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 2024-11-10.

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% 2 6% 4 4 4 3 5% 7% 52%	105347
	Night	7% 8% 3 7% 4 4 4 4 6% 7% 47%	102411
Transparency	Day	33% 5% 11% 22% 19% 10%	105116
	Night	37% 6% 11% 21% 16% 8%	102288
Seeing	Day	8% 7% 20% 28% 25% 12%	14402
	Night	10% 6% 22% 35% 22% 3	23437

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	11% 19% 6% 11% 7% 5% 5% 5% 6% 6% 18%	2539
	11th to 20th	Day	17% 16% 7% 11% 6% 4 5% 3 5% 5% 20%	2516
	21st to end	Day	18% 21% 7% 15% 7% 6% 5% 3 4 4 10%	2794
February	1st to 10th	Day	10% 13% 4 11% 8% 5% 7% 3 7% 7% 26%	1657
	11th to 20th	Day	8% 11% 3 9% 5% 6% 6% 4 5% 8% 35%	1716
	21st to end	Day	8% 9% 3 7% 5% 5% 6% 5% 8% 9% 35%	1570
March	1st to 10th	Day	5% 7% 2 6% 5% 5% 6% 5% 8% 11% 40%	3138
	11th to 20th	Day	3 5% 2 5% 4 5% 5% 4 5% 6% 57%	3325
	21st to end	Day	6% 9% 3 9% 6% 6% 7% 5% 8% 8% 33%	4126
April	1st to 10th	Day	6% 8% 3 7% 6% 5% 6% 5% 6% 8% 40%	2280
	11th to 20th	Day	5% 6% 2 6% 5% 4 5% 4 6% 8% 48%	2252
	21st to end	Day	5% 6% 2 5% 5% 5% 5% 5% 6% 9% 48%	2442
May	1st to 10th	Day	8% 5% 1 4 3 3 3 3 5% 8% 57%	4420
	11th to 20th	Day	3 3 1 4 3 3 4 3 5% 7% 63%	4377
	21st to end	Day	6% 6% 2 6% 5% 5% 6% 4 7% 7% 45%	4813
June	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 5% 8% 49%	2660
	11th to 20th	Day	5% 4 1 5% 4 4 3 4 6% 8% 55%	2660
	21st to end	Day	3 2 1 4 3 3 3 3 5% 7% 66%	2660
July	1st to 10th	Day	1 1 2 2 1 1 2 3 4 83%	4420
	11th to 20th	Day	1 1 2 1 2 2 2 2 4 83%	4420
	21st to end	Day	1 1 1 2 2 2 2 2 4 4 81%	4790
August	1st to 10th	Day	2 2 1 3 2 2 2 2 4 5% 75%	4401
	11th to 20th	Day	1 1 1 2 1 2 3 2 4 6% 76%	4070
	21st to end	Day	3 4 2 4 2 3 3 2 4 5% 69%	4180
September	1st to 10th	Day	2 2 1 2 2 2 2 3 5% 5% 73%	2280
	11th to 20th	Day	4 4 1 5% 4 3 4 4 6% 7% 59%	2246
	21st to end	Day	5% 5% 2 5% 3 4 3 3 6% 7% 58%	2090
October	1st to 10th	Day	6% 3 1 3 3 2 2 3 5% 8% 65%	3192
	11th to 20th	Day	3 3 1 5% 4 5% 7% 5% 7% 9% 49%	3140
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 4 6% 7% 44%	3101
November	1st to 10th	Day	9% 8% 3 9% 5% 5% 7% 4 6% 8% 36%	1755
	11th to 20th	Day	8% 14% 5% 10% 6% 7% 6% 3 6% 7% 29%	1558
	21st to end	Day	8% 12% 5% 12% 8% 8% 6% 5% 6% 6% 24%	1520
December	1st to 10th	Day	7% 14% 5% 15% 8% 6% 6% 5% 6% 6% 23%	1520
	11th to 20th	Day	12% 18% 6% 15% 8% 7% 6% 4 5% 4 15%	1925
	21st to end	Day	11% 16% 5% 18% 9% 7% 6% 4 4 4 15%	2794
January	1st to 10th	Night	11% 17% 6% 11% 7% 5% 5% 5% 6% 6% 19%	4406
	11th to 20th	Night	15% 16% 5% 8% 5% 5% 5% 4 5% 8% 23%	4382
	21st to end	Night	13% 17% 8% 16% 7% 6% 4 3 5% 5% 16%	4862
