Hanford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hanford clear sky chart from 2005-11-06 to 2025-06-10.

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	4 3 1 4 3 3 3 2 4 5% 68%	123236
	Night	6% 3 1 4 3 3 3 2 4 5% 66%	123891
Transparency	Day	20% 3 8% 18% 26% 25%	122988
	Night	22% 4 9% 19% 25% 21%	123726
Seeing	Day	5% 3 12% 25% 33% 22%	16115
	Night	6% 3 15% 39% 33% 4	25728

By Month

Percentage Cloud Cover SW (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	7% 10% 3 10% 7% 5% 4 3 5% 7% 38%	3639
	11th to 20th	Day	6% 6% 3 7% 5% 6% 4 3 4 5% 50%	4026
	21st to end	Day	11% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	4510
February	1st to 10th	Day	13% 6% 2 5% 4 3 4 2 5% 7% 48%	1922
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 7% 51%	1950
	21st to end	Day	11% 7% 2 5% 4 4 4 3 5% 7% 49%	1650
March	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 42%	4518
	11th to 20th	Day	8% 4 2 6% 4 4 4 3 5% 7% 53%	4912
	21st to end	Day	9% 4 2 5% 3 4 5% 4 5% 8% 51%	5854
April	1st to 10th	Day	7% 4 1 4 3 3 3 3 5% 7% 62%	2400
	11th to 20th	Day	6% 3 1 4 3 3 3 3 5% 5% 65%	2369
	21st to end	Day	4 3 1 2 2 2 3 2 4 6% 71%	2400
May	1st to 10th	Day	3 2 1 2 1 2 2 1 2 4 81%	5501
	11th to 20th	Day	1 1 1 1 2 2 2 3 4 84%	5841
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	6833
June	1st to 10th	Day	2 1 1 2 1 2 2 2 4 5% 78%	2789
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	2660
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 2 1 1 2 2 2 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4461
August	1st to 10th	Day	1 1 1 1 1 1 1 2 3 87%	3849
	11th to 20th	Day	1 1 1 1 1 1 1 1 94%	3724
	21st to end	Day	1 1 1 1 1 1 2 92%	4180
September	1st to 10th	Day	1 1 1 1 2 1 1 3 88%	2280
	11th to 20th	Day	1 1 1 1 1 1 1 2 4 86%	2228
	21st to end	Day	1 1 2 1 2 2 2 3 4 84%	2090
October	1st to 10th	Day	1 1 1 1 1 2 2 4 5% 80%	3312
	11th to 20th	Day	1 2 1 3 2 2 2 2 3 4 78%	3161
	21st to end	Day	2 2 1 3 3 2 3 3 4 7% 71%	3448
November	1st to 10th	Day	5% 3 1 3 3 3 3 3 5% 7% 63%	1774
	11th to 20th	Day	7% 5% 2 5% 3 4 4 2 4 7% 56%	1800
	21st to end	Day	8% 6% 2 6% 4 4 4 4 5% 6% 53%	1800
December	1st to 10th	Day	5% 7% 3 10% 7% 6% 6% 4 5% 6% 40%	2823
	11th to 20th	Day	11% 8% 3 9% 7% 6% 5% 4 6% 9% 32%	3028
	21st to end	Day	13% 12% 5% 12% 6% 5% 5% 3 5% 8% 25%	4004
January	1st to 10th	Night	9% 7% 2 7% 6% 5% 4 3 5% 7% 44%	6306
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 4 5% 56%	5946
	21st to end	Night	12% 6% 2 6% 4 4 5% 3 5% 6% 46%	6468
February	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2475
	11th to 20th	Night	10% 4 2 3 3 3 4 3 4 7% 58%	2338
	21st to end	Night	10% 5% 1 5% 3 3 3 2 4 6% 58%	1980
