Mount Tamalpais State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Tamalpais State Park clear sky chart from 2005-11-06 to 2025-11-03.

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% 4 2 5% 3 3 4 3 5% 9% 55%	135290
	Night	8% 5% 2 6% 4 4 4 3 5% 7% 52%	132214
Transparency	Day	15% 38% 5% 14% 16% 12%	130201
	Night	18% 38% 6% 13% 14% 11%	128292
Seeing	Day	9% 10% 25% 31% 20% 6%	16745
	Night	10% 6% 17% 33% 29% 5%	26018

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	11% 6% 2 5% 4 4 5% 4 7% 8% 43%	3942
	11th to 20th	Day	14% 7% 3 7% 4 4 4 3 4 5% 47%	4100
	21st to end	Day	16% 7% 3 8% 4 4 4 3 5% 6% 39%	4510
February	1st to 10th	Day	18% 6% 2 5% 5% 3 3 3 5% 6% 44%	1800
	11th to 20th	Day	12% 8% 2 7% 5% 5% 4 4 7% 8% 39%	1810
	21st to end	Day	12% 7% 3 6% 4 4 4 3 5% 7% 46%	1650
March	1st to 10th	Day	12% 9% 3 9% 5% 5% 5% 4 6% 8% 34%	4942
	11th to 20th	Day	13% 4 2 4 4 4 4 3 6% 9% 48%	5000
	21st to end	Day	13% 7% 2 6% 5% 4 5% 5% 7% 10% 35%	5520
April	1st to 10th	Day	10% 7% 2 6% 4 4 5% 4 7% 7% 44%	2400
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 5% 8% 52%	2369
	21st to end	Day	4 4 2 5% 4 3 3 3 6% 11% 55%	2400
May	1st to 10th	Day	3 2 1 3 2 2 2 2 3 5% 76%	5735
	11th to 20th	Day	2 2 2 4 3 3 3 3 4 10% 65%	5899
	21st to end	Day	1 3 1 3 2 2 2 2 4 7% 74%	6902
June	1st to 10th	Day	4 5% 2 4 3 3 3 3 6% 9% 57%	2800
	11th to 20th	Day	1 3 1 3 3 2 3 3 5% 9% 67%	2800
	21st to end	Day	2 3 1 4 2 3 3 3 5% 9% 65%	2800
July	1st to 10th	Day	2 1 4 2 2 2 2 5% 9% 71%	6262
	11th to 20th	Day	1 3 2 4 4 5% 4 3 6% 11% 59%	5880
	21st to end	Day	2 1 6% 5% 4 4 3 7% 13% 54%	6402
August	1st to 10th	Day	1 2 1 3 3 2 4 3 7% 10% 64%	5777
	11th to 20th	Day	2 1 3 3 3 3 2 5% 9% 70%	5587
	21st to end	Day	1 3 1 4 3 3 4 3 7% 13% 59%	5984
September	1st to 10th	Day	1 3 1 4 2 3 3 2 5% 11% 64%	2400
	11th to 20th	Day	3 5% 2 5% 3 3 4 4 5% 9% 57%	2399
	21st to end	Day	3 3 2 3 2 2 2 3 4 8% 68%	2101
October	1st to 10th	Day	2 3 1 4 3 2 3 2 4 7% 69%	3280
	11th to 20th	Day	5% 6% 2 5% 3 3 3 2 5% 6% 60%	3261
	21st to end	Day	5% 4 2 5% 4 3 4 4 7% 9% 53%	3558
November	1st to 10th	Day	8% 5% 2 5% 4 3 4 3 5% 8% 54%	1973
	11th to 20th	Day	11% 8% 3 7% 4 4 3 3 5% 6% 45%	1795
	21st to end	Day	12% 5% 2 4 4 3 4 3 5% 8% 51%	1600
December	1st to 10th	Day	9% 5% 2 4 3 3 4 4 7% 9% 51%	2620
	11th to 20th	Day	23% 9% 3 7% 5% 5% 6% 6% 6% 6% 24%	3028
	21st to end	Day	26% 9% 3 6% 5% 4 4 4 6% 8% 27%	4004
January	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 6% 9% 46%	6002
	11th to 20th	Night	13% 6% 3 7% 4 4 4 3 4 6% 46%	5880
	21st to end	Night	15% 8% 3 6% 4 4 4 3 5% 6% 42%	6468
February	1st to 10th	Night	18% 6% 2 4 4 4 4 3 5% 5% 47%	2600
	11th to 20th	Night	11% 7% 3 7% 4 4 4 4 5% 8% 44%	2476
	21st to end	Night	11% 5% 2 4 4 4 4 3 5% 6% 51%	1980
