Mount Shasta
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Shasta clear sky chart from 2005-11-06 to 2026-08-30.

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	7% 7% 2 4 3 3 3 3 5% 6% 57%	153093
	Night	9% 8% 2 4 3 3 3 3 5% 6% 54%	146549
Transparency	Day	26% 6% 10% 20% 22% 16%	147947
	Night	29% 7% 11% 20% 20% 12%	142690
Seeing	Day	16% 18% 29% 24% 10% 2	17317
	Night	15% 10% 25% 32% 16% 2	26385

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	16% 20% 3 6% 4 4 5% 3 5% 7% 28%	4722
	11th to 20th	Day	11% 11% 2 3 2 2 2 2 3 4 59%	5077
	21st to end	Day	12% 10% 2 5% 3 4 4 4 5% 6% 45%	5843
February	1st to 10th	Day	17% 11% 3 4 4 3 4 4 6% 7% 38%	1890
	11th to 20th	Day	11% 16% 4 6% 4 4 3 4 6% 6% 37%	1850
	21st to end	Day	12% 14% 4 6% 4 4 5% 3 6% 6% 37%	1730
March	1st to 10th	Day	10% 23% 7% 8% 3 2 2 3 4 4 33%	4886
	11th to 20th	Day	11% 17% 3 6% 3 3 4 3 4 5% 41%	5110
	21st to end	Day	13% 22% 4 7% 4 3 4 3 5% 7% 28%	5728
April	1st to 10th	Day	14% 13% 2 7% 5% 4 3 3 5% 8% 36%	2520
	11th to 20th	Day	12% 10% 2 5% 4 4 3 4 6% 8% 42%	2489
	21st to end	Day	6% 8% 2 6% 4 3 4 4 6% 9% 47%	2555
May	1st to 10th	Day	3 4 1 4 3 3 4 3 5% 8% 62%	7717
	11th to 20th	Day	4 3 1 3 2 2 3 3 5% 7% 66%	8160
	21st to end	Day	5% 3 1 3 3 3 4 3 6% 9% 60%	8992
June	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 7% 51%	2940
	11th to 20th	Day	4 3 2 4 3 3 3 3 5% 5% 64%	2940
	21st to end	Day	4 2 1 2 2 2 3 3 4 4 74%	2940
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 82%	8219
	11th to 20th	Day	3 2 1 3 2 2 2 3 4 4 74%	8220
	21st to end	Day	1 1 1 2 2 2 3 2 4 5% 77%	8457
August	1st to 10th	Day	2 1 1 2 2 2 3 3 5% 7% 72%	5907
	11th to 20th	Day	1 1 1 2 1 2 2 2 4 6% 77%	5864
	21st to end	Day	2 2 1 3 2 2 2 2 3 5% 76%	6093
September	1st to 10th	Day	1 1 1 2 2 2 2 2 3 5% 79%	2400
	11th to 20th	Day	4 3 1 3 3 3 3 4 5% 6% 66%	2400
	21st to end	Day	4 3 1 3 2 2 2 3 4 6% 70%	2114
October	1st to 10th	Day	5% 2 1 2 2 2 2 2 4 5% 73%	3280
	11th to 20th	Day	7% 3 1 4 3 3 3 3 6% 7% 59%	3261
	21st to end	Day	9% 7% 2 5% 4 4 5% 3 7% 8% 46%	3558
November	1st to 10th	Day	15% 8% 2 4 4 4 3 4 5% 8% 42%	1898
	11th to 20th	Day	16% 9% 2 6% 4 4 4 4 7% 9% 36%	1753
	21st to end	Day	11% 8% 2 5% 4 4 4 3 4 9% 46%	1680
December	1st to 10th	Day	9% 8% 1 4 3 3 4 4 5% 6% 52%	2700
	11th to 20th	Day	20% 17% 3 6% 4 4 5% 5% 6% 7% 23%	3108
	21st to end	Day	23% 18% 3 7% 5% 4 4 3 5% 6% 23%	4092
January	1st to 10th	Night	15% 18% 3 7% 5% 5% 5% 5% 5% 6% 26%	8172
	11th to 20th	Night	10% 9% 2 4 2 3 3 3 4 6% 56%	7895
	21st to end	Night	12% 12% 3 6% 4 4 4 3 5% 5% 41%	8371
February	1st to 10th	Night	15% 10% 3 5% 3 3 4 4 5% 8% 39%	2730
	11th to 20th	Night	12% 16% 2 5% 4 4 4 3 6% 7% 37%	2655
	21st to end	Night	13% 12% 4 5% 4 4 4 3 6% 8% 39%	2076
March	1st to 10th	Night	10% 19% 5% 9% 4 3 3 4 5% 7% 30%	5226
