Burlington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Burlington clear sky chart from 2005-11-06 to 2024-01-22.

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	13% 8% 4 7% 7% 4 6% 5% 6% 8% 33%	74723
	Night	14% 9% 4 7% 6% 4 5% 5% 5% 7% 34%	75202
Transparency	Day	46% 8% 14% 20% 10% 1	74440
	Night	48% 7% 13% 20% 10% 1	75019
Seeing	Day	16% 9% 27% 27% 15% 6%	13350
	Night	18% 8% 29% 29% 14% 2	22945

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% 11% 6% 9% 8% 5% 6% 5% 5% 6% 25%	1517
	11th to 20th	Day	14% 13% 6% 11% 9% 4 7% 5% 6% 7% 18%	1705
	21st to end	Day	10% 13% 6% 12% 10% 6% 7% 7% 6% 6% 18%	1791
February	1st to 10th	Day	15% 15% 5% 10% 8% 5% 7% 6% 5% 5% 19%	1620
	11th to 20th	Day	12% 11% 7% 12% 9% 4 7% 5% 5% 6% 23%	1573
	21st to end	Day	18% 12% 4 10% 8% 3 6% 5% 5% 6% 24%	1398
March	1st to 10th	Day	12% 10% 5% 8% 9% 4 6% 4 6% 6% 29%	1977
	11th to 20th	Day	12% 12% 5% 8% 6% 4 6% 5% 4 6% 32%	1980
	21st to end	Day	16% 9% 5% 7% 6% 4 5% 4 6% 6% 32%	2178
April	1st to 10th	Day	15% 8% 4 8% 7% 5% 5% 5% 5% 6% 30%	2109
	11th to 20th	Day	16% 7% 4 6% 7% 3 5% 4 5% 6% 36%	2212
	21st to end	Day	18% 8% 3 7% 6% 4 4 5% 5% 6% 34%	2340
May	1st to 10th	Day	13% 9% 4 7% 6% 4 4 5% 6% 7% 35%	2340
	11th to 20th	Day	14% 7% 3 7% 7% 3 5% 4 5% 7% 38%	2420
	21st to end	Day	10% 5% 3 5% 5% 3 5% 5% 6% 9% 44%	2725
June	1st to 10th	Day	13% 5% 2 5% 6% 4 6% 5% 6% 8% 42%	2520
	11th to 20th	Day	10% 5% 2 5% 5% 4 5% 7% 6% 9% 41%	2520
	21st to end	Day	11% 4 2 5% 5% 3 5% 5% 6% 10% 42%	2520
July	1st to 10th	Day	7% 3 2 4 4 3 6% 6% 7% 10% 47%	2520
	11th to 20th	Day	11% 5% 2 4 5% 4 6% 6% 7% 10% 39%	2495
	21st to end	Day	9% 4 2 5% 6% 4 6% 6% 8% 10% 39%	2547
August	1st to 10th	Day	7% 5% 2 6% 5% 3 7% 6% 7% 10% 41%	2327
	11th to 20th	Day	7% 5% 2 6% 6% 4 6% 7% 8% 9% 39%	2303
	21st to end	Day	10% 6% 3 4 6% 3 5% 5% 6% 9% 43%	2486
September	1st to 10th	Day	12% 6% 3 5% 6% 4 5% 5% 6% 8% 40%	2160
	11th to 20th	Day	9% 5% 3 5% 5% 3 4 4 6% 8% 47%	2030
	21st to end	Day	13% 5% 3 6% 7% 3 5% 6% 6% 8% 37%	1958
October	1st to 10th	Day	17% 8% 3 6% 6% 4 5% 4 5% 7% 35%	1800
	11th to 20th	Day	18% 7% 4 8% 8% 3 4 5% 6% 7% 30%	1800
	21st to end	Day	22% 9% 4 8% 7% 4 5% 5% 5% 6% 25%	1980
November	1st to 10th	Day	11% 8% 3 7% 7% 3 5% 6% 6% 9% 34%	1634
	11th to 20th	Day	12% 10% 4 9% 8% 4 7% 6% 6% 8% 28%	1520
	21st to end	Day	18% 12% 5% 7% 8% 4 5% 6% 6% 8% 23%	1520
December	1st to 10th	Day	14% 13% 6% 10% 9% 4 6% 5% 5% 7% 21%	1520
	11th to 20th	Day	17% 12% 5% 9% 8% 5% 6% 6% 5% 6% 21%	1906
	21st to end	Day	26% 17% 8% 11% 7% 4 5% 5% 4 4 10%	2772
January	1st to 10th	Night	14% 12% 5% 10% 9% 4 6% 5% 5% 6% 24%	2649
	11th to 20th	Night	18% 13% 5% 10% 8% 4 6% 6% 6% 6% 18%	2470
	21st to end	Night	11% 11% 6% 10% 9% 4 6% 5% 6% 7% 26%	2599
February	1st to 10th	Night	15% 13% 5% 9% 7% 4 6% 5% 6% 7% 23%	2340
