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 2026-09-29.

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	16% 8% 3 8% 5% 5% 5% 4 5% 7% 35%	154282
	Night	18% 9% 3 8% 5% 4 4 4 5% 6% 34%	145805
Transparency	Day	31% 35% 11% 15% 7% 1	153999
	Night	35% 36% 10% 14% 5% 1	145670
Seeing	Day	20% 10% 20% 25% 18% 6%	17538
	Night	24% 8% 17% 28% 19% 2	25787

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	31% 13% 4 13% 8% 6% 5% 4 4 4 8%	4338
	11th to 20th	Day	18% 11% 4 10% 6% 5% 5% 3 5% 8% 25%	4716
	21st to end	Day	26% 12% 4 9% 4 4 5% 4 4 6% 23%	5407
February	1st to 10th	Day	27% 12% 4 10% 6% 5% 4 4 5% 7% 15%	1890
	11th to 20th	Day	29% 12% 4 9% 6% 4 4 4 3 6% 18%	1845
	21st to end	Day	26% 12% 4 9% 6% 5% 4 4 5% 6% 18%	1625
March	1st to 10th	Day	17% 11% 4 11% 7% 7% 7% 6% 7% 6% 17%	4753
	11th to 20th	Day	19% 13% 4 9% 5% 5% 5% 3 5% 6% 26%	5110
	21st to end	Day	22% 13% 4 11% 7% 7% 6% 5% 6% 6% 13%	5776
April	1st to 10th	Day	23% 11% 4 9% 6% 6% 5% 5% 6% 7% 18%	2520
	11th to 20th	Day	19% 11% 3 7% 6% 5% 5% 4 7% 6% 27%	2518
	21st to end	Day	17% 10% 4 9% 6% 6% 5% 5% 6% 7% 26%	2756
May	1st to 10th	Day	11% 7% 2 6% 5% 4 5% 4 6% 7% 42%	8195
	11th to 20th	Day	14% 10% 3 9% 5% 6% 5% 4 7% 9% 29%	8160
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 4 7% 9% 32%	8993
June	1st to 10th	Day	16% 8% 3 8% 6% 5% 4 4 6% 9% 31%	2963
	11th to 20th	Day	16% 9% 3 8% 7% 6% 6% 5% 6% 8% 26%	3150
	21st to end	Day	10% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	3150
July	1st to 10th	Day	6% 5% 2 6% 3 3 3 3 4 5% 59%	8820
	11th to 20th	Day	4 4 1 4 3 3 3 2 3 5% 68%	8365
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 6% 57%	8916
August	1st to 10th	Day	3 3 2 5% 3 3 3 3 4 7% 65%	6270
	11th to 20th	Day	6% 5% 2 6% 5% 4 5% 4 6% 9% 48%	5923
	21st to end	Day	12% 7% 3 6% 5% 4 4 4 6% 8% 43%	6301
September	1st to 10th	Day	9% 6% 2 7% 4 4 4 3 5% 7% 48%	2520
	11th to 20th	Day	12% 8% 3 8% 5% 4 5% 5% 5% 7% 39%	2520
	21st to end	Day	16% 7% 2 7% 4 4 3 4 6% 8% 37%	2197
October	1st to 10th	Day	14% 7% 3 6% 5% 5% 4 4 7% 12% 34%	3280
	11th to 20th	Day	23% 10% 3 9% 5% 4 4 3 5% 8% 25%	3261
	21st to end	Day	25% 9% 3 10% 5% 5% 5% 4 6% 7% 20%	3378
November	1st to 10th	Day	29% 11% 4 8% 6% 4 5% 4 5% 6% 17%	1694
	11th to 20th	Day	31% 10% 3 8% 5% 5% 4 3 4 5% 20%	1680
	21st to end	Day	28% 11% 4 9% 6% 5% 5% 4 6% 6% 18%	1680
December	1st to 10th	Day	22% 8% 4 8% 9% 6% 5% 4 6% 7% 22%	2700
	11th to 20th	Day	32% 12% 3 10% 7% 6% 6% 4 5% 5% 10%	3105
	21st to end	Day	28% 13% 5% 11% 7% 6% 5% 4 4 5% 11%	3807
January	1st to 10th	Night	31% 15% 6% 12% 7% 6% 4 3 4 3 10%	8534
	11th to 20th	Night	18% 9% 4 9% 5% 4 4 3 5% 6% 32%	8182
	21st to end	Night	24% 10% 4 9% 5% 4 4 3 4 7% 26%	8770
February	1st to 10th	Night	27% 12% 3 10% 6% 5% 4 3 4 6% 20%	2730
	11th to 20th	Night	25% 12% 4 8% 5% 4 4 3 5% 6% 24%	2662
