The University of Puget Sound Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The University of Puget Sound Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	18% 9% 3 8% 5% 5% 5% 4 5% 7% 31%	118330
	Night	20% 9% 3 8% 5% 4 4 4 5% 6% 31%	119609
Transparency	Day	30% 43% 8% 11% 6% 1	118088
	Night	34% 42% 8% 11% 4 1	119497
Seeing	Day	21% 10% 20% 25% 18% 6%	15715
	Night	25% 8% 17% 28% 19% 3	24205

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	33% 13% 4 12% 8% 6% 5% 4 4 4 8%	3336
	11th to 20th	Day	18% 11% 4 11% 7% 5% 5% 3 5% 8% 23%	3616
	21st to end	Day	27% 12% 4 9% 5% 4 4 3 4 6% 24%	4170
February	1st to 10th	Day	27% 13% 4 10% 6% 5% 4 4 5% 7% 15%	1800
	11th to 20th	Day	30% 12% 4 9% 6% 4 4 4 3 6% 18%	1755
	21st to end	Day	26% 12% 4 9% 6% 5% 4 4 5% 6% 19%	1550
March	1st to 10th	Day	17% 11% 4 11% 7% 7% 7% 6% 7% 6% 17%	4650
	11th to 20th	Day	18% 13% 4 9% 5% 5% 5% 3 5% 6% 26%	5000
	21st to end	Day	22% 13% 4 11% 7% 6% 6% 5% 6% 6% 13%	5651
April	1st to 10th	Day	24% 11% 4 9% 6% 6% 5% 5% 6% 7% 17%	2400
	11th to 20th	Day	19% 11% 3 7% 6% 5% 4 4 7% 6% 27%	2397
	21st to end	Day	17% 10% 4 9% 6% 5% 5% 5% 6% 7% 26%	2625
May	1st to 10th	Day	15% 8% 3 6% 4 4 5% 4 5% 7% 38%	4560
	11th to 20th	Day	12% 8% 2 6% 4 5% 4 4 7% 9% 38%	4517
	21st to end	Day	20% 9% 4 9% 6% 5% 5% 4 7% 8% 22%	4886
June	1st to 10th	Day	18% 9% 3 8% 6% 5% 5% 4 6% 9% 29%	2681
	11th to 20th	Day	17% 9% 4 9% 7% 6% 5% 5% 6% 8% 24%	2850
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 4 7% 8% 40%	2850
July	1st to 10th	Day	6% 4 2 4 3 3 3 3 4 5% 64%	4740
	11th to 20th	Day	5% 3 1 3 3 2 3 2 4 5% 68%	4495
	21st to end	Day	8% 5% 2 6% 4 3 3 4 5% 5% 56%	4790
August	1st to 10th	Day	4 4 2 5% 3 3 3 3 4 8% 62%	4286
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	4026
	21st to end	Day	14% 7% 3 7% 5% 4 4 3 5% 6% 43%	4337
September	1st to 10th	Day	9% 5% 2 7% 4 4 4 3 5% 7% 50%	2280
	11th to 20th	Day	12% 8% 3 8% 5% 4 5% 5% 5% 7% 38%	2280
	21st to end	Day	16% 7% 2 7% 4 4 3 4 6% 8% 37%	2006
October	1st to 10th	Day	14% 7% 3 6% 5% 5% 4 4 7% 12% 34%	3180
	11th to 20th	Day	24% 10% 3 9% 5% 4 4 3 5% 8% 25%	3161
	21st to end	Day	25% 9% 3 9% 5% 5% 5% 4 5% 7% 21%	3273
November	1st to 10th	Day	29% 11% 4 8% 6% 4 5% 4 5% 5% 17%	1612
	11th to 20th	Day	30% 11% 3 8% 5% 5% 4 4 4 5% 20%	1600
	21st to end	Day	28% 11% 4 9% 6% 5% 5% 4 6% 6% 18%	1600
December	1st to 10th	Day	20% 7% 4 8% 9% 6% 5% 4 6% 7% 23%	2620
	11th to 20th	Day	32% 12% 3 9% 7% 6% 6% 4 5% 5% 10%	3025
	21st to end	Day	28% 14% 5% 11% 7% 6% 5% 4 4 5% 11%	3725
January	1st to 10th	Night	32% 15% 6% 12% 6% 5% 4 3 4 3 10%	6586
	11th to 20th	Night	19% 9% 3 9% 5% 4 4 3 5% 7% 31%	6282
	21st to end	Night	25% 11% 4 9% 4 4 4 3 4 6% 27%	6779
February	1st to 10th	Night	27% 13% 3 10% 6% 5% 4 2 4 6% 20%	2600
	11th to 20th	Night	25% 11% 4 9% 5% 4 4 3 5% 6% 24%	2532
