Cabot Head
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cabot Head 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	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	126636
	Night	14% 13% 5% 12% 7% 5% 4 3 5% 6% 27%	123175
Transparency	Day	49% 7% 12% 20% 11% 1	130154
	Night	57% 7% 10% 16% 9% 1	127744
Seeing	Day	17% 7% 20% 30% 21% 5%	16505
	Night	21% 9% 25% 27% 15% 3	25655

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	13% 19% 10% 16% 14% 7% 4 3 3 4 8%	3158
	11th to 20th	Day	11% 20% 9% 27% 12% 7% 4 3 2 2 4	3585
	21st to end	Day	14% 18% 9% 19% 8% 7% 5% 3 4 4 9%	4422
February	1st to 10th	Day	13% 19% 7% 17% 10% 5% 4 3 3 4 15%	1800
	11th to 20th	Day	10% 17% 7% 14% 8% 7% 5% 3 5% 5% 18%	1753
	21st to end	Day	12% 18% 5% 13% 7% 6% 6% 3 4 5% 21%	1610
March	1st to 10th	Day	15% 12% 4 10% 5% 5% 4 3 5% 6% 30%	4748
	11th to 20th	Day	10% 13% 7% 12% 7% 5% 5% 4 5% 6% 26%	4980
	21st to end	Day	18% 13% 5% 9% 5% 4 4 3 4 5% 29%	5465
April	1st to 10th	Day	17% 12% 4 9% 5% 4 4 4 5% 5% 32%	2322
	11th to 20th	Day	17% 10% 2 6% 4 4 3 3 5% 5% 40%	2572
	21st to end	Day	18% 10% 3 6% 4 3 4 3 5% 7% 37%	2600
May	1st to 10th	Day	15% 7% 3 6% 4 3 4 3 5% 7% 41%	5888
	11th to 20th	Day	11% 8% 3 8% 4 4 4 4 5% 6% 44%	6134
	21st to end	Day	17% 8% 3 6% 4 3 4 3 4 7% 41%	6985
June	1st to 10th	Day	12% 6% 3 5% 5% 4 6% 4 6% 7% 42%	3000
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 6% 9% 44%	3000
	21st to end	Day	13% 6% 2 6% 5% 4 4 4 5% 8% 43%	3000
July	1st to 10th	Day	12% 6% 2 7% 4 5% 4 4 6% 7% 44%	6819
	11th to 20th	Day	9% 5% 2 5% 3 3 4 3 5% 7% 54%	6734
	21st to end	Day	7% 5% 2 5% 4 3 4 3 5% 8% 55%	6525
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 45%	4234
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 6% 9% 43%	4174
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 47%	4675
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 3 5% 7% 43%	2365
	11th to 20th	Day	12% 4 2 6% 4 4 5% 4 5% 8% 46%	2200
	21st to end	Day	18% 8% 2 8% 4 4 5% 3 6% 7% 36%	2200
October	1st to 10th	Day	16% 9% 3 7% 5% 4 4 3 5% 7% 35%	2035
	11th to 20th	Day	21% 14% 3 9% 5% 5% 4 3 5% 6% 24%	1980
	21st to end	Day	24% 16% 4 10% 4 4 4 4 4 6% 19%	1990
November	1st to 10th	Day	17% 18% 5% 9% 4 5% 5% 4 5% 5% 23%	1712
	11th to 20th	Day	16% 17% 5% 12% 7% 7% 5% 3 6% 5% 19%	1600
	21st to end	Day	19% 22% 6% 13% 7% 6% 6% 3 5% 3 10%	1600
December	1st to 10th	Day	15% 26% 9% 18% 6% 5% 4 3 4 3 7%	2600
	11th to 20th	Day	19% 22% 7% 21% 7% 4 3 2 3 3 9%	2695
	21st to end	Day	25% 23% 7% 19% 5% 6% 2 2 3 3 5%	3476
January	1st to 10th	Night	14% 17% 7% 21% 13% 7% 4 3 4 3 7%	6808
	11th to 20th	Night	11% 19% 8% 22% 11% 8% 5% 4 4 3 7%	6316
	21st to end	Night	11% 16% 9% 19% 9% 7% 5% 3 4 4 12%	6521
February	1st to 10th	Night	10% 19% 7% 15% 9% 6% 4 4 5% 5% 16%	2600
	11th to 20th	Night	12% 16% 6% 14% 7% 6% 5% 4 5% 4 21%	2533
