Shelburne
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shelburne clear sky chart from 2005-11-06 to 2026-07-01.

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% 12% 5% 10% 6% 5% 5% 4 5% 6% 29%	138972
	Night	13% 12% 5% 11% 6% 5% 5% 4 5% 6% 28%	133350
Transparency	Day	52% 7% 11% 19% 10% 1	142551
	Night	52% 7% 11% 18% 10% 1	137860
Seeing	Day	23% 19% 31% 17% 7% 2	17032
	Night	21% 19% 32% 20% 7% 1	25882

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	14% 14% 7% 21% 9% 5% 5% 3 4 5% 14%	4661
	11th to 20th	Day	8% 20% 12% 23% 10% 11% 5% 2 2 2 5%	5186
	21st to end	Day	10% 16% 7% 20% 10% 9% 8% 3 4 4 9%	5780
February	1st to 10th	Day	11% 18% 6% 17% 12% 6% 5% 4 4 3 14%	1890
	11th to 20th	Day	11% 14% 6% 18% 11% 6% 4 4 5% 6% 15%	1843
	21st to end	Day	14% 17% 7% 12% 8% 6% 6% 3 5% 5% 18%	1639
March	1st to 10th	Day	15% 14% 5% 13% 7% 5% 3 3 4 5% 26%	4997
	11th to 20th	Day	10% 14% 7% 13% 6% 5% 4 4 6% 6% 26%	5090
	21st to end	Day	16% 15% 7% 9% 5% 4 4 3 4 5% 28%	5586
April	1st to 10th	Day	16% 16% 4 9% 5% 4 4 3 5% 6% 26%	2448
	11th to 20th	Day	18% 13% 3 6% 4 4 4 3 5% 6% 34%	2634
	21st to end	Day	19% 11% 4 7% 4 4 4 4 5% 6% 31%	2730
May	1st to 10th	Day	16% 10% 3 8% 5% 5% 4 4 5% 7% 34%	7642
	11th to 20th	Day	15% 12% 3 8% 5% 5% 5% 4 6% 7% 33%	8061
	21st to end	Day	17% 7% 2 6% 4 3 4 3 4 7% 42%	9060
June	1st to 10th	Day	14% 7% 3 6% 5% 5% 5% 5% 6% 8% 36%	2940
	11th to 20th	Day	12% 6% 3 7% 5% 4 6% 4 6% 9% 39%	3113
	21st to end	Day	15% 6% 3 6% 5% 5% 5% 4 7% 8% 37%	3150
July	1st to 10th	Day	11% 7% 3 8% 5% 5% 6% 5% 7% 8% 35%	6820
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	6378
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 6% 8% 45%	6424
August	1st to 10th	Day	14% 6% 3 8% 5% 6% 6% 4 6% 8% 34%	4234
	11th to 20th	Day	12% 8% 3 8% 6% 6% 6% 5% 8% 9% 29%	4174
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 8% 38%	4555
September	1st to 10th	Day	15% 8% 2 7% 4 5% 5% 4 5% 7% 38%	2400
	11th to 20th	Day	12% 7% 2 7% 3 5% 5% 3 6% 8% 42%	2232
	21st to end	Day	16% 8% 3 8% 6% 5% 5% 4 6% 7% 32%	2190
October	1st to 10th	Day	16% 11% 4 8% 4 4 4 4 5% 6% 32%	2000
	11th to 20th	Day	19% 14% 4 10% 5% 5% 5% 4 6% 6% 24%	2000
	21st to end	Day	24% 17% 4 9% 5% 4 4 4 4 5% 18%	2195
November	1st to 10th	Day	14% 17% 5% 9% 5% 4 5% 3 5% 6% 26%	1766
	11th to 20th	Day	13% 18% 5% 11% 6% 5% 5% 4 6% 6% 21%	1680
	21st to end	Day	13% 22% 7% 13% 6% 6% 5% 4 5% 6% 14%	1680
December	1st to 10th	Day	14% 24% 11% 15% 6% 4 5% 3 4 4 8%	2680
	11th to 20th	Day	18% 22% 7% 14% 6% 5% 3 3 3 4 14%	3066
	21st to end	Day	20% 21% 7% 17% 7% 6% 4 2 3 3 9%	4048
January	1st to 10th	Night	13% 13% 8% 19% 9% 6% 5% 4 6% 6% 12%	8234
	11th to 20th	Night	9% 16% 10% 22% 14% 10% 5% 3 3 3 7%	7681
	21st to end	Night	10% 12% 7% 18% 10% 8% 8% 5% 6% 6% 11%	8401
February	1st to 10th	Night	11% 15% 7% 13% 9% 7% 5% 5% 5% 6% 17%	2730
