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 2003-08-01 to 2005-11-07.

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	9% 6% 9% 47%	8694
	Night	13% 7% 48%	8477
Transparency	Day	10% 9% 27% 31% 20%	6187
	Night	13% 10% 25% 26% 23%	5661
Seeing	Day	9% 29% 37% 20%	662
	Night	5% 19% 29% 36% 8%	2695

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	21% 5% 7% 9% 34%	542
	Night	23% 7% 6% 34%	812
February	Day	27% 8% 6% 5% 6% 27%	527
	Night	28% 6% 5% 5% 31%	718
March	Day	16% 8% 46%	682
	Night	12% 6% 57%	680
April	Day	10% 5% 5% 8% 53%	720
	Night	8% 5% 6% 62%	600
May	Day	9% 9% 49%	824
	Night	10% 7% 57%	540
June	Day	5% 5% 11% 56%	840
	Night	10% 59%	478
July	Day	5% 6% 9% 12% 13% 40%	812
	Night	6% 6% 8% 6% 5% 6% 17% 35%	539
August	Day	5% 5% 6% 12% 51%	1159
	Night	5% 5% 5% 9% 51%	884
September	Day	5% 9% 60%	1016
	Night	7% 66%	930
October	Day	7% 5% 5% 7% 7% 48%	620
	Night	9% 6% 5% 7% 53%	744
November	Day	15% 7% 7% 45%	464
	Night	16% 5% 5% 6% 44%	702
December	Day	27% 5% 6% 5% 5% 33%	488
	Night	32% 6% 5% 5% 28%	850

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	26% 11% 44% 16%	277
	Night	28% 20% 31% 16%	363
February	Day	36% 15% 19% 19% 7%	206
	Night	30% 14% 19% 21% 13%	276
March	Day	10% 9% 29% 37% 13%	421
	Night	11% 11% 35% 21% 20%	443
April	Day	14% 19% 25% 16% 23%	498
	Night	11% 17% 25% 22% 24%	416
May	Day	13% 10% 38% 24% 13%	539
	Night	7% 14% 29% 31% 16%	375
June	Day	5% 7% 37% 44%	631
	Night	5% 5% 41% 41% 5%	358
July	Day	7% 11% 35% 41%	640
	Night	18% 16% 24% 37%	389
August	Day	7% 20% 37% 29%	926
	Night	13% 5% 16% 34% 30%	632
September	Day	22% 37% 33%	818
	Night	9% 16% 25% 46%	781
October	Day	5% 16% 25% 32% 19%	665
	Night	5% 17% 22% 27% 26%	792
November	Day	19% 20% 34% 21%	345
	Night	18% 12% 38% 18% 10%	475
December	Day	18% 12% 37% 19% 11%	221
	Night	19% 9% 31% 30% 9%	361

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Day	24% 28% 40% 6%	45
	Night	6% 18% 33% 40%	198
February	Day	9% 35% 33% 14% 7%	42
	Night	5% 30% 34% 24% 5%	176
March	Day	15% 30% 18% 35%	53
	Night	7% 15% 29% 41%	227
April	Day	20% 33% 25% 18%	54
	Night	7% 27% 22% 37% 5%	227
May	Day	24% 33% 31% 10%	57
	Night	7% 27% 31% 29%	151
June	Day	10% 36% 45% 7%	57
	Night	6% 25% 33% 28% 5%	118
July	Day	25% 61% 12%	62
	Night	13% 29% 43% 13%	165
August	Day	17% 52% 28%	91
	Night	12% 28% 45% 13%	318
September	Day	11% 26% 38% 21%	90
	Night	16% 29% 36% 13%	357
October	Day	10% 34% 32% 21%	89
	Night	7% 14% 26% 40% 11%	349
November	Day	13% 45% 22% 18%	22
	Night	7% 19% 39% 29%	228
December	Night	7% 24% 28% 32% 6%	181

Page generated at 2008-05-11 01:45:37. Version ab11