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 2024-01-17.

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	24% 8% 3 6% 5% 3 4 4 5% 6% 33%	74766
	Night	28% 8% 3 6% 5% 3 4 4 4 5% 30%	75073
Transparency	Day	58% 6% 13% 16% 7% 1	74454
	Night	59% 5% 12% 15% 7% 1	74903
Seeing	Day	22% 4 15% 26% 26% 8%	14259
	Night	29% 4 13% 24% 23% 6%	22601

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	47% 9% 4 7% 5% 2 2 2 4 5% 12%	1329
	11th to 20th	Day	35% 11% 4 7% 5% 3 5% 4 4 6% 18%	1302
	21st to end	Day	37% 11% 4 6% 6% 3 3 4 5% 6% 15%	1580
February	1st to 10th	Day	35% 10% 3 7% 6% 3 4 3 6% 6% 17%	1620
	11th to 20th	Day	31% 9% 3 6% 6% 3 5% 4 5% 5% 23%	1575
	21st to end	Day	24% 11% 5% 7% 6% 3 4 5% 4 5% 24%	1340
March	1st to 10th	Day	29% 12% 4 8% 6% 4 4 4 4 6% 21%	1865
	11th to 20th	Day	34% 13% 3 6% 5% 3 4 3 3 4 22%	1980
	21st to end	Day	29% 11% 4 7% 6% 2 4 5% 4 5% 23%	2227
April	1st to 10th	Day	27% 10% 3 7% 6% 3 6% 5% 5% 6% 21%	2160
	11th to 20th	Day	22% 9% 4 7% 7% 3 4 5% 5% 6% 28%	2227
	21st to end	Day	22% 9% 4 6% 6% 3 5% 4 5% 6% 29%	2387
May	1st to 10th	Day	16% 7% 3 6% 6% 3 5% 5% 6% 6% 37%	2520
	11th to 20th	Day	16% 7% 3 6% 6% 4 5% 5% 6% 7% 36%	2477
	21st to end	Day	18% 8% 4 7% 5% 4 5% 4 5% 6% 35%	2787
June	1st to 10th	Day	21% 6% 2 7% 6% 3 5% 4 5% 7% 33%	2700
	11th to 20th	Day	20% 8% 3 7% 6% 3 5% 5% 6% 7% 30%	2700
	21st to end	Day	14% 6% 2 6% 5% 3 5% 5% 6% 7% 41%	2700
July	1st to 10th	Day	11% 5% 2 5% 4 2 4 4 4 7% 51%	2700
	11th to 20th	Day	11% 4 2 4 4 2 3 4 5% 6% 55%	2700
	21st to end	Day	8% 2 1 3 3 2 3 4 4 6% 65%	2804
August	1st to 10th	Day	10% 4 2 4 4 2 4 3 4 5% 58%	2483
	11th to 20th	Day	8% 5% 2 4 4 2 3 3 4 6% 58%	2298
	21st to end	Day	12% 4 2 5% 5% 3 4 4 5% 8% 48%	2525
September	1st to 10th	Day	14% 6% 3 4 6% 3 3 4 4 6% 48%	2160
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 4 5% 6% 39%	2152
	21st to end	Day	24% 6% 2 5% 4 2 4 5% 5% 7% 37%	1900
October	1st to 10th	Day	24% 7% 3 5% 4 3 4 4 5% 7% 35%	1800
	11th to 20th	Day	33% 7% 4 6% 5% 3 4 4 4 7% 23%	1800
	21st to end	Day	35% 8% 3 6% 5% 3 4 4 5% 6% 23%	1829
November	1st to 10th	Day	41% 8% 2 5% 5% 2 4 4 4 6% 18%	1488
	11th to 20th	Day	37% 10% 4 5% 5% 3 3 4 3 5% 21%	1520
	21st to end	Day	41% 9% 3 6% 4 3 4 3 5% 5% 19%	1520
December	1st to 10th	Day	33% 12% 3 5% 5% 2 4 4 3 4 24%	1484
	11th to 20th	Day	37% 14% 4 9% 6% 3 4 4 4 5% 12%	1685
	21st to end	Day	44% 11% 4 7% 6% 3 3 3 3 3 11%	2442
January	1st to 10th	Night	44% 11% 3 6% 5% 3 4 3 3 3 16%	2836
	11th to 20th	Night	40% 9% 3 5% 4 2 4 4 4 7% 19%	2802
	21st to end	Night	34% 11% 4 6% 5% 3 3 4 4 7% 20%	2773
February	1st to 10th	Night	35% 10% 4 7% 5% 3 4 4 3 5% 21%	2340
	11th to 20th	Night	30% 12% 3 6% 5% 3 3 4 3 6% 26%	2272
	21st to end	Night	29% 9% 4 7% 5% 3 4 4 4 5% 27%	1916
