Shasta Dam
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shasta Dam clear sky chart from 2005-11-06 to 2024-01-13.

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	12% 3 1 2 2 2 2 3 4 6% 63%	74728
	Night	15% 3 1 3 3 2 3 3 4 6% 58%	75035
Transparency	Day	24% 5% 9% 18% 23% 21%	74442
	Night	29% 5% 9% 19% 22% 16%	74842
Seeing	Day	13% 11% 27% 31% 16% 3	13931
	Night	15% 5% 23% 35% 21% 2	23204

By Month

Percentage Cloud Cover SW (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	26% 6% 2 3 3 2 3 5% 5% 8% 36%	1519
	11th to 20th	Day	25% 5% 1 3 2 2 3 3 3 6% 48%	1594
	21st to end	Day	20% 7% 2 3 3 3 3 3 3 6% 47%	1782
February	1st to 10th	Day	20% 5% 1 3 3 2 3 4 5% 7% 46%	1620
	11th to 20th	Day	18% 4 1 3 3 2 3 4 4 6% 52%	1584
	21st to end	Day	20% 5% 2 3 3 2 4 3 5% 7% 46%	1480
March	1st to 10th	Day	23% 8% 3 4 5% 2 4 4 4 8% 35%	1920
	11th to 20th	Day	22% 5% 2 3 3 3 3 3 4 9% 44%	1980
	21st to end	Day	22% 6% 2 4 3 2 3 4 4 7% 43%	2214
April	1st to 10th	Day	19% 5% 2 3 3 2 3 4 5% 8% 45%	2160
	11th to 20th	Day	15% 4 1 3 4 2 3 3 5% 5% 55%	2129
	21st to end	Day	6% 3 2 3 3 2 3 4 4 7% 62%	2178
May	1st to 10th	Day	8% 3 1 3 3 1 3 3 4 6% 64%	2340
	11th to 20th	Day	7% 3 2 3 4 2 3 3 4 8% 62%	2472
	21st to end	Day	9% 3 2 2 3 2 3 4 4 9% 60%	2723
June	1st to 10th	Day	7% 2 1 3 3 2 4 4 5% 7% 61%	2520
	11th to 20th	Day	3 1 1 1 2 1 2 2 4 6% 78%	2520
	21st to end	Day	4 1 1 1 2 1 1 2 2 3 80%	2520
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 4 85%	2520
	11th to 20th	Day	2 1 1 1 1 1 1 2 2 3 86%	2520
	21st to end	Day	1 1 1 1 1 1 2 2 3 4 85%	2557
August	1st to 10th	Day	2 1 1 2 1 1 2 2 5% 82%	2328
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 88%	2268
	21st to end	Day	1 1 1 1 1 1 2 3 88%	2376
September	1st to 10th	Day	1 1 1 1 1 1 2 4 88%	2160
	11th to 20th	Day	4 1 1 1 2 1 3 3 4 6% 74%	2160
	21st to end	Day	3 2 2 2 1 2 2 2 6% 77%	1901
October	1st to 10th	Day	5% 2 1 2 2 2 2 3 3 4 74%	1800
	11th to 20th	Day	8% 3 1 2 2 1 2 3 3 6% 68%	1800
	21st to end	Day	13% 3 1 3 3 1 2 2 4 6% 62%	1980
November	1st to 10th	Day	15% 5% 1 4 3 2 3 4 5% 5% 53%	1738
	11th to 20th	Day	15% 4 1 4 3 2 3 5% 5% 9% 50%	1603
	21st to end	Day	13% 4 2 3 3 3 3 3 5% 7% 55%	1520
December	1st to 10th	Day	25% 4 2 3 2 2 2 3 3 5% 50%	1520
	11th to 20th	Day	35% 4 2 4 4 2 3 4 4 6% 32%	1928
	21st to end	Day	28% 5% 2 4 3 2 3 4 4 6% 38%	2794
January	1st to 10th	Night	26% 5% 1 3 4 2 3 4 5% 7% 40%	2646
	11th to 20th	Night	25% 4 2 3 2 2 3 4 4 6% 46%	2422
	21st to end	Night	22% 5% 1 3 3 2 3 4 4 5% 47%	2574
February	1st to 10th	Night	21% 3 1 3 4 2 3 3 5% 7% 48%	2340
	11th to 20th	Night	21% 4 2 3 2 2 3 4 3 7% 50%	2262
	21st to end	Night	18% 4 1 3 4 3 3 4 5% 6% 48%	1776
March	1st to 10th	Night	26% 7% 2 4 4 2 3 3 3 6% 38%	2038
	11th to 20th	Night	21% 6% 2 4 3 3 4 4 5% 8% 41%	1980
