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 2026-10-14.

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 3 2 2 3 2 3 5% 71%	157214
	Night	5% 3 1 4 3 3 3 2 4 5% 67%	152089
Transparency	Day	19% 3 7% 17% 27% 27%	152264
	Night	21% 3 8% 19% 26% 23%	148016
Seeing	Day	5% 3 12% 26% 33% 21%	17946
	Night	6% 3 15% 40% 32% 4	27451

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	9% 10% 4 10% 7% 5% 4 3 5% 7% 35%	4722
	11th to 20th	Day	5% 5% 3 9% 6% 6% 4 3 5% 6% 49%	5238
	21st to end	Day	9% 8% 4 9% 6% 5% 4 4 6% 7% 38%	5843
February	1st to 10th	Day	13% 7% 2 6% 4 3 4 2 5% 6% 47%	2018
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 7% 50%	2050
	21st to end	Day	11% 7% 2 5% 4 4 4 3 5% 7% 50%	1730
March	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 42%	4618
	11th to 20th	Day	8% 4 2 5% 4 4 4 3 5% 7% 54%	5020
	21st to end	Day	9% 4 2 5% 3 4 5% 4 5% 8% 51%	5983
April	1st to 10th	Day	7% 3 1 4 3 3 3 3 5% 7% 62%	2520
	11th to 20th	Day	5% 3 1 4 3 3 3 3 5% 5% 65%	2489
	21st to end	Day	4 3 1 2 2 2 3 2 4 6% 70%	2520
May	1st to 10th	Day	3 2 1 2 1 2 2 1 2 4 80%	7131
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	7596
	21st to end	Day	2 1 1 1 1 1 1 1 2 3 85%	8897
June	1st to 10th	Day	2 1 1 2 1 2 2 2 4 5% 78%	2940
	11th to 20th	Day	1 1 1 1 1 1 1 3 91%	2940
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	2940
July	1st to 10th	Day	1 1 1 1 1 94%	8219
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 86%	8220
	21st to end	Day	1 1 1 1 1 1 2 3 90%	8310
August	1st to 10th	Day	1 1 1 1 1 1 1 2 91%	7104
	11th to 20th	Day	1 1 1 1 1 2 3 90%	7001
	21st to end	Day	2 1 1 1 1 1 1 1 2 4 84%	7750
September	1st to 10th	Day	1 1 1 1 2 1 1 3 89%	2520
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 85%	2462
	21st to end	Day	1 1 2 1 2 1 2 3 4 84%	2310
October	1st to 10th	Day	1 1 1 1 1 2 2 4 4 81%	3520
	11th to 20th	Day	1 2 1 3 2 2 2 2 3 4 78%	3292
	21st to end	Day	2 2 1 3 3 2 3 3 4 7% 70%	3558
November	1st to 10th	Day	4 3 1 3 3 3 3 3 5% 7% 63%	1864
	11th to 20th	Day	9% 6% 2 5% 3 4 4 2 4 7% 54%	1890
	21st to end	Day	8% 7% 2 6% 4 4 4 4 5% 6% 51%	1890
December	1st to 10th	Day	5% 7% 3 11% 7% 6% 6% 4 5% 6% 39%	2909
	11th to 20th	Day	11% 8% 3 9% 7% 6% 5% 4 6% 8% 32%	3108
	21st to end	Day	13% 13% 5% 12% 6% 6% 5% 3 5% 8% 24%	4092
January	1st to 10th	Night	12% 8% 2 7% 6% 5% 4 3 5% 6% 41%	8172
	11th to 20th	Night	6% 4 2 6% 6% 5% 5% 3 5% 6% 54%	7729
	21st to end	Night	10% 7% 3 8% 6% 5% 5% 4 5% 6% 42%	8371
February	1st to 10th	Night	11% 5% 2 5% 4 4 3 2 5% 6% 53%	2599
	11th to 20th	Night	10% 4 2 3 3 3 4 3 4 8% 56%	2458
	21st to end	Night	10% 5% 1 5% 3 3 3 3 4 7% 58%	2076
March	1st to 10th	Night	9% 4 2 6% 4 4 5% 4 6% 8% 47%	5496
