Alliston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alliston 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	14% 11% 4 9% 6% 5% 5% 4 6% 7% 30%	109408
	Night	14% 11% 4 10% 6% 5% 5% 4 5% 6% 30%	109226
Transparency	Day	51% 8% 12% 18% 10% 1	113043
	Night	51% 8% 12% 18% 10% 1	113756
Seeing	Day	19% 18% 31% 20% 9% 3	15138
	Night	19% 16% 32% 24% 9% 1	24225

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% 11% 5% 17% 12% 7% 5% 4 5% 5% 16%	3597
	11th to 20th	Day	10% 16% 7% 19% 11% 7% 6% 4 5% 5% 11%	3916
	21st to end	Day	15% 18% 7% 16% 7% 5% 5% 4 5% 5% 14%	4466
February	1st to 10th	Day	12% 17% 4 14% 9% 6% 7% 4 5% 5% 18%	1800
	11th to 20th	Day	10% 15% 7% 13% 8% 7% 5% 4 5% 6% 20%	1753
	21st to end	Day	16% 17% 5% 11% 6% 5% 4 4 5% 5% 23%	1502
March	1st to 10th	Day	17% 13% 5% 9% 6% 4 4 3 4 5% 29%	3427
	11th to 20th	Day	10% 17% 6% 10% 6% 5% 5% 4 4 5% 27%	3480
	21st to end	Day	14% 12% 4 8% 5% 4 5% 3 4 5% 35%	3815
April	1st to 10th	Day	15% 16% 4 9% 5% 4 5% 3 5% 5% 30%	2229
	11th to 20th	Day	19% 12% 3 6% 4 4 4 3 5% 6% 35%	2361
	21st to end	Day	19% 10% 4 7% 5% 3 4 3 5% 5% 34%	2470
May	1st to 10th	Day	16% 9% 3 7% 4 4 4 4 5% 8% 35%	4108
	11th to 20th	Day	16% 10% 3 7% 5% 5% 4 4 6% 8% 32%	4357
	21st to end	Day	11% 6% 2 6% 4 4 4 3 6% 7% 46%	4837
June	1st to 10th	Day	15% 8% 2 5% 5% 4 5% 5% 6% 9% 36%	2660
	11th to 20th	Day	11% 6% 1 5% 5% 5% 5% 5% 6% 9% 41%	2775
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	2850
July	1st to 10th	Day	10% 5% 2 7% 5% 6% 6% 5% 8% 10% 36%	4735
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	4440
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 4 6% 8% 43%	4498
August	1st to 10th	Day	13% 8% 2 7% 4 5% 5% 4 7% 8% 37%	4104
	11th to 20th	Day	10% 7% 3 9% 5% 6% 5% 5% 8% 10% 33%	4044
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	4390
September	1st to 10th	Day	14% 6% 2 7% 5% 4 5% 4 5% 7% 40%	2280
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 3 6% 8% 44%	2143
	21st to end	Day	13% 7% 3 8% 6% 5% 6% 3 7% 8% 33%	2066
October	1st to 10th	Day	18% 9% 2 8% 5% 5% 5% 3 5% 7% 33%	1900
	11th to 20th	Day	18% 10% 3 10% 6% 5% 5% 4 5% 6% 28%	1900
	21st to end	Day	24% 16% 3 9% 5% 4 5% 3 4 6% 20%	2090
November	1st to 10th	Day	12% 13% 5% 9% 6% 5% 4 4 5% 8% 28%	1669
	11th to 20th	Day	11% 16% 5% 10% 7% 6% 6% 5% 6% 6% 22%	1600
	21st to end	Day	16% 19% 5% 13% 6% 5% 6% 5% 5% 5% 16%	1600
December	1st to 10th	Day	18% 19% 7% 12% 8% 6% 6% 4 5% 5% 9%	2600
	11th to 20th	Day	16% 20% 6% 13% 5% 4 5% 3 5% 4 18%	2986
	21st to end	Day	23% 18% 7% 16% 6% 6% 4 3 3 4 9%	3960
January	1st to 10th	Night	13% 12% 5% 15% 11% 6% 5% 3 5% 6% 18%	6349
	11th to 20th	Night	11% 15% 6% 18% 12% 8% 6% 4 5% 5% 11%	5952
	21st to end	Night	13% 14% 7% 13% 8% 6% 5% 4 5% 6% 19%	6490
February	1st to 10th	Night	12% 16% 5% 13% 8% 6% 4 4 6% 6% 21%	2600
	11th to 20th	Night	10% 15% 6% 12% 8% 6% 5% 5% 5% 6% 22%	2533
