Colliers Mills WMA
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Colliers Mills WMA clear sky chart from 2005-11-06 to 2025-12-18.

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	16% 7% 2 6% 4 4 4 4 5% 7% 40%	127136
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	123855
Transparency	Day	42% 5% 11% 22% 17% 3	130676
	Night	41% 4 11% 23% 17% 4	128402
Seeing	Day	21% 14% 29% 22% 11% 3	16450
	Night	19% 11% 31% 29% 10% 1	25690

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	13% 8% 2 5% 3 3 3 3 4 5% 52%	3597
	11th to 20th	Day	14% 8% 3 8% 5% 4 3 3 4 5% 43%	3570
	21st to end	Day	19% 9% 3 7% 4 4 3 2 4 5% 41%	4445
February	1st to 10th	Day	15% 9% 2 7% 5% 4 4 3 5% 6% 40%	1800
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 4 6% 41%	1948
	21st to end	Day	17% 8% 3 8% 4 3 4 3 6% 5% 39%	1650
March	1st to 10th	Day	21% 7% 2 4 3 3 3 3 4 5% 45%	4476
	11th to 20th	Day	11% 8% 2 6% 4 4 5% 5% 6% 7% 44%	4860
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 6% 43%	5873
April	1st to 10th	Day	18% 8% 2 6% 4 5% 4 3 5% 6% 39%	2400
	11th to 20th	Day	15% 6% 2 6% 5% 4 4 3 6% 7% 41%	2374
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 5% 7% 40%	2400
May	1st to 10th	Day	24% 9% 3 6% 4 4 4 4 5% 7% 29%	5833
	11th to 20th	Day	24% 9% 3 7% 3 4 4 3 4 7% 34%	6272
	21st to end	Day	20% 7% 2 6% 4 5% 4 4 6% 8% 35%	6949
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 38%	2800
	11th to 20th	Day	13% 7% 3 6% 5% 4 5% 4 6% 9% 39%	2800
	21st to end	Day	11% 5% 1 7% 5% 5% 6% 4 6% 8% 41%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 6% 6% 5% 7% 10% 36%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 5% 8% 10% 30%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 7% 10% 38%	6926
August	1st to 10th	Day	19% 6% 3 8% 5% 5% 6% 4 7% 8% 31%	4175
	11th to 20th	Day	13% 7% 2 6% 5% 5% 5% 5% 6% 9% 38%	3850
	21st to end	Day	10% 5% 2 6% 5% 4 5% 5% 7% 10% 43%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 5% 4 3 5% 7% 44%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 8% 49%	2332
	21st to end	Day	18% 8% 3 7% 3 4 5% 4 5% 6% 38%	2200
October	1st to 10th	Day	17% 7% 2 6% 4 3 3 2 5% 6% 45%	2060
	11th to 20th	Day	15% 7% 2 4 3 3 4 3 4 6% 49%	2000
	21st to end	Day	17% 6% 2 6% 3 3 3 3 5% 6% 47%	2084
November	1st to 10th	Day	13% 6% 2 5% 4 3 3 2 4 7% 52%	1851
	11th to 20th	Day	14% 6% 2 5% 4 3 3 4 5% 6% 47%	1890
	21st to end	Day	15% 5% 2 6% 4 4 3 3 5% 6% 46%	1807
December	1st to 10th	Day	12% 8% 2 6% 4 4 4 3 6% 6% 45%	2680
	11th to 20th	Day	21% 9% 2 4 3 3 3 3 3 5% 45%	3042
	21st to end	Day	16% 6% 2 5% 4 4 4 4 6% 7% 41%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 4 3 3 4 6% 51%	6349
	11th to 20th	Night	15% 7% 2 6% 4 3 3 3 4 5% 49%	6315
	21st to end	Night	17% 9% 2 6% 4 3 4 3 5% 6% 41%	6508
February	1st to 10th	Night	16% 9% 3 6% 4 4 3 3 5% 7% 40%	2598
	11th to 20th	Night	18% 9% 2 6% 4 3 3 3 5% 6% 41%	2338
