Oakville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oakville clear sky chart from 2005-11-06 to 2025-07-06.

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% 5% 5% 4 6% 7% 33%	120145
	Night	15% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	118616
Transparency	Day	46% 9% 14% 20% 11% 1	123782
	Night	47% 8% 13% 20% 11% 1	123139
Seeing	Day	16% 9% 26% 28% 15% 5%	15245
	Night	18% 9% 29% 29% 14% 2	24705

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	10% 13% 4 12% 8% 6% 5% 4 5% 6% 26%	3597
	11th to 20th	Day	10% 17% 7% 18% 8% 6% 5% 4 4 6% 15%	3980
	21st to end	Day	17% 14% 6% 14% 6% 5% 5% 3 4 5% 19%	4466
February	1st to 10th	Day	13% 16% 4 11% 8% 6% 6% 4 5% 6% 22%	1800
	11th to 20th	Day	12% 14% 6% 11% 7% 5% 5% 4 5% 6% 24%	1753
	21st to end	Day	18% 12% 4 8% 6% 5% 5% 4 6% 6% 26%	1554
March	1st to 10th	Day	18% 9% 4 10% 6% 5% 5% 3 5% 5% 31%	4932
	11th to 20th	Day	10% 11% 4 9% 6% 5% 4 4 6% 9% 31%	4980
	21st to end	Day	16% 8% 3 9% 6% 5% 5% 3 5% 6% 33%	5465
April	1st to 10th	Day	15% 11% 3 8% 6% 6% 5% 4 5% 6% 32%	2350
	11th to 20th	Day	16% 9% 3 7% 4 4 4 4 4 7% 36%	2458
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 3 5% 6% 35%	2600
May	1st to 10th	Day	18% 9% 3 7% 4 4 4 4 6% 7% 33%	5888
	11th to 20th	Day	15% 10% 4 8% 5% 4 5% 4 7% 7% 31%	6217
	21st to end	Day	16% 6% 3 6% 5% 4 5% 4 6% 7% 39%	6949
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	2800
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 9% 40%	2800
	21st to end	Day	12% 5% 2 6% 4 4 5% 5% 7% 9% 41%	2800
July	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 5% 8% 11% 43%	4510
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 5% 7% 9% 46%	4418
	21st to end	Day	6% 4 2 5% 5% 6% 6% 5% 8% 10% 42%	4466
August	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 6% 8% 39%	4104
	11th to 20th	Day	8% 6% 3 8% 5% 6% 6% 5% 7% 9% 37%	4044
	21st to end	Day	7% 6% 2 6% 4 5% 5% 4 6% 9% 46%	4372
September	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 8% 43%	2280
	11th to 20th	Day	9% 6% 2 6% 4 3 4 5% 5% 8% 48%	2151
	21st to end	Day	13% 6% 3 6% 5% 5% 6% 4 6% 9% 37%	2062
October	1st to 10th	Day	16% 8% 2 7% 5% 4 5% 4 5% 7% 36%	1900
	11th to 20th	Day	18% 8% 3 9% 4 5% 5% 4 6% 6% 31%	1900
	21st to end	Day	22% 10% 3 8% 6% 5% 6% 3 6% 6% 24%	2090
November	1st to 10th	Day	12% 8% 3 10% 5% 6% 5% 4 7% 8% 33%	1713
	11th to 20th	Day	13% 10% 3 10% 7% 6% 6% 4 6% 8% 27%	1600
	21st to end	Day	18% 12% 4 9% 6% 5% 5% 4 7% 7% 23%	1600
December	1st to 10th	Day	18% 15% 4 12% 6% 6% 5% 5% 7% 6% 16%	2600
	11th to 20th	Day	19% 17% 3 9% 5% 5% 4 4 5% 6% 24%	2986
	21st to end	Day	26% 16% 6% 11% 6% 5% 5% 3 5% 5% 12%	3960
January	1st to 10th	Night	13% 11% 3 11% 6% 6% 5% 4 5% 6% 29%	6349
	11th to 20th	Night	11% 13% 6% 14% 9% 7% 6% 4 5% 6% 19%	5887
	21st to end	Night	17% 13% 5% 10% 6% 5% 5% 4 6% 7% 24%	6490
February	1st to 10th	Night	14% 14% 4 10% 7% 5% 5% 5% 6% 7% 24%	2600