	11th to 20th	Night	11% 15% 3 6% 3 3 3 3 5% 6% 42%	5110
	21st to end	Night	12% 17% 4 7% 5% 4 4 4 7% 8% 28%	5488
April	1st to 10th	Night	13% 12% 2 5% 4 4 4 5% 6% 8% 37%	2100
	11th to 20th	Night	9% 10% 2 4 4 3 4 3 5% 7% 49%	2079
	21st to end	Night	9% 8% 1 4 4 3 3 3 6% 9% 50%	2061
May	1st to 10th	Night	6% 3 1 4 3 3 3 3 4 7% 63%	5226
	11th to 20th	Night	4 3 1 4 2 2 3 3 6% 7% 64%	4700
	21st to end	Night	6% 3 1 4 2 3 2 3 4 8% 64%	5181
June	1st to 10th	Night	7% 6% 2 5% 3 3 3 4 6% 7% 54%	1680
	11th to 20th	Night	4 2 1 3 2 2 2 2 4 6% 71%	1680
	21st to end	Night	3 3 1 2 1 2 2 2 3 5% 76%	1680
July	1st to 10th	Night	1 1 2 1 1 2 1 3 4 84%	4740
	11th to 20th	Night	3 3 1 3 2 2 1 2 2 4 78%	4740
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 79%	5627
August	1st to 10th	Night	2 1 1 2 1 1 2 2 3 5% 79%	4125
	11th to 20th	Night	1 1 1 2 1 2 2 1 3 4 82%	4207
	21st to end	Night	5% 2 1 2 2 1 2 2 3 4 77%	5105
September	1st to 10th	Night	2 1 1 1 1 1 1 2 3 4 84%	2000
	11th to 20th	Night	4 3 1 3 2 2 2 2 5% 7% 70%	2000
	21st to end	Night	4 2 1 3 2 2 3 2 4 6% 71%	2281
October	1st to 10th	Night	4 2 1 2 2 3 2 2 3 3 76%	3920
	11th to 20th	Night	6% 3 1 3 3 3 3 3 6% 7% 63%	3897
	21st to end	Night	8% 4 1 3 3 4 4 6% 7% 8% 53%	4253
November	1st to 10th	Night	14% 8% 2 6% 3 4 4 4 6% 7% 43%	2620
	11th to 20th	Night	16% 10% 1 5% 4 3 4 5% 6% 9% 37%	2862
	21st to end	Night	9% 9% 2 5% 4 4 4 3 5% 7% 46%	2940
December	1st to 10th	Night	9% 7% 2 3 2 3 3 2 5% 7% 57%	4700
	11th to 20th	Night	21% 16% 3 7% 4 5% 5% 4 5% 7% 24%	5316
	21st to end	Night	26% 15% 3 5% 3 3 3 3 4 6% 28%	7106

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	50% 10% 12% 18% 9% 1	4723
	11th to 20th	Day	30% 5% 6% 15% 22% 23%	5077
	21st to end	Day	35% 9% 11% 13% 18% 14%	5843
February	1st to 10th	Day	43% 6% 13% 19% 13% 5%	1890
	11th to 20th	Day	44% 6% 14% 19% 14% 3	1859
	21st to end	Day	44% 7% 13% 18% 12% 5%	1730
March	1st to 10th	Day	51% 7% 11% 17% 10% 3	4886
	11th to 20th	Day	43% 8% 14% 17% 18% 1	5110
	21st to end	Day	51% 8% 16% 19% 5% 1	5728
April	1st to 10th	Day	45% 6% 15% 19% 13% 2	2520
	11th to 20th	Day	37% 7% 14% 24% 14% 5%	2492
	21st to end	Day	30% 7% 16% 25% 18% 3	2555
May	1st to 10th	Day	18% 7% 11% 28% 30% 6%	7717
	11th to 20th	Day	14% 5% 12% 26% 31% 11%	8163
	21st to end	Day	15% 7% 17% 35% 24% 2	8994
June	1st to 10th	Day	28% 6% 14% 25% 24% 4	2940
	11th to 20th	Day	19% 5% 10% 24% 31% 11%	2940
	21st to end	Day	13% 3 8% 20% 32% 23%	2940
July	1st to 10th	Day	6% 4 5% 14% 30% 41%	8151
	11th to 20th	Day	13% 3 5% 10% 24% 46%	8145
	21st to end	Day	9% 5% 6% 17% 26% 37%	8269
August	1st to 10th	Day	13% 3 5% 12% 22% 45%	4336
	11th to 20th	Day	8% 3 7% 21% 39% 23%	4275
	21st to end	Day	11% 4 6% 20% 36% 24%	4420
September	1st to 10th	Day	8% 2 5% 15% 27% 42%	2400
	11th to 20th	Day	18% 5% 8% 19% 33% 17%	2400
