Norfolk County
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Norfolk County clear sky chart from 2005-11-06 to 2024-01-22.

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	15% 9% 4 7% 6% 3 5% 5% 6% 7% 33%	74840
	Night	15% 10% 4 7% 6% 3 5% 5% 5% 7% 34%	75052
Transparency	Day	48% 8% 14% 19% 10% 1	74559
	Night	49% 8% 12% 19% 11% 1	74868
Seeing	Day	18% 11% 27% 25% 14% 5%	13507
	Night	19% 10% 29% 28% 12% 2	23051

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	16% 16% 5% 10% 6% 3 6% 5% 5% 6% 21%	1587
	11th to 20th	Day	16% 18% 8% 11% 7% 4 5% 4 5% 6% 17%	1710
	21st to end	Day	14% 16% 7% 12% 8% 5% 6% 5% 5% 4 18%	1791
February	1st to 10th	Day	17% 17% 5% 10% 9% 5% 5% 5% 4 4 18%	1620
	11th to 20th	Day	15% 12% 9% 13% 8% 4 4 4 5% 6% 21%	1573
	21st to end	Day	20% 12% 5% 11% 8% 3 5% 4 4 6% 22%	1435
March	1st to 10th	Day	17% 12% 5% 9% 8% 4 5% 4 5% 7% 26%	1963
	11th to 20th	Day	14% 13% 6% 9% 6% 3 3 4 4 7% 30%	1980
	21st to end	Day	20% 12% 4 8% 6% 3 5% 3 4 5% 30%	2178
April	1st to 10th	Day	17% 11% 5% 8% 6% 3 5% 4 6% 7% 29%	2078
	11th to 20th	Day	17% 9% 5% 7% 5% 3 3 3 5% 7% 34%	2240
	21st to end	Day	20% 9% 3 6% 6% 3 5% 4 5% 6% 33%	2340
May	1st to 10th	Day	16% 7% 3 6% 7% 3 5% 5% 5% 7% 33%	2340
	11th to 20th	Day	17% 7% 3 6% 6% 3 5% 4 5% 6% 37%	2354
	21st to end	Day	11% 5% 2 4 5% 3 6% 6% 6% 9% 43%	2725
June	1st to 10th	Day	12% 6% 2 5% 6% 3 5% 5% 6% 8% 42%	2520
	11th to 20th	Day	12% 6% 3 6% 5% 3 5% 5% 7% 9% 39%	2520
	21st to end	Day	12% 5% 2 5% 4 3 5% 5% 6% 11% 41%	2638
July	1st to 10th	Day	9% 4 2 4 5% 3 5% 6% 7% 9% 46%	2534
	11th to 20th	Day	12% 5% 2 5% 5% 3 6% 6% 7% 10% 38%	2409
	21st to end	Day	8% 5% 2 5% 6% 4 6% 7% 8% 10% 38%	2547
August	1st to 10th	Day	9% 5% 2 4 5% 3 6% 6% 7% 10% 43%	2327
	11th to 20th	Day	9% 4 3 6% 5% 5% 7% 8% 8% 10% 36%	2303
	21st to end	Day	11% 6% 3 6% 6% 3 5% 5% 6% 8% 42%	2505
September	1st to 10th	Day	14% 5% 2 6% 5% 4 6% 5% 6% 8% 40%	2160
	11th to 20th	Day	10% 4 3 4 5% 3 5% 5% 7% 8% 46%	1984
	21st to end	Day	15% 5% 3 6% 6% 4 5% 5% 6% 7% 38%	1980
October	1st to 10th	Day	17% 7% 3 6% 6% 4 5% 5% 6% 6% 36%	1805
	11th to 20th	Day	18% 10% 4 7% 6% 4 5% 5% 5% 6% 30%	1800
	21st to end	Day	23% 10% 4 8% 7% 3 5% 4 6% 7% 24%	1980
November	1st to 10th	Day	14% 8% 3 7% 6% 3 5% 5% 5% 7% 36%	1676
	11th to 20th	Day	16% 10% 3 7% 7% 4 5% 5% 5% 8% 30%	1520
	21st to end	Day	21% 13% 4 8% 5% 3 5% 6% 6% 7% 23%	1520
December	1st to 10th	Day	16% 17% 5% 9% 8% 4 5% 5% 5% 6% 22%	1520
	11th to 20th	Day	19% 15% 6% 9% 8% 4 5% 6% 5% 5% 19%	1906
	21st to end	Day	28% 19% 6% 8% 7% 3 4 4 4 4 12%	2772
January	1st to 10th	Night	17% 13% 6% 10% 7% 4 5% 5% 5% 5% 23%	2577
	11th to 20th	Night	19% 15% 6% 9% 6% 4 5% 6% 5% 6% 18%	2470
	21st to end	Night	12% 14% 7% 11% 9% 3 6% 5% 5% 6% 23%	2599
February	1st to 10th	Night	17% 16% 5% 9% 7% 3 5% 5% 5% 5% 22%	2340