	11th to 20th	Night	11% 13% 6% 16% 9% 6% 4 4 6% 6% 19%	2663
	21st to end	Night	11% 13% 5% 14% 7% 6% 5% 4 5% 6% 21%	2165
March	1st to 10th	Night	12% 12% 5% 11% 5% 5% 6% 4 6% 7% 26%	5051
	11th to 20th	Night	8% 10% 6% 11% 5% 5% 6% 4 6% 7% 32%	5089
	21st to end	Night	17% 11% 4 9% 5% 4 5% 3 5% 5% 33%	5588
April	1st to 10th	Night	17% 13% 5% 9% 6% 5% 4 3 5% 6% 28%	2171
	11th to 20th	Night	15% 11% 3 7% 5% 5% 4 3 4 6% 37%	1953
	21st to end	Night	13% 12% 4 8% 5% 4 5% 4 5% 6% 33%	1890
May	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 7% 39%	5257
	11th to 20th	Night	14% 9% 3 7% 4 4 4 4 5% 6% 40%	4784
	21st to end	Night	17% 7% 2 5% 3 3 3 3 5% 6% 47%	5114
June	1st to 10th	Night	12% 6% 3 7% 5% 4 4 4 5% 10% 42%	1680
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 5% 8% 8% 42%	1501
	21st to end	Night	11% 7% 2 7% 4 4 4 4 6% 8% 44%	1470
July	1st to 10th	Night	7% 9% 3 6% 4 4 4 4 5% 8% 46%	3160
	11th to 20th	Night	10% 7% 2 5% 4 3 4 4 5% 7% 49%	3577
	21st to end	Night	8% 7% 2 6% 4 4 4 4 6% 8% 49%	4428
August	1st to 10th	Night	10% 6% 2 6% 5% 4 4 3 5% 7% 48%	2915
	11th to 20th	Night	13% 5% 2 6% 4 4 4 3 5% 7% 46%	2875
	21st to end	Night	11% 6% 2 7% 4 4 4 3 5% 7% 47%	3308
September	1st to 10th	Night	9% 7% 2 7% 4 4 3 3 6% 8% 47%	2000
	11th to 20th	Night	10% 5% 2 5% 3 4 3 3 5% 6% 53%	2165
	21st to end	Night	15% 10% 3 8% 4 4 4 3 5% 6% 39%	2210
October	1st to 10th	Night	15% 8% 3 8% 5% 4 4 3 6% 6% 38%	2400
	11th to 20th	Night	18% 13% 4 8% 4 5% 4 3 4 6% 31%	2400
	21st to end	Night	18% 14% 4 9% 6% 5% 4 4 5% 7% 24%	2642
November	1st to 10th	Night	14% 14% 4 9% 6% 5% 4 4 6% 6% 29%	2750
	11th to 20th	Night	14% 15% 4 10% 6% 5% 6% 4 6% 7% 23%	2940
	21st to end	Night	15% 18% 7% 11% 6% 5% 4 4 6% 6% 17%	2940
December	1st to 10th	Night	14% 19% 8% 18% 7% 5% 5% 3 5% 5% 11%	4720
	11th to 20th	Night	11% 22% 9% 17% 8% 5% 4 3 4 3 13%	5348
	21st to end	Night	20% 21% 8% 18% 7% 6% 4 3 3 3 8%	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	66% 9% 10% 10% 5%	4662
	11th to 20th	Day	81% 6% 6% 4 3	5220
	21st to end	Day	72% 9% 8% 7% 3	5780
February	1st to 10th	Day	72% 5% 7% 13% 4 1	1890
	11th to 20th	Day	67% 5% 10% 12% 5% 1	1852
	21st to end	Day	65% 5% 9% 14% 7% 1	1639
March	1st to 10th	Day	59% 5% 12% 13% 11%	5000
	11th to 20th	Day	54% 5% 13% 19% 7% 3	5090
	21st to end	Day	56% 5% 10% 16% 12% 2	5586
April	1st to 10th	Day	59% 4 8% 17% 10% 1	2448
	11th to 20th	Day	50% 5% 9% 21% 14% 1	2634
	21st to end	Day	52% 5% 10% 19% 11% 2	2730
May	1st to 10th	Day	46% 6% 13% 19% 11% 5%	7651
	11th to 20th	Day	47% 8% 10% 18% 16% 1	8065
	21st to end	Day	41% 5% 9% 27% 16% 3	9061
June	1st to 10th	Day	43% 7% 11% 21% 16% 1	2940
	11th to 20th	Day	40% 6% 13% 25% 15% 1	3113
	21st to end	Day	43% 6% 13% 24% 13% 1	3150
July	1st to 10th	Day	41% 9% 13% 21% 14% 1	6747
	11th to 20th	Day	34% 8% 11% 31% 16% 1	6301
	21st to end	Day	33% 8% 13% 29% 16% 1	6240
