Sarnia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sarnia 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	14% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	125728
	Night	14% 11% 4 8% 5% 4 4 3 5% 6% 34%	123068
Transparency	Day	48% 5% 13% 21% 12% 1	129300
	Night	49% 5% 12% 20% 12% 1	127565
Seeing	Day	17% 8% 24% 28% 17% 6%	16355
	Night	20% 8% 27% 30% 14% 2	25761

By Month

Percentage Cloud Cover SE (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	11% 16% 7% 15% 5% 4 4 4 5% 5% 24%	3617
	11th to 20th	Day	13% 18% 6% 18% 8% 7% 4 3 4 4 14%	3730
	21st to end	Day	21% 16% 6% 12% 6% 5% 5% 3 4 5% 17%	4466
February	1st to 10th	Day	15% 17% 4 13% 8% 5% 5% 3 4 5% 21%	1800
	11th to 20th	Day	14% 15% 5% 11% 7% 4 5% 3 5% 6% 25%	1823
	21st to end	Day	14% 14% 5% 14% 6% 5% 4 4 4 5% 25%	1650
March	1st to 10th	Day	15% 13% 6% 9% 7% 4 4 3 4 5% 30%	4699
	11th to 20th	Day	12% 13% 4 9% 5% 5% 4 3 5% 8% 31%	4980
	21st to end	Day	18% 15% 5% 9% 5% 5% 4 3 5% 5% 27%	5465
April	1st to 10th	Day	16% 12% 2 8% 5% 5% 4 4 5% 6% 32%	2281
	11th to 20th	Day	17% 11% 3 6% 4 4 4 3 4 7% 36%	2572
	21st to end	Day	18% 9% 3 7% 4 5% 4 4 5% 6% 35%	2600
May	1st to 10th	Day	18% 8% 3 7% 5% 4 4 3 6% 6% 36%	5862
	11th to 20th	Day	16% 8% 3 6% 5% 5% 5% 4 5% 7% 35%	5861
	21st to end	Day	15% 7% 3 7% 5% 5% 4 3 5% 6% 41%	6657
June	1st to 10th	Day	10% 7% 3 7% 5% 4 4 4 7% 8% 42%	2920
	11th to 20th	Day	9% 5% 2 6% 4 4 6% 5% 7% 9% 42%	3000
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	3000
July	1st to 10th	Day	10% 5% 3 6% 4 5% 5% 4 7% 10% 41%	6509
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 47%	6319
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 7% 10% 47%	6424
August	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 5% 7% 9% 40%	4234
	11th to 20th	Day	9% 6% 2 7% 4 5% 6% 5% 7% 9% 38%	4174
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	4675
September	1st to 10th	Day	12% 5% 2 7% 4 5% 4 5% 7% 8% 41%	2201
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 47%	2090
	21st to end	Day	16% 7% 2 7% 4 5% 4 4 6% 9% 35%	2090
October	1st to 10th	Day	13% 7% 2 7% 5% 5% 4 4 5% 7% 40%	1990
	11th to 20th	Day	18% 10% 3 7% 5% 4 5% 4 4 7% 33%	1900
	21st to end	Day	20% 10% 3 9% 6% 5% 4 3 6% 6% 28%	1986
November	1st to 10th	Day	12% 10% 4 8% 5% 4 5% 5% 6% 8% 33%	1762
	11th to 20th	Day	13% 11% 4 9% 6% 5% 5% 4 6% 5% 31%	1743
	21st to end	Day	18% 13% 5% 10% 6% 6% 5% 3 4 6% 24%	1600
December	1st to 10th	Day	16% 15% 4 10% 5% 5% 5% 3 6% 8% 24%	2600
	11th to 20th	Day	20% 21% 4 9% 5% 3 4 3 5% 5% 21%	2902
	21st to end	Day	26% 20% 6% 11% 5% 4 4 2 4 5% 13%	3546
January	1st to 10th	Night	13% 14% 5% 13% 5% 4 4 3 4 6% 29%	6348
	11th to 20th	Night	12% 18% 6% 14% 8% 6% 5% 4 4 5% 18%	6241
	21st to end	Night	18% 17% 8% 10% 5% 5% 5% 3 4 4 22%	6490
February	1st to 10th	Night	16% 16% 5% 12% 7% 5% 5% 3 4 6% 22%	2600
	11th to 20th	Night	15% 15% 5% 10% 5% 5% 5% 4 5% 6% 26%	2461
