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 2024-07-03.

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	16% 11% 4 8% 5% 5% 4 4 5% 7% 31%	89387
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 30%	89843
Transparency	Day	53% 3 12% 20% 10% 1	89097
	Night	55% 4 12% 19% 9% 1	89688
Seeing	Day	21% 12% 30% 24% 10% 3	14760
	Night	22% 10% 30% 28% 9% 1	23952

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 4 6% 7% 32%	2850
July	1st to 10th	Day	11% 5% 2 6% 6% 5% 4 5% 6% 9% 41%	2731
	11th to 20th	Day	12% 7% 2 6% 6% 6% 5% 5% 6% 7% 37%	2700
	21st to end	Day	12% 6% 2 7% 6% 5% 5% 5% 7% 8% 36%	2708
August	1st to 10th	Day	14% 7% 2 6% 4 6% 6% 4 8% 9% 34%	2327
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 7% 8% 39%	2303
	21st to end	Day	13% 7% 2 6% 5% 5% 5% 4 6% 9% 35%	2574
September	1st to 10th	Day	13% 9% 3 7% 6% 5% 4 3 5% 8% 38%	2160
	11th to 20th	Day	14% 5% 2 6% 5% 4 5% 4 6% 8% 41%	1989
	21st to end	Day	22% 8% 3 7% 4 4 4 4 5% 6% 34%	1980
October	1st to 10th	Day	19% 9% 3 8% 5% 5% 4 3 4 6% 34%	1810
	11th to 20th	Day	24% 12% 3 9% 6% 5% 4 4 5% 6% 22%	1800
	21st to end	Day	26% 14% 3 9% 5% 4 4 4 5% 5% 21%	1875
November	1st to 10th	Day	18% 13% 4 9% 5% 5% 4 4 6% 6% 27%	1554
	11th to 20th	Day	16% 14% 4 12% 6% 5% 4 4 5% 6% 23%	1520
	21st to end	Day	23% 20% 4 10% 7% 5% 3 3 3 4 18%	1520
December	1st to 10th	Day	14% 21% 6% 10% 6% 5% 5% 3 5% 5% 19%	1492
	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% 6% 3 6% 5% 4 4 4 6% 8% 42%	1330
July	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 9% 48%	1274
	11th to 20th	Night	9% 6% 2 6% 4 5% 4 3 6% 8% 47%	1260
	21st to end	Night	9% 5% 3 6% 4 3 4 6% 7% 7% 45%	1604
August	1st to 10th	Night	10% 6% 3 7% 6% 5% 4 3 5% 7% 44%	1611
	11th to 20th	Night	12% 7% 2 5% 5% 3 4 3 6% 8% 45%	1598
	21st to end	Night	15% 6% 2 6% 5% 5% 4 4 5% 7% 41%	1782
September	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 5% 6% 47%	1782
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 3 5% 6% 47%	1969
	21st to end	Night	18% 7% 2 7% 4 3 4 4 4 8% 38%	1980
October	1st to 10th	Night	21% 8% 3 6% 4 4 3 4 5% 5% 37%	2150
	11th to 20th	Night	25% 10% 3 7% 5% 4 3 3 6% 6% 28%	2160
	21st to end	Night	25% 12% 4 9% 4 5% 5% 3 4 5% 23%	2479
November	1st to 10th	Night	17% 12% 4 7% 4 4 4 4 5% 7% 31%	2523
	11th to 20th	Night	18% 14% 4 9% 6% 5% 5% 3 5% 5% 25%	2660
	21st to end	Night	22% 16% 4 8% 5% 5% 5% 4 4 6% 21%	2660
December	1st to 10th	Night	18% 19% 5% 10% 6% 4 4 3 5% 5% 20%	2683
	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	39% 4 13% 27% 16% 1	2657
	11th to 20th	Day	43% 5% 11% 29% 12% 1	2619
	21st to end	Day	43% 5% 13% 28% 10%	2557
August	1st to 10th	Day	44% 6% 16% 24% 9% 1	2314
	11th to 20th	Day	43% 5% 18% 26% 7% 1	2303
	21st to end	Day	44% 5% 15% 27% 8% 1	2574
September	1st to 10th	Day	45% 3 15% 26% 10% 1	2160
	11th to 20th	Day	41% 4 17% 28% 9%	1989
