Ottawa
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ottawa clear sky chart from 2005-11-06 to 2025-11-03.

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% 3 8% 5% 5% 5% 4 6% 7% 32%	127137
	Night	17% 11% 4 9% 5% 5% 4 4 5% 6% 30%	122895
Transparency	Day	49% 7% 12% 20% 10% 1	130602
	Night	52% 7% 12% 18% 10% 1	127517
Seeing	Day	21% 15% 31% 21% 10% 3	15737
	Night	22% 12% 30% 27% 9% 1	24771

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	14% 17% 6% 16% 6% 5% 4 4 4 4 20%	3597
	11th to 20th	Day	13% 14% 6% 13% 8% 6% 5% 3 5% 5% 22%	3570
	21st to end	Day	10% 15% 6% 13% 7% 6% 5% 5% 6% 6% 21%	4120
February	1st to 10th	Day	13% 15% 5% 11% 7% 6% 5% 4 5% 6% 25%	1800
	11th to 20th	Day	13% 14% 5% 10% 6% 5% 4 4 6% 5% 28%	1763
	21st to end	Day	18% 13% 3 10% 6% 5% 6% 4 6% 5% 25%	1650
March	1st to 10th	Day	18% 12% 4 11% 6% 5% 4 4 5% 6% 25%	4476
	11th to 20th	Day	12% 11% 4 9% 6% 5% 5% 4 6% 7% 30%	4970
	21st to end	Day	17% 10% 4 8% 5% 4 4 3 4 5% 36%	5781
April	1st to 10th	Day	22% 10% 3 7% 5% 3 4 3 5% 6% 33%	2400
	11th to 20th	Day	21% 9% 3 7% 4 4 4 3 4 5% 37%	2374
	21st to end	Day	23% 9% 3 7% 6% 5% 4 2 5% 4 32%	2540
May	1st to 10th	Day	23% 10% 3 8% 5% 5% 4 4 6% 6% 25%	6346
	11th to 20th	Day	17% 9% 3 9% 6% 5% 5% 4 5% 6% 31%	6318
	21st to end	Day	21% 8% 2 7% 5% 4 4 3 5% 6% 33%	6949
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 4 6% 7% 30%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 38%	2800
	21st to end	Day	18% 7% 2 7% 5% 4 5% 5% 7% 8% 31%	2800
July	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 5% 6% 10% 35%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 43%	6360
	21st to end	Day	9% 5% 2 5% 4 4 4 4 7% 10% 46%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 4 6% 4 6% 8% 37%	4507
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 6% 9% 38%	4115
	21st to end	Day	14% 5% 3 6% 4 5% 4 4 6% 9% 40%	4312
September	1st to 10th	Day	13% 5% 2 6% 4 3 5% 5% 6% 8% 43%	2400
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 6% 7% 49%	2396
	21st to end	Day	18% 6% 2 6% 4 4 3 4 5% 8% 39%	2140
October	1st to 10th	Day	19% 8% 3 6% 4 4 4 3 5% 7% 37%	2000
	11th to 20th	Day	20% 8% 3 7% 5% 5% 5% 4 6% 6% 30%	2000
	21st to end	Day	24% 10% 3 7% 5% 5% 4 4 4 6% 29%	2032
November	1st to 10th	Day	15% 9% 3 7% 5% 4 5% 5% 6% 7% 34%	1779
	11th to 20th	Day	19% 12% 3 8% 5% 5% 4 4 6% 6% 28%	1610
	21st to end	Day	21% 14% 4 9% 6% 6% 5% 4 4 5% 21%	1600
December	1st to 10th	Day	20% 20% 6% 10% 5% 4 5% 5% 6% 6% 14%	2600
	11th to 20th	Day	17% 15% 4 11% 7% 5% 4 4 4 5% 25%	2986
	21st to end	Day	23% 16% 4 11% 6% 5% 5% 4 3 3 20%	3960
January	1st to 10th	Night	14% 20% 9% 15% 5% 3 3 2 4 4 20%	6349
	11th to 20th	Night	11% 14% 6% 15% 8% 7% 6% 4 4 5% 21%	6315
	21st to end	Night	13% 19% 6% 12% 6% 6% 5% 4 5% 6% 20%	6835
February	1st to 10th	Night	13% 14% 5% 13% 6% 6% 5% 4 5% 5% 24%	2600
