NAA
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NAA 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	18% 6% 5% 5% 5% 7% 34%	12261
	Night	19% 7% 5% 6% 37%	12132
Cloud Cover	Day	16% 7% 5% 5% 5% 7% 33%	12279
	Night	18% 7% 5% 6% 36%	12156
Transparency	Day	5% 19% 26% 16%	8384
	Night	18% 27% 17%	8354
Seeing	Day	6% 22% 33% 22% 8%	764
	Night	11% 30% 33% 12%	3632

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	23% 11% 7% 7% 5% 6% 6% 20%	753
	Night	22% 13% 7% 6% 5% 29%	1288
February	Day	22% 12% 5% 5% 6% 6% 6% 24%	784
	Night	22% 10% 7% 7% 5% 29%	1005
March	Day	25% 8% 6% 5% 5% 6% 8% 21%	984
	Night	23% 11% 5% 5% 5% 29%	1015
April	Day	21% 7% 6% 5% 35%	1090
	Night	20% 5% 6% 5% 5% 38%	887
May	Day	19% 5% 5% 6% 9% 32%	1264
	Night	15% 5% 6% 6% 9% 40%	782
June	Day	16% 6% 6% 6% 8% 36%	1250
	Night	14% 5% 5% 6% 43%	719
July	Day	10% 5% 5% 5% 9% 43%	1302
	Night	9% 6% 8% 10% 47%	741
August	Day	12% 5% 5% 5% 5% 5% 9% 41%	1157
	Night	10% 5% 6% 8% 49%	879
September	Day	16% 5% 7% 45%	1032
	Night	12% 5% 54%	942
October	Day	16% 7% 5% 8% 42%	910
	Night	20% 5% 6% 46%	1123
November	Day	20% 9% 5% 5% 5% 36%	991
	Night	21% 8% 5% 5% 37%	1454
December	Day	25% 11% 5% 5% 6% 7% 7% 21%	744
	Night	29% 11% 6% 7% 5% 5% 20%	1297

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% 13% 9% 7% 5% 6% 5% 19%	752
	Night	21% 13% 7% 7% 5% 27%	1288
February	Day	19% 13% 5% 6% 6% 6% 7% 21%	782
	Night	20% 10% 8% 7% 5% 28%	996
March	Day	24% 8% 7% 6% 5% 5% 7% 7% 20%	1005
	Night	23% 9% 6% 5% 5% 5% 28%	1040
April	Day	20% 8% 5% 5% 5% 5% 34%	1089
	Night	19% 5% 5% 6% 36%	887
May	Day	17% 5% 6% 6% 6% 6% 7% 31%	1263
	Night	14% 6% 5% 5% 8% 39%	782
June	Day	13% 5% 6% 7% 6% 8% 34%	1260
	Night	13% 5% 5% 5% 5% 5% 42%	720
July	Day	9% 7% 5% 6% 9% 42%	1302
	Night	7% 6% 8% 11% 46%	742
August	Day	11% 5% 6% 6% 5% 9% 39%	1152
	Night	9% 5% 5% 5% 10% 46%	880
September	Day	14% 6% 5% 5% 7% 44%	1031
	Night	11% 6% 53%	943
October	Day	14% 7% 5% 9% 41%	910
	Night	19% 5% 5% 7% 44%	1123
November	Day	19% 9% 5% 6% 5% 5% 35%	989
	Night	20% 8% 5% 5% 6% 36%	1454
December	Day	24% 10% 7% 7% 7% 7% 19%	744
	Night	27% 11% 7% 8% 5% 5% 20%	1301

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	5% 23% 19% 8%	424
	Night	26% 22% 15%	734
February	Day	6% 28% 17% 7%	493
	Night	5% 20% 25% 7%	636
March	Day	8% 29% 25%	582
	Night	28% 25% 6%	620
April	Day	19% 26% 17%	741
	Night	14% 33% 19%	604
May	Day	6% 18% 32% 15%	841
	Night	7% 18% 35% 20%	545
June	Day	6% 17% 30% 22%	877
	Night	6% 16% 32% 20% 5%	516
July	Day	22% 27% 19%	717
	Night	8% 22% 26% 24%	437
August	Day	13% 22% 26%	951
	Night	5% 15% 24% 28%	728
September	Day	12% 29% 22% 6%	859
	Night	11% 29% 24% 7%	820
October	Day	14% 29% 17%	768
	Night	10% 31% 23%	922
November	Day	20% 30% 10%	706
	Night	5% 19% 25% 15%	1071
December	Day	5% 28% 18%	425
	Night	21% 22%	721

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	22% 40% 24%	395
February	Night	10% 45% 24%	306
March	Day	23% 34% 15% 7%	26
	Night	19% 36% 25% 7%	282
April	Day	14% 23% 39% 13%	81
	Night	16% 31% 37% 6%	237
May	Day	9% 29% 31% 20%	133
	Night	6% 32% 41% 11%	251
June	Day	8% 28% 26% 21% 6%	160
	Night	6% 24% 44% 17%	239
July	Day	22% 31% 24% 12%	153
	Night	5% 15% 46% 27%	237
August	Day	10% 40% 28% 13%	110
	Night	8% 49% 28%	257
September	Day	15% 32% 27% 14%	83
	Night	19% 30% 31% 6%	293
October	Day	22% 50% 16% 5%	18
	Night	9% 29% 33% 12%	337
November	Night	14% 35% 30% 5%	401
December	Night	14% 33% 29%	397

Page generated at 2008-11-27 12:47:49. Version ab11