August	1st to 10th	Day	43% 8% 14% 24% 11% 1	4221
	11th to 20th	Day	45% 8% 15% 23% 8%	4213
	21st to end	Day	38% 8% 14% 30% 10% 1	4555
September	1st to 10th	Day	43% 7% 12% 25% 10% 2	2400
	11th to 20th	Day	38% 5% 12% 30% 14%	2232
	21st to end	Day	49% 7% 12% 23% 9% 1	2190
October	1st to 10th	Day	45% 7% 12% 23% 11% 1	3260
	11th to 20th	Day	49% 6% 11% 20% 14% 1	3240
	21st to end	Day	61% 6% 11% 16% 6%	3521
November	1st to 10th	Day	57% 7% 10% 17% 8% 1	1766
	11th to 20th	Day	62% 6% 11% 14% 7% 1	1680
	21st to end	Day	69% 7% 11% 10% 3	1680
December	1st to 10th	Day	75% 8% 8% 7% 2	2680
	11th to 20th	Day	71% 6% 8% 10% 5%	3066
	21st to end	Day	79% 6% 5% 6% 4	4048
January	1st to 10th	Night	64% 11% 11% 10% 4	8234
	11th to 20th	Night	76% 8% 7% 6% 3	7722
	21st to end	Night	60% 14% 10% 9% 6%	8401
February	1st to 10th	Night	63% 6% 9% 16% 5%	2730
	11th to 20th	Night	62% 6% 11% 14% 7%	2665
	21st to end	Night	59% 7% 11% 15% 8% 1	2165
March	1st to 10th	Night	51% 9% 13% 16% 10% 1	5061
	11th to 20th	Night	47% 5% 18% 19% 9% 2	5090
	21st to end	Night	50% 5% 11% 18% 14% 2	5588
April	1st to 10th	Night	56% 4 9% 17% 13% 1	2171
	11th to 20th	Night	48% 5% 8% 21% 16% 2	1953
	21st to end	Night	50% 4 11% 17% 16% 2	1890
May	1st to 10th	Night	43% 6% 11% 19% 15% 6%	5264
	11th to 20th	Night	42% 7% 11% 23% 15% 2	4785
	21st to end	Night	37% 5% 11% 29% 17% 2	5115
June	1st to 10th	Night	39% 5% 11% 24% 20% 2	1680
	11th to 20th	Night	34% 6% 15% 26% 18% 1	1501
	21st to end	Night	37% 6% 13% 27% 15% 1	1470
July	1st to 10th	Night	33% 8% 15% 24% 18% 2	3125
	11th to 20th	Night	31% 7% 10% 32% 19% 1	3531
	21st to end	Night	30% 7% 12% 32% 16% 2	4297
August	1st to 10th	Night	35% 5% 13% 29% 16% 2	2904
	11th to 20th	Night	34% 6% 16% 29% 14% 1	2902
	21st to end	Night	34% 7% 13% 32% 13% 1	3308
September	1st to 10th	Night	33% 6% 16% 31% 13% 1	2000
	11th to 20th	Night	30% 5% 13% 33% 19% 1	2165
	21st to end	Night	45% 6% 13% 26% 11% 1	2210
October	1st to 10th	Night	36% 7% 10% 26% 21% 1	3920
	11th to 20th	Night	47% 5% 11% 22% 14% 2	3899
	21st to end	Night	47% 8% 14% 22% 8%	4266
November	1st to 10th	Night	52% 7% 11% 20% 9% 1	2750
	11th to 20th	Night	56% 7% 13% 15% 8% 1	2940
	21st to end	Night	63% 8% 13% 12% 3 1	2940
December	1st to 10th	Night	70% 9% 10% 8% 2	4720
	11th to 20th	Night	69% 8% 8% 10% 5%	5348
	21st to end	Night	79% 6% 5% 7% 3	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	27% 32% 26% 13% 2	220
	11th to 20th	Day	24% 35% 30% 8% 2	319
	21st to end	Day	30% 26% 32% 11% 1	363
February	1st to 10th	Day	32% 30% 30% 7% 2	330
	11th to 20th	Day	29% 32% 34% 5% 1	330
	21st to end	Day	37% 30% 29% 4	273
March	1st to 10th	Day	28% 26% 32% 11% 3	519
	11th to 20th	Day	31% 21% 33% 11% 3	540
	21st to end	Day	34% 23% 28% 12% 3	594
April	1st to 10th	Day	30% 22% 34% 10% 3	540
	11th to 20th	Day	34% 22% 24% 11% 6% 3	532
