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 2025-12-18.

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% 65%	136084
	Night	14% 4 1 3 2 2 3 2 4 5% 60%	132355
Transparency	Day	24% 4 7% 19% 24% 22%	130951
	Night	28% 4 8% 19% 23% 17%	128478
Seeing	Day	13% 11% 27% 31% 16% 3	16466
	Night	15% 6% 23% 35% 20% 2	25654

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	24% 7% 3 5% 3 3 3 3 5% 6% 38%	3639
	11th to 20th	Day	24% 5% 1 3 2 2 2 1 1 3 55%	3985
	21st to end	Day	25% 5% 1 2 1 2 2 2 3 5% 51%	4510
February	1st to 10th	Day	22% 6% 1 4 3 3 3 3 5% 6% 44%	1800
	11th to 20th	Day	21% 4 1 3 3 2 3 3 4 6% 49%	1765
	21st to end	Day	19% 6% 2 3 3 3 4 2 5% 7% 47%	1650
March	1st to 10th	Day	23% 10% 2 5% 4 3 3 2 3 5% 39%	4819
	11th to 20th	Day	22% 7% 1 4 3 3 2 2 4 6% 47%	5000
	21st to end	Day	23% 7% 2 5% 3 3 3 3 5% 7% 38%	5573
April	1st to 10th	Day	18% 6% 2 4 3 3 3 3 6% 8% 46%	2400
	11th to 20th	Day	14% 4 1 4 3 3 3 2 5% 5% 56%	2369
	21st to end	Day	6% 4 1 4 3 3 3 4 4 7% 62%	2420
May	1st to 10th	Day	5% 3 1 3 2 2 2 3 4 6% 69%	5880
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 6% 73%	6272
	21st to end	Day	4 1 1 1 1 1 2 2 4 6% 78%	6902
June	1st to 10th	Day	7% 3 1 3 3 3 4 3 6% 6% 63%	2800
	11th to 20th	Day	3 1 1 1 2 2 2 4 5% 78%	2800
	21st to end	Day	4 2 1 1 1 1 1 2 3 3 81%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 2 90%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6320
	21st to end	Day	1 1 1 1 1 2 2 4 5% 83%	6413
August	1st to 10th	Day	2 1 2 1 1 2 2 3 4 82%	5777
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	5714
	21st to end	Day	1 1 1 1 1 1 1 2 4 84%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 3 4 86%	2400
	11th to 20th	Day	4 2 1 2 2 2 2 2 4 6% 74%	2400
	21st to end	Day	4 2 2 2 1 2 2 2 6% 77%	2111
October	1st to 10th	Day	4 1 1 2 1 2 2 2 3 4 79%	3280
	11th to 20th	Day	6% 2 1 2 1 2 3 3 4 7% 68%	3261
	21st to end	Day	12% 4 1 5% 3 3 4 3 4 7% 54%	3558
November	1st to 10th	Day	15% 5% 1 4 3 2 4 3 5% 6% 53%	1924
	11th to 20th	Day	18% 4 1 4 3 3 3 4 5% 8% 47%	1771
	21st to end	Day	14% 4 1 3 3 3 3 3 4 7% 56%	1680
December	1st to 10th	Day	14% 3 1 3 3 3 3 3 4 6% 57%	2700
	11th to 20th	Day	34% 5% 2 5% 3 4 4 3 5% 7% 28%	3084
	21st to end	Day	35% 5% 1 4 3 3 3 3 4 5% 33%	4004
January	1st to 10th	Night	23% 4 2 5% 4 4 4 4 5% 6% 39%	6306
	11th to 20th	Night	23% 4 1 3 2 2 2 2 3 4 54%	5985
	21st to end	Night	22% 6% 2 4 3 3 3 2 3 4 47%	6468
February	1st to 10th	Night	23% 4 1 4 3 2 3 3 5% 5% 46%	2600
	11th to 20th	Night	22% 5% 1 3 2 3 3 3 3 6% 47%	2521
	21st to end	Night	18% 5% 1 4 3 4 3 3 5% 6% 48%	1980
March	1st to 10th	Night	21% 7% 2 6% 4 3 3 2 3 6% 41%	5073
	11th to 20th	Night	21% 7% 2 4 2 2 2 2 4 5% 48%	5000
	21st to end	Night	20% 8% 2 6% 5% 4 4 3 5% 6% 38%	5402
