Norwich
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Norwich 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	19% 10% 5% 7% 6% 5% 5% 6% 24%	16460
	Night	18% 9% 6% 8% 6% 5% 6% 25%	16472
Cloud Cover	Day	19% 10% 6% 7% 6% 5% 5% 6% 24%	16471
	Night	18% 10% 6% 8% 6% 5% 5% 6% 25%	16491
Transparency	Day	6% 14% 20% 10%	12022
	Night	7% 15% 19% 9%	12119
Seeing	Day	10% 23% 26% 15% 6%	976
	Night	17% 30% 26% 7%	4656

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	17% 13% 7% 13% 9% 6% 14%	1014
	Night	19% 14% 9% 12% 8% 6% 13%	1685
February	Day	14% 13% 8% 15% 13% 5% 13%	1031
	Night	12% 11% 11% 14% 9% 9% 5% 5% 5% 12%	1376
March	Day	17% 12% 9% 7% 5% 6% 26%	1329
	Night	18% 8% 9% 9% 5% 5% 6% 26%	1349
April	Day	20% 12% 5% 5% 6% 32%	1418
	Night	19% 9% 5% 6% 6% 6% 31%	1186
May	Day	17% 7% 6% 5% 7% 32%	1716
	Night	17% 6% 5% 6% 6% 7% 35%	997
June	Day	21% 6% 6% 6% 6% 5% 5% 7% 23%	1663
	Night	19% 6% 6% 5% 6% 8% 31%	956
July	Day	17% 6% 6% 6% 7% 6% 6% 8% 25%	1716
	Night	13% 5% 5% 5% 5% 6% 5% 7% 38%	974
August	Day	16% 6% 7% 6% 5% 5% 6% 8% 30%	1548
	Night	14% 6% 6% 5% 6% 39%	1164
September	Day	21% 7% 5% 5% 5% 6% 32%	1373
	Night	18% 5% 5% 6% 42%	1230
October	Day	25% 11% 7% 5% 6% 25%	1200
	Night	26% 9% 6% 5% 27%	1493
November	Day	21% 15% 5% 7% 5% 5% 5% 19%	1235
	Night	19% 14% 6% 7% 6% 5% 5% 6% 20%	1936
December	Day	19% 18% 12% 10% 7% 5% 11%	1217
	Night	19% 13% 9% 12% 7% 5% 5% 14%	2126

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	17% 12% 8% 12% 9% 6% 14%	1012
	Night	19% 13% 9% 12% 8% 6% 14%	1686
February	Day	15% 13% 8% 16% 12% 6% 13%	1029
	Night	13% 11% 10% 15% 8% 7% 5% 5% 5% 12%	1367
March	Day	18% 12% 8% 8% 6% 26%	1350
	Night	19% 9% 8% 8% 5% 5% 5% 26%	1373
April	Day	20% 12% 5% 5% 5% 32%	1417
	Night	19% 9% 5% 6% 5% 5% 31%	1186
May	Day	17% 7% 7% 5% 5% 7% 32%	1710
	Night	17% 5% 6% 6% 6% 7% 35%	991
June	Day	21% 6% 5% 5% 6% 6% 6% 5% 7% 23%	1672
	Night	19% 5% 6% 5% 6% 9% 31%	958
July	Day	16% 7% 5% 7% 6% 6% 7% 8% 25%	1716
	Night	13% 5% 6% 5% 5% 6% 39%	975
August	Day	16% 6% 7% 6% 6% 5% 6% 8% 30%	1543
	Night	14% 6% 6% 5% 6% 6% 38%	1166
September	Day	22% 7% 6% 5% 5% 5% 6% 32%	1372
	Night	19% 5% 5% 5% 6% 41%	1231
October	Day	24% 12% 5% 6% 5% 5% 6% 26%	1200
	Night	25% 10% 6% 5% 27%	1493
November	Day	21% 15% 5% 7% 6% 5% 5% 20%	1233
	Night	19% 15% 5% 7% 6% 5% 5% 5% 20%	1935
December	Day	20% 17% 12% 10% 7% 5% 12%	1217
	Night	19% 13% 9% 12% 8% 5% 5% 14%	2130

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	8% 18% 12%	631
	Night	7% 17% 12% 5%	1020
February	Day	8% 12% 12%	646
	Night	10% 15% 13%	878
March	Day	7% 12% 22% 14%	905
	Night	6% 17% 20% 10%	958
April	Day	12% 25% 17%	991
	Night	6% 13% 21% 17%	836
May	Day	7% 16% 23% 12%	1307
	Night	7% 16% 22% 15%	772
June	Day	9% 18% 21% 5%	1200
	Night	9% 21% 32% 5%	686
July	Day	8% 18% 21% 6%	1105
	Night	8% 20% 24% 11%	644
August	Day	6% 11% 23% 13%	1356
	Night	6% 15% 27% 14%	1006
September	Day	6% 12% 24% 14%	1132
	Night	12% 30% 16%	1052
October	Day	9% 19% 12%	1009
	Night	5% 11% 17% 13%	1268
November	Day	5% 17% 13% 8%	920
	Night	6% 15% 15% 7%	1465
December	Day	5% 13% 11%	820
	Night	8% 16% 9%	1534

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	25% 42% 14%	413
February	Day	23% 28% 14% 14%	42
	Night	28% 38% 15%	381
March	Day	20% 26% 24% 8%	112
	Night	26% 30% 22%	442
April	Day	14% 26% 20% 9% 9%	104
	Night	20% 32% 26% 7%	420
May	Day	10% 23% 30% 14% 6%	116
	Night	8% 31% 35% 8%	237
June	Day	6% 24% 27% 18%	101
	Night	5% 24% 46% 13%	208
July	Day	5% 22% 37% 22%	100
	Night	23% 49% 15%	208
August	Day	20% 22% 24% 13%	115
	Night	23% 36% 18%	390
September	Day	23% 22% 20% 10%	105
	Night	6% 21% 34% 17%	439
October	Day	9% 18% 34% 10%	111
	Night	11% 25% 29% 8%	450
November	Day	18% 22% 28% 5%	70
	Night	19% 31% 23% 5%	519
December	Night	27% 33% 14%	549

Page generated at 2009-12-30 05:20:38. Version ab11