Gordon MacMillan Southam Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gordon MacMillan Southam Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	22% 8% 3 7% 4 4 4 3 5% 6% 34%	135431
	Night	27% 9% 3 7% 4 4 4 3 5% 6% 30%	135786
Transparency	Day	54% 7% 14% 17% 8% 1	135138
	Night	57% 6% 12% 16% 8% 1	135657
Seeing	Day	22% 5% 15% 26% 25% 7%	17224
	Night	29% 5% 13% 25% 22% 6%	25535

By Month

Percentage Cloud Cover NW (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	41% 12% 4 8% 5% 5% 4 3 5% 5% 9%	4125
	11th to 20th	Day	22% 9% 3 5% 4 4 4 3 8% 10% 28%	4119
	21st to end	Day	42% 8% 3 6% 4 3 3 2 4 5% 20%	5196
February	1st to 10th	Day	32% 11% 4 8% 5% 5% 4 3 6% 6% 16%	1890
	11th to 20th	Day	29% 10% 3 7% 6% 4 5% 3 5% 4 24%	1845
	21st to end	Day	26% 13% 4 8% 6% 4 5% 3 4 5% 22%	1567
March	1st to 10th	Day	25% 14% 4 11% 6% 5% 5% 4 4 5% 17%	4791
	11th to 20th	Day	25% 13% 4 7% 4 4 4 3 4 5% 26%	5110
	21st to end	Day	30% 12% 3 9% 5% 5% 4 4 4 5% 18%	5736
April	1st to 10th	Day	25% 11% 3 8% 5% 5% 5% 4 6% 7% 21%	2520
	11th to 20th	Day	20% 9% 4 7% 6% 4 4 4 6% 6% 29%	2605
	21st to end	Day	22% 9% 4 7% 5% 4 4 3 5% 7% 31%	2786
May	1st to 10th	Day	13% 8% 3 6% 4 4 4 3 4 6% 45%	6335
	11th to 20th	Day	16% 8% 3 8% 4 4 4 4 6% 8% 33%	6277
	21st to end	Day	21% 8% 3 7% 4 5% 5% 4 7% 8% 29%	7079
June	1st to 10th	Day	20% 6% 2 8% 5% 4 4 4 6% 7% 34%	3000
	11th to 20th	Day	19% 8% 3 8% 5% 4 5% 4 6% 7% 30%	3000
	21st to end	Day	14% 7% 2 7% 5% 5% 4 4 6% 7% 39%	3000
July	1st to 10th	Day	8% 4 2 4 3 3 3 3 4 6% 60%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 4 6% 71%	6780
	21st to end	Day	8% 4 2 4 2 3 3 3 4 7% 61%	7021
August	1st to 10th	Day	6% 3 1 3 2 3 3 2 4 5% 68%	4489
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 3 4 7% 47%	4156
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 7% 44%	4573
September	1st to 10th	Day	12% 5% 3 6% 5% 4 4 3 4 7% 48%	2400
	11th to 20th	Day	17% 8% 3 7% 4 4 4 3 5% 6% 38%	2390
	21st to end	Day	24% 6% 2 6% 4 4 4 3 5% 7% 35%	2110
October	1st to 10th	Day	22% 8% 3 8% 5% 5% 4 4 6% 6% 29%	3280
	11th to 20th	Day	35% 7% 3 6% 3 4 3 3 4 6% 24%	3261
	21st to end	Day	30% 11% 4 9% 6% 4 4 3 5% 6% 17%	3278
November	1st to 10th	Day	39% 9% 2 7% 5% 3 4 4 4 6% 17%	1648
	11th to 20th	Day	38% 11% 3 6% 4 4 3 3 4 5% 20%	1680
	21st to end	Day	41% 10% 2 6% 3 4 3 3 5% 5% 18%	1680
December	1st to 10th	Day	26% 10% 3 8% 6% 6% 4 5% 6% 6% 20%	2634
	11th to 20th	Day	42% 13% 3 10% 4 4 3 3 3 4 10%	2715
	21st to end	Day	42% 13% 4 9% 6% 5% 4 3 3 3 9%	3575
January	1st to 10th	Night	39% 13% 4 9% 5% 4 4 3 4 4 12%	8770
	11th to 20th	Night	25% 7% 2 6% 4 4 5% 5% 8% 8% 27%	8779
	21st to end	Night	38% 8% 2 6% 4 3 3 3 6% 6% 22%	8986
February	1st to 10th	Night	32% 12% 4 8% 5% 4 4 4 4 5% 20%	2730
	11th to 20th	Night	29% 12% 3 7% 4 4 3 3 4 6% 26%	2662
	21st to end	Night	30% 11% 3 8% 5% 3 3 3 4 5% 25%	2239