April	1st to 10th	Night	17% 6% 2 4 4 3 3 2 5% 7% 48%	2000
	11th to 20th	Night	11% 4 1 4 3 3 3 3 6% 6% 56%	1979
	21st to end	Night	9% 4 1 3 2 2 2 2 5% 7% 62%	1976
May	1st to 10th	Night	7% 3 1 3 1 1 2 2 3 6% 72%	4100
	11th to 20th	Night	4 2 1 3 2 2 3 2 5% 5% 71%	3621
	21st to end	Night	3 2 1 2 1 2 2 2 4 5% 77%	3972
June	1st to 10th	Night	6% 4 1 3 4 3 4 4 6% 7% 59%	1600
	11th to 20th	Night	3 3 1 1 1 2 2 2 3 5% 79%	1600
	21st to end	Night	3 1 2 2 1 2 1 3 3 82%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 2 2 90%	3640
	11th to 20th	Night	1 1 1 1 1 2 1 2 2 88%	3640
	21st to end	Night	1 1 1 2 2 3 5% 83%	4444
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 3 82%	4035
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 87%	4136
	21st to end	Night	4 1 1 1 1 1 1 2 4 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 2 4 86%	2000
	11th to 20th	Night	4 2 2 2 2 2 2 3 5% 76%	2000
	21st to end	Night	4 2 1 2 1 2 2 2 3 5% 77%	2285
October	1st to 10th	Night	4 2 1 2 2 2 2 2 2 3 79%	3920
	11th to 20th	Night	6% 2 1 2 2 2 3 2 3 5% 72%	3897
	21st to end	Night	9% 2 1 2 2 2 3 2 6% 9% 62%	4253
November	1st to 10th	Night	15% 4 1 4 3 2 3 3 5% 7% 52%	2595
	11th to 20th	Night	17% 4 2 4 3 3 4 3 5% 8% 48%	2846
	21st to end	Night	15% 4 1 3 3 2 3 2 5% 6% 55%	2940
December	1st to 10th	Night	15% 2 1 2 2 2 3 2 4 7% 60%	4700
	11th to 20th	Night	35% 6% 1 4 3 4 4 3 6% 6% 28%	5274
	21st to end	Night	38% 5% 1 3 3 2 3 3 3 4 35%	6952

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% 8% 12% 20% 13% 2	3639
	11th to 20th	Day	39% 3 6% 20% 22% 10%	3985
	21st to end	Day	38% 4 10% 11% 21% 16%	4510
February	1st to 10th	Day	40% 7% 11% 18% 17% 9%	1800
	11th to 20th	Day	37% 5% 9% 21% 22% 5%	1774
	21st to end	Day	37% 6% 12% 21% 17% 7%	1650
March	1st to 10th	Day	48% 6% 8% 17% 18% 4	4819
	11th to 20th	Day	41% 4 12% 19% 20% 4	5000
	21st to end	Day	45% 9% 12% 23% 9% 2	5573
April	1st to 10th	Day	36% 6% 14% 20% 19% 5%	2400
	11th to 20th	Day	29% 5% 13% 22% 19% 11%	2372
	21st to end	Day	22% 7% 12% 26% 25% 8%	2420
May	1st to 10th	Day	16% 5% 8% 23% 29% 19%	5880
	11th to 20th	Day	15% 3 5% 23% 32% 23%	6275
	21st to end	Day	10% 3 9% 30% 38% 11%	6904
June	1st to 10th	Day	20% 6% 10% 23% 26% 15%	2800
	11th to 20th	Day	10% 4 9% 19% 33% 25%	2800
	21st to end	Day	10% 2 4 15% 30% 39%	2800
July	1st to 10th	Day	4 1 3 10% 23% 59%	6251
	11th to 20th	Day	6% 1 2 6% 22% 63%	6245
	21st to end	Day	7% 1 5% 22% 28% 37%	6227
August	1st to 10th	Day	11% 1 2 11% 23% 52%	4206
	11th to 20th	Day	6% 1 2 17% 38% 36%	4137
	21st to end	Day	8% 1 4 16% 36% 35%	4311
September	1st to 10th	Day	6% 1 4 12% 23% 54%	2400
	11th to 20th	Day	13% 3 8% 20% 32% 23%	2400
	21st to end	Day	13% 3 5% 17% 31% 31%	2111