	21st to end	Day	16% 4 8% 21% 29% 22%	2114
October	1st to 10th	Day	14% 3 8% 17% 24% 34%	3280
	11th to 20th	Day	21% 5% 13% 32% 22% 7%	3261
	21st to end	Day	28% 11% 14% 21% 18% 9%	3558
November	1st to 10th	Day	37% 6% 14% 21% 16% 6%	1898
	11th to 20th	Day	42% 9% 14% 17% 13% 6%	1753
	21st to end	Day	36% 5% 13% 19% 17% 9%	1680
December	1st to 10th	Day	30% 6% 8% 20% 27% 9%	2700
	11th to 20th	Day	53% 12% 12% 13% 7% 2	3108
	21st to end	Day	54% 12% 11% 14% 7% 1	4092
January	1st to 10th	Night	53% 10% 12% 17% 7% 1	8172
	11th to 20th	Night	28% 6% 7% 14% 18% 26%	7895
	21st to end	Night	40% 7% 10% 11% 20% 12%	8371
February	1st to 10th	Night	40% 7% 15% 18% 13% 7%	2730
	11th to 20th	Night	44% 6% 14% 20% 13% 3	2657
	21st to end	Night	40% 7% 16% 18% 14% 4	2076
March	1st to 10th	Night	46% 9% 15% 20% 6% 4	5226
	11th to 20th	Night	39% 8% 13% 22% 16% 2	5110
	21st to end	Night	46% 12% 18% 17% 7% 1	5488
April	1st to 10th	Night	40% 8% 19% 19% 11% 2	2100
	11th to 20th	Night	33% 8% 18% 23% 14% 5%	2079
	21st to end	Night	27% 9% 21% 26% 15% 2	2061
May	1st to 10th	Night	17% 7% 13% 30% 29% 4	5224
	11th to 20th	Night	15% 8% 17% 25% 29% 7%	4700
	21st to end	Night	17% 8% 18% 41% 16% 1	5182
June	1st to 10th	Night	27% 7% 15% 30% 20% 1	1680
	11th to 20th	Night	14% 5% 11% 31% 30% 9%	1680
	21st to end	Night	12% 3 8% 26% 38% 13%	1680
July	1st to 10th	Night	6% 2 4 20% 31% 36%	4700
	11th to 20th	Night	12% 2 3 11% 31% 41%	4692
	21st to end	Night	10% 4 7% 19% 29% 31%	5504
August	1st to 10th	Night	11% 2 4 16% 26% 41%	3019
	11th to 20th	Night	6% 2 6% 28% 37% 20%	3071
	21st to end	Night	11% 3 5% 28% 33% 19%	3698
September	1st to 10th	Night	7% 2 6% 15% 29% 42%	2000
	11th to 20th	Night	15% 5% 8% 22% 33% 16%	2000
	21st to end	Night	15% 4 11% 22% 25% 23%	2281
October	1st to 10th	Night	14% 4 7% 12% 32% 30%	3920
	11th to 20th	Night	19% 6% 12% 36% 20% 7%	3897
	21st to end	Night	21% 9% 16% 23% 23% 8%	4253
November	1st to 10th	Night	37% 8% 16% 18% 15% 6%	2620
	11th to 20th	Night	41% 9% 15% 18% 13% 5%	2862
	21st to end	Night	36% 6% 14% 17% 18% 9%	2940
December	1st to 10th	Night	26% 5% 10% 20% 28% 11%	4700
	11th to 20th	Night	54% 10% 11% 15% 9% 2	5316
	21st to end	Night	53% 10% 12% 13% 10% 2	7106

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	38% 22% 23% 12% 4 1	220
	11th to 20th	Day	26% 20% 21% 21% 10% 3	336
	21st to end	Day	23% 19% 28% 18% 10% 1	473
February	1st to 10th	Day	26% 19% 29% 19% 7%	430
	11th to 20th	Day	30% 25% 25% 15% 5%	428
	21st to end	Day	24% 22% 32% 18% 4	446
March	1st to 10th	Day	34% 26% 25% 11% 5%	540
	11th to 20th	Day	26% 24% 27% 15% 7% 1	540
	21st to end	Day	30% 29% 22% 15% 4	594
April	1st to 10th	Day	29% 23% 28% 15% 4 1	540
	11th to 20th	Day	28% 20% 26% 19% 5% 2	532
	21st to end	Day	15% 27% 28% 20% 9% 1	540
May	1st to 10th	Day	14% 28% 32% 17% 7% 1	543
