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 2025-01-07.

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%	108849
	Night	28% 9% 3 7% 4 4 4 3 4 5% 30%	108788
Transparency	Day	55% 6% 13% 17% 7% 1	108597
	Night	59% 6% 12% 15% 7% 1	108686
Seeing	Day	22% 4 15% 26% 25% 8%	15540
	Night	29% 5% 13% 24% 22% 6%	23817

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	44% 11% 3 8% 5% 4 3 3 5% 5% 10%	2261
	11th to 20th	Day	30% 13% 3 7% 4 3 3 2 3 6% 26%	2199
	21st to end	Day	57% 10% 3 5% 3 2 3 2 3 3 9%	2776
February	1st to 10th	Day	34% 11% 4 8% 5% 4 3 3 6% 6% 16%	1710
	11th to 20th	Day	30% 10% 3 7% 6% 4 5% 3 5% 4 23%	1665
	21st to end	Day	25% 13% 4 8% 6% 4 5% 3 4 6% 23%	1422
March	1st to 10th	Day	24% 14% 4 10% 6% 4 5% 4 5% 6% 18%	3255
	11th to 20th	Day	23% 10% 2 7% 4 4 4 3 4 6% 34%	3490
	21st to end	Day	27% 13% 3 8% 5% 4 3 3 4 6% 24%	3912
April	1st to 10th	Day	26% 11% 3 9% 5% 5% 5% 4 5% 6% 20%	2280
	11th to 20th	Day	21% 10% 4 7% 6% 4 4 4 5% 6% 29%	2353
	21st to end	Day	23% 10% 4 7% 5% 4 4 3 5% 7% 29%	2520
May	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 4 5% 44%	4420
	11th to 20th	Day	11% 6% 3 7% 4 4 4 4 7% 9% 40%	4377
	21st to end	Day	26% 9% 3 7% 4 5% 4 3 5% 6% 26%	4933
June	1st to 10th	Day	21% 6% 2 8% 5% 4 4 4 6% 7% 32%	2850
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 4 6% 7% 30%	2850
	21st to end	Day	14% 7% 2 7% 5% 5% 4 4 6% 7% 40%	2850
July	1st to 10th	Day	7% 4 2 4 3 2 3 2 3 6% 64%	4740
	11th to 20th	Day	6% 3 1 2 2 2 2 2 3 5% 71%	4740
	21st to end	Day	11% 4 2 4 3 3 3 3 5% 7% 55%	4895
August	1st to 10th	Day	6% 3 1 3 2 2 3 2 4 5% 69%	4351
	11th to 20th	Day	11% 6% 3 6% 4 4 5% 3 4 7% 47%	4026
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 7% 44%	4433
September	1st to 10th	Day	13% 5% 3 6% 4 3 4 3 4 7% 48%	2280
	11th to 20th	Day	18% 8% 3 7% 4 4 4 3 5% 6% 38%	2271
	21st to end	Day	24% 6% 1 6% 4 4 4 3 5% 7% 36%	2005
October	1st to 10th	Day	22% 7% 3 8% 5% 5% 4 4 6% 6% 29%	3180
	11th to 20th	Day	36% 7% 3 6% 3 4 3 3 4 6% 24%	3161
	21st to end	Day	30% 11% 4 10% 6% 4 4 3 5% 6% 17%	3177
November	1st to 10th	Day	40% 9% 2 7% 5% 3 4 4 4 5% 17%	1568
	11th to 20th	Day	37% 12% 3 6% 4 4 3 3 4 5% 21%	1600
	21st to end	Day	40% 10% 2 7% 3 4 4 3 5% 5% 18%	1600
December	1st to 10th	Day	25% 10% 3 8% 6% 6% 5% 5% 6% 6% 20%	2556
	11th to 20th	Day	42% 13% 3 10% 4 4 3 3 3 4 10%	2645
	21st to end	Day	42% 13% 4 9% 6% 5% 4 3 3 3 9%	3498
January	1st to 10th	Night	40% 13% 3 8% 5% 4 3 3 3 4 14%	4816
	11th to 20th	Night	34% 9% 3 5% 3 3 4 4 5% 8% 24%	4699
	21st to end	Night	53% 11% 2 6% 3 2 2 2 3 4 12%	4878
February	1st to 10th	Night	33% 11% 4 8% 4 4 4 3 4 5% 20%	2470
	11th to 20th	Night	30% 12% 3 7% 4 4 3 3 3 6% 26%	2402
	21st to end	Night	30% 11% 3 7% 4 3 3 3 4 5% 27%	2032
March	1st to 10th	Night	29% 14% 3 8% 5% 4 4 3 4 5% 21%	3617
