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 2024-11-04.

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	14% 9% 3 8% 5% 4 5% 4 5% 7% 37%	105534
	Night	16% 9% 3 7% 5% 5% 4 3 5% 6% 36%	102087
Transparency	Day	43% 10% 18% 19% 8% 1	105284
	Night	48% 10% 15% 18% 8% 1	101984
Seeing	Day	16% 8% 22% 27% 21% 5%	15421
	Night	20% 9% 18% 26% 22% 5%	23582

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	25% 15% 5% 10% 7% 6% 5% 4 5% 5% 15%	2219
	11th to 20th	Day	22% 15% 4 9% 5% 5% 5% 3 4 6% 23%	2199
	21st to end	Day	29% 21% 5% 12% 6% 5% 3 3 3 4 8%	2900
February	1st to 10th	Day	22% 15% 5% 9% 7% 6% 5% 3 6% 6% 16%	1710
	11th to 20th	Day	20% 16% 4 10% 5% 4 4 3 4 6% 23%	1665
	21st to end	Day	17% 15% 4 10% 7% 6% 5% 4 6% 6% 19%	1420
March	1st to 10th	Day	12% 12% 5% 12% 7% 7% 7% 5% 8% 8% 18%	3288
	11th to 20th	Day	13% 10% 4 8% 5% 5% 5% 4 5% 6% 35%	3490
	21st to end	Day	18% 14% 4 10% 7% 5% 6% 3 6% 6% 22%	3899
April	1st to 10th	Day	17% 13% 3 10% 7% 6% 6% 5% 7% 7% 20%	2280
	11th to 20th	Day	15% 11% 3 9% 6% 6% 6% 5% 6% 7% 26%	2353
	21st to end	Day	17% 10% 4 8% 6% 5% 5% 4 7% 7% 28%	2488
May	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 4 6% 6% 41%	4420
	11th to 20th	Day	7% 6% 2 6% 5% 4 6% 4 7% 11% 42%	4377
	21st to end	Day	16% 10% 4 9% 5% 5% 5% 4 6% 7% 29%	4871
June	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 5% 7% 7% 35%	2850
	11th to 20th	Day	10% 6% 3 9% 7% 6% 6% 4 7% 8% 34%	2850
	21st to end	Day	9% 6% 2 5% 5% 5% 6% 5% 7% 8% 43%	2850
July	1st to 10th	Day	4 3 1 3 2 2 3 3 4 5% 69%	4740
	11th to 20th	Day	4 2 1 3 2 2 2 3 3 5% 73%	4740
	21st to end	Day	8% 4 1 3 3 3 3 3 4 7% 63%	4853
August	1st to 10th	Day	4 2 1 3 2 2 3 3 3 6% 72%	4282
	11th to 20th	Day	7% 6% 3 8% 5% 5% 4 4 6% 8% 44%	4026
	21st to end	Day	10% 5% 2 6% 5% 4 4 3 6% 8% 46%	4431
September	1st to 10th	Day	7% 5% 2 5% 3 4 4 4 5% 9% 52%	2280
	11th to 20th	Day	11% 6% 2 7% 5% 5% 4 4 6% 7% 43%	2253
	21st to end	Day	16% 7% 2 7% 4 4 4 4 6% 7% 40%	2014
October	1st to 10th	Day	15% 7% 2 7% 6% 5% 7% 6% 7% 8% 31%	3180
	11th to 20th	Day	23% 11% 4 9% 5% 5% 5% 4 5% 6% 24%	3161
	21st to end	Day	17% 11% 4 11% 6% 5% 5% 5% 6% 7% 24%	3239
November	1st to 10th	Day	25% 13% 4 9% 6% 5% 5% 5% 6% 5% 16%	1512
	11th to 20th	Day	24% 13% 3 8% 6% 5% 4 4 5% 5% 23%	1520
	21st to end	Day	25% 13% 4 9% 6% 4 5% 4 4 6% 20%	1520
December	1st to 10th	Day	20% 17% 4 8% 5% 4 4 3 5% 6% 23%	1520
	11th to 20th	Day	26% 14% 4 10% 7% 6% 5% 5% 5% 6% 12%	1692
	21st to end	Day	20% 18% 6% 12% 7% 6% 5% 3 4 5% 13%	2442
January	1st to 10th	Night	23% 14% 4 12% 7% 6% 6% 4 5% 5% 14%	4726
	11th to 20th	Night	24% 12% 4 8% 5% 4 4 3 5% 6% 26%	4699
	21st to end	Night	30% 16% 5% 10% 6% 4 4 3 4 4 14%	4755
February	1st to 10th	Night	22% 13% 4 9% 5% 6% 5% 3 4 6% 22%	2470
	11th to 20th	Night	18% 12% 4 9% 5% 5% 5% 3 5% 6% 29%	2402
	21st to end	Night	19% 14% 4 9% 6% 5% 5% 3 5% 6% 25%	2033
March	1st to 10th	Night	13% 10% 3 8% 7% 7% 6% 5% 7% 8% 26%	3583
	11th to 20th	Night	11% 7% 3 5% 4 5% 5% 4 7% 8% 40%	3489
	21st to end	Night	18% 13% 4 10% 6% 5% 5% 3 4 6% 25%	3753
