Strathroy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Strathroy clear sky chart from 2005-11-06 to 2026-07-01.

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	15% 10% 4 9% 5% 5% 4 4 5% 7% 32%	137913
	Night	15% 12% 4 9% 5% 4 4 4 5% 6% 32%	134419
Transparency	Day	50% 6% 12% 19% 11% 1	141509
	Night	51% 6% 12% 18% 11% 1	138894
Seeing	Day	18% 12% 28% 23% 13% 5%	17228
	Night	20% 10% 29% 29% 11% 1	26504

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	15% 18% 6% 22% 6% 5% 4 3 4 4 13%	4573
	11th to 20th	Day	12% 22% 9% 22% 9% 8% 4 2 3 2 7%	5237
	21st to end	Day	17% 17% 7% 12% 7% 6% 4 3 4 5% 18%	5780
February	1st to 10th	Day	16% 16% 4 14% 7% 5% 4 4 4 5% 19%	1890
	11th to 20th	Day	15% 15% 6% 14% 7% 5% 4 3 4 6% 20%	1866
	21st to end	Day	17% 16% 6% 12% 6% 4 5% 3 6% 5% 22%	1730
March	1st to 10th	Day	18% 12% 6% 12% 6% 4 3 3 4 5% 27%	4880
	11th to 20th	Day	13% 13% 5% 9% 4 4 4 3 5% 8% 32%	5090
	21st to end	Day	19% 13% 5% 9% 5% 4 4 4 4 4 29%	5586
April	1st to 10th	Day	16% 14% 4 7% 6% 4 4 3 5% 6% 30%	2376
	11th to 20th	Day	18% 11% 3 6% 4 4 4 3 6% 7% 33%	2702
	21st to end	Day	19% 10% 3 6% 5% 4 4 3 5% 6% 34%	2730
May	1st to 10th	Day	18% 8% 3 7% 5% 4 5% 3 5% 6% 36%	7616
	11th to 20th	Day	16% 8% 2 7% 4 4 4 3 6% 8% 36%	7626
	21st to end	Day	16% 7% 2 5% 4 4 4 3 4 7% 44%	8928
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 7% 41%	2965
	11th to 20th	Day	11% 6% 3 6% 5% 5% 6% 5% 6% 9% 39%	3150
	21st to end	Day	12% 5% 3 6% 5% 4 5% 4 6% 9% 41%	3150
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 41%	6733
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	6112
	21st to end	Day	9% 5% 2 6% 4 4 5% 5% 6% 9% 45%	6424
August	1st to 10th	Day	14% 4 2 5% 5% 5% 6% 5% 7% 9% 39%	4234
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 10% 36%	4174
	21st to end	Day	9% 6% 2 7% 4 5% 4 4 6% 9% 43%	4645
September	1st to 10th	Day	11% 6% 3 7% 4 5% 5% 5% 5% 8% 42%	2315
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	2200
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 6% 9% 35%	2200
October	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 4 6% 7% 39%	2060
	11th to 20th	Day	20% 9% 4 7% 4 4 4 4 6% 8% 30%	2000
	21st to end	Day	23% 11% 4 9% 5% 5% 4 4 5% 6% 25%	2140
November	1st to 10th	Day	15% 12% 3 8% 5% 4 4 4 5% 6% 32%	1852
	11th to 20th	Day	16% 15% 4 8% 6% 6% 5% 5% 5% 7% 26%	1758
	21st to end	Day	20% 18% 5% 9% 6% 5% 4 5% 5% 6% 17%	1680
December	1st to 10th	Day	19% 17% 6% 11% 5% 4 4 4 6% 7% 16%	2680
	11th to 20th	Day	21% 20% 6% 9% 5% 4 4 3 5% 5% 16%	3070
	21st to end	Day	27% 21% 8% 12% 5% 4 3 3 4 3 10%	3761
January	1st to 10th	Night	15% 16% 6% 18% 6% 5% 4 3 4 5% 17%	8332
	11th to 20th	Night	11% 20% 8% 16% 8% 6% 5% 4 4 4 14%	7705
	21st to end	Night	16% 15% 7% 11% 6% 5% 5% 4 5% 5% 22%	8401
February	1st to 10th	Night	16% 16% 5% 11% 7% 5% 5% 4 5% 5% 21%	2730
