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 2024-11-11.

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	11% 3 1 3 2 2 2 2 4 5% 64%	105337
	Night	14% 4 1 3 2 2 3 2 4 5% 60%	102565
Transparency	Day	24% 4 8% 17% 23% 24%	105091
	Night	28% 5% 9% 19% 22% 18%	102400
Seeing	Day	13% 11% 27% 31% 16% 3	15095
	Night	14% 6% 23% 35% 20% 2	24231

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	25% 7% 3 5% 3 3 3 3 4 6% 37%	2539
	11th to 20th	Day	35% 8% 2 5% 3 3 2 2 2 4 35%	2773
	21st to end	Day	33% 7% 2 3 2 3 3 2 4 6% 35%	3146
February	1st to 10th	Day	20% 6% 1 4 3 3 3 3 6% 6% 45%	1710
	11th to 20th	Day	20% 4 1 3 3 2 3 3 4 6% 50%	1674
	21st to end	Day	19% 6% 2 3 3 3 4 2 5% 7% 46%	1570
March	1st to 10th	Day	29% 13% 2 5% 4 3 3 3 3 5% 30%	3348
	11th to 20th	Day	15% 5% 1 3 2 2 2 2 4 6% 58%	3490
	21st to end	Day	25% 7% 2 5% 3 3 3 3 5% 6% 37%	3886
April	1st to 10th	Day	18% 6% 2 4 3 3 3 3 6% 8% 46%	2280
	11th to 20th	Day	15% 4 1 4 3 3 3 2 5% 5% 55%	2249
	21st to end	Day	6% 4 1 3 3 3 3 4 4 7% 62%	2299
May	1st to 10th	Day	7% 3 1 3 3 2 3 2 4 5% 67%	4110
	11th to 20th	Day	4 2 1 2 2 1 2 2 3 5% 76%	4372
	21st to end	Day	5% 2 1 2 1 2 2 2 4 7% 72%	4813
June	1st to 10th	Day	7% 3 1 3 3 3 4 3 6% 6% 62%	2660
	11th to 20th	Day	3 1 1 2 2 2 2 4 5% 78%	2660
	21st to end	Day	4 2 1 1 1 1 1 2 3 3 81%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 91%	4420
	11th to 20th	Day	2 2 2 1 1 1 2 2 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 2 2 90%	4466
August	1st to 10th	Day	2 2 1 2 1 1 2 2 3 5% 79%	4091
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	3970
	21st to end	Day	2 1 1 1 1 1 1 1 2 4 85%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 4 88%	2280
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 6% 74%	2280
	21st to end	Day	3 2 2 2 1 2 2 2 6% 78%	2006
October	1st to 10th	Day	3 1 2 1 2 2 2 3 4 79%	3180
	11th to 20th	Day	6% 2 1 2 1 2 3 3 4 7% 69%	3161
	21st to end	Day	11% 4 1 5% 4 3 4 3 4 7% 54%	3448
November	1st to 10th	Day	15% 5% 1 4 3 2 4 3 5% 5% 54%	1831
	11th to 20th	Day	15% 4 1 4 3 3 3 4 5% 9% 49%	1603
	21st to end	Day	13% 4 1 3 3 3 3 3 4 7% 55%	1520
December	1st to 10th	Day	24% 4 1 3 2 2 2 3 4 5% 49%	1520
	11th to 20th	Day	34% 5% 2 5% 3 3 3 3 4 6% 31%	1928
	21st to end	Day	28% 6% 1 5% 3 3 3 3 4 6% 38%	2794
January	1st to 10th	Night	25% 4 1 4 3 4 3 4 5% 7% 38%	4406
	11th to 20th	Night	33% 6% 2 4 3 3 3 3 4 5% 35%	4189
	21st to end	Night	31% 8% 2 6% 4 3 4 3 4 4 31%	4521
February	1st to 10th	Night	22% 4 1 4 3 3 3 3 5% 6% 47%	2470
