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 2024-01-16.

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% 4 7% 6% 4 5% 5% 6% 7% 32%	74777
	Night	15% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	74969
Transparency	Day	50% 4 12% 22% 11% 1	74492
	Night	50% 3 11% 22% 12% 1	74789
Seeing	Day	19% 9% 28% 26% 13% 5%	13399
	Night	19% 7% 29% 31% 13% 1	22833

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	15% 11% 5% 11% 8% 4 6% 5% 5% 7% 23%	1517
	11th to 20th	Day	16% 15% 6% 9% 8% 5% 6% 4 5% 6% 20%	1608
	21st to end	Day	12% 14% 8% 10% 9% 4 7% 5% 5% 7% 20%	1782
February	1st to 10th	Day	16% 16% 5% 10% 8% 5% 6% 5% 5% 5% 20%	1620
	11th to 20th	Day	12% 14% 7% 11% 8% 3 4 5% 5% 6% 26%	1573
	21st to end	Day	18% 15% 5% 10% 7% 3 5% 5% 3 5% 26%	1381
March	1st to 10th	Day	15% 10% 5% 9% 7% 4 5% 5% 5% 6% 30%	1977
	11th to 20th	Day	14% 11% 5% 7% 7% 5% 5% 5% 5% 4 32%	1980
	21st to end	Day	19% 10% 3 7% 7% 3 5% 5% 5% 5% 33%	2178
April	1st to 10th	Day	17% 11% 5% 8% 5% 3 4 5% 6% 6% 29%	2128
	11th to 20th	Day	19% 8% 3 7% 5% 3 4 4 5% 6% 36%	2202
	21st to end	Day	21% 7% 3 6% 7% 4 5% 4 5% 5% 33%	2340
May	1st to 10th	Day	17% 8% 3 8% 5% 3 5% 5% 6% 7% 34%	2340
	11th to 20th	Day	14% 7% 3 6% 5% 3 5% 5% 5% 7% 39%	2468
	21st to end	Day	12% 5% 3 6% 5% 3 5% 5% 6% 8% 43%	2725
June	1st to 10th	Day	14% 6% 3 6% 6% 3 6% 5% 6% 9% 37%	2520
	11th to 20th	Day	13% 5% 3 5% 5% 3 4 6% 7% 10% 39%	2520
	21st to end	Day	14% 4 2 5% 6% 4 5% 5% 6% 8% 40%	2612
July	1st to 10th	Day	8% 4 2 5% 5% 3 7% 6% 7% 9% 45%	2520
	11th to 20th	Day	11% 5% 2 5% 6% 4 6% 6% 7% 9% 40%	2520
	21st to end	Day	9% 5% 3 7% 7% 4 6% 6% 8% 10% 34%	2581
August	1st to 10th	Day	9% 5% 2 7% 6% 4 5% 6% 8% 10% 38%	2327
	11th to 20th	Day	9% 6% 2 6% 7% 3 5% 7% 8% 9% 38%	2303
	21st to end	Day	11% 5% 3 6% 5% 4 6% 5% 6% 10% 38%	2479
September	1st to 10th	Day	14% 6% 2 6% 5% 3 5% 6% 6% 7% 41%	2160
	11th to 20th	Day	12% 5% 3 5% 5% 3 5% 5% 5% 9% 43%	2057
	21st to end	Day	13% 5% 4 7% 6% 4 6% 5% 5% 8% 37%	1940
October	1st to 10th	Day	19% 8% 3 6% 6% 3 4 5% 5% 7% 34%	1800
	11th to 20th	Day	21% 8% 3 8% 7% 4 5% 4 5% 7% 28%	1800
	21st to end	Day	24% 11% 4 8% 6% 4 6% 5% 6% 5% 22%	1980
November	1st to 10th	Day	14% 10% 4 8% 5% 3 5% 5% 6% 8% 31%	1601
	11th to 20th	Day	15% 10% 3 9% 8% 4 5% 7% 6% 7% 24%	1520
	21st to end	Day	18% 13% 5% 9% 6% 5% 6% 5% 6% 7% 21%	1520
December	1st to 10th	Day	17% 14% 7% 11% 8% 4 6% 6% 5% 6% 17%	1520
	11th to 20th	Day	19% 13% 5% 8% 7% 4 5% 5% 5% 7% 22%	1906
	21st to end	Day	28% 16% 6% 12% 7% 4 4 3 3 4 12%	2772
January	1st to 10th	Night	18% 13% 5% 10% 7% 4 5% 5% 5% 6% 22%	2649
	11th to 20th	Night	19% 14% 5% 9% 7% 5% 6% 4 5% 6% 20%	2459
	21st to end	Night	13% 12% 6% 9% 7% 4 6% 5% 5% 6% 27%	2574
