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 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	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	109517
	Night	16% 14% 5% 10% 5% 4 4 3 4 6% 28%	109140
Transparency	Day	52% 5% 12% 20% 10% 1	113144
	Night	56% 5% 11% 18% 10% 1	113677
Seeing	Day	23% 14% 30% 20% 9% 3	15156
	Night	23% 13% 29% 25% 10% 1	24105

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	13% 16% 9% 11% 5% 5% 4 3 5% 5% 24%	3597
	11th to 20th	Day	11% 20% 8% 18% 8% 7% 5% 4 4 4 12%	3769
	21st to end	Day	13% 19% 8% 15% 6% 6% 6% 3 4 4 16%	4466
February	1st to 10th	Day	12% 20% 6% 15% 10% 6% 3 3 3 3 19%	1800
	11th to 20th	Day	14% 15% 7% 14% 7% 5% 4 3 4 5% 23%	1753
	21st to end	Day	17% 18% 6% 12% 8% 5% 5% 3 3 4 20%	1472
March	1st to 10th	Day	19% 15% 5% 8% 4 4 4 3 4 5% 29%	3432
	11th to 20th	Day	11% 18% 7% 10% 6% 4 5% 3 4 6% 25%	3480
	21st to end	Day	15% 11% 5% 7% 4 3 4 3 4 5% 39%	3815
April	1st to 10th	Day	20% 15% 4 7% 5% 4 3 3 4 6% 29%	2256
	11th to 20th	Day	23% 11% 2 6% 4 4 4 3 4 4 36%	2347
	21st to end	Day	23% 10% 4 7% 4 4 3 3 4 5% 33%	2470
May	1st to 10th	Day	20% 9% 2 7% 5% 4 5% 4 5% 7% 32%	4171
	11th to 20th	Day	15% 9% 3 6% 4 4 4 4 6% 9% 37%	4398
	21st to end	Day	17% 7% 2 5% 4 4 4 3 5% 7% 42%	4837
June	1st to 10th	Day	19% 8% 3 6% 4 4 4 4 5% 7% 35%	2660
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 7% 9% 38%	2810
	21st to end	Day	19% 7% 2 6% 6% 4 4 4 6% 7% 35%	2850
July	1st to 10th	Day	13% 6% 3 6% 5% 6% 5% 4 7% 9% 36%	4760
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 5% 8% 44%	4454
	21st to end	Day	10% 6% 2 5% 4 4 4 4 6% 9% 45%	4630
August	1st to 10th	Day	15% 7% 2 8% 4 4 5% 4 6% 7% 37%	4104
	11th to 20th	Day	16% 7% 3 6% 4 5% 5% 4 8% 9% 34%	4044
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	4381
September	1st to 10th	Day	15% 7% 2 7% 5% 4 4 3 5% 7% 39%	2280
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 44%	2181
	21st to end	Day	19% 7% 3 7% 5% 4 5% 5% 5% 6% 35%	2043
October	1st to 10th	Day	22% 9% 3 7% 4 4 4 3 4 7% 33%	1900
	11th to 20th	Day	25% 10% 4 10% 5% 5% 4 3 5% 5% 23%	1900
	21st to end	Day	29% 12% 3 9% 5% 4 4 3 4 5% 22%	2090
November	1st to 10th	Day	19% 14% 4 8% 5% 4 4 3 5% 7% 27%	1621
	11th to 20th	Day	17% 16% 4 10% 5% 5% 5% 4 5% 5% 22%	1600
	21st to end	Day	19% 22% 5% 11% 6% 6% 5% 3 4 5% 15%	1600
December	1st to 10th	Day	17% 27% 6% 10% 4 4 4 4 5% 5% 13%	2600
	11th to 20th	Day	18% 23% 7% 15% 5% 4 3 2 2 4 18%	2986
	21st to end	Day	24% 20% 6% 14% 5% 3 3 3 3 3 16%	3960
January	1st to 10th	Night	14% 20% 8% 12% 6% 6% 5% 3 4 4 18%	6349
	11th to 20th	Night	12% 17% 8% 19% 8% 6% 5% 3 4 4 13%	6115
	21st to end	Night	13% 19% 8% 14% 5% 6% 4 3 4 4 18%	6490
February	1st to 10th	Night	12% 18% 7% 14% 9% 5% 4 3 4 5% 19%	2600
	11th to 20th	Night	13% 17% 6% 12% 6% 5% 4 4 4 5% 24%	2533
