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 2026-05-17.

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	16% 10% 4 9% 5% 5% 5% 4 5% 7% 30%	131829
	Night	17% 14% 5% 11% 6% 5% 4 3 5% 5% 26%	130992
Transparency	Day	36% 37% 7% 13% 6%	135297
	Night	42% 36% 6% 10% 6%	135611
Seeing	Day	26% 17% 29% 19% 8% 2	16567
	Night	26% 14% 26% 24% 9% 1	25629

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% 17% 8% 17% 8% 5% 5% 3 4 4 15%	4661
	11th to 20th	Day	10% 16% 8% 18% 9% 6% 5% 4 4 4 16%	4650
	21st to end	Day	10% 16% 5% 12% 10% 8% 6% 4 4 6% 19%	5124
February	1st to 10th	Day	13% 14% 5% 12% 9% 6% 4 4 5% 6% 21%	1853
	11th to 20th	Day	11% 15% 6% 14% 7% 5% 4 4 5% 6% 23%	2006
	21st to end	Day	15% 16% 5% 10% 7% 5% 5% 4 4 6% 24%	1730
March	1st to 10th	Day	19% 13% 5% 12% 6% 5% 5% 3 4 5% 23%	4576
	11th to 20th	Day	13% 16% 4 9% 5% 6% 4 4 5% 5% 28%	4761
	21st to end	Day	20% 12% 4 9% 4 4 4 3 5% 5% 30%	6102
April	1st to 10th	Day	20% 13% 3 8% 5% 4 4 3 4 7% 30%	2520
	11th to 20th	Day	18% 11% 3 8% 6% 5% 4 3 4 5% 35%	2494
	21st to end	Day	22% 9% 3 7% 5% 4 4 3 4 6% 33%	2691
May	1st to 10th	Day	28% 10% 3 9% 5% 5% 4 3 5% 6% 22%	6220
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 8% 30%	6402
	21st to end	Day	19% 9% 2 7% 4 5% 5% 5% 6% 7% 31%	6949
June	1st to 10th	Day	21% 7% 2 7% 5% 4 5% 5% 6% 8% 29%	2800
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 7% 10% 37%	2800
	21st to end	Day	18% 7% 3 7% 5% 5% 5% 5% 6% 8% 30%	2800
July	1st to 10th	Day	11% 5% 3 7% 6% 6% 6% 5% 8% 10% 33%	6360
	11th to 20th	Day	11% 5% 2 7% 4 5% 5% 5% 8% 10% 37%	6360
	21st to end	Day	13% 6% 2 6% 4 4 4 4 7% 9% 42%	6926
August	1st to 10th	Day	15% 6% 3 8% 5% 4 5% 4 6% 8% 35%	4563
	11th to 20th	Day	14% 7% 3 7% 5% 5% 4 5% 5% 8% 36%	4028
	21st to end	Day	12% 6% 3 6% 4 4 4 4 6% 9% 43%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	14% 6% 2 5% 3 4 4 3 5% 5% 50%	2290
	21st to end	Day	14% 7% 3 6% 5% 5% 4 4 5% 8% 39%	2200
October	1st to 10th	Day	20% 8% 3 7% 5% 4 4 4 5% 6% 34%	2109
	11th to 20th	Day	18% 10% 3 7% 5% 5% 4 3 5% 7% 33%	1950
	21st to end	Day	24% 13% 3 8% 5% 5% 4 3 4 4 26%	1980
November	1st to 10th	Day	17% 14% 4 8% 5% 5% 4 3 5% 6% 30%	1851
	11th to 20th	Day	17% 15% 3 10% 5% 5% 4 4 5% 5% 26%	1887
	21st to end	Day	19% 16% 5% 9% 5% 4 5% 3 5% 4 23%	1680
December	1st to 10th	Day	16% 18% 9% 16% 7% 6% 4 3 4 5% 11%	2680
	11th to 20th	Day	19% 15% 5% 10% 4 5% 5% 4 5% 6% 22%	3066
	21st to end	Day	20% 16% 4 11% 6% 5% 6% 4 5% 6% 17%	4048
January	1st to 10th	Night	13% 21% 10% 18% 6% 4 3 2 3 3 16%	8234
	11th to 20th	Night	11% 20% 9% 19% 7% 6% 5% 3 4 4 12%	8235
	21st to end	Night	11% 15% 6% 14% 12% 7% 6% 4 5% 4 17%	9057
February	1st to 10th	Night	13% 17% 6% 15% 8% 6% 4 4 4 4 20%	2766
	11th to 20th	Night	13% 15% 5% 15% 8% 5% 5% 4 4 5% 22%	2500
