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 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	9% 6% 5% 8% 44%	16564
	Night	10% 7% 5% 5% 5% 5% 5% 7% 40%	16395
Cloud Cover	Day	10% 6% 5% 5% 7% 44%	16571
	Night	11% 7% 5% 5% 5% 7% 40%	16391
Transparency	Day	6% 17% 25% 17% 6%	13550
	Night	6% 17% 25% 15%	13196
Seeing	Day	8% 29% 33% 18%	1098
	Night	8% 30% 37% 13%	5310

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	11% 5% 7% 8% 5% 9% 35%	1023
	Night	10% 9% 6% 9% 5% 6% 5% 7% 31%	1679
February	Day	7% 7% 7% 5% 6% 7% 5% 5% 10% 31%	986
	Night	9% 10% 7% 7% 5% 6% 5% 7% 6% 29%	1408
March	Day	10% 10% 5% 6% 5% 5% 5% 6% 10% 32%	1349
	Night	9% 9% 6% 6% 6% 5% 5% 8% 33%	1372
April	Day	14% 8% 5% 5% 5% 6% 5% 7% 31%	1464
	Night	15% 8% 6% 6% 6% 5% 7% 32%	1136
May	Day	15% 5% 5% 5% 8% 41%	1708
	Night	16% 5% 5% 5% 5% 8% 36%	991
June	Day	9% 5% 5% 7% 8% 46%	1740
	Night	10% 5% 5% 5% 7% 9% 44%	888
July	Day	5% 10% 60%	1757
	Night	7% 6% 59%	931
August	Day	6% 7% 61%	1583
	Night	5% 5% 5% 8% 57%	1135
September	Day	11% 6% 56%	1362
	Night	13% 6% 54%	1242
October	Day	15% 10% 5% 43%	1214
	Night	17% 7% 5% 6% 41%	1476
November	Day	11% 7% 5% 5% 5% 6% 44%	1162
	Night	11% 7% 5% 5% 5% 7% 41%	2007
December	Day	7% 10% 7% 7% 6% 5% 5% 6% 7% 29%	1216
	Night	7% 9% 5% 9% 6% 5% 5% 6% 35%	2130

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% 12% 5% 7% 6% 6% