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 2025-02-25.

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% 11% 6% 5% 5% 4 5% 6% 27%	109513
	Night	13% 12% 5% 12% 6% 5% 5% 4 5% 6% 28%	109132
Transparency	Day	53% 7% 11% 18% 10% 1	113135
	Night	52% 7% 11% 18% 10% 1	113674
Seeing	Day	22% 19% 32% 18% 8% 3	15212
	Night	20% 19% 32% 20% 8% 1	24250

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	12% 13% 7% 21% 8% 6% 5% 3 5% 5% 15%	3597
	11th to 20th	Day	9% 19% 10% 23% 10% 10% 5% 3 3 3 6%	3980
	21st to end	Day	13% 18% 8% 20% 8% 6% 6% 3 4 4 10%	4466
February	1st to 10th	Day	11% 18% 6% 17% 12% 6% 5% 4 4 3 14%	1800
	11th to 20th	Day	10% 15% 7% 18% 11% 5% 4 4 5% 6% 15%	1753
	21st to end	Day	15% 16% 7% 12% 8% 6% 6% 3 5% 5% 18%	1523
March	1st to 10th	Day	15% 17% 5% 12% 6% 5% 4 3 3 5% 26%	3402
	11th to 20th	Day	9% 16% 9% 14% 6% 5% 4 3 4 4 25%	3480
	21st to end	Day	14% 13% 5% 9% 5% 5% 5% 3 4 6% 31%	3815
April	1st to 10th	Day	16% 16% 5% 9% 5% 5% 4 3 5% 7% 26%	2214
	11th to 20th	Day	17% 13% 3 6% 4 4 5% 3 5% 6% 34%	2380
	21st to end	Day	20% 11% 4 7% 4 4 3 3 5% 6% 31%	2470
May	1st to 10th	Day	17% 9% 3 8% 5% 4 4 4 6% 7% 33%	4108
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 8% 30%	4320
	21st to end	Day	12% 6% 2 6% 4 4 4 3 5% 8% 45%	4837
June	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 8% 34%	2660
	11th to 20th	Day	12% 6% 2 7% 5% 4 6% 4 6% 9% 39%	2817
	21st to end	Day	15% 6% 2 6% 5% 4 5% 4 7% 8% 37%	2850
July	1st to 10th	Day	12% 6% 3 8% 6% 6% 6% 5% 8% 8% 33%	4760
	11th to 20th	Day	11% 6% 2 6% 6% 5% 6% 4 7% 8% 39%	4458
	21st to end	Day	9% 7% 3 7% 5% 6% 5% 4 7% 9% 38%	4466
August	1st to 10th	Day	15% 6% 3 8% 5% 6% 6% 4 6% 8% 34%	4104
	11th to 20th	Day	12% 8% 3 8% 6% 6% 6% 5% 8% 9% 29%	4044
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 8% 38%	4416
September	1st to 10th	Day	15% 8% 2 8% 4 5% 5% 4 5% 7% 37%	2280
	11th to 20th	Day	12% 8% 2 7% 3 5% 5% 3 6% 7% 41%	2121
	21st to end	Day	16% 8% 3 8% 6% 5% 5% 4 6% 8% 32%	2081
October	1st to 10th	Day	17% 12% 5% 9% 4 5% 4 4 5% 6% 31%	1900
	11th to 20th	Day	19% 15% 4 10% 5% 5% 4 4 5% 6% 24%	1900
	21st to end	Day	25% 18% 5% 9% 5% 4 4 4 4 5% 18%	2085
November	1st to 10th	Day	13% 17% 5% 9% 6% 4 5% 4 5% 6% 26%	1680
	11th to 20th	Day	13% 18% 5% 11% 6% 6% 5% 4 6% 6% 21%	1600
	21st to end	Day	13% 21% 7% 13% 6% 6% 5% 4 5% 6% 14%	1600
December	1st to 10th	Day	14% 25% 11% 15% 6% 4 4 3 4 4 8%	2600
	11th to 20th	Day	18% 22% 7% 14% 6% 5% 3 3 3 4 14%	2986
	21st to end	Day	21% 21% 7% 17% 8% 6% 4 2 3 3 9%	3960
January	1st to 10th	Night	13% 13% 8% 18% 8% 5% 5% 4 6% 7% 13%	6349
	11th to 20th	Night	10% 13% 8% 22% 15% 11% 6% 3 3 3 7%	5887
	21st to end	Night	12% 13% 7% 19% 8% 6% 6% 5% 6% 6% 12%	6490
February	1st to 10th	Night	11% 15% 7% 14% 9% 7% 5% 4 6% 6% 16%	2600
	11th to 20th	Night	11% 13% 6% 15% 9% 6% 5% 4 6% 6% 19%	2533
