Halton Hills
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Halton Hills 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% 10% 4 9% 6% 5% 5% 4 5% 7% 31%	138629
	Night	13% 10% 4 9% 6% 5% 5% 4 5% 6% 32%	133687
Transparency	Day	50% 6% 13% 20% 11% 1	142225
	Night	49% 6% 13% 20% 12% 1	138180
Seeing	Day	20% 18% 31% 20% 9% 3	16875
	Night	19% 17% 31% 23% 9% 1	25941

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% 15% 7% 16% 7% 6% 5% 3 3 4 20%	4661
	11th to 20th	Day	8% 20% 10% 21% 9% 8% 5% 3 5% 4 8%	5227
	21st to end	Day	13% 14% 6% 16% 9% 8% 6% 3 4 4 15%	5780
February	1st to 10th	Day	13% 17% 6% 14% 9% 6% 5% 4 4 5% 19%	1890
	11th to 20th	Day	12% 14% 6% 14% 10% 5% 4 3 5% 6% 21%	1843
	21st to end	Day	17% 14% 6% 11% 7% 5% 5% 3 4 4 23%	1636
March	1st to 10th	Day	17% 11% 5% 11% 7% 4 4 3 4 5% 30%	5036
	11th to 20th	Day	11% 12% 4 10% 6% 5% 5% 4 6% 7% 30%	5090
	21st to end	Day	16% 13% 4 10% 4 4 5% 3 5% 5% 31%	5586
April	1st to 10th	Day	17% 13% 4 9% 6% 5% 3 3 5% 7% 28%	2452
	11th to 20th	Day	18% 10% 3 7% 5% 4 3 4 5% 6% 35%	2610
	21st to end	Day	19% 10% 3 8% 5% 4 4 4 5% 7% 32%	2730
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 5% 7% 33%	7642
	11th to 20th	Day	15% 9% 3 8% 6% 5% 5% 4 5% 7% 34%	8039
	21st to end	Day	17% 5% 2 6% 4 4 4 4 5% 7% 43%	9060
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 4 4 6% 9% 38%	2940
	11th to 20th	Day	11% 6% 3 8% 5% 4 6% 4 7% 9% 39%	3000
	21st to end	Day	13% 6% 2 6% 6% 4 5% 5% 6% 8% 39%	3150
July	1st to 10th	Day	10% 5% 3 6% 5% 5% 5% 5% 7% 10% 40%	6646
	11th to 20th	Day	7% 4 2 6% 5% 5% 5% 4 7% 9% 45%	6282
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 8% 47%	6424
August	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 4 6% 9% 36%	4234
	11th to 20th	Day	11% 7% 3 8% 6% 5% 6% 5% 7% 10% 33%	4174
	21st to end	Day	10% 5% 3 6% 4 6% 5% 5% 7% 8% 40%	4527
September	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 5% 8% 40%	2400
	11th to 20th	Day	12% 6% 2 6% 4 3 4 4 6% 9% 44%	2241
	21st to end	Day	15% 7% 3 7% 6% 6% 6% 3 6% 8% 34%	2184
October	1st to 10th	Day	17% 10% 3 7% 5% 4 4 3 5% 7% 35%	2000
	11th to 20th	Day	20% 10% 4 9% 5% 5% 4 4 5% 5% 29%	2000
	21st to end	Day	23% 13% 4 9% 6% 5% 5% 3 5% 6% 21%	2200
November	1st to 10th	Day	14% 13% 3 9% 6% 5% 5% 4 6% 6% 30%	1791
	11th to 20th	Day	13% 14% 4 11% 6% 6% 5% 4 6% 6% 26%	1680
	21st to end	Day	16% 17% 6% 12% 6% 6% 4 3 5% 6% 19%	1680
December	1st to 10th	Day	15% 19% 7% 14% 8% 5% 5% 3 5% 5% 13%	2680
	11th to 20th	Day	21% 18% 5% 10% 6% 7% 4 3 4 5% 18%	3066
	21st to end	Day	25% 19% 7% 14% 6% 5% 4 2 3 4 11%	4048
January	1st to 10th	Night	13% 12% 6% 13% 7% 6% 5% 3 5% 6% 22%	8234
	11th to 20th	Night	9% 14% 8% 17% 13% 9% 7% 4 5% 4 11%	7657
	21st to end	Night	12% 11% 6% 14% 9% 7% 7% 4 6% 6% 18%	8401
February	1st to 10th	Night	12% 14% 4 11% 8% 6% 5% 5% 5% 6% 23%	2730
