Montreal
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montreal clear sky chart from 2003-08-01 to 2005-11-07.

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% 6% 5% 5% 6% 5% 7% 29%	8748
	Night	25% 6% 5% 5% 6% 30%	8422
Transparency	Day	23% 21% 36% 16%	3960
	Night	20% 19% 40% 16%	3747
Seeing	Day	12% 25% 33% 21% 6%	538
	Night	15% 31% 36% 15%	2310

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	23% 13% 9% 7% 5% 5% 27%	490
	Night	20% 9% 8% 5% 5% 6% 29%	863
February	Day	21% 7% 6% 6% 6% 7% 30%	536
	Night	22% 6% 7% 6% 5% 5% 32%	718
March	Day	21% 7% 5% 5% 5% 5% 35%	670
	Night	24% 11% 8% 6% 6% 25%	692
April	Day	29% 5% 5% 5% 5% 30%	737
	Night	29% 5% 5% 5% 35%	582
May	Day	22% 5% 5% 5% 5% 5% 5% 8% 8% 25%	859
	Night	27% 5% 5% 6% 7% 5% 28%	496
June	Day	15% 9% 5% 6% 9% 34%	840
	Night	14% 6% 10% 5% 10% 34%	476
July	Day	18% 5% 5% 7% 7% 9% 31%	866
	Night	19% 5% 6% 6% 5% 9% 32%	496
August	Day	19% 5% 6% 8% 5% 5% 9% 29%	1202
	Night	27% 5% 5% 5% 6% 30%	836
September	Day	17% 6% 5% 8% 40%	1017
	Night	17% 5% 5% 7% 42%	928
October	Day	27% 7% 6% 5% 5% 5% 6% 26%	597
	Night	31% 7% 5% 27%	764
November	Day	32% 9% 5% 5% 6% 7% 18%	448
	Night	29% 5% 5% 5% 6% 5% 5% 23%	719
December	Day	39% 8% 5% 6% 5% 15%	486
	Night	35% 7% 7% 5% 6% 19%	852

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	13% 20% 53% 12%	171
	Night	16% 15% 48% 20%	340
February	Day	26% 12% 42% 18%	221
	Night	21% 18% 47% 12%	303
March	Day	18% 35% 33% 8%	294
	Night	22% 22% 39% 9% 6%	253
April	Day	20% 11% 37% 29%	287
	Night	13% 7% 39% 30% 9%	255
May	Day	27% 23% 31% 14%	373
	Night	20% 16% 35% 20% 6%	178
June	Day	21% 13% 44% 14% 5%	415
	Night	31% 17% 28% 20%	259
July	Day	22% 18% 37% 20%	422
	Night	11% 16% 38% 27% 5%	226
August	Day	23% 24% 27% 20%	541
	Night	24% 18% 35% 20%	349
September	Day	16% 24% 39% 19%	589
	Night	15% 23% 40% 19%	561
October	Day	33% 29% 25% 10%	342
	Night	24% 27% 33% 10%	438
November	Day	36% 20% 37% 5%	188
	Night	27% 21% 42% 8%	318
December	Day	35% 20% 32% 11%	117
	Night	19% 19% 55% 5%	267

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Night	17% 42% 32% 7%	207
February	Day	12% 24% 36% 24%	25
	Night	14% 32% 33% 14%	194
March	Day	18% 24% 38% 14%	49
	Night	15% 32% 36% 13%	202
April	Day	16% 27% 33% 20%	48
	Night	15% 38% 34% 11%	167
May	Day	18% 25% 29% 22%	48
	Night	13% 43% 29% 11%	108
June	Day	10% 29% 34% 14% 10%	55
	Night	5% 39% 35% 19%	110
July	Day	13% 15% 35% 30% 5%	53
	Night	8% 18% 55% 16%	106
August	Day	12% 21% 32% 25% 7%	82
	Night	11% 28% 40% 18%	231
September	Day	6% 23% 27% 25% 17%	76
	Night	10% 25% 33% 28%	313
October	Day	5% 35% 32% 22%	71
	Night	18% 26% 35% 17%	289
November	Day	22% 25% 38% 12%	31
	Night	19% 30% 41% 8%	206
December	Night	24% 33% 28% 13%	177

Page generated at 2008-06-05 23:07:22. Version ab11