	11th to 20th	Night	16% 12% 6% 8% 7% 4 6% 5% 5% 6% 25%	2273
	21st to end	Night	18% 13% 5% 8% 7% 3 5% 5% 5% 6% 24%	1817
March	1st to 10th	Night	14% 8% 3 8% 7% 4 6% 5% 5% 7% 34%	1984
	11th to 20th	Night	16% 9% 5% 7% 5% 3 4 4 5% 7% 35%	1979
	21st to end	Night	17% 10% 4 6% 5% 3 5% 5% 4 5% 35%	2178
April	1st to 10th	Night	18% 9% 4 6% 5% 3 6% 4 5% 7% 33%	1882
	11th to 20th	Night	17% 8% 3 6% 4 3 4 6% 5% 7% 37%	1684
	21st to end	Night	15% 6% 3 6% 6% 4 5% 5% 6% 8% 35%	1620
May	1st to 10th	Night	17% 8% 3 6% 5% 3 4 5% 6% 8% 36%	1620
	11th to 20th	Night	13% 6% 3 6% 6% 3 5% 4 6% 7% 41%	1539
	21st to end	Night	11% 5% 2 5% 4 3 3 4 6% 8% 50%	1550
June	1st to 10th	Night	11% 5% 3 5% 5% 4 3 6% 6% 8% 43%	1440
	11th to 20th	Night	10% 5% 2 5% 6% 4 5% 5% 6% 10% 42%	1440
	21st to end	Night	10% 5% 2 5% 5% 2 5% 4 6% 9% 47%	1304
July	1st to 10th	Night	7% 4 2 5% 5% 2 4 3 4 7% 57%	1408
	11th to 20th	Night	8% 4 2 4 5% 3 5% 5% 7% 9% 47%	1547
	21st to end	Night	7% 4 2 6% 6% 3 4 5% 6% 10% 48%	1771
August	1st to 10th	Night	6% 5% 2 5% 6% 3 5% 5% 7% 8% 48%	1611
	11th to 20th	Night	8% 4 2 5% 5% 3 3 5% 6% 11% 49%	1598
	21st to end	Night	8% 4 2 5% 5% 3 5% 4 6% 9% 49%	1831
September	1st to 10th	Night	12% 7% 3 6% 5% 3 4 4 5% 7% 45%	1800
	11th to 20th	Night	10% 4 2 4 3 3 3 4 6% 7% 54%	1973
	21st to end	Night	15% 5% 2 6% 6% 3 4 5% 6% 6% 41%	1980
October	1st to 10th	Night	15% 6% 3 6% 5% 2 4 4 5% 8% 43%	2155
	11th to 20th	Night	17% 11% 3 6% 6% 3 4 4 5% 6% 35%	2160
	21st to end	Night	22% 8% 3 6% 6% 3 4 5% 5% 6% 31%	2376
November	1st to 10th	Night	15% 7% 3 5% 5% 3 4 4 5% 8% 41%	2394
	11th to 20th	Night	15% 10% 3 7% 6% 4 5% 5% 6% 8% 31%	2660
	21st to end	Night	19% 12% 4 7% 6% 3 5% 5% 5% 7% 27%	2660
December	1st to 10th	Night	19% 15% 5% 8% 6% 3 4 5% 5% 6% 23%	2660
	11th to 20th	Night	16% 16% 6% 9% 7% 3 5% 4 5% 6% 23%	3288
	21st to end	Night	28% 18% 5% 9% 6% 3 4 4 4 5% 14%	4884

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	63% 7% 12% 13% 5%	1588
	11th to 20th	Day	67% 8% 10% 9% 4	1710
	21st to end	Day	69% 6% 9% 11% 4 1	1791
February	1st to 10th	Day	68% 5% 11% 10% 4 1	1620
	11th to 20th	Day	64% 5% 10% 14% 5% 1	1582
	21st to end	Day	64% 6% 10% 14% 5% 1	1435
March	1st to 10th	Day	56% 7% 14% 16% 6% 1	1966
	11th to 20th	Day	55% 5% 11% 18% 9% 2	1980
	21st to end	Day	57% 4 9% 18% 10% 2	2178
April	1st to 10th	Day	52% 6% 12% 17% 12% 1	2078
	11th to 20th	Day	49% 5% 12% 19% 13% 1	2240
	21st to end	Day	49% 6% 13% 18% 11% 2	2340
May	1st to 10th	Day	48% 7% 12% 21% 11% 2	2340
	11th to 20th	Day	45% 8% 13% 20% 12% 2	2358
	21st to end	Day	35% 9% 17% 23% 14% 2	2725
June	1st to 10th	Day	38% 8% 14% 25% 14% 2	2520
	11th to 20th	Day	39% 9% 15% 21% 14% 2	2520
	21st to end	Day	36% 8% 14% 26% 16% 1	2638
July	1st to 10th	Day	31% 8% 14% 26% 17% 3	2464
	11th to 20th	Day	36% 8% 18% 24% 12% 1	2336
	21st to end	Day	35% 10% 17% 23% 13% 1	2405