	21st to end	Night	17% 17% 4 11% 6% 5% 4 3 4 5% 23%	2075
March	1st to 10th	Night	17% 12% 5% 9% 5% 4 4 3 4 6% 30%	3408
	11th to 20th	Night	12% 17% 7% 9% 5% 5% 4 3 5% 5% 28%	3479
	21st to end	Night	15% 10% 6% 6% 4 4 4 4 6% 6% 35%	3817
April	1st to 10th	Night	22% 14% 4 8% 5% 3 3 3 4 4 29%	1924
	11th to 20th	Night	19% 10% 3 7% 4 4 4 3 4 6% 35%	1797
	21st to end	Night	19% 10% 3 8% 5% 5% 4 3 4 6% 32%	1710
May	1st to 10th	Night	18% 6% 2 7% 4 5% 5% 3 4 7% 38%	2769
	11th to 20th	Night	15% 8% 2 6% 4 4 4 4 6% 7% 39%	2495
	21st to end	Night	12% 7% 2 5% 4 4 4 4 5% 8% 44%	2738
June	1st to 10th	Night	16% 9% 2 7% 5% 4 5% 4 5% 8% 35%	1520
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 4 5% 9% 43%	1363
	21st to end	Night	15% 7% 3 8% 5% 4 4 5% 4 8% 37%	1330
July	1st to 10th	Night	9% 6% 2 8% 6% 5% 5% 4 5% 7% 43%	2210
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 5% 7% 47%	2489
	21st to end	Night	9% 6% 2 5% 4 4 4 3 6% 8% 49%	2919
August	1st to 10th	Night	11% 6% 2 5% 4 4 5% 4 6% 7% 46%	2825
	11th to 20th	Night	16% 7% 3 6% 4 3 4 3 4 7% 44%	2785
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 8% 47%	3242
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 6% 8% 45%	1900
	11th to 20th	Night	14% 5% 2 5% 5% 4 3 3 4 7% 49%	1995
	21st to end	Night	18% 6% 3 7% 5% 4 4 3 4 6% 40%	2136
October	1st to 10th	Night	18% 8% 3 7% 5% 4 4 4 5% 6% 36%	2280
	11th to 20th	Night	25% 11% 3 8% 4 4 4 3 5% 6% 27%	2280
	21st to end	Night	25% 12% 5% 8% 5% 3 4 4 4 4 25%	2508
November	1st to 10th	Night	20% 13% 3 7% 5% 4 3 3 5% 6% 31%	2675
	11th to 20th	Night	18% 17% 4 9% 5% 4 4 3 5% 6% 24%	2800
	21st to end	Night	22% 19% 5% 10% 5% 5% 5% 3 4 6% 16%	2800
December	1st to 10th	Night	19% 25% 6% 12% 5% 4 4 3 4 5% 12%	4580
	11th to 20th	Night	16% 22% 8% 17% 7% 4 4 2 4 4 11%	5208
	21st to end	Night	23% 21% 6% 15% 6% 4 3 2 3 3 13%	6996

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% 16% 9%	3598
	11th to 20th	Day	75% 4 6% 8% 5%	3801
	21st to end	Day	71% 5% 8% 8% 7%	4466
February	1st to 10th	Day	72% 3 7% 13% 4 1	1800
	11th to 20th	Day	65% 4 8% 13% 8% 1	1762
	21st to end	Day	69% 3 8% 14% 6%	1472
March	1st to 10th	Day	59% 4 14% 13% 10%	3435
	11th to 20th	Day	60% 5% 11% 16% 7% 1	3480
	21st to end	Day	46% 5% 9% 21% 16% 2	3815
April	1st to 10th	Day	57% 4 9% 19% 10% 1	2256
	11th to 20th	Day	52% 4 7% 23% 13% 1	2347
	21st to end	Day	53% 4 8% 19% 13% 3	2470
May	1st to 10th	Day	50% 6% 10% 21% 12% 1	4183
	11th to 20th	Day	44% 7% 13% 26% 10% 1	4402
	21st to end	Day	41% 6% 13% 20% 16% 3	4837
June	1st to 10th	Day	48% 5% 11% 23% 13%	2660
	11th to 20th	Day	40% 6% 13% 24% 15% 1	2810
	21st to end	Day	47% 5% 14% 22% 11% 1	2850
July	1st to 10th	Day	41% 6% 17% 23% 12% 1	4686
	11th to 20th	Day	38% 6% 15% 31% 9%	4378
	21st to end	Day	35% 6% 12% 30% 16% 1	4482
August	1st to 10th	Day	45% 6% 15% 24% 10% 1	4091