	11th to 20th	Night	13% 13% 5% 11% 6% 6% 4 3 4 5% 28%	2523
	21st to end	Night	16% 13% 4 9% 6% 4 5% 3 5% 6% 28%	1980
March	1st to 10th	Night	22% 14% 4 9% 5% 5% 4 2 4 4 28%	5364
	11th to 20th	Night	13% 12% 4 9% 6% 5% 4 4 6% 8% 30%	4984
	21st to end	Night	19% 10% 3 8% 5% 4 4 3 5% 6% 33%	5151
April	1st to 10th	Night	21% 11% 3 7% 4 4 4 3 5% 6% 32%	2000
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 4 4 5% 36%	1975
	21st to end	Night	22% 8% 3 7% 5% 4 4 3 4 7% 34%	1874
May	1st to 10th	Night	23% 9% 3 8% 6% 5% 6% 5% 6% 7% 24%	3594
	11th to 20th	Night	17% 7% 3 7% 4 4 4 4 5% 7% 37%	3575
	21st to end	Night	18% 9% 3 8% 6% 5% 5% 4 6% 6% 31%	3926
June	1st to 10th	Night	15% 6% 2 7% 6% 5% 6% 5% 6% 7% 34%	1600
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 7% 42%	1600
	21st to end	Night	18% 7% 2 7% 5% 4 4 5% 6% 7% 35%	1600
July	1st to 10th	Night	14% 7% 2 7% 5% 5% 6% 5% 7% 8% 33%	3600
	11th to 20th	Night	12% 6% 2 7% 4 4 4 4 7% 7% 42%	3600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 6% 7% 46%	3926
August	1st to 10th	Night	12% 7% 3 8% 4 4 5% 4 5% 9% 39%	2640
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 4 6% 7% 41%	2899
	21st to end	Night	13% 6% 3 6% 5% 5% 4 4 6% 7% 41%	3586
September	1st to 10th	Night	11% 4 2 7% 4 5% 4 4 6% 6% 48%	2000
	11th to 20th	Night	13% 5% 1 6% 4 3 4 3 5% 6% 50%	2003
	21st to end	Night	17% 6% 2 7% 4 4 4 3 4 6% 44%	2256
October	1st to 10th	Night	18% 7% 2 6% 4 4 4 3 5% 6% 41%	2400
	11th to 20th	Night	20% 8% 3 7% 6% 5% 4 3 6% 6% 32%	2400
	21st to end	Night	22% 9% 3 7% 4 5% 4 4 4 7% 29%	2807
November	1st to 10th	Night	20% 10% 4 6% 5% 4 3 4 5% 6% 34%	2564
	11th to 20th	Night	18% 10% 3 9% 5% 5% 5% 4 5% 5% 30%	2785
	21st to end	Night	21% 13% 4 9% 6% 4 4 4 5% 6% 24%	2800
December	1st to 10th	Night	19% 17% 5% 11% 7% 6% 5% 4 5% 6% 16%	4580
	11th to 20th	Night	23% 14% 4 9% 5% 4 4 3 4 5% 25%	5208
	21st to end	Night	27% 17% 4 9% 4 4 4 3 4 5% 19%	6996

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	64% 10% 8% 13% 4 1	3598
	11th to 20th	Day	63% 6% 11% 12% 8%	3600
	21st to end	Day	62% 7% 11% 11% 7% 1	4120
February	1st to 10th	Day	60% 6% 11% 17% 5% 1	1800
	11th to 20th	Day	56% 7% 11% 16% 8% 1	1772
	21st to end	Day	59% 7% 11% 13% 9% 2	1650
March	1st to 10th	Day	58% 6% 9% 12% 12% 3	4479
	11th to 20th	Day	49% 9% 17% 15% 9% 1	4970
	21st to end	Day	49% 5% 8% 19% 16% 2	5781
April	1st to 10th	Day	52% 6% 8% 19% 12% 3	2400
	11th to 20th	Day	50% 5% 7% 21% 14% 2	2374
	21st to end	Day	54% 6% 9% 14% 14% 4	2540
May	1st to 10th	Day	55% 7% 10% 17% 8% 2	6357
	11th to 20th	Day	50% 9% 12% 17% 10% 1	6322
	21st to end	Day	49% 8% 11% 19% 12% 1	6949
June	1st to 10th	Day	52% 8% 10% 18% 12% 1	2800
	11th to 20th	Day	41% 10% 11% 23% 13% 2	2800
	21st to end	Day	47% 10% 13% 19% 10% 1	2800
July	1st to 10th	Day	43% 7% 15% 25% 9%	6290
	11th to 20th	Day	36% 7% 13% 30% 13%	6285
