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 2025-09-04.

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	13% 9% 3 8% 5% 5% 5% 4 5% 7% 37%	130327
	Night	15% 9% 3 7% 5% 5% 5% 4 5% 6% 36%	127110
Transparency	Day	43% 10% 19% 19% 8% 1	130035
	Night	48% 9% 16% 19% 8% 1	126980
Seeing	Day	16% 9% 22% 27% 21% 5%	16491
	Night	20% 9% 18% 26% 22% 5%	24601

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% 15% 4 10% 6% 5% 5% 4 5% 5% 14%	3179
	11th to 20th	Day	15% 11% 4 9% 5% 6% 5% 4 6% 7% 27%	3159
	21st to end	Day	22% 16% 4 11% 6% 5% 4 4 4 4 19%	4166
February	1st to 10th	Day	21% 16% 5% 10% 7% 6% 6% 3 6% 6% 16%	1800
	11th to 20th	Day	20% 16% 4 10% 5% 4 4 3 4 6% 23%	1755
	21st to end	Day	17% 15% 5% 10% 7% 6% 5% 4 6% 6% 18%	1493
March	1st to 10th	Day	14% 14% 5% 12% 7% 7% 6% 5% 6% 7% 18%	4733
	11th to 20th	Day	14% 12% 4 9% 6% 5% 6% 4 5% 6% 28%	5000
	21st to end	Day	17% 14% 5% 11% 7% 6% 7% 4 6% 6% 17%	5586
April	1st to 10th	Day	17% 12% 3 10% 7% 6% 6% 5% 7% 7% 20%	2400
	11th to 20th	Day	15% 11% 3 8% 6% 6% 6% 5% 6% 7% 27%	2479
	21st to end	Day	16% 9% 3 8% 6% 5% 5% 4 7% 7% 29%	2619
May	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 7% 43%	6320
	11th to 20th	Day	9% 8% 3 8% 5% 5% 6% 4 7% 10% 35%	6277
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 4 7% 9% 34%	6989
June	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 5% 7% 8% 36%	3000
	11th to 20th	Day	9% 7% 3 9% 7% 6% 6% 4 7% 8% 34%	3000
	21st to end	Day	9% 6% 2 5% 5% 5% 6% 5% 7% 8% 43%	3000
July	1st to 10th	Day	4 3 1 3 2 2 3 3 4 6% 68%	6780
	11th to 20th	Day	3 2 1 2 2 2 2 3 3 5% 74%	6780
	21st to end	Day	5% 3 1 3 2 3 3 3 4 7% 66%	6965
August	1st to 10th	Day	4 2 1 3 2 2 3 3 4 6% 71%	4418
	11th to 20th	Day	7% 6% 3 8% 5% 5% 5% 4 6% 8% 43%	4156
	21st to end	Day	10% 5% 2 6% 4 4 4 3 6% 8% 46%	4571
September	1st to 10th	Day	7% 5% 2 5% 3 4 4 4 6% 9% 52%	2317
	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% 6% 5% 5% 6% 5% 16%	1568
	11th to 20th	Day	24% 12% 4 8% 6% 5% 4 4 5% 5% 23%	1600
	21st to end	Day	24% 14% 4 10% 6% 5% 5% 4 4 6% 19%	1600
December	1st to 10th	Day	15% 15% 5% 11% 6% 5% 5% 2 5% 7% 24%	2620
	11th to 20th	Day	28% 14% 3 9% 6% 6% 5% 4 6% 6% 13%	2652
	21st to end	Day	22% 20% 6% 11% 6% 5% 5% 3 4 5% 11%	3498
January	1st to 10th	Night	25% 17% 5% 12% 6% 6% 5% 4 4 5% 14%	6766
	11th to 20th	Night	17% 9% 3 6% 4 4 4 4 6% 7% 35%	6725
	21st to end	Night	23% 12% 4 8% 5% 5% 5% 3 5% 5% 26%	6793
February	1st to 10th	Night	21% 14% 4 9% 5% 6% 5% 3 4 6% 22%	2600