	21st to end	Night	23% 4 2 3 3 2 3 4 5% 5% 45%	2142
April	1st to 10th	Night	18% 6% 2 4 4 2 3 3 5% 6% 47%	1800
	11th to 20th	Night	11% 4 1 4 3 2 4 3 6% 6% 55%	1779
	21st to end	Night	9% 4 1 3 2 1 3 3 4 7% 63%	1779
May	1st to 10th	Night	9% 3 1 2 2 1 2 4 3 7% 64%	1620
	11th to 20th	Night	8% 4 1 3 2 2 4 3 5% 7% 59%	1421
	21st to end	Night	8% 4 2 3 3 3 4 3 5% 7% 59%	1552
June	1st to 10th	Night	7% 4 2 3 4 2 4 4 5% 8% 57%	1440
	11th to 20th	Night	3 3 1 1 2 1 2 3 3 5% 77%	1440
	21st to end	Night	3 1 2 2 1 2 2 3 3 81%	1440
July	1st to 10th	Night	1 1 1 1 1 2 2 3 4 84%	1440
	11th to 20th	Night	2 2 1 1 2 1 2 2 2 3 83%	1440
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	1759
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 4 82%	1611
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 85%	1630
	21st to end	Night	1 1 1 1 1 2 2 3 88%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 2 4 88%	1800
	11th to 20th	Night	3 1 1 2 1 2 2 3 5% 78%	1800
	21st to end	Night	4 2 1 1 2 2 2 3 3 6% 76%	2055
October	1st to 10th	Night	6% 2 1 2 2 1 2 3 3 4 73%	2160
	11th to 20th	Night	8% 2 1 2 2 1 2 2 3 5% 72%	2160
	21st to end	Night	12% 2 1 2 2 1 2 2 3 6% 66%	2376
November	1st to 10th	Night	14% 4 2 3 3 2 3 4 5% 7% 52%	2341
	11th to 20th	Night	15% 4 2 3 3 2 4 4 5% 9% 50%	2574
	21st to end	Night	15% 3 2 3 3 2 3 3 5% 6% 55%	2660
December	1st to 10th	Night	25% 3 1 3 2 1 2 3 4 7% 49%	2660
	11th to 20th	Night	35% 5% 1 3 3 2 3 3 5% 5% 34%	3276
	21st to end	Night	28% 6% 2 4 4 2 3 4 4 5% 38%	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	46% 8% 13% 21% 8% 4	1519
	11th to 20th	Day	40% 5% 10% 18% 15% 12%	1594
	21st to end	Day	41% 5% 7% 17% 22% 8%	1782
February	1st to 10th	Day	38% 6% 11% 18% 17% 9%	1620
	11th to 20th	Day	34% 6% 10% 21% 24% 5%	1593
	21st to end	Day	38% 6% 12% 21% 18% 6%	1480
March	1st to 10th	Day	48% 8% 11% 15% 13% 5%	1920
	11th to 20th	Day	39% 6% 10% 21% 18% 5%	1980
	21st to end	Day	41% 8% 13% 19% 15% 4	2214
April	1st to 10th	Day	37% 6% 14% 19% 18% 5%	2160
	11th to 20th	Day	30% 6% 14% 21% 19% 10%	2132
	21st to end	Day	21% 7% 12% 26% 25% 8%	2178
May	1st to 10th	Day	21% 5% 10% 22% 29% 13%	2340
	11th to 20th	Day	22% 6% 10% 24% 26% 12%	2475
	21st to end	Day	22% 6% 14% 26% 24% 9%	2724
June	1st to 10th	Day	22% 6% 10% 22% 24% 15%	2520
	11th to 20th	Day	9% 4 10% 19% 34% 25%	2520
	21st to end	Day	11% 2 5% 15% 29% 39%	2520
July	1st to 10th	Day	6% 3 4 12% 27% 47%	2451
	11th to 20th	Day	6% 2 5% 12% 27% 49%	2445
	21st to end	Day	6% 2 4 10% 25% 53%	2415
August	1st to 10th	Day	8% 1 4 17% 30% 40%	2312
	11th to 20th	Day	6% 1 3 11% 31% 48%	2268
	21st to end	Day	5% 1 3 14% 34% 43%	2376
September	1st to 10th	Day	5% 1 4 11% 22% 56%	2160
	11th to 20th	Day	12% 4 8% 19% 33% 24%	2160
