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Nantucket Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nantucket Island 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	22% 6% 5% 6% 5% 7% 31%	8742
	Night	25% 7% 5% 5% 7% 28%	8429
Transparency	Day	17% 16% 41% 21%	4390
	Night	19% 18% 37% 21%	3877
Seeing	Day	13% 22% 36% 21% 5%	560
	Night	12% 28% 33% 21%	2243

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	31% 16% 10% 8% 7% 12%	500
	Night	29% 14% 10% 11% 5% 12%	852
February	Day	17% 7% 7% 6% 5% 9% 32%	557
	Night	21% 6% 6% 5% 8% 34%	696
March	Day	28% 6% 5% 36%	664
	Night	26% 7% 5% 5% 5% 8% 29%	700
April	Day	23% 5% 5% 5% 39%	746
	Night	28% 5% 5% 5% 7% 30%	574
May	Day	23% 9% 6% 5% 5% 5% 7% 29%	809
	Night	28% 10% 7% 5% 5% 6% 22%	546
June	Day	19% 7% 5% 5% 5% 7% 6% 9% 26%	840
	Night	20% 8% 9% 5% 5% 5% 5% 8% 25%	476
July	Day	19% 5% 6% 5% 5% 6% 5% 7% 7% 26%	854
	Night	24% 8% 7% 5% 5% 5% 7% 22%	506
August	Day	19% 6% 5% 7% 5% 7% 7% 30%	1180
	Night	25% 5% 6% 5% 7% 6% 6% 27%	860
September	Day	16% 6% 7% 43%	991
	Night	19% 5% 7% 41%	954
October	Day	31% 7% 7% 8% 27%	617
	Night	33% 7% 8% 5% 5% 24%	744
November	Day	17% 5% 7% 9% 37%	475
	Night	21% 5% 5% 6% 10% 33%	692
December	Day	27% 6% 8% 7% 25%	509
	Night	28% 6% 6% 30%	829

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	32% 18% 41% 6%	129
	Night	33% 22% 32% 11%	212
February	Day	16% 7% 54% 22%	281
	Night	15% 14% 50% 16%	352
March	Day	18% 16% 54% 9%	324
	Night	25% 25% 33% 15%	332
April	Day	12% 16% 36% 29%	397
	Night	13% 15% 38% 31%	230
May	Day	23% 14% 44% 16%	356
	Night	22% 23% 36% 15%	204
June	Day	8% 17% 43% 28%	373
	Night	13% 18% 38% 27%	199
July	Day	27% 11% 36% 21%	404
	Night	25% 25% 36% 13%	183
August	Day	18% 22% 35% 17% 6%	577
	Night	19% 20% 35% 18% 5%	348
September	Day	11% 13% 35% 35%	618
	Night	9% 10% 39% 36%	553
October	Day	17% 17% 44% 20%	404
	Night	15% 19% 38% 25%	438
November	Day	19% 23% 35% 14% 6%	309
	Night	24% 20% 31% 19%	430
December	Day	21% 18% 46% 11%	218
	Night	23% 20% 36% 15%	396

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	Day	25% 50% 25%	4
	Night	29% 46% 20%	175
February	Day	15% 48% 26% 8%	45
	Night	16% 40% 29% 11%	179
March	Day	19% 23% 47% 8%	46
	Night	8% 43% 39% 8%	189
April	Day	5% 21% 42% 26% 5%	38
	Night	8% 29% 46% 15%	158
May	Day	20% 27% 27% 25%	48
	Night	7% 23% 32% 33%	134
June	Day	12% 8% 59% 17%	47
	Night	11% 17% 47% 23%	102
July	Day	6% 6% 37% 37% 11%	45
	Night	10% 14% 32% 33% 8%	110
August	Day	6% 13% 39% 31% 9%	74
	Night	9% 10% 36% 35% 8%	246
September	Day	7% 18% 38% 28% 6%	76
	Night	6% 10% 35% 40% 6%	306
October	Day	17% 29% 25% 13% 14%	68
	Night	14% 29% 21% 23% 9%	267
November	Day	17% 27% 35% 17%	51
	Night	13% 31% 39% 15%	195
December	Day	33% 44% 22%	18
	Night	19% 44% 28% 6%	182

Page generated at 2008-09-30 05:41:18. Version ab11