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 2024-11-06.

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	15% 10% 4 9% 5% 5% 5% 4 5% 6% 32%	105753
	Night	17% 10% 3 8% 5% 5% 4 3 5% 6% 33%	101902
Transparency	Day	49% 9% 14% 17% 9% 2	105510
	Night	49% 9% 15% 18% 7% 1	101791
Seeing	Day	20% 13% 22% 25% 16% 5%	15184
	Night	22% 10% 20% 28% 18% 2	23529

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	28% 15% 6% 12% 6% 5% 6% 4 4 4 9%	2252
	11th to 20th	Day	26% 15% 5% 10% 5% 4 4 3 4 5% 19%	2516
	21st to end	Day	31% 15% 5% 12% 7% 6% 5% 3 4 4 8%	2958
February	1st to 10th	Day	24% 15% 4 11% 7% 6% 5% 4 5% 6% 12%	1710
	11th to 20th	Day	26% 15% 5% 10% 7% 4 4 4 5% 5% 16%	1665
	21st to end	Day	23% 15% 5% 10% 7% 6% 5% 3 5% 5% 18%	1460
March	1st to 10th	Day	15% 12% 5% 14% 8% 8% 7% 5% 5% 5% 16%	3279
	11th to 20th	Day	18% 11% 4 8% 5% 5% 4 3 3 5% 34%	3490
	21st to end	Day	19% 16% 5% 12% 7% 6% 6% 4 5% 5% 15%	3910
April	1st to 10th	Day	22% 14% 4 10% 7% 5% 5% 4 7% 5% 16%	2280
	11th to 20th	Day	20% 12% 3 10% 7% 6% 5% 4 5% 6% 23%	2293
	21st to end	Day	16% 11% 5% 11% 6% 6% 6% 4 6% 7% 22%	2470
May	1st to 10th	Day	16% 8% 3 8% 4 5% 4 3 5% 6% 37%	4419
	11th to 20th	Day	10% 7% 3 6% 5% 6% 6% 5% 8% 9% 36%	4377
	21st to end	Day	19% 11% 4 10% 6% 7% 6% 5% 6% 6% 20%	4813
June	1st to 10th	Day	17% 11% 4 8% 6% 5% 5% 5% 6% 8% 25%	2702
	11th to 20th	Day	14% 12% 4 10% 6% 7% 7% 5% 7% 7% 23%	2850
	21st to end	Day	9% 8% 3 8% 5% 5% 5% 4 6% 7% 40%	2850
July	1st to 10th	Day	5% 5% 2 5% 3 2 3 2 4 5% 65%	4740
	11th to 20th	Day	5% 4 2 4 3 2 3 2 4 5% 66%	4530
	21st to end	Day	7% 5% 2 6% 4 4 3 3 4 6% 56%	4790
August	1st to 10th	Day	4 4 2 5% 3 4 4 3 5% 7% 60%	4226
	11th to 20th	Day	6% 5% 3 8% 5% 5% 6% 5% 8% 10% 39%	4026
	21st to end	Day	13% 7% 3 7% 4 4 4 3 5% 6% 44%	4349
September	1st to 10th	Day	6% 7% 3 7% 5% 5% 3 4 5% 7% 48%	2280
	11th to 20th	Day	10% 9% 3 9% 6% 6% 5% 4 6% 7% 37%	2279
	21st to end	Day	15% 8% 3 8% 5% 5% 5% 4 5% 6% 36%	1999
October	1st to 10th	Day	12% 7% 3 8% 5% 5% 5% 5% 7% 9% 34%	3180
	11th to 20th	Day	19% 10% 4 10% 5% 5% 5% 4 6% 8% 24%	3161
	21st to end	Day	20% 9% 4 11% 6% 6% 6% 5% 6% 7% 21%	3323
November	1st to 10th	Day	25% 14% 5% 11% 6% 6% 5% 4 4 6% 15%	1557
	11th to 20th	Day	28% 13% 4 9% 6% 4 5% 4 5% 6% 16%	1520
	21st to end	Day	25% 14% 4 10% 6% 6% 5% 3 5% 5% 18%	1520
December	1st to 10th	Day	23% 13% 5% 11% 6% 4 4 4 5% 5% 21%	1520
	11th to 20th	Day	27% 14% 5% 12% 9% 6% 5% 4 4 4 9%	1925
	21st to end	Day	23% 15% 5% 14% 7% 6% 6% 3 4 5% 11%	2534
January	1st to 10th	Night	28% 17% 5% 12% 7% 5% 4 3 4 4 10%	4687
	11th to 20th	Night	25% 14% 4 9% 4 4 4 3 5% 8% 21%	4382