	11th to 20th	Night	12% 12% 5% 13% 8% 6% 6% 3 5% 5% 25%	2663
	21st to end	Night	13% 13% 4 11% 6% 6% 5% 5% 6% 6% 26%	2167
March	1st to 10th	Night	14% 9% 3 10% 5% 5% 5% 4 6% 6% 32%	5023
	11th to 20th	Night	9% 8% 4 8% 5% 5% 5% 5% 7% 8% 36%	5089
	21st to end	Night	18% 9% 3 7% 4 4 4 3 5% 5% 38%	5588
April	1st to 10th	Night	18% 11% 4 7% 5% 4 4 3 5% 7% 30%	2167
	11th to 20th	Night	14% 9% 3 7% 6% 5% 4 4 5% 5% 37%	1977
	21st to end	Night	14% 9% 3 7% 5% 4 5% 4 5% 6% 37%	1890
May	1st to 10th	Night	18% 7% 2 7% 4 5% 4 4 5% 6% 37%	5257
	11th to 20th	Night	14% 8% 3 8% 5% 4 5% 3 5% 7% 38%	4817
	21st to end	Night	17% 5% 2 5% 4 3 4 4 4 7% 46%	5114
June	1st to 10th	Night	11% 5% 2 6% 5% 4 4 5% 6% 8% 44%	1680
	11th to 20th	Night	10% 5% 3 6% 4 4 5% 5% 7% 8% 43%	1610
	21st to end	Night	10% 6% 2 6% 5% 4 3 3 5% 6% 48%	1470
July	1st to 10th	Night	6% 5% 2 6% 4 4 4 4 6% 9% 49%	3307
	11th to 20th	Night	9% 3 2 6% 4 4 4 3 5% 8% 50%	3661
	21st to end	Night	7% 5% 2 5% 4 4 4 3 6% 8% 52%	4428
August	1st to 10th	Night	10% 5% 2 7% 4 4 4 4 5% 7% 47%	2915
	11th to 20th	Night	11% 5% 2 5% 3 4 3 4 6% 8% 47%	2875
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 5% 7% 48%	3336
September	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 7% 48%	2000
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 5% 8% 55%	2155
	21st to end	Night	16% 7% 2 7% 5% 4 3 3 5% 6% 41%	2215
October	1st to 10th	Night	15% 7% 3 7% 3 5% 4 3 5% 7% 42%	2400
	11th to 20th	Night	17% 10% 2 8% 5% 5% 4 3 5% 6% 35%	2400
	21st to end	Night	20% 9% 3 8% 5% 5% 5% 4 5% 6% 30%	2640
November	1st to 10th	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 36%	2723
	11th to 20th	Night	14% 12% 4 10% 6% 5% 5% 4 6% 7% 27%	2940
	21st to end	Night	17% 12% 4 11% 6% 5% 4 3 6% 7% 24%	2940
December	1st to 10th	Night	13% 15% 6% 14% 7% 7% 6% 4 7% 6% 15%	4720
	11th to 20th	Night	15% 19% 6% 12% 7% 6% 6% 3 4 5% 18%	5348
	21st to end	Night	23% 19% 7% 13% 6% 5% 4 3 4 4 13%	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	64% 5% 11% 10% 9% 1	4662
	11th to 20th	Day	76% 4 8% 8% 3	5261
	21st to end	Day	69% 4 11% 10% 6%	5780
February	1st to 10th	Day	66% 7% 8% 14% 4 1	1890
	11th to 20th	Day	62% 6% 10% 14% 7% 2	1852
	21st to end	Day	61% 6% 11% 14% 7% 1	1636
March	1st to 10th	Day	56% 4 14% 14% 11% 1	5039
	11th to 20th	Day	50% 5% 17% 18% 6% 3	5090
	21st to end	Day	53% 5% 11% 19% 11% 1	5586
April	1st to 10th	Day	55% 5% 11% 17% 11% 1	2452
	11th to 20th	Day	49% 6% 10% 20% 13% 1	2610
	21st to end	Day	51% 5% 11% 19% 11% 2	2730
May	1st to 10th	Day	48% 5% 13% 19% 11% 4	7651
	11th to 20th	Day	47% 6% 12% 20% 13% 1	8043
	21st to end	Day	41% 4 11% 26% 16% 2	9061
June	1st to 10th	Day	41% 8% 11% 23% 17% 1	2940
	11th to 20th	Day	40% 7% 14% 24% 14% 1	3000
	21st to end	Day	40% 7% 14% 24% 14%	3150
July	1st to 10th	Day	37% 5% 15% 25% 16% 2	6576
	11th to 20th	Day	32% 5% 14% 31% 16% 1	6207
	21st to end	Day	33% 6% 15% 29% 17% 1	6240