March	1st to 10th	Night	31% 10% 3 6% 6% 3 4 4 4 5% 24%	2093
	11th to 20th	Night	35% 9% 3 6% 5% 2 4 3 5% 5% 23%	1979
	21st to end	Night	31% 11% 4 7% 6% 4 4 4 4 4 21%	2128
April	1st to 10th	Night	28% 11% 4 6% 6% 3 5% 4 4 6% 24%	1800
	11th to 20th	Night	23% 10% 3 7% 6% 3 4 4 5% 6% 28%	1679
	21st to end	Night	23% 9% 4 7% 6% 4 4 4 6% 6% 28%	1571
May	1st to 10th	Night	17% 5% 3 6% 5% 3 5% 4 7% 6% 40%	1440
	11th to 20th	Night	16% 6% 3 5% 6% 3 5% 6% 6% 9% 36%	1416
	21st to end	Night	17% 7% 3 7% 6% 3 5% 4 5% 7% 36%	1485
June	1st to 10th	Night	21% 7% 2 6% 5% 4 5% 4 5% 5% 35%	1260
	11th to 20th	Night	18% 8% 3 6% 5% 4 6% 6% 5% 7% 32%	1260
	21st to end	Night	12% 5% 3 5% 5% 4 6% 4 5% 9% 43%	1260
July	1st to 10th	Night	11% 4 2 5% 5% 2 3 3 4 6% 55%	1260
	11th to 20th	Night	7% 4 2 3 4 2 4 3 4 7% 59%	1260
	21st to end	Night	4 3 1 3 3 2 3 4 4 5% 68%	1510
August	1st to 10th	Night	8% 4 1 3 3 2 3 3 4 5% 64%	1453
	11th to 20th	Night	6% 3 1 3 3 1 3 4 4 8% 64%	1602
	21st to end	Night	10% 4 3 5% 5% 3 4 5% 6% 9% 46%	1831
September	1st to 10th	Night	9% 3 2 4 4 3 4 4 5% 7% 56%	1800
	11th to 20th	Night	19% 6% 2 5% 4 3 3 4 5% 5% 43%	1806
	21st to end	Night	22% 6% 3 5% 5% 3 3 3 5% 5% 41%	2060
October	1st to 10th	Night	20% 6% 3 6% 4 3 4 4 5% 5% 40%	2160
	11th to 20th	Night	31% 8% 3 5% 3 2 3 4 4 6% 31%	2160
	21st to end	Night	35% 7% 3 5% 4 2 3 3 4 6% 29%	2524
November	1st to 10th	Night	39% 8% 3 6% 5% 3 4 4 4 5% 19%	2590
	11th to 20th	Night	37% 9% 3 4 4 2 4 4 3 5% 24%	2660
	21st to end	Night	37% 9% 4 7% 5% 3 4 3 3 4 21%	2660
December	1st to 10th	Night	33% 10% 3 6% 5% 2 4 4 3 5% 24%	2693
	11th to 20th	Night	45% 12% 4 7% 5% 2 3 3 3 4 13%	3520
	21st to end	Night	43% 13% 3 6% 4 2 3 2 3 3 17%	5214

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	77% 4 7% 7% 4 1	1329
	11th to 20th	Day	71% 6% 7% 10% 3 2	1302
	21st to end	Day	71% 4 9% 12% 4	1580
February	1st to 10th	Day	72% 5% 9% 8% 4 2	1620
	11th to 20th	Day	67% 5% 10% 12% 7%	1584
	21st to end	Day	68% 4 11% 11% 3 2	1340
March	1st to 10th	Day	69% 7% 11% 9% 4	1865
	11th to 20th	Day	73% 3 8% 10% 4 1	1980
	21st to end	Day	72% 4 11% 11% 3	2227
April	1st to 10th	Day	69% 6% 13% 9% 2	2160
	11th to 20th	Day	65% 6% 11% 11% 7% 1	2227
	21st to end	Day	64% 6% 12% 11% 6%	2387
May	1st to 10th	Day	55% 6% 17% 16% 5% 1	2520
	11th to 20th	Day	55% 7% 15% 19% 4	2480
	21st to end	Day	56% 7% 15% 18% 3	2788
June	1st to 10th	Day	59% 4 18% 12% 4 2	2700
	11th to 20th	Day	64% 6% 16% 11% 3	2700
	21st to end	Day	49% 7% 17% 17% 8% 2	2700
July	1st to 10th	Day	40% 7% 16% 23% 11% 3	2626
	11th to 20th	Day	36% 6% 19% 28% 10% 1	2619
	21st to end	Day	26% 5% 19% 31% 16% 3	2648
August	1st to 10th	Day	35% 6% 16% 30% 12% 2	2466
	11th to 20th	Day	31% 6% 20% 25% 15% 2	2298
	21st to end	Day	40% 6% 20% 26% 6% 1	2525
September	1st to 10th	Day	42% 6% 14% 24% 10% 4	2160
