Dominion Astrophysical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dominion Astrophysical Observatory clear sky chart from 2005-11-06 to 2024-03-28.

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% 9% 4 7% 6% 4 5% 5% 5% 7% 33%	78365
	Night	18% 10% 4 7% 6% 3 5% 4 5% 6% 33%	81582
Transparency	Day	47% 11% 17% 17% 6% 1	78076
	Night	51% 10% 15% 16% 7% 1	81452
Seeing	Day	17% 8% 22% 27% 21% 5%	14497
	Night	20% 9% 18% 26% 22% 5%	22931

By Month

Percentage Cloud Cover NW (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	26% 14% 5% 9% 8% 5% 5% 5% 5% 5% 15%	2219
	11th to 20th	Day	22% 14% 4 8% 5% 3 6% 4 4 6% 23%	2199
	21st to end	Day	29% 20% 6% 11% 8% 3 4 4 3 4 8%	2900
February	1st to 10th	Day	23% 15% 5% 8% 8% 4 6% 5% 5% 6% 16%	1710
	11th to 20th	Day	20% 15% 5% 9% 6% 4 4 4 4 6% 23%	1665
	21st to end	Day	17% 14% 5% 9% 8% 5% 6% 5% 6% 6% 19%	1420
March	1st to 10th	Day	17% 11% 6% 10% 6% 4 6% 6% 7% 7% 20%	1990
	11th to 20th	Day	20% 13% 4 8% 7% 4 5% 6% 5% 5% 23%	2090
	21st to end	Day	20% 14% 5% 8% 8% 4 6% 5% 6% 5% 18%	2292
April	1st to 10th	Day	18% 11% 3 9% 7% 5% 6% 6% 7% 7% 21%	2160
	11th to 20th	Day	16% 10% 4 8% 7% 5% 6% 6% 6% 7% 24%	2227
	21st to end	Day	17% 9% 4 7% 7% 3 6% 5% 7% 8% 27%	2357
May	1st to 10th	Day	12% 6% 3 7% 7% 5% 5% 6% 7% 7% 34%	2520
	11th to 20th	Day	12% 7% 4 6% 7% 4 6% 6% 6% 7% 36%	2477
	21st to end	Day	13% 8% 4 8% 6% 3 5% 5% 6% 7% 35%	2753
June	1st to 10th	Day	15% 7% 4 6% 5% 3 5% 5% 7% 8% 35%	2700
	11th to 20th	Day	10% 6% 3 8% 8% 4 7% 5% 7% 9% 33%	2700
	21st to end	Day	9% 6% 2 4 6% 4 5% 6% 7% 8% 44%	2700
July	1st to 10th	Day	7% 4 2 5% 4 3 5% 5% 5% 7% 54%	2700
	11th to 20th	Day	6% 3 2 4 4 3 4 5% 4 7% 58%	2700
	21st to end	Day	5% 2 1 2 3 2 3 3 4 6% 71%	2776
August	1st to 10th	Day	6% 4 1 4 3 2 3 4 4 7% 61%	2440
	11th to 20th	Day	6% 3 1 4 4 3 3 4 5% 8% 59%	2298
	21st to end	Day	7% 3 2 4 5% 3 5% 4 6% 9% 50%	2523
September	1st to 10th	Day	7% 6% 2 5% 4 3 3 5% 5% 8% 52%	2160
	11th to 20th	Day	11% 6% 3 6% 6% 4 4 4 6% 7% 44%	2135
	21st to end	Day	16% 7% 2 5% 5% 3 4 5% 5% 7% 41%	1908
October	1st to 10th	Day	18% 6% 3 5% 7% 2 5% 5% 5% 7% 37%	1800
	11th to 20th	Day	20% 10% 5% 8% 6% 3 5% 6% 5% 7% 25%	1800
	21st to end	Day	22% 11% 4 7% 6% 3 4 5% 5% 7% 26%	1864
November	1st to 10th	Day	26% 12% 5% 8% 7% 4 5% 6% 5% 6% 16%	1488
	11th to 20th	Day	24% 11% 5% 7% 7% 3 5% 4 5% 6% 23%	1520
	21st to end	Day	25% 12% 5% 9% 6% 3 5% 5% 4 6% 20%	1520
December	1st to 10th	Day	21% 15% 5% 7% 6% 4 4 4 5% 6% 24%	1520
	11th to 20th	Day	27% 13% 4 8% 8% 5% 6% 5% 5% 6% 12%	1692
	21st to end	Day	21% 17% 7% 11% 8% 4 6% 4 4 5% 13%	2442
January	1st to 10th	Night	23% 14% 5% 10% 8% 4 6% 6% 5% 5% 15%	4726
	11th to 20th	Night	24% 11% 4 6% 6% 3 4 4 5% 6% 26%	4699
	21st to end	Night	30% 16% 5% 8% 7% 3 4 3 4 5% 14%	4755
February	1st to 10th	Night	23% 13% 4 7% 6% 5% 6% 4 4 6% 22%	2470
