Francis E. Walter Dam
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Francis E. Walter Dam clear sky chart from 2005-11-06 to 2024-03-23.

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	16% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	78659
	Night	15% 9% 4 7% 6% 3 5% 5% 5% 6% 35%	81219
Transparency	Day	48% 6% 12% 20% 12% 2	78366
	Night	45% 6% 14% 21% 13% 2	81046
Seeing	Day	25% 19% 28% 17% 8% 3	13968
	Night	22% 18% 28% 23% 8% 1	23384

By Month

Percentage Cloud Cover NE (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	14% 14% 7% 8% 6% 3 5% 3 4 4 32%	2517
	11th to 20th	Day	14% 14% 10% 14% 7% 3 4 4 4 5% 22%	2561
	21st to end	Day	19% 13% 7% 10% 8% 3 4 4 4 5% 23%	3124
February	1st to 10th	Day	16% 11% 6% 8% 6% 4 5% 5% 5% 5% 29%	1710
	11th to 20th	Day	15% 10% 4 9% 6% 4 6% 5% 5% 5% 31%	1757
	21st to end	Day	18% 12% 4 8% 7% 3 4 3 4 6% 29%	1570
March	1st to 10th	Day	16% 11% 5% 7% 7% 4 4 4 5% 6% 33%	1921
	11th to 20th	Day	14% 10% 4 8% 7% 3 4 4 5% 7% 33%	2090
	21st to end	Day	19% 11% 5% 7% 6% 3 4 4 4 6% 33%	2308
April	1st to 10th	Day	16% 11% 5% 6% 7% 4 5% 4 4 6% 33%	2160
	11th to 20th	Day	15% 11% 4 6% 6% 3 4 4 5% 6% 37%	2134
	21st to end	Day	15% 9% 4 7% 5% 3 5% 6% 5% 6% 35%	2160
May	1st to 10th	Day	20% 8% 4 6% 5% 4 5% 5% 5% 6% 31%	2382
	11th to 20th	Day	20% 8% 4 6% 6% 3 5% 4 6% 6% 33%	2478
	21st to end	Day	15% 7% 3 6% 5% 3 4 5% 6% 7% 38%	2725
June	1st to 10th	Day	16% 6% 3 6% 6% 3 7% 5% 6% 9% 34%	2520
	11th to 20th	Day	15% 8% 3 6% 5% 3 6% 6% 6% 9% 32%	2520
	21st to end	Day	13% 6% 3 7% 6% 3 5% 5% 6% 8% 38%	2520
July	1st to 10th	Day	12% 6% 3 5% 6% 4 6% 6% 6% 9% 35%	2520
	11th to 20th	Day	9% 6% 2 6% 5% 3 6% 6% 7% 9% 40%	2520
	21st to end	Day	13% 6% 3 6% 6% 4 6% 7% 7% 8% 35%	2716
August	1st to 10th	Day	12% 7% 3 7% 6% 4 6% 6% 7% 9% 32%	2327
	11th to 20th	Day	14% 7% 3 6% 7% 4 5% 6% 7% 9% 34%	2179
	21st to end	Day	16% 6% 3 5% 5% 3 5% 6% 7% 8% 36%	2376
September	1st to 10th	Day	18% 5% 3 6% 6% 3 5% 5% 5% 8% 38%	2160
	11th to 20th	Day	14% 5% 3 5% 5% 2 4 3 5% 7% 46%	2152
	21st to end	Day	18% 6% 3 7% 5% 3 5% 5% 5% 6% 37%	1944
October	1st to 10th	Day	18% 8% 3 5% 4 3 4 5% 5% 7% 38%	1800
	11th to 20th	Day	17% 8% 3 5% 6% 3 4 4 4 6% 39%	1800
	21st to end	Day	20% 10% 4 7% 4 3 5% 4 4 6% 32%	1918
November	1st to 10th	Day	14% 8% 3 6% 5% 2 3 3 4 6% 45%	1671
	11th to 20th	Day	16% 11% 4 7% 5% 3 4 5% 4 6% 36%	1701