October	1st to 10th	Day	12% 3 4 15% 22% 44%	3280
	11th to 20th	Day	18% 2 8% 24% 32% 15%	3261
	21st to end	Day	32% 2 8% 20% 24% 13%	3558
November	1st to 10th	Day	32% 5% 9% 25% 17% 11%	1924
	11th to 20th	Day	35% 8% 12% 22% 14% 9%	1771
	21st to end	Day	31% 5% 9% 21% 22% 11%	1680
December	1st to 10th	Day	31% 3 5% 17% 32% 11%	2700
	11th to 20th	Day	58% 4 12% 14% 9% 3	3084
	21st to end	Day	54% 6% 9% 18% 11% 2	4004
January	1st to 10th	Night	44% 8% 14% 18% 15% 2	6306
	11th to 20th	Night	37% 5% 6% 18% 24% 10%	5985
	21st to end	Night	42% 5% 7% 9% 18% 19%	6468
February	1st to 10th	Night	39% 5% 9% 20% 18% 8%	2600
	11th to 20th	Night	39% 4 10% 20% 21% 6%	2523
	21st to end	Night	35% 5% 12% 21% 20% 7%	1980
March	1st to 10th	Night	44% 7% 10% 19% 14% 5%	5073
	11th to 20th	Night	40% 3 11% 23% 18% 5%	5000
	21st to end	Night	44% 9% 14% 23% 9% 2	5402
April	1st to 10th	Night	37% 5% 15% 21% 16% 5%	2000
	11th to 20th	Night	28% 7% 14% 23% 18% 10%	1979
	21st to end	Night	22% 6% 15% 30% 21% 6%	1976
May	1st to 10th	Night	17% 4 7% 23% 33% 16%	4097
	11th to 20th	Night	17% 3 6% 23% 33% 18%	3621
	21st to end	Night	12% 4 10% 34% 35% 6%	3972
June	1st to 10th	Night	23% 6% 11% 25% 27% 9%	1600
	11th to 20th	Night	11% 3 8% 21% 36% 21%	1600
	21st to end	Night	10% 3 4 15% 38% 31%	1600
July	1st to 10th	Night	5% 1 2 12% 27% 53%	3600
	11th to 20th	Night	6% 2 2 5% 23% 63%	3592
	21st to end	Night	7% 1 7% 22% 28% 35%	4318
August	1st to 10th	Night	13% 2 2 11% 26% 45%	2929
	11th to 20th	Night	5% 1 3 17% 42% 33%	2987
	21st to end	Night	10% 1 2 17% 38% 33%	3608
September	1st to 10th	Night	7% 1 3 11% 24% 54%	2000
	11th to 20th	Night	13% 3 7% 21% 35% 22%	2000
	21st to end	Night	13% 3 5% 19% 26% 34%	2285
October	1st to 10th	Night	13% 2 4 12% 22% 47%	3920
	11th to 20th	Night	17% 2 7% 31% 28% 14%	3897
	21st to end	Night	21% 2 8% 28% 27% 14%	4253
November	1st to 10th	Night	31% 6% 12% 24% 16% 10%	2595
	11th to 20th	Night	36% 6% 13% 22% 15% 8%	2846
	21st to end	Night	31% 5% 10% 21% 22% 11%	2940
December	1st to 10th	Night	26% 3 7% 21% 31% 13%	4700
	11th to 20th	Night	57% 3 8% 17% 12% 3	5274
	21st to end	Night	55% 5% 10% 16% 12% 2	6952

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% 9% 26% 17% 7% 1	200
	11th to 20th	Day	29% 9% 23% 24% 12% 3	350
	21st to end	Day	25% 10% 24% 25% 12% 3	440
February	1st to 10th	Day	25% 13% 25% 25% 12% 1	400
	11th to 20th	Day	25% 14% 31% 24% 5% 1	400
	21st to end	Day	19% 14% 30% 30% 7%	414
March	1st to 10th	Day	30% 18% 28% 17% 6% 1	500
	11th to 20th	Day	28% 16% 26% 19% 10% 1	500
	21st to end	Day	25% 17% 30% 20% 8%	550
April	1st to 10th	Day	23% 17% 26% 26% 7% 1	500
	11th to 20th	Day	18% 18% 29% 23% 11% 2	492
	21st to end	Day	8% 19% 33% 25% 13% 2	500
May	1st to 10th	Day	10% 22% 32% 24% 11% 2	503