	11th to 20th	Night	19% 12% 4 9% 5% 5% 4 3 5% 6% 29%	2532
	21st to end	Night	19% 14% 4 9% 6% 4 5% 3 5% 6% 24%	2136
March	1st to 10th	Night	12% 10% 3 8% 6% 6% 6% 5% 8% 8% 28%	5158
	11th to 20th	Night	11% 10% 4 7% 6% 5% 5% 4 7% 7% 32%	4999
	21st to end	Night	19% 14% 5% 11% 6% 5% 5% 4 5% 6% 20%	5388
April	1st to 10th	Night	17% 9% 4 8% 6% 5% 4 4 6% 7% 29%	2000
	11th to 20th	Night	13% 9% 3 7% 5% 4 5% 4 6% 8% 35%	1870
	21st to end	Night	16% 8% 4 7% 5% 4 5% 4 5% 8% 34%	1780
May	1st to 10th	Night	11% 7% 2 5% 4 4 3 2 4 5% 53%	3640
	11th to 20th	Night	9% 7% 3 5% 4 4 4 4 6% 8% 45%	3616
	21st to end	Night	10% 7% 3 7% 5% 6% 7% 5% 7% 7% 35%	3883
June	1st to 10th	Night	12% 8% 3 6% 6% 3 5% 4 6% 6% 42%	1400
	11th to 20th	Night	10% 6% 2 7% 4 4 4 6% 7% 8% 42%	1400
	21st to end	Night	9% 4 2 5% 3 4 4 4 6% 10% 50%	1400
July	1st to 10th	Night	4 3 1 3 2 2 3 2 3 6% 70%	3180
	11th to 20th	Night	2 2 1 2 1 1 2 2 3 4 80%	3180
	21st to end	Night	5% 3 1 2 2 2 3 2 3 6% 71%	3875
August	1st to 10th	Night	3 2 1 2 2 2 2 2 4 6% 75%	2733
	11th to 20th	Night	8% 6% 2 7% 4 4 4 4 5% 6% 51%	2902
	21st to end	Night	9% 3 1 4 3 3 4 3 5% 8% 56%	3340
September	1st to 10th	Night	5% 2 1 4 2 3 4 3 5% 6% 64%	1930
	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	25% 12% 3 8% 6% 5% 5% 4 4 5% 23%	2730
	11th to 20th	Night	23% 11% 3 8% 6% 5% 5% 4 5% 5% 25%	2800
	21st to end	Night	20% 13% 5% 8% 6% 5% 5% 4 5% 5% 23%	2800
December	1st to 10th	Night	15% 12% 5% 10% 6% 5% 6% 5% 7% 7% 25%	4560
	11th to 20th	Night	28% 16% 5% 10% 6% 6% 5% 3 4 4 13%	5550
	21st to end	Night	20% 14% 4 10% 6% 6% 6% 4 5% 6% 18%	7458

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	69% 11% 8% 9% 3	3179
	11th to 20th	Day	53% 11% 18% 13% 4	3180
	21st to end	Day	64% 11% 10% 12% 3	4166
February	1st to 10th	Day	66% 11% 10% 9% 4 1	1800
	11th to 20th	Day	61% 11% 10% 12% 5%	1764
	21st to end	Day	62% 11% 14% 9% 3 1	1493
March	1st to 10th	Day	55% 15% 14% 12% 4	4733
	11th to 20th	Day	51% 9% 14% 22% 3 1	5000
	21st to end	Day	60% 12% 16% 9% 2	5586
April	1st to 10th	Day	55% 13% 19% 11% 2	2400
	11th to 20th	Day	48% 12% 16% 16% 8% 2	2479
	21st to end	Day	48% 12% 19% 15% 6% 1	2619
May	1st to 10th	Day	36% 8% 21% 25% 10% 1	6320
	11th to 20th	Day	38% 12% 28% 18% 4	6280
	21st to end	Day	42% 11% 25% 19% 3 1	6990
June	1st to 10th	Day	41% 11% 24% 17% 6% 1	3000
	11th to 20th	Day	40% 13% 25% 17% 4	3000
	21st to end	Day	35% 10% 26% 20% 8% 2	3000
July	1st to 10th	Day	18% 6% 23% 26% 21% 7%	6706
	11th to 20th	Day	12% 5% 21% 33% 26% 3	6699