August	1st to 10th	Day	32% 10% 18% 26% 12% 1	2314
	11th to 20th	Day	37% 11% 18% 24% 10% 1	2303
	21st to end	Day	37% 8% 14% 26% 12% 2	2505
September	1st to 10th	Day	38% 9% 17% 24% 11% 1	2160
	11th to 20th	Day	33% 8% 16% 28% 15%	1984
	21st to end	Day	42% 8% 15% 23% 11% 1	1980
October	1st to 10th	Day	46% 8% 14% 19% 12% 1	1805
	11th to 20th	Day	53% 8% 14% 16% 9% 1	1800
	21st to end	Day	58% 8% 14% 14% 6% 1	1980
November	1st to 10th	Day	45% 8% 13% 23% 8% 2	1676
	11th to 20th	Day	50% 9% 16% 15% 9%	1520
	21st to end	Day	59% 9% 14% 13% 4	1520
December	1st to 10th	Day	63% 9% 13% 12% 2	1520
	11th to 20th	Day	63% 12% 11% 9% 4	1906
	21st to end	Day	75% 6% 10% 7% 2	2772
January	1st to 10th	Night	62% 8% 10% 12% 8%	2577
	11th to 20th	Night	65% 9% 11% 10% 5%	2470
	21st to end	Night	62% 7% 10% 14% 6% 1	2599
February	1st to 10th	Night	62% 8% 11% 13% 5% 1	2340
	11th to 20th	Night	59% 7% 11% 16% 7%	2275
	21st to end	Night	59% 6% 11% 16% 7% 1	1817
March	1st to 10th	Night	48% 8% 12% 21% 9% 1	1994
	11th to 20th	Night	49% 6% 10% 21% 12% 2	1980
	21st to end	Night	51% 5% 8% 19% 13% 3	2178
April	1st to 10th	Night	50% 6% 12% 18% 13% 2	1882
	11th to 20th	Night	46% 6% 10% 20% 16% 2	1684
	21st to end	Night	45% 7% 11% 19% 15% 3	1620
May	1st to 10th	Night	45% 6% 11% 21% 13% 3	1620
	11th to 20th	Night	42% 6% 12% 22% 14% 4	1540
	21st to end	Night	32% 7% 15% 25% 19% 3	1551
June	1st to 10th	Night	38% 7% 11% 25% 16% 3	1440
	11th to 20th	Night	36% 8% 13% 23% 17% 2	1440
	21st to end	Night	34% 8% 13% 25% 18% 2	1304
July	1st to 10th	Night	28% 6% 11% 28% 22% 4	1368
	11th to 20th	Night	32% 8% 13% 27% 18% 3	1498
	21st to end	Night	29% 10% 16% 24% 17% 3	1672
August	1st to 10th	Night	31% 9% 13% 32% 14% 1	1600
	11th to 20th	Night	30% 8% 17% 30% 14% 1	1598
	21st to end	Night	32% 8% 14% 29% 16% 2	1831
September	1st to 10th	Night	37% 8% 13% 24% 16% 2	1800
	11th to 20th	Night	30% 6% 13% 32% 19% 1	1973
	21st to end	Night	42% 7% 12% 25% 13% 1	1980
October	1st to 10th	Night	40% 7% 12% 24% 15% 1	2155
	11th to 20th	Night	49% 7% 13% 20% 11%	2160
	21st to end	Night	53% 7% 11% 18% 10% 1	2376
November	1st to 10th	Night	41% 7% 13% 24% 14% 1	2394
	11th to 20th	Night	50% 9% 13% 17% 10% 1	2660
	21st to end	Night	56% 9% 16% 14% 4 1	2660
December	1st to 10th	Night	62% 8% 14% 13% 3	2660
	11th to 20th	Night	61% 10% 12% 12% 4	3288
	21st to end	Night	73% 8% 11% 6% 2	4884

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	25% 19% 32% 13% 8% 2	213
	11th to 20th	Day	26% 17% 37% 14% 5% 1	270
	21st to end	Day	25% 18% 34% 18% 6% 1	267
February	1st to 10th	Day	25% 17% 45% 13% 1	240
	11th to 20th	Day	23% 25% 35% 16% 1	240
	21st to end	Day	26% 26% 34% 13% 2	198
March	1st to 10th	Day	24% 11% 33% 25% 7%	406
	11th to 20th	Day	21% 15% 36% 22% 5% 1	420
	21st to end	Day	28% 12% 31% 21% 7% 1	462
April	1st to 10th	Day	18% 16% 32% 25% 8% 1	420
