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 2025-04-26.

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	8% 9% 2 4 3 3 3 3 5% 6% 54%	117818
	Night	10% 9% 2 4 3 3 3 3 5% 6% 51%	119645
Transparency	Day	29% 6% 10% 18% 21% 16%	117570
	Night	31% 7% 12% 19% 19% 12%	119483
Seeing	Day	16% 18% 29% 24% 11% 2	15525
	Night	15% 10% 24% 32% 17% 2	24800

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	12% 17% 4 7% 5% 4 5% 3 5% 8% 29%	3639
	11th to 20th	Day	14% 14% 2 4 2 3 2 2 2 3 52%	3893
	21st to end	Day	16% 13% 2 4 3 3 3 2 3 5% 47%	4510
February	1st to 10th	Day	17% 11% 3 4 4 3 4 4 6% 7% 37%	1800
	11th to 20th	Day	10% 16% 4 5% 4 4 3 4 6% 6% 38%	1760
	21st to end	Day	12% 14% 4 6% 4 4 4 3 6% 6% 38%	1650
March	1st to 10th	Day	10% 24% 7% 8% 3 2 2 3 4 4 32%	4780
	11th to 20th	Day	11% 18% 3 6% 3 3 4 3 4 5% 40%	5000
	21st to end	Day	13% 23% 4 7% 4 3 4 3 5% 7% 28%	5605
April	1st to 10th	Day	14% 14% 2 7% 5% 4 3 4 5% 8% 35%	2400
	11th to 20th	Day	12% 10% 2 5% 4 4 4 4 6% 8% 42%	2369
	21st to end	Day	6% 8% 2 6% 5% 3 4 4 6% 9% 47%	2373
May	1st to 10th	Day	6% 6% 2 4 3 3 4 3 5% 6% 59%	4162
	11th to 20th	Day	6% 4 1 3 2 3 3 3 5% 6% 65%	4377
	21st to end	Day	7% 5% 2 4 3 3 3 3 5% 8% 57%	4813
June	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 7% 8% 49%	2660
	11th to 20th	Day	5% 3 2 3 3 3 3 3 5% 6% 64%	2660
	21st to end	Day	5% 2 1 2 2 1 2 3 4 4 74%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 2 3 86%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 77%	4420
	21st to end	Day	1 1 1 1 1 2 2 3 4 83%	4532
August	1st to 10th	Day	3 2 1 3 2 2 3 3 4 6% 72%	4091
	11th to 20th	Day	1 1 1 2 1 2 3 2 5% 5% 77%	3978
	21st to end	Day	3 2 1 3 1 1 2 2 3 4 78%	4180
September	1st to 10th	Day	1 1 1 1 2 2 2 2 3 4 80%	2280
	11th to 20th	Day	4 3 1 3 3 3 3 4 4 6% 66%	2280
	21st to end	Day	4 3 1 3 2 2 2 3 4 7% 70%	2009
October	1st to 10th	Day	4 2 1 2 2 2 2 2 4 5% 74%	3180
	11th to 20th	Day	7% 3 2 4 3 2 3 3 6% 7% 59%	3161
	21st to end	Day	9% 7% 2 5% 4 4 5% 3 7% 8% 46%	3448
November	1st to 10th	Day	14% 8% 2 4 5% 4 3 5% 5% 8% 42%	1806
	11th to 20th	Day	16% 9% 2 6% 4 4 4 4 7% 10% 36%	1670
	21st to end	Day	11% 9% 3 6% 4 4 4 3 4 8% 45%	1600
December	1st to 10th	Day	9% 8% 1 4 3 3 4 4 5% 6% 52%	2620
	11th to 20th	Day	20% 18% 3 6% 4 4 5% 5% 6% 6% 22%	3028
	21st to end	Day	23% 18% 3 7% 5% 4 4 3 5% 6% 22%	4004
January	1st to 10th	Night	10% 16% 3 8% 6% 6% 5% 6% 6% 6% 29%	6306
	11th to 20th	Night	13% 12% 3 5% 2 3 3 3 4 6% 47%	6073
	21st to end	Night	15% 14% 3 5% 3 3 3 2 3 4 43%	6468
February	1st to 10th	Night	16% 11% 3 6% 3 3 4 4 5% 8% 38%	2600
	11th to 20th	Night	12% 16% 2 5% 4 4 3 3 6% 7% 38%	2526
	21st to end	Night	12% 11% 3 5% 4 4 4 3 6% 7% 41%	1980
March	1st to 10th	Night	10% 19% 6% 9% 4 3 3 4 5% 7% 30%	5112
