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 2024-02-05.

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 7% 6% 3 4 5% 5% 6% 31%	78686
	Night	17% 13% 5% 8% 6% 3 4 4 4 6% 28%	80162
Transparency	Day	54% 5% 11% 19% 10% 1	78392
	Night	57% 5% 10% 17% 9% 1	79991
Seeing	Day	23% 14% 30% 20% 9% 3	13798
	Night	23% 12% 29% 25% 10% 1	22773

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	17% 20% 12% 10% 6% 2 3 3 3 3 20%	2517
	11th to 20th	Day	12% 17% 7% 16% 11% 4 5% 5% 4 5% 14%	2651
	21st to end	Day	19% 21% 9% 11% 7% 3 4 3 3 4 16%	3124
February	1st to 10th	Day	13% 20% 7% 13% 12% 5% 5% 3 3 3 18%	1656
	11th to 20th	Day	15% 14% 7% 13% 7% 4 4 4 3 5% 25%	1573
	21st to end	Day	17% 18% 6% 10% 8% 4 5% 4 3 4 20%	1353
March	1st to 10th	Day	15% 13% 5% 8% 7% 4 5% 5% 4 5% 29%	1977
	11th to 20th	Day	12% 14% 6% 9% 8% 3 4 4 5% 6% 30%	1980
	21st to end	Day	19% 10% 5% 6% 5% 3 4 4 5% 5% 35%	2178
April	1st to 10th	Day	21% 13% 4 6% 5% 3 3 4 4 6% 30%	2137
	11th to 20th	Day	21% 10% 3 6% 5% 3 4 4 4 4 36%	2222
	21st to end	Day	22% 10% 4 6% 5% 3 4 4 4 5% 33%	2340
May	1st to 10th	Day	20% 8% 2 6% 6% 3 5% 5% 5% 7% 33%	2375
	11th to 20th	Day	16% 9% 3 5% 5% 3 5% 4 4 7% 40%	2478
	21st to end	Day	14% 7% 2 5% 5% 3 5% 5% 5% 8% 41%	2725
June	1st to 10th	Day	19% 7% 3 5% 5% 3 4 5% 5% 8% 37%	2520
	11th to 20th	Day	15% 6% 2 5% 5% 3 5% 5% 7% 9% 39%	2662
	21st to end	Day	18% 6% 3 5% 6% 3 5% 5% 6% 7% 36%	2700
July	1st to 10th	Day	12% 4 2 4 4 3 6% 6% 6% 9% 44%	2700
	11th to 20th	Day	13% 7% 3 6% 5% 3 4 6% 5% 8% 40%	2534
	21st to end	Day	12% 6% 3 5% 6% 4 5% 6% 6% 10% 37%	2641
August	1st to 10th	Day	13% 6% 2 5% 5% 3 5% 5% 6% 7% 42%	2327
	11th to 20th	Day	12% 5% 3 5% 6% 4 5% 5% 7% 9% 39%	2303
	21st to end	Day	13% 6% 3 6% 6% 4 5% 5% 6% 9% 37%	2493
September	1st to 10th	Day	15% 6% 2 6% 6% 4 4 4 5% 8% 39%	2160
	11th to 20th	Day	15% 5% 2 5% 5% 3 4 5% 6% 7% 42%	2067
	21st to end	Day	19% 7% 3 6% 6% 3 5% 5% 5% 6% 35%	1936
October	1st to 10th	Day	22% 9% 3 6% 6% 3 4 4 4 7% 32%	1800
	11th to 20th	Day	26% 9% 4 8% 7% 4 5% 4 4 5% 22%	1800
	21st to end	Day	30% 12% 4 8% 6% 3 4 4 4 5% 21%	1980
November	1st to 10th	Day	19% 13% 4 7% 6% 3 4 5% 5% 8% 26%	1539
	11th to 20th	Day	18% 14% 5% 9% 6% 4 6% 5% 5% 5% 22%	1520
	21st to end	Day	20% 20% 6% 9% 7% 4 6% 5% 4 4 16%	1520
December	1st to 10th	Day	17% 26% 8% 10% 5% 4 4 4 4 4 15%	1520
	11th to 20th	Day	20% 19% 7% 10% 7% 3 4 3 3 4 20%	1906
	21st to end	Day	26% 20% 7% 12% 6% 3 3 3 3 3 13%	2772
January	1st to 10th	Night	18% 22% 10% 12% 7% 3 5% 3 3 3 13%	4429
	11th to 20th	Night	14% 15% 7% 17% 10% 4 5% 4 4 4 16%	4308
	21st to end	Night	18% 19% 8% 10% 7% 4 4 4 4 4 17%	4532
February	1st to 10th	Night	13% 17% 8% 13% 10% 4 5% 4 4 5% 18%	2399
	11th to 20th	Night	13% 15% 7% 9% 8% 4 5% 5% 4 5% 25%	2273
	21st to end	Night	18% 15% 6% 9% 7% 4 4 3 4 5% 24%	1901
