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 2025-02-25.

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% 11% 4 9% 5% 5% 4 4 5% 7% 31%	108853
	Night	15% 12% 4 9% 5% 4 4 4 5% 6% 31%	109793
Transparency	Day	50% 7% 13% 18% 10% 1	112493
	Night	51% 7% 13% 18% 11% 1	114293
Seeing	Day	18% 12% 28% 24% 13% 5%	15391
	Night	20% 10% 29% 29% 11% 1	24837

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	13% 18% 6% 22% 5% 5% 4 3 4 4 15%	3525
	11th to 20th	Day	12% 21% 8% 21% 9% 8% 5% 3 3 3 8%	4030
	21st to end	Day	21% 18% 8% 12% 6% 5% 3 3 3 4 17%	4466
February	1st to 10th	Day	17% 16% 4 15% 7% 5% 4 4 4 5% 19%	1800
	11th to 20th	Day	15% 15% 6% 13% 7% 5% 4 3 4 6% 20%	1775
	21st to end	Day	18% 15% 6% 11% 6% 4 4 3 5% 4 23%	1610
March	1st to 10th	Day	20% 14% 5% 10% 5% 4 3 3 3 4 28%	3330
	11th to 20th	Day	13% 14% 6% 9% 4 4 3 4 5% 7% 31%	3480
	21st to end	Day	18% 12% 4 9% 5% 4 4 4 4 5% 32%	3815
April	1st to 10th	Day	15% 14% 4 7% 6% 4 4 3 6% 6% 31%	2149
	11th to 20th	Day	19% 11% 3 6% 4 4 4 3 6% 7% 33%	2442
	21st to end	Day	20% 10% 4 6% 5% 4 4 3 5% 6% 33%	2470
May	1st to 10th	Day	18% 8% 3 7% 4 4 5% 4 6% 7% 36%	4082
	11th to 20th	Day	17% 9% 2 8% 5% 4 4 4 6% 8% 32%	4081
	21st to end	Day	13% 4 2 5% 4 4 4 4 5% 8% 47%	4761
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 5% 7% 7% 40%	2683
	11th to 20th	Day	10% 6% 3 6% 5% 4 6% 5% 6% 9% 41%	2850
	21st to end	Day	13% 5% 3 6% 5% 4 5% 4 7% 9% 40%	2850
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 10% 39%	4700
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 6% 10% 44%	4262
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	4466
August	1st to 10th	Day	15% 5% 2 5% 5% 5% 5% 5% 7% 9% 38%	4104
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 10% 36%	4044
	21st to end	Day	9% 6% 2 7% 4 5% 4 4 6% 9% 42%	4503
September	1st to 10th	Day	11% 6% 3 7% 4 5% 5% 5% 5% 8% 41%	2199
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 46%	2090
	21st to end	Day	17% 7% 2 7% 4 4 5% 4 6% 9% 34%	2090
October	1st to 10th	Day	15% 7% 3 6% 5% 4 5% 4 6% 7% 37%	1957
	11th to 20th	Day	20% 10% 4 7% 5% 4 4 4 6% 7% 29%	1900
	21st to end	Day	24% 11% 4 9% 5% 5% 4 4 5% 6% 24%	2033
November	1st to 10th	Day	15% 12% 3 8% 6% 5% 4 4 6% 6% 32%	1762
	11th to 20th	Day	16% 15% 4 8% 6% 6% 5% 5% 5% 7% 25%	1675
	21st to end	Day	20% 18% 5% 9% 6% 5% 4 5% 5% 5% 17%	1600
December	1st to 10th	Day	19% 17% 6% 12% 5% 4 4 4 6% 7% 16%	2600
	11th to 20th	Day	22% 20% 7% 9% 5% 4 4 3 5% 5% 17%	2990
	21st to end	Day	28% 21% 8% 11% 5% 4 3 3 4 3 10%	3679
January	1st to 10th	Night	14% 15% 6% 18% 5% 5% 4 3 5% 6% 19%	6422
	11th to 20th	Night	12% 19% 8% 16% 8% 7% 6% 4 4 4 12%	5925
	21st to end	Night	19% 16% 7% 10% 5% 4 5% 4 5% 5% 20%	6490
February	1st to 10th	Night	17% 16% 5% 11% 7% 5% 5% 4 5% 5% 20%	2600
	11th to 20th	Night	16% 14% 5% 9% 6% 5% 5% 4 6% 6% 24%	2508
