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 2024-04-10.

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	25% 10% 3 6% 5% 3 4 4 4 6% 29%	83572
	Night	27% 11% 3 6% 5% 3 4 4 4 5% 27%	85222
Transparency	Day	60% 5% 11% 16% 7% 1	83293
	Night	61% 4 11% 15% 7% 1	85081
Seeing	Day	26% 11% 20% 25% 15% 3	14147
	Night	30% 9% 15% 26% 18% 2	22722

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 7% 5% 3 4 3 3 4 11%	2539
	11th to 20th	Day	27% 12% 4 8% 6% 3 4 4 4 6% 23%	2516
	21st to end	Day	40% 14% 4 8% 5% 3 3 3 3 5% 12%	2862
February	1st to 10th	Day	35% 12% 4 7% 5% 3 5% 4 3 6% 17%	1710
	11th to 20th	Day	34% 13% 3 7% 5% 2 4 4 3 5% 19%	1665
	21st to end	Day	32% 14% 5% 7% 5% 3 3 4 4 4 19%	1490
March	1st to 10th	Day	21% 14% 4 9% 8% 4 6% 4 4 5% 20%	3195
	11th to 20th	Day	23% 11% 4 5% 5% 2 4 3 3 4 35%	3490
	21st to end	Day	28% 15% 4 8% 7% 4 5% 6% 4 5% 14%	3969
April	1st to 10th	Day	29% 14% 4 7% 6% 4 6% 5% 5% 6% 15%	2270
	11th to 20th	Day	26% 12% 4 7% 6% 3 5% 5% 5% 5% 21%	2136
	21st to end	Day	25% 11% 5% 8% 7% 4 5% 5% 4 6% 20%	2385
May	1st to 10th	Day	21% 11% 3 6% 6% 3 5% 5% 5% 6% 29%	2520
	11th to 20th	Day	22% 9% 4 6% 6% 3 5% 5% 6% 7% 27%	2477
	21st to end	Day	29% 10% 4 7% 6% 3 5% 4 4 6% 23%	2723
June	1st to 10th	Day	26% 8% 4 7% 6% 4 4 5% 6% 7% 24%	2520
	11th to 20th	Day	27% 11% 5% 8% 5% 4 5% 4 5% 6% 20%	2686
	21st to end	Day	16% 6% 3 7% 6% 3 5% 5% 5% 7% 38%	2700
July	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 5% 6% 47%	2700
	11th to 20th	Day	13% 5% 3 4 3 2 4 4 5% 7% 48%	2534
	21st to end	Day	10% 4 2 3 3 2 4 3 5% 5% 60%	2741
August	1st to 10th	Day	13% 5% 2 4 4 2 3 3 5% 5% 52%	2473
	11th to 20th	Day	10% 5% 2 3 4 3 3 4 5% 8% 55%	2298
	21st to end	Day	15% 7% 3 5% 4 3 4 4 4 7% 44%	2453
September	1st to 10th	Day	14% 5% 2 4 4 3 3 4 4 7% 50%	2160
	11th to 20th	Day	17% 8% 3 6% 6% 2 4 4 5% 6% 39%	2160
	21st to end	Day	21% 8% 4 5% 5% 3 4 3 4 6% 38%	1910
October	1st to 10th	Day	23% 8% 3 6% 5% 3 5% 5% 6% 7% 30%	1800
	11th to 20th	Day	28% 9% 3 5% 5% 3 4 5% 4 7% 28%	1800
	21st to end	Day	33% 8% 3 6% 5% 3 4 4 5% 5% 25%	1864
November	1st to 10th	Day	38% 9% 3 7% 4 3 5% 5% 4 5% 18%	1547
	11th to 20th	Day	36% 10% 3 6% 5% 3 3 4 3 5% 22%	1520
	21st to end	Day	36% 9% 4 6% 4 4 4 4 4 4 21%	1520
December	1st to 10th	Day	32% 10% 4 5% 4 3 4 4 4 5% 23%	1520
	11th to 20th	Day	36% 11% 3 7% 6% 3 3 3 6% 4 18%	1925
	21st to end	Day	27% 14% 5% 8% 8% 4 5% 4 4 4 17%	2794
January	1st to 10th	Night	41% 17% 3 6% 5% 3 3 3 3 3 11%	4406
	11th to 20th	Night	29% 13% 4 6% 5% 3 4 4 5% 6% 22%	4382
	21st to end	Night	39% 11% 5% 6% 5% 3 4 4 4 4 14%	4792
February	1st to 10th	Night	34% 13% 4 6% 6% 3 5% 4 3 5% 18%	2470
	11th to 20th	Night	31% 13% 3 6% 6% 3 4 4 4 4 23%	2402
	21st to end	Night	32% 15% 4 6% 4 2 4 4 4 5% 19%	1963
March	1st to 10th	Night	21% 14% 5% 8% 6% 5% 5% 5% 6% 6% 19%	3678
	11th to 20th	Night	23% 11% 3 4 4 2 3 3 4 5% 38%	3489
