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 2024-11-04.

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	16% 10% 3 8% 5% 4 4 4 5% 7% 33%	101414
	Night	16% 10% 3 8% 5% 4 4 4 5% 6% 35%	97603
Transparency	Day	47% 5% 12% 21% 13% 2	104963
	Night	45% 5% 14% 21% 13% 2	102142
Seeing	Day	25% 19% 28% 18% 8% 3	14865
	Night	22% 18% 28% 23% 8% 1	24101

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	13% 15% 6% 9% 5% 5% 4 3 4 4 32%	2517
	11th to 20th	Day	14% 15% 9% 15% 6% 5% 4 3 3 5% 22%	2561
	21st to end	Day	19% 14% 7% 12% 6% 4 4 3 4 5% 23%	3124
February	1st to 10th	Day	16% 12% 5% 9% 6% 5% 5% 4 5% 5% 29%	1710
	11th to 20th	Day	15% 11% 3 9% 5% 5% 6% 4 5% 5% 31%	1757
	21st to end	Day	17% 13% 4 10% 7% 4 4 3 4 5% 29%	1570
March	1st to 10th	Day	24% 14% 3 8% 5% 4 3 3 4 5% 28%	3166
	11th to 20th	Day	9% 10% 4 10% 6% 4 4 4 5% 7% 37%	3480
	21st to end	Day	19% 10% 3 7% 4 3 3 3 4 6% 38%	4006
April	1st to 10th	Day	16% 12% 4 8% 6% 5% 4 4 4 5% 32%	2280
	11th to 20th	Day	15% 12% 3 7% 5% 5% 4 3 5% 6% 36%	2254
	21st to end	Day	15% 9% 3 8% 5% 4 5% 5% 5% 6% 35%	2280
May	1st to 10th	Day	27% 9% 3 6% 4 4 4 3 5% 6% 29%	4205
	11th to 20th	Day	23% 10% 4 9% 6% 5% 4 4 5% 6% 25%	4398
	21st to end	Day	13% 7% 2 6% 4 4 4 4 7% 8% 41%	4837
June	1st to 10th	Day	15% 7% 2 7% 6% 5% 6% 4 6% 9% 34%	2660
	11th to 20th	Day	15% 8% 3 7% 4 5% 6% 4 7% 9% 33%	2660
	21st to end	Day	13% 7% 2 8% 5% 5% 5% 4 7% 8% 37%	2660
July	1st to 10th	Day	9% 6% 3 7% 6% 6% 6% 5% 7% 10% 35%	4440
	11th to 20th	Day	10% 7% 2 7% 5% 5% 5% 5% 8% 10% 38%	4440
	21st to end	Day	14% 9% 3 8% 5% 5% 6% 4 6% 7% 31%	4760
August	1st to 10th	Day	20% 8% 3 8% 5% 4 5% 4 6% 8% 29%	4104
	11th to 20th	Day	15% 7% 2 6% 5% 5% 4 5% 7% 9% 36%	3822
	21st to end	Day	17% 6% 2 4 4 4 4 3 6% 8% 42%	4180
September	1st to 10th	Day	17% 5% 2 6% 5% 5% 5% 3 5% 7% 39%	2280
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 6% 7% 47%	2271
	21st to end	Day	20% 7% 3 8% 4 5% 4 3 5% 5% 36%	2051
October	1st to 10th	Day	18% 8% 2 6% 3 4 4 4 5% 7% 38%	1900
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 3 5% 6% 40%	1900
	21st to end	Day	19% 10% 4 7% 4 4 5% 3 5% 6% 34%	2024
November	1st to 10th	Day	14% 9% 3 7% 4 3 3 3 4 6% 45%	1698
	11th to 20th	Day	15% 13% 3 7% 5% 4 5% 4 4 6% 35%	1701
	21st to end	Day	19% 10% 3 7% 4 5% 5% 3 5% 6% 34%	1520
December	1st to 10th	Day	18% 15% 6% 9% 5% 5% 4 3 5% 5% 27%	1520
	11th to 20th	Day	19% 12% 6% 8% 5% 4 3 3 5% 5% 30%	1906
	21st to end	Day	17% 15% 7% 11% 8% 6% 5% 4 4 5% 19%	2772
January	1st to 10th	Night	14% 11% 5% 10% 6% 5% 5% 4 5% 6% 30%	4429
	11th to 20th	Night	13% 15% 6% 14% 6% 4 4 3 4 5% 26%	4397
	21st to end	Night	20% 14% 5% 9% 7% 6% 4 4 4 4 23%	4532
February	1st to 10th	Night	16% 11% 4 8% 6% 4 4 4 5% 6% 34%	2470
	11th to 20th	Night	15% 11% 4 9% 6% 5% 5% 5% 6% 6% 29%	2309
	21st to end	Night	16% 12% 3 8% 7% 5% 5% 4 4 6% 31%	1884
March	1st to 10th	Night	20% 12% 5% 9% 4 4 4 3 4 4 31%	3671
	11th to 20th	Night	9% 8% 5% 9% 6% 5% 5% 4 5% 6% 37%	3479
	21st to end	Night	13% 11% 4 9% 5% 5% 4 3 4 4 40%	3612
April	1st to 10th	Night	16% 12% 4 9% 6% 5% 4 4 5% 5% 31%	1900