March	1st to 10th	Night	15% 12% 5% 9% 6% 4 4 3 4 6% 33%	1970
	11th to 20th	Night	13% 12% 6% 7% 6% 3 4 4 5% 5% 33%	1979
	21st to end	Night	17% 10% 4 7% 6% 2 4 4 5% 6% 36%	2178
April	1st to 10th	Night	22% 12% 5% 6% 6% 3 3 4 4 4 29%	1823
	11th to 20th	Night	19% 10% 3 6% 5% 3 4 4 4 6% 36%	1702
	21st to end	Night	19% 9% 3 8% 6% 4 5% 4 4 6% 32%	1620
May	1st to 10th	Night	19% 6% 3 6% 4 4 5% 5% 4 8% 38%	1584
	11th to 20th	Night	15% 7% 2 4 4 3 4 4 5% 7% 43%	1415
	21st to end	Night	11% 6% 2 4 4 3 4 5% 6% 9% 46%	1550
June	1st to 10th	Night	16% 7% 3 5% 5% 3 6% 5% 5% 8% 36%	1440
	11th to 20th	Night	12% 6% 2 5% 5% 2 5% 5% 5% 9% 43%	1292
	21st to end	Night	14% 7% 3 7% 5% 3 5% 6% 4 9% 37%	1260
July	1st to 10th	Night	8% 5% 2 4 5% 3 4 4 5% 7% 52%	1260
	11th to 20th	Night	12% 6% 3 5% 5% 2 4 4 6% 9% 44%	1422
	21st to end	Night	11% 5% 2 4 5% 3 5% 5% 6% 8% 46%	1673
August	1st to 10th	Night	10% 5% 2 4 5% 3 5% 5% 5% 8% 48%	1611
	11th to 20th	Night	9% 6% 3 6% 5% 3 6% 4 5% 7% 46%	1598
	21st to end	Night	14% 6% 2 6% 6% 3 5% 4 5% 7% 43%	1844
September	1st to 10th	Night	12% 6% 3 4 5% 3 4 5% 6% 8% 44%	1800
	11th to 20th	Night	14% 5% 2 4 5% 3 4 4 5% 7% 47%	1890
	21st to end	Night	17% 6% 3 6% 6% 3 3 4 4 6% 41%	2023
October	1st to 10th	Night	18% 8% 3 6% 6% 3 4 5% 5% 6% 34%	2160
	11th to 20th	Night	26% 11% 3 7% 5% 3 5% 4 5% 6% 25%	2160
	21st to end	Night	26% 12% 5% 7% 7% 3 3 5% 4 4 24%	2376
November	1st to 10th	Night	21% 12% 3 6% 6% 3 4 3 5% 6% 31%	2538
	11th to 20th	Night	19% 15% 5% 8% 6% 4 4 4 5% 6% 24%	2660
	21st to end	Night	23% 17% 6% 9% 6% 4 5% 4 4 6% 16%	2660
December	1st to 10th	Night	21% 22% 8% 10% 7% 3 5% 3 4 4 13%	2660
	11th to 20th	Night	19% 20% 8% 11% 7% 3 4 3 4 5% 17%	3288
	21st to end	Night	26% 21% 7% 13% 7% 3 3 3 3 4 10%	4884

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	71% 3 11% 9% 6%	2518
	11th to 20th	Day	73% 5% 8% 8% 6%	2651
	21st to end	Day	73% 4 8% 8% 6% 1	3124
February	1st to 10th	Day	74% 3 7% 11% 4 1	1656
	11th to 20th	Day	64% 4 9% 13% 9% 2	1582
	21st to end	Day	68% 3 7% 15% 6%	1353
March	1st to 10th	Day	58% 5% 13% 14% 10%	1980
	11th to 20th	Day	56% 4 10% 18% 11% 2	1980
	21st to end	Day	52% 4 9% 18% 15% 3	2178
April	1st to 10th	Day	57% 4 9% 19% 10% 1	2137
	11th to 20th	Day	51% 4 8% 23% 13% 1	2222
	21st to end	Day	53% 5% 8% 19% 12% 3	2340
May	1st to 10th	Day	50% 5% 10% 20% 13% 2	2375
	11th to 20th	Day	45% 5% 8% 26% 14% 1	2482
	21st to end	Day	42% 6% 14% 21% 14% 3	2725
June	1st to 10th	Day	46% 5% 11% 24% 13%	2520
	11th to 20th	Day	40% 6% 13% 24% 15% 1	2662
	21st to end	Day	46% 5% 14% 23% 11% 1	2700
July	1st to 10th	Day	35% 6% 14% 28% 16% 1	2626
	11th to 20th	Day	40% 7% 14% 28% 10% 1	2458
	21st to end	Day	42% 7% 13% 27% 11% 1	2493
August	1st to 10th	Day	41% 6% 15% 25% 12% 1	2314
	11th to 20th	Day	40% 7% 17% 26% 10%	2303