	11th to 20th	Day	43% 7% 16% 25% 8%	4083
	21st to end	Day	35% 6% 14% 35% 10% 1	4381
September	1st to 10th	Day	43% 6% 13% 27% 11% 1	2280
	11th to 20th	Day	37% 7% 14% 30% 12%	2181
	21st to end	Day	48% 7% 12% 24% 9%	2043
October	1st to 10th	Day	45% 6% 14% 26% 9% 1	3160
	11th to 20th	Day	52% 5% 13% 22% 8%	3140
	21st to end	Day	58% 6% 14% 17% 5%	3428
November	1st to 10th	Day	57% 6% 13% 18% 6%	1621
	11th to 20th	Day	62% 7% 13% 13% 5% 1	1600
	21st to end	Day	72% 5% 9% 11% 3	1600
December	1st to 10th	Day	72% 5% 12% 9% 2	2600
	11th to 20th	Day	73% 3 6% 13% 5%	2986
	21st to end	Day	74% 4 6% 10% 6%	3960
January	1st to 10th	Night	69% 5% 7% 12% 7%	6349
	11th to 20th	Night	73% 6% 8% 9% 5%	6158
	21st to end	Night	68% 5% 9% 9% 9% 1	6490
February	1st to 10th	Night	68% 4 8% 14% 4 1	2600
	11th to 20th	Night	63% 5% 9% 15% 8%	2535
	21st to end	Night	63% 3 11% 16% 7%	2075
March	1st to 10th	Night	56% 4 12% 16% 10% 1	3418
	11th to 20th	Night	58% 5% 12% 16% 7% 2	3480
	21st to end	Night	47% 6% 8% 22% 15% 1	3817
April	1st to 10th	Night	59% 4 8% 18% 10% 2	1924
	11th to 20th	Night	49% 5% 7% 23% 15% 2	1797
	21st to end	Night	53% 5% 9% 16% 16% 1	1710
May	1st to 10th	Night	44% 6% 13% 23% 13% 2	2776
	11th to 20th	Night	41% 6% 14% 25% 13% 1	2496
	21st to end	Night	36% 7% 12% 25% 18% 2	2739
June	1st to 10th	Night	45% 7% 11% 21% 15% 1	1520
	11th to 20th	Night	37% 5% 14% 24% 19% 1	1363
	21st to end	Night	45% 7% 13% 23% 12% 1	1330
July	1st to 10th	Night	38% 7% 14% 23% 17% 2	2175
	11th to 20th	Night	36% 6% 12% 33% 13%	2442
	21st to end	Night	32% 6% 12% 33% 17% 1	2826
August	1st to 10th	Night	36% 5% 17% 25% 16% 1	2814
	11th to 20th	Night	40% 6% 17% 24% 13%	2812
	21st to end	Night	36% 6% 12% 34% 11% 1	3242
September	1st to 10th	Night	37% 7% 16% 27% 12% 1	1900
	11th to 20th	Night	35% 6% 14% 32% 14%	1995
	21st to end	Night	44% 5% 12% 27% 11%	2136
October	1st to 10th	Night	38% 6% 11% 28% 17%	3800
	11th to 20th	Night	49% 7% 14% 22% 8%	3779
	21st to end	Night	53% 6% 16% 19% 6%	4120
November	1st to 10th	Night	54% 6% 12% 21% 7%	2675
	11th to 20th	Night	59% 6% 12% 15% 7% 1	2800
	21st to end	Night	69% 6% 10% 11% 3	2800
December	1st to 10th	Night	74% 5% 12% 7% 2	4580
	11th to 20th	Night	77% 5% 7% 8% 3	5208
	21st to end	Night	77% 4 5% 9% 5%	6996

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	21% 27% 35% 12% 6%	200
	11th to 20th	Day	26% 21% 35% 12% 5% 1	243
	21st to end	Day	26% 18% 35% 17% 4	330
February	1st to 10th	Day	30% 27% 31% 8% 4	300
	11th to 20th	Day	28% 24% 36% 10% 1	300
	21st to end	Day	39% 22% 31% 8%	246
March	1st to 10th	Day	24% 18% 38% 16% 3	460
	11th to 20th	Day	23% 21% 32% 18% 5%	460
	21st to end	Day	32% 15% 29% 16% 7%	506
April	1st to 10th	Day	28% 21% 29% 15% 7%	460
	11th to 20th	Day	36% 17% 22% 16% 7% 3	452
	21st to end	Day	31% 14% 32% 13% 7% 2	460
