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 2025-11-03.

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	14% 10% 4 9% 6% 5% 5% 4 5% 6% 32%	131370
	Night	16% 10% 3 9% 5% 5% 4 4 5% 6% 33%	127357
Transparency	Day	49% 8% 14% 18% 8% 1	131085
	Night	50% 8% 15% 18% 7% 1	127219
Seeing	Day	20% 13% 22% 25% 16% 5%	16468
	Night	22% 10% 20% 28% 18% 2	24762

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	26% 14% 7% 14% 8% 6% 6% 4 4 3 7%	3240
	11th to 20th	Day	18% 12% 5% 15% 7% 5% 5% 4 4 6% 20%	3616
	21st to end	Day	23% 13% 4 10% 7% 6% 5% 3 4 4 21%	4252
February	1st to 10th	Day	23% 15% 4 11% 8% 6% 5% 4 6% 6% 11%	1800
	11th to 20th	Day	25% 15% 5% 10% 7% 4 4 4 5% 5% 16%	1755
	21st to end	Day	23% 16% 5% 10% 7% 6% 5% 3 5% 5% 18%	1535
March	1st to 10th	Day	14% 12% 5% 13% 8% 8% 7% 6% 6% 5% 16%	4711
	11th to 20th	Day	17% 15% 5% 10% 6% 5% 5% 3 4 5% 25%	5000
	21st to end	Day	19% 16% 6% 14% 8% 7% 6% 5% 5% 5% 11%	5610
April	1st to 10th	Day	22% 13% 4 10% 7% 5% 5% 5% 7% 6% 16%	2400
	11th to 20th	Day	19% 11% 3 9% 7% 5% 5% 4 5% 6% 25%	2415
	21st to end	Day	16% 11% 4 11% 6% 6% 5% 4 6% 7% 24%	2600
May	1st to 10th	Day	13% 8% 3 8% 4 5% 5% 4 5% 6% 40%	6319
	11th to 20th	Day	10% 9% 4 8% 6% 7% 6% 5% 8% 9% 28%	6277
	21st to end	Day	14% 9% 3 8% 5% 6% 6% 5% 6% 8% 29%	6903
June	1st to 10th	Day	16% 10% 4 8% 6% 5% 5% 5% 6% 8% 28%	2844
	11th to 20th	Day	13% 11% 4 10% 7% 7% 7% 5% 7% 7% 23%	3000
	21st to end	Day	9% 8% 3 8% 5% 5% 5% 4 6% 7% 39%	3000
July	1st to 10th	Day	4 5% 2 5% 3 2 3 2 5% 6% 64%	6780
	11th to 20th	Day	4 4 1 3 2 2 3 2 4 5% 70%	6486
	21st to end	Day	5% 5% 2 5% 4 4 4 3 5% 7% 58%	6874
August	1st to 10th	Day	4 5% 2 5% 3 4 4 3 5% 7% 59%	4360
	11th to 20th	Day	6% 5% 3 8% 5% 5% 7% 5% 8% 10% 39%	4156
	21st to end	Day	12% 7% 3 7% 4 4 4 3 5% 6% 44%	4487
September	1st to 10th	Day	7% 7% 3 7% 5% 5% 4 4 5% 7% 47%	2400
	11th to 20th	Day	9% 9% 3 8% 6% 5% 5% 4 6% 7% 38%	2399
	21st to end	Day	14% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	2104
October	1st to 10th	Day	11% 7% 3 8% 5% 6% 5% 5% 7% 9% 34%	3280
	11th to 20th	Day	19% 10% 4 10% 5% 5% 5% 4 6% 8% 24%	3261
	21st to end	Day	20% 9% 4 11% 6% 6% 6% 5% 7% 7% 20%	3429
November	1st to 10th	Day	25% 14% 5% 11% 7% 6% 5% 4 4 6% 14%	1614
	11th to 20th	Day	27% 13% 4 9% 6% 4 5% 4 5% 6% 16%	1600
	21st to end	Day	24% 14% 4 10% 6% 6% 5% 4 5% 6% 18%	1600
December	1st to 10th	Day	17% 10% 4 13% 8% 5% 5% 5% 5% 7% 20%	2620
	11th to 20th	Day	27% 13% 5% 13% 9% 7% 5% 4 5% 4 8%	3025
	21st to end	Day	25% 15% 5% 15% 7% 6% 6% 3 4 5% 9%	3618
January	1st to 10th	Night	27% 18% 6% 13% 8% 5% 4 3 4 3 9%	6699
	11th to 20th	Night	17% 11% 4 12% 6% 4 4 3 4 7% 27%	6282
	21st to end	Night	22% 11% 3 9% 5% 5% 4 3 5% 7% 23%	6711
February	1st to 10th	Night	24% 14% 4 9% 6% 5% 6% 4 5% 6% 18%	2600
	11th to 20th	Night	21% 14% 3 10% 6% 5% 4 3 5% 6% 24%	2532