	21st to end	Night	17% 12% 4 12% 7% 6% 5% 4 5% 5% 23%	1939
March	1st to 10th	Night	17% 14% 4 8% 5% 4 4 3 4 5% 32%	3523
	11th to 20th	Night	11% 9% 4 7% 5% 5% 5% 4 5% 8% 36%	3480
	21st to end	Night	17% 10% 3 8% 5% 5% 4 3 4 4 37%	3817
April	1st to 10th	Night	18% 12% 3 5% 5% 4 4 3 6% 6% 33%	2038
	11th to 20th	Night	18% 9% 2 6% 4 4 4 3 5% 8% 36%	1697
	21st to end	Night	16% 10% 3 8% 4 5% 4 3 5% 8% 34%	1710
May	1st to 10th	Night	19% 8% 3 7% 4 4 4 3 5% 8% 37%	2813
	11th to 20th	Night	12% 9% 3 7% 5% 5% 5% 4 7% 8% 36%	2812
	21st to end	Night	12% 4 1 4 4 4 5% 4 5% 7% 50%	2780
June	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 9% 43%	1492
	11th to 20th	Night	10% 4 2 6% 5% 3 6% 5% 7% 10% 43%	1330
	21st to end	Night	10% 5% 2 6% 5% 5% 4 4 6% 8% 47%	1330
July	1st to 10th	Night	8% 4 2 4 4 3 4 3 5% 10% 51%	2264
	11th to 20th	Night	9% 4 2 5% 3 3 3 3 5% 9% 53%	2685
	21st to end	Night	6% 5% 1 5% 4 5% 6% 5% 7% 9% 48%	3087
August	1st to 10th	Night	12% 5% 3 6% 4 4 4 4 5% 7% 46%	2825
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 3 5% 7% 49%	2785
	21st to end	Night	9% 5% 2 5% 4 4 5% 3 6% 8% 49%	3124
September	1st to 10th	Night	9% 6% 3 6% 4 4 4 4 5% 8% 47%	1973
	11th to 20th	Night	10% 5% 1 4 3 3 4 4 4 8% 52%	2090
	21st to end	Night	17% 7% 2 7% 4 5% 4 3 5% 6% 41%	2090
October	1st to 10th	Night	14% 7% 3 7% 5% 4 4 4 5% 6% 43%	2223
	11th to 20th	Night	21% 9% 3 7% 4 3 3 4 5% 6% 35%	2280
	21st to end	Night	23% 10% 3 7% 4 5% 4 3 6% 6% 27%	2561
November	1st to 10th	Night	15% 10% 3 6% 4 4 4 3 6% 7% 39%	2536
	11th to 20th	Night	17% 12% 4 8% 5% 5% 5% 5% 7% 7% 27%	2725
	21st to end	Night	21% 17% 4 7% 5% 4 4 4 6% 5% 22%	2800
December	1st to 10th	Night	15% 18% 7% 11% 6% 5% 6% 5% 6% 7% 15%	4580
	11th to 20th	Night	16% 22% 7% 9% 6% 4 4 3 4 4 21%	5205
	21st to end	Night	25% 20% 7% 11% 5% 4 4 3 4 5% 14%	7254

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	72% 8% 9% 8% 2	3526
	11th to 20th	Day	81% 5% 6% 3 3	4030
	21st to end	Day	72% 5% 12% 6% 6% 1	4466
February	1st to 10th	Day	67% 6% 11% 11% 5% 1	1800
	11th to 20th	Day	66% 5% 10% 13% 6%	1784
	21st to end	Day	63% 7% 11% 12% 6% 1	1610
March	1st to 10th	Day	61% 5% 10% 20% 4	3330
	11th to 20th	Day	53% 7% 15% 17% 7% 1	3480
	21st to end	Day	54% 6% 11% 16% 11% 1	3815
April	1st to 10th	Day	54% 6% 12% 19% 10% 1	2149
	11th to 20th	Day	49% 6% 12% 19% 13% 1	2442
	21st to end	Day	50% 6% 13% 17% 12% 2	2470
May	1st to 10th	Day	45% 7% 13% 21% 12% 3	4120
	11th to 20th	Day	47% 9% 15% 20% 8% 1	4085
	21st to end	Day	35% 7% 12% 20% 20% 6%	4775
June	1st to 10th	Day	40% 8% 13% 25% 13% 2	2683
	11th to 20th	Day	38% 9% 13% 22% 17% 2	2850
	21st to end	Day	40% 8% 12% 24% 15% 1	2850
July	1st to 10th	Day	39% 7% 14% 23% 15% 2	4627
	11th to 20th	Day	33% 7% 14% 32% 14% 1	4188
	21st to end	Day	36% 8% 15% 19% 20% 1	4324
August	1st to 10th	Day	40% 8% 15% 23% 15% 1	4091