	11th to 20th	Night	11% 15% 4 6% 3 3 3 3 5% 6% 41%	5000
	21st to end	Night	13% 17% 4 7% 5% 4 4 4 6% 8% 28%	5369
April	1st to 10th	Night	13% 12% 2 5% 4 4 4 4 6% 8% 37%	2000
	11th to 20th	Night	9% 10% 2 4 4 3 4 3 5% 7% 50%	1979
	21st to end	Night	9% 7% 1 4 3 3 3 3 6% 9% 51%	1919
May	1st to 10th	Night	8% 3 2 4 3 3 3 3 4 5% 63%	2806
	11th to 20th	Night	5% 3 1 3 2 2 3 2 5% 5% 69%	2516
	21st to end	Night	6% 3 1 3 2 2 2 3 4 9% 64%	2762
June	1st to 10th	Night	7% 6% 2 5% 3 4 3 4 6% 7% 52%	1520
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 6% 71%	1520
	21st to end	Night	3 3 1 2 1 2 2 2 3 5% 76%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 1 2 3 88%	2540
	11th to 20th	Night	2 3 1 3 2 2 2 2 3 4 77%	2540
	21st to end	Night	1 1 1 2 1 1 1 2 3 87%	3012
August	1st to 10th	Night	3 2 1 2 2 2 2 3 3 4 76%	2848
	11th to 20th	Night	1 1 2 1 2 2 2 3 4 82%	2862
	21st to end	Night	6% 2 1 1 1 1 1 1 2 3 80%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 3 3 86%	1900
	11th to 20th	Night	4 2 1 3 2 2 2 2 5% 7% 71%	1900
	21st to end	Night	4 2 1 3 2 3 3 2 4 6% 71%	2166
October	1st to 10th	Night	3 2 1 2 2 3 2 2 3 3 77%	3800
	11th to 20th	Night	6% 3 1 3 3 3 3 3 6% 7% 63%	3777
	21st to end	Night	8% 4 1 3 3 4 4 6% 7% 8% 53%	4121
November	1st to 10th	Night	13% 7% 2 6% 3 4 4 4 6% 7% 43%	2492
	11th to 20th	Night	15% 10% 1 5% 4 3 4 5% 6% 9% 38%	2725
	21st to end	Night	10% 10% 2 6% 4 4 5% 3 4 7% 45%	2800
December	1st to 10th	Night	10% 7% 2 3 2 3 3 3 5% 6% 57%	4560
	11th to 20th	Night	21% 16% 3 6% 4 5% 5% 4 5% 7% 23%	5176
	21st to end	Night	26% 15% 3 5% 3 3 3 3 4 6% 28%	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	47% 13% 11% 18% 11% 1	3639
	11th to 20th	Day	38% 6% 7% 19% 25% 5%	3893
	21st to end	Day	39% 7% 9% 10% 20% 14%	4510
February	1st to 10th	Day	44% 6% 14% 19% 12% 5%	1800
	11th to 20th	Day	43% 6% 13% 20% 15% 3	1769
	21st to end	Day	44% 7% 13% 19% 13% 5%	1650
March	1st to 10th	Day	52% 7% 11% 17% 10% 3	4780
	11th to 20th	Day	44% 8% 13% 17% 18% 1	5000
	21st to end	Day	52% 8% 15% 18% 5% 1	5605
April	1st to 10th	Day	45% 6% 15% 18% 13% 2	2400
	11th to 20th	Day	37% 7% 14% 25% 13% 5%	2372
	21st to end	Day	31% 7% 16% 25% 18% 3	2373
May	1st to 10th	Day	24% 7% 10% 25% 26% 7%	4162
	11th to 20th	Day	18% 4 9% 19% 34% 17%	4380
	21st to end	Day	21% 6% 16% 30% 25% 2	4814
June	1st to 10th	Day	30% 6% 14% 24% 22% 4	2660
	11th to 20th	Day	19% 5% 10% 25% 31% 10%	2660
	21st to end	Day	14% 2 7% 20% 32% 25%	2660
July	1st to 10th	Day	6% 2 3 9% 16% 63%	4351
	11th to 20th	Day	11% 2 5% 9% 21% 51%	4345
	21st to end	Day	6% 2 5% 14% 28% 45%	4389
August	1st to 10th	Day	13% 3 5% 11% 22% 46%	4076
	11th to 20th	Day	8% 3 7% 21% 38% 23%	4017
	21st to end	Day	11% 3 6% 20% 36% 24%	4179
September	1st to 10th	Day	8% 2 5% 14% 28% 44%	2280
	11th to 20th	Day	18% 5% 8% 19% 33% 17%	2280