	11th to 20th	Day	24% 13% 28% 21% 10% 4	412
	21st to end	Day	22% 9% 35% 23% 9% 2	420
May	1st to 10th	Day	23% 11% 29% 23% 8% 6%	423
	11th to 20th	Day	19% 8% 23% 27% 19% 4	447
	21st to end	Day	14% 4 22% 32% 20% 8%	485
June	1st to 10th	Day	12% 5% 31% 36% 13% 3	450
	11th to 20th	Day	14% 8% 23% 31% 19% 4	450
	21st to end	Day	14% 5% 28% 35% 15% 3	568
July	1st to 10th	Day	9% 3 19% 30% 28% 12%	459
	11th to 20th	Day	16% 4 20% 30% 19% 11%	444
	21st to end	Day	9% 4 22% 37% 22% 7%	488
August	1st to 10th	Day	8% 3 18% 34% 24% 13%	446
	11th to 20th	Day	9% 5% 17% 34% 22% 12%	439
	21st to end	Day	12% 5% 21% 29% 23% 11%	495
September	1st to 10th	Day	15% 5% 19% 29% 22% 11%	450
	11th to 20th	Day	8% 3 19% 27% 28% 14%	450
	21st to end	Day	15% 9% 24% 28% 16% 7%	450
October	1st to 10th	Day	18% 10% 25% 21% 19% 6%	450
	11th to 20th	Day	18% 17% 34% 23% 7% 2	450
	21st to end	Day	26% 15% 30% 21% 7%	495
November	1st to 10th	Day	19% 18% 33% 18% 10% 2	282
	11th to 20th	Day	23% 23% 29% 16% 7% 2	180
	21st to end	Day	28% 17% 35% 17% 3	180
December	1st to 10th	Day	26% 24% 28% 16% 5% 1	180
	11th to 20th	Day	27% 24% 32% 13% 4	180
	21st to end	Day	28% 21% 33% 13% 5%	198
January	1st to 10th	Night	24% 18% 36% 17% 6%	760
	11th to 20th	Night	27% 16% 33% 19% 4	760
	21st to end	Night	19% 14% 36% 25% 6%	800
February	1st to 10th	Night	27% 14% 36% 21% 2	720
	11th to 20th	Night	22% 14% 34% 25% 4	703
	21st to end	Night	24% 15% 36% 22% 2	592
March	1st to 10th	Night	18% 11% 37% 28% 7%	720
	11th to 20th	Night	21% 10% 35% 25% 8%	720
	21st to end	Night	23% 9% 31% 27% 9% 1	792
April	1st to 10th	Night	22% 11% 34% 27% 6% 1	720
	11th to 20th	Night	22% 9% 32% 27% 9% 1	620
	21st to end	Night	17% 8% 31% 35% 9%	540
May	1st to 10th	Night	18% 6% 29% 30% 16% 1	540
	11th to 20th	Night	15% 5% 27% 38% 14% 1	478
	21st to end	Night	14% 2 24% 33% 24% 3	389
June	1st to 10th	Night	13% 4 27% 40% 16% 1	360
	11th to 20th	Night	12% 3 24% 42% 18%	360
	21st to end	Night	11% 5% 22% 43% 18% 1	360
July	1st to 10th	Night	7% 3 18% 43% 25% 4	350
	11th to 20th	Night	11% 2 19% 39% 24% 5%	454
	21st to end	Night	8% 2 19% 44% 24% 3	591
August	1st to 10th	Night	8% 3 13% 42% 30% 5%	537
	11th to 20th	Night	9% 4 16% 34% 33% 4	534
	21st to end	Night	10% 3 19% 38% 26% 4	643
September	1st to 10th	Night	14% 6% 19% 32% 24% 5%	720
	11th to 20th	Night	12% 4 16% 33% 30% 5%	720
	21st to end	Night	16% 7% 21% 30% 21% 5%	720
October	1st to 10th	Night	18% 7% 23% 29% 19% 3	720
	11th to 20th	Night	22% 13% 27% 28% 9% 1	720
	21st to end	Night	26% 11% 32% 26% 5%	792
November	1st to 10th	Night	20% 11% 29% 28% 12% 1	740
	11th to 20th	Night	20% 19% 31% 22% 7% 2	760
	21st to end	Night	26% 14% 33% 21% 6%	760
December	1st to 10th	Night	30% 17% 32% 16% 5% 1	760
	11th to 20th	Night	24% 18% 35% 19% 4	760
	21st to end	Night	31% 13% 34% 20% 2	836

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