May	1st to 10th	Day	28% 11% 28% 23% 8% 3	463
	11th to 20th	Day	21% 12% 31% 21% 12% 3	487
	21st to end	Day	19% 12% 31% 22% 11% 4	529
June	1st to 10th	Day	22% 12% 30% 27% 7% 2	490
	11th to 20th	Day	16% 13% 31% 24% 12% 3	640
	21st to end	Day	22% 10% 36% 23% 8% 2	680
July	1st to 10th	Day	12% 10% 32% 28% 14% 4	671
	11th to 20th	Day	15% 10% 28% 28% 13% 7%	497
	21st to end	Day	11% 8% 33% 29% 13% 6%	532
August	1st to 10th	Day	15% 8% 26% 28% 16% 7%	486
	11th to 20th	Day	16% 5% 24% 29% 16% 9%	479
	21st to end	Day	17% 9% 28% 24% 15% 8%	539
September	1st to 10th	Day	17% 8% 29% 26% 13% 7%	490
	11th to 20th	Day	13% 7% 26% 23% 20% 11%	490
	21st to end	Day	22% 11% 25% 23% 15% 3	490
October	1st to 10th	Day	24% 13% 28% 18% 11% 5%	490
	11th to 20th	Day	25% 19% 32% 18% 6% 1	490
	21st to end	Day	33% 17% 32% 15% 4	539
November	1st to 10th	Day	33% 16% 31% 15% 5%	237
	11th to 20th	Day	29% 20% 37% 11% 5%	200
	21st to end	Day	27% 23% 36% 13% 2	200
December	1st to 10th	Day	33% 18% 32% 14% 3 1	200
	11th to 20th	Day	35% 24% 25% 14% 4 1	200
	21st to end	Day	38% 21% 25% 12% 3	220
January	1st to 10th	Night	29% 19% 31% 17% 5%	800
	11th to 20th	Night	30% 15% 31% 18% 5% 1	800
	21st to end	Night	22% 12% 33% 25% 6% 1	880
February	1st to 10th	Night	23% 19% 34% 21% 2	800
	11th to 20th	Night	23% 18% 35% 19% 5%	783
	21st to end	Night	27% 19% 31% 19% 4	647
March	1st to 10th	Night	23% 17% 34% 22% 3	760
	11th to 20th	Night	22% 16% 35% 21% 5% 1	760
	21st to end	Night	22% 14% 32% 25% 8%	836
April	1st to 10th	Night	29% 15% 29% 23% 4	760
	11th to 20th	Night	26% 14% 29% 21% 10% 1	673
	21st to end	Night	25% 10% 30% 25% 9% 1	570
May	1st to 10th	Night	21% 6% 25% 33% 12% 2	532
	11th to 20th	Night	18% 8% 32% 31% 11%	374
	21st to end	Night	13% 6% 32% 36% 13%	411
June	1st to 10th	Night	17% 8% 32% 33% 10% 1	380
	11th to 20th	Night	13% 11% 25% 38% 13%	380
	21st to end	Night	17% 6% 26% 38% 12% 1	380
July	1st to 10th	Night	9% 4 20% 48% 17% 1	370
	11th to 20th	Night	14% 4 24% 37% 20% 1	368
	21st to end	Night	12% 4 24% 38% 22% 1	521
August	1st to 10th	Night	13% 4 18% 37% 26% 2	567
	11th to 20th	Night	11% 5% 18% 36% 28% 3	564
	21st to end	Night	15% 6% 24% 31% 22% 2	693
September	1st to 10th	Night	14% 7% 26% 31% 19% 3	760
	11th to 20th	Night	16% 5% 23% 28% 23% 4	760
	21st to end	Night	22% 7% 19% 33% 16% 3	760
October	1st to 10th	Night	21% 10% 29% 25% 13% 2	760
	11th to 20th	Night	30% 13% 26% 21% 9% 1	760
	21st to end	Night	31% 15% 31% 20% 4	836
November	1st to 10th	Night	28% 14% 27% 23% 8% 1	780
	11th to 20th	Night	27% 18% 31% 18% 5% 1	800
	21st to end	Night	33% 16% 27% 19% 4	800
December	1st to 10th	Night	34% 18% 30% 14% 4	800
	11th to 20th	Night	27% 19% 35% 15% 4 1	800
	21st to end	Night	34% 16% 30% 17% 3	880

Page generated at 2025-02-22 06:51:39. Version ac07

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