	11th to 20th	Night	13% 13% 5% 10% 6% 7% 5% 4 5% 6% 27%	2533
	21st to end	Night	14% 11% 4 10% 6% 5% 5% 5% 6% 7% 28%	2072
March	1st to 10th	Night	17% 6% 4 8% 6% 5% 5% 4 5% 7% 33%	4908
	11th to 20th	Night	10% 8% 3 8% 5% 4 4 4 6% 9% 40%	4979
	21st to end	Night	18% 9% 3 6% 4 4 4 3 4 6% 39%	5467
April	1st to 10th	Night	17% 10% 3 8% 5% 5% 5% 4 5% 5% 33%	2050
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 3 6% 7% 36%	1907
	21st to end	Night	16% 9% 2 7% 5% 3 5% 4 4 7% 39%	1800
May	1st to 10th	Night	20% 10% 3 7% 4 4 5% 3 4 6% 34%	4053
	11th to 20th	Night	14% 7% 3 8% 5% 6% 5% 4 5% 7% 33%	3672
	21st to end	Night	16% 5% 2 5% 3 4 4 3 5% 8% 43%	3926
June	1st to 10th	Night	12% 4 2 5% 5% 6% 4 4 6% 9% 43%	1600
	11th to 20th	Night	11% 5% 2 6% 4 5% 5% 4 8% 8% 43%	1600
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 8% 44%	1600
July	1st to 10th	Night	5% 4 2 6% 4 4 4 4 5% 9% 52%	2560
	11th to 20th	Night	10% 4 2 4 3 4 4 4 6% 8% 51%	2538
	21st to end	Night	6% 4 2 5% 4 4 5% 4 7% 8% 51%	3087
August	1st to 10th	Night	10% 6% 2 7% 5% 5% 4 4 6% 7% 45%	2825
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 5% 8% 48%	2785
	21st to end	Night	9% 5% 2 7% 4 4 4 4 7% 9% 46%	3252
September	1st to 10th	Night	10% 6% 2 6% 4 5% 5% 4 5% 7% 47%	1900
	11th to 20th	Night	9% 5% 2 4 4 3 4 3 4 7% 55%	2026
	21st to end	Night	15% 7% 2 6% 5% 4 5% 4 5% 8% 40%	2118
October	1st to 10th	Night	17% 6% 2 7% 4 3 4 4 5% 7% 42%	2280
	11th to 20th	Night	17% 8% 2 8% 4 5% 6% 4 5% 6% 35%	2280
	21st to end	Night	20% 9% 3 7% 5% 5% 5% 4 5% 6% 31%	2508
November	1st to 10th	Night	16% 6% 2 7% 4 5% 5% 3 6% 6% 40%	2580
	11th to 20th	Night	14% 10% 4 8% 5% 5% 5% 5% 7% 8% 30%	2800
	21st to end	Night	18% 11% 4 7% 6% 5% 5% 4 5% 8% 29%	2800
December	1st to 10th	Night	15% 13% 4 10% 6% 6% 7% 5% 7% 7% 20%	4580
	11th to 20th	Night	18% 15% 4 8% 6% 5% 5% 4 5% 6% 25%	5208
	21st to end	Night	25% 16% 6% 10% 6% 5% 5% 3 4 6% 15%	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	54% 11% 15% 11% 9% 1	3598
	11th to 20th	Day	69% 7% 11% 10% 3	4014
	21st to end	Day	67% 7% 11% 10% 5%	4466
February	1st to 10th	Day	62% 8% 13% 13% 3 1	1800
	11th to 20th	Day	59% 8% 13% 15% 5% 1	1762
	21st to end	Day	55% 10% 15% 13% 6% 1	1554
March	1st to 10th	Day	50% 8% 15% 15% 11% 1	4935
	11th to 20th	Day	46% 8% 19% 18% 7% 2	4980
	21st to end	Day	46% 8% 12% 22% 11% 1	5465
April	1st to 10th	Day	50% 7% 13% 17% 12% 1	2350
	11th to 20th	Day	46% 7% 11% 21% 14% 1	2458
	21st to end	Day	48% 7% 12% 18% 12% 3	2600
May	1st to 10th	Day	46% 7% 14% 16% 11% 5%	5897
	11th to 20th	Day	47% 11% 11% 17% 13% 2	6221
	21st to end	Day	41% 8% 13% 21% 15% 3	6949
June	1st to 10th	Day	39% 9% 12% 22% 16% 2	2800
	11th to 20th	Day	37% 9% 14% 24% 15% 1	2800
	21st to end	Day	35% 9% 15% 26% 15% 1	2800
July	1st to 10th	Day	30% 9% 16% 28% 15% 2	4440
	11th to 20th	Day	29% 8% 16% 34% 12% 1	4343