	21st to end	Day	42% 7% 15% 25% 10% 1	2493
September	1st to 10th	Day	43% 6% 13% 27% 11% 1	2160
	11th to 20th	Day	39% 7% 14% 30% 10%	2067
	21st to end	Day	48% 7% 12% 25% 7%	1936
October	1st to 10th	Day	52% 6% 12% 19% 10% 1	1800
	11th to 20th	Day	64% 5% 12% 13% 6% 1	1800
	21st to end	Day	66% 5% 10% 13% 6%	1980
November	1st to 10th	Day	57% 6% 14% 17% 6%	1539
	11th to 20th	Day	62% 7% 13% 12% 5% 1	1520
	21st to end	Day	71% 5% 9% 11% 3	1520
December	1st to 10th	Day	73% 4 9% 10% 3	1520
	11th to 20th	Day	71% 4 7% 12% 7%	1906
	21st to end	Day	77% 3 7% 9% 3	2772
January	1st to 10th	Night	78% 4 6% 8% 4	4429
	11th to 20th	Night	71% 6% 9% 10% 4 1	4308
	21st to end	Night	70% 4 10% 9% 6% 1	4532
February	1st to 10th	Night	70% 4 8% 13% 4 1	2399
	11th to 20th	Night	62% 4 10% 16% 7%	2275
	21st to end	Night	63% 3 10% 16% 7%	1901
March	1st to 10th	Night	54% 4 13% 16% 12% 2	1980
	11th to 20th	Night	54% 5% 9% 20% 11% 2	1980
	21st to end	Night	49% 4 9% 21% 14% 3	2178
April	1st to 10th	Night	59% 4 8% 18% 10% 2	1823
	11th to 20th	Night	48% 5% 6% 23% 15% 2	1702
	21st to end	Night	54% 5% 9% 16% 16% 1	1620
May	1st to 10th	Night	45% 4 10% 20% 17% 3	1584
	11th to 20th	Night	40% 5% 10% 24% 19% 2	1416
	21st to end	Night	34% 7% 16% 24% 17% 3	1551
June	1st to 10th	Night	44% 7% 11% 22% 15% 1	1440
	11th to 20th	Night	38% 4 13% 24% 20% 1	1292
	21st to end	Night	44% 7% 13% 23% 12% 1	1260
July	1st to 10th	Night	30% 5% 14% 26% 22% 3	1225
	11th to 20th	Night	36% 7% 12% 30% 15% 1	1375
	21st to end	Night	35% 7% 13% 29% 15% 1	1580
August	1st to 10th	Night	32% 6% 20% 28% 13% 1	1600
	11th to 20th	Night	35% 7% 16% 25% 15% 1	1598
	21st to end	Night	40% 7% 13% 28% 10% 1	1844
September	1st to 10th	Night	37% 7% 16% 26% 12% 2	1800
	11th to 20th	Night	37% 6% 14% 32% 12%	1890
	21st to end	Night	45% 5% 12% 28% 10%	2023
October	1st to 10th	Night	50% 7% 11% 20% 12%	2160
	11th to 20th	Night	59% 7% 11% 18% 5%	2160
	21st to end	Night	63% 5% 10% 13% 9%	2376
November	1st to 10th	Night	54% 6% 12% 20% 8%	2538
	11th to 20th	Night	60% 6% 12% 15% 7% 1	2660
	21st to end	Night	69% 7% 11% 11% 3	2660
December	1st to 10th	Night	74% 5% 9% 9% 3	2660
	11th to 20th	Night	71% 6% 9% 11% 4	3288
	21st to end	Night	80% 3 6% 8% 2	4884

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% 33% 13% 7%	180
	11th to 20th	Day	27% 22% 34% 11% 5% 1	218
	21st to end	Day	28% 15% 34% 18% 5%	297
February	1st to 10th	Day	30% 26% 31% 8% 5%	252
	11th to 20th	Day	28% 23% 39% 10% 1	240
	21st to end	Day	41% 22% 31% 7%	206
March	1st to 10th	Day	24% 19% 38% 16% 3	420
	11th to 20th	Day	23% 20% 33% 19% 5%	420
	21st to end	Day	32% 15% 29% 16% 7%	462
April	1st to 10th	Day	28% 20% 29% 16% 7%	420
	11th to 20th	Day	32% 17% 24% 17% 7% 3	412
	21st to end	Day	30% 14% 33% 14% 8% 2	420
May	1st to 10th	Day	28% 12% 27% 22% 7% 3	423