	11th to 20th	Night	22% 5% 1 3 2 3 3 3 3 6% 48%	2392
	21st to end	Night	18% 5% 1 4 3 4 3 3 5% 6% 48%	1884
March	1st to 10th	Night	27% 9% 2 6% 4 3 3 2 3 5% 36%	3524
	11th to 20th	Night	18% 5% 1 3 2 2 2 2 4 5% 57%	3490
	21st to end	Night	19% 7% 2 5% 5% 4 4 4 5% 6% 38%	3767
April	1st to 10th	Night	18% 6% 2 4 4 2 3 2 5% 6% 48%	1900
	11th to 20th	Night	12% 4 1 4 3 3 3 3 6% 6% 55%	1879
	21st to end	Night	9% 4 1 3 2 2 3 2 5% 7% 63%	1877
May	1st to 10th	Night	8% 3 1 4 2 2 2 2 2 5% 71%	2860
	11th to 20th	Night	5% 2 1 2 1 2 2 2 4 4 76%	2521
	21st to end	Night	5% 2 1 2 2 2 2 2 4 5% 72%	2762
June	1st to 10th	Night	6% 4 2 4 4 3 4 4 6% 7% 57%	1520
	11th to 20th	Night	3 3 1 1 1 2 2 2 3 4 78%	1520
	21st to end	Night	3 1 2 2 1 2 1 3 3 82%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 2 2 91%	2540
	11th to 20th	Night	2 2 1 1 2 2 2 1 2 3 83%	2540
	21st to end	Night	1 1 1 1 1 2 3 91%	3095
August	1st to 10th	Night	3 2 1 2 1 2 2 2 3 4 79%	2848
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 89%	2868
	21st to end	Night	6% 1 1 1 1 1 1 1 3 85%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 4 88%	1900
	11th to 20th	Night	4 1 1 2 2 2 2 3 5% 77%	1900
	21st to end	Night	3 2 1 2 1 2 2 2 3 6% 77%	2170
October	1st to 10th	Night	3 1 1 2 2 2 3 2 2 3 80%	3800
	11th to 20th	Night	6% 2 1 2 2 2 3 2 3 5% 72%	3777
	21st to end	Night	9% 2 1 2 2 2 3 3 6% 9% 62%	4121
November	1st to 10th	Night	14% 4 1 4 3 2 3 3 5% 7% 53%	2468
	11th to 20th	Night	15% 4 2 4 3 3 4 3 6% 9% 49%	2580
	21st to end	Night	15% 4 2 3 3 2 3 2 5% 6% 55%	2660
December	1st to 10th	Night	25% 3 1 3 2 2 2 2 4 7% 48%	2660
	11th to 20th	Night	35% 5% 1 3 3 3 3 2 5% 5% 34%	3276
	21st to end	Night	28% 6% 2 4 3 3 3 3 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	45% 9% 14% 24% 7% 2	2539
	11th to 20th	Day	56% 4 9% 15% 9% 7%	2773
	21st to end	Day	51% 6% 14% 12% 12% 5%	3146
February	1st to 10th	Day	39% 7% 11% 18% 17% 8%	1710
	11th to 20th	Day	36% 5% 9% 21% 23% 5%	1683
	21st to end	Day	38% 6% 13% 21% 17% 6%	1570
March	1st to 10th	Day	56% 8% 8% 10% 15% 3	3348
	11th to 20th	Day	29% 5% 10% 23% 27% 6%	3490
	21st to end	Day	46% 9% 13% 20% 10% 3	3886
April	1st to 10th	Day	36% 6% 14% 20% 18% 5%	2280
	11th to 20th	Day	30% 5% 14% 23% 18% 10%	2252
	21st to end	Day	21% 7% 12% 26% 25% 8%	2299
May	1st to 10th	Day	17% 7% 11% 22% 28% 16%	4110
	11th to 20th	Day	13% 3 6% 15% 32% 31%	4375
	21st to end	Day	13% 4 13% 29% 31% 11%	4814
June	1st to 10th	Day	21% 6% 11% 22% 25% 15%	2660
	11th to 20th	Day	9% 4 9% 18% 33% 26%	2660
	21st to end	Day	11% 2 5% 14% 29% 40%	2660
