Guelph
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Guelph clear sky chart from 2005-11-06 to 2026-08-30.

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% 31%	144911
	Night	14% 11% 4 10% 5% 5% 5% 4 5% 6% 31%	137278
Transparency	Day	51% 4 12% 21% 11% 1	148506
	Night	52% 4 11% 20% 12% 1	141773
Seeing	Day	22% 14% 30% 21% 10% 4	17184
	Night	20% 14% 30% 25% 10% 1	26133

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% 17% 8% 17% 7% 4 4 3 3 4 19%	4703
	11th to 20th	Day	9% 23% 11% 21% 9% 8% 4 3 4 3 7%	5246
	21st to end	Day	13% 16% 6% 18% 9% 7% 5% 4 4 4 13%	5780
February	1st to 10th	Day	15% 17% 6% 13% 8% 6% 5% 3 4 4 18%	1890
	11th to 20th	Day	14% 13% 6% 16% 7% 6% 4 3 5% 6% 19%	1843
	21st to end	Day	16% 16% 5% 11% 7% 5% 6% 3 5% 6% 20%	1651
March	1st to 10th	Day	19% 12% 6% 10% 6% 4 4 2 4 5% 28%	5013
	11th to 20th	Day	12% 13% 5% 10% 5% 5% 5% 3 5% 8% 28%	5090
	21st to end	Day	18% 13% 4 9% 5% 5% 4 3 5% 5% 29%	5586
April	1st to 10th	Day	17% 14% 4 9% 5% 4 4 3 5% 6% 28%	2441
	11th to 20th	Day	18% 12% 3 7% 5% 4 4 3 5% 6% 33%	2624
	21st to end	Day	20% 10% 3 7% 6% 5% 4 3 5% 6% 33%	2730
May	1st to 10th	Day	17% 9% 3 8% 5% 4 5% 4 6% 7% 33%	7642
	11th to 20th	Day	15% 10% 4 8% 4 5% 5% 4 6% 8% 33%	7978
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 42%	9060
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 5% 7% 8% 38%	2940
	11th to 20th	Day	11% 6% 3 7% 5% 4 5% 5% 7% 9% 38%	3033
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	3150
July	1st to 10th	Day	10% 7% 2 7% 5% 4 5% 5% 7% 10% 38%	8759
	11th to 20th	Day	7% 5% 2 5% 5% 4 5% 5% 7% 10% 46%	8107
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 7% 8% 45%	8340
August	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 4 6% 8% 37%	4364
	11th to 20th	Day	10% 8% 3 8% 5% 5% 5% 5% 7% 9% 33%	4304
	21st to end	Day	9% 6% 3 7% 4 5% 6% 5% 7% 9% 40%	4665
September	1st to 10th	Day	12% 6% 2 7% 6% 4 5% 4 6% 8% 40%	2400
	11th to 20th	Day	11% 5% 3 6% 4 3 4 4 6% 9% 45%	2224
	21st to end	Day	15% 7% 3 8% 4 5% 6% 4 6% 8% 35%	2194
October	1st to 10th	Day	18% 9% 3 7% 4 4 5% 4 5% 6% 35%	2000
	11th to 20th	Day	21% 9% 4 8% 5% 4 4 4 5% 5% 30%	2000
	21st to end	Day	23% 13% 3 8% 5% 4 5% 5% 5% 7% 21%	2200
November	1st to 10th	Day	16% 11% 4 8% 6% 4 4 4 6% 7% 30%	1800
	11th to 20th	Day	15% 15% 5% 9% 5% 5% 4 4 7% 7% 25%	1680
	21st to end	Day	18% 18% 7% 11% 6% 4 4 3 5% 6% 17%	1680
December	1st to 10th	Day	17% 22% 7% 14% 6% 5% 5% 3 4 5% 12%	2680
	11th to 20th	Day	21% 20% 5% 11% 7% 4 4 3 4 4 16%	3066
	21st to end	Day	26% 18% 6% 12% 6% 5% 4 3 4 3 11%	4048
January	1st to 10th	Night	14% 15% 7% 17% 7% 5% 5% 4 5% 5% 17%	8174
	11th to 20th	Night	10% 18% 8% 18% 11% 7% 6% 4 5% 4 9%	7639
	21st to end	Night	13% 13% 7% 15% 8% 7% 5% 4 5% 7% 16%	8401
February	1st to 10th	Night	14% 15% 6% 11% 8% 6% 5% 4 5% 5% 21%	2730
