This page made possible by the sponsorship of: Amateur Telescope Makers of Boston, .

Boston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boston 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	18% 6% 5% 5% 7% 36%	16488
	Night	19% 6% 6% 37%	16444
Cloud Cover	Day	18% 5% 5% 5% 7% 35%	16497
	Night	18% 6% 5% 7% 37%	16465
Transparency	Day	9% 16% 24% 15%	12812
	Night	8% 16% 23% 15%	13037
Seeing	Day	46% 15% 14% 6%	1207
	Night	47% 19% 14%	5142

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% 5% 6% 7% 38%	979
	Night	18% 7% 5% 6% 36%	1720
February	Day	13% 7% 5% 5% 6% 6% 5% 7% 35%	1067
	Night	16% 6% 5% 5% 5% 6% 39%	1339
March	Day	17% 5% 5% 5% 7% 39%	1308
	Night	16% 7% 5% 5% 5% 40%	1370
April	Day	18% 6% 6% 5% 7% 35%	1463
	Night	22% 8% 5% 6% 35%	1140
May	Day	23% 6% 5% 5% 5% 6% 34%	1650
	Night	24% 6% 5% 5% 5% 8% 31%	1063
June	Day	27% 6% 6% 5% 6% 8% 22%	1663
	Night	27% 6% 5% 5% 5% 5% 8% 24%	956
July	Day	12% 5% 5% 6% 7% 7% 8% 9% 29%	1716
	Night	15% 5% 5% 5% 8% 5% 6% 9% 29%	974
August	Day	11% 5% 6% 6% 5% 8% 40%	1561
	Night	12% 5% 5% 5% 5% 7% 7% 39%	1151
September	Day	13% 6% 51%	1340
	Night	17% 7% 50%	1264
October	Day	15% 6% 5% 7% 41%	1210
	Night	16% 7% 5% 5% 6% 41%	1483
November	Day	23% 7% 7% 35%	1299
	Night	23% 7% 5% 35%	1873
December	Day	20% 7% 5% 6% 37%	1232
	Night	18% 8% 5% 6% 39%	2111

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% 5% 5% 5% 6% 7% 37%	977
	Night	18% 8% 5% 7% 36%	1721
February	Day	13% 7% 5% 7% 6% 5% 7% 35%	1064
	Night	15% 7% 5% 6% 6% 39%	1331
March	Day	17% 5% 5% 5% 7% 39%	1329
	Night	15% 8% 5% 5% 6% 39%	1394
April	Day	18% 5% 6% 5% 5% 7% 35%	1462
	Night	22% 7% 5% 6% 35%	1140
May	Day	22% 7% 5% 5% 6% 33%	1644
	Night	23% 7% 5% 5% 8% 31%	1057
June	Day	27% 6% 6% 6% 6% 9% 22%	1672
	Night	27% 6% 5% 6% 5% 5% 7% 24%	958
July	Day	12% 5% 5% 9% 6% 7% 10% 29%	1716
	Night	15% 5% 6% 6% 8% 5% 6% 9% 29%	975
August	Day	12% 5% 5% 5% 5% 7% 8% 39%	1555
	Night	11% 5% 5% 5% 5% 6% 8% 39%	1154
September	Day	14% 5% 5% 6% 50%	1339
	Night	16% 7% 49%	1265
October	Day	16% 6% 5% 6% 42%	1210
	Night	16% 6% 5% 5% 5% 6% 40%	1483
November	Day	23% 6% 6% 35%	1297
	Night	23% 7% 5% 5% 35%	1872
December	Day	20% 7% 5% 5% 6% 37%	1232
	Night	18% 7% 6% 38%	2115

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	11% 20% 29% 13%	736
	Night	8% 19% 24% 14%	1257
February	Day	12% 19% 25% 9%	835
	Night	9% 17% 29% 11%	1054
March	Day	7% 18% 26% 16%	1014
	Night	6% 15% 24% 18%	1087
April	Day	9% 11% 24% 17% 7%	1070
	Night	5% 13% 22% 21%	824
May	Day	7% 15% 23% 14%	1208
	Night	12% 14% 20% 13%	807
June	Day	14% 21% 19%	1174
	Night	12% 20% 16% 8%	658
July	Day	11% 19% 22% 12%	1103
	Night	9% 18% 25% 8%	637
August	Day	11% 14% 22% 18%	1391
	Night	9% 13% 26% 16%	1007
September	Day	5% 12% 25% 28%	1181
	Night	5% 11% 25% 27%	1130
October	Day	6% 16% 24% 14%	1099
	Night	7% 16% 23% 15%	1333
November	Day	6% 14% 26% 13%	1014
	Night	6% 17% 20% 14%	1483
December	Day	5% 21% 19% 15%	987
	Night	7% 19% 24% 12%	1760

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	56% 24% 9%	491
February	Day	59% 15% 11%	102
	Night	58% 14% 11%	430
March	Day	62% 11% 10%	124
	Night	59% 19% 7%	495
April	Day	55% 15% 11% 7%	119
	Night	57% 17% 11%	428
May	Day	54% 15% 9%	122
	Night	56% 18% 9%	323
June	Day	47% 11% 15%	113
	Night	50% 11% 17%	228
July	Day	45% 11% 21% 10%	110
	Night	45% 16% 22% 5%	220
August	Day	32% 9% 28% 11% 5%	121
	Night	28% 17% 32% 9%	398
September	Day	31% 23% 16% 6% 9%	116
	Night	28% 19% 25% 10%	463
October	Day	33% 25% 15% 8%	121
	Night	32% 25% 19% 5%	487
November	Day	45% 18% 9% 5%	144
	Night	46% 20% 12%	573
December	Day	53% 6% 13%	15
	Night	54% 20% 5%	606

Page generated at 2009-12-30 05:20:40. Version ab11