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 2024-06-17.

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	15% 11% 4 8% 5% 5% 5% 4 5% 7% 32%	89429
	Night	15% 11% 4 8% 5% 5% 5% 4 5% 6% 32%	89436
Transparency	Day	49% 9% 13% 18% 10% 1	89153
	Night	49% 9% 13% 18% 10% 1	89265
Seeing	Day	20% 15% 30% 22% 10% 3	13877
	Night	19% 12% 31% 27% 10% 1	23301

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% 16% 5% 8% 5% 5% 4 3 4 6% 28%	2517
	11th to 20th	Day	10% 18% 8% 15% 8% 6% 5% 3 4 4 18%	2676
	21st to end	Day	20% 20% 6% 12% 6% 4 4 3 4 4 17%	3124
February	1st to 10th	Day	14% 17% 5% 12% 8% 6% 5% 4 4 4 21%	1710
	11th to 20th	Day	12% 15% 7% 14% 7% 4 4 3 5% 5% 25%	1663
	21st to end	Day	16% 16% 5% 11% 6% 6% 5% 3 3 4 26%	1451
March	1st to 10th	Day	17% 13% 5% 9% 5% 4 4 3 4 5% 31%	3432
	11th to 20th	Day	11% 16% 5% 10% 5% 5% 6% 4 5% 5% 29%	3480
	21st to end	Day	16% 9% 3 8% 5% 5% 4 4 4 5% 37%	3815
April	1st to 10th	Day	19% 13% 3 8% 5% 4 4 4 5% 6% 29%	2269
	11th to 20th	Day	20% 11% 3 7% 4 3 4 4 4 6% 35%	2300
	21st to end	Day	21% 9% 3 8% 5% 3 4 3 4 6% 34%	2470
May	1st to 10th	Day	18% 9% 2 8% 4 5% 5% 4 6% 7% 33%	4163
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 6% 8% 32%	4398
	21st to end	Day	13% 6% 2 6% 4 4 4 4 5% 7% 45%	4837
June	1st to 10th	Day	15% 8% 3 7% 4 5% 5% 5% 6% 7% 35%	2660
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 4 7% 8% 39%	2607
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	2520
July	1st to 10th	Day	10% 4 2 5% 5% 5% 5% 4 7% 9% 44%	2520
	11th to 20th	Day	12% 6% 2 7% 6% 5% 4 4 7% 10% 37%	2520
	21st to end	Day	9% 6% 3 7% 6% 6% 6% 6% 8% 9% 35%	2629
August	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	2327
	11th to 20th	Day	9% 5% 3 7% 6% 5% 5% 5% 7% 10% 38%	2303
	21st to end	Day	12% 7% 2 6% 5% 4 5% 5% 6% 8% 39%	2462
September	1st to 10th	Day	13% 6% 3 7% 5% 4 5% 3 6% 8% 40%	2160
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 6% 7% 44%	2090
	21st to end	Day	14% 7% 2 8% 4 4 6% 4 5% 7% 39%	1922
October	1st to 10th	Day	19% 9% 3 8% 5% 4 4 4 5% 6% 33%	1800
	11th to 20th	Day	21% 10% 3 9% 6% 4 5% 3 5% 6% 28%	1800
	21st to end	Day	24% 14% 4 8% 5% 3 5% 3 4 5% 23%	1980
November	1st to 10th	Day	16% 11% 3 8% 6% 4 5% 4 5% 7% 32%	1586
	11th to 20th	Day	14% 15% 4 10% 5% 5% 5% 5% 6% 7% 24%	1520
	21st to end	Day	17% 17% 5% 9% 7% 4 4 4 4 6% 23%	1520
December	1st to 10th	Day	16% 16% 6% 12% 6% 6% 6% 5% 5% 6% 15%	1520
	11th to 20th	Day	17% 15% 5% 9% 5% 4 5% 3 6% 7% 23%	1906
	21st to end	Day	25% 18% 5% 11% 6% 4 5% 3 4 4 14%	2772
January	1st to 10th	Night	16% 15% 5% 11% 7% 6% 4 3 4 6% 22%	4429
	11th to 20th	Night	13% 14% 6% 14% 9% 6% 5% 4 5% 5% 19%	4280