	21st to end	Day	19% 9% 3 6% 4 3 5% 4 5% 6% 34%	1520
December	1st to 10th	Day	18% 14% 6% 8% 5% 4 4 4 4 6% 27%	1520
	11th to 20th	Day	19% 12% 6% 7% 6% 3 3 4 4 5% 30%	1906
	21st to end	Day	17% 14% 7% 10% 9% 4 5% 5% 4 5% 20%	2772
January	1st to 10th	Night	14% 10% 5% 8% 7% 4 5% 5% 5% 6% 30%	4429
	11th to 20th	Night	13% 15% 6% 12% 7% 3 4 4 4 5% 26%	4397
	21st to end	Night	21% 13% 5% 8% 8% 4 5% 5% 4 4 23%	4532
February	1st to 10th	Night	16% 10% 5% 7% 6% 3 4 4 5% 6% 34%	2470
	11th to 20th	Night	15% 10% 5% 8% 7% 4 6% 5% 6% 6% 29%	2309
	21st to end	Night	16% 11% 3 7% 7% 4 6% 5% 4 6% 31%	1884
March	1st to 10th	Night	16% 9% 4 7% 5% 3 5% 5% 5% 6% 35%	2243
	11th to 20th	Night	14% 9% 4 6% 6% 4 5% 4 5% 6% 37%	2089
	21st to end	Night	15% 9% 3 7% 6% 4 5% 4 4 6% 36%	2093
April	1st to 10th	Night	15% 11% 4 8% 7% 3 5% 5% 5% 5% 32%	1800
	11th to 20th	Night	15% 9% 3 6% 5% 3 4 5% 6% 7% 38%	1775
	21st to end	Night	18% 8% 3 7% 5% 2 5% 5% 5% 5% 38%	1800
May	1st to 10th	Night	21% 9% 3 6% 6% 3 4 4 4 6% 33%	1590
	11th to 20th	Night	15% 7% 4 4 6% 4 5% 5% 6% 7% 38%	1415
	21st to end	Night	11% 7% 4 6% 6% 3 6% 5% 5% 7% 39%	1550
June	1st to 10th	Night	15% 5% 3 5% 5% 3 5% 5% 5% 8% 42%	1440
	11th to 20th	Night	13% 6% 3 6% 5% 3 4 6% 6% 9% 39%	1440
	21st to end	Night	12% 5% 2 5% 5% 2 4 5% 5% 10% 45%	1440
July	1st to 10th	Night	12% 6% 3 5% 6% 4 4 6% 6% 9% 40%	1440
	11th to 20th	Night	10% 6% 3 5% 5% 3 5% 6% 6% 9% 41%	1440
	21st to end	Night	12% 5% 2 4 5% 3 5% 6% 7% 9% 42%	1599
August	1st to 10th	Night	12% 4 2 5% 5% 3 4 6% 8% 9% 42%	1611
	11th to 20th	Night	13% 6% 3 5% 5% 3 5% 5% 6% 10% 39%	1702
	21st to end	Night	13% 6% 2 5% 5% 3 5% 5% 5% 7% 44%	1980
September	1st to 10th	Night	15% 6% 3 5% 5% 2 4 4 5% 7% 43%	1800
	11th to 20th	Night	12% 6% 3 5% 4 3 4 3 4 7% 49%	1806
	21st to end	Night	16% 6% 3 6% 5% 2 4 5% 5% 5% 43%	2014
October	1st to 10th	Night	18% 8% 3 4 4 3 3 4 4 6% 42%	2160
	11th to 20th	Night	15% 8% 3 6% 5% 3 4 3 4 5% 43%	2160
	21st to end	Night	19% 10% 2 5% 5% 3 4 5% 5% 6% 35%	2436
November	1st to 10th	Night	12% 7% 3 6% 5% 3 4 4 4 5% 46%	2407
	11th to 20th	Night	17% 8% 4 6% 5% 3 4 4 5% 7% 36%	2476
	21st to end	Night	14% 10% 3 7% 5% 4 5% 4 5% 7% 36%	2660
December	1st to 10th	Night	19% 11% 5% 9% 6% 4 5% 5% 5% 6% 26%	2660
	11th to 20th	Night	18% 11% 5% 8% 6% 3 5% 5% 5% 6% 29%	3288
	21st to end	Night	19% 13% 4 10% 7% 4 5% 4 4 6% 24%	4884