	21st to end	Night	15% 17% 5% 11% 6% 5% 4 3 4 4 27%	2076
March	1st to 10th	Night	20% 16% 5% 10% 7% 4 3 3 3 4 24%	5484
	11th to 20th	Night	15% 17% 4 9% 5% 4 4 3 5% 5% 28%	5400
	21st to end	Night	19% 18% 5% 11% 4 3 4 3 3 3 27%	5072
April	1st to 10th	Night	21% 12% 4 9% 5% 4 4 3 4 5% 29%	2100
	11th to 20th	Night	23% 13% 4 7% 6% 4 3 3 4 4 30%	2075
	21st to end	Night	21% 12% 4 6% 4 4 4 3 4 5% 34%	1927
May	1st to 10th	Night	28% 12% 4 9% 5% 5% 4 3 4 6% 21%	3971
	11th to 20th	Night	20% 10% 4 9% 5% 5% 5% 3 4 6% 29%	3623
	21st to end	Night	19% 10% 3 9% 4 4 5% 4 6% 7% 30%	3926
June	1st to 10th	Night	23% 9% 2 6% 4 4 4 4 5% 7% 31%	1600
	11th to 20th	Night	16% 6% 2 8% 5% 3 5% 4 6% 8% 37%	1600
	21st to end	Night	19% 7% 3 7% 6% 4 4 4 6% 6% 34%	1600
July	1st to 10th	Night	12% 5% 2 8% 5% 5% 6% 5% 7% 9% 35%	3600
	11th to 20th	Night	15% 5% 2 8% 5% 5% 4 4 6% 7% 38%	3600
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 5% 6% 8% 39%	3926
August	1st to 10th	Night	17% 8% 3 7% 4 5% 5% 3 5% 7% 36%	2586
	11th to 20th	Night	15% 8% 3 9% 6% 4 5% 4 5% 7% 37%	2983
	21st to end	Night	13% 6% 3 6% 4 4 4 4 7% 8% 41%	3586
September	1st to 10th	Night	13% 5% 2 7% 5% 4 4 3 6% 7% 44%	2000
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 5% 6% 48%	2110
	21st to end	Night	17% 6% 2 6% 4 4 5% 4 5% 7% 41%	2200
October	1st to 10th	Night	22% 8% 2 6% 4 4 5% 3 4 7% 33%	2290
	11th to 20th	Night	22% 9% 3 8% 4 3 4 4 5% 6% 33%	2450
	21st to end	Night	26% 11% 3 8% 5% 4 5% 3 5% 5% 27%	2860
November	1st to 10th	Night	20% 13% 3 7% 4 3 3 4 6% 5% 31%	2667
	11th to 20th	Night	18% 14% 4 8% 5% 5% 4 3 5% 6% 27%	2730
	21st to end	Night	19% 17% 5% 10% 5% 4 4 3 5% 5% 21%	2940
December	1st to 10th	Night	17% 29% 10% 13% 5% 4 4 2 3 4 10%	4720
	11th to 20th	Night	23% 16% 6% 11% 5% 5% 4 3 4 4 20%	5348
	21st to end	Night	21% 17% 6% 12% 7% 6% 4 3 4 4 16%	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	60% 22% 7% 9% 2	4662
	11th to 20th	Day	59% 22% 5% 8% 4 1	4680
	21st to end	Day	55% 25% 5% 9% 7%	5124
February	1st to 10th	Day	28% 59% 4 6% 2 1	1853
	11th to 20th	Day	27% 58% 4 8% 3	2015
	21st to end	Day	26% 60% 3 5% 4 1	1730
March	1st to 10th	Day	49% 32% 4 6% 6% 3	4579
	11th to 20th	Day	43% 29% 10% 10% 8% 1	4761
	21st to end	Day	44% 29% 5% 9% 13% 1	6102
April	1st to 10th	Day	25% 59% 3 8% 4 1	2520
	11th to 20th	Day	24% 60% 2 8% 6% 1	2494
	21st to end	Day	23% 59% 4 8% 6%	2691
May	1st to 10th	Day	51% 31% 5% 11% 3	6231
	11th to 20th	Day	40% 32% 11% 12% 5%	6406
	21st to end	Day	39% 31% 9% 17% 5%	6949
June	1st to 10th	Day	20% 63% 3 8% 6%	2800
	11th to 20th	Day	16% 62% 5% 10% 6% 1	2800
	21st to end	Day	19% 61% 5% 10% 5% 1	2800
July	1st to 10th	Day	35% 26% 12% 19% 8%	6290
	11th to 20th	Day	32% 26% 10% 23% 8%	6285
	21st to end	Day	30% 26% 8% 30% 6%	6728
August	1st to 10th	Day	32% 36% 10% 15% 5%	4549