	21st to end	Day	29% 21% 32% 11% 5% 1	540
May	1st to 10th	Day	25% 19% 33% 16% 6% 2	543
	11th to 20th	Day	21% 16% 34% 18% 8% 3	567
	21st to end	Day	19% 17% 32% 21% 8% 3	617
June	1st to 10th	Day	18% 17% 35% 21% 8% 1	570
	11th to 20th	Day	16% 17% 36% 22% 8% 2	743
	21st to end	Day	17% 16% 37% 20% 7% 2	780
July	1st to 10th	Day	13% 11% 33% 28% 12% 3	722
	11th to 20th	Day	15% 10% 34% 24% 11% 5%	552
	21st to end	Day	10% 10% 40% 24% 11% 4	576
August	1st to 10th	Day	10% 11% 29% 30% 14% 6%	526
	11th to 20th	Day	11% 9% 30% 27% 16% 7%	519
	21st to end	Day	15% 12% 30% 27% 12% 4	583
September	1st to 10th	Day	17% 13% 28% 25% 12% 6%	530
	11th to 20th	Day	11% 10% 24% 27% 18% 9%	530
	21st to end	Day	18% 16% 28% 21% 13% 4	530
October	1st to 10th	Day	22% 19% 26% 20% 11% 3	530
	11th to 20th	Day	25% 26% 32% 13% 4	530
	21st to end	Day	32% 24% 28% 14% 2	578
November	1st to 10th	Day	32% 25% 28% 12% 2	284
	11th to 20th	Day	30% 23% 34% 9% 3 1	220
	21st to end	Day	31% 22% 40% 6% 1	220
December	1st to 10th	Day	30% 30% 33% 8%	220
	11th to 20th	Day	35% 27% 28% 8% 2	220
	21st to end	Day	31% 32% 29% 5% 2	242
January	1st to 10th	Night	22% 32% 31% 13% 2	840
	11th to 20th	Night	27% 26% 30% 14% 2	840
	21st to end	Night	20% 21% 38% 17% 4	924
February	1st to 10th	Night	21% 31% 33% 14% 1	840
	11th to 20th	Night	24% 25% 36% 13% 2	823
	21st to end	Night	25% 28% 34% 12% 2	692
March	1st to 10th	Night	23% 24% 34% 16% 2	840
	11th to 20th	Night	24% 24% 34% 14% 4	840
	21st to end	Night	23% 21% 33% 19% 5%	924
April	1st to 10th	Night	27% 21% 35% 16% 1	840
	11th to 20th	Night	24% 19% 32% 18% 7% 1	709
	21st to end	Night	23% 16% 31% 23% 6%	630
May	1st to 10th	Night	19% 13% 31% 28% 7% 2	630
	11th to 20th	Night	19% 12% 34% 24% 10%	470
	21st to end	Night	14% 13% 32% 29% 11%	455
June	1st to 10th	Night	15% 10% 39% 27% 8% 1	420
	11th to 20th	Night	14% 11% 30% 35% 10%	420
	21st to end	Night	15% 11% 34% 28% 12% 1	420
July	1st to 10th	Night	7% 7% 33% 36% 17% 1	390
	11th to 20th	Night	10% 7% 35% 29% 18% 1	397
	21st to end	Night	10% 7% 33% 34% 15% 1	657
August	1st to 10th	Night	10% 4 30% 34% 19% 2	597
	11th to 20th	Night	8% 6% 27% 34% 22% 2	594
	21st to end	Night	14% 8% 30% 28% 19% 1	706
September	1st to 10th	Night	12% 10% 30% 28% 16% 4	800
	11th to 20th	Night	13% 8% 27% 28% 19% 5%	800
	21st to end	Night	20% 11% 24% 26% 16% 3	800
October	1st to 10th	Night	19% 15% 31% 21% 12% 2	800
	11th to 20th	Night	26% 17% 32% 18% 6% 1	800
	21st to end	Night	25% 21% 32% 18% 4	880
November	1st to 10th	Night	25% 22% 31% 17% 5%	820
	11th to 20th	Night	23% 29% 27% 15% 6% 1	840
	21st to end	Night	26% 26% 32% 14% 2	840
December	1st to 10th	Night	26% 26% 33% 11% 3	840
	11th to 20th	Night	23% 30% 34% 10% 2	840
	21st to end	Night	31% 27% 28% 13% 2	924

Page generated at 2026-06-28 02:10:45. Version ac07

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