	11th to 20th	Night	14% 14% 6% 10% 7% 4 6% 3 6% 6% 23%	2663
	21st to end	Night	14% 13% 6% 13% 6% 5% 5% 4 5% 6% 24%	2150
March	1st to 10th	Night	14% 11% 4 9% 6% 5% 5% 3 6% 7% 30%	5047
	11th to 20th	Night	10% 9% 3 9% 5% 5% 5% 5% 6% 8% 35%	5089
	21st to end	Night	19% 10% 3 7% 5% 4 4 3 5% 4 36%	5588
April	1st to 10th	Night	19% 11% 4 8% 5% 4 5% 4 5% 6% 31%	2179
	11th to 20th	Night	17% 10% 2 7% 5% 4 4 3 5% 6% 37%	1962
	21st to end	Night	14% 10% 3 8% 4 5% 4 3 6% 7% 36%	1890
May	1st to 10th	Night	17% 8% 3 6% 4 4 5% 4 5% 7% 37%	5257
	11th to 20th	Night	14% 9% 3 8% 4 4 4 4 5% 7% 37%	4881
	21st to end	Night	18% 6% 2 5% 4 3 3 3 4 7% 46%	5114
June	1st to 10th	Night	12% 6% 2 6% 3 5% 4 4 6% 8% 45%	1680
	11th to 20th	Night	10% 5% 3 6% 3 6% 5% 4 7% 9% 42%	1579
	21st to end	Night	11% 6% 2 5% 5% 5% 4 3 5% 8% 46%	1470
July	1st to 10th	Night	6% 6% 2 6% 4 5% 4 4 6% 9% 48%	4217
	11th to 20th	Night	8% 5% 2 5% 3 4 4 3 5% 9% 50%	4835
	21st to end	Night	9% 6% 2 6% 4 4 4 4 5% 7% 50%	5737
August	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 46%	3005
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 6% 9% 45%	2965
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 7% 48%	3397
September	1st to 10th	Night	9% 6% 2 6% 4 3 4 4 7% 7% 48%	2000
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 5% 8% 54%	2172
	21st to end	Night	16% 7% 3 6% 5% 4 4 3 6% 7% 40%	2205
October	1st to 10th	Night	14% 7% 3 7% 4 4 4 3 5% 7% 42%	2400
	11th to 20th	Night	18% 9% 3 8% 5% 4 5% 3 5% 6% 36%	2400
	21st to end	Night	21% 10% 3 8% 5% 5% 4 4 5% 6% 29%	2640
November	1st to 10th	Night	14% 9% 4 8% 4 4 4 4 6% 7% 36%	2714
	11th to 20th	Night	15% 13% 4 10% 5% 5% 5% 4 6% 7% 26%	2940
	21st to end	Night	19% 14% 5% 9% 5% 5% 5% 4 5% 7% 22%	2940
December	1st to 10th	Night	14% 19% 8% 13% 6% 6% 5% 4 5% 6% 15%	4720
	11th to 20th	Night	16% 21% 6% 12% 6% 5% 4 3 5% 4 18%	5348
	21st to end	Night	23% 19% 7% 13% 6% 5% 4 3 4 4 12%	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	69% 3 9% 9% 9% 1	4704
	11th to 20th	Day	83% 2 6% 6% 3	5280
	21st to end	Day	73% 3 10% 8% 5%	5780
February	1st to 10th	Day	70% 4 7% 14% 4 1	1890
	11th to 20th	Day	66% 3 7% 15% 7% 2	1852
	21st to end	Day	65% 2 10% 14% 7% 1	1651
March	1st to 10th	Day	59% 3 13% 15% 10% 1	5016
	11th to 20th	Day	53% 4 15% 19% 7% 3	5090
	21st to end	Day	57% 3 8% 20% 10% 1	5586
April	1st to 10th	Day	57% 3 10% 18% 11% 1	2441
	11th to 20th	Day	52% 3 10% 21% 12% 1	2624
	21st to end	Day	53% 4 9% 21% 11% 2	2730
May	1st to 10th	Day	50% 4 12% 20% 10% 4	7651
	11th to 20th	Day	50% 4 11% 21% 13% 1	7982
	21st to end	Day	43% 3 10% 26% 16% 3	9061
June	1st to 10th	Day	43% 5% 11% 23% 17% 1	2940
	11th to 20th	Day	41% 6% 14% 25% 14% 1	3033
	21st to end	Day	42% 5% 13% 25% 15%	3150
July	1st to 10th	Day	39% 6% 15% 25% 13% 1	8688
	11th to 20th	Day	31% 5% 13% 35% 15% 1	8032
	21st to end	Day	35% 6% 12% 28% 18% 1	8156
