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 2026-07-01.

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 5% 7% 31%	138917
	Night	13% 11% 4 10% 6% 5% 5% 4 5% 6% 31%	133407
Transparency	Day	51% 7% 12% 19% 10% 1	142509
	Night	51% 7% 12% 19% 10% 1	137905
Seeing	Day	20% 17% 31% 20% 9% 3	16952
	Night	19% 16% 32% 24% 9% 1	25854

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	14% 12% 5% 17% 11% 7% 5% 3 5% 5% 15%	4661
	11th to 20th	Day	9% 19% 7% 19% 11% 8% 6% 4 4 4 9%	5108
	21st to end	Day	12% 16% 6% 15% 9% 7% 6% 4 5% 6% 13%	5780
February	1st to 10th	Day	12% 16% 4 14% 9% 5% 6% 4 5% 5% 19%	1890
	11th to 20th	Day	11% 15% 7% 13% 8% 7% 5% 4 5% 6% 19%	1843
	21st to end	Day	15% 17% 5% 11% 6% 5% 5% 4 5% 5% 23%	1616
March	1st to 10th	Day	17% 11% 5% 10% 7% 4 4 3 4 5% 29%	5037
	11th to 20th	Day	10% 14% 5% 10% 6% 5% 5% 4 5% 7% 28%	5090
	21st to end	Day	17% 14% 5% 9% 5% 4 4 3 4 5% 31%	5586
April	1st to 10th	Day	15% 15% 4 10% 5% 4 5% 3 5% 5% 29%	2464
	11th to 20th	Day	18% 12% 3 6% 4 4 4 3 5% 6% 35%	2613
	21st to end	Day	19% 11% 4 7% 5% 3 4 3 6% 5% 34%	2730
May	1st to 10th	Day	16% 9% 3 8% 5% 4 4 4 5% 7% 34%	7642
	11th to 20th	Day	14% 10% 3 8% 4 5% 4 4 5% 8% 36%	8153
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 43%	9060
June	1st to 10th	Day	14% 7% 2 5% 5% 4 5% 5% 6% 9% 37%	2940
	11th to 20th	Day	11% 6% 2 5% 5% 5% 5% 5% 6% 9% 41%	3067
	21st to end	Day	13% 7% 2 6% 5% 5% 5% 4 6% 8% 39%	3150
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 9% 37%	6781
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	6360
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 47%	6470
August	1st to 10th	Day	12% 8% 2 7% 4 5% 5% 4 7% 8% 37%	4234
	11th to 20th	Day	10% 7% 3 8% 5% 6% 5% 5% 8% 10% 34%	4174
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 6% 9% 42%	4529
September	1st to 10th	Day	14% 6% 2 7% 5% 4 6% 4 5% 7% 41%	2400
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 3 7% 8% 45%	2256
	21st to end	Day	13% 7% 3 8% 6% 5% 5% 3 6% 8% 34%	2175
October	1st to 10th	Day	18% 9% 2 7% 4 5% 5% 3 6% 7% 35%	2000
	11th to 20th	Day	18% 10% 3 10% 6% 5% 5% 4 5% 6% 28%	2000
	21st to end	Day	23% 15% 3 9% 5% 4 5% 3 4 6% 21%	2200
November	1st to 10th	Day	13% 14% 5% 9% 5% 5% 4 4 5% 8% 28%	1754
	11th to 20th	Day	11% 16% 4 10% 6% 6% 6% 5% 6% 6% 22%	1680
	21st to end	Day	15% 19% 5% 13% 6% 5% 6% 5% 5% 5% 16%	1680
December	1st to 10th	Day	18% 19% 7% 12% 8% 6% 6% 4 5% 5% 9%	2680
	11th to 20th	Day	16% 20% 6% 13% 5% 5% 5% 3 5% 4 17%	3066
	21st to end	Day	23% 18% 7% 16% 6% 6% 4 3 4 4 9%	4048
January	1st to 10th	Night	12% 13% 5% 16% 10% 7% 6% 3 5% 5% 17%	8234
	11th to 20th	Night	9% 18% 7% 18% 12% 7% 6% 4 5% 4 10%	7760
	21st to end	Night	11% 13% 6% 13% 9% 7% 7% 5% 6% 7% 17%	8401
February	1st to 10th	Night	11% 16% 5% 12% 8% 6% 4 4 6% 6% 23%	2730
	11th to 20th	Night	10% 15% 6% 12% 7% 6% 5% 5% 5% 6% 23%	2663
