Arch Cape
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arch Cape clear sky chart from 2005-11-05 to 2008-11-27.

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	29% 8% 5% 5% 8% 20%	12308
	Night	32% 10% 5% 6% 5% 20%	12117
Cloud Cover	Day	31% 10% 5% 6% 6% 19%	12315
	Night	33% 10% 5% 5% 5% 21%	12113
Transparency	Day	11% 14% 15% 10%	7583
	Night	11% 15% 13% 11%	7432
Seeing	Day	58% 9% 12%	916
	Night	56% 6% 9% 8%	3724

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	46% 8% 5% 6% 18%	749
	Night	45% 12% 5% 5% 16%	1289
February	Day	41% 7% 6% 5% 6% 15%	728
	Night	40% 9% 5% 5% 5% 19%	1049
March	Day	41% 11% 8% 6% 5% 5% 6% 6%	1012
	Night	48% 14% 5% 6% 6%	1029
April	Day	33% 9% 5% 6% 5% 7% 8% 14%	1118
	Night	33% 12% 6% 6% 6% 13%	855
May	Day	25% 9% 6% 7% 6% 6% 5% 9% 17%	1287
	Night	31% 10% 6% 5% 5% 8% 17%	759
June	Day	24% 9% 5% 7% 5% 5% 6% 8% 19%	1323
	Night	25% 12% 9% 8% 5% 6% 19%	656
July	Day	19% 5% 5% 6% 10% 34%	1334
	Night	13% 6% 10% 5% 5% 10% 32%	704
August	Day	19% 8% 6% 5% 5% 13% 25%	1188
	Night	16% 9% 6% 6% 7% 31%	848
September	Day	11% 7% 6% 6% 12% 36%	1030
	Night	13% 7% 5% 6% 7% 41%	947
October	Day	28% 8% 5% 5% 5% 6% 25%	920
	Night	25% 7% 5% 5% 30%	1114
November	Day	43% 6% 5% 16%	893
	Night	36% 9% 5% 5% 5% 19%	1550
December	Day	43% 11% 6% 13%	726
	Night	41% 10% 5% 5% 5% 6% 11%	1317

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	46% 13% 6% 17%	750
	Night	47% 12% 5% 16%	1286
February	Day	39% 10% 6% 5% 14%	730
	Night	39% 11% 5% 5% 19%	1048
March	Day	43% 14% 7% 8% 5%	1014
	Night	46% 16% 5% 5% 6%	1027
April	Day	33% 13% 8% 6% 5% 6% 13%	1119
	Night	32% 14% 8% 7% 6% 13%	857
May	Day	29% 10% 9% 5% 6% 5% 8% 14%	1287
	Night	32% 8% 6% 5% 6% 8% 17%	759
June	Day	28% 12% 6% 8% 5% 6% 16%	1323
	Night	26% 9% 7% 9% 5% 5% 19%	656
July	Day	19% 5% 7% 9% 33%	1334
	Night	14% 5% 7% 8% 5% 8% 35%	705
August	Day	20% 9% 8% 6% 9% 25%	1189
	Night	17% 6% 7% 5% 5% 5% 5% 6% 33%	848
September	Day	13% 8% 7% 7% 10% 37%	1028
	Night	14% 6% 6% 6% 6% 7% 41%	947
October	Day	29% 10% 6% 6% 23%	920
	Night	26% 7% 6% 30%	1113
November	Day	43% 7% 5% 15%	895
	Night	38% 8% 6% 19%	1550
December	Day	45% 12% 5% 5% 14%	726
	Night	42% 11% 5% 6% 11%	1317

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	5% 10% 12% 19%	346
	Night	7% 11% 9% 14%	604
February	Day	14% 9% 15% 11%	374
	Night	7% 10% 14% 14%	545
March	Day	15% 8%	419
	Night	12% 9% 6%	445
April	Day	16% 14% 11%	548
	Night	14% 14% 11%	401
May	Day	15% 17% 17% 5%	753
	Night	17% 21% 9% 10%	451
June	Day	19% 20% 13% 5%	757
	Night	18% 21% 15% 5%	364
July	Day	11% 20% 21% 20%	763
	Night	15% 23% 16% 15% 10%	402
August	Day	9% 18% 17% 9%	1042
	Night	11% 18% 22% 9%	743
September	Day	7% 16% 19% 13% 9%	917
	Night	10% 15% 17% 15% 11%	852
October	Day	7% 9% 15% 12%	758
	Night	8% 14% 12% 14%	934
November	Day	11% 8% 13% 7%	559
	Night	8% 13% 12% 10%	1006
December	Day	6% 12% 12% 6%	347
	Night	12% 11% 10% 5%	685

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	71% 6% 6%	32
	Night	67%	372
February	Day	67% 8%	80
	Night	68% 6% 7%	319
March	Day	69% 5%	93
	Night	69% 6%	371
April	Day	66% 8%	90
	Night	67% 7%	317
May	Day	67% 5%	92
	Night	66% 5%	201
June	Day	67% 8% 14%	150
	Night	65% 6% 10% 7%	170
July	Day	61% 12% 20%	109
	Night	61% 16% 15%	167
August	Day	41% 15% 19% 5%	89
	Night	38% 8% 14% 17%	278
September	Day	35% 20% 20% 6%	88
	Night	34% 5% 18% 23%	350
October	Day	34% 14% 16% 6%	92
	Night	35% 12% 15% 13%	366
November	Day		1
	Night	51% 9% 10% 8%	444
December	Night	67%	369

Page generated at 2008-11-27 10:47:41. Version ab11