	21st to end	Night	30% 15% 4 10% 6% 6% 4 4 5% 5% 11%	4699
February	1st to 10th	Night	25% 14% 4 9% 5% 5% 5% 4 5% 6% 18%	2470
	11th to 20th	Night	20% 14% 3 10% 6% 5% 4 3 5% 5% 23%	2402
	21st to end	Night	21% 13% 4 10% 7% 6% 5% 4 6% 5% 20%	1994
March	1st to 10th	Night	15% 13% 6% 12% 7% 6% 7% 4 6% 6% 19%	3593
	11th to 20th	Night	14% 9% 3 8% 4 5% 3 3 4 6% 40%	3489
	21st to end	Night	23% 16% 5% 11% 5% 6% 5% 3 5% 5% 16%	3742
April	1st to 10th	Night	19% 11% 3 10% 7% 5% 5% 4 5% 6% 25%	1900
	11th to 20th	Night	16% 8% 4 8% 7% 4 5% 5% 6% 8% 30%	1835
	21st to end	Night	16% 11% 3 7% 5% 5% 5% 4 6% 7% 31%	1710
May	1st to 10th	Night	14% 8% 2 7% 5% 3 4 2 4 5% 44%	2540
	11th to 20th	Night	8% 5% 2 5% 3 4 5% 5% 7% 8% 49%	2516
	21st to end	Night	8% 10% 3 9% 7% 6% 5% 5% 6% 8% 33%	2762
June	1st to 10th	Night	13% 9% 2 7% 4 5% 5% 4 7% 8% 39%	1474
	11th to 20th	Night	10% 7% 3 7% 5% 5% 4 5% 6% 10% 39%	1330
	21st to end	Night	6% 5% 2 5% 4 4 5% 3 7% 6% 53%	1330
July	1st to 10th	Night	3 2 1 3 2 2 2 2 4 4 75%	2220
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 5% 75%	2428
	21st to end	Night	3 3 1 4 2 2 3 3 5% 6% 68%	2762
August	1st to 10th	Night	2 1 1 3 2 3 2 2 4 7% 74%	2706
	11th to 20th	Night	6% 4 1 4 3 3 4 4 6% 8% 58%	2812
	21st to end	Night	12% 5% 2 5% 3 3 4 3 5% 6% 52%	3323
September	1st to 10th	Night	5% 4 2 4 3 4 3 2 4 6% 62%	1900
	11th to 20th	Night	11% 5% 2 7% 4 5% 4 4 5% 7% 45%	1900
	21st to end	Night	11% 8% 3 7% 4 4 5% 3 4 7% 45%	2179
October	1st to 10th	Night	10% 5% 2 7% 4 5% 4 4 7% 8% 44%	3800
	11th to 20th	Night	15% 9% 4 9% 6% 5% 4 4 5% 7% 32%	3777
	21st to end	Night	17% 9% 4 10% 7% 5% 4 4 6% 7% 29%	4241
November	1st to 10th	Night	27% 10% 3 8% 5% 6% 5% 4 5% 6% 21%	2639
	11th to 20th	Night	24% 13% 4 10% 7% 5% 4 4 4 5% 21%	2660
	21st to end	Night	22% 11% 3 9% 7% 5% 5% 4 5% 6% 23%	2660
December	1st to 10th	Night	25% 13% 4 9% 4 5% 4 4 4 6% 23%	2660
	11th to 20th	Night	24% 14% 4 10% 8% 6% 5% 4 5% 5% 15%	3280
	21st to end	Night	24% 14% 6% 12% 7% 6% 4 3 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	74% 12% 7% 5% 2 1	2252
	11th to 20th	Day	65% 9% 10% 13% 3 1	2540
	21st to end	Day	76% 10% 6% 6% 2	2958
February	1st to 10th	Day	69% 11% 10% 7% 4	1710
	11th to 20th	Day	68% 10% 9% 8% 5%	1674
	21st to end	Day	65% 10% 10% 10% 4	1460
March	1st to 10th	Day	60% 13% 16% 9% 2	3279
	11th to 20th	Day	50% 9% 13% 26% 2 1	3490
	21st to end	Day	65% 11% 13% 10% 1	3910
April	1st to 10th	Day	64% 11% 15% 9% 1	2280
	11th to 20th	Day	59% 10% 12% 12% 7% 1	2293
	21st to end	Day	58% 11% 14% 12% 5%	2470
May	1st to 10th	Day	43% 11% 16% 20% 10%	4419
	11th to 20th	Day	37% 15% 27% 18% 4	4380