	21st to end	Day	13% 3 6% 19% 30% 30%	1901
October	1st to 10th	Day	16% 4 6% 20% 27% 27%	1800
	11th to 20th	Day	19% 4 11% 17% 27% 23%	1800
	21st to end	Day	24% 4 11% 19% 21% 21%	1980
November	1st to 10th	Day	33% 6% 8% 24% 17% 12%	1738
	11th to 20th	Day	33% 9% 12% 23% 14% 9%	1603
	21st to end	Day	31% 5% 10% 20% 22% 11%	1520
December	1st to 10th	Day	38% 6% 8% 17% 22% 9%	1520
	11th to 20th	Day	55% 5% 9% 14% 13% 4	1928
	21st to end	Day	47% 8% 11% 20% 11% 3	2794
January	1st to 10th	Night	43% 8% 15% 19% 10% 5%	2646
	11th to 20th	Night	41% 6% 10% 17% 15% 11%	2422
	21st to end	Night	40% 5% 8% 16% 23% 8%	2574
February	1st to 10th	Night	37% 5% 9% 22% 18% 9%	2340
	11th to 20th	Night	36% 5% 10% 21% 22% 6%	2264
	21st to end	Night	36% 5% 12% 21% 19% 7%	1776
March	1st to 10th	Night	50% 5% 10% 15% 14% 5%	2038
	11th to 20th	Night	40% 7% 11% 20% 16% 6%	1980
	21st to end	Night	41% 7% 12% 23% 14% 4	2142
April	1st to 10th	Night	39% 5% 15% 19% 16% 6%	1800
	11th to 20th	Night	29% 7% 14% 23% 18% 9%	1779
	21st to end	Night	22% 6% 15% 30% 21% 6%	1779
May	1st to 10th	Night	21% 5% 10% 24% 30% 10%	1620
	11th to 20th	Night	24% 7% 13% 24% 23% 10%	1421
	21st to end	Night	25% 7% 11% 31% 19% 7%	1552
June	1st to 10th	Night	25% 6% 11% 23% 26% 8%	1440
	11th to 20th	Night	12% 4 9% 21% 34% 21%	1440
	21st to end	Night	10% 3 5% 15% 37% 30%	1440
July	1st to 10th	Night	7% 3 4 15% 31% 40%	1400
	11th to 20th	Night	8% 3 4 10% 27% 47%	1392
	21st to end	Night	6% 2 3 13% 27% 50%	1661
August	1st to 10th	Night	9% 2 4 18% 34% 33%	1602
	11th to 20th	Night	6% 2 5% 15% 33% 40%	1630
	21st to end	Night	5% 1 3 18% 32% 41%	1980
September	1st to 10th	Night	5% 2 4 10% 25% 55%	1800
	11th to 20th	Night	11% 3 7% 21% 35% 23%	1800
	21st to end	Night	12% 4 5% 20% 25% 33%	2055
October	1st to 10th	Night	17% 3 6% 19% 28% 27%	2160
	11th to 20th	Night	18% 4 7% 22% 25% 24%	2160
	21st to end	Night	21% 4 9% 21% 23% 21%	2376
November	1st to 10th	Night	31% 7% 12% 22% 17% 11%	2341
	11th to 20th	Night	33% 7% 13% 22% 16% 9%	2574
	21st to end	Night	31% 5% 11% 20% 21% 12%	2660
December	1st to 10th	Night	37% 5% 10% 19% 21% 8%	2660
	11th to 20th	Night	52% 5% 8% 15% 17% 3	3276
	21st to end	Night	49% 7% 11% 18% 13% 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% 8% 25% 18% 8% 1	180
	11th to 20th	Day	30% 10% 22% 24% 12% 2	300
	21st to end	Day	25% 10% 25% 24% 13% 3	374
February	1st to 10th	Day	21% 14% 23% 27% 14% 1	340
	11th to 20th	Day	23% 14% 32% 25% 6% 1	339
	21st to end	Day	21% 14% 31% 27% 7%	346
March	1st to 10th	Day	30% 18% 30% 16% 5% 1	420
	11th to 20th	Day	27% 16% 27% 19% 10% 1	420
	21st to end	Day	24% 18% 30% 20% 8%	462
April	1st to 10th	Day	25% 18% 25% 25% 7%	420
	11th to 20th	Day	20% 19% 29% 21% 9% 2	412
	21st to end	Day	8% 20% 33% 24% 13% 2	420
May	1st to 10th	Day	10% 21% 31% 26% 10% 1	423
