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 2024-01-13.

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	15% 6% 3 5% 5% 3 5% 5% 5% 7% 41%	74752
	Night	15% 6% 3 5% 5% 3 4 4 5% 7% 43%	74977
Transparency	Day	42% 6% 12% 22% 16% 3	74457
	Night	41% 5% 12% 23% 17% 3	74806
Seeing	Day	20% 13% 29% 23% 11% 4	13905
	Night	18% 11% 31% 29% 10% 1	23236

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% 7% 3 6% 6% 3 4 4 5% 5% 44%	1517
	11th to 20th	Day	16% 8% 3 5% 6% 3 4 5% 5% 7% 40%	1456
	21st to end	Day	16% 7% 2 4 5% 2 5% 4 5% 7% 42%	1775
February	1st to 10th	Day	16% 8% 2 6% 6% 4 4 3 4 6% 39%	1620
	11th to 20th	Day	17% 7% 3 6% 5% 3 5% 4 4 6% 41%	1748
	21st to end	Day	18% 8% 3 7% 5% 3 4 4 6% 4 38%	1480
March	1st to 10th	Day	14% 7% 2 5% 5% 2 4 4 4 7% 46%	1797
	11th to 20th	Day	15% 7% 2 5% 5% 2 5% 5% 6% 7% 40%	1930
	21st to end	Day	21% 7% 2 5% 5% 2 4 4 5% 6% 40%	2335
April	1st to 10th	Day	16% 8% 2 5% 4 3 5% 4 5% 6% 41%	2160
	11th to 20th	Day	15% 5% 2 4 5% 3 4 4 7% 8% 42%	2134
	21st to end	Day	17% 6% 2 5% 5% 4 6% 5% 5% 6% 39%	2160
May	1st to 10th	Day	20% 7% 3 5% 5% 3 4 5% 6% 7% 36%	2315
	11th to 20th	Day	18% 7% 2 5% 5% 3 5% 5% 5% 7% 37%	2460
	21st to end	Day	14% 7% 3 4 6% 3 5% 4 5% 8% 41%	2725
June	1st to 10th	Day	13% 5% 2 5% 5% 4 6% 6% 6% 9% 38%	2520
	11th to 20th	Day	13% 6% 3 5% 6% 3 5% 5% 5% 9% 38%	2520
	21st to end	Day	12% 5% 2 6% 6% 4 6% 6% 6% 8% 41%	2520
July	1st to 10th	Day	10% 5% 3 5% 5% 5% 7% 7% 7% 9% 38%	2520
	11th to 20th	Day	8% 5% 3 5% 6% 4 6% 6% 7% 11% 39%	2520
	21st to end	Day	10% 6% 3 7% 6% 4 6% 5% 7% 9% 36%	2744
August	1st to 10th	Day	10% 5% 3 6% 6% 4 6% 6% 8% 9% 37%	2310
	11th to 20th	Day	12% 5% 2 6% 6% 4 6% 6% 7% 8% 38%	2126
	21st to end	Day	13% 6% 3 6% 5% 3 5% 6% 6% 9% 38%	2376
September	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 4 5% 7% 43%	2160
	11th to 20th	Day	11% 6% 2 5% 4 3 4 5% 4 8% 48%	2100
	21st to end	Day	16% 7% 3 5% 4 3 5% 5% 5% 6% 40%	1980
October	1st to 10th	Day	18% 7% 2 5% 5% 2 3 3 5% 6% 43%	1854
	11th to 20th	Day	15% 6% 2 4 4 3 3 5% 4 6% 48%	1800
	21st to end	Day	18% 6% 3 6% 3 2 3 3 5% 6% 45%	1876
November	1st to 10th	Day	14% 5% 2 4 4 2 3 3 4 6% 52%	1671
	11th to 20th	Day	16% 5% 2 4 5% 2 4 4 5% 6% 47%	1710
	21st to end	Day	14% 4 2 6% 4 3 4 4 5% 7% 47%	1635
December	1st to 10th	Day	17% 8% 3 6% 5% 2 4 4 5% 6% 40%	1520
	11th to 20th	Day	20% 7% 3 4 3 3 4 4 4 5% 43%	1906
	21st to end	Day	17% 7% 2 5% 5% 4 5% 6% 6% 7% 37%	2772
January	1st to 10th	Night	16% 6% 2 5% 4 2 4 4 5% 6% 46%	2649
	11th to 20th	Night	19% 5% 3 5% 4 3 5% 5% 5% 6% 40%	2560
	21st to end	Night	15% 7% 2 4 4 3 4 5% 4 7% 44%	2578
February	1st to 10th	Night	16% 9% 3 5% 5% 3 4 4 5% 7% 40%	2339
	11th to 20th	Night	18% 9% 3 6% 4 2 4 3 5% 6% 40%	2098
	21st to end	Night	18% 8% 2 6% 4 2 3 4 4 6% 42%	1776