	21st to end	Night	25% 12% 3 6% 5% 5% 5% 4 5% 5% 24%	2080
March	1st to 10th	Night	17% 12% 4 9% 6% 6% 7% 5% 7% 8% 20%	5230
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 3 4 7% 33%	4999
	21st to end	Night	25% 13% 4 10% 7% 5% 6% 4 5% 5% 15%	5323
April	1st to 10th	Night	20% 11% 3 8% 5% 5% 5% 4 6% 7% 24%	2000
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 5% 6% 8% 32%	1949
	21st to end	Night	18% 8% 3 8% 7% 4 5% 4 5% 8% 31%	1773
May	1st to 10th	Night	16% 8% 3 8% 4 4 4 3 5% 6% 40%	2620
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 4 6% 7% 43%	2596
	21st to end	Night	9% 12% 3 9% 6% 7% 6% 5% 7% 7% 30%	2805
June	1st to 10th	Night	14% 6% 2 7% 5% 5% 4 4 6% 8% 39%	1494
	11th to 20th	Night	13% 8% 3 7% 5% 4 5% 5% 5% 10% 35%	1330
	21st to end	Night	8% 6% 2 6% 5% 4 4 5% 5% 7% 49%	1330
July	1st to 10th	Night	4 3 1 3 2 2 3 3 5% 5% 69%	2220
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 75%	2462
	21st to end	Night	5% 3 2 3 2 2 3 3 5% 6% 66%	2762
August	1st to 10th	Night	3 2 1 3 3 2 2 2 4 6% 72%	2646
	11th to 20th	Night	7% 4 2 4 3 5% 5% 3 5% 8% 54%	2812
	21st to end	Night	12% 6% 2 6% 3 3 3 3 5% 6% 49%	3334
September	1st to 10th	Night	7% 4 2 4 3 3 4 3 5% 6% 59%	1900
	11th to 20th	Night	14% 6% 3 6% 5% 4 3 3 5% 6% 45%	1900
	21st to end	Night	14% 7% 3 6% 4 4 4 2 4 8% 44%	2171
October	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 6% 8% 43%	3800
	11th to 20th	Night	24% 9% 3 6% 4 4 4 4 4 6% 31%	3777
	21st to end	Night	22% 11% 4 8% 5% 4 4 4 6% 6% 27%	4290
November	1st to 10th	Night	30% 9% 3 7% 5% 5% 5% 4 5% 5% 22%	2682
	11th to 20th	Night	30% 10% 3 8% 5% 5% 3 3 5% 5% 23%	2800
	21st to end	Night	26% 11% 4 8% 6% 5% 4 4 6% 5% 22%	2800
December	1st to 10th	Night	21% 9% 3 9% 7% 5% 5% 4 5% 7% 26%	4560
	11th to 20th	Night	31% 13% 5% 11% 7% 5% 5% 4 4 4 11%	5180
	21st to end	Night	29% 14% 5% 11% 6% 6% 5% 4 4 5% 13%	7205

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	60% 28% 6% 5% 1	3336
	11th to 20th	Day	42% 29% 10% 14% 4	3640
	21st to end	Day	45% 29% 7% 12% 7%	4170
February	1st to 10th	Day	28% 62% 4 3 3 1	1800
	11th to 20th	Day	31% 61% 2 3 3	1764
	21st to end	Day	25% 64% 3 6% 3	1550
March	1st to 10th	Day	43% 32% 11% 12% 3	4650
	11th to 20th	Day	40% 31% 10% 17% 2	5000
	21st to end	Day	50% 31% 10% 8% 1	5651
April	1st to 10th	Day	24% 65% 5% 6%	2400
	11th to 20th	Day	18% 65% 6% 8% 3	2397
	21st to end	Day	20% 64% 6% 7% 3	2625
May	1st to 10th	Day	24% 40% 9% 17% 9% 1	4560
	11th to 20th	Day	24% 41% 16% 15% 4	4520
	21st to end	Day	36% 42% 14% 6% 1	4887
June	1st to 10th	Day	14% 67% 9% 7% 3	2681
	11th to 20th	Day	18% 67% 6% 6% 3	2850
	21st to end	Day	14% 66% 8% 9% 3	2850
July	1st to 10th	Day	12% 37% 3 12% 20% 16%	4666
	11th to 20th	Day	7% 37% 5% 18% 26% 7%	4417
	21st to end	Day	19% 37% 9% 24% 9% 1	4639
August	1st to 10th	Day	11% 37% 6% 26% 16% 3	4269