March	1st to 10th	Night	9% 4 2 6% 4 4 5% 4 6% 8% 47%	5376
	11th to 20th	Night	7% 3 1 5% 3 4 4 4 5% 7% 56%	5071
	21st to end	Night	12% 6% 2 6% 4 4 3 3 4 5% 51%	5121
April	1st to 10th	Night	7% 3 2 3 3 3 3 3 4 6% 64%	2000
	11th to 20th	Night	4 4 1 3 3 2 2 2 3 6% 70%	1979
	21st to end	Night	4 3 1 2 3 2 3 2 4 6% 72%	2000
May	1st to 10th	Night	3 1 2 1 2 2 2 2 5% 80%	4481
	11th to 20th	Night	2 1 1 1 1 1 1 1 2 4 84%	4074
	21st to end	Night	1 1 1 1 1 1 1 1 2 91%	4026
June	1st to 10th	Night	2 2 1 2 1 2 2 2 3 5% 79%	1595
	11th to 20th	Night	1 1 1 1 1 1 2 3 89%	1520
	21st to end	Night	1 1 1 1 2 3 91%	1520
July	1st to 10th	Night	1 1 1 1 2 93%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 3 86%	2540
	21st to end	Night	1 1 1 1 1 2 1 3 3 87%	3099
August	1st to 10th	Night	1 1 1 2 1 1 2 1 2 3 85%	3097
	11th to 20th	Night	1 1 1 2 2 92%	3126
	21st to end	Night	1 1 1 1 1 1 2 92%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	1 1 1 1 2 1 1 2 3 86%	1951
	21st to end	Night	1 1 1 1 1 1 1 2 4 85%	2090
October	1st to 10th	Night	1 1 2 1 1 1 1 2 4 84%	3665
	11th to 20th	Night	1 1 1 2 2 2 2 3 2 4 81%	3777
	21st to end	Night	3 2 2 1 2 2 2 3 4 79%	4121
November	1st to 10th	Night	4 3 1 4 3 2 3 3 4 6% 68%	2525
	11th to 20th	Night	5% 3 1 3 2 3 4 2 4 6% 66%	2600
	21st to end	Night	7% 5% 2 6% 3 3 3 3 5% 6% 58%	2600
December	1st to 10th	Night	6% 4 2 6% 7% 7% 5% 4 5% 6% 48%	4358
	11th to 20th	Night	13% 6% 2 8% 6% 6% 5% 4 7% 8% 36%	5176
	21st to end	Night	10% 7% 4 10% 7% 6% 5% 4 7% 8% 32%	6952

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	43% 7% 19% 17% 12% 1	3639
	11th to 20th	Day	35% 6% 10% 14% 23% 12%	4026
	21st to end	Day	42% 6% 11% 13% 23% 4	4510
February	1st to 10th	Day	37% 5% 11% 25% 17% 5%	1922
	11th to 20th	Day	30% 6% 12% 27% 18% 6%	1960
	21st to end	Day	35% 6% 13% 26% 16% 4	1650
March	1st to 10th	Day	35% 7% 15% 20% 21% 3	4518
	11th to 20th	Day	30% 5% 14% 30% 15% 7%	4912
	21st to end	Day	30% 6% 14% 33% 15% 3	5854
April	1st to 10th	Day	24% 4 11% 30% 23% 8%	2400
	11th to 20th	Day	20% 5% 12% 28% 26% 10%	2372
	21st to end	Day	16% 4 11% 31% 29% 8%	2400
May	1st to 10th	Day	11% 2 8% 21% 39% 19%	5501
	11th to 20th	Day	8% 1 4 25% 42% 19%	5844
	21st to end	Day	7% 2 4 21% 47% 21%	6834
June	1st to 10th	Day	10% 3 8% 20% 34% 25%	2789
	11th to 20th	Day	4 2 4 13% 38% 40%	2660
	21st to end	Day	6% 1 2 8% 27% 57%	2660
July	1st to 10th	Day	3 1 1 3 13% 80%	4351
	11th to 20th	Day	8% 1 2 5% 16% 69%	4345
	21st to end	Day	5% 1 2 6% 29% 56%	4320
August	1st to 10th	Day	6% 1 1 5% 30% 56%	3834
	11th to 20th	Day	3 1 5% 24% 67%	3760
	21st to end	Day	3 1 2 7% 28% 59%	4179
September	1st to 10th	Day	5% 1 3 7% 28% 56%	2280
	11th to 20th	Day	6% 1 4 18% 41% 31%	2228