April	1st to 10th	Night	18% 9% 4 8% 6% 5% 4 4 6% 7% 29%	1900
	11th to 20th	Night	14% 9% 3 7% 5% 4 5% 5% 6% 7% 34%	1776
	21st to end	Night	17% 8% 4 8% 5% 4 5% 4 5% 8% 33%	1691
May	1st to 10th	Night	12% 7% 2 5% 4 3 3 3 4 5% 52%	2540
	11th to 20th	Night	6% 4 2 5% 4 4 4 3 6% 8% 54%	2516
	21st to end	Night	13% 8% 3 7% 5% 6% 6% 4 6% 7% 35%	2701
June	1st to 10th	Night	13% 8% 3 6% 6% 3 5% 4 6% 6% 40%	1330
	11th to 20th	Night	10% 6% 2 7% 5% 4 4 6% 7% 9% 42%	1330
	21st to end	Night	8% 4 2 5% 3 4 3 4 6% 10% 51%	1330
July	1st to 10th	Night	4 3 1 3 2 3 3 2 4 5% 72%	2220
	11th to 20th	Night	3 2 1 2 2 1 2 2 3 4 79%	2220
	21st to end	Night	7% 4 1 3 2 3 4 3 3 5% 66%	2696
August	1st to 10th	Night	2 2 1 2 2 1 2 2 4 6% 76%	2649
	11th to 20th	Night	7% 6% 2 7% 4 4 4 4 5% 7% 51%	2812
	21st to end	Night	10% 3 1 4 3 4 4 3 5% 8% 55%	3238
September	1st to 10th	Night	5% 2 1 4 2 3 4 3 5% 6% 64%	1900
	11th to 20th	Night	12% 5% 2 5% 5% 3 4 3 4 6% 49%	1924
	21st to end	Night	13% 5% 2 6% 4 4 4 4 5% 7% 46%	2165
October	1st to 10th	Night	11% 6% 2 7% 5% 5% 4 4 5% 7% 44%	3800
	11th to 20th	Night	20% 8% 4 7% 5% 5% 4 4 6% 6% 32%	3777
	21st to end	Night	18% 10% 3 9% 5% 5% 5% 3 5% 6% 31%	4320
November	1st to 10th	Night	24% 12% 3 8% 6% 5% 5% 4 5% 5% 23%	2638
	11th to 20th	Night	23% 11% 3 8% 5% 5% 5% 4 5% 5% 26%	2660
	21st to end	Night	20% 13% 5% 8% 6% 5% 5% 4 5% 5% 24%	2660
December	1st to 10th	Night	19% 15% 4 8% 6% 5% 5% 4 5% 6% 25%	2660
	11th to 20th	Night	24% 15% 5% 11% 6% 6% 5% 4 4 5% 15%	3510
	21st to end	Night	21% 15% 5% 11% 6% 5% 5% 3 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	49% 18% 16% 11% 4	3288
	11th to 20th	Day	45% 11% 16% 25% 3 1	3490
	21st to end	Day	56% 13% 18% 11% 2	3899
April	1st to 10th	Day	55% 13% 18% 11% 2	2280
	11th to 20th	Day	49% 12% 15% 14% 7% 2	2353
	21st to end	Day	49% 12% 17% 14% 6% 1	2488
May	1st to 10th	Day	37% 9% 22% 23% 9% 1	4420
	11th to 20th	Day	29% 13% 32% 20% 6%	4380
	21st to end	Day	48% 13% 23% 14% 3	4872
June	1st to 10th	Day	43% 11% 24% 15% 6% 1	2850
	11th to 20th	Day	40% 13% 26% 17% 4	2850
	21st to end	Day	34% 10% 25% 21% 8% 2	2850
July	1st to 10th	Day	17% 7% 12% 29% 26% 10%	4666
	11th to 20th	Day	14% 5% 14% 36% 29% 2	4659
	21st to end	Day	21% 6% 27% 29% 15% 2	4700
August	1st to 10th	Day	14% 6% 17% 43% 18% 2	4265
	11th to 20th	Day	33% 10% 27% 20% 8% 1	4065
	21st to end	Day	34% 8% 22% 26% 9%	4430
September	1st to 10th	Day	28% 11% 23% 23% 13% 3	2280
	11th to 20th	Day	39% 11% 18% 19% 13% 1	2253
	21st to end	Day	43% 11% 18% 18% 9% 1	2014
October	1st to 10th	Day	46% 10% 19% 21% 4	3180
	11th to 20th	Day	60% 9% 12% 14% 5%	3161
	21st to end	Day	55% 10% 16% 12% 6%	3239
November	1st to 10th	Day	66% 9% 13% 8% 3 1	1512
	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	50% 16% 15% 14% 5%	3583
	11th to 20th	Night	38% 12% 18% 27% 4 1	3490
	21st to end	Night	59% 12% 14% 14% 1	3753
April	1st to 10th	Night	53% 11% 17% 15% 4	1900
	11th to 20th	Night	46% 10% 15% 19% 7% 2	1776
	21st to end	Night	50% 10% 14% 17% 9% 1	1691
May	1st to 10th	Night	33% 9% 16% 27% 14% 1	2540
	11th to 20th	Night	26% 10% 28% 28% 8%	2516