March	1st to 10th	Night	13% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	4947
	11th to 20th	Night	13% 6% 2 4 4 4 3 3 5% 6% 51%	5000
	21st to end	Night	13% 8% 2 7% 5% 4 4 3 4 8% 42%	5454
April	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 5% 7% 50%	2000
	11th to 20th	Night	8% 5% 2 6% 4 3 3 3 4 6% 56%	1979
	21st to end	Night	4 4 2 4 3 3 4 3 6% 9% 59%	2000
May	1st to 10th	Night	3 3 1 4 2 2 2 2 3 6% 72%	4248
	11th to 20th	Night	1 3 3 9% 4 5% 4 2 4 6% 59%	4010
	21st to end	Night	2 2 1 3 2 3 2 2 3 4 78%	3972
June	1st to 10th	Night	3 5% 2 6% 5% 4 4 4 6% 8% 53%	1600
	11th to 20th	Night	1 2 2 4 3 4 3 3 5% 8% 67%	1600
	21st to end	Night	2 4 1 5% 4 3 3 2 5% 8% 64%	1600
July	1st to 10th	Night	2 1 5% 5% 4 4 2 3 7% 66%	3681
	11th to 20th	Night	1 4 2 7% 6% 9% 7% 4 5% 7% 47%	4100
	21st to end	Night	4 3 8% 9% 10% 6% 4 5% 8% 43%	4472
August	1st to 10th	Night	2 4 2 4 4 4 4 3 6% 9% 58%	4035
	11th to 20th	Night	2 1 5% 4 5% 3 3 4 8% 65%	4272
	21st to end	Night	1 3 1 5% 4 4 5% 4 6% 8% 60%	5015
September	1st to 10th	Night	1 3 1 4 4 3 4 3 5% 10% 63%	2000
	11th to 20th	Night	3 5% 2 6% 4 4 4 3 5% 8% 55%	2000
	21st to end	Night	2 5% 2 5% 4 4 3 3 4 7% 63%	2293
October	1st to 10th	Night	2 3 1 4 3 2 3 2 5% 6% 68%	3920
	11th to 20th	Night	3 5% 2 5% 4 4 3 2 4 7% 61%	3897
	21st to end	Night	5% 3 2 6% 3 3 4 3 5% 7% 59%	4253
November	1st to 10th	Night	8% 4 2 5% 4 3 3 3 5% 8% 55%	2369
	11th to 20th	Night	10% 6% 2 5% 4 3 3 4 5% 7% 51%	2603
	21st to end	Night	9% 6% 1 5% 4 4 4 3 5% 7% 54%	2800
December	1st to 10th	Night	10% 4 2 5% 3 3 4 4 6% 7% 52%	4560
	11th to 20th	Night	22% 9% 3 6% 5% 4 5% 5% 7% 7% 27%	5176
	21st to end	Night	26% 10% 3 7% 4 4 4 3 4 5% 31%	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	25% 32% 13% 16% 10% 5%	3942
	11th to 20th	Day	31% 29% 6% 9% 14% 11%	4100
	21st to end	Day	33% 31% 4 10% 6% 15%	4510
February	1st to 10th	Day	20% 61% 4 6% 7% 1	1800
	11th to 20th	Day	19% 61% 3 7% 7% 2	1819
	21st to end	Day	14% 61% 4 11% 6% 3	1650
March	1st to 10th	Day	33% 32% 5% 13% 15% 1	4942
	11th to 20th	Day	20% 32% 12% 22% 7% 7%	5000
	21st to end	Day	29% 34% 13% 19% 5% 1	5520
April	1st to 10th	Day	15% 62% 5% 10% 7% 1	2400
	11th to 20th	Day	11% 63% 5% 12% 8% 1	2372
	21st to end	Day	10% 61% 7% 12% 9% 1	2400
May	1st to 10th	Day	7% 28% 6% 22% 23% 15%	5735
	11th to 20th	Day	8% 28% 6% 25% 29% 5%	5902
	21st to end	Day	5% 29% 4 23% 36% 3	6904
June	1st to 10th	Day	6% 61% 3 12% 13% 4	2800
	11th to 20th	Day	4 60% 2 11% 16% 6%	2800
	21st to end	Day	5% 59% 2 6% 16% 13%	2800
July	1st to 10th	Day	5% 24% 2 5% 18% 45%	6195
	11th to 20th	Day	7% 25% 3 7% 13% 45%	5811
	21st to end	Day	9% 27% 3 14% 22% 24%	6217
August	1st to 10th	Day	10% 35% 2 7% 19% 26%	4206
	11th to 20th	Day	4 35% 3 9% 28% 20%	4046
	21st to end	Day	8% 35% 3 13% 24% 17%	4311
September	1st to 10th	Day	7% 56% 2 8% 18% 10%	2400