	21st to end	Day	32% 10% 17% 21% 19% 1	4324
August	1st to 10th	Day	39% 8% 15% 22% 14% 1	4091
	11th to 20th	Day	37% 10% 17% 24% 11% 1	4083
	21st to end	Day	30% 8% 15% 35% 12% 1	4372
September	1st to 10th	Day	36% 9% 16% 27% 11% 1	2280
	11th to 20th	Day	30% 9% 16% 29% 15% 1	2151
	21st to end	Day	41% 10% 15% 25% 8%	2062
October	1st to 10th	Day	35% 8% 16% 27% 13% 1	3160
	11th to 20th	Day	37% 6% 17% 24% 15%	3140
	21st to end	Day	49% 8% 16% 21% 6%	3428
November	1st to 10th	Day	45% 9% 16% 21% 8% 1	1713
	11th to 20th	Day	52% 10% 16% 15% 7% 1	1600
	21st to end	Day	56% 11% 16% 14% 3	1600
December	1st to 10th	Day	65% 8% 17% 8% 2	2600
	11th to 20th	Day	60% 9% 13% 10% 7%	2986
	21st to end	Day	69% 11% 9% 7% 4	3960
January	1st to 10th	Night	53% 10% 12% 19% 6%	6349
	11th to 20th	Night	62% 10% 12% 13% 3	5928
	21st to end	Night	59% 8% 13% 12% 7%	6490
February	1st to 10th	Night	58% 8% 14% 16% 4	2600
	11th to 20th	Night	56% 7% 13% 17% 7%	2535
	21st to end	Night	54% 8% 14% 17% 7%	2072
March	1st to 10th	Night	48% 7% 14% 18% 10% 2	4918
	11th to 20th	Night	40% 7% 21% 21% 9% 2	4980
	21st to end	Night	46% 6% 10% 21% 16% 2	5467
April	1st to 10th	Night	51% 7% 10% 18% 12% 2	2050
	11th to 20th	Night	46% 7% 10% 20% 15% 2	1907
	21st to end	Night	44% 8% 10% 19% 16% 2	1800
May	1st to 10th	Night	51% 6% 11% 13% 13% 6%	4060
	11th to 20th	Night	47% 9% 12% 18% 12% 2	3673
	21st to end	Night	39% 7% 13% 22% 17% 2	3927
June	1st to 10th	Night	37% 8% 13% 24% 16% 2	1600
	11th to 20th	Night	35% 8% 14% 23% 19% 2	1600
	21st to end	Night	37% 7% 13% 23% 18% 2	1600
July	1st to 10th	Night	29% 7% 14% 23% 24% 3	2520
	11th to 20th	Night	31% 6% 14% 32% 16% 1	2490
	21st to end	Night	29% 8% 18% 27% 18% 2	2988
August	1st to 10th	Night	36% 7% 13% 27% 15% 2	2814
	11th to 20th	Night	34% 7% 16% 31% 11% 1	2812
	21st to end	Night	33% 8% 14% 30% 14% 1	3252
September	1st to 10th	Night	35% 8% 15% 27% 14% 2	1900
	11th to 20th	Night	30% 6% 14% 33% 17%	2026
	21st to end	Night	42% 8% 12% 26% 11% 1	2118
October	1st to 10th	Night	33% 6% 11% 26% 22% 1	3800
	11th to 20th	Night	43% 5% 13% 28% 11%	3779
	21st to end	Night	38% 8% 17% 26% 10% 1	4120
November	1st to 10th	Night	44% 7% 14% 23% 12% 1	2580
	11th to 20th	Night	49% 9% 15% 19% 8% 1	2800
	21st to end	Night	53% 10% 18% 16% 3	2800
December	1st to 10th	Night	61% 10% 16% 11% 3	4580
	11th to 20th	Night	60% 8% 11% 13% 8% 1	5208
	21st to end	Night	69% 10% 10% 8% 3	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	17% 15% 34% 25% 9% 2	200
	11th to 20th	Day	21% 20% 35% 16% 7% 1	290
	21st to end	Day	22% 15% 38% 20% 5%	330
February	1st to 10th	Day	20% 16% 41% 18% 5%	300
	11th to 20th	Day	21% 20% 34% 23% 2	300
	21st to end	Day	22% 23% 33% 19% 3	260
March	1st to 10th	Day	18% 13% 35% 26% 8%	500
	11th to 20th	Day	18% 16% 34% 25% 7% 1	500
	21st to end	Day	25% 10% 30% 25% 8% 1	550
April	1st to 10th	Day	16% 13% 33% 28% 8% 1	500
	11th to 20th	Day	22% 14% 27% 25% 9% 3	492
