Chicago
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chicago 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	15% 7% 6% 5% 5% 5% 5% 7% 33%	16473
	Night	18% 8% 5% 6% 34%	16461
Cloud Cover	Day	15% 8% 6% 5% 5% 5% 5% 7% 33%	16487
	Night	18% 8% 5% 5% 6% 34%	16477
Transparency	Day	8% 17% 21% 13%	12675
	Night	7% 16% 23% 14%	12672
Seeing	Day	23% 35% 19% 7%	1058
	Night	10% 31% 30% 12%	5133

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	19% 15% 6% 8% 5% 6% 6% 20%	984
	Night	21% 12% 6% 6% 5% 5% 5% 24%	1715
February	Day	18% 11% 7% 6% 6% 7% 5% 24%	1065
	Night	20% 9% 6% 6% 32%	1341
March	Day	19% 9% 5% 7% 6% 5% 5% 5% 6% 26%	1316
	Night	20% 10% 7% 6% 6% 28%	1367
April	Day	21% 6% 5% 6% 5% 33%	1443
	Night	21% 6% 5% 5% 38%	1160
May	Day	12% 6% 6% 5% 6% 5% 6% 8% 34%	1666
	Night	15% 5% 5% 5% 9% 37%	1043
June	Day	11% 5% 5% 5% 6% 5% 6% 9% 37%	1661
	Night	10% 7% 6% 6% 6% 8% 38%	958
July	Day	8% 5% 5% 6% 6% 8% 45%	1718
	Night	8% 5% 7% 7% 8% 46%	974
August	Day	10% 6% 5% 5% 10% 41%	1552
	Night	8% 5% 5% 7% 49%	1165
September	Day	12% 5% 8% 47%	1354
	Night	12% 5% 54%	1246
October	Day	20% 7% 5% 6% 7% 6% 31%	1207
	Night	22% 7% 6% 35%	1486
November	Day	20% 9% 5% 6% 5% 35%	1290
	Night	19% 9% 5% 5% 5% 35%	1880
December	Day	22% 14% 6% 7% 5% 6% 8% 5% 15%	1217
	Night	27% 12% 7% 5% 6% 5% 17%	2126

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	19% 15% 7% 9% 5% 7% 5% 20%	983
	Night	21% 13% 6% 6% 5% 6% 24%	1714
February	Day	17% 11% 7% 6% 5% 7% 5% 24%	1063
	Night	19% 10% 7% 6% 5% 32%	1332
March	Day	19% 10% 5% 8% 5% 5% 5% 7% 25%	1337
	Night	21% 9% 5% 6% 6% 6% 28%	1392
April	Day	21% 6% 5% 5% 5% 5% 33%	1442
	Night	20% 6% 6% 38%	1160
May	Day	12% 6% 7% 5% 6% 5% 6% 8% 34%	1661
	Night	15% 6% 5% 9% 36%	1036
June	Day	12% 5% 5% 5% 6% 6% 9% 37%	1671
	Night	11% 6% 5% 6% 6% 7% 8% 37%	959
July	Day	8% 5% 5% 5% 6% 8% 44%	1718
	Night	8% 5% 7% 6% 9% 46%	975
August	Day	10% 5% 5% 5% 5% 5% 9% 42%	1547
	Night	9% 5% 5% 5% 7% 49%	1166
September	Day	12% 5% 8% 47%	1353
	Night	11% 6% 54%	1247
October	Day	19% 8% 6% 7% 6% 31%	1207
	Night	22% 6% 5% 5% 35%	1486
November	Day	19% 10% 6% 5% 35%	1288
	Night	19% 9% 5% 5% 5% 5% 35%	1880
December	Day	22% 14% 5% 7% 6% 5% 8% 5% 15%	1217
	Night	27% 12% 7% 5% 6% 5% 5% 17%	2130

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	7% 22% 18% 6%	668
	Night	7% 20% 23% 7%	1152
February	Day	10% 19% 19% 5%	761
	Night	7% 21% 24% 6%	970
March	Day	8% 23% 21%	912
	Night	11% 19% 21% 6%	977
April	Day	9% 16% 21% 13%	1092
	Night	5% 14% 23% 20%	894
May	Day	10% 15% 26% 14%	1249
	Night	8% 16% 29% 18%	776
June	Day	10% 18% 23% 17%	1320
	Night	9% 18% 23% 18%	757
July	Day	8% 19% 22% 17%	1157
	Night	11% 20% 22% 21%	668
August	Day	8% 13% 23% 20%	1361
	Night	6% 14% 24% 26%	1020
September	Day	7% 13% 26% 22%	1180
	Night	6% 11% 25% 25% 6%	1117
October	Day	6% 15% 17% 12%	1063
	Night	14% 23% 13%	1281
November	Day	7% 19% 25% 9%	1007
	Night	6% 18% 23% 11%	1476
December	Day	11% 19% 11%	905
	Night	10% 15% 15%	1584

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% 42% 25%	543
February	Night	10% 47% 24%	420
March	Day	8% 31% 29% 8%	47
	Night	12% 39% 24% 9%	402
April	Day	10% 24% 33% 11%	109
	Night	10% 31% 34% 9%	331
May	Day	6% 28% 38% 16%	177
	Night	27% 41% 18%	336
June	Day	5% 28% 34% 17% 6%	223
	Night	22% 42% 22%	330
July	Day	22% 40% 20% 7%	213
	Night	14% 45% 27%	323
August	Day	14% 39% 26% 12%	141
	Night	13% 44% 26% 5%	353
September	Day	17% 26% 30% 16%	113
	Night	5% 16% 27% 32% 11%	399
October	Day	5% 31% 28% 22%	35
	Night	13% 25% 27% 12%	459
November	Night	12% 35% 25% 9%	537
December	Night	14% 37% 20%	700

Page generated at 2009-12-30 04:20:40. Version ab11