Maple Valley Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maple Valley Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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	23% 10% 3 7% 4 4 4 3 5% 6% 32%	106093
	Night	26% 11% 3 7% 4 4 4 3 4 5% 29%	101517
Transparency	Day	56% 5% 12% 17% 9% 2	105853
	Night	59% 4 11% 16% 8% 2	101403
Seeing	Day	26% 11% 20% 25% 15% 3	14998
	Night	30% 9% 15% 26% 18% 2	23322

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	40% 16% 4 9% 4 4 3 2 3 4 11%	2539
	11th to 20th	Day	27% 12% 3 9% 5% 4 4 3 4 5% 23%	2516
	21st to end	Day	40% 14% 4 9% 4 4 2 2 3 4 12%	2862
February	1st to 10th	Day	34% 13% 4 7% 5% 4 4 3 3 6% 17%	1710
	11th to 20th	Day	34% 13% 3 9% 4 3 4 3 4 5% 19%	1665
	21st to end	Day	32% 16% 4 8% 5% 4 3 3 4 4 19%	1490
March	1st to 10th	Day	20% 15% 4 11% 6% 6% 6% 3 4 5% 19%	3195
	11th to 20th	Day	22% 12% 3 6% 4 3 4 2 3 4 35%	3490
	21st to end	Day	28% 15% 4 9% 6% 5% 5% 5% 4 5% 14%	3969
April	1st to 10th	Day	28% 15% 3 8% 5% 5% 5% 4 5% 5% 15%	2280
	11th to 20th	Day	24% 13% 4 7% 5% 5% 5% 4 5% 5% 23%	2257
	21st to end	Day	25% 12% 4 9% 7% 5% 4 4 4 5% 19%	2518
May	1st to 10th	Day	21% 10% 3 6% 5% 5% 5% 4 4 5% 33%	4420
	11th to 20th	Day	20% 8% 2 6% 4 4 4 3 6% 8% 35%	4377
	21st to end	Day	31% 12% 4 9% 5% 4 4 3 4 5% 18%	4813
June	1st to 10th	Day	25% 9% 3 7% 5% 5% 4 4 5% 7% 24%	2660
	11th to 20th	Day	27% 12% 4 8% 5% 5% 4 4 5% 6% 20%	2836
	21st to end	Day	16% 8% 2 7% 5% 5% 4 4 5% 7% 37%	2850
July	1st to 10th	Day	9% 4 1 4 3 3 3 2 3 5% 61%	4740
	11th to 20th	Day	8% 4 2 3 2 2 3 2 4 5% 65%	4434
	21st to end	Day	13% 5% 2 5% 3 3 3 2 5% 5% 54%	4790
August	1st to 10th	Day	7% 4 1 4 3 3 3 3 5% 6% 60%	4341
	11th to 20th	Day	11% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	4026
	21st to end	Day	19% 9% 3 6% 3 3 3 2 4 5% 42%	4309
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 4 6% 50%	2280
	11th to 20th	Day	17% 8% 3 7% 5% 3 4 4 5% 7% 37%	2280
	21st to end	Day	20% 8% 3 6% 4 4 4 3 5% 6% 37%	2015
October	1st to 10th	Day	20% 7% 2 5% 3 4 4 3 6% 10% 37%	3180
	11th to 20th	Day	32% 9% 2 7% 4 3 4 3 4 6% 26%	3161
	21st to end	Day	32% 11% 3 8% 4 4 4 3 5% 5% 19%	3237
November	1st to 10th	Day	38% 10% 3 7% 4 4 4 4 4 5% 17%	1574
	11th to 20th	Day	36% 11% 3 7% 4 4 3 3 4 5% 21%	1520
	21st to end	Day	36% 11% 3 6% 4 4 4 3 4 4 21%	1520
December	1st to 10th	Day	32% 11% 3 6% 4 4 4 4 5% 5% 23%	1520
	11th to 20th	Day	35% 12% 3 8% 5% 4 3 3 6% 4 18%	1925
	21st to end	Day	27% 15% 4 9% 6% 6% 5% 3 4 4 17%	2794
January	1st to 10th	Night	40% 18% 3 7% 4 4 3 3 3 3 11%	4406
	11th to 20th	Night	29% 14% 4 7% 4 4 4 3 5% 6% 22%	4382
	21st to end	Night	39% 12% 4 7% 4 5% 4 3 4 4 13%	4792
February	1st to 10th	Night	34% 14% 4 7% 5% 4 4 3 3 5% 17%	2470
	11th to 20th	Night	31% 13% 3 7% 5% 4 4 3 4 4 23%	2402
