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 2024-03-06.

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	20% 9% 4 7% 6% 4 5% 5% 5% 7% 30%	78588
	Night	22% 9% 3 7% 6% 3 5% 4 5% 6% 30%	80822
Transparency	Day	38% 35% 10% 12% 4 1	78307
	Night	40% 34% 10% 11% 4 1	80683
Seeing	Day	21% 10% 19% 25% 18% 6%	14106
	Night	25% 8% 17% 27% 20% 3	22703

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	36% 13% 4 8% 8% 4 5% 4 3 4 10%	2320
	11th to 20th	Day	25% 12% 4 7% 7% 3 4 4 5% 7% 22%	2516
	21st to end	Day	36% 15% 5% 9% 6% 3 5% 4 3 4 9%	2904
February	1st to 10th	Day	29% 11% 5% 9% 6% 4 5% 5% 5% 7% 15%	1710
	11th to 20th	Day	30% 12% 4 7% 7% 3 5% 5% 4 6% 18%	1665
	21st to end	Day	26% 11% 4 8% 7% 4 5% 5% 5% 6% 19%	1475
March	1st to 10th	Day	24% 9% 4 10% 7% 5% 6% 6% 7% 6% 18%	1905
	11th to 20th	Day	30% 10% 4 7% 7% 4 5% 4 4 6% 19%	1980
	21st to end	Day	24% 11% 4 8% 8% 4 7% 6% 6% 6% 14%	2240
April	1st to 10th	Day	25% 9% 4 8% 7% 5% 5% 6% 5% 7% 17%	2160
	11th to 20th	Day	22% 10% 4 7% 7% 4 5% 5% 7% 6% 24%	2153
	21st to end	Day	18% 9% 4 8% 7% 4 6% 6% 6% 7% 25%	2362
May	1st to 10th	Day	14% 8% 4 6% 6% 4 6% 7% 5% 7% 32%	2520
	11th to 20th	Day	17% 8% 4 7% 6% 4 7% 5% 7% 8% 28%	2477
	21st to end	Day	19% 8% 5% 8% 7% 4 5% 5% 6% 7% 26%	2723
June	1st to 10th	Day	18% 8% 3 7% 6% 4 5% 5% 6% 9% 29%	2539
	11th to 20th	Day	18% 8% 4 8% 8% 5% 6% 6% 6% 8% 23%	2700
	21st to end	Day	11% 6% 3 6% 5% 3 5% 6% 6% 8% 41%	2700
July	1st to 10th	Day	10% 5% 2 5% 5% 3 4 4 5% 6% 51%	2700
	11th to 20th	Day	10% 5% 2 4 4 3 5% 4 5% 7% 52%	2567
	21st to end	Day	6% 3 1 3 4 2 3 4 4 6% 63%	2741
August	1st to 10th	Day	6% 5% 2 5% 4 2 4 5% 4 8% 54%	2444
	11th to 20th	Day	6% 4 2 4 4 3 3 4 5% 8% 57%	2298
	21st to end	Day	10% 5% 2 6% 6% 3 4 4 6% 8% 46%	2468
September	1st to 10th	Day	10% 5% 2 6% 5% 3 4 4 4 7% 50%	2160
	11th to 20th	Day	12% 7% 4 6% 6% 3 5% 6% 4 7% 39%	2160
	21st to end	Day	16% 8% 3 6% 6% 3 3 4 5% 8% 38%	1901
October	1st to 10th	Day	19% 9% 4 7% 6% 4 4 4 6% 7% 31%	1800
	11th to 20th	Day	22% 9% 4 7% 6% 4 5% 4 5% 7% 27%	1800
	21st to end	Day	28% 10% 4 7% 5% 3 4 5% 5% 6% 23%	1886
November	1st to 10th	Day	30% 10% 4 7% 7% 3 5% 5% 5% 5% 18%	1530
	11th to 20th	Day	30% 10% 4 7% 6% 3 5% 4 4 6% 22%	1520
	21st to end	Day	28% 10% 4 8% 7% 3 5% 5% 6% 6% 19%	1520
December	1st to 10th	Day	27% 9% 5% 6% 8% 4 4 4 5% 5% 23%	1520
	11th to 20th	Day	31% 13% 4 8% 8% 5% 7% 4 5% 5% 11%	1925
	21st to end	Day	25% 12% 6% 10% 9% 4 6% 6% 5% 6% 12%	2599
January	1st to 10th	Night	33% 13% 5% 9% 8% 4 5% 4 4 4 12%	4602
	11th to 20th	Night	27% 10% 4 7% 5% 3 4 4 5% 7% 25%	4382
	21st to end	Night	34% 13% 4 8% 6% 3 4 4 4 5% 13%	4741
February	1st to 10th	Night	29% 12% 3 8% 7% 3 4 3 4 6% 20%	2470
	11th to 20th	Night	25% 11% 4 8% 6% 3 4 4 5% 6% 24%	2402
	21st to end	Night	26% 11% 4 6% 5% 4 5% 5% 5% 5% 23%	1979