	21st to end	Night	19% 13% 5% 10% 6% 5% 5% 4 5% 5% 22%	4532
February	1st to 10th	Night	12% 16% 5% 10% 7% 7% 5% 4 5% 5% 23%	2470
	11th to 20th	Night	14% 13% 6% 11% 7% 4 4 3 5% 5% 28%	2403
	21st to end	Night	12% 13% 4 11% 6% 5% 5% 4 6% 6% 28%	1998
March	1st to 10th	Night	16% 11% 4 8% 6% 5% 4 4 4 5% 33%	3408
	11th to 20th	Night	13% 11% 4 9% 6% 5% 5% 4 5% 6% 33%	3479
	21st to end	Night	15% 8% 3 6% 4 4 5% 4 5% 7% 39%	3817
April	1st to 10th	Night	18% 12% 3 7% 5% 4 4 4 5% 7% 31%	1910
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 4 5% 6% 37%	1844
	21st to end	Night	16% 11% 3 7% 4 5% 4 4 5% 7% 34%	1710
May	1st to 10th	Night	17% 8% 3 6% 4 4 5% 3 5% 6% 38%	2775
	11th to 20th	Night	14% 9% 3 6% 5% 4 4 5% 5% 8% 37%	2495
	21st to end	Night	10% 5% 2 5% 4 3 4 3 6% 8% 49%	2738
June	1st to 10th	Night	12% 7% 2 6% 5% 5% 5% 4 6% 8% 39%	1520
	11th to 20th	Night	9% 6% 2 7% 5% 5% 4 5% 6% 8% 43%	1496
	21st to end	Night	11% 7% 3 5% 4 5% 4 4 6% 8% 43%	1440
July	1st to 10th	Night	6% 5% 2 4 5% 4 4 4 7% 7% 53%	1440
	11th to 20th	Night	8% 6% 2 6% 5% 4 4 4 7% 8% 48%	1440
	21st to end	Night	8% 5% 1 5% 4 4 5% 4 6% 9% 49%	1686
August	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 7% 8% 50%	1611
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 4 6% 10% 50%	1598
	21st to end	Night	9% 6% 2 7% 5% 4 5% 4 5% 7% 47%	1876
September	1st to 10th	Night	10% 6% 2 7% 5% 4 4 4 6% 7% 45%	1800
	11th to 20th	Night	11% 4 2 5% 4 4 3 3 4 8% 52%	1867
	21st to end	Night	15% 7% 3 7% 4 4 3 3 5% 6% 43%	2038
October	1st to 10th	Night	15% 7% 2 8% 5% 4 4 4 5% 7% 38%	2160
	11th to 20th	Night	20% 9% 3 7% 6% 5% 4 3 6% 6% 31%	2160
	21st to end	Night	21% 11% 4 8% 5% 4 5% 3 5% 7% 28%	2376
November	1st to 10th	Night	18% 9% 2 7% 5% 4 4 4 6% 6% 34%	2488
	11th to 20th	Night	17% 11% 3 9% 6% 6% 5% 4 6% 8% 27%	2660
	21st to end	Night	19% 13% 3 9% 5% 6% 5% 5% 5% 7% 23%	2660
December	1st to 10th	Night	17% 16% 5% 10% 6% 5% 4 4 6% 6% 20%	2660
	11th to 20th	Night	15% 18% 5% 11% 6% 6% 5% 4 6% 5% 20%	3288
	21st to end	Night	26% 17% 5% 12% 5% 5% 5% 3 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	55% 11% 15% 12% 7% 1	2518
	11th to 20th	Day	67% 8% 10% 9% 5%	2676
	21st to end	Day	69% 8% 10% 9% 4 1	3124
February	1st to 10th	Day	64% 7% 9% 14% 5% 1	1710
	11th to 20th	Day	60% 6% 10% 15% 7% 1	1672
	21st to end	Day	60% 6% 10% 16% 7% 1	1451
March	1st to 10th	Day	52% 8% 14% 16% 10%	3435
	11th to 20th	Day	51% 9% 14% 16% 8% 1	3480
	21st to end	Day	43% 9% 11% 20% 15% 2	3815
April	1st to 10th	Day	54% 6% 11% 17% 10% 1	2269
	11th to 20th	Day	49% 6% 10% 20% 14% 1	2300
	21st to end	Day	50% 7% 10% 17% 13% 2	2470
May	1st to 10th	Day	47% 10% 12% 18% 12% 2	4175