July	1st to 10th	Day	4 2 2 7% 15% 70%	4351
	11th to 20th	Day	9% 2 3 7% 21% 59%	4345
	21st to end	Day	4 1 2 11% 30% 52%	4325
August	1st to 10th	Day	11% 1 2 10% 22% 52%	4076
	11th to 20th	Day	6% 1 2 17% 38% 36%	4009
	21st to end	Day	8% 1 4 17% 35% 35%	4179
September	1st to 10th	Day	5% 1 4 11% 23% 57%	2280
	11th to 20th	Day	13% 3 8% 19% 33% 24%	2280
	21st to end	Day	12% 3 5% 18% 31% 31%	2006
October	1st to 10th	Day	11% 3 4 15% 22% 45%	3180
	11th to 20th	Day	18% 2 8% 23% 32% 16%	3161
	21st to end	Day	32% 2 8% 20% 24% 14%	3448
November	1st to 10th	Day	32% 6% 9% 24% 17% 12%	1831
	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% 9% 18% 20% 8% 3	4406
	11th to 20th	Night	52% 7% 9% 14% 12% 7%	4189
	21st to end	Night	55% 7% 9% 11% 13% 4	4521
February	1st to 10th	Night	38% 6% 9% 21% 18% 8%	2470
	11th to 20th	Night	38% 5% 9% 20% 21% 6%	2394
	21st to end	Night	36% 6% 12% 21% 19% 6%	1884
March	1st to 10th	Night	48% 10% 12% 15% 12% 3	3524
	11th to 20th	Night	31% 5% 9% 26% 22% 8%	3490
	21st to end	Night	40% 11% 15% 22% 9% 2	3767
April	1st to 10th	Night	38% 6% 16% 20% 16% 5%	1900
	11th to 20th	Night	29% 7% 14% 24% 18% 9%	1879
	21st to end	Night	22% 6% 15% 30% 21% 6%	1877
May	1st to 10th	Night	19% 5% 10% 23% 30% 14%	2860
	11th to 20th	Night	14% 4 7% 15% 37% 23%	2521
	21st to end	Night	14% 6% 13% 34% 26% 7%	2762
June	1st to 10th	Night	24% 7% 11% 24% 26% 8%	1520
	11th to 20th	Night	11% 4 8% 20% 36% 21%	1520
	21st to end	Night	10% 3 4 14% 37% 31%	1520
July	1st to 10th	Night	4 2 2 9% 17% 67%	2500
	11th to 20th	Night	9% 2 2 6% 20% 60%	2492
	21st to end	Night	4 1 2 11% 32% 50%	2998
August	1st to 10th	Night	14% 2 2 10% 26% 46%	2839
	11th to 20th	Night	5% 1 3 17% 42% 33%	2895
	21st to end	Night	10% 1 2 17% 37% 33%	3498
September	1st to 10th	Night	5% 2 3 9% 25% 56%	1900
	11th to 20th	Night	12% 3 6% 21% 35% 22%	1900
	21st to end	Night	12% 3 5% 20% 26% 33%	2170
October	1st to 10th	Night	12% 2 4 12% 22% 48%	3800
	11th to 20th	Night	17% 2 8% 30% 29% 14%	3777
	21st to end	Night	21% 2 7% 28% 27% 15%	4121
November	1st to 10th	Night	30% 6% 12% 23% 17% 11%	2468
	11th to 20th	Night	33% 7% 13% 22% 16% 9%	2580
	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	31% 10% 22% 24% 12% 2	322
	21st to end	Day	26% 10% 25% 24% 12% 3	407
February	1st to 10th	Day	23% 13% 24% 26% 13% 1	370
	11th to 20th	Day	25% 14% 30% 25% 6% 1	369
	21st to end	Day	20% 14% 30% 28% 8%	382
March	1st to 10th	Day	32% 19% 28% 15% 5% 1	460
	11th to 20th	Day	26% 16% 26% 19% 11% 1	460
	21st to end	Day	25% 18% 30% 19% 8%	506