	21st to end	Night	17% 12% 4 11% 6% 6% 4 4 5% 5% 26%	1980
March	1st to 10th	Night	15% 13% 3 8% 5% 4 4 3 4 5% 35%	5153
	11th to 20th	Night	9% 9% 3 9% 5% 5% 5% 4 6% 7% 38%	4980
	21st to end	Night	16% 10% 3 8% 5% 5% 4 3 4 4 37%	5467
April	1st to 10th	Night	17% 13% 3 8% 4 4 4 4 5% 5% 33%	2135
	11th to 20th	Night	18% 9% 3 5% 5% 4 4 4 5% 7% 38%	1777
	21st to end	Night	17% 11% 3 5% 4 4 4 4 5% 8% 36%	1800
May	1st to 10th	Night	19% 8% 4 8% 4 4 4 3 5% 7% 34%	4033
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 4 5% 6% 39%	4032
	21st to end	Night	17% 6% 2 5% 4 4 4 4 5% 7% 43%	4193
June	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 7% 7% 46%	1475
	11th to 20th	Night	9% 5% 2 6% 5% 5% 4 4 6% 9% 44%	1400
	21st to end	Night	9% 5% 3 6% 4 4 4 4 5% 7% 49%	1400
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 6% 11% 45%	3450
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 9% 48%	3654
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 8% 51%	4427
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 46%	2915
	11th to 20th	Night	9% 8% 3 6% 4 4 4 4 5% 7% 46%	2875
	21st to end	Night	8% 6% 2 7% 5% 4 5% 4 5% 7% 47%	3223
September	1st to 10th	Night	10% 5% 3 7% 4 4 5% 4 5% 7% 46%	2023
	11th to 20th	Night	9% 5% 2 6% 4 3 4 3 4 6% 54%	2090
	21st to end	Night	16% 7% 3 7% 4 4 3 3 4 6% 42%	2090
October	1st to 10th	Night	14% 7% 2 6% 5% 4 3 3 4 7% 45%	2190
	11th to 20th	Night	19% 9% 3 8% 4 4 4 3 4 5% 35%	2280
	21st to end	Night	21% 9% 4 7% 5% 5% 4 4 5% 6% 30%	2608
November	1st to 10th	Night	15% 10% 3 6% 4 4 4 2 6% 7% 39%	2536
	11th to 20th	Night	15% 13% 4 8% 5% 5% 4 4 6% 6% 30%	2655
	21st to end	Night	20% 14% 4 8% 5% 4 4 3 5% 6% 28%	2800
December	1st to 10th	Night	13% 15% 6% 11% 5% 5% 5% 4 5% 7% 23%	4580
	11th to 20th	Night	15% 19% 6% 10% 5% 4 4 3 5% 5% 23%	5279
	21st to end	Night	23% 19% 6% 10% 5% 5% 4 3 4 4 15%	7428

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	60% 4 14% 13% 8% 1	3618
	11th to 20th	Day	75% 3 10% 6% 5% 1	3730
	21st to end	Day	70% 3 12% 8% 6% 1	4466
February	1st to 10th	Day	66% 4 11% 14% 5%	1800
	11th to 20th	Day	60% 4 10% 17% 7% 1	1832
	21st to end	Day	61% 5% 12% 14% 7% 2	1650
March	1st to 10th	Day	56% 4 14% 17% 8% 1	4699
	11th to 20th	Day	50% 5% 15% 22% 7% 1	4980
	21st to end	Day	59% 5% 11% 16% 9% 1	5465
April	1st to 10th	Day	52% 6% 12% 20% 10% 1	2281
	11th to 20th	Day	47% 5% 11% 20% 14% 2	2572
	21st to end	Day	48% 6% 13% 19% 12% 2	2600
May	1st to 10th	Day	47% 5% 11% 18% 13% 5%	5897
	11th to 20th	Day	47% 6% 12% 22% 11% 1	5865
	21st to end	Day	44% 4 10% 20% 17% 5%	6672
June	1st to 10th	Day	38% 6% 13% 27% 14% 2	2920
	11th to 20th	Day	36% 7% 14% 26% 15% 2	3000
	21st to end	Day	37% 7% 14% 25% 16% 1	3000
July	1st to 10th	Day	37% 6% 13% 26% 16% 1	6440
	11th to 20th	Day	32% 6% 12% 31% 17% 1	6244
	21st to end	Day	32% 6% 15% 25% 21% 1	6240