	21st to end	Day	51% 4 16% 24% 6%	1980
October	1st to 10th	Day	52% 4 12% 20% 11%	1810
	11th to 20th	Day	63% 4 10% 15% 7%	1800
	21st to end	Day	65% 3 12% 12% 7%	1875
November	1st to 10th	Day	57% 4 14% 18% 7%	1554
	11th to 20th	Day	62% 3 12% 16% 6%	1520
	21st to end	Day	72% 3 10% 10% 5%	1520
December	1st to 10th	Day	67% 3 10% 14% 5%	1492
	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	30% 5% 15% 30% 17% 2	1239
	11th to 20th	Night	35% 4 15% 32% 14% 1	1218
	21st to end	Night	36% 4 19% 28% 12% 1	1516
August	1st to 10th	Night	40% 4 19% 23% 12% 1	1600
	11th to 20th	Night	37% 5% 17% 28% 11% 2	1598
	21st to end	Night	42% 4 15% 26% 11% 1	1782
September	1st to 10th	Night	37% 5% 14% 30% 11% 3	1782
	11th to 20th	Night	39% 4 15% 33% 9%	1969
	21st to end	Night	44% 5% 15% 28% 7%	1980
October	1st to 10th	Night	49% 4 13% 22% 12%	2150
	11th to 20th	Night	58% 3 11% 22% 5%	2160
	21st to end	Night	64% 3 10% 15% 7% 1	2479
November	1st to 10th	Night	51% 5% 14% 22% 8%	2523
	11th to 20th	Night	61% 4 11% 16% 8% 1	2660
	21st to end	Night	65% 5% 12% 13% 5%	2660
December	1st to 10th	Night	66% 4 11% 16% 4	2683
	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% 33% 25% 9% 2	680
July	1st to 10th	Day	11% 11% 31% 31% 13% 3	632
	11th to 20th	Day	15% 10% 31% 26% 14% 5%	618
	21st to end	Day	13% 6% 29% 31% 14% 6%	576
August	1st to 10th	Day	19% 7% 23% 25% 20% 7%	446
	11th to 20th	Day	15% 5% 20% 28% 20% 11%	439
	21st to end	Day	18% 6% 28% 24% 16% 8%	495
September	1st to 10th	Day	16% 11% 26% 27% 15% 6%	450
	11th to 20th	Day	16% 6% 28% 31% 14% 5%	450
	21st to end	Day	27% 11% 22% 24% 12% 4	450
October	1st to 10th	Day	24% 14% 25% 23% 12% 3	450
	11th to 20th	Day	28% 16% 33% 17% 5% 1	450
	21st to end	Day	31% 14% 30% 21% 4 1	390
November	1st to 10th	Day	29% 12% 37% 18% 4	214
	11th to 20th	Day	18% 20% 36% 21% 5% 1	180
	21st to end	Day	31% 15% 29% 17% 8% 1	180
December	1st to 10th	Day	24% 12% 31% 22% 9% 1	180
	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	16% 7% 27% 34% 14% 1	380
July	1st to 10th	Night	8% 4 34% 45% 9% 1	354
	11th to 20th	Night	11% 4 32% 39% 15%	348
	21st to end	Night	10% 3 30% 41% 14% 2	514
August	1st to 10th	Night	12% 4 25% 37% 20% 1	537
	11th to 20th	Night	15% 4 22% 35% 21% 3	534
	21st to end	Night	17% 4 25% 34% 19% 1	594
September	1st to 10th	Night	16% 4 29% 31% 20% 1	702
	11th to 20th	Night	16% 8% 26% 31% 16% 2	720
	21st to end	Night	22% 5% 26% 29% 16% 3	720
October	1st to 10th	Night	26% 7% 28% 28% 10% 1	720
	11th to 20th	Night	32% 10% 28% 23% 7% 1	720
	21st to end	Night	32% 13% 26% 24% 4 1	792
November	1st to 10th	Night	26% 11% 30% 25% 7% 1	740
	11th to 20th	Night	27% 12% 29% 25% 7%	760
	21st to end	Night	30% 11% 28% 24% 6%	760
December	1st to 10th	Night	29% 11% 31% 22% 5% 1	783
	11th to 20th	Night	28% 12% 32% 22% 6%	950
	21st to end	Night	30% 12% 29% 22% 6%	1045

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