
	21st to end	Night	22% 4 11% 29% 24% 10%	1980
October	1st to 10th	Night	30% 4 10% 26% 28% 2	2146
	11th to 20th	Night	32% 7% 11% 29% 19% 3	2168
	21st to end	Night	35% 7% 12% 27% 16% 2	2574
November	1st to 10th	Night	45% 8% 15% 21% 10% 1	2405
	11th to 20th	Night	49% 10% 14% 19% 6% 1	2617
	21st to end	Night	47% 10% 18% 17% 7% 1	2660
December	1st to 10th	Night	58% 10% 13% 15% 4	2660
	11th to 20th	Night	70% 9% 12% 7% 2	3276
	21st to end	Night	61% 10% 14% 9% 6%	4862

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	21% 8% 24% 24% 17% 5%	180
	11th to 20th	Day	20% 5% 25% 31% 15% 4	164
	21st to end	Day	16% 5% 23% 31% 23% 3	176
February	1st to 10th	Day	18% 7% 26% 32% 14% 3	290
	11th to 20th	Day	13% 10% 25% 28% 20% 4	357
	21st to end	Day	10% 12% 25% 32% 18% 3	346
March	1st to 10th	Day	13% 11% 28% 27% 17% 4	420
	11th to 20th	Day	9% 12% 23% 29% 20% 7%	420
	21st to end	Day	12% 13% 27% 29% 16% 3	462
April	1st to 10th	Day	12% 13% 27% 25% 17% 6%	420
	11th to 20th	Day	10% 17% 30% 23% 14% 7%	412
	21st to end	Day	7% 11% 28% 28% 18% 8%	420
May	1st to 10th	Day	7% 10% 24% 27% 23% 10%	423
	11th to 20th	Day	9% 8% 21% 27% 23% 12%	448
	21st to end	Day	9% 5% 17% 27% 26% 15%	485
June	1st to 10th	Day	8% 8% 19% 26% 25% 14%	450
	11th to 20th	Day	8% 7% 24% 36% 18% 8%	450
	21st to end	Day	4 5% 16% 29% 28% 18%	450
July	1st to 10th	Day	1 3 11% 30% 43% 13%	446
	11th to 20th	Day	1 3 14% 30% 37% 15%	444
	21st to end	Day	1 2 10% 22% 41% 24%	487
August	1st to 10th	Day	3 3 11% 23% 35% 24%	447
	11th to 20th	Day	1 2 8% 23% 34% 31%	437
	21st to end	Day	3 5% 15% 28% 27% 22%	495
September	1st to 10th	Day	3 2 14% 31% 35% 16%	450
	11th to 20th	Day	6% 5% 14% 26% 31% 18%	450
	21st to end	Day	8% 8% 18% 25% 30% 11%	450
October	1st to 10th	Day	10% 6% 21% 26% 24% 13%	450
	11th to 20th	Day	9% 8% 16% 28% 26% 13%	442
	21st to end	Day	15% 8% 25% 29% 15% 8%	297
November	1st to 10th	Day	15% 5% 30% 33% 14% 3	270
	11th to 20th	Day	16% 11% 25% 26% 15% 7%	198
	21st to end	Day	11% 4 23% 31% 23% 8%	180
December	1st to 10th	Day	12% 4 26% 29% 16% 13%	180
	11th to 20th	Day	17% 5% 24% 38% 13% 2	180
	21st to end	Day	17% 9% 19% 36% 15% 3	198
January	1st to 10th	Night	20% 8% 24% 29% 17% 2	760
	11th to 20th	Night	17% 7% 18% 35% 20% 3	728
	21st to end	Night	15% 5% 17% 38% 23% 3	792
February	1st to 10th	Night	14% 8% 23% 35% 18% 2	720
	11th to 20th	Night	11% 8% 24% 35% 20% 2	704
	21st to end	Night	10% 10% 24% 40% 15% 2	592
March	1st to 10th	Night	12% 7% 26% 35% 17% 3	720
	11th to 20th	Night	9% 11% 23% 35% 21% 2	720
	21st to end	Night	10% 10% 31% 32% 15% 1	792
April	1st to 10th	Night	14% 9% 23% 33% 19% 2	720
	11th to 20th	Night	10% 11% 29% 28% 17% 4	711
	21st to end	Night	9% 10% 30% 30% 20% 2	564
May	1st to 10th	Night	5% 9% 28% 34% 19% 4	360
	11th to 20th	Night	8% 6% 22% 33% 25% 6%	354
	21st to end	Night	6% 6% 21% 38% 25% 4	389
June	1st to 10th	Night	9% 5% 26% 35% 23% 2	360
	11th to 20th	Night	9% 6% 35% 33% 15% 2	360
	21st to end	Night	5% 5% 29% 34% 25% 2	360
July	1st to 10th	Night	3 2 19% 45% 29% 3	350
	11th to 20th	Night	3 4 24% 43% 25% 2	348
	21st to end	Night	1 3 19% 41% 32% 3	394
August	1st to 10th	Night	3 2 18% 37% 35% 5%	358
	11th to 20th	Night	3 1 16% 33% 37% 10%	496
	21st to end	Night	2 3 18% 36% 34% 6%	792
September	1st to 10th	Night	3 3 18% 38% 32% 6%	720
	11th to 20th	Night	4 3 16% 32% 37% 8%	720
	21st to end	Night	5% 4 20% 33% 31% 5%	720
October	1st to 10th	Night	10% 5% 18% 35% 29% 4	720
	11th to 20th	Night	8% 6% 20% 34% 24% 8%	720
	21st to end	Night	10% 6% 26% 31% 21% 5%	792
November	1st to 10th	Night	14% 7% 20% 41% 17% 1	740
	11th to 20th	Night	13% 8% 25% 33% 17% 4	760
	21st to end	Night	13% 6% 21% 35% 21% 4	760
December	1st to 10th	Night	13% 6% 18% 37% 23% 3	760
	11th to 20th	Night	20% 6% 16% 40% 17% 2	760
	21st to end	Night	19% 7% 23% 35% 13% 2	836

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