Tillsonburg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tillsonburg clear sky chart from 2005-11-06 to 2026-06-01.

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% 5% 5% 5% 4 6% 7% 31%	132808
	Night	14% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	130307
Transparency	Day	50% 6% 12% 19% 11% 1	136419
	Night	50% 6% 12% 19% 12% 1	134785
Seeing	Day	18% 12% 28% 23% 13% 5%	16808
	Night	19% 11% 29% 29% 12% 1	26050

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% 16% 6% 6% 5% 3 4 5% 18%	4943
	11th to 20th	Day	9% 21% 10% 20% 9% 6% 4 3 3 4 10%	5246
	21st to end	Day	14% 16% 6% 17% 8% 6% 5% 3 4 4 17%	5780
February	1st to 10th	Day	15% 17% 5% 12% 8% 6% 5% 4 5% 4 20%	1890
	11th to 20th	Day	14% 15% 7% 14% 7% 5% 5% 4 4 6% 19%	1843
	21st to end	Day	17% 15% 4 12% 6% 5% 5% 4 4 7% 21%	1698
March	1st to 10th	Day	17% 12% 6% 12% 6% 4 4 3 4 5% 26%	4985
	11th to 20th	Day	11% 12% 4 11% 5% 5% 4 3 6% 8% 31%	5090
	21st to end	Day	19% 11% 4 10% 6% 5% 4 3 4 5% 29%	5586
April	1st to 10th	Day	17% 12% 4 9% 5% 4 5% 4 5% 7% 28%	2410
	11th to 20th	Day	16% 10% 3 9% 5% 5% 4 3 5% 7% 32%	2644
	21st to end	Day	19% 10% 3 7% 5% 4 4 3 5% 6% 34%	2730
May	1st to 10th	Day	20% 9% 3 8% 5% 4 4 4 5% 7% 31%	6018
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 4 6% 8% 32%	6079
	21st to end	Day	16% 7% 2 6% 4 4 4 4 6% 7% 39%	7103
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 7% 7% 39%	2800
	11th to 20th	Day	11% 7% 2 7% 5% 5% 5% 5% 7% 9% 38%	2860
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 7% 10% 41%	3000
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	6681
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	6002
	21st to end	Day	8% 5% 2 6% 5% 5% 4 4 6% 8% 46%	6424
August	1st to 10th	Day	15% 6% 2 5% 4 5% 5% 5% 6% 9% 38%	4234
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 10% 35%	4174
	21st to end	Day	8% 6% 3 7% 5% 5% 5% 5% 6% 9% 41%	4576
September	1st to 10th	Day	11% 6% 2 7% 4 5% 5% 5% 6% 8% 41%	2380
	11th to 20th	Day	9% 6% 2 5% 4 4 5% 3 6% 9% 47%	2200
	21st to end	Day	14% 8% 2 6% 5% 4 6% 4 6% 9% 35%	2200
October	1st to 10th	Day	15% 7% 4 7% 5% 5% 5% 3 6% 5% 39%	2020
	11th to 20th	Day	17% 9% 3 8% 5% 5% 6% 4 5% 7% 32%	2000
	21st to end	Day	22% 10% 3 9% 6% 4 4 4 6% 6% 25%	2200
November	1st to 10th	Day	15% 10% 3 8% 5% 5% 5% 4 5% 8% 34%	1852
	11th to 20th	Day	15% 12% 4 9% 5% 4 4 3 6% 8% 29%	1686
	21st to end	Day	21% 13% 5% 9% 6% 5% 6% 4 6% 5% 20%	1680
December	1st to 10th	Day	18% 17% 5% 13% 6% 5% 5% 4 6% 6% 16%	2680
	11th to 20th	Day	21% 17% 5% 10% 4 5% 5% 3 5% 5% 20%	3066
	21st to end	Day	27% 19% 6% 11% 6% 5% 4 3 4 5% 11%	4048
January	1st to 10th	Night	14% 14% 5% 15% 7% 6% 5% 4 5% 6% 18%	7937
	11th to 20th	Night	10% 17% 7% 16% 9% 7% 5% 3 5% 5% 15%	7639
	21st to end	Night	15% 14% 6% 15% 7% 5% 5% 3 4 5% 20%	8401
February	1st to 10th	Night	15% 15% 4 11% 7% 5% 5% 5% 5% 5% 23%	2730
