Brockville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brockville clear sky chart from 2005-11-06 to 2026-06-16.

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	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	138078
	Night	18% 12% 4 9% 5% 4 4 3 5% 6% 29%	133928
Transparency	Day	52% 5% 11% 21% 10% 1	141558
	Night	55% 5% 11% 18% 9% 1	138537
Seeing	Day	25% 15% 30% 19% 8% 2	16530
	Night	25% 14% 29% 23% 8% 1	25678

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	16% 18% 6% 15% 6% 5% 4 3 5% 5% 18%	4661
	11th to 20th	Day	12% 20% 8% 17% 8% 5% 4 3 4 4 15%	4650
	21st to end	Day	11% 17% 7% 14% 9% 8% 6% 3 4 4 17%	5423
February	1st to 10th	Day	17% 17% 5% 13% 7% 4 4 4 4 6% 21%	1890
	11th to 20th	Day	15% 15% 5% 12% 6% 5% 4 3 5% 4 24%	1870
	21st to end	Day	19% 17% 6% 11% 7% 5% 4 3 4 4 20%	1730
March	1st to 10th	Day	20% 15% 4 10% 5% 4 4 3 4 5% 27%	4576
	11th to 20th	Day	11% 14% 4 10% 5% 5% 5% 4 5% 6% 30%	5085
	21st to end	Day	20% 10% 3 9% 5% 4 4 3 4 4 35%	5906
April	1st to 10th	Day	21% 14% 3 7% 5% 5% 3 2 3 5% 31%	2520
	11th to 20th	Day	23% 9% 3 7% 4 4 3 3 4 5% 36%	2494
	21st to end	Day	22% 10% 3 7% 4 4 4 3 4 6% 32%	2615
May	1st to 10th	Day	24% 10% 3 8% 5% 4 4 3 5% 6% 27%	8237
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 3 5% 7% 34%	8221
	21st to end	Day	22% 8% 3 7% 4 4 4 3 6% 7% 33%	9060
June	1st to 10th	Day	21% 7% 3 7% 5% 5% 4 4 6% 8% 32%	2940
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 8% 37%	2870
	21st to end	Day	19% 8% 2 6% 5% 4 5% 4 6% 7% 34%	2800
July	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 5% 7% 9% 37%	6360
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 4 6% 8% 42%	6360
	21st to end	Day	12% 5% 2 5% 4 4 5% 4 6% 9% 44%	6926
August	1st to 10th	Day	16% 7% 3 7% 4 4 4 4 6% 8% 37%	4447
	11th to 20th	Day	17% 6% 3 5% 4 4 4 3 6% 8% 39%	4083
	21st to end	Day	17% 5% 2 6% 4 3 4 4 6% 8% 40%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 3 5% 4 6% 6% 44%	2400
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 5% 7% 49%	2395
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 5% 7% 38%	2145
October	1st to 10th	Day	21% 8% 3 7% 4 4 4 3 5% 6% 36%	2000
	11th to 20th	Day	20% 11% 3 9% 6% 4 4 3 5% 7% 30%	2000
	21st to end	Day	27% 9% 2 8% 5% 4 5% 4 4 6% 27%	2050
November	1st to 10th	Day	17% 10% 4 8% 6% 6% 3 4 6% 7% 30%	1851
	11th to 20th	Day	16% 13% 4 10% 6% 4 5% 4 5% 5% 29%	1727
	21st to end	Day	21% 17% 5% 10% 5% 5% 4 3 4 5% 21%	1680
December	1st to 10th	Day	22% 19% 5% 12% 6% 4 5% 3 5% 5% 14%	2680
	11th to 20th	Day	19% 17% 6% 11% 6% 5% 4 2 4 5% 21%	3066
	21st to end	Day	22% 17% 4 12% 5% 5% 4 3 4 4 22%	4048
January	1st to 10th	Night	16% 22% 7% 12% 6% 4 4 3 4 4 19%	8234
	11th to 20th	Night	15% 20% 8% 16% 8% 6% 4 3 4 4 14%	8235
	21st to end	Night	14% 15% 7% 12% 8% 7% 5% 4 4 5% 18%	8742
February	1st to 10th	Night	18% 16% 5% 13% 7% 4 5% 3 4 5% 20%	2730