	11th to 20th	Night	13% 12% 5% 10% 7% 4 5% 6% 5% 6% 27%	2273
	21st to end	Night	15% 12% 4 9% 8% 4 6% 4 5% 6% 28%	1854
March	1st to 10th	Night	13% 7% 4 8% 6% 4 6% 5% 5% 7% 34%	1970
	11th to 20th	Night	15% 11% 4 7% 6% 3 4 4 4 6% 34%	1979
	21st to end	Night	15% 10% 4 7% 6% 3 4 4 4 4 38%	2178
April	1st to 10th	Night	16% 8% 3 8% 6% 4 5% 4 5% 7% 34%	1850
	11th to 20th	Night	15% 7% 4 7% 5% 3 5% 4 5% 7% 38%	1712
	21st to end	Night	15% 8% 4 5% 6% 3 5% 4 5% 6% 38%	1620
May	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 5% 8% 38%	1620
	11th to 20th	Night	14% 5% 2 5% 5% 3 4 5% 6% 7% 41%	1469
	21st to end	Night	10% 4 2 4 5% 3 4 4 6% 8% 50%	1550
June	1st to 10th	Night	10% 5% 2 5% 5% 4 6% 7% 5% 9% 43%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 3 6% 5% 7% 8% 44%	1440
	21st to end	Night	11% 3 2 5% 6% 3 5% 5% 6% 8% 47%	1440
July	1st to 10th	Night	6% 4 2 4 3 3 4 3 5% 9% 57%	1440
	11th to 20th	Night	9% 4 2 5% 4 2 4 5% 5% 9% 50%	1462
	21st to end	Night	7% 4 2 5% 5% 2 4 5% 7% 8% 50%	1771
August	1st to 10th	Night	7% 4 2 5% 5% 3 5% 6% 7% 8% 48%	1611
	11th to 20th	Night	7% 3 2 4 5% 3 4 5% 6% 10% 51%	1598
	21st to end	Night	8% 6% 2 5% 5% 3 5% 5% 6% 8% 47%	1851
September	1st to 10th	Night	11% 6% 2 5% 6% 3 5% 5% 5% 8% 43%	1800
	11th to 20th	Night	10% 4 2 4 3 3 5% 4 5% 7% 54%	1927
	21st to end	Night	13% 7% 2 6% 6% 3 5% 5% 6% 7% 39%	2002
October	1st to 10th	Night	17% 7% 3 5% 5% 3 3 4 5% 6% 42%	2160
	11th to 20th	Night	17% 8% 4 6% 6% 3 5% 5% 6% 7% 34%	2160
	21st to end	Night	21% 9% 3 7% 6% 3 5% 5% 5% 5% 30%	2376
November	1st to 10th	Night	14% 6% 3 5% 4 4 5% 4 6% 8% 40%	2439
	11th to 20th	Night	14% 10% 4 7% 6% 4 6% 5% 6% 8% 31%	2660
	21st to end	Night	17% 9% 4 7% 7% 4 5% 5% 6% 8% 28%	2660
December	1st to 10th	Night	17% 13% 5% 8% 7% 5% 6% 6% 6% 7% 23%	2660
	11th to 20th	Night	15% 13% 5% 9% 7% 4 6% 5% 6% 6% 23%	3288
	21st to end	Night	28% 16% 6% 11% 7% 4 5% 4 4 4 11%	4884

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	61% 7% 15% 12% 4	1518
	11th to 20th	Day	64% 8% 12% 12% 4	1705
	21st to end	Day	66% 6% 11% 12% 5%	1791
February	1st to 10th	Day	64% 6% 13% 12% 3 1	1620
	11th to 20th	Day	59% 7% 11% 17% 5% 1	1582
	21st to end	Day	56% 8% 14% 14% 7% 1	1398
March	1st to 10th	Day	52% 9% 14% 17% 8% 1	1980
	11th to 20th	Day	51% 6% 11% 18% 10% 2	1980
	21st to end	Day	49% 7% 10% 21% 11% 2	2178
April	1st to 10th	Day	50% 7% 13% 17% 12% 1	2109
	11th to 20th	Day	46% 6% 12% 20% 15% 2	2212
	21st to end	Day	48% 7% 12% 18% 12% 3	2340
May	1st to 10th	Day	44% 7% 12% 22% 12% 1	2340
	11th to 20th	Day	43% 8% 11% 22% 13% 2	2424
	21st to end	Day	34% 8% 17% 24% 14% 2	2725
June	1st to 10th	Day	39% 7% 13% 24% 15% 2	2520
	11th to 20th	Day	36% 9% 14% 24% 16% 1	2520
	21st to end	Day	35% 8% 15% 26% 16% 1	2520
July	1st to 10th	Day	26% 11% 14% 27% 18% 3	2450
	11th to 20th	Day	34% 9% 17% 26% 13% 1	2420
	21st to end	Day	34% 11% 17% 23% 14% 2	2405
