Sudbury
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sudbury clear sky chart from 2005-11-06 to 2026-03-03.

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	15% 11% 4 9% 5% 4 4 4 5% 7% 32%	130950
	Night	15% 12% 4 9% 5% 5% 5% 4 5% 6% 30%	130359
Transparency	Day	52% 3 11% 22% 10% 1	134445
	Night	54% 3 11% 21% 10% 1	134952
Seeing	Day	21% 12% 31% 23% 10% 3	16870
	Night	22% 9% 30% 28% 9% 1	26172

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% 19% 7% 11% 5% 4 4 4 4 6% 23%	4094
	11th to 20th	Day	10% 17% 7% 18% 8% 4 5% 4 6% 5% 18%	4378
	21st to end	Day	10% 14% 8% 15% 8% 6% 5% 3 5% 4 22%	5569
February	1st to 10th	Day	10% 17% 5% 10% 8% 5% 6% 4 5% 6% 24%	1890
	11th to 20th	Day	12% 17% 4 11% 6% 5% 4 3 5% 5% 27%	1843
	21st to end	Day	13% 16% 5% 11% 6% 5% 5% 4 6% 5% 24%	1637
March	1st to 10th	Day	19% 14% 4 8% 4 4 3 3 4 6% 29%	4787
	11th to 20th	Day	14% 12% 5% 11% 6% 5% 5% 3 5% 7% 27%	4980
	21st to end	Day	14% 14% 5% 11% 5% 4 4 3 4 5% 32%	5465
April	1st to 10th	Day	18% 12% 3 7% 5% 4 5% 4 5% 7% 29%	2352
	11th to 20th	Day	20% 11% 2 5% 5% 4 3 4 5% 6% 35%	2572
	21st to end	Day	23% 10% 3 7% 4 4 3 3 4 6% 35%	2600
May	1st to 10th	Day	18% 8% 3 7% 5% 3 4 3 5% 6% 38%	5924
	11th to 20th	Day	14% 8% 3 7% 4 4 4 4 5% 7% 41%	6318
	21st to end	Day	20% 8% 2 7% 4 3 4 3 4 6% 37%	7131
June	1st to 10th	Day	15% 8% 3 7% 5% 5% 4 5% 6% 8% 33%	3000
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	3000
	21st to end	Day	18% 8% 3 6% 6% 4 4 4 7% 7% 32%	3000
July	1st to 10th	Day	15% 7% 3 8% 6% 5% 5% 4 6% 8% 33%	6819
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 8% 44%	6820
	21st to end	Day	10% 5% 2 6% 4 4 4 4 6% 9% 46%	6837
August	1st to 10th	Day	11% 7% 2 7% 4 5% 6% 4 7% 9% 37%	4234
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 6% 8% 39%	4174
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 5% 8% 41%	4675
September	1st to 10th	Day	12% 9% 3 7% 6% 5% 4 3 5% 8% 38%	2400
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 3 5% 8% 44%	2209
	21st to end	Day	21% 8% 3 7% 4 4 4 4 5% 6% 36%	2200
October	1st to 10th	Day	18% 8% 3 8% 5% 4 4 3 4 6% 36%	2010
	11th to 20th	Day	23% 11% 3 9% 6% 5% 4 4 5% 6% 24%	2000
	21st to end	Day	26% 14% 3 9% 5% 4 4 4 5% 5% 22%	2083
November	1st to 10th	Day	19% 12% 4 9% 5% 5% 4 4 6% 6% 27%	1721
	11th to 20th	Day	17% 15% 4 11% 6% 5% 4 4 5% 6% 23%	1680
	21st to end	Day	22% 20% 5% 11% 7% 5% 3 3 3 4 18%	1680
December	1st to 10th	Day	13% 20% 6% 13% 7% 6% 6% 3 5% 5% 16%	2624
	11th to 20th	Day	16% 16% 6% 14% 7% 4 4 2 4 5% 22%	2691
	21st to end	Day	23% 21% 7% 13% 4 3 3 2 3 5% 15%	3553
January	1st to 10th	Night	11% 14% 6% 13% 7% 6% 5% 4 5% 6% 21%	8831
	11th to 20th	Night	12% 19% 6% 12% 6% 5% 5% 4 5% 5% 21%	8534
	21st to end	Night	8% 14% 7% 14% 7% 6% 6% 4 6% 6% 22%	8608
February	1st to 10th	Night	11% 15% 5% 12% 8% 5% 5% 4 5% 6% 24%	2730