	21st to end	Night	11% 13% 5% 12% 7% 6% 5% 4 6% 7% 24%	2186
March	1st to 10th	Night	15% 11% 5% 10% 6% 6% 5% 3 5% 6% 27%	5023
	11th to 20th	Night	9% 10% 3 8% 6% 5% 6% 5% 6% 8% 33%	5089
	21st to end	Night	17% 10% 4 9% 5% 5% 4 3 5% 5% 32%	5588
April	1st to 10th	Night	17% 12% 3 9% 5% 4 4 4 5% 6% 31%	2154
	11th to 20th	Night	16% 9% 2 7% 4 5% 5% 3 5% 6% 37%	1974
	21st to end	Night	14% 10% 3 8% 5% 4 5% 4 4 6% 37%	1890
May	1st to 10th	Night	17% 7% 2 6% 4 4 5% 4 5% 7% 39%	5257
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 5% 7% 41%	4703
	21st to end	Night	17% 5% 2 5% 3 3 3 3 5% 7% 47%	5114
June	1st to 10th	Night	13% 5% 2 6% 4 4 6% 4 6% 7% 43%	1680
	11th to 20th	Night	9% 4 1 7% 5% 4 6% 6% 8% 8% 43%	1545
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 6% 7% 46%	1470
July	1st to 10th	Night	6% 6% 3 7% 5% 4 4 4 5% 8% 47%	3192
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 5% 7% 51%	3600
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 51%	4378
August	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 48%	2915
	11th to 20th	Night	11% 6% 2 6% 3 4 3 3 5% 8% 49%	2875
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 8% 49%	3322
September	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 7% 8% 47%	2000
	11th to 20th	Night	10% 5% 1 4 4 3 3 3 6% 7% 55%	2140
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 5% 6% 42%	2225
October	1st to 10th	Night	17% 6% 2 6% 5% 4 4 4 5% 7% 40%	2400
	11th to 20th	Night	17% 9% 3 7% 4 5% 5% 4 6% 6% 33%	2400
	21st to end	Night	20% 12% 3 9% 6% 4 5% 4 5% 5% 27%	2640
November	1st to 10th	Night	15% 11% 3 7% 5% 5% 5% 4 6% 7% 33%	2761
	11th to 20th	Night	14% 14% 5% 9% 6% 5% 4 4 6% 6% 27%	2940
	21st to end	Night	16% 16% 6% 10% 6% 5% 6% 5% 5% 6% 20%	2940
December	1st to 10th	Night	17% 16% 6% 13% 8% 6% 5% 5% 6% 5% 12%	4720
	11th to 20th	Night	13% 18% 6% 14% 7% 6% 5% 4 5% 5% 18%	5348
	21st to end	Night	21% 20% 8% 15% 6% 5% 4 3 4 4 11%	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	67% 8% 11% 11% 4	4662
	11th to 20th	Day	77% 7% 7% 7% 3	5142
	21st to end	Day	69% 7% 9% 10% 5%	5780
February	1st to 10th	Day	67% 8% 8% 13% 4	1890
	11th to 20th	Day	65% 7% 10% 13% 4 1	1852
	21st to end	Day	63% 5% 10% 15% 7% 1	1616
March	1st to 10th	Day	56% 6% 12% 13% 12%	5040
	11th to 20th	Day	54% 5% 14% 19% 6% 2	5090
	21st to end	Day	55% 5% 9% 19% 11% 1	5586
April	1st to 10th	Day	56% 6% 10% 18% 9% 1	2464
	11th to 20th	Day	49% 5% 9% 22% 14% 1	2613
	21st to end	Day	51% 6% 11% 18% 12% 2	2730
May	1st to 10th	Day	47% 6% 13% 19% 11% 4	7651
	11th to 20th	Day	46% 8% 10% 19% 16% 1	8157
	21st to end	Day	41% 5% 10% 26% 16% 2	9061
June	1st to 10th	Day	41% 9% 13% 20% 16% 1	2940
	11th to 20th	Day	38% 8% 13% 24% 16% 1	3067
	21st to end	Day	41% 7% 15% 23% 13% 1	3150
July	1st to 10th	Day	41% 6% 15% 22% 14% 1	6708
	11th to 20th	Day	34% 6% 12% 31% 16% 1	6285
