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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 2026-03-18.

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 9% 5% 5% 5% 4 6% 7% 32%	131565
	Night	14% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	129846
Transparency	Day	50% 4 12% 22% 12% 1	135158
	Night	49% 4 11% 22% 13% 1	134340
Seeing	Day	19% 10% 29% 25% 12% 5%	16158
	Night	19% 8% 29% 31% 12% 1	25593

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% 10% 4 13% 6% 5% 5% 4 6% 6% 24%	4661
	11th to 20th	Day	9% 17% 9% 17% 9% 7% 6% 3 4 4 13%	5066
	21st to end	Day	13% 15% 6% 13% 8% 6% 6% 4 5% 6% 19%	5780
February	1st to 10th	Day	15% 17% 4 12% 7% 6% 5% 4 5% 4 22%	1890
	11th to 20th	Day	12% 15% 6% 13% 7% 4 4 4 5% 6% 24%	1843
	21st to end	Day	16% 15% 4 11% 6% 4 5% 4 4 5% 26%	1615
March	1st to 10th	Day	19% 10% 4 8% 6% 4 4 4 4 5% 32%	5042
	11th to 20th	Day	11% 12% 5% 9% 6% 5% 5% 4 6% 7% 31%	5057
	21st to end	Day	18% 11% 3 8% 5% 4 4 3 5% 5% 33%	5465
April	1st to 10th	Day	17% 13% 3 9% 5% 4 4 4 6% 6% 29%	2365
	11th to 20th	Day	19% 10% 3 8% 4 4 4 3 5% 5% 35%	2450
	21st to end	Day	20% 8% 2 8% 5% 4 5% 3 5% 6% 33%	2600
May	1st to 10th	Day	20% 8% 3 7% 4 4 5% 4 6% 7% 33%	5888
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 5% 8% 34%	6308
	21st to end	Day	18% 6% 2 6% 5% 4 4 4 5% 8% 38%	6949
June	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 5% 6% 8% 36%	2800
	11th to 20th	Day	12% 5% 3 6% 4 4 5% 5% 7% 10% 38%	2800
	21st to end	Day	14% 5% 2 5% 6% 5% 5% 4 7% 8% 38%	2902
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 4 7% 10% 38%	6360
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 6% 9% 46%	6360
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 8% 45%	6500
August	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 9% 37%	4234
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 5% 8% 9% 35%	4174
	21st to end	Day	9% 5% 2 7% 5% 6% 5% 4 7% 9% 41%	4490
September	1st to 10th	Day	13% 6% 2 7% 4 5% 4 4 6% 7% 43%	2400
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 5% 9% 46%	2285
	21st to end	Day	14% 5% 4 8% 5% 5% 6% 4 5% 8% 36%	2155
October	1st to 10th	Day	17% 8% 2 7% 4 4 4 4 5% 7% 37%	2000
	11th to 20th	Day	20% 8% 3 8% 5% 5% 4 4 6% 7% 30%	2000
	21st to end	Day	23% 11% 4 9% 5% 5% 5% 4 6% 6% 22%	2200
November	1st to 10th	Day	14% 11% 4 9% 5% 5% 5% 4 6% 7% 31%	1772
	11th to 20th	Day	15% 11% 3 11% 6% 6% 6% 5% 6% 7% 25%	1680
	21st to end	Day	17% 15% 5% 11% 5% 6% 5% 4 6% 6% 20%	1680
December	1st to 10th	Day	18% 17% 5% 12% 7% 6% 6% 3 6% 6% 15%	2680
	11th to 20th	Day	20% 17% 5% 10% 6% 4 4 3 5% 5% 21%	3066
	21st to end	Day	26% 16% 6% 14% 6% 4 4 2 3 4 14%	4048
January	1st to 10th	Night	14% 11% 5% 11% 6% 6% 6% 4 6% 7% 25%	8234
	11th to 20th	Night	11% 15% 6% 14% 10% 7% 6% 4 5% 5% 16%	7801
	21st to end	Night	12% 12% 5% 11% 7% 6% 6% 5% 6% 7% 22%	8401
