Gravenhurst
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gravenhurst 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	16% 11% 4 9% 5% 4 4 3 5% 6% 32%	139134
	Night	16% 13% 5% 10% 5% 4 4 3 5% 6% 29%	133203
Transparency	Day	52% 5% 11% 21% 10% 1	142717
	Night	55% 5% 10% 18% 10% 1	137709
Seeing	Day	24% 14% 30% 20% 9% 3	16971
	Night	23% 13% 29% 25% 10% 1	25727

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% 15% 8% 11% 5% 5% 4 3 5% 5% 23%	4661
	11th to 20th	Day	10% 22% 8% 19% 9% 7% 5% 3 3 4 10%	4905
	21st to end	Day	10% 16% 8% 17% 7% 6% 6% 4 4 5% 16%	5780
February	1st to 10th	Day	12% 20% 6% 15% 9% 6% 3 3 3 3 19%	1890
	11th to 20th	Day	14% 15% 7% 15% 7% 5% 4 3 4 5% 22%	1843
	21st to end	Day	16% 19% 5% 12% 7% 5% 5% 3 3 5% 20%	1583
March	1st to 10th	Day	18% 14% 5% 10% 5% 4 4 3 4 5% 29%	5042
	11th to 20th	Day	12% 15% 6% 10% 6% 5% 6% 4 6% 6% 25%	5090
	21st to end	Day	19% 15% 5% 8% 4 3 3 2 4 4 33%	5586
April	1st to 10th	Day	21% 15% 4 7% 5% 4 4 3 4 6% 29%	2494
	11th to 20th	Day	23% 12% 2 6% 4 4 3 3 4 4 35%	2597
	21st to end	Day	23% 10% 4 7% 4 3 3 3 4 5% 33%	2730
May	1st to 10th	Day	18% 9% 2 7% 4 4 5% 4 6% 7% 35%	7759
	11th to 20th	Day	17% 9% 3 7% 4 4 4 3 5% 8% 36%	8221
	21st to end	Day	20% 8% 2 5% 4 3 4 3 4 7% 39%	9060
June	1st to 10th	Day	19% 8% 2 6% 4 4 4 4 5% 7% 36%	2940
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 7% 10% 37%	3106
	21st to end	Day	18% 7% 2 6% 5% 4 5% 5% 6% 7% 35%	3150
July	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 10% 38%	6820
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 5% 7% 47%	6388
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 9% 48%	6658
August	1st to 10th	Day	14% 7% 2 7% 4 4 5% 4 6% 8% 38%	4234
	11th to 20th	Day	15% 7% 3 6% 4 5% 5% 4 8% 9% 34%	4174
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	4519
September	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 3 5% 7% 39%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 7% 45%	2296
	21st to end	Day	19% 7% 3 8% 5% 4 5% 4 5% 6% 35%	2150
October	1st to 10th	Day	21% 8% 3 7% 4 4 4 3 4 7% 35%	2000
	11th to 20th	Day	24% 10% 4 10% 5% 5% 4 4 5% 6% 24%	2000
	21st to end	Day	28% 12% 3 9% 5% 4 4 3 4 5% 23%	2200
November	1st to 10th	Day	19% 14% 4 8% 5% 4 4 3 5% 7% 27%	1704
	11th to 20th	Day	17% 16% 4 10% 5% 6% 5% 4 5% 5% 22%	1680
	21st to end	Day	19% 23% 5% 10% 6% 5% 5% 4 3 4 16%	1680
December	1st to 10th	Day	17% 27% 6% 10% 4 4 4 4 5% 5% 13%	2680
	11th to 20th	Day	18% 23% 7% 15% 5% 4 3 2 3 4 18%	3066
	21st to end	Day	23% 20% 6% 14% 5% 3 3 2 3 3 16%	4048
January	1st to 10th	Night	13% 19% 9% 15% 6% 5% 5% 3 4 4 17%	8234
	11th to 20th	Night	11% 20% 8% 18% 8% 6% 4 3 4 4 13%	7965
	21st to end	Night	10% 17% 7% 15% 8% 6% 4 3 4 4 20%	8401
February	1st to 10th	Night	12% 18% 7% 13% 9% 5% 4 3 4 5% 20%	2730
	11th to 20th	Night	13% 17% 7% 12% 6% 5% 4 4 4 5% 23%	2663
