Emory University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Emory University Observatory clear sky chart from 2005-11-05 to 2009-12-29.

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	13% 5% 5% 5% 8% 41%	16478
	Night	12% 5% 5% 8% 44%	16485
Transparency	Day	5% 14% 23% 22%	13546
	Night	12% 22% 25% 8%	13555
Seeing	Day	14% 22% 27% 19%	891
	Night	7% 24% 32% 22%	5081

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	17% 6% 5% 6% 43%	1118
	Night	18% 7% 5% 6% 42%	1579
February	Day	14% 5% 5% 5% 5% 5% 6% 42%	1089
	Night	15% 6% 47%	1306
March	Day	15% 5% 5% 9% 42%	1334
	Night	10% 5% 7% 50%	1390
April	Day	12% 5% 5% 5% 8% 45%	1417
	Night	10% 5% 6% 5% 8% 49%	1188
May	Day	11% 5% 5% 5% 5% 7% 44%	1598
	Night	8% 5% 5% 5% 6% 5% 6% 8% 42%	1100
June	Day	5% 5% 6% 6% 8% 13% 44%	1552
	Night	6% 7% 5% 7% 11% 43%	1078
July	Day	11% 5% 6% 6% 8% 8% 8% 11% 26%	1595
	Night	6% 7% 6% 6% 7% 9% 12% 32%	1097
August	Day	11% 5% 5% 6% 7% 6% 8% 10% 30%	1520
	Night	9% 5% 5% 6% 6% 7% 8% 9% 33%	1193
September	Day	17% 5% 5% 5% 5% 5% 7% 35%	1305
	Night	14% 6% 5% 5% 6% 5% 9% 37%	1295
October	Day	17% 50%	1280
	Night	14% 5% 5% 5% 5% 49%	1414
November	Day	13% 6% 52%	1301
	Night	12% 7% 55%	1867
December	Day	20% 6% 5% 5% 6% 38%	1369
	Night	18% 6% 5% 6% 41%	1978

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	12% 20% 33% 6%	863
	Night	8% 23% 29% 10%	1174
February	Day	13% 23% 28% 6%	839
	Night	10% 23% 30% 11%	1011
March	Day	5% 17% 28% 21%	1093
	Night	15% 28% 26% 6%	1139
April	Day	12% 30% 26%	1134
	Night	12% 26% 26% 9%	1012
May	Day	6% 17% 23% 20%	1396
	Night	6% 15% 25% 20%	931
June	Day	9% 25% 30% 18%	1292
	Night	8% 22% 26% 22%	851
July	Day	10% 19% 13% 16%	1020
	Night	8% 20% 23% 16% 5%	703
August	Day	10% 17% 22% 10%	1323
	Night	8% 17% 25% 12%	1010
September	Day	6% 9% 17% 21%	1145
	Night	7% 14% 16% 16% 7%	1173
October	Day	6% 18% 28% 8%	1193
	Night	7% 14% 30% 11%	1300
November	Day	7% 23% 34% 9%	1088
	Night	7% 22% 34% 11%	1602
December	Day	9% 24% 16% 7%	1160
	Night	9% 22% 24% 6%	1649

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% 37% 31% 5%	435
February	Night	11% 42% 29% 6%	397
March	Day	11% 19% 37% 19%	89
	Night	12% 28% 38% 13%	475
April	Day	7% 31% 26% 23%	113
	Night	7% 32% 38% 11%	374
May	Day	21% 27% 19% 16%	116
	Night	6% 25% 32% 24%	361
June	Day	22% 24% 43%	112
	Night	8% 35% 44% 7%	331
July	Day	12% 33% 39%	106
	Night	5% 32% 48% 8%	325
August	Day	5% 12% 31% 33%	119
	Night	5% 21% 48% 16%	354
September	Day	10% 22% 38% 6%	115
	Night	8% 34% 35%	447
October	Day	6% 20% 19% 27% 6%	117
	Night	23% 31% 20%	473
November	Day	25% 25% 25% 25%	4
	Night	8% 26% 34% 20%	544
December	Night	10% 33% 30% 12%	565

Page generated at 2009-12-30 05:20:35. Version ab11