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 2024-02-25.

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% 10% 4 8% 6% 3 5% 5% 5% 7% 30%	78803
	Night	15% 11% 5% 8% 6% 3 5% 5% 5% 6% 31%	80458
Transparency	Day	54% 4 12% 20% 10% 1	78518
	Night	54% 4 11% 20% 10% 1	80279
Seeing	Day	21% 13% 30% 22% 11% 4	13821
	Night	20% 14% 30% 25% 10% 1	23067

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	16% 19% 8% 11% 7% 3 4 3 3 5% 22%	2531
	11th to 20th	Day	12% 19% 9% 16% 10% 4 4 5% 6% 4 11%	2840
	21st to end	Day	20% 20% 8% 12% 8% 4 5% 3 3 4 13%	3124
February	1st to 10th	Day	16% 17% 6% 11% 10% 5% 6% 4 4 5% 17%	1710
	11th to 20th	Day	14% 12% 7% 14% 8% 5% 5% 4 5% 6% 20%	1663
	21st to end	Day	18% 15% 6% 10% 8% 3 5% 4 5% 6% 21%	1448
March	1st to 10th	Day	15% 12% 6% 9% 7% 3 6% 4 5% 6% 29%	1963
	11th to 20th	Day	13% 12% 5% 8% 7% 3 5% 4 4 6% 31%	1980
	21st to end	Day	18% 11% 5% 7% 6% 3 5% 5% 5% 5% 31%	2178
April	1st to 10th	Day	18% 12% 6% 8% 6% 3 4 4 5% 6% 28%	2092
	11th to 20th	Day	18% 11% 3 6% 5% 3 5% 4 5% 6% 35%	2244
	21st to end	Day	21% 9% 4 6% 7% 3 4 4 4 6% 32%	2340
May	1st to 10th	Day	17% 8% 4 7% 6% 3 4 4 6% 7% 33%	2340
	11th to 20th	Day	15% 7% 4 7% 5% 3 4 5% 5% 6% 37%	2401
	21st to end	Day	12% 6% 2 6% 6% 3 5% 5% 6% 9% 41%	2725
June	1st to 10th	Day	14% 6% 3 5% 5% 3 5% 6% 6% 9% 38%	2520
	11th to 20th	Day	12% 5% 3 5% 6% 3 5% 6% 6% 10% 39%	2599
	21st to end	Day	14% 5% 2 5% 5% 3 5% 5% 6% 9% 40%	2700
July	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 5% 7% 9% 45%	2664
	11th to 20th	Day	11% 6% 2 6% 6% 3 6% 6% 7% 10% 37%	2473
	21st to end	Day	10% 6% 3 6% 6% 3 6% 6% 7% 10% 37%	2547
August	1st to 10th	Day	10% 5% 3 6% 6% 4 5% 7% 7% 10% 38%	2327
	11th to 20th	Day	10% 7% 3 6% 7% 4 6% 6% 6% 9% 36%	2303
	21st to end	Day	13% 6% 3 6% 5% 4 5% 5% 6% 9% 38%	2509
September	1st to 10th	Day	13% 5% 3 6% 7% 3 4 5% 6% 9% 39%	2160
	11th to 20th	Day	12% 5% 3 5% 6% 2 4 5% 6% 9% 42%	2002
	21st to end	Day	15% 7% 3 7% 5% 4 5% 5% 6% 8% 35%	1976
October	1st to 10th	Day	19% 10% 3 6% 6% 3 5% 5% 5% 6% 33%	1800
	11th to 20th	Day	22% 9% 5% 7% 7% 3 4 4 5% 5% 28%	1800
	21st to end	Day	25% 13% 3 7% 6% 3 5% 5% 5% 7% 21%	1980
November	1st to 10th	Day	16% 11% 4 8% 7% 3 5% 5% 6% 7% 30%	1626
	11th to 20th	Day	16% 13% 5% 8% 6% 4 4 6% 7% 7% 24%	1520
	21st to end	Day	19% 16% 8% 10% 6% 3 4 4 5% 7% 18%	1520
December	1st to 10th	Day	16% 18% 6% 13% 7% 4 6% 4 6% 6% 15%	1520
	11th to 20th	Day	18% 17% 7% 10% 7% 4 5% 6% 4 5% 17%	1906
	21st to end	Day	29% 18% 7% 11% 8% 4 5% 4 4 3 9%	2772
January	1st to 10th	Night	17% 17% 8% 12% 8% 4 4 5% 4 5% 16%	4397
	11th to 20th	Night	14% 14% 8% 14% 12% 6% 7% 6% 5% 5% 10%	4120
	21st to end	Night	21% 15% 9% 11% 7% 3 4 4 4 6% 17%	4532
February	1st to 10th	Night	15% 14% 6% 10% 9% 4 6% 5% 5% 5% 20%	2470
	11th to 20th	Night	14% 13% 6% 9% 8% 3 6% 5% 6% 6% 24%	2403
