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 2025-08-05.

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% 3 8% 5% 4 5% 4 5% 7% 33%	125977
	Night	16% 10% 3 8% 5% 4 4 4 5% 6% 35%	122075
Transparency	Day	48% 5% 12% 21% 13% 2	129508
	Night	46% 4 13% 21% 14% 2	126630
Seeing	Day	25% 19% 28% 17% 8% 2	15797
	Night	23% 18% 28% 22% 8% 1	25016

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	11% 14% 5% 11% 5% 5% 4 4 5% 6% 30%	3597
	11th to 20th	Day	12% 13% 8% 16% 7% 5% 4 3 3 5% 24%	3639
	21st to end	Day	16% 13% 7% 11% 5% 3 4 3 3 5% 31%	4466
February	1st to 10th	Day	16% 12% 5% 10% 6% 5% 5% 4 5% 5% 28%	1800
	11th to 20th	Day	15% 12% 4 10% 5% 5% 6% 4 5% 5% 30%	1852
	21st to end	Day	17% 13% 4 9% 6% 4 4 3 5% 6% 30%	1650
March	1st to 10th	Day	20% 12% 4 8% 5% 4 3 3 4 4 33%	4540
	11th to 20th	Day	12% 8% 3 9% 5% 4 4 3 5% 7% 38%	4980
	21st to end	Day	16% 8% 3 7% 4 4 5% 4 5% 6% 37%	5740
April	1st to 10th	Day	17% 12% 4 8% 6% 5% 4 4 4 5% 31%	2400
	11th to 20th	Day	15% 12% 3 7% 5% 5% 4 3 5% 6% 35%	2374
	21st to end	Day	15% 9% 3 8% 5% 4 5% 5% 5% 6% 36%	2400
May	1st to 10th	Day	27% 9% 3 7% 4 4 5% 4 5% 6% 26%	6027
	11th to 20th	Day	21% 10% 3 8% 5% 5% 4 4 5% 6% 28%	6318
	21st to end	Day	19% 8% 3 6% 5% 5% 5% 5% 7% 8% 30%	6949
June	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 4 6% 9% 34%	2800
	11th to 20th	Day	16% 8% 3 7% 4 5% 5% 4 7% 9% 32%	2800
	21st to end	Day	14% 7% 2 7% 5% 4 5% 4 7% 8% 37%	2800
July	1st to 10th	Day	9% 6% 3 8% 6% 6% 6% 5% 7% 9% 36%	6360
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 5% 8% 10% 31%	6360
	21st to end	Day	13% 7% 3 7% 4 5% 6% 5% 6% 8% 35%	6844
August	1st to 10th	Day	20% 8% 3 8% 5% 4 5% 4 6% 8% 30%	4156
	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	13% 9% 3 7% 4 3 3 3 4 6% 46%	1761
	11th to 20th	Day	15% 13% 3 7% 5% 3 5% 3 4 6% 36%	1790
	21st to end	Day	19% 11% 3 7% 4 5% 5% 3 5% 6% 33%	1600
December	1st to 10th	Day	14% 13% 5% 9% 5% 5% 4 3 4 6% 33%	2600
	11th to 20th	Day	19% 14% 5% 8% 4 3 3 3 4 5% 31%	2986
	21st to end	Day	16% 14% 6% 11% 6% 5% 4 4 4 6% 25%	3960
January	1st to 10th	Night	11% 9% 4 11% 6% 6% 6% 4 6% 7% 30%	6349
	11th to 20th	Night	11% 14% 6% 14% 6% 5% 5% 3 4 5% 27%	6244
	21st to end	Night	15% 12% 4 9% 6% 6% 5% 4 5% 5% 29%	6490
February	1st to 10th	Night	16% 11% 4 8% 6% 4 4 4 5% 5% 33%	2600
	11th to 20th	Night	15% 11% 4 9% 6% 5% 5% 5% 6% 6% 28%	2434
	21st to end	Night	15% 12% 3 8% 7% 5% 5% 4 5% 6% 31%	1980
March	1st to 10th	Night	17% 10% 4 9% 5% 5% 4 3 4 4 35%	5283
	11th to 20th	Night	11% 6% 4 8% 5% 5% 4 4 5% 7% 42%	4979
	21st to end	Night	14% 10% 4 8% 5% 5% 4 3 5% 5% 37%	5178
April	1st to 10th	Night	17% 12% 4 9% 6% 5% 4 4 5% 5% 31%	2000
	11th to 20th	Night	15% 10% 2 7% 5% 4 4 4 6% 7% 36%	1975
	21st to end	Night	17% 8% 3 7% 4 3 4 4 6% 5% 39%	2000
May	1st to 10th	Night	28% 10% 2 7% 4 4 4 3 5% 7% 26%	3954
	11th to 20th	Night	19% 11% 4 7% 5% 5% 4 4 5% 6% 31%	3575
	21st to end	Night	20% 9% 3 6% 4 4 5% 4 5% 7% 32%	3926
June	1st to 10th	Night	15% 7% 2 5% 5% 4 4 4 6% 7% 42%	1600
	11th to 20th	Night	13% 7% 3 6% 4 4 4 5% 6% 9% 38%	1600
	21st to end	Night	13% 5% 2 6% 3 3 4 3 6% 10% 43%	1600
July	1st to 10th	Night	11% 7% 3 7% 6% 5% 4 4 6% 7% 39%	3600
	11th to 20th	Night	15% 9% 3 6% 5% 5% 5% 5% 7% 8% 32%	3600
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 7% 46%	4005
August	1st to 10th	Night	21% 8% 3 6% 4 4 5% 4 6% 7% 33%	2861
	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 7% 4 4 4 4 4 6% 46%	2537
	11th to 20th	Night	17% 9% 4 7% 4 4 4 3 5% 7% 36%	2607
	21st to end	Night	14% 11% 3 8% 4 5% 5% 4 5% 6% 35%	2800
December	1st to 10th	Night	13% 13% 5% 10% 5% 4 3 3 5% 6% 34%	4580
	11th to 20th	Night	23% 12% 4 7% 4 3 4 3 4 5% 32%	5208
	21st to end	Night	18% 14% 4 10% 6% 5% 4 3 4 5% 27%	6996