	11th to 20th	Day	34% 37% 11% 16% 2	4067
	21st to end	Day	25% 36% 13% 15% 10%	4312
September	1st to 10th	Day	13% 58% 5% 16% 7%	2400
	11th to 20th	Day	14% 57% 6% 15% 8% 1	2290
	21st to end	Day	16% 58% 7% 13% 5% 1	2200
October	1st to 10th	Day	27% 35% 7% 23% 7%	3434
	11th to 20th	Day	31% 36% 9% 16% 8%	3153
	21st to end	Day	31% 36% 10% 20% 3	3180
November	1st to 10th	Day	30% 56% 4 7% 3	1851
	11th to 20th	Day	27% 55% 6% 8% 4	1887
	21st to end	Day	32% 55% 6% 5% 2	1680
December	1st to 10th	Day	56% 35% 5% 4 1	2680
	11th to 20th	Day	47% 31% 5% 13% 3	3066
	21st to end	Day	51% 27% 6% 10% 5%	4048
January	1st to 10th	Night	61% 21% 8% 7% 2	8234
	11th to 20th	Night	63% 23% 5% 6% 3 1	8280
	21st to end	Night	60% 23% 5% 6% 6%	9057
February	1st to 10th	Night	29% 59% 3 6% 2	2766
	11th to 20th	Night	27% 58% 4 7% 4	2502
	21st to end	Night	24% 59% 4 8% 5% 1	2076
March	1st to 10th	Night	49% 29% 5% 7% 9% 1	5494
	11th to 20th	Night	42% 28% 11% 9% 7% 1	5401
	21st to end	Night	49% 28% 3 9% 10% 1	5072
April	1st to 10th	Night	25% 59% 3 6% 6%	2100
	11th to 20th	Night	26% 60% 2 6% 6%	2075
	21st to end	Night	23% 59% 3 7% 7% 1	1927
May	1st to 10th	Night	51% 31% 6% 10% 2	3977
	11th to 20th	Night	42% 31% 8% 14% 5%	3624
	21st to end	Night	42% 30% 7% 13% 7% 1	3927
June	1st to 10th	Night	21% 62% 3 7% 7% 1	1600
	11th to 20th	Night	17% 63% 5% 8% 6% 1	1600
	21st to end	Night	19% 61% 4 11% 5%	1600
July	1st to 10th	Night	34% 25% 14% 19% 7%	3560
	11th to 20th	Night	36% 26% 8% 21% 9%	3552
	21st to end	Night	34% 26% 9% 23% 7%	3809
August	1st to 10th	Night	36% 36% 8% 14% 5%	2576
	11th to 20th	Night	35% 37% 11% 12% 5%	3010
	21st to end	Night	27% 36% 10% 16% 10% 1	3586
September	1st to 10th	Night	15% 58% 6% 13% 7% 1	2000
	11th to 20th	Night	15% 58% 6% 14% 7%	2110
	21st to end	Night	15% 58% 6% 12% 8% 1	2200
October	1st to 10th	Night	37% 35% 5% 16% 7%	3745
	11th to 20th	Night	32% 36% 7% 16% 9%	3986
	21st to end	Night	35% 35% 8% 17% 5%	4610
November	1st to 10th	Night	28% 54% 6% 8% 3	2667
	11th to 20th	Night	29% 54% 6% 7% 4	2730
	21st to end	Night	31% 55% 6% 5% 2	2940
December	1st to 10th	Night	57% 35% 4 4	4720
	11th to 20th	Night	51% 32% 4 7% 6%	5348
	21st to end	Night	54% 27% 4 9% 6%	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% 23% 29% 18% 3	220
	11th to 20th	Day	31% 28% 30% 6% 3 1	220
	21st to end	Day	26% 23% 35% 12% 4	242
February	1st to 10th	Day	31% 29% 28% 10% 2	311
	11th to 20th	Day	27% 27% 34% 10% 2	494
	21st to end	Day	31% 21% 35% 11% 2	446
March	1st to 10th	Day	32% 19% 29% 16% 3	540
	11th to 20th	Day	29% 26% 28% 14% 4 1	540
	21st to end	Day	34% 22% 25% 12% 6% 1	594
April	1st to 10th	Day	37% 19% 24% 15% 5% 1	540
	11th to 20th	Day	35% 23% 20% 13% 6% 2	532
	21st to end	Day	30% 21% 28% 13% 6% 1	540
May	1st to 10th	Day	32% 18% 26% 15% 7% 1	543