	21st to end	Night	25% 12% 3 7% 5% 5% 5% 4 6% 5% 24%	2181
March	1st to 10th	Night	17% 12% 4 9% 6% 6% 7% 5% 7% 8% 20%	5347
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 3 4 6% 32%	5109
	21st to end	Night	25% 13% 4 10% 7% 5% 6% 4 5% 5% 15%	5440
April	1st to 10th	Night	19% 11% 3 8% 5% 6% 5% 4 6% 7% 25%	2100
	11th to 20th	Night	15% 8% 3 9% 5% 5% 5% 5% 6% 8% 31%	2048
	21st to end	Night	18% 8% 3 8% 6% 4 5% 4 5% 8% 32%	1862
May	1st to 10th	Night	11% 7% 2 7% 4 4 5% 3 5% 6% 47%	4724
	11th to 20th	Night	12% 8% 3 9% 5% 5% 6% 4 6% 7% 34%	4700
	21st to end	Night	7% 9% 3 8% 5% 6% 5% 5% 7% 9% 36%	5178
June	1st to 10th	Night	14% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	1651
	11th to 20th	Night	12% 7% 3 6% 5% 4 5% 5% 5% 9% 37%	1470
	21st to end	Night	8% 6% 2 7% 5% 4 4 5% 5% 7% 48%	1470
July	1st to 10th	Night	4 3 1 3 2 3 3 3 5% 7% 65%	4140
	11th to 20th	Night	2 2 1 2 2 2 2 3 4 5% 75%	4578
	21st to end	Night	4 3 1 4 3 3 3 3 5% 7% 63%	5152
August	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 76%	3872
	11th to 20th	Night	5% 3 1 4 3 4 5% 3 5% 9% 56%	4142
	21st to end	Night	11% 6% 2 6% 3 3 3 3 5% 6% 52%	4857
September	1st to 10th	Night	7% 4 2 4 3 4 4 3 5% 7% 58%	2100
	11th to 20th	Night	13% 6% 3 6% 5% 4 3 3 5% 6% 46%	2100
	21st to end	Night	14% 7% 3 6% 4 4 4 2 4 7% 44%	2377
October	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 6% 8% 43%	3920
	11th to 20th	Night	23% 9% 3 6% 4 4 4 4 4 6% 31%	3897
	21st to end	Night	22% 11% 4 8% 5% 4 4 4 6% 6% 27%	4427
November	1st to 10th	Night	30% 9% 3 8% 5% 5% 5% 4 5% 5% 21%	2820
	11th to 20th	Night	31% 10% 3 9% 5% 5% 3 3 5% 5% 22%	2940
	21st to end	Night	27% 11% 4 8% 6% 5% 4 4 6% 5% 22%	2940
December	1st to 10th	Night	22% 9% 3 9% 7% 5% 5% 4 5% 6% 25%	4700
	11th to 20th	Night	31% 13% 5% 11% 7% 5% 5% 4 4 4 11%	5320
	21st to end	Night	28% 14% 5% 11% 6% 6% 5% 4 4 5% 13%	7365

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	64% 23% 6% 5% 1	4338
	11th to 20th	Day	44% 23% 9% 14% 9%	4740
	21st to end	Day	49% 23% 7% 13% 7%	5408
February	1st to 10th	Day	30% 59% 4 3 3 1	1890
	11th to 20th	Day	32% 59% 3 4 3	1854
	21st to end	Day	27% 61% 4 6% 2	1625
March	1st to 10th	Day	43% 31% 11% 12% 3	4753
	11th to 20th	Day	41% 30% 10% 17% 2	5110
	21st to end	Day	50% 31% 10% 9% 1	5776
April	1st to 10th	Day	24% 62% 7% 7% 1	2520
	11th to 20th	Day	20% 62% 7% 8% 3	2518
	21st to end	Day	21% 61% 7% 8% 3	2756
May	1st to 10th	Day	27% 24% 14% 25% 9% 1	8195
	11th to 20th	Day	37% 28% 17% 16% 3	8163
	21st to end	Day	33% 25% 22% 16% 4	8994
June	1st to 10th	Day	15% 61% 9% 10% 4	2963
	11th to 20th	Day	19% 61% 7% 10% 4	3150
	21st to end	Day	17% 61% 8% 10% 4	3150
July	1st to 10th	Day	19% 23% 14% 16% 18% 9%	8746
	11th to 20th	Day	13% 22% 9% 24% 26% 6%	8287
	21st to end	Day	21% 22% 17% 28% 10% 1	8723
August	1st to 10th	Day	13% 26% 9% 25% 21% 6%	6253