March	1st to 10th	Night	16% 5% 2 5% 4 2 4 4 5% 6% 47%	2150
	11th to 20th	Night	17% 7% 3 6% 5% 3 3 4 4 5% 43%	2028
	21st to end	Night	18% 7% 3 6% 4 3 4 3 5% 5% 42%	2020
April	1st to 10th	Night	16% 8% 3 6% 4 2 4 4 5% 7% 42%	1800
	11th to 20th	Night	15% 7% 2 6% 5% 3 4 4 5% 6% 43%	1775
	21st to end	Night	19% 6% 2 3 4 2 4 4 6% 7% 43%	1800
May	1st to 10th	Night	18% 7% 3 8% 5% 3 4 5% 4 7% 36%	1643
	11th to 20th	Night	17% 8% 3 5% 5% 3 4 5% 6% 7% 37%	1447
	21st to end	Night	14% 7% 2 5% 5% 3 5% 5% 5% 8% 40%	1550
June	1st to 10th	Night	14% 6% 2 5% 5% 3 5% 5% 6% 8% 42%	1440
	11th to 20th	Night	11% 7% 3 5% 6% 4 5% 5% 6% 8% 40%	1440
	21st to end	Night	8% 5% 3 5% 6% 4 6% 6% 7% 10% 41%	1440
July	1st to 10th	Night	10% 5% 3 6% 5% 4 5% 6% 6% 10% 38%	1440
	11th to 20th	Night	7% 5% 3 5% 5% 3 5% 5% 8% 9% 44%	1440
	21st to end	Night	8% 5% 3 5% 4 3 5% 6% 7% 11% 42%	1574
August	1st to 10th	Night	9% 6% 2 7% 4 4 5% 6% 7% 9% 41%	1608
	11th to 20th	Night	10% 5% 4 6% 6% 5% 5% 6% 6% 7% 40%	1774
	21st to end	Night	12% 6% 3 6% 4 3 4 5% 6% 9% 41%	1980
September	1st to 10th	Night	14% 6% 2 5% 6% 3 4 4 5% 6% 44%	1800
	11th to 20th	Night	11% 5% 2 4 4 2 4 4 4 7% 53%	1858
	21st to end	Night	17% 7% 2 6% 4 2 4 4 5% 7% 42%	1980
October	1st to 10th	Night	19% 6% 2 4 4 2 5% 4 4 6% 43%	2101
	11th to 20th	Night	14% 5% 2 4 5% 2 4 4 4 6% 50%	2160
	21st to end	Night	18% 8% 2 4 4 1 4 3 4 5% 46%	2479
November	1st to 10th	Night	12% 5% 2 4 4 2 3 3 5% 6% 52%	2407
	11th to 20th	Night	17% 6% 2 5% 4 2 4 3 4 6% 47%	2470
	21st to end	Night	13% 5% 2 5% 4 3 4 5% 5% 7% 46%	2541
December	1st to 10th	Night	21% 7% 2 4 4 3 4 4 6% 6% 40%	2660
	11th to 20th	Night	18% 5% 3 5% 5% 3 4 4 5% 7% 42%	3288
	21st to end	Night	18% 8% 3 5% 5% 3 4 4 5% 6% 39%	4884

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	42% 3 12% 23% 15% 5%	1518
	11th to 20th	Day	44% 5% 13% 22% 13% 3	1456
	21st to end	Day	41% 4 11% 25% 15% 3	1775
February	1st to 10th	Day	46% 4 12% 24% 12% 2	1620
	11th to 20th	Day	45% 5% 9% 24% 15% 3	1757
	21st to end	Day	47% 4 9% 21% 14% 5%	1480
March	1st to 10th	Day	39% 5% 12% 27% 14% 3	1800
	11th to 20th	Day	42% 5% 11% 24% 15% 3	1930
	21st to end	Day	47% 5% 7% 20% 16% 4	2335
April	1st to 10th	Day	43% 6% 9% 21% 16% 5%	2160
	11th to 20th	Day	38% 6% 12% 24% 17% 3	2134
	21st to end	Day	44% 5% 10% 19% 18% 4	2160
May	1st to 10th	Day	48% 5% 10% 19% 15% 3	2315
	11th to 20th	Day	46% 5% 12% 21% 14% 2	2464
	21st to end	Day	43% 6% 14% 22% 13% 3	2725
June	1st to 10th	Day	41% 7% 14% 22% 14% 2	2520
	11th to 20th	Day	42% 7% 12% 24% 12% 2	2520
	21st to end	Day	39% 9% 13% 22% 16% 1	2520
July	1st to 10th	Day	40% 9% 15% 20% 15% 2	2450
	11th to 20th	Day	36% 8% 16% 24% 15% 2	2445
	21st to end	Day	42% 7% 11% 22% 15% 2	2591
August	1st to 10th	Day	40% 8% 14% 22% 15% 1	2296
	11th to 20th	Day	42% 8% 14% 20% 15% 2	2126
