Pittsburgh
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pittsburgh clear sky chart from 2005-11-05 to 2008-11-27.

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	19% 8% 5% 5% 5% 5% 5% 7% 29%	12223
	Night	20% 8% 5% 6% 5% 5% 6% 30%	12170
Cloud Cover	Day	19% 8% 5% 5% 5% 5% 5% 7% 30%	12242
	Night	20% 8% 5% 5% 5% 6% 31%	12193
Transparency	Day	9% 18% 24% 13%	8071
	Night	8% 16% 24% 14%	8085
Seeing	Day	8% 22% 24% 22% 11%	673
	Night	12% 27% 32% 13%	3422

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% 16% 7% 5% 6% 5% 18%	833
	Night	30% 11% 7% 7% 5% 5% 21%	1207
February	Day	22% 14% 8% 8% 7% 5% 6% 17%	756
	Night	23% 9% 6% 7% 5% 8% 5% 6% 19%	1034
March	Day	25% 9% 6% 5% 5% 5% 7% 7% 21%	999
	Night	23% 11% 7% 6% 5% 5% 6% 23%	998
April	Day	19% 7% 7% 5% 5% 6% 32%	1098
	Night	18% 9% 6% 6% 6% 5% 5% 33%	882
May	Day	18% 5% 5% 6% 6% 5% 7% 32%	1233
	Night	15% 5% 5% 5% 8% 39%	813
June	Day	14% 5% 7% 5% 5% 10% 34%	1251
	Night	9% 5% 7% 7% 5% 10% 40%	718
July	Day	15% 5% 7% 6% 6% 8% 33%	1226
	Night	10% 6% 5% 7% 10% 39%	815
August	Day	13% 5% 6% 6% 7% 9% 35%	1180
	Night	15% 5% 5% 5% 6% 42%	852
September	Day	17% 5% 5% 43%	1019
	Night	13% 5% 6% 5% 6% 46%	958
October	Day	16% 8% 6% 6% 36%	923
	Night	18% 6% 5% 5% 5% 6% 37%	1111
November	Day	22% 13% 6% 6% 6% 6% 24%	961
	Night	25% 10% 6% 6% 5% 5% 24%	1485
December	Day	23% 15% 8% 7% 6% 7% 17%	744
	Night	25% 11% 9% 7% 5% 7% 18%	1297

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% 17% 6% 5% 6% 5% 19%	832
	Night	30% 12% 7% 5% 21%	1208
February	Day	23% 12% 9% 7% 7% 6% 18%	754
	Night	22% 9% 7% 6% 5% 7% 5% 5% 21%	1025
March	Day	26% 9% 6% 5% 7% 7% 21%	1022
	Night	25% 10% 6% 6% 6% 23%	1020
April	Day	19% 8% 7% 5% 5% 33%	1097
	Night	19% 9% 6% 5% 5% 5% 32%	882
May	Day	19% 5% 7% 5% 5% 8% 32%	1232
	Night	15% 5% 5% 5% 8% 39%	813
June	Day	15% 5% 7% 5% 7% 8% 35%	1260
	Night	9% 5% 5% 7% 6% 6% 9% 41%	720
July	Day	15% 5% 8% 5% 6% 9% 33%	1226
	Night	10% 7% 5% 7% 11% 39%	816
August	Day	14% 6% 7% 6% 7% 8% 35%	1175
	Night	14% 5% 5% 5% 6% 43%	854
September	Day	17% 5% 5% 5% 43%	1018
	Night	13% 5% 5% 5% 6% 46%	959
October	Day	17% 8% 7% 7% 36%	923
	Night	18% 5% 6% 5% 5% 6% 37%	1111
November	Day	22% 13% 5% 6% 5% 6% 24%	959
	Night	25% 10% 5% 6% 5% 5% 23%	1484
December	Day	23% 14% 8% 7% 5% 7% 18%	744
	Night	25% 11% 9% 7% 5% 6% 19%	1301

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	8% 21% 25%	416
	Night	7% 22% 23% 6%	619
February	Day	11% 22% 10% 7%	434
	Night	11% 23% 16% 8%	600
March	Day	12% 24% 19% 8%	594
	Night	10% 17% 28% 6%	604
April	Day	8% 19% 19% 20% 7%	674
	Night	8% 20% 15% 18% 11%	536
May	Day	10% 17% 29% 14%	855
	Night	7% 15% 29% 24%	576
June	Day	12% 24% 29% 11%	843
	Night	13% 24% 29% 15%	528
July	Day	11% 18% 28% 12%	631
	Night	10% 16% 35% 17%	454
August	Day	9% 15% 26% 15%	977
	Night	6% 10% 26% 21%	711
September	Day	6% 13% 28% 22%	802
	Night	7% 8% 28% 25% 5%	778
October	Day	6% 11% 24% 18%	761
	Night	7% 9% 24% 20%	920
November	Day	7% 17% 17% 8%	668
	Night	7% 13% 19% 9%	1028
December	Day	11% 25% 16%	416
	Night	8% 27% 17%	731

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	25% 36% 22%	292
February	Day	60% 20% 20%	5
	Night	20% 44% 21%	275
March	Day	11% 43% 23% 7%	80
	Night	17% 35% 27%	317
April	Day	13% 26% 29% 17%	79
	Night	15% 32% 34% 10%	282
May	Day	12% 22% 16% 20% 13%	86
	Night	11% 23% 39% 12%	243
June	Day	21% 35% 21% 12%	79
	Night	20% 49% 22%	163
July	Day	6% 15% 21% 30% 19%	73
	Night	19% 43% 29%	220
August	Day	11% 15% 29% 22%	86
	Night	15% 41% 25%	269
September	Day	17% 27% 32% 17%	81
	Night	15% 31% 35%	322
October	Day	12% 18% 29% 22%	85
	Night	8% 24% 33% 16%	339
November	Day	5% 15% 36% 31%	19
	Night	14% 25% 30% 7%	389
December	Night	19% 33% 26%	311

Page generated at 2008-11-27 13:47:45. Version ab11