	21st to end	Night	13% 14% 6% 11% 7% 5% 5% 3 6% 5% 24%	2016
March	1st to 10th	Night	12% 10% 4 10% 7% 5% 5% 4 5% 6% 31%	5091
	11th to 20th	Night	9% 11% 5% 10% 5% 5% 5% 4 6% 7% 32%	4979
	21st to end	Night	18% 12% 5% 9% 6% 5% 4 3 4 5% 30%	5467
April	1st to 10th	Night	19% 11% 3 8% 5% 4 4 3 5% 6% 33%	2094
	11th to 20th	Night	17% 8% 3 6% 3 5% 4 3 5% 7% 37%	1777
	21st to end	Night	19% 9% 3 7% 6% 4 4 4 6% 6% 32%	1800
May	1st to 10th	Night	14% 8% 4 8% 5% 4 5% 4 5% 7% 36%	4053
	11th to 20th	Night	10% 6% 3 8% 5% 5% 5% 4 5% 9% 41%	3759
	21st to end	Night	18% 7% 3 7% 4 5% 4 3 4 6% 39%	3888
June	1st to 10th	Night	12% 7% 3 7% 6% 5% 5% 3 5% 9% 40%	1400
	11th to 20th	Night	10% 6% 2 6% 5% 4 5% 5% 6% 7% 43%	1400
	21st to end	Night	11% 7% 3 6% 5% 5% 6% 4 7% 7% 41%	1400
July	1st to 10th	Night	11% 6% 3 6% 3 4 4 3 5% 9% 46%	3160
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 6% 8% 49%	3242
	21st to end	Night	7% 6% 3 6% 4 3 4 4 5% 8% 51%	4331
August	1st to 10th	Night	9% 7% 2 6% 4 4 4 4 6% 8% 45%	2915
	11th to 20th	Night	13% 6% 3 6% 5% 4 4 4 5% 8% 42%	2875
	21st to end	Night	10% 5% 2 6% 4 3 5% 4 6% 7% 47%	3223
September	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 7% 47%	2009
	11th to 20th	Night	11% 5% 1 5% 4 4 4 4 5% 7% 49%	2200
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 7% 38%	2200
October	1st to 10th	Night	17% 8% 3 9% 4 4 4 4 5% 6% 35%	2365
	11th to 20th	Night	21% 12% 3 9% 5% 5% 4 3 5% 6% 25%	2377
	21st to end	Night	23% 14% 5% 9% 6% 5% 5% 3 5% 6% 20%	2605
November	1st to 10th	Night	18% 14% 4 10% 6% 4 5% 4 5% 5% 26%	2584
	11th to 20th	Night	15% 18% 5% 12% 7% 4 5% 4 5% 6% 19%	2800
	21st to end	Night	20% 20% 5% 13% 7% 5% 5% 3 5% 5% 12%	2800
December	1st to 10th	Night	15% 22% 7% 18% 7% 6% 5% 3 4 3 8%	4580
	11th to 20th	Night	14% 21% 8% 19% 8% 6% 4 3 4 4 8%	5505
	21st to end	Night	22% 20% 7% 18% 8% 7% 4 2 3 3 7%	7502

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	83% 7% 5% 4 1	3159
	11th to 20th	Day	88% 6% 3 2	3616
	21st to end	Day	75% 9% 8% 6% 2	4422
February	1st to 10th	Day	72% 7% 9% 11% 1	1800
	11th to 20th	Day	67% 8% 11% 9% 5%	1762
	21st to end	Day	61% 6% 11% 17% 5%	1610
March	1st to 10th	Day	53% 7% 14% 11% 13% 2	4751
	11th to 20th	Day	55% 7% 13% 18% 6% 1	4980
	21st to end	Day	51% 7% 16% 17% 8% 1	5465
April	1st to 10th	Day	51% 6% 11% 20% 11% 1	2322
	11th to 20th	Day	44% 5% 9% 24% 16% 2	2572
	21st to end	Day	45% 6% 12% 21% 13% 3	2600
May	1st to 10th	Day	39% 8% 13% 20% 16% 4	5897
	11th to 20th	Day	40% 9% 11% 19% 20% 2	6138
	21st to end	Day	43% 7% 11% 18% 17% 4	6985
June	1st to 10th	Day	38% 8% 12% 26% 15% 1	3000
	11th to 20th	Day	36% 6% 12% 27% 17% 1	3000
	21st to end	Day	38% 7% 14% 26% 13% 2	3000
July	1st to 10th	Day	37% 8% 16% 26% 13% 1	6746
	11th to 20th	Day	27% 7% 14% 38% 13% 1	6653
	21st to end	Day	27% 7% 14% 33% 18% 1	6339
