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 2025-04-26.

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	15% 11% 4 9% 6% 5% 5% 4 6% 7% 30%	114315
	Night	14% 11% 4 10% 5% 5% 5% 4 5% 6% 31%	114268
Transparency	Day	53% 4 12% 20% 10% 1	117953
	Night	52% 4 12% 20% 11% 1	118795
Seeing	Day	22% 13% 30% 21% 10% 4	15359
	Night	20% 14% 30% 25% 10% 1	24559

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	12% 17% 8% 16% 7% 4 4 3 3 4 22%	3625
	11th to 20th	Day	10% 21% 9% 22% 9% 7% 4 3 5% 3 8%	4026
	21st to end	Day	16% 18% 7% 18% 7% 5% 4 3 4 4 13%	4466
February	1st to 10th	Day	15% 17% 6% 13% 9% 6% 5% 3 5% 4 17%	1800
	11th to 20th	Day	14% 14% 6% 16% 7% 6% 4 3 5% 6% 19%	1753
	21st to end	Day	17% 16% 5% 11% 7% 5% 5% 3 5% 6% 21%	1575
March	1st to 10th	Day	18% 12% 6% 11% 6% 4 4 2 4 5% 28%	4904
	11th to 20th	Day	12% 13% 5% 9% 5% 5% 5% 3 6% 8% 29%	4980
	21st to end	Day	18% 13% 4 9% 5% 4 4 3 5% 5% 29%	5465
April	1st to 10th	Day	17% 14% 4 9% 5% 4 4 3 5% 6% 28%	2325
	11th to 20th	Day	18% 12% 2 7% 4 4 4 3 5% 6% 34%	2498
	21st to end	Day	20% 10% 3 7% 6% 4 4 3 4 6% 32%	2535
May	1st to 10th	Day	18% 8% 3 7% 5% 4 4 3 6% 7% 34%	4108
	11th to 20th	Day	15% 9% 4 9% 5% 5% 5% 4 6% 7% 31%	4265
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 7% 46%	4837
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 5% 7% 8% 36%	2660
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 4 8% 9% 39%	2744
	21st to end	Day	14% 6% 2 6% 5% 4 5% 4 6% 8% 38%	2850
July	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 5% 8% 10% 38%	4696
	11th to 20th	Day	10% 6% 2 5% 5% 4 5% 5% 8% 10% 42%	4351
	21st to end	Day	8% 6% 2 6% 5% 5% 6% 5% 7% 9% 41%	4466
August	1st to 10th	Day	16% 7% 2 6% 5% 5% 5% 4 6% 8% 36%	4104
	11th to 20th	Day	11% 9% 3 8% 6% 5% 5% 5% 7% 9% 33%	4044
	21st to end	Day	9% 5% 3 7% 5% 5% 6% 5% 7% 9% 40%	4411
September	1st to 10th	Day	12% 6% 2 7% 6% 4 5% 4 6% 8% 40%	2280
	11th to 20th	Day	12% 5% 3 6% 5% 3 4 4 6% 9% 43%	2113
	21st to end	Day	15% 7% 3 8% 5% 5% 6% 4 6% 8% 34%	2085
October	1st to 10th	Day	18% 10% 3 7% 5% 4 5% 4 5% 6% 33%	1900
	11th to 20th	Day	21% 9% 4 8% 5% 4 4 4 5% 5% 30%	1900
	21st to end	Day	24% 13% 3 8% 5% 4 5% 4 5% 7% 21%	2090
November	1st to 10th	Day	15% 11% 4 8% 6% 4 4 4 6% 7% 30%	1713
	11th to 20th	Day	16% 15% 5% 9% 5% 5% 4 4 7% 7% 24%	1600
	21st to end	Day	18% 18% 7% 11% 6% 4 4 3 5% 6% 18%	1600
December	1st to 10th	Day	17% 22% 7% 13% 6% 5% 5% 3 4 5% 11%	2600
	11th to 20th	Day	21% 20% 5% 10% 7% 4 4 4 4 4 17%	2986
	21st to end	Day	26% 18% 6% 12% 6% 5% 4 3 4 3 11%	3960
January	1st to 10th	Night	14% 14% 6% 17% 7% 5% 5% 4 5% 5% 19%	6303
	11th to 20th	Night	11% 15% 7% 19% 11% 7% 6% 4 5% 4 10%	5859
	21st to end	Night	16% 14% 8% 14% 6% 6% 5% 4 5% 6% 17%	6490
February	1st to 10th	Night	14% 15% 6% 11% 8% 6% 5% 4 5% 5% 20%	2600
	11th to 20th	Night	14% 14% 6% 10% 7% 5% 6% 3 6% 6% 23%	2533