	21st to end	Day	32% 8% 16% 35% 10%	6728
August	1st to 10th	Day	44% 7% 16% 23% 9%	4493
	11th to 20th	Day	41% 7% 16% 28% 8% 1	4154
	21st to end	Day	39% 7% 17% 26% 11% 1	4312
September	1st to 10th	Day	37% 9% 15% 28% 11%	2400
	11th to 20th	Day	33% 8% 14% 32% 13% 1	2396
	21st to end	Day	43% 7% 15% 25% 10%	2140
October	1st to 10th	Day	46% 6% 12% 29% 7%	3260
	11th to 20th	Day	45% 7% 16% 23% 8% 1	3240
	21st to end	Day	49% 6% 18% 20% 7%	3257
November	1st to 10th	Day	46% 9% 16% 22% 6% 1	1779
	11th to 20th	Day	55% 7% 13% 15% 8% 1	1610
	21st to end	Day	64% 8% 11% 10% 6% 1	1600
December	1st to 10th	Day	70% 7% 13% 9% 2	2600
	11th to 20th	Day	62% 6% 11% 15% 6%	2986
	21st to end	Day	68% 6% 8% 15% 4	3960
January	1st to 10th	Night	68% 6% 11% 12% 4	6349
	11th to 20th	Night	63% 8% 12% 12% 5%	6360
	21st to end	Night	64% 7% 9% 9% 11% 1	6835
February	1st to 10th	Night	60% 6% 11% 15% 7% 1	2600
	11th to 20th	Night	57% 6% 10% 18% 7% 1	2525
	21st to end	Night	56% 6% 10% 18% 10% 1	1980
March	1st to 10th	Night	59% 5% 8% 14% 11% 3	5374
	11th to 20th	Night	50% 9% 16% 15% 7% 3	4985
	21st to end	Night	50% 6% 8% 22% 12% 2	5151
April	1st to 10th	Night	53% 6% 9% 17% 13% 2	2000
	11th to 20th	Night	50% 7% 7% 20% 15% 1	1975
	21st to end	Night	51% 5% 9% 17% 14% 3	1874
May	1st to 10th	Night	54% 10% 10% 17% 8% 3	3600
	11th to 20th	Night	44% 9% 13% 19% 14% 1	3576
	21st to end	Night	49% 9% 12% 15% 14% 1	3927
June	1st to 10th	Night	45% 9% 13% 22% 10% 2	1600
	11th to 20th	Night	39% 8% 11% 23% 16% 2	1600
	21st to end	Night	46% 9% 12% 19% 14% 1	1600
July	1st to 10th	Night	45% 7% 16% 19% 11% 1	3560
	11th to 20th	Night	37% 7% 13% 31% 11%	3552
	21st to end	Night	35% 7% 13% 30% 16%	3809
August	1st to 10th	Night	41% 8% 16% 24% 11% 1	2630
	11th to 20th	Night	41% 7% 15% 30% 8%	2926
	21st to end	Night	40% 7% 15% 24% 12% 2	3586
September	1st to 10th	Night	36% 7% 13% 30% 14% 1	2000
	11th to 20th	Night	33% 8% 14% 34% 10% 1	2003
	21st to end	Night	41% 6% 12% 28% 12%	2256
October	1st to 10th	Night	40% 6% 14% 27% 12% 1	3920
	11th to 20th	Night	45% 6% 15% 25% 9%	3899
	21st to end	Night	48% 7% 17% 20% 9%	4532
November	1st to 10th	Night	51% 6% 15% 19% 8% 1	2564
	11th to 20th	Night	55% 6% 12% 17% 9% 1	2785
	21st to end	Night	60% 8% 13% 15% 4	2800
December	1st to 10th	Night	67% 6% 13% 11% 2	4580
	11th to 20th	Night	62% 5% 9% 13% 10% 1	5208
	21st to end	Night	68% 5% 7% 12% 9%	6996

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	22% 20% 34% 21% 4 1	200
	11th to 20th	Day	27% 14% 39% 17% 4 1	200
	21st to end	Day	21% 16% 35% 23% 5%	255
February	1st to 10th	Day	30% 21% 34% 14% 2	300
	11th to 20th	Day	26% 28% 35% 10% 1	310
	21st to end	Day	28% 20% 32% 18% 3	414
March	1st to 10th	Day	25% 17% 36% 18% 5%	500
	11th to 20th	Day	25% 17% 36% 18% 3 1	500
	21st to end	Day	29% 22% 26% 15% 7% 1	550
April	1st to 10th	Day	28% 21% 30% 15% 6% 1	500