	11th to 20th	Night	15% 15% 6% 11% 6% 4 4 3 4 5% 26%	2635
	21st to end	Night	18% 16% 5% 10% 6% 4 4 3 4 6% 24%	2076
March	1st to 10th	Night	21% 14% 4 9% 5% 5% 3 3 3 4 29%	5484
	11th to 20th	Night	17% 14% 4 7% 4 3 4 3 6% 8% 31%	5090
	21st to end	Night	21% 13% 4 9% 5% 4 3 3 4 4 31%	5267
April	1st to 10th	Night	22% 11% 3 8% 4 3 4 3 5% 5% 31%	2100
	11th to 20th	Night	22% 10% 2 7% 5% 3 3 2 4 5% 35%	2075
	21st to end	Night	23% 9% 3 7% 5% 4 4 3 4 5% 34%	2021
May	1st to 10th	Night	25% 11% 3 8% 5% 4 5% 4 5% 6% 23%	4660
	11th to 20th	Night	18% 10% 3 8% 4 4 4 3 5% 8% 33%	4639
	21st to end	Night	20% 7% 2 6% 4 4 5% 4 5% 8% 34%	5114
June	1st to 10th	Night	19% 7% 2 7% 6% 4 4 4 6% 8% 33%	1680
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 4 5% 8% 40%	1640
	21st to end	Night	18% 7% 3 6% 4 5% 5% 4 6% 8% 34%	1600
July	1st to 10th	Night	13% 6% 2 7% 4 5% 5% 4 7% 9% 38%	3600
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 8% 43%	3600
	21st to end	Night	10% 6% 2 7% 4 5% 4 4 6% 7% 46%	3926
August	1st to 10th	Night	14% 8% 3 6% 4 4 5% 4 6% 9% 39%	2699
	11th to 20th	Night	15% 8% 2 7% 4 4 4 4 6% 7% 41%	2932
	21st to end	Night	10% 5% 3 7% 5% 5% 6% 4 7% 8% 41%	3586
September	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 4 6% 46%	2000
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 7% 50%	2005
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 6% 40%	2252
October	1st to 10th	Night	19% 8% 3 6% 5% 4 5% 3 5% 6% 37%	2400
	11th to 20th	Night	19% 9% 3 8% 5% 4 4 4 6% 6% 33%	2400
	21st to end	Night	26% 8% 3 8% 5% 4 4 3 5% 6% 28%	2790
November	1st to 10th	Night	20% 10% 4 7% 4 4 3 3 5% 6% 34%	2667
	11th to 20th	Night	18% 12% 4 7% 5% 4 4 4 5% 6% 31%	2891
	21st to end	Night	21% 16% 4 9% 5% 5% 4 4 5% 6% 22%	2940
December	1st to 10th	Night	20% 21% 6% 13% 6% 5% 4 3 4 4 14%	4720
	11th to 20th	Night	23% 16% 5% 9% 6% 4 3 3 3 4 25%	5348
	21st to end	Night	27% 15% 5% 12% 5% 4 4 3 5% 4 17%	7150

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	67% 6% 10% 14% 3	4662
	11th to 20th	Day	73% 4 9% 7% 7% 1	4680
	21st to end	Day	71% 4 8% 8% 8% 1	5423
February	1st to 10th	Day	65% 4 9% 16% 4 1	1890
	11th to 20th	Day	63% 5% 10% 15% 7% 1	1879
	21st to end	Day	68% 5% 7% 12% 8% 1	1730
March	1st to 10th	Day	59% 4 10% 14% 10% 2	4579
	11th to 20th	Day	53% 5% 17% 13% 11% 1	5085
	21st to end	Day	52% 3 8% 18% 16% 2	5906
April	1st to 10th	Day	57% 4 7% 18% 13% 2	2520
	11th to 20th	Day	51% 4 8% 19% 15% 3	2494
	21st to end	Day	54% 5% 8% 16% 14% 3	2615
May	1st to 10th	Day	56% 6% 9% 20% 9% 1	8249
	11th to 20th	Day	48% 7% 12% 23% 10% 1	8225
	21st to end	Day	50% 5% 11% 24% 9% 1	9061
June	1st to 10th	Day	50% 6% 10% 20% 13% 1	2940
	11th to 20th	Day	43% 7% 12% 25% 11% 2	2870
	21st to end	Day	48% 8% 12% 20% 11% 1	2800
July	1st to 10th	Day	41% 6% 13% 28% 11%	6290
	11th to 20th	Day	38% 6% 14% 27% 14%	6285
