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David Dunlap Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the David Dunlap Observatory clear sky chart from 2005-11-06 to 2024-12-06.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	101810
	Night	15% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	97856
Transparency	Day	49% 4 12% 23% 12% 1	105414
	Night	48% 3 12% 23% 12% 1	102341
Seeing	Day	19% 9% 29% 25% 13% 5%	14595
	Night	19% 7% 29% 31% 12% 1	23931

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	17% 12% 4 11% 5% 4 4 3 5% 6% 29%	2517
	11th to 20th	Day	11% 17% 8% 16% 8% 6% 5% 3 4 5% 17%	2744
	21st to end	Day	20% 20% 6% 10% 6% 4 4 3 4 5% 16%	3124
February	1st to 10th	Day	15% 17% 4 12% 7% 6% 5% 4 5% 4 21%	1710
	11th to 20th	Day	11% 15% 6% 13% 7% 4 4 4 4 6% 25%	1663
	21st to end	Day	17% 15% 4 10% 6% 4 5% 4 4 5% 26%	1465
March	1st to 10th	Day	19% 11% 5% 8% 6% 4 3 4 4 5% 31%	3432
	11th to 20th	Day	11% 14% 5% 9% 6% 6% 5% 4 6% 5% 29%	3480
	21st to end	Day	16% 10% 2 7% 5% 4 5% 3 5% 6% 36%	3815
April	1st to 10th	Day	17% 13% 3 9% 5% 4 4 4 6% 6% 29%	2247
	11th to 20th	Day	20% 9% 2 7% 4 4 4 4 5% 6% 35%	2326
	21st to end	Day	21% 8% 2 8% 5% 4 5% 3 5% 5% 33%	2470
May	1st to 10th	Day	18% 8% 3 8% 4 4 5% 4 6% 8% 33%	4108
	11th to 20th	Day	15% 9% 3 8% 5% 5% 4 4 5% 8% 32%	4388
	21st to end	Day	12% 5% 2 5% 5% 4 4 4 5% 8% 46%	4837
June	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 5% 6% 8% 35%	2660
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 5% 7% 10% 38%	2660
	21st to end	Day	14% 5% 2 5% 6% 5% 5% 4 7% 8% 39%	2757
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 11% 38%	4440
	11th to 20th	Day	10% 5% 2 5% 4 5% 6% 4 7% 9% 44%	4440
	21st to end	Day	7% 6% 2 7% 5% 6% 6% 5% 8% 9% 40%	4514
August	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 9% 36%	4104
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 5% 8% 9% 35%	4044
	21st to end	Day	9% 5% 2 7% 5% 6% 5% 4 7% 9% 41%	4353
September	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 4 6% 7% 42%	2280
	11th to 20th	Day	11% 5% 2 5% 5% 4 5% 4 5% 9% 45%	2171
	21st to end	Day	14% 5% 4 8% 6% 5% 6% 4 5% 8% 36%	2047
October	1st to 10th	Day	18% 8% 3 7% 5% 4 4 4 5% 7% 35%	1900
	11th to 20th	Day	20% 8% 3 8% 5% 5% 4 4 6% 7% 30%	1900
	21st to end	Day	23% 11% 4 9% 5% 5% 5% 4 6% 6% 22%	2090
November	1st to 10th	Day	13% 10% 4 9% 5% 5% 5% 4 6% 7% 31%	1686
	11th to 20th	Day	15% 11% 3 11% 6% 6% 6% 5% 6% 7% 25%	1600
	21st to end	Day	17% 14% 5% 10% 5% 6% 5% 4 6% 6% 21%	1600
December	1st to 10th	Day	17% 15% 6% 12% 7% 5% 6% 4 6% 6% 16%	1560
	11th to 20th	Day	19% 14% 4 10% 6% 5% 5% 4 6% 6% 22%	1906
	21st to end	Day	28% 17% 5% 14% 5% 4 4 2 4 4 12%	2772
January	1st to 10th	Night	17% 14% 5% 10% 6% 5% 5% 4 5% 6% 22%	4429
	11th to 20th	Night	14% 13% 5% 13% 9% 7% 6% 4 5% 6% 18%	4212
	21st to end	Night	20% 13% 6% 10% 8% 6% 5% 4 5% 5% 19%	4532