	11th to 20th	Night	15% 12% 4 11% 5% 6% 6% 4 6% 5% 25%	2663
	21st to end	Night	16% 13% 6% 10% 7% 4 5% 4 6% 6% 23%	2106
March	1st to 10th	Night	15% 10% 4 8% 6% 4 5% 4 6% 6% 34%	5074
	11th to 20th	Night	11% 9% 3 8% 5% 4 5% 4 6% 9% 38%	5089
	21st to end	Night	17% 10% 4 8% 5% 4 3 3 5% 4 36%	5588
April	1st to 10th	Night	18% 10% 4 7% 5% 5% 5% 4 6% 6% 30%	2209
	11th to 20th	Night	17% 8% 3 8% 4 4 5% 4 5% 6% 36%	1944
	21st to end	Night	14% 7% 2 8% 6% 5% 5% 5% 6% 8% 35%	1890
May	1st to 10th	Night	21% 9% 3 7% 4 4 4 3 5% 6% 34%	4143
	11th to 20th	Night	13% 8% 3 9% 5% 5% 5% 4 5% 6% 37%	4029
	21st to end	Night	18% 5% 2 5% 4 3 4 4 5% 7% 44%	4014
June	1st to 10th	Night	12% 5% 2 6% 5% 4 5% 4 7% 7% 44%	1600
	11th to 20th	Night	8% 6% 2 6% 5% 5% 5% 5% 7% 10% 42%	1531
	21st to end	Night	11% 5% 2 6% 4 3 4 4 6% 9% 46%	1400
July	1st to 10th	Night	6% 5% 2 5% 4 5% 4 4 6% 11% 48%	3285
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 49%	3940
	21st to end	Night	7% 5% 2 5% 4 5% 5% 3 6% 7% 51%	4428
August	1st to 10th	Night	10% 6% 2 6% 4 5% 4 3 6% 9% 45%	2915
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 6% 9% 47%	2875
	21st to end	Night	7% 4 2 6% 5% 4 5% 3 6% 9% 49%	3289
September	1st to 10th	Night	11% 6% 2 7% 5% 5% 4 4 4 6% 47%	2015
	11th to 20th	Night	9% 4 2 4 3 3 3 3 6% 6% 56%	2200
	21st to end	Night	17% 5% 2 7% 5% 5% 4 4 5% 7% 40%	2200
October	1st to 10th	Night	14% 6% 3 6% 4 4 4 3 4 6% 45%	2380
	11th to 20th	Night	16% 8% 4 8% 4 4 4 3 5% 7% 38%	2400
	21st to end	Night	20% 9% 3 8% 5% 5% 4 4 5% 6% 32%	2640
November	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 6% 7% 40%	2664
	11th to 20th	Night	15% 11% 3 8% 5% 5% 5% 5% 6% 8% 30%	2931
	21st to end	Night	21% 13% 4 8% 5% 5% 5% 4 5% 7% 24%	2940
December	1st to 10th	Night	14% 15% 5% 12% 6% 5% 5% 5% 6% 7% 19%	4720
	11th to 20th	Night	16% 19% 5% 10% 6% 4 4 4 4 5% 23%	5348
	21st to end	Night	24% 18% 5% 10% 5% 4 4 3 5% 5% 17%	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% 6% 10% 11% 5%	4944
	11th to 20th	Day	79% 6% 6% 5% 3	5280
	21st to end	Day	70% 5% 10% 8% 6%	5780
February	1st to 10th	Day	66% 6% 10% 12% 5%	1890
	11th to 20th	Day	66% 6% 9% 13% 7%	1852
	21st to end	Day	63% 7% 11% 13% 5% 1	1698
March	1st to 10th	Day	59% 5% 13% 17% 6%	4988
	11th to 20th	Day	51% 5% 17% 19% 7% 2	5090
	21st to end	Day	56% 5% 10% 18% 10% 1	5586
April	1st to 10th	Day	55% 5% 11% 18% 10% 1	2410
	11th to 20th	Day	52% 4 11% 20% 11% 1	2644
	21st to end	Day	50% 5% 11% 20% 11% 2	2730
May	1st to 10th	Day	52% 6% 11% 17% 9% 5%	6027
	11th to 20th	Day	49% 7% 13% 20% 11% 1	6083
	21st to end	Day	44% 6% 10% 20% 16% 4	7103
June	1st to 10th	Day	40% 7% 13% 25% 13% 2	2800
	11th to 20th	Day	41% 7% 15% 21% 14% 2	2860
	21st to end	Day	37% 7% 15% 25% 15% 1	3000
July	1st to 10th	Day	36% 7% 13% 26% 16% 2	6611
	11th to 20th	Day	35% 6% 14% 29% 14% 1	5930
	21st to end	Day	33% 7% 13% 25% 20% 1	6240