	21st to end	Day	18% 6% 32% 32% 11% 1	6770
August	1st to 10th	Day	15% 6% 17% 43% 17% 2	4401
	11th to 20th	Day	34% 10% 27% 20% 8% 1	4195
	21st to end	Day	34% 8% 22% 26% 9% 1	4570
September	1st to 10th	Day	28% 11% 23% 23% 13% 3	2317
	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	67% 9% 12% 8% 3 1	1568
	11th to 20th	Day	62% 8% 12% 12% 4 2	1600
	21st to end	Day	65% 11% 10% 10% 4	1600
December	1st to 10th	Day	61% 10% 11% 15% 4	2620
	11th to 20th	Day	69% 12% 11% 7% 1	2655
	21st to end	Day	70% 11% 10% 6% 2	3498
January	1st to 10th	Night	70% 11% 9% 8% 2	6766
	11th to 20th	Night	46% 9% 21% 19% 5% 1	6766
	21st to end	Night	59% 10% 11% 15% 5%	6793
February	1st to 10th	Night	63% 9% 13% 9% 5% 1	2600
	11th to 20th	Night	57% 9% 12% 15% 6% 1	2534
	21st to end	Night	58% 9% 11% 15% 5% 1	2136
March	1st to 10th	Night	48% 14% 16% 16% 6%	5158
	11th to 20th	Night	46% 11% 15% 24% 4	5000
	21st to end	Night	62% 11% 13% 13% 1	5388
April	1st to 10th	Night	53% 10% 17% 15% 4	2000
	11th to 20th	Night	45% 9% 16% 20% 8% 2	1870
	21st to end	Night	48% 10% 15% 18% 9% 1	1780
May	1st to 10th	Night	34% 7% 14% 28% 16% 1	3637
	11th to 20th	Night	35% 9% 26% 24% 6%	3616
	21st to end	Night	44% 13% 20% 21% 3	3883
June	1st to 10th	Night	42% 8% 18% 22% 8% 2	1400
	11th to 20th	Night	36% 12% 23% 22% 6%	1400
	21st to end	Night	30% 10% 25% 23% 9% 3	1400
July	1st to 10th	Night	18% 5% 19% 32% 17% 9%	3145
	11th to 20th	Night	11% 3 19% 33% 30% 3	3138
	21st to end	Night	17% 5% 22% 36% 18% 2	3766
August	1st to 10th	Night	13% 5% 16% 37% 27% 3	2725
	11th to 20th	Night	32% 8% 26% 21% 9% 4	2929
	21st to end	Night	27% 7% 20% 32% 13% 1	3340
September	1st to 10th	Night	20% 9% 20% 29% 17% 5%	1930
	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	62% 10% 12% 12% 3 1	2730
	11th to 20th	Night	60% 10% 13% 10% 6% 2	2800
	21st to end	Night	62% 10% 12% 10% 6%	2800
December	1st to 10th	Night	56% 11% 14% 15% 4	4560
	11th to 20th	Night	73% 10% 9% 5% 2	5546
	21st to end	Night	63% 11% 13% 10% 2	7458

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	29% 20% 25% 15% 10% 2	200
	11th to 20th	Day	32% 13% 23% 17% 14% 2	200
	21st to end	Day	30% 14% 23% 20% 10% 2	292
February	1st to 10th	Day	27% 18% 27% 19% 7% 2	400
	11th to 20th	Day	29% 12% 32% 18% 8% 1	395
	21st to end	Day	25% 18% 26% 22% 7% 1	337
March	1st to 10th	Day	20% 15% 26% 23% 14% 2	500
	11th to 20th	Day	23% 16% 21% 24% 14% 3	500
	21st to end	Day	25% 11% 27% 24% 12% 1	550
April	1st to 10th	Day	22% 12% 27% 24% 13% 2	500
	11th to 20th	Day	18% 10% 28% 26% 15% 3	492