February	1st to 10th	Night	13% 15% 4 10% 7% 5% 5% 4 5% 6% 26%	2730
	11th to 20th	Night	14% 13% 4 11% 6% 5% 5% 4 5% 7% 26%	2663
	21st to end	Night	13% 13% 4 10% 6% 5% 5% 4 6% 6% 28%	2188
March	1st to 10th	Night	15% 10% 4 9% 5% 5% 5% 4 5% 5% 31%	5018
	11th to 20th	Night	10% 8% 3 8% 5% 5% 5% 5% 7% 8% 36%	5057
	21st to end	Night	21% 7% 3 6% 4 4 4 3 5% 6% 38%	5467
April	1st to 10th	Night	18% 10% 3 8% 4 5% 5% 4 6% 6% 31%	2035
	11th to 20th	Night	16% 9% 3 7% 4 5% 5% 4 5% 6% 38%	1917
	21st to end	Night	15% 8% 3 8% 4 4 5% 4 7% 7% 37%	1800
May	1st to 10th	Night	21% 8% 3 6% 4 4 4 4 5% 6% 35%	4053
	11th to 20th	Night	13% 8% 4 7% 4 4 5% 4 6% 7% 39%	3585
	21st to end	Night	17% 6% 2 5% 3 3 4 3 5% 7% 44%	3926
June	1st to 10th	Night	13% 6% 2 6% 5% 5% 4 3 6% 8% 42%	1600
	11th to 20th	Night	8% 6% 3 6% 4 5% 5% 5% 6% 9% 43%	1600
	21st to end	Night	11% 5% 2 6% 5% 5% 4 4 7% 7% 44%	1460
July	1st to 10th	Night	6% 6% 2 6% 4 4 5% 4 6% 10% 47%	3577
	11th to 20th	Night	9% 4 2 5% 4 4 4 3 6% 8% 50%	3600
	21st to end	Night	8% 4 2 5% 3 4 4 4 5% 8% 52%	4335
August	1st to 10th	Night	9% 5% 2 6% 5% 4 4 3 6% 7% 48%	2915
	11th to 20th	Night	11% 4 2 5% 4 3 4 4 5% 7% 50%	2875
	21st to end	Night	9% 6% 2 5% 4 4 5% 4 6% 8% 47%	3374
September	1st to 10th	Night	9% 5% 2 6% 4 5% 4 4 6% 8% 48%	2000
	11th to 20th	Night	10% 4 1 4 3 3 3 3 5% 7% 55%	2110
	21st to end	Night	16% 6% 3 7% 3 4 4 3 5% 7% 42%	2243
October	1st to 10th	Night	15% 7% 3 7% 4 5% 4 4 5% 7% 40%	2400
	11th to 20th	Night	18% 8% 3 8% 5% 5% 4 4 6% 6% 34%	2400
	21st to end	Night	19% 9% 3 7% 5% 4 4 4 6% 7% 31%	2640
November	1st to 10th	Night	16% 9% 2 6% 5% 4 5% 3 6% 7% 36%	2744
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 5% 6% 7% 29%	2940
	21st to end	Night	20% 12% 3 8% 6% 5% 5% 4 6% 7% 24%	2940
December	1st to 10th	Night	16% 15% 5% 11% 7% 5% 6% 4 6% 7% 18%	4720
	11th to 20th	Night	15% 17% 4 10% 6% 5% 5% 4 5% 5% 22%	5348
	21st to end	Night	25% 14% 6% 13% 6% 4 4 3 5% 5% 15%	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	57% 4 12% 15% 10%	4662
	11th to 20th	Day	73% 3 8% 11% 5%	5100
	21st to end	Day	66% 3 10% 13% 9%	5780
February	1st to 10th	Day	64% 2 9% 18% 6% 1	1890
	11th to 20th	Day	62% 2 8% 19% 8% 1	1852
	21st to end	Day	61% 3 10% 18% 7% 1	1615
March	1st to 10th	Day	54% 3 13% 16% 14% 1	5045
	11th to 20th	Day	52% 3 14% 19% 9% 3	5057
	21st to end	Day	53% 2 8% 20% 15% 1	5465
April	1st to 10th	Day	55% 3 10% 19% 12% 1	2365
	11th to 20th	Day	51% 2 9% 22% 14% 1	2450
	21st to end	Day	52% 3 10% 20% 12% 3	2600
May	1st to 10th	Day	51% 3 11% 19% 12% 5%	5897
	11th to 20th	Day	50% 4 11% 18% 16% 1	6312
	21st to end	Day	45% 3 12% 24% 14% 3	6949
June	1st to 10th	Day	45% 4 11% 23% 16% 1	2800
	11th to 20th	Day	39% 5% 13% 26% 16% 1	2800
	21st to end	Day	42% 4 13% 25% 15% 1	2902
July	1st to 10th	Day	39% 6% 13% 26% 15% 2	6290
	11th to 20th	Day	33% 5% 11% 34% 16% 1	6285