	11th to 20th	Day	39% 9% 16% 27% 9% 1	4083
	21st to end	Day	37% 6% 14% 30% 13% 1	4503
September	1st to 10th	Day	40% 8% 15% 23% 13% 1	2199
	11th to 20th	Day	34% 7% 13% 30% 16%	2090
	21st to end	Day	45% 7% 13% 23% 11% 1	2090
October	1st to 10th	Day	36% 8% 14% 23% 18% 1	3256
	11th to 20th	Day	43% 7% 12% 21% 17%	3140
	21st to end	Day	50% 5% 17% 21% 7%	3335
November	1st to 10th	Day	51% 6% 13% 20% 8% 1	1762
	11th to 20th	Day	56% 7% 16% 12% 8% 1	1675
	21st to end	Day	67% 6% 13% 12% 2	1600
December	1st to 10th	Day	67% 9% 13% 8% 2	2600
	11th to 20th	Day	70% 6% 11% 7% 6%	2986
	21st to end	Day	79% 6% 8% 6% 1	3679
January	1st to 10th	Night	67% 7% 13% 11% 3	6422
	11th to 20th	Night	75% 7% 9% 6% 3	5925
	21st to end	Night	65% 6% 11% 10% 8%	6490
February	1st to 10th	Night	65% 8% 10% 12% 5%	2600
	11th to 20th	Night	59% 6% 14% 15% 6%	2510
	21st to end	Night	61% 7% 11% 15% 6%	1939
March	1st to 10th	Night	53% 6% 13% 22% 5%	3523
	11th to 20th	Night	43% 8% 18% 19% 11% 1	3480
	21st to end	Night	49% 6% 11% 20% 13% 1	3817
April	1st to 10th	Night	50% 7% 10% 19% 12% 1	2038
	11th to 20th	Night	47% 5% 10% 20% 16% 2	1697
	21st to end	Night	48% 6% 11% 17% 15% 3	1710
May	1st to 10th	Night	45% 6% 11% 18% 16% 5%	2840
	11th to 20th	Night	43% 9% 13% 22% 11% 2	2813
	21st to end	Night	32% 8% 11% 23% 22% 4	2787
June	1st to 10th	Night	37% 6% 12% 26% 16% 2	1492
	11th to 20th	Night	34% 9% 14% 22% 19% 2	1330
	21st to end	Night	34% 7% 13% 26% 18% 1	1330
July	1st to 10th	Night	29% 6% 13% 28% 18% 7%	2228
	11th to 20th	Night	29% 6% 14% 31% 19% 2	2636
	21st to end	Night	29% 8% 18% 21% 21% 3	2988
August	1st to 10th	Night	36% 7% 14% 25% 17% 1	2814
	11th to 20th	Night	35% 6% 13% 32% 12% 1	2812
	21st to end	Night	32% 6% 12% 31% 17% 1	3124
September	1st to 10th	Night	35% 6% 15% 26% 16% 2	1973
	11th to 20th	Night	29% 6% 14% 31% 19% 1	2090
	21st to end	Night	44% 7% 13% 23% 13%	2090
October	1st to 10th	Night	30% 5% 12% 27% 26% 1	3704
	11th to 20th	Night	45% 5% 14% 20% 15% 1	3779
	21st to end	Night	45% 6% 15% 22% 11%	4209
November	1st to 10th	Night	45% 6% 13% 24% 12% 1	2536
	11th to 20th	Night	54% 7% 15% 15% 8% 1	2725
	21st to end	Night	61% 8% 16% 12% 3 1	2800
December	1st to 10th	Night	68% 9% 14% 7% 2	4580
	11th to 20th	Night	66% 6% 10% 10% 7% 1	5208
	21st to end	Night	74% 6% 11% 7% 2	7254

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% 23% 32% 18% 5% 1	280
	11th to 20th	Day	21% 21% 43% 11% 4 1	300
	21st to end	Day	30% 15% 34% 18% 2 1	330
February	1st to 10th	Day	24% 22% 39% 11% 4	300
	11th to 20th	Day	26% 22% 38% 13% 1	300
	21st to end	Day	27% 25% 35% 11% 1	237
March	1st to 10th	Day	26% 15% 37% 16% 5% 1	389
	11th to 20th	Day	21% 16% 32% 24% 5% 1	460
	21st to end	Day	28% 14% 32% 21% 5% 1	506
April	1st to 10th	Day	18% 19% 35% 20% 8% 1	460
	11th to 20th	Day	27% 13% 27% 20% 7% 4	452
	21st to end	Day	23% 12% 32% 22% 9% 2	460