	11th to 20th	Night	11% 16% 5% 11% 8% 5% 5% 3 5% 6% 26%	2663
	21st to end	Night	13% 17% 5% 9% 6% 6% 4 4 5% 5% 26%	2165
March	1st to 10th	Night	18% 13% 3 8% 4 4 4 3 4 6% 32%	5097
	11th to 20th	Night	15% 11% 4 11% 6% 5% 6% 4 5% 6% 27%	4979
	21st to end	Night	17% 11% 5% 10% 5% 5% 4 4 4 5% 31%	5467
April	1st to 10th	Night	18% 12% 4 8% 5% 4 4 3 5% 6% 31%	2062
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 3 4 7% 36%	1777
	21st to end	Night	19% 10% 3 7% 5% 5% 5% 3 5% 6% 33%	1800
May	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 4 5% 5% 37%	4017
	11th to 20th	Night	12% 8% 3 6% 5% 4 4 3 5% 7% 42%	3575
	21st to end	Night	15% 9% 3 7% 4 3 5% 4 5% 7% 38%	3737
June	1st to 10th	Night	12% 7% 3 7% 5% 6% 4 4 7% 8% 36%	1400
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 3 8% 9% 41%	1400
	21st to end	Night	13% 6% 3 6% 5% 4 4 4 6% 8% 42%	1400
July	1st to 10th	Night	15% 7% 2 6% 4 5% 4 4 5% 7% 41%	3160
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 5% 9% 47%	3160
	21st to end	Night	9% 5% 2 5% 3 4 4 4 6% 7% 51%	4019
August	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	2915
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 8% 43%	2875
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 7% 47%	3223
September	1st to 10th	Night	13% 6% 2 6% 3 4 4 3 6% 6% 46%	1980
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 50%	2189
	21st to end	Night	18% 7% 2 6% 4 3 4 4 4 8% 40%	2200
October	1st to 10th	Night	20% 8% 2 6% 4 4 3 4 5% 6% 38%	2390
	11th to 20th	Night	24% 10% 3 7% 5% 4 3 3 6% 6% 28%	2400
	21st to end	Night	24% 12% 4 9% 4 5% 5% 3 5% 5% 25%	2755
November	1st to 10th	Night	17% 12% 4 7% 5% 4 4 4 5% 7% 30%	2796
	11th to 20th	Night	18% 14% 4 10% 6% 5% 4 3 4 5% 26%	2940
	21st to end	Night	22% 16% 4 9% 5% 4 4 4 5% 6% 20%	2940
December	1st to 10th	Night	16% 19% 5% 11% 6% 5% 5% 4 5% 6% 18%	4772
	11th to 20th	Night	15% 16% 5% 13% 6% 5% 4 3 5% 7% 23%	5736
	21st to end	Night	22% 23% 5% 13% 4 4 3 2 3 4 16%	7667

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	64% 1 9% 19% 5% 1	4095
	11th to 20th	Day	70% 2 8% 11% 8% 1	4407
	21st to end	Day	66% 2 6% 16% 10%	5569
February	1st to 10th	Day	61% 3 11% 18% 6% 1	1890
	11th to 20th	Day	60% 3 11% 15% 10% 1	1852
	21st to end	Day	61% 3 8% 18% 9% 1	1637
March	1st to 10th	Day	57% 2 11% 12% 15% 3	4790
	11th to 20th	Day	58% 3 13% 16% 9% 1	4980
	21st to end	Day	56% 2 11% 19% 11% 2	5465
April	1st to 10th	Day	55% 3 11% 20% 9% 2	2352
	11th to 20th	Day	50% 3 7% 25% 13% 1	2572
	21st to end	Day	52% 4 8% 20% 14% 3	2600
May	1st to 10th	Day	48% 2 8% 23% 15% 4	5933
	11th to 20th	Day	44% 3 11% 23% 17% 3	6322
	21st to end	Day	49% 2 10% 19% 16% 3	7131
June	1st to 10th	Day	48% 4 12% 22% 13% 1	3000
	11th to 20th	Day	43% 5% 11% 24% 17% 1	3000
	21st to end	Day	50% 5% 11% 22% 11% 1	3000
July	1st to 10th	Day	49% 2 14% 25% 9%	6746
	11th to 20th	Day	38% 3 13% 38% 7%	6739
