Austin
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Austin 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% 5% 6% 5% 6% 8% 35%	8680
	Night	17% 6% 5% 7% 41%	8493
Transparency	Day	17% 13% 29% 27% 10%	4696
	Night	17% 14% 27% 26% 14%	4949
Seeing	Day	7% 18% 32% 35% 5%	462
	Night	8% 23% 37% 27%	2557

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	27% 7% 5% 6% 5% 5% 29%	559
	Night	28% 5% 5% 5% 34%	795
February	Day	32% 5% 6% 5% 30%	570
	Night	30% 6% 5% 5% 30%	684
March	Day	23% 6% 5% 5% 37%	682
	Night	28% 45%	682
April	Day	22% 5% 6% 37%	696
	Night	14% 6% 8% 49%	624
May	Day	14% 5% 8% 5% 7% 11% 31%	803
	Night	11% 6% 8% 10% 42%	550
June	Day	16% 5% 5% 8% 6% 6% 8% 31%	780
	Night	14% 5% 6% 11% 40%	537
July	Day	6% 6% 5% 6% 5% 9% 7% 9% 10% 31%	801
	Night	5% 5% 5% 5% 7% 6% 8% 9% 38%	558
August	Day	9% 8% 6% 8% 10% 34%	1134
	Night	10% 5% 7% 5% 7% 10% 35%	908
September	Day	12% 6% 8% 6% 5% 8% 37%	970
	Night	12% 7% 6% 6% 9% 37%	975
October	Day	17% 7% 7% 5% 5% 9% 32%	641
	Night	13% 5% 5% 8% 5% 8% 38%	718
November	Day	22% 5% 46%	496
	Night	21% 49%	671
December	Day	18% 56%	548
	Night	15% 58%	791

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	17% 9% 33% 34%	224
	Night	12% 10% 37% 28% 10%	347
February	Day	12% 10% 43% 29%	224
	Night	15% 13% 35% 25% 9%	274
March	Day	15% 9% 22% 37% 14%	350
	Night	13% 5% 26% 34% 20%	372
April	Day	12% 7% 33% 28% 18%	356
	Night	17% 9% 27% 23% 21%	384
May	Day	24% 17% 34% 17% 5%	408
	Night	22% 21% 39% 15%	335
June	Day	22% 13% 30% 28% 5%	373
	Night	20% 15% 36% 25%	306
July	Day	22% 7% 30% 30% 9%	380
	Night	32% 12% 26% 21% 6%	296
August	Day	30% 29% 28% 9%	627
	Night	31% 22% 26% 15%	480
September	Day	15% 14% 27% 30% 12%	548
	Night	19% 21% 23% 21% 15%	578
October	Day	12% 15% 31% 32% 7%	513
	Night	14% 18% 21% 30% 15%	627
November	Day	11% 7% 21% 30% 27%	326
	Night	11% 9% 16% 25% 36%	421
December	Day	5% 24% 40% 25%	367
	Night	6% 5% 23% 42% 22%	529

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	13% 39% 35% 10%	231
February	Night	12% 35% 44% 7%	165
March	Night	12% 45% 37%	146
April	Day	17% 58% 20%	29
	Night	9% 53% 34%	163
May	Day	11% 23% 41% 22%	109
	Night	8% 25% 45% 20%	175
June	Day	7% 19% 33% 36%	106
	Night	6% 16% 39% 36%	164
July	Day	8% 34% 43% 10%	118
	Night	13% 36% 44%	183
August	Day	5% 13% 22% 48% 12%	100
	Night	9% 27% 52% 8%	258
September	Night	7% 26% 53% 9%	250
October	Night	5% 13% 44% 31% 5%	306
November	Night	15% 25% 35% 20%	239
December	Night	14% 24% 40% 19%	277

Page generated at 2008-10-12 19:06:10. Version ab11