	11th to 20th	Day	30% 20% 23% 16% 9% 2	492
	21st to end	Day	31% 19% 31% 12% 6% 1	500
May	1st to 10th	Day	24% 15% 33% 20% 7% 2	503
	11th to 20th	Day	17% 17% 35% 18% 10% 2	527
	21st to end	Day	20% 15% 29% 23% 11% 2	573
June	1st to 10th	Day	23% 13% 31% 23% 9% 2	530
	11th to 20th	Day	15% 14% 32% 26% 11% 2	530
	21st to end	Day	21% 12% 32% 22% 10% 2	530
July	1st to 10th	Day	12% 8% 28% 34% 13% 4	525
	11th to 20th	Day	11% 9% 34% 28% 15% 3	524
	21st to end	Day	11% 7% 33% 30% 14% 4	576
August	1st to 10th	Day	13% 6% 27% 33% 17% 4	526
	11th to 20th	Day	14% 8% 23% 28% 19% 8%	519
	21st to end	Day	13% 10% 30% 26% 14% 7%	583
September	1st to 10th	Day	15% 11% 30% 20% 15% 9%	530
	11th to 20th	Day	10% 8% 24% 24% 22% 12%	530
	21st to end	Day	18% 12% 29% 23% 13% 5%	530
October	1st to 10th	Day	22% 14% 25% 22% 12% 5%	530
	11th to 20th	Day	22% 20% 32% 18% 7% 1	530
	21st to end	Day	30% 12% 30% 22% 5%	489
November	1st to 10th	Day	24% 17% 30% 20% 9% 1	401
	11th to 20th	Day	25% 20% 36% 13% 5% 1	210
	21st to end	Day	30% 19% 32% 15% 4 1	200
December	1st to 10th	Day	30% 17% 32% 18% 4	200
	11th to 20th	Day	28% 24% 32% 14% 2 1	200
	21st to end	Day	30% 19% 32% 14% 3 1	220
January	1st to 10th	Night	25% 15% 36% 20% 3	800
	11th to 20th	Night	25% 17% 31% 22% 5% 1	800
	21st to end	Night	19% 11% 37% 27% 5%	880
February	1st to 10th	Night	24% 13% 36% 25% 3	800
	11th to 20th	Night	20% 17% 39% 20% 4	783
	21st to end	Night	24% 16% 30% 27% 3	660
March	1st to 10th	Night	23% 15% 36% 22% 4	800
	11th to 20th	Night	21% 19% 34% 22% 5%	800
	21st to end	Night	24% 16% 29% 24% 8%	880
April	1st to 10th	Night	27% 15% 30% 24% 5%	800
	11th to 20th	Night	25% 15% 30% 23% 7%	791
	21st to end	Night	26% 14% 30% 23% 7% 1	674
May	1st to 10th	Night	22% 9% 31% 28% 9% 2	400
	11th to 20th	Night	21% 10% 27% 32% 10%	394
	21st to end	Night	15% 7% 39% 28% 11%	433
June	1st to 10th	Night	18% 4 31% 38% 10%	400
	11th to 20th	Night	14% 8% 31% 34% 12% 2	400
	21st to end	Night	21% 4 32% 30% 13% 1	400
July	1st to 10th	Night	10% 5% 28% 43% 14%	390
	11th to 20th	Night	11% 5% 24% 44% 15% 1	388
	21st to end	Night	8% 4 28% 43% 16% 2	438
August	1st to 10th	Night	12% 4 22% 40% 21% 1	426
	11th to 20th	Night	13% 4 19% 37% 24% 2	605
	21st to end	Night	12% 7% 25% 36% 18% 2	880
September	1st to 10th	Night	13% 6% 29% 33% 19% 1	800
	11th to 20th	Night	15% 6% 20% 32% 23% 4	800
	21st to end	Night	19% 9% 25% 26% 20% 2	800
October	1st to 10th	Night	20% 9% 28% 26% 16% 1	800
	11th to 20th	Night	23% 12% 31% 26% 8% 1	800
	21st to end	Night	26% 13% 28% 27% 6%	880
November	1st to 10th	Night	25% 14% 27% 25% 8% 1	789
	11th to 20th	Night	24% 15% 34% 17% 9% 1	800
	21st to end	Night	29% 17% 27% 22% 6%	800
December	1st to 10th	Night	30% 14% 30% 22% 4	800
	11th to 20th	Night	28% 16% 30% 20% 5% 1	800
	21st to end	Night	33% 15% 29% 20% 3	880

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