	21st to end	Day	33% 7% 12% 30% 18% 1	6314
August	1st to 10th	Day	43% 5% 13% 23% 14% 1	4221
	11th to 20th	Day	41% 6% 15% 27% 11%	4213
	21st to end	Day	36% 5% 12% 36% 11% 1	4490
September	1st to 10th	Day	39% 5% 14% 30% 11% 1	2400
	11th to 20th	Day	35% 4 12% 33% 15%	2285
	21st to end	Day	46% 4 13% 27% 10%	2155
October	1st to 10th	Day	38% 5% 13% 28% 15% 1	3260
	11th to 20th	Day	40% 4 14% 27% 15%	3240
	21st to end	Day	53% 5% 13% 21% 7%	3538
November	1st to 10th	Day	51% 4 12% 23% 9% 1	1772
	11th to 20th	Day	56% 4 14% 17% 9% 1	1680
	21st to end	Day	61% 4 13% 18% 4	1680
December	1st to 10th	Day	67% 5% 13% 11% 4	2680
	11th to 20th	Day	64% 4 12% 11% 9%	3066
	21st to end	Day	75% 3 5% 10% 7%	4048
January	1st to 10th	Night	56% 5% 12% 20% 7%	8234
	11th to 20th	Night	66% 4 9% 15% 6% 1	7842
	21st to end	Night	58% 4 10% 14% 13% 1	8401
February	1st to 10th	Night	58% 3 9% 22% 7% 1	2730
	11th to 20th	Night	58% 3 9% 20% 10% 1	2665
	21st to end	Night	55% 3 11% 21% 9%	2188
March	1st to 10th	Night	54% 3 9% 19% 13% 2	5028
	11th to 20th	Night	45% 2 16% 23% 11% 2	5058
	21st to end	Night	48% 2 8% 21% 19% 3	5467
April	1st to 10th	Night	52% 3 9% 20% 14% 1	2035
	11th to 20th	Night	49% 3 8% 22% 16% 2	1917
	21st to end	Night	47% 3 10% 21% 17% 2	1800
May	1st to 10th	Night	50% 3 10% 16% 14% 6%	4060
	11th to 20th	Night	45% 3 12% 22% 14% 3	3586
	21st to end	Night	41% 3 14% 25% 16% 2	3927
June	1st to 10th	Night	39% 3 10% 27% 18% 2	1600
	11th to 20th	Night	37% 3 11% 25% 22% 2	1600
	21st to end	Night	38% 3 14% 26% 18% 1	1460
July	1st to 10th	Night	32% 5% 14% 26% 20% 3	3537
	11th to 20th	Night	30% 6% 11% 33% 19% 1	3552
	21st to end	Night	29% 6% 11% 31% 21% 1	4207
August	1st to 10th	Night	35% 4 13% 29% 16% 2	2904
	11th to 20th	Night	33% 4 15% 35% 13% 1	2902
	21st to end	Night	33% 5% 13% 34% 14% 1	3374
September	1st to 10th	Night	33% 5% 14% 32% 15% 1	2000
	11th to 20th	Night	29% 3 10% 38% 19% 1	2110
	21st to end	Night	42% 3 12% 29% 13% 1	2243
October	1st to 10th	Night	33% 4 8% 30% 25% 1	3920
	11th to 20th	Night	42% 4 13% 28% 13%	3899
	21st to end	Night	39% 5% 15% 28% 12% 1	4252
November	1st to 10th	Night	47% 2 12% 26% 13% 1	2744
	11th to 20th	Night	52% 4 12% 20% 11% 1	2940
	21st to end	Night	58% 4 16% 17% 4	2940
December	1st to 10th	Night	62% 6% 12% 16% 5%	4720
	11th to 20th	Night	61% 5% 10% 14% 9% 1	5348
	21st to end	Night	71% 3 10% 10% 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	21% 19% 32% 20% 7% 1	220
	11th to 20th	Day	24% 17% 35% 15% 7% 2	297
	21st to end	Day	23% 16% 36% 20% 4 1	363
February	1st to 10th	Day	23% 17% 38% 18% 4 1	330
	11th to 20th	Day	25% 18% 42% 14% 2	330
	21st to end	Day	30% 20% 31% 17% 2	294
March	1st to 10th	Day	22% 13% 36% 22% 7%	540
	11th to 20th	Day	22% 16% 35% 21% 5% 1	528
	21st to end	Day	29% 11% 33% 19% 7% 1	550
April	1st to 10th	Day	24% 15% 33% 20% 7% 1	500
	11th to 20th	Day	28% 14% 29% 18% 9% 3	492