	11th to 20th	Night	18% 12% 4 7% 6% 4 5% 4 4 6% 29%	2402
	21st to end	Night	20% 13% 4 8% 6% 3 6% 4 5% 6% 25%	2033
March	1st to 10th	Night	17% 10% 4 6% 7% 4 5% 4 6% 7% 29%	2187
	11th to 20th	Night	17% 10% 5% 6% 7% 4 7% 5% 7% 7% 26%	2089
	21st to end	Night	19% 12% 5% 9% 7% 4 5% 5% 5% 6% 25%	2218
April	1st to 10th	Night	19% 8% 4 7% 7% 3 5% 5% 6% 7% 30%	1800
	11th to 20th	Night	15% 8% 3 6% 6% 3 5% 5% 6% 8% 33%	1682
	21st to end	Night	17% 7% 4 7% 5% 3 5% 5% 5% 8% 34%	1602
May	1st to 10th	Night	11% 6% 3 5% 6% 3 5% 5% 5% 6% 46%	1440
	11th to 20th	Night	10% 7% 3 6% 5% 3 5% 6% 7% 8% 42%	1416
	21st to end	Night	12% 7% 3 5% 5% 3 5% 5% 6% 8% 41%	1519
June	1st to 10th	Night	13% 7% 3 6% 6% 3 5% 5% 6% 6% 40%	1260
	11th to 20th	Night	10% 6% 2 6% 5% 3 4 7% 7% 10% 41%	1260
	21st to end	Night	8% 3 2 4 4 3 3 5% 6% 10% 51%	1260
July	1st to 10th	Night	7% 4 2 3 3 3 4 3 4 6% 61%	1260
	11th to 20th	Night	5% 3 2 3 3 2 4 3 4 5% 67%	1260
	21st to end	Night	3 2 1 2 2 1 2 2 3 6% 77%	1538
August	1st to 10th	Night	4 3 1 3 4 2 3 3 4 5% 68%	1496
	11th to 20th	Night	3 2 1 3 2 1 3 4 4 6% 69%	1602
	21st to end	Night	7% 4 1 3 4 2 3 5% 6% 8% 57%	1832
September	1st to 10th	Night	6% 3 1 4 3 2 3 4 5% 6% 64%	1800
	11th to 20th	Night	12% 5% 2 4 5% 3 4 4 4 6% 50%	1822
	21st to end	Night	13% 5% 2 5% 4 3 4 5% 5% 7% 46%	2051
October	1st to 10th	Night	15% 7% 2 6% 6% 3 4 4 4 6% 44%	2160
	11th to 20th	Night	19% 8% 3 5% 6% 3 4 5% 6% 6% 35%	2160
	21st to end	Night	22% 8% 3 5% 6% 3 5% 4 4 6% 35%	2489
November	1st to 10th	Night	25% 11% 3 6% 7% 4 5% 5% 4 5% 24%	2590
	11th to 20th	Night	23% 10% 4 7% 6% 4 5% 5% 5% 5% 26%	2660
	21st to end	Night	20% 11% 5% 8% 7% 4 5% 5% 5% 5% 24%	2660
December	1st to 10th	Night	20% 13% 5% 8% 6% 3 6% 5% 5% 6% 25%	2660
	11th to 20th	Night	24% 14% 5% 9% 7% 5% 5% 5% 4 5% 15%	3510
	21st to end	Night	22% 14% 5% 9% 7% 4 5% 4 4 5% 20%	5214

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	68% 13% 8% 8% 3	2219
	11th to 20th	Day	62% 9% 15% 10% 4	2220
	21st to end	Day	76% 12% 6% 5% 1	2900
February	1st to 10th	Day	66% 11% 10% 9% 4 1	1710
	11th to 20th	Day	61% 12% 11% 12% 5%	1674
	21st to end	Day	61% 11% 14% 9% 4 1	1420
March	1st to 10th	Day	56% 14% 16% 10% 4 1	1990
	11th to 20th	Day	58% 10% 13% 14% 3 1	2090
	21st to end	Day	61% 11% 17% 10% 2	2292
April	1st to 10th	Day	55% 13% 19% 11% 2	2160
	11th to 20th	Day	51% 12% 15% 14% 7% 1	2227
	21st to end	Day	49% 12% 17% 14% 6% 1	2357
May	1st to 10th	Day	42% 12% 22% 17% 6% 1	2520
	11th to 20th	Day	43% 10% 21% 19% 6%	2480
	21st to end	Day	44% 10% 23% 20% 3	2754
June	1st to 10th	Day	43% 11% 24% 15% 6% 1	2700
	11th to 20th	Day	41% 13% 26% 16% 4	2700
	21st to end	Day	34% 10% 25% 21% 8% 2	2700
July	1st to 10th	Day	28% 10% 21% 27% 12% 3	2626
	11th to 20th	Day	23% 9% 25% 30% 13% 1	2619
	21st to end	Day	13% 7% 23% 40% 15% 3	2622
August	1st to 10th	Day	22% 9% 23% 31% 13% 2	2423