	21st to end	Night	21% 13% 4 9% 6% 6% 5% 4 6% 5% 21%	2095
March	1st to 10th	Night	12% 11% 5% 11% 7% 6% 7% 5% 7% 7% 21%	5168
	11th to 20th	Night	15% 12% 4 9% 5% 5% 4 4 5% 6% 30%	4999
	21st to end	Night	22% 16% 5% 13% 5% 6% 5% 3 5% 6% 13%	5364
April	1st to 10th	Night	18% 11% 3 10% 7% 6% 5% 4 5% 6% 25%	2000
	11th to 20th	Night	15% 8% 3 7% 6% 4 5% 5% 6% 8% 31%	1932
	21st to end	Night	16% 11% 3 7% 5% 4 4 4 6% 7% 33%	1800
May	1st to 10th	Night	11% 8% 2 7% 4 3 4 3 4 5% 48%	3640
	11th to 20th	Night	9% 7% 3 6% 4 5% 5% 5% 7% 8% 40%	3616
	21st to end	Night	7% 9% 3 9% 6% 5% 5% 4 7% 10% 36%	3972
June	1st to 10th	Night	12% 8% 2 7% 3 4 5% 4 6% 8% 41%	1551
	11th to 20th	Night	9% 7% 3 7% 5% 5% 4 5% 6% 9% 40%	1400
	21st to end	Night	6% 6% 2 5% 4 4 5% 3 7% 7% 52%	1400
July	1st to 10th	Night	2 2 1 3 2 2 2 2 4 6% 75%	3180
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 5% 78%	3472
	21st to end	Night	2 2 1 3 2 2 3 3 6% 8% 67%	3969
August	1st to 10th	Night	3 2 1 3 2 3 2 2 4 7% 73%	2792
	11th to 20th	Night	6% 4 1 4 3 3 4 4 6% 8% 57%	2902
	21st to end	Night	12% 5% 2 5% 3 3 4 3 5% 6% 51%	3427
September	1st to 10th	Night	5% 4 2 4 3 4 4 3 4 7% 61%	2000
	11th to 20th	Night	11% 5% 2 7% 4 4 5% 4 5% 8% 46%	2000
	21st to end	Night	11% 8% 3 7% 4 4 5% 3 4 7% 45%	2294
October	1st to 10th	Night	10% 5% 2 7% 4 5% 4 4 7% 8% 44%	3920
	11th to 20th	Night	15% 9% 4 9% 6% 5% 4 4 5% 7% 32%	3897
	21st to end	Night	17% 8% 3 10% 7% 5% 4 4 6% 7% 28%	4377
November	1st to 10th	Night	26% 11% 3 9% 5% 6% 5% 4 5% 6% 21%	2724
	11th to 20th	Night	25% 13% 4 10% 7% 5% 4 4 4 5% 21%	2800
	21st to end	Night	22% 12% 3 9% 7% 5% 5% 4 5% 6% 22%	2800
December	1st to 10th	Night	16% 11% 4 14% 6% 6% 4 4 5% 7% 23%	4560
	11th to 20th	Night	26% 16% 5% 12% 8% 6% 5% 3 4 4 11%	5180
	21st to end	Night	25% 14% 6% 12% 7% 6% 5% 3 4 4 12%	7302

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	76% 9% 7% 5% 2	3240
	11th to 20th	Day	62% 8% 11% 14% 3 1	3640
	21st to end	Day	65% 9% 7% 13% 7%	4252
February	1st to 10th	Day	68% 11% 9% 8% 4	1800
	11th to 20th	Day	68% 10% 8% 8% 5%	1764
	21st to end	Day	65% 10% 10% 10% 4	1535
March	1st to 10th	Day	61% 10% 15% 12% 2	4711
	11th to 20th	Day	59% 7% 12% 20% 2	5000
	21st to end	Day	70% 8% 13% 9%	5610
April	1st to 10th	Day	64% 10% 15% 10% 1	2400
	11th to 20th	Day	56% 10% 12% 14% 7% 1	2415
	21st to end	Day	56% 10% 15% 14% 5%	2600
May	1st to 10th	Day	41% 8% 16% 24% 10%	6319
	11th to 20th	Day	46% 12% 23% 17% 3	6280
	21st to end	Day	47% 13% 21% 17% 3	6904
June	1st to 10th	Day	51% 10% 19% 13% 6%	2844
	11th to 20th	Day	55% 11% 16% 13% 4	3000
	21st to end	Day	41% 10% 19% 21% 7% 2	3000
July	1st to 10th	Day	22% 5% 17% 24% 20% 12%	6706
	11th to 20th	Day	18% 4 12% 31% 32% 4	6407
	21st to end	Day	26% 5% 20% 35% 13% 1	6681
August	1st to 10th	Day	24% 6% 12% 35% 21% 3	4343
	11th to 20th	Day	37% 7% 22% 24% 8% 2	4195
