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 2024-08-15.

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% 9% 3 8% 6% 5% 5% 4 6% 7% 33%	95497
	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	93204
Transparency	Day	24% 52% 7% 11% 6% 1	95226
	Night	25% 52% 7% 10% 6%	93028
Seeing	Day	16% 9% 26% 28% 16% 6%	14137
	Night	18% 8% 29% 29% 14% 2	23582

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% 12% 5% 10% 7% 6% 5% 4 5% 6% 27%	2517
	11th to 20th	Day	10% 17% 7% 17% 8% 6% 5% 3 5% 6% 17%	2822
	21st to end	Day	20% 17% 7% 12% 7% 6% 4 4 4 5% 15%	3124
February	1st to 10th	Day	13% 16% 4 11% 7% 6% 7% 4 4 5% 21%	1710
	11th to 20th	Day	11% 13% 5% 13% 8% 6% 6% 4 5% 6% 24%	1663
	21st to end	Day	17% 12% 4 12% 6% 5% 5% 4 5% 6% 24%	1484
March	1st to 10th	Day	18% 10% 4 10% 7% 5% 5% 3 5% 5% 27%	3432
	11th to 20th	Day	9% 12% 5% 11% 6% 6% 6% 4 6% 6% 30%	3480
	21st to end	Day	15% 9% 3 7% 5% 5% 5% 4 7% 6% 33%	3815
April	1st to 10th	Day	15% 10% 3 9% 6% 6% 4 4 5% 6% 31%	2227
	11th to 20th	Day	16% 8% 3 8% 5% 5% 4 4 6% 6% 36%	2337
	21st to end	Day	18% 9% 3 8% 5% 5% 4 4 5% 6% 34%	2470
May	1st to 10th	Day	15% 8% 3 8% 4 5% 4 4 6% 7% 35%	4108
	11th to 20th	Day	14% 11% 3 9% 6% 4 5% 4 6% 8% 30%	4298
	21st to end	Day	9% 6% 3 6% 4 4 4 4 6% 8% 47%	4837
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	2660
	11th to 20th	Day	9% 5% 2 6% 5% 6% 5% 5% 7% 9% 40%	2660
	21st to end	Day	11% 5% 2 7% 5% 5% 5% 4 6% 10% 41%	2660
July	1st to 10th	Day	9% 4 2 5% 4 5% 6% 5% 7% 12% 42%	4440
	11th to 20th	Day	8% 5% 2 5% 5% 5% 6% 4 8% 9% 43%	4387
	21st to end	Day	6% 4 2 6% 5% 5% 6% 4 8% 10% 43%	4466
August	1st to 10th	Day	8% 5% 2 7% 5% 5% 7% 5% 7% 10% 40%	2457
	11th to 20th	Day	7% 6% 2 7% 4 6% 6% 5% 8% 9% 39%	2357
	21st to end	Day	10% 6% 3 5% 5% 4 5% 4 6% 9% 43%	2486
September	1st to 10th	Day	12% 6% 3 6% 5% 5% 5% 4 6% 8% 40%	2160
	11th to 20th	Day	9% 6% 3 6% 5% 4 4 3 6% 9% 47%	2030
	21st to end	Day	13% 5% 3 7% 6% 5% 5% 5% 6% 8% 37%	1958
October	1st to 10th	Day	17% 8% 3 7% 5% 5% 4 3 5% 7% 35%	1800
	11th to 20th	Day	17% 8% 4 10% 6% 4 4 4 6% 7% 30%	1800
	21st to end	Day	22% 9% 4 9% 6% 5% 5% 4 5% 6% 25%	1980
November	1st to 10th	Day	11% 9% 3 8% 6% 5% 5% 6% 6% 8% 34%	1634
	11th to 20th	Day	12% 11% 3 11% 7% 5% 6% 5% 6% 8% 27%	1520
	21st to end	Day	17% 13% 4 8% 6% 5% 4 5% 6% 8% 23%	1520
December	1st to 10th	Day	14% 14% 5% 12% 8% 6% 5% 3 5% 7% 20%	1520
	11th to 20th	Day	16% 13% 4 10% 7% 6% 6% 5% 5% 6% 21%	1906