	21st to end	Day	36% 3 14% 35% 12%	6643
August	1st to 10th	Day	42% 4 12% 32% 9% 1	4221
	11th to 20th	Day	44% 3 15% 29% 9%	4213
	21st to end	Day	41% 3 13% 33% 9% 1	4674
September	1st to 10th	Day	45% 3 15% 26% 10% 1	2400
	11th to 20th	Day	39% 4 17% 30% 11% 1	2209
	21st to end	Day	50% 4 15% 24% 9%	2200
October	1st to 10th	Day	43% 3 10% 31% 13%	3270
	11th to 20th	Day	51% 4 12% 23% 11%	3240
	21st to end	Day	64% 2 13% 14% 6% 1	3346
November	1st to 10th	Day	58% 4 13% 19% 7%	1721
	11th to 20th	Day	62% 3 12% 16% 6%	1680
	21st to end	Day	72% 3 10% 10% 5%	1680
December	1st to 10th	Day	71% 2 13% 10% 3	2624
	11th to 20th	Day	66% 2 6% 19% 6%	2691
	21st to end	Day	76% 1 5% 12% 6%	3553
January	1st to 10th	Night	64% 2 10% 18% 7%	8831
	11th to 20th	Night	66% 2 10% 13% 9%	8580
	21st to end	Night	63% 3 7% 16% 10% 1	8608
February	1st to 10th	Night	60% 3 10% 19% 7% 1	2730
	11th to 20th	Night	60% 3 11% 19% 7% 1	2665
	21st to end	Night	61% 3 10% 16% 10% 1	2165
March	1st to 10th	Night	54% 2 8% 16% 14% 4	5107
	11th to 20th	Night	58% 2 14% 17% 8% 1	4980
	21st to end	Night	56% 3 8% 18% 12% 3	5467
April	1st to 10th	Night	55% 3 7% 20% 13% 1	2062
	11th to 20th	Night	50% 3 6% 23% 17% 2	1777
	21st to end	Night	53% 3 10% 16% 16% 2	1800
May	1st to 10th	Night	47% 4 7% 21% 17% 3	4024
	11th to 20th	Night	41% 3 11% 23% 17% 4	3576
	21st to end	Night	46% 3 9% 24% 13% 5%	3738
June	1st to 10th	Night	45% 4 12% 24% 13% 3	1400
	11th to 20th	Night	39% 4 13% 23% 20% 2	1400
	21st to end	Night	41% 4 16% 23% 14% 2	1400
July	1st to 10th	Night	43% 3 13% 30% 11% 1	3125
	11th to 20th	Night	36% 2 15% 37% 10% 1	3118
	21st to end	Night	33% 2 14% 34% 16%	3900
August	1st to 10th	Night	41% 3 15% 28% 12% 1	2904
	11th to 20th	Night	41% 3 17% 29% 9% 1	2902
	21st to end	Night	38% 3 14% 32% 12% 1	3223
September	1st to 10th	Night	38% 5% 14% 31% 10% 2	1980
	11th to 20th	Night	37% 3 15% 34% 11%	2189
	21st to end	Night	44% 5% 14% 27% 10%	2200
October	1st to 10th	Night	43% 2 11% 32% 12%	3910
	11th to 20th	Night	48% 2 12% 31% 7%	3899
	21st to end	Night	57% 3 18% 16% 6% 1	4442
November	1st to 10th	Night	52% 4 14% 22% 8%	2796
	11th to 20th	Night	61% 3 12% 16% 7% 1	2940
	21st to end	Night	65% 4 12% 14% 5%	2940
December	1st to 10th	Night	67% 2 13% 15% 3	4772
	11th to 20th	Night	63% 3 9% 14% 11% 1	5735
	21st to end	Night	73% 2 7% 13% 6%	7667

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	24% 17% 30% 20% 7%	220
	11th to 20th	Day	19% 11% 35% 26% 7% 1	274
	21st to end	Day	24% 7% 32% 23% 11% 3	363
February	1st to 10th	Day	23% 19% 32% 19% 5% 1	330
	11th to 20th	Day	21% 15% 39% 20% 4	330
	21st to end	Day	24% 21% 36% 16% 3	273
March	1st to 10th	Day	22% 14% 36% 23% 5%	427
	11th to 20th	Day	22% 14% 32% 24% 8% 1	500
	21st to end	Day	24% 15% 31% 23% 5% 1	550
April	1st to 10th	Day	23% 11% 30% 24% 9% 2	500
	11th to 20th	Day	31% 13% 24% 17% 11% 4	492
