WCAC
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WCAC 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	24% 7% 6% 5% 6% 7% 26%	8699
	Night	23% 7% 6% 6% 6% 30%	8474
Transparency	Day	24% 19% 38% 16%	3698
	Night	21% 16% 37% 22%	3900
Seeing	Day	9% 20% 27% 25% 16%	481
	Night	13% 27% 33% 22%	2390

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	33% 13% 12% 5% 6% 10%	551
	Night	35% 10% 9% 5% 5% 6% 12%	802
February	Day	27% 10% 9% 5% 6% 22%	537
	Night	24% 9% 10% 6% 7% 23%	717
March	Day	31% 11% 7% 5% 6% 7% 14%	677
	Night	28% 9% 8% 5% 5% 6% 6% 19%	687
April	Day	29% 5% 6% 5% 5% 5% 30%	740
	Night	26% 5% 5% 6% 35%	581
May	Day	18% 5% 5% 5% 6% 6% 6% 7% 30%	802
	Night	16% 5% 6% 6% 5% 7% 35%	551
June	Day	13% 6% 6% 5% 5% 8% 5% 6% 9% 29%	822
	Night	14% 5% 5% 5% 8% 6% 7% 37%	494
July	Day	23% 6% 5% 5% 8% 7% 24%	804
	Night	13% 5% 8% 5% 9% 10% 33%	558
August	Day	18% 5% 5% 5% 6% 7% 8% 29%	1201
	Night	13% 6% 5% 5% 7% 40%	838
September	Day	18% 7% 7% 41%	983
	Night	15% 5% 6% 48%	962
October	Day	26% 5% 5% 5% 10% 27%	625
	Night	25% 5% 6% 10% 33%	737
November	Day	33% 7% 8% 5% 5% 22%	457
	Night	29% 8% 6% 5% 25%	710
December	Day	34% 12% 6% 5% 5% 6% 15%	500
	Night	38% 11% 6% 5% 5% 5% 16%	837

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	34% 27% 30% 7%	114
	Night	43% 14% 27% 15%	186
February	Day	30% 20% 33% 14%	183
	Night	25% 15% 36% 19%	233
March	Day	39% 26% 30%	185
	Night	24% 21% 40% 11%	214
April	Day	21% 9% 46% 20%	309
	Night	9% 6% 45% 30% 7%	279
May	Day	30% 20% 41% 7%	369
	Night	25% 17% 34% 22%	275
June	Day	25% 15% 38% 20%	390
	Night	16% 16% 38% 27%	258
July	Day	24% 13% 36% 19% 6%	300
	Night	19% 12% 41% 20% 6%	281
August	Day	22% 21% 42% 12%	524
	Night	26% 15% 40% 17%	467
September	Day	16% 15% 38% 26%	570
	Night	11% 11% 32% 42%	596
October	Day	22% 24% 39% 13%	404
	Night	20% 21% 35% 20%	573
November	Day	24% 26% 35% 13%	204
	Night	21% 25% 39% 10%	328
December	Day	37% 32% 18% 11%	146
	Night	33% 11% 39% 14%	210

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	30% 43% 21%	173
February	Night	20% 37% 31% 9%	182
March	Day	23% 31% 40%	47
	Night	23% 35% 36%	185
April	Day	15% 34% 29% 18%	44
	Night	12% 37% 32% 14%	158
May	Day	12% 27% 27% 25% 5%	54
	Night	7% 29% 41% 20%	165
June	Day	15% 35% 22% 22%	53
	Night	26% 42% 25%	124
July	Day	5% 12% 29% 27% 24%	54
	Night	16% 45% 30%	174
August	Day	8% 22% 32% 32%	80
	Night	11% 38% 39% 6%	259
September	Day	17% 20% 35% 22%	74
	Night	14% 30% 42% 9%	311
October	Day	12% 25% 25% 26% 10%	75
	Night	10% 25% 30% 30%	296
November	Night	16% 41% 30% 11%	195
December	Night	29% 34% 29% 5%	168

Page generated at 2008-06-05 23:07:34. Version ab11