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 2024-12-02.

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% 11% 3 8% 5% 4 4 4 5% 7% 33%	101892
	Night	15% 12% 4 9% 5% 5% 4 3 5% 6% 31%	97807
Transparency	Day	51% 3 12% 23% 10% 1	105403
	Night	53% 3 12% 21% 10% 1	102385
Seeing	Day	21% 12% 30% 24% 10% 3	15325
	Night	22% 9% 30% 28% 9% 1	24481

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	16% 20% 7% 11% 6% 4 4 3 4 5% 23%	2208
	11th to 20th	Day	9% 14% 6% 17% 10% 5% 6% 4 6% 5% 18%	2367
	21st to end	Day	16% 20% 9% 11% 7% 5% 4 3 5% 4 15%	3004
February	1st to 10th	Day	10% 16% 5% 10% 8% 6% 6% 4 5% 6% 24%	1710
	11th to 20th	Day	12% 17% 4 10% 6% 5% 4 3 5% 6% 29%	1663
	21st to end	Day	13% 17% 5% 10% 6% 5% 5% 4 6% 5% 24%	1485
March	1st to 10th	Day	18% 14% 4 8% 5% 4 4 3 5% 7% 29%	3323
	11th to 20th	Day	9% 14% 5% 12% 7% 5% 5% 3 5% 6% 28%	3480
	21st to end	Day	12% 13% 4 11% 5% 4 4 3 4 6% 33%	3815
April	1st to 10th	Day	19% 12% 4 7% 5% 4 5% 4 5% 7% 29%	2234
	11th to 20th	Day	20% 11% 2 5% 5% 4 3 4 5% 6% 34%	2442
	21st to end	Day	23% 10% 3 7% 4 3 3 3 4 6% 35%	2470
May	1st to 10th	Day	18% 8% 3 6% 5% 4 4 3 6% 7% 36%	4130
	11th to 20th	Day	13% 8% 3 7% 4 5% 5% 4 5% 7% 39%	4398
	21st to end	Day	19% 6% 2 6% 4 3 4 4 4 7% 42%	4963
June	1st to 10th	Day	16% 8% 3 7% 5% 5% 4 5% 6% 8% 32%	2850
	11th to 20th	Day	12% 6% 3 7% 6% 5% 5% 4 5% 8% 39%	2850
	21st to end	Day	17% 8% 3 6% 7% 4 4 5% 6% 7% 32%	2850
July	1st to 10th	Day	16% 8% 3 8% 6% 5% 4 4 6% 8% 33%	4760
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 7% 42%	4760
	21st to end	Day	11% 6% 2 6% 5% 4 4 4 6% 8% 46%	4753
August	1st to 10th	Day	11% 7% 2 7% 4 5% 6% 4 7% 9% 37%	4104
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 6% 8% 39%	4044
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 5% 8% 41%	4532
September	1st to 10th	Day	13% 9% 3 7% 5% 4 4 3 5% 8% 39%	2280
	11th to 20th	Day	14% 5% 2 6% 5% 4 5% 3 5% 8% 43%	2099
	21st to end	Day	22% 8% 3 7% 4 4 4 4 5% 6% 35%	2090
October	1st to 10th	Day	19% 9% 3 8% 5% 4 4 3 4 6% 35%	1910
	11th to 20th	Day	23% 11% 3 9% 6% 5% 4 4 5% 6% 23%	1900
	21st to end	Day	27% 14% 3 9% 5% 4 4 4 5% 6% 21%	1979
November	1st to 10th	Day	19% 12% 4 9% 5% 5% 4 4 6% 6% 27%	1637
	11th to 20th	Day	17% 15% 5% 11% 6% 5% 4 4 4 6% 23%	1600
	21st to end	Day	22% 20% 5% 11% 7% 5% 4 3 3 4 18%	1600
December	1st to 10th	Day	14% 21% 6% 10% 6% 5% 5% 3 5% 5% 19%	1500
	11th to 20th	Day	17% 18% 6% 12% 6% 4 4 3 4 5% 22%	1671
	21st to end	Day	23% 21% 6% 14% 5% 3 4 2 3 4 14%	2431
January	1st to 10th	Night	15% 15% 6% 13% 7% 5% 5% 4 5% 6% 21%	4748
	11th to 20th	Night	12% 15% 5% 11% 7% 6% 6% 4 6% 7% 21%	4600
	21st to end	Night	14% 19% 9% 12% 6% 5% 5% 3 4 4 18%	4655
February	1st to 10th	Night	11% 15% 5% 12% 8% 5% 5% 4 5% 6% 24%	2470
	11th to 20th	Night	11% 15% 4 11% 8% 5% 5% 3 5% 5% 27%	2403