	21st to end	Night	14% 12% 6% 13% 6% 5% 5% 4 5% 6% 25%	2051
March	1st to 10th	Night	14% 11% 4 10% 6% 5% 5% 3 6% 7% 30%	4936
	11th to 20th	Night	9% 9% 3 9% 5% 5% 5% 5% 6% 8% 36%	4979
	21st to end	Night	19% 10% 3 7% 5% 4 4 3 5% 4 36%	5467
April	1st to 10th	Night	19% 12% 4 7% 5% 4 4 3 5% 5% 31%	2075
	11th to 20th	Night	17% 10% 2 7% 5% 4 4 3 5% 6% 37%	1867
	21st to end	Night	15% 10% 3 8% 4 5% 4 3 6% 7% 35%	1761
May	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 40%	2833
	11th to 20th	Night	12% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	2627
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 50%	2738
June	1st to 10th	Night	12% 6% 2 6% 3 5% 5% 4 7% 8% 42%	1520
	11th to 20th	Night	10% 5% 3 5% 3 6% 6% 4 7% 8% 43%	1429
	21st to end	Night	12% 6% 2 5% 5% 5% 4 3 5% 7% 46%	1330
July	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 5% 9% 53%	2265
	11th to 20th	Night	10% 5% 2 5% 3 3 3 4 5% 9% 52%	2605
	21st to end	Night	6% 5% 2 6% 4 4 5% 5% 6% 8% 49%	3087
August	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 46%	2825
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 6% 9% 45%	2785
	21st to end	Night	10% 5% 2 5% 4 4 4 4 6% 7% 49%	3212
September	1st to 10th	Night	9% 6% 2 6% 5% 3 4 4 7% 7% 47%	1900
	11th to 20th	Night	11% 5% 1 5% 3 3 4 3 5% 8% 53%	2063
	21st to end	Night	16% 7% 3 6% 5% 4 4 3 6% 7% 40%	2095
October	1st to 10th	Night	14% 8% 3 8% 4 4 4 4 5% 7% 40%	2280
	11th to 20th	Night	18% 9% 3 8% 5% 4 5% 3 5% 6% 35%	2280
	21st to end	Night	21% 10% 3 8% 5% 5% 4 4 5% 6% 28%	2508
November	1st to 10th	Night	15% 9% 4 8% 4 4 4 4 6% 6% 36%	2581
	11th to 20th	Night	16% 13% 4 10% 5% 5% 5% 4 6% 7% 26%	2800
	21st to end	Night	19% 14% 5% 9% 5% 5% 5% 4 5% 7% 23%	2800
December	1st to 10th	Night	14% 19% 8% 13% 6% 6% 5% 4 5% 6% 14%	4580
	11th to 20th	Night	16% 21% 6% 12% 6% 5% 4 3 5% 4 17%	5208
	21st to end	Night	23% 20% 7% 13% 6% 5% 4 3 4 4 12%	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% 3 9% 10% 10% 1	3626
	11th to 20th	Day	80% 2 7% 8% 3	4060
	21st to end	Day	74% 3 10% 7% 6%	4466
February	1st to 10th	Day	71% 4 7% 14% 4 1	1800
	11th to 20th	Day	66% 3 7% 15% 7% 2	1762
	21st to end	Day	66% 2 10% 14% 7% 1	1575
March	1st to 10th	Day	59% 3 13% 15% 10% 1	4907
	11th to 20th	Day	53% 4 15% 19% 7% 3	4980
	21st to end	Day	57% 3 8% 20% 11% 1	5465
April	1st to 10th	Day	57% 3 10% 18% 11% 1	2325
	11th to 20th	Day	52% 3 10% 21% 13% 1	2498
	21st to end	Day	53% 4 10% 20% 11% 2	2535
May	1st to 10th	Day	50% 3 12% 21% 12% 2	4120
	11th to 20th	Day	52% 4 14% 22% 7% 1	4269
	21st to end	Day	39% 3 13% 23% 18% 4	4837
June	1st to 10th	Day	44% 5% 12% 23% 15% 1	2660
	11th to 20th	Day	40% 5% 14% 25% 15% 1	2744
	21st to end	Day	42% 4 13% 25% 15%	2850
July	1st to 10th	Day	38% 6% 15% 25% 15% 2	4624
	11th to 20th	Day	34% 5% 14% 35% 11% 1	4276
	21st to end	Day	37% 8% 12% 24% 18% 1	4324