	11th to 20th	Night	24% 7% 2 6% 4 4 4 2 5% 6% 36%	3489
	21st to end	Night	31% 14% 4 8% 5% 4 3 3 3 4 22%	3740
April	1st to 10th	Night	26% 12% 4 7% 6% 5% 5% 3 4 5% 23%	1900
	11th to 20th	Night	22% 11% 3 8% 5% 4 4 3 5% 6% 29%	1773
	21st to end	Night	23% 10% 3 8% 5% 5% 4 4 6% 6% 27%	1658
May	1st to 10th	Night	17% 7% 2 6% 3 3 3 2 5% 4 47%	2540
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 6% 10% 45%	2516
	21st to end	Night	22% 10% 4 9% 6% 4 4 3 4 5% 29%	2639
June	1st to 10th	Night	21% 8% 2 7% 5% 5% 5% 3 5% 5% 34%	1330
	11th to 20th	Night	17% 9% 2 7% 5% 6% 5% 5% 5% 8% 31%	1330
	21st to end	Night	12% 7% 2 7% 4 6% 4 3 5% 9% 42%	1330
July	1st to 10th	Night	6% 3 1 3 3 3 3 2 3 6% 66%	2220
	11th to 20th	Night	4 3 1 2 2 2 2 2 3 5% 75%	2220
	21st to end	Night	7% 6% 2 5% 3 3 4 3 5% 5% 56%	2654
August	1st to 10th	Night	4 2 2 2 2 2 2 4 6% 73%	2578
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 7% 51%	2812
	21st to end	Night	13% 5% 3 6% 5% 4 4 4 6% 8% 44%	3237
September	1st to 10th	Night	9% 3 2 4 3 4 4 3 5% 7% 56%	1900
	11th to 20th	Night	19% 6% 2 6% 4 3 3 4 5% 5% 42%	1907
	21st to end	Night	22% 6% 3 6% 5% 4 3 2 5% 5% 40%	2175
October	1st to 10th	Night	19% 8% 3 8% 5% 5% 5% 4 5% 6% 31%	3800
	11th to 20th	Night	30% 9% 3 6% 4 4 4 3 4 6% 28%	3777
	21st to end	Night	32% 9% 4 7% 3 4 4 3 4 5% 25%	4380
November	1st to 10th	Night	39% 9% 3 8% 4 4 3 3 3 5% 18%	2730
	11th to 20th	Night	37% 10% 3 5% 4 4 3 3 3 5% 22%	2800
	21st to end	Night	37% 11% 3 7% 5% 4 4 3 3 4 20%	2800
December	1st to 10th	Night	27% 11% 3 7% 5% 5% 7% 5% 6% 6% 19%	4621
	11th to 20th	Night	45% 12% 3 8% 4 3 3 3 3 4 11%	5560
	21st to end	Night	41% 14% 4 8% 4 4 3 2 3 3 14%	7458

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% 5% 7% 7% 2 1	2261
	11th to 20th	Day	63% 5% 11% 11% 5% 5%	2220
	21st to end	Day	83% 3 5% 7% 2	2776
February	1st to 10th	Day	72% 5% 9% 8% 4 2	1710
	11th to 20th	Day	66% 6% 9% 12% 6% 1	1674
	21st to end	Day	69% 4 11% 11% 3 2	1422
March	1st to 10th	Day	67% 9% 11% 9% 4	3255
	11th to 20th	Day	56% 7% 11% 24% 2 1	3490
	21st to end	Day	68% 4 15% 9% 3	3912
April	1st to 10th	Day	69% 7% 13% 9% 2	2280
	11th to 20th	Day	62% 6% 11% 12% 7% 2	2353
	21st to end	Day	64% 6% 12% 12% 6%	2520
May	1st to 10th	Day	47% 6% 16% 23% 7%	4420
	11th to 20th	Day	45% 8% 22% 21% 4	4380
	21st to end	Day	63% 7% 16% 13% 2	4934
June	1st to 10th	Day	59% 5% 18% 13% 4 2	2850
	11th to 20th	Day	62% 7% 16% 12% 3	2850
	21st to end	Day	50% 7% 16% 17% 8% 1	2850
July	1st to 10th	Day	25% 6% 11% 25% 24% 9%	4666
	11th to 20th	Day	21% 4 14% 38% 21% 2	4659
	21st to end	Day	32% 10% 20% 23% 13% 2	4740
August	1st to 10th	Day	22% 5% 10% 47% 15% 1	4334
	11th to 20th	Day	43% 9% 21% 17% 8% 1	4065
	21st to end	Day	45% 7% 18% 19% 11% 1	4432
September	1st to 10th	Day	41% 6% 14% 23% 11% 4	2280