	21st to end	Day	8% 2 3 16% 42% 30%	2090
October	1st to 10th	Day	9% 2 3 13% 24% 48%	3312
	11th to 20th	Day	13% 2 5% 25% 36% 19%	3161
	21st to end	Day	15% 3 5% 21% 37% 19%	3448
November	1st to 10th	Day	20% 6% 10% 28% 27% 9%	1774
	11th to 20th	Day	28% 5% 14% 26% 22% 4	1800
	21st to end	Day	32% 5% 10% 24% 22% 8%	1800
December	1st to 10th	Day	44% 5% 10% 21% 16% 4	2823
	11th to 20th	Day	48% 7% 10% 21% 12% 2	3028
	21st to end	Day	55% 6% 16% 16% 6% 1	4004
January	1st to 10th	Night	38% 7% 16% 25% 13% 1	6306
	11th to 20th	Night	29% 7% 11% 17% 27% 9%	5946
	21st to end	Night	37% 6% 10% 20% 21% 6%	6468
February	1st to 10th	Night	31% 4 10% 27% 20% 7%	2475
	11th to 20th	Night	25% 4 12% 27% 24% 7%	2339
	21st to end	Night	29% 4 12% 29% 20% 6%	1980
March	1st to 10th	Night	32% 7% 14% 22% 23% 3	5376
	11th to 20th	Night	27% 4 15% 31% 18% 5%	5071
	21st to end	Night	35% 4 14% 27% 16% 3	5121
April	1st to 10th	Night	23% 4 11% 30% 22% 10%	2000
	11th to 20th	Night	18% 3 9% 29% 31% 10%	1979
	21st to end	Night	15% 3 11% 30% 34% 8%	2000
May	1st to 10th	Night	11% 2 8% 25% 35% 19%	4478
	11th to 20th	Night	8% 1 3 21% 48% 18%	4075
	21st to end	Night	5% 1 3 18% 51% 21%	4026
June	1st to 10th	Night	10% 2 6% 20% 34% 28%	1595
	11th to 20th	Night	4 1 4 12% 37% 41%	1520
	21st to end	Night	3 1 1 6% 28% 61%	1520
July	1st to 10th	Night	2 1 1 3 12% 82%	2500
	11th to 20th	Night	7% 1 1 4 13% 73%	2492
	21st to end	Night	6% 1 2 6% 26% 60%	3002
August	1st to 10th	Night	8% 1 1 5% 28% 58%	3088
	11th to 20th	Night	3 2 4 21% 70%	3156
	21st to end	Night	4 2 7% 23% 64%	3498
September	1st to 10th	Night	5% 1 2 6% 27% 60%	1900
	11th to 20th	Night	7% 1 3 15% 39% 35%	1951
	21st to end	Night	7% 1 2 15% 39% 36%	2090
October	1st to 10th	Night	7% 2 3 11% 23% 54%	3665
	11th to 20th	Night	9% 2 5% 22% 40% 22%	3777
	21st to end	Night	12% 2 3 19% 42% 22%	4121
November	1st to 10th	Night	18% 4 9% 27% 29% 13%	2525
	11th to 20th	Night	20% 4 13% 27% 30% 8%	2600
	21st to end	Night	28% 3 11% 22% 28% 9%	2600
December	1st to 10th	Night	35% 5% 11% 25% 17% 6%	4358
	11th to 20th	Night	44% 5% 15% 17% 15% 3	5176
	21st to end	Night	48% 7% 18% 19% 7% 1	6952

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	19% 8% 12% 31% 23% 9%	200
	11th to 20th	Day	12% 3 14% 28% 27% 17%	365
	21st to end	Day	14% 3 12% 24% 30% 17%	440
February	1st to 10th	Day	12% 5% 14% 26% 24% 18%	462
	11th to 20th	Day	11% 6% 17% 30% 25% 12%	495
	21st to end	Day	9% 7% 16% 32% 27% 9%	414
March	1st to 10th	Day	11% 7% 17% 28% 26% 10%	500
	11th to 20th	Day	10% 6% 21% 26% 25% 13%	500
	21st to end	Day	10% 7% 23% 29% 21% 10%	550
April	1st to 10th	Day	9% 5% 19% 29% 23% 17%	500
	11th to 20th	Day	6% 7% 21% 29% 26% 11%	492
	21st to end	Day	2 6% 19% 29% 24% 19%	500
