Port Perry Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Perry Astronomical Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	15% 9% 4 7% 6% 3 5% 5% 5% 7% 32%	74678
	Night	15% 10% 4 7% 6% 4 5% 5% 5% 7% 33%	75069
Transparency	Day	49% 8% 13% 19% 10% 1	74390
	Night	49% 8% 13% 19% 10% 1	74891
Seeing	Day	20% 15% 30% 22% 10% 4	13303
	Night	19% 12% 31% 27% 10% 1	22780

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	15% 15% 6% 8% 8% 4 6% 5% 5% 6% 23%	1517
	11th to 20th	Day	16% 18% 7% 10% 8% 3 5% 5% 4 5% 20%	1550
	21st to end	Day	13% 13% 7% 11% 10% 4 6% 5% 5% 5% 21%	1782
February	1st to 10th	Day	15% 16% 6% 11% 9% 4 6% 5% 4 4 20%	1620
	11th to 20th	Day	12% 13% 7% 13% 8% 3 4 4 5% 5% 25%	1573
	21st to end	Day	17% 15% 6% 10% 7% 4 5% 3 4 4 26%	1368
March	1st to 10th	Day	14% 11% 4 10% 8% 4 5% 5% 5% 6% 30%	1977
	11th to 20th	Day	14% 13% 5% 7% 6% 4 5% 4 5% 4 33%	1980
	21st to end	Day	18% 10% 4 6% 6% 4 4 4 3 5% 34%	2178
April	1st to 10th	Day	19% 12% 4 7% 6% 3 4 5% 5% 6% 29%	2149
	11th to 20th	Day	19% 10% 4 6% 5% 3 4 5% 4 6% 36%	2177
	21st to end	Day	21% 9% 3 7% 6% 3 4 4 4 6% 34%	2340
May	1st to 10th	Day	18% 8% 3 8% 6% 4 5% 4 5% 6% 33%	2367
	11th to 20th	Day	15% 8% 2 6% 5% 3 4 5% 5% 7% 40%	2478
	21st to end	Day	12% 5% 3 5% 5% 3 5% 5% 6% 8% 42%	2725
June	1st to 10th	Day	16% 6% 3 6% 4 4 5% 5% 6% 7% 37%	2520
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 5% 6% 8% 39%	2520
	21st to end	Day	15% 6% 2 5% 5% 4 6% 5% 7% 9% 37%	2520
July	1st to 10th	Day	10% 4 2 4 5% 4 5% 5% 7% 9% 45%	2520
	11th to 20th	Day	12% 5% 3 6% 6% 3 5% 5% 7% 10% 37%	2520
	21st to end	Day	9% 5% 3 6% 7% 4 6% 8% 8% 9% 36%	2629
August	1st to 10th	Day	9% 6% 3 6% 6% 4 6% 6% 7% 9% 39%	2327
	11th to 20th	Day	9% 5% 3 6% 6% 4 5% 7% 7% 10% 38%	2303
	21st to end	Day	12% 7% 2 5% 6% 3 5% 6% 6% 8% 39%	2462
September	1st to 10th	Day	13% 5% 3 6% 6% 3 5% 5% 6% 8% 40%	2160
	11th to 20th	Day	12% 6% 2 5% 5% 3 5% 5% 6% 7% 44%	2090
	21st to end	Day	14% 7% 3 7% 5% 3 5% 5% 5% 7% 39%	1922
October	1st to 10th	Day	19% 8% 4 6% 6% 3 4 4 5% 6% 33%	1800
	11th to 20th	Day	21% 10% 4 8% 7% 3 5% 4 5% 6% 28%	1800
	21st to end	Day	25% 13% 5% 7% 6% 3 4 5% 4 5% 23%	1980
November	1st to 10th	Day	16% 10% 4 7% 6% 4 5% 5% 5% 7% 32%	1586
	11th to 20th	Day	14% 14% 5% 8% 7% 3 6% 6% 6% 7% 24%	1520
	21st to end	Day	18% 16% 6% 9% 8% 3 4 4 4 6% 23%	1520
December	1st to 10th	Day	17% 15% 6% 11% 8% 4 6% 6% 5% 7% 15%	1520
	11th to 20th	Day	17% 14% 5% 9% 6% 4 5% 4 6% 8% 23%	1906
	21st to end	Day	25% 17% 6% 10% 8% 3 5% 4 3 4 14%	2772
January	1st to 10th	Night	16% 14% 6% 10% 7% 5% 4 4 4 6% 23%	2649
	11th to 20th	Night	18% 16% 6% 10% 7% 4 5% 5% 4 5% 19%	2479
	21st to end	Night	12% 13% 6% 10% 7% 4 6% 6% 5% 6% 26%	2574
February	1st to 10th	Night	13% 16% 6% 9% 8% 5% 7% 5% 5% 5% 21%	2340
	11th to 20th	Night	15% 11% 6% 9% 8% 3 4 4 5% 5% 28%	2273