	11th to 20th	Day	23% 10% 22% 29% 14% 2	2298
	21st to end	Day	27% 10% 25% 30% 7% 1	2523
September	1st to 10th	Day	28% 11% 22% 24% 13% 3	2160
	11th to 20th	Day	39% 10% 16% 20% 13% 1	2135
	21st to end	Day	43% 11% 18% 18% 9% 1	1908
October	1st to 10th	Day	46% 11% 17% 19% 7%	1800
	11th to 20th	Day	58% 12% 15% 13% 3	1800
	21st to end	Day	56% 11% 13% 13% 7%	1864
November	1st to 10th	Day	66% 9% 13% 8% 3 1	1488
	11th to 20th	Day	63% 8% 12% 11% 4 2	1520
	21st to end	Day	64% 12% 10% 10% 4	1520
December	1st to 10th	Day	62% 11% 10% 11% 5%	1520
	11th to 20th	Day	69% 14% 9% 5% 2 1	1695
	21st to end	Day	69% 13% 10% 5% 3	2442
January	1st to 10th	Night	67% 14% 9% 7% 3	4726
	11th to 20th	Night	57% 9% 16% 14% 3 1	4740
	21st to end	Night	72% 10% 7% 9% 2	4755
February	1st to 10th	Night	63% 9% 13% 9% 5% 1	2470
	11th to 20th	Night	57% 9% 12% 14% 7%	2404
	21st to end	Night	58% 9% 11% 15% 6% 1	2033
March	1st to 10th	Night	54% 11% 15% 16% 5%	2187
	11th to 20th	Night	54% 10% 13% 17% 4 1	2090
	21st to end	Night	60% 10% 14% 14% 2	2218
April	1st to 10th	Night	53% 11% 17% 15% 4	1800
	11th to 20th	Night	48% 10% 15% 19% 7% 2	1682
	21st to end	Night	49% 10% 14% 17% 9% 1	1602
May	1st to 10th	Night	37% 11% 20% 21% 11% 1	1440
	11th to 20th	Night	39% 10% 19% 24% 8% 1	1416
	21st to end	Night	41% 11% 20% 22% 6%	1519
June	1st to 10th	Night	44% 9% 17% 22% 6% 2	1260
	11th to 20th	Night	37% 13% 24% 21% 5%	1260
	21st to end	Night	28% 10% 26% 23% 10% 3	1260
July	1st to 10th	Night	26% 9% 18% 28% 15% 4	1225
	11th to 20th	Night	22% 7% 20% 32% 18% 2	1218
	21st to end	Night	10% 6% 17% 36% 26% 4	1453
August	1st to 10th	Night	20% 7% 20% 30% 20% 3	1488
	11th to 20th	Night	15% 8% 23% 31% 16% 6%	1602
	21st to end	Night	24% 8% 22% 33% 12% 1	1832
September	1st to 10th	Night	21% 9% 20% 29% 17% 4	1800
	11th to 20th	Night	36% 8% 15% 19% 18% 4	1822
	21st to end	Night	39% 9% 16% 23% 11% 2	2051
October	1st to 10th	Night	42% 8% 17% 22% 11%	2160
	11th to 20th	Night	49% 9% 18% 17% 8%	2160
	21st to end	Night	51% 9% 14% 17% 9%	2489
November	1st to 10th	Night	61% 10% 12% 12% 3 1	2590
	11th to 20th	Night	59% 10% 13% 10% 6% 2	2660
	21st to end	Night	61% 10% 12% 10% 6%	2660
December	1st to 10th	Night	61% 8% 13% 12% 6%	2660
	11th to 20th	Night	69% 14% 9% 5% 3	3506
	21st to end	Night	64% 12% 12% 9% 3	5214

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	27% 20% 25% 15% 11% 2	180
	11th to 20th	Day	35% 14% 20% 16% 13% 2	180
	21st to end	Day	33% 14% 24% 20% 8% 1	266
February	1st to 10th	Day	29% 16% 27% 20% 8% 1	370
	11th to 20th	Day	28% 13% 33% 17% 8% 1	365
	21st to end	Day	25% 18% 26% 23% 7% 2	312
March	1st to 10th	Day	20% 15% 28% 22% 13% 2	460
	11th to 20th	Day	22% 15% 22% 24% 14% 3	460
	21st to end	Day	25% 10% 26% 24% 13% 1	491
April	1st to 10th	Day	24% 13% 25% 22% 14% 2	420
	11th to 20th	Day	20% 8% 28% 24% 16% 4	412
	21st to end	Day	20% 7% 28% 25% 17% 3	420