	21st to end	Night	16% 17% 5% 11% 6% 6% 4 3 4 5% 23%	2221
March	1st to 10th	Night	18% 12% 5% 10% 4 4 4 3 4 6% 29%	5018
	11th to 20th	Night	11% 14% 6% 9% 6% 6% 5% 4 6% 7% 28%	5089
	21st to end	Night	21% 11% 6% 7% 4 4 4 3 5% 5% 30%	5588
April	1st to 10th	Night	21% 13% 4 8% 5% 3 3 3 5% 5% 29%	2126
	11th to 20th	Night	19% 10% 3 7% 4 5% 4 3 5% 6% 34%	1989
	21st to end	Night	19% 9% 3 8% 5% 5% 4 3 4 6% 33%	1890
May	1st to 10th	Night	18% 7% 3 6% 4 5% 5% 4 5% 8% 35%	5139
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 41%	4639
	21st to end	Night	19% 7% 2 5% 3 4 4 3 5% 7% 41%	5114
June	1st to 10th	Night	16% 9% 2 6% 5% 4 5% 4 5% 8% 36%	1680
	11th to 20th	Night	12% 7% 2 6% 5% 4 4 4 5% 9% 42%	1507
	21st to end	Night	15% 7% 3 8% 5% 4 4 5% 4 8% 37%	1470
July	1st to 10th	Night	12% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	3160
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 7% 47%	3556
	21st to end	Night	10% 6% 2 5% 4 3 4 3 5% 8% 51%	4190
August	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 6% 7% 46%	2915
	11th to 20th	Night	16% 7% 3 6% 4 3 4 3 4 7% 44%	2875
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 8% 47%	3345
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 8% 45%	2000
	11th to 20th	Night	13% 5% 2 5% 5% 4 3 3 4 7% 50%	2100
	21st to end	Night	18% 7% 3 7% 5% 4 3 3 4 6% 40%	2249
October	1st to 10th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 37%	2400
	11th to 20th	Night	25% 11% 3 8% 4 4 4 3 6% 6% 27%	2400
	21st to end	Night	24% 12% 5% 8% 5% 3 4 4 4 4 26%	2640
November	1st to 10th	Night	20% 13% 3 7% 5% 4 3 3 5% 6% 30%	2812
	11th to 20th	Night	18% 17% 4 9% 5% 4 4 3 5% 6% 24%	2940
	21st to end	Night	22% 20% 5% 10% 5% 5% 5% 3 4 6% 15%	2940
December	1st to 10th	Night	19% 25% 6% 12% 5% 4 4 3 5% 5% 13%	4720
	11th to 20th	Night	16% 22% 8% 17% 7% 4 4 3 4 4 12%	5348
	21st to end	Night	23% 20% 7% 15% 6% 4 3 2 3 3 13%	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	61% 4 11% 15% 8%	4662
	11th to 20th	Day	79% 4 5% 7% 5%	4937
	21st to end	Day	69% 5% 9% 9% 8%	5780
February	1st to 10th	Day	72% 3 7% 13% 5% 1	1890
	11th to 20th	Day	65% 4 8% 13% 8% 1	1852
	21st to end	Day	68% 3 8% 15% 6%	1583
March	1st to 10th	Day	60% 4 12% 10% 14% 1	5045
	11th to 20th	Day	58% 5% 13% 16% 6% 1	5090
	21st to end	Day	54% 4 7% 19% 13% 2	5586
April	1st to 10th	Day	57% 4 8% 19% 10% 1	2494
	11th to 20th	Day	53% 4 7% 22% 13% 1	2597
	21st to end	Day	53% 4 8% 20% 12% 3	2730
May	1st to 10th	Day	46% 6% 11% 21% 12% 4	7768
	11th to 20th	Day	46% 6% 11% 22% 15% 1	8225
	21st to end	Day	46% 5% 11% 23% 14% 2	9061
June	1st to 10th	Day	47% 5% 10% 23% 14%	2940
	11th to 20th	Day	41% 7% 13% 25% 14% 1	3106
	21st to end	Day	46% 5% 15% 22% 11% 1	3150
July	1st to 10th	Day	41% 6% 15% 26% 11%	6747
	11th to 20th	Day	36% 6% 13% 32% 14%	6312
	21st to end	Day	32% 6% 14% 33% 14%	6467
