Redwood Mountain View
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Redwood Mountain View 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	6% 5% 1 3 2 2 3 2 3 5% 67%	135373
	Night	7% 4 1 3 2 2 3 2 4 5% 66%	133080
Transparency	Day	21% 3 8% 19% 28% 21%	130442
	Night	21% 3 9% 20% 27% 19%	128985
Seeing	Day	10% 11% 31% 33% 12% 3	16734
	Night	9% 3 19% 39% 27% 4	26322

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	8% 8% 2 4 3 3 3 2 5% 8% 54%	3639
	11th to 20th	Day	8% 5% 2 5% 3 4 4 3 4 5% 57%	3852
	21st to end	Day	11% 8% 2 6% 4 4 4 4 5% 7% 45%	4510
February	1st to 10th	Day	15% 10% 2 3 3 3 3 3 5% 5% 48%	1965
	11th to 20th	Day	11% 13% 3 5% 3 4 3 3 4 6% 46%	1950
	21st to end	Day	15% 12% 3 5% 3 3 3 2 3 6% 42%	1650
March	1st to 10th	Day	12% 16% 3 6% 4 5% 4 3 5% 6% 35%	4518
	11th to 20th	Day	11% 11% 2 5% 3 4 3 3 5% 7% 46%	4780
	21st to end	Day	15% 13% 2 6% 4 4 3 2 4 5% 40%	5925
April	1st to 10th	Day	10% 10% 2 5% 4 4 3 2 5% 7% 50%	2400
	11th to 20th	Day	11% 9% 2 5% 4 3 4 2 5% 5% 50%	2369
	21st to end	Day	9% 8% 2 5% 4 3 3 3 5% 6% 53%	2400
May	1st to 10th	Day	6% 5% 1 4 2 2 2 2 3 5% 67%	5652
	11th to 20th	Day	3 4 1 3 2 3 3 2 3 5% 70%	5841
	21st to end	Day	4 2 1 2 2 2 2 2 3 4 77%	6728
June	1st to 10th	Day	4 3 1 3 3 2 3 3 5% 7% 67%	2800
	11th to 20th	Day	2 1 1 1 1 1 2 1 3 4 83%	2800
	21st to end	Day	1 1 1 2 1 1 2 2 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 92%	6319
	11th to 20th	Day	1 1 1 1 2 2 1 2 3 85%	6320
	21st to end	Day	1 1 1 1 1 2 2 3 3 86%	6604
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 2 87%	5351
	11th to 20th	Day	1 1 1 1 1 1 2 92%	5364
	21st to end	Day	1 1 2 2 2 2 2 3 4 82%	5984
September	1st to 10th	Day	2 1 2 1 2 1 1 3 4 84%	2400
	11th to 20th	Day	3 1 1 2 2 2 2 1 2 4 80%	2312
	21st to end	Day	3 2 1 2 2 2 2 1 3 3 80%	2200
October	1st to 10th	Day	4 3 1 2 2 2 3 2 3 4 74%	3507
	11th to 20th	Day	3 2 1 3 2 3 2 2 2 3 77%	3261
	21st to end	Day	5% 4 1 4 3 3 3 3 4 6% 63%	3473
November	1st to 10th	Day	6% 8% 2 4 3 2 3 2 5% 6% 59%	1854
	11th to 20th	Day	9% 4 2 3 3 3 2 3 5% 6% 61%	1890
	21st to end	Day	9% 6% 2 4 3 3 4 3 5% 6% 56%	1890
December	1st to 10th	Day	7% 4 1 3 3 3 3 3 4 6% 62%	2977
	11th to 20th	Day	15% 7% 2 5% 3 4 4 4 4 8% 45%	3084
	21st to end	Day	14% 9% 2 6% 4 4 4 3 6% 9% 41%	4004
January	1st to 10th	Night	10% 7% 2 3 2 3 3 2 5% 7% 56%	6306
	11th to 20th	Night	10% 7% 2 5% 3 3 3 2 4 5% 57%	6100
	21st to end	Night	11% 9% 2 5% 4 4 4 3 5% 6% 47%	6468
February	1st to 10th	Night	15% 8% 2 4 3 4 3 2 5% 5% 50%	2432
	11th to 20th	Night	12% 8% 2 4 2 3 4 3 4 6% 53%	2338
	21st to end	Night	13% 9% 3 4 3 3 3 2 5% 5% 50%	1980
March	1st to 10th	Night	12% 13% 2 5% 3 4 4 4 5% 6% 41%	5376
	11th to 20th	Night	8% 8% 2 5% 4 4 4 3 5% 6% 52%	5203
