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 2026-05-02.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	132012
	Night	14% 11% 4 9% 5% 4 4 4 5% 7% 33%	130368
Transparency	Day	50% 6% 12% 20% 11% 1	135610
	Night	50% 6% 12% 19% 12% 1	134858
Seeing	Day	19% 11% 28% 25% 13% 5%	16451
	Night	19% 10% 29% 28% 12% 1	25967

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	15% 16% 7% 13% 6% 4 5% 3 4 5% 22%	4882
	11th to 20th	Day	9% 22% 10% 21% 9% 7% 4 2 3 3 9%	5246
	21st to end	Day	15% 15% 6% 17% 7% 6% 5% 3 4 4 17%	5780
February	1st to 10th	Day	15% 18% 4 12% 8% 6% 5% 4 4 4 20%	1890
	11th to 20th	Day	14% 13% 8% 14% 7% 5% 4 3 5% 5% 21%	1843
	21st to end	Day	18% 14% 4 12% 7% 4 5% 4 5% 6% 21%	1678
March	1st to 10th	Day	18% 11% 5% 12% 6% 4 4 3 4 5% 28%	5013
	11th to 20th	Day	11% 13% 5% 10% 5% 5% 4 3 5% 8% 31%	5090
	21st to end	Day	19% 12% 3 10% 5% 5% 4 3 4 5% 30%	5586
April	1st to 10th	Day	18% 13% 4 9% 5% 4 4 3 6% 7% 29%	2425
	11th to 20th	Day	16% 11% 4 8% 5% 4 3 3 5% 7% 33%	2618
	21st to end	Day	19% 9% 3 7% 6% 4 4 3 5% 6% 34%	2730
May	1st to 10th	Day	20% 9% 3 8% 5% 4 5% 4 5% 6% 31%	5901
	11th to 20th	Day	17% 10% 3 8% 5% 4 4 4 6% 7% 32%	5998
	21st to end	Day	16% 7% 2 7% 5% 4 5% 4 5% 8% 38%	6949
June	1st to 10th	Day	12% 7% 2 6% 5% 4 4 4 6% 7% 41%	2800
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 4 8% 9% 38%	2800
	21st to end	Day	12% 6% 2 6% 4 4 4 4 7% 10% 41%	2932
July	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	6387
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 6% 10% 42%	6067
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 47%	6424
August	1st to 10th	Day	16% 5% 2 5% 4 4 5% 4 7% 9% 39%	4234
	11th to 20th	Day	11% 6% 3 7% 5% 6% 7% 5% 7% 9% 34%	4174
	21st to end	Day	9% 6% 2 7% 4 4 5% 4 7% 9% 42%	4546
September	1st to 10th	Day	13% 5% 2 6% 4 5% 6% 4 7% 8% 42%	2400
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 6% 8% 49%	2204
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 6% 7% 36%	2200
October	1st to 10th	Day	16% 7% 3 6% 5% 5% 4 4 6% 6% 39%	2005
	11th to 20th	Day	17% 10% 4 7% 5% 5% 4 4 5% 6% 32%	2000
	21st to end	Day	22% 10% 4 9% 5% 4 4 3 6% 7% 26%	2200
November	1st to 10th	Day	14% 9% 3 8% 5% 4 5% 4 5% 8% 36%	1856
	11th to 20th	Day	15% 12% 3 8% 6% 5% 4 4 6% 8% 29%	1680
	21st to end	Day	20% 15% 3 9% 4 5% 5% 5% 7% 6% 21%	1680
December	1st to 10th	Day	18% 18% 5% 10% 7% 5% 6% 4 6% 7% 15%	2680
	11th to 20th	Day	21% 18% 5% 9% 5% 4 4 4 5% 5% 21%	3066
	21st to end	Day	26% 20% 5% 9% 6% 5% 4 3 4 5% 13%	4048
January	1st to 10th	Night	15% 15% 5% 13% 6% 5% 5% 4 6% 6% 22%	7998
	11th to 20th	Night	11% 17% 7% 15% 7% 6% 6% 4 6% 6% 17%	7639
	21st to end	Night	16% 14% 6% 15% 8% 5% 5% 3 4 4 20%	8401
February	1st to 10th	Night	15% 16% 5% 10% 6% 5% 5% 4 5% 5% 24%	2730
	11th to 20th	Night	15% 13% 5% 9% 6% 6% 5% 4 6% 6% 25%	2663