	21st to end	Night	14% 16% 4 9% 6% 6% 4 4 5% 5% 27%	1965
March	1st to 10th	Night	16% 13% 3 8% 4 4 4 3 4 6% 34%	3516
	11th to 20th	Night	11% 12% 5% 11% 6% 5% 4 3 4 6% 32%	3479
	21st to end	Night	13% 8% 5% 9% 6% 5% 5% 4 5% 5% 34%	3817
April	1st to 10th	Night	18% 12% 4 8% 4 4 4 3 5% 6% 30%	1959
	11th to 20th	Night	20% 9% 3 6% 5% 4 4 2 4 7% 36%	1687
	21st to end	Night	19% 10% 3 7% 5% 4 5% 3 5% 6% 34%	1710
May	1st to 10th	Night	16% 8% 3 8% 6% 5% 6% 4 4 5% 35%	2811
	11th to 20th	Night	12% 8% 3 7% 6% 5% 4 4 6% 8% 38%	2495
	21st to end	Night	13% 9% 2 7% 4 3 4 4 5% 7% 43%	2601
June	1st to 10th	Night	13% 7% 3 7% 5% 6% 4 4 7% 8% 36%	1330
	11th to 20th	Night	13% 6% 2 5% 5% 4 4 3 8% 9% 41%	1330
	21st to end	Night	13% 5% 3 6% 5% 4 4 4 6% 8% 42%	1330
July	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	2210
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 8% 45%	2210
	21st to end	Night	9% 5% 2 5% 3 3 4 4 6% 6% 50%	2799
August	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	2825
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 8% 43%	2785
	21st to end	Night	12% 5% 2 5% 4 5% 4 3 5% 7% 47%	3124
September	1st to 10th	Night	13% 6% 2 6% 3 4 4 4 5% 6% 47%	1881
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 49%	2079
	21st to end	Night	18% 7% 2 6% 4 3 4 4 4 8% 39%	2090
October	1st to 10th	Night	20% 8% 3 6% 4 4 3 4 5% 6% 37%	2270
	11th to 20th	Night	24% 10% 3 7% 5% 4 3 3 6% 6% 28%	2280
	21st to end	Night	25% 12% 4 9% 4 5% 5% 3 5% 5% 24%	2617
November	1st to 10th	Night	18% 12% 4 7% 4 4 4 4 6% 7% 30%	2660
	11th to 20th	Night	19% 14% 4 10% 6% 5% 5% 3 4 5% 26%	2800
	21st to end	Night	22% 16% 4 9% 5% 5% 5% 4 4 6% 21%	2800
December	1st to 10th	Night	17% 19% 5% 10% 6% 4 4 3 5% 5% 20%	2709
	11th to 20th	Night	17% 17% 4 11% 6% 4 4 2 5% 6% 24%	3526
	21st to end	Night	22% 22% 5% 14% 5% 4 3 2 4 4 15%	5236

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	67% 2 16% 13% 3	2209
	11th to 20th	Day	68% 2 13% 12% 4 1	2367
	21st to end	Day	73% 2 9% 11% 5%	3004
February	1st to 10th	Day	60% 3 12% 18% 6% 1	1710
	11th to 20th	Day	58% 4 11% 16% 11% 1	1672
	21st to end	Day	61% 3 8% 18% 10% 1	1485
March	1st to 10th	Day	56% 2 15% 16% 10% 1	3326
	11th to 20th	Day	57% 3 12% 20% 8% 1	3480
	21st to end	Day	53% 2 11% 19% 12% 2	3815
April	1st to 10th	Day	55% 4 12% 19% 9% 1	2234
	11th to 20th	Day	51% 3 8% 25% 12% 1	2442
	21st to end	Day	52% 4 8% 19% 14% 3	2470
May	1st to 10th	Day	47% 3 10% 25% 13% 2	4142
	11th to 20th	Day	44% 4 12% 27% 12% 1	4402
	21st to end	Day	43% 3 13% 19% 19% 3	4963
June	1st to 10th	Day	49% 4 12% 22% 12% 1	2850
	11th to 20th	Day	43% 5% 11% 23% 17% 1	2850
	21st to end	Day	50% 5% 11% 22% 11% 2	2850
July	1st to 10th	Day	49% 3 14% 23% 10% 1	4686
	11th to 20th	Day	39% 4 13% 34% 9% 1	4679
	21st to end	Day	37% 3 12% 34% 14%	4602