August	1st to 10th	Day	43% 6% 13% 23% 13% 1	4351
	11th to 20th	Day	44% 5% 15% 26% 9%	4343
	21st to end	Day	38% 6% 13% 33% 10% 1	4665
September	1st to 10th	Day	40% 5% 14% 27% 12% 1	2400
	11th to 20th	Day	36% 5% 13% 32% 14% 1	2224
	21st to end	Day	46% 5% 13% 25% 10% 1	2194
October	1st to 10th	Day	42% 6% 13% 25% 13% 1	3260
	11th to 20th	Day	43% 5% 14% 21% 17%	3240
	21st to end	Day	53% 5% 14% 20% 7%	3538
November	1st to 10th	Day	53% 5% 12% 21% 8% 1	1800
	11th to 20th	Day	58% 5% 13% 16% 8% 1	1680
	21st to end	Day	67% 3 12% 13% 5%	1680
December	1st to 10th	Day	74% 5% 10% 9% 3	2680
	11th to 20th	Day	71% 3 9% 10% 6%	3066
	21st to end	Day	79% 3 6% 7% 5%	4048
January	1st to 10th	Night	66% 5% 10% 13% 6%	8174
	11th to 20th	Night	77% 3 7% 8% 4	7680
	21st to end	Night	67% 4 11% 11% 7%	8401
February	1st to 10th	Night	62% 4 9% 18% 5% 1	2730
	11th to 20th	Night	60% 4 10% 17% 9% 1	2665
	21st to end	Night	60% 3 11% 18% 8%	2150
March	1st to 10th	Night	52% 4 13% 18% 11% 2	5057
	11th to 20th	Night	45% 4 16% 22% 11% 2	5090
	21st to end	Night	51% 3 8% 19% 17% 2	5588
April	1st to 10th	Night	54% 3 9% 19% 13% 1	2179
	11th to 20th	Night	48% 4 8% 22% 16% 2	1962
	21st to end	Night	48% 3 10% 22% 15% 2	1890
May	1st to 10th	Night	47% 4 11% 17% 15% 6%	5264
	11th to 20th	Night	48% 5% 11% 22% 13% 2	4882
	21st to end	Night	40% 3 11% 26% 18% 2	5115
June	1st to 10th	Night	37% 5% 10% 27% 19% 2	1680
	11th to 20th	Night	37% 5% 13% 26% 18% 2	1579
	21st to end	Night	37% 4 14% 27% 17% 1	1470
July	1st to 10th	Night	32% 6% 16% 26% 18% 3	4181
	11th to 20th	Night	30% 4 11% 31% 21% 2	4787
	21st to end	Night	33% 6% 11% 27% 21% 2	5606
August	1st to 10th	Night	37% 5% 14% 26% 16% 3	2994
	11th to 20th	Night	35% 5% 16% 31% 12% 1	2992
	21st to end	Night	33% 5% 13% 33% 15% 1	3397
September	1st to 10th	Night	35% 5% 13% 31% 15% 1	2000
	11th to 20th	Night	29% 4 12% 34% 20% 1	2172
	21st to end	Night	44% 4 13% 26% 13% 1	2205
October	1st to 10th	Night	33% 4 10% 29% 23% 1	3920
	11th to 20th	Night	45% 4 11% 25% 15%	3899
	21st to end	Night	43% 5% 15% 27% 10%	4252
November	1st to 10th	Night	47% 4 12% 23% 13% 1	2714
	11th to 20th	Night	56% 4 13% 18% 8% 1	2940
	21st to end	Night	60% 5% 15% 14% 5%	2940
December	1st to 10th	Night	69% 5% 11% 13% 3	4720
	11th to 20th	Night	69% 4 9% 12% 6%	5348
	21st to end	Night	76% 3 9% 8% 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	28% 20% 34% 14% 4 1	228
	11th to 20th	Day	29% 27% 30% 11% 3	330
	21st to end	Day	29% 23% 31% 15% 3	363
February	1st to 10th	Day	33% 20% 32% 11% 4	330
	11th to 20th	Day	27% 26% 39% 7%	330
	21st to end	Day	36% 23% 32% 8% 1	273
March	1st to 10th	Day	30% 16% 31% 18% 4	524
	11th to 20th	Day	30% 18% 32% 16% 4 1	540
	21st to end	Day	32% 15% 32% 15% 4 1	594
April	1st to 10th	Day	27% 21% 31% 16% 5%	540
	11th to 20th	Day	33% 16% 24% 17% 8% 2	532
	21st to end	Day	24% 14% 36% 17% 7% 1	540