February	1st to 10th	Night	13% 15% 4 10% 7% 5% 5% 4 5% 6% 25%	2470
	11th to 20th	Night	14% 13% 4 10% 6% 5% 5% 4 5% 6% 27%	2403
	21st to end	Night	14% 13% 4 9% 6% 5% 5% 4 6% 6% 29%	1986
March	1st to 10th	Night	17% 10% 4 8% 5% 5% 5% 3 5% 5% 32%	3408
	11th to 20th	Night	13% 10% 3 8% 5% 5% 5% 4 5% 7% 36%	3479
	21st to end	Night	16% 7% 3 6% 4 3 4 3 5% 7% 42%	3817
April	1st to 10th	Night	18% 11% 3 7% 4 5% 4 4 6% 6% 32%	1933
	11th to 20th	Night	16% 8% 3 7% 4 5% 5% 4 5% 6% 38%	1820
	21st to end	Night	16% 8% 3 8% 4 4 5% 3 7% 7% 37%	1710
May	1st to 10th	Night	18% 7% 2 6% 4 4 4 4 5% 6% 39%	2833
	11th to 20th	Night	14% 9% 3 6% 4 4 5% 4 5% 7% 38%	2505
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 51%	2738
June	1st to 10th	Night	13% 6% 2 6% 5% 5% 4 3 6% 8% 41%	1520
	11th to 20th	Night	8% 6% 3 6% 4 5% 5% 5% 7% 10% 43%	1520
	21st to end	Night	12% 5% 2 5% 5% 5% 4 4 7% 7% 45%	1387
July	1st to 10th	Night	6% 5% 2 6% 4 4 4 4 6% 10% 50%	2510
	11th to 20th	Night	11% 4 2 5% 3 3 4 3 6% 8% 51%	2520
	21st to end	Night	7% 4 2 5% 3 4 5% 4 6% 8% 51%	3022
August	1st to 10th	Night	9% 5% 2 6% 5% 4 4 3 6% 7% 47%	2825
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 5% 8% 50%	2785
	21st to end	Night	10% 6% 2 5% 4 4 5% 4 6% 8% 47%	3270
September	1st to 10th	Night	9% 5% 2 6% 4 5% 4 4 6% 8% 47%	1900
	11th to 20th	Night	10% 4 1 5% 3 3 4 3 5% 7% 54%	2004
	21st to end	Night	15% 6% 3 7% 3 4 4 4 5% 7% 42%	2131
October	1st to 10th	Night	16% 7% 3 7% 4 5% 4 4 5% 7% 38%	2280
	11th to 20th	Night	18% 8% 3 8% 5% 5% 5% 4 6% 6% 34%	2280
	21st to end	Night	20% 9% 4 7% 5% 4 4 4 6% 7% 30%	2508
November	1st to 10th	Night	16% 9% 2 6% 5% 4 5% 3 6% 7% 37%	2610
	11th to 20th	Night	17% 10% 3 8% 5% 5% 5% 5% 6% 7% 29%	2800
	21st to end	Night	20% 11% 3 8% 6% 5% 5% 4 6% 7% 25%	2800
December	1st to 10th	Night	19% 15% 5% 10% 6% 4 5% 4 5% 7% 21%	2737
	11th to 20th	Night	15% 18% 4 9% 6% 6% 5% 4 5% 6% 22%	3288
	21st to end	Night	29% 14% 4 10% 6% 5% 5% 4 5% 5% 13%	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	57% 2 19% 14% 8% 1	2518
	11th to 20th	Day	71% 2 10% 12% 5%	2744
	21st to end	Day	72% 2 10% 11% 6% 1	3124
February	1st to 10th	Day	65% 2 9% 18% 5% 1	1710
	11th to 20th	Day	60% 2 9% 19% 8% 1	1672
	21st to end	Day	61% 3 10% 18% 7% 1	1465
March	1st to 10th	Day	57% 2 12% 19% 10% 1	3435
	11th to 20th	Day	56% 2 10% 20% 10% 2	3480
	21st to end	Day	50% 1 10% 21% 16% 2	3815
April	1st to 10th	Day	54% 3 11% 19% 12% 1	2247
	11th to 20th	Day	51% 2 9% 22% 15% 1	2326
	21st to end	Day	53% 2 10% 20% 12% 3	2470
May	1st to 10th	Day	50% 2 10% 21% 15% 2	4120
	11th to 20th	Day	52% 3 14% 22% 9% 1	4392
	21st to end	Day	37% 2 14% 26% 17% 3	4837
June	1st to 10th	Day	45% 4 11% 23% 15% 1	2660
	11th to 20th	Day	38% 5% 13% 26% 17% 2	2660
	21st to end	Day	42% 4 13% 25% 15% 1	2757
July	1st to 10th	Day	38% 5% 14% 26% 16% 2	4370
