Grenville Base Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grenville Base Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	138092
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	134603
Transparency	Day	38% 7% 12% 23% 16% 4	141691
	Night	38% 6% 11% 23% 18% 4	139076
Seeing	Day	19% 14% 26% 23% 13% 5%	17145
	Night	17% 11% 30% 29% 12% 1	26699

By Month

Percentage Cloud Cover SE (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	14% 4 2 6% 5% 4 5% 4 5% 5% 47%	4938
	11th to 20th	Day	14% 8% 3 6% 3 3 3 2 3 4 50%	5280
	21st to end	Day	14% 6% 2 6% 5% 4 4 4 5% 6% 43%	5780
February	1st to 10th	Day	13% 7% 2 6% 5% 5% 4 5% 5% 6% 41%	1890
	11th to 20th	Day	19% 8% 2 5% 5% 4 4 3 5% 6% 39%	1873
	21st to end	Day	18% 8% 2 6% 5% 5% 4 3 5% 5% 41%	1730
March	1st to 10th	Day	18% 6% 2 5% 5% 4 4 3 4 7% 42%	5040
	11th to 20th	Day	10% 6% 2 6% 3 4 4 4 4 6% 50%	5090
	21st to end	Day	16% 6% 2 6% 4 4 5% 3 5% 7% 40%	5624
April	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 5% 7% 43%	2520
	11th to 20th	Day	13% 5% 2 6% 5% 3 4 4 4 6% 47%	2494
	21st to end	Day	18% 6% 2 6% 4 4 5% 3 6% 6% 41%	2520
May	1st to 10th	Day	17% 7% 2 6% 4 5% 5% 4 6% 9% 35%	7592
	11th to 20th	Day	18% 5% 2 5% 4 4 5% 4 6% 9% 37%	7863
	21st to end	Day	22% 5% 2 6% 4 3 4 3 6% 7% 38%	9045
June	1st to 10th	Day	10% 5% 2 5% 5% 4 4 5% 6% 9% 44%	2940
	11th to 20th	Day	11% 5% 3 6% 4 4 5% 5% 8% 8% 42%	2940
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 6% 11% 45%	2940
July	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 5% 7% 10% 44%	6500
	11th to 20th	Day	8% 6% 2 7% 6% 6% 6% 6% 9% 12% 32%	6092
	21st to end	Day	8% 5% 2 6% 6% 5% 6% 5% 8% 9% 40%	6424
August	1st to 10th	Day	16% 5% 2 6% 4 5% 6% 4 8% 10% 34%	4234
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 6% 8% 9% 40%	4042
	21st to end	Day	11% 4 2 5% 4 5% 4 4 6% 8% 49%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 4 7% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 3 4 5% 8% 49%	2399
	21st to end	Day	22% 8% 2 5% 4 4 3 4 5% 5% 38%	2085
October	1st to 10th	Day	17% 4 2 5% 4 4 3 3 5% 6% 48%	2000
	11th to 20th	Day	16% 4 1 4 4 3 4 3 4 5% 50%	2000
	21st to end	Day	14% 5% 2 4 4 3 3 3 4 7% 51%	2200
November	1st to 10th	Day	11% 6% 1 5% 4 3 3 3 5% 8% 51%	1991
	11th to 20th	Day	14% 6% 1 4 3 3 4 4 5% 6% 50%	1836
	21st to end	Day	15% 5% 2 4 3 4 4 4 5% 7% 46%	1680
December	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 6% 50%	2680
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 4 6% 42%	3070
	21st to end	Day	17% 8% 3 6% 4 4 5% 3 6% 7% 37%	4048
January	1st to 10th	Night	11% 6% 2 5% 4 4 5% 4 6% 6% 45%	7951
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 4 5% 52%	7680
	21st to end	Night	13% 6% 2 6% 3 3 4 3 5% 5% 49%	8401
February	1st to 10th	Night	14% 7% 2 7% 5% 5% 5% 4 5% 6% 41%	2730
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 5% 6% 42%	2631
