Norwell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Norwell 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	16% 8% 2 6% 4 4 4 4 5% 7% 39%	126268
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	123430
Transparency	Day	44% 5% 12% 22% 15% 3	129815
	Night	45% 5% 11% 21% 16% 2	127970
Seeing	Day	20% 13% 32% 24% 9% 2	16366
	Night	21% 12% 33% 26% 8% 1	25177

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	11% 7% 3 7% 4 5% 4 3 5% 6% 45%	3597
	11th to 20th	Day	15% 9% 3 5% 3 2 3 3 4 5% 48%	3570
	21st to end	Day	18% 9% 2 7% 4 3 4 3 4 5% 41%	3960
February	1st to 10th	Day	20% 9% 3 5% 4 4 5% 4 5% 5% 35%	1891
	11th to 20th	Day	14% 8% 2 7% 3 4 4 3 5% 7% 41%	1948
	21st to end	Day	17% 8% 3 6% 4 5% 4 3 5% 7% 37%	1650
March	1st to 10th	Day	21% 12% 3 6% 3 3 4 3 4 6% 35%	4476
	11th to 20th	Day	15% 11% 3 7% 4 4 4 3 5% 7% 37%	4833
	21st to end	Day	26% 9% 2 6% 5% 5% 4 4 4 5% 30%	5684
April	1st to 10th	Day	19% 10% 3 5% 4 3 4 3 6% 5% 37%	2400
	11th to 20th	Day	17% 8% 2 6% 4 4 4 4 5% 6% 40%	2410
	21st to end	Day	18% 7% 1 6% 4 4 4 3 5% 7% 41%	2600
May	1st to 10th	Day	26% 12% 4 7% 3 3 4 3 5% 6% 26%	5886
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 3 5% 7% 32%	5861
	21st to end	Day	18% 8% 3 6% 4 3 4 4 6% 7% 37%	6919
June	1st to 10th	Day	20% 6% 2 5% 4 3 5% 5% 7% 9% 34%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 5% 6% 9% 37%	2800
	21st to end	Day	14% 7% 2 6% 4 5% 4 4 7% 9% 37%	2800
July	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 4 8% 11% 33%	6360
	11th to 20th	Day	8% 7% 3 7% 5% 5% 6% 5% 8% 11% 36%	6360
	21st to end	Day	14% 6% 2 5% 4 4 4 4 5% 8% 44%	6885
August	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 5% 7% 9% 32%	4235
	11th to 20th	Day	11% 9% 3 7% 5% 5% 6% 4 6% 7% 38%	4175
	21st to end	Day	8% 4 1 5% 4 6% 5% 5% 6% 10% 46%	4315
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 47%	2370
	11th to 20th	Day	14% 6% 1 5% 3 3 3 2 4 6% 53%	2200
	21st to end	Day	18% 6% 2 5% 3 4 3 4 5% 7% 42%	2200
October	1st to 10th	Day	22% 6% 2 5% 3 3 3 3 5% 6% 44%	2200
	11th to 20th	Day	16% 6% 2 5% 3 3 4 3 4 7% 47%	1952
	21st to end	Day	19% 7% 2 5% 4 3 4 4 4 6% 42%	1881
November	1st to 10th	Day	17% 6% 2 6% 4 2 4 3 4 6% 46%	1761
	11th to 20th	Day	16% 7% 2 5% 4 3 3 2 5% 6% 46%	1800
	21st to end	Day	19% 7% 2 5% 4 4 4 3 4 6% 43%	1800
December	1st to 10th	Day	14% 10% 2 4 3 3 4 3 4 5% 48%	2743
	11th to 20th	Day	17% 9% 2 5% 4 2 3 3 4 5% 45%	2986
	21st to end	Day	19% 12% 2 7% 5% 4 3 3 4 5% 35%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 7% 51%	6349
	11th to 20th	Night	14% 10% 4 9% 4 4 4 3 4 5% 39%	6315
	21st to end	Night	16% 11% 4 8% 4 4 4 3 4 6% 36%	6996
February	1st to 10th	Night	17% 9% 3 7% 4 4 4 3 5% 6% 37%	2505
	11th to 20th	Night	17% 9% 3 6% 4 4 4 4 5% 5% 38%	2338
	21st to end	Night	17% 8% 2 6% 4 4 3 3 5% 6% 42%	1980
