Dunham Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dunham Hill Observatory 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	16% 9% 5% 6% 6% 5% 5% 7% 29%	16490
	Night	18% 9% 5% 7% 5% 5% 7% 29%	16443
Cloud Cover	Day	16% 9% 5% 6% 5% 5% 5% 7% 30%	16504
	Night	19% 9% 5% 6% 5% 5% 6% 29%	16458
Transparency	Day	8% 15% 21% 12%	12294
	Night	7% 15% 21% 12%	12226
Seeing	Day	7% 26% 29% 16% 8%	1051
	Night	10% 32% 29% 11%	5092

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	19% 17% 12% 11% 8% 6% 8%	986
	Night	22% 14% 10% 10% 7% 5% 13%	1713
February	Day	18% 13% 9% 11% 8% 8% 5% 13%	1057
	Night	17% 11% 6% 10% 9% 7% 5% 5% 16%	1349
March	Day	16% 12% 6% 7% 6% 5% 5% 7% 26%	1303
	Night	17% 9% 6% 6% 6% 33%	1379
April	Day	19% 8% 6% 5% 5% 5% 5% 33%	1475
	Night	21% 5% 5% 6% 36%	1128
May	Day	15% 6% 5% 7% 6% 6% 36%	1632
	Night	15% 6% 5% 6% 5% 5% 8% 36%	1077
June	Day	10% 5% 5% 6% 6% 6% 5% 10% 35%	1661
	Night	9% 5% 5% 6% 7% 11% 38%	958
July	Day	8% 5% 6% 6% 5% 9% 41%	1712
	Night	7% 5% 5% 6% 10% 48%	980
August	Day	11% 7% 5% 5% 6% 5% 9% 39%	1576
	Night	10% 5% 5% 7% 47%	1141
September	Day	15% 6% 7% 42%	1331
	Night	15% 5% 7% 47%	1269
October	Day	23% 8% 5% 5% 6% 5% 5% 28%	1211
	Night	23% 7% 6% 7% 31%	1482
November	Day	19% 14% 6% 7% 5% 7% 23%	1299
	Night	20% 14% 6% 6% 6% 6% 23%	1872
December	Day	21% 15% 11% 11% 7% 5% 11%	1247
	Night	25% 15% 10% 11% 6% 5% 10%	2095

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	20% 17% 12% 9% 8% 6% 8%	985
	Night	22% 13% 9% 11% 7% 5% 13%	1712
February	Day	17% 14% 9% 11% 8% 6% 6% 13%	1055
	Night	18% 10% 6% 9% 9% 5% 6% 5% 5% 17%	1340
March	Day	16% 11% 7% 6% 6% 5% 5% 7% 25%	1324
	Night	19% 8% 5% 6% 5% 6% 33%	1403
April	Day	19% 8% 5% 5% 5% 5% 6% 34%	1474
	Night	21% 6% 6% 36%	1128
May	Day	17% 6% 5% 5% 5% 6% 37%	1627
	Night	16% 6% 6% 5% 5% 7% 38%	1070
June	Day	11% 5% 5% 6% 5% 6% 9% 36%	1671
	Night	10% 5% 5% 6% 7% 11% 38%	959
July	Day	9% 6% 6% 6% 9% 41%	1712
	Night	7% 5% 5% 6% 9% 49%	981
August	Day	13% 5% 5% 5% 5% 8% 41%	1571
	Night	10% 5% 5% 7% 48%	1142
September	Day	16% 5% 5% 8% 42%	1330
	Night	16% 5% 7% 47%	1270
October	Day	24% 9% 7% 5% 5% 28%	1211
	Night	25% 6% 5% 7% 31%	1482
November	Day	21% 14% 7% 5% 7% 23%	1297
	Night	20% 14% 5% 5% 5% 5% 6% 24%	1872
December	Day	21% 16% 10% 11% 6% 5% 12%	1247
	Night	25% 16% 10% 10% 6% 5% 11%	2099

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	7% 14% 6%	591
	Night	7% 17% 12%	1048
February	Day	7% 11% 15%	660
	Night	12% 16% 17%	882
March	Day	11% 21% 19% 7%	906
	Night	8% 18% 27% 9%	998
April	Day	8% 13% 23% 17%	1074
	Night	6% 17% 21% 22%	836
May	Day	7% 16% 28% 14%	1232
	Night	5% 16% 26% 18%	817
June	Day	10% 20% 24% 13%	1333
	Night	9% 24% 25% 14%	772
July	Day	10% 15% 23% 18% 5%	1144
	Night	9% 19% 32% 17%	664
August	Day	7% 13% 23% 19%	1378
	Night	6% 9% 24% 27%	1010
September	Day	7% 13% 27% 20%	1137
	Night	5% 14% 26% 22%	1098
October	Day	5% 12% 16% 14%	1018
	Night	5% 13% 22% 12%	1251
November	Day	7% 14% 20% 7%	963
	Night	8% 15% 18% 6%	1420
December	Day	7% 15% 9%	858
	Night	8% 13% 8%	1430

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	15% 39% 26%	529
February	Night	17% 42% 20%	403
March	Day	9% 40% 18% 12%	71
	Night	15% 37% 28% 6%	448
April	Day	12% 27% 29% 13%	112
	Night	11% 32% 33% 7%	327
May	Day	7% 31% 26% 19%	142
	Night	5% 36% 37% 10%	340
June	Day	7% 29% 32% 12% 8%	226
	Night	27% 37% 24%	336
July	Day	7% 28% 30% 17% 9%	205
	Night	25% 42% 25%	318
August	Day	15% 31% 21% 15%	120
	Night	21% 47% 16%	353
September	Day	14% 24% 21% 20%	110
	Night	15% 28% 30% 6%	408
October	Day	10% 13% 33% 18%	65
	Night	8% 31% 26% 10%	461
November	Night	12% 32% 28% 7%	521
December	Night	16% 39% 19%	648

Page generated at 2009-12-30 05:20:50. Version ab11