	11th to 20th	Night	15% 10% 2 7% 5% 4 4 4 6% 6% 37%	1875
	21st to end	Night	17% 8% 3 7% 4 3 5% 4 5% 5% 39%	1900
May	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 5% 7% 29%	2762
	11th to 20th	Night	21% 11% 4 7% 5% 4 4 4 5% 6% 28%	2495
	21st to end	Night	12% 9% 4 7% 5% 5% 5% 4 5% 8% 37%	2738
June	1st to 10th	Night	15% 6% 2 5% 5% 4 4 4 5% 8% 42%	1520
	11th to 20th	Night	12% 6% 3 6% 4 4 4 5% 6% 9% 39%	1520
	21st to end	Night	13% 5% 2 6% 4 4 4 3 6% 10% 43%	1520
July	1st to 10th	Night	10% 7% 3 7% 6% 5% 5% 5% 6% 8% 39%	2520
	11th to 20th	Night	11% 7% 2 6% 5% 4 5% 5% 7% 9% 38%	2520
	21st to end	Night	16% 7% 2 5% 4 5% 4 4 6% 7% 39%	2790
August	1st to 10th	Night	21% 8% 3 6% 4 4 5% 4 6% 7% 33%	2825
	11th to 20th	Night	16% 7% 3 6% 4 3 3 3 5% 7% 43%	2973
	21st to end	Night	12% 5% 1 5% 3 5% 4 4 5% 7% 49%	3476
September	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 7% 44%	1900
	11th to 20th	Night	11% 6% 2 6% 4 4 3 3 5% 6% 50%	1907
	21st to end	Night	19% 7% 3 7% 4 3 3 4 5% 5% 41%	2126
October	1st to 10th	Night	18% 9% 2 5% 4 4 3 3 4 7% 41%	2280
	11th to 20th	Night	15% 8% 3 7% 4 3 4 3 5% 5% 44%	2280
	21st to end	Night	19% 10% 2 6% 4 4 4 4 5% 6% 37%	2572
November	1st to 10th	Night	11% 8% 3 6% 4 4 4 4 4 6% 46%	2453
	11th to 20th	Night	17% 9% 4 7% 5% 4 4 3 5% 7% 36%	2476
	21st to end	Night	14% 10% 3 7% 4 5% 5% 4 6% 7% 36%	2660
December	1st to 10th	Night	19% 12% 4 10% 6% 5% 4 4 5% 5% 26%	2660
	11th to 20th	Night	18% 12% 4 8% 5% 4 5% 4 5% 6% 29%	3288
	21st to end	Night	19% 13% 4 11% 6% 6% 5% 3 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	55% 4 12% 20% 7% 2	3169
	11th to 20th	Day	40% 5% 16% 25% 11% 3	3480
	21st to end	Day	47% 5% 10% 15% 20% 4	4006
April	1st to 10th	Day	50% 6% 11% 19% 13% 3	2280
	11th to 20th	Day	48% 4 10% 22% 13% 3	2254
	21st to end	Day	45% 6% 10% 20% 14% 4	2280
May	1st to 10th	Day	54% 7% 10% 19% 9% 1	4218
	11th to 20th	Day	57% 8% 11% 15% 8% 1	4402
	21st to end	Day	37% 8% 13% 24% 16% 2	4837
June	1st to 10th	Day	45% 7% 13% 21% 12% 1	2660
	11th to 20th	Day	45% 7% 13% 21% 11% 2	2660
	21st to end	Day	42% 8% 13% 25% 12%	2660
July	1st to 10th	Day	45% 7% 13% 22% 12% 1	4370
	11th to 20th	Day	41% 6% 16% 24% 13%	4365
	21st to end	Day	52% 5% 10% 18% 14% 1	4608
August	1st to 10th	Day	53% 6% 13% 19% 9%	4091
	11th to 20th	Day	43% 5% 14% 28% 10%	3861
	21st to end	Day	39% 4 11% 32% 13% 1	4180
September	1st to 10th	Day	42% 7% 11% 20% 17% 2	2280
	11th to 20th	Day	35% 5% 12% 22% 24% 2	2271
	21st to end	Day	49% 5% 10% 17% 18% 2	2051
October	1st to 10th	Day	43% 3 8% 21% 21% 4	3160
	11th to 20th	Day	38% 3 8% 27% 16% 7%	3140
	21st to end	Day	33% 3 16% 25% 21% 3	3314
November	1st to 10th	Day	38% 4 14% 24% 18% 3	1698
	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	52% 4 12% 20% 10% 2	3681
	11th to 20th	Night	39% 6% 19% 23% 12% 1	3480
	21st to end	Night	44% 5% 10% 16% 19% 5%	3612
April	1st to 10th	Night	50% 5% 11% 19% 12% 2	1900
	11th to 20th	Night	45% 6% 10% 20% 17% 1	1875
	21st to end	Night	43% 6% 11% 20% 17% 3	1900
May	1st to 10th	Night	53% 6% 10% 22% 8% 1	2768
	11th to 20th	Night	52% 9% 13% 15% 10% 1	2496