August	1st to 10th	Day	42% 4 12% 32% 9% 1	4091
	11th to 20th	Day	44% 3 15% 28% 9%	4083
	21st to end	Day	41% 3 12% 33% 9% 1	4531
September	1st to 10th	Day	45% 3 15% 27% 10% 1	2280
	11th to 20th	Day	40% 4 17% 29% 10% 1	2099
	21st to end	Day	50% 4 15% 23% 8%	2090
October	1st to 10th	Day	43% 3 10% 31% 13%	3170
	11th to 20th	Day	51% 4 12% 23% 10%	3140
	21st to end	Day	65% 2 13% 13% 6%	3242
November	1st to 10th	Day	57% 4 13% 18% 7%	1637
	11th to 20th	Day	63% 3 12% 17% 6%	1600
	21st to end	Day	72% 3 10% 10% 5%	1600
December	1st to 10th	Day	67% 3 10% 14% 5%	1500
	11th to 20th	Day	66% 3 8% 18% 4 1	1671
	21st to end	Day	77% 2 6% 12% 3	2431
January	1st to 10th	Night	65% 3 14% 13% 6%	4748
	11th to 20th	Night	63% 3 15% 14% 5%	4600
	21st to end	Night	70% 3 8% 13% 6%	4655
February	1st to 10th	Night	61% 3 10% 18% 7% 1	2470
	11th to 20th	Night	59% 3 11% 18% 7% 1	2405
	21st to end	Night	60% 3 11% 15% 10% 1	1965
March	1st to 10th	Night	52% 3 11% 19% 13% 1	3526
	11th to 20th	Night	54% 3 13% 18% 10% 2	3480
	21st to end	Night	50% 4 10% 20% 13% 3	3817
April	1st to 10th	Night	55% 3 8% 20% 13% 1	1959
	11th to 20th	Night	50% 3 6% 23% 16% 2	1687
	21st to end	Night	53% 3 11% 16% 16% 2	1710
May	1st to 10th	Night	49% 6% 8% 23% 13% 2	2818
	11th to 20th	Night	42% 5% 11% 27% 14% 1	2496
	21st to end	Night	41% 4 12% 24% 14% 6%	2602
June	1st to 10th	Night	45% 5% 12% 24% 12% 3	1330
	11th to 20th	Night	39% 5% 13% 23% 19% 2	1330
	21st to end	Night	40% 5% 16% 23% 14% 2	1330
July	1st to 10th	Night	41% 3 14% 27% 14% 1	2175
	11th to 20th	Night	38% 3 16% 34% 9% 1	2168
	21st to end	Night	34% 3 13% 31% 19%	2711
August	1st to 10th	Night	42% 3 15% 27% 13% 1	2814
	11th to 20th	Night	41% 3 17% 29% 9% 1	2812
	21st to end	Night	38% 3 14% 31% 13% 1	3124
September	1st to 10th	Night	37% 5% 14% 31% 11% 2	1881
	11th to 20th	Night	37% 3 15% 33% 11%	2079
	21st to end	Night	44% 5% 15% 27% 9%	2090
October	1st to 10th	Night	44% 2 10% 32% 12%	3790
	11th to 20th	Night	48% 2 12% 31% 6%	3779
	21st to end	Night	58% 3 18% 15% 6% 1	4304
November	1st to 10th	Night	51% 4 14% 22% 8%	2660
	11th to 20th	Night	60% 4 12% 16% 7% 1	2800
	21st to end	Night	65% 5% 12% 14% 5%	2800
December	1st to 10th	Night	66% 4 11% 16% 4	2709
	11th to 20th	Night	62% 3 10% 15% 8% 1	3525
	21st to end	Night	75% 2 9% 10% 4	5236

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% 21% 27% 21% 7% 1	180
	11th to 20th	Day	17% 10% 38% 27% 8% 2	224
	21st to end	Day	27% 5% 30% 23% 11% 3	297
February	1st to 10th	Day	24% 19% 33% 19% 5% 1	270
	11th to 20th	Day	20% 14% 39% 23% 4	270
	21st to end	Day	24% 22% 36% 15% 2	225
March	1st to 10th	Day	21% 14% 36% 24% 6%	385
	11th to 20th	Day	20% 14% 32% 25% 8% 1	460
	21st to end	Day	23% 14% 32% 24% 5% 2	506
April	1st to 10th	Day	23% 11% 29% 25% 10% 2	460
	11th to 20th	Day	32% 13% 24% 17% 10% 4	452