March	1st to 10th	Night	25% 9% 2 5% 3 4 3 3 4 5% 37%	5364
	11th to 20th	Night	16% 11% 4 6% 4 4 4 3 4 6% 37%	5124
	21st to end	Night	26% 10% 3 7% 3 4 4 3 4 4 32%	5243
April	1st to 10th	Night	18% 10% 3 7% 4 4 4 2 6% 6% 35%	2000
	11th to 20th	Night	19% 8% 2 5% 5% 4 5% 3 5% 6% 37%	1937
	21st to end	Night	18% 8% 3 7% 4 4 4 3 5% 5% 38%	1800
May	1st to 10th	Night	27% 11% 4 7% 4 3 4 3 4 6% 26%	4054
	11th to 20th	Night	19% 9% 3 7% 5% 5% 5% 5% 6% 7% 28%	4033
	21st to end	Night	19% 7% 4 8% 5% 5% 5% 4 5% 6% 31%	3984
June	1st to 10th	Night	18% 8% 3 6% 4 4 5% 4 6% 7% 36%	1600
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 5% 5% 8% 36%	1600
	21st to end	Night	16% 7% 2 6% 3 4 4 4 6% 9% 39%	1600
July	1st to 10th	Night	10% 9% 3 8% 5% 4 5% 4 7% 10% 36%	3600
	11th to 20th	Night	10% 7% 3 8% 5% 4 5% 4 7% 10% 36%	3600
	21st to end	Night	13% 7% 3 6% 4 4 4 4 6% 7% 42%	3926
August	1st to 10th	Night	17% 8% 2 8% 5% 5% 5% 5% 7% 8% 30%	2910
	11th to 20th	Night	8% 9% 3 8% 4 5% 5% 5% 7% 8% 37%	2872
	21st to end	Night	8% 4 2 4 4 4 4 4 6% 8% 53%	3580
September	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 6% 6% 46%	2030
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 7% 52%	2200
	21st to end	Night	19% 5% 2 6% 4 4 4 3 6% 6% 42%	2200
October	1st to 10th	Night	20% 6% 2 6% 3 3 3 4 4 5% 44%	2200
	11th to 20th	Night	16% 6% 2 6% 3 3 4 3 6% 6% 46%	2399
	21st to end	Night	20% 7% 3 6% 4 4 4 3 4 6% 39%	2717
November	1st to 10th	Night	15% 5% 2 6% 4 4 4 4 5% 7% 46%	2537
	11th to 20th	Night	16% 8% 3 5% 3 3 3 3 4 5% 46%	2600
	21st to end	Night	16% 7% 2 5% 5% 3 4 3 5% 7% 43%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 4 3 3 4 5% 45%	4433
	11th to 20th	Night	20% 9% 2 6% 4 3 3 3 5% 5% 40%	5208
	21st to end	Night	17% 9% 3 7% 5% 3 3 3 5% 8% 36%	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	40% 3 7% 27% 21% 2	3598
	11th to 20th	Day	39% 4 10% 23% 18% 6%	3600
	21st to end	Day	46% 4 9% 20% 18% 2	3960
February	1st to 10th	Day	49% 6% 13% 17% 14% 2	1891
	11th to 20th	Day	43% 5% 13% 21% 16% 2	1957
	21st to end	Day	46% 5% 12% 20% 15% 3	1650
March	1st to 10th	Day	51% 3 11% 13% 12% 10%	4479
	11th to 20th	Day	45% 5% 13% 25% 11% 2	4833
	21st to end	Day	56% 3 7% 14% 13% 7%	5684
April	1st to 10th	Day	48% 4 10% 17% 17% 4	2400
	11th to 20th	Day	44% 6% 9% 21% 16% 3	2410
	21st to end	Day	44% 5% 9% 22% 16% 4	2600
May	1st to 10th	Day	59% 6% 13% 14% 7% 1	5897
	11th to 20th	Day	52% 6% 13% 17% 10% 2	5865
	21st to end	Day	46% 6% 12% 22% 13% 1	6919
June	1st to 10th	Day	45% 7% 13% 22% 12% 1	2800
	11th to 20th	Day	42% 7% 15% 20% 15% 2	2800
	21st to end	Day	41% 9% 13% 22% 14% 1	2800
July	1st to 10th	Day	46% 6% 13% 20% 13% 2	6290
	11th to 20th	Day	40% 6% 17% 24% 11% 1	6285
	21st to end	Day	39% 6% 12% 28% 15% 1	6687
August	1st to 10th	Day	48% 6% 15% 21% 9% 1	4222
	11th to 20th	Day	43% 6% 14% 27% 10% 1	4214
