Port Perry
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Perry clear sky chart from 2005-11-06 to 2026-01-17.

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	14% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	127663
	Night	14% 10% 4 9% 5% 5% 5% 4 5% 6% 33%	124024
Transparency	Day	49% 7% 13% 19% 11% 1	131251
	Night	49% 7% 13% 19% 11% 1	128522
Seeing	Day	20% 15% 31% 21% 10% 3	15856
	Night	19% 13% 31% 27% 10% 1	25302

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% 13% 5% 11% 6% 6% 5% 4 4 5% 28%	3677
	11th to 20th	Day	9% 18% 8% 18% 9% 6% 5% 3 4 5% 16%	3844
	21st to end	Day	15% 18% 6% 14% 7% 5% 5% 4 4 5% 18%	4466
February	1st to 10th	Day	14% 17% 5% 12% 8% 6% 5% 4 5% 4 21%	1800
	11th to 20th	Day	12% 15% 7% 15% 7% 4 4 3 5% 5% 24%	1753
	21st to end	Day	16% 16% 5% 10% 6% 7% 5% 3 3 4 25%	1526
March	1st to 10th	Day	17% 12% 5% 10% 5% 4 4 3 4 5% 31%	4932
	11th to 20th	Day	11% 13% 5% 10% 5% 5% 6% 4 6% 7% 29%	4980
	21st to end	Day	18% 12% 3 8% 5% 4 4 3 4 5% 34%	5465
April	1st to 10th	Day	19% 13% 3 8% 5% 4 4 3 5% 6% 29%	2388
	11th to 20th	Day	19% 12% 3 7% 4 3 4 4 4 6% 35%	2424
	21st to end	Day	20% 10% 3 8% 5% 4 4 3 4 6% 34%	2600
May	1st to 10th	Day	20% 9% 3 8% 4 5% 5% 3 5% 7% 32%	5971
	11th to 20th	Day	15% 10% 4 9% 5% 5% 4 4 5% 7% 34%	6318
	21st to end	Day	18% 8% 3 6% 4 4 5% 3 5% 7% 37%	6949
June	1st to 10th	Day	15% 8% 3 7% 4 5% 5% 5% 6% 7% 35%	2800
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 7% 8% 39%	2800
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	2800
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 5% 6% 10% 39%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	6360
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 9% 46%	6617
August	1st to 10th	Day	15% 8% 3 7% 4 4 5% 4 6% 7% 37%	4234
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 4 6% 10% 36%	4174
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 6% 9% 43%	4459
September	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 3 6% 8% 41%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 4 5% 7% 46%	2321
	21st to end	Day	15% 7% 2 8% 4 4 6% 4 5% 6% 39%	2135
October	1st to 10th	Day	18% 8% 3 7% 5% 4 4 4 5% 6% 37%	2000
	11th to 20th	Day	20% 10% 3 9% 6% 4 5% 3 5% 6% 30%	2000
	21st to end	Day	23% 13% 4 8% 6% 4 5% 4 5% 6% 24%	2200
November	1st to 10th	Day	16% 12% 3 8% 5% 4 4 4 5% 6% 32%	1756
	11th to 20th	Day	14% 15% 4 10% 5% 5% 5% 5% 6% 7% 25%	1680
	21st to end	Day	16% 18% 5% 10% 7% 4 4 4 4 6% 21%	1680
December	1st to 10th	Day	16% 19% 6% 13% 7% 6% 6% 4 5% 6% 13%	2680
	11th to 20th	Day	15% 18% 6% 11% 5% 4 4 3 5% 6% 22%	3066
	21st to end	Day	21% 18% 6% 14% 6% 5% 4 3 4 4 15%	4048
January	1st to 10th	Night	13% 13% 6% 14% 6% 6% 4 3 5% 6% 24%	6489
	11th to 20th	Night	11% 14% 6% 15% 9% 6% 5% 4 5% 6% 18%	6171
	21st to end	Night	14% 14% 5% 12% 6% 5% 5% 4 5% 6% 24%	6490
February	1st to 10th	Night	12% 16% 5% 10% 8% 7% 5% 4 5% 5% 23%	2600