August	1st to 10th	Day	44% 6% 15% 25% 10% 1	4221
	11th to 20th	Day	43% 7% 16% 25% 8%	4213
	21st to end	Day	35% 6% 14% 35% 10% 1	4519
September	1st to 10th	Day	43% 6% 14% 27% 10% 1	2400
	11th to 20th	Day	36% 7% 14% 31% 12%	2296
	21st to end	Day	49% 7% 12% 24% 9%	2150
October	1st to 10th	Day	44% 6% 14% 26% 10% 1	3260
	11th to 20th	Day	52% 5% 13% 22% 8%	3240
	21st to end	Day	57% 6% 13% 17% 6%	3538
November	1st to 10th	Day	57% 6% 13% 18% 6%	1704
	11th to 20th	Day	61% 7% 13% 13% 5% 1	1680
	21st to end	Day	72% 5% 9% 11% 3	1680
December	1st to 10th	Day	71% 5% 12% 9% 2	2680
	11th to 20th	Day	73% 3 6% 12% 5%	3066
	21st to end	Day	74% 4 6% 10% 6%	4048
January	1st to 10th	Night	71% 5% 7% 11% 6%	8234
	11th to 20th	Night	74% 5% 7% 8% 6%	8008
	21st to end	Night	66% 5% 8% 10% 10% 1	8401
February	1st to 10th	Night	67% 4 8% 15% 5% 1	2730
	11th to 20th	Night	63% 5% 9% 15% 7%	2665
	21st to end	Night	64% 4 11% 16% 6%	2221
March	1st to 10th	Night	56% 4 11% 14% 11% 4	5028
	11th to 20th	Night	55% 5% 15% 16% 8% 1	5090
	21st to end	Night	54% 5% 7% 19% 13% 2	5588
April	1st to 10th	Night	59% 4 8% 18% 10% 1	2126
	11th to 20th	Night	49% 5% 7% 23% 14% 2	1989
	21st to end	Night	52% 5% 9% 18% 15% 1	1890
May	1st to 10th	Night	46% 5% 10% 22% 13% 3	5146
	11th to 20th	Night	42% 5% 11% 24% 17% 1	4640
	21st to end	Night	42% 5% 10% 26% 15% 2	5115
June	1st to 10th	Night	43% 7% 10% 21% 17% 1	1680
	11th to 20th	Night	38% 4 14% 24% 18% 1	1507
	21st to end	Night	45% 6% 13% 23% 12% 1	1470
July	1st to 10th	Night	40% 7% 13% 25% 14% 2	3125
	11th to 20th	Night	36% 6% 11% 32% 15%	3509
	21st to end	Night	32% 5% 10% 36% 16% 1	4066
August	1st to 10th	Night	36% 6% 17% 26% 15% 1	2904
	11th to 20th	Night	40% 5% 17% 24% 13%	2902
	21st to end	Night	36% 6% 12% 34% 11% 1	3345
September	1st to 10th	Night	36% 7% 17% 27% 12% 1	2000
	11th to 20th	Night	34% 5% 13% 33% 14%	2100
	21st to end	Night	45% 5% 12% 27% 11%	2249
October	1st to 10th	Night	37% 6% 11% 29% 17%	3920
	11th to 20th	Night	49% 7% 13% 22% 8%	3899
	21st to end	Night	52% 6% 15% 19% 7%	4252
November	1st to 10th	Night	55% 6% 11% 20% 7%	2812
	11th to 20th	Night	60% 6% 12% 15% 7% 1	2940
	21st to end	Night	70% 6% 10% 11% 3	2940
December	1st to 10th	Night	74% 5% 12% 8% 2	4720
	11th to 20th	Night	76% 5% 7% 8% 3	5348
	21st to end	Night	77% 4 5% 9% 5%	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	22% 26% 34% 13% 5%	220
	11th to 20th	Day	25% 23% 35% 12% 4 1	267
	21st to end	Day	26% 18% 34% 18% 4	363
February	1st to 10th	Day	28% 25% 32% 11% 4	330
	11th to 20th	Day	28% 24% 36% 11% 1	330
	21st to end	Day	37% 21% 33% 10%	283
March	1st to 10th	Day	25% 19% 38% 15% 3	540
	11th to 20th	Day	26% 21% 31% 16% 4	540
	21st to end	Day	34% 15% 29% 15% 6%	594
April	1st to 10th	Day	31% 21% 28% 14% 6%	540
	11th to 20th	Day	37% 16% 22% 15% 7% 3	532
	21st to end	Day	31% 15% 33% 13% 7% 2	540