	21st to end	Day	16% 3 8% 22% 29% 22%	2009
October	1st to 10th	Day	14% 3 8% 17% 24% 35%	3180
	11th to 20th	Day	21% 5% 13% 31% 22% 7%	3161
	21st to end	Day	28% 10% 14% 21% 19% 9%	3448
November	1st to 10th	Day	38% 6% 14% 20% 16% 6%	1806
	11th to 20th	Day	41% 9% 14% 17% 13% 6%	1670
	21st to end	Day	38% 6% 13% 18% 16% 10%	1600
December	1st to 10th	Day	31% 6% 8% 20% 27% 9%	2620
	11th to 20th	Day	54% 12% 12% 13% 7% 1	3028
	21st to end	Day	54% 12% 11% 13% 7% 2	4004
January	1st to 10th	Night	48% 11% 13% 18% 8% 2	6306
	11th to 20th	Night	34% 8% 9% 18% 22% 8%	6073
	21st to end	Night	43% 6% 7% 9% 19% 15%	6468
February	1st to 10th	Night	42% 6% 14% 18% 13% 6%	2600
	11th to 20th	Night	44% 6% 14% 20% 13% 3	2528
	21st to end	Night	39% 7% 16% 19% 15% 4	1980
March	1st to 10th	Night	46% 9% 15% 20% 6% 4	5112
	11th to 20th	Night	40% 9% 13% 22% 16% 1	5000
	21st to end	Night	47% 12% 18% 16% 6% 1	5369
April	1st to 10th	Night	41% 8% 19% 19% 10% 2	2000
	11th to 20th	Night	33% 7% 18% 23% 14% 5%	1979
	21st to end	Night	27% 9% 21% 27% 14% 2	1919
May	1st to 10th	Night	20% 5% 15% 33% 20% 6%	2806
	11th to 20th	Night	16% 5% 11% 22% 33% 13%	2516
	21st to end	Night	17% 9% 22% 34% 18% 1	2762
June	1st to 10th	Night	29% 7% 16% 29% 18% 1	1520
	11th to 20th	Night	15% 5% 12% 32% 30% 8%	1520
	21st to end	Night	13% 2 8% 25% 39% 14%	1520
July	1st to 10th	Night	5% 2 4 12% 18% 59%	2500
	11th to 20th	Night	11% 3 4 11% 21% 50%	2492
	21st to end	Night	6% 1 4 15% 35% 38%	2918
August	1st to 10th	Night	12% 2 4 15% 26% 41%	2839
	11th to 20th	Night	6% 2 6% 28% 37% 21%	2889
	21st to end	Night	11% 3 5% 28% 33% 19%	3498
September	1st to 10th	Night	6% 1 5% 14% 29% 44%	1900
	11th to 20th	Night	14% 5% 8% 22% 34% 17%	1900
	21st to end	Night	15% 4 11% 22% 26% 23%	2166
October	1st to 10th	Night	13% 4 7% 13% 32% 31%	3800
	11th to 20th	Night	19% 6% 12% 35% 20% 8%	3777
	21st to end	Night	21% 9% 16% 22% 22% 9%	4121
November	1st to 10th	Night	36% 8% 17% 16% 16% 6%	2492
	11th to 20th	Night	40% 9% 15% 18% 13% 5%	2725
	21st to end	Night	38% 6% 13% 16% 18% 10%	2800
December	1st to 10th	Night	27% 4 8% 19% 29% 12%	4560
	11th to 20th	Night	55% 9% 11% 15% 9% 1	5176
	21st to end	Night	53% 10% 12% 13% 10% 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	35% 24% 25% 12% 4 2	200
	11th to 20th	Day	28% 22% 23% 18% 7% 3	310
	21st to end	Day	25% 19% 26% 18% 10% 1	440
February	1st to 10th	Day	27% 19% 28% 20% 6%	400
	11th to 20th	Day	29% 24% 26% 15% 6%	398
	21st to end	Day	22% 22% 33% 19% 5%	414
March	1st to 10th	Day	34% 25% 25% 11% 5%	500
	11th to 20th	Day	28% 24% 25% 15% 8% 1	500
	21st to end	Day	30% 29% 21% 15% 4	550
April	1st to 10th	Day	29% 23% 28% 15% 4 1	500
	11th to 20th	Day	27% 20% 27% 19% 5% 2	492
	21st to end	Day	15% 27% 28% 21% 8% 1	481