	11th to 20th	Day	12% 15% 30% 27% 13% 3	448
	21st to end	Day	9% 15% 26% 32% 14% 4	485
June	1st to 10th	Day	7% 16% 25% 33% 17% 2	450
	11th to 20th	Day	7% 18% 28% 33% 12% 2	450
	21st to end	Day	4 8% 27% 37% 21% 3	450
July	1st to 10th	Day	1 6% 31% 40% 18% 3	446
	11th to 20th	Day	1 4 25% 43% 25% 3	444
	21st to end	Day	3 20% 47% 24% 6%	487
August	1st to 10th	Day	2 2 28% 37% 25% 6%	447
	11th to 20th	Day	2 5% 21% 39% 27% 6%	437
	21st to end	Day	2 5% 26% 40% 22% 4	495
September	1st to 10th	Day	1 4 25% 42% 23% 4	450
	11th to 20th	Day	4 9% 23% 41% 21% 4	450
	21st to end	Day	4 10% 30% 38% 16% 3	450
October	1st to 10th	Day	5% 13% 29% 30% 19% 4	450
	11th to 20th	Day	8% 7% 28% 33% 20% 4	450
	21st to end	Day	15% 10% 25% 28% 18% 4	495
November	1st to 10th	Day	17% 10% 25% 26% 20% 2	334
	11th to 20th	Day	16% 5% 27% 29% 20% 3	219
	21st to end	Day	15% 10% 28% 32% 14% 1	180
December	1st to 10th	Day	27% 10% 25% 22% 14% 2	180
	11th to 20th	Day	26% 5% 32% 26% 8% 3	180
	21st to end	Day	20% 13% 29% 28% 10%	198
January	1st to 10th	Night	27% 4 24% 27% 15% 2	760
	11th to 20th	Night	26% 5% 26% 30% 11% 2	732
	21st to end	Night	25% 4 26% 30% 14% 2	792
February	1st to 10th	Night	21% 5% 25% 33% 14% 1	720
	11th to 20th	Night	22% 7% 26% 31% 12% 2	704
	21st to end	Night	20% 9% 28% 35% 7% 1	592
March	1st to 10th	Night	29% 9% 24% 24% 12% 2	720
	11th to 20th	Night	24% 6% 21% 32% 17% 1	720
	21st to end	Night	23% 10% 26% 31% 9%	792
April	1st to 10th	Night	19% 10% 29% 27% 13% 2	720
	11th to 20th	Night	13% 12% 30% 31% 13% 2	711
	21st to end	Night	10% 9% 31% 32% 18% 1	699
May	1st to 10th	Night	11% 10% 31% 31% 17% 1	540
	11th to 20th	Night	11% 6% 23% 37% 22% 1	359
	21st to end	Night	10% 9% 25% 35% 20% 1	389
June	1st to 10th	Night	8% 6% 26% 40% 19% 1	360
	11th to 20th	Night	5% 8% 28% 41% 18%	360
	21st to end	Night	4 2 19% 52% 23%	360
July	1st to 10th	Night	1 2 13% 46% 37% 1	350
	11th to 20th	Night	1 1 13% 44% 40% 1	348
	21st to end	Night	1 1 6% 44% 47% 2	577
August	1st to 10th	Night	3 1 9% 39% 45% 4	537
	11th to 20th	Night	1 1 12% 42% 40% 3	562
	21st to end	Night	2 1 12% 42% 41% 3	792
September	1st to 10th	Night	1 2 18% 42% 35% 2	720
	11th to 20th	Night	4 3 14% 38% 38% 4	720
	21st to end	Night	4 5% 21% 35% 32% 3	720
October	1st to 10th	Night	6% 8% 21% 39% 23% 3	720
	11th to 20th	Night	9% 3 22% 37% 25% 3	720
	21st to end	Night	13% 6% 24% 33% 21% 3	792
November	1st to 10th	Night	14% 6% 20% 37% 19% 3	740
	11th to 20th	Night	16% 4 28% 37% 15% 1	760
	21st to end	Night	15% 6% 28% 33% 16% 1	760
December	1st to 10th	Night	26% 4 24% 33% 11% 2	760
	11th to 20th	Night	30% 4 26% 27% 12% 2	760
	21st to end	Night	26% 5% 31% 30% 8% 1	836

Page generated at 2024-01-10 16:35:22. Version ac07

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