May	1st to 10th	Day	11% 6% 23% 35% 21% 3	423
	11th to 20th	Day	14% 5% 19% 27% 27% 8%	448
	21st to end	Day	12% 5% 23% 30% 25% 5%	515
June	1st to 10th	Day	14% 6% 24% 32% 21% 3	630
	11th to 20th	Day	10% 6% 30% 33% 18% 3	630
	21st to end	Day	8% 3 20% 36% 29% 5%	630
July	1st to 10th	Day	9% 2 15% 29% 37% 8%	621
	11th to 20th	Day	5% 3 17% 34% 32% 9%	618
	21st to end	Day	4 2 12% 28% 42% 13%	522
August	1st to 10th	Day	6% 2 13% 24% 37% 17%	447
	11th to 20th	Day	4 1 12% 31% 42% 10%	437
	21st to end	Day	8% 4 17% 34% 27% 10%	495
September	1st to 10th	Day	10% 4 19% 40% 24% 5%	450
	11th to 20th	Day	13% 6% 18% 30% 26% 7%	450
	21st to end	Day	15% 8% 21% 28% 24% 3	450
October	1st to 10th	Day	19% 6% 23% 25% 22% 4	450
	11th to 20th	Day	24% 11% 19% 26% 17% 4	450
	21st to end	Day	24% 16% 26% 20% 10% 4	397
November	1st to 10th	Day	28% 12% 30% 21% 8% 1	180
	11th to 20th	Day	27% 11% 24% 22% 12% 4	180
	21st to end	Day	29% 14% 23% 16% 10% 8%	180
December	1st to 10th	Day	34% 13% 19% 15% 16% 3	180
	11th to 20th	Day	33% 11% 26% 21% 8% 1	180
	21st to end	Day	31% 20% 21% 20% 7% 2	198
January	1st to 10th	Night	29% 16% 23% 17% 11% 3	850
	11th to 20th	Night	31% 12% 15% 24% 17% 2	850
	21st to end	Night	29% 9% 19% 24% 16% 2	836
February	1st to 10th	Night	29% 15% 19% 21% 12% 3	760
	11th to 20th	Night	23% 13% 21% 26% 15% 1	744
	21st to end	Night	24% 15% 21% 21% 14% 4	628
March	1st to 10th	Night	20% 12% 22% 26% 16% 3	760
	11th to 20th	Night	22% 12% 22% 26% 15% 3	760
	21st to end	Night	22% 14% 18% 27% 16% 3	820
April	1st to 10th	Night	20% 10% 21% 26% 18% 5%	720
	11th to 20th	Night	16% 10% 21% 29% 19% 4	614
	21st to end	Night	19% 5% 22% 34% 18% 3	522
May	1st to 10th	Night	12% 7% 15% 34% 27% 5%	360
	11th to 20th	Night	12% 4 14% 29% 32% 9%	354
	21st to end	Night	12% 1 19% 33% 29% 6%	389
June	1st to 10th	Night	16% 8% 14% 31% 28% 4	360
	11th to 20th	Night	12% 5% 22% 35% 23% 4	360
	21st to end	Night	8% 4 15% 32% 33% 8%	360
July	1st to 10th	Night	8% 1 12% 29% 39% 11%	350
	11th to 20th	Night	5% 1 13% 32% 41% 8%	348
	21st to end	Night	3 1 8% 29% 46% 12%	394
August	1st to 10th	Night	5% 2 7% 25% 40% 21%	422
	11th to 20th	Night	4 1 5% 23% 51% 15%	534
	21st to end	Night	8% 2 12% 31% 38% 9%	644
September	1st to 10th	Night	7% 2 10% 37% 34% 10%	720
	11th to 20th	Night	14% 3 13% 22% 33% 16%	720
	21st to end	Night	15% 4 15% 28% 31% 6%	720
October	1st to 10th	Night	17% 5% 14% 28% 28% 8%	720
	11th to 20th	Night	23% 6% 17% 26% 20% 8%	720
	21st to end	Night	26% 9% 19% 27% 16% 4	792
November	1st to 10th	Night	31% 12% 17% 27% 12% 1	740
	11th to 20th	Night	28% 13% 22% 21% 14% 2	760
	21st to end	Night	25% 12% 21% 23% 15% 4	760
December	1st to 10th	Night	26% 12% 17% 25% 17% 2	760
	11th to 20th	Night	30% 13% 23% 23% 10% 2	845
	21st to end	Night	34% 16% 23% 20% 7% 1	935

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