Norwich
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Norwich clear sky chart from 2005-11-06 to 2025-10-19.

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% 11% 4 9% 5% 4 4 4 5% 6% 29%	126942
	Night	18% 12% 4 10% 5% 4 4 3 5% 6% 29%	122735
Transparency	Day	54% 6% 11% 19% 9% 1	130446
	Night	56% 6% 11% 17% 9% 1	127318
Seeing	Day	27% 15% 27% 19% 9% 3	15966
	Night	25% 15% 26% 24% 9% 1	25092

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 16% 11% 24% 7% 3 3 2 3 3 17%	3597
	11th to 20th	Day	13% 18% 10% 19% 9% 5% 4 3 4 3 13%	3570
	21st to end	Day	15% 17% 9% 11% 7% 4 4 3 5% 5% 21%	4356
February	1st to 10th	Day	16% 16% 5% 15% 9% 5% 5% 4 5% 4 17%	1800
	11th to 20th	Day	13% 14% 7% 16% 10% 5% 5% 4 3 4 19%	1831
	21st to end	Day	20% 14% 3 13% 8% 4 3 3 3 5% 22%	1650
March	1st to 10th	Day	21% 15% 7% 11% 7% 4 4 3 3 3 23%	4476
	11th to 20th	Day	12% 17% 5% 8% 5% 4 4 2 4 6% 34%	4980
	21st to end	Day	18% 12% 4 11% 5% 4 4 3 4 4 29%	5780
April	1st to 10th	Day	19% 16% 4 9% 4 4 4 3 4 5% 30%	2400
	11th to 20th	Day	18% 11% 3 7% 5% 5% 5% 3 4 5% 34%	2374
	21st to end	Day	20% 13% 3 7% 5% 4 4 3 4 6% 32%	2400
May	1st to 10th	Day	32% 10% 3 6% 4 3 4 3 4 6% 25%	6244
	11th to 20th	Day	20% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	6318
	21st to end	Day	22% 9% 3 7% 5% 4 4 4 6% 8% 29%	6949
June	1st to 10th	Day	19% 8% 2 7% 5% 4 5% 4 5% 8% 32%	2800
	11th to 20th	Day	18% 7% 3 6% 5% 5% 5% 4 6% 9% 32%	2800
	21st to end	Day	18% 6% 3 7% 5% 5% 5% 5% 6% 9% 32%	2800
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 10% 37%	6360
	11th to 20th	Day	11% 5% 2 7% 6% 5% 6% 6% 8% 11% 32%	6360
	21st to end	Day	14% 5% 2 6% 5% 5% 5% 4 6% 7% 40%	6926
August	1st to 10th	Day	20% 6% 3 7% 5% 5% 5% 4 6% 8% 32%	4314
	11th to 20th	Day	16% 7% 3 8% 5% 6% 6% 4 7% 9% 29%	3987
	21st to end	Day	18% 6% 2 7% 4 4 4 4 6% 8% 37%	4312
September	1st to 10th	Day	17% 7% 2 5% 5% 4 4 4 6% 8% 39%	2400
	11th to 20th	Day	15% 6% 2 6% 3 4 4 3 5% 7% 45%	2385
	21st to end	Day	20% 6% 3 7% 5% 5% 5% 4 6% 7% 33%	2165
October	1st to 10th	Day	20% 9% 3 8% 4 4 3 3 6% 6% 33%	2000
	11th to 20th	Day	20% 9% 3 7% 5% 4 5% 4 5% 6% 33%	1980
	21st to end	Day	23% 15% 3 6% 4 3 4 3 6% 6% 26%	1981
November	1st to 10th	Day	13% 13% 3 7% 4 5% 4 3 5% 6% 35%	1761
	11th to 20th	Day	17% 17% 3 10% 4 4 4 4 4 5% 27%	1740
	21st to end	Day	18% 18% 5% 9% 7% 5% 5% 3 4 6% 21%	1600
December	1st to 10th	Day	18% 19% 10% 16% 6% 4 4 3 4 4 12%	2600
	11th to 20th	Day	18% 18% 8% 13% 5% 4 3 2 3 3 22%	2986
	21st to end	Day	18% 19% 8% 11% 5% 4 4 3 5% 5% 19%	3960
January	1st to 10th	Night	14% 13% 8% 27% 7% 4 4 3 4 4 12%	6349
	11th to 20th	Night	10% 17% 8% 20% 8% 5% 3 3 4 5% 16%	6315
	21st to end	Night	16% 14% 6% 14% 6% 6% 5% 3 5% 5% 20%	6585
February	1st to 10th	Night	14% 16% 5% 14% 7% 5% 5% 3 4 5% 20%	2600
	11th to 20th	Night	12% 13% 7% 14% 9% 6% 6% 5% 5% 5% 18%	2454