	21st to end	Night	16% 9% 2 4 3 3 4 3 5% 7% 44%	5050
April	1st to 10th	Night	10% 6% 1 4 2 4 3 3 4 6% 58%	2000
	11th to 20th	Night	8% 5% 1 4 4 3 4 3 4 5% 59%	1979
	21st to end	Night	7% 5% 1 3 3 3 3 3 4 6% 62%	2000
May	1st to 10th	Night	5% 3 1 3 2 2 3 2 3 5% 71%	4335
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 5% 78%	4074
	21st to end	Night	2 1 1 2 1 1 1 1 2 3 85%	4136
June	1st to 10th	Night	4 3 1 2 2 1 2 2 3 7% 73%	1600
	11th to 20th	Night	1 1 1 2 1 2 2 1 2 4 83%	1600
	21st to end	Night	1 1 1 1 1 1 1 2 2 2 88%	1600
July	1st to 10th	Night	1 1 1 1 1 94%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 3 86%	3640
	21st to end	Night	1 1 1 1 1 1 1 3 3 86%	4242
August	1st to 10th	Night	1 1 2 1 1 2 1 2 3 86%	4479
	11th to 20th	Night	1 1 1 1 1 94%	4506
	21st to end	Night	1 1 2 2 2 2 1 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	2000
	11th to 20th	Night	1 2 1 2 1 2 2 2 3 2 82%	2086
	21st to end	Night	2 1 1 2 2 2 2 2 3 4 81%	2200
October	1st to 10th	Night	2 1 1 1 1 2 2 1 2 3 83%	3689
	11th to 20th	Night	2 2 1 2 2 2 3 2 3 3 79%	3897
	21st to end	Night	4 2 1 3 1 1 2 2 3 5% 76%	4332
November	1st to 10th	Night	7% 5% 1 2 2 2 3 3 4 5% 66%	2665
	11th to 20th	Night	7% 4 2 4 2 2 3 3 5% 8% 61%	2730
	21st to end	Night	9% 6% 2 4 4 2 3 3 4 7% 57%	2730
December	1st to 10th	Night	7% 3 1 4 3 3 4 3 4 5% 62%	4426
	11th to 20th	Night	15% 6% 2 4 3 4 4 4 7% 8% 44%	5274
	21st to end	Night	11% 7% 3 7% 4 5% 5% 4 6% 9% 39%	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	29% 6% 21% 23% 17% 4	3639
	11th to 20th	Day	27% 8% 12% 12% 24% 17%	3852
	21st to end	Day	36% 7% 10% 17% 23% 8%	4510
February	1st to 10th	Day	36% 3 11% 23% 18% 8%	1965
	11th to 20th	Day	38% 5% 10% 20% 20% 7%	1960
	21st to end	Day	44% 5% 9% 25% 15% 3	1650
March	1st to 10th	Day	46% 5% 10% 14% 22% 3	4518
	11th to 20th	Day	35% 5% 16% 29% 12% 4	4780
	21st to end	Day	41% 5% 16% 21% 14% 2	5925
April	1st to 10th	Day	33% 3 15% 27% 19% 3	2400
	11th to 20th	Day	33% 3 14% 24% 22% 4	2372
	21st to end	Day	29% 4 12% 34% 19% 2	2400
May	1st to 10th	Day	19% 2 10% 30% 33% 6%	5652
	11th to 20th	Day	16% 2 7% 34% 35% 5%	5844
	21st to end	Day	12% 2 6% 29% 46% 5%	6729
June	1st to 10th	Day	16% 4 12% 22% 36% 10%	2800
	11th to 20th	Day	7% 2 6% 18% 42% 25%	2800
	21st to end	Day	8% 2 3 10% 39% 38%	2800
July	1st to 10th	Day	3 1 1 5% 20% 70%	6251
	11th to 20th	Day	7% 1 2 5% 27% 58%	6245
	21st to end	Day	6% 1 3 15% 37% 38%	6413
August	1st to 10th	Day	8% 1 3 7% 38% 43%	3894
	11th to 20th	Day	4 1 2 7% 31% 55%	3880
	21st to end	Day	6% 1 2 10% 35% 46%	4311
September	1st to 10th	Day	7% 1 5% 10% 37% 41%	2400
	11th to 20th	Day	10% 2 5% 20% 39% 24%	2312
	21st to end	Day	11% 2 5% 18% 38% 27%	2200