	11th to 20th	Night	7% 3 1 4 3 4 4 4 5% 7% 57%	5183
	21st to end	Night	12% 6% 2 6% 4 4 3 3 4 5% 51%	5234
April	1st to 10th	Night	7% 3 2 3 3 3 3 3 4 6% 63%	2100
	11th to 20th	Night	4 4 1 3 3 2 2 2 3 6% 70%	2079
	21st to end	Night	4 3 1 2 2 2 3 2 4 6% 72%	2100
May	1st to 10th	Night	3 2 1 2 2 2 2 1 2 5% 78%	5827
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 86%	5296
	21st to end	Night	1 1 1 2 1 1 1 1 2 2 88%	5263
June	1st to 10th	Night	2 2 1 1 1 1 1 2 3 5% 80%	1680
	11th to 20th	Night	1 1 1 1 1 1 2 3 90%	1680
	21st to end	Night	1 1 1 1 2 3 92%	1680
July	1st to 10th	Night	1 1 1 2 95%	4740
	11th to 20th	Night	2 2 1 2 1 1 1 1 2 2 86%	4740
	21st to end	Night	1 1 1 1 1 2 2 90%	5782
August	1st to 10th	Night	1 1 1 1 1 1 1 2 2 91%	5741
	11th to 20th	Night	1 1 1 1 1 2 3 89%	5886
	21st to end	Night	1 1 1 1 2 2 2 3 5% 82%	6499
September	1st to 10th	Night	1 1 1 1 1 1 1 2 90%	2100
	11th to 20th	Night	1 1 1 1 2 1 1 2 3 85%	2157
	21st to end	Night	2 1 1 1 1 2 1 3 4 84%	2310
October	1st to 10th	Night	1 1 2 1 1 1 1 2 4 84%	3896
	11th to 20th	Night	2 1 1 2 2 2 2 3 2 4 80%	3933
	21st to end	Night	3 2 2 1 2 2 2 3 4 79%	4253
November	1st to 10th	Night	4 3 1 4 3 2 3 3 4 6% 68%	2655
	11th to 20th	Night	6% 3 1 4 3 3 4 2 4 6% 64%	2730
	21st to end	Night	6% 6% 2 6% 3 3 3 3 5% 6% 56%	2730
December	1st to 10th	Night	6% 5% 2 6% 7% 7% 5% 4 5% 6% 48%	4492
	11th to 20th	Night	12% 6% 3 8% 6% 6% 5% 4 7% 8% 36%	5316
	21st to end	Night	10% 8% 4 10% 7% 6% 5% 4 7% 8% 31%	7106

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	47% 6% 17% 18% 11% 1	4723
	11th to 20th	Day	36% 5% 8% 14% 25% 12%	5238
	21st to end	Day	44% 5% 9% 16% 21% 4	5843
February	1st to 10th	Day	38% 5% 11% 24% 17% 5%	2018
	11th to 20th	Day	31% 6% 12% 27% 18% 5%	2060
	21st to end	Day	34% 6% 15% 27% 16% 3	1730
March	1st to 10th	Day	34% 7% 15% 20% 21% 3	4618
	11th to 20th	Day	30% 5% 13% 30% 15% 7%	5020
	21st to end	Day	29% 6% 14% 33% 15% 3	5983
April	1st to 10th	Day	23% 3 12% 30% 24% 8%	2520
	11th to 20th	Day	20% 5% 12% 28% 26% 9%	2492
	21st to end	Day	17% 4 11% 31% 30% 8%	2520
May	1st to 10th	Day	12% 2 8% 22% 39% 18%	7131
	11th to 20th	Day	6% 1 3 21% 45% 24%	7599
	21st to end	Day	9% 1 4 19% 47% 20%	8898
June	1st to 10th	Day	10% 3 7% 20% 34% 26%	2940
	11th to 20th	Day	4 1 3 12% 36% 44%	2940
	21st to end	Day	6% 1 1 8% 27% 58%	2940
July	1st to 10th	Day	2 1 1 3 19% 74%	8151
	11th to 20th	Day	7% 1 1 7% 17% 67%	8145
	21st to end	Day	4 1 1 10% 31% 53%	8124
August	1st to 10th	Day	5% 1 1 5% 33% 55%	5622
	11th to 20th	Day	6% 1 1 14% 31% 48%	5517
	21st to end	Day	8% 1 2 12% 33% 44%	6077
September	1st to 10th	Day	5% 1 3 8% 30% 52%	2520
	11th to 20th	Day	7% 1 3 17% 41% 31%	2462
