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 2024-11-06.

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% 5% 2 5% 4 3 4 3 5% 8% 54%	105047
	Night	9% 5% 2 6% 4 4 4 3 5% 7% 51%	102756
Transparency	Day	14% 46% 4 12% 13% 11%	104809
	Night	16% 46% 5% 11% 11% 11%	102585
Seeing	Day	9% 10% 24% 31% 20% 6%	15442
	Night	10% 6% 16% 33% 29% 6%	24729

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	14% 9% 2 7% 5% 4 5% 4 6% 7% 37%	2744
	11th to 20th	Day	20% 9% 4 9% 5% 4 4 3 5% 6% 31%	2860
	21st to end	Day	20% 8% 3 9% 5% 4 5% 4 5% 7% 30%	3146
February	1st to 10th	Day	16% 6% 2 5% 5% 3 4 3 5% 6% 44%	1710
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	1717
	21st to end	Day	12% 7% 3 6% 4 3 4 3 4 7% 47%	1570
March	1st to 10th	Day	15% 10% 3 10% 6% 6% 5% 4 5% 7% 29%	3432
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 5% 8% 53%	3490
	21st to end	Day	15% 7% 2 6% 5% 4 5% 5% 7% 9% 35%	3846
April	1st to 10th	Day	11% 7% 2 6% 4 4 4 4 6% 7% 43%	2280
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 5% 8% 53%	2249
	21st to end	Day	4 4 2 5% 3 3 4 3 6% 11% 56%	2280
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 3 5% 71%	4004
	11th to 20th	Day	3 3 2 5% 3 3 4 3 5% 11% 58%	4116
	21st to end	Day	2 4 1 4 3 2 3 2 4 6% 69%	4813
June	1st to 10th	Day	5% 5% 2 4 3 3 3 3 6% 9% 57%	2660
	11th to 20th	Day	1 3 1 3 3 3 3 3 5% 9% 66%	2660
	21st to end	Day	2 3 1 4 2 3 3 3 5% 9% 64%	2660
July	1st to 10th	Day	1 3 1 3 2 1 2 2 4 7% 73%	4376
	11th to 20th	Day	1 3 1 3 3 3 4 3 5% 9% 65%	4110
	21st to end	Day	3 1 4 3 4 3 3 7% 12% 60%	4455
August	1st to 10th	Day	1 3 2 5% 4 3 6% 4 9% 12% 52%	4091
	11th to 20th	Day	2 1 3 2 3 3 2 5% 8% 70%	3882
	21st to end	Day	1 4 2 4 4 4 4 3 6% 12% 57%	4180
September	1st to 10th	Day	1 3 1 4 2 3 3 2 5% 11% 65%	2280
	11th to 20th	Day	3 5% 2 5% 3 3 4 4 5% 9% 57%	2279
	21st to end	Day	2 3 1 3 2 2 2 3 5% 8% 69%	1996
October	1st to 10th	Day	2 3 1 4 3 2 3 2 4 7% 69%	3180
	11th to 20th	Day	5% 6% 2 5% 3 3 3 2 5% 6% 60%	3161
	21st to end	Day	5% 4 2 5% 4 3 4 4 6% 9% 53%	3448
November	1st to 10th	Day	8% 5% 2 5% 4 3 4 3 5% 8% 54%	1904
	11th to 20th	Day	10% 8% 3 7% 4 4 3 3 5% 6% 46%	1706
	21st to end	Day	11% 6% 2 4 4 3 3 3 5% 8% 52%	1520
December	1st to 10th	Day	15% 7% 3 6% 4 4 4 3 5% 6% 43%	1520
	11th to 20th	Day	28% 11% 3 6% 5% 3 3 3 5% 4 29%	1928
	21st to end	Day	23% 7% 2 6% 5% 3 3 4 6% 8% 33%	2794
January	1st to 10th	Night	14% 7% 3 7% 5% 4 3 4 5% 6% 42%	4193
	11th to 20th	Night	19% 9% 3 9% 5% 5% 4 4 5% 6% 32%	4110
	21st to end	Night	21% 10% 4 7% 5% 4 4 4 5% 6% 30%	4521
February	1st to 10th	Night	16% 6% 2 4 4 3 4 3 5% 5% 48%	2470
	11th to 20th	Night	11% 7% 3 7% 4 4 4 4 5% 8% 44%	2349
	21st to end	Night	11% 6% 2 4 4 4 4 3 5% 6% 51%	1884
March	1st to 10th	Night	15% 9% 3 8% 5% 5% 5% 3 5% 7% 36%	3437