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	49% 3 11% 22% 14% 2	3598
	11th to 20th	Day	60% 3 9% 15% 10% 3	3669
	21st to end	Day	54% 4 9% 20% 12% 1	4466
February	1st to 10th	Day	53% 6% 13% 17% 11% 1	1800
	11th to 20th	Day	51% 5% 11% 21% 11% 1	1861
	21st to end	Day	53% 4 11% 21% 9% 2	1650
March	1st to 10th	Day	52% 3 11% 19% 10% 6%	4543
	11th to 20th	Day	41% 4 17% 25% 9% 4	4980
	21st to end	Day	46% 5% 10% 12% 22% 4	5740
April	1st to 10th	Day	51% 5% 10% 18% 12% 3	2400
	11th to 20th	Day	48% 4 10% 21% 14% 2	2374
	21st to end	Day	45% 6% 10% 21% 14% 4	2400
May	1st to 10th	Day	58% 6% 11% 16% 8% 1	6037
	11th to 20th	Day	55% 6% 9% 20% 10% 1	6322
	21st to end	Day	49% 6% 11% 21% 12% 2	6949
June	1st to 10th	Day	45% 7% 13% 21% 13% 1	2800
	11th to 20th	Day	46% 7% 13% 20% 11% 2	2800
	21st to end	Day	42% 7% 13% 24% 13%	2800
July	1st to 10th	Day	45% 7% 12% 22% 13%	6290
	11th to 20th	Day	47% 5% 16% 23% 10%	6285
	21st to end	Day	46% 3 10% 21% 18% 1	6647
August	1st to 10th	Day	52% 6% 13% 19% 9%	4143
	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	37% 4 14% 23% 19% 2	1761
	11th to 20th	Day	47% 5% 11% 21% 15% 2	1790
	21st to end	Day	47% 4 15% 24% 8% 1	1600
December	1st to 10th	Day	51% 3 14% 23% 7% 1	2600
	11th to 20th	Day	53% 3 15% 19% 10% 1	2986
	21st to end	Day	56% 4 13% 14% 12%	3960
January	1st to 10th	Night	47% 3 13% 20% 15% 2	6349
	11th to 20th	Night	55% 3 14% 14% 9% 4	6289
	21st to end	Night	52% 3 13% 15% 16% 2	6490
February	1st to 10th	Night	48% 5% 15% 21% 9% 1	2600
	11th to 20th	Night	49% 4 14% 19% 12% 2	2436
	21st to end	Night	50% 4 11% 20% 13% 3	1980
March	1st to 10th	Night	50% 3 13% 18% 11% 5%	5293
	11th to 20th	Night	37% 4 21% 23% 10% 4	4980
	21st to end	Night	46% 4 9% 15% 21% 5%	5178
April	1st to 10th	Night	52% 5% 11% 19% 11% 3	2000
	11th to 20th	Night	46% 6% 10% 20% 17% 1	1975
	21st to end	Night	43% 6% 11% 21% 17% 3	2000
May	1st to 10th	Night	57% 5% 11% 17% 8% 2	3960
	11th to 20th	Night	52% 7% 11% 17% 11% 1	3576
	21st to end	Night	49% 6% 13% 20% 11% 2	3927
June	1st to 10th	Night	40% 7% 10% 25% 18% 1	1600
	11th to 20th	Night	39% 8% 17% 25% 11% 1	1600
	21st to end	Night	34% 8% 15% 30% 13%	1600
July	1st to 10th	Night	44% 6% 14% 19% 16% 1	3560
	11th to 20th	Night	49% 5% 15% 21% 10%	3552
	21st to end	Night	38% 4 14% 25% 19% 1	3887
August	1st to 10th	Night	49% 6% 16% 20% 9%	2850
	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	2537
	11th to 20th	Night	44% 4 12% 22% 15% 3	2607
	21st to end	Night	43% 6% 16% 23% 11% 1	2800
December	1st to 10th	Night	49% 3 15% 28% 5% 1	4580
	11th to 20th	Night	52% 3 13% 17% 13% 1	5208
	21st to end	Night	54% 5% 11% 16% 13% 1	6996