	21st to end	Night	17% 9% 2 6% 3 3 4 3 5% 6% 42%	1980
March	1st to 10th	Night	19% 8% 2 6% 3 3 3 2 4 5% 45%	5364
	11th to 20th	Night	12% 10% 2 7% 4 3 3 3 4 5% 48%	5084
	21st to end	Night	16% 8% 3 7% 4 4 4 3 5% 5% 42%	5058
April	1st to 10th	Night	17% 8% 3 7% 4 4 3 3 5% 7% 40%	2000
	11th to 20th	Night	16% 8% 2 7% 4 4 4 3 5% 6% 42%	1975
	21st to end	Night	17% 6% 2 4 4 3 4 3 6% 7% 44%	2000
May	1st to 10th	Night	21% 10% 3 8% 4 4 4 4 6% 8% 28%	4105
	11th to 20th	Night	22% 8% 3 7% 5% 3 4 3 5% 7% 33%	3649
	21st to end	Night	21% 8% 2 6% 4 4 4 3 5% 8% 36%	3926
June	1st to 10th	Night	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	1600
	11th to 20th	Night	11% 8% 3 6% 5% 5% 5% 4 6% 8% 40%	1600
	21st to end	Night	8% 5% 3 6% 5% 5% 6% 5% 7% 11% 40%	1600
July	1st to 10th	Night	8% 6% 3 8% 6% 5% 6% 5% 7% 10% 35%	3600
	11th to 20th	Night	12% 8% 4 7% 5% 5% 5% 4 7% 11% 32%	3600
	21st to end	Night	11% 7% 3 6% 4 4 5% 4 6% 9% 42%	3926
August	1st to 10th	Night	23% 6% 2 6% 4 4 5% 4 7% 8% 31%	2912
	11th to 20th	Night	12% 6% 3 7% 4 5% 5% 5% 5% 7% 42%	3197
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 8% 51%	3586
September	1st to 10th	Night	13% 7% 2 6% 5% 4 5% 3 5% 6% 45%	2000
	11th to 20th	Night	10% 5% 2 5% 3 3 4 4 4 7% 52%	2066
	21st to end	Night	18% 7% 2 7% 3 3 4 3 5% 7% 40%	2200
October	1st to 10th	Night	18% 6% 2 4 3 4 4 3 5% 6% 45%	2335
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 4 6% 51%	2400
	21st to end	Night	17% 7% 2 5% 3 3 3 3 5% 5% 47%	2755
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2667
	11th to 20th	Night	16% 7% 2 6% 4 3 3 2 4 5% 47%	2730
	21st to end	Night	13% 7% 2 6% 4 4 4 3 5% 6% 46%	2809
December	1st to 10th	Night	14% 7% 2 5% 3 4 3 3 5% 6% 48%	4720
	11th to 20th	Night	22% 7% 2 5% 4 3 3 2 4 6% 41%	5307
	21st to end	Night	19% 8% 2 6% 4 3 3 3 5% 5% 42%	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	35% 3 8% 28% 23% 3	3598
	11th to 20th	Day	43% 4 8% 23% 17% 5%	3600
	21st to end	Day	47% 3 10% 23% 14% 2	4445
February	1st to 10th	Day	45% 4 13% 24% 12% 2	1800
	11th to 20th	Day	45% 5% 9% 23% 16% 3	1957
	21st to end	Day	46% 4 9% 22% 14% 4	1650
March	1st to 10th	Day	42% 3 9% 23% 15% 7%	4479
	11th to 20th	Day	38% 5% 9% 28% 15% 5%	4860
	21st to end	Day	43% 3 9% 16% 23% 6%	5873
April	1st to 10th	Day	45% 6% 9% 20% 16% 5%	2400
	11th to 20th	Day	40% 5% 12% 23% 17% 3	2374
	21st to end	Day	42% 5% 10% 20% 19% 4	2400
May	1st to 10th	Day	54% 5% 14% 17% 9% 1	5844
	11th to 20th	Day	51% 4 10% 22% 12% 1	6276
	21st to end	Day	47% 5% 10% 22% 14% 2	6949
June	1st to 10th	Day	41% 7% 14% 22% 15% 2	2800
	11th to 20th	Day	42% 7% 12% 25% 12% 3	2800
	21st to end	Day	38% 8% 13% 22% 17% 2	2800
July	1st to 10th	Day	41% 9% 15% 21% 13% 1	6290
	11th to 20th	Day	46% 8% 19% 18% 10% 1	6285
	21st to end	Day	40% 6% 10% 22% 21% 1	6728
August	1st to 10th	Day	49% 8% 13% 20% 10% 1	4161