	11th to 20th	Day	46% 10% 15% 19% 10% 1	4402
	21st to end	Day	35% 9% 15% 23% 15% 3	4837
June	1st to 10th	Day	45% 8% 14% 20% 13% 1	2660
	11th to 20th	Day	39% 8% 13% 23% 15% 1	2607
	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	60% 10% 15% 11% 4	4429
	11th to 20th	Night	63% 10% 13% 11% 3 1	4280
	21st to end	Night	61% 9% 13% 11% 5% 1	4532
February	1st to 10th	Night	61% 8% 10% 15% 5% 1	2470
	11th to 20th	Night	56% 7% 12% 17% 8%	2405
	21st to end	Night	54% 6% 13% 18% 7% 1	1998
March	1st to 10th	Night	51% 7% 12% 21% 8% 1	3418
	11th to 20th	Night	46% 9% 16% 19% 8% 2	3480
	21st to end	Night	38% 9% 12% 24% 15% 2	3817
April	1st to 10th	Night	51% 6% 11% 18% 13% 1	1910
	11th to 20th	Night	46% 8% 10% 19% 16% 2	1844
	21st to end	Night	47% 7% 13% 15% 16% 2	1710
May	1st to 10th	Night	43% 9% 13% 17% 16% 3	2782
	11th to 20th	Night	41% 10% 15% 21% 10% 2	2496
	21st to end	Night	29% 10% 14% 24% 20% 2	2739
June	1st to 10th	Night	40% 8% 13% 22% 15% 2	1520
	11th to 20th	Night	36% 7% 12% 22% 20% 2	1496
	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	21% 29% 32% 13% 4 1	223
	21st to end	Day	25% 23% 35% 14% 2	297
February	1st to 10th	Day	26% 29% 29% 13% 3	270
	11th to 20th	Day	24% 29% 38% 8% 1	270
	21st to end	Day	33% 26% 32% 8%	263
March	1st to 10th	Day	21% 22% 38% 16% 3	460
	11th to 20th	Day	24% 24% 32% 17% 2 1	460
	21st to end	Day	30% 18% 31% 15% 5% 1	506
April	1st to 10th	Day	25% 19% 36% 14% 5% 1	460
	11th to 20th	Day	28% 21% 26% 15% 8% 3	452
	21st to end	Day	24% 18% 34% 15% 7% 1	460
May	1st to 10th	Day	22% 14% 30% 23% 8% 4	463
	11th to 20th	Day	19% 13% 30% 25% 10% 3	487
	21st to end	Day	15% 11% 28% 28% 13% 5%	529
June	1st to 10th	Day	18% 12% 33% 26% 9% 2	490
	11th to 20th	Day	15% 12% 28% 29% 13% 3	475
	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% 17% 33% 21% 4 1	760
	21st to end	Night	19% 14% 36% 25% 6%	836
February	1st to 10th	Night	20% 18% 41% 20% 2	760
	11th to 20th	Night	22% 16% 40% 18% 4	743
	21st to end	Night	19% 18% 39% 20% 4	628
March	1st to 10th	Night	20% 17% 37% 23% 3	760
	11th to 20th	Night	20% 21% 33% 21% 4 1	760
	21st to end	Night	19% 13% 35% 25% 7%	836
April	1st to 10th	Night	22% 17% 35% 23% 3	760
	11th to 20th	Night	19% 14% 36% 21% 9% 1	720
	21st to end	Night	21% 11% 33% 26% 9%	570
May	1st to 10th	Night	20% 7% 28% 32% 12% 2	538
	11th to 20th	Night	16% 6% 32% 33% 11%	374
	21st to end	Night	10% 7% 26% 40% 17% 1	411
June	1st to 10th	Night	12% 7% 33% 35% 13%	380
	11th to 20th	Night	10% 9% 26% 40% 14% 1	374
	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

Page generated at 2024-06-14 13:06:09. Version ac07

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