	11th to 20th	Day	8% 57% 3 13% 14% 4	2399
	21st to end	Day	8% 56% 3 10% 16% 7%	2101
October	1st to 10th	Day	8% 35% 2 7% 17% 32%	3280
	11th to 20th	Day	14% 36% 4 15% 21% 10%	3261
	21st to end	Day	14% 37% 7% 18% 17% 7%	3558
November	1st to 10th	Day	8% 55% 5% 16% 12% 4	1973
	11th to 20th	Day	18% 56% 4 12% 7% 2	1795
	21st to end	Day	17% 57% 5% 9% 9% 4	1600
December	1st to 10th	Day	16% 36% 5% 22% 16% 3	2620
	11th to 20th	Day	39% 37% 5% 9% 8% 2	3028
	21st to end	Day	40% 29% 9% 14% 6% 1	4004
January	1st to 10th	Night	22% 30% 17% 17% 10% 3	6002
	11th to 20th	Night	30% 29% 7% 11% 15% 9%	5880
	21st to end	Night	31% 31% 4 8% 14% 12%	6468
February	1st to 10th	Night	19% 61% 3 7% 6% 3	2600
	11th to 20th	Night	16% 61% 4 9% 7% 3	2478
	21st to end	Night	11% 61% 3 14% 8% 3	1980
March	1st to 10th	Night	29% 33% 8% 16% 13% 2	4947
	11th to 20th	Night	21% 32% 8% 18% 14% 7%	5000
	21st to end	Night	28% 34% 11% 19% 8%	5454
April	1st to 10th	Night	13% 61% 4 11% 9% 2	2000
	11th to 20th	Night	12% 63% 4 12% 7% 3	1979
	21st to end	Night	8% 62% 6% 13% 9% 2	2000
May	1st to 10th	Night	9% 28% 4 19% 25% 15%	4245
	11th to 20th	Night	14% 28% 8% 22% 21% 7%	4011
	21st to end	Night	5% 28% 4 21% 38% 4	3972
June	1st to 10th	Night	6% 61% 4 9% 15% 5%	1600
	11th to 20th	Night	4 60% 2 8% 17% 8%	1600
	21st to end	Night	5% 59% 1 4 15% 16%	1600
July	1st to 10th	Night	7% 25% 3 8% 16% 41%	3641
	11th to 20th	Night	9% 26% 6% 16% 10% 33%	4046
	21st to end	Night	9% 30% 7% 11% 17% 26%	4345
August	1st to 10th	Night	12% 36% 2 9% 18% 22%	2929
	11th to 20th	Night	6% 35% 3 6% 26% 25%	3084
	21st to end	Night	9% 35% 3 10% 24% 19%	3608
September	1st to 10th	Night	6% 56% 2 6% 16% 13%	2000
	11th to 20th	Night	8% 56% 3 11% 14% 7%	2000
	21st to end	Night	9% 56% 2 10% 13% 10%	2293
October	1st to 10th	Night	8% 35% 2 4 12% 39%	3920
	11th to 20th	Night	11% 37% 6% 16% 17% 13%	3897
	21st to end	Night	15% 36% 4 15% 18% 12%	4253
November	1st to 10th	Night	10% 55% 3 15% 11% 6%	2369
	11th to 20th	Night	14% 57% 5% 14% 8% 3	2603
	21st to end	Night	16% 56% 4 9% 9% 5%	2800
December	1st to 10th	Night	17% 37% 5% 19% 16% 6%	4560
	11th to 20th	Night	37% 34% 6% 11% 9% 2	5176
	21st to end	Night	42% 29% 8% 14% 6% 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	26% 8% 29% 21% 13% 3	334
	11th to 20th	Day	16% 13% 24% 29% 15% 4	400
	21st to end	Day	13% 9% 27% 30% 18% 3	440
February	1st to 10th	Day	21% 11% 21% 23% 19% 4	400
	11th to 20th	Day	19% 16% 26% 22% 14% 3	422
	21st to end	Day	13% 15% 27% 27% 15% 3	414
March	1st to 10th	Day	21% 15% 29% 22% 11% 2	500
	11th to 20th	Day	21% 11% 29% 21% 13% 6%	500
	21st to end	Day	16% 18% 31% 23% 11% 2	550
April	1st to 10th	Day	15% 13% 30% 25% 13% 4	500
	11th to 20th	Day	11% 18% 30% 22% 13% 6%	492
	21st to end	Day	7% 18% 29% 26% 14% 6%	500
May	1st to 10th	Day	7% 17% 29% 29% 15% 3	503