	21st to end	Night	13% 12% 5% 10% 8% 4 5% 5% 5% 6% 27%	1884
March	1st to 10th	Night	14% 8% 5% 8% 8% 4 5% 5% 4 5% 35%	1970
	11th to 20th	Night	15% 11% 5% 7% 7% 3 4 4 3 5% 34%	1979
	21st to end	Night	15% 7% 3 6% 6% 3 5% 5% 4 6% 39%	2178
April	1st to 10th	Night	19% 10% 3 7% 5% 3 4 4 5% 7% 32%	1810
	11th to 20th	Night	16% 8% 3 6% 6% 3 5% 5% 5% 6% 38%	1746
	21st to end	Night	16% 10% 4 6% 5% 3 5% 4 5% 7% 34%	1620
May	1st to 10th	Night	18% 5% 3 5% 6% 3 5% 5% 5% 7% 38%	1590
	11th to 20th	Night	13% 5% 3 6% 5% 3 4 5% 5% 8% 44%	1415
	21st to end	Night	11% 4 2 4 4 3 4 5% 6% 8% 50%	1550
June	1st to 10th	Night	12% 6% 2 4 6% 4 5% 5% 6% 9% 40%	1440
	11th to 20th	Night	9% 5% 2 5% 6% 4 4 5% 6% 8% 44%	1440
	21st to end	Night	11% 7% 3 5% 5% 3 5% 5% 5% 8% 43%	1440
July	1st to 10th	Night	6% 5% 2 3 5% 3 5% 4 6% 7% 53%	1440
	11th to 20th	Night	9% 5% 2 5% 5% 3 4 4 7% 8% 48%	1440
	21st to end	Night	8% 4 2 4 4 3 5% 5% 6% 9% 50%	1686
August	1st to 10th	Night	8% 5% 2 4 5% 3 4 5% 6% 8% 50%	1611
	11th to 20th	Night	7% 4 3 5% 4 3 4 5% 6% 10% 50%	1598
	21st to end	Night	9% 6% 2 6% 5% 3 5% 5% 5% 7% 47%	1876
September	1st to 10th	Night	10% 6% 2 5% 6% 3 5% 5% 6% 7% 46%	1800
	11th to 20th	Night	11% 4 2 4 4 4 3 4 4 8% 52%	1867
	21st to end	Night	15% 6% 3 6% 5% 3 4 4 5% 6% 44%	2038
October	1st to 10th	Night	15% 7% 2 7% 6% 3 4 4 5% 7% 38%	2160
	11th to 20th	Night	20% 9% 3 5% 7% 4 4 4 6% 6% 31%	2160
	21st to end	Night	21% 10% 4 7% 6% 3 5% 5% 5% 7% 28%	2376
November	1st to 10th	Night	18% 8% 3 6% 6% 3 4 5% 6% 7% 34%	2488
	11th to 20th	Night	17% 10% 4 8% 6% 5% 5% 5% 6% 8% 27%	2660
	21st to end	Night	19% 12% 4 8% 6% 5% 6% 6% 5% 7% 23%	2660
December	1st to 10th	Night	18% 15% 6% 10% 7% 4 4 5% 5% 7% 20%	2660
	11th to 20th	Night	15% 17% 6% 10% 7% 4 5% 5% 6% 6% 20%	3288
	21st to end	Night	26% 16% 5% 11% 7% 4 5% 5% 4 4 14%	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	60% 8% 13% 13% 6%	1518
	11th to 20th	Day	65% 9% 10% 9% 7%	1550
	21st to end	Day	63% 6% 9% 12% 8% 2	1782
February	1st to 10th	Day	66% 7% 9% 13% 4 1	1620
	11th to 20th	Day	60% 6% 10% 15% 7% 1	1582
	21st to end	Day	61% 6% 9% 17% 7% 1	1368
March	1st to 10th	Day	54% 7% 14% 14% 10% 1	1980
	11th to 20th	Day	53% 6% 10% 17% 12% 2	1980
	21st to end	Day	51% 6% 9% 16% 16% 3	2178
April	1st to 10th	Day	54% 6% 11% 17% 10% 1	2149
	11th to 20th	Day	48% 6% 10% 21% 14% 1	2177
	21st to end	Day	50% 7% 10% 17% 13% 2	2340
May	1st to 10th	Day	50% 7% 10% 17% 13% 2	2367
	11th to 20th	Day	41% 7% 12% 24% 15% 1	2482
	21st to end	Day	39% 9% 15% 22% 12% 2	2725
June	1st to 10th	Day	44% 8% 14% 20% 14% 1	2520
	11th to 20th	Day	40% 8% 13% 23% 14% 1	2520
	21st to end	Day	41% 10% 15% 22% 12% 1	2520
July	1st to 10th	Day	33% 8% 15% 24% 18% 2	2450
	11th to 20th	Day	38% 9% 17% 25% 10% 1	2445
	21st to end	Day	40% 11% 16% 22% 11% 1	2482
