Olympia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Olympia clear sky chart from 2005-11-06 to 2024-01-16.

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	17% 10% 4 8% 7% 4 5% 5% 5% 6% 28%	74981
	Night	18% 9% 4 7% 6% 4 5% 5% 5% 6% 32%	74797
Transparency	Day	53% 10% 13% 16% 7% 1	74675
	Night	51% 10% 14% 16% 7% 1	74621
Seeing	Day	20% 13% 22% 25% 16% 5%	14034
	Night	23% 10% 20% 28% 18% 2	22571

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% 6% 10% 7% 4 5% 5% 4 4 11%	1349
	11th to 20th	Day	26% 14% 5% 10% 7% 3 5% 5% 4 5% 17%	1480
	21st to end	Day	26% 13% 5% 10% 8% 4 6% 5% 4 5% 13%	1678
February	1st to 10th	Day	25% 14% 4 10% 8% 4 6% 5% 5% 6% 12%	1620
	11th to 20th	Day	26% 14% 5% 9% 7% 3 4 5% 5% 6% 17%	1575
	21st to end	Day	23% 14% 5% 9% 8% 4 5% 4 5% 5% 19%	1376
March	1st to 10th	Day	24% 11% 5% 10% 8% 5% 5% 5% 5% 5% 17%	1876
	11th to 20th	Day	26% 13% 6% 8% 7% 4 5% 5% 3 5% 18%	1980
	21st to end	Day	23% 14% 6% 10% 9% 5% 7% 5% 5% 5% 12%	2225
April	1st to 10th	Day	23% 12% 5% 10% 8% 4 5% 5% 6% 6% 16%	2160
	11th to 20th	Day	22% 11% 4 9% 7% 5% 6% 5% 5% 6% 20%	2170
	21st to end	Day	16% 11% 5% 9% 8% 5% 6% 5% 5% 7% 23%	2340
May	1st to 10th	Day	15% 8% 4 8% 7% 5% 6% 5% 5% 7% 31%	2519
	11th to 20th	Day	16% 9% 4 7% 7% 5% 7% 6% 7% 7% 26%	2477
	21st to end	Day	17% 10% 4 9% 7% 5% 6% 6% 6% 6% 25%	2723
June	1st to 10th	Day	18% 10% 5% 7% 7% 4 5% 6% 6% 8% 25%	2559
	11th to 20th	Day	15% 11% 5% 9% 7% 5% 7% 6% 7% 7% 22%	2700
	21st to end	Day	10% 7% 3 7% 6% 4 4 5% 6% 7% 41%	2700
July	1st to 10th	Day	7% 7% 3 6% 5% 3 4 4 5% 7% 49%	2700
	11th to 20th	Day	8% 6% 3 5% 4 2 4 5% 6% 7% 50%	2588
	21st to end	Day	5% 4 2 4 4 2 4 4 4 6% 61%	2741
August	1st to 10th	Day	6% 6% 2 5% 4 3 5% 5% 5% 9% 50%	2410
	11th to 20th	Day	6% 4 2 4 4 3 4 4 6% 9% 54%	2298
	21st to end	Day	9% 5% 3 7% 6% 3 5% 5% 6% 7% 45%	2480
September	1st to 10th	Day	7% 6% 3 6% 6% 4 4 4 5% 7% 48%	2160
	11th to 20th	Day	10% 8% 3 7% 7% 4 5% 5% 5% 7% 38%	2160
	21st to end	Day	15% 7% 4 7% 6% 3 5% 6% 5% 6% 36%	1894
October	1st to 10th	Day	17% 10% 4 8% 7% 4 5% 4 5% 6% 29%	1800
	11th to 20th	Day	19% 10% 4 9% 7% 4 5% 5% 5% 7% 24%	1800
	21st to end	Day	24% 10% 4 10% 6% 4 5% 4 5% 6% 23%	1908
November	1st to 10th	Day	25% 14% 6% 9% 8% 4 6% 5% 4 5% 15%	1516
	11th to 20th	Day	28% 11% 5% 9% 7% 3 6% 5% 5% 6% 17%	1520
	21st to end	Day	25% 13% 4 9% 7% 4 5% 5% 5% 6% 18%	1520
December	1st to 10th	Day	23% 12% 5% 10% 7% 3 4 4 5% 5% 21%	1520
	11th to 20th	Day	27% 13% 6% 10% 10% 5% 6% 5% 4 4 9%	1925
	21st to end	Day	23% 14% 5% 13% 9% 5% 7% 5% 4 5% 11%	2534
January	1st to 10th	Night	34% 12% 4 9% 8% 3 5% 4 4 4 14%	2810
	11th to 20th	Night	26% 13% 4 8% 6% 4 5% 4 5% 8% 18%	2590
	21st to end	Night	21% 12% 4 8% 7% 4 5% 5% 5% 7% 19%	2674
February	1st to 10th	Night	25% 13% 5% 8% 6% 4 5% 5% 5% 6% 19%	2340
	11th to 20th	Night	20% 12% 4 8% 7% 4 5% 4 5% 6% 24%	2272