February	1st to 10th	Night	14% 14% 5% 9% 8% 4 5% 5% 4 6% 24%	2340
	11th to 20th	Night	15% 12% 5% 9% 7% 4 5% 5% 5% 6% 26%	2273
	21st to end	Night	14% 12% 5% 9% 6% 4 5% 5% 6% 6% 28%	1872
March	1st to 10th	Night	13% 8% 5% 8% 7% 4 5% 5% 5% 5% 36%	1970
	11th to 20th	Night	16% 11% 4 8% 5% 3 5% 4 3 7% 35%	1979
	21st to end	Night	16% 8% 4 6% 5% 3 4 4 4 6% 39%	2178
April	1st to 10th	Night	19% 9% 3 6% 5% 3 5% 5% 6% 6% 32%	1832
	11th to 20th	Night	15% 8% 4 6% 5% 4 5% 5% 4 6% 38%	1723
	21st to end	Night	16% 7% 4 7% 5% 3 5% 4 6% 7% 36%	1620
May	1st to 10th	Night	17% 5% 3 5% 5% 3 4 6% 5% 7% 39%	1620
	11th to 20th	Night	14% 5% 3 5% 4 2 5% 4 5% 8% 46%	1425
	21st to end	Night	10% 4 2 4 4 2 4 4 6% 8% 52%	1550
June	1st to 10th	Night	13% 5% 2 5% 6% 3 5% 4 6% 9% 42%	1440
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 5% 6% 9% 44%	1440
	21st to end	Night	12% 5% 2 4 5% 3 4 5% 6% 8% 46%	1315
July	1st to 10th	Night	5% 4 2 4 4 3 4 4 5% 9% 56%	1430
	11th to 20th	Night	10% 4 2 4 4 3 5% 5% 6% 7% 50%	1440
	21st to end	Night	7% 4 2 4 5% 3 5% 5% 6% 8% 50%	1734
August	1st to 10th	Night	7% 5% 2 5% 6% 3 5% 4 7% 8% 50%	1611
	11th to 20th	Night	7% 4 2 4 4 3 4 5% 6% 9% 52%	1598
	21st to end	Night	10% 5% 3 4 4 3 4 5% 6% 7% 49%	1858
September	1st to 10th	Night	10% 4 3 5% 5% 4 4 6% 6% 8% 46%	1800
	11th to 20th	Night	11% 4 2 4 3 2 3 4 5% 7% 53%	1898
	21st to end	Night	15% 6% 3 6% 5% 2 5% 4 5% 7% 43%	2018
October	1st to 10th	Night	16% 7% 3 7% 4 4 5% 5% 5% 7% 37%	2160
	11th to 20th	Night	19% 8% 3 6% 6% 4 5% 5% 6% 7% 32%	2160
	21st to end	Night	21% 9% 4 7% 7% 3 4 4 6% 7% 28%	2376
November	1st to 10th	Night	17% 8% 3 5% 6% 3 5% 4 6% 7% 37%	2475
	11th to 20th	Night	17% 9% 4 7% 6% 3 6% 5% 6% 7% 29%	2660
	21st to end	Night	20% 11% 4 7% 6% 3 6% 5% 5% 7% 26%	2660
December	1st to 10th	Night	20% 13% 5% 9% 6% 3 5% 5% 5% 7% 22%	2660
	11th to 20th	Night	15% 17% 5% 8% 7% 4 5% 5% 5% 6% 22%	3288
	21st to end	Night	30% 13% 5% 9% 7% 3 5% 5% 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	60% 3 13% 15% 8%	1518
	11th to 20th	Day	65% 4 11% 13% 7%	1608
	21st to end	Day	64% 3 7% 14% 10% 1	1782
February	1st to 10th	Day	67% 2 9% 16% 5% 1	1620
	11th to 20th	Day	60% 2 9% 19% 9% 2	1582
	21st to end	Day	61% 3 10% 18% 7% 1	1381
March	1st to 10th	Day	55% 3 12% 18% 11% 1	1980
	11th to 20th	Day	54% 4 8% 20% 13% 2	1980
	21st to end	Day	53% 3 9% 18% 14% 3	2178
April	1st to 10th	Day	54% 3 11% 19% 12% 1	2128
	11th to 20th	Day	50% 2 9% 22% 15% 1	2202
	21st to end	Day	53% 3 10% 20% 12% 3	2340
May	1st to 10th	Day	49% 3 9% 22% 15% 2	2340
	11th to 20th	Day	44% 4 10% 26% 15% 1	2472
	21st to end	Day	39% 4 16% 25% 14% 2	2725
June	1st to 10th	Day	44% 4 12% 23% 16% 1	2520
	11th to 20th	Day	39% 5% 12% 27% 16% 2	2520
	21st to end	Day	41% 4 13% 26% 15% 1	2612