March	1st to 10th	Night	27% 13% 3 8% 5% 4 4 3 5% 5% 22%	5321
	11th to 20th	Night	28% 10% 3 6% 4 4 4 3 5% 6% 27%	5109
	21st to end	Night	36% 12% 4 8% 4 4 3 3 3 4 17%	5480
April	1st to 10th	Night	26% 12% 4 7% 5% 5% 5% 3 4 5% 24%	2100
	11th to 20th	Night	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	1961
	21st to end	Night	21% 9% 3 7% 5% 5% 3 4 6% 6% 31%	1832
May	1st to 10th	Night	16% 7% 3 6% 3 3 3 2 5% 4 49%	3648
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 7% 9% 36%	3616
	21st to end	Night	18% 9% 4 10% 6% 5% 5% 3 5% 6% 29%	3793
June	1st to 10th	Night	20% 8% 2 7% 5% 5% 5% 3 5% 5% 36%	1400
	11th to 20th	Night	16% 9% 3 7% 5% 6% 5% 4 5% 8% 32%	1400
	21st to end	Night	12% 7% 2 7% 4 6% 4 3 5% 9% 41%	1400
July	1st to 10th	Night	9% 4 2 4 3 3 3 2 4 6% 62%	3180
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 5% 77%	3180
	21st to end	Night	6% 4 1 4 3 3 4 3 5% 6% 62%	3819
August	1st to 10th	Night	5% 2 2 2 2 2 2 4 6% 73%	2659
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 7% 51%	2902
	21st to end	Night	13% 5% 2 6% 5% 4 4 4 6% 8% 44%	3339
September	1st to 10th	Night	8% 3 2 4 3 4 4 3 6% 8% 56%	2000
	11th to 20th	Night	18% 6% 2 5% 4 3 3 4 5% 5% 43%	2008
	21st to end	Night	22% 6% 3 6% 5% 4 3 2 4 5% 39%	2290
October	1st to 10th	Night	19% 8% 3 8% 5% 5% 5% 3 5% 6% 31%	3920
	11th to 20th	Night	30% 8% 3 6% 4 4 4 3 4 6% 29%	3897
	21st to end	Night	32% 9% 4 7% 4 4 4 3 4 5% 25%	4520
November	1st to 10th	Night	39% 10% 3 8% 4 4 3 3 4 5% 18%	2870
	11th to 20th	Night	38% 10% 2 5% 4 4 3 3 4 5% 22%	2940
	21st to end	Night	37% 10% 3 7% 5% 4 4 3 3 4 20%	2940
December	1st to 10th	Night	28% 11% 3 7% 5% 5% 7% 5% 5% 5% 19%	4763
	11th to 20th	Night	45% 12% 3 8% 4 3 3 3 3 4 11%	5710
	21st to end	Night	41% 14% 4 8% 4 4 3 2 3 3 14%	7623

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	78% 6% 6% 7% 2	4125
	11th to 20th	Day	49% 7% 11% 18% 13% 3	4140
	21st to end	Day	67% 4 5% 16% 8%	5197
February	1st to 10th	Day	71% 5% 9% 8% 4 2	1890
	11th to 20th	Day	65% 6% 9% 13% 6% 1	1854
	21st to end	Day	70% 5% 10% 10% 3 2	1567
March	1st to 10th	Day	69% 8% 9% 10% 4	4791
	11th to 20th	Day	64% 7% 8% 18% 2 1	5110
	21st to end	Day	73% 6% 11% 8% 2	5736
April	1st to 10th	Day	67% 8% 14% 10% 2	2520
	11th to 20th	Day	60% 7% 11% 13% 8% 1	2605
	21st to end	Day	61% 7% 13% 14% 6%	2786
May	1st to 10th	Day	44% 6% 17% 22% 9%	6335
	11th to 20th	Day	52% 10% 18% 17% 3	6280
	21st to end	Day	55% 9% 18% 16% 2	7080
June	1st to 10th	Day	56% 5% 19% 14% 4 2	3000
	11th to 20th	Day	62% 7% 16% 12% 3	3000
	21st to end	Day	51% 8% 16% 16% 8% 1	3000
July	1st to 10th	Day	28% 8% 18% 22% 18% 6%	6706
	11th to 20th	Day	19% 7% 19% 33% 20% 1	6699
	21st to end	Day	26% 10% 28% 23% 12% 2	6824
August	1st to 10th	Day	22% 5% 11% 46% 15% 1	4472
	11th to 20th	Day	43% 9% 21% 17% 8% 1	4195