August	1st to 10th	Day	43% 7% 13% 23% 13% 1	4221
	11th to 20th	Day	43% 7% 16% 25% 9%	4213
	21st to end	Day	37% 6% 15% 32% 9% 1	4527
September	1st to 10th	Day	40% 7% 14% 27% 11% 1	2400
	11th to 20th	Day	35% 7% 13% 31% 13%	2241
	21st to end	Day	46% 8% 13% 23% 9% 1	2184
October	1st to 10th	Day	42% 5% 13% 26% 13% 1	3260
	11th to 20th	Day	43% 5% 14% 21% 16%	3240
	21st to end	Day	55% 5% 13% 19% 7%	3538
November	1st to 10th	Day	51% 7% 13% 21% 8% 1	1791
	11th to 20th	Day	54% 8% 15% 15% 8%	1680
	21st to end	Day	62% 8% 12% 13% 4	1680
December	1st to 10th	Day	69% 6% 13% 9% 3	2680
	11th to 20th	Day	67% 5% 11% 10% 6%	3066
	21st to end	Day	76% 6% 6% 7% 5%	4048
January	1st to 10th	Night	57% 6% 13% 16% 7%	8234
	11th to 20th	Night	69% 6% 9% 11% 5%	7698
	21st to end	Night	61% 4 12% 12% 9% 1	8401
February	1st to 10th	Night	58% 8% 11% 17% 6% 1	2730
	11th to 20th	Night	58% 5% 12% 17% 8%	2665
	21st to end	Night	55% 8% 12% 17% 8%	2167
March	1st to 10th	Night	48% 5% 15% 18% 12% 2	5033
	11th to 20th	Night	42% 5% 20% 20% 11% 2	5090
	21st to end	Night	46% 4 11% 19% 18% 2	5588
April	1st to 10th	Night	53% 5% 10% 17% 14% 1	2167
	11th to 20th	Night	47% 5% 10% 20% 16% 2	1977
	21st to end	Night	45% 6% 11% 20% 16% 2	1890
May	1st to 10th	Night	46% 5% 12% 16% 15% 6%	5264
	11th to 20th	Night	44% 6% 13% 21% 13% 2	4818
	21st to end	Night	38% 5% 11% 26% 18% 2	5115
June	1st to 10th	Night	35% 7% 11% 25% 19% 1	1680
	11th to 20th	Night	36% 6% 13% 25% 19% 2	1610
	21st to end	Night	36% 6% 14% 27% 17% 1	1470
July	1st to 10th	Night	31% 4 16% 25% 21% 3	3269
	11th to 20th	Night	31% 4 13% 30% 20% 2	3613
	21st to end	Night	30% 6% 13% 30% 20% 2	4297
August	1st to 10th	Night	36% 5% 13% 27% 16% 3	2904
	11th to 20th	Night	33% 6% 16% 31% 13% 1	2902
	21st to end	Night	34% 5% 14% 32% 14% 1	3336
September	1st to 10th	Night	32% 8% 14% 30% 14% 1	2000
	11th to 20th	Night	28% 6% 11% 34% 19% 1	2155
	21st to end	Night	44% 6% 11% 27% 12% 1	2215
October	1st to 10th	Night	33% 5% 9% 28% 24% 1	3920
	11th to 20th	Night	45% 5% 11% 24% 15%	3899
	21st to end	Night	42% 5% 15% 26% 11% 1	4252
November	1st to 10th	Night	45% 7% 13% 22% 12% 1	2723
	11th to 20th	Night	52% 8% 14% 17% 8% 1	2940
	21st to end	Night	55% 9% 17% 14% 5%	2940
December	1st to 10th	Night	63% 7% 15% 12% 3	4720
	11th to 20th	Night	64% 6% 10% 13% 6%	5348
	21st to end	Night	73% 6% 9% 8% 4	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	25% 28% 31% 13% 3	220
	11th to 20th	Day	27% 31% 29% 11% 2	327
	21st to end	Day	26% 29% 31% 12% 2	363
February	1st to 10th	Day	27% 31% 29% 11% 2	330
	11th to 20th	Day	27% 31% 35% 7%	330
	21st to end	Day	36% 26% 33% 4	273
March	1st to 10th	Day	24% 23% 34% 16% 4	534
	11th to 20th	Day	27% 24% 33% 13% 4	540
	21st to end	Day	30% 20% 31% 15% 4 1	594
April	1st to 10th	Day	24% 28% 30% 12% 5%	540
	11th to 20th	Day	30% 21% 26% 14% 6% 2	532