	21st to end	Day	19% 8% 27% 24% 18% 4	500
May	1st to 10th	Day	11% 7% 22% 36% 21% 3	503
	11th to 20th	Day	13% 7% 20% 27% 25% 7%	528
	21st to end	Day	13% 6% 24% 29% 24% 5%	607
June	1st to 10th	Day	13% 7% 22% 34% 21% 3	730
	11th to 20th	Day	9% 8% 29% 33% 18% 3	730
	21st to end	Day	8% 4 19% 36% 28% 5%	730
July	1st to 10th	Day	8% 2 16% 30% 37% 7%	721
	11th to 20th	Day	4 3 17% 35% 33% 8%	718
	21st to end	Day	4 3 12% 30% 39% 13%	614
August	1st to 10th	Day	5% 2 13% 24% 39% 16%	527
	11th to 20th	Day	4 3 13% 31% 39% 10%	517
	21st to end	Day	9% 4 16% 33% 28% 10%	583
September	1st to 10th	Day	9% 3 17% 38% 27% 5%	503
	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% 13% 29% 22% 9% 1	200
	11th to 20th	Day	27% 13% 23% 21% 13% 4	200
	21st to end	Day	29% 16% 22% 16% 10% 8%	200
December	1st to 10th	Day	32% 13% 18% 17% 19% 3	200
	11th to 20th	Day	33% 13% 26% 20% 7% 1	200
	21st to end	Day	33% 21% 20% 18% 6% 1	220
January	1st to 10th	Night	30% 16% 23% 18% 11% 3	900
	11th to 20th	Night	30% 11% 15% 25% 17% 2	899
	21st to end	Night	28% 9% 19% 25% 16% 3	880
February	1st to 10th	Night	28% 15% 20% 22% 12% 3	800
	11th to 20th	Night	24% 13% 21% 26% 15% 1	784
	21st to end	Night	24% 16% 21% 21% 14% 3	660
March	1st to 10th	Night	20% 12% 22% 27% 16% 3	800
	11th to 20th	Night	22% 13% 22% 26% 15% 2	800
	21st to end	Night	21% 14% 18% 26% 17% 3	880
April	1st to 10th	Night	19% 11% 21% 28% 18% 5%	800
	11th to 20th	Night	15% 10% 23% 30% 19% 3	682
	21st to end	Night	18% 6% 21% 33% 19% 3	580
May	1st to 10th	Night	12% 7% 16% 35% 26% 5%	400
	11th to 20th	Night	12% 5% 16% 29% 30% 8%	394
	21st to end	Night	12% 2 19% 34% 28% 5%	433
June	1st to 10th	Night	15% 8% 15% 31% 28% 4	400
	11th to 20th	Night	11% 5% 21% 36% 23% 5%	400
	21st to end	Night	9% 5% 16% 32% 32% 8%	400
July	1st to 10th	Night	7% 1 12% 30% 39% 10%	390
	11th to 20th	Night	5% 1 12% 33% 42% 8%	388
	21st to end	Night	4 1 8% 29% 46% 12%	438
August	1st to 10th	Night	6% 2 6% 25% 43% 19%	470
	11th to 20th	Night	5% 2 5% 24% 50% 15%	594
	21st to end	Night	8% 2 11% 30% 39% 9%	716
September	1st to 10th	Night	6% 2 9% 35% 36% 11%	772
	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% 17% 27% 12% 1	780
	11th to 20th	Night	28% 13% 22% 22% 14% 2	800
	21st to end	Night	26% 12% 21% 22% 15% 4	800
December	1st to 10th	Night	25% 12% 17% 25% 18% 3	800
	11th to 20th	Night	31% 13% 23% 23% 9% 1	895
	21st to end	Night	33% 16% 23% 20% 7% 1	990

Page generated at 2025-08-31 22:51:05. Version ac07

Copyright © 2025 Attilla Danko.