	21st to end	Night	12% 12% 5% 12% 7% 6% 5% 4 6% 7% 25%	2043
March	1st to 10th	Night	17% 11% 4 10% 6% 5% 5% 3 5% 6% 30%	3413
	11th to 20th	Night	12% 13% 4 9% 6% 4 4 3 5% 6% 35%	3479
	21st to end	Night	14% 9% 4 7% 5% 5% 4 4 5% 6% 37%	3817
April	1st to 10th	Night	18% 12% 3 9% 5% 4 4 4 5% 6% 31%	1949
	11th to 20th	Night	17% 8% 2 7% 4 4 5% 3 6% 7% 37%	1784
	21st to end	Night	14% 11% 3 8% 5% 5% 5% 4 4 6% 36%	1710
May	1st to 10th	Night	16% 7% 3 7% 5% 4 4 3 5% 6% 41%	2833
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 6% 7% 40%	2532
	21st to end	Night	9% 5% 2 5% 3 4 3 3 5% 8% 52%	2738
June	1st to 10th	Night	13% 5% 2 6% 5% 5% 6% 4 6% 7% 42%	1520
	11th to 20th	Night	9% 4 1 6% 4 4 6% 6% 7% 9% 44%	1398
	21st to end	Night	11% 5% 2 5% 4 5% 5% 4 5% 7% 47%	1330
July	1st to 10th	Night	5% 5% 3 6% 5% 5% 4 4 5% 8% 51%	2229
	11th to 20th	Night	11% 4 2 4 3 4 4 4 5% 7% 52%	2520
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 6% 8% 50%	3051
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 48%	2825
	11th to 20th	Night	11% 6% 2 6% 3 4 3 3 5% 8% 49%	2785
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 8% 48%	3220
September	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 7% 8% 47%	1900
	11th to 20th	Night	10% 5% 1 4 4 3 3 3 6% 7% 54%	2033
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 5% 6% 41%	2114
October	1st to 10th	Night	17% 7% 3 7% 5% 4 4 4 6% 6% 38%	2280
	11th to 20th	Night	17% 9% 3 7% 4 5% 5% 4 6% 6% 33%	2280
	21st to end	Night	20% 13% 3 9% 6% 5% 5% 4 5% 5% 25%	2508
November	1st to 10th	Night	15% 10% 3 7% 5% 5% 5% 4 6% 7% 33%	2627
	11th to 20th	Night	15% 14% 4 9% 6% 5% 4 4 6% 6% 27%	2800
	21st to end	Night	16% 16% 6% 10% 6% 5% 6% 5% 5% 6% 20%	2800
December	1st to 10th	Night	18% 16% 6% 13% 8% 6% 5% 5% 6% 5% 12%	4580
	11th to 20th	Night	13% 18% 6% 14% 6% 5% 5% 4 5% 5% 18%	5208
	21st to end	Night	21% 20% 7% 15% 6% 5% 4 3 4 4 11%	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	66% 9% 11% 12% 3	3598
	11th to 20th	Day	75% 7% 8% 6% 3 1	3950
	21st to end	Day	70% 8% 9% 9% 4	4466
February	1st to 10th	Day	68% 8% 9% 13% 3	1800
	11th to 20th	Day	64% 7% 10% 13% 5% 1	1762
	21st to end	Day	63% 5% 10% 14% 7% 1	1502
March	1st to 10th	Day	57% 7% 12% 15% 9%	3430
	11th to 20th	Day	58% 6% 10% 18% 7% 1	3480
	21st to end	Day	49% 7% 11% 19% 12% 2	3815
April	1st to 10th	Day	56% 7% 10% 17% 9% 1	2229
	11th to 20th	Day	49% 6% 9% 21% 15% 1	2361
	21st to end	Day	51% 6% 11% 18% 12% 2	2470
May	1st to 10th	Day	44% 8% 12% 20% 14% 1	4120
	11th to 20th	Day	46% 12% 14% 19% 8% 1	4361
	21st to end	Day	36% 8% 13% 21% 19% 3	4837
June	1st to 10th	Day	41% 9% 14% 20% 15% 1	2660
	11th to 20th	Day	38% 8% 13% 23% 17% 1	2775
	21st to end	Day	41% 7% 14% 23% 14% 1	2850
July	1st to 10th	Day	41% 8% 16% 20% 15% 1	4661
	11th to 20th	Day	36% 7% 13% 33% 10% 1	4365
	21st to end	Day	38% 7% 14% 24% 17% 1	4355