	21st to end	Day	41% 7% 12% 20% 17% 3	2376
September	1st to 10th	Day	40% 6% 12% 19% 19% 3	2160
	11th to 20th	Day	35% 5% 10% 23% 24% 3	2100
	21st to end	Day	44% 5% 9% 18% 21% 2	1980
October	1st to 10th	Day	42% 5% 11% 22% 18% 2	1854
	11th to 20th	Day	37% 4 10% 24% 23% 2	1800
	21st to end	Day	41% 4 10% 23% 20% 2	1876
November	1st to 10th	Day	35% 3 12% 28% 19% 2	1671
	11th to 20th	Day	38% 4 9% 23% 23% 3	1710
	21st to end	Day	38% 4 13% 30% 13% 3	1635
December	1st to 10th	Day	45% 4 15% 21% 14% 1	1520
	11th to 20th	Day	42% 5% 11% 26% 13% 2	1906
	21st to end	Day	44% 6% 12% 24% 13% 1	2772
January	1st to 10th	Night	39% 5% 10% 25% 18% 4	2649
	11th to 20th	Night	44% 5% 14% 23% 12% 2	2560
	21st to end	Night	40% 5% 9% 25% 18% 3	2578
February	1st to 10th	Night	45% 4 14% 22% 13% 2	2339
	11th to 20th	Night	45% 4 9% 20% 18% 5%	2100
	21st to end	Night	44% 3 7% 23% 18% 4	1776
March	1st to 10th	Night	38% 4 11% 25% 18% 4	2160
	11th to 20th	Night	44% 4 9% 21% 17% 4	2029
	21st to end	Night	45% 3 9% 19% 18% 6%	2020
April	1st to 10th	Night	43% 4 10% 20% 18% 5%	1800
	11th to 20th	Night	42% 5% 9% 21% 20% 3	1775
	21st to end	Night	41% 5% 10% 19% 20% 5%	1800
May	1st to 10th	Night	48% 5% 9% 19% 17% 2	1643
	11th to 20th	Night	46% 6% 11% 17% 18% 3	1448
	21st to end	Night	44% 7% 14% 19% 13% 2	1551
June	1st to 10th	Night	40% 8% 12% 20% 19% 1	1440
	11th to 20th	Night	41% 7% 14% 23% 13% 1	1440
	21st to end	Night	34% 8% 17% 24% 15% 2	1440
July	1st to 10th	Night	40% 9% 16% 19% 13% 2	1400
	11th to 20th	Night	34% 8% 16% 26% 15% 2	1392
	21st to end	Night	34% 7% 17% 26% 14% 3	1486
August	1st to 10th	Night	38% 7% 17% 21% 15% 1	1598
	11th to 20th	Night	40% 8% 14% 23% 14% 1	1774
	21st to end	Night	39% 7% 14% 18% 19% 3	1980
September	1st to 10th	Night	41% 6% 11% 22% 18% 2	1800
	11th to 20th	Night	32% 5% 12% 23% 24% 4	1858
	21st to end	Night	42% 5% 7% 23% 21% 2	1980
October	1st to 10th	Night	42% 6% 10% 23% 16% 2	2101
	11th to 20th	Night	36% 4 12% 28% 18% 3	2160
	21st to end	Night	41% 3 10% 24% 19% 2	2479
November	1st to 10th	Night	33% 5% 11% 25% 21% 4	2407
	11th to 20th	Night	40% 4 8% 23% 19% 5%	2470
	21st to end	Night	37% 5% 13% 27% 17% 2	2541
December	1st to 10th	Night	45% 4 12% 23% 14% 2	2660
	11th to 20th	Night	41% 5% 12% 20% 18% 4	3288
	21st to end	Night	46% 4 13% 25% 12% 1	4884

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	23% 24% 34% 14% 4 1	180
	11th to 20th	Day	30% 27% 26% 16% 1	164
	21st to end	Day	25% 24% 31% 15% 4 1	262
February	1st to 10th	Day	28% 28% 31% 10% 3	240
	11th to 20th	Day	28% 22% 38% 11% 2	416
	21st to end	Day	29% 21% 32% 16% 2	346
March	1st to 10th	Day	24% 21% 32% 19% 4	420
	11th to 20th	Day	23% 23% 34% 14% 4 1	420
	21st to end	Day	26% 19% 32% 16% 5% 1	462
April	1st to 10th	Day	22% 24% 34% 13% 6% 1	420
	11th to 20th	Day	20% 17% 34% 18% 8% 3	412