August	1st to 10th	Day	38% 11% 18% 22% 11% 1	2314
	11th to 20th	Day	37% 10% 18% 24% 11%	2303
	21st to end	Day	40% 9% 15% 24% 11% 1	2462
September	1st to 10th	Day	41% 7% 14% 27% 10% 1	2160
	11th to 20th	Day	36% 8% 15% 28% 13%	2090
	21st to end	Day	42% 10% 14% 25% 8%	1922
October	1st to 10th	Day	49% 9% 13% 18% 10% 1	1800
	11th to 20th	Day	55% 8% 13% 17% 5% 1	1800
	21st to end	Day	62% 7% 12% 13% 6% 1	1980
November	1st to 10th	Day	50% 8% 16% 17% 8% 1	1586
	11th to 20th	Day	55% 11% 13% 13% 8% 1	1520
	21st to end	Day	61% 8% 13% 14% 3	1520
December	1st to 10th	Day	64% 11% 14% 9% 2	1520
	11th to 20th	Day	59% 10% 13% 12% 6%	1906
	21st to end	Day	71% 10% 8% 9% 2	2772
January	1st to 10th	Night	61% 7% 11% 14% 7%	2649
	11th to 20th	Night	67% 7% 10% 10% 5% 1	2479
	21st to end	Night	57% 7% 11% 15% 8% 2	2574
February	1st to 10th	Night	63% 8% 9% 14% 5% 1	2340
	11th to 20th	Night	56% 7% 11% 17% 8%	2275
	21st to end	Night	56% 6% 13% 18% 7% 1	1884
March	1st to 10th	Night	50% 7% 14% 18% 9% 2	1980
	11th to 20th	Night	52% 5% 11% 21% 10% 2	1980
	21st to end	Night	44% 7% 10% 20% 15% 3	2178
April	1st to 10th	Night	51% 6% 11% 18% 13% 1	1810
	11th to 20th	Night	46% 8% 10% 19% 16% 2	1746
	21st to end	Night	48% 7% 12% 16% 15% 2	1620
May	1st to 10th	Night	44% 7% 11% 20% 16% 2	1590
	11th to 20th	Night	38% 6% 11% 23% 17% 4	1416
	21st to end	Night	30% 8% 16% 24% 17% 4	1551
June	1st to 10th	Night	39% 8% 13% 23% 15% 2	1440
	11th to 20th	Night	36% 7% 12% 22% 20% 3	1440
	21st to end	Night	38% 8% 14% 23% 15% 1	1440
July	1st to 10th	Night	27% 8% 13% 27% 23% 2	1400
	11th to 20th	Night	32% 8% 14% 28% 17% 1	1392
	21st to end	Night	29% 8% 16% 30% 15% 2	1592
August	1st to 10th	Night	32% 7% 20% 27% 14% 1	1600
	11th to 20th	Night	29% 7% 20% 29% 14% 1	1598
	21st to end	Night	34% 9% 14% 27% 14% 1	1876
September	1st to 10th	Night	36% 8% 17% 25% 13% 1	1800
	11th to 20th	Night	32% 6% 13% 31% 17% 1	1867
	21st to end	Night	40% 8% 13% 27% 11%	2038
October	1st to 10th	Night	44% 9% 14% 18% 13% 1	2160
	11th to 20th	Night	51% 10% 14% 18% 7%	2160
	21st to end	Night	55% 9% 12% 14% 9%	2376
November	1st to 10th	Night	46% 8% 13% 21% 11% 1	2488
	11th to 20th	Night	54% 8% 14% 15% 8% 2	2660
	21st to end	Night	57% 11% 16% 12% 4	2660
December	1st to 10th	Night	62% 9% 14% 12% 3	2660
	11th to 20th	Night	62% 9% 12% 13% 4	3288
	21st to end	Night	70% 10% 10% 8% 3	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% 24% 30% 18% 5% 1	180
	11th to 20th	Day	22% 28% 32% 13% 4 1	204
	21st to end	Day	23% 24% 35% 15% 2	264
February	1st to 10th	Day	30% 31% 28% 10% 2	240
	11th to 20th	Day	24% 30% 38% 7% 1	240
	21st to end	Day	34% 26% 31% 8%	234
March	1st to 10th	Day	21% 22% 38% 15% 3	420
	11th to 20th	Day	23% 23% 33% 17% 3 1	420
	21st to end	Day	30% 17% 30% 16% 5% 1	462
April	1st to 10th	Day	24% 19% 36% 15% 6% 1	420
	11th to 20th	Day	26% 20% 26% 16% 9% 3	412