	21st to end	Night	12% 13% 5% 14% 7% 6% 6% 4 5% 7% 22%	2024
March	1st to 10th	Night	14% 12% 6% 9% 5% 5% 5% 4 5% 7% 28%	3426
	11th to 20th	Night	9% 13% 6% 11% 5% 5% 5% 4 4 6% 33%	3479
	21st to end	Night	13% 9% 4 9% 5% 5% 5% 4 5% 6% 36%	3817
April	1st to 10th	Night	17% 13% 4 8% 6% 5% 4 3 4 5% 30%	1965
	11th to 20th	Night	15% 11% 3 7% 5% 4 4 3 4 7% 37%	1765
	21st to end	Night	14% 13% 4 8% 5% 4 5% 4 5% 6% 32%	1710
May	1st to 10th	Night	15% 9% 3 5% 5% 4 4 4 5% 6% 41%	2833
	11th to 20th	Night	15% 9% 2 7% 4 4 5% 4 5% 7% 39%	2572
	21st to end	Night	9% 6% 2 6% 3 3 4 4 5% 7% 51%	2738
June	1st to 10th	Night	12% 7% 3 8% 5% 4 4 4 5% 10% 40%	1520
	11th to 20th	Night	9% 5% 2 6% 4 5% 6% 4 8% 8% 43%	1359
	21st to end	Night	12% 7% 2 7% 4 4 4 3 6% 8% 44%	1330
July	1st to 10th	Night	6% 7% 2 6% 4 4 4 4 5% 7% 49%	2210
	11th to 20th	Night	9% 7% 2 5% 4 3 4 3 5% 6% 52%	2496
	21st to end	Night	7% 6% 1 7% 5% 4 4 4 6% 8% 47%	3087
August	1st to 10th	Night	11% 6% 2 6% 5% 4 4 3 5% 7% 47%	2825
	11th to 20th	Night	13% 5% 2 6% 4 4 4 3 5% 7% 46%	2785
	21st to end	Night	11% 6% 2 7% 4 4 4 3 5% 7% 47%	3206
September	1st to 10th	Night	9% 7% 2 7% 4 4 3 3 6% 8% 46%	1900
	11th to 20th	Night	10% 5% 2 5% 3 4 3 3 6% 6% 52%	2057
	21st to end	Night	15% 9% 3 8% 4 4 4 3 5% 6% 39%	2099
October	1st to 10th	Night	16% 8% 3 8% 5% 4 5% 3 6% 6% 37%	2280
	11th to 20th	Night	18% 13% 4 8% 4 5% 4 3 4 6% 30%	2280
	21st to end	Night	18% 14% 4 9% 6% 5% 4 4 5% 7% 23%	2510
November	1st to 10th	Night	15% 13% 4 9% 6% 5% 4 4 6% 6% 29%	2616
	11th to 20th	Night	14% 15% 4 10% 6% 5% 6% 4 6% 7% 23%	2800
	21st to end	Night	15% 17% 7% 11% 6% 6% 5% 4 6% 7% 18%	2800
December	1st to 10th	Night	14% 20% 8% 18% 7% 5% 5% 3 5% 5% 10%	4580
	11th to 20th	Night	11% 23% 9% 17% 8% 5% 4 3 4 3 13%	5208
	21st to end	Night	20% 21% 8% 18% 7% 6% 4 3 3 3 8%	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	65% 8% 10% 11% 5%	3598
	11th to 20th	Day	79% 6% 7% 5% 3	4014
	21st to end	Day	72% 9% 9% 7% 3	4466
February	1st to 10th	Day	72% 5% 7% 12% 3 1	1800
	11th to 20th	Day	67% 5% 10% 11% 5% 1	1762
	21st to end	Day	65% 5% 9% 14% 7% 1	1523
March	1st to 10th	Day	61% 4 11% 15% 9%	3405
	11th to 20th	Day	60% 4 9% 19% 7% 1	3480
	21st to end	Day	51% 5% 12% 16% 13% 2	3815
April	1st to 10th	Day	59% 4 8% 17% 10% 1	2214
	11th to 20th	Day	50% 5% 10% 20% 15% 1	2380
	21st to end	Day	53% 4 10% 18% 12% 2	2470
May	1st to 10th	Day	47% 5% 13% 20% 14% 1	4120
	11th to 20th	Day	48% 9% 13% 20% 8% 1	4324
	21st to end	Day	37% 6% 12% 22% 19% 4	4837
June	1st to 10th	Day	44% 7% 12% 21% 14% 1	2660
	11th to 20th	Day	40% 6% 12% 25% 16% 1	2817
	21st to end	Day	43% 6% 13% 24% 14% 1	2850
July	1st to 10th	Day	43% 9% 14% 19% 15% 1	4686
	11th to 20th	Day	37% 8% 13% 32% 11% 1	4381
	21st to end	Day	38% 8% 13% 22% 17% 1	4324
August	1st to 10th	Day	43% 8% 14% 24% 11% 1	4091