	21st to end	Day	33% 6% 14% 30% 16% 1	6285
August	1st to 10th	Day	43% 7% 15% 23% 12% 1	4221
	11th to 20th	Day	42% 9% 17% 22% 10%	4213
	21st to end	Day	37% 7% 13% 32% 10% 1	4529
September	1st to 10th	Day	42% 7% 14% 25% 10% 2	2400
	11th to 20th	Day	37% 7% 15% 29% 12%	2256
	21st to end	Day	47% 8% 13% 23% 9%	2175
October	1st to 10th	Day	44% 6% 12% 26% 11% 1	3260
	11th to 20th	Day	44% 6% 15% 22% 13%	3240
	21st to end	Day	60% 7% 12% 16% 6%	3538
November	1st to 10th	Day	54% 9% 14% 17% 6%	1754
	11th to 20th	Day	58% 10% 13% 12% 6% 1	1680
	21st to end	Day	68% 9% 12% 9% 2	1680
December	1st to 10th	Day	75% 6% 11% 5% 2	2680
	11th to 20th	Day	69% 8% 9% 8% 6%	3066
	21st to end	Day	79% 5% 5% 7% 4	4048
January	1st to 10th	Night	65% 8% 12% 12% 3	8234
	11th to 20th	Night	75% 8% 7% 8% 2	7801
	21st to end	Night	63% 8% 9% 11% 9%	8401
February	1st to 10th	Night	60% 7% 12% 16% 4	2730
	11th to 20th	Night	60% 8% 11% 16% 5%	2665
	21st to end	Night	56% 8% 13% 15% 7%	2186
March	1st to 10th	Night	55% 7% 10% 17% 9% 1	5033
	11th to 20th	Night	44% 7% 21% 19% 8% 1	5090
	21st to end	Night	51% 5% 11% 19% 12% 2	5588
April	1st to 10th	Night	54% 6% 10% 18% 11% 1	2154
	11th to 20th	Night	47% 6% 10% 21% 15% 1	1974
	21st to end	Night	47% 7% 11% 18% 15% 2	1890
May	1st to 10th	Night	44% 6% 11% 19% 15% 5%	5264
	11th to 20th	Night	42% 7% 12% 23% 15% 1	4704
	21st to end	Night	37% 6% 11% 28% 16% 2	5115
June	1st to 10th	Night	38% 8% 11% 23% 18% 2	1680
	11th to 20th	Night	34% 8% 14% 24% 18% 1	1545
	21st to end	Night	36% 8% 14% 27% 15% 1	1470
July	1st to 10th	Night	35% 5% 15% 26% 17% 2	3157
	11th to 20th	Night	32% 6% 11% 31% 18% 1	3552
	21st to end	Night	31% 6% 12% 32% 18% 1	4249
August	1st to 10th	Night	35% 6% 14% 28% 15% 1	2904
	11th to 20th	Night	34% 6% 17% 29% 13% 1	2902
	21st to end	Night	34% 6% 14% 33% 13% 1	3322
September	1st to 10th	Night	32% 8% 18% 28% 12% 1	2000
	11th to 20th	Night	28% 7% 15% 33% 17% 1	2140
	21st to end	Night	44% 6% 13% 27% 10% 1	2225
October	1st to 10th	Night	35% 6% 9% 29% 20% 1	3920
	11th to 20th	Night	42% 6% 16% 23% 13%	3899
	21st to end	Night	46% 8% 16% 22% 8%	4252
November	1st to 10th	Night	48% 9% 15% 20% 8% 1	2761
	11th to 20th	Night	55% 9% 14% 14% 7% 1	2940
	21st to end	Night	63% 10% 14% 11% 1 1	2940
December	1st to 10th	Night	71% 7% 13% 7% 2	4720
	11th to 20th	Night	67% 8% 9% 10% 6%	5348
	21st to end	Night	78% 6% 7% 7% 3	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	19% 25% 36% 15% 4 1	220
	11th to 20th	Day	24% 27% 33% 11% 4 1	305
	21st to end	Day	25% 25% 33% 15% 2	363
February	1st to 10th	Day	26% 31% 28% 12% 4	330
	11th to 20th	Day	22% 32% 38% 8% 1	330
	21st to end	Day	31% 32% 29% 8%	273
March	1st to 10th	Day	23% 22% 37% 15% 3	535
	11th to 20th	Day	24% 24% 34% 14% 4	540
	21st to end	Day	32% 19% 29% 15% 4 1	594
April	1st to 10th	Day	26% 24% 33% 12% 5% 1	540
	11th to 20th	Day	31% 21% 24% 14% 7% 3	532