	21st to end	Day	26% 17% 34% 15% 6% 2	540
May	1st to 10th	Day	24% 17% 31% 17% 7% 3	543
	11th to 20th	Day	17% 15% 34% 22% 10% 3	567
	21st to end	Day	15% 14% 33% 23% 12% 4	617
June	1st to 10th	Day	14% 15% 36% 25% 7% 2	570
	11th to 20th	Day	15% 16% 30% 27% 11% 2	630
	21st to end	Day	16% 11% 35% 27% 8% 2	780
July	1st to 10th	Day	10% 7% 31% 31% 16% 5%	655
	11th to 20th	Day	11% 8% 31% 26% 16% 6%	524
	21st to end	Day	9% 7% 35% 31% 14% 5%	576
August	1st to 10th	Day	11% 6% 29% 29% 18% 6%	526
	11th to 20th	Day	11% 7% 28% 28% 19% 8%	519
	21st to end	Day	14% 8% 27% 28% 17% 6%	583
September	1st to 10th	Day	12% 14% 24% 24% 17% 8%	530
	11th to 20th	Day	10% 7% 23% 28% 22% 12%	530
	21st to end	Day	18% 13% 25% 24% 14% 5%	530
October	1st to 10th	Day	21% 17% 26% 19% 13% 3	530
	11th to 20th	Day	23% 21% 35% 13% 7% 1	530
	21st to end	Day	29% 21% 29% 17% 4	583
November	1st to 10th	Day	26% 23% 33% 14% 4 1	307
	11th to 20th	Day	24% 29% 30% 10% 5% 1	220
	21st to end	Day	28% 25% 36% 10% 1	220
December	1st to 10th	Day	31% 30% 25% 13%	220
	11th to 20th	Day	32% 29% 30% 9% 1	220
	21st to end	Day	33% 29% 26% 10% 1 1	242
January	1st to 10th	Night	21% 29% 33% 13% 3	840
	11th to 20th	Night	27% 24% 31% 15% 2	840
	21st to end	Night	18% 22% 37% 18% 5%	924
February	1st to 10th	Night	20% 30% 31% 18% 2	840
	11th to 20th	Night	22% 24% 35% 17% 3	823
	21st to end	Night	21% 26% 35% 16% 3	692
March	1st to 10th	Night	20% 21% 36% 19% 4	840
	11th to 20th	Night	22% 21% 35% 17% 5%	840
	21st to end	Night	21% 18% 34% 21% 6%	924
April	1st to 10th	Night	23% 18% 35% 21% 3	840
	11th to 20th	Night	20% 17% 35% 20% 8% 1	733
	21st to end	Night	18% 13% 33% 27% 8%	630
May	1st to 10th	Night	17% 12% 31% 27% 11% 2	630
	11th to 20th	Night	16% 9% 32% 30% 12% 1	477
	21st to end	Night	13% 9% 30% 33% 15%	455
June	1st to 10th	Night	12% 8% 34% 33% 12% 1	420
	11th to 20th	Night	12% 9% 33% 34% 11%	420
	21st to end	Night	12% 6% 31% 39% 12%	420
July	1st to 10th	Night	6% 5% 28% 42% 18% 1	390
	11th to 20th	Night	9% 5% 31% 35% 18% 1	410
	21st to end	Night	9% 3 28% 38% 19% 2	657
August	1st to 10th	Night	9% 2 25% 38% 24% 2	597
	11th to 20th	Night	8% 4 22% 38% 25% 3	594
	21st to end	Night	13% 5% 28% 30% 22% 2	721
September	1st to 10th	Night	11% 9% 26% 31% 19% 4	800
	11th to 20th	Night	11% 7% 24% 29% 25% 5%	800
	21st to end	Night	19% 11% 21% 26% 18% 4	800
October	1st to 10th	Night	18% 10% 31% 23% 15% 3	800
	11th to 20th	Night	22% 17% 30% 23% 8% 1	800
	21st to end	Night	23% 19% 33% 21% 4	880
November	1st to 10th	Night	22% 17% 29% 23% 8% 1	820
	11th to 20th	Night	21% 22% 30% 20% 5% 1	840
	21st to end	Night	23% 23% 33% 17% 4	840
December	1st to 10th	Night	26% 24% 31% 14% 4	840
	11th to 20th	Night	23% 27% 35% 12% 3	840
	21st to end	Night	29% 24% 28% 16% 2	924

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

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