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Boston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boston 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	19% 6% 5% 7% 35%	8739
	Night	22% 5% 6% 5% 6% 35%	8430
Transparency	Day	20% 14% 40% 20%	4544
	Night	20% 15% 39% 22%	4235
Seeing	Day	14% 39% 27% 13% 5%	556
	Night	18% 41% 27% 11%	2294

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	24% 5% 7% 37%	490
	Night	22% 8% 6% 36%	863
February	Day	14% 6% 7% 43%	557
	Night	20% 5% 45%	696
March	Day	25% 6% 5% 7% 37%	666
	Night	22% 5% 6% 6% 35%	698
April	Day	27% 6% 38%	743
	Night	30% 5% 6% 5% 33%	576
May	Day	23% 5% 7% 5% 5% 7% 29%	825
	Night	26% 7% 5% 5% 6% 7% 8% 22%	530
June	Day	12% 5% 5% 7% 7% 6% 10% 33%	840
	Night	18% 9% 5% 6% 5% 36%	476
July	Day	16% 5% 5% 8% 6% 7% 9% 27%	866
	Night	21% 5% 10% 7% 6% 10% 22%	496
August	Day	15% 5% 5% 5% 9% 5% 6% 9% 29%	1174
	Night	17% 5% 6% 5% 8% 5% 6% 7% 29%	863
September	Day	17% 7% 46%	1000
	Night	16% 5% 5% 44%	945
October	Day	27% 5% 6% 5% 5% 32%	611
	Night	32% 5% 5% 7% 29%	750
November	Day	15% 5% 6% 7% 8% 37%	475
	Night	18% 6% 5% 7% 39%	692
December	Day	22% 5% 5% 7% 6% 34%	492
	Night	21% 5% 5% 8% 37%	845

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	19% 13% 37% 25%	243
	Night	19% 11% 44% 22%	408
February	Day	22% 10% 39% 25%	330
	Night	17% 9% 48% 19%	384
March	Day	23% 17% 49% 8%	325
	Night	27% 17% 34% 19%	341
April	Day	15% 11% 34% 33% 6%	363
	Night	10% 16% 43% 28%	240
May	Day	27% 13% 42% 14%	355
	Night	38% 9% 26% 25%	203
June	Day	20% 12% 42% 23%	409
	Night	18% 14% 34% 29%	224
July	Day	25% 18% 32% 19%	408
	Night	29% 10% 37% 18%	193
August	Day	28% 22% 29% 14% 5%	535
	Night	24% 19% 32% 21%	351
September	Day	9% 9% 44% 33%	592
	Night	13% 13% 43% 25%	547
October	Day	20% 12% 52% 15%	440
	Night	18% 16% 47% 17%	494
November	Day	19% 16% 37% 21% 6%	300
	Night	18% 21% 33% 23%	413
December	Day	22% 18% 46% 10%	244
	Night	19% 16% 40% 18%	437

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	35% 51% 10%	184
February	Day	20% 52% 18% 6%	44
	Night	19% 50% 22% 7%	185
March	Day	15% 47% 32%	46
	Night	27% 40% 25% 6%	195
April	Day	23% 28% 33% 12%	39
	Night	19% 48% 26%	154
May	Day	20% 27% 34% 18%	44
	Night	13% 43% 36% 6%	129
June	Day	13% 49% 30% 7%	53
	Night	13% 57% 25%	107
July	Day	5% 44% 25% 19% 5%	52
	Night	8% 39% 37% 12%	108
August	Day	6% 40% 23% 20% 10%	80
	Night	8% 41% 31% 17%	268
September	Day	8% 27% 35% 15% 13%	73
	Night	9% 30% 35% 22%	311
October	Day	18% 45% 12% 14% 8%	70
	Night	20% 32% 25% 19%	268
November	Day	22% 30% 36% 12%	50
	Night	22% 40% 29% 6%	194
December	Day	40% 40% 20%	5
	Night	25% 45% 24% 5%	191

Page generated at 2008-08-29 16:03:13. Version ab11