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	48% 4 16% 23% 8% 2	2518
	11th to 20th	Day	61% 4 12% 14% 8% 1	2561
	21st to end	Day	58% 6% 11% 17% 8% 1	3124
February	1st to 10th	Day	52% 6% 14% 17% 11% 1	1710
	11th to 20th	Day	50% 5% 12% 21% 11% 1	1766
	21st to end	Day	54% 4 11% 20% 9% 2	1570
March	1st to 10th	Day	48% 5% 14% 20% 10% 2	1924
	11th to 20th	Day	48% 5% 11% 22% 12% 2	2090
	21st to end	Day	51% 4 10% 16% 15% 3	2308
April	1st to 10th	Day	49% 6% 10% 19% 13% 3	2160
	11th to 20th	Day	47% 4 10% 22% 14% 3	2134
	21st to end	Day	46% 6% 10% 20% 13% 5%	2160
May	1st to 10th	Day	51% 7% 9% 18% 13% 2	2382
	11th to 20th	Day	51% 6% 10% 18% 13% 2	2482
	21st to end	Day	44% 9% 13% 19% 13% 2	2725
June	1st to 10th	Day	46% 7% 13% 21% 12% 1	2520
	11th to 20th	Day	47% 8% 13% 20% 11% 2	2520
	21st to end	Day	42% 8% 13% 25% 12%	2520
July	1st to 10th	Day	43% 9% 13% 21% 14% 1	2450
	11th to 20th	Day	37% 9% 15% 25% 14% 1	2445
	21st to end	Day	45% 7% 11% 20% 15% 2	2564
August	1st to 10th	Day	44% 9% 13% 21% 12%	2314
	11th to 20th	Day	43% 9% 15% 21% 12% 1	2179
	21st to end	Day	42% 7% 11% 21% 16% 3	2376
September	1st to 10th	Day	44% 7% 12% 19% 16% 2	2160
	11th to 20th	Day	36% 5% 12% 21% 23% 2	2152
	21st to end	Day	47% 5% 10% 17% 19% 2	1944
October	1st to 10th	Day	45% 6% 13% 21% 14% 2	1800
	11th to 20th	Day	44% 5% 11% 24% 14% 1	1800
	21st to end	Day	50% 5% 12% 19% 12% 2	1918
November	1st to 10th	Day	38% 4 14% 23% 18% 3	1671
	11th to 20th	Day	47% 5% 11% 20% 15% 2	1701
	21st to end	Day	46% 5% 15% 25% 8% 1	1520
December	1st to 10th	Day	56% 5% 14% 15% 8% 1	1520
	11th to 20th	Day	52% 5% 17% 20% 7% 1	1906
	21st to end	Day	60% 6% 13% 15% 6%	2772
January	1st to 10th	Night	44% 5% 17% 24% 8% 2	4429
	11th to 20th	Night	54% 4 16% 16% 8% 2	4397
	21st to end	Night	55% 5% 14% 16% 8% 1	4532
February	1st to 10th	Night	47% 5% 15% 22% 9% 1	2470
	11th to 20th	Night	48% 5% 15% 19% 12% 2	2311
	21st to end	Night	49% 5% 11% 20% 12% 3	1884
March	1st to 10th	Night	43% 4 16% 22% 11% 3	2253
	11th to 20th	Night	43% 5% 14% 22% 14% 2	2090
	21st to end	Night	46% 6% 12% 17% 16% 4	2093
April	1st to 10th	Night	50% 5% 12% 19% 12% 3	1800
	11th to 20th	Night	44% 6% 11% 20% 17% 1	1775
	21st to end	Night	44% 6% 11% 20% 16% 3	1800
May	1st to 10th	Night	50% 5% 12% 19% 12% 2	1590
	11th to 20th	Night	43% 7% 11% 20% 17% 2	1416
	21st to end	Night	42% 9% 16% 19% 12% 3	1551
June	1st to 10th	Night	40% 7% 11% 25% 17% 1	1440
	11th to 20th	Night	39% 9% 17% 24% 11% 1	1440
	21st to end	Night	33% 8% 15% 31% 13%	1440
July	1st to 10th	Night	40% 10% 12% 23% 13% 2	1400
	11th to 20th	Night	36% 9% 16% 25% 13% 1	1392
	21st to end	Night	36% 8% 19% 19% 17% 2	1510
August	1st to 10th	Night	35% 10% 16% 25% 14% 1	1600
	11th to 20th	Night	41% 9% 15% 20% 14% 1	1702
	21st to end	Night	37% 9% 12% 22% 19% 2	1980
September	1st to 10th	Night	40% 6% 12% 24% 16% 2	1800
	11th to 20th	Night	35% 5% 10% 25% 22% 2	1806
	21st to end	Night	42% 5% 9% 22% 21% 2	2014
October	1st to 10th	Night	43% 6% 12% 23% 14% 2	2160
	11th to 20th	Night	41% 5% 13% 26% 13% 2	2160
	21st to end	Night	44% 5% 15% 22% 13% 1	2436
November	1st to 10th	Night	37% 5% 14% 22% 19% 3	2407
	11th to 20th	Night	44% 4 13% 22% 15% 3	2476
	21st to end	Night	42% 6% 16% 24% 11% 1	2660
December	1st to 10th	Night	52% 5% 16% 18% 8% 1	2660
	11th to 20th	Night	49% 5% 15% 17% 12% 1	3288
	21st to end	Night	53% 7% 15% 18% 6%	4884