April	1st to 10th	Day	23% 17% 26% 25% 7% 1	460
	11th to 20th	Day	20% 18% 27% 23% 10% 2	452
	21st to end	Day	8% 20% 33% 25% 13% 2	460
May	1st to 10th	Day	10% 23% 32% 24% 10% 1	463
	11th to 20th	Day	11% 14% 30% 29% 14% 3	488
	21st to end	Day	9% 15% 28% 31% 14% 4	529
June	1st to 10th	Day	6% 16% 24% 34% 17% 2	490
	11th to 20th	Day	6% 18% 29% 32% 12% 2	490
	21st to end	Day	4 8% 28% 37% 20% 3	490
July	1st to 10th	Day	1 6% 31% 38% 20% 4	486
	11th to 20th	Day	1 4 25% 42% 24% 3	484
	21st to end	Day	3 21% 46% 23% 7%	531
August	1st to 10th	Day	2 2 27% 38% 24% 7%	487
	11th to 20th	Day	2 5% 23% 39% 26% 6%	477
	21st to end	Day	3 5% 26% 40% 22% 5%	539
September	1st to 10th	Day	1 4 25% 43% 23% 3	490
	11th to 20th	Day	5% 8% 23% 40% 20% 3	490
	21st to end	Day	3 9% 29% 39% 16% 3	490
October	1st to 10th	Day	5% 12% 27% 31% 20% 4	490
	11th to 20th	Day	9% 8% 28% 33% 19% 3	490
	21st to end	Day	15% 11% 27% 28% 16% 3	539
November	1st to 10th	Day	17% 10% 26% 26% 19% 2	367
	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	760
	21st to end	Night	26% 4 26% 29% 14% 2	836
February	1st to 10th	Night	22% 6% 26% 32% 14% 1	760
	11th to 20th	Night	23% 8% 25% 31% 11% 2	744
	21st to end	Night	20% 9% 29% 35% 7% 1	628
March	1st to 10th	Night	30% 9% 24% 24% 12% 1	760
	11th to 20th	Night	24% 7% 21% 31% 17% 1	760
	21st to end	Night	22% 11% 27% 31% 9%	836
April	1st to 10th	Night	18% 11% 29% 28% 13% 1	760
	11th to 20th	Night	13% 11% 30% 31% 13% 1	751
	21st to end	Night	9% 9% 31% 31% 18% 1	737
May	1st to 10th	Night	11% 11% 31% 30% 16% 1	570
	11th to 20th	Night	11% 6% 23% 39% 21% 1	379
	21st to end	Night	9% 9% 26% 35% 20%	411
June	1st to 10th	Night	8% 7% 26% 40% 18% 1	380
	11th to 20th	Night	5% 9% 28% 41% 17%	380
	21st to end	Night	3 2 19% 52% 22%	380
July	1st to 10th	Night	1 2 15% 46% 36% 1	370
	11th to 20th	Night	1 1 14% 44% 39% 1	368
	21st to end	Night	1 1 6% 45% 47% 2	610
August	1st to 10th	Night	3 1 9% 41% 42% 4	567
	11th to 20th	Night	1 1 13% 44% 38% 3	594
	21st to end	Night	2 1 12% 41% 41% 3	836
September	1st to 10th	Night	1 2 18% 44% 34% 2	760
	11th to 20th	Night	4 3 15% 38% 37% 3	760
	21st to end	Night	4 5% 22% 36% 31% 3	760
October	1st to 10th	Night	6% 8% 21% 41% 23% 2	760
	11th to 20th	Night	9% 4 22% 38% 25% 3	760
	21st to end	Night	12% 6% 25% 33% 20% 3	836
November	1st to 10th	Night	14% 6% 21% 36% 19% 3	780
	11th to 20th	Night	15% 4 28% 37% 15% 1	762
	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 2024-11-08 03:44:45. Version ac07

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