August	1st to 10th	Day	43% 7% 15% 23% 11% 1	4091
	11th to 20th	Day	43% 9% 17% 22% 10%	4083
	21st to end	Day	37% 7% 13% 31% 10% 1	4390
September	1st to 10th	Day	43% 7% 14% 24% 10% 2	2280
	11th to 20th	Day	38% 7% 15% 27% 13%	2143
	21st to end	Day	47% 9% 13% 23% 8%	2066
October	1st to 10th	Day	45% 6% 13% 26% 10% 1	3160
	11th to 20th	Day	44% 6% 15% 22% 13%	3140
	21st to end	Day	60% 7% 12% 16% 5%	3428
November	1st to 10th	Day	53% 9% 15% 16% 6%	1669
	11th to 20th	Day	59% 10% 12% 12% 6% 1	1600
	21st to end	Day	68% 9% 13% 9% 2	1600
December	1st to 10th	Day	76% 7% 11% 5% 2	2600
	11th to 20th	Day	69% 8% 10% 8% 6%	2986
	21st to end	Day	79% 6% 5% 7% 4	3960
January	1st to 10th	Night	64% 8% 12% 13% 2	6349
	11th to 20th	Night	73% 9% 8% 7% 2	5993
	21st to end	Night	64% 8% 10% 10% 8% 1	6490
February	1st to 10th	Night	61% 7% 13% 15% 4	2600
	11th to 20th	Night	60% 8% 11% 16% 5%	2535
	21st to end	Night	56% 8% 13% 15% 7%	2043
March	1st to 10th	Night	53% 7% 12% 21% 6% 1	3423
	11th to 20th	Night	45% 8% 16% 20% 9% 2	3480
	21st to end	Night	44% 6% 13% 24% 12% 1	3817
April	1st to 10th	Night	54% 6% 10% 19% 11% 1	1949
	11th to 20th	Night	46% 7% 10% 20% 16% 2	1784
	21st to end	Night	49% 7% 11% 16% 15% 2	1710
May	1st to 10th	Night	42% 8% 13% 18% 16% 4	2840
	11th to 20th	Night	40% 10% 14% 22% 12% 2	2533
	21st to end	Night	29% 9% 14% 26% 20% 2	2739
June	1st to 10th	Night	39% 8% 12% 23% 16% 2	1520
	11th to 20th	Night	33% 9% 14% 24% 19% 1	1398
	21st to end	Night	35% 8% 14% 27% 15% 1	1330
July	1st to 10th	Night	31% 6% 15% 26% 19% 3	2194
	11th to 20th	Night	31% 7% 12% 34% 15% 1	2472
	21st to end	Night	32% 7% 14% 29% 16% 1	2954
August	1st to 10th	Night	35% 6% 14% 29% 15% 1	2814
	11th to 20th	Night	34% 6% 17% 29% 13% 1	2812
	21st to end	Night	34% 6% 14% 32% 13% 1	3220
September	1st to 10th	Night	33% 8% 18% 27% 13% 1	1900
	11th to 20th	Night	29% 7% 14% 32% 17% 1	2033
	21st to end	Night	44% 6% 13% 27% 10%	2114
October	1st to 10th	Night	36% 6% 9% 28% 20% 1	3800
	11th to 20th	Night	42% 6% 16% 23% 13%	3779
	21st to end	Night	47% 8% 16% 21% 8%	4120
November	1st to 10th	Night	48% 8% 15% 19% 8% 1	2627
	11th to 20th	Night	55% 9% 14% 14% 7% 1	2800
	21st to end	Night	62% 11% 15% 11% 1 1	2800
December	1st to 10th	Night	71% 7% 13% 7% 2	4580
	11th to 20th	Night	67% 8% 9% 10% 6%	5208
	21st to end	Night	77% 6% 7% 7% 3	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	18% 26% 35% 17% 5% 1	200
	11th to 20th	Day	23% 26% 34% 12% 5% 1	277
	21st to end	Day	25% 25% 35% 13% 2	330
February	1st to 10th	Day	27% 33% 27% 10% 3	300
	11th to 20th	Day	22% 34% 37% 7% 1	300
	21st to end	Day	32% 32% 29% 7%	237
March	1st to 10th	Day	21% 23% 37% 15% 3	455
	11th to 20th	Day	20% 24% 36% 15% 5%	460
	21st to end	Day	31% 19% 29% 16% 4 1	506
April	1st to 10th	Day	23% 23% 34% 13% 6% 1	460
	11th to 20th	Day	31% 22% 24% 14% 7% 3	452