	21st to end	Night	17% 13% 5% 10% 7% 4 5% 3 6% 6% 24%	2124
March	1st to 10th	Night	15% 9% 3 9% 6% 5% 5% 4 5% 6% 33%	5047
	11th to 20th	Night	11% 9% 3 8% 4 4 5% 4 6% 7% 39%	5089
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 4 5% 37%	5588
April	1st to 10th	Night	19% 10% 4 7% 5% 4 5% 4 6% 6% 30%	2195
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 5% 5% 6% 35%	1968
	21st to end	Night	15% 6% 2 8% 5% 5% 4 4 6% 7% 36%	1890
May	1st to 10th	Night	21% 9% 3 8% 5% 4 4 3 5% 6% 34%	4062
	11th to 20th	Night	13% 9% 3 8% 5% 5% 5% 4 5% 6% 36%	3881
	21st to end	Night	17% 6% 2 5% 3 4 4 4 6% 8% 42%	3926
June	1st to 10th	Night	11% 6% 2 6% 5% 5% 3 5% 6% 8% 43%	1600
	11th to 20th	Night	9% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	1600
	21st to end	Night	10% 5% 2 5% 4 4 5% 3 7% 8% 46%	1448
July	1st to 10th	Night	7% 5% 2 6% 4 5% 4 3 6% 10% 49%	3529
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 3 7% 8% 48%	3875
	21st to end	Night	7% 5% 2 6% 5% 3 5% 4 5% 7% 51%	4428
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 45%	2915
	11th to 20th	Night	11% 5% 2 6% 4 3 4 4 5% 9% 47%	2875
	21st to end	Night	7% 4 2 6% 4 4 5% 4 6% 9% 49%	3317
September	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 5% 6% 47%	2000
	11th to 20th	Night	9% 4 2 5% 3 4 3 3 6% 6% 56%	2193
	21st to end	Night	16% 6% 2 7% 5% 4 4 4 6% 6% 40%	2200
October	1st to 10th	Night	14% 6% 3 7% 4 3 4 3 5% 7% 45%	2395
	11th to 20th	Night	16% 11% 3 7% 5% 4 3 3 5% 6% 37%	2400
	21st to end	Night	20% 8% 3 7% 5% 4 4 4 5% 6% 34%	2640
November	1st to 10th	Night	14% 8% 2 6% 4 4 4 3 5% 8% 41%	2654
	11th to 20th	Night	15% 11% 3 8% 6% 5% 5% 4 6% 8% 30%	2940
	21st to end	Night	19% 13% 4 8% 5% 4 5% 4 5% 7% 26%	2940
December	1st to 10th	Night	14% 17% 4 10% 6% 5% 5% 5% 6% 7% 20%	4720
	11th to 20th	Night	17% 19% 5% 10% 5% 4 4 3 4 5% 23%	5348
	21st to end	Night	24% 18% 5% 10% 5% 4 4 3 5% 5% 18%	7150

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	65% 5% 10% 12% 8%	4883
	11th to 20th	Day	81% 6% 5% 4 4	5280
	21st to end	Day	71% 4 9% 9% 7% 1	5780
February	1st to 10th	Day	66% 5% 10% 12% 5%	1890
	11th to 20th	Day	64% 5% 10% 15% 6%	1852
	21st to end	Day	63% 6% 11% 14% 5% 1	1678
March	1st to 10th	Day	58% 5% 13% 17% 6%	5016
	11th to 20th	Day	50% 5% 18% 18% 6% 2	5090
	21st to end	Day	55% 4 9% 20% 10% 1	5586
April	1st to 10th	Day	53% 6% 12% 16% 12% 1	2425
	11th to 20th	Day	50% 5% 12% 20% 12% 1	2618
	21st to end	Day	49% 6% 12% 20% 12% 2	2730
May	1st to 10th	Day	52% 7% 11% 17% 9% 5%	5910
	11th to 20th	Day	49% 7% 13% 20% 10% 1	6002
	21st to end	Day	44% 7% 11% 20% 15% 3	6949
June	1st to 10th	Day	39% 8% 13% 24% 14% 2	2800
	11th to 20th	Day	39% 9% 16% 22% 13% 2	2800
	21st to end	Day	37% 8% 14% 26% 16% 1	2932
July	1st to 10th	Day	37% 4 13% 27% 17% 2	6318
	11th to 20th	Day	37% 4 14% 28% 16% 1	5994
	21st to end	Day	34% 5% 14% 25% 21% 1	6240