August	1st to 10th	Day	30% 11% 18% 26% 13% 2	2314
	11th to 20th	Day	34% 12% 17% 25% 11% 1	2303
	21st to end	Day	32% 10% 14% 29% 14% 1	2486
September	1st to 10th	Day	37% 9% 16% 26% 10% 1	2160
	11th to 20th	Day	31% 9% 16% 29% 15%	2030
	21st to end	Day	41% 9% 15% 26% 9%	1958
October	1st to 10th	Day	46% 8% 15% 20% 10% 1	1800
	11th to 20th	Day	51% 7% 15% 20% 6% 1	1800
	21st to end	Day	58% 7% 14% 15% 5% 1	1980
November	1st to 10th	Day	45% 8% 16% 22% 8% 1	1634
	11th to 20th	Day	52% 9% 16% 16% 7% 1	1520
	21st to end	Day	55% 9% 17% 14% 4	1520
December	1st to 10th	Day	61% 8% 16% 11% 3	1520
	11th to 20th	Day	60% 9% 15% 12% 4	1906
	21st to end	Day	73% 9% 9% 6% 2	2772
January	1st to 10th	Night	61% 7% 13% 14% 4	2649
	11th to 20th	Night	65% 8% 14% 10% 3	2470
	21st to end	Night	56% 6% 16% 16% 5%	2599
February	1st to 10th	Night	61% 6% 13% 15% 4 1	2340
	11th to 20th	Night	57% 7% 12% 17% 7%	2275
	21st to end	Night	54% 7% 15% 16% 7%	1854
March	1st to 10th	Night	48% 7% 15% 20% 8% 2	1980
	11th to 20th	Night	52% 5% 10% 20% 11% 2	1980
	21st to end	Night	48% 5% 8% 22% 14% 3	2178
April	1st to 10th	Night	49% 7% 11% 20% 12% 1	1850
	11th to 20th	Night	45% 6% 9% 21% 16% 3	1712
	21st to end	Night	46% 7% 10% 20% 15% 3	1620
May	1st to 10th	Night	44% 6% 9% 22% 16% 3	1620
	11th to 20th	Night	40% 6% 11% 25% 15% 4	1470
	21st to end	Night	32% 6% 15% 24% 19% 4	1551
June	1st to 10th	Night	37% 8% 13% 24% 17% 2	1440
	11th to 20th	Night	36% 7% 12% 24% 19% 2	1440
	21st to end	Night	35% 8% 12% 27% 16% 2	1440
July	1st to 10th	Night	25% 7% 11% 25% 27% 4	1400
	11th to 20th	Night	32% 6% 14% 29% 18% 2	1414
	21st to end	Night	29% 8% 17% 26% 18% 3	1672
August	1st to 10th	Night	32% 8% 16% 29% 13% 2	1600
	11th to 20th	Night	28% 7% 18% 32% 14% 1	1598
	21st to end	Night	33% 8% 14% 28% 15% 2	1851
September	1st to 10th	Night	36% 7% 15% 26% 14% 2	1800
	11th to 20th	Night	31% 5% 13% 33% 17% 1	1927
	21st to end	Night	41% 7% 13% 27% 12% 1	2002
October	1st to 10th	Night	43% 6% 14% 23% 13% 1	2160
	11th to 20th	Night	49% 6% 14% 22% 9%	2160
	21st to end	Night	54% 7% 13% 16% 9% 1	2376
November	1st to 10th	Night	43% 7% 14% 23% 12% 1	2439
	11th to 20th	Night	49% 8% 15% 19% 8% 1	2660
	21st to end	Night	53% 8% 19% 16% 4	2660
December	1st to 10th	Night	60% 8% 15% 14% 4	2660
	11th to 20th	Night	60% 8% 15% 14% 4	3288
	21st to end	Night	73% 8% 11% 6% 2	4884

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% 33% 24% 9% 2	180
	11th to 20th	Day	22% 16% 39% 13% 7% 2	267
	21st to end	Day	21% 12% 37% 24% 6%	267
February	1st to 10th	Day	21% 13% 48% 15% 3	240
	11th to 20th	Day	20% 18% 40% 20% 1	240
	21st to end	Day	25% 23% 33% 16% 2	205
March	1st to 10th	Day	18% 11% 38% 25% 7%	420
	11th to 20th	Day	18% 16% 33% 25% 7% 1	420
	21st to end	Day	25% 10% 29% 24% 10% 1	462
April	1st to 10th	Day	15% 12% 33% 30% 9% 1	420