	21st to end	Night	16% 11% 6% 11% 7% 4 5% 5% 5% 6% 25%	1899
March	1st to 10th	Night	13% 7% 5% 8% 6% 4 7% 5% 6% 5% 32%	1984
	11th to 20th	Night	14% 11% 3 8% 7% 4 4 5% 4 5% 34%	1979
	21st to end	Night	17% 11% 4 6% 6% 3 4 3 5% 4 37%	2178
April	1st to 10th	Night	19% 9% 5% 6% 5% 2 5% 5% 5% 6% 33%	1868
	11th to 20th	Night	16% 10% 3 6% 6% 3 4 4 5% 6% 38%	1679
	21st to end	Night	15% 10% 4 7% 5% 3 5% 4 5% 7% 36%	1620
May	1st to 10th	Night	15% 8% 3 6% 4 3 5% 5% 6% 7% 38%	1620
	11th to 20th	Night	14% 5% 2 5% 5% 3 5% 4 5% 7% 43%	1491
	21st to end	Night	9% 5% 2 4 4 3 5% 4 6% 8% 51%	1550
June	1st to 10th	Night	11% 5% 2 5% 4 3 5% 5% 7% 8% 43%	1440
	11th to 20th	Night	11% 5% 3 4 4 4 6% 5% 6% 9% 44%	1354
	21st to end	Night	12% 5% 2 4 5% 4 4 4 5% 8% 47%	1260
July	1st to 10th	Night	5% 4 2 4 4 3 4 4 5% 8% 58%	1289
	11th to 20th	Night	9% 4 2 4 4 3 4 5% 5% 9% 50%	1483
	21st to end	Night	8% 5% 3 4 4 3 5% 5% 6% 8% 48%	1771
August	1st to 10th	Night	7% 5% 2 5% 6% 3 4 4 7% 8% 49%	1611
	11th to 20th	Night	7% 4 2 5% 5% 3 4 4 7% 9% 51%	1598
	21st to end	Night	10% 5% 3 5% 5% 3 4 5% 6% 7% 47%	1828
September	1st to 10th	Night	10% 6% 2 5% 5% 3 4 5% 7% 7% 46%	1800
	11th to 20th	Night	12% 5% 2 4 3 2 4 4 5% 8% 52%	1954
	21st to end	Night	15% 7% 3 5% 5% 3 4 4 6% 7% 41%	1984
October	1st to 10th	Night	15% 8% 3 7% 6% 3 4 4 5% 7% 39%	2160
	11th to 20th	Night	19% 9% 3 7% 6% 3 4 5% 5% 6% 34%	2160
	21st to end	Night	22% 10% 4 7% 6% 4 5% 5% 5% 6% 27%	2376
November	1st to 10th	Night	15% 8% 4 7% 5% 3 4 5% 5% 7% 36%	2448
	11th to 20th	Night	16% 12% 5% 9% 6% 3 5% 5% 6% 7% 27%	2660
	21st to end	Night	19% 13% 5% 8% 6% 4 5% 5% 5% 7% 24%	2660
December	1st to 10th	Night	19% 15% 6% 9% 7% 4 5% 5% 5% 6% 17%	2660
	11th to 20th	Night	17% 18% 7% 11% 6% 4 5% 4 5% 5% 19%	3288
	21st to end	Night	27% 20% 7% 11% 7% 3 4 4 4 4 9%	4884

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% 2 11% 11% 8%	2532
	11th to 20th	Day	75% 2 8% 10% 4	2840
	21st to end	Day	77% 2 8% 8% 4 1	3124
February	1st to 10th	Day	71% 4 7% 14% 4 1	1710
	11th to 20th	Day	65% 3 7% 16% 8% 2	1672
	21st to end	Day	65% 2 9% 15% 8% 1	1448
March	1st to 10th	Day	57% 4 11% 16% 10% 1	1966
	11th to 20th	Day	56% 4 10% 17% 11% 3	1980
	21st to end	Day	56% 4 9% 19% 11% 2	2178
April	1st to 10th	Day	57% 4 10% 17% 11% 1	2092
	11th to 20th	Day	51% 3 11% 21% 13% 1	2244
	21st to end	Day	54% 4 9% 20% 11% 2	2340
May	1st to 10th	Day	50% 4 11% 23% 12% 1	2340
	11th to 20th	Day	48% 4 10% 24% 12% 1	2405
	21st to end	Day	40% 4 16% 23% 15% 2	2725
June	1st to 10th	Day	43% 5% 12% 24% 15% 1	2520
	11th to 20th	Day	40% 5% 13% 25% 15% 1	2599
	21st to end	Day	41% 4 13% 25% 15%	2700
July	1st to 10th	Day	35% 5% 13% 27% 18% 3	2592
	11th to 20th	Day	40% 5% 15% 28% 11% 1	2398
	21st to end	Day	41% 7% 12% 24% 14% 2	2405