	21st to end	Night	45% 13% 20% 17% 4	2701
June	1st to 10th	Night	44% 9% 17% 22% 7% 2	1330
	11th to 20th	Night	36% 13% 24% 21% 6%	1330
	21st to end	Night	29% 10% 25% 23% 10% 3	1330
July	1st to 10th	Night	17% 6% 12% 29% 23% 13%	2185
	11th to 20th	Night	12% 4 12% 34% 36% 2	2178
	21st to end	Night	22% 5% 15% 31% 24% 3	2611
August	1st to 10th	Night	12% 5% 16% 37% 27% 3	2641
	11th to 20th	Night	31% 8% 27% 21% 9% 3	2839
	21st to end	Night	28% 7% 21% 32% 12%	3238
September	1st to 10th	Night	20% 9% 20% 29% 18% 5%	1900
	11th to 20th	Night	37% 8% 15% 20% 17% 4	1924
	21st to end	Night	39% 9% 17% 23% 11% 2	2165
October	1st to 10th	Night	40% 7% 21% 23% 8%	3800
	11th to 20th	Night	52% 6% 17% 20% 5%	3777
	21st to end	Night	52% 8% 15% 15% 8%	4320
November	1st to 10th	Night	61% 10% 12% 12% 3 1	2638
	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% 27% 24% 13% 2	506
April	1st to 10th	Day	23% 13% 27% 23% 13% 2	460
	11th to 20th	Day	19% 9% 29% 24% 15% 3	452
	21st to end	Day	21% 8% 28% 24% 16% 3	460
May	1st to 10th	Day	11% 6% 22% 36% 21% 3	463
	11th to 20th	Day	13% 6% 19% 29% 26% 8%	488
	21st to end	Day	13% 6% 23% 29% 24% 5%	561
June	1st to 10th	Day	14% 7% 23% 33% 20% 3	680
	11th to 20th	Day	10% 7% 29% 33% 18% 3	680
	21st to end	Day	8% 4 20% 36% 28% 5%	680
July	1st to 10th	Day	9% 1 16% 30% 36% 8%	671
	11th to 20th	Day	4 3 16% 34% 34% 9%	668
	21st to end	Day	4 3 13% 29% 40% 12%	568
August	1st to 10th	Day	5% 2 12% 24% 39% 16%	487
	11th to 20th	Day	4 2 12% 31% 41% 10%	477
	21st to end	Day	9% 4 17% 33% 27% 10%	539
September	1st to 10th	Day	9% 3 18% 38% 27% 6%	490
	11th to 20th	Day	13% 6% 19% 31% 24% 7%	490
	21st to end	Day	15% 9% 22% 29% 23% 3	490
October	1st to 10th	Day	18% 7% 24% 25% 22% 4	490
	11th to 20th	Day	24% 11% 20% 25% 17% 3	490
	21st to end	Day	23% 17% 27% 21% 9% 4	434
November	1st to 10th	Day	28% 11% 31% 21% 8% 1	186
	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	21% 14% 18% 27% 17% 3	836
April	1st to 10th	Night	20% 11% 21% 27% 17% 5%	760
	11th to 20th	Night	16% 10% 23% 30% 18% 4	648
	21st to end	Night	18% 7% 22% 33% 18% 3	551
May	1st to 10th	Night	12% 7% 16% 34% 27% 5%	380
	11th to 20th	Night	11% 4 16% 29% 32% 8%	374
	21st to end	Night	12% 2 19% 33% 28% 5%	411
June	1st to 10th	Night	16% 8% 15% 31% 27% 4	380
	11th to 20th	Night	11% 5% 22% 35% 22% 5%	380
	21st to end	Night	8% 4 16% 32% 32% 8%	380
July	1st to 10th	Night	7% 1 12% 31% 39% 11%	370
	11th to 20th	Night	5% 1 12% 32% 41% 9%	368
	21st to end	Night	4 1 8% 29% 46% 12%	416
August	1st to 10th	Night	5% 2 6% 24% 42% 20%	446
	11th to 20th	Night	5% 1 5% 23% 50% 16%	564
	21st to end	Night	9% 3 12% 30% 38% 8%	680
September	1st to 10th	Night	6% 2 9% 35% 36% 11%	760
	11th to 20th	Night	14% 3 12% 22% 33% 15%	760
	21st to end	Night	15% 4 16% 29% 30% 6%	760
October	1st to 10th	Night	16% 5% 15% 29% 27% 8%	760
	11th to 20th	Night	23% 6% 17% 26% 21% 7%	760
	21st to end	Night	25% 9% 19% 27% 16% 4	836
November	1st to 10th	Night	31% 12% 18% 27% 12% 1	754
	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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