May	1st to 10th	Day	3 5% 18% 32% 28% 15%	503
	11th to 20th	Day	3 3 15% 33% 29% 17%	528
	21st to end	Day	2 4 15% 25% 33% 21%	573
June	1st to 10th	Day	1 2 11% 28% 33% 25%	527
	11th to 20th	Day	1 2 14% 20% 42% 21%	490
	21st to end	Day	5% 28% 38% 29%	490
July	1st to 10th	Day	1 3 17% 48% 30%	486
	11th to 20th	Day	2 1 15% 34% 48%	484
	21st to end	Day	1 1 2 12% 47% 38%	531
August	1st to 10th	Day	2 7% 20% 45% 26%	487
	11th to 20th	Day	1 3 23% 41% 32%	477
	21st to end	Day	1 5% 19% 41% 34%	539
September	1st to 10th	Day	1 1 3 23% 45% 28%	490
	11th to 20th	Day	1 1 9% 28% 42% 19%	490
	21st to end	Day	1 10% 21% 42% 25%	490
October	1st to 10th	Day	2 1 12% 27% 33% 25%	490
	11th to 20th	Day	3 3 11% 23% 36% 25%	490
	21st to end	Day	3 3 10% 24% 33% 27%	539
November	1st to 10th	Day	4 3 12% 23% 32% 26%	310
	11th to 20th	Day	4 3 10% 22% 38% 23%	300
	21st to end	Day	11% 1 11% 27% 30% 20%	300
December	1st to 10th	Day	13% 2 11% 29% 27% 17%	263
	11th to 20th	Day	9% 6% 12% 24% 27% 24%	200
	21st to end	Day	14% 6% 16% 31% 25% 8%	220
January	1st to 10th	Night	17% 3 9% 37% 27% 7%	800
	11th to 20th	Night	9% 2 10% 42% 32% 5%	800
	21st to end	Night	14% 2 11% 37% 31% 5%	880
February	1st to 10th	Night	13% 3 14% 35% 30% 6%	800
	11th to 20th	Night	12% 3 14% 36% 30% 5%	784
	21st to end	Night	11% 3 15% 45% 23% 3	660
March	1st to 10th	Night	11% 3 17% 41% 26% 2	800
	11th to 20th	Night	9% 5% 17% 38% 28% 4	800
	21st to end	Night	10% 4 23% 39% 22% 1	880
April	1st to 10th	Night	8% 4 27% 36% 23% 2	800
	11th to 20th	Night	4 7% 26% 38% 23% 1	791
	21st to end	Night	4 5% 28% 35% 24% 4	800
May	1st to 10th	Night	3 5% 27% 41% 23% 1	773
	11th to 20th	Night	3 4 28% 41% 23%	591
	21st to end	Night	3 5% 30% 39% 22%	462
June	1st to 10th	Night	3 5% 27% 49% 16% 1	399
	11th to 20th	Night	1 3 24% 51% 21% 1	380
	21st to end	Night	1 2 21% 56% 20% 1	380
July	1st to 10th	Night	13% 54% 33%	370
	11th to 20th	Night	2 7% 51% 40%	368
	21st to end	Night	1 6% 40% 49% 4	614
August	1st to 10th	Night	2 11% 43% 41% 4	712
	11th to 20th	Night	1 9% 40% 47% 4	752
	21st to end	Night	1 5% 33% 56% 4	836
September	1st to 10th	Night	1 1 6% 37% 51% 4	760
	11th to 20th	Night	1 1 13% 41% 39% 5%	760
	21st to end	Night	1 2 10% 37% 44% 5%	760
October	1st to 10th	Night	3 2 13% 39% 39% 5%	760
	11th to 20th	Night	3 2 11% 35% 43% 7%	760
	21st to end	Night	4 2 9% 31% 40% 14%	836
November	1st to 10th	Night	5% 2 10% 33% 40% 11%	780
	11th to 20th	Night	5% 1 9% 41% 35% 9%	800
	21st to end	Night	8% 2 11% 38% 35% 6%	800
December	1st to 10th	Night	10% 3 11% 41% 31% 5%	800
	11th to 20th	Night	14% 3 9% 42% 27% 6%	800
	21st to end	Night	12% 5% 13% 42% 25% 3	880

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