	21st to end	Day	31% 14% 24% 18% 10% 3	500
May	1st to 10th	Day	23% 10% 30% 24% 11% 2	503
	11th to 20th	Day	19% 14% 30% 21% 12% 4	527
	21st to end	Day	21% 12% 31% 24% 10% 3	651
June	1st to 10th	Day	17% 15% 35% 25% 6% 1	730
	11th to 20th	Day	18% 15% 34% 23% 8% 3	730
	21st to end	Day	23% 9% 33% 24% 9% 2	730
July	1st to 10th	Day	12% 11% 33% 30% 13% 2	721
	11th to 20th	Day	14% 10% 31% 27% 13% 4	718
	21st to end	Day	13% 6% 30% 31% 14% 6%	674
August	1st to 10th	Day	17% 7% 25% 26% 18% 6%	526
	11th to 20th	Day	15% 6% 23% 27% 19% 10%	519
	21st to end	Day	17% 7% 28% 25% 15% 7%	583
September	1st to 10th	Day	16% 12% 26% 27% 14% 5%	530
	11th to 20th	Day	14% 5% 26% 31% 17% 8%	530
	21st to end	Day	26% 9% 22% 25% 14% 4	530
October	1st to 10th	Day	22% 13% 27% 23% 12% 3	530
	11th to 20th	Day	26% 15% 35% 17% 5% 1	530
	21st to end	Day	31% 14% 29% 21% 4 1	466
November	1st to 10th	Day	29% 14% 35% 18% 4	261
	11th to 20th	Day	21% 17% 38% 19% 5% 1	220
	21st to end	Day	30% 15% 31% 16% 6%	220
December	1st to 10th	Day	23% 13% 32% 22% 9%	220
	11th to 20th	Day	31% 14% 34% 14% 6%	220
	21st to end	Day	30% 11% 36% 16% 6%	242
January	1st to 10th	Night	23% 10% 32% 27% 7% 1	1050
	11th to 20th	Night	27% 11% 29% 26% 7%	1050
	21st to end	Night	20% 9% 29% 32% 9% 1	1001
February	1st to 10th	Night	18% 13% 38% 27% 4	840
	11th to 20th	Night	19% 14% 35% 26% 6%	823
	21st to end	Night	21% 15% 35% 25% 3	692
March	1st to 10th	Night	20% 14% 36% 24% 5%	809
	11th to 20th	Night	22% 13% 33% 25% 6% 1	800
	21st to end	Night	21% 10% 35% 26% 7% 1	880
April	1st to 10th	Night	24% 12% 31% 28% 5%	785
	11th to 20th	Night	24% 11% 29% 28% 8% 1	593
	21st to end	Night	26% 9% 29% 26% 9%	600
May	1st to 10th	Night	18% 7% 32% 34% 8%	590
	11th to 20th	Night	17% 9% 31% 35% 8% 1	394
	21st to end	Night	17% 6% 36% 32% 9%	433
June	1st to 10th	Night	16% 7% 37% 32% 9%	400
	11th to 20th	Night	18% 8% 31% 36% 8%	400
	21st to end	Night	15% 7% 30% 34% 14% 1	400
July	1st to 10th	Night	9% 4 33% 43% 9% 1	390
	11th to 20th	Night	10% 4 32% 39% 14%	388
	21st to end	Night	10% 3 30% 42% 14% 2	572
August	1st to 10th	Night	13% 4 24% 39% 20% 1	597
	11th to 20th	Night	15% 3 23% 35% 21% 2	594
	21st to end	Night	17% 4 25% 35% 18% 1	660
September	1st to 10th	Night	16% 5% 28% 31% 19% 1	780
	11th to 20th	Night	15% 7% 24% 32% 19% 3	800
	21st to end	Night	22% 5% 25% 29% 16% 3	800
October	1st to 10th	Night	25% 7% 28% 29% 10% 1	800
	11th to 20th	Night	31% 10% 28% 24% 7% 1	800
	21st to end	Night	31% 12% 26% 26% 5% 1	880
November	1st to 10th	Night	27% 11% 30% 25% 7%	820
	11th to 20th	Night	27% 11% 30% 25% 7%	840
	21st to end	Night	31% 10% 29% 24% 6%	840
December	1st to 10th	Night	28% 11% 31% 23% 6% 1	866
	11th to 20th	Night	27% 12% 32% 22% 6%	1050
	21st to end	Night	31% 12% 29% 22% 6%	1155

Page generated at 2026-02-28 01:40:19. Version ac07

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