	21st to end	Day	8% 1 3 17% 42% 29%	2310
October	1st to 10th	Day	9% 2 3 13% 25% 48%	3520
	11th to 20th	Day	13% 2 5% 25% 36% 19%	3292
	21st to end	Day	16% 3 5% 21% 37% 19%	3558
November	1st to 10th	Day	20% 6% 10% 28% 27% 9%	1864
	11th to 20th	Day	30% 5% 14% 26% 21% 4	1890
	21st to end	Day	34% 5% 9% 24% 21% 8%	1890
December	1st to 10th	Day	45% 5% 10% 21% 16% 4	2909
	11th to 20th	Day	49% 6% 10% 21% 12% 2	3108
	21st to end	Day	56% 6% 16% 16% 6% 1	4092
January	1st to 10th	Night	43% 6% 15% 23% 12% 1	8172
	11th to 20th	Night	31% 6% 9% 17% 28% 10%	7729
	21st to end	Night	41% 5% 9% 22% 19% 5%	8371
February	1st to 10th	Night	32% 4 10% 27% 20% 7%	2599
	11th to 20th	Night	27% 4 12% 27% 24% 6%	2459
	21st to end	Night	28% 4 14% 29% 20% 6%	2076
March	1st to 10th	Night	31% 7% 14% 22% 24% 3	5496
	11th to 20th	Night	27% 4 14% 31% 18% 6%	5183
	21st to end	Night	35% 4 14% 27% 17% 3	5234
April	1st to 10th	Night	23% 4 11% 29% 23% 10%	2100
	11th to 20th	Night	19% 3 9% 30% 31% 9%	2079
	21st to end	Night	15% 3 11% 30% 34% 8%	2100
May	1st to 10th	Night	13% 2 8% 24% 36% 18%	5825
	11th to 20th	Night	7% 1 2 18% 44% 27%	5297
	21st to end	Night	7% 1 3 18% 51% 19%	5264
June	1st to 10th	Night	10% 2 6% 19% 35% 28%	1680
	11th to 20th	Night	3 1 4 11% 35% 45%	1680
	21st to end	Night	3 1 1 6% 27% 62%	1680
July	1st to 10th	Night	2 1 1 2 17% 78%	4700
	11th to 20th	Night	8% 1 1 6% 14% 71%	4692
	21st to end	Night	4 1 1 9% 30% 55%	5656
August	1st to 10th	Night	5% 1 1 4 29% 60%	4538
	11th to 20th	Night	5% 1 2 14% 29% 50%	4636
	21st to end	Night	8% 2 12% 28% 51%	5092
September	1st to 10th	Night	6% 1 2 7% 29% 56%	2100
	11th to 20th	Night	7% 1 3 14% 39% 35%	2157
	21st to end	Night	7% 1 3 15% 39% 34%	2310
October	1st to 10th	Night	7% 2 3 11% 23% 54%	3896
	11th to 20th	Night	10% 2 4 22% 39% 22%	3933
	21st to end	Night	13% 2 4 19% 42% 21%	4253
November	1st to 10th	Night	17% 4 9% 28% 29% 13%	2655
	11th to 20th	Night	22% 4 12% 26% 28% 8%	2730
	21st to end	Night	29% 3 10% 22% 27% 8%	2730
December	1st to 10th	Night	36% 5% 11% 25% 17% 6%	4492
	11th to 20th	Night	45% 5% 15% 17% 15% 3	5316
	21st to end	Night	48% 7% 18% 18% 7% 1	7106

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	18% 7% 14% 30% 22% 9%	220
	11th to 20th	Day	11% 3 13% 26% 30% 18%	393
	21st to end	Day	14% 3 11% 25% 31% 17%	473
February	1st to 10th	Day	12% 5% 13% 26% 25% 18%	498
	11th to 20th	Day	11% 6% 17% 30% 25% 11%	535
	21st to end	Day	9% 7% 16% 32% 28% 9%	446
March	1st to 10th	Day	10% 7% 17% 29% 26% 10%	540
	11th to 20th	Day	9% 6% 20% 25% 26% 14%	540
	21st to end	Day	10% 7% 22% 29% 22% 10%	594
April	1st to 10th	Day	9% 4 18% 29% 24% 16%	540
	11th to 20th	Day	6% 7% 21% 29% 26% 11%	532
	21st to end	Day	3 7% 20% 28% 24% 18%	540