	21st to end	Night	16% 8% 2 6% 4 4 4 3 5% 6% 43%	2076
March	1st to 10th	Night	17% 6% 2 5% 3 3 4 3 5% 5% 47%	5028
	11th to 20th	Night	10% 6% 3 4 4 3 4 3 5% 7% 50%	5090
	21st to end	Night	11% 7% 2 6% 4 4 5% 4 6% 7% 44%	5548
April	1st to 10th	Night	15% 8% 2 5% 5% 5% 4 3 6% 6% 42%	2100
	11th to 20th	Night	13% 5% 2 5% 5% 4 4 4 5% 9% 45%	2075
	21st to end	Night	16% 4 2 4 4 4 4 4 6% 7% 44%	2100
May	1st to 10th	Night	19% 8% 3 7% 4 4 5% 4 5% 7% 33%	5303
	11th to 20th	Night	21% 6% 2 6% 4 4 4 4 6% 8% 35%	4996
	21st to end	Night	23% 6% 3 7% 4 4 4 3 5% 7% 34%	5108
June	1st to 10th	Night	9% 5% 3 5% 4 5% 4 4 7% 8% 46%	1680
	11th to 20th	Night	12% 5% 2 6% 5% 4 5% 4 6% 8% 42%	1680
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 6% 10% 44%	1680
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 6% 4 7% 9% 41%	3678
	11th to 20th	Night	9% 7% 4 8% 7% 6% 6% 5% 8% 10% 30%	3957
	21st to end	Night	7% 6% 3 7% 4 5% 5% 5% 7% 10% 41%	4427
August	1st to 10th	Night	16% 5% 3 7% 5% 6% 5% 4 7% 9% 33%	2915
	11th to 20th	Night	10% 6% 3 8% 5% 7% 5% 4 6% 9% 38%	2972
	21st to end	Night	12% 5% 2 5% 3 4 4 4 5% 8% 49%	3586
September	1st to 10th	Night	14% 5% 2 5% 5% 4 4 3 6% 6% 46%	2000
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 49%	2000
	21st to end	Night	21% 7% 3 6% 4 4 4 3 4 6% 38%	2310
October	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 4 5% 48%	2400
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 5% 52%	2400
	21st to end	Night	14% 5% 2 5% 3 4 3 3 4 6% 51%	2640
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 54%	2524
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 6% 50%	2782
	21st to end	Night	14% 4 2 5% 4 4 4 4 6% 7% 48%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 53%	4720
	11th to 20th	Night	20% 6% 2 5% 4 4 4 3 5% 7% 41%	5345
	21st to end	Night	16% 7% 2 6% 4 5% 5% 4 6% 8% 36%	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	37% 5% 10% 25% 18% 6%	4939
	11th to 20th	Day	37% 4 6% 22% 19% 12%	5280
	21st to end	Day	40% 5% 9% 30% 13% 3	5780
February	1st to 10th	Day	41% 7% 12% 24% 13% 3	1890
	11th to 20th	Day	44% 4 11% 22% 17% 2	1882
	21st to end	Day	45% 5% 12% 20% 15% 4	1730
March	1st to 10th	Day	41% 5% 9% 20% 16% 8%	5040
	11th to 20th	Day	33% 6% 11% 25% 20% 6%	5090
	21st to end	Day	42% 7% 11% 18% 17% 6%	5624
April	1st to 10th	Day	39% 6% 12% 20% 18% 5%	2520
	11th to 20th	Day	36% 6% 11% 21% 20% 5%	2494
	21st to end	Day	42% 7% 10% 23% 14% 4	2520
May	1st to 10th	Day	43% 9% 13% 20% 13% 1	7627
	11th to 20th	Day	41% 8% 12% 27% 11% 1	7867
	21st to end	Day	44% 8% 11% 18% 17% 3	9061
June	1st to 10th	Day	34% 9% 15% 23% 17% 2	2940
	11th to 20th	Day	36% 10% 15% 23% 14% 2	2940
	21st to end	Day	31% 11% 16% 25% 17% 1	2940
July	1st to 10th	Day	33% 11% 17% 20% 14% 4	6430
	11th to 20th	Day	40% 12% 17% 23% 7% 1	6020
	21st to end	Day	36% 8% 10% 23% 21% 1	6240
August	1st to 10th	Day	42% 9% 18% 24% 7%	4221
