Billings
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Billings clear sky chart from 2005-11-05 to 2008-11-27.

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	9% 5% 5% 5% 8% 45%	12332
	Night	11% 6% 5% 5% 5% 5% 7% 39%	12096
Cloud Cover	Day	10% 6% 5% 5% 8% 46%	12339
	Night	12% 6% 5% 5% 5% 7% 40%	12092
Transparency	Day	7% 19% 25% 19% 7%	9319
	Night	7% 20% 26% 16%	8892
Seeing	Day	9% 30% 34% 17%	813
	Night	9% 31% 37% 13%	3845

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	9% 5% 6% 6% 5% 6% 10% 35%	772
	Night	8% 8% 7% 9% 5% 7% 5% 6% 7% 30%	1270
February	Day	8% 7% 9% 5% 6% 8% 5% 5% 9% 28%	732
	Night	11% 10% 7% 7% 5% 7% 6% 6% 28%	1046
March	Day	10% 9% 5% 5% 5% 5% 6% 11% 32%	1010
	Night	10% 8% 5% 6% 6% 5% 5% 9% 33%	1029
April	Day	12% 7% 5% 5% 5% 6% 5% 6% 8% 32%	1113
	Night	13% 6% 7% 6% 7% 6% 8% 31%	860
May	Day	17% 6% 5% 8% 39%	1291
	Night	19% 5% 5% 5% 6% 7% 33%	755
June	Day	7% 5% 5% 8% 9% 49%	1313
	Night	7% 6% 6% 8% 10% 45%	666
July	Day	5% 10% 66%	1330
	Night	8% 7% 58%	709
August	Day	5% 6% 66%	1185
	Night	5% 5% 6% 59%	851
September	Day	14% 5% 5% 53%	1031
	Night	15% 6% 50%	946
October	Day	15% 8% 5% 47%	916
	Night	17% 5% 7% 43%	1115
November	Day	13% 7% 6% 5% 6% 6% 38%	896
	Night	13% 8% 5% 5% 5% 5% 7% 36%	1548
December	Day	8% 9% 8% 7% 5% 6% 5% 5% 7% 32%	743
	Night	7% 7% 6% 7% 6% 6% 5% 5% 6% 36%	1301

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	5% 10% 5% 5% 6% 5% 7% 9% 36%	773
	Night	8% 9% 7% 8% 5% 7% 5% 7% 31%	1267
February	Day	9% 9% 7% 5% 6% 8% 6% 9% 28%	734
	Night	12% 10% 7% 8% 5% 5% 5% 6% 6% 28%	1045
March	Day	10% 9% 5% 5% 6% 6% 11% 33%	1012
	Night	11% 7% 5% 6% 6% 6% 5% 6% 9% 33%	1027
April	Day	14% 7% 5% 5% 5% 6% 5% 7% 34%	1114
	Night	14% 6% 6% 7% 6% 5% 7% 32%	862
May	Day	18% 6% 5% 7% 41%	1291
	Night	20% 5% 5% 6% 5% 7% 34%	755
June	Day	7% 6% 8% 8% 51%	1313
	Night	9% 5% 5% 7% 11% 46%	666
July	Day	5% 8% 67%	1330
	Night	6% 7% 60%	711
August	Day	7% 67%	1186
	Night	5% 6% 5% 60%	850
September	Day	15% 6% 53%	1029
	Night	17% 5% 7% 50%	946
October	Day	17% 7% 6% 46%	915
	Night	18% 5% 6% 44%	1115
November	Day	13% 9% 5% 5% 6% 6% 38%	898
	Night	14% 7% 5% 5% 5% 7% 37%	1548
December	Day	8% 9% 8% 7% 5% 7% 5% 5% 7% 32%	744
	Night	8% 8% 5% 7% 5% 6% 5% 5% 6% 36%	1300

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	9% 32% 28% 5%	571
	Night	8% 27% 26% 9%	833
February	Day	10% 30% 30%	469
	Night	8% 26% 30%	650
March	Day	11% 31% 25% 8%	715
	Night	12% 30% 25% 7%	715
April	Day	11% 25% 25% 14%	732
	Night	11% 27% 26% 13%	543
May	Day	7% 23% 28% 13%	969
	Night	7% 25% 24% 14%	544
June	Day	8% 23% 26% 23% 5%	1053
	Night	8% 28% 22% 23%	524
July	Day	8% 21% 34% 25%	887
	Night	10% 23% 29% 19%	486
August	Day	7% 24% 30% 21%	1104
	Night	8% 23% 30% 17%	767
September	Day	11% 22% 25% 13%	906
	Night	11% 22% 26% 10%	829
October	Day	6% 11% 24% 23% 5%	763
	Night	7% 12% 26% 19% 5%	928
November	Day	8% 22% 23% 16%	687
	Night	5% 20% 26% 14%	1194
December	Day	8% 23% 32% 9%	463
	Night	10% 20% 34% 10%	879

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	10% 53% 23% 5%	414
February	Night	17% 40% 30% 5%	299
March	Day	17% 34% 30% 9%	75
	Night	7% 29% 40% 17%	344
April	Day	18% 46% 24% 6%	83
	Night	9% 25% 45% 14%	242
May	Day	9% 29% 22% 22%	163
	Night	8% 21% 36% 20%	265
June	Day	6% 26% 38% 21%	171
	Night	24% 45% 19%	196
July	Day	5% 25% 41% 24%	158
	Night	21% 49% 22%	218
August	Day	7% 35% 43% 6%	88
	Night	5% 21% 49% 17%	263
September	Day	9% 20% 40% 17%	75
	Night	13% 41% 23%	281
October	Night	6% 27% 34% 17%	348
November	Night	10% 35% 37%	530
December	Night	14% 40% 30% 8%	445

Page generated at 2008-11-27 11:47:46. Version ab11