August	1st to 10th	Day	37% 9% 14% 24% 16% 1	4221
	11th to 20th	Day	38% 8% 15% 31% 9% 1	4213
	21st to end	Day	38% 6% 12% 31% 12% 1	4674
September	1st to 10th	Day	38% 7% 15% 26% 13% 1	2201
	11th to 20th	Day	33% 6% 14% 31% 15% 1	2090
	21st to end	Day	45% 6% 14% 24% 11% 1	2090
October	1st to 10th	Day	33% 5% 12% 31% 17% 2	3302
	11th to 20th	Day	43% 6% 12% 24% 15%	3140
	21st to end	Day	46% 5% 15% 23% 10%	3249
November	1st to 10th	Day	48% 6% 13% 22% 9% 1	1762
	11th to 20th	Day	52% 6% 16% 16% 10% 1	1743
	21st to end	Day	61% 6% 12% 17% 4	1600
December	1st to 10th	Day	58% 4 20% 13% 4	2600
	11th to 20th	Day	66% 3 13% 11% 6%	2898
	21st to end	Day	76% 4 10% 7% 4	3546
January	1st to 10th	Night	57% 3 12% 19% 7% 2	6348
	11th to 20th	Night	69% 5% 11% 10% 5%	6241
	21st to end	Night	66% 3 10% 14% 7%	6490
February	1st to 10th	Night	64% 5% 9% 14% 7% 1	2600
	11th to 20th	Night	58% 5% 11% 17% 8% 1	2463
	21st to end	Night	59% 4 10% 19% 7% 1	1980
March	1st to 10th	Night	52% 4 13% 20% 10% 1	5153
	11th to 20th	Night	43% 5% 16% 24% 11% 1	4980
	21st to end	Night	50% 4 12% 20% 14% 1	5467
April	1st to 10th	Night	52% 5% 10% 20% 12% 2	2135
	11th to 20th	Night	46% 5% 10% 20% 18% 2	1777
	21st to end	Night	45% 6% 11% 19% 15% 4	1800
May	1st to 10th	Night	50% 5% 11% 16% 13% 6%	4060
	11th to 20th	Night	45% 6% 11% 25% 12% 2	4033
	21st to end	Night	40% 5% 11% 23% 16% 5%	4200
June	1st to 10th	Night	35% 5% 11% 28% 18% 2	1475
	11th to 20th	Night	34% 7% 14% 24% 18% 2	1400
	21st to end	Night	33% 6% 13% 26% 21% 1	1400
July	1st to 10th	Night	34% 6% 14% 26% 17% 4	3412
	11th to 20th	Night	33% 6% 12% 31% 17% 2	3609
	21st to end	Night	30% 6% 13% 23% 25% 2	4297
August	1st to 10th	Night	35% 6% 14% 26% 17% 2	2904
	11th to 20th	Night	36% 7% 14% 30% 11% 2	2902
	21st to end	Night	36% 6% 12% 30% 15% 1	3223
September	1st to 10th	Night	36% 7% 13% 28% 14% 2	2023
	11th to 20th	Night	32% 5% 12% 30% 19% 2	2090
	21st to end	Night	42% 5% 12% 28% 12% 1	2090
October	1st to 10th	Night	28% 5% 11% 31% 25% 1	3658
	11th to 20th	Night	46% 4 10% 22% 16% 1	3779
	21st to end	Night	44% 3 13% 26% 12% 1	4295
November	1st to 10th	Night	44% 6% 12% 25% 12% 1	2536
	11th to 20th	Night	52% 5% 15% 18% 8% 1	2655
	21st to end	Night	57% 5% 17% 16% 5% 1	2800
December	1st to 10th	Night	61% 6% 19% 13% 2	4580
	11th to 20th	Night	63% 4 12% 14% 7% 1	5282
	21st to end	Night	72% 4 11% 9% 3	7428

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	20% 11% 41% 19% 8% 1	300
	11th to 20th	Day	22% 14% 41% 19% 3 1	300
	21st to end	Day	26% 10% 33% 23% 6% 1	330
February	1st to 10th	Day	25% 12% 40% 18% 4 1	300
	11th to 20th	Day	21% 12% 35% 27% 4	300
	21st to end	Day	18% 15% 42% 19% 6%	249
March	1st to 10th	Day	22% 13% 34% 24% 6% 1	389
	11th to 20th	Day	17% 14% 28% 32% 8% 1	500
	21st to end	Day	24% 11% 30% 25% 8% 1	550
April	1st to 10th	Day	20% 10% 33% 26% 9% 2	500
	11th to 20th	Day	22% 11% 27% 24% 12% 4	492
