University of Maryland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Maryland Observatory 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% 7% 5% 7% 34%	8726
	Night	20% 5% 6% 5% 6% 37%	8446
Transparency	Day	16% 15% 40% 24%	4617
	Night	16% 13% 41% 25%	4631
Seeing	Day	9% 25% 26% 26% 11%	535
	Night	12% 28% 35% 21%	2433

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	21% 5% 5% 5% 6% 36%	535
	Night	20% 5% 5% 6% 41%	818
February	Day	18% 5% 5% 8% 8% 39%	537
	Night	17% 7% 5% 41%	717
March	Day	20% 5% 5% 5% 8% 5% 5% 31%	682
	Night	23% 5% 5% 38%	681
April	Day	20% 8% 5% 6% 38%	720
	Night	23% 5% 5% 5% 5% 37%	600
May	Day	16% 5% 5% 7% 6% 7% 35%	833
	Night	17% 8% 7% 9% 7% 33%	521
June	Day	15% 10% 6% 8% 10% 27%	840
	Night	14% 6% 5% 8% 5% 5% 7% 35%	476
July	Day	15% 6% 7% 7% 9% 5% 8% 8% 23%	838
	Night	18% 7% 5% 6% 6% 8% 8% 27%	524
August	Day	18% 6% 7% 7% 5% 5% 8% 26%	1157
	Night	15% 5% 5% 5% 5% 8% 7% 10% 29%	882
September	Day	21% 5% 6% 6% 38%	1019
	Night	20% 5% 5% 5% 41%	926
October	Day	27% 5% 6% 39%	618
	Night	29% 6% 37%	744
November	Day	19% 6% 9% 39%	461
	Night	23% 5% 5% 6% 37%	705
December	Day	17% 5% 44%	486
	Night	15% 5% 5% 5% 5% 42%	852

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	35% 12% 33% 16%	248
	Night	25% 15% 41% 16%	426
February	Day	15% 17% 33% 30%	323
	Night	9% 12% 46% 25% 6%	367
March	Day	17% 24% 38% 19%	294
	Night	16% 12% 44% 25%	335
April	Day	10% 6% 42% 34% 6%	387
	Night	8% 6% 45% 33% 5%	301
May	Day	17% 18% 43% 19%	420
	Night	32% 13% 36% 16%	288
June	Day	24% 15% 41% 18%	413
	Night	20% 13% 53% 13%	245
July	Day	23% 13% 40% 19%	355
	Night	18% 19% 36% 21%	256
August	Day	14% 21% 46% 16%	515
	Night	22% 22% 36% 17%	437
September	Day	9% 12% 30% 38% 8%	573
	Night	6% 10% 35% 39% 7%	501
October	Day	9% 12% 44% 32%	475
	Night	13% 13% 37% 34%	572
November	Day	14% 20% 41% 22%	327
	Night	10% 14% 47% 26%	423
December	Day	21% 11% 45% 20%	287
	Night	16% 13% 44% 24%	480

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	18% 51% 25%	198
February	Day	19% 47% 28%	21
	Night	14% 49% 29% 6%	195
March	Day	6% 42% 34% 16%	49
	Night	17% 43% 34% 5%	193
April	Day	8% 39% 31% 14% 6%	48
	Night	17% 33% 37% 9%	193
May	Day	12% 24% 31% 22% 9%	54
	Night	9% 27% 41% 21%	134
June	Day	5% 24% 37% 24% 9%	54
	Night	8% 14% 54% 21%	107
July	Day	7% 15% 22% 39% 15%	53
	Night	8% 7% 38% 41%	125
August	Day	10% 8% 24% 36% 20%	79
	Night	8% 11% 34% 41%	290
September	Day	7% 22% 16% 32% 21%	71
	Night	7% 16% 35% 36%	297
October	Day	13% 29% 20% 25% 11%	68
	Night	12% 21% 35% 26%	287
November	Day	15% 23% 28% 31%	38
	Night	12% 25% 34% 27%	203
December	Night	18% 37% 34% 9%	211

Page generated at 2008-06-05 23:07:38. Version ab11