	11th to 20th	Night	16% 14% 5% 9% 6% 5% 5% 4 6% 6% 24%	2637
	21st to end	Night	17% 14% 4 12% 7% 5% 5% 4 5% 5% 23%	2076
March	1st to 10th	Night	16% 12% 4 9% 6% 4 4 4 5% 5% 31%	5193
	11th to 20th	Night	10% 9% 4 8% 5% 5% 5% 4 6% 8% 37%	5090
	21st to end	Night	18% 10% 3 8% 5% 5% 4 3 4 4 36%	5588
April	1st to 10th	Night	18% 12% 4 6% 5% 4 5% 3 6% 6% 32%	2252
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 3 5% 8% 36%	1877
	21st to end	Night	15% 9% 3 8% 4 5% 4 3 5% 9% 35%	1890
May	1st to 10th	Night	19% 7% 3 8% 4 4 4 3 5% 7% 35%	5237
	11th to 20th	Night	14% 9% 3 7% 5% 4 5% 4 6% 6% 38%	5234
	21st to end	Night	19% 8% 2 4 3 3 4 3 5% 6% 43%	5212
June	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 45%	1649
	11th to 20th	Night	10% 4 2 6% 5% 3 5% 5% 7% 10% 42%	1470
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 6% 7% 48%	1470
July	1st to 10th	Night	8% 5% 2 6% 5% 4 4 3 6% 10% 47%	3240
	11th to 20th	Night	10% 5% 2 5% 4 4 3 4 5% 9% 50%	3837
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 8% 51%	4427
August	1st to 10th	Night	11% 5% 3 6% 4 4 4 4 5% 8% 46%	2915
	11th to 20th	Night	10% 7% 2 6% 4 3 4 3 5% 7% 49%	2875
	21st to end	Night	9% 5% 2 5% 4 4 5% 3 6% 8% 49%	3223
September	1st to 10th	Night	9% 6% 3 6% 4 4 4 4 5% 7% 48%	2077
	11th to 20th	Night	10% 5% 1 4 3 3 4 4 4 8% 54%	2200
	21st to end	Night	17% 7% 2 7% 4 5% 4 3 5% 6% 41%	2200
October	1st to 10th	Night	13% 7% 3 7% 5% 4 4 3 5% 6% 44%	2340
	11th to 20th	Night	20% 9% 3 7% 4 3 3 4 4 6% 36%	2400
	21st to end	Night	22% 10% 3 7% 5% 5% 4 3 6% 6% 28%	2696
November	1st to 10th	Night	15% 10% 3 6% 4 4 4 3 6% 7% 39%	2666
	11th to 20th	Night	16% 12% 4 7% 5% 5% 4 5% 7% 7% 27%	2861
	21st to end	Night	21% 17% 4 8% 5% 5% 4 4 6% 5% 22%	2940
December	1st to 10th	Night	15% 18% 7% 11% 6% 5% 6% 5% 6% 7% 15%	4720
	11th to 20th	Night	16% 21% 7% 10% 6% 4 4 3 4 5% 21%	5345
	21st to end	Night	25% 19% 6% 11% 5% 4 4 3 4 5% 14%	7414

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	74% 7% 9% 8% 2	4574
	11th to 20th	Day	84% 5% 5% 3 3	5237
	21st to end	Day	70% 5% 11% 7% 7% 1	5780
February	1st to 10th	Day	67% 6% 11% 11% 5%	1890
	11th to 20th	Day	66% 5% 10% 13% 6%	1875
	21st to end	Day	64% 6% 11% 12% 6% 1	1730
March	1st to 10th	Day	61% 4 14% 17% 5%	4880
	11th to 20th	Day	51% 5% 16% 19% 7% 1	5090
	21st to end	Day	59% 5% 11% 15% 9% 1	5586
April	1st to 10th	Day	55% 5% 11% 19% 9% 1	2376
	11th to 20th	Day	49% 5% 12% 19% 13% 1	2702
	21st to end	Day	49% 6% 13% 18% 12% 2	2730
May	1st to 10th	Day	47% 6% 12% 20% 11% 4	7651
	11th to 20th	Day	45% 7% 13% 22% 12% 1	7630
	21st to end	Day	41% 5% 9% 21% 20% 4	8943
June	1st to 10th	Day	39% 7% 13% 25% 14% 2	2965
	11th to 20th	Day	39% 9% 13% 22% 16% 2	3150
	21st to end	Day	39% 8% 13% 25% 15% 1	3150
July	1st to 10th	Day	38% 6% 13% 24% 17% 2	6660
	11th to 20th	Day	34% 7% 13% 29% 16% 1	6038
	21st to end	Day	33% 7% 14% 24% 20% 1	6240
