Brodie Road Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brodie Road Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	17% 10% 4 8% 5% 5% 4 4 5% 7% 31%	127175
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	123176
Transparency	Day	34% 38% 8% 14% 6% 1	130605
	Night	36% 37% 7% 13% 7% 1	127833
Seeing	Day	24% 16% 30% 19% 8% 2	15838
	Night	24% 14% 29% 24% 9% 1	24829

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% 19% 6% 15% 5% 4 4 3 3 4 21%	3597
	11th to 20th	Day	13% 16% 7% 12% 6% 5% 5% 3 5% 5% 23%	3570
	21st to end	Day	12% 19% 7% 10% 6% 5% 5% 3 5% 5% 23%	3978
February	1st to 10th	Day	15% 17% 4 12% 7% 4 4 4 4 5% 25%	1800
	11th to 20th	Day	14% 14% 5% 11% 7% 5% 4 3 4 6% 25%	1818
	21st to end	Day	20% 14% 5% 9% 5% 5% 4 3 4 5% 25%	1650
March	1st to 10th	Day	19% 14% 5% 12% 6% 5% 4 3 4 5% 24%	4476
	11th to 20th	Day	11% 14% 4 10% 6% 5% 4 4 5% 6% 30%	4796
	21st to end	Day	19% 10% 4 9% 6% 4 3 3 4 5% 34%	5894
April	1st to 10th	Day	21% 12% 4 7% 5% 4 3 2 4 5% 32%	2400
	11th to 20th	Day	21% 10% 3 7% 5% 3 3 3 5% 5% 34%	2374
	21st to end	Day	22% 10% 4 8% 4 4 4 4 5% 5% 31%	2528
May	1st to 10th	Day	26% 10% 4 9% 5% 5% 5% 4 5% 6% 22%	6346
	11th to 20th	Day	19% 9% 4 9% 5% 5% 4 3 5% 7% 30%	6318
	21st to end	Day	22% 9% 2 7% 4 4 4 4 5% 7% 31%	6949
June	1st to 10th	Day	22% 8% 3 7% 5% 4 4 4 5% 8% 30%	2800
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 37%	2800
	21st to end	Day	20% 7% 3 7% 5% 5% 5% 4 6% 8% 31%	2800
July	1st to 10th	Day	12% 7% 3 8% 6% 6% 5% 5% 6% 9% 32%	6360
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 7% 8% 40%	6360
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 10% 46%	6926
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 9% 34%	4563
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 7% 8% 39%	4088
	21st to end	Day	16% 4 2 6% 4 5% 5% 4 7% 9% 38%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 6% 8% 43%	2400
	11th to 20th	Day	12% 5% 2 5% 3 3 4 3 5% 7% 50%	2350
	21st to end	Day	17% 8% 2 5% 4 4 4 3 6% 7% 41%	2200
October	1st to 10th	Day	20% 7% 3 7% 4 4 4 4 4 6% 38%	2014
	11th to 20th	Day	19% 10% 3 9% 6% 5% 4 3 4 7% 31%	1995
	21st to end	Day	24% 11% 3 7% 4 4 5% 4 5% 6% 28%	1980
November	1st to 10th	Day	17% 9% 4 8% 6% 5% 5% 4 4 6% 32%	1851
	11th to 20th	Day	18% 12% 3 9% 6% 5% 5% 3 5% 6% 28%	1736
	21st to end	Day	21% 14% 4 9% 6% 6% 4 3 5% 6% 21%	1600
December	1st to 10th	Day	21% 21% 6% 10% 5% 5% 4 3 6% 6% 15%	2600
	11th to 20th	Day	19% 16% 6% 10% 4 4 5% 3 4 5% 24%	2986
	21st to end	Day	24% 16% 4 10% 4 4 4 3 4 4 23%	3960
January	1st to 10th	Night	14% 22% 8% 13% 5% 4 4 3 4 5% 19%	6349
	11th to 20th	Night	11% 16% 7% 17% 7% 6% 5% 3 5% 5% 18%	6315
	21st to end	Night	14% 17% 6% 11% 7% 6% 4 3 4 5% 23%	6977
February	1st to 10th	Night	15% 13% 5% 11% 8% 6% 5% 4 6% 5% 23%	2600
	11th to 20th	Night	14% 12% 6% 11% 6% 6% 5% 4 5% 5% 27%	2466