March	1st to 10th	Night	25% 11% 4 7% 6% 4 5% 5% 6% 6% 23%	2170
	11th to 20th	Night	26% 10% 4 8% 6% 3 4 4 4 7% 23%	1979
	21st to end	Night	25% 11% 4 8% 8% 4 6% 6% 6% 5% 18%	2115
April	1st to 10th	Night	21% 10% 4 7% 6% 4 6% 5% 5% 8% 24%	1800
	11th to 20th	Night	17% 7% 4 7% 8% 3 6% 6% 6% 8% 30%	1753
	21st to end	Night	18% 8% 3 7% 8% 3 5% 5% 6% 8% 30%	1597
May	1st to 10th	Night	14% 7% 3 7% 6% 4 5% 5% 5% 7% 36%	1440
	11th to 20th	Night	14% 6% 2 7% 6% 5% 6% 5% 6% 8% 34%	1416
	21st to end	Night	11% 7% 3 8% 6% 5% 6% 5% 6% 6% 35%	1552
June	1st to 10th	Night	14% 6% 2 7% 6% 4 5% 5% 5% 8% 39%	1416
	11th to 20th	Night	14% 7% 4 6% 6% 4 5% 6% 5% 10% 34%	1260
	21st to end	Night	8% 6% 2 5% 5% 3 4 5% 5% 7% 50%	1260
July	1st to 10th	Night	6% 4 2 4 3 2 4 4 5% 7% 58%	1260
	11th to 20th	Night	4 3 2 4 3 2 4 3 4 6% 64%	1390
	21st to end	Night	4 2 1 2 2 1 3 3 4 6% 72%	1576
August	1st to 10th	Night	4 4 2 3 4 2 3 4 4 9% 62%	1493
	11th to 20th	Night	5% 3 1 2 2 2 3 3 4 7% 68%	1602
	21st to end	Night	7% 5% 2 5% 5% 2 4 4 6% 6% 55%	1886
September	1st to 10th	Night	7% 4 2 4 3 2 4 4 5% 6% 59%	1800
	11th to 20th	Night	12% 6% 3 6% 6% 3 4 4 5% 6% 47%	1800
	21st to end	Night	15% 7% 3 5% 5% 3 3 4 4 8% 43%	2056
October	1st to 10th	Night	18% 6% 3 6% 6% 3 5% 6% 5% 6% 38%	2160
	11th to 20th	Night	22% 7% 3 6% 6% 4 5% 5% 4 6% 33%	2160
	21st to end	Night	26% 8% 3 5% 4 2 4 4 5% 6% 32%	2470
November	1st to 10th	Night	30% 9% 3 6% 6% 3 5% 5% 5% 6% 22%	2544
	11th to 20th	Night	30% 9% 3 8% 6% 3 4 4 4 5% 24%	2660
	21st to end	Night	27% 10% 4 7% 6% 4 5% 5% 5% 5% 23%	2660
December	1st to 10th	Night	27% 10% 4 7% 6% 3 4 4 4 5% 25%	2660
	11th to 20th	Night	28% 11% 5% 9% 8% 4 5% 5% 5% 5% 15%	3280
	21st to end	Night	27% 13% 6% 10% 8% 4 5% 4 4 4 15%	5031

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% 24% 6% 5%	2320
	11th to 20th	Day	49% 25% 10% 13% 3 1	2540
	21st to end	Day	65% 21% 7% 5% 2	2904
February	1st to 10th	Day	45% 36% 10% 7% 2	1710
	11th to 20th	Day	43% 35% 8% 10% 4	1674
	21st to end	Day	47% 35% 8% 7% 3	1475
March	1st to 10th	Day	45% 36% 10% 7% 1	1905
	11th to 20th	Day	47% 36% 8% 8% 2	1980
	21st to end	Day	48% 37% 8% 7% 1	2240
April	1st to 10th	Day	45% 36% 10% 8% 1	2160
	11th to 20th	Day	40% 36% 8% 10% 5% 1	2153
	21st to end	Day	40% 37% 10% 9% 3 1	2362
May	1st to 10th	Day	37% 36% 12% 12% 3	2520
	11th to 20th	Day	32% 38% 15% 11% 4	2480
	21st to end	Day	42% 37% 10% 9% 2	2724
June	1st to 10th	Day	39% 36% 12% 9% 4	2539
	11th to 20th	Day	40% 38% 13% 8% 2	2700
	21st to end	Day	27% 36% 17% 14% 5% 2	2700
July	1st to 10th	Day	21% 38% 11% 19% 9% 1	2626
	11th to 20th	Day	21% 37% 9% 22% 11% 1	2489
	21st to end	Day	15% 37% 9% 24% 12% 3	2589
August	1st to 10th	Day	21% 36% 13% 22% 8% 1	2427
	11th to 20th	Day	19% 35% 14% 20% 10% 3	2298
	21st to end	Day	23% 37% 10% 22% 7% 1	2468
