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 2024-11-02.

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% 4 4 4 5% 7% 33%	101942
	Night	20% 9% 3 8% 5% 4 4 3 5% 6% 33%	97039
Transparency	Day	25% 49% 8% 11% 6% 1	101700
	Night	28% 49% 7% 10% 4 1	96927
Seeing	Day	21% 10% 20% 25% 18% 6%	15096
	Night	25% 8% 17% 27% 20% 3	23436

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% 3 10% 7% 5% 5% 3 3 4 10%	2320
	11th to 20th	Day	25% 12% 4 8% 6% 4 4 3 5% 7% 22%	2516
	21st to end	Day	36% 16% 4 10% 5% 4 5% 3 4 4 9%	2904
February	1st to 10th	Day	29% 13% 4 10% 5% 5% 4 4 5% 8% 15%	1710
	11th to 20th	Day	30% 12% 4 8% 6% 5% 4 4 3 6% 18%	1665
	21st to end	Day	26% 12% 4 9% 6% 5% 4 4 5% 6% 19%	1475
March	1st to 10th	Day	16% 10% 4 12% 7% 7% 7% 5% 7% 6% 19%	3231
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 3 4 7% 34%	3490
	21st to end	Day	21% 13% 4 11% 7% 6% 6% 4 6% 7% 16%	3938
April	1st to 10th	Day	24% 11% 4 9% 6% 6% 5% 5% 5% 6% 18%	2280
	11th to 20th	Day	20% 11% 3 7% 6% 5% 4 4 7% 6% 26%	2275
	21st to end	Day	18% 10% 4 10% 6% 5% 5% 5% 6% 7% 24%	2494
May	1st to 10th	Day	15% 9% 3 6% 4 4 5% 4 5% 7% 38%	4420
	11th to 20th	Day	12% 8% 2 6% 4 5% 4 3 7% 9% 39%	4377
	21st to end	Day	20% 9% 4 9% 6% 5% 5% 4 7% 8% 21%	4813
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	18% 9% 3 8% 5% 5% 4 4 6% 7% 31%	1900
	11th to 20th	Day	22% 10% 4 8% 4 4 5% 3 5% 7% 27%	1900
	21st to end	Day	28% 10% 4 8% 5% 4 4 4 5% 6% 23%	1990
November	1st to 10th	Day	30% 11% 4 8% 6% 4 5% 4 5% 5% 18%	1539
	11th to 20th	Day	30% 11% 3 8% 5% 5% 4 3 4 5% 21%	1520
	21st to end	Day	28% 11% 3 8% 6% 5% 4 4 6% 6% 19%	1520
December	1st to 10th	Day	27% 10% 4 7% 7% 5% 3 3 5% 5% 23%	1520
	11th to 20th	Day	31% 13% 4 9% 7% 6% 6% 3 5% 4 11%	1925
	21st to end	Day	25% 13% 6% 12% 7% 6% 6% 4 5% 6% 12%	2599
January	1st to 10th	Night	32% 14% 5% 11% 6% 5% 4 3 4 4 12%	4602
	11th to 20th	Night	27% 10% 4 7% 4 4 4 3 5% 7% 25%	4382
	21st to end	Night	34% 13% 4 10% 5% 4 4 3 4 5% 13%	4741
February	1st to 10th	Night	29% 12% 3 9% 6% 4 4 2 4 6% 20%	2470
	11th to 20th	Night	25% 11% 4 9% 5% 4 4 3 5% 6% 24%	2402
	21st to end	Night	25% 12% 3 7% 5% 5% 6% 4 6% 5% 23%	1979
March	1st to 10th	Night	18% 13% 4 10% 6% 6% 6% 5% 6% 7% 17%	3629
	11th to 20th	Night	17% 8% 2 7% 4 4 3 3 4 7% 41%	3489
	21st to end	Night	24% 14% 4 10% 7% 5% 6% 4 5% 5% 16%	3714
April	1st to 10th	Night	21% 12% 3 8% 5% 6% 5% 4 6% 7% 23%	1900
	11th to 20th	Night	16% 8% 3 8% 6% 5% 5% 5% 6% 8% 30%	1851
	21st to end	Night	18% 9% 3 8% 7% 4 5% 4 6% 8% 29%	1685
May	1st to 10th	Night	16% 8% 3 8% 4 4 4 3 4 6% 40%	2540
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 4 6% 7% 44%	2516
	21st to end	Night	9% 12% 3 9% 6% 7% 6% 5% 7% 7% 30%	2762
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	17% 6% 2 6% 5% 5% 4 5% 5% 6% 39%	2280
	11th to 20th	Night	23% 8% 3 7% 5% 5% 4 4 4 6% 33%	2280
	21st to end	Night	25% 9% 2 7% 4 3 3 4 6% 6% 31%	2608
November	1st to 10th	Night	30% 9% 3 7% 5% 4 5% 4 5% 6% 22%	2557
	11th to 20th	Night	30% 9% 3 8% 5% 5% 3 3 5% 5% 24%	2660
	21st to end	Night	27% 10% 4 7% 6% 5% 4 3 5% 5% 23%	2660
December	1st to 10th	Night	27% 11% 3 8% 6% 4 4 3 4 4 25%	2660
	11th to 20th	Night	27% 12% 4 11% 7% 6% 5% 4 5% 5% 14%	3280