	21st to end	Day	41% 6% 12% 28% 12% 1	4486
September	1st to 10th	Day	35% 8% 16% 23% 13% 5%	2400
	11th to 20th	Day	43% 10% 14% 17% 14% 2	2399
	21st to end	Day	45% 9% 16% 19% 10% 1	2104
October	1st to 10th	Day	44% 6% 16% 28% 6%	3280
	11th to 20th	Day	56% 7% 13% 16% 8%	3261
	21st to end	Day	59% 6% 13% 13% 8% 1	3429
November	1st to 10th	Day	69% 9% 11% 8% 4	1614
	11th to 20th	Day	67% 10% 10% 8% 5% 1	1600
	21st to end	Day	65% 9% 9% 11% 7%	1600
December	1st to 10th	Day	62% 7% 7% 14% 9%	2620
	11th to 20th	Day	74% 10% 9% 7% 1	3028
	21st to end	Day	74% 10% 10% 5% 2	3618
January	1st to 10th	Night	78% 8% 7% 5% 2 1	6699
	11th to 20th	Night	54% 9% 11% 21% 4	6320
	21st to end	Night	59% 10% 9% 14% 7%	6711
February	1st to 10th	Night	64% 10% 11% 10% 5%	2600
	11th to 20th	Night	60% 10% 13% 10% 6% 1	2534
	21st to end	Night	61% 11% 12% 11% 5%	2095
March	1st to 10th	Night	53% 12% 18% 13% 5%	5168
	11th to 20th	Night	50% 9% 15% 23% 3 1	5000
	21st to end	Night	68% 10% 12% 9% 1	5364
April	1st to 10th	Night	58% 11% 15% 13% 2	2000
	11th to 20th	Night	48% 11% 17% 16% 7% 1	1932
	21st to end	Night	49% 10% 18% 17% 6% 1	1800
May	1st to 10th	Night	37% 8% 12% 32% 10%	3637
	11th to 20th	Night	38% 10% 25% 22% 5%	3616
	21st to end	Night	42% 12% 26% 19% 2	3972
June	1st to 10th	Night	40% 9% 21% 23% 7%	1551
	11th to 20th	Night	39% 13% 24% 21% 3	1400
	21st to end	Night	30% 10% 27% 23% 7% 3	1400
July	1st to 10th	Night	13% 5% 22% 32% 15% 13%	3145
	11th to 20th	Night	11% 3 13% 33% 36% 3	3428
	21st to end	Night	16% 4 23% 37% 16% 3	3857
August	1st to 10th	Night	15% 5% 14% 38% 26% 3	2784
	11th to 20th	Night	26% 6% 33% 22% 9% 3	2929
	21st to end	Night	34% 6% 17% 30% 13% 1	3427
September	1st to 10th	Night	24% 8% 21% 26% 16% 5%	2000
	11th to 20th	Night	37% 9% 15% 19% 17% 4	2000
	21st to end	Night	38% 9% 16% 23% 12% 1	2294
October	1st to 10th	Night	38% 5% 21% 31% 6%	3920
	11th to 20th	Night	51% 6% 16% 22% 5%	3897
	21st to end	Night	52% 6% 14% 19% 8% 1	4377
November	1st to 10th	Night	62% 9% 12% 12% 4	2724
	11th to 20th	Night	64% 9% 11% 9% 5% 2	2800
	21st to end	Night	60% 11% 12% 9% 8% 1	2800
December	1st to 10th	Night	61% 7% 10% 18% 5%	4560
	11th to 20th	Night	73% 8% 10% 6% 2	5176
	21st to end	Night	70% 9% 12% 8% 2	7302

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% 19% 20% 9% 4	200
	11th to 20th	Day	31% 15% 20% 18% 13% 4	200
	21st to end	Day	32% 15% 18% 17% 14% 3	325
February	1st to 10th	Day	32% 19% 18% 19% 11% 2	400
	11th to 20th	Day	35% 15% 23% 17% 8% 1	395
	21st to end	Day	34% 17% 24% 16% 8% 1	357
March	1st to 10th	Day	29% 17% 23% 23% 8% 1	500
	11th to 20th	Day	27% 14% 20% 20% 13% 4	500
	21st to end	Day	26% 19% 26% 19% 9% 2	550
April	1st to 10th	Day	30% 15% 21% 22% 8% 4	500
	11th to 20th	Day	23% 15% 27% 21% 9% 5%	492
	21st to end	Day	20% 15% 31% 20% 13% 2	500