	21st to end	Day	35% 6% 13% 36% 9%	6728
August	1st to 10th	Day	44% 6% 15% 24% 10%	4433
	11th to 20th	Day	42% 6% 14% 27% 9% 1	4122
	21st to end	Day	41% 6% 15% 28% 10%	4312
September	1st to 10th	Day	38% 7% 14% 28% 12% 1	2400
	11th to 20th	Day	36% 5% 12% 31% 15%	2395
	21st to end	Day	46% 6% 12% 25% 11%	2145
October	1st to 10th	Day	46% 4 12% 28% 10%	3260
	11th to 20th	Day	49% 6% 15% 23% 7% 1	3240
	21st to end	Day	49% 5% 15% 24% 7%	3288
November	1st to 10th	Day	51% 6% 15% 21% 7% 1	1851
	11th to 20th	Day	57% 5% 12% 16% 8% 2	1727
	21st to end	Day	65% 6% 13% 10% 6%	1680
December	1st to 10th	Day	71% 6% 12% 10% 1	2680
	11th to 20th	Day	68% 4 10% 13% 5%	3066
	21st to end	Day	66% 5% 9% 16% 4	4048
January	1st to 10th	Night	68% 5% 11% 11% 4	8234
	11th to 20th	Night	74% 4 8% 8% 5%	8280
	21st to end	Night	67% 5% 7% 12% 8% 1	8742
February	1st to 10th	Night	67% 5% 9% 13% 6% 1	2730
	11th to 20th	Night	60% 5% 10% 18% 7%	2637
	21st to end	Night	62% 5% 9% 16% 8% 1	2076
March	1st to 10th	Night	60% 4 7% 17% 10% 3	5494
	11th to 20th	Night	52% 4 18% 15% 8% 3	5091
	21st to end	Night	57% 3 7% 21% 11% 2	5267
April	1st to 10th	Night	54% 4 9% 19% 13% 1	2100
	11th to 20th	Night	52% 5% 9% 18% 15% 1	2075
	21st to end	Night	53% 4 9% 17% 15% 3	2021
May	1st to 10th	Night	60% 5% 9% 16% 8% 2	4666
	11th to 20th	Night	49% 6% 12% 20% 12% 1	4640
	21st to end	Night	48% 5% 11% 20% 15% 1	5115
June	1st to 10th	Night	47% 7% 12% 20% 13% 1	1680
	11th to 20th	Night	42% 6% 13% 20% 16% 2	1640
	21st to end	Night	46% 8% 12% 21% 13% 1	1600
July	1st to 10th	Night	42% 6% 16% 22% 14% 1	3560
	11th to 20th	Night	40% 5% 13% 30% 11% 1	3552
	21st to end	Night	36% 7% 12% 32% 13% 1	3809
August	1st to 10th	Night	41% 7% 15% 26% 11% 1	2689
	11th to 20th	Night	42% 6% 13% 29% 9% 1	2959
	21st to end	Night	38% 7% 15% 29% 11% 1	3586
September	1st to 10th	Night	39% 5% 13% 29% 13% 1	2000
	11th to 20th	Night	35% 6% 13% 32% 14% 1	2005
	21st to end	Night	43% 6% 11% 26% 13% 1	2252
October	1st to 10th	Night	43% 5% 13% 27% 12% 1	3920
	11th to 20th	Night	44% 6% 15% 24% 10%	3899
	21st to end	Night	50% 4 16% 23% 7%	4502
November	1st to 10th	Night	50% 5% 14% 19% 10% 1	2667
	11th to 20th	Night	53% 6% 13% 18% 9% 2	2891
	21st to end	Night	63% 6% 12% 15% 5%	2940
December	1st to 10th	Night	73% 5% 11% 9% 2	4720
	11th to 20th	Night	64% 5% 9% 14% 8%	5348
	21st to end	Night	69% 4 8% 11% 7%	7150

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	27% 20% 35% 13% 5%	220
	11th to 20th	Day	39% 18% 29% 12% 2	220
	21st to end	Day	25% 23% 33% 15% 4	297
February	1st to 10th	Day	33% 22% 31% 11% 2	330
	11th to 20th	Day	32% 24% 34% 8% 1	357
	21st to end	Day	35% 21% 32% 12% 2	446
March	1st to 10th	Day	29% 21% 31% 15% 4	540
	11th to 20th	Day	32% 21% 30% 14% 4 1	540
	21st to end	Day	35% 19% 26% 14% 5% 1	594
April	1st to 10th	Day	37% 14% 31% 14% 4	540
	11th to 20th	Day	34% 19% 25% 13% 6% 3	532