	11th to 20th	Day	23% 32% 20% 21% 4	5962
	21st to end	Day	29% 29% 12% 23% 6%	6300
September	1st to 10th	Day	19% 57% 10% 9% 4 1	2520
	11th to 20th	Day	22% 57% 5% 8% 7%	2520
	21st to end	Day	20% 57% 9% 10% 4	2197
October	1st to 10th	Day	24% 40% 12% 20% 3	3280
	11th to 20th	Day	38% 38% 9% 9% 6%	3261
	21st to end	Day	41% 38% 8% 7% 5% 1	3378
November	1st to 10th	Day	31% 55% 7% 6% 1	1694
	11th to 20th	Day	29% 56% 5% 6% 3 1	1680
	21st to end	Day	27% 56% 5% 6% 5%	1680
December	1st to 10th	Day	40% 39% 5% 10% 6%	2700
	11th to 20th	Day	53% 33% 7% 6% 1	3108
	21st to end	Day	56% 27% 10% 5% 2	3807
January	1st to 10th	Night	65% 22% 5% 5% 1	8534
	11th to 20th	Night	38% 25% 7% 19% 10% 1	8220
	21st to end	Night	48% 25% 7% 15% 5%	8770
February	1st to 10th	Night	28% 60% 5% 4 4	2730
	11th to 20th	Night	29% 59% 4 4 3	2664
	21st to end	Night	24% 61% 7% 6% 2	2181
March	1st to 10th	Night	41% 32% 11% 12% 4	5347
	11th to 20th	Night	37% 30% 13% 19% 1	5110
	21st to end	Night	51% 31% 10% 8% 1	5440
April	1st to 10th	Night	23% 63% 7% 7%	2100
	11th to 20th	Night	20% 62% 8% 6% 3	2048
	21st to end	Night	19% 61% 8% 9% 3	1862
May	1st to 10th	Night	28% 24% 10% 30% 7%	4721
	11th to 20th	Night	38% 29% 14% 17% 2	4700
	21st to end	Night	36% 27% 20% 16% 1	5182
June	1st to 10th	Night	16% 61% 8% 12% 3	1651
	11th to 20th	Night	15% 61% 8% 14% 3	1470
	21st to end	Night	15% 60% 10% 12% 3	1470
July	1st to 10th	Night	18% 25% 17% 16% 14% 9%	4105
	11th to 20th	Night	10% 22% 11% 21% 33% 4	4533
	21st to end	Night	20% 23% 17% 29% 9% 1	5040
August	1st to 10th	Night	8% 26% 7% 33% 21% 4	3864
	11th to 20th	Night	21% 32% 22% 21% 3 1	4169
	21st to end	Night	28% 29% 15% 22% 6%	4857
September	1st to 10th	Night	16% 56% 11% 11% 6%	2100
	11th to 20th	Night	21% 56% 5% 10% 6% 1	2100
	21st to end	Night	20% 57% 7% 13% 3	2377
October	1st to 10th	Night	24% 39% 15% 21% 1	3920
	11th to 20th	Night	36% 38% 9% 13% 4	3897
	21st to end	Night	40% 38% 7% 10% 5%	4427
November	1st to 10th	Night	30% 56% 6% 7% 1	2820
	11th to 20th	Night	30% 55% 4 5% 3 2	2940
	21st to end	Night	28% 56% 5% 6% 5% 1	2940
December	1st to 10th	Night	40% 39% 6% 11% 4	4700
	11th to 20th	Night	56% 32% 6% 5% 1	5316
	21st to end	Night	56% 29% 8% 7% 1	7365

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	34% 14% 19% 19% 10% 5%	220
	11th to 20th	Day	29% 13% 19% 19% 15% 5%	220
	21st to end	Day	32% 12% 21% 16% 15% 5%	321
February	1st to 10th	Day	36% 13% 16% 20% 12% 4	430
	11th to 20th	Day	38% 12% 21% 18% 9% 1	425
	21st to end	Day	32% 16% 22% 19% 8% 1	391
March	1st to 10th	Day	28% 15% 25% 21% 10% 1	540
	11th to 20th	Day	31% 15% 16% 21% 12% 4	540
	21st to end	Day	28% 14% 25% 21% 9% 2	594
April	1st to 10th	Day	27% 15% 20% 23% 11% 4	540
	11th to 20th	Day	20% 14% 30% 21% 9% 6%	532
	21st to end	Day	18% 13% 28% 25% 14% 2	540
May	1st to 10th	Day	14% 11% 24% 31% 15% 5%	543