	11th to 20th	Night	11% 5% 1 3 3 3 3 3 5% 5% 57%	3490
	21st to end	Night	13% 8% 2 7% 5% 5% 4 3 5% 7% 41%	3806
April	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 5% 7% 50%	1900
	11th to 20th	Night	8% 5% 2 5% 3 3 3 3 4 6% 57%	1879
	21st to end	Night	4 4 2 4 3 3 3 3 6% 9% 59%	1900
May	1st to 10th	Night	3 3 1 4 3 2 2 2 3 6% 71%	2966
	11th to 20th	Night	2 4 4 12% 4 4 3 2 3 7% 55%	2784
	21st to end	Night	2 3 1 3 3 3 2 2 3 5% 73%	2762
June	1st to 10th	Night	3 5% 2 6% 5% 4 4 4 6% 8% 53%	1520
	11th to 20th	Night	1 2 2 4 3 4 3 3 5% 8% 66%	1520
	21st to end	Night	2 4 1 5% 5% 3 3 3 5% 8% 63%	1520
July	1st to 10th	Night	1 2 4 4 2 2 2 3 8% 71%	2567
	11th to 20th	Night	1 3 2 5% 3 6% 6% 3 5% 8% 57%	2860
	21st to end	Night	1 4 2 7% 6% 7% 6% 4 6% 9% 48%	3109
August	1st to 10th	Night	2 5% 2 6% 5% 6% 6% 4 7% 11% 44%	2848
	11th to 20th	Night	3 2 6% 4 4 3 3 4 7% 64%	2962
	21st to end	Night	1 3 2 6% 4 4 4 4 5% 7% 60%	3498
September	1st to 10th	Night	1 3 1 4 4 3 4 3 5% 10% 64%	1900
	11th to 20th	Night	3 5% 2 6% 4 4 5% 3 5% 8% 55%	1900
	21st to end	Night	2 4 2 5% 4 4 4 3 4 7% 63%	2178
October	1st to 10th	Night	2 3 1 4 3 2 3 2 5% 6% 69%	3800
	11th to 20th	Night	3 5% 2 5% 4 4 3 2 4 7% 61%	3777
	21st to end	Night	5% 3 2 6% 3 3 4 3 5% 7% 59%	4121
November	1st to 10th	Night	8% 4 2 5% 4 3 3 3 5% 8% 55%	2295
	11th to 20th	Night	10% 6% 2 5% 4 3 3 4 6% 7% 50%	2472
	21st to end	Night	8% 6% 1 5% 4 4 4 2 5% 7% 55%	2660
December	1st to 10th	Night	17% 6% 2 6% 4 3 4 3 4 6% 45%	2660
	11th to 20th	Night	30% 9% 2 5% 4 3 3 3 5% 6% 31%	3276
	21st to end	Night	21% 8% 2 7% 5% 4 5% 4 5% 6% 35%	4862

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	27% 40% 8% 20% 3 1	2744
	11th to 20th	Day	43% 39% 7% 7% 2 2	2860
	21st to end	Day	38% 41% 5% 12% 3 1	3146
February	1st to 10th	Day	18% 64% 4 6% 6% 1	1710
	11th to 20th	Day	17% 64% 2 7% 8% 2	1726
	21st to end	Day	13% 65% 4 11% 5% 3	1570
March	1st to 10th	Day	37% 43% 5% 7% 7%	3432
	11th to 20th	Day	12% 42% 7% 22% 7% 10%	3490
	21st to end	Day	26% 43% 10% 16% 4 1	3846
April	1st to 10th	Day	15% 65% 4 9% 6% 1	2280
	11th to 20th	Day	11% 66% 5% 11% 7%	2252
	21st to end	Day	8% 64% 7% 11% 9% 1	2280
May	1st to 10th	Day	8% 39% 8% 22% 13% 11%	4004
	11th to 20th	Day	9% 39% 5% 17% 25% 5%	4119
	21st to end	Day	7% 41% 5% 23% 23% 2	4814
June	1st to 10th	Day	7% 64% 3 12% 11% 4	2660
	11th to 20th	Day	4 63% 2 11% 15% 4	2660
	21st to end	Day	5% 62% 2 5% 14% 12%	2660
July	1st to 10th	Day	4 34% 2 4 7% 48%	4308
	11th to 20th	Day	6% 34% 3 6% 10% 41%	4041
	21st to end	Day	4 36% 1 7% 21% 31%	4315
August	1st to 10th	Day	11% 37% 2 7% 17% 26%	4076
	11th to 20th	Day	4 36% 3 9% 28% 19%	3921
	21st to end	Day	8% 36% 3 13% 23% 16%	4179
September	1st to 10th	Day	6% 58% 1 7% 17% 11%	2280
	11th to 20th	Day	8% 60% 3 12% 13% 4	2279