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	37% 27% 29% 6% 2 1	200
	11th to 20th	Day	43% 26% 25% 6%	215
	21st to end	Day	28% 35% 25% 9% 2 1	330
February	1st to 10th	Day	35% 34% 25% 5% 1	300
	11th to 20th	Day	38% 30% 26% 5% 1	399
	21st to end	Day	33% 30% 27% 8% 1	414
March	1st to 10th	Day	38% 22% 27% 10% 2	500
	11th to 20th	Day	33% 26% 28% 9% 3 1	500
	21st to end	Day	34% 28% 23% 10% 4 1	550
April	1st to 10th	Day	35% 26% 24% 10% 4	500
	11th to 20th	Day	29% 26% 26% 11% 6% 2	492
	21st to end	Day	27% 23% 34% 10% 5% 2	500
May	1st to 10th	Day	33% 22% 27% 10% 7% 2	503
	11th to 20th	Day	25% 18% 33% 16% 6% 2	527
	21st to end	Day	22% 12% 30% 21% 12% 3	573
June	1st to 10th	Day	20% 17% 29% 23% 9% 2	530
	11th to 20th	Day	21% 16% 27% 25% 10% 1	530
	21st to end	Day	16% 17% 33% 23% 10% 1	530
July	1st to 10th	Day	13% 9% 30% 28% 15% 5%	525
	11th to 20th	Day	12% 7% 29% 31% 17% 3	524
	21st to end	Day	15% 12% 32% 26% 12% 3	576
August	1st to 10th	Day	15% 8% 29% 25% 17% 5%	502
	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% 23% 26% 20% 7% 2	395
	11th to 20th	Day	28% 25% 30% 13% 3	390
	21st to end	Day	37% 24% 31% 8% 1	200
December	1st to 10th	Day	37% 29% 24% 6% 4 1	200
	11th to 20th	Day	38% 29% 23% 10% 2	200
	21st to end	Day	31% 26% 31% 10% 2	220
January	1st to 10th	Night	28% 26% 32% 12% 2	800
	11th to 20th	Night	28% 23% 31% 16% 2	800
	21st to end	Night	21% 28% 34% 14% 2	880
February	1st to 10th	Night	29% 24% 31% 15% 1	800
	11th to 20th	Night	31% 28% 31% 9% 1	783
	21st to end	Night	29% 27% 31% 12% 2	660
March	1st to 10th	Night	29% 25% 29% 14% 3	800
	11th to 20th	Night	27% 26% 29% 14% 4 1	800
	21st to end	Night	24% 26% 32% 13% 4 1	880
April	1st to 10th	Night	28% 23% 29% 17% 4	800
	11th to 20th	Night	23% 22% 30% 19% 5%	791
	21st to end	Night	25% 19% 29% 22% 6%	800
May	1st to 10th	Night	27% 16% 26% 22% 8%	565
	11th to 20th	Night	19% 11% 37% 27% 6% 1	394
	21st to end	Night	17% 9% 27% 34% 12%	433
June	1st to 10th	Night	17% 8% 25% 35% 16% 1	400
	11th to 20th	Night	15% 9% 29% 35% 11% 1	400
	21st to end	Night	14% 6% 28% 39% 13%	400
July	1st to 10th	Night	14% 5% 23% 40% 17% 2	390
	11th to 20th	Night	12% 6% 20% 44% 17% 2	388
	21st to end	Night	15% 5% 25% 38% 16% 2	467
August	1st to 10th	Night	15% 4 19% 39% 21% 2	579
	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	780
	11th to 20th	Night	27% 19% 29% 19% 5% 1	800
	21st to end	Night	22% 24% 32% 17% 5%	800
December	1st to 10th	Night	29% 23% 29% 14% 4	800
	11th to 20th	Night	27% 24% 31% 16% 3	800
	21st to end	Night	26% 25% 30% 16% 2	880

Page generated at 2025-08-02 01:32:29. Version ac07

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