	11th to 20th	Day	35% 11% 13% 27% 14% 1	4081
	21st to end	Day	32% 7% 15% 27% 17% 2	4312
September	1st to 10th	Day	36% 8% 13% 18% 21% 5%	2400
	11th to 20th	Day	34% 6% 9% 28% 20% 3	2399
	21st to end	Day	46% 7% 10% 19% 15% 2	2085
October	1st to 10th	Day	35% 5% 7% 18% 29% 5%	3260
	11th to 20th	Day	25% 4 7% 28% 25% 10%	3240
	21st to end	Day	22% 3 10% 30% 26% 8%	3538
November	1st to 10th	Day	32% 4 13% 24% 21% 5%	1991
	11th to 20th	Day	34% 6% 10% 22% 21% 5%	1836
	21st to end	Day	37% 6% 11% 27% 17% 2	1680
December	1st to 10th	Day	34% 6% 12% 27% 17% 4	2680
	11th to 20th	Day	42% 4 14% 26% 10% 3	3066
	21st to end	Day	42% 7% 15% 21% 14% 1	4048
January	1st to 10th	Night	37% 6% 9% 25% 20% 3	7952
	11th to 20th	Night	35% 4 7% 22% 24% 8%	7680
	21st to end	Night	36% 4 8% 30% 18% 3	8401
February	1st to 10th	Night	41% 6% 13% 22% 15% 2	2730
	11th to 20th	Night	43% 4 11% 22% 16% 5%	2633
	21st to end	Night	42% 5% 11% 21% 16% 5%	2076
March	1st to 10th	Night	38% 6% 9% 24% 15% 8%	5028
	11th to 20th	Night	32% 6% 11% 23% 21% 6%	5090
	21st to end	Night	38% 6% 10% 22% 19% 5%	5548
April	1st to 10th	Night	41% 7% 10% 20% 19% 4	2100
	11th to 20th	Night	35% 7% 12% 21% 19% 5%	2075
	21st to end	Night	36% 9% 12% 24% 14% 4	2100
May	1st to 10th	Night	49% 7% 12% 19% 11% 3	5330
	11th to 20th	Night	44% 8% 12% 24% 11% 1	4997
	21st to end	Night	49% 8% 11% 18% 12% 3	5115
June	1st to 10th	Night	34% 9% 13% 24% 18% 2	1680
	11th to 20th	Night	38% 9% 15% 22% 14% 2	1680
	21st to end	Night	34% 8% 16% 22% 18% 3	1680
July	1st to 10th	Night	38% 11% 16% 17% 15% 3	3640
	11th to 20th	Night	46% 12% 17% 16% 8% 1	3910
	21st to end	Night	36% 9% 15% 23% 18%	4297
August	1st to 10th	Night	45% 9% 16% 21% 8% 1	2904
	11th to 20th	Night	40% 8% 12% 24% 14% 1	2999
	21st to end	Night	32% 6% 13% 30% 17% 2	3586
September	1st to 10th	Night	38% 7% 10% 21% 19% 5%	2000
	11th to 20th	Night	34% 6% 10% 25% 22% 3	2000
	21st to end	Night	48% 7% 10% 18% 15% 3	2310
October	1st to 10th	Night	39% 5% 10% 20% 21% 5%	3920
	11th to 20th	Night	24% 4 11% 22% 31% 8%	3899
	21st to end	Night	22% 4 9% 31% 26% 7%	4252
November	1st to 10th	Night	32% 4 10% 25% 22% 6%	2524
	11th to 20th	Night	35% 5% 8% 24% 22% 6%	2782
	21st to end	Night	35% 5% 11% 25% 20% 4	2940
December	1st to 10th	Night	31% 5% 13% 28% 21% 2	4720
	11th to 20th	Night	43% 5% 12% 19% 16% 5%	5348
	21st to end	Night	42% 9% 15% 20% 14% 1	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	32% 24% 28% 14% 1	270
	11th to 20th	Day	33% 26% 27% 13% 1	330
	21st to end	Day	23% 33% 30% 12% 2 1	363
February	1st to 10th	Day	22% 28% 34% 12% 4	330
	11th to 20th	Day	39% 24% 30% 5% 2	360
	21st to end	Day	28% 23% 31% 17% 2	446
March	1st to 10th	Day	24% 24% 30% 19% 3	540
	11th to 20th	Day	25% 24% 29% 15% 5% 1	540
	21st to end	Day	22% 21% 35% 17% 5% 1	594
April	1st to 10th	Day	23% 26% 25% 19% 6% 1	540
	11th to 20th	Day	18% 20% 29% 18% 12% 2	532