	11th to 20th	Day	41% 7% 13% 25% 13% 1	3886
	21st to end	Day	35% 6% 13% 25% 19% 2	4312
September	1st to 10th	Day	39% 6% 12% 20% 21% 3	2400
	11th to 20th	Day	34% 5% 10% 24% 24% 3	2332
	21st to end	Day	47% 5% 9% 18% 19% 2	2200
October	1st to 10th	Day	36% 4 11% 19% 26% 5%	3359
	11th to 20th	Day	26% 3 10% 23% 32% 7%	3240
	21st to end	Day	27% 3 12% 26% 27% 5%	3348
November	1st to 10th	Day	34% 3 13% 27% 20% 3	1851
	11th to 20th	Day	38% 4 9% 24% 22% 3	1890
	21st to end	Day	39% 4 13% 29% 13% 2	1807
December	1st to 10th	Day	38% 4 15% 26% 15% 2	2680
	11th to 20th	Day	43% 4 9% 29% 12% 3	3042
	21st to end	Day	41% 6% 11% 23% 18% 2	3960
January	1st to 10th	Night	34% 4 8% 25% 22% 6%	6349
	11th to 20th	Night	39% 3 10% 24% 17% 7%	6360
	21st to end	Night	44% 3 9% 23% 18% 3	6508
February	1st to 10th	Night	44% 4 14% 23% 13% 2	2598
	11th to 20th	Night	44% 3 9% 20% 19% 5%	2340
	21st to end	Night	44% 3 7% 23% 18% 5%	1980
March	1st to 10th	Night	42% 3 8% 24% 13% 10%	5374
	11th to 20th	Night	39% 4 13% 25% 14% 4	5085
	21st to end	Night	45% 4 7% 16% 23% 5%	5058
April	1st to 10th	Night	45% 5% 9% 20% 18% 5%	2000
	11th to 20th	Night	43% 5% 9% 20% 20% 3	1975
	21st to end	Night	39% 5% 10% 20% 21% 5%	2000
May	1st to 10th	Night	53% 5% 12% 17% 10% 2	4111
	11th to 20th	Night	52% 5% 11% 17% 14% 2	3650
	21st to end	Night	47% 5% 12% 22% 13% 1	3927
June	1st to 10th	Night	40% 8% 11% 20% 19% 1	1600
	11th to 20th	Night	41% 7% 14% 24% 13% 1	1600
	21st to end	Night	36% 8% 16% 23% 15% 2	1600
July	1st to 10th	Night	43% 8% 15% 20% 12% 1	3560
	11th to 20th	Night	44% 8% 18% 21% 9% 1	3552
	21st to end	Night	38% 5% 15% 22% 18% 1	3809
August	1st to 10th	Night	49% 7% 18% 16% 9% 1	2902
	11th to 20th	Night	39% 6% 12% 31% 11% 1	3227
	21st to end	Night	32% 6% 10% 29% 22% 2	3586
September	1st to 10th	Night	40% 5% 10% 22% 20% 2	2000
	11th to 20th	Night	32% 5% 11% 23% 25% 4	2066
	21st to end	Night	44% 5% 8% 22% 20% 2	2200
October	1st to 10th	Night	37% 5% 10% 24% 22% 3	3816
	11th to 20th	Night	25% 3 10% 27% 29% 6%	3899
	21st to end	Night	27% 3 11% 30% 24% 4	4441
November	1st to 10th	Night	33% 5% 11% 25% 22% 4	2667
	11th to 20th	Night	40% 4 8% 23% 20% 5%	2730
	21st to end	Night	37% 5% 12% 27% 17% 2	2809
December	1st to 10th	Night	38% 3 12% 30% 15% 2	4720
	11th to 20th	Night	44% 3 10% 24% 14% 4	5307
	21st to end	Night	45% 3 11% 20% 18% 3	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	26% 25% 33% 13% 4 1	200
	11th to 20th	Day	33% 26% 28% 13% 1	200
	21st to end	Day	27% 25% 29% 15% 4 1	327
February	1st to 10th	Day	27% 27% 33% 11% 3	300
	11th to 20th	Day	31% 22% 34% 11% 1	496
	21st to end	Day	29% 20% 32% 16% 2	414
March	1st to 10th	Day	27% 20% 32% 17% 4	500
	11th to 20th	Day	23% 25% 32% 14% 4 1	500
	21st to end	Day	26% 21% 32% 16% 5% 1	550
April	1st to 10th	Day	26% 23% 33% 12% 5% 1	500
	11th to 20th	Day	23% 18% 34% 16% 7% 2	492