	21st to end	Day	34% 5% 13% 28% 19% 1	4315
September	1st to 10th	Day	37% 6% 10% 24% 21% 1	2370
	11th to 20th	Day	34% 5% 9% 28% 22% 3	2200
	21st to end	Day	43% 5% 9% 21% 21% 2	2200
October	1st to 10th	Day	39% 4 9% 26% 18% 3	3590
	11th to 20th	Day	33% 3 7% 29% 24% 3	3168
	21st to end	Day	32% 4 13% 29% 19% 3	3081
November	1st to 10th	Day	39% 4 14% 24% 17% 2	1761
	11th to 20th	Day	40% 3 10% 22% 22% 3	1800
	21st to end	Day	42% 5% 13% 25% 13% 2	1800
December	1st to 10th	Day	40% 3 14% 28% 15% 1	2743
	11th to 20th	Day	42% 3 16% 15% 19% 5%	2986
	21st to end	Day	51% 4 8% 21% 13% 4	3960
January	1st to 10th	Night	34% 3 9% 34% 17% 2	6349
	11th to 20th	Night	47% 3 11% 21% 14% 4	6360
	21st to end	Night	50% 4 8% 16% 17% 4	6996
February	1st to 10th	Night	47% 5% 13% 20% 14% 2	2505
	11th to 20th	Night	47% 5% 9% 20% 17% 3	2340
	21st to end	Night	44% 5% 9% 23% 16% 4	1980
March	1st to 10th	Night	51% 3 8% 15% 15% 7%	5374
	11th to 20th	Night	48% 6% 14% 18% 13% 2	5125
	21st to end	Night	57% 2 5% 11% 19% 6%	5243
April	1st to 10th	Night	49% 5% 8% 19% 16% 4	2000
	11th to 20th	Night	47% 5% 8% 20% 17% 3	1937
	21st to end	Night	47% 5% 10% 19% 16% 4	1800
May	1st to 10th	Night	59% 6% 13% 13% 7% 2	4060
	11th to 20th	Night	53% 7% 14% 15% 8% 2	4034
	21st to end	Night	52% 7% 12% 18% 10% 1	3985
June	1st to 10th	Night	46% 7% 14% 20% 12% 1	1600
	11th to 20th	Night	46% 6% 12% 20% 15% 1	1600
	21st to end	Night	41% 8% 14% 24% 13% 1	1600
July	1st to 10th	Night	44% 6% 11% 22% 17%	3560
	11th to 20th	Night	43% 5% 17% 23% 11% 1	3552
	21st to end	Night	42% 6% 10% 27% 14% 1	3809
August	1st to 10th	Night	50% 7% 14% 18% 10% 1	2899
	11th to 20th	Night	42% 7% 15% 26% 9%	2899
	21st to end	Night	28% 5% 15% 28% 23% 1	3580
September	1st to 10th	Night	38% 6% 11% 25% 19% 1	2030
	11th to 20th	Night	35% 4 11% 24% 25% 3	2200
	21st to end	Night	43% 6% 9% 21% 19% 2	2200
October	1st to 10th	Night	40% 4 11% 26% 15% 3	3590
	11th to 20th	Night	28% 4 9% 26% 30% 3	3922
	21st to end	Night	34% 3 12% 29% 20% 2	4467
November	1st to 10th	Night	38% 6% 12% 23% 18% 3	2537
	11th to 20th	Night	41% 4 11% 21% 20% 4	2600
	21st to end	Night	40% 5% 13% 26% 14% 2	2600
December	1st to 10th	Night	43% 3 11% 28% 14% 1	4433
	11th to 20th	Night	47% 3 15% 14% 19% 1	5208
	21st to end	Night	46% 4 10% 23% 16% 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	29% 22% 27% 19% 5%	200
	11th to 20th	Day	23% 24% 38% 14% 2 1	200
	21st to end	Day	26% 20% 36% 13% 4	220
February	1st to 10th	Day	26% 20% 39% 15% 1	443
	11th to 20th	Day	24% 24% 38% 13% 1	496
	21st to end	Day	21% 22% 38% 14% 4 1	414
March	1st to 10th	Day	24% 21% 35% 16% 4	500
	11th to 20th	Day	22% 21% 38% 13% 5% 1	500
	21st to end	Day	26% 19% 34% 13% 7% 1	550
April	1st to 10th	Day	28% 20% 29% 17% 5% 1	500
	11th to 20th	Day	26% 14% 37% 19% 4	492
	21st to end	Day	24% 12% 36% 23% 3 1	500