August	1st to 10th	Day	39% 8% 15% 23% 15% 1	4221
	11th to 20th	Day	39% 9% 15% 27% 9% 1	4213
	21st to end	Day	36% 6% 14% 30% 12% 1	4645
September	1st to 10th	Day	39% 8% 15% 24% 13% 1	2315
	11th to 20th	Day	32% 7% 13% 32% 15%	2200
	21st to end	Day	45% 7% 13% 23% 11% 1	2200
October	1st to 10th	Day	35% 7% 14% 24% 18% 1	3359
	11th to 20th	Day	43% 6% 12% 22% 17%	3240
	21st to end	Day	50% 5% 16% 20% 8%	3442
November	1st to 10th	Day	51% 6% 13% 21% 8% 1	1852
	11th to 20th	Day	56% 7% 15% 13% 8% 1	1758
	21st to end	Day	67% 6% 13% 12% 2	1680
December	1st to 10th	Day	67% 9% 13% 9% 2	2680
	11th to 20th	Day	70% 6% 11% 7% 5%	3066
	21st to end	Day	79% 6% 8% 6% 1	3761
January	1st to 10th	Night	69% 7% 12% 9% 3	8333
	11th to 20th	Night	74% 6% 8% 8% 4	7705
	21st to end	Night	63% 6% 10% 12% 8%	8401
February	1st to 10th	Night	63% 8% 10% 13% 5% 1	2730
	11th to 20th	Night	58% 6% 14% 15% 6%	2639
	21st to end	Night	61% 6% 11% 15% 6%	2076
March	1st to 10th	Night	53% 5% 14% 18% 8% 1	5193
	11th to 20th	Night	43% 6% 18% 21% 10% 1	5090
	21st to end	Night	51% 5% 11% 17% 14% 1	5588
April	1st to 10th	Night	52% 6% 11% 18% 12% 2	2252
	11th to 20th	Night	47% 5% 10% 20% 16% 2	1877
	21st to end	Night	47% 6% 12% 17% 16% 3	1890
May	1st to 10th	Night	48% 5% 11% 15% 15% 6%	5264
	11th to 20th	Night	45% 7% 11% 23% 12% 3	5235
	21st to end	Night	42% 5% 9% 23% 18% 3	5219
June	1st to 10th	Night	36% 6% 11% 25% 18% 3	1649
	11th to 20th	Night	34% 9% 14% 23% 18% 2	1470
	21st to end	Night	33% 7% 13% 26% 19% 1	1470
July	1st to 10th	Night	33% 6% 14% 25% 17% 5%	3206
	11th to 20th	Night	32% 6% 13% 27% 20% 2	3791
	21st to end	Night	30% 7% 14% 22% 25% 3	4297
August	1st to 10th	Night	36% 7% 14% 24% 18% 1	2904
	11th to 20th	Night	35% 6% 13% 32% 13% 1	2902
	21st to end	Night	32% 7% 12% 32% 16% 1	3223
September	1st to 10th	Night	35% 6% 15% 27% 15% 2	2077
	11th to 20th	Night	28% 5% 14% 32% 19% 1	2200
	21st to end	Night	44% 6% 13% 23% 14% 1	2200
October	1st to 10th	Night	29% 5% 11% 27% 26% 1	3821
	11th to 20th	Night	45% 5% 14% 19% 16% 1	3899
	21st to end	Night	45% 6% 15% 22% 11%	4344
November	1st to 10th	Night	45% 6% 13% 24% 12% 1	2666
	11th to 20th	Night	54% 7% 15% 16% 8% 1	2861
	21st to end	Night	62% 7% 16% 12% 3	2940
December	1st to 10th	Night	68% 9% 14% 7% 2	4720
	11th to 20th	Night	66% 5% 11% 10% 7% 1	5348
	21st to end	Night	73% 6% 11% 8% 2	7414

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% 22% 34% 17% 4 1	308
	11th to 20th	Day	22% 22% 42% 10% 3 1	330
	21st to end	Day	29% 17% 33% 19% 2 1	363
February	1st to 10th	Day	24% 21% 38% 13% 4	330
	11th to 20th	Day	26% 22% 37% 14% 1	330
	21st to end	Day	25% 27% 36% 11% 1	273
March	1st to 10th	Day	26% 17% 36% 16% 4 1	461
	11th to 20th	Day	24% 18% 31% 22% 4 1	540
	21st to end	Day	28% 15% 32% 20% 4 1	594
April	1st to 10th	Day	21% 19% 34% 18% 7% 1	540
	11th to 20th	Day	26% 15% 28% 19% 8% 4	532