	21st to end	Night	21% 12% 4 9% 7% 4 7% 5% 5% 6% 21%	1880
March	1st to 10th	Night	22% 11% 5% 8% 6% 3 6% 5% 5% 5% 23%	2082
	11th to 20th	Night	22% 11% 5% 8% 7% 4 5% 5% 5% 6% 22%	1979
	21st to end	Night	24% 11% 4 9% 7% 5% 5% 5% 5% 6% 18%	2130
April	1st to 10th	Night	19% 10% 4 8% 8% 4 6% 6% 5% 6% 25%	1800
	11th to 20th	Night	16% 8% 4 7% 7% 3 6% 6% 6% 8% 29%	1738
	21st to end	Night	16% 10% 3 6% 6% 3 5% 5% 6% 7% 32%	1620
May	1st to 10th	Night	10% 7% 2 7% 7% 4 6% 4 5% 7% 40%	1440
	11th to 20th	Night	14% 6% 4 6% 5% 4 6% 6% 8% 7% 36%	1416
	21st to end	Night	10% 7% 3 7% 6% 4 5% 5% 6% 10% 36%	1552
June	1st to 10th	Night	13% 7% 2 6% 5% 3 5% 5% 7% 8% 40%	1397
	11th to 20th	Night	10% 6% 3 6% 6% 3 5% 5% 6% 10% 37%	1260
	21st to end	Night	6% 5% 2 4 4 2 5% 4 7% 7% 54%	1260
July	1st to 10th	Night	5% 3 2 3 4 3 3 4 6% 6% 61%	1260
	11th to 20th	Night	4 3 2 4 4 2 4 3 4 7% 65%	1370
	21st to end	Night	3 1 1 2 2 1 2 3 4 7% 74%	1576
August	1st to 10th	Night	4 2 1 3 2 2 3 4 5% 9% 65%	1525
	11th to 20th	Night	4 2 1 2 2 2 2 3 4 7% 69%	1602
	21st to end	Night	7% 4 2 5% 4 2 4 4 5% 6% 57%	1875
September	1st to 10th	Night	5% 4 2 3 4 3 4 3 4 6% 62%	1800
	11th to 20th	Night	11% 5% 2 6% 5% 3 5% 4 5% 7% 47%	1800
	21st to end	Night	11% 8% 3 6% 5% 3 5% 4 4 7% 45%	2064
October	1st to 10th	Night	16% 6% 3 6% 6% 3 5% 5% 5% 7% 39%	2160
	11th to 20th	Night	19% 8% 3 7% 6% 3 4 5% 4 6% 35%	2160
	21st to end	Night	22% 9% 3 7% 6% 3 4 4 5% 6% 32%	2447
November	1st to 10th	Night	27% 10% 3 8% 6% 4 5% 5% 5% 6% 22%	2558
	11th to 20th	Night	25% 11% 5% 8% 8% 3 5% 5% 4 5% 21%	2660
	21st to end	Night	22% 10% 4 7% 8% 4 5% 5% 5% 6% 23%	2660
December	1st to 10th	Night	25% 11% 5% 8% 5% 3 5% 4 4 6% 23%	2660
	11th to 20th	Night	24% 13% 5% 9% 9% 5% 5% 5% 5% 5% 15%	3280
	21st to end	Night	25% 14% 6% 10% 8% 4 5% 4 4 4 15%	5100

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	75% 9% 6% 7% 3 1	1349
	11th to 20th	Day	68% 9% 10% 7% 4 2	1480
	21st to end	Day	69% 9% 9% 10% 3	1678
February	1st to 10th	Day	70% 10% 9% 7% 4	1620
	11th to 20th	Day	67% 10% 9% 8% 5%	1584
	21st to end	Day	65% 10% 11% 10% 5%	1376
March	1st to 10th	Day	66% 9% 12% 11% 3	1876
	11th to 20th	Day	67% 8% 9% 12% 3 1	1980
	21st to end	Day	71% 10% 10% 9% 1	2225
April	1st to 10th	Day	65% 10% 14% 10% 1	2160
	11th to 20th	Day	61% 10% 11% 12% 6% 1	2170
	21st to end	Day	58% 11% 14% 12% 5%	2340
May	1st to 10th	Day	50% 10% 18% 16% 6%	2519
	11th to 20th	Day	52% 13% 18% 13% 4	2480
	21st to end	Day	55% 13% 15% 15% 3	2724
June	1st to 10th	Day	55% 10% 19% 11% 6%	2559
	11th to 20th	Day	57% 12% 17% 11% 3	2700
	21st to end	Day	40% 11% 19% 20% 8% 2	2700
July	1st to 10th	Day	35% 8% 16% 25% 14% 3	2626
	11th to 20th	Day	31% 8% 15% 28% 17% 2	2509
	21st to end	Day	22% 7% 14% 35% 19% 3	2589
August	1st to 10th	Day	29% 9% 18% 27% 15% 3	2393
	11th to 20th	Day	26% 8% 21% 28% 13% 4	2298