July	1st to 10th	Day	35% 5% 12% 27% 19% 3	2450
	11th to 20th	Day	39% 4 13% 31% 12% 1	2445
	21st to end	Day	43% 5% 13% 23% 14% 1	2437
August	1st to 10th	Day	39% 4 15% 26% 14% 2	2314
	11th to 20th	Day	40% 3 16% 29% 12% 1	2303
	21st to end	Day	40% 5% 13% 30% 13% 1	2479
September	1st to 10th	Day	40% 5% 14% 29% 11% 1	2160
	11th to 20th	Day	37% 4 13% 31% 15% 1	2057
	21st to end	Day	45% 4 13% 28% 9%	1940
October	1st to 10th	Day	49% 3 13% 20% 13% 1	1800
	11th to 20th	Day	55% 3 12% 22% 7% 1	1800
	21st to end	Day	61% 3 12% 17% 6% 1	1980
November	1st to 10th	Day	50% 3 13% 23% 10% 1	1601
	11th to 20th	Day	57% 4 14% 16% 9% 1	1520
	21st to end	Day	60% 3 14% 18% 4	1520
December	1st to 10th	Day	66% 3 12% 14% 4	1520
	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% 2 10% 17% 8%	2649
	11th to 20th	Night	66% 3 11% 13% 6% 1	2459
	21st to end	Night	57% 3 10% 19% 9% 2	2574
February	1st to 10th	Night	61% 3 9% 19% 7% 1	2340
	11th to 20th	Night	58% 3 9% 19% 10% 1	2275
	21st to end	Night	56% 2 11% 21% 9%	1872
March	1st to 10th	Night	51% 2 12% 22% 12% 2	1980
	11th to 20th	Night	53% 2 8% 22% 13% 2	1980
	21st to end	Night	47% 3 8% 24% 16% 3	2178
April	1st to 10th	Night	51% 3 9% 21% 15% 1	1832
	11th to 20th	Night	48% 2 8% 22% 16% 2	1723
	21st to end	Night	47% 3 10% 22% 16% 2	1620
May	1st to 10th	Night	44% 4 9% 24% 17% 3	1620
	11th to 20th	Night	38% 3 9% 26% 19% 4	1426
	21st to end	Night	31% 3 17% 26% 18% 5%	1551
June	1st to 10th	Night	39% 4 10% 27% 18% 2	1440
	11th to 20th	Night	38% 3 11% 24% 23% 2	1440
	21st to end	Night	37% 3 13% 27% 18% 1	1315
July	1st to 10th	Night	27% 3 11% 30% 25% 4	1390
	11th to 20th	Night	31% 4 10% 33% 19% 2	1392
	21st to end	Night	31% 4 15% 31% 16% 3	1638
August	1st to 10th	Night	32% 4 18% 31% 15% 2	1600
	11th to 20th	Night	29% 4 15% 37% 14% 2	1598
	21st to end	Night	35% 3 13% 31% 17% 2	1858
September	1st to 10th	Night	35% 5% 15% 30% 15% 1	1800
	11th to 20th	Night	31% 3 10% 38% 18% 1	1898
	21st to end	Night	41% 3 13% 30% 13% 1	2018
October	1st to 10th	Night	46% 4 10% 23% 15% 1	2160
	11th to 20th	Night	51% 3 11% 25% 9%	2160
	21st to end	Night	55% 3 11% 19% 11%	2376
November	1st to 10th	Night	46% 2 12% 26% 13% 1	2475
	11th to 20th	Night	53% 3 13% 20% 11% 1	2660
	21st to end	Night	57% 4 17% 18% 4	2660
December	1st to 10th	Night	62% 3 11% 17% 6%	2660
	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% 17% 35% 14% 9% 2	228
	21st to end	Day	20% 16% 38% 21% 5% 1	264
February	1st to 10th	Day	23% 17% 43% 14% 3	240
	11th to 20th	Day	26% 14% 44% 14% 3	240
	21st to end	Day	31% 23% 26% 19%	216
March	1st to 10th	Day	21% 13% 37% 21% 8%	420
	11th to 20th	Day	21% 12% 37% 22% 6% 1	420
	21st to end	Day	28% 10% 33% 20% 8% 1	462
April	1st to 10th	Day	21% 13% 35% 22% 8% 1	420