	11th to 20th	Day	34% 5% 12% 36% 12% 1	4365
	21st to end	Day	38% 8% 13% 23% 18% 1	4370
August	1st to 10th	Day	44% 5% 13% 24% 14% 1	4091
	11th to 20th	Day	41% 5% 15% 27% 11%	4083
	21st to end	Day	36% 5% 12% 35% 11% 1	4353
September	1st to 10th	Day	40% 5% 14% 30% 11% 1	2280
	11th to 20th	Day	36% 4 12% 32% 16% 1	2171
	21st to end	Day	46% 4 13% 27% 10%	2047
October	1st to 10th	Day	39% 5% 14% 27% 15% 1	3160
	11th to 20th	Day	40% 4 14% 27% 15%	3140
	21st to end	Day	54% 5% 13% 21% 7%	3428
November	1st to 10th	Day	50% 4 13% 22% 10% 1	1686
	11th to 20th	Day	57% 4 14% 16% 9% 1	1600
	21st to end	Day	61% 3 14% 18% 4	1600
December	1st to 10th	Day	65% 3 12% 15% 5%	1560
	11th to 20th	Day	62% 4 13% 13% 7%	1906
	21st to end	Day	77% 2 6% 12% 3	2772
January	1st to 10th	Night	62% 1 14% 16% 6% 1	4429
	11th to 20th	Night	67% 2 12% 14% 4 1	4212
	21st to end	Night	67% 2 12% 13% 5% 1	4532
February	1st to 10th	Night	60% 2 9% 21% 6% 1	2470
	11th to 20th	Night	57% 3 9% 20% 10% 1	2405
	21st to end	Night	55% 2 11% 22% 9%	1986
March	1st to 10th	Night	55% 1 10% 24% 9% 1	3418
	11th to 20th	Night	50% 1 11% 23% 13% 2	3480
	21st to end	Night	43% 1 8% 24% 21% 2	3817
April	1st to 10th	Night	51% 3 9% 21% 14% 1	1933
	11th to 20th	Night	49% 3 8% 22% 16% 2	1820
	21st to end	Night	47% 3 11% 21% 16% 2	1710
May	1st to 10th	Night	46% 2 11% 20% 17% 4	2840
	11th to 20th	Night	47% 2 14% 24% 12% 2	2506
	21st to end	Night	33% 2 14% 27% 22% 3	2739
June	1st to 10th	Night	40% 3 10% 27% 18% 2	1520
	11th to 20th	Night	37% 3 12% 24% 22% 2	1520
	21st to end	Night	38% 3 13% 27% 18% 1	1387
July	1st to 10th	Night	30% 4 14% 27% 21% 4	2470
	11th to 20th	Night	30% 5% 12% 36% 16% 1	2472
	21st to end	Night	30% 7% 14% 31% 16% 2	2926
August	1st to 10th	Night	36% 4 13% 30% 15% 2	2814
	11th to 20th	Night	33% 4 15% 35% 12% 1	2812
	21st to end	Night	34% 5% 13% 34% 14% 1	3270
September	1st to 10th	Night	34% 5% 14% 31% 15% 1	1900
	11th to 20th	Night	30% 3 10% 37% 19% 1	2004
	21st to end	Night	42% 3 13% 29% 13% 1	2131
October	1st to 10th	Night	34% 4 8% 29% 25% 1	3800
	11th to 20th	Night	42% 4 13% 28% 13%	3779
	21st to end	Night	40% 5% 15% 28% 11% 1	4120
November	1st to 10th	Night	46% 2 12% 26% 13% 1	2610
	11th to 20th	Night	52% 3 13% 20% 11% 1	2800
	21st to end	Night	57% 4 17% 17% 4	2800
December	1st to 10th	Night	62% 4 11% 17% 6%	2737
	11th to 20th	Night	62% 3 13% 16% 5%	3288
	21st to end	Night	73% 2 11% 10% 3	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	21% 16% 32% 22% 8% 1	180
	11th to 20th	Day	23% 16% 35% 15% 9% 2	243
	21st to end	Day	23% 14% 37% 20% 5% 1	297
February	1st to 10th	Day	21% 16% 42% 16% 4	270
	11th to 20th	Day	24% 15% 43% 15% 3	270
	21st to end	Day	30% 22% 28% 18% 2	244
March	1st to 10th	Day	22% 12% 36% 22% 8%	460
	11th to 20th	Day	21% 13% 36% 23% 6% 1	460
	21st to end	Day	28% 10% 33% 21% 7% 1	506
April	1st to 10th	Day	23% 13% 34% 21% 8% 1	460
