Montpelier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Montpelier Observatory 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	21% 6% 7% 35%	8713
	Night	20% 5% 6% 5% 7% 36%	8458
Transparency	Day	16% 14% 38% 26%	4499
	Night	17% 13% 36% 28%	4588
Seeing	Day	9% 27% 26% 26% 10%	530
	Night	12% 33% 32% 20%	2502

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	21% 5% 5% 6% 38%	548
	Night	19% 5% 5% 5% 41%	805
February	Day	23% 5% 5% 5% 5% 38%	537
	Night	19% 7% 6% 5% 6% 40%	717
March	Day	23% 7% 5% 7% 36%	683
	Night	22% 5% 6% 6% 36%	681
April	Day	23% 6% 43%	720
	Night	25% 5% 5% 38%	600
May	Day	19% 5% 5% 8% 37%	820
	Night	14% 6% 5% 7% 5% 7% 11% 31%	534
June	Day	15% 5% 7% 6% 11% 5% 6% 8% 25%	840
	Night	18% 8% 6% 8% 10% 27%	476
July	Day	17% 8% 7% 5% 8% 5% 7% 9% 22%	820
	Night	20% 6% 7% 8% 5% 7% 30%	542
August	Day	22% 5% 6% 6% 5% 9% 29%	1159
	Night	19% 5% 5% 6% 7% 5% 10% 27%	879
September	Day	24% 6% 7% 38%	1014
	Night	20% 5% 5% 8% 37%	930
October	Day	26% 5% 6% 7% 35%	618
	Night	25% 5% 5% 5% 6% 34%	744
November	Day	22% 5% 5% 6% 40%	468
	Night	22% 5% 6% 5% 39%	698
December	Day	17% 5% 47%	486
	Night	17% 5% 5% 46%	852

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	20% 13% 47% 15%	268
	Night	18% 13% 40% 25%	402
February	Day	15% 12% 32% 35%	275
	Night	12% 11% 39% 28% 8%	371
March	Day	17% 22% 33% 25%	336
	Night	16% 10% 38% 32%	337
April	Day	10% 8% 38% 34% 6%	382
	Night	13% 5% 38% 35% 6%	295
May	Day	21% 14% 47% 12%	407
	Night	38% 11% 30% 17%	281
June	Day	30% 10% 42% 15%	321
	Night	28% 23% 34% 14%	228
July	Day	24% 16% 39% 18%	311
	Night	18% 21% 37% 19%	247
August	Day	19% 23% 37% 18%	566
	Night	22% 26% 37% 13%	393
September	Day	9% 9% 32% 39% 8%	551
	Night	14% 8% 33% 34% 8%	525
October	Day	12% 13% 39% 31%	489
	Night	14% 10% 37% 34%	590
November	Day	11% 19% 35% 32%	320
	Night	10% 12% 38% 35%	430
December	Day	9% 11% 40% 33%	273
	Night	13% 15% 31% 36%	489

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	20% 55% 20%	205
February	Day	18% 25% 56%	16
	Night	16% 48% 33%	192
March	Day	19% 39% 37%	51
	Night	24% 43% 27%	198
April	Day	21% 40% 23% 6% 8%	47
	Night	16% 39% 33% 10%	193
May	Day	5% 32% 32% 30%	53
	Night	35% 44% 15%	148
June	Day	7% 18% 33% 32% 7%	53
	Night	21% 54% 19%	103
July	Day	5% 16% 28% 33% 15%	53
	Night	5% 14% 36% 40%	149
August	Day	7% 15% 17% 37% 21%	78
	Night	6% 16% 27% 45%	297
September	Day	7% 21% 21% 30% 20%	70
	Night	6% 20% 38% 32%	303
October	Day	8% 32% 25% 26% 7%	71
	Night	10% 34% 28% 23%	296
November	Day	5% 39% 18% 34%	38
	Night	11% 31% 34% 21%	210
December	Night	16% 43% 29% 10%	208

Page generated at 2008-08-29 16:03:17. Version ab11