	21st to end	Day	6% 59% 3 10% 16% 7%	1996
October	1st to 10th	Day	7% 36% 1 7% 17% 32%	3180
	11th to 20th	Day	14% 37% 4 14% 21% 10%	3161
	21st to end	Day	14% 38% 7% 18% 17% 7%	3448
November	1st to 10th	Day	8% 57% 5% 16% 10% 4	1904
	11th to 20th	Day	17% 59% 4 11% 7% 2	1706
	21st to end	Day	15% 59% 4 8% 9% 4	1520
December	1st to 10th	Day	21% 60% 4 8% 4 3	1520
	11th to 20th	Day	34% 48% 3 4 8% 3	1928
	21st to end	Day	30% 39% 9% 13% 7% 2	2794
January	1st to 10th	Night	25% 39% 13% 18% 4 2	4193
	11th to 20th	Night	41% 39% 8% 6% 3 2	4110
	21st to end	Night	38% 43% 5% 8% 4 2	4521
February	1st to 10th	Night	18% 64% 3 7% 6% 3	2470
	11th to 20th	Night	15% 64% 3 8% 7% 3	2351
	21st to end	Night	10% 64% 3 14% 7% 2	1884
March	1st to 10th	Night	29% 45% 9% 10% 7% 1	3437
	11th to 20th	Night	13% 42% 5% 15% 16% 9%	3490
	21st to end	Night	27% 44% 9% 14% 5% 1	3806
April	1st to 10th	Night	12% 64% 4 9% 8% 2	1900
	11th to 20th	Night	10% 65% 4 12% 6% 3	1879
	21st to end	Night	7% 65% 5% 12% 9% 2	1900
May	1st to 10th	Night	9% 39% 5% 20% 18% 9%	2966
	11th to 20th	Night	15% 38% 9% 10% 18% 10%	2785
	21st to end	Night	5% 39% 5% 26% 23% 2	2762
June	1st to 10th	Night	6% 64% 3 8% 14% 5%	1520
	11th to 20th	Night	4 63% 2 9% 16% 6%	1520
	21st to end	Night	5% 62% 1 3 13% 15%	1520
July	1st to 10th	Night	5% 34% 1 5% 7% 48%	2527
	11th to 20th	Night	7% 34% 3 10% 10% 36%	2806
	21st to end	Night	5% 36% 1 9% 14% 35%	3011
August	1st to 10th	Night	12% 37% 2 9% 17% 23%	2839
	11th to 20th	Night	6% 35% 2 5% 27% 25%	2989
	21st to end	Night	8% 36% 3 10% 24% 19%	3498
September	1st to 10th	Night	5% 59% 1 5% 16% 13%	1900
	11th to 20th	Night	7% 59% 3 10% 14% 7%	1900
	21st to end	Night	7% 58% 2 11% 12% 10%	2178
October	1st to 10th	Night	7% 36% 2 4 12% 40%	3800
	11th to 20th	Night	11% 38% 6% 16% 16% 13%	3777
	21st to end	Night	15% 37% 4 14% 18% 12%	4121
November	1st to 10th	Night	10% 56% 3 15% 11% 6%	2295
	11th to 20th	Night	14% 60% 4 13% 7% 3	2472
	21st to end	Night	15% 59% 4 9% 9% 5%	2660
December	1st to 10th	Night	20% 59% 4 8% 5% 4	2660
	11th to 20th	Night	31% 48% 3 7% 8% 3	3276
	21st to end	Night	29% 40% 9% 13% 6% 2	4862

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	28% 8% 30% 18% 14% 3	307
	11th to 20th	Day	17% 12% 24% 28% 15% 4	370
	21st to end	Day	14% 8% 28% 30% 17% 3	407
February	1st to 10th	Day	18% 12% 21% 25% 20% 4	370
	11th to 20th	Day	18% 16% 26% 22% 15% 3	389
	21st to end	Day	13% 16% 27% 25% 15% 3	382
March	1st to 10th	Day	21% 15% 29% 23% 11% 2	460
	11th to 20th	Day	19% 11% 29% 21% 13% 6%	460
	21st to end	Day	15% 18% 31% 23% 10% 2	506
April	1st to 10th	Day	16% 13% 29% 25% 13% 3	460
	11th to 20th	Day	12% 18% 29% 22% 13% 6%	452
	21st to end	Day	7% 18% 29% 27% 13% 6%	460
May	1st to 10th	Day	7% 17% 29% 29% 15% 3	463