	21st to end	Day	44% 7% 18% 19% 11% 1	4572
September	1st to 10th	Day	40% 7% 15% 23% 11% 4	2400
	11th to 20th	Day	49% 6% 11% 18% 14% 2	2390
	21st to end	Day	53% 7% 15% 16% 8% 2	2110
October	1st to 10th	Day	59% 7% 14% 15% 5%	3280
	11th to 20th	Day	63% 9% 9% 12% 6%	3261
	21st to end	Day	70% 9% 8% 9% 4 1	3278
November	1st to 10th	Day	73% 5% 8% 9% 4 1	1648
	11th to 20th	Day	73% 4 7% 8% 5% 2	1680
	21st to end	Day	71% 4 7% 10% 6% 1	1680
December	1st to 10th	Day	59% 8% 9% 17% 8% 1	2634
	11th to 20th	Day	81% 5% 6% 6% 2 1	2718
	21st to end	Day	83% 5% 5% 5% 2	3575
January	1st to 10th	Night	77% 6% 5% 9% 3	8770
	11th to 20th	Night	46% 7% 13% 18% 14% 2	8820
	21st to end	Night	62% 4 7% 19% 8%	8986
February	1st to 10th	Night	69% 5% 9% 9% 5% 3	2730
	11th to 20th	Night	63% 7% 9% 12% 8% 2	2664
	21st to end	Night	64% 4 11% 13% 6% 2	2239
March	1st to 10th	Night	64% 8% 9% 12% 7%	5321
	11th to 20th	Night	62% 6% 9% 18% 4	5110
	21st to end	Night	73% 5% 11% 8% 2	5480
April	1st to 10th	Night	64% 5% 14% 13% 3	2100
	11th to 20th	Night	57% 7% 11% 16% 7% 2	1961
	21st to end	Night	57% 5% 14% 16% 8%	1832
May	1st to 10th	Night	41% 6% 15% 26% 12%	3645
	11th to 20th	Night	43% 11% 21% 21% 4	3616
	21st to end	Night	56% 7% 18% 16% 2	3793
June	1st to 10th	Night	52% 6% 19% 14% 6% 3	1400
	11th to 20th	Night	53% 8% 19% 15% 5%	1400
	21st to end	Night	44% 9% 20% 16% 8% 3	1400
July	1st to 10th	Night	25% 6% 20% 26% 16% 7%	3145
	11th to 20th	Night	12% 6% 22% 33% 24% 3	3138
	21st to end	Night	23% 7% 25% 28% 14% 2	3711
August	1st to 10th	Night	16% 4 15% 37% 26% 2	2651
	11th to 20th	Night	34% 11% 27% 16% 8% 4	2929
	21st to end	Night	40% 6% 20% 23% 10% 1	3339
September	1st to 10th	Night	30% 7% 18% 26% 13% 5%	2000
	11th to 20th	Night	44% 8% 12% 17% 15% 4	2008
	21st to end	Night	51% 5% 15% 19% 8% 2	2290
October	1st to 10th	Night	56% 8% 15% 14% 6%	3920
	11th to 20th	Night	61% 5% 9% 16% 9%	3897
	21st to end	Night	66% 7% 8% 14% 5%	4520
November	1st to 10th	Night	73% 4 8% 9% 4 1	2870
	11th to 20th	Night	70% 4 9% 9% 6% 2	2940
	21st to end	Night	70% 4 8% 9% 8% 1	2940
December	1st to 10th	Night	59% 7% 12% 16% 5% 1	4763
	11th to 20th	Night	80% 5% 6% 6% 2 1	5706
	21st to end	Night	80% 5% 5% 7% 3	7623

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	40% 7% 15% 20% 11% 6%	220
	11th to 20th	Day	36% 10% 12% 19% 16% 7%	220
	21st to end	Day	44% 5% 11% 18% 16% 5%	274
February	1st to 10th	Day	37% 6% 16% 22% 16% 3	430
	11th to 20th	Day	33% 3 21% 24% 17% 2	425
	21st to end	Day	29% 8% 19% 27% 12% 4	361
March	1st to 10th	Day	30% 7% 16% 26% 15% 5%	540
	11th to 20th	Day	32% 8% 16% 19% 20% 4	540
	21st to end	Day	31% 8% 20% 23% 14% 4	594
April	1st to 10th	Day	26% 6% 19% 24% 20% 4	540
	11th to 20th	Day	21% 5% 21% 29% 17% 6%	532
	21st to end	Day	20% 6% 23% 27% 18% 6%	540
May	1st to 10th	Day	15% 5% 16% 31% 29% 5%	508
