Austin
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Austin clear sky chart from 2005-11-05 to 2009-10-21.

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% 5% 7% 41%	15852
	Night	12% 7% 49%	15613
Transparency	Day	12% 21% 22% 12%	12379
	Night	15% 21% 21% 15%	12732
Seeing	Day	13% 40% 30% 5%	863
	Night	6% 26% 37% 19%	4752

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	18% 5% 49%	1121
	Night	21% 5% 5% 44%	1580
February	Day	16% 5% 54%	1089
	Night	13% 5% 57%	1306
March	Day	22% 6% 5% 5% 40%	1362
	Night	18% 7% 5% 43%	1362
April	Day	15% 6% 5% 5% 6% 41%	1365
	Night	16% 5% 46%	1237
May	Day	14% 6% 5% 7% 5% 6% 8% 34%	1599
	Night	11% 5% 6% 9% 46%	1099
June	Day	10% 5% 6% 6% 6% 6% 8% 39%	1551
	Night	5% 5% 5% 8% 56%	1079
July	Day	12% 5% 5% 7% 8% 5% 7% 8% 31%	1596
	Night	7% 5% 7% 12% 45%	1096
August	Day	5% 5% 6% 7% 7% 8% 13% 33%	1500
	Night	6% 7% 7% 12% 44%	1213
September	Day	10% 5% 7% 6% 6% 6% 6% 8% 35%	1304
	Night	9% 6% 6% 6% 5% 6% 44%	1300
October	Day	11% 5% 6% 53%	1158
	Night	10% 6% 60%	1291
November	Day	11% 6% 6% 50%	1091
	Night	10% 6% 57%	1439
December	Day	12% 5% 6% 50%	1116
	Night	15% 6% 50%	1611

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	5% 16% 27% 24%	874
	Night	6% 14% 23% 27%	1204
February	Day	8% 19% 27% 24%	863
	Night	7% 16% 27% 30%	1058
March	Day	10% 17% 22% 15%	992
	Night	5% 13% 17% 22% 17%	1017
April	Day	9% 19% 21% 19%	1008
	Night	12% 18% 20% 22%	926
May	Day	6% 15% 21% 23% 5%	1203
	Night	5% 21% 27% 19% 7%	895
June	Day	7% 16% 23% 27% 5%	1150
	Night	7% 18% 30% 31%	862
July	Day	7% 18% 25% 19%	956
	Night	8% 30% 28% 16%	746
August	Day	9% 21% 24% 11%	1347
	Night	10% 29% 25% 7%	1148
September	Day	5% 13% 20% 16% 7%	1132
	Night	5% 18% 20% 15% 7%	1165
October	Day	6% 19% 25% 19%	1041
	Night	5% 7% 23% 21% 25%	1149
November	Day	9% 27% 20% 16%	912
	Night	12% 25% 22% 17%	1257
December	Day	9% 19% 33% 12%	901
	Night	8% 18% 29% 15%	1305

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	8% 36% 30% 11%	523
February	Night	10% 49% 30%	394
March	Night	14% 38% 33% 5%	346
April	Day	14% 32% 29% 9%	61
	Night	9% 45% 32%	335
May	Day	26% 39% 19%	231
	Night	5% 27% 42% 16%	351
June	Day	10% 49% 27%	223
	Night	19% 51% 23%	330
July	Day	35% 45% 8%	204
	Night	5% 43% 39% 5%	319
August	Day	36% 42% 9%	144
	Night	40% 45% 6%	358
September	Night	8% 35% 40% 5%	342
October	Night	5% 16% 40% 27%	379
November	Night	23% 41% 21%	498
December	Night	9% 31% 36% 9%	577

Page generated at 2009-10-21 18:31:56. Version ab11