	11th to 20th	Day	25% 42% 13% 15% 4	4065
	21st to end	Day	30% 39% 6% 17% 8%	4336
September	1st to 10th	Day	16% 61% 8% 9% 4 1	2280
	11th to 20th	Day	21% 63% 4 5% 6%	2280
	21st to end	Day	18% 61% 8% 8% 4	2006
October	1st to 10th	Day	24% 41% 12% 20% 3	3180
	11th to 20th	Day	38% 39% 9% 9% 6%	3161
	21st to end	Day	40% 39% 8% 7% 5% 1	3273
November	1st to 10th	Day	31% 57% 6% 5% 1	1612
	11th to 20th	Day	27% 58% 5% 6% 3 1	1600
	21st to end	Day	25% 59% 4 6% 5%	1600
December	1st to 10th	Day	38% 40% 5% 10% 6%	2620
	11th to 20th	Day	53% 34% 7% 6% 1	3028
	21st to end	Day	56% 28% 10% 4 2	3725
January	1st to 10th	Night	61% 28% 5% 4 1 1	6586
	11th to 20th	Night	36% 31% 8% 20% 5%	6320
	21st to end	Night	44% 31% 8% 14% 4	6779
February	1st to 10th	Night	27% 62% 4 3 3	2600
	11th to 20th	Night	28% 62% 4 4 2 1	2534
	21st to end	Night	23% 63% 6% 6% 2	2080
March	1st to 10th	Night	40% 33% 11% 12% 4	5230
	11th to 20th	Night	36% 30% 13% 19% 1	5000
	21st to end	Night	51% 32% 10% 8%	5323
April	1st to 10th	Night	22% 66% 6% 6%	2000
	11th to 20th	Night	18% 64% 7% 6% 3	1949
	21st to end	Night	19% 63% 7% 8% 2	1773
May	1st to 10th	Night	26% 40% 7% 21% 6%	2620
	11th to 20th	Night	22% 44% 14% 16% 4	2596
	21st to end	Night	36% 42% 14% 6% 1	2805
June	1st to 10th	Night	14% 66% 7% 10% 3	1494
	11th to 20th	Night	15% 67% 8% 9% 2	1330
	21st to end	Night	13% 65% 10% 10% 3	1330
July	1st to 10th	Night	14% 37% 5% 12% 15% 17%	2185
	11th to 20th	Night	7% 36% 4 17% 32% 4	2417
	21st to end	Night	19% 38% 10% 20% 12% 2	2674
August	1st to 10th	Night	9% 37% 6% 28% 19% 1	2638
	11th to 20th	Night	23% 42% 18% 13% 4 1	2839
	21st to end	Night	26% 39% 8% 19% 7%	3334
September	1st to 10th	Night	14% 61% 8% 10% 6%	1900
	11th to 20th	Night	20% 62% 5% 8% 5% 1	1900
	21st to end	Night	18% 61% 6% 11% 3	2171
October	1st to 10th	Night	23% 40% 15% 21% 1	3800
	11th to 20th	Night	36% 39% 9% 13% 3	3777
	21st to end	Night	40% 39% 7% 10% 5%	4290
November	1st to 10th	Night	28% 58% 6% 6% 1	2682
	11th to 20th	Night	28% 58% 4 5% 3 2	2800
	21st to end	Night	26% 58% 4 5% 6% 1	2800
December	1st to 10th	Night	39% 40% 6% 12% 4	4560
	11th to 20th	Night	56% 33% 6% 5% 1	5176
	21st to end	Night	56% 29% 7% 6% 1	7205

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	35% 13% 18% 19% 12% 5%	200
	11th to 20th	Day	30% 14% 18% 19% 15% 5%	200
	21st to end	Day	32% 12% 21% 16% 15% 4	295
February	1st to 10th	Day	36% 14% 16% 19% 11% 4	400
	11th to 20th	Day	39% 12% 20% 18% 9% 2	395
	21st to end	Day	32% 16% 21% 21% 9% 1	364
March	1st to 10th	Day	26% 15% 24% 22% 10% 1	500
	11th to 20th	Day	27% 16% 17% 22% 13% 5%	500
	21st to end	Day	29% 15% 25% 20% 9% 2	550
April	1st to 10th	Day	29% 16% 20% 22% 10% 4	500
	11th to 20th	Day	21% 15% 29% 21% 9% 6%	492
	21st to end	Day	19% 13% 27% 24% 15% 2	500
May	1st to 10th	Day	15% 11% 24% 30% 15% 5%	503