September	1st to 10th	Day	19% 35% 12% 21% 9% 4	2160
	11th to 20th	Day	27% 37% 11% 15% 8% 2	2160
	21st to end	Day	31% 36% 12% 13% 7% 2	1901
October	1st to 10th	Day	34% 37% 9% 14% 5%	1800
	11th to 20th	Day	39% 36% 11% 11% 3 1	1800
	21st to end	Day	40% 37% 8% 10% 4	1886
November	1st to 10th	Day	42% 39% 7% 7% 4	1530
	11th to 20th	Day	43% 40% 7% 7% 3	1520
	21st to end	Day	44% 40% 6% 8% 2	1520
December	1st to 10th	Day	40% 39% 8% 8% 4	1520
	11th to 20th	Day	54% 31% 7% 7% 1	1928
	21st to end	Day	57% 25% 12% 6% 1	2599
January	1st to 10th	Night	64% 25% 6% 5%	4602
	11th to 20th	Night	44% 25% 7% 19% 3 1	4420
	21st to end	Night	62% 21% 7% 6% 2 1	4741
February	1st to 10th	Night	46% 35% 9% 8% 3	2470
	11th to 20th	Night	39% 36% 10% 9% 6% 1	2404
	21st to end	Night	45% 35% 8% 8% 4	1979
March	1st to 10th	Night	41% 36% 11% 11% 2	2170
	11th to 20th	Night	44% 37% 8% 8% 3	1980
	21st to end	Night	46% 37% 9% 7%	2115
April	1st to 10th	Night	41% 38% 10% 9% 2	1800
	11th to 20th	Night	34% 38% 10% 11% 5% 1	1753
	21st to end	Night	38% 37% 11% 9% 4 1	1597
May	1st to 10th	Night	36% 37% 12% 12% 4	1440
	11th to 20th	Night	31% 37% 14% 12% 5% 1	1416
	21st to end	Night	37% 38% 12% 11% 2	1552
June	1st to 10th	Night	34% 38% 11% 14% 3	1416
	11th to 20th	Night	34% 39% 17% 9% 1	1260
	21st to end	Night	25% 35% 18% 13% 6% 3	1260
July	1st to 10th	Night	17% 38% 13% 21% 8% 4	1225
	11th to 20th	Night	16% 37% 9% 26% 11% 1	1345
	21st to end	Night	9% 38% 10% 24% 16% 4	1488
August	1st to 10th	Night	16% 36% 13% 22% 11% 1	1485
	11th to 20th	Night	14% 36% 13% 21% 12% 4	1602
	21st to end	Night	21% 38% 11% 21% 9% 1	1886
September	1st to 10th	Night	16% 37% 11% 22% 11% 5%	1800
	11th to 20th	Night	26% 36% 10% 16% 11% 3	1800
	21st to end	Night	28% 36% 11% 15% 8% 2	2056
October	1st to 10th	Night	31% 37% 10% 15% 7% 1	2160
	11th to 20th	Night	35% 37% 10% 14% 4	2160
	21st to end	Night	34% 37% 10% 12% 7%	2470
November	1st to 10th	Night	40% 41% 8% 7% 3 1	2544
	11th to 20th	Night	41% 39% 9% 8% 3	2660
	21st to end	Night	41% 40% 9% 7% 3	2660
December	1st to 10th	Night	40% 40% 8% 9% 4	2660
	11th to 20th	Night	51% 32% 10% 6% 1	3276
	21st to end	Night	58% 26% 7% 8% 2	5031

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% 12% 16% 19% 12% 6%	180
	11th to 20th	Day	33% 14% 18% 16% 14% 5%	180
	21st to end	Day	34% 12% 22% 15% 14% 3	269
February	1st to 10th	Day	38% 13% 15% 19% 11% 4	370
	11th to 20th	Day	40% 13% 19% 18% 9% 2	365
	21st to end	Day	31% 17% 21% 20% 9% 1	337
March	1st to 10th	Day	26% 16% 25% 22% 10% 1	441
	11th to 20th	Day	29% 14% 18% 21% 14% 5%	420
	21st to end	Day	29% 14% 25% 20% 10% 2	462
April	1st to 10th	Day	30% 16% 20% 21% 10% 4	420
	11th to 20th	Day	24% 13% 29% 19% 8% 7%	412
	21st to end	Day	20% 13% 28% 24% 14% 1	420
May	1st to 10th	Day	15% 12% 23% 30% 15% 5%	423