	21st to end	Day	26% 17% 31% 19% 4 2	500
May	1st to 10th	Day	30% 17% 29% 16% 7% 2	503
	11th to 20th	Day	23% 13% 34% 20% 8% 2	527
	21st to end	Day	23% 8% 23% 27% 16% 3	573
June	1st to 10th	Day	17% 10% 28% 28% 13% 3	530
	11th to 20th	Day	16% 12% 30% 26% 13% 2	530
	21st to end	Day	13% 8% 29% 32% 15% 3	530
July	1st to 10th	Day	12% 5% 23% 33% 21% 6%	525
	11th to 20th	Day	9% 5% 25% 32% 24% 6%	524
	21st to end	Day	13% 9% 26% 32% 16% 5%	576
August	1st to 10th	Day	12% 5% 27% 32% 17% 7%	526
	11th to 20th	Day	15% 4 21% 32% 21% 7%	519
	21st to end	Day	11% 5% 27% 30% 17% 10%	583
September	1st to 10th	Day	17% 8% 28% 23% 17% 8%	530
	11th to 20th	Day	13% 5% 28% 31% 14% 9%	530
	21st to end	Day	24% 12% 24% 25% 12% 5%	530
October	1st to 10th	Day	23% 8% 25% 23% 13% 8%	530
	11th to 20th	Day	20% 14% 26% 26% 10% 3	530
	21st to end	Day	19% 14% 34% 20% 11% 2	519
November	1st to 10th	Day	16% 14% 35% 23% 12% 1	425
	11th to 20th	Day	24% 17% 30% 21% 7%	430
	21st to end	Day	23% 15% 35% 20% 7% 1	347
December	1st to 10th	Day	25% 27% 31% 9% 6% 1	220
	11th to 20th	Day	34% 20% 29% 13% 5%	214
	21st to end	Day	26% 17% 30% 21% 5% 1	220
January	1st to 10th	Night	20% 19% 37% 20% 4	800
	11th to 20th	Night	25% 15% 34% 22% 4	800
	21st to end	Night	20% 17% 39% 22% 2	880
February	1st to 10th	Night	21% 15% 37% 25% 2	800
	11th to 20th	Night	26% 20% 37% 16% 1	783
	21st to end	Night	23% 20% 35% 18% 2	660
March	1st to 10th	Night	22% 17% 36% 21% 4	800
	11th to 20th	Night	21% 17% 36% 20% 5% 1	800
	21st to end	Night	20% 13% 40% 21% 5%	880
April	1st to 10th	Night	21% 19% 37% 18% 5%	800
	11th to 20th	Night	20% 13% 37% 26% 4	791
	21st to end	Night	20% 12% 36% 26% 5%	800
May	1st to 10th	Night	20% 14% 33% 27% 6%	625
	11th to 20th	Night	21% 10% 35% 28% 5%	429
	21st to end	Night	18% 6% 22% 39% 14% 1	433
June	1st to 10th	Night	17% 5% 29% 39% 11% 1	400
	11th to 20th	Night	14% 5% 28% 40% 14%	400
	21st to end	Night	11% 4 25% 47% 13%	400
July	1st to 10th	Night	11% 4 22% 44% 19% 1	390
	11th to 20th	Night	7% 4 21% 46% 22% 1	388
	21st to end	Night	11% 3 22% 47% 17% 1	438
August	1st to 10th	Night	13% 2 18% 44% 21% 2	594
	11th to 20th	Night	12% 4 17% 41% 23% 2	790
	21st to end	Night	13% 5% 18% 39% 23% 2	880
September	1st to 10th	Night	16% 3 25% 36% 19% 3	800
	11th to 20th	Night	13% 5% 20% 35% 23% 4	800
	21st to end	Night	22% 5% 20% 36% 16% 1	800
October	1st to 10th	Night	22% 5% 26% 26% 17% 4	800
	11th to 20th	Night	17% 11% 27% 33% 11% 1	800
	21st to end	Night	21% 9% 34% 27% 9%	880
November	1st to 10th	Night	15% 9% 36% 29% 11% 1	820
	11th to 20th	Night	19% 16% 30% 27% 9%	840
	21st to end	Night	17% 14% 35% 26% 7% 1	840
December	1st to 10th	Night	25% 14% 35% 19% 6% 1	840
	11th to 20th	Night	22% 14% 36% 24% 5%	829
	21st to end	Night	21% 13% 35% 28% 3	880

Page generated at 2025-12-15 01:13:56. Version ac07

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