February	1st to 10th	Night	10% 10% 3 9% 7% 5% 6% 4 6% 7% 32%	2523
	11th to 20th	Night	7% 9% 3 8% 6% 5% 6% 4 6% 7% 38%	2350
	21st to end	Night	6% 10% 3 8% 6% 6% 5% 6% 7% 8% 37%	1884
March	1st to 10th	Night	5% 6% 2 7% 5% 4 5% 5% 9% 10% 41%	3736
	11th to 20th	Night	5% 5% 2 5% 3 3 3 4 5% 6% 59%	3649
	21st to end	Night	5% 7% 3 8% 6% 7% 7% 6% 8% 9% 33%	3526
April	1st to 10th	Night	9% 9% 3 7% 6% 5% 4 4 6% 9% 37%	1900
	11th to 20th	Night	6% 7% 3 6% 5% 5% 4 4 5% 6% 50%	1879
	21st to end	Night	5% 6% 2 6% 5% 5% 5% 4 6% 9% 47%	1734
May	1st to 10th	Night	6% 5% 2 5% 4 4 4 3 6% 6% 54%	2540
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 7% 64%	2516
	21st to end	Night	4 4 1 4 4 4 5% 4 7% 9% 53%	2762
June	1st to 10th	Night	7% 4 2 4 4 4 4 4 6% 8% 53%	1520
	11th to 20th	Night	8% 5% 2 5% 3 3 3 3 4 6% 57%	1520
	21st to end	Night	3 3 1 4 3 3 4 3 5% 6% 64%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 3 3 84%	2540
	11th to 20th	Night	1 1 1 1 1 2 1 3 4 83%	2540
	21st to end	Night	1 1 1 1 2 2 2 2 4 83%	2762
August	1st to 10th	Night	2 2 1 3 2 3 3 2 3 6% 74%	2529
	11th to 20th	Night	2 2 1 2 2 2 3 3 4 5% 74%	2763
	21st to end	Night	1 2 1 3 3 3 3 2 5% 6% 71%	3498
September	1st to 10th	Night	2 2 1 3 2 3 2 2 3 6% 75%	1900
	11th to 20th	Night	3 3 2 4 3 3 4 4 5% 7% 63%	1933
	21st to end	Night	4 3 2 5% 2 3 4 2 6% 9% 61%	2090
October	1st to 10th	Night	5% 3 1 4 2 3 3 3 5% 6% 66%	3786
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 7% 9% 53%	3797
	21st to end	Night	8% 4 2 6% 4 5% 5% 3 7% 8% 47%	4457
November	1st to 10th	Night	10% 8% 3 7% 5% 5% 6% 4 5% 7% 39%	2528
	11th to 20th	Night	10% 9% 3 8% 6% 6% 6% 4 7% 6% 34%	2617
	21st to end	Night	9% 10% 3 9% 6% 5% 6% 5% 6% 8% 34%	2660
December	1st to 10th	Night	9% 13% 4 11% 8% 6% 6% 4 6% 6% 28%	2660
	11th to 20th	Night	13% 17% 7% 14% 7% 5% 6% 4 5% 5% 17%	3280
	21st to end	Night	11% 18% 5% 11% 6% 6% 5% 4 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	63% 10% 11% 11% 5%	2539
	11th to 20th	Day	66% 7% 15% 10% 2	2540
	21st to end	Day	78% 8% 6% 6% 2	2794
February	1st to 10th	Day	59% 9% 17% 11% 5%	1657
	11th to 20th	Day	48% 9% 14% 20% 8% 1	1725
	21st to end	Day	45% 9% 15% 21% 9% 1	1570
March	1st to 10th	Day	35% 8% 15% 25% 17% 1	3138
	11th to 20th	Day	28% 5% 14% 40% 12%	3325
	21st to end	Day	44% 10% 17% 18% 10% 1	4126
April	1st to 10th	Day	41% 6% 15% 22% 14% 2	2280
	11th to 20th	Day	33% 6% 15% 23% 17% 5%	2252
	21st to end	Day	31% 7% 13% 27% 19% 4	2442
May	1st to 10th	Day	25% 4 10% 27% 27% 7%	4420
	11th to 20th	Day	21% 4 9% 29% 28% 10%	4380
	21st to end	Day	34% 7% 17% 27% 13% 3	4814
