Durham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Durham 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% 35%	8673
	Night	20% 6% 5% 6% 36%	8501
Cloud Cover	Day	19% 5% 5% 6% 7% 34%	8672
	Night	19% 5% 5% 6% 5% 6% 35%	8503
Transparency	Day	17% 14% 32% 28% 6%	4437
	Night	17% 13% 33% 29% 6%	4449
Seeing	Day	10% 26% 27% 25% 10%	530
	Night	13% 29% 33% 22%	2591

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	18% 5% 7% 7% 37%	551
	Night	18% 5% 8% 44%	802
February	Day	26% 5% 5% 37%	547
	Night	23% 5% 7% 5% 36%	707
March	Day	20% 6% 6% 41%	682
	Night	19% 6% 5% 6% 41%	682
April	Day	18% 5% 8% 42%	713
	Night	18% 5% 5% 5% 5% 6% 38%	607
May	Day	15% 5% 5% 5% 7% 5% 6% 6% 37%	800
	Night	19% 7% 6% 5% 6% 7% 30%	554
June	Day	26% 7% 7% 7% 5% 6% 5% 19%	780
	Night	21% 7% 5% 7% 6% 8% 6% 5% 10% 17%	536
July	Day	14% 7% 6% 5% 5% 10% 5% 7% 8% 23%	804
	Night	15% 8% 7% 7% 6% 9% 5% 9% 23%	558
August	Day	22% 6% 5% 8% 7% 7% 27%	1164
	Night	22% 6% 7% 9% 5% 6% 7% 25%	875
September	Day	22% 5% 6% 39%	1006
	Night	17% 6% 5% 5% 7% 37%	939
October	Day	22% 5% 5% 8% 37%	618
	Night	25% 5% 5% 6% 36%	744
November	Day	20% 6% 45%	494
	Night	22% 5% 44%	673
December	Day	16% 51%	514
	Night	17% 48%	824

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	18% 5% 5% 7% 5% 6% 37%	551
	Night	18% 5% 8% 43%	802
February	Day	26% 5% 5% 37%	547
	Night	22% 5% 8% 5% 5% 36%	707
March	Day	19% 7% 6% 40%	682
	Night	17% 5% 6% 5% 6% 41%	682
April	Day	16% 5% 8% 42%	713
	Night	18% 5% 7% 5% 39%	607
May	Day	14% 5% 5% 5% 7% 5% 6% 8% 35%	800
	Night	16% 5% 5% 6% 7% 6% 6% 6% 8% 28%	554
June	Day	25% 7% 7% 6% 7% 8% 5% 5% 18%	780
	Night	20% 5% 7% 7% 5% 7% 7% 7% 9% 16%	537
July	Day	13% 6% 7% 6% 7% 5% 9% 6% 5% 8% 22%	801
	Night	16% 7% 7% 6% 8% 7% 6% 9% 21%	558
August	Day	20% 7% 5% 5% 7% 5% 7% 10% 23%	1165
	Night	20% 7% 6% 5% 9% 5% 6% 7% 25%	875
September	Day	20% 5% 6% 7% 37%	1006
	Night	16% 5% 5% 6% 5% 6% 8% 36%	940
October	Day	21% 5% 6% 5% 5% 7% 36%	618
	Night	22% 5% 5% 5% 6% 6% 36%	744
November	Day	20% 5% 6% 45%	495
	Night	23% 5% 44%	673
December	Day	16% 51%	514
	Night	17% 5% 48%	824

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	16% 15% 36% 28%	279
	Night	21% 12% 39% 21%	457
February	Day	13% 17% 24% 36% 6%	265
	Night	10% 15% 34% 33% 6%	309
March	Day	15% 13% 42% 21% 7%	347
	Night	17% 13% 36% 30%	366
April	Day	15% 7% 31% 33% 11%	411
	Night	17% 28% 42% 8%	323
May	Day	26% 13% 35% 17% 6%	406
	Night	27% 15% 38% 16%	234
June	Day	33% 22% 29% 13%	232
	Night	38% 16% 32% 12%	182
July	Day	30% 20% 33% 16%	295
	Night	32% 16% 38% 13%	206
August	Day	27% 18% 36% 16%	496
	Night	22% 24% 36% 16%	350
September	Day	11% 11% 28% 38% 9%	546
	Night	18% 14% 26% 31% 9%	527
October	Day	11% 12% 35% 30% 10%	486
	Night	8% 14% 34% 33% 9%	549
November	Day	11% 20% 22% 38% 6%	349
	Night	12% 8% 28% 38% 12%	468
December	Day	6% 7% 34% 40% 11%	325
	Night	7% 7% 29% 42% 13%	478

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	21% 42% 29% 5%	206
February	Day	18% 36% 27% 18%	11
	Night	17% 46% 32%	176
March	Day	16% 37% 35% 9%	53
	Night	19% 47% 29%	207
April	Day	21% 32% 25% 19%	52
	Night	18% 40% 27% 12%	205
May	Day	10% 27% 25% 30% 5%	55
	Night	10% 24% 40% 24%	165
June	Day	24% 35% 31% 6%	45
	Night	8% 21% 40% 29%	157
July	Day	5% 18% 21% 40% 14%	55
	Night	14% 39% 38%	164
August	Day	8% 18% 13% 22% 36%	74
	Night	6% 17% 23% 42% 9%	272
September	Day	8% 19% 36% 25% 11%	72
	Night	6% 22% 36% 31%	314
October	Day	13% 27% 31% 23%	76
	Night	10% 29% 37% 22%	299
November	Day	8% 32% 21% 29% 8%	37
	Night	18% 21% 27% 29%	217
December	Night	19% 31% 34% 14%	209

Page generated at 2008-09-30 05:41:08. Version ab11