	21st to end	Night	32% 16% 3 7% 5% 3 5% 5% 5% 5% 14%	3684
April	1st to 10th	Night	27% 13% 3 7% 6% 3 5% 5% 5% 6% 18%	1900
	11th to 20th	Night	23% 11% 4 7% 5% 3 5% 5% 4 6% 25%	1774
	21st to end	Night	27% 10% 3 8% 7% 3 5% 4 6% 5% 21%	1574
May	1st to 10th	Night	22% 10% 4 5% 5% 3 4 5% 5% 6% 31%	1440
	11th to 20th	Night	20% 9% 3 5% 5% 4 5% 6% 5% 6% 30%	1416
	21st to end	Night	23% 9% 5% 7% 6% 4 5% 4 5% 6% 27%	1552
June	1st to 10th	Night	21% 9% 5% 6% 6% 4 5% 4 4 6% 28%	1440
	11th to 20th	Night	23% 10% 4 8% 8% 4 4 5% 4 5% 24%	1270
	21st to end	Night	15% 6% 3 5% 5% 3 4 5% 5% 7% 44%	1260
July	1st to 10th	Night	10% 8% 4 4 4 3 3 4 4 7% 50%	1260
	11th to 20th	Night	10% 4 3 4 4 2 4 4 4 5% 56%	1422
	21st to end	Night	7% 3 2 3 3 2 3 3 3 6% 65%	1576
August	1st to 10th	Night	9% 4 2 4 3 2 2 3 5% 8% 58%	1462
	11th to 20th	Night	8% 4 1 3 3 1 3 4 4 7% 62%	1602
	21st to end	Night	14% 6% 2 4 4 2 3 3 4 6% 51%	1901
September	1st to 10th	Night	13% 4 2 5% 4 2 4 4 4 6% 53%	1800
	11th to 20th	Night	19% 7% 4 5% 4 2 4 4 4 5% 42%	1800
	21st to end	Night	19% 8% 3 6% 4 3 4 3 4 6% 40%	2046
October	1st to 10th	Night	22% 7% 3 6% 5% 2 5% 4 5% 6% 35%	2160
	11th to 20th	Night	27% 9% 3 6% 4 3 4 4 4 5% 32%	2160
	21st to end	Night	30% 9% 3 5% 5% 2 4 4 4 5% 29%	2491
November	1st to 10th	Night	35% 10% 3 6% 5% 3 4 4 5% 5% 20%	2528
	11th to 20th	Night	35% 9% 3 6% 5% 3 4 4 4 3 23%	2660
	21st to end	Night	34% 11% 3 6% 5% 3 4 3 4 4 22%	2660
December	1st to 10th	Night	32% 10% 4 6% 5% 2 4 3 4 5% 24%	2660
	11th to 20th	Night	33% 13% 4 7% 6% 3 4 4 4 4 18%	3280
	21st to end	Night	30% 13% 5% 8% 7% 4 6% 5% 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	73% 6% 13% 8% 1	2270
	11th to 20th	Day	67% 5% 10% 13% 5% 1	2136
	21st to end	Day	71% 5% 10% 10% 4 1	2385
May	1st to 10th	Day	60% 5% 15% 14% 5%	2520
	11th to 20th	Day	59% 6% 17% 13% 5%	2480
	21st to end	Day	68% 6% 12% 13% 1	2724
June	1st to 10th	Day	63% 6% 15% 11% 5%	2520
	11th to 20th	Day	69% 6% 13% 10% 2	2686
	21st to end	Day	49% 5% 18% 18% 8% 2	2700
July	1st to 10th	Day	42% 7% 12% 24% 12% 2	2626
	11th to 20th	Day	40% 5% 10% 29% 16% 1	2458
	21st to end	Day	27% 4 12% 36% 17% 4	2589
August	1st to 10th	Day	36% 5% 14% 29% 14% 2	2456
	11th to 20th	Day	30% 4 20% 29% 14% 2	2298
	21st to end	Day	43% 5% 12% 30% 9% 1	2453
September	1st to 10th	Day	37% 4 15% 26% 13% 5%	2160
	11th to 20th	Day	48% 5% 11% 18% 14% 3	2160
	21st to end	Day	51% 4 15% 17% 11% 3	1910
October	1st to 10th	Day	55% 4 10% 22% 8% 1	1800
	11th to 20th	Day	57% 4 14% 17% 7% 1	1800
	21st to end	Day	64% 4 9% 13% 11%	1864
November	1st to 10th	Day	71% 3 10% 11% 4	1547
	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	63% 5% 11% 14% 6% 1	1774
	21st to end	Night	68% 6% 9% 11% 6% 1	1574
May	1st to 10th	Night	56% 6% 14% 16% 7% 1	1440
	11th to 20th	Night	56% 6% 18% 13% 6% 1	1416
	21st to end	Night	61% 7% 15% 14% 2	1552
June	1st to 10th	Night	62% 4 15% 14% 5%	1440