May	1st to 10th	Day	3 6% 17% 31% 29% 14%	543
	11th to 20th	Day	3 3 15% 33% 30% 16%	568
	21st to end	Day	2 4 15% 25% 33% 21%	617
June	1st to 10th	Day	1 2 11% 29% 33% 23%	570
	11th to 20th	Day	1 2 15% 23% 40% 19%	570
	21st to end	Day	1 1 7% 27% 38% 26%	570
July	1st to 10th	Day	1 4 21% 47% 27%	566
	11th to 20th	Day	2 2 16% 35% 44%	564
	21st to end	Day	1 1 16% 46% 35%	619
August	1st to 10th	Day	2 7% 20% 43% 28%	567
	11th to 20th	Day	1 3 24% 43% 29%	557
	21st to end	Day	2 5% 22% 41% 31%	627
September	1st to 10th	Day	1 1 4 26% 43% 26%	570
	11th to 20th	Day	2 1 10% 29% 40% 18%	570
	21st to end	Day	1 1 11% 23% 39% 24%	570
October	1st to 10th	Day	1 1 11% 29% 33% 25%	570
	11th to 20th	Day	3 4 13% 22% 34% 24%	543
	21st to end	Day	3 3 10% 24% 33% 27%	583
November	1st to 10th	Day	4 3 12% 22% 31% 28%	340
	11th to 20th	Day	8% 4 9% 23% 35% 21%	330
	21st to end	Day	11% 1 10% 26% 31% 21%	330
December	1st to 10th	Day	12% 2 10% 27% 29% 18%	289
	11th to 20th	Day	9% 5% 10% 23% 29% 24%	220
	21st to end	Day	16% 7% 15% 31% 25% 7%	242
January	1st to 10th	Night	18% 3 10% 37% 27% 6%	840
	11th to 20th	Night	8% 2 9% 41% 34% 5%	840
	21st to end	Night	14% 2 10% 37% 31% 5%	924
February	1st to 10th	Night	13% 3 13% 35% 30% 6%	840
	11th to 20th	Night	12% 4 15% 36% 29% 4	824
	21st to end	Night	10% 3 15% 46% 24% 3	692
March	1st to 10th	Night	10% 4 17% 42% 25% 2	840
	11th to 20th	Night	8% 4 17% 39% 28% 4	840
	21st to end	Night	9% 4 23% 40% 23% 1	924
April	1st to 10th	Night	8% 4 27% 37% 23% 2	840
	11th to 20th	Night	5% 6% 26% 38% 23% 1	831
	21st to end	Night	4 5% 28% 35% 23% 3	840
May	1st to 10th	Night	4 5% 26% 42% 23% 1	812
	11th to 20th	Night	3 4 29% 42% 23%	621
	21st to end	Night	3 6% 29% 40% 22%	486
June	1st to 10th	Night	3 5% 28% 49% 15% 1	420
	11th to 20th	Night	1 4 26% 49% 20%	420
	21st to end	Night	1 3 22% 55% 19% 1	420
July	1st to 10th	Night	16% 53% 30%	410
	11th to 20th	Night	2 8% 51% 38%	408
	21st to end	Night	1 1 7% 43% 45% 4	679
August	1st to 10th	Night	2 11% 43% 40% 4	788
	11th to 20th	Night	1 10% 41% 45% 3	832
	21st to end	Night	1 6% 36% 53% 4	924
September	1st to 10th	Night	1 1 8% 38% 48% 4	840
	11th to 20th	Night	2 1 15% 41% 37% 5%	840
	21st to end	Night	2 2 10% 39% 42% 5%	840
October	1st to 10th	Night	2 2 13% 39% 39% 5%	840
	11th to 20th	Night	3 2 12% 35% 41% 7%	812
	21st to end	Night	4 2 9% 31% 40% 14%	880
November	1st to 10th	Night	4 2 10% 33% 40% 11%	820
	11th to 20th	Night	6% 2 9% 40% 35% 8%	840
	21st to end	Night	8% 3 11% 38% 35% 6%	840
December	1st to 10th	Night	10% 3 10% 40% 31% 6%	840
	11th to 20th	Night	13% 3 9% 41% 28% 6%	840
	21st to end	Night	12% 5% 13% 42% 25% 3	924

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