	21st to end	Day	36% 9% 15% 30% 10% 1	2480
September	1st to 10th	Day	34% 8% 16% 24% 14% 5%	2160
	11th to 20th	Day	43% 10% 14% 17% 14% 3	2160
	21st to end	Day	46% 10% 15% 17% 10% 2	1894
October	1st to 10th	Day	53% 8% 12% 19% 8% 1	1800
	11th to 20th	Day	56% 9% 14% 15% 5% 1	1800
	21st to end	Day	60% 8% 11% 13% 7%	1908
November	1st to 10th	Day	69% 9% 10% 8% 3	1516
	11th to 20th	Day	66% 10% 10% 8% 5% 1	1520
	21st to end	Day	65% 9% 9% 11% 7%	1520
December	1st to 10th	Day	64% 8% 8% 11% 8% 1	1520
	11th to 20th	Day	72% 12% 8% 6% 2	1928
	21st to end	Day	72% 12% 8% 5% 2	2534
January	1st to 10th	Night	73% 8% 8% 7% 3 1	2810
	11th to 20th	Night	65% 9% 12% 10% 3 1	2590
	21st to end	Night	62% 11% 11% 11% 4 1	2674
February	1st to 10th	Night	64% 10% 12% 9% 5%	2340
	11th to 20th	Night	60% 10% 13% 11% 6% 1	2274
	21st to end	Night	61% 11% 11% 11% 6%	1880
March	1st to 10th	Night	60% 9% 12% 14% 5%	2082
	11th to 20th	Night	60% 9% 11% 14% 3 1	1980
	21st to end	Night	64% 12% 14% 9% 1	2130
April	1st to 10th	Night	58% 12% 15% 13% 3	1800
	11th to 20th	Night	51% 12% 15% 16% 5% 1	1738
	21st to end	Night	50% 10% 18% 15% 7% 1	1620
May	1st to 10th	Night	41% 11% 18% 21% 8% 1	1440
	11th to 20th	Night	45% 10% 20% 19% 6% 1	1416
	21st to end	Night	43% 14% 20% 18% 4	1552
June	1st to 10th	Night	42% 9% 20% 23% 6% 1	1397
	11th to 20th	Night	41% 14% 25% 17% 3	1260
	21st to end	Night	29% 10% 27% 23% 8% 3	1260
July	1st to 10th	Night	23% 10% 21% 28% 13% 5%	1225
	11th to 20th	Night	21% 7% 16% 35% 18% 2	1326
	21st to end	Night	11% 7% 17% 38% 21% 7%	1488
August	1st to 10th	Night	19% 8% 20% 30% 20% 3	1517
	11th to 20th	Night	15% 7% 24% 32% 16% 5%	1602
	21st to end	Night	28% 8% 17% 32% 14% 2	1875
September	1st to 10th	Night	23% 9% 20% 27% 17% 5%	1800
	11th to 20th	Night	36% 10% 15% 19% 17% 4	1800
	21st to end	Night	39% 9% 14% 23% 13% 2	2064
October	1st to 10th	Night	44% 8% 14% 24% 10% 1	2160
	11th to 20th	Night	49% 8% 14% 21% 7% 1	2160
	21st to end	Night	52% 7% 14% 17% 10%	2447
November	1st to 10th	Night	61% 9% 12% 12% 4 1	2558
	11th to 20th	Night	63% 9% 12% 9% 5% 2	2660
	21st to end	Night	59% 11% 12% 9% 8% 1	2660
December	1st to 10th	Night	62% 8% 11% 13% 5%	2660
	11th to 20th	Night	66% 12% 12% 7% 3	3276
	21st to end	Night	68% 10% 11% 8% 2	5100

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	36% 13% 18% 19% 10% 4	180
	11th to 20th	Day	34% 17% 18% 16% 12% 3	170
	21st to end	Day	34% 14% 19% 18% 13% 2	270
February	1st to 10th	Day	36% 17% 17% 19% 10% 1	340
	11th to 20th	Day	36% 15% 22% 18% 8% 1	335
	21st to end	Day	32% 18% 25% 15% 8% 2	300
March	1st to 10th	Day	30% 17% 23% 22% 7% 1	420
	11th to 20th	Day	28% 14% 21% 18% 14% 4	420
	21st to end	Day	27% 19% 24% 19% 8% 2	462
April	1st to 10th	Day	30% 16% 21% 21% 8% 4	420
	11th to 20th	Day	27% 15% 25% 19% 9% 5%	412
	21st to end	Day	21% 14% 32% 20% 11% 1	420