August	1st to 10th	Day	40% 6% 16% 26% 11% 2	2314
	11th to 20th	Day	44% 5% 16% 25% 11%	2303
	21st to end	Day	42% 5% 13% 27% 12% 1	2509
September	1st to 10th	Day	42% 5% 14% 26% 11% 1	2160
	11th to 20th	Day	38% 5% 13% 30% 13% 1	2002
	21st to end	Day	45% 5% 14% 26% 9% 1	1976
October	1st to 10th	Day	52% 5% 12% 18% 12% 2	1800
	11th to 20th	Day	57% 4 11% 19% 8% 1	1800
	21st to end	Day	63% 4 11% 15% 6%	1980
November	1st to 10th	Day	53% 4 12% 21% 8% 1	1626
	11th to 20th	Day	58% 5% 13% 15% 8% 1	1520
	21st to end	Day	66% 3 12% 13% 5%	1520
December	1st to 10th	Day	71% 4 10% 11% 4	1520
	11th to 20th	Day	69% 4 11% 10% 6%	1906
	21st to end	Day	83% 2 6% 8% 2	2772
January	1st to 10th	Night	69% 3 10% 12% 5% 1	4397
	11th to 20th	Night	76% 3 9% 9% 3	4120
	21st to end	Night	70% 2 11% 13% 4 1	4532
February	1st to 10th	Night	64% 4 9% 18% 4 1	2470
	11th to 20th	Night	60% 3 10% 17% 9% 1	2405
	21st to end	Night	60% 3 11% 18% 9%	1899
March	1st to 10th	Night	52% 3 12% 21% 10% 2	1994
	11th to 20th	Night	53% 3 9% 22% 12% 2	1980
	21st to end	Night	52% 3 7% 21% 15% 3	2178
April	1st to 10th	Night	52% 3 8% 21% 15% 1	1868
	11th to 20th	Night	47% 5% 8% 22% 16% 2	1679
	21st to end	Night	49% 3 9% 22% 15% 2	1620
May	1st to 10th	Night	46% 4 8% 24% 15% 3	1620
	11th to 20th	Night	41% 4 10% 26% 15% 4	1492
	21st to end	Night	32% 5% 14% 27% 18% 4	1551
June	1st to 10th	Night	37% 5% 11% 27% 18% 1	1440
	11th to 20th	Night	37% 4 11% 26% 19% 2	1354
	21st to end	Night	36% 4 15% 28% 17% 1	1260
July	1st to 10th	Night	26% 5% 12% 28% 26% 4	1253
	11th to 20th	Night	32% 4 13% 32% 17% 2	1435
	21st to end	Night	32% 5% 16% 28% 16% 3	1672
August	1st to 10th	Night	33% 5% 17% 32% 12% 2	1600
	11th to 20th	Night	29% 5% 15% 34% 15% 1	1598
	21st to end	Night	36% 5% 12% 31% 15% 1	1828
September	1st to 10th	Night	36% 5% 14% 28% 15% 1	1800
	11th to 20th	Night	31% 4 11% 33% 19% 1	1954
	21st to end	Night	42% 4 14% 27% 13% 1	1984
October	1st to 10th	Night	46% 4 10% 24% 15% 1	2160
	11th to 20th	Night	51% 3 12% 24% 9% 1	2160
	21st to end	Night	57% 4 9% 19% 10%	2376
November	1st to 10th	Night	47% 4 12% 23% 13% 1	2448
	11th to 20th	Night	56% 4 13% 18% 8% 1	2660
	21st to end	Night	59% 5% 16% 14% 5% 1	2660
December	1st to 10th	Night	66% 4 12% 14% 4	2660
	11th to 20th	Night	66% 5% 11% 14% 4	3288
	21st to end	Night	81% 2 8% 7% 2	4884

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% 18% 34% 16% 5% 1	186
	11th to 20th	Day	26% 27% 32% 11% 4	270
	21st to end	Day	28% 22% 31% 16% 3	297
February	1st to 10th	Day	31% 22% 34% 10% 3	270
	11th to 20th	Day	24% 27% 41% 8%	270
	21st to end	Day	36% 26% 29% 8% 1	210
March	1st to 10th	Day	28% 17% 30% 19% 5%	406
	11th to 20th	Day	24% 17% 35% 18% 4 1	420
	21st to end	Day	31% 13% 33% 17% 5% 1	462
April	1st to 10th	Day	24% 20% 32% 18% 6%	420
	11th to 20th	Day	30% 16% 25% 17% 9% 2	412