	21st to end	Day	26% 17% 8% 13% 5% 5% 5% 4 4 4 10%	2772
January	1st to 10th	Night	15% 13% 5% 11% 7% 6% 5% 4 5% 6% 23%	4429
	11th to 20th	Night	13% 12% 5% 11% 8% 7% 7% 5% 7% 7% 19%	4120
	21st to end	Night	21% 14% 5% 10% 6% 4 5% 3 4 5% 21%	4532
February	1st to 10th	Night	14% 14% 4 10% 7% 5% 6% 4 6% 6% 24%	2470
	11th to 20th	Night	12% 12% 4 11% 6% 5% 5% 5% 6% 6% 27%	2403
	21st to end	Night	14% 12% 4 10% 6% 6% 5% 3 6% 6% 28%	1966
March	1st to 10th	Night	19% 9% 3 9% 5% 5% 5% 4 5% 6% 32%	3408
	11th to 20th	Night	12% 10% 3 7% 5% 4 4 3 5% 8% 38%	3479
	21st to end	Night	15% 9% 3 7% 5% 4 4 3 4 5% 41%	3817
April	1st to 10th	Night	16% 9% 3 9% 5% 5% 4 3 5% 6% 33%	1952
	11th to 20th	Night	16% 8% 3 8% 5% 4 4 3 5% 7% 37%	1808
	21st to end	Night	15% 9% 3 6% 6% 4 4 4 5% 5% 38%	1710
May	1st to 10th	Night	17% 9% 3 7% 4 4 4 2 5% 7% 37%	2833
	11th to 20th	Night	14% 9% 3 8% 6% 5% 6% 5% 7% 7% 31%	2591
	21st to end	Night	9% 4 1 5% 4 4 4 4 7% 9% 49%	2738
June	1st to 10th	Night	10% 5% 2 6% 4 5% 6% 6% 5% 8% 42%	1520
	11th to 20th	Night	10% 5% 2 5% 5% 4 6% 4 8% 8% 43%	1520
	21st to end	Night	11% 4 2 6% 5% 4 5% 4 6% 8% 45%	1520
July	1st to 10th	Night	6% 4 2 6% 4 5% 4 4 6% 10% 51%	2520
	11th to 20th	Night	9% 4 2 5% 3 3 4 4 6% 9% 50%	2570
	21st to end	Night	6% 4 2 5% 4 4 4 4 8% 9% 49%	3087
August	1st to 10th	Night	7% 5% 2 6% 4 5% 6% 4 6% 9% 47%	1701
	11th to 20th	Night	7% 3 2 4 4 4 5% 4 6% 10% 51%	1643
	21st to end	Night	8% 6% 2 5% 4 4 5% 4 6% 8% 47%	1851
September	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 4 5% 8% 43%	1800
	11th to 20th	Night	10% 5% 1 5% 3 3 4 3 5% 7% 54%	1927
	21st to end	Night	13% 7% 2 7% 5% 5% 5% 4 6% 7% 39%	2002
October	1st to 10th	Night	17% 7% 2 7% 4 4 3 3 5% 6% 42%	2160
	11th to 20th	Night	17% 8% 3 7% 5% 5% 5% 3 6% 7% 34%	2160
	21st to end	Night	21% 9% 3 8% 5% 5% 5% 4 5% 5% 30%	2376
November	1st to 10th	Night	14% 7% 3 6% 4 5% 5% 3 6% 8% 39%	2439
	11th to 20th	Night	13% 11% 4 8% 5% 5% 6% 4 6% 8% 31%	2660
	21st to end	Night	17% 10% 4 8% 6% 5% 5% 5% 6% 7% 28%	2660
December	1st to 10th	Night	17% 14% 5% 9% 6% 6% 5% 5% 6% 6% 22%	2660
	11th to 20th	Night	15% 14% 5% 10% 6% 6% 6% 4 6% 6% 23%	3288
	21st to end	Night	27% 17% 6% 12% 5% 5% 4 3 4 5% 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	34% 41% 10% 9% 6%	2518
	11th to 20th	Day	44% 38% 8% 8% 2	2822
	21st to end	Day	45% 42% 7% 5% 2	3124
February	1st to 10th	Day	23% 66% 4 5% 2	1710
	11th to 20th	Day	19% 65% 4 9% 2	1672
	21st to end	Day	17% 66% 6% 6% 4 1	1484