May	1st to 10th	Day	29% 16% 34% 17% 4 1	503
	11th to 20th	Day	20% 11% 36% 23% 8% 2	527
	21st to end	Day	18% 8% 30% 32% 10% 2	573
June	1st to 10th	Day	21% 6% 30% 31% 10% 2	530
	11th to 20th	Day	16% 9% 36% 29% 8% 2	530
	21st to end	Day	15% 8% 31% 32% 13% 1	530
July	1st to 10th	Day	18% 6% 23% 34% 18% 2	525
	11th to 20th	Day	14% 5% 24% 36% 19% 3	524
	21st to end	Day	11% 5% 27% 40% 14% 2	576
August	1st to 10th	Day	13% 3 29% 37% 16% 2	526
	11th to 20th	Day	14% 4 25% 34% 18% 4	519
	21st to end	Day	12% 7% 28% 31% 15% 7%	583
September	1st to 10th	Day	17% 8% 26% 29% 16% 5%	530
	11th to 20th	Day	17% 8% 25% 26% 15% 8%	530
	21st to end	Day	22% 8% 28% 26% 11% 5%	530
October	1st to 10th	Day	25% 9% 23% 26% 13% 4	530
	11th to 20th	Day	18% 12% 31% 24% 13% 2	450
	21st to end	Day	23% 12% 34% 21% 8% 2	429
November	1st to 10th	Day	17% 17% 30% 21% 14% 1	395
	11th to 20th	Day	23% 17% 37% 16% 6% 1	400
	21st to end	Day	22% 20% 33% 17% 8% 1	400
December	1st to 10th	Day	21% 16% 40% 18% 5%	291
	11th to 20th	Day	27% 23% 36% 12% 3	200
	21st to end	Day	22% 19% 38% 16% 4 2	220
January	1st to 10th	Night	23% 18% 40% 16% 3	800
	11th to 20th	Night	26% 19% 37% 16% 3	800
	21st to end	Night	20% 18% 39% 20% 4	880
February	1st to 10th	Night	24% 18% 38% 20% 1	800
	11th to 20th	Night	25% 19% 41% 14% 2	783
	21st to end	Night	22% 19% 37% 19% 4	660
March	1st to 10th	Night	25% 18% 35% 19% 4	800
	11th to 20th	Night	22% 15% 39% 19% 5% 1	800
	21st to end	Night	24% 13% 37% 20% 6%	880
April	1st to 10th	Night	24% 13% 36% 21% 5%	800
	11th to 20th	Night	24% 11% 37% 22% 5%	753
	21st to end	Night	24% 11% 34% 27% 4	600
May	1st to 10th	Night	28% 9% 33% 23% 6% 1	600
	11th to 20th	Night	21% 8% 39% 24% 6% 1	592
	21st to end	Night	20% 6% 28% 37% 8%	465
June	1st to 10th	Night	19% 6% 30% 35% 10% 1	400
	11th to 20th	Night	20% 4 29% 38% 9% 1	400
	21st to end	Night	17% 4 28% 42% 10%	400
July	1st to 10th	Night	13% 3 25% 46% 11% 1	390
	11th to 20th	Night	16% 3 22% 47% 11% 1	388
	21st to end	Night	14% 2 24% 47% 13%	438
August	1st to 10th	Night	11% 4 23% 48% 13%	593
	11th to 20th	Night	13% 3 26% 42% 15% 1	592
	21st to end	Night	14% 4 21% 40% 20% 2	874
September	1st to 10th	Night	13% 3 27% 37% 18% 2	800
	11th to 20th	Night	16% 4 24% 32% 20% 3	800
	21st to end	Night	22% 4 26% 31% 15% 3	800
October	1st to 10th	Night	23% 8% 26% 27% 15% 2	800
	11th to 20th	Night	19% 9% 33% 26% 11% 2	793
	21st to end	Night	24% 12% 34% 22% 8% 1	836
November	1st to 10th	Night	17% 12% 33% 27% 9% 1	780
	11th to 20th	Night	21% 18% 35% 19% 7% 1	800
	21st to end	Night	20% 20% 34% 21% 5%	800
December	1st to 10th	Night	25% 15% 38% 18% 4	800
	11th to 20th	Night	23% 18% 35% 20% 3	800
	21st to end	Night	23% 15% 40% 18% 3	880

Page generated at 2025-10-16 01:50:29. Version ac07

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