May	1st to 10th	Day	16% 27% 32% 16% 7% 1	463
	11th to 20th	Day	22% 21% 27% 18% 9% 3	488
	21st to end	Day	16% 19% 27% 25% 12% 3	529
June	1st to 10th	Day	13% 17% 28% 27% 12% 2	490
	11th to 20th	Day	15% 23% 29% 23% 8% 1	490
	21st to end	Day	5% 14% 33% 31% 13% 3	490
July	1st to 10th	Day	3 12% 36% 34% 12% 3	486
	11th to 20th	Day	1 11% 35% 35% 14% 3	484
	21st to end	Day	1 7% 32% 38% 18% 4	531
August	1st to 10th	Day	2 8% 38% 33% 15% 5%	487
	11th to 20th	Day	4 12% 27% 33% 17% 6%	477
	21st to end	Day	5% 9% 35% 30% 17% 3	539
September	1st to 10th	Day	3 13% 32% 36% 13% 3	490
	11th to 20th	Day	7% 15% 33% 30% 13% 2	490
	21st to end	Day	8% 19% 29% 30% 12% 1	490
October	1st to 10th	Day	10% 18% 27% 28% 14% 2	490
	11th to 20th	Day	15% 14% 25% 29% 15% 2	490
	21st to end	Day	16% 20% 26% 25% 10% 2	539
November	1st to 10th	Day	23% 15% 26% 23% 12% 1	342
	11th to 20th	Day	22% 14% 26% 23% 13% 3	235
	21st to end	Day	22% 12% 25% 27% 14% 1	200
December	1st to 10th	Day	25% 14% 24% 19% 13% 6%	200
	11th to 20th	Day	28% 20% 24% 16% 9% 4	200
	21st to end	Day	25% 20% 26% 18% 11% 1	220
January	1st to 10th	Night	24% 11% 27% 24% 11% 3	800
	11th to 20th	Night	22% 12% 27% 23% 13% 3	800
	21st to end	Night	20% 13% 24% 28% 13% 3	880
February	1st to 10th	Night	21% 12% 29% 27% 10% 1	800
	11th to 20th	Night	24% 12% 23% 28% 12% 1	784
	21st to end	Night	18% 15% 28% 30% 7%	660
March	1st to 10th	Night	29% 17% 24% 20% 8% 2	800
	11th to 20th	Night	23% 14% 22% 25% 15% 1	800
	21st to end	Night	25% 17% 26% 23% 8% 1	880
April	1st to 10th	Night	22% 16% 27% 25% 10%	800
	11th to 20th	Night	15% 16% 31% 25% 12% 2	791
	21st to end	Night	14% 13% 30% 29% 13% 1	747
May	1st to 10th	Night	11% 14% 33% 25% 16%	542
	11th to 20th	Night	13% 10% 24% 32% 20% 1	374
	21st to end	Night	14% 11% 26% 32% 16% 1	411
June	1st to 10th	Night	10% 8% 24% 37% 20% 1	380
	11th to 20th	Night	7% 13% 30% 39% 9% 1	380
	21st to end	Night	5% 4 27% 49% 15%	380
July	1st to 10th	Night	2 5% 19% 48% 26%	370
	11th to 20th	Night	3 2 18% 51% 25% 1	368
	21st to end	Night	1 1 11% 50% 35% 2	566
August	1st to 10th	Night	3 1 12% 49% 32% 3	567
	11th to 20th	Night	3 1 17% 45% 31% 3	588
	21st to end	Night	3 3 19% 44% 29% 2	836
September	1st to 10th	Night	2 4 21% 43% 27% 3	760
	11th to 20th	Night	5% 6% 19% 34% 32% 4	760
	21st to end	Night	5% 8% 21% 37% 25% 5%	760
October	1st to 10th	Night	7% 7% 26% 36% 21% 3	760
	11th to 20th	Night	12% 7% 19% 36% 22% 4	760
	21st to end	Night	15% 8% 21% 34% 17% 5%	836
November	1st to 10th	Night	17% 11% 23% 34% 13% 4	780
	11th to 20th	Night	21% 8% 29% 29% 11% 2	800
	21st to end	Night	15% 10% 28% 31% 14% 2	800
December	1st to 10th	Night	23% 9% 24% 29% 13% 3	800
	11th to 20th	Night	28% 10% 25% 23% 12% 2	800
	21st to end	Night	27% 16% 27% 23% 6% 1	880

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