May	1st to 10th	Day	28% 14% 28% 21% 7% 2	543
	11th to 20th	Day	21% 13% 32% 21% 11% 3	567
	21st to end	Day	22% 14% 30% 20% 10% 4	617
June	1st to 10th	Day	23% 13% 30% 26% 7% 2	570
	11th to 20th	Day	18% 15% 32% 23% 10% 3	736
	21st to end	Day	21% 10% 36% 23% 8% 2	780
July	1st to 10th	Day	12% 10% 32% 29% 14% 4	722
	11th to 20th	Day	14% 10% 29% 28% 12% 6%	538
	21st to end	Day	11% 8% 34% 29% 13% 5%	576
August	1st to 10th	Day	14% 8% 26% 29% 17% 7%	526
	11th to 20th	Day	15% 5% 25% 30% 16% 9%	519
	21st to end	Day	17% 9% 28% 24% 15% 8%	583
September	1st to 10th	Day	18% 8% 28% 25% 13% 8%	530
	11th to 20th	Day	12% 6% 25% 24% 20% 12%	530
	21st to end	Day	22% 11% 27% 23% 15% 3	530
October	1st to 10th	Day	23% 13% 28% 19% 12% 4	530
	11th to 20th	Day	25% 19% 33% 17% 5% 1	530
	21st to end	Day	32% 16% 33% 15% 4	583
November	1st to 10th	Day	35% 17% 30% 14% 5%	260
	11th to 20th	Day	28% 20% 37% 11% 5%	220
	21st to end	Day	29% 21% 36% 12% 2	220
December	1st to 10th	Day	33% 19% 30% 14% 4	220
	11th to 20th	Day	36% 22% 25% 14% 3	220
	21st to end	Day	38% 22% 25% 12% 3	242
January	1st to 10th	Night	28% 18% 31% 17% 5%	840
	11th to 20th	Night	30% 17% 31% 17% 5% 1	840
	21st to end	Night	22% 13% 34% 25% 6% 1	924
February	1st to 10th	Night	23% 20% 34% 21% 3	840
	11th to 20th	Night	23% 18% 35% 20% 5%	823
	21st to end	Night	26% 20% 30% 20% 4	692
March	1st to 10th	Night	23% 17% 35% 22% 3	840
	11th to 20th	Night	23% 16% 36% 20% 5% 1	840
	21st to end	Night	23% 14% 32% 24% 7%	924
April	1st to 10th	Night	29% 16% 30% 21% 4	840
	11th to 20th	Night	26% 14% 30% 21% 9% 1	745
	21st to end	Night	24% 11% 31% 26% 9%	630
May	1st to 10th	Night	21% 8% 25% 32% 12% 2	588
	11th to 20th	Night	18% 9% 34% 29% 10%	414
	21st to end	Night	15% 6% 32% 35% 12%	455
June	1st to 10th	Night	17% 8% 31% 34% 10% 1	420
	11th to 20th	Night	14% 12% 26% 37% 12%	420
	21st to end	Night	18% 6% 26% 39% 11% 1	420
July	1st to 10th	Night	9% 4 21% 48% 17% 1	390
	11th to 20th	Night	14% 5% 23% 38% 19% 1	388
	21st to end	Night	12% 4 25% 38% 21% 1	549
August	1st to 10th	Night	12% 4 18% 37% 26% 2	597
	11th to 20th	Night	12% 4 17% 37% 27% 3	594
	21st to end	Night	14% 6% 25% 32% 22% 2	730
September	1st to 10th	Night	14% 7% 26% 31% 19% 3	800
	11th to 20th	Night	15% 5% 22% 29% 24% 5%	800
	21st to end	Night	22% 7% 19% 33% 16% 3	800
October	1st to 10th	Night	20% 10% 29% 26% 13% 2	800
	11th to 20th	Night	30% 13% 27% 21% 8% 1	800
	21st to end	Night	30% 14% 31% 21% 4	880
November	1st to 10th	Night	28% 14% 27% 23% 7% 1	820
	11th to 20th	Night	27% 18% 31% 18% 5% 1	840
	21st to end	Night	33% 17% 27% 18% 4	840
December	1st to 10th	Night	34% 18% 30% 15% 4	840
	11th to 20th	Night	26% 19% 35% 15% 4 1	840
	21st to end	Night	34% 16% 30% 17% 3	924

Page generated at 2026-06-28 02:10:45. Version ac07

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