Mansfield Dam
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mansfield Dam 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	17% 6% 5% 7% 36%	8680
	Night	16% 6% 5% 7% 41%	8495
Transparency	Day	17% 13% 30% 28% 10%	4763
	Night	17% 14% 27% 26% 14%	5037
Seeing	Day	7% 23% 37% 27%	463
	Night	10% 28% 36% 21%	2556

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	25% 5% 6% 5% 31%	561
	Night	27% 5% 5% 36%	794
February	Day	32% 6% 31%	570
	Night	31% 6% 5% 31%	684
March	Day	21% 6% 6% 5% 6% 37%	680
	Night	27% 5% 42%	683
April	Day	19% 5% 5% 5% 6% 41%	696
	Night	13% 6% 9% 47%	624
May	Day	11% 5% 5% 7% 5% 7% 13% 30%	803
	Night	11% 6% 6% 5% 11% 41%	550
June	Day	18% 6% 5% 6% 7% 34%	780
	Night	14% 5% 5% 11% 43%	537
July	Day	9% 6% 10% 6% 5% 11% 32%	801
	Night	9% 5% 5% 8% 5% 8% 11% 37%	558
August	Day	10% 5% 6% 6% 8% 5% 7% 8% 33%	1135
	Night	10% 5% 8% 8% 12% 34%	908
September	Day	11% 5% 5% 5% 7% 5% 7% 38%	970
	Night	12% 7% 6% 7% 8% 39%	975
October	Day	21% 5% 5% 5% 5% 5% 34%	642
	Night	11% 5% 5% 7% 6% 5% 6% 39%	718
November	Day	22% 5% 45%	494
	Night	20% 5% 47%	673
December	Day	15% 57%	548
	Night	11% 59%	791

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	16% 9% 32% 37%	225
	Night	12% 11% 35% 28% 10%	364
February	Day	14% 12% 40% 28%	235
	Night	11% 16% 35% 27% 9%	271
March	Day	13% 11% 24% 37% 14%	362
	Night	7% 6% 28% 37% 20%	359
April	Day	13% 8% 33% 26% 16%	384
	Night	16% 10% 27% 23% 21%	392
May	Day	25% 17% 36% 15% 5%	410
	Night	23% 22% 35% 16%	334
June	Day	18% 16% 33% 26% 5%	384
	Night	22% 14% 34% 25%	316
July	Day	22% 6% 30% 31% 8%	376
	Night	34% 10% 26% 21% 6%	303
August	Day	31% 26% 29% 11%	620
	Night	32% 21% 28% 13%	499
September	Day	12% 17% 26% 29% 12%	547
	Night	18% 23% 22% 20% 15%	592
October	Day	12% 13% 30% 35% 8%	511
	Night	14% 16% 22% 30% 15%	637
November	Day	13% 7% 23% 29% 26%	338
	Night	11% 7% 18% 24% 37%	415
December	Day	5% 24% 41% 24%	371
	Night	8% 5% 22% 41% 21%	555

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	Night	16% 44% 30% 8%	230
February	Night	15% 41% 36% 6%	165
March	Night	14% 49% 31%	147
April	Day	13% 55% 27%	29
	Night	19% 47% 31%	163
May	Day	12% 25% 44% 16%	109
	Night	7% 33% 41% 17%	175
June	Day	8% 23% 39% 25%	106
	Night	6% 25% 38% 28%	164
July	Day	18% 39% 34%	118
	Night	6% 17% 42% 31%	183
August	Day	17% 28% 39% 8%	101
	Night	10% 32% 46% 6%	258
September	Night	9% 38% 41% 7%	250
October	Night	8% 19% 42% 25%	305
November	Night	20% 29% 34% 14%	239
December	Night	14% 29% 38% 15%	277

Page generated at 2008-06-05 22:07:38. Version ab11