August	1st to 10th	Day	41% 6% 15% 23% 14% 1	4221
	11th to 20th	Day	43% 6% 16% 25% 9% 1	4213
	21st to end	Day	38% 5% 12% 32% 12% 1	4546
September	1st to 10th	Day	36% 8% 16% 26% 13% 1	2400
	11th to 20th	Day	31% 7% 16% 30% 15%	2204
	21st to end	Day	44% 7% 14% 23% 11% 1	2200
October	1st to 10th	Day	39% 4 13% 25% 18% 1	3265
	11th to 20th	Day	41% 4 14% 24% 16%	3240
	21st to end	Day	48% 4 16% 24% 8%	3538
November	1st to 10th	Day	46% 7% 13% 24% 8% 2	1856
	11th to 20th	Day	52% 8% 15% 16% 9%	1680
	21st to end	Day	61% 8% 13% 13% 4	1680
December	1st to 10th	Day	69% 6% 14% 10% 2	2680
	11th to 20th	Day	65% 7% 12% 7% 8%	3066
	21st to end	Day	75% 4 8% 8% 5%	4048
January	1st to 10th	Night	62% 5% 10% 17% 5%	7998
	11th to 20th	Night	67% 9% 6% 9% 8% 1	7680
	21st to end	Night	69% 4 6% 12% 10%	8401
February	1st to 10th	Night	61% 7% 11% 15% 6% 1	2730
	11th to 20th	Night	58% 7% 12% 17% 6%	2665
	21st to end	Night	59% 6% 11% 16% 7% 1	2124
March	1st to 10th	Night	51% 5% 14% 18% 10% 2	5057
	11th to 20th	Night	42% 6% 17% 21% 13% 1	5090
	21st to end	Night	49% 5% 11% 18% 16% 2	5588
April	1st to 10th	Night	53% 6% 11% 16% 12% 2	2195
	11th to 20th	Night	48% 6% 9% 20% 15% 2	1968
	21st to end	Night	45% 7% 10% 21% 14% 3	1890
May	1st to 10th	Night	51% 5% 10% 15% 11% 8%	4069
	11th to 20th	Night	46% 7% 12% 22% 10% 2	3882
	21st to end	Night	40% 5% 13% 23% 18% 2	3927
June	1st to 10th	Night	38% 7% 11% 24% 15% 4	1600
	11th to 20th	Night	36% 8% 14% 24% 16% 2	1600
	21st to end	Night	35% 7% 13% 24% 19% 2	1448
July	1st to 10th	Night	34% 3 14% 26% 19% 4	3489
	11th to 20th	Night	34% 4 12% 30% 17% 2	3826
	21st to end	Night	32% 5% 13% 21% 25% 3	4297
August	1st to 10th	Night	37% 6% 13% 26% 17% 2	2904
	11th to 20th	Night	35% 5% 17% 31% 11% 1	2902
	21st to end	Night	32% 5% 13% 34% 16% 1	3317
September	1st to 10th	Night	35% 7% 13% 27% 16% 2	2000
	11th to 20th	Night	28% 6% 13% 33% 19% 1	2193
	21st to end	Night	44% 6% 11% 24% 13% 1	2200
October	1st to 10th	Night	32% 4 7% 30% 25% 2	3915
	11th to 20th	Night	42% 4 13% 24% 17%	3899
	21st to end	Night	40% 4 15% 25% 15% 1	4252
November	1st to 10th	Night	42% 6% 13% 24% 14% 1	2654
	11th to 20th	Night	51% 8% 13% 17% 10% 1	2940
	21st to end	Night	57% 8% 15% 15% 4 1	2940
December	1st to 10th	Night	63% 6% 15% 12% 3	4720
	11th to 20th	Night	64% 6% 9% 12% 9% 1	5348
	21st to end	Night	68% 6% 10% 11% 5%	7150

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	24% 21% 35% 12% 7% 2	261
	11th to 20th	Day	27% 20% 35% 12% 4 1	330
	21st to end	Day	27% 19% 31% 18% 4 1	363
February	1st to 10th	Day	22% 18% 40% 15% 4 1	330
	11th to 20th	Day	23% 24% 33% 18% 2	330
	21st to end	Day	25% 25% 37% 12% 1	273
March	1st to 10th	Day	25% 12% 33% 23% 6%	524
	11th to 20th	Day	24% 17% 34% 19% 4 1	540
	21st to end	Day	28% 14% 32% 19% 6% 1	594
April	1st to 10th	Day	23% 16% 32% 22% 6% 1	540
	11th to 20th	Day	26% 15% 28% 20% 9% 3	532