May	1st to 10th	Day	25% 14% 32% 18% 7% 3	543
	11th to 20th	Day	18% 13% 31% 23% 10% 4	567
	21st to end	Day	17% 12% 30% 24% 11% 5%	617
June	1st to 10th	Day	16% 14% 33% 24% 11% 2	570
	11th to 20th	Day	16% 12% 32% 27% 11% 3	663
	21st to end	Day	17% 9% 34% 28% 10% 2	780
July	1st to 10th	Day	11% 6% 29% 31% 17% 6%	731
	11th to 20th	Day	13% 7% 30% 25% 16% 7%	564
	21st to end	Day	10% 6% 33% 28% 17% 6%	620
August	1st to 10th	Day	14% 7% 23% 30% 18% 8%	566
	11th to 20th	Day	13% 6% 25% 27% 18% 10%	559
	21st to end	Day	15% 6% 25% 28% 19% 7%	619
September	1st to 10th	Day	13% 11% 24% 23% 19% 10%	530
	11th to 20th	Day	11% 5% 20% 28% 22% 14%	530
	21st to end	Day	17% 14% 25% 25% 12% 7%	530
October	1st to 10th	Day	22% 13% 22% 23% 14% 5%	530
	11th to 20th	Day	25% 15% 36% 15% 7% 1	530
	21st to end	Day	31% 16% 30% 17% 6%	583
November	1st to 10th	Day	32% 18% 31% 13% 5%	306
	11th to 20th	Day	29% 21% 31% 14% 3 3	220
	21st to end	Day	29% 24% 35% 10% 3	220
December	1st to 10th	Day	32% 22% 30% 12% 4	220
	11th to 20th	Day	35% 22% 29% 13% 1	220
	21st to end	Day	32% 26% 30% 10% 2	242
January	1st to 10th	Night	23% 24% 34% 15% 4	840
	11th to 20th	Night	29% 21% 29% 17% 3	840
	21st to end	Night	21% 19% 37% 18% 5%	924
February	1st to 10th	Night	23% 24% 31% 20% 2	840
	11th to 20th	Night	24% 21% 34% 18% 4	823
	21st to end	Night	23% 22% 35% 17% 3	692
March	1st to 10th	Night	21% 18% 36% 21% 5%	840
	11th to 20th	Night	25% 17% 35% 17% 5%	840
	21st to end	Night	23% 16% 32% 21% 7% 1	924
April	1st to 10th	Night	27% 15% 32% 22% 3	840
	11th to 20th	Night	24% 14% 33% 20% 8% 1	718
	21st to end	Night	17% 13% 34% 29% 7%	630
May	1st to 10th	Night	19% 11% 30% 27% 11% 2	630
	11th to 20th	Night	17% 7% 32% 32% 11% 1	502
	21st to end	Night	14% 9% 25% 35% 18%	455
June	1st to 10th	Night	14% 8% 30% 35% 13% 1	420
	11th to 20th	Night	12% 10% 34% 32% 12%	420
	21st to end	Night	14% 5% 29% 38% 14%	420
July	1st to 10th	Night	5% 4 25% 45% 19% 1	410
	11th to 20th	Night	9% 5% 29% 36% 19% 2	459
	21st to end	Night	11% 2 25% 41% 19% 2	690
August	1st to 10th	Night	9% 4 21% 41% 23% 2	627
	11th to 20th	Night	8% 4 22% 37% 26% 3	624
	21st to end	Night	13% 5% 24% 33% 23% 3	741
September	1st to 10th	Night	12% 7% 23% 34% 20% 5%	800
	11th to 20th	Night	12% 5% 21% 33% 24% 5%	800
	21st to end	Night	19% 9% 21% 28% 19% 5%	800
October	1st to 10th	Night	17% 9% 27% 27% 17% 3	800
	11th to 20th	Night	23% 13% 29% 26% 9% 1	800
	21st to end	Night	25% 16% 32% 23% 4	880
November	1st to 10th	Night	23% 14% 30% 24% 8% 1	820
	11th to 20th	Night	23% 22% 28% 20% 6% 1	840
	21st to end	Night	29% 19% 30% 17% 4	840
December	1st to 10th	Night	29% 21% 29% 16% 5%	840
	11th to 20th	Night	26% 24% 32% 15% 3	840
	21st to end	Night	32% 17% 33% 15% 2	924

Page generated at 2026-08-27 04:07:12. Version ac07

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