	11th to 20th	Day	45% 8% 15% 23% 8%	4083
	21st to end	Day	39% 8% 13% 30% 10% 1	4416
September	1st to 10th	Day	44% 7% 13% 24% 10% 2	2280
	11th to 20th	Day	40% 6% 12% 29% 14%	2121
	21st to end	Day	49% 7% 12% 23% 8% 1	2081
October	1st to 10th	Day	46% 8% 12% 23% 11% 1	3160
	11th to 20th	Day	49% 6% 11% 20% 14% 1	3140
	21st to end	Day	61% 6% 11% 16% 6%	3411
November	1st to 10th	Day	57% 7% 11% 17% 8% 1	1680
	11th to 20th	Day	63% 5% 11% 14% 7% 1	1600
	21st to end	Day	68% 7% 11% 10% 3	1600
December	1st to 10th	Day	76% 8% 8% 6% 2	2600
	11th to 20th	Day	72% 6% 8% 10% 5%	2986
	21st to end	Day	79% 6% 5% 6% 4	3960
January	1st to 10th	Night	64% 10% 10% 11% 5%	6349
	11th to 20th	Night	76% 7% 8% 6% 2	5928
	21st to end	Night	62% 13% 10% 9% 6% 1	6490
February	1st to 10th	Night	64% 6% 9% 16% 5%	2600
	11th to 20th	Night	62% 6% 11% 14% 7%	2535
	21st to end	Night	59% 6% 11% 15% 8% 1	2024
March	1st to 10th	Night	53% 7% 12% 20% 8% 1	3436
	11th to 20th	Night	48% 5% 15% 20% 10% 2	3480
	21st to end	Night	46% 6% 10% 22% 15% 2	3817
April	1st to 10th	Night	56% 3 9% 18% 13% 1	1965
	11th to 20th	Night	48% 5% 9% 21% 16% 2	1765
	21st to end	Night	51% 4 11% 15% 16% 2	1710
May	1st to 10th	Night	43% 6% 12% 19% 16% 5%	2840
	11th to 20th	Night	42% 7% 14% 24% 11% 2	2573
	21st to end	Night	30% 7% 13% 25% 22% 3	2739
June	1st to 10th	Night	40% 5% 11% 24% 18% 2	1520
	11th to 20th	Night	34% 6% 15% 25% 19% 1	1359
	21st to end	Night	37% 6% 13% 28% 15% 1	1330
July	1st to 10th	Night	31% 7% 15% 24% 19% 4	2175
	11th to 20th	Night	30% 6% 12% 36% 16% 1	2450
	21st to end	Night	31% 8% 15% 29% 15% 2	2988
August	1st to 10th	Night	35% 5% 13% 29% 16% 2	2814
	11th to 20th	Night	34% 6% 16% 28% 14% 1	2812
	21st to end	Night	34% 6% 13% 32% 14% 1	3206
September	1st to 10th	Night	34% 6% 16% 29% 13% 1	1900
	11th to 20th	Night	31% 5% 12% 32% 19% 1	2057
	21st to end	Night	45% 6% 13% 26% 10%	2099
October	1st to 10th	Night	37% 7% 10% 25% 21% 1	3800
	11th to 20th	Night	47% 5% 11% 22% 14% 2	3779
	21st to end	Night	48% 8% 15% 22% 7%	4134
November	1st to 10th	Night	52% 7% 11% 20% 10% 1	2616
	11th to 20th	Night	57% 7% 13% 14% 7% 2	2800
	21st to end	Night	63% 7% 14% 12% 3 1	2800
December	1st to 10th	Night	70% 9% 10% 8% 2	4580
	11th to 20th	Night	69% 8% 8% 10% 4	5208
	21st to end	Night	79% 6% 5% 7% 3	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	25% 31% 28% 15% 3	200
	11th to 20th	Day	22% 35% 31% 9% 2	290
	21st to end	Day	29% 26% 32% 12% 1	330
February	1st to 10th	Day	32% 31% 29% 6% 2	300
	11th to 20th	Day	30% 32% 32% 5% 1	300
	21st to end	Day	39% 27% 29% 4	237
March	1st to 10th	Day	27% 25% 33% 12% 3	441
	11th to 20th	Day	26% 21% 37% 12% 4	460
	21st to end	Day	32% 21% 30% 14% 3	506
April	1st to 10th	Day	27% 22% 36% 11% 4	460
	11th to 20th	Day	33% 23% 25% 10% 6% 3	452
	21st to end	Day	30% 20% 31% 12% 5% 2	460