	21st to end	Day	22% 8% 27% 27% 14% 2	500
May	1st to 10th	Day	21% 9% 22% 31% 14% 3	503
	11th to 20th	Day	18% 5% 21% 29% 21% 7%	527
	21st to end	Day	15% 2 20% 28% 27% 8%	573
June	1st to 10th	Day	12% 6% 23% 34% 19% 5%	650
	11th to 20th	Day	13% 4 22% 35% 20% 6%	730
	21st to end	Day	13% 4 22% 35% 21% 5%	730
July	1st to 10th	Day	10% 4 17% 31% 26% 12%	721
	11th to 20th	Day	12% 3 17% 34% 23% 10%	703
	21st to end	Day	8% 2 13% 36% 31% 11%	576
August	1st to 10th	Day	10% 3 11% 30% 31% 14%	526
	11th to 20th	Day	12% 3 12% 33% 28% 12%	519
	21st to end	Day	13% 5% 14% 27% 29% 12%	583
September	1st to 10th	Day	14% 4 16% 27% 27% 12%	502
	11th to 20th	Day	10% 3 13% 25% 33% 17%	490
	21st to end	Day	16% 9% 22% 25% 18% 11%	490
October	1st to 10th	Day	16% 7% 22% 27% 18% 9%	490
	11th to 20th	Day	23% 11% 27% 25% 11% 2	490
	21st to end	Day	23% 14% 27% 26% 9% 1	435
November	1st to 10th	Day	19% 13% 31% 25% 9% 4	300
	11th to 20th	Day	23% 15% 30% 25% 6% 2	271
	21st to end	Day	27% 12% 37% 19% 5%	200
December	1st to 10th	Day	22% 17% 33% 22% 6% 1	200
	11th to 20th	Day	25% 21% 31% 21% 3 1	200
	21st to end	Day	28% 14% 36% 17% 4	236
January	1st to 10th	Night	24% 12% 38% 19% 6% 1	1000
	11th to 20th	Night	27% 13% 32% 24% 4 1	946
	21st to end	Night	21% 10% 34% 28% 7%	880
February	1st to 10th	Night	24% 7% 39% 26% 4	800
	11th to 20th	Night	23% 10% 33% 29% 5% 1	783
	21st to end	Night	22% 11% 39% 25% 3	660
March	1st to 10th	Night	19% 10% 33% 30% 7%	800
	11th to 20th	Night	18% 9% 35% 31% 6% 1	800
	21st to end	Night	22% 8% 28% 32% 9% 1	880
April	1st to 10th	Night	22% 10% 31% 30% 7%	755
	11th to 20th	Night	22% 6% 30% 28% 11% 3	593
	21st to end	Night	23% 7% 23% 36% 10% 2	600
May	1st to 10th	Night	19% 6% 22% 35% 16% 3	600
	11th to 20th	Night	18% 4 23% 36% 18% 1	591
	21st to end	Night	12% 3 17% 39% 23% 5%	551
June	1st to 10th	Night	13% 4 21% 40% 22% 1	400
	11th to 20th	Night	12% 2 19% 40% 27% 2	400
	21st to end	Night	11% 4 17% 41% 25% 2	400
July	1st to 10th	Night	9% 3 10% 40% 34% 5%	508
	11th to 20th	Night	12% 2 12% 40% 30% 5%	582
	21st to end	Night	8% 3 11% 46% 29% 3	657
August	1st to 10th	Night	10% 2 11% 39% 32% 6%	597
	11th to 20th	Night	11% 2 11% 37% 34% 6%	594
	21st to end	Night	11% 3 15% 36% 31% 5%	660
September	1st to 10th	Night	12% 5% 17% 31% 27% 7%	733
	11th to 20th	Night	11% 4 13% 35% 30% 6%	760
	21st to end	Night	20% 6% 16% 33% 19% 6%	760
October	1st to 10th	Night	15% 9% 24% 30% 19% 3	760
	11th to 20th	Night	23% 11% 26% 27% 11% 1	760
	21st to end	Night	26% 11% 30% 27% 6%	836
November	1st to 10th	Night	22% 10% 28% 27% 12% 1	780
	11th to 20th	Night	23% 13% 36% 21% 7% 1	800
	21st to end	Night	27% 13% 31% 23% 6%	800
December	1st to 10th	Night	25% 14% 36% 20% 5%	800
	11th to 20th	Night	23% 14% 36% 22% 4	835
	21st to end	Night	30% 11% 33% 23% 3	1100

Page generated at 2025-09-01 01:51:09. Version ac07

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