	21st to end	Day	56% 17% 16% 9% 2	4814
June	1st to 10th	Day	54% 10% 19% 11% 6%	2702
	11th to 20th	Day	56% 11% 17% 12% 4	2850
	21st to end	Day	40% 11% 19% 21% 8% 2	2850
July	1st to 10th	Day	23% 6% 9% 21% 24% 17%	4666
	11th to 20th	Day	20% 5% 9% 27% 35% 5%	4451
	21st to end	Day	28% 6% 13% 34% 17% 2	4639
August	1st to 10th	Day	23% 6% 11% 35% 22% 3	4209
	11th to 20th	Day	37% 7% 22% 24% 8% 2	4065
	21st to end	Day	40% 6% 13% 28% 12% 1	4348
September	1st to 10th	Day	34% 8% 16% 23% 14% 5%	2280
	11th to 20th	Day	44% 10% 14% 16% 14% 2	2279
	21st to end	Day	46% 10% 16% 17% 10% 1	1999
October	1st to 10th	Day	44% 6% 16% 28% 6%	3180
	11th to 20th	Day	56% 7% 13% 16% 8%	3161
	21st to end	Day	58% 6% 13% 13% 8% 1	3323
November	1st to 10th	Day	68% 9% 11% 8% 4	1557
	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	75% 11% 7% 5% 2 1	4687
	11th to 20th	Night	58% 10% 13% 16% 2 1	4420
	21st to end	Night	71% 11% 8% 7% 2	4699
February	1st to 10th	Night	64% 10% 12% 9% 5%	2470
	11th to 20th	Night	60% 10% 13% 10% 6%	2404
	21st to end	Night	61% 11% 12% 11% 6%	1994
March	1st to 10th	Night	57% 15% 14% 12% 3	3593
	11th to 20th	Night	41% 11% 19% 26% 2 1	3490
	21st to end	Night	67% 12% 11% 9% 1	3742
April	1st to 10th	Night	58% 12% 16% 12% 2	1900
	11th to 20th	Night	49% 11% 16% 15% 6% 1	1835
	21st to end	Night	51% 10% 17% 15% 6% 1	1710
May	1st to 10th	Night	40% 10% 15% 26% 9%	2540
	11th to 20th	Night	30% 12% 27% 23% 7% 1	2516
	21st to end	Night	46% 16% 23% 13% 2	2762
June	1st to 10th	Night	42% 9% 20% 23% 6%	1474
	11th to 20th	Night	40% 13% 25% 19% 3	1330
	21st to end	Night	30% 10% 27% 23% 8% 3	1330
July	1st to 10th	Night	14% 6% 13% 30% 18% 18%	2185
	11th to 20th	Night	13% 4 9% 32% 39% 2	2384
	21st to end	Night	17% 5% 16% 36% 22% 4	2674
August	1st to 10th	Night	14% 5% 13% 39% 27% 3	2698
	11th to 20th	Night	26% 6% 34% 22% 9% 3	2839
	21st to end	Night	34% 6% 17% 30% 12% 1	3323
September	1st to 10th	Night	23% 8% 20% 27% 17% 5%	1900
	11th to 20th	Night	38% 9% 15% 18% 16% 4	1900
	21st to end	Night	38% 9% 16% 23% 13% 2	2179
October	1st to 10th	Night	38% 5% 20% 31% 6%	3800
	11th to 20th	Night	52% 6% 16% 22% 4	3777
	21st to end	Night	52% 6% 14% 19% 8% 1	4241
November	1st to 10th	Night	61% 9% 13% 12% 4	2639
	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% 17% 17% 12% 3	180
	21st to end	Day	35% 16% 19% 16% 12% 2	297
February	1st to 10th	Day	34% 17% 17% 19% 11% 2	370
	11th to 20th	Day	35% 16% 22% 18% 8% 1	365
	21st to end	Day	33% 19% 24% 15% 8% 2	330
March	1st to 10th	Day	29% 17% 23% 22% 8% 1	460
	11th to 20th	Day	26% 14% 20% 20% 15% 5%	460