	21st to end	Night	27% 14% 5% 12% 7% 6% 4 3 4 4 14%	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	51% 37% 6% 5% 1 1	2320
	11th to 20th	Day	39% 37% 9% 12% 2	2540
	21st to end	Day	52% 39% 5% 3	2904
February	1st to 10th	Day	27% 65% 4 2 2 1	1710
	11th to 20th	Day	29% 65% 2 3 2	1674
	21st to end	Day	22% 67% 3 5% 3	1475
March	1st to 10th	Day	35% 41% 10% 12% 2	3231
	11th to 20th	Day	24% 41% 11% 22% 1 1	3490
	21st to end	Day	39% 42% 9% 9% 1	3938
April	1st to 10th	Day	22% 68% 5% 6%	2280
	11th to 20th	Day	18% 68% 5% 6% 3	2275
	21st to end	Day	20% 66% 5% 5% 3	2494
May	1st to 10th	Day	24% 41% 9% 16% 9% 1	4420
	11th to 20th	Day	23% 42% 16% 15% 4	4380
	21st to end	Day	37% 42% 14% 6% 1	4814
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	20% 61% 6% 10% 3	1900
	11th to 20th	Day	23% 61% 7% 6% 3	1900
	21st to end	Day	26% 60% 4 5% 4	1990
November	1st to 10th	Day	29% 60% 6% 5%	1539
	11th to 20th	Day	24% 60% 5% 6% 3 1	1520
	21st to end	Day	22% 61% 4 6% 6%	1520
December	1st to 10th	Day	26% 61% 3 6% 4 1	1520
	11th to 20th	Day	41% 48% 5% 5% 1	1928
	21st to end	Day	46% 37% 11% 4 2	2599
January	1st to 10th	Night	51% 37% 5% 4 2 1	4602
	11th to 20th	Night	35% 38% 6% 18% 3	4420
	21st to end	Night	50% 39% 6% 4 1	4741
February	1st to 10th	Night	25% 65% 4 3 3	2470
	11th to 20th	Night	26% 65% 4 3 2	2404
	21st to end	Night	21% 66% 5% 5% 2	1979
March	1st to 10th	Night	38% 42% 7% 10% 3	3629
	11th to 20th	Night	19% 40% 16% 23% 1 1	3490
	21st to end	Night	44% 41% 8% 6%	3714
April	1st to 10th	Night	20% 68% 6% 5%	1900
	11th to 20th	Night	18% 67% 7% 5% 3	1851
	21st to end	Night	20% 66% 6% 6% 2	1685
May	1st to 10th	Night	26% 41% 7% 20% 5%	2540
	11th to 20th	Night	21% 45% 15% 16% 4	2516
	21st to end	Night	36% 43% 14% 6% 1	2762
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	18% 60% 7% 12% 2	2280
	11th to 20th	Night	23% 61% 6% 8% 3	2280
	21st to end	Night	24% 59% 5% 8% 4	2608
November	1st to 10th	Night	26% 60% 6% 6% 1	2557
	11th to 20th	Night	25% 60% 4 5% 3 2	2660
	21st to end	Night	23% 61% 4 5% 6% 1	2660
December	1st to 10th	Night	27% 61% 3 7% 3 1	2660
	11th to 20th	Night	39% 50% 6% 4 2	3276
	21st to end	Night	46% 38% 7% 6% 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	27% 15% 25% 22% 10% 1	460
	11th to 20th	Day	28% 15% 16% 22% 14% 5%	460
	21st to end	Day	28% 15% 25% 20% 10% 2	506
April	1st to 10th	Day	29% 16% 20% 21% 10% 3	460
	11th to 20th	Day	22% 15% 30% 19% 8% 6%	452
	21st to end	Day	20% 14% 28% 22% 14% 2	460
May	1st to 10th	Day	16% 12% 24% 29% 15% 5%	463
	11th to 20th	Day	21% 8% 21% 24% 19% 8%	488
	21st to end	Day	19% 9% 20% 28% 17% 6%	529
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% 18% 12% 5%	439
November	1st to 10th	Day	36% 13% 18% 21% 10% 2	204
	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	760
	11th to 20th	Night	28% 12% 19% 24% 15% 2	760
	21st to end	Night	29% 12% 20% 27% 10% 1	836
April	1st to 10th	Night	26% 9% 22% 27% 14% 2	760
	11th to 20th	Night	20% 10% 23% 29% 15% 3	723
	21st to end	Night	22% 9% 23% 32% 12% 1	545
May	1st to 10th	Night	17% 11% 18% 34% 17% 1	380
	11th to 20th	Night	15% 6% 18% 32% 26% 3	374
	21st to end	Night	13% 7% 18% 34% 25% 3	411
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%	744
	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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