	11th to 20th	Day	11% 15% 30% 28% 13% 2	528
	21st to end	Day	8% 14% 28% 32% 15% 3	573
June	1st to 10th	Day	6% 15% 25% 35% 17% 2	530
	11th to 20th	Day	6% 18% 29% 32% 13% 2	530
	21st to end	Day	4 8% 28% 36% 21% 4	530
July	1st to 10th	Day	1 6% 31% 38% 20% 4	526
	11th to 20th	Day	1 4 25% 42% 25% 4	524
	21st to end	Day	3 21% 45% 24% 7%	575
August	1st to 10th	Day	2 2 26% 39% 24% 7%	527
	11th to 20th	Day	2 5% 22% 38% 26% 7%	517
	21st to end	Day	3 4 24% 40% 23% 5%	583
September	1st to 10th	Day	2 5% 25% 41% 23% 4	530
	11th to 20th	Day	5% 8% 23% 40% 20% 4	530
	21st to end	Day	5% 9% 28% 39% 16% 3	530
October	1st to 10th	Day	6% 12% 28% 31% 19% 4	530
	11th to 20th	Day	8% 8% 29% 32% 19% 3	530
	21st to end	Day	15% 10% 26% 28% 17% 4	583
November	1st to 10th	Day	17% 9% 26% 27% 20% 2	400
	11th to 20th	Day	22% 5% 28% 25% 18% 3	267
	21st to end	Day	16% 9% 27% 31% 16% 1	220
December	1st to 10th	Day	24% 9% 23% 25% 16% 4	220
	11th to 20th	Day	26% 5% 30% 25% 11% 4	214
	21st to end	Day	25% 13% 28% 26% 9%	220
January	1st to 10th	Night	28% 5% 25% 26% 14% 2	800
	11th to 20th	Night	25% 6% 26% 30% 11% 2	800
	21st to end	Night	26% 4 25% 29% 14% 2	880
February	1st to 10th	Night	24% 6% 26% 31% 13% 1	800
	11th to 20th	Night	23% 9% 26% 30% 11% 2	784
	21st to end	Night	20% 9% 30% 34% 7% 1	660
March	1st to 10th	Night	28% 9% 25% 24% 12% 1	800
	11th to 20th	Night	25% 7% 21% 31% 16% 1	800
	21st to end	Night	23% 10% 27% 31% 9%	880
April	1st to 10th	Night	18% 10% 30% 28% 12% 1	800
	11th to 20th	Night	13% 11% 31% 32% 12% 1	791
	21st to end	Night	10% 9% 31% 31% 17% 1	776
May	1st to 10th	Night	10% 11% 32% 31% 16% 1	600
	11th to 20th	Night	11% 6% 25% 37% 20% 1	399
	21st to end	Night	9% 9% 27% 35% 20%	433
June	1st to 10th	Night	7% 7% 26% 41% 19% 1	400
	11th to 20th	Night	5% 9% 28% 42% 16%	400
	21st to end	Night	4 3 19% 53% 22%	400
July	1st to 10th	Night	1 2 16% 47% 35% 1	390
	11th to 20th	Night	1 1 13% 45% 38% 1	388
	21st to end	Night	1 1 5% 44% 48% 2	642
August	1st to 10th	Night	3 1 9% 41% 42% 3	597
	11th to 20th	Night	1 1 13% 43% 39% 3	626
	21st to end	Night	2 1 12% 42% 40% 2	880
September	1st to 10th	Night	1 2 17% 45% 34% 2	800
	11th to 20th	Night	4 3 15% 37% 37% 3	800
	21st to end	Night	4 5% 22% 36% 30% 3	800
October	1st to 10th	Night	7% 8% 21% 40% 22% 2	800
	11th to 20th	Night	9% 4 23% 37% 25% 3	800
	21st to end	Night	13% 6% 25% 34% 19% 3	880
November	1st to 10th	Night	15% 6% 22% 35% 19% 3	820
	11th to 20th	Night	18% 5% 28% 36% 14%	840
	21st to end	Night	16% 6% 27% 34% 15% 1	840
December	1st to 10th	Night	24% 5% 25% 33% 11% 2	840
	11th to 20th	Night	29% 4 25% 28% 12% 1	828
	21st to end	Night	27% 5% 31% 28% 8% 1	880

Page generated at 2025-12-14 22:13:52. Version ac07

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