	21st to end	Day	32% 18% 30% 13% 5% 2	540
May	1st to 10th	Day	27% 18% 28% 19% 6% 1	543
	11th to 20th	Day	20% 16% 35% 19% 8% 2	567
	21st to end	Day	22% 15% 30% 22% 9% 1	617
June	1st to 10th	Day	26% 12% 31% 22% 8% 1	570
	11th to 20th	Day	18% 14% 34% 22% 11% 2	550
	21st to end	Day	21% 18% 31% 19% 9% 2	530
July	1st to 10th	Day	14% 9% 32% 27% 14% 2	525
	11th to 20th	Day	16% 9% 31% 29% 12% 2	524
	21st to end	Day	13% 10% 34% 30% 11% 2	576
August	1st to 10th	Day	16% 8% 26% 31% 15% 4	526
	11th to 20th	Day	15% 7% 26% 30% 15% 6%	519
	21st to end	Day	16% 11% 31% 22% 14% 6%	583
September	1st to 10th	Day	16% 12% 30% 23% 13% 7%	530
	11th to 20th	Day	12% 8% 23% 25% 18% 13%	530
	21st to end	Day	21% 10% 28% 23% 15% 3	530
October	1st to 10th	Day	23% 15% 26% 19% 12% 5%	530
	11th to 20th	Day	26% 19% 30% 17% 6% 1	530
	21st to end	Day	34% 15% 27% 18% 6%	500
November	1st to 10th	Day	28% 20% 28% 17% 6% 1	425
	11th to 20th	Day	29% 15% 34% 13% 7% 1	267
	21st to end	Day	35% 19% 34% 11% 1	220
December	1st to 10th	Day	36% 19% 27% 15% 4	220
	11th to 20th	Day	32% 24% 35% 7% 2	220
	21st to end	Day	38% 18% 26% 14% 3 1	242
January	1st to 10th	Night	32% 17% 29% 18% 4	840
	11th to 20th	Night	32% 17% 28% 18% 4	840
	21st to end	Night	24% 18% 32% 21% 6%	924
February	1st to 10th	Night	27% 20% 31% 19% 3	840
	11th to 20th	Night	25% 19% 36% 17% 3	823
	21st to end	Night	28% 18% 30% 20% 3	692
March	1st to 10th	Night	28% 16% 30% 21% 4	840
	11th to 20th	Night	28% 18% 31% 19% 4 1	840
	21st to end	Night	28% 15% 32% 18% 6%	924
April	1st to 10th	Night	29% 15% 31% 23% 2	840
	11th to 20th	Night	29% 18% 28% 19% 6%	831
	21st to end	Night	28% 11% 31% 22% 8%	761
May	1st to 10th	Night	28% 9% 26% 27% 9% 1	420
	11th to 20th	Night	17% 11% 35% 30% 8%	414
	21st to end	Night	17% 9% 35% 29% 9% 1	455
June	1st to 10th	Night	20% 7% 28% 37% 7% 1	420
	11th to 20th	Night	16% 12% 29% 30% 11% 1	410
	21st to end	Night	19% 8% 28% 32% 13% 1	400
July	1st to 10th	Night	11% 6% 31% 37% 14%	390
	11th to 20th	Night	13% 5% 27% 36% 18% 1	388
	21st to end	Night	13% 5% 27% 41% 11% 2	438
August	1st to 10th	Night	13% 3 25% 38% 20%	459
	11th to 20th	Night	18% 4 20% 34% 22% 3	625
	21st to end	Night	14% 8% 27% 34% 15% 2	880
September	1st to 10th	Night	16% 9% 27% 30% 17% 1	800
	11th to 20th	Night	17% 6% 20% 30% 22% 6%	800
	21st to end	Night	18% 10% 26% 26% 18% 2	800
October	1st to 10th	Night	24% 12% 26% 21% 16% 1	800
	11th to 20th	Night	24% 15% 30% 23% 8% 1	800
	21st to end	Night	31% 13% 29% 22% 5%	880
November	1st to 10th	Night	27% 15% 29% 20% 7%	820
	11th to 20th	Night	25% 19% 30% 17% 9%	840
	21st to end	Night	34% 17% 27% 16% 6%	840
December	1st to 10th	Night	34% 18% 27% 17% 4	840
	11th to 20th	Night	31% 18% 32% 17% 2 1	840
	21st to end	Night	35% 16% 30% 16% 3	924

Page generated at 2026-06-13 02:31:42. Version ac07

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