	21st to end	Day	24% 18% 34% 15% 7% 1	420
May	1st to 10th	Day	23% 14% 29% 23% 8% 4	423
	11th to 20th	Day	20% 13% 31% 24% 9% 4	447
	21st to end	Day	15% 10% 28% 29% 13% 5%	485
June	1st to 10th	Day	17% 11% 34% 27% 10% 2	450
	11th to 20th	Day	16% 12% 26% 30% 13% 3	450
	21st to end	Day	18% 8% 31% 29% 11% 4	450
July	1st to 10th	Day	9% 6% 28% 32% 20% 5%	445
	11th to 20th	Day	14% 7% 26% 28% 18% 7%	444
	21st to end	Day	9% 6% 34% 34% 12% 6%	488
August	1st to 10th	Day	12% 6% 26% 32% 15% 9%	446
	11th to 20th	Day	10% 6% 24% 29% 22% 8%	439
	21st to end	Day	14% 9% 26% 27% 17% 8%	495
September	1st to 10th	Day	13% 8% 28% 26% 16% 8%	450
	11th to 20th	Day	11% 6% 29% 25% 17% 12%	450
	21st to end	Day	13% 14% 28% 25% 13% 6%	450
October	1st to 10th	Day	22% 11% 29% 19% 16% 4	450
	11th to 20th	Day	21% 21% 33% 18% 5% 2	450
	21st to end	Day	31% 19% 27% 19% 4	495
November	1st to 10th	Day	24% 20% 29% 19% 7%	262
	11th to 20th	Day	24% 18% 41% 12% 4 1	180
	21st to end	Day	26% 25% 37% 9% 3	180
December	1st to 10th	Day	25% 22% 32% 17% 4 1	180
	11th to 20th	Day	26% 27% 29% 15% 2 1	180
	21st to end	Day	30% 24% 32% 13% 1	198
January	1st to 10th	Night	21% 20% 36% 17% 5%	760
	11th to 20th	Night	26% 16% 32% 21% 4 1	735
	21st to end	Night	18% 14% 36% 25% 6% 1	792
February	1st to 10th	Night	21% 18% 41% 18% 2	720
	11th to 20th	Night	22% 16% 40% 18% 4	703
	21st to end	Night	20% 17% 40% 19% 3	592
March	1st to 10th	Night	20% 17% 37% 23% 3	720
	11th to 20th	Night	20% 20% 33% 22% 4 1	720
	21st to end	Night	19% 12% 36% 26% 7%	792
April	1st to 10th	Night	22% 17% 34% 23% 4	720
	11th to 20th	Night	18% 14% 36% 22% 10% 1	682
	21st to end	Night	21% 11% 33% 26% 9%	540
May	1st to 10th	Night	20% 7% 28% 31% 13% 2	510
	11th to 20th	Night	16% 6% 33% 32% 11%	354
	21st to end	Night	11% 6% 26% 39% 17% 1	389
June	1st to 10th	Night	13% 6% 33% 35% 13%	360
	11th to 20th	Night	11% 9% 25% 40% 15% 1	360
	21st to end	Night	12% 4 29% 41% 13%	360
July	1st to 10th	Night	5% 4 27% 42% 21% 1	350
	11th to 20th	Night	9% 5% 26% 37% 22% 1	348
	21st to end	Night	9% 2 25% 44% 19% 1	506
August	1st to 10th	Night	10% 3 17% 41% 26% 2	537
	11th to 20th	Night	7% 2 18% 41% 28% 4	534
	21st to end	Night	11% 4 24% 37% 22% 2	688
September	1st to 10th	Night	12% 5% 26% 34% 18% 4	720
	11th to 20th	Night	12% 5% 23% 32% 22% 5%	720
	21st to end	Night	17% 9% 22% 32% 17% 4	720
October	1st to 10th	Night	19% 7% 31% 27% 15% 2	720
	11th to 20th	Night	23% 14% 29% 24% 8% 1	720
	21st to end	Night	27% 13% 32% 24% 4	792
November	1st to 10th	Night	24% 13% 29% 25% 9% 1	740
	11th to 20th	Night	21% 16% 35% 20% 6% 1	760
	21st to end	Night	25% 16% 31% 23% 5%	760
December	1st to 10th	Night	27% 18% 34% 18% 3	760
	11th to 20th	Night	21% 17% 38% 20% 3 1	760
	21st to end	Night	30% 15% 35% 18% 2	836

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