	11th to 20th	Day	50% 5% 11% 18% 14% 2	2152
	21st to end	Day	52% 7% 15% 16% 8% 2	1900
October	1st to 10th	Day	53% 5% 15% 18% 9%	1800
	11th to 20th	Day	63% 7% 11% 15% 4 1	1800
	21st to end	Day	64% 6% 11% 11% 7% 1	1829
November	1st to 10th	Day	74% 4 8% 9% 4 1	1488
	11th to 20th	Day	72% 3 8% 9% 6% 2	1520
	21st to end	Day	70% 4 7% 11% 7% 1	1520
December	1st to 10th	Day	66% 5% 8% 12% 8% 1	1484
	11th to 20th	Day	77% 4 7% 8% 2 2	1688
	21st to end	Day	80% 5% 6% 7% 2 1	2442
January	1st to 10th	Night	76% 3 5% 11% 4 1	2836
	11th to 20th	Night	67% 5% 12% 9% 5% 2	2802
	21st to end	Night	67% 5% 9% 14% 4 1	2773
February	1st to 10th	Night	68% 5% 10% 10% 4 3	2340
	11th to 20th	Night	65% 7% 9% 11% 7% 1	2274
	21st to end	Night	62% 4 11% 14% 6% 2	1916
March	1st to 10th	Night	65% 6% 12% 12% 5%	2093
	11th to 20th	Night	70% 4 8% 12% 6% 1	1980
	21st to end	Night	70% 6% 12% 10% 2	2128
April	1st to 10th	Night	65% 5% 15% 13% 3	1800
	11th to 20th	Night	60% 6% 11% 15% 6% 2	1679
	21st to end	Night	61% 5% 13% 13% 8%	1571
May	1st to 10th	Night	45% 10% 18% 18% 8% 1	1440
	11th to 20th	Night	47% 8% 21% 18% 6% 1	1416
	21st to end	Night	49% 7% 20% 19% 5%	1485
June	1st to 10th	Night	54% 5% 18% 15% 4 3	1260
	11th to 20th	Night	53% 9% 20% 14% 4	1260
	21st to end	Night	42% 9% 21% 17% 8% 3	1260
July	1st to 10th	Night	35% 6% 17% 26% 13% 3	1225
	11th to 20th	Night	28% 7% 23% 29% 13% 1	1218
	21st to end	Night	20% 6% 15% 34% 19% 5%	1426
August	1st to 10th	Night	27% 5% 20% 28% 18% 3	1445
	11th to 20th	Night	21% 8% 25% 26% 14% 6%	1602
	21st to end	Night	36% 5% 21% 26% 9% 1	1831
September	1st to 10th	Night	31% 7% 17% 27% 14% 4	1800
	11th to 20th	Night	45% 7% 12% 16% 17% 5%	1806
	21st to end	Night	50% 5% 15% 20% 8% 2	2060
October	1st to 10th	Night	49% 6% 14% 19% 11% 1	2160
	11th to 20th	Night	59% 4 13% 17% 6%	2160
	21st to end	Night	61% 6% 10% 14% 9%	2524
November	1st to 10th	Night	73% 4 9% 9% 4 1	2590
	11th to 20th	Night	69% 4 9% 10% 6% 2	2660
	21st to end	Night	70% 3 9% 9% 8% 1	2660
December	1st to 10th	Night	65% 5% 11% 11% 7% 1	2693
	11th to 20th	Night	78% 4 7% 6% 4 1	3516
	21st to end	Night	77% 3 6% 9% 4 1	5214

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	38% 7% 17% 19% 12% 7%	180
	11th to 20th	Day	40% 12% 13% 16% 14% 6%	172
	21st to end	Day	44% 4 11% 20% 17% 4	203
February	1st to 10th	Day	41% 6% 14% 21% 16% 1	340
	11th to 20th	Day	35% 2 21% 23% 17% 2	335
	21st to end	Day	26% 6% 18% 31% 14% 5%	284
March	1st to 10th	Day	30% 5% 15% 28% 16% 5%	420
	11th to 20th	Day	30% 5% 19% 20% 21% 5%	420
	21st to end	Day	31% 7% 18% 24% 16% 4	462
April	1st to 10th	Day	27% 6% 20% 22% 21% 4	420
	11th to 20th	Day	25% 3 19% 27% 18% 7%	412
	21st to end	Day	21% 5% 20% 29% 20% 5%	420
May	1st to 10th	Day	15% 4 15% 32% 30% 5%	423
	11th to 20th	Day	16% 3 9% 26% 33% 12%	448