August	1st to 10th	Day	41% 7% 15% 22% 14% 1	4221
	11th to 20th	Day	40% 8% 16% 26% 9% 1	4213
	21st to end	Day	37% 7% 13% 30% 12% 1	4576
September	1st to 10th	Day	38% 8% 14% 26% 13% 1	2380
	11th to 20th	Day	33% 7% 15% 31% 15%	2200
	21st to end	Day	43% 8% 13% 23% 11% 1	2200
October	1st to 10th	Day	38% 6% 15% 23% 17% 1	3293
	11th to 20th	Day	39% 5% 14% 24% 18%	3240
	21st to end	Day	48% 5% 16% 23% 7%	3538
November	1st to 10th	Day	49% 6% 11% 24% 8% 2	1852
	11th to 20th	Day	52% 6% 17% 15% 9% 1	1686
	21st to end	Day	63% 6% 14% 13% 4	1680
December	1st to 10th	Day	68% 6% 14% 10% 2	2680
	11th to 20th	Day	67% 5% 11% 8% 8%	3066
	21st to end	Day	75% 6% 8% 8% 3	4048
January	1st to 10th	Night	64% 7% 11% 14% 4	7937
	11th to 20th	Night	69% 6% 9% 10% 6%	7680
	21st to end	Night	66% 5% 7% 13% 9%	8401
February	1st to 10th	Night	61% 6% 10% 16% 6%	2730
	11th to 20th	Night	58% 5% 12% 16% 7%	2665
	21st to end	Night	60% 6% 12% 16% 7% 1	2106
March	1st to 10th	Night	50% 5% 15% 19% 10% 1	5084
	11th to 20th	Night	43% 6% 17% 21% 12% 1	5090
	21st to end	Night	50% 5% 10% 18% 16% 2	5588
April	1st to 10th	Night	53% 6% 9% 18% 12% 1	2209
	11th to 20th	Night	48% 5% 9% 20% 16% 2	1944
	21st to end	Night	44% 7% 10% 21% 14% 3	1890
May	1st to 10th	Night	51% 5% 10% 15% 12% 8%	4150
	11th to 20th	Night	47% 6% 13% 22% 10% 2	4030
	21st to end	Night	39% 5% 12% 23% 20% 2	4015
June	1st to 10th	Night	37% 7% 11% 26% 15% 4	1600
	11th to 20th	Night	34% 7% 14% 25% 17% 2	1531
	21st to end	Night	34% 8% 14% 24% 20% 1	1400
July	1st to 10th	Night	30% 6% 14% 27% 19% 4	3248
	11th to 20th	Night	33% 5% 12% 29% 18% 2	3889
	21st to end	Night	31% 6% 13% 22% 24% 4	4297
August	1st to 10th	Night	36% 5% 15% 24% 18% 2	2904
	11th to 20th	Night	34% 6% 17% 31% 11% 1	2902
	21st to end	Night	30% 7% 13% 33% 16% 1	3289
September	1st to 10th	Night	37% 5% 13% 27% 17% 2	2015
	11th to 20th	Night	28% 6% 12% 35% 19% 1	2200
	21st to end	Night	43% 6% 11% 25% 13% 1	2200
October	1st to 10th	Night	31% 5% 8% 30% 25% 2	3887
	11th to 20th	Night	41% 4 13% 24% 17%	3899
	21st to end	Night	40% 5% 15% 26% 14% 1	4252
November	1st to 10th	Night	42% 5% 13% 26% 13% 1	2664
	11th to 20th	Night	51% 7% 13% 19% 9% 1	2931
	21st to end	Night	59% 6% 16% 14% 5%	2940
December	1st to 10th	Night	62% 7% 15% 12% 4	4720
	11th to 20th	Night	62% 6% 10% 13% 8% 1	5348
	21st to end	Night	69% 6% 10% 10% 5%	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	22% 22% 34% 15% 6% 1	272
	11th to 20th	Day	24% 22% 38% 11% 4 1	330
	21st to end	Day	25% 21% 32% 17% 4 1	363
February	1st to 10th	Day	23% 19% 39% 15% 3 1	330
	11th to 20th	Day	27% 22% 33% 17% 2	330
	21st to end	Day	26% 26% 36% 11% 1	273
March	1st to 10th	Day	23% 16% 36% 19% 5% 1	507
	11th to 20th	Day	24% 18% 34% 17% 6% 1	540
	21st to end	Day	29% 16% 30% 19% 6%	594
April	1st to 10th	Day	24% 19% 30% 20% 7% 1	540
	11th to 20th	Day	24% 18% 28% 17% 9% 4	532