May	1st to 10th	Day	15% 13% 25% 29% 15% 3	503
	11th to 20th	Day	22% 13% 21% 23% 16% 6%	528
	21st to end	Day	17% 10% 23% 28% 15% 6%	573
June	1st to 10th	Day	17% 11% 26% 23% 18% 5%	574
	11th to 20th	Day	15% 16% 30% 25% 12% 2	730
	21st to end	Day	9% 12% 25% 30% 19% 4	730
July	1st to 10th	Day	11% 7% 19% 36% 23% 5%	721
	11th to 20th	Day	6% 5% 21% 35% 25% 8%	595
	21st to end	Day	7% 4 14% 31% 35% 9%	575
August	1st to 10th	Day	8% 6% 16% 31% 29% 10%	527
	11th to 20th	Day	8% 3 16% 34% 28% 11%	517
	21st to end	Day	11% 9% 20% 30% 22% 7%	583
September	1st to 10th	Day	10% 9% 20% 35% 18% 8%	530
	11th to 20th	Day	14% 9% 18% 30% 23% 5%	530
	21st to end	Day	16% 11% 21% 29% 21% 2	530
October	1st to 10th	Day	20% 11% 23% 22% 19% 5%	530
	11th to 20th	Day	22% 16% 19% 23% 15% 5%	530
	21st to end	Day	29% 18% 21% 20% 10% 3	503
November	1st to 10th	Day	30% 23% 18% 15% 11% 2	220
	11th to 20th	Day	36% 18% 18% 12% 14% 3	200
	21st to end	Day	27% 17% 25% 16% 9% 7%	200
December	1st to 10th	Day	32% 15% 20% 18% 13% 4	200
	11th to 20th	Day	41% 15% 23% 16% 5% 2	200
	21st to end	Day	36% 20% 19% 16% 7% 1	220
January	1st to 10th	Night	40% 14% 16% 17% 11% 2	885
	11th to 20th	Night	31% 13% 14% 24% 18% 1	800
	21st to end	Night	30% 9% 18% 24% 17% 2	880
February	1st to 10th	Night	30% 14% 20% 22% 12% 2	800
	11th to 20th	Night	26% 15% 21% 25% 12% 1	784
	21st to end	Night	29% 15% 22% 22% 10% 2	660
March	1st to 10th	Night	24% 15% 21% 26% 14% 2	800
	11th to 20th	Night	27% 14% 20% 25% 13% 2	800
	21st to end	Night	29% 12% 25% 25% 9% 1	880
April	1st to 10th	Night	24% 13% 22% 26% 14% 1	800
	11th to 20th	Night	16% 13% 25% 30% 14% 2	744
	21st to end	Night	21% 13% 22% 30% 14% 1	600
May	1st to 10th	Night	13% 11% 23% 36% 17% 1	400
	11th to 20th	Night	16% 7% 24% 31% 20% 2	394
	21st to end	Night	11% 8% 24% 38% 18% 2	433
June	1st to 10th	Night	13% 7% 27% 34% 19%	400
	11th to 20th	Night	9% 9% 33% 32% 17%	400
	21st to end	Night	8% 5% 24% 40% 21% 2	400
July	1st to 10th	Night	6% 2 19% 46% 27% 1	390
	11th to 20th	Night	4 3 20% 44% 28% 2	388
	21st to end	Night	3 1 17% 44% 33% 1	438
August	1st to 10th	Night	6% 2 15% 34% 39% 4	503
	11th to 20th	Night	7% 2 10% 35% 42% 4	594
	21st to end	Night	9% 5% 18% 38% 28% 2	764
September	1st to 10th	Night	7% 3 19% 37% 29% 5%	800
	11th to 20th	Night	13% 4 17% 29% 33% 4	800
	21st to end	Night	15% 7% 19% 34% 23% 3	800
October	1st to 10th	Night	16% 7% 18% 29% 27% 3	800
	11th to 20th	Night	23% 11% 16% 28% 20% 3	800
	21st to end	Night	26% 9% 23% 25% 16% 1	880
November	1st to 10th	Night	32% 11% 20% 27% 10%	788
	11th to 20th	Night	32% 13% 22% 20% 12% 1	800
	21st to end	Night	28% 13% 21% 23% 13% 3	800
December	1st to 10th	Night	30% 10% 16% 27% 16% 2	800
	11th to 20th	Night	35% 13% 20% 22% 9% 1	800
	21st to end	Night	38% 17% 18% 19% 8% 1	957

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