	21st to end	Day	25% 19% 25% 19% 9% 2	506
April	1st to 10th	Day	29% 16% 22% 21% 8% 4	460
	11th to 20th	Day	25% 16% 27% 19% 8% 4	452
	21st to end	Day	21% 16% 31% 20% 12% 1	460
May	1st to 10th	Day	16% 13% 25% 29% 15% 3	463
	11th to 20th	Day	22% 13% 19% 24% 16% 6%	488
	21st to end	Day	18% 10% 23% 28% 15% 5%	529
June	1st to 10th	Day	18% 11% 27% 23% 16% 5%	532
	11th to 20th	Day	16% 16% 29% 25% 12% 2	680
	21st to end	Day	9% 12% 25% 30% 19% 4	680
July	1st to 10th	Day	11% 7% 19% 36% 23% 5%	671
	11th to 20th	Day	6% 6% 20% 35% 25% 8%	551
	21st to end	Day	7% 4 15% 31% 35% 8%	531
August	1st to 10th	Day	7% 6% 16% 30% 29% 11%	487
	11th to 20th	Day	7% 3 14% 35% 30% 11%	477
	21st to end	Day	11% 10% 21% 29% 21% 7%	539
September	1st to 10th	Day	10% 9% 21% 36% 17% 7%	490
	11th to 20th	Day	14% 10% 18% 30% 23% 5%	490
	21st to end	Day	16% 12% 21% 29% 20% 2	490
October	1st to 10th	Day	21% 11% 22% 22% 18% 6%	490
	11th to 20th	Day	22% 16% 18% 23% 16% 5%	490
	21st to end	Day	28% 18% 20% 21% 11% 3	463
November	1st to 10th	Day	30% 24% 19% 16% 10% 1	205
	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	760
	21st to end	Night	31% 9% 19% 23% 17% 2	836
February	1st to 10th	Night	31% 13% 20% 22% 12% 2	760
	11th to 20th	Night	25% 15% 22% 25% 13% 1	744
	21st to end	Night	29% 15% 23% 22% 10% 2	628
March	1st to 10th	Night	24% 15% 21% 24% 14% 2	760
	11th to 20th	Night	26% 14% 19% 25% 13% 2	760
	21st to end	Night	28% 12% 25% 25% 9% 1	836
April	1st to 10th	Night	25% 13% 22% 26% 14% 1	760
	11th to 20th	Night	17% 13% 25% 28% 14% 2	707
	21st to end	Night	21% 13% 23% 30% 13% 1	570
May	1st to 10th	Night	13% 11% 23% 34% 18% 1	380
	11th to 20th	Night	16% 7% 23% 32% 21% 2	374
	21st to end	Night	11% 8% 25% 37% 17% 2	411
June	1st to 10th	Night	14% 7% 28% 34% 18%	380
	11th to 20th	Night	10% 10% 34% 30% 16%	380
	21st to end	Night	9% 5% 24% 39% 21% 2	380
July	1st to 10th	Night	6% 2 19% 46% 26% 1	370
	11th to 20th	Night	4 3 21% 43% 28% 1	368
	21st to end	Night	3 1 18% 45% 32% 1	416
August	1st to 10th	Night	5% 2 16% 34% 39% 4	477
	11th to 20th	Night	6% 2 10% 35% 42% 5%	564
	21st to end	Night	9% 5% 19% 38% 26% 2	726
September	1st to 10th	Night	7% 3 20% 37% 29% 4	760
	11th to 20th	Night	14% 4 17% 28% 33% 4	760
	21st to end	Night	15% 7% 19% 34% 22% 3	760
October	1st to 10th	Night	17% 7% 17% 28% 28% 3	760
	11th to 20th	Night	22% 11% 15% 28% 21% 4	760
	21st to end	Night	27% 9% 23% 24% 16% 1	836
November	1st to 10th	Night	32% 11% 20% 27% 10%	764
	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

Page generated at 2024-11-03 10:19:03. Version ac07

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