June	1st to 10th	Day	33% 6% 9% 26% 21% 5%	2660
	11th to 20th	Day	26% 6% 11% 27% 24% 6%	2660
	21st to end	Day	19% 4 7% 29% 29% 11%	2660
July	1st to 10th	Day	8% 2 4 13% 26% 47%	4351
	11th to 20th	Day	9% 2 4 8% 24% 54%	4345
	21st to end	Day	9% 2 4 16% 36% 34%	4639
August	1st to 10th	Day	14% 2 5% 12% 38% 30%	4384
	11th to 20th	Day	12% 3 4 34% 36% 11%	4112
	21st to end	Day	20% 3 7% 27% 32% 11%	4179
September	1st to 10th	Day	13% 4 8% 27% 31% 16%	2280
	11th to 20th	Day	24% 6% 11% 24% 27% 7%	2246
	21st to end	Day	25% 7% 14% 31% 18% 5%	2090
October	1st to 10th	Day	21% 3 12% 36% 25% 3	3192
	11th to 20th	Day	31% 6% 21% 27% 15% 1	3140
	21st to end	Day	40% 6% 14% 27% 12% 1	3101
November	1st to 10th	Day	47% 9% 14% 20% 8% 1	1755
	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	63% 10% 13% 10% 4	4406
	11th to 20th	Night	58% 8% 15% 16% 3	4420
	21st to end	Night	70% 9% 10% 8% 2 1	4862
February	1st to 10th	Night	51% 8% 19% 16% 6% 1	2523
	11th to 20th	Night	44% 9% 17% 21% 9% 1	2352
	21st to end	Night	42% 9% 16% 20% 10% 2	1884
March	1st to 10th	Night	34% 6% 15% 29% 15% 1	3736
	11th to 20th	Night	26% 6% 11% 38% 19% 1	3650
	21st to end	Night	42% 11% 15% 25% 7%	3526
April	1st to 10th	Night	43% 6% 15% 22% 13% 2	1900
	11th to 20th	Night	33% 7% 10% 26% 19% 5%	1879
	21st to end	Night	34% 7% 13% 23% 18% 5%	1734
May	1st to 10th	Night	28% 4 10% 22% 30% 5%	2540
	11th to 20th	Night	20% 4 9% 28% 34% 6%	2516
	21st to end	Night	24% 9% 19% 35% 11% 2	2762
June	1st to 10th	Night	29% 6% 10% 26% 22% 7%	1520
	11th to 20th	Night	29% 5% 7% 28% 25% 5%	1520
	21st to end	Night	21% 3 8% 24% 30% 14%	1520
July	1st to 10th	Night	7% 2 4 13% 24% 51%	2500
	11th to 20th	Night	8% 2 3 9% 22% 57%	2492
	21st to end	Night	8% 1 3 13% 32% 43%	2674
August	1st to 10th	Night	15% 2 3 8% 35% 37%	2521
	11th to 20th	Night	14% 3 4 28% 34% 16%	2787
	21st to end	Night	16% 2 6% 26% 31% 19%	3498
September	1st to 10th	Night	13% 3 7% 25% 30% 22%	1900
	11th to 20th	Night	22% 5% 10% 23% 26% 14%	1933
	21st to end	Night	21% 4 11% 31% 24% 10%	2090
October	1st to 10th	Night	20% 3 9% 35% 31% 2	3786
	11th to 20th	Night	28% 5% 14% 29% 22% 2	3797
	21st to end	Night	35% 5% 11% 31% 16% 2	4457
November	1st to 10th	Night	45% 8% 15% 21% 10% 1	2528
	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	21% 7% 24% 30% 16% 3	180
	21st to end	Day	17% 5% 22% 32% 22% 3	198
February	1st to 10th	Day	17% 7% 25% 32% 15% 4	317
	11th to 20th	Day	14% 11% 24% 28% 20% 4	389
	21st to end	Day	10% 12% 26% 31% 18% 3	382
March	1st to 10th	Day	12% 11% 28% 28% 17% 4	460
	11th to 20th	Day	8% 12% 22% 29% 20% 9%	460