	21st to end	Night	17% 14% 5% 14% 7% 4 3 3 5% 6% 22%	1980
March	1st to 10th	Night	17% 13% 5% 12% 8% 4 4 4 5% 5% 23%	5347
	11th to 20th	Night	17% 14% 5% 8% 4 4 3 2 4 5% 33%	4979
	21st to end	Night	19% 13% 4 11% 5% 4 4 3 4 5% 28%	5151
April	1st to 10th	Night	20% 15% 4 7% 5% 5% 3 3 4 6% 28%	2000
	11th to 20th	Night	19% 13% 3 6% 5% 4 4 3 5% 6% 34%	1975
	21st to end	Night	20% 10% 4 7% 4 4 4 3 5% 6% 34%	2000
May	1st to 10th	Night	33% 11% 3 7% 4 3 3 2 4 5% 25%	3709
	11th to 20th	Night	20% 9% 2 7% 4 4 4 3 6% 7% 33%	3575
	21st to end	Night	22% 10% 3 7% 4 5% 5% 4 5% 6% 30%	3926
June	1st to 10th	Night	17% 8% 2 6% 4 4 4 4 5% 6% 39%	1600
	11th to 20th	Night	18% 7% 2 5% 4 3 4 4 7% 9% 37%	1600
	21st to end	Night	14% 5% 2 5% 4 4 4 4 6% 9% 43%	1600
July	1st to 10th	Night	10% 6% 3 7% 5% 4 5% 4 6% 9% 39%	3600
	11th to 20th	Night	16% 6% 2 7% 4 4 4 4 5% 7% 40%	3600
	21st to end	Night	13% 6% 2 5% 4 4 4 3 5% 7% 48%	3926
August	1st to 10th	Night	22% 7% 2 6% 4 3 4 3 6% 6% 37%	2832
	11th to 20th	Night	18% 6% 3 7% 4 4 4 3 5% 5% 41%	3029
	21st to end	Night	16% 6% 2 5% 4 3 4 3 5% 7% 45%	3586
September	1st to 10th	Night	16% 6% 2 6% 3 3 4 3 5% 7% 45%	2000
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 7% 50%	2015
	21st to end	Night	18% 8% 3 6% 4 4 5% 3 5% 7% 37%	2230
October	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 5% 38%	2400
	11th to 20th	Night	20% 9% 3 7% 4 4 5% 3 5% 6% 35%	2377
	21st to end	Night	25% 13% 3 8% 4 4 4 3 4 7% 26%	2615
November	1st to 10th	Night	13% 13% 4 7% 4 4 4 4 5% 7% 36%	2537
	11th to 20th	Night	19% 14% 3 8% 4 4 4 3 5% 6% 30%	2659
	21st to end	Night	15% 16% 5% 9% 6% 5% 4 4 6% 7% 25%	2800
December	1st to 10th	Night	15% 17% 8% 14% 6% 5% 5% 4 5% 5% 17%	4580
	11th to 20th	Night	23% 15% 6% 10% 4 4 3 2 3 4 25%	5208
	21st to end	Night	22% 18% 7% 11% 7% 4 3 3 4 4 18%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	74% 3 7% 12% 3	3598
	11th to 20th	Day	75% 5% 7% 8% 4 1	3600
	21st to end	Day	66% 4 9% 14% 6%	4356
February	1st to 10th	Day	71% 5% 7% 12% 4	1800
	11th to 20th	Day	69% 5% 5% 14% 6%	1840
	21st to end	Day	65% 5% 8% 14% 7%	1650
March	1st to 10th	Day	66% 5% 9% 12% 7% 1	4479
	11th to 20th	Day	53% 4 17% 15% 8% 3	4980
	21st to end	Day	57% 5% 6% 14% 15% 2	5780
April	1st to 10th	Day	59% 4 6% 20% 9% 2	2400
	11th to 20th	Day	53% 5% 9% 19% 13% 1	2374
	21st to end	Day	54% 4 8% 19% 12% 2	2400
May	1st to 10th	Day	61% 5% 8% 15% 10% 1	6255
	11th to 20th	Day	53% 7% 9% 21% 9% 1	6322
	21st to end	Day	52% 7% 11% 20% 10% 1	6949
June	1st to 10th	Day	49% 7% 10% 20% 12% 1	2800
	11th to 20th	Day	49% 6% 13% 22% 9% 1	2800
	21st to end	Day	48% 7% 13% 21% 10% 1	2800
July	1st to 10th	Day	41% 8% 16% 24% 11%	6290
	11th to 20th	Day	41% 10% 16% 24% 9%	6285
	21st to end	Day	41% 6% 12% 27% 13%	6728
August	1st to 10th	Day	50% 7% 14% 18% 11%	4300