	11th to 20th	Day	50% 6% 11% 17% 14% 2	2271
	21st to end	Day	53% 7% 15% 16% 8% 2	2005
October	1st to 10th	Day	59% 7% 14% 15% 5%	3180
	11th to 20th	Day	63% 9% 9% 12% 6%	3161
	21st to end	Day	69% 9% 8% 9% 4 1	3177
November	1st to 10th	Day	73% 5% 8% 9% 4 1	1568
	11th to 20th	Day	73% 4 8% 8% 5% 2	1600
	21st to end	Day	71% 4 7% 11% 7% 1	1600
December	1st to 10th	Day	58% 8% 9% 17% 8% 1	2556
	11th to 20th	Day	81% 5% 6% 6% 2 1	2648
	21st to end	Day	83% 5% 5% 5% 2	3498
January	1st to 10th	Night	76% 5% 5% 11% 2	4816
	11th to 20th	Night	59% 5% 14% 12% 8% 3	4740
	21st to end	Night	81% 3 5% 8% 2 1	4878
February	1st to 10th	Night	69% 5% 10% 10% 4 2	2470
	11th to 20th	Night	64% 7% 10% 11% 7% 2	2404
	21st to end	Night	63% 4 11% 14% 6% 2	2032
March	1st to 10th	Night	65% 6% 9% 13% 7%	3617
	11th to 20th	Night	53% 6% 13% 23% 5% 1	3490
	21st to end	Night	70% 5% 12% 10% 2	3740
April	1st to 10th	Night	65% 5% 14% 13% 3	1900
	11th to 20th	Night	58% 6% 11% 15% 7% 2	1773
	21st to end	Night	61% 5% 12% 14% 8%	1658
May	1st to 10th	Night	41% 7% 15% 26% 10%	2540
	11th to 20th	Night	34% 11% 25% 25% 5%	2516
	21st to end	Night	60% 6% 18% 13% 3	2639
June	1st to 10th	Night	54% 6% 18% 15% 4 3	1330
	11th to 20th	Night	53% 8% 20% 14% 4	1330
	21st to end	Night	43% 8% 20% 16% 8% 3	1330
July	1st to 10th	Night	22% 4 12% 30% 22% 10%	2185
	11th to 20th	Night	16% 4 14% 39% 26% 2	2178
	21st to end	Night	29% 7% 16% 29% 15% 3	2570
August	1st to 10th	Night	16% 4 15% 38% 27% 2	2570
	11th to 20th	Night	34% 10% 28% 17% 8% 3	2839
	21st to end	Night	41% 6% 20% 23% 10% 1	3237
September	1st to 10th	Night	31% 7% 17% 26% 14% 5%	1900
	11th to 20th	Night	45% 7% 12% 16% 16% 4	1907
	21st to end	Night	51% 5% 15% 19% 8% 2	2175
October	1st to 10th	Night	56% 8% 15% 14% 6%	3800
	11th to 20th	Night	61% 5% 9% 16% 8%	3777
	21st to end	Night	66% 7% 8% 14% 5%	4380
November	1st to 10th	Night	73% 4 8% 9% 4 1	2730
	11th to 20th	Night	70% 4 9% 9% 6% 2	2800
	21st to end	Night	70% 4 9% 9% 8% 1	2800
December	1st to 10th	Night	58% 7% 12% 17% 5% 1	4621
	11th to 20th	Night	79% 5% 7% 6% 3 1	5556
	21st to end	Night	81% 4 5% 7% 3	7458

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	39% 7% 17% 20% 11% 7%	192
	11th to 20th	Day	40% 12% 13% 16% 14% 6%	180
	21st to end	Day	47% 5% 11% 18% 15% 4	226
February	1st to 10th	Day	39% 6% 15% 23% 16% 2	370
	11th to 20th	Day	34% 2 21% 23% 18% 2	365
	21st to end	Day	27% 7% 18% 29% 13% 5%	312
March	1st to 10th	Day	29% 7% 15% 28% 16% 5%	460
	11th to 20th	Day	29% 5% 17% 21% 23% 5%	460
	21st to end	Day	31% 8% 18% 23% 16% 4	506
April	1st to 10th	Day	27% 6% 20% 23% 21% 4	460
	11th to 20th	Day	23% 4 20% 28% 17% 7%	452
	21st to end	Day	22% 5% 20% 28% 19% 5%	460
May	1st to 10th	Day	15% 4 14% 31% 30% 5%	463
	11th to 20th	Day	16% 3 10% 26% 33% 11%	488