	21st to end	Day	21% 10% 36% 23% 8% 2	540
May	1st to 10th	Day	23% 11% 30% 23% 8% 5%	507
	11th to 20th	Day	18% 9% 25% 26% 18% 4	527
	21st to end	Day	15% 5% 25% 30% 18% 7%	573
June	1st to 10th	Day	13% 7% 32% 34% 13% 3	530
	11th to 20th	Day	14% 9% 24% 32% 17% 4	530
	21st to end	Day	14% 5% 31% 33% 14% 3	662
July	1st to 10th	Day	10% 4 21% 29% 26% 10%	540
	11th to 20th	Day	15% 5% 22% 29% 19% 10%	524
	21st to end	Day	9% 4 23% 36% 22% 7%	576
August	1st to 10th	Day	9% 4 18% 34% 23% 12%	526
	11th to 20th	Day	10% 5% 19% 33% 22% 11%	519
	21st to end	Day	11% 4 22% 31% 22% 9%	583
September	1st to 10th	Day	14% 6% 20% 27% 22% 11%	530
	11th to 20th	Day	8% 3 17% 27% 29% 16%	530
	21st to end	Day	17% 8% 23% 30% 16% 7%	530
October	1st to 10th	Day	17% 10% 25% 24% 18% 6%	530
	11th to 20th	Day	18% 15% 34% 24% 7% 3	530
	21st to end	Day	25% 16% 31% 20% 8%	583
November	1st to 10th	Day	20% 19% 35% 16% 8% 2	342
	11th to 20th	Day	24% 23% 30% 16% 6% 2	220
	21st to end	Day	31% 19% 33% 15% 3	220
December	1st to 10th	Day	27% 23% 27% 17% 5% 1	220
	11th to 20th	Day	29% 23% 33% 12% 3	220
	21st to end	Day	32% 19% 32% 13% 4	242
January	1st to 10th	Night	23% 18% 36% 17% 5%	840
	11th to 20th	Night	27% 19% 33% 18% 4	840
	21st to end	Night	20% 16% 36% 23% 5%	924
February	1st to 10th	Night	25% 14% 35% 24% 2	840
	11th to 20th	Night	22% 15% 35% 25% 4	823
	21st to end	Night	22% 17% 36% 22% 3	692
March	1st to 10th	Night	19% 13% 35% 27% 6%	840
	11th to 20th	Night	22% 11% 36% 23% 7%	840
	21st to end	Night	22% 11% 32% 26% 8% 1	924
April	1st to 10th	Night	22% 11% 35% 26% 5%	840
	11th to 20th	Night	22% 10% 31% 27% 8% 1	724
	21st to end	Night	17% 8% 30% 37% 8%	630
May	1st to 10th	Night	19% 6% 29% 31% 14% 1	603
	11th to 20th	Night	14% 6% 26% 38% 14% 1	531
	21st to end	Night	15% 3 25% 33% 22% 2	433
June	1st to 10th	Night	13% 4 27% 40% 15% 1	400
	11th to 20th	Night	11% 3 25% 42% 18%	400
	21st to end	Night	11% 5% 23% 42% 18% 1	400
July	1st to 10th	Night	8% 2 19% 44% 24% 3	390
	11th to 20th	Night	11% 3 19% 39% 25% 4	506
	21st to end	Night	9% 2 18% 44% 25% 3	657
August	1st to 10th	Night	8% 3 14% 42% 30% 4	597
	11th to 20th	Night	10% 3 16% 35% 32% 4	594
	21st to end	Night	10% 3 19% 37% 26% 4	715
September	1st to 10th	Night	13% 6% 20% 32% 25% 5%	800
	11th to 20th	Night	11% 3 15% 34% 32% 5%	800
	21st to end	Night	18% 7% 20% 31% 20% 5%	800
October	1st to 10th	Night	18% 7% 24% 29% 19% 4	800
	11th to 20th	Night	22% 12% 27% 28% 9% 2	800
	21st to end	Night	24% 11% 33% 27% 5%	880
November	1st to 10th	Night	20% 12% 29% 27% 11% 1	820
	11th to 20th	Night	21% 18% 30% 22% 6% 1	840
	21st to end	Night	27% 14% 33% 20% 6%	840
December	1st to 10th	Night	30% 16% 31% 17% 5%	840
	11th to 20th	Night	25% 19% 34% 19% 3	840
	21st to end	Night	31% 13% 33% 20% 2	924

Page generated at 2026-04-29 02:21:47. Version ac07

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