	11th to 20th	Night	14% 13% 6% 11% 7% 4 4 3 5% 5% 27%	2533
	21st to end	Night	12% 13% 4 11% 6% 5% 5% 4 6% 6% 28%	2100
March	1st to 10th	Night	15% 11% 4 11% 5% 5% 4 4 5% 5% 31%	4908
	11th to 20th	Night	9% 8% 3 8% 6% 5% 6% 4 6% 8% 35%	4979
	21st to end	Night	18% 9% 3 6% 4 4 5% 4 5% 6% 36%	5467
April	1st to 10th	Night	18% 12% 4 8% 5% 4 4 4 5% 7% 30%	2010
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 4 5% 6% 37%	1942
	21st to end	Night	15% 10% 3 7% 5% 5% 4 4 5% 7% 34%	1800
May	1st to 10th	Night	22% 9% 3 7% 4 4 4 3 5% 5% 34%	3968
	11th to 20th	Night	11% 8% 3 7% 5% 4 4 5% 5% 7% 40%	3575
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 7% 40%	3926
June	1st to 10th	Night	12% 7% 2 6% 5% 5% 5% 4 7% 8% 40%	1600
	11th to 20th	Night	9% 6% 2 7% 5% 5% 4 5% 7% 8% 43%	1600
	21st to end	Night	11% 7% 3 6% 4 5% 4 4 5% 8% 42%	1600
July	1st to 10th	Night	7% 6% 3 5% 5% 4 5% 4 7% 9% 46%	3600
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 5% 8% 50%	3600
	21st to end	Night	9% 4 2 5% 3 4 4 4 5% 8% 53%	4231
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 7% 49%	2915
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 7% 50%	2875
	21st to end	Night	9% 6% 2 6% 4 4 4 4 5% 7% 49%	3406
September	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 7% 47%	2000
	11th to 20th	Night	10% 4 1 5% 3 4 3 3 4 7% 54%	2075
	21st to end	Night	16% 7% 3 7% 4 4 3 3 5% 6% 42%	2265
October	1st to 10th	Night	14% 6% 2 7% 5% 4 4 3 5% 7% 42%	2400
	11th to 20th	Night	19% 9% 3 7% 6% 5% 4 3 6% 6% 33%	2400
	21st to end	Night	19% 10% 4 8% 5% 4 5% 3 5% 7% 30%	2640
November	1st to 10th	Night	17% 9% 2 8% 5% 4 4 4 6% 6% 34%	2761
	11th to 20th	Night	16% 12% 4 9% 6% 6% 5% 4 6% 7% 26%	2940
	21st to end	Night	19% 14% 3 9% 5% 6% 5% 5% 5% 6% 22%	2940
December	1st to 10th	Night	16% 16% 5% 12% 7% 6% 5% 4 6% 7% 16%	4720
	11th to 20th	Night	14% 17% 6% 12% 6% 6% 5% 4 6% 5% 20%	5348
	21st to end	Night	21% 16% 5% 15% 5% 5% 5% 3 4 4 15%	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	57% 8% 11% 13% 10% 1	3678
	11th to 20th	Day	71% 6% 9% 9% 5%	3876
	21st to end	Day	68% 6% 10% 9% 7% 1	4466
February	1st to 10th	Day	64% 7% 9% 15% 5% 1	1800
	11th to 20th	Day	61% 6% 10% 15% 7% 1	1762
	21st to end	Day	61% 6% 10% 16% 6% 1	1526
March	1st to 10th	Day	54% 6% 13% 13% 14%	4935
	11th to 20th	Day	50% 7% 16% 17% 7% 2	4980
	21st to end	Day	49% 7% 9% 18% 15% 1	5465
April	1st to 10th	Day	55% 6% 11% 18% 10% 1	2388
	11th to 20th	Day	49% 6% 10% 20% 14% 1	2424
	21st to end	Day	51% 7% 10% 17% 13% 2	2600
May	1st to 10th	Day	50% 8% 12% 16% 10% 5%	5980
	11th to 20th	Day	48% 8% 12% 15% 18% 1	6322
	21st to end	Day	46% 7% 12% 21% 12% 3	6949
June	1st to 10th	Day	44% 8% 13% 20% 14% 1	2800
	11th to 20th	Day	40% 8% 14% 23% 14% 1	2800
	21st to end	Day	42% 9% 15% 22% 12% 1	2800
July	1st to 10th	Day	39% 6% 15% 24% 14% 1	6290
	11th to 20th	Day	34% 6% 14% 30% 15% 1	6285