	21st to end	Day	16% 3 15% 31% 26% 9%	597
June	1st to 10th	Day	19% 3 15% 31% 27% 5%	680
	11th to 20th	Day	17% 4 17% 32% 25% 5%	680
	21st to end	Day	12% 3 16% 28% 33% 8%	680
July	1st to 10th	Day	11% 2 11% 29% 36% 11%	671
	11th to 20th	Day	6% 2 10% 32% 40% 10%	668
	21st to end	Day	5% 2 9% 24% 46% 13%	597
August	1st to 10th	Day	9% 2 8% 22% 40% 18%	487
	11th to 20th	Day	6% 2 9% 27% 41% 14%	477
	21st to end	Day	11% 3 15% 29% 30% 11%	539
September	1st to 10th	Day	12% 2 14% 33% 28% 9%	490
	11th to 20th	Day	19% 4 14% 29% 26% 10%	490
	21st to end	Day	23% 4 16% 23% 26% 8%	490
October	1st to 10th	Day	27% 4 14% 25% 21% 9%	490
	11th to 20th	Day	34% 6% 14% 19% 19% 8%	490
	21st to end	Day	32% 8% 18% 24% 14% 5%	400
November	1st to 10th	Day	39% 6% 19% 20% 15% 3	200
	11th to 20th	Day	37% 7% 16% 15% 19% 8%	200
	21st to end	Day	40% 9% 14% 17% 9% 12%	200
December	1st to 10th	Day	40% 4 19% 11% 17% 11%	200
	11th to 20th	Day	40% 11% 14% 19% 16% 2	200
	21st to end	Day	42% 6% 19% 20% 10% 3	220
January	1st to 10th	Night	45% 8% 15% 17% 11% 3	880
	11th to 20th	Night	43% 6% 10% 20% 18% 2	850
	21st to end	Night	40% 5% 14% 24% 15% 3	850
February	1st to 10th	Night	35% 7% 17% 25% 14% 3	760
	11th to 20th	Night	33% 5% 17% 28% 15% 2	744
	21st to end	Night	33% 5% 18% 25% 16% 3	628
March	1st to 10th	Night	32% 6% 17% 22% 19% 3	760
	11th to 20th	Night	34% 6% 16% 23% 18% 4	760
	21st to end	Night	34% 6% 15% 23% 17% 6%	836
April	1st to 10th	Night	30% 5% 11% 29% 21% 4	760
	11th to 20th	Night	23% 6% 15% 32% 18% 6%	645
	21st to end	Night	24% 4 17% 31% 19% 5%	518
May	1st to 10th	Night	17% 3 13% 29% 32% 7%	380
	11th to 20th	Night	18% 2 7% 25% 37% 10%	374
	21st to end	Night	18% 4 8% 29% 35% 6%	411
June	1st to 10th	Night	21% 4 10% 29% 32% 5%	380
	11th to 20th	Night	16% 5% 14% 34% 26% 7%	380
	21st to end	Night	12% 3 8% 32% 38% 7%	380
July	1st to 10th	Night	12% 1 8% 25% 44% 10%	370
	11th to 20th	Night	7% 3 7% 26% 45% 12%	368
	21st to end	Night	5% 1 6% 24% 48% 15%	416
August	1st to 10th	Night	8% 1 5% 19% 45% 20%	401
	11th to 20th	Night	7% 4 21% 49% 18%	564
	21st to end	Night	13% 3 8% 23% 41% 12%	679
September	1st to 10th	Night	10% 1 7% 31% 36% 15%	760
	11th to 20th	Night	20% 3 9% 21% 31% 16%	760
	21st to end	Night	22% 4 13% 24% 29% 8%	760
October	1st to 10th	Night	21% 3 13% 26% 25% 11%	760
	11th to 20th	Night	33% 4 12% 23% 22% 7%	760
	21st to end	Night	36% 6% 13% 25% 13% 6%	836
November	1st to 10th	Night	41% 6% 14% 23% 15% 1	780
	11th to 20th	Night	39% 7% 16% 21% 13% 4	800
	21st to end	Night	40% 5% 16% 21% 14% 5%	800
December	1st to 10th	Night	34% 5% 14% 26% 17% 4	817
	11th to 20th	Night	49% 6% 15% 21% 9% 1	900
	21st to end	Night	45% 7% 19% 19% 8% 1	990

Page generated at 2025-01-04 00:24:02. Version ac07

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