October	1st to 10th	Day	14% 3 4 11% 25% 44%	3507
	11th to 20th	Day	15% 2 3 24% 36% 20%	3261
	21st to end	Day	23% 1 4 17% 35% 19%	3473
November	1st to 10th	Day	25% 4 9% 20% 26% 15%	1854
	11th to 20th	Day	25% 3 11% 23% 27% 11%	1890
	21st to end	Day	29% 4 8% 18% 26% 15%	1890
December	1st to 10th	Day	23% 4 8% 24% 26% 15%	2977
	11th to 20th	Day	38% 4 11% 19% 22% 6%	3084
	21st to end	Day	40% 5% 17% 24% 10% 4	4004
January	1st to 10th	Night	27% 8% 16% 24% 18% 7%	6306
	11th to 20th	Night	29% 6% 13% 14% 27% 11%	6100
	21st to end	Night	35% 7% 9% 18% 23% 8%	6468
February	1st to 10th	Night	36% 3 9% 24% 19% 8%	2432
	11th to 20th	Night	31% 6% 11% 24% 22% 6%	2339
	21st to end	Night	35% 4 13% 27% 16% 5%	1980
March	1st to 10th	Night	39% 5% 11% 19% 22% 4	5376
	11th to 20th	Night	30% 4 17% 30% 13% 5%	5203
	21st to end	Night	38% 5% 22% 19% 14% 2	5050
April	1st to 10th	Night	28% 4 15% 30% 18% 6%	2000
	11th to 20th	Night	26% 5% 13% 28% 21% 6%	1979
	21st to end	Night	20% 5% 15% 36% 23% 1	2000
May	1st to 10th	Night	17% 2 11% 34% 31% 6%	4332
	11th to 20th	Night	11% 2 8% 35% 39% 6%	4075
	21st to end	Night	8% 1 6% 28% 52% 5%	4136
June	1st to 10th	Night	14% 3 10% 25% 38% 10%	1600
	11th to 20th	Night	7% 2 6% 21% 37% 28%	1600
	21st to end	Night	5% 1 4 11% 41% 39%	1600
July	1st to 10th	Night	2 1 4 22% 70%	3600
	11th to 20th	Night	7% 1 3 5% 28% 56%	3592
	21st to end	Night	5% 1 2 17% 34% 41%	4122
August	1st to 10th	Night	9% 1 1 12% 34% 43%	3250
	11th to 20th	Night	3 1 2 7% 25% 63%	3256
	21st to end	Night	6% 1 2 7% 35% 49%	3608
September	1st to 10th	Night	5% 1 3 11% 32% 47%	2000
	11th to 20th	Night	9% 2 4 17% 43% 25%	2086
	21st to end	Night	9% 2 5% 18% 39% 27%	2200
October	1st to 10th	Night	9% 2 4 12% 26% 48%	3689
	11th to 20th	Night	12% 2 4 20% 43% 19%	3897
	21st to end	Night	13% 1 4 21% 38% 23%	4332
November	1st to 10th	Night	21% 3 11% 21% 27% 17%	2665
	11th to 20th	Night	23% 3 11% 22% 28% 14%	2730
	21st to end	Night	28% 3 9% 18% 31% 11%	2730
December	1st to 10th	Night	25% 3 9% 24% 25% 16%	4426
	11th to 20th	Night	36% 5% 12% 20% 20% 7%	5274
	21st to end	Night	40% 5% 19% 22% 11% 4	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	26% 9% 35% 21% 8% 2	200
	11th to 20th	Day	15% 14% 31% 26% 12% 2	295
	21st to end	Day	18% 10% 32% 26% 11% 2	440
February	1st to 10th	Day	21% 11% 34% 23% 9% 2	483
	11th to 20th	Day	19% 14% 38% 22% 7% 1	495
	21st to end	Day	17% 17% 39% 22% 4	414
March	1st to 10th	Day	24% 14% 37% 19% 5% 1	500
	11th to 20th	Day	19% 18% 33% 21% 8% 1	500
	21st to end	Day	21% 21% 31% 23% 4	550
April	1st to 10th	Day	15% 17% 32% 26% 9% 1	500
	11th to 20th	Day	20% 17% 31% 25% 6% 1	492
	21st to end	Day	11% 14% 33% 27% 12% 2	500
May	1st to 10th	Day	12% 18% 36% 22% 10% 2	503