	11th to 20th	Day	26% 12% 29% 19% 9% 3	412
	21st to end	Day	25% 10% 32% 21% 10% 3	420
May	1st to 10th	Day	24% 7% 28% 24% 10% 6%	423
	11th to 20th	Day	18% 7% 27% 30% 13% 6%	447
	21st to end	Day	15% 7% 25% 30% 16% 7%	485
June	1st to 10th	Day	15% 6% 29% 34% 13% 3	450
	11th to 20th	Day	18% 7% 23% 31% 18% 3	450
	21st to end	Day	17% 5% 24% 34% 15% 5%	542
July	1st to 10th	Day	10% 2 20% 34% 26% 8%	445
	11th to 20th	Day	14% 2 21% 31% 21% 10%	444
	21st to end	Day	10% 2 24% 36% 18% 9%	488
August	1st to 10th	Day	12% 3 19% 32% 21% 14%	446
	11th to 20th	Day	10% 3 19% 30% 27% 12%	439
	21st to end	Day	14% 5% 20% 29% 23% 10%	495
September	1st to 10th	Day	14% 5% 23% 27% 19% 12%	450
	11th to 20th	Day	12% 4 22% 27% 19% 17%	450
	21st to end	Day	13% 8% 28% 25% 17% 8%	450
October	1st to 10th	Day	23% 6% 25% 23% 16% 7%	450
	11th to 20th	Day	21% 10% 36% 23% 8% 2	450
	21st to end	Day	28% 11% 31% 24% 6%	495
November	1st to 10th	Day	20% 17% 27% 24% 11% 1	260
	11th to 20th	Day	21% 16% 33% 22% 6% 2	180
	21st to end	Day	25% 12% 40% 21% 2	180
December	1st to 10th	Day	24% 12% 38% 18% 6% 1	180
	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	27% 11% 30% 26% 6% 1	741
	21st to end	Night	18% 7% 37% 29% 8%	792
February	1st to 10th	Night	20% 12% 39% 25% 3	720
	11th to 20th	Night	22% 12% 34% 27% 5% 1	703
	21st to end	Night	20% 11% 40% 24% 5%	592
March	1st to 10th	Night	19% 11% 35% 27% 8%	720
	11th to 20th	Night	24% 11% 34% 24% 7% 1	720
	21st to end	Night	21% 9% 31% 29% 9% 1	792
April	1st to 10th	Night	24% 11% 33% 27% 6%	720
	11th to 20th	Night	19% 8% 35% 27% 10% 1	659
	21st to end	Night	20% 7% 31% 31% 11%	540
May	1st to 10th	Night	19% 3 27% 33% 16% 2	540
	11th to 20th	Night	17% 2 30% 37% 14% 1	364
	21st to end	Night	11% 4 26% 37% 21% 1	389
June	1st to 10th	Night	13% 4 26% 40% 16% 1	360
	11th to 20th	Night	14% 5% 26% 37% 17% 1	360
	21st to end	Night	13% 4 21% 45% 16% 1	360
July	1st to 10th	Night	7% 1 21% 45% 23% 2	350
	11th to 20th	Night	11% 24% 41% 22% 2	348
	21st to end	Night	8% 2 19% 47% 22% 3	554
August	1st to 10th	Night	10% 1 12% 43% 32% 2	537
	11th to 20th	Night	6% 2 14% 44% 31% 3	534
	21st to end	Night	13% 1 20% 36% 26% 3	670
September	1st to 10th	Night	13% 3 20% 36% 22% 5%	720
	11th to 20th	Night	14% 3 19% 33% 27% 5%	720
	21st to end	Night	16% 4 21% 33% 22% 4	720
October	1st to 10th	Night	21% 4 23% 30% 19% 2	720
	11th to 20th	Night	21% 6% 29% 33% 10% 2	720
	21st to end	Night	26% 7% 27% 33% 6%	792
November	1st to 10th	Night	21% 9% 26% 32% 11% 1	740
	11th to 20th	Night	22% 9% 33% 26% 9% 2	760
	21st to end	Night	26% 8% 31% 27% 8%	760
December	1st to 10th	Night	27% 8% 35% 23% 6% 1	760
	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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