	21st to end	Day	27% 18% 30% 16% 7% 2	460
May	1st to 10th	Day	21% 16% 34% 17% 10% 2	463
	11th to 20th	Day	18% 13% 32% 23% 10% 3	487
	21st to end	Day	15% 14% 30% 25% 12% 4	529
June	1st to 10th	Day	17% 15% 32% 25% 9% 2	490
	11th to 20th	Day	13% 13% 32% 27% 12% 2	605
	21st to end	Day	16% 14% 35% 24% 8% 3	680
July	1st to 10th	Day	10% 9% 31% 31% 15% 4	659
	11th to 20th	Day	13% 9% 30% 28% 13% 8%	484
	21st to end	Day	9% 8% 33% 31% 13% 6%	532
August	1st to 10th	Day	9% 8% 28% 29% 17% 8%	486
	11th to 20th	Day	8% 8% 28% 27% 21% 9%	479
	21st to end	Day	14% 10% 27% 26% 17% 6%	539
September	1st to 10th	Day	14% 11% 28% 24% 14% 8%	490
	11th to 20th	Day	9% 11% 23% 24% 19% 14%	490
	21st to end	Day	15% 14% 28% 22% 16% 5%	490
October	1st to 10th	Day	21% 17% 26% 20% 12% 4	490
	11th to 20th	Day	20% 23% 33% 17% 5% 2	490
	21st to end	Day	31% 22% 27% 15% 4	539
November	1st to 10th	Day	25% 25% 29% 16% 4 1	259
	11th to 20th	Day	24% 23% 36% 13% 4 1	200
	21st to end	Day	28% 24% 37% 10% 1	200
December	1st to 10th	Day	29% 28% 28% 13% 3	200
	11th to 20th	Day	29% 29% 28% 13% 2 1	200
	21st to end	Day	26% 26% 35% 10% 2 1	220
January	1st to 10th	Night	21% 26% 34% 15% 4	800
	11th to 20th	Night	22% 23% 32% 20% 4	800
	21st to end	Night	18% 18% 37% 20% 6%	880
February	1st to 10th	Night	22% 23% 37% 17% 1	800
	11th to 20th	Night	19% 23% 38% 16% 3	783
	21st to end	Night	20% 23% 36% 18% 2	647
March	1st to 10th	Night	20% 21% 36% 19% 3	760
	11th to 20th	Night	20% 25% 31% 18% 5%	760
	21st to end	Night	20% 17% 34% 22% 6%	836
April	1st to 10th	Night	24% 19% 35% 20% 2	760
	11th to 20th	Night	19% 18% 35% 19% 8% 1	660
	21st to end	Night	20% 15% 33% 24% 8%	570
May	1st to 10th	Night	17% 9% 30% 32% 11% 1	570
	11th to 20th	Night	16% 8% 34% 31% 10% 1	398
	21st to end	Night	9% 9% 32% 37% 13%	411
June	1st to 10th	Night	14% 7% 35% 33% 10% 1	380
	11th to 20th	Night	12% 7% 31% 38% 13%	380
	21st to end	Night	12% 7% 31% 34% 14% 1	380
July	1st to 10th	Night	6% 5% 26% 43% 19% 1	370
	11th to 20th	Night	10% 7% 29% 35% 18% 1	368
	21st to end	Night	8% 5% 28% 39% 19% 1	601
August	1st to 10th	Night	10% 4 25% 37% 24% 1	567
	11th to 20th	Night	7% 5% 21% 39% 26% 2	564
	21st to end	Night	13% 6% 29% 29% 22% 1	684
September	1st to 10th	Night	12% 7% 28% 30% 19% 4	760
	11th to 20th	Night	12% 8% 25% 28% 22% 6%	760
	21st to end	Night	18% 11% 22% 28% 17% 3	760
October	1st to 10th	Night	20% 10% 32% 23% 14% 2	760
	11th to 20th	Night	23% 16% 31% 22% 8% 1	760
	21st to end	Night	29% 17% 31% 19% 5%	836
November	1st to 10th	Night	22% 17% 31% 21% 8% 1	780
	11th to 20th	Night	22% 24% 30% 18% 5% 1	800
	21st to end	Night	24% 23% 33% 17% 3	800
December	1st to 10th	Night	26% 22% 36% 13% 3 1	800
	11th to 20th	Night	20% 25% 36% 16% 3	800
	21st to end	Night	30% 18% 34% 16% 2	880

Page generated at 2025-02-22 06:51:33. Version ac07

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