	21st to end	Day	32% 8% 14% 31% 14%	6428
August	1st to 10th	Day	44% 7% 15% 21% 12% 1	4221
	11th to 20th	Day	41% 7% 17% 27% 9%	4213
	21st to end	Day	36% 7% 14% 32% 10% 1	4459
September	1st to 10th	Day	40% 7% 14% 28% 10% 1	2400
	11th to 20th	Day	34% 7% 15% 29% 14%	2321
	21st to end	Day	43% 10% 13% 24% 9%	2135
October	1st to 10th	Day	39% 7% 15% 26% 13% 1	3260
	11th to 20th	Day	43% 6% 16% 21% 13%	3240
	21st to end	Day	53% 6% 15% 19% 7%	3538
November	1st to 10th	Day	50% 8% 14% 18% 8% 1	1756
	11th to 20th	Day	55% 10% 13% 13% 8% 1	1680
	21st to end	Day	63% 7% 13% 14% 3	1680
December	1st to 10th	Day	70% 8% 13% 8% 2	2680
	11th to 20th	Day	62% 7% 11% 11% 8%	3066
	21st to end	Day	70% 9% 7% 8% 6%	4048
January	1st to 10th	Night	60% 7% 12% 14% 8%	6489
	11th to 20th	Night	65% 7% 12% 10% 5% 1	6214
	21st to end	Night	58% 7% 13% 10% 11% 1	6490
February	1st to 10th	Night	61% 8% 11% 15% 6% 1	2600
	11th to 20th	Night	57% 7% 11% 17% 8%	2535
	21st to end	Night	54% 6% 13% 19% 7% 1	2100
March	1st to 10th	Night	53% 5% 11% 16% 12% 2	4918
	11th to 20th	Night	43% 8% 18% 21% 8% 2	4980
	21st to end	Night	46% 7% 11% 20% 15% 2	5467
April	1st to 10th	Night	52% 6% 11% 17% 12% 1	2010
	11th to 20th	Night	46% 8% 10% 19% 16% 2	1942
	21st to end	Night	47% 7% 12% 16% 16% 2	1800
May	1st to 10th	Night	50% 7% 11% 13% 14% 5%	3975
	11th to 20th	Night	40% 9% 14% 20% 15% 2	3576
	21st to end	Night	41% 8% 13% 22% 14% 2	3927
June	1st to 10th	Night	39% 8% 12% 23% 15% 2	1600
	11th to 20th	Night	36% 7% 13% 22% 20% 2	1600
	21st to end	Night	39% 8% 14% 23% 15% 1	1600
July	1st to 10th	Night	34% 6% 16% 24% 18% 1	3560
	11th to 20th	Night	32% 6% 14% 30% 17% 1	3552
	21st to end	Night	30% 6% 12% 32% 19% 1	4105
August	1st to 10th	Night	34% 5% 15% 28% 16% 2	2904
	11th to 20th	Night	34% 5% 18% 29% 12% 1	2902
	21st to end	Night	33% 7% 13% 33% 13% 1	3406
September	1st to 10th	Night	34% 7% 16% 28% 13% 1	2000
	11th to 20th	Night	30% 6% 13% 32% 18% 1	2075
	21st to end	Night	41% 7% 12% 26% 12% 1	2265
October	1st to 10th	Night	32% 6% 11% 28% 23% 1	3920
	11th to 20th	Night	42% 7% 16% 25% 11%	3899
	21st to end	Night	43% 7% 16% 26% 8%	4252
November	1st to 10th	Night	47% 8% 13% 21% 10% 1	2761
	11th to 20th	Night	54% 8% 13% 15% 8% 1	2940
	21st to end	Night	59% 10% 15% 12% 4	2940
December	1st to 10th	Night	65% 7% 15% 10% 3	4720
	11th to 20th	Night	64% 8% 10% 11% 7%	5348
	21st to end	Night	69% 8% 10% 8% 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	21% 25% 32% 17% 4	220
	11th to 20th	Day	22% 29% 31% 13% 4 1	262
	21st to end	Day	24% 25% 35% 14% 2	330
February	1st to 10th	Day	27% 29% 29% 12% 2	300
	11th to 20th	Day	26% 28% 37% 7% 1	300
	21st to end	Day	34% 24% 33% 9%	289
March	1st to 10th	Day	22% 23% 37% 16% 3	500
	11th to 20th	Day	24% 25% 31% 17% 2 1	500
	21st to end	Day	31% 18% 31% 15% 5% 1	550
April	1st to 10th	Day	26% 20% 35% 14% 5% 1	500
