Dark Horse Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Horse 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	20% 5% 6% 5% 6% 37%	8731
	Night	21% 5% 5% 7% 37%	8440
Transparency	Day	19% 14% 40% 22%	4724
	Night	17% 14% 41% 23%	4566
Seeing	Day	14% 30% 29% 18% 7%	532
	Night	18% 32% 32% 14%	2393

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	22% 6% 7% 35%	519
	Night	26% 5% 38%	834
February	Day	16% 5% 5% 6% 42%	543
	Night	16% 6% 5% 5% 42%	711
March	Day	24% 5% 6% 6% 6% 31%	677
	Night	24% 6% 5% 6% 32%	686
April	Day	22% 5% 43%	720
	Night	29% 5% 6% 6% 33%	600
May	Day	18% 5% 7% 6% 7% 34%	841
	Night	16% 6% 5% 5% 6% 6% 37%	513
June	Day	14% 5% 6% 5% 6% 8% 36%	840
	Night	14% 5% 7% 6% 5% 8% 37%	476
July	Day	16% 5% 6% 6% 7% 7% 33%	858
	Night	14% 5% 5% 5% 6% 6% 10% 33%	503
August	Day	20% 5% 6% 8% 6% 5% 6% 33%	1149
	Night	19% 6% 6% 5% 9% 33%	890
September	Day	25% 6% 6% 40%	1026
	Night	21% 5% 7% 42%	918
October	Day	28% 6% 6% 39%	612
	Night	29% 5% 7% 35%	750
November	Day	19% 6% 9% 6% 6% 38%	460
	Night	25% 5% 6% 6% 34%	707
December	Day	21% 5% 6% 7% 39%	486
	Night	17% 5% 5% 6% 6% 6% 40%	852

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	29% 14% 42% 13%	227
	Night	24% 15% 44% 15%	386
February	Day	16% 9% 45% 23%	290
	Night	15% 7% 45% 27%	384
March	Day	26% 19% 37% 14%	304
	Night	17% 18% 40% 22%	317
April	Day	13% 11% 37% 30% 6%	410
	Night	10% 11% 41% 31% 5%	278
May	Day	23% 16% 48% 11%	432
	Night	21% 21% 40% 15%	270
June	Day	19% 9% 45% 24%	453
	Night	15% 13% 49% 21%	253
July	Day	18% 9% 44% 23%	396
	Night	22% 14% 42% 16%	262
August	Day	21% 24% 37% 15%	563
	Night	25% 14% 41% 16%	446
September	Day	10% 10% 30% 41% 6%	566
	Night	8% 13% 33% 40%	522
October	Day	16% 15% 43% 25%	499
	Night	15% 10% 44% 27%	560
November	Day	20% 21% 40% 14%	325
	Night	15% 21% 40% 22%	416
December	Day	25% 15% 44% 13%	259
	Night	21% 13% 43% 19%	472

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	38% 37% 20%	182
February	Day	11% 62% 25%	27
	Night	25% 40% 29% 5%	193
March	Day	22% 44% 24% 8%	45
	Night	31% 41% 25%	196
April	Day	27% 33% 27% 8%	48
	Night	20% 43% 27% 9%	181
May	Day	11% 40% 36% 9%	52
	Night	13% 41% 34% 9%	131
June	Day	5% 30% 39% 20%	53
	Night	10% 27% 52% 8%	104
July	Day	7% 31% 19% 35% 5%	51
	Night	6% 19% 45% 26%	120
August	Day	9% 15% 32% 28% 13%	73
	Night	6% 24% 32% 33%	290
September	Day	7% 21% 31% 24% 14%	69
	Night	10% 22% 42% 22%	300
October	Day	26% 28% 19% 13% 12%	73
	Night	19% 27% 29% 19%	298
November	Day	19% 21% 36% 21%	41
	Night	18% 30% 37% 14%	197
December	Night	25% 41% 27% 5%	201

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