	21st to end	Night	32% 16% 3 7% 4 3 4 4 5% 5% 19%	1963
March	1st to 10th	Night	20% 15% 4 9% 6% 6% 5% 5% 6% 6% 18%	3678
	11th to 20th	Night	23% 11% 2 5% 3 3 3 2 4 5% 38%	3489
	21st to end	Night	32% 16% 3 8% 4 4 5% 3 5% 5% 14%	3684
April	1st to 10th	Night	27% 13% 3 8% 5% 5% 5% 4 6% 6% 18%	1900
	11th to 20th	Night	22% 12% 3 7% 5% 4 5% 4 5% 6% 27%	1873
	21st to end	Night	27% 12% 3 9% 6% 4 5% 3 5% 5% 20%	1661
May	1st to 10th	Night	23% 11% 3 5% 4 3 3 2 4 4 37%	2540
	11th to 20th	Night	18% 10% 3 7% 4 4 5% 4 5% 7% 33%	2516
	21st to end	Night	28% 14% 5% 9% 5% 5% 4 3 3 4 19%	2762
June	1st to 10th	Night	21% 11% 4 8% 5% 5% 5% 3 4 6% 29%	1520
	11th to 20th	Night	22% 11% 3 8% 8% 5% 4 4 5% 5% 25%	1340
	21st to end	Night	16% 7% 2 6% 4 4 4 4 5% 7% 42%	1330
July	1st to 10th	Night	8% 7% 3 4 3 3 2 2 3 5% 60%	2220
	11th to 20th	Night	6% 3 1 3 2 2 3 2 3 3 71%	2508
	21st to end	Night	13% 5% 2 4 3 3 3 3 3 5% 57%	2762
August	1st to 10th	Night	5% 3 2 4 2 2 2 2 4 7% 67%	2587
	11th to 20th	Night	14% 6% 3 6% 3 3 4 3 4 7% 46%	2812
	21st to end	Night	22% 7% 2 4 3 2 2 2 3 5% 47%	3363
September	1st to 10th	Night	12% 4 2 6% 3 3 4 3 4 6% 53%	1900
	11th to 20th	Night	19% 8% 4 6% 3 3 4 3 4 5% 41%	1900
	21st to end	Night	19% 8% 3 7% 3 4 4 2 4 6% 40%	2161
October	1st to 10th	Night	19% 6% 3 6% 4 3 5% 4 6% 8% 37%	3800
	11th to 20th	Night	30% 9% 2 6% 3 4 4 3 4 5% 30%	3777
	21st to end	Night	34% 13% 3 6% 4 3 4 3 4 5% 22%	4324
November	1st to 10th	Night	36% 10% 3 7% 4 4 4 3 5% 5% 19%	2573
	11th to 20th	Night	35% 10% 3 7% 4 4 4 3 4 3 23%	2660
	21st to end	Night	33% 12% 3 7% 5% 3 4 3 4 5% 22%	2660
December	1st to 10th	Night	32% 11% 3 7% 5% 3 4 3 4 5% 24%	2660
	11th to 20th	Night	33% 13% 4 8% 5% 4 4 3 4 4 18%	3280
	21st to end	Night	30% 14% 4 9% 6% 6% 5% 3 4 4 15%	4862

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	79% 3 6% 8% 3 1	2539
	11th to 20th	Day	64% 4 10% 13% 7% 2	2540
	21st to end	Day	79% 2 6% 8% 4 1	2862
February	1st to 10th	Day	72% 3 11% 8% 4 1	1710
	11th to 20th	Day	72% 3 8% 11% 6% 1	1674
	21st to end	Day	72% 4 8% 11% 4	1490
March	1st to 10th	Day	66% 6% 10% 13% 5%	3195
	11th to 20th	Day	55% 4 10% 27% 3 1	3490
	21st to end	Day	73% 4 12% 9% 1	3969
April	1st to 10th	Day	72% 6% 13% 8% 1	2280
	11th to 20th	Day	64% 6% 10% 12% 6% 1	2257
	21st to end	Day	71% 5% 9% 10% 4 1	2518
May	1st to 10th	Day	53% 7% 11% 19% 9%	4420
	11th to 20th	Day	50% 8% 21% 17% 3	4380
	21st to end	Day	72% 5% 15% 8% 1	4814
June	1st to 10th	Day	62% 6% 15% 12% 5%	2660
	11th to 20th	Day	68% 6% 12% 11% 3	2836
	21st to end	Day	49% 5% 18% 19% 8% 2	2850
July	1st to 10th	Day	28% 5% 8% 20% 21% 16%	4666
	11th to 20th	Day	25% 4 6% 27% 35% 4	4358
	21st to end	Day	32% 4 13% 34% 15% 2	4639
August	1st to 10th	Day	25% 5% 10% 38% 20% 2	4324
	11th to 20th	Day	46% 6% 18% 20% 8% 1	4065
	21st to end	Day	47% 4 11% 26% 12% 1	4308
