Vancouver
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vancouver clear sky chart from 2005-11-05 to 2009-12-29.

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	25% 7% 5% 5% 6% 30%	16485
	Night	29% 7% 5% 5% 29%	16474
Cloud Cover	Day	27% 7% 5% 6% 29%	16491
	Night	31% 7% 5% 29%	16470
Transparency	Day	11% 18% 18% 6%	11898
	Night	9% 18% 17% 7%	11905
Seeing	Day	23% 34% 23%	1168
	Night	8% 14% 30% 21%	4522

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	38% 9% 5% 5% 17%	902
	Night	42% 9% 5% 20%	1795
February	Day	25% 8% 6% 6% 5% 5% 6% 22%	980
	Night	31% 10% 5% 5% 25%	1414
March	Day	35% 10% 6% 6% 5% 19%	1343
	Night	36% 8% 7% 5% 19%	1378
April	Day	23% 9% 7% 6% 5% 5% 6% 24%	1485
	Night	25% 8% 5% 6% 6% 6% 5% 5% 22%	1114
May	Day	18% 6% 6% 5% 5% 5% 6% 7% 32%	1737
	Night	17% 5% 6% 5% 5% 7% 35%	963
June	Day	18% 6% 5% 5% 6% 36%	1789
	Night	20% 5% 5% 6% 6% 38%	840
July	Day	12% 5% 5% 7% 50%	1812
	Night	7% 5% 6% 57%	878
August	Day	12% 5% 5% 6% 49%	1626
	Night	10% 7% 53%	1092
September	Day	16% 6% 5% 5% 6% 40%	1365
	Night	14% 5% 6% 52%	1238
October	Day	33% 6% 6% 5% 5% 6% 21%	1187
	Night	31% 7% 5% 32%	1507
November	Day	49% 9% 5% 6% 8%	1154
	Night	43% 7% 5% 5% 14%	2016
December	Day	40% 10% 18%	1105
	Night	38% 12% 5% 19%	2239

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	42% 8% 18%	902
	Night	45% 9% 19%	1792
February	Day	28% 7% 5% 6% 6% 6% 7% 21%	982
	Night	33% 10% 5% 25%	1413
March	Day	37% 10% 6% 5% 17%	1345
	Night	37% 8% 6% 5% 5% 18%	1376
April	Day	26% 9% 5% 6% 5% 5% 6% 22%	1486
	Night	26% 8% 6% 6% 5% 5% 6% 22%	1116
May	Day	21% 5% 5% 5% 5% 5% 8% 30%	1737
	Night	19% 5% 5% 5% 5% 7% 35%	963
June	Day	22% 5% 5% 6% 7% 32%	1789
	Night	22% 6% 7% 37%	840
July	Day	13% 5% 6% 50%	1812
	Night	8% 6% 59%	879
August	Day	14% 5% 5% 7% 47%	1627
	Night	11% 5% 7% 52%	1092
September	Day	19% 6% 7% 41%	1363
	Night	15% 5% 6% 52%	1238
October	Day	34% 9% 6% 5% 6% 20%	1187
	Night	33% 6% 5% 32%	1506
November	Day	51% 7% 5% 8%	1156
	Night	45% 6% 6% 13%	2016
December	Day	41% 11% 19%	1105
	Night	41% 11% 19%	2239

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	10% 17% 14%	548
	Night	9% 11% 18%	1110
February	Day	12% 16% 14% 7%	672
	Night	7% 16% 20% 8%	951
March	Day	10% 15% 14%	824
	Night	12% 17% 14%	857
April	Day	12% 20% 18% 7%	912
	Night	12% 21% 18% 7%	700
May	Day	13% 23% 24%	1272
	Night	10% 29% 22% 7%	736
June	Day	14% 25% 19% 6%	1238
	Night	11% 29% 19% 7% 5%	578
July	Day	13% 22% 27% 12%	1206
	Night	9% 23% 29% 18%	595
August	Day	9% 21% 27% 6%	1488
	Night	6% 25% 23% 13%	1015
September	Day	10% 16% 18% 12%	1197
	Night	9% 19% 24% 12% 6%	1124
October	Day	12% 13% 10% 5%	1025
	Night	8% 17% 16% 7%	1327
November	Day	9% 7% 8%	798
	Night	10% 12% 8%	1410
December	Day	6% 10% 13% 6%	718
	Night	7% 13% 11% 6%	1502

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	Day	20% 20%	5
	Night	14% 17% 23% 13%	410
February	Day	10% 21% 30% 20%	95
	Night	10% 18% 32% 16%	376
March	Day	9% 31% 25% 8%	104
	Night	14% 21% 30% 11%	426
April	Day	32% 31% 19%	101
	Night	9% 14% 38% 21%	351
May	Day	8% 24% 30% 26%	131
	Night	8% 39% 30%	231
June	Day	5% 27% 35% 21%	218
	Night	8% 11% 37% 29% 6%	213
July	Day	15% 44% 33%	184
	Night	6% 30% 47% 8%	212
August	Day	15% 36% 29% 5%	119
	Night	5% 22% 44% 9%	335
September	Day	20% 33% 30%	115
	Night	8% 29% 36% 9%	455
October	Day	22% 31% 14%	96
	Night	16% 34% 13%	464
November	Night	11% 19% 24% 9%	515
December	Night	11% 17% 27% 10%	534

Page generated at 2009-12-30 02:20:37. Version ab11