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Richmond
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Richmond 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	21% 6% 5% 7% 34%	8714
	Night	20% 5% 6% 5% 7% 36%	8457
Transparency	Day	17% 13% 37% 27%	4461
	Night	17% 13% 36% 28%	4572
Seeing	Day	8% 28% 27% 25% 9%	541
	Night	14% 32% 32% 19%	2503

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% 6% 5% 39%	547
	Night	18% 6% 5% 6% 41%	805
February	Day	23% 5% 5% 39%	537
	Night	20% 6% 5% 7% 40%	716
March	Day	21% 6% 6% 8% 35%	684
	Night	20% 7% 5% 6% 7% 33%	680
April	Day	20% 5% 44%	720
	Night	23% 7% 6% 5% 36%	600
May	Day	18% 6% 6% 9% 34%	818
	Night	17% 5% 8% 6% 7% 9% 28%	537
June	Day	18% 5% 6% 7% 5% 9% 5% 6% 8% 22%	840
	Night	21% 8% 6% 6% 10% 27%	476
July	Day	19% 7% 6% 5% 7% 7% 8% 8% 22%	820
	Night	20% 6% 6% 6% 5% 9% 30%	541
August	Day	21% 6% 6% 5% 6% 5% 7% 29%	1156
	Night	17% 6% 5% 7% 6% 6% 7% 9% 25%	883
September	Day	24% 5% 8% 38%	1016
	Night	21% 6% 5% 7% 39%	928
October	Day	26% 5% 5% 7% 35%	618
	Night	23% 5% 6% 7% 34%	744
November	Day	23% 5% 7% 42%	472
	Night	23% 5% 6% 5% 40%	695
December	Day	18% 5% 6% 46%	486
	Night	16% 5% 6% 6% 47%	852

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	17% 13% 50% 15%	270
	Night	19% 10% 41% 26%	412
February	Day	13% 11% 32% 36% 5%	279
	Night	12% 11% 39% 28% 8%	382
March	Day	15% 20% 37% 26%	326
	Night	14% 12% 35% 34%	316
April	Day	9% 9% 40% 32% 8%	389
	Night	12% 6% 34% 38% 8%	283
May	Day	31% 12% 37% 14%	408
	Night	40% 11% 29% 14%	261
June	Day	31% 11% 38% 18%	305
	Night	32% 22% 28% 14%	227
July	Day	22% 17% 37% 20%	306
	Night	22% 17% 40% 17%	245
August	Day	18% 19% 43% 16%	515
	Night	21% 27% 37% 13%	384
September	Day	12% 9% 28% 41% 8%	563
	Night	13% 9% 35% 33% 7%	516
October	Day	17% 12% 35% 30%	484
	Night	14% 10% 38% 32%	590
November	Day	15% 15% 33% 33%	340
	Night	11% 14% 37% 33%	460
December	Day	10% 9% 39% 35%	276
	Night	12% 14% 31% 37%	496

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	22% 53% 21%	202
February	Day	11% 38% 50%	18
	Night	20% 44% 31%	188
March	Day	12% 42% 40%	49
	Night	31% 41% 25%	199
April	Day	15% 46% 23% 7% 7%	52
	Night	19% 39% 31% 8%	192
May	Day	41% 27% 27%	51
	Night	9% 34% 46% 9%	149
June	Day	17% 44% 32%	52
	Night	7% 22% 50% 18%	102
July	Day	7% 12% 25% 42% 11%	54
	Night	5% 15% 35% 39%	146
August	Day	9% 14% 20% 32% 23%	77
	Night	6% 14% 29% 47%	295
September	Day	13% 20% 20% 17% 27%	73
	Night	9% 20% 39% 28%	304
October	Day	6% 32% 28% 26% 5%	73
	Night	10% 34% 29% 23%	302
November	Day	33% 11% 50%	42
	Night	15% 31% 31% 21%	216
December	Night	21% 37% 31% 10%	208

Page generated at 2008-06-05 23:07:39. Version ab11