	21st to end	Day	27% 18% 32% 15% 7% 1	540
May	1st to 10th	Day	21% 17% 35% 17% 8% 2	543
	11th to 20th	Day	18% 15% 33% 21% 10% 3	567
	21st to end	Day	18% 15% 30% 23% 11% 4	617
June	1st to 10th	Day	16% 15% 33% 25% 9% 2	570
	11th to 20th	Day	14% 14% 34% 25% 11% 1	697
	21st to end	Day	16% 12% 35% 25% 9% 3	780
July	1st to 10th	Day	10% 9% 30% 31% 15% 4	709
	11th to 20th	Day	13% 9% 29% 27% 14% 8%	524
	21st to end	Day	9% 8% 34% 31% 12% 6%	576
August	1st to 10th	Day	9% 8% 28% 30% 18% 8%	526
	11th to 20th	Day	8% 7% 28% 27% 21% 9%	519
	21st to end	Day	14% 10% 26% 26% 16% 7%	583
September	1st to 10th	Day	14% 12% 27% 23% 15% 9%	530
	11th to 20th	Day	9% 10% 22% 24% 20% 15%	530
	21st to end	Day	15% 14% 28% 22% 16% 5%	530
October	1st to 10th	Day	20% 16% 26% 20% 14% 4	530
	11th to 20th	Day	20% 23% 33% 17% 5% 2	530
	21st to end	Day	30% 21% 28% 16% 4	583
November	1st to 10th	Day	27% 25% 29% 15% 4 1	284
	11th to 20th	Day	25% 23% 35% 13% 4 1	220
	21st to end	Day	28% 24% 36% 10% 1	220
December	1st to 10th	Day	27% 26% 30% 14% 3	220
	11th to 20th	Day	29% 28% 28% 13% 1 1	220
	21st to end	Day	28% 27% 33% 10% 2 1	242
January	1st to 10th	Night	21% 25% 34% 16% 4	840
	11th to 20th	Night	22% 23% 32% 20% 3	840
	21st to end	Night	18% 18% 38% 20% 6%	924
February	1st to 10th	Night	21% 23% 36% 18% 2	840
	11th to 20th	Night	19% 23% 38% 17% 3	823
	21st to end	Night	21% 23% 35% 18% 2	692
March	1st to 10th	Night	20% 22% 36% 20% 3	840
	11th to 20th	Night	21% 24% 31% 19% 5%	840
	21st to end	Night	21% 17% 34% 21% 6%	924
April	1st to 10th	Night	24% 19% 36% 20% 2	840
	11th to 20th	Night	20% 17% 35% 19% 8% 1	730
	21st to end	Night	20% 14% 31% 26% 8%	630
May	1st to 10th	Night	17% 9% 30% 32% 11% 1	630
	11th to 20th	Night	18% 7% 35% 30% 9%	440
	21st to end	Night	11% 9% 31% 38% 11%	455
June	1st to 10th	Night	14% 7% 35% 33% 10%	420
	11th to 20th	Night	12% 7% 31% 37% 12%	420
	21st to end	Night	12% 7% 29% 36% 15% 1	420
July	1st to 10th	Night	6% 6% 25% 44% 18% 1	390
	11th to 20th	Night	10% 7% 28% 36% 18% 1	388
	21st to end	Night	8% 5% 27% 40% 19% 1	633
August	1st to 10th	Night	9% 4 23% 38% 25% 1	597
	11th to 20th	Night	7% 5% 20% 40% 26% 2	594
	21st to end	Night	12% 5% 28% 30% 23% 1	720
September	1st to 10th	Night	12% 7% 28% 31% 20% 4	800
	11th to 20th	Night	11% 7% 24% 29% 23% 6%	800
	21st to end	Night	19% 11% 22% 29% 17% 3	800
October	1st to 10th	Night	19% 10% 31% 24% 15% 2	800
	11th to 20th	Night	23% 15% 31% 23% 7% 1	800
	21st to end	Night	28% 16% 30% 21% 5%	880
November	1st to 10th	Night	22% 18% 31% 21% 7%	820
	11th to 20th	Night	22% 24% 30% 18% 6% 1	840
	21st to end	Night	24% 23% 33% 17% 3	840
December	1st to 10th	Night	25% 21% 36% 14% 3	840
	11th to 20th	Night	20% 25% 37% 16% 3	840
	21st to end	Night	30% 18% 34% 16% 2	924

Page generated at 2026-06-28 02:10:47. Version ac07

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