	11th to 20th	Day	17% 4 12% 26% 31% 10%	528
	21st to end	Day	16% 3 17% 31% 25% 8%	645
June	1st to 10th	Day	18% 3 16% 32% 27% 5%	730
	11th to 20th	Day	17% 4 17% 32% 25% 5%	730
	21st to end	Day	13% 3 16% 28% 32% 8%	730
July	1st to 10th	Day	11% 2 13% 30% 35% 10%	721
	11th to 20th	Day	6% 3 11% 33% 38% 10%	718
	21st to end	Day	5% 2 9% 25% 46% 13%	644
August	1st to 10th	Day	9% 3 9% 23% 39% 17%	527
	11th to 20th	Day	7% 3 11% 27% 39% 13%	517
	21st to end	Day	10% 3 14% 29% 31% 12%	583
September	1st to 10th	Day	12% 2 14% 33% 30% 10%	530
	11th to 20th	Day	18% 4 14% 29% 26% 10%	530
	21st to end	Day	23% 5% 18% 23% 24% 7%	530
October	1st to 10th	Day	26% 5% 15% 24% 21% 8%	530
	11th to 20th	Day	34% 6% 15% 18% 18% 8%	530
	21st to end	Day	34% 9% 17% 24% 13% 4	435
November	1st to 10th	Day	37% 8% 18% 20% 15% 3	220
	11th to 20th	Day	40% 7% 15% 14% 18% 7%	220
	21st to end	Day	42% 8% 14% 16% 9% 11%	220
December	1st to 10th	Day	41% 5% 18% 11% 15% 10%	220
	11th to 20th	Day	42% 11% 13% 18% 14% 2	220
	21st to end	Day	43% 7% 17% 20% 11% 3	242
January	1st to 10th	Night	45% 8% 15% 18% 11% 3	950
	11th to 20th	Night	39% 6% 10% 22% 19% 2	950
	21st to end	Night	38% 4 13% 24% 17% 4	940
February	1st to 10th	Night	34% 7% 17% 25% 14% 2	840
	11th to 20th	Night	33% 5% 17% 27% 16% 2	824
	21st to end	Night	33% 7% 19% 24% 15% 3	692
March	1st to 10th	Night	33% 6% 16% 23% 19% 3	840
	11th to 20th	Night	36% 6% 16% 22% 16% 4	840
	21st to end	Night	33% 7% 15% 23% 16% 5%	924
April	1st to 10th	Night	30% 6% 11% 28% 21% 5%	840
	11th to 20th	Night	22% 6% 16% 31% 19% 6%	713
	21st to end	Night	22% 4 17% 32% 20% 5%	572
May	1st to 10th	Night	17% 3 12% 30% 32% 7%	402
	11th to 20th	Night	19% 3 7% 25% 36% 10%	394
	21st to end	Night	18% 3 9% 29% 35% 6%	433
June	1st to 10th	Night	20% 4 11% 30% 31% 5%	400
	11th to 20th	Night	15% 5% 14% 34% 26% 7%	400
	21st to end	Night	13% 3 8% 32% 38% 7%	400
July	1st to 10th	Night	12% 1 9% 25% 43% 10%	390
	11th to 20th	Night	6% 3 8% 28% 44% 11%	388
	21st to end	Night	5% 1 7% 24% 48% 16%	438
August	1st to 10th	Night	8% 1 6% 20% 44% 20%	422
	11th to 20th	Night	8% 5% 23% 47% 17%	594
	21st to end	Night	12% 3 8% 22% 43% 12%	715
September	1st to 10th	Night	10% 1 7% 31% 36% 15%	800
	11th to 20th	Night	19% 3 10% 21% 32% 16%	800
	21st to end	Night	22% 4 13% 25% 28% 7%	800
October	1st to 10th	Night	21% 4 13% 27% 25% 11%	800
	11th to 20th	Night	32% 4 13% 23% 21% 7%	800
	21st to end	Night	36% 6% 14% 25% 14% 5%	880
November	1st to 10th	Night	41% 7% 14% 24% 14% 1	820
	11th to 20th	Night	40% 7% 16% 22% 13% 3	840
	21st to end	Night	40% 5% 16% 22% 13% 4	840
December	1st to 10th	Night	35% 5% 14% 26% 17% 4	859
	11th to 20th	Night	50% 6% 14% 20% 8% 1	950
	21st to end	Night	44% 7% 19% 20% 8% 1	1045

Page generated at 2026-04-28 23:21:44. Version ac07

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