	11th to 20th	Day	29% 12% 29% 18% 9% 3	452
	21st to end	Day	25% 10% 33% 20% 10% 2	460
May	1st to 10th	Day	24% 7% 28% 24% 11% 6%	463
	11th to 20th	Day	18% 6% 26% 31% 13% 6%	487
	21st to end	Day	15% 7% 26% 29% 16% 7%	529
June	1st to 10th	Day	16% 7% 28% 32% 13% 3	490
	11th to 20th	Day	17% 7% 24% 32% 17% 3	490
	21st to end	Day	18% 6% 25% 32% 14% 4	587
July	1st to 10th	Day	11% 3 22% 33% 24% 7%	485
	11th to 20th	Day	14% 3 22% 31% 21% 9%	484
	21st to end	Day	10% 3 23% 37% 18% 9%	532
August	1st to 10th	Day	14% 3 20% 30% 19% 13%	486
	11th to 20th	Day	10% 4 20% 28% 26% 11%	479
	21st to end	Day	13% 5% 20% 30% 22% 10%	539
September	1st to 10th	Day	14% 5% 24% 27% 18% 12%	490
	11th to 20th	Day	11% 3 20% 27% 20% 18%	490
	21st to end	Day	15% 8% 28% 25% 17% 8%	490
October	1st to 10th	Day	22% 7% 27% 23% 15% 6%	490
	11th to 20th	Day	20% 10% 37% 22% 9% 2	490
	21st to end	Day	27% 11% 31% 24% 6% 1	539
November	1st to 10th	Day	21% 16% 28% 24% 10% 1	285
	11th to 20th	Day	21% 19% 33% 21% 6% 2	200
	21st to end	Day	24% 17% 39% 20% 2	200
December	1st to 10th	Day	23% 12% 39% 18% 6% 1	190
	11th to 20th	Day	25% 21% 27% 19% 7% 1	180
	21st to end	Day	28% 13% 40% 18% 2	198
January	1st to 10th	Night	24% 11% 35% 23% 6% 1	760
	11th to 20th	Night	26% 11% 30% 26% 6% 1	760
	21st to end	Night	19% 7% 37% 29% 8%	836
February	1st to 10th	Night	20% 12% 38% 27% 3	760
	11th to 20th	Night	21% 12% 34% 28% 4 1	743
	21st to end	Night	19% 11% 40% 24% 6%	628
March	1st to 10th	Night	20% 10% 35% 27% 7%	760
	11th to 20th	Night	24% 11% 34% 24% 6% 1	760
	21st to end	Night	21% 9% 32% 28% 9% 1	836
April	1st to 10th	Night	24% 10% 33% 27% 5%	760
	11th to 20th	Night	20% 8% 36% 26% 9% 1	696
	21st to end	Night	20% 7% 31% 32% 11%	570
May	1st to 10th	Night	19% 3 28% 34% 15% 2	570
	11th to 20th	Night	17% 2 29% 37% 15% 1	384
	21st to end	Night	11% 4 27% 37% 20% 1	411
June	1st to 10th	Night	14% 4 26% 40% 16% 1	380
	11th to 20th	Night	13% 4 26% 37% 18% 1	380
	21st to end	Night	13% 4 21% 45% 16% 1	380
July	1st to 10th	Night	8% 1 22% 45% 22% 2	370
	11th to 20th	Night	11% 23% 42% 22% 2	368
	21st to end	Night	8% 2 18% 48% 22% 2	585
August	1st to 10th	Night	10% 1 13% 43% 31% 2	567
	11th to 20th	Night	7% 2 15% 43% 30% 3	564
	21st to end	Night	13% 1 20% 36% 26% 3	708
September	1st to 10th	Night	12% 4 21% 36% 22% 5%	760
	11th to 20th	Night	13% 2 18% 33% 29% 5%	760
	21st to end	Night	17% 4 21% 34% 21% 4	760
October	1st to 10th	Night	20% 4 24% 30% 19% 3	760
	11th to 20th	Night	20% 6% 28% 33% 11% 2	760
	21st to end	Night	25% 8% 28% 33% 6%	836
November	1st to 10th	Night	22% 9% 27% 31% 11% 1	780
	11th to 20th	Night	22% 9% 33% 26% 9% 2	800
	21st to end	Night	26% 9% 32% 26% 8%	800
December	1st to 10th	Night	26% 9% 35% 23% 6% 1	783
	11th to 20th	Night	22% 11% 37% 24% 5% 1	760
	21st to end	Night	32% 8% 34% 23% 3	836

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