	11th to 20th	Day	47% 8% 16% 23% 6%	4026
	21st to end	Day	44% 6% 12% 30% 7% 1	4312
September	1st to 10th	Day	43% 6% 12% 26% 12% 1	2400
	11th to 20th	Day	40% 4 11% 25% 19% 1	2385
	21st to end	Day	49% 6% 11% 21% 12% 1	2165
October	1st to 10th	Day	46% 6% 11% 25% 12% 1	3260
	11th to 20th	Day	46% 5% 12% 20% 16% 1	3220
	21st to end	Day	44% 6% 15% 24% 9% 1	3245
November	1st to 10th	Day	49% 5% 13% 19% 12% 1	1761
	11th to 20th	Day	60% 4 10% 16% 8% 2	1740
	21st to end	Day	65% 4 13% 14% 5%	1600
December	1st to 10th	Day	77% 5% 8% 8% 2	2600
	11th to 20th	Day	69% 3 9% 11% 7%	2986
	21st to end	Day	69% 4 9% 11% 7% 1	3960
January	1st to 10th	Night	76% 5% 9% 7% 3	6349
	11th to 20th	Night	71% 5% 8% 9% 6% 1	6360
	21st to end	Night	66% 5% 9% 9% 10% 1	6585
February	1st to 10th	Night	67% 5% 9% 13% 6%	2600
	11th to 20th	Night	67% 6% 8% 13% 5% 1	2456
	21st to end	Night	63% 5% 10% 14% 8% 1	1980
March	1st to 10th	Night	63% 5% 11% 13% 8%	5357
	11th to 20th	Night	54% 5% 17% 15% 6% 3	4980
	21st to end	Night	58% 5% 5% 18% 12% 1	5151
April	1st to 10th	Night	59% 5% 9% 17% 8% 2	2000
	11th to 20th	Night	52% 5% 10% 18% 15% 1	1975
	21st to end	Night	51% 4 9% 19% 13% 3	2000
May	1st to 10th	Night	63% 5% 8% 13% 9% 3	3715
	11th to 20th	Night	50% 8% 13% 19% 9% 1	3576
	21st to end	Night	55% 6% 10% 17% 11% 1	3927
June	1st to 10th	Night	44% 6% 11% 24% 14% 1	1600
	11th to 20th	Night	42% 8% 15% 24% 11% 1	1600
	21st to end	Night	38% 8% 14% 26% 12% 1	1600
July	1st to 10th	Night	40% 8% 16% 21% 14% 1	3560
	11th to 20th	Night	43% 8% 12% 23% 13% 1	3552
	21st to end	Night	36% 6% 13% 29% 16% 1	3809
August	1st to 10th	Night	47% 7% 14% 23% 9% 1	2822
	11th to 20th	Night	45% 5% 15% 27% 9%	3056
	21st to end	Night	38% 6% 13% 32% 11%	3586
September	1st to 10th	Night	40% 5% 14% 26% 14% 1	2000
	11th to 20th	Night	35% 5% 11% 30% 18% 1	2015
	21st to end	Night	47% 5% 11% 23% 13% 1	2230
October	1st to 10th	Night	50% 6% 10% 22% 11% 1	3920
	11th to 20th	Night	46% 5% 13% 18% 14% 3	3876
	21st to end	Night	45% 6% 15% 26% 8%	4301
November	1st to 10th	Night	49% 5% 10% 20% 14% 2	2537
	11th to 20th	Night	56% 5% 9% 19% 9% 2	2659
	21st to end	Night	59% 5% 15% 16% 5%	2800
December	1st to 10th	Night	68% 6% 11% 13% 2	4580
	11th to 20th	Night	65% 4 9% 13% 9%	5208
	21st to end	Night	70% 5% 7% 9% 7% 1	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 20% 27% 13% 4 1	200
	11th to 20th	Day	44% 24% 22% 8% 2 1	200
	21st to end	Day	33% 26% 29% 10% 3	307
February	1st to 10th	Day	38% 29% 27% 6%	300
	11th to 20th	Day	39% 27% 26% 7%	378
	21st to end	Day	39% 19% 28% 11% 3	414
March	1st to 10th	Day	33% 21% 29% 13% 4	500
	11th to 20th	Day	36% 21% 25% 14% 3	500
	21st to end	Day	37% 21% 24% 11% 4 2	550
April	1st to 10th	Day	39% 19% 24% 13% 4 1	500
	11th to 20th	Day	35% 21% 24% 11% 6% 3	492
	21st to end	Day	33% 17% 28% 14% 6% 2	500