	11th to 20th	Day	24% 22% 27% 17% 8% 2	568
	21st to end	Day	15% 19% 28% 24% 11% 2	617
June	1st to 10th	Day	13% 19% 28% 26% 12% 2	570
	11th to 20th	Day	15% 22% 29% 23% 9% 2	570
	21st to end	Day	6% 14% 34% 30% 14% 3	570
July	1st to 10th	Day	3 13% 36% 33% 12% 3	566
	11th to 20th	Day	2 13% 34% 35% 13% 3	564
	21st to end	Day	2 9% 30% 37% 17% 4	619
August	1st to 10th	Day	2 7% 37% 33% 15% 5%	567
	11th to 20th	Day	4 11% 29% 33% 17% 6%	557
	21st to end	Day	5% 11% 34% 30% 16% 3	620
September	1st to 10th	Day	3 14% 32% 35% 12% 3	530
	11th to 20th	Day	7% 15% 32% 31% 13% 2	530
	21st to end	Day	8% 18% 29% 30% 13% 1	530
October	1st to 10th	Day	12% 19% 26% 27% 14% 2	530
	11th to 20th	Day	15% 16% 25% 28% 14% 2	530
	21st to end	Day	17% 19% 26% 25% 11% 2	583
November	1st to 10th	Day	24% 14% 25% 23% 12% 1	374
	11th to 20th	Day	23% 16% 24% 22% 12% 3	258
	21st to end	Day	20% 12% 27% 26% 14% 1	220
December	1st to 10th	Day	23% 15% 25% 19% 12% 5%	220
	11th to 20th	Day	28% 20% 24% 15% 10% 3	220
	21st to end	Day	27% 19% 25% 17% 10% 2	242
January	1st to 10th	Night	26% 11% 26% 23% 11% 3	840
	11th to 20th	Night	21% 11% 26% 25% 13% 3	840
	21st to end	Night	19% 13% 25% 27% 13% 3	924
February	1st to 10th	Night	20% 12% 30% 27% 10% 1	840
	11th to 20th	Night	24% 13% 23% 28% 11% 1	824
	21st to end	Night	20% 15% 28% 29% 7%	692
March	1st to 10th	Night	29% 17% 25% 19% 8% 2	840
	11th to 20th	Night	22% 15% 24% 25% 14% 1	840
	21st to end	Night	24% 17% 27% 22% 8% 1	924
April	1st to 10th	Night	21% 16% 27% 26% 10%	840
	11th to 20th	Night	16% 16% 30% 25% 11% 2	831
	21st to end	Night	14% 13% 30% 29% 13% 1	801
May	1st to 10th	Night	12% 14% 33% 26% 16% 1	599
	11th to 20th	Night	14% 12% 25% 29% 18% 1	414
	21st to end	Night	14% 11% 27% 32% 15% 1	455
June	1st to 10th	Night	10% 11% 26% 35% 18% 1	420
	11th to 20th	Night	7% 13% 32% 38% 9% 1	420
	21st to end	Night	5% 5% 29% 47% 13%	420
July	1st to 10th	Night	2 5% 22% 47% 23%	410
	11th to 20th	Night	3 3 21% 49% 24% 1	408
	21st to end	Night	1 1 13% 49% 35% 2	626
August	1st to 10th	Night	2 2 14% 48% 31% 3	627
	11th to 20th	Night	3 1 19% 46% 29% 2	650
	21st to end	Night	3 3 20% 45% 27% 2	916
September	1st to 10th	Night	2 5% 21% 43% 27% 3	800
	11th to 20th	Night	5% 7% 19% 34% 32% 4	800
	21st to end	Night	5% 8% 21% 37% 25% 4	800
October	1st to 10th	Night	9% 7% 25% 35% 21% 3	800
	11th to 20th	Night	12% 7% 21% 35% 22% 4	800
	21st to end	Night	15% 8% 20% 35% 17% 5%	880
November	1st to 10th	Night	17% 10% 23% 34% 12% 3	820
	11th to 20th	Night	22% 9% 28% 28% 10% 2	840
	21st to end	Night	14% 11% 29% 31% 13% 2	840
December	1st to 10th	Night	22% 9% 25% 28% 13% 3	840
	11th to 20th	Night	28% 10% 25% 24% 12% 2	840
	21st to end	Night	28% 16% 27% 23% 6% 1	924

Page generated at 2026-08-27 01:07:13. Version ac07

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