	21st to end	Day	29% 17% 32% 16% 4 2	420
May	1st to 10th	Day	28% 17% 28% 17% 8% 2	423
	11th to 20th	Day	22% 13% 34% 20% 8% 3	447
	21st to end	Day	22% 6% 21% 29% 17% 4	485
June	1st to 10th	Day	16% 10% 29% 28% 14% 4	450
	11th to 20th	Day	15% 12% 30% 27% 13% 3	450
	21st to end	Day	13% 7% 28% 33% 15% 3	450
July	1st to 10th	Day	12% 4 21% 32% 23% 7%	445
	11th to 20th	Day	8% 5% 24% 31% 25% 7%	444
	21st to end	Day	14% 9% 24% 32% 16% 5%	488
August	1st to 10th	Day	11% 4 26% 34% 17% 8%	446
	11th to 20th	Day	14% 4 21% 32% 23% 7%	439
	21st to end	Day	12% 4 26% 29% 18% 11%	495
September	1st to 10th	Day	17% 6% 26% 22% 19% 9%	450
	11th to 20th	Day	12% 6% 28% 29% 15% 10%	450
	21st to end	Day	22% 12% 24% 24% 13% 5%	450
October	1st to 10th	Day	26% 8% 23% 23% 14% 6%	450
	11th to 20th	Day	19% 13% 26% 29% 11% 3	450
	21st to end	Day	20% 14% 33% 20% 10% 2	443
November	1st to 10th	Day	15% 13% 33% 25% 13% 2	365
	11th to 20th	Day	24% 16% 30% 22% 8%	370
	21st to end	Day	20% 13% 36% 22% 8% 1	295
December	1st to 10th	Day	25% 26% 31% 9% 8% 1	180
	11th to 20th	Day	33% 18% 30% 14% 5%	180
	21st to end	Day	27% 16% 31% 21% 4 1	198
January	1st to 10th	Night	19% 18% 38% 21% 4	760
	11th to 20th	Night	25% 13% 35% 23% 4	732
	21st to end	Night	19% 18% 38% 23% 2	792
February	1st to 10th	Night	21% 16% 37% 24% 2	720
	11th to 20th	Night	24% 20% 38% 17% 1	703
	21st to end	Night	23% 21% 36% 18% 2	592
March	1st to 10th	Night	21% 16% 37% 22% 4	720
	11th to 20th	Night	22% 16% 36% 20% 5% 1	720
	21st to end	Night	20% 12% 39% 23% 6%	792
April	1st to 10th	Night	21% 18% 38% 18% 5%	720
	11th to 20th	Night	18% 12% 38% 28% 5%	711
	21st to end	Night	22% 11% 35% 27% 5%	720
May	1st to 10th	Night	20% 14% 33% 27% 6%	563
	11th to 20th	Night	22% 9% 35% 29% 5%	386
	21st to end	Night	17% 5% 22% 40% 15% 1	389
June	1st to 10th	Night	17% 4 27% 39% 12% 1	360
	11th to 20th	Night	13% 5% 28% 41% 14%	360
	21st to end	Night	10% 2 25% 50% 13%	360
July	1st to 10th	Night	11% 4 21% 43% 20% 1	350
	11th to 20th	Night	6% 4 20% 47% 22% 1	348
	21st to end	Night	10% 2 22% 48% 17% 1	394
August	1st to 10th	Night	12% 1 19% 45% 21% 3	534
	11th to 20th	Night	12% 3 17% 41% 24% 3	710
	21st to end	Night	13% 5% 16% 40% 24% 2	792
September	1st to 10th	Night	16% 2 23% 36% 20% 3	720
	11th to 20th	Night	13% 5% 20% 35% 22% 5%	720
	21st to end	Night	20% 5% 21% 36% 17% 1	720
October	1st to 10th	Night	23% 5% 26% 26% 16% 4	720
	11th to 20th	Night	17% 10% 27% 34% 11% 1	720
	21st to end	Night	23% 8% 35% 25% 9% 1	792
November	1st to 10th	Night	15% 8% 36% 28% 12% 1	740
	11th to 20th	Night	19% 14% 29% 28% 9% 1	760
	21st to end	Night	16% 13% 36% 26% 8% 1	760
December	1st to 10th	Night	25% 14% 34% 19% 7% 1	760
	11th to 20th	Night	20% 14% 37% 24% 5%	760
	21st to end	Night	21% 14% 36% 27% 2	836

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