	11th to 20th	Day	7% 15% 29% 28% 16% 5%	488
	21st to end	Day	6% 11% 26% 29% 23% 5%	529
June	1st to 10th	Day	5% 13% 27% 33% 17% 5%	490
	11th to 20th	Day	2 12% 32% 38% 13% 4	490
	21st to end	Day	1 5% 28% 40% 22% 4	490
July	1st to 10th	Day	2 3 20% 49% 23% 3	486
	11th to 20th	Day	1 18% 44% 30% 6%	484
	21st to end	Day	2 2 15% 53% 24% 5%	531
August	1st to 10th	Day	2 3 22% 45% 24% 4	487
	11th to 20th	Day	2 3 24% 39% 24% 8%	477
	21st to end	Day	1 2 18% 43% 26% 9%	539
September	1st to 10th	Day	1 4 19% 37% 28% 11%	490
	11th to 20th	Day	3 9% 21% 35% 24% 8%	490
	21st to end	Day	2 7% 22% 31% 31% 8%	490
October	1st to 10th	Day	4 6% 23% 28% 25% 13%	490
	11th to 20th	Day	6% 9% 20% 22% 29% 13%	490
	21st to end	Day	9% 8% 22% 26% 25% 10%	539
November	1st to 10th	Day	10% 9% 20% 23% 29% 9%	470
	11th to 20th	Day	11% 8% 15% 31% 27% 8%	268
	21st to end	Day	10% 9% 24% 32% 21% 3	180
December	1st to 10th	Day	19% 12% 19% 27% 16% 7%	180
	11th to 20th	Day	21% 14% 20% 26% 17% 3	180
	21st to end	Day	21% 15% 21% 28% 14% 1	198
January	1st to 10th	Night	21% 8% 11% 30% 25% 6%	760
	11th to 20th	Night	18% 6% 13% 34% 24% 5%	760
	21st to end	Night	18% 6% 14% 32% 26% 4	836
February	1st to 10th	Night	18% 6% 16% 33% 25% 4	760
	11th to 20th	Night	15% 9% 19% 30% 21% 6%	744
	21st to end	Night	14% 11% 19% 32% 20% 4	628
March	1st to 10th	Night	20% 9% 18% 29% 21% 3	760
	11th to 20th	Night	17% 8% 18% 30% 24% 3	760
	21st to end	Night	14% 10% 26% 32% 17% 2	836
April	1st to 10th	Night	11% 10% 27% 31% 19% 3	760
	11th to 20th	Night	11% 13% 23% 27% 22% 4	751
	21st to end	Night	6% 13% 26% 27% 24% 4	760
May	1st to 10th	Night	5% 7% 28% 29% 25% 5%	637
	11th to 20th	Night	5% 4 22% 33% 33% 4	538
	21st to end	Night	5% 9% 20% 37% 27% 3	411
June	1st to 10th	Night	4 5% 22% 41% 27% 2	380
	11th to 20th	Night	2 7% 19% 48% 22% 2	380
	21st to end	Night	2 1 16% 47% 32% 2	380
July	1st to 10th	Night	2 1 11% 42% 41% 3	384
	11th to 20th	Night	3 1 5% 40% 43% 9%	552
	21st to end	Night	3 1 6% 42% 42% 7%	624
August	1st to 10th	Night	5% 1 8% 41% 40% 6%	567
	11th to 20th	Night	2 13% 41% 36% 7%	649
	21st to end	Night	3 1 10% 35% 44% 8%	836
September	1st to 10th	Night	1 1 9% 38% 41% 9%	760
	11th to 20th	Night	5% 3 14% 36% 36% 7%	760
	21st to end	Night	3 3 13% 34% 38% 9%	760
October	1st to 10th	Night	6% 3 15% 32% 33% 10%	760
	11th to 20th	Night	7% 4 15% 30% 35% 9%	760
	21st to end	Night	9% 4 11% 30% 34% 12%	836
November	1st to 10th	Night	9% 5% 16% 27% 33% 10%	764
	11th to 20th	Night	12% 3 12% 33% 32% 8%	760
	21st to end	Night	10% 6% 15% 36% 27% 6%	760
December	1st to 10th	Night	19% 5% 14% 31% 26% 6%	760
	11th to 20th	Night	23% 7% 14% 31% 22% 4	760
	21st to end	Night	20% 9% 19% 29% 21% 2	836

Page generated at 2024-11-03 10:19:03. Version ac07

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