	21st to end	Night	41% 9% 15% 20% 13% 2	2739
June	1st to 10th	Night	39% 7% 11% 25% 17% 1	1520
	11th to 20th	Night	37% 8% 17% 25% 11% 1	1520
	21st to end	Night	34% 8% 14% 30% 13%	1520
July	1st to 10th	Night	43% 8% 14% 18% 16% 2	2480
	11th to 20th	Night	40% 6% 17% 25% 12% 1	2472
	21st to end	Night	44% 5% 15% 17% 18% 1	2701
August	1st to 10th	Night	49% 7% 16% 20% 8%	2814
	11th to 20th	Night	42% 5% 14% 24% 14%	3000
	21st to end	Night	34% 5% 10% 33% 16% 1	3476
September	1st to 10th	Night	40% 5% 12% 24% 17% 2	1900
	11th to 20th	Night	35% 4 10% 25% 23% 2	1907
	21st to end	Night	44% 5% 9% 21% 19% 2	2126
October	1st to 10th	Night	45% 4 9% 22% 19% 1	3800
	11th to 20th	Night	34% 3 10% 27% 20% 7%	3779
	21st to end	Night	32% 3 17% 27% 19% 2	4232
November	1st to 10th	Night	37% 5% 14% 22% 19% 3	2453
	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% 10% 4 1	506
April	1st to 10th	Day	33% 25% 25% 11% 4	460
	11th to 20th	Day	27% 27% 27% 11% 6% 2	452
	21st to end	Day	27% 22% 34% 10% 5% 2	460
May	1st to 10th	Day	33% 22% 26% 10% 7% 2	463
	11th to 20th	Day	24% 17% 34% 16% 7% 2	487
	21st to end	Day	20% 12% 31% 22% 13% 3	529
June	1st to 10th	Day	20% 17% 29% 24% 9% 2	490
	11th to 20th	Day	18% 17% 26% 27% 10% 1	490
	21st to end	Day	15% 17% 33% 22% 10% 1	490
July	1st to 10th	Day	13% 9% 29% 28% 16% 5%	485
	11th to 20th	Day	12% 7% 30% 31% 17% 3	484
	21st to end	Day	16% 13% 32% 25% 12% 3	532
August	1st to 10th	Day	15% 8% 29% 25% 17% 5%	486
	11th to 20th	Day	14% 9% 27% 28% 16% 5%	479
	21st to end	Day	16% 11% 27% 25% 14% 7%	539
September	1st to 10th	Day	22% 12% 22% 23% 14% 7%	490
	11th to 20th	Day	13% 9% 30% 22% 17% 10%	490
	21st to end	Day	26% 14% 26% 20% 11% 3	490
October	1st to 10th	Day	24% 14% 26% 20% 12% 4	490
	11th to 20th	Day	22% 21% 28% 21% 6% 2	490
	21st to end	Day	27% 20% 30% 17% 6% 1	504
November	1st to 10th	Day	23% 22% 24% 21% 7% 2	374
	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	24% 26% 31% 14% 4 1	836
April	1st to 10th	Night	26% 23% 30% 17% 4	760
	11th to 20th	Night	22% 22% 31% 19% 6%	751
	21st to end	Night	25% 19% 29% 21% 6%	760
May	1st to 10th	Night	27% 17% 26% 22% 8%	537
	11th to 20th	Night	18% 10% 38% 26% 6% 1	374
	21st to end	Night	16% 9% 26% 35% 13%	411
June	1st to 10th	Night	16% 8% 25% 35% 16% 1	380
	11th to 20th	Night	14% 9% 29% 36% 12% 1	380
	21st to end	Night	14% 6% 28% 40% 12%	380
July	1st to 10th	Night	13% 6% 23% 40% 16% 2	370
	11th to 20th	Night	11% 6% 20% 44% 17% 2	368
	21st to end	Night	15% 5% 24% 39% 15% 2	443
August	1st to 10th	Night	15% 4 19% 40% 21% 2	567
	11th to 20th	Night	15% 5% 16% 40% 23% 2	674
	21st to end	Night	16% 6% 21% 33% 23% 2	836
September	1st to 10th	Night	18% 8% 23% 32% 17% 2	760
	11th to 20th	Night	13% 6% 25% 29% 22% 5%	760
	21st to end	Night	22% 10% 22% 30% 14% 2	760
October	1st to 10th	Night	23% 11% 24% 24% 15% 2	760
	11th to 20th	Night	19% 14% 32% 27% 6% 1	760
	21st to end	Night	25% 18% 28% 22% 5%	836
November	1st to 10th	Night	18% 20% 31% 22% 8% 1	755
	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

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