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	33% 26% 32% 7% 2 1	180
	11th to 20th	Day	41% 27% 24% 7% 1	193
	21st to end	Day	28% 33% 27% 9% 2 1	297
February	1st to 10th	Day	34% 33% 26% 6% 1	270
	11th to 20th	Day	35% 30% 27% 6% 1	364
	21st to end	Day	34% 30% 27% 8% 1	382
March	1st to 10th	Day	37% 23% 27% 11% 3	460
	11th to 20th	Day	33% 26% 28% 10% 3 1	460
	21st to end	Day	34% 28% 22% 11% 4 1	470
April	1st to 10th	Day	32% 26% 26% 11% 4	420
	11th to 20th	Day	26% 26% 27% 12% 7% 2	412
	21st to end	Day	28% 23% 33% 10% 5% 2	420
May	1st to 10th	Day	32% 22% 26% 10% 7% 2	423
	11th to 20th	Day	24% 18% 33% 16% 6% 2	447
	21st to end	Day	21% 12% 29% 22% 13% 3	485
June	1st to 10th	Day	20% 16% 29% 24% 9% 2	450
	11th to 20th	Day	20% 18% 26% 25% 10% 1	450
	21st to end	Day	14% 17% 33% 24% 10% 2	450
July	1st to 10th	Day	13% 10% 28% 27% 17% 5%	445
	11th to 20th	Day	12% 6% 29% 31% 18% 4	444
	21st to end	Day	15% 12% 32% 26% 12% 3	488
August	1st to 10th	Day	14% 7% 29% 26% 18% 6%	446
	11th to 20th	Day	14% 8% 27% 28% 17% 6%	439
	21st to end	Day	16% 12% 26% 25% 14% 7%	495
September	1st to 10th	Day	22% 12% 22% 24% 15% 7%	450
	11th to 20th	Day	14% 10% 31% 20% 16% 9%	450
	21st to end	Day	24% 14% 26% 20% 12% 4	450
October	1st to 10th	Day	23% 13% 26% 21% 13% 4	450
	11th to 20th	Day	22% 21% 28% 22% 6% 1	450
	21st to end	Day	29% 20% 30% 15% 6% 1	464
November	1st to 10th	Day	23% 22% 24% 21% 8% 2	365
	11th to 20th	Day	29% 25% 29% 14% 4	361
	21st to end	Day	33% 24% 33% 8% 1	180
December	1st to 10th	Day	37% 30% 22% 6% 4 1	180
	11th to 20th	Day	37% 30% 23% 9% 2	180
	21st to end	Day	33% 26% 31% 9% 2	198
January	1st to 10th	Night	27% 26% 32% 13% 3	760
	11th to 20th	Night	27% 23% 31% 16% 2	760
	21st to end	Night	21% 29% 34% 15% 2	836
February	1st to 10th	Night	28% 24% 31% 16% 1	760
	11th to 20th	Night	29% 29% 32% 9% 1	743
	21st to end	Night	30% 27% 30% 12% 2	628
March	1st to 10th	Night	29% 25% 30% 14% 3	760
	11th to 20th	Night	27% 26% 29% 14% 4 1	760
	21st to end	Night	23% 26% 32% 13% 4 1	801
April	1st to 10th	Night	26% 23% 30% 17% 4	720
	11th to 20th	Night	21% 22% 32% 20% 6%	711
	21st to end	Night	26% 18% 30% 21% 6%	720
May	1st to 10th	Night	27% 17% 25% 22% 8%	510
	11th to 20th	Night	18% 10% 38% 27% 6% 1	354
	21st to end	Night	16% 9% 25% 35% 14% 1	389
June	1st to 10th	Night	16% 7% 25% 35% 16%	360
	11th to 20th	Night	14% 9% 29% 35% 12% 1	360
	21st to end	Night	13% 6% 27% 42% 12%	360
July	1st to 10th	Night	13% 6% 23% 39% 17% 2	350
	11th to 20th	Night	11% 6% 20% 44% 17% 2	348
	21st to end	Night	15% 4 24% 39% 16% 2	419
August	1st to 10th	Night	14% 4 18% 40% 21% 2	537
	11th to 20th	Night	15% 4 15% 40% 24% 2	638
	21st to end	Night	15% 6% 22% 33% 22% 2	792
September	1st to 10th	Night	18% 8% 23% 32% 17% 2	720
	11th to 20th	Night	13% 7% 26% 28% 20% 5%	720
	21st to end	Night	19% 10% 23% 31% 15% 2	720
October	1st to 10th	Night	23% 11% 24% 25% 15% 3	720
	11th to 20th	Night	20% 14% 32% 28% 6% 1	720
	21st to end	Night	27% 19% 28% 21% 5% 1	792
November	1st to 10th	Night	18% 20% 30% 22% 9% 1	740
	11th to 20th	Night	26% 19% 29% 20% 5% 1	760
	21st to end	Night	21% 23% 33% 18% 5%	760
December	1st to 10th	Night	29% 23% 29% 14% 4	760
	11th to 20th	Night	26% 24% 32% 15% 3	760
	21st to end	Night	26% 26% 31% 15% 2	836

Page generated at 2024-03-19 23:31:08. Version ac07

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