May	1st to 10th	Day	25% 16% 32% 18% 7% 2	463
	11th to 20th	Day	21% 14% 34% 20% 8% 3	487
	21st to end	Day	16% 15% 32% 23% 10% 3	529
June	1st to 10th	Day	19% 16% 34% 22% 8% 1	490
	11th to 20th	Day	15% 15% 35% 24% 9% 2	647
	21st to end	Day	17% 16% 36% 21% 8% 3	680
July	1st to 10th	Day	14% 11% 32% 27% 13% 3	671
	11th to 20th	Day	16% 10% 33% 24% 11% 6%	511
	21st to end	Day	11% 9% 40% 24% 11% 4	532
August	1st to 10th	Day	11% 11% 30% 28% 14% 6%	486
	11th to 20th	Day	11% 9% 30% 25% 17% 8%	479
	21st to end	Day	15% 11% 29% 27% 13% 4	539
September	1st to 10th	Day	17% 13% 29% 24% 11% 6%	490
	11th to 20th	Day	11% 11% 25% 26% 18% 9%	490
	21st to end	Day	17% 17% 28% 20% 14% 4	490
October	1st to 10th	Day	22% 20% 26% 18% 10% 3	490
	11th to 20th	Day	25% 25% 31% 14% 4	490
	21st to end	Day	33% 23% 28% 13% 2	534
November	1st to 10th	Day	29% 26% 28% 13% 3	258
	11th to 20th	Day	29% 23% 36% 9% 3 2	200
	21st to end	Day	30% 23% 40% 7% 1	200
December	1st to 10th	Day	30% 31% 34% 7%	200
	11th to 20th	Day	33% 29% 28% 8% 2 1	200
	21st to end	Day	28% 33% 30% 5% 3	220
January	1st to 10th	Night	22% 31% 31% 13% 2	800
	11th to 20th	Night	26% 26% 31% 15% 2 1	800
	21st to end	Night	19% 21% 38% 17% 5%	880
February	1st to 10th	Night	22% 31% 33% 14% 1	800
	11th to 20th	Night	24% 25% 37% 12% 2	783
	21st to end	Night	23% 29% 34% 12% 2	647
March	1st to 10th	Night	22% 24% 35% 17% 2	760
	11th to 20th	Night	23% 24% 34% 14% 4	760
	21st to end	Night	21% 20% 33% 20% 5%	836
April	1st to 10th	Night	26% 20% 36% 17% 1	760
	11th to 20th	Night	23% 19% 32% 18% 7% 1	641
	21st to end	Night	23% 17% 31% 22% 7%	570
May	1st to 10th	Night	18% 12% 31% 29% 8% 2	570
	11th to 20th	Night	19% 10% 35% 24% 11%	425
	21st to end	Night	13% 12% 33% 30% 12%	411
June	1st to 10th	Night	16% 10% 39% 28% 7% 1	380
	11th to 20th	Night	13% 9% 30% 37% 11%	380
	21st to end	Night	15% 10% 33% 28% 13% 1	380
July	1st to 10th	Night	7% 6% 32% 35% 18% 1	370
	11th to 20th	Night	10% 7% 35% 29% 19% 1	376
	21st to end	Night	10% 7% 32% 34% 15% 1	624
August	1st to 10th	Night	11% 4 31% 32% 19% 2	567
	11th to 20th	Night	8% 6% 26% 35% 23% 2	564
	21st to end	Night	14% 7% 30% 28% 19% 1	670
September	1st to 10th	Night	13% 9% 30% 28% 15% 5%	760
	11th to 20th	Night	13% 9% 28% 28% 18% 5%	760
	21st to end	Night	20% 11% 24% 26% 16% 3	760
October	1st to 10th	Night	20% 15% 31% 19% 12% 2	760
	11th to 20th	Night	26% 17% 31% 18% 6% 1	760
	21st to end	Night	26% 22% 32% 17% 4	836
November	1st to 10th	Night	24% 21% 31% 17% 6% 1	780
	11th to 20th	Night	22% 29% 27% 15% 6% 1	800
	21st to end	Night	25% 26% 32% 15% 2	800
December	1st to 10th	Night	27% 26% 33% 11% 3	800
	11th to 20th	Night	22% 29% 35% 11% 3 1	800
	21st to end	Night	30% 28% 27% 13% 2	880

Page generated at 2025-02-22 06:51:37. Version ac07

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