	21st to end	Day	24% 14% 35% 17% 8% 2	420
May	1st to 10th	Day	24% 14% 30% 20% 8% 4	423
	11th to 20th	Day	19% 12% 29% 26% 10% 4	447
	21st to end	Day	15% 9% 29% 27% 13% 7%	485
June	1st to 10th	Day	16% 13% 33% 24% 11% 3	450
	11th to 20th	Day	15% 11% 28% 30% 13% 3	529
	21st to end	Day	17% 8% 32% 29% 12% 3	630
July	1st to 10th	Day	10% 5% 28% 32% 19% 7%	587
	11th to 20th	Day	15% 7% 27% 25% 18% 8%	444
	21st to end	Day	10% 5% 34% 30% 15% 6%	488
August	1st to 10th	Day	11% 7% 23% 30% 20% 9%	446
	11th to 20th	Day	13% 4 23% 29% 20% 11%	439
	21st to end	Day	16% 6% 24% 27% 20% 8%	495
September	1st to 10th	Day	14% 9% 25% 26% 17% 9%	450
	11th to 20th	Day	12% 6% 23% 27% 20% 12%	450
	21st to end	Day	17% 14% 25% 23% 14% 8%	450
October	1st to 10th	Day	24% 14% 21% 22% 14% 5%	450
	11th to 20th	Day	26% 15% 36% 15% 7% 1	450
	21st to end	Day	33% 15% 29% 17% 6%	495
November	1st to 10th	Day	29% 18% 31% 15% 6%	252
	11th to 20th	Day	27% 19% 32% 14% 3 3	180
	21st to end	Day	27% 22% 36% 12% 3 1	180
December	1st to 10th	Day	32% 22% 31% 12% 4	180
	11th to 20th	Day	34% 20% 31% 14% 2	180
	21st to end	Day	30% 26% 32% 11% 1	198
January	1st to 10th	Night	23% 24% 33% 15% 4 1	760
	11th to 20th	Night	29% 20% 29% 18% 3	760
	21st to end	Night	21% 17% 37% 19% 6%	836
February	1st to 10th	Night	23% 24% 31% 20% 2	760
	11th to 20th	Night	23% 20% 35% 18% 4	743
	21st to end	Night	22% 23% 33% 18% 3	611
March	1st to 10th	Night	20% 17% 36% 23% 5%	720
	11th to 20th	Night	24% 18% 34% 18% 6%	720
	21st to end	Night	22% 15% 31% 23% 8% 1	792
April	1st to 10th	Night	25% 16% 32% 23% 4	720
	11th to 20th	Night	22% 15% 32% 21% 8% 1	615
	21st to end	Night	17% 12% 34% 28% 8%	540
May	1st to 10th	Night	19% 11% 29% 26% 13% 2	540
	11th to 20th	Night	16% 6% 33% 33% 11% 1	430
	21st to end	Night	11% 7% 26% 36% 20%	389
June	1st to 10th	Night	13% 9% 29% 36% 13% 1	360
	11th to 20th	Night	12% 8% 34% 33% 14% 1	360
	21st to end	Night	14% 4 29% 37% 15%	360
July	1st to 10th	Night	6% 3 25% 43% 21% 2	350
	11th to 20th	Night	9% 5% 29% 35% 20% 2	391
	21st to end	Night	11% 2 26% 41% 19% 2	591
August	1st to 10th	Night	8% 4 21% 41% 23% 3	537
	11th to 20th	Night	8% 4 20% 37% 28% 3	534
	21st to end	Night	13% 5% 24% 32% 23% 3	640
September	1st to 10th	Night	13% 6% 23% 34% 19% 5%	720
	11th to 20th	Night	13% 5% 23% 33% 22% 5%	720
	21st to end	Night	18% 9% 21% 27% 19% 5%	720
October	1st to 10th	Night	18% 10% 28% 26% 16% 3	720
	11th to 20th	Night	23% 13% 30% 25% 8% 1	720
	21st to end	Night	26% 16% 31% 23% 4	792
November	1st to 10th	Night	22% 13% 30% 25% 9% 1	740
	11th to 20th	Night	23% 21% 28% 20% 7% 1	760
	21st to end	Night	28% 19% 30% 18% 5% 1	760
December	1st to 10th	Night	29% 20% 29% 16% 4	760
	11th to 20th	Night	26% 23% 32% 15% 4	760
	21st to end	Night	32% 18% 33% 15% 2	836

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