March	1st to 10th	Day	35% 45% 6% 11% 4	3435
	11th to 20th	Day	29% 43% 9% 13% 5% 1	3480
	21st to end	Day	25% 44% 8% 15% 8% 1	3815
April	1st to 10th	Day	20% 67% 3 6% 4	2227
	11th to 20th	Day	17% 66% 4 8% 4 1	2337
	21st to end	Day	18% 66% 4 7% 4 1	2470
May	1st to 10th	Day	28% 43% 9% 12% 7% 2	4120
	11th to 20th	Day	33% 44% 10% 11% 2	4302
	21st to end	Day	20% 42% 7% 15% 13% 2	4837
June	1st to 10th	Day	15% 66% 4 9% 5% 1	2660
	11th to 20th	Day	15% 65% 5% 8% 6% 1	2660
	21st to end	Day	14% 65% 3 10% 8%	2660
July	1st to 10th	Day	23% 36% 10% 20% 9% 2	4370
	11th to 20th	Day	20% 36% 8% 28% 8%	4312
	21st to end	Day	22% 38% 10% 15% 15% 1	4324
August	1st to 10th	Day	13% 62% 8% 10% 6% 1	2444
	11th to 20th	Day	15% 65% 8% 11% 1	2357
	21st to end	Day	11% 65% 5% 12% 6%	2486
September	1st to 10th	Day	14% 65% 6% 10% 5%	2160
	11th to 20th	Day	10% 65% 4 11% 9%	2030
	21st to end	Day	13% 64% 7% 11% 5%	1958
October	1st to 10th	Day	18% 63% 6% 7% 5% 1	1800
	11th to 20th	Day	18% 65% 5% 7% 4	1800
	21st to end	Day	22% 62% 6% 6% 4	1980
November	1st to 10th	Day	22% 60% 6% 8% 3	1634
	11th to 20th	Day	20% 62% 6% 8% 4 1	1520
	21st to end	Day	23% 62% 7% 5% 3	1520
December	1st to 10th	Day	25% 61% 6% 5% 3	1520
	11th to 20th	Day	33% 49% 7% 8% 3	1906
	21st to end	Day	49% 41% 6% 3 1	2772
January	1st to 10th	Night	35% 43% 10% 9% 3	4429
	11th to 20th	Night	37% 40% 11% 10% 2	4120
	21st to end	Night	43% 40% 8% 6% 2	4532
February	1st to 10th	Night	22% 65% 5% 7% 1	2470
	11th to 20th	Night	18% 64% 5% 10% 4	2405
	21st to end	Night	18% 66% 6% 6% 4	1966
March	1st to 10th	Night	32% 43% 8% 13% 3	3418
	11th to 20th	Night	23% 41% 14% 12% 8% 1	3480
	21st to end	Night	22% 44% 5% 16% 12% 1	3817
April	1st to 10th	Night	21% 65% 2 7% 4	1952
	11th to 20th	Night	17% 66% 3 9% 4 1	1808
	21st to end	Night	15% 66% 4 8% 6% 1	1710
May	1st to 10th	Night	31% 42% 8% 9% 9% 2	2840
	11th to 20th	Night	34% 45% 10% 8% 3	2592
	21st to end	Night	18% 43% 8% 14% 16% 1	2739
June	1st to 10th	Night	14% 66% 3 9% 7%	1520
	11th to 20th	Night	14% 65% 4 9% 7% 1	1520
	21st to end	Night	15% 64% 3 10% 8% 1	1520
July	1st to 10th	Night	22% 35% 9% 16% 14% 3	2480
	11th to 20th	Night	21% 35% 8% 24% 11%	2522
	21st to end	Night	21% 36% 13% 19% 11% 1	2988
August	1st to 10th	Night	12% 61% 6% 13% 7% 1	1690
	11th to 20th	Night	14% 63% 7% 13% 3	1643
	21st to end	Night	14% 64% 6% 10% 7%	1851
September	1st to 10th	Night	13% 63% 8% 9% 7%	1800
	11th to 20th	Night	11% 63% 5% 10% 10% 1	1927
	21st to end	Night	15% 63% 5% 12% 5%	2002