	21st to end	Day	14% 3 13% 33% 27% 9%	549
June	1st to 10th	Day	19% 2 15% 31% 28% 6%	630
	11th to 20th	Day	18% 3 17% 33% 25% 5%	630
	21st to end	Day	12% 3 14% 29% 34% 8%	630
July	1st to 10th	Day	11% 1 10% 29% 37% 11%	621
	11th to 20th	Day	6% 2 11% 31% 40% 10%	618
	21st to end	Day	5% 1 7% 24% 49% 14%	550
August	1st to 10th	Day	10% 2 9% 20% 41% 18%	447
	11th to 20th	Day	6% 2 9% 27% 41% 15%	437
	21st to end	Day	11% 3 15% 30% 31% 11%	495
September	1st to 10th	Day	14% 2 15% 33% 28% 8%	450
	11th to 20th	Day	19% 3 13% 28% 27% 10%	450
	21st to end	Day	22% 4 16% 23% 27% 8%	450
October	1st to 10th	Day	27% 4 13% 25% 22% 10%	450
	11th to 20th	Day	34% 5% 15% 18% 19% 9%	450
	21st to end	Day	33% 8% 17% 23% 14% 5%	365
November	1st to 10th	Day	38% 4 19% 20% 14% 3	180
	11th to 20th	Day	37% 6% 15% 14% 19% 9%	180
	21st to end	Day	39% 8% 14% 17% 9% 12%	180
December	1st to 10th	Day	42% 4 20% 10% 14% 10%	180
	11th to 20th	Day	37% 12% 12% 21% 16% 2	180
	21st to end	Day	41% 5% 19% 22% 10% 3	198
January	1st to 10th	Night	45% 9% 15% 17% 11% 4	850
	11th to 20th	Night	43% 6% 10% 21% 18% 2	834
	21st to end	Night	37% 4 14% 25% 16% 4	804
February	1st to 10th	Night	36% 6% 17% 24% 13% 3	720
	11th to 20th	Night	34% 5% 17% 28% 15% 2	704
	21st to end	Night	31% 6% 18% 26% 16% 3	592
March	1st to 10th	Night	32% 6% 17% 23% 19% 3	720
	11th to 20th	Night	35% 6% 16% 24% 16% 4	720
	21st to end	Night	34% 5% 15% 23% 17% 6%	792
April	1st to 10th	Night	31% 5% 11% 28% 21% 5%	720
	11th to 20th	Night	24% 5% 16% 31% 18% 6%	611
	21st to end	Night	24% 4 16% 31% 20% 5%	491
May	1st to 10th	Night	17% 3 13% 28% 32% 7%	360
	11th to 20th	Night	18% 2 8% 24% 38% 11%	354
	21st to end	Night	19% 3 7% 29% 35% 7%	389
June	1st to 10th	Night	21% 4 10% 28% 32% 5%	360
	11th to 20th	Night	16% 5% 14% 33% 26% 6%	360
	21st to end	Night	13% 2 8% 32% 39% 7%	360
July	1st to 10th	Night	12% 1 8% 24% 45% 11%	350
	11th to 20th	Night	7% 3 7% 26% 45% 12%	348
	21st to end	Night	5% 1 6% 23% 49% 16%	394
August	1st to 10th	Night	8% 1 6% 19% 44% 21%	379
	11th to 20th	Night	7% 4 22% 50% 18%	534
	21st to end	Night	12% 2 8% 22% 42% 13%	643
September	1st to 10th	Night	10% 1 7% 32% 36% 14%	720
	11th to 20th	Night	20% 3 9% 20% 31% 17%	720
	21st to end	Night	22% 4 13% 24% 29% 8%	720
October	1st to 10th	Night	22% 3 14% 26% 25% 12%	720
	11th to 20th	Night	34% 3 12% 23% 21% 7%	720
	21st to end	Night	37% 5% 13% 25% 14% 6%	792
November	1st to 10th	Night	40% 6% 15% 23% 15% 1	740
	11th to 20th	Night	38% 7% 16% 21% 14% 4	760
	21st to end	Night	40% 4 16% 21% 14% 5%	760
December	1st to 10th	Night	35% 5% 14% 26% 16% 4	775
	11th to 20th	Night	49% 6% 14% 21% 9% 1	850
	21st to end	Night	45% 7% 19% 19% 9% 1	935

Page generated at 2024-01-14 06:55:00. Version ac07

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