	21st to end	Night	15% 15% 5% 10% 6% 4 4 4 4 5% 29%	1980
March	1st to 10th	Night	21% 14% 4 8% 5% 4 4 3 4 4 29%	5364
	11th to 20th	Night	15% 13% 4 8% 5% 4 5% 3 6% 7% 29%	5145
	21st to end	Night	18% 10% 4 9% 6% 5% 5% 3 4 5% 32%	5036
April	1st to 10th	Night	22% 11% 3 7% 5% 4 3 4 4 6% 31%	2000
	11th to 20th	Night	22% 10% 3 7% 5% 4 4 3 5% 4 34%	1975
	21st to end	Night	22% 8% 2 7% 5% 4 4 3 5% 6% 34%	1880
May	1st to 10th	Night	23% 10% 3 9% 5% 6% 5% 4 5% 7% 24%	3604
	11th to 20th	Night	18% 10% 3 7% 4 4 4 4 5% 7% 34%	3575
	21st to end	Night	18% 11% 3 9% 5% 4 5% 3 5% 8% 30%	3926
June	1st to 10th	Night	20% 7% 2 6% 5% 5% 5% 5% 7% 8% 30%	1600
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 4 5% 7% 40%	1600
	21st to end	Night	19% 6% 3 7% 5% 4 5% 4 5% 8% 35%	1600
July	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 5% 7% 9% 33%	3600
	11th to 20th	Night	13% 6% 3 7% 4 4 4 3 5% 8% 42%	3600
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	3926
August	1st to 10th	Night	14% 7% 3 7% 4 4 4 4 6% 8% 39%	2586
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 5% 7% 42%	2924
	21st to end	Night	14% 6% 3 7% 5% 5% 5% 3 6% 8% 40%	3586
September	1st to 10th	Night	10% 6% 2 7% 5% 4 4 4 5% 7% 45%	2000
	11th to 20th	Night	14% 5% 2 5% 4 3 3 2 5% 7% 50%	2050
	21st to end	Night	17% 5% 2 5% 4 4 4 3 5% 6% 43%	2200
October	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 6% 6% 39%	2383
	11th to 20th	Night	19% 9% 3 8% 5% 5% 4 4 5% 5% 34%	2403
	21st to end	Night	23% 8% 4 8% 5% 4 5% 3 4 5% 30%	2860
November	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 6% 33%	2667
	11th to 20th	Night	19% 10% 3 8% 5% 5% 5% 4 4 6% 31%	2815
	21st to end	Night	21% 13% 4 10% 6% 5% 5% 3 4 6% 24%	2800
December	1st to 10th	Night	18% 17% 6% 12% 7% 5% 5% 4 4 5% 17%	4580
	11th to 20th	Night	23% 16% 5% 10% 5% 4 4 2 3 4 24%	5208
	21st to end	Night	26% 17% 4 9% 4 3 3 3 4 5% 22%	6996

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	52% 26% 8% 11% 3 1	3598
	11th to 20th	Day	48% 26% 8% 11% 6% 1	3600
	21st to end	Day	48% 28% 9% 8% 6% 1	3978
February	1st to 10th	Day	24% 61% 3 8% 3 1	1800
	11th to 20th	Day	24% 60% 4 7% 5%	1827
	21st to end	Day	23% 62% 3 5% 5% 1	1650
March	1st to 10th	Day	47% 28% 5% 9% 8% 3	4479
	11th to 20th	Day	40% 28% 11% 11% 8% 1	4796
	21st to end	Day	41% 27% 3 13% 14% 1	5894
April	1st to 10th	Day	21% 61% 3 8% 6% 1	2400
	11th to 20th	Day	20% 62% 2 9% 6% 1	2374
	21st to end	Day	22% 62% 4 7% 6%	2528
May	1st to 10th	Day	50% 28% 4 14% 3	6357
	11th to 20th	Day	43% 28% 11% 12% 5%	6322
	21st to end	Day	41% 28% 7% 16% 7%	6949
June	1st to 10th	Day	21% 61% 2 9% 6% 1	2800
	11th to 20th	Day	17% 62% 4 9% 6% 1	2800
	21st to end	Day	19% 61% 4 10% 5% 1	2800
July	1st to 10th	Day	37% 27% 10% 19% 6%	6290
	11th to 20th	Day	29% 27% 10% 24% 10%	6285
	21st to end	Day	24% 27% 11% 32% 6%	6728