	21st to end	Day	25% 12% 33% 20% 9% 2	500
May	1st to 10th	Day	24% 9% 29% 23% 10% 5%	503
	11th to 20th	Day	17% 8% 27% 30% 13% 6%	527
	21st to end	Day	16% 8% 26% 28% 16% 6%	573
June	1st to 10th	Day	16% 8% 30% 31% 12% 3	530
	11th to 20th	Day	16% 7% 26% 31% 16% 3	530
	21st to end	Day	18% 7% 26% 32% 14% 4	632
July	1st to 10th	Day	10% 4 23% 32% 23% 7%	525
	11th to 20th	Day	13% 4 22% 31% 20% 9%	524
	21st to end	Day	9% 3 25% 37% 18% 9%	576
August	1st to 10th	Day	13% 3 21% 32% 19% 12%	526
	11th to 20th	Day	11% 4 21% 28% 26% 11%	519
	21st to end	Day	13% 5% 20% 31% 21% 9%	583
September	1st to 10th	Day	14% 6% 24% 26% 19% 12%	530
	11th to 20th	Day	10% 3 19% 28% 22% 18%	530
	21st to end	Day	15% 7% 27% 26% 16% 7%	530
October	1st to 10th	Day	21% 7% 26% 24% 16% 6%	530
	11th to 20th	Day	21% 11% 37% 22% 8% 2	530
	21st to end	Day	26% 12% 31% 25% 5% 1	583
November	1st to 10th	Day	22% 16% 30% 23% 9% 1	311
	11th to 20th	Day	19% 21% 33% 19% 6% 2	220
	21st to end	Day	24% 17% 38% 18% 2	220
December	1st to 10th	Day	25% 13% 37% 18% 5% 1	220
	11th to 20th	Day	29% 20% 27% 18% 6%	220
	21st to end	Day	29% 14% 37% 17% 2	242
January	1st to 10th	Night	23% 11% 36% 23% 6% 1	840
	11th to 20th	Night	24% 13% 31% 25% 5%	840
	21st to end	Night	19% 9% 37% 28% 7%	924
February	1st to 10th	Night	20% 12% 37% 28% 3	840
	11th to 20th	Night	22% 13% 34% 27% 4	823
	21st to end	Night	19% 12% 40% 24% 5%	692
March	1st to 10th	Night	20% 12% 34% 27% 7%	840
	11th to 20th	Night	24% 12% 34% 24% 6% 1	829
	21st to end	Night	21% 10% 31% 28% 9% 1	880
April	1st to 10th	Night	25% 11% 33% 26% 5%	800
	11th to 20th	Night	20% 9% 35% 27% 9% 1	733
	21st to end	Night	20% 7% 31% 31% 10%	600
May	1st to 10th	Night	20% 3 28% 33% 15% 2	600
	11th to 20th	Night	17% 2 29% 37% 14%	404
	21st to end	Night	12% 4 27% 36% 19% 1	433
June	1st to 10th	Night	14% 4 26% 40% 15% 1	400
	11th to 20th	Night	13% 5% 27% 37% 18% 1	400
	21st to end	Night	13% 5% 22% 45% 16% 1	400
July	1st to 10th	Night	7% 1 23% 45% 22% 2	390
	11th to 20th	Night	11% 1 23% 43% 21% 2	388
	21st to end	Night	8% 2 19% 47% 22% 2	616
August	1st to 10th	Night	10% 1 12% 43% 32% 2	597
	11th to 20th	Night	7% 2 16% 43% 30% 3	594
	21st to end	Night	13% 2 21% 36% 26% 3	746
September	1st to 10th	Night	12% 4 21% 36% 23% 5%	800
	11th to 20th	Night	13% 2 17% 34% 29% 5%	800
	21st to end	Night	18% 4 21% 34% 21% 4	800
October	1st to 10th	Night	19% 4 24% 31% 20% 3	800
	11th to 20th	Night	21% 6% 29% 33% 10% 2	800
	21st to end	Night	24% 8% 28% 34% 6%	880
November	1st to 10th	Night	21% 10% 27% 31% 10% 1	820
	11th to 20th	Night	21% 10% 33% 26% 8% 2	840
	21st to end	Night	26% 9% 32% 25% 7%	840
December	1st to 10th	Night	26% 9% 35% 24% 6% 1	840
	11th to 20th	Night	22% 12% 38% 23% 5%	840
	21st to end	Night	31% 9% 34% 23% 3	924

Page generated at 2026-03-15 02:23:56. Version ac07

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