	11th to 20th	Day	21% 9% 22% 22% 18% 7%	528
	21st to end	Day	19% 9% 20% 28% 18% 7%	550
June	1st to 10th	Day	20% 9% 23% 24% 18% 6%	511
	11th to 20th	Day	17% 13% 22% 28% 16% 3	680
	21st to end	Day	11% 7% 20% 34% 22% 7%	680
July	1st to 10th	Day	10% 4 17% 33% 28% 8%	671
	11th to 20th	Day	6% 4 14% 34% 31% 10%	530
	21st to end	Day	5% 4 12% 31% 37% 11%	531
August	1st to 10th	Day	8% 4 12% 30% 34% 13%	487
	11th to 20th	Day	8% 4 10% 30% 33% 16%	477
	21st to end	Day	13% 8% 19% 29% 23% 8%	539
September	1st to 10th	Day	10% 6% 23% 33% 21% 6%	490
	11th to 20th	Day	16% 8% 18% 27% 23% 8%	490
	21st to end	Day	18% 9% 22% 28% 19% 4	490
October	1st to 10th	Day	20% 12% 19% 22% 19% 8%	490
	11th to 20th	Day	26% 12% 16% 24% 17% 6%	490
	21st to end	Day	30% 15% 19% 19% 12% 5%	439
November	1st to 10th	Day	34% 13% 19% 21% 11% 3	223
	11th to 20th	Day	33% 16% 23% 12% 10% 8%	200
	21st to end	Day	32% 10% 22% 18% 9% 11%	200
December	1st to 10th	Day	35% 14% 16% 14% 15% 9%	200
	11th to 20th	Day	35% 19% 16% 21% 8% 2	200
	21st to end	Day	39% 10% 20% 16% 12% 2	220
January	1st to 10th	Night	41% 11% 16% 17% 13% 2	862
	11th to 20th	Night	34% 10% 13% 25% 17% 2	800
	21st to end	Night	31% 9% 16% 25% 16% 3	880
February	1st to 10th	Night	34% 12% 17% 22% 13% 2	800
	11th to 20th	Night	30% 11% 19% 24% 14% 2	784
	21st to end	Night	33% 11% 20% 22% 12% 2	660
March	1st to 10th	Night	28% 12% 19% 25% 15% 2	800
	11th to 20th	Night	29% 12% 19% 25% 14% 2	800
	21st to end	Night	30% 12% 20% 26% 10% 1	880
April	1st to 10th	Night	26% 9% 22% 28% 14% 2	800
	11th to 20th	Night	19% 10% 23% 30% 14% 3	761
	21st to end	Night	22% 9% 22% 32% 13% 2	573
May	1st to 10th	Night	17% 11% 18% 36% 18% 1	400
	11th to 20th	Night	15% 7% 19% 31% 25% 3	394
	21st to end	Night	13% 7% 18% 34% 25% 3	422
June	1st to 10th	Night	14% 7% 25% 32% 22% 1	380
	11th to 20th	Night	15% 8% 25% 32% 18% 1	380
	21st to end	Night	11% 7% 15% 39% 26% 2	380
July	1st to 10th	Night	7% 4 15% 37% 35% 2	370
	11th to 20th	Night	7% 2 11% 46% 31% 3	368
	21st to end	Night	5% 4 10% 38% 40% 3	416
August	1st to 10th	Night	5% 2 10% 34% 41% 8%	443
	11th to 20th	Night	7% 2 9% 29% 48% 6%	564
	21st to end	Night	13% 2 13% 35% 33% 4	737
September	1st to 10th	Night	11% 3 14% 36% 31% 4	760
	11th to 20th	Night	16% 4 14% 25% 34% 6%	760
	21st to end	Night	18% 6% 16% 35% 22% 3	760
October	1st to 10th	Night	19% 5% 15% 29% 27% 4	760
	11th to 20th	Night	26% 7% 15% 27% 21% 4	760
	21st to end	Night	28% 9% 20% 25% 16% 2	836
November	1st to 10th	Night	33% 9% 20% 26% 12%	780
	11th to 20th	Night	35% 11% 19% 21% 12% 1	800
	21st to end	Night	31% 9% 20% 22% 15% 3	800
December	1st to 10th	Night	30% 9% 16% 26% 18% 1	800
	11th to 20th	Night	35% 11% 19% 24% 11% 2	800
	21st to end	Night	42% 11% 18% 19% 9% 1	935

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