August	1st to 10th	Day	44% 6% 13% 24% 13% 1	4091
	11th to 20th	Day	45% 5% 15% 25% 9%	4083
	21st to end	Day	38% 6% 13% 33% 10% 1	4411
September	1st to 10th	Day	41% 5% 14% 27% 12% 1	2280
	11th to 20th	Day	37% 5% 13% 31% 14% 1	2113
	21st to end	Day	46% 6% 13% 25% 9%	2085
October	1st to 10th	Day	43% 6% 14% 24% 13% 1	3160
	11th to 20th	Day	43% 5% 14% 21% 17% 1	3140
	21st to end	Day	53% 5% 14% 21% 7%	3428
November	1st to 10th	Day	53% 4 12% 21% 8% 1	1713
	11th to 20th	Day	58% 5% 13% 16% 8% 1	1600
	21st to end	Day	67% 3 12% 13% 5%	1600
December	1st to 10th	Day	74% 5% 10% 8% 3	2600
	11th to 20th	Day	71% 3 10% 10% 6%	2986
	21st to end	Day	78% 3 6% 7% 6%	3960
January	1st to 10th	Night	64% 4 9% 15% 8% 1	6303
	11th to 20th	Night	76% 4 8% 9% 4	5900
	21st to end	Night	67% 3 12% 12% 6% 1	6490
February	1st to 10th	Night	64% 4 9% 18% 5% 1	2600
	11th to 20th	Night	60% 3 10% 17% 9% 1	2535
	21st to end	Night	59% 3 11% 18% 8%	2051
March	1st to 10th	Night	53% 4 13% 17% 11% 2	4946
	11th to 20th	Night	45% 4 17% 22% 11% 2	4980
	21st to end	Night	51% 3 8% 20% 17% 2	5467
April	1st to 10th	Night	55% 3 8% 19% 14% 1	2075
	11th to 20th	Night	49% 4 7% 22% 16% 2	1867
	21st to end	Night	49% 3 10% 21% 15% 2	1761
May	1st to 10th	Night	45% 3 11% 21% 15% 4	2840
	11th to 20th	Night	48% 4 14% 23% 9% 2	2628
	21st to end	Night	33% 4 14% 24% 23% 3	2739
June	1st to 10th	Night	38% 5% 11% 27% 17% 1	1520
	11th to 20th	Night	37% 4 12% 26% 18% 2	1429
	21st to end	Night	37% 4 14% 28% 17% 1	1330
July	1st to 10th	Night	29% 5% 13% 26% 22% 5%	2229
	11th to 20th	Night	31% 4 13% 33% 18% 1	2557
	21st to end	Night	31% 7% 15% 28% 16% 2	2988
August	1st to 10th	Night	37% 5% 14% 27% 16% 2	2814
	11th to 20th	Night	36% 5% 16% 31% 12% 1	2812
	21st to end	Night	34% 5% 12% 33% 15% 1	3212
September	1st to 10th	Night	35% 5% 14% 29% 16% 1	1900
	11th to 20th	Night	30% 4 11% 33% 20% 1	2063
	21st to end	Night	44% 4 13% 27% 12%	2095
October	1st to 10th	Night	34% 4 10% 28% 23% 1	3800
	11th to 20th	Night	45% 4 11% 25% 15%	3779
	21st to end	Night	43% 5% 15% 27% 10%	4120
November	1st to 10th	Night	47% 4 12% 23% 13% 1	2581
	11th to 20th	Night	56% 4 13% 18% 8% 1	2800
	21st to end	Night	60% 5% 16% 14% 5% 1	2800
December	1st to 10th	Night	69% 5% 11% 13% 2	4580
	11th to 20th	Night	69% 4 9% 12% 5%	5208
	21st to end	Night	76% 3 9% 8% 5%	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	26% 19% 34% 15% 5% 1	207
	11th to 20th	Day	26% 27% 31% 12% 3	300
	21st to end	Day	28% 22% 32% 15% 3	330
February	1st to 10th	Day	33% 21% 32% 10% 3	300
	11th to 20th	Day	28% 25% 39% 7%	300
	21st to end	Day	36% 24% 31% 8% 1	249
March	1st to 10th	Day	30% 16% 32% 18% 5%	485
	11th to 20th	Day	27% 18% 34% 16% 4 1	500
	21st to end	Day	32% 14% 32% 16% 4 1	550
April	1st to 10th	Day	26% 19% 32% 16% 6%	500
	11th to 20th	Day	32% 16% 24% 17% 8% 2	492
	21st to end	Day	24% 14% 35% 18% 7% 1	480