May	1st to 10th	Day	21% 12% 27% 27% 10% 3	463
	11th to 20th	Day	18% 8% 27% 25% 17% 6%	487
	21st to end	Day	12% 8% 22% 31% 19% 8%	529
June	1st to 10th	Day	12% 8% 31% 30% 15% 4	513
	11th to 20th	Day	12% 9% 29% 33% 15% 4	680
	21st to end	Day	13% 8% 29% 30% 16% 4	680
July	1st to 10th	Day	10% 7% 21% 30% 24% 9%	671
	11th to 20th	Day	15% 4 27% 26% 20% 8%	569
	21st to end	Day	9% 5% 20% 31% 25% 11%	532
August	1st to 10th	Day	10% 4 19% 29% 22% 16%	486
	11th to 20th	Day	10% 5% 19% 28% 22% 16%	479
	21st to end	Day	10% 6% 19% 27% 27% 10%	539
September	1st to 10th	Day	12% 5% 23% 27% 22% 11%	490
	11th to 20th	Day	8% 4 20% 23% 29% 15%	490
	21st to end	Day	18% 11% 22% 27% 14% 8%	490
October	1st to 10th	Day	19% 10% 25% 24% 18% 5%	490
	11th to 20th	Day	20% 15% 33% 22% 9% 2	490
	21st to end	Day	26% 16% 33% 21% 4	482
November	1st to 10th	Day	20% 18% 32% 19% 8% 2	300
	11th to 20th	Day	23% 23% 32% 16% 5% 2	237
	21st to end	Day	27% 20% 35% 15% 4	200
December	1st to 10th	Day	29% 18% 31% 16% 6% 1	200
	11th to 20th	Day	31% 22% 26% 19% 4	200
	21st to end	Day	29% 20% 33% 15% 3	220
January	1st to 10th	Night	24% 17% 37% 16% 5% 1	1000
	11th to 20th	Night	27% 16% 31% 21% 4	812
	21st to end	Night	23% 12% 35% 24% 6%	880
February	1st to 10th	Night	25% 13% 37% 22% 2	800
	11th to 20th	Night	24% 15% 31% 25% 5%	783
	21st to end	Night	24% 15% 38% 19% 3	647
March	1st to 10th	Night	19% 13% 35% 27% 6%	760
	11th to 20th	Night	18% 15% 35% 24% 7% 1	760
	21st to end	Night	24% 12% 30% 27% 8%	836
April	1st to 10th	Night	23% 14% 31% 27% 5%	760
	11th to 20th	Night	21% 12% 32% 24% 10% 1	573
	21st to end	Night	20% 9% 33% 30% 7% 1	570
May	1st to 10th	Night	19% 7% 29% 32% 11% 1	570
	11th to 20th	Night	14% 4 28% 40% 12% 2	561
	21st to end	Night	13% 3 26% 41% 17% 1	447
June	1st to 10th	Night	14% 4 28% 40% 14%	380
	11th to 20th	Night	12% 3 26% 44% 14%	380
	21st to end	Night	10% 6% 22% 46% 16% 1	380
July	1st to 10th	Night	7% 1 18% 47% 25% 2	397
	11th to 20th	Night	12% 3 17% 41% 25% 2	552
	21st to end	Night	9% 2 16% 48% 23% 2	624
August	1st to 10th	Night	8% 2 13% 44% 29% 3	567
	11th to 20th	Night	10% 2 14% 38% 32% 4	564
	21st to end	Night	12% 2 16% 37% 30% 3	627
September	1st to 10th	Night	10% 5% 22% 35% 24% 5%	756
	11th to 20th	Night	11% 4 16% 37% 28% 4	760
	21st to end	Night	18% 7% 21% 33% 16% 5%	760
October	1st to 10th	Night	17% 6% 26% 31% 17% 2	760
	11th to 20th	Night	24% 11% 26% 27% 11% 1	760
	21st to end	Night	27% 11% 31% 26% 5%	836
November	1st to 10th	Night	20% 12% 29% 29% 9% 1	780
	11th to 20th	Night	23% 13% 36% 21% 6% 1	800
	21st to end	Night	28% 13% 29% 24% 5%	800
December	1st to 10th	Night	28% 15% 34% 18% 5%	800
	11th to 20th	Night	24% 17% 36% 19% 4	800
	21st to end	Night	29% 12% 35% 21% 3	995

Page generated at 2025-02-22 06:51:38. Version ac07

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