October	1st to 10th	Night	17% 64% 4 9% 7%	2160
	11th to 20th	Night	19% 63% 5% 9% 4	2160
	21st to end	Night	20% 62% 5% 7% 5%	2376
November	1st to 10th	Night	21% 60% 5% 8% 5%	2439
	11th to 20th	Night	18% 61% 5% 11% 4	2660
	21st to end	Night	22% 61% 8% 7% 3	2660
December	1st to 10th	Night	24% 60% 6% 6% 3	2660
	11th to 20th	Night	30% 51% 7% 9% 3	3288
	21st to end	Night	48% 41% 6% 4 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% 40% 13% 7% 2	267
	21st to end	Day	22% 13% 35% 24% 5%	297
February	1st to 10th	Day	19% 14% 44% 17% 5%	270
	11th to 20th	Day	19% 19% 39% 22% 2	270
	21st to end	Day	24% 24% 34% 16% 2	233
March	1st to 10th	Day	18% 12% 36% 25% 8%	460
	11th to 20th	Day	18% 17% 33% 25% 6% 1	460
	21st to end	Day	24% 10% 30% 25% 9% 1	506
April	1st to 10th	Day	17% 11% 33% 30% 8% 1	460
	11th to 20th	Day	21% 15% 27% 23% 10% 4	452
	21st to end	Day	22% 9% 30% 27% 10% 2	460
May	1st to 10th	Day	17% 9% 25% 28% 15% 5%	463
	11th to 20th	Day	15% 6% 22% 30% 21% 6%	487
	21st to end	Day	12% 4 22% 34% 21% 8%	529
June	1st to 10th	Day	11% 6% 28% 36% 17% 4	490
	11th to 20th	Day	12% 5% 19% 32% 27% 5%	490
	21st to end	Day	13% 3 21% 35% 20% 7%	490
July	1st to 10th	Day	7% 2 13% 36% 29% 13%	485
	11th to 20th	Day	13% 3 15% 32% 26% 11%	484
	21st to end	Day	8% 2 16% 37% 27% 10%	532
August	1st to 10th	Day	10% 3 17% 31% 26% 14%	486
	11th to 20th	Day	7% 3 18% 30% 26% 15%	456
	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	18% 11% 39% 25% 7%	836
February	1st to 10th	Night	22% 14% 40% 22% 3	760
	11th to 20th	Night	20% 13% 38% 25% 4	743
	21st to end	Night	21% 14% 38% 24% 4	628
March	1st to 10th	Night	19% 9% 35% 28% 8%	760
	11th to 20th	Night	22% 11% 35% 25% 7%	760
	21st to end	Night	20% 8% 29% 32% 9% 1	836
April	1st to 10th	Night	20% 9% 32% 32% 6%	760
	11th to 20th	Night	20% 9% 32% 28% 11% 2	684
	21st to end	Night	19% 6% 27% 35% 12%	570
May	1st to 10th	Night	16% 4 26% 33% 18% 3	570
	11th to 20th	Night	18% 4 22% 38% 18% 1	431
	21st to end	Night	11% 3 22% 36% 27% 1	411
June	1st to 10th	Night	13% 2 24% 41% 18% 2	380
	11th to 20th	Night	11% 2 25% 38% 23% 1	380
	21st to end	Night	11% 3 19% 43% 24% 1	380
July	1st to 10th	Night	7% 1 16% 45% 28% 3	370
	11th to 20th	Night	10% 3 20% 39% 25% 4	392
	21st to end	Night	8% 2 12% 48% 27% 3	624
August	1st to 10th	Night	9% 2 12% 40% 32% 4	567
	11th to 20th	Night	7% 2 12% 38% 37% 5%	549
	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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