	21st to end	Day	31% 14% 23% 19% 10% 3	460
May	1st to 10th	Day	22% 9% 29% 25% 12% 3	463
	11th to 20th	Day	20% 14% 29% 21% 12% 5%	487
	21st to end	Day	20% 12% 31% 23% 10% 3	603
June	1st to 10th	Day	17% 15% 35% 25% 6% 1	680
	11th to 20th	Day	18% 14% 32% 24% 8% 3	680
	21st to end	Day	22% 9% 32% 25% 9% 2	680
July	1st to 10th	Day	11% 11% 32% 30% 13% 3	671
	11th to 20th	Day	15% 9% 31% 27% 14% 4	668
	21st to end	Day	13% 6% 29% 31% 14% 7%	625
August	1st to 10th	Day	17% 7% 25% 25% 19% 7%	486
	11th to 20th	Day	16% 5% 21% 27% 20% 11%	479
	21st to end	Day	17% 6% 28% 25% 16% 8%	539
September	1st to 10th	Day	16% 12% 26% 27% 14% 5%	490
	11th to 20th	Day	15% 5% 26% 31% 16% 7%	490
	21st to end	Day	27% 10% 22% 24% 13% 4	490
October	1st to 10th	Day	23% 14% 26% 22% 12% 3	490
	11th to 20th	Day	26% 16% 34% 17% 5% 1	490
	21st to end	Day	32% 14% 30% 20% 3 1	428
November	1st to 10th	Day	30% 12% 35% 18% 4	237
	11th to 20th	Day	22% 19% 35% 20% 5% 1	200
	21st to end	Day	30% 15% 32% 16% 8% 1	200
December	1st to 10th	Day	24% 12% 31% 23% 10% 1	182
	11th to 20th	Day	32% 13% 35% 13% 7%	180
	21st to end	Day	30% 11% 38% 15% 6%	198
January	1st to 10th	Night	24% 11% 31% 26% 7% 1	950
	11th to 20th	Night	26% 11% 28% 26% 8%	950
	21st to end	Night	21% 9% 29% 31% 9% 1	907
February	1st to 10th	Night	18% 13% 38% 27% 4	760
	11th to 20th	Night	19% 14% 35% 24% 7% 1	743
	21st to end	Night	21% 16% 35% 25% 4	628
March	1st to 10th	Night	20% 15% 35% 25% 6%	760
	11th to 20th	Night	21% 13% 33% 25% 7% 1	760
	21st to end	Night	20% 10% 36% 26% 7% 1	836
April	1st to 10th	Night	24% 11% 31% 29% 5%	746
	11th to 20th	Night	24% 11% 28% 28% 7% 1	563
	21st to end	Night	25% 9% 29% 27% 10%	570
May	1st to 10th	Night	18% 7% 32% 34% 9%	561
	11th to 20th	Night	17% 9% 31% 35% 8% 1	374
	21st to end	Night	16% 6% 36% 32% 9%	411
June	1st to 10th	Night	16% 6% 37% 32% 9%	380
	11th to 20th	Night	18% 7% 31% 35% 8%	380
	21st to end	Night	15% 7% 28% 34% 14% 1	380
July	1st to 10th	Night	9% 4 33% 44% 9% 1	370
	11th to 20th	Night	10% 4 32% 39% 15%	368
	21st to end	Night	10% 3 30% 41% 15% 2	543
August	1st to 10th	Night	13% 4 25% 37% 20% 1	567
	11th to 20th	Night	15% 4 23% 35% 21% 2	564
	21st to end	Night	17% 4 25% 35% 18% 1	627
September	1st to 10th	Night	16% 4 28% 32% 19% 1	741
	11th to 20th	Night	16% 8% 25% 30% 19% 3	760
	21st to end	Night	22% 5% 25% 30% 16% 3	760
October	1st to 10th	Night	25% 7% 29% 28% 10% 1	760
	11th to 20th	Night	31% 10% 28% 23% 7% 1	760
	21st to end	Night	32% 13% 27% 24% 4 1	836
November	1st to 10th	Night	27% 11% 30% 25% 7% 1	780
	11th to 20th	Night	28% 11% 29% 25% 7%	800
	21st to end	Night	31% 10% 28% 24% 6%	800
December	1st to 10th	Night	29% 11% 30% 23% 6% 1	791
	11th to 20th	Night	28% 12% 32% 22% 6%	950
	21st to end	Night	30% 12% 29% 22% 6%	1045

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