	21st to end	Day	24% 14% 32% 21% 8% 2	540
May	1st to 10th	Day	23% 13% 30% 20% 10% 3	543
	11th to 20th	Day	17% 11% 30% 24% 14% 4	567
	21st to end	Day	14% 9% 27% 27% 17% 7%	617
June	1st to 10th	Day	14% 7% 33% 30% 13% 3	530
	11th to 20th	Day	14% 8% 27% 32% 15% 4	590
	21st to end	Day	12% 8% 30% 32% 14% 3	730
July	1st to 10th	Day	9% 5% 22% 30% 24% 8%	664
	11th to 20th	Day	12% 6% 26% 28% 18% 10%	524
	21st to end	Day	9% 4 22% 34% 22% 9%	576
August	1st to 10th	Day	9% 3 21% 32% 21% 14%	526
	11th to 20th	Day	11% 4 21% 28% 22% 13%	519
	21st to end	Day	10% 5% 22% 29% 23% 10%	583
September	1st to 10th	Day	12% 5% 25% 25% 20% 13%	530
	11th to 20th	Day	7% 6% 15% 27% 29% 16%	530
	21st to end	Day	17% 10% 24% 26% 16% 7%	530
October	1st to 10th	Day	18% 8% 26% 24% 18% 7%	530
	11th to 20th	Day	19% 15% 32% 24% 7% 3	530
	21st to end	Day	26% 15% 32% 21% 6% 1	583
November	1st to 10th	Day	18% 22% 36% 15% 5% 3	330
	11th to 20th	Day	24% 22% 28% 17% 7% 2	223
	21st to end	Day	27% 21% 31% 18% 2	220
December	1st to 10th	Day	28% 22% 28% 16% 5% 1	220
	11th to 20th	Day	28% 23% 31% 13% 4	220
	21st to end	Day	32% 21% 31% 12% 5%	242
January	1st to 10th	Night	22% 19% 36% 16% 6% 1	840
	11th to 20th	Night	26% 20% 31% 20% 3	840
	21st to end	Night	20% 15% 38% 23% 5%	924
February	1st to 10th	Night	23% 16% 36% 23% 2	840
	11th to 20th	Night	22% 15% 35% 24% 4	823
	21st to end	Night	23% 18% 35% 22% 3	692
March	1st to 10th	Night	18% 16% 34% 26% 5%	840
	11th to 20th	Night	22% 12% 37% 23% 6%	840
	21st to end	Night	22% 14% 30% 26% 8% 1	924
April	1st to 10th	Night	23% 14% 33% 26% 5%	840
	11th to 20th	Night	21% 12% 32% 26% 8% 1	700
	21st to end	Night	17% 10% 33% 32% 8%	630
May	1st to 10th	Night	21% 8% 27% 31% 13% 1	630
	11th to 20th	Night	16% 5% 29% 37% 12% 1	580
	21st to end	Night	12% 5% 29% 36% 17% 1	455
June	1st to 10th	Night	14% 4 28% 41% 13% 1	400
	11th to 20th	Night	10% 6% 30% 41% 14%	400
	21st to end	Night	12% 5% 25% 43% 15% 1	400
July	1st to 10th	Night	6% 2 22% 44% 24% 3	390
	11th to 20th	Night	10% 4 18% 39% 25% 3	530
	21st to end	Night	9% 2 17% 47% 24% 1	657
August	1st to 10th	Night	7% 4 13% 44% 30% 3	597
	11th to 20th	Night	10% 2 14% 37% 33% 4	594
	21st to end	Night	8% 3 21% 38% 28% 2	700
September	1st to 10th	Night	12% 5% 19% 34% 25% 4	800
	11th to 20th	Night	10% 3 15% 37% 32% 4	800
	21st to end	Night	18% 6% 20% 33% 18% 5%	800
October	1st to 10th	Night	16% 6% 25% 33% 18% 3	800
	11th to 20th	Night	20% 12% 30% 27% 10% 1	800
	21st to end	Night	24% 10% 31% 29% 5%	880
November	1st to 10th	Night	19% 11% 30% 28% 10% 1	820
	11th to 20th	Night	22% 16% 33% 21% 7% 1	840
	21st to end	Night	29% 12% 32% 21% 6%	840
December	1st to 10th	Night	27% 17% 33% 17% 5% 1	840
	11th to 20th	Night	22% 18% 37% 18% 4	840
	21st to end	Night	29% 14% 33% 22% 2	924

Page generated at 2026-05-29 01:53:26. Version ac07

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