	11th to 20th	Day	10% 13% 34% 27% 13% 2	528
	21st to end	Day	11% 11% 31% 33% 12% 2	573
June	1st to 10th	Day	5% 11% 28% 38% 16% 3	530
	11th to 20th	Day	4 12% 31% 37% 13% 3	530
	21st to end	Day	2 8% 32% 38% 16% 4	530
July	1st to 10th	Day	1 3 30% 45% 16% 4	526
	11th to 20th	Day	2 1 17% 53% 20% 7%	524
	21st to end	Day	2 1 22% 51% 17% 7%	575
August	1st to 10th	Day	2 5% 34% 40% 12% 6%	527
	11th to 20th	Day	2 5% 26% 45% 14% 8%	517
	21st to end	Day	2 2 27% 47% 15% 6%	583
September	1st to 10th	Day	2 4 32% 44% 15% 3	530
	11th to 20th	Day	5% 9% 33% 38% 13% 2	530
	21st to end	Day	5% 11% 31% 40% 13% 2	530
October	1st to 10th	Day	5% 12% 32% 37% 13%	530
	11th to 20th	Day	6% 10% 33% 37% 13% 1	530
	21st to end	Day	6% 10% 32% 37% 15% 1	534
November	1st to 10th	Day	8% 10% 32% 32% 18% 1	330
	11th to 20th	Day	12% 9% 34% 30% 13% 2	330
	21st to end	Day	12% 11% 31% 31% 13% 2	330
December	1st to 10th	Day	14% 9% 32% 30% 13% 2	311
	11th to 20th	Day	14% 9% 31% 27% 15% 3	214
	21st to end	Day	22% 10% 33% 23% 10% 2	220
January	1st to 10th	Night	19% 4 23% 32% 18% 5%	800
	11th to 20th	Night	12% 5% 26% 36% 19% 3	800
	21st to end	Night	18% 3 25% 35% 16% 3	880
February	1st to 10th	Night	19% 4 23% 36% 17% 3	800
	11th to 20th	Night	14% 4 27% 33% 18% 4	784
	21st to end	Night	18% 5% 25% 37% 12% 2	660
March	1st to 10th	Night	17% 6% 24% 38% 14% 2	800
	11th to 20th	Night	14% 6% 23% 33% 21% 4	800
	21st to end	Night	17% 5% 23% 36% 18% 1	880
April	1st to 10th	Night	12% 3 23% 43% 18% 1	800
	11th to 20th	Night	10% 5% 24% 40% 21% 1	791
	21st to end	Night	7% 4 20% 42% 25% 3	800
May	1st to 10th	Night	7% 4 21% 43% 24% 2	705
	11th to 20th	Night	6% 3 18% 43% 27% 2	591
	21st to end	Night	6% 3 21% 40% 28% 3	507
June	1st to 10th	Night	5% 2 14% 44% 32% 4	400
	11th to 20th	Night	1 2 18% 45% 31% 4	400
	21st to end	Night	1 1 15% 41% 39% 4	400
July	1st to 10th	Night	1 1 6% 39% 50% 3	390
	11th to 20th	Night	1 3 38% 52% 5%	388
	21st to end	Night	1 1 6% 34% 50% 8%	557
August	1st to 10th	Night	2 1 9% 41% 43% 4	789
	11th to 20th	Night	1 7% 36% 51% 4	792
	21st to end	Night	2 6% 36% 51% 5%	880
September	1st to 10th	Night	1 1 9% 38% 45% 6%	800
	11th to 20th	Night	2 1 12% 47% 33% 6%	800
	21st to end	Night	2 1 15% 43% 35% 4	800
October	1st to 10th	Night	3 1 17% 49% 24% 5%	800
	11th to 20th	Night	4 1 20% 40% 31% 4	800
	21st to end	Night	5% 2 20% 34% 31% 8%	880
November	1st to 10th	Night	8% 3 16% 40% 27% 6%	820
	11th to 20th	Night	10% 3 20% 40% 23% 4	840
	21st to end	Night	11% 4 23% 39% 20% 4	840
December	1st to 10th	Night	12% 4 26% 39% 17% 4	840
	11th to 20th	Night	18% 3 25% 35% 16% 2	828
	21st to end	Night	16% 6% 27% 34% 13% 3	880

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