	21st to end	Day	23% 12% 32% 23% 8% 2	540
May	1st to 10th	Day	22% 13% 29% 26% 8% 3	543
	11th to 20th	Day	18% 9% 29% 23% 15% 5%	567
	21st to end	Day	14% 9% 23% 30% 17% 7%	617
June	1st to 10th	Day	12% 9% 32% 30% 14% 4	595
	11th to 20th	Day	12% 10% 30% 31% 13% 3	780
	21st to end	Day	12% 7% 29% 32% 16% 4	780
July	1st to 10th	Day	10% 7% 22% 29% 23% 9%	722
	11th to 20th	Day	14% 5% 27% 27% 20% 8%	614
	21st to end	Day	8% 6% 20% 31% 24% 10%	576
August	1st to 10th	Day	9% 4 18% 30% 23% 15%	526
	11th to 20th	Day	11% 5% 19% 28% 22% 15%	519
	21st to end	Day	10% 7% 19% 28% 26% 10%	583
September	1st to 10th	Day	12% 5% 24% 25% 22% 12%	530
	11th to 20th	Day	8% 4 19% 23% 31% 16%	530
	21st to end	Day	18% 10% 21% 27% 15% 9%	530
October	1st to 10th	Day	18% 9% 24% 25% 19% 5%	530
	11th to 20th	Day	19% 15% 34% 22% 8% 2	530
	21st to end	Day	25% 16% 34% 20% 4 1	523
November	1st to 10th	Day	22% 18% 32% 19% 8% 2	330
	11th to 20th	Day	24% 25% 30% 16% 5% 2	260
	21st to end	Day	28% 20% 33% 15% 3	220
December	1st to 10th	Day	30% 18% 30% 17% 5% 1	220
	11th to 20th	Day	30% 24% 25% 18% 4	220
	21st to end	Day	30% 21% 31% 14% 3	242
January	1st to 10th	Night	24% 17% 37% 16% 5% 1	1050
	11th to 20th	Night	27% 17% 32% 20% 4	852
	21st to end	Night	22% 13% 36% 23% 6%	924
February	1st to 10th	Night	24% 13% 37% 23% 3	840
	11th to 20th	Night	24% 15% 31% 25% 5%	823
	21st to end	Night	24% 17% 37% 20% 3	692
March	1st to 10th	Night	20% 14% 35% 26% 5%	840
	11th to 20th	Night	20% 15% 35% 23% 6% 1	840
	21st to end	Night	23% 13% 30% 26% 7%	924
April	1st to 10th	Night	23% 14% 31% 26% 5%	840
	11th to 20th	Night	21% 12% 33% 24% 10% 1	633
	21st to end	Night	19% 10% 32% 31% 7% 1	630
May	1st to 10th	Night	19% 8% 30% 31% 11% 1	630
	11th to 20th	Night	14% 5% 30% 39% 11% 2	621
	21st to end	Night	15% 3 27% 39% 15% 1	495
June	1st to 10th	Night	14% 4 27% 40% 14%	420
	11th to 20th	Night	12% 4 26% 44% 13%	420
	21st to end	Night	10% 5% 23% 46% 15% 1	420
July	1st to 10th	Night	7% 1 18% 47% 25% 1	419
	11th to 20th	Night	12% 3 17% 41% 25% 2	582
	21st to end	Night	9% 2 16% 48% 23% 2	657
August	1st to 10th	Night	8% 2 13% 44% 29% 4	597
	11th to 20th	Night	10% 2 13% 38% 32% 4	594
	21st to end	Night	11% 2 16% 38% 30% 2	660
September	1st to 10th	Night	10% 5% 22% 34% 25% 5%	796
	11th to 20th	Night	10% 4 15% 37% 30% 4	800
	21st to end	Night	18% 7% 20% 34% 16% 5%	800
October	1st to 10th	Night	17% 6% 25% 32% 18% 2	800
	11th to 20th	Night	23% 11% 27% 27% 11% 1	800
	21st to end	Night	26% 11% 31% 27% 5%	880
November	1st to 10th	Night	20% 13% 29% 29% 9% 1	820
	11th to 20th	Night	22% 14% 35% 21% 6% 1	840
	21st to end	Night	29% 13% 30% 23% 5%	840
December	1st to 10th	Night	28% 15% 33% 18% 5%	840
	11th to 20th	Night	24% 18% 37% 18% 3	840
	21st to end	Night	29% 12% 35% 21% 3	1045

Page generated at 2026-06-28 02:10:46. Version ac07

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