August	1st to 10th	Day	30% 37% 11% 16% 6%	4549
	11th to 20th	Day	30% 37% 10% 20% 3	4129
	21st to end	Day	27% 37% 12% 16% 7%	4312
September	1st to 10th	Day	15% 57% 7% 14% 6%	2400
	11th to 20th	Day	14% 57% 6% 16% 7% 1	2350
	21st to end	Day	16% 57% 6% 13% 7% 1	2200
October	1st to 10th	Day	28% 36% 8% 22% 5%	3274
	11th to 20th	Day	26% 36% 12% 21% 5%	3223
	21st to end	Day	28% 36% 15% 17% 4	3180
November	1st to 10th	Day	28% 54% 6% 8% 4 1	1851
	11th to 20th	Day	24% 56% 4 11% 6%	1736
	21st to end	Day	28% 58% 6% 5% 3	1600
December	1st to 10th	Day	46% 37% 8% 8% 1	2600
	11th to 20th	Day	46% 32% 6% 10% 6%	2986
	21st to end	Day	49% 26% 6% 14% 6%	3960
January	1st to 10th	Night	54% 25% 10% 9% 2	6349
	11th to 20th	Night	53% 26% 7% 10% 3	6360
	21st to end	Night	49% 28% 7% 7% 9% 1	6977
February	1st to 10th	Night	24% 61% 4 8% 2	2600
	11th to 20th	Night	23% 60% 3 8% 5%	2468
	21st to end	Night	21% 61% 3 8% 7% 1	1980
March	1st to 10th	Night	43% 29% 3 12% 10% 3	5374
	11th to 20th	Night	39% 28% 11% 11% 7% 3	5146
	21st to end	Night	42% 28% 3 17% 9% 1	5036
April	1st to 10th	Night	23% 61% 3 7% 7%	2000
	11th to 20th	Night	22% 62% 2 7% 6% 1	1975
	21st to end	Night	21% 62% 4 7% 6% 1	1880
May	1st to 10th	Night	48% 29% 6% 14% 3	3610
	11th to 20th	Night	40% 28% 10% 14% 8%	3576
	21st to end	Night	43% 28% 7% 14% 8%	3927
June	1st to 10th	Night	19% 62% 3 8% 7% 1	1600
	11th to 20th	Night	16% 62% 4 10% 8% 1	1600
	21st to end	Night	20% 60% 4 9% 7% 1	1600
July	1st to 10th	Night	36% 28% 13% 15% 7%	3560
	11th to 20th	Night	31% 26% 11% 24% 7%	3552
	21st to end	Night	26% 27% 10% 25% 12%	3809
August	1st to 10th	Night	28% 38% 10% 19% 5%	2576
	11th to 20th	Night	31% 38% 11% 18% 3	2949
	21st to end	Night	28% 37% 10% 16% 9% 1	3586
September	1st to 10th	Night	15% 57% 6% 15% 8%	2000
	11th to 20th	Night	14% 57% 6% 16% 6% 1	2050
	21st to end	Night	16% 56% 6% 14% 8%	2200
October	1st to 10th	Night	25% 36% 11% 22% 6% 1	3903
	11th to 20th	Night	26% 36% 10% 22% 7%	3914
	21st to end	Night	29% 36% 12% 14% 7%	4610
November	1st to 10th	Night	28% 53% 6% 8% 4 1	2667
	11th to 20th	Night	26% 55% 4 10% 5%	2815
	21st to end	Night	27% 57% 5% 7% 3	2800
December	1st to 10th	Night	45% 36% 9% 7% 2	4580
	11th to 20th	Night	47% 32% 3 9% 8%	5208
	21st to end	Night	50% 25% 4 11% 10%	6996

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% 18% 32% 16% 3	200
	11th to 20th	Day	31% 26% 29% 10% 4 1	200
	21st to end	Day	19% 22% 39% 17% 3	223
February	1st to 10th	Day	33% 30% 26% 9% 2	300
	11th to 20th	Day	29% 27% 36% 7%	365
	21st to end	Day	31% 20% 34% 14% 1	414
March	1st to 10th	Day	30% 21% 31% 14% 4	500
	11th to 20th	Day	28% 23% 33% 13% 2 1	500
	21st to end	Day	32% 23% 24% 14% 6% 1	550
April	1st to 10th	Day	33% 20% 29% 13% 5%	500
	11th to 20th	Day	35% 19% 26% 13% 6% 1	492
	21st to end	Day	32% 21% 29% 12% 6% 1	500