	11th to 20th	Day	27% 22% 26% 15% 8% 3	492
	21st to end	Day	24% 18% 34% 15% 7% 1	500
May	1st to 10th	Day	20% 15% 32% 22% 7% 4	503
	11th to 20th	Day	18% 13% 32% 24% 10% 3	527
	21st to end	Day	17% 11% 28% 27% 13% 4	573
June	1st to 10th	Day	18% 12% 35% 25% 9% 2	530
	11th to 20th	Day	15% 12% 29% 29% 12% 3	530
	21st to end	Day	18% 10% 32% 26% 10% 3	530
July	1st to 10th	Day	10% 7% 29% 33% 18% 4	525
	11th to 20th	Day	13% 8% 27% 27% 18% 6%	524
	21st to end	Day	9% 6% 34% 32% 13% 6%	576
August	1st to 10th	Day	13% 5% 27% 31% 16% 8%	526
	11th to 20th	Day	11% 7% 24% 29% 22% 8%	519
	21st to end	Day	14% 9% 25% 28% 17% 8%	583
September	1st to 10th	Day	14% 8% 27% 25% 16% 9%	530
	11th to 20th	Day	10% 5% 25% 24% 20% 16%	530
	21st to end	Day	15% 12% 28% 26% 14% 5%	530
October	1st to 10th	Day	20% 11% 30% 19% 16% 3	530
	11th to 20th	Day	20% 20% 35% 17% 6% 2	530
	21st to end	Day	28% 19% 28% 21% 4	583
November	1st to 10th	Day	25% 21% 29% 18% 6%	312
	11th to 20th	Day	24% 20% 40% 11% 5% 1	220
	21st to end	Day	26% 25% 35% 10% 3	220
December	1st to 10th	Day	27% 21% 31% 16% 4	220
	11th to 20th	Day	27% 26% 31% 13% 2	220
	21st to end	Day	29% 23% 33% 12% 2	242
January	1st to 10th	Night	21% 21% 37% 17% 5%	840
	11th to 20th	Night	25% 17% 33% 20% 4 1	825
	21st to end	Night	18% 16% 37% 24% 5%	880
February	1st to 10th	Night	20% 18% 40% 20% 2	800
	11th to 20th	Night	22% 17% 39% 18% 4	783
	21st to end	Night	19% 19% 39% 20% 3	660
March	1st to 10th	Night	20% 17% 38% 22% 3	800
	11th to 20th	Night	19% 21% 34% 21% 4 1	800
	21st to end	Night	20% 14% 35% 25% 7%	880
April	1st to 10th	Night	23% 17% 34% 23% 3	800
	11th to 20th	Night	19% 14% 36% 21% 9% 1	758
	21st to end	Night	20% 11% 33% 27% 9%	600
May	1st to 10th	Night	20% 8% 27% 31% 11% 2	567
	11th to 20th	Night	15% 7% 33% 33% 11%	394
	21st to end	Night	12% 6% 26% 39% 16% 1	433
June	1st to 10th	Night	12% 7% 33% 35% 12%	400
	11th to 20th	Night	10% 9% 27% 40% 14% 1	400
	21st to end	Night	12% 5% 29% 41% 13%	400
July	1st to 10th	Night	5% 4 27% 43% 19% 1	390
	11th to 20th	Night	9% 5% 24% 39% 22% 1	388
	21st to end	Night	10% 2 24% 43% 19% 2	564
August	1st to 10th	Night	9% 3 17% 41% 27% 2	597
	11th to 20th	Night	9% 2 18% 40% 27% 3	594
	21st to end	Night	11% 4 24% 37% 22% 2	765
September	1st to 10th	Night	12% 5% 26% 34% 19% 4	800
	11th to 20th	Night	11% 5% 22% 33% 24% 7%	800
	21st to end	Night	18% 8% 21% 32% 17% 4	800
October	1st to 10th	Night	17% 7% 30% 28% 16% 2	800
	11th to 20th	Night	22% 15% 30% 25% 8% 1	800
	21st to end	Night	25% 12% 32% 25% 4	880
November	1st to 10th	Night	23% 14% 29% 24% 8% 1	820
	11th to 20th	Night	21% 17% 34% 21% 6% 1	840
	21st to end	Night	26% 16% 31% 22% 4	840
December	1st to 10th	Night	26% 18% 34% 19% 3	840
	11th to 20th	Night	21% 18% 38% 19% 3 1	840
	21st to end	Night	29% 16% 36% 18% 2	924

Page generated at 2026-01-14 01:14:26. Version ac07

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