August	1st to 10th	Day	32% 7% 16% 30% 14% 1	4221
	11th to 20th	Day	35% 8% 18% 27% 11%	4213
	21st to end	Day	31% 7% 17% 34% 11% 1	4674
September	1st to 10th	Day	38% 8% 15% 27% 12% 1	2365
	11th to 20th	Day	33% 7% 17% 30% 12% 1	2200
	21st to end	Day	45% 8% 15% 23% 9%	2200
October	1st to 10th	Day	39% 7% 13% 26% 15%	3308
	11th to 20th	Day	53% 5% 12% 22% 8%	3220
	21st to end	Day	58% 7% 13% 16% 4	3253
November	1st to 10th	Day	61% 7% 12% 15% 5%	1712
	11th to 20th	Day	67% 9% 11% 9% 4	1600
	21st to end	Day	77% 7% 9% 6% 1	1600
December	1st to 10th	Day	84% 7% 6% 2 1	2600
	11th to 20th	Day	80% 8% 7% 4 1	2695
	21st to end	Day	85% 7% 4 3 1	3476
January	1st to 10th	Night	84% 6% 5% 4 1	6808
	11th to 20th	Night	84% 8% 5% 3 1	6360
	21st to end	Night	75% 9% 8% 5% 3	6521
February	1st to 10th	Night	69% 6% 10% 12% 3	2600
	11th to 20th	Night	65% 7% 10% 13% 5%	2535
	21st to end	Night	59% 9% 12% 14% 7%	2016
March	1st to 10th	Night	52% 7% 14% 14% 10% 3	5101
	11th to 20th	Night	49% 7% 16% 19% 8% 1	4980
	21st to end	Night	54% 7% 10% 16% 11% 2	5467
April	1st to 10th	Night	51% 5% 10% 21% 11% 2	2094
	11th to 20th	Night	46% 6% 7% 22% 18% 2	1777
	21st to end	Night	52% 6% 10% 15% 15% 3	1800
May	1st to 10th	Night	45% 7% 10% 17% 15% 6%	4060
	11th to 20th	Night	40% 8% 9% 21% 20% 2	3760
	21st to end	Night	45% 7% 10% 21% 13% 3	3889
June	1st to 10th	Night	41% 7% 9% 22% 18% 3	1400
	11th to 20th	Night	35% 8% 12% 24% 20% 1	1400
	21st to end	Night	39% 7% 15% 24% 13% 2	1400
July	1st to 10th	Night	35% 6% 14% 28% 14% 3	3125
	11th to 20th	Night	31% 7% 10% 33% 17% 1	3200
	21st to end	Night	31% 7% 11% 30% 20% 1	4203
August	1st to 10th	Night	35% 7% 15% 27% 15% 1	2904
	11th to 20th	Night	39% 8% 17% 22% 13% 1	2902
	21st to end	Night	34% 7% 14% 32% 12% 1	3223
September	1st to 10th	Night	35% 8% 14% 28% 14% 1	2009
	11th to 20th	Night	35% 7% 15% 30% 13%	2200
	21st to end	Night	45% 7% 14% 23% 10%	2200
October	1st to 10th	Night	39% 7% 10% 30% 15%	3872
	11th to 20th	Night	52% 6% 14% 20% 8%	3876
	21st to end	Night	56% 8% 15% 16% 5%	4292
November	1st to 10th	Night	59% 7% 11% 18% 5%	2584
	11th to 20th	Night	68% 8% 13% 7% 4 1	2800
	21st to end	Night	77% 8% 10% 6% 1	2800
December	1st to 10th	Night	83% 7% 7% 3 1	4580
	11th to 20th	Night	79% 8% 8% 4 1	5504
	21st to end	Night	83% 7% 7% 2 1	7502

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	18% 14% 36% 24% 8% 1	200
	11th to 20th	Day	20% 14% 36% 20% 9% 1	298
	21st to end	Day	27% 9% 25% 30% 9% 1	330
February	1st to 10th	Day	22% 13% 33% 25% 7% 1	300
	11th to 20th	Day	16% 11% 37% 30% 5% 1	300
	21st to end	Day	17% 16% 35% 28% 4	249
March	1st to 10th	Day	19% 7% 30% 33% 10% 1	415
	11th to 20th	Day	17% 12% 27% 32% 11% 1	500
	21st to end	Day	24% 7% 25% 32% 10% 2	550
April	1st to 10th	Day	19% 7% 24% 35% 12% 2	500
	11th to 20th	Day	22% 7% 25% 28% 12% 7%	492
	21st to end	Day	24% 6% 22% 31% 15% 2	500
