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 2024-10-25.

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% 11% 4 8% 5% 5% 5% 4 6% 7% 31%	101834
	Night	15% 11% 4 9% 5% 5% 4 4 5% 6% 33%	96955
Transparency	Day	52% 4 12% 21% 10% 1	101600
	Night	51% 4 12% 21% 11% 1	96810
Seeing	Day	21% 13% 30% 21% 11% 4	14814
	Night	20% 14% 30% 25% 10% 1	23840

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% 20% 7% 12% 6% 4 3 3 3 5% 21%	2531
	11th to 20th	Day	11% 20% 9% 18% 8% 5% 4 4 6% 4 11%	2840
	21st to end	Day	20% 21% 7% 13% 7% 5% 4 3 3 4 12%	3124
February	1st to 10th	Day	15% 18% 5% 12% 9% 6% 5% 3 5% 4 17%	1710
	11th to 20th	Day	13% 14% 6% 16% 6% 6% 4 3 6% 6% 20%	1663
	21st to end	Day	17% 15% 5% 11% 7% 5% 5% 3 5% 6% 21%	1498
March	1st to 10th	Day	20% 14% 5% 9% 5% 4 4 2 4 5% 27%	3418
	11th to 20th	Day	11% 15% 6% 10% 5% 5% 5% 4 5% 6% 28%	3480
	21st to end	Day	16% 11% 3 9% 5% 5% 5% 4 6% 5% 31%	3815
April	1st to 10th	Day	17% 14% 4 9% 5% 4 4 3 5% 7% 28%	2209
	11th to 20th	Day	18% 12% 2 7% 4 4 4 3 5% 6% 34%	2371
	21st to end	Day	20% 10% 3 7% 6% 5% 4 3 4 6% 32%	2470
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% 14% 3 8% 5% 4 5% 4 5% 7% 21%	2027
November	1st to 10th	Day	15% 11% 4 8% 6% 4 4 4 6% 7% 30%	1626
	11th to 20th	Day	16% 14% 5% 9% 5% 5% 4 4 7% 7% 24%	1520
	21st to end	Day	19% 17% 7% 10% 6% 4 4 3 6% 7% 18%	1520
December	1st to 10th	Day	16% 20% 6% 14% 7% 5% 5% 3 5% 6% 15%	1520
	11th to 20th	Day	18% 18% 6% 11% 6% 5% 5% 5% 5% 5% 17%	1906
	21st to end	Day	28% 19% 7% 12% 7% 5% 4 3 4 3 9%	2772
January	1st to 10th	Night	17% 18% 8% 13% 6% 5% 4 4 5% 5% 16%	4397
	11th to 20th	Night	14% 15% 8% 16% 10% 7% 7% 5% 5% 4 10%	4120
	21st to end	Night	21% 15% 9% 12% 5% 5% 3 3 4 6% 17%	4532
February	1st to 10th	Night	15% 15% 6% 11% 8% 6% 5% 4 5% 5% 20%	2470
	11th to 20th	Night	14% 14% 6% 10% 7% 4 6% 3 6% 6% 24%	2403
	21st to end	Night	15% 12% 6% 12% 6% 5% 5% 4 5% 6% 25%	1952
March	1st to 10th	Night	16% 11% 4 8% 5% 5% 5% 3 6% 6% 31%	3422
	11th to 20th	Night	11% 11% 3 9% 5% 4 4 4 5% 7% 36%	3479
	21st to end	Night	16% 9% 3 7% 5% 5% 4 3 5% 5% 39%	3817
April	1st to 10th	Night	19% 12% 3 7% 5% 4 4 4 5% 6% 32%	1971
	11th to 20th	Night	17% 10% 2 7% 5% 4 4 3 5% 6% 37%	1773
	21st to end	Night	15% 10% 3 8% 4 5% 4 3 6% 7% 36%	1710
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	22% 10% 3 8% 5% 5% 4 4 5% 6% 27%	2435
November	1st to 10th	Night	15% 9% 4 8% 4 4 4 4 6% 6% 36%	2448
	11th to 20th	Night	15% 13% 4 10% 5% 5% 5% 4 6% 7% 27%	2660
	21st to end	Night	19% 13% 5% 8% 5% 5% 5% 4 5% 7% 24%	2660
December	1st to 10th	Night	19% 17% 6% 10% 6% 5% 5% 4 5% 6% 17%	2660
	11th to 20th	Night	16% 19% 6% 11% 6% 5% 4 3 5% 5% 19%	3288
	21st to end	Night	27% 21% 7% 13% 5% 4 4 3 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 10% 15% 8% 1	1498
March	1st to 10th	Day	61% 3 10% 19% 7% 1	3421
	11th to 20th	Day	57% 4 12% 19% 7% 2	3480
	21st to end	Day	53% 3 9% 21% 11% 2	3815
April	1st to 10th	Day	57% 4 10% 18% 11% 1	2209
	11th to 20th	Day	51% 3 11% 21% 13% 1	2371
	21st to end	Day	54% 4 9% 20% 11% 2	2470
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	51% 4 12% 19% 12% 2	1900
	11th to 20th	Day	55% 4 12% 19% 9% 1	1900
	21st to end	Day	62% 4 11% 16% 6%	2027
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	59% 3 11% 18% 8%	1952
March	1st to 10th	Night	53% 4 13% 22% 7% 1	3432
	11th to 20th	Night	47% 3 14% 22% 12% 2	3480
	21st to end	Night	47% 2 8% 24% 17% 2	3817
April	1st to 10th	Night	53% 3 8% 20% 14% 1	1971
	11th to 20th	Night	48% 5% 8% 22% 16% 2	1773
	21st to end	Night	49% 3 9% 21% 15% 2	1710
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	45% 4 10% 24% 16% 1	2280
	11th to 20th	Night	50% 3 11% 24% 10%	2280
	21st to end	Night	57% 4 9% 20% 10%	2435
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	37% 24% 30% 8% 1	225
March	1st to 10th	Day	29% 16% 31% 19% 5%	446
	11th to 20th	Day	26% 17% 35% 16% 4 1	460
	21st to end	Day	31% 14% 33% 17% 5% 1	506
April	1st to 10th	Day	25% 19% 33% 17% 6%	460
	11th to 20th	Day	33% 16% 25% 17% 8% 2	452
	21st to end	Day	25% 13% 35% 18% 7% 2	460
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% 18% 6%	514
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	23% 23% 34% 17% 3	628
March	1st to 10th	Night	20% 17% 36% 22% 5%	760
	11th to 20th	Night	24% 18% 35% 18% 6%	760
	21st to end	Night	22% 16% 32% 22% 8% 1	836
April	1st to 10th	Night	26% 15% 33% 23% 3	760
	11th to 20th	Night	23% 14% 32% 21% 8% 1	649
	21st to end	Night	18% 12% 35% 27% 8%	570
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	26% 16% 31% 23% 4	812
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

Page generated at 2024-10-22 20:21:04. Version ac07

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