	11th to 20th	Day	20% 14% 27% 24% 11% 4	412
	21st to end	Day	22% 8% 30% 28% 10% 2	420
May	1st to 10th	Day	18% 9% 26% 27% 14% 6%	423
	11th to 20th	Day	15% 6% 23% 30% 19% 6%	447
	21st to end	Day	12% 4 20% 34% 21% 9%	485
June	1st to 10th	Day	10% 5% 27% 37% 17% 4	450
	11th to 20th	Day	13% 5% 21% 31% 25% 4	450
	21st to end	Day	13% 4 22% 34% 20% 7%	450
July	1st to 10th	Day	7% 2 14% 35% 28% 14%	445
	11th to 20th	Day	13% 3 17% 31% 25% 11%	444
	21st to end	Day	8% 2 17% 39% 25% 8%	488
August	1st to 10th	Day	8% 3 17% 31% 26% 14%	446
	11th to 20th	Day	8% 3 18% 31% 26% 14%	439
	21st to end	Day	11% 4 20% 32% 22% 12%	495
September	1st to 10th	Day	13% 2 20% 31% 22% 12%	450
	11th to 20th	Day	10% 3 18% 28% 26% 14%	450
	21st to end	Day	12% 8% 26% 28% 17% 9%	450
October	1st to 10th	Day	20% 9% 24% 23% 16% 8%	450
	11th to 20th	Day	19% 12% 33% 23% 9% 2	450
	21st to end	Day	28% 11% 29% 22% 9% 1	495
November	1st to 10th	Day	14% 18% 30% 25% 10% 3	272
	11th to 20th	Day	22% 17% 28% 24% 8% 1	180
	21st to end	Day	21% 14% 37% 21% 8%	180
December	1st to 10th	Day	19% 17% 36% 21% 7%	180
	11th to 20th	Day	24% 23% 30% 19% 3	180
	21st to end	Day	23% 16% 43% 15% 4	198
January	1st to 10th	Night	22% 15% 38% 18% 7%	760
	11th to 20th	Night	26% 14% 34% 20% 5%	760
	21st to end	Night	17% 10% 40% 26% 7%	800
February	1st to 10th	Night	23% 14% 41% 20% 3	720
	11th to 20th	Night	20% 13% 38% 24% 4	703
	21st to end	Night	21% 13% 38% 24% 3	592
March	1st to 10th	Night	19% 9% 36% 27% 8%	720
	11th to 20th	Night	22% 11% 35% 25% 7%	720
	21st to end	Night	20% 8% 28% 33% 10% 1	792
April	1st to 10th	Night	20% 8% 32% 32% 7%	720
	11th to 20th	Night	19% 8% 32% 28% 11% 2	648
	21st to end	Night	20% 5% 27% 35% 13%	540
May	1st to 10th	Night	17% 3 26% 33% 18% 3	540
	11th to 20th	Night	18% 3 22% 38% 17% 1	408
	21st to end	Night	11% 2 23% 36% 27% 2	389
June	1st to 10th	Night	12% 2 24% 41% 19% 2	360
	11th to 20th	Night	12% 2 26% 38% 22% 1	360
	21st to end	Night	11% 3 19% 43% 24% 1	360
July	1st to 10th	Night	7% 1 17% 45% 28% 3	350
	11th to 20th	Night	10% 3 20% 39% 24% 4	370
	21st to end	Night	9% 2 13% 49% 25% 3	591
August	1st to 10th	Night	9% 2 12% 40% 32% 4	537
	11th to 20th	Night	7% 2 12% 38% 36% 4	534
	21st to end	Night	10% 2 19% 34% 30% 4	663
September	1st to 10th	Night	12% 4 18% 32% 27% 7%	720
	11th to 20th	Night	11% 4 18% 29% 30% 7%	720
	21st to end	Night	14% 6% 20% 32% 23% 5%	720
October	1st to 10th	Night	19% 6% 25% 25% 21% 3	720
	11th to 20th	Night	21% 9% 31% 28% 10% 2	720
	21st to end	Night	26% 10% 29% 28% 7%	792
November	1st to 10th	Night	19% 9% 28% 31% 11% 2	740
	11th to 20th	Night	19% 13% 35% 23% 9% 2	760
	21st to end	Night	23% 12% 35% 24% 6%	760
December	1st to 10th	Night	26% 14% 34% 19% 6%	760
	11th to 20th	Night	22% 16% 36% 21% 5%	760
	21st to end	Night	30% 12% 34% 21% 3	836

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