May	1st to 10th	Day	20% 3 18% 36% 20% 3	503
	11th to 20th	Day	16% 2 13% 35% 25% 9%	527
	21st to end	Day	15% 2 10% 34% 31% 9%	583
June	1st to 10th	Day	12% 3 16% 36% 28% 5%	730
	11th to 20th	Day	12% 3 15% 35% 30% 5%	730
	21st to end	Day	16% 4 14% 34% 26% 5%	730
July	1st to 10th	Day	8% 2 9% 40% 33% 8%	721
	11th to 20th	Day	10% 2 14% 32% 33% 8%	718
	21st to end	Day	11% 2 11% 35% 34% 6%	625
August	1st to 10th	Day	14% 3 11% 27% 34% 11%	526
	11th to 20th	Day	14% 1 10% 25% 38% 13%	519
	21st to end	Day	13% 5% 15% 31% 27% 9%	583
September	1st to 10th	Day	15% 5% 15% 29% 27% 10%	530
	11th to 20th	Day	11% 4 12% 26% 33% 15%	530
	21st to end	Day	18% 6% 16% 27% 24% 8%	530
October	1st to 10th	Day	21% 7% 22% 27% 17% 7%	530
	11th to 20th	Day	24% 11% 28% 29% 8% 1	522
	21st to end	Day	29% 14% 25% 25% 7% 1	439
November	1st to 10th	Day	24% 11% 28% 27% 9% 1	275
	11th to 20th	Day	23% 15% 33% 23% 7% 1	200
	21st to end	Day	25% 15% 34% 25% 2	200
December	1st to 10th	Day	25% 19% 27% 22% 8%	200
	11th to 20th	Day	29% 18% 31% 17% 5%	200
	21st to end	Day	30% 16% 34% 17% 2 1	220
January	1st to 10th	Night	24% 17% 31% 21% 7% 1	1000
	11th to 20th	Night	23% 16% 31% 23% 6% 1	1000
	21st to end	Night	20% 9% 33% 27% 10% 1	898
February	1st to 10th	Night	19% 14% 39% 23% 6% 1	800
	11th to 20th	Night	20% 11% 34% 26% 9% 1	783
	21st to end	Night	20% 8% 35% 31% 6% 1	660
March	1st to 10th	Night	17% 9% 35% 30% 9% 1	800
	11th to 20th	Night	16% 9% 36% 28% 10% 2	800
	21st to end	Night	19% 6% 28% 31% 15% 1	880
April	1st to 10th	Night	24% 9% 24% 32% 10% 1	785
	11th to 20th	Night	22% 8% 22% 29% 15% 4	593
	21st to end	Night	24% 6% 23% 29% 16% 3	600
May	1st to 10th	Night	16% 3 19% 37% 22% 3	600
	11th to 20th	Night	16% 3 16% 36% 25% 4	474
	21st to end	Night	17% 2 12% 39% 23% 6%	433
June	1st to 10th	Night	16% 3 16% 40% 20% 6%	400
	11th to 20th	Night	12% 3 16% 39% 27% 4	400
	21st to end	Night	13% 3 13% 39% 29% 4	400
July	1st to 10th	Night	7% 3 10% 44% 29% 6%	390
	11th to 20th	Night	11% 2 12% 35% 32% 8%	418
	21st to end	Night	8% 3 12% 41% 33% 3	657
August	1st to 10th	Night	12% 3 14% 31% 34% 6%	597
	11th to 20th	Night	13% 3 14% 28% 33% 10%	594
	21st to end	Night	16% 3 17% 30% 28% 6%	660
September	1st to 10th	Night	12% 6% 18% 29% 24% 11%	779
	11th to 20th	Night	15% 4 15% 26% 29% 12%	800
	21st to end	Night	21% 6% 17% 25% 24% 8%	800
October	1st to 10th	Night	23% 7% 23% 25% 18% 4	800
	11th to 20th	Night	29% 12% 23% 24% 10% 2	793
	21st to end	Night	34% 11% 26% 22% 6% 1	836
November	1st to 10th	Night	26% 14% 25% 23% 10% 1	780
	11th to 20th	Night	26% 18% 30% 18% 9%	800
	21st to end	Night	32% 14% 28% 21% 5% 1	800
December	1st to 10th	Night	32% 17% 30% 15% 6% 1	800
	11th to 20th	Night	27% 19% 35% 13% 5% 1	945
	21st to end	Night	32% 14% 32% 16% 5%	1100

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