	21st to end	Day	20% 9% 30% 30% 10% 2	500
May	1st to 10th	Day	16% 9% 26% 29% 15% 5%	503
	11th to 20th	Day	15% 6% 21% 32% 20% 6%	527
	21st to end	Day	12% 5% 20% 35% 20% 8%	573
June	1st to 10th	Day	11% 6% 26% 38% 16% 4	530
	11th to 20th	Day	11% 5% 22% 33% 25% 4	530
	21st to end	Day	13% 5% 19% 35% 22% 6%	530
July	1st to 10th	Day	7% 2 12% 38% 28% 12%	505
	11th to 20th	Day	12% 3 16% 32% 27% 10%	484
	21st to end	Day	8% 2 17% 37% 28% 8%	532
August	1st to 10th	Day	10% 4 17% 31% 27% 12%	486
	11th to 20th	Day	8% 4 18% 31% 25% 14%	479
	21st to end	Day	9% 4 20% 30% 25% 12%	539
September	1st to 10th	Day	11% 4 20% 30% 21% 13%	490
	11th to 20th	Day	8% 3 19% 26% 26% 18%	490
	21st to end	Day	14% 8% 28% 28% 16% 8%	490
October	1st to 10th	Day	19% 8% 26% 22% 17% 8%	490
	11th to 20th	Day	19% 12% 30% 24% 12% 4	490
	21st to end	Day	26% 11% 28% 24% 10% 1	539
November	1st to 10th	Day	17% 16% 30% 27% 9% 2	296
	11th to 20th	Day	22% 18% 28% 25% 8% 1	200
	21st to end	Day	23% 12% 41% 19% 5% 1	200
December	1st to 10th	Day	21% 16% 35% 23% 6%	200
	11th to 20th	Day	26% 22% 30% 21% 3	200
	21st to end	Day	24% 15% 42% 15% 5%	220
January	1st to 10th	Night	22% 16% 37% 19% 6%	800
	11th to 20th	Night	26% 14% 34% 22% 4	800
	21st to end	Night	19% 11% 39% 24% 7%	880
February	1st to 10th	Night	22% 14% 39% 22% 3	800
	11th to 20th	Night	21% 14% 38% 24% 3	783
	21st to end	Night	19% 15% 38% 25% 4	660
March	1st to 10th	Night	19% 10% 37% 27% 7%	800
	11th to 20th	Night	21% 11% 34% 27% 7%	800
	21st to end	Night	20% 10% 29% 31% 9% 1	880
April	1st to 10th	Night	20% 9% 34% 31% 6% 1	800
	11th to 20th	Night	19% 9% 32% 28% 10% 2	723
	21st to end	Night	19% 6% 29% 34% 12%	600
May	1st to 10th	Night	18% 4 25% 33% 18% 3	600
	11th to 20th	Night	16% 4 22% 41% 17% 1	439
	21st to end	Night	12% 2 21% 37% 25% 2	433
June	1st to 10th	Night	13% 3 22% 43% 19% 1	400
	11th to 20th	Night	13% 2 25% 37% 23% 1	400
	21st to end	Night	10% 4 20% 43% 24% 1	400
July	1st to 10th	Night	6% 2 14% 46% 29% 3	380
	11th to 20th	Night	11% 2 18% 37% 29% 3	373
	21st to end	Night	8% 2 14% 47% 26% 3	624
August	1st to 10th	Night	9% 3 12% 40% 32% 4	567
	11th to 20th	Night	8% 2 14% 36% 34% 5%	564
	21st to end	Night	11% 2 18% 33% 30% 4	703
September	1st to 10th	Night	11% 5% 18% 32% 27% 7%	760
	11th to 20th	Night	11% 4 18% 29% 32% 8%	760
	21st to end	Night	16% 5% 21% 30% 23% 5%	760
October	1st to 10th	Night	19% 4 27% 25% 21% 4	760
	11th to 20th	Night	20% 10% 31% 27% 10% 2	760
	21st to end	Night	25% 9% 30% 30% 6%	836
November	1st to 10th	Night	20% 8% 27% 32% 11% 1	780
	11th to 20th	Night	19% 12% 36% 22% 9% 2	800
	21st to end	Night	23% 11% 36% 24% 6%	800
December	1st to 10th	Night	26% 14% 35% 19% 7%	800
	11th to 20th	Night	23% 14% 39% 20% 5%	800
	21st to end	Night	28% 13% 34% 23% 3	880

Page generated at 2025-07-03 01:30:13. Version ac07

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