	11th to 20th	Day	21% 7% 21% 23% 19% 7%	448
	21st to end	Day	18% 8% 20% 28% 18% 7%	485
June	1st to 10th	Day	20% 9% 23% 23% 18% 7%	469
	11th to 20th	Day	17% 12% 23% 28% 17% 3	630
	21st to end	Day	10% 7% 19% 34% 23% 7%	630
July	1st to 10th	Day	11% 4 16% 32% 29% 9%	621
	11th to 20th	Day	7% 5% 15% 34% 31% 9%	488
	21st to end	Day	5% 3 12% 30% 39% 11%	487
August	1st to 10th	Day	8% 4 13% 27% 34% 14%	447
	11th to 20th	Day	7% 3 8% 31% 34% 16%	437
	21st to end	Day	11% 8% 19% 30% 23% 8%	495
September	1st to 10th	Day	11% 6% 24% 33% 21% 6%	450
	11th to 20th	Day	15% 8% 17% 27% 24% 9%	450
	21st to end	Day	18% 10% 22% 28% 19% 4	450
October	1st to 10th	Day	21% 12% 18% 22% 19% 8%	450
	11th to 20th	Day	26% 11% 15% 25% 17% 6%	450
	21st to end	Day	32% 15% 17% 19% 12% 5%	401
November	1st to 10th	Day	35% 13% 18% 21% 10% 2	201
	11th to 20th	Day	35% 14% 21% 11% 11% 8%	180
	21st to end	Day	33% 10% 23% 14% 9% 11%	180
December	1st to 10th	Day	37% 14% 17% 12% 12% 7%	180
	11th to 20th	Day	34% 19% 16% 20% 9% 2	180
	21st to end	Day	37% 10% 21% 17% 13% 2	198
January	1st to 10th	Night	42% 11% 16% 16% 13% 2	816
	11th to 20th	Night	36% 10% 13% 23% 16% 2	760
	21st to end	Night	32% 9% 16% 25% 16% 3	836
February	1st to 10th	Night	34% 11% 17% 22% 14% 2	760
	11th to 20th	Night	29% 11% 19% 24% 15% 2	744
	21st to end	Night	33% 11% 21% 21% 12% 2	628
March	1st to 10th	Night	28% 12% 19% 24% 15% 2	742
	11th to 20th	Night	29% 11% 19% 24% 14% 2	720
	21st to end	Night	30% 12% 19% 28% 10% 1	792
April	1st to 10th	Night	27% 8% 22% 27% 14% 2	720
	11th to 20th	Night	21% 9% 22% 29% 16% 3	685
	21st to end	Night	23% 7% 23% 33% 12% 1	517
May	1st to 10th	Night	18% 11% 18% 34% 18% 1	360
	11th to 20th	Night	16% 5% 18% 32% 26% 3	354
	21st to end	Night	13% 6% 17% 35% 25% 3	389
June	1st to 10th	Night	13% 6% 26% 31% 23% 1	360
	11th to 20th	Night	15% 9% 24% 32% 18% 1	360
	21st to end	Night	11% 7% 15% 39% 27% 2	360
July	1st to 10th	Night	7% 4 14% 37% 35% 2	350
	11th to 20th	Night	7% 3 11% 46% 30% 3	348
	21st to end	Night	5% 3 10% 37% 42% 3	394
August	1st to 10th	Night	5% 2 10% 34% 41% 8%	419
	11th to 20th	Night	6% 2 9% 29% 49% 6%	534
	21st to end	Night	12% 2 13% 35% 34% 4	698
September	1st to 10th	Night	11% 3 14% 37% 31% 4	720
	11th to 20th	Night	15% 4 14% 25% 35% 7%	720
	21st to end	Night	19% 6% 15% 35% 22% 3	720
October	1st to 10th	Night	20% 5% 14% 29% 28% 5%	720
	11th to 20th	Night	26% 7% 15% 27% 21% 4	720
	21st to end	Night	29% 8% 19% 25% 17% 2	792
November	1st to 10th	Night	33% 9% 20% 26% 12%	740
	11th to 20th	Night	34% 11% 19% 22% 13% 1	760
	21st to end	Night	32% 9% 20% 21% 16% 3	760
December	1st to 10th	Night	31% 9% 17% 26% 16% 1	760
	11th to 20th	Night	35% 11% 19% 23% 11% 2	760
	21st to end	Night	42% 11% 17% 19% 10% 1	885

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