	11th to 20th	Day	8% 15% 29% 27% 16% 5%	528
	21st to end	Day	6% 11% 27% 29% 22% 5%	573
June	1st to 10th	Day	5% 12% 28% 34% 16% 5%	530
	11th to 20th	Day	2 13% 32% 37% 12% 4	530
	21st to end	Day	1 5% 28% 39% 23% 5%	530
July	1st to 10th	Day	2 3 22% 48% 22% 3	526
	11th to 20th	Day	1 18% 44% 30% 6%	524
	21st to end	Day	1 2 16% 51% 24% 5%	575
August	1st to 10th	Day	2 3 23% 43% 24% 4	527
	11th to 20th	Day	2 3 24% 40% 24% 8%	517
	21st to end	Day	1 2 18% 44% 26% 10%	583
September	1st to 10th	Day	1 4 21% 37% 27% 10%	530
	11th to 20th	Day	4 8% 21% 34% 24% 9%	530
	21st to end	Day	3 7% 22% 31% 29% 8%	530
October	1st to 10th	Day	5% 7% 23% 26% 25% 14%	530
	11th to 20th	Day	7% 10% 20% 22% 29% 12%	530
	21st to end	Day	9% 8% 22% 25% 26% 9%	583
November	1st to 10th	Day	9% 9% 20% 22% 30% 10%	497
	11th to 20th	Day	12% 10% 16% 30% 25% 7%	297
	21st to end	Day	12% 11% 25% 30% 20% 3	200
December	1st to 10th	Day	18% 11% 20% 29% 17% 7%	200
	11th to 20th	Day	22% 14% 21% 26% 16% 3	200
	21st to end	Day	23% 16% 21% 26% 13% 1	220
January	1st to 10th	Night	20% 8% 12% 31% 24% 5%	800
	11th to 20th	Night	17% 6% 15% 34% 24% 5%	800
	21st to end	Night	17% 5% 14% 32% 27% 4	880
February	1st to 10th	Night	19% 6% 16% 32% 24% 4	800
	11th to 20th	Night	16% 9% 19% 30% 20% 5%	784
	21st to end	Night	14% 11% 19% 33% 20% 3	660
March	1st to 10th	Night	20% 9% 19% 29% 20% 3	800
	11th to 20th	Night	17% 9% 19% 30% 23% 3	800
	21st to end	Night	14% 10% 26% 31% 17% 2	880
April	1st to 10th	Night	11% 11% 27% 31% 18% 3	800
	11th to 20th	Night	10% 13% 24% 27% 21% 4	791
	21st to end	Night	6% 13% 26% 28% 24% 4	800
May	1st to 10th	Night	5% 7% 28% 30% 25% 4	670
	11th to 20th	Night	5% 5% 23% 33% 31% 4	567
	21st to end	Night	5% 9% 21% 36% 27% 3	433
June	1st to 10th	Night	4 5% 21% 42% 27% 2	400
	11th to 20th	Night	2 7% 21% 48% 21% 2	400
	21st to end	Night	2 2 16% 46% 32% 2	400
July	1st to 10th	Night	1 1 12% 43% 40% 3	405
	11th to 20th	Night	2 1 5% 40% 44% 9%	582
	21st to end	Night	2 1 6% 42% 41% 7%	657
August	1st to 10th	Night	4 1 9% 40% 40% 6%	597
	11th to 20th	Night	2 13% 42% 36% 7%	684
	21st to end	Night	3 1 10% 35% 44% 8%	880
September	1st to 10th	Night	1 2 10% 39% 40% 9%	800
	11th to 20th	Night	5% 3 15% 35% 36% 7%	800
	21st to end	Night	3 3 15% 34% 37% 8%	800
October	1st to 10th	Night	6% 4 16% 33% 33% 10%	800
	11th to 20th	Night	7% 5% 16% 30% 34% 9%	800
	21st to end	Night	9% 4 12% 30% 33% 12%	880
November	1st to 10th	Night	9% 4 16% 27% 34% 10%	788
	11th to 20th	Night	12% 5% 13% 32% 31% 7%	800
	21st to end	Night	11% 6% 16% 36% 26% 6%	800
December	1st to 10th	Night	18% 5% 14% 31% 26% 6%	800
	11th to 20th	Night	23% 7% 15% 31% 21% 3	800
	21st to end	Night	21% 10% 19% 28% 20% 1	880

Page generated at 2025-10-30 23:02:38. Version ac07

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