May	1st to 10th	Day	15% 13% 25% 29% 15% 3	423
	11th to 20th	Day	23% 12% 20% 24% 16% 6%	448
	21st to end	Day	17% 10% 23% 28% 16% 6%	485
June	1st to 10th	Day	18% 11% 28% 22% 16% 5%	489
	11th to 20th	Day	15% 17% 30% 25% 12% 2	630
	21st to end	Day	8% 11% 26% 30% 20% 5%	630
July	1st to 10th	Day	12% 7% 18% 36% 23% 5%	621
	11th to 20th	Day	7% 6% 21% 35% 25% 6%	508
	21st to end	Day	6% 3 14% 31% 36% 9%	487
August	1st to 10th	Day	8% 6% 17% 28% 29% 11%	447
	11th to 20th	Day	6% 3 14% 35% 31% 11%	437
	21st to end	Day	9% 9% 22% 31% 21% 7%	495
September	1st to 10th	Day	10% 10% 23% 36% 16% 6%	450
	11th to 20th	Day	13% 9% 18% 30% 25% 5%	450
	21st to end	Day	17% 12% 20% 29% 20% 2	450
October	1st to 10th	Day	22% 11% 22% 21% 18% 6%	450
	11th to 20th	Day	21% 16% 18% 23% 16% 6%	450
	21st to end	Day	29% 18% 20% 19% 11% 3	423
November	1st to 10th	Day	29% 25% 18% 16% 11% 2	194
	11th to 20th	Day	34% 18% 18% 11% 15% 3	180
	21st to end	Day	27% 17% 27% 13% 9% 7%	180
December	1st to 10th	Day	33% 16% 21% 16% 12% 2	180
	11th to 20th	Day	39% 17% 21% 16% 5% 2	180
	21st to end	Day	35% 20% 19% 17% 8% 2	198
January	1st to 10th	Night	40% 15% 16% 17% 11% 2	837
	11th to 20th	Night	32% 13% 14% 23% 17% 1	740
	21st to end	Night	30% 9% 19% 23% 18% 2	792
February	1st to 10th	Night	32% 14% 20% 22% 11% 2	720
	11th to 20th	Night	26% 15% 21% 24% 13% 1	704
	21st to end	Night	28% 15% 23% 23% 10% 2	592
March	1st to 10th	Night	25% 15% 21% 23% 14% 2	720
	11th to 20th	Night	28% 14% 19% 25% 12% 2	720
	21st to end	Night	29% 11% 23% 26% 10% 1	792
April	1st to 10th	Night	25% 12% 22% 26% 14% 1	720
	11th to 20th	Night	18% 13% 25% 28% 14% 2	670
	21st to end	Night	21% 12% 22% 31% 13% 1	540
May	1st to 10th	Night	13% 11% 22% 35% 18% 1	360
	11th to 20th	Night	16% 6% 23% 32% 20% 2	354
	21st to end	Night	12% 7% 24% 38% 17% 2	389
June	1st to 10th	Night	13% 7% 28% 34% 18%	360
	11th to 20th	Night	10% 10% 34% 29% 16%	360
	21st to end	Night	8% 5% 24% 39% 21% 2	360
July	1st to 10th	Night	7% 2 20% 45% 26% 1	350
	11th to 20th	Night	4 3 22% 43% 26% 1	348
	21st to end	Night	3 1 18% 44% 32% 1	394
August	1st to 10th	Night	6% 2 16% 33% 38% 4	451
	11th to 20th	Night	6% 2 11% 35% 43% 4	534
	21st to end	Night	9% 5% 19% 39% 26% 2	687
September	1st to 10th	Night	8% 3 21% 37% 27% 4	720
	11th to 20th	Night	12% 4 17% 28% 34% 4	720
	21st to end	Night	15% 7% 19% 34% 22% 3	720
October	1st to 10th	Night	17% 8% 16% 27% 28% 3	720
	11th to 20th	Night	23% 10% 15% 27% 21% 4	720
	21st to end	Night	27% 9% 23% 23% 17% 1	792
November	1st to 10th	Night	31% 11% 20% 27% 10%	740
	11th to 20th	Night	31% 13% 22% 21% 13% 1	760
	21st to end	Night	28% 12% 21% 24% 13% 3	760
December	1st to 10th	Night	31% 10% 16% 27% 15% 1	760
	11th to 20th	Night	34% 13% 20% 21% 10% 1	760
	21st to end	Night	38% 16% 18% 19% 8% 1	905

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