September	1st to 10th	Day	37% 4 15% 25% 14% 5%	2280
	11th to 20th	Day	49% 6% 11% 17% 14% 3	2280
	21st to end	Day	50% 4 16% 17% 10% 3	2015
October	1st to 10th	Day	44% 5% 12% 30% 8% 1	3180
	11th to 20th	Day	60% 4 12% 15% 9% 1	3161
	21st to end	Day	67% 4 10% 9% 8% 2	3237
November	1st to 10th	Day	72% 3 10% 11% 4	1574
	11th to 20th	Day	70% 4 7% 9% 6% 3	1520
	21st to end	Day	69% 4 5% 10% 9% 3	1520
December	1st to 10th	Day	65% 5% 9% 12% 8% 1	1520
	11th to 20th	Day	70% 3 10% 11% 5% 1	1928
	21st to end	Day	72% 3 11% 10% 3	2794
January	1st to 10th	Night	81% 3 6% 7% 2 1	4406
	11th to 20th	Night	64% 4 7% 16% 8% 1	4420
	21st to end	Night	76% 2 8% 7% 5% 1	4792
February	1st to 10th	Night	72% 4 10% 8% 5% 1	2470
	11th to 20th	Night	66% 5% 10% 10% 6% 3	2404
	21st to end	Night	70% 5% 8% 11% 5%	1963
March	1st to 10th	Night	64% 6% 12% 13% 5%	3678
	11th to 20th	Night	52% 4 14% 25% 4 1	3490
	21st to end	Night	74% 4 12% 9% 1	3684
April	1st to 10th	Night	68% 8% 12% 10% 1	1900
	11th to 20th	Night	62% 5% 11% 13% 8% 2	1873
	21st to end	Night	68% 6% 9% 11% 5% 1	1661
May	1st to 10th	Night	53% 5% 11% 20% 10%	2540
	11th to 20th	Night	52% 6% 20% 18% 3 1	2516
	21st to end	Night	73% 5% 14% 8% 1	2762
June	1st to 10th	Night	62% 4 15% 15% 5%	1520
	11th to 20th	Night	63% 6% 16% 12% 2	1340
	21st to end	Night	45% 5% 21% 20% 7% 2	1330
July	1st to 10th	Night	31% 5% 9% 19% 15% 21%	2185
	11th to 20th	Night	21% 3 8% 27% 40% 2	2461
	21st to end	Night	32% 5% 12% 30% 18% 3	2674
August	1st to 10th	Night	20% 4 11% 40% 23% 2	2579
	11th to 20th	Night	40% 6% 21% 22% 8% 2	2839
	21st to end	Night	44% 3 12% 28% 12% 1	3363
September	1st to 10th	Night	35% 4 15% 26% 15% 5%	1900
	11th to 20th	Night	50% 5% 9% 19% 13% 5%	1900
	21st to end	Night	49% 5% 12% 19% 12% 3	2161
October	1st to 10th	Night	46% 4 13% 29% 8% 1	3800
	11th to 20th	Night	59% 3 10% 16% 11%	3777
	21st to end	Night	66% 4 10% 11% 8%	4324
November	1st to 10th	Night	69% 5% 10% 11% 4 1	2573
	11th to 20th	Night	69% 3 9% 10% 6% 3	2660
	21st to end	Night	68% 4 7% 9% 11% 1	2660
December	1st to 10th	Night	66% 4 9% 11% 8% 2	2660
	11th to 20th	Night	71% 3 10% 10% 5% 1	3276
	21st to end	Night	74% 4 10% 9% 3	4862

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	41% 13% 16% 15% 11% 5%	180
	11th to 20th	Day	35% 15% 16% 21% 9% 4	180
	21st to end	Day	42% 14% 15% 16% 10% 3	240
February	1st to 10th	Day	41% 11% 20% 17% 9% 2	370
	11th to 20th	Day	45% 12% 21% 14% 8% 1	365
	21st to end	Day	36% 15% 20% 23% 5% 1	344
March	1st to 10th	Day	32% 17% 23% 19% 9%	460
	11th to 20th	Day	32% 16% 18% 21% 10% 3	460
	21st to end	Day	34% 15% 25% 16% 8% 1	506
April	1st to 10th	Day	34% 17% 21% 16% 12% 1	460
	11th to 20th	Day	26% 19% 28% 16% 7% 4	452
	21st to end	Day	27% 16% 26% 22% 8% 1	460
May	1st to 10th	Day	20% 13% 25% 30% 11% 2	463
	11th to 20th	Day	27% 10% 19% 25% 15% 4	488