May	1st to 10th	Day	26% 16% 34% 16% 6% 1	503
	11th to 20th	Day	20% 17% 33% 19% 9% 2	527
	21st to end	Day	22% 17% 30% 23% 7% 1	573
June	1st to 10th	Day	27% 11% 32% 22% 6% 2	530
	11th to 20th	Day	17% 16% 33% 25% 7% 2	530
	21st to end	Day	24% 12% 35% 18% 9% 2	530
July	1st to 10th	Day	16% 9% 33% 26% 13% 2	525
	11th to 20th	Day	14% 11% 33% 27% 12% 2	524
	21st to end	Day	16% 9% 34% 28% 11% 2	576
August	1st to 10th	Day	15% 8% 27% 32% 14% 3	526
	11th to 20th	Day	15% 7% 27% 31% 14% 7%	519
	21st to end	Day	17% 10% 32% 23% 13% 5%	583
September	1st to 10th	Day	18% 10% 30% 21% 16% 6%	530
	11th to 20th	Day	14% 7% 25% 23% 21% 10%	530
	21st to end	Day	19% 14% 31% 19% 14% 4	530
October	1st to 10th	Day	22% 15% 27% 21% 12% 3	530
	11th to 20th	Day	24% 20% 28% 20% 6% 1	527
	21st to end	Day	33% 14% 30% 18% 6%	462
November	1st to 10th	Day	31% 18% 27% 17% 6% 1	425
	11th to 20th	Day	28% 22% 29% 17% 4	294
	21st to end	Day	31% 21% 28% 16% 5% 1	200
December	1st to 10th	Day	30% 18% 33% 16% 4	200
	11th to 20th	Day	31% 26% 30% 11% 1 1	200
	21st to end	Day	32% 22% 28% 15% 2	220
January	1st to 10th	Night	27% 19% 35% 14% 4	800
	11th to 20th	Night	28% 20% 26% 20% 5% 1	800
	21st to end	Night	21% 18% 35% 21% 5%	880
February	1st to 10th	Night	23% 20% 33% 21% 3	800
	11th to 20th	Night	23% 22% 34% 17% 3	783
	21st to end	Night	25% 18% 31% 22% 2	660
March	1st to 10th	Night	25% 19% 30% 22% 4	800
	11th to 20th	Night	27% 18% 33% 19% 4 1	800
	21st to end	Night	26% 17% 27% 24% 6% 1	880
April	1st to 10th	Night	31% 16% 28% 21% 4	800
	11th to 20th	Night	27% 18% 29% 20% 5%	791
	21st to end	Night	29% 14% 29% 20% 8%	680
May	1st to 10th	Night	26% 11% 30% 23% 7% 2	410
	11th to 20th	Night	23% 11% 30% 25% 9% 1	394
	21st to end	Night	16% 11% 37% 27% 9%	433
June	1st to 10th	Night	22% 9% 29% 32% 9% 1	400
	11th to 20th	Night	21% 9% 27% 31% 12% 1	400
	21st to end	Night	18% 9% 31% 27% 15% 1	400
July	1st to 10th	Night	13% 7% 30% 34% 15% 1	390
	11th to 20th	Night	14% 5% 29% 37% 15%	388
	21st to end	Night	13% 4 31% 37% 13% 1	438
August	1st to 10th	Night	14% 6% 23% 39% 18% 1	398
	11th to 20th	Night	14% 2 26% 34% 22% 2	615
	21st to end	Night	17% 6% 28% 31% 17% 2	880
September	1st to 10th	Night	14% 6% 31% 33% 15% 1	800
	11th to 20th	Night	18% 6% 21% 31% 22% 3	800
	21st to end	Night	20% 9% 28% 24% 18% 2	800
October	1st to 10th	Night	23% 10% 27% 28% 12% 1	800
	11th to 20th	Night	24% 12% 31% 24% 10%	800
	21st to end	Night	28% 15% 28% 25% 4	880
November	1st to 10th	Night	29% 17% 24% 23% 7% 1	820
	11th to 20th	Night	24% 19% 32% 16% 8%	829
	21st to end	Night	29% 19% 26% 20% 7%	800
December	1st to 10th	Night	31% 18% 29% 19% 4	800
	11th to 20th	Night	32% 21% 26% 16% 5% 1	800
	21st to end	Night	37% 17% 26% 15% 4	880

Page generated at 2025-11-15 00:56:01. Version ac07

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