	11th to 20th	Night	64% 6% 16% 11% 2	1270
	21st to end	Night	44% 5% 21% 21% 7% 2	1260
July	1st to 10th	Night	39% 7% 14% 24% 13% 3	1225
	11th to 20th	Night	36% 3 12% 32% 16% 1	1375
	21st to end	Night	25% 5% 10% 36% 20% 5%	1488
August	1st to 10th	Night	29% 5% 13% 33% 18% 3	1454
	11th to 20th	Night	27% 5% 18% 32% 14% 4	1602
	21st to end	Night	38% 4 15% 31% 12% 1	1901
September	1st to 10th	Night	36% 4 15% 26% 15% 5%	1800
	11th to 20th	Night	49% 5% 8% 19% 14% 5%	1800
	21st to end	Night	49% 5% 11% 19% 12% 3	2046
October	1st to 10th	Night	52% 3 13% 22% 9% 2	2160
	11th to 20th	Night	58% 4 10% 18% 9% 1	2160
	21st to end	Night	60% 4 10% 14% 11%	2491
November	1st to 10th	Night	69% 5% 10% 12% 4 1	2528
	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	35% 16% 21% 16% 11% 1	457
	11th to 20th	Day	27% 18% 27% 16% 8% 4	412
	21st to end	Day	26% 15% 27% 23% 8% 1	420
May	1st to 10th	Day	20% 13% 24% 31% 11% 2	423
	11th to 20th	Day	28% 9% 19% 26% 14% 4	448
	21st to end	Day	27% 9% 22% 26% 13% 3	485
June	1st to 10th	Day	26% 10% 21% 25% 15% 3	450
	11th to 20th	Day	25% 14% 21% 26% 11% 2	616
	21st to end	Day	16% 7% 19% 34% 21% 3	630
July	1st to 10th	Day	14% 5% 17% 35% 26% 2	621
	11th to 20th	Day	11% 6% 18% 39% 22% 5%	457
	21st to end	Day	8% 3 15% 39% 29% 5%	487
August	1st to 10th	Day	14% 5% 14% 32% 28% 9%	447
	11th to 20th	Day	8% 3 14% 35% 32% 8%	437
	21st to end	Day	16% 7% 22% 31% 18% 6%	495
September	1st to 10th	Day	16% 5% 25% 36% 15% 3	450
	11th to 20th	Day	18% 8% 18% 31% 20% 4	450
	21st to end	Day	20% 12% 18% 28% 19% 3	450
October	1st to 10th	Day	25% 10% 20% 21% 20% 5%	450
	11th to 20th	Day	32% 9% 15% 24% 15% 5%	450
	21st to end	Day	37% 13% 17% 20% 9% 4	379
November	1st to 10th	Day	39% 12% 17% 20% 11%	210
	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% 19% 27% 13% 2	706
	21st to end	Night	30% 11% 16% 27% 13% 2	494
May	1st to 10th	Night	23% 11% 14% 30% 21% 1	360
	11th to 20th	Night	22% 6% 13% 30% 26% 2	354
	21st to end	Night	24% 7% 15% 24% 26% 4	389
June	1st to 10th	Night	23% 8% 15% 28% 24% 2	360
	11th to 20th	Night	24% 13% 14% 29% 19% 1	360
	21st to end	Night	16% 3 13% 34% 31% 3	360
July	1st to 10th	Night	13% 5% 11% 29% 38% 4	350
	11th to 20th	Night	12% 3 8% 36% 36% 3	348
	21st to end	Night	8% 3 8% 30% 46% 5%	394
August	1st to 10th	Night	11% 3 9% 25% 44% 8%	388
	11th to 20th	Night	10% 1 7% 25% 46% 11%	534
	21st to end	Night	16% 5% 9% 27% 39% 5%	713
September	1st to 10th	Night	13% 3 11% 39% 30% 4	720
	11th to 20th	Night	20% 4 10% 25% 35% 6%	720
	21st to end	Night	22% 5% 12% 32% 26% 3	720
October	1st to 10th	Night	24% 6% 13% 28% 26% 3	720
	11th to 20th	Night	29% 9% 13% 26% 18% 4	720
	21st to end	Night	32% 9% 16% 27% 12% 2	792
November	1st to 10th	Night	38% 10% 20% 25% 7%	740
	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 2024-04-07 06:28:11. Version ac07

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