	21st to end	Day	28% 10% 23% 25% 12% 3	529
June	1st to 10th	Day	26% 9% 21% 26% 15% 3	490
	11th to 20th	Day	25% 14% 21% 26% 11% 2	666
	21st to end	Day	16% 8% 20% 33% 20% 3	680
July	1st to 10th	Day	13% 6% 17% 36% 26% 2	671
	11th to 20th	Day	10% 6% 17% 39% 23% 6%	497
	21st to end	Day	9% 3 15% 39% 28% 5%	531
August	1st to 10th	Day	13% 5% 13% 34% 27% 9%	487
	11th to 20th	Day	8% 3 14% 35% 31% 7%	477
	21st to end	Day	17% 7% 22% 30% 18% 6%	539
September	1st to 10th	Day	16% 5% 23% 36% 16% 4	490
	11th to 20th	Day	19% 8% 20% 30% 20% 4	490
	21st to end	Day	20% 11% 19% 28% 19% 3	490
October	1st to 10th	Day	24% 10% 21% 21% 19% 5%	490
	11th to 20th	Day	32% 9% 15% 24% 14% 4	490
	21st to end	Day	36% 14% 17% 20% 10% 4	416
November	1st to 10th	Day	41% 12% 17% 19% 11%	219
	11th to 20th	Day	38% 16% 15% 11% 15% 6%	180
	21st to end	Day	37% 12% 15% 18% 9% 9%	180
December	1st to 10th	Day	45% 10% 13% 14% 12% 6%	180
	11th to 20th	Day	43% 9% 23% 17% 7% 2	180
	21st to end	Day	38% 13% 16% 21% 11% 2	198
January	1st to 10th	Night	44% 14% 16% 16% 9% 1	760
	11th to 20th	Night	38% 12% 14% 24% 11% 1	760
	21st to end	Night	38% 9% 15% 27% 10%	836
February	1st to 10th	Night	39% 9% 20% 23% 8% 1	760
	11th to 20th	Night	36% 11% 15% 27% 10% 1	744
	21st to end	Night	37% 13% 17% 23% 9% 1	628
March	1st to 10th	Night	32% 13% 19% 22% 14% 1	760
	11th to 20th	Night	32% 11% 18% 26% 12% 1	760
	21st to end	Night	35% 13% 17% 25% 8% 1	836
April	1st to 10th	Night	33% 11% 17% 25% 11% 2	760
	11th to 20th	Night	27% 12% 20% 27% 13% 2	745
	21st to end	Night	31% 12% 16% 27% 12% 2	521
May	1st to 10th	Night	23% 12% 14% 30% 21% 1	380
	11th to 20th	Night	22% 7% 13% 30% 26% 2	374
	21st to end	Night	25% 8% 15% 24% 25% 3	411
June	1st to 10th	Night	23% 8% 15% 28% 23% 2	380
	11th to 20th	Night	24% 13% 14% 28% 19% 1	380
	21st to end	Night	17% 3 12% 34% 30% 3	380
July	1st to 10th	Night	13% 5% 11% 30% 37% 4	370
	11th to 20th	Night	12% 3 8% 37% 37% 3	368
	21st to end	Night	9% 3 8% 30% 44% 5%	416
August	1st to 10th	Night	10% 3 9% 27% 44% 8%	410
	11th to 20th	Night	11% 1 7% 25% 45% 11%	564
	21st to end	Night	16% 5% 9% 27% 38% 5%	753
September	1st to 10th	Night	13% 3 10% 38% 32% 4	760
	11th to 20th	Night	20% 5% 10% 25% 34% 6%	760
	21st to end	Night	22% 6% 12% 31% 26% 3	760
October	1st to 10th	Night	23% 6% 13% 28% 26% 3	760
	11th to 20th	Night	30% 9% 13% 27% 18% 4	760
	21st to end	Night	34% 9% 16% 27% 12% 2	836
November	1st to 10th	Night	39% 10% 20% 24% 7%	754
	11th to 20th	Night	38% 11% 17% 24% 9% 1	760
	21st to end	Night	36% 11% 19% 23% 11%	760
December	1st to 10th	Night	33% 9% 21% 26% 10% 1	760
	11th to 20th	Night	42% 10% 19% 21% 7% 1	760
	21st to end	Night	47% 11% 18% 18% 5%	836

Page generated at 2024-11-01 19:47:38. Version ac07

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