May	1st to 10th	Day	36% 16% 26% 13% 7% 3	503
	11th to 20th	Day	24% 14% 32% 20% 7% 4	527
	21st to end	Day	21% 11% 32% 24% 9% 4	573
June	1st to 10th	Day	26% 14% 25% 23% 9% 2	530
	11th to 20th	Day	21% 12% 30% 25% 11% 2	530
	21st to end	Day	22% 12% 34% 23% 7% 2	530
July	1st to 10th	Day	14% 9% 30% 27% 15% 5%	525
	11th to 20th	Day	15% 6% 27% 33% 14% 4	524
	21st to end	Day	18% 6% 34% 27% 12% 3	576
August	1st to 10th	Day	13% 8% 28% 27% 18% 6%	526
	11th to 20th	Day	16% 8% 26% 28% 14% 7%	519
	21st to end	Day	16% 11% 28% 23% 14% 7%	583
September	1st to 10th	Day	18% 10% 24% 24% 15% 9%	530
	11th to 20th	Day	14% 7% 26% 21% 20% 12%	530
	21st to end	Day	23% 11% 26% 22% 13% 5%	530
October	1st to 10th	Day	25% 12% 24% 19% 12% 7%	530
	11th to 20th	Day	30% 16% 27% 16% 9% 2	522
	21st to end	Day	36% 14% 26% 20% 4	482
November	1st to 10th	Day	27% 22% 23% 18% 9% 2	395
	11th to 20th	Day	34% 22% 23% 15% 5% 1	340
	21st to end	Day	38% 20% 30% 12% 1	200
December	1st to 10th	Day	38% 22% 26% 11% 3 1	200
	11th to 20th	Day	38% 22% 30% 10% 2	200
	21st to end	Day	37% 19% 34% 8% 2 1	220
January	1st to 10th	Night	27% 25% 32% 14% 2	800
	11th to 20th	Night	28% 22% 28% 18% 3	800
	21st to end	Night	23% 23% 33% 17% 3	880
February	1st to 10th	Night	29% 21% 30% 18% 2	800
	11th to 20th	Night	28% 23% 32% 15% 2	783
	21st to end	Night	30% 22% 30% 16% 2	660
March	1st to 10th	Night	28% 22% 29% 18% 3	800
	11th to 20th	Night	29% 20% 29% 19% 3	800
	21st to end	Night	27% 22% 25% 19% 6% 1	880
April	1st to 10th	Night	31% 15% 30% 21% 3	800
	11th to 20th	Night	27% 18% 27% 20% 7% 1	791
	21st to end	Night	29% 13% 27% 23% 7%	800
May	1st to 10th	Night	32% 11% 21% 28% 9%	463
	11th to 20th	Night	21% 10% 31% 30% 8% 1	394
	21st to end	Night	15% 6% 29% 37% 11% 1	433
June	1st to 10th	Night	19% 8% 25% 36% 11% 1	400
	11th to 20th	Night	20% 8% 24% 34% 15%	400
	21st to end	Night	14% 5% 27% 40% 14% 1	400
July	1st to 10th	Night	10% 3 23% 44% 18% 2	390
	11th to 20th	Night	13% 3 20% 42% 20% 2	388
	21st to end	Night	16% 3 22% 45% 13% 2	438
August	1st to 10th	Night	15% 3 22% 36% 23% 2	540
	11th to 20th	Night	17% 4 13% 37% 25% 4	683
	21st to end	Night	19% 6% 18% 35% 22% 2	880
September	1st to 10th	Night	18% 6% 22% 33% 18% 2	800
	11th to 20th	Night	17% 6% 19% 27% 26% 4	800
	21st to end	Night	21% 9% 24% 27% 17% 2	800
October	1st to 10th	Night	23% 11% 23% 25% 16% 2	800
	11th to 20th	Night	25% 12% 28% 26% 7% 1	793
	21st to end	Night	32% 12% 28% 22% 5%	836
November	1st to 10th	Night	21% 17% 30% 24% 8%	780
	11th to 20th	Night	30% 17% 28% 18% 6% 1	800
	21st to end	Night	26% 20% 30% 19% 5%	800
December	1st to 10th	Night	31% 23% 25% 17% 3	800
	11th to 20th	Night	31% 21% 27% 18% 3	800
	21st to end	Night	30% 23% 29% 16% 3	880

Page generated at 2025-10-16 01:50:23. Version ac07

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