May	1st to 10th	Day	24% 13% 30% 21% 9% 4	463
	11th to 20th	Day	19% 11% 29% 26% 11% 4	487
	21st to end	Day	15% 10% 30% 27% 12% 6%	529
June	1st to 10th	Day	17% 14% 31% 24% 11% 3	490
	11th to 20th	Day	14% 10% 30% 30% 13% 3	574
	21st to end	Day	18% 8% 33% 28% 11% 3	680
July	1st to 10th	Day	10% 5% 29% 31% 18% 6%	635
	11th to 20th	Day	14% 7% 29% 25% 17% 8%	484
	21st to end	Day	10% 5% 33% 29% 17% 6%	532
August	1st to 10th	Day	13% 7% 23% 29% 19% 8%	486
	11th to 20th	Day	14% 5% 22% 28% 19% 11%	479
	21st to end	Day	15% 6% 25% 27% 20% 7%	539
September	1st to 10th	Day	13% 10% 25% 24% 18% 10%	490
	11th to 20th	Day	11% 5% 21% 27% 21% 14%	490
	21st to end	Day	17% 14% 26% 23% 13% 7%	490
October	1st to 10th	Day	24% 14% 22% 22% 13% 5%	490
	11th to 20th	Day	25% 15% 35% 16% 8% 1	490
	21st to end	Day	32% 15% 29% 17% 6%	539
November	1st to 10th	Day	30% 18% 31% 15% 5%	279
	11th to 20th	Day	28% 22% 32% 14% 3 3	200
	21st to end	Day	28% 24% 35% 11% 3 1	200
December	1st to 10th	Day	32% 22% 31% 12% 4	200
	11th to 20th	Day	34% 23% 29% 13% 2	200
	21st to end	Day	31% 25% 31% 10% 2	220
January	1st to 10th	Night	23% 24% 34% 15% 4 1	800
	11th to 20th	Night	29% 21% 30% 18% 3	800
	21st to end	Night	21% 19% 37% 18% 6%	880
February	1st to 10th	Night	23% 25% 31% 20% 2	800
	11th to 20th	Night	24% 20% 34% 17% 4	783
	21st to end	Night	22% 23% 35% 17% 3	660
March	1st to 10th	Night	21% 18% 36% 21% 5%	800
	11th to 20th	Night	24% 18% 36% 17% 6%	800
	21st to end	Night	23% 16% 31% 22% 7% 1	880
April	1st to 10th	Night	27% 15% 33% 22% 3	800
	11th to 20th	Night	24% 15% 32% 20% 8% 1	683
	21st to end	Night	18% 13% 35% 27% 7%	587
May	1st to 10th	Night	19% 11% 29% 27% 12% 2	570
	11th to 20th	Night	16% 7% 31% 33% 12% 1	454
	21st to end	Night	11% 8% 25% 36% 19%	411
June	1st to 10th	Night	14% 9% 28% 36% 12% 1	380
	11th to 20th	Night	11% 8% 34% 34% 13% 1	380
	21st to end	Night	14% 6% 29% 36% 15%	380
July	1st to 10th	Night	5% 4 26% 44% 20% 2	370
	11th to 20th	Night	9% 5% 29% 35% 20% 2	414
	21st to end	Night	11% 2 25% 40% 20% 2	624
August	1st to 10th	Night	9% 4 22% 41% 22% 2	567
	11th to 20th	Night	9% 4 21% 36% 27% 3	564
	21st to end	Night	13% 5% 24% 33% 23% 3	676
September	1st to 10th	Night	12% 7% 23% 34% 19% 5%	760
	11th to 20th	Night	13% 5% 21% 33% 23% 5%	760
	21st to end	Night	19% 9% 21% 28% 19% 5%	760
October	1st to 10th	Night	17% 10% 28% 26% 16% 3	760
	11th to 20th	Night	23% 13% 29% 26% 9% 1	760
	21st to end	Night	25% 16% 31% 23% 4	836
November	1st to 10th	Night	22% 14% 30% 24% 9% 1	780
	11th to 20th	Night	23% 21% 28% 20% 6% 1	800
	21st to end	Night	28% 19% 30% 18% 5% 1	800
December	1st to 10th	Night	29% 21% 30% 16% 5%	800
	11th to 20th	Night	26% 23% 32% 15% 4	800
	21st to end	Night	31% 17% 33% 16% 3	880

Page generated at 2025-04-23 09:25:03. Version ac07

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