	21st to end	Day	11% 13% 29% 28% 15% 3	506
April	1st to 10th	Day	11% 13% 28% 25% 16% 6%	460
	11th to 20th	Day	9% 16% 30% 24% 14% 7%	452
	21st to end	Day	8% 10% 28% 27% 19% 8%	460
May	1st to 10th	Day	8% 10% 25% 26% 22% 10%	463
	11th to 20th	Day	9% 8% 21% 27% 24% 12%	488
	21st to end	Day	9% 6% 16% 28% 26% 15%	529
June	1st to 10th	Day	9% 8% 19% 27% 26% 13%	490
	11th to 20th	Day	7% 7% 23% 36% 19% 8%	490
	21st to end	Day	3 5% 16% 29% 29% 18%	490
July	1st to 10th	Day	1 2 10% 29% 44% 13%	486
	11th to 20th	Day	1 3 13% 30% 36% 17%	484
	21st to end	Day	1 2 11% 23% 40% 24%	531
August	1st to 10th	Day	3 3 10% 24% 36% 24%	487
	11th to 20th	Day	2 2 9% 23% 34% 31%	477
	21st to end	Day	3 5% 14% 27% 27% 23%	539
September	1st to 10th	Day	3 2 13% 29% 34% 19%	490
	11th to 20th	Day	5% 5% 13% 28% 31% 18%	490
	21st to end	Day	7% 8% 18% 25% 31% 11%	490
October	1st to 10th	Day	9% 6% 20% 27% 25% 13%	490
	11th to 20th	Day	8% 7% 17% 29% 26% 13%	481
	21st to end	Day	15% 8% 24% 30% 16% 7%	330
November	1st to 10th	Day	15% 5% 29% 33% 15% 3	297
	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% 6% 18% 35% 20% 3	760
	21st to end	Night	15% 5% 16% 39% 23% 3	836
February	1st to 10th	Night	14% 8% 23% 35% 18% 2	760
	11th to 20th	Night	11% 8% 24% 34% 20% 2	744
	21st to end	Night	10% 11% 23% 39% 15% 1	628
March	1st to 10th	Night	12% 7% 26% 36% 17% 3	760
	11th to 20th	Night	8% 11% 23% 35% 22% 2	760
	21st to end	Night	10% 10% 31% 33% 15% 1	836
April	1st to 10th	Night	14% 9% 24% 32% 19% 2	760
	11th to 20th	Night	10% 11% 30% 29% 17% 3	751
	21st to end	Night	9% 9% 31% 30% 20% 2	594
May	1st to 10th	Night	5% 9% 28% 34% 18% 4	380
	11th to 20th	Night	8% 7% 23% 33% 24% 5%	374
	21st to end	Night	6% 6% 21% 38% 25% 4	411
June	1st to 10th	Night	9% 6% 26% 35% 23% 2	380
	11th to 20th	Night	9% 6% 35% 33% 15% 2	380
	21st to end	Night	4 5% 29% 35% 25% 2	380
July	1st to 10th	Night	3 2 20% 44% 28% 2	370
	11th to 20th	Night	3 4 23% 43% 25% 2	368
	21st to end	Night	1 3 18% 43% 32% 3	416
August	1st to 10th	Night	2 2 18% 39% 35% 5%	378
	11th to 20th	Night	3 1 16% 33% 37% 10%	525
	21st to end	Night	2 3 18% 37% 34% 6%	836
September	1st to 10th	Night	3 3 17% 38% 32% 7%	760
	11th to 20th	Night	4 3 16% 33% 36% 8%	760
	21st to end	Night	5% 4 20% 34% 31% 6%	760
October	1st to 10th	Night	9% 4 19% 35% 29% 4	760
	11th to 20th	Night	7% 6% 21% 34% 25% 7%	760
	21st to end	Night	10% 6% 26% 33% 21% 5%	836
November	1st to 10th	Night	14% 7% 20% 41% 18% 1	778
	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

Page generated at 2024-11-07 03:55:48. Version ac07

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