	11th to 20th	Day	21% 12% 32% 21% 11% 4	447
	21st to end	Day	19% 12% 30% 23% 11% 5%	485
June	1st to 10th	Day	22% 12% 30% 27% 6% 2	450
	11th to 20th	Day	18% 13% 30% 24% 12% 3	592
	21st to end	Day	20% 9% 36% 24% 9% 2	630
July	1st to 10th	Day	11% 10% 31% 29% 15% 4	621
	11th to 20th	Day	15% 9% 27% 28% 13% 7%	457
	21st to end	Day	12% 8% 33% 30% 12% 5%	488
August	1st to 10th	Day	15% 7% 26% 28% 16% 8%	446
	11th to 20th	Day	14% 6% 23% 31% 16% 10%	439
	21st to end	Day	18% 9% 28% 23% 15% 8%	495
September	1st to 10th	Day	16% 8% 30% 26% 13% 8%	450
	11th to 20th	Day	14% 7% 28% 24% 19% 8%	450
	21st to end	Day	21% 12% 26% 23% 14% 4	450
October	1st to 10th	Day	25% 13% 27% 19% 11% 5%	450
	11th to 20th	Day	26% 19% 31% 18% 4 1	450
	21st to end	Day	33% 17% 31% 14% 4	495
November	1st to 10th	Day	32% 16% 31% 16% 5%	215
	11th to 20th	Day	27% 20% 37% 12% 4	180
	21st to end	Day	28% 22% 36% 13% 2	180
December	1st to 10th	Day	32% 19% 32% 14% 3	180
	11th to 20th	Day	34% 25% 23% 14% 4 1	180
	21st to end	Day	38% 22% 27% 11% 3	198
January	1st to 10th	Night	30% 18% 29% 17% 5%	760
	11th to 20th	Night	31% 15% 31% 18% 5% 1	760
	21st to end	Night	22% 11% 33% 26% 7% 1	836
February	1st to 10th	Night	24% 19% 33% 21% 3	739
	11th to 20th	Night	22% 18% 35% 19% 6%	703
	21st to end	Night	27% 19% 30% 19% 4	592
March	1st to 10th	Night	23% 17% 34% 23% 3	720
	11th to 20th	Night	23% 16% 35% 20% 5% 1	720
	21st to end	Night	22% 13% 31% 26% 8%	792
April	1st to 10th	Night	29% 15% 29% 23% 5%	720
	11th to 20th	Night	25% 14% 28% 21% 11% 1	638
	21st to end	Night	25% 10% 30% 26% 9% 1	540
May	1st to 10th	Night	22% 6% 25% 32% 13% 2	504
	11th to 20th	Night	18% 8% 33% 30% 11%	354
	21st to end	Night	13% 5% 33% 35% 13%	389
June	1st to 10th	Night	16% 8% 32% 34% 10% 1	360
	11th to 20th	Night	14% 11% 25% 38% 13%	360
	21st to end	Night	16% 6% 27% 38% 13% 1	360
July	1st to 10th	Night	9% 4 19% 48% 18% 1	350
	11th to 20th	Night	15% 4 24% 37% 19% 1	348
	21st to end	Night	12% 3 25% 38% 21% 1	493
August	1st to 10th	Night	12% 4 18% 37% 26% 2	537
	11th to 20th	Night	10% 5% 17% 37% 28% 3	534
	21st to end	Night	14% 6% 25% 31% 21% 2	656
September	1st to 10th	Night	14% 6% 26% 32% 19% 3	720
	11th to 20th	Night	17% 6% 24% 28% 21% 4	720
	21st to end	Night	21% 8% 19% 32% 17% 3	720
October	1st to 10th	Night	22% 10% 29% 24% 13% 2	720
	11th to 20th	Night	31% 13% 27% 21% 8% 1	720
	21st to end	Night	32% 15% 30% 20% 4	792
November	1st to 10th	Night	27% 14% 27% 24% 8% 1	740
	11th to 20th	Night	26% 18% 31% 18% 6% 1	760
	21st to end	Night	33% 15% 27% 20% 5%	760
December	1st to 10th	Night	35% 18% 29% 14% 4	760
	11th to 20th	Night	27% 19% 34% 15% 4 1	760
	21st to end	Night	35% 16% 30% 17% 2	836

Page generated at 2024-02-02 05:20:27. Version ac07

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