	11th to 20th	Day	23% 15% 32% 23% 7% 1	551
	21st to end	Day	21% 17% 31% 21% 8% 1	573
June	1st to 10th	Day	28% 13% 32% 20% 7% 1	530
	11th to 20th	Day	19% 15% 34% 21% 9% 2	530
	21st to end	Day	23% 15% 32% 21% 8% 1	530
July	1st to 10th	Day	17% 10% 31% 29% 11% 3	525
	11th to 20th	Day	14% 9% 34% 29% 12% 2	524
	21st to end	Day	19% 7% 34% 30% 10% 1	576
August	1st to 10th	Day	15% 9% 28% 28% 17% 2	526
	11th to 20th	Day	18% 9% 24% 29% 14% 5%	519
	21st to end	Day	16% 14% 28% 26% 11% 5%	583
September	1st to 10th	Day	17% 14% 29% 23% 12% 5%	530
	11th to 20th	Day	16% 13% 20% 23% 20% 8%	530
	21st to end	Day	16% 14% 30% 22% 14% 5%	530
October	1st to 10th	Day	26% 13% 26% 17% 15% 4	530
	11th to 20th	Day	29% 16% 26% 20% 7% 1	502
	21st to end	Day	38% 16% 25% 17% 3	462
November	1st to 10th	Day	34% 18% 26% 16% 5%	425
	11th to 20th	Day	32% 25% 22% 14% 6% 1	427
	21st to end	Day	39% 21% 27% 12% 1	220
December	1st to 10th	Day	28% 25% 29% 17% 2	220
	11th to 20th	Day	31% 27% 31% 9% 1	220
	21st to end	Day	29% 26% 30% 12% 3	242
January	1st to 10th	Night	29% 20% 32% 16% 3	840
	11th to 20th	Night	31% 19% 29% 17% 4	840
	21st to end	Night	23% 18% 31% 24% 5% 1	924
February	1st to 10th	Night	23% 17% 34% 23% 3	840
	11th to 20th	Night	26% 19% 33% 19% 3	823
	21st to end	Night	27% 18% 29% 21% 5%	692
March	1st to 10th	Night	30% 16% 27% 23% 3	840
	11th to 20th	Night	28% 22% 25% 19% 5%	840
	21st to end	Night	32% 16% 25% 19% 7% 1	924
April	1st to 10th	Night	30% 15% 26% 24% 4	840
	11th to 20th	Night	33% 17% 22% 21% 7%	831
	21st to end	Night	27% 18% 27% 21% 7%	667
May	1st to 10th	Night	31% 11% 25% 24% 7% 1	561
	11th to 20th	Night	24% 9% 33% 25% 8% 1	406
	21st to end	Night	19% 9% 27% 33% 10% 1	433
June	1st to 10th	Night	27% 7% 27% 31% 8% 1	400
	11th to 20th	Night	19% 7% 28% 33% 13% 1	400
	21st to end	Night	22% 5% 26% 37% 10% 1	400
July	1st to 10th	Night	16% 3 23% 43% 15% 1	390
	11th to 20th	Night	12% 4 23% 42% 18% 2	388
	21st to end	Night	16% 6% 21% 42% 15%	438
August	1st to 10th	Night	16% 4 17% 41% 22% 1	398
	11th to 20th	Night	18% 4 20% 33% 22% 3	650
	21st to end	Night	16% 7% 23% 32% 21% 1	880
September	1st to 10th	Night	17% 8% 24% 34% 16% 2	800
	11th to 20th	Night	18% 8% 17% 30% 23% 4	800
	21st to end	Night	21% 9% 27% 25% 17% 2	800
October	1st to 10th	Night	28% 11% 20% 25% 15% 1	800
	11th to 20th	Night	31% 11% 23% 26% 8% 1	800
	21st to end	Night	32% 15% 27% 21% 5%	880
November	1st to 10th	Night	31% 17% 25% 20% 6% 1	820
	11th to 20th	Night	31% 16% 27% 17% 8% 1	840
	21st to end	Night	33% 20% 26% 17% 4 1	840
December	1st to 10th	Night	34% 19% 28% 14% 4	840
	11th to 20th	Night	29% 22% 28% 17% 4	840
	21st to end	Night	33% 17% 29% 17% 3	924

Page generated at 2026-05-14 01:58:55. Version ac07

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