	11th to 20th	Day	21% 10% 22% 23% 18% 7%	568
	21st to end	Day	18% 9% 21% 28% 18% 7%	617
June	1st to 10th	Day	19% 10% 22% 25% 18% 7%	593
	11th to 20th	Day	16% 12% 23% 30% 16% 3	780
	21st to end	Day	11% 8% 20% 34% 21% 7%	780
July	1st to 10th	Day	9% 4 17% 34% 28% 8%	771
	11th to 20th	Day	6% 5% 16% 35% 28% 9%	615
	21st to end	Day	5% 4 12% 31% 36% 11%	619
August	1st to 10th	Day	8% 4 12% 31% 33% 12%	567
	11th to 20th	Day	8% 4 10% 30% 33% 16%	557
	21st to end	Day	12% 8% 19% 30% 25% 7%	627
September	1st to 10th	Day	10% 5% 21% 32% 24% 7%	570
	11th to 20th	Day	16% 7% 18% 28% 23% 9%	570
	21st to end	Day	17% 8% 23% 29% 19% 4	563
October	1st to 10th	Day	19% 11% 20% 23% 19% 7%	530
	11th to 20th	Day	26% 12% 17% 23% 16% 5%	530
	21st to end	Day	31% 14% 21% 18% 11% 5%	478
November	1st to 10th	Day	33% 13% 20% 20% 11% 2	245
	11th to 20th	Day	35% 14% 20% 14% 10% 7%	220
	21st to end	Day	33% 9% 23% 17% 9% 10%	220
December	1st to 10th	Day	37% 14% 15% 12% 14% 8%	220
	11th to 20th	Day	38% 19% 15% 20% 7% 1	220
	21st to end	Day	38% 10% 20% 17% 14% 2	242
January	1st to 10th	Night	41% 11% 16% 17% 12% 2	908
	11th to 20th	Night	33% 10% 13% 25% 17% 2	840
	21st to end	Night	30% 9% 16% 26% 16% 3	924
February	1st to 10th	Night	32% 12% 17% 24% 13% 2	840
	11th to 20th	Night	29% 11% 20% 24% 15% 2	824
	21st to end	Night	33% 11% 21% 22% 11% 1	692
March	1st to 10th	Night	28% 12% 19% 24% 15% 2	840
	11th to 20th	Night	31% 11% 18% 24% 14% 2	840
	21st to end	Night	30% 13% 21% 27% 9% 1	924
April	1st to 10th	Night	25% 9% 22% 28% 15% 2	840
	11th to 20th	Night	19% 10% 23% 30% 15% 3	800
	21st to end	Night	21% 9% 22% 33% 13% 1	602
May	1st to 10th	Night	16% 10% 18% 36% 19% 1	420
	11th to 20th	Night	15% 8% 18% 32% 24% 2	414
	21st to end	Night	13% 7% 17% 35% 24% 3	455
June	1st to 10th	Night	14% 7% 24% 32% 22% 1	420
	11th to 20th	Night	14% 8% 24% 36% 17% 1	420
	21st to end	Night	10% 7% 16% 40% 26% 2	420
July	1st to 10th	Night	7% 4 14% 39% 34% 2	410
	11th to 20th	Night	6% 4 13% 45% 30% 3	408
	21st to end	Night	5% 4 10% 38% 40% 3	460
August	1st to 10th	Night	5% 2 10% 34% 41% 7%	490
	11th to 20th	Night	7% 2 8% 29% 47% 6%	624
	21st to end	Night	12% 2 13% 35% 33% 4	815
September	1st to 10th	Night	10% 2 14% 37% 31% 5%	840
	11th to 20th	Night	16% 5% 14% 26% 33% 7%	840
	21st to end	Night	18% 6% 17% 35% 22% 3	832
October	1st to 10th	Night	19% 6% 15% 30% 26% 4	800
	11th to 20th	Night	26% 8% 16% 27% 20% 4	800
	21st to end	Night	28% 9% 20% 25% 16% 2	880
November	1st to 10th	Night	33% 10% 19% 26% 11%	820
	11th to 20th	Night	35% 12% 19% 21% 12% 1	840
	21st to end	Night	32% 9% 20% 23% 15% 3	840
December	1st to 10th	Night	31% 9% 15% 26% 17% 1	840
	11th to 20th	Night	36% 11% 18% 23% 10% 1	840
	21st to end	Night	41% 12% 18% 20% 10% 1	985

Page generated at 2026-09-26 01:18:40. Version ac07

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