	21st to end	Day	23% 16% 32% 18% 10% 1	540
May	1st to 10th	Day	22% 19% 29% 18% 9% 3	543
	11th to 20th	Day	20% 12% 30% 22% 13% 4	567
	21st to end	Day	22% 8% 23% 24% 17% 5%	617
June	1st to 10th	Day	11% 8% 29% 29% 18% 5%	570
	11th to 20th	Day	13% 10% 27% 30% 15% 5%	570
	21st to end	Day	10% 6% 27% 32% 18% 6%	570
July	1st to 10th	Day	12% 4 17% 31% 25% 11%	565
	11th to 20th	Day	5% 3 21% 33% 28% 10%	544
	21st to end	Day	11% 4 20% 34% 23% 9%	576
August	1st to 10th	Day	15% 3 20% 25% 26% 11%	526
	11th to 20th	Day	13% 4 19% 32% 22% 11%	519
	21st to end	Day	9% 4 20% 33% 23% 12%	583
September	1st to 10th	Day	15% 8% 21% 27% 18% 11%	530
	11th to 20th	Day	14% 4 20% 30% 20% 12%	530
	21st to end	Day	28% 11% 20% 25% 13% 3	530
October	1st to 10th	Day	20% 7% 20% 26% 19% 7%	530
	11th to 20th	Day	18% 15% 24% 27% 13% 2	530
	21st to end	Day	16% 13% 34% 24% 10% 3	583
November	1st to 10th	Day	16% 15% 33% 24% 9% 3	499
	11th to 20th	Day	20% 16% 35% 20% 9% 1	376
	21st to end	Day	23% 23% 29% 19% 5% 1	220
December	1st to 10th	Day	20% 22% 37% 18% 3	220
	11th to 20th	Day	21% 24% 30% 20% 5%	220
	21st to end	Day	24% 23% 31% 16% 5% 1	242
January	1st to 10th	Night	20% 20% 39% 18% 3	840
	11th to 20th	Night	23% 16% 34% 23% 4	840
	21st to end	Night	19% 16% 44% 20% 1	924
February	1st to 10th	Night	19% 16% 39% 24% 2	840
	11th to 20th	Night	25% 21% 36% 16% 2	823
	21st to end	Night	25% 17% 39% 17% 3	692
March	1st to 10th	Night	20% 19% 37% 21% 3	840
	11th to 20th	Night	23% 19% 32% 21% 5% 1	840
	21st to end	Night	19% 19% 39% 19% 4	924
April	1st to 10th	Night	19% 19% 36% 22% 4	840
	11th to 20th	Night	16% 14% 35% 26% 7%	831
	21st to end	Night	18% 11% 34% 29% 8%	840
May	1st to 10th	Night	22% 9% 27% 31% 11%	657
	11th to 20th	Night	20% 6% 29% 33% 11% 1	539
	21st to end	Night	17% 3 23% 38% 18% 1	455
June	1st to 10th	Night	9% 4 27% 40% 20%	420
	11th to 20th	Night	12% 5% 23% 40% 18% 1	420
	21st to end	Night	9% 2 23% 47% 18% 2	420
July	1st to 10th	Night	11% 3 17% 38% 27% 3	410
	11th to 20th	Night	6% 1 14% 40% 35% 3	534
	21st to end	Night	9% 3 16% 41% 28% 3	657
August	1st to 10th	Night	12% 2 12% 41% 30% 3	597
	11th to 20th	Night	10% 3 14% 41% 29% 3	652
	21st to end	Night	11% 3 17% 40% 25% 4	880
September	1st to 10th	Night	16% 2 19% 32% 27% 4	800
	11th to 20th	Night	12% 4 18% 36% 28% 3	800
	21st to end	Night	24% 5% 21% 31% 18% 1	800
October	1st to 10th	Night	20% 5% 23% 29% 20% 4	800
	11th to 20th	Night	17% 11% 26% 32% 13% 1	800
	21st to end	Night	16% 11% 32% 29% 11%	880
November	1st to 10th	Night	14% 11% 34% 30% 10% 1	820
	11th to 20th	Night	18% 14% 34% 27% 7% 1	840
	21st to end	Night	17% 15% 33% 28% 7% 1	840
December	1st to 10th	Night	20% 16% 35% 22% 6%	840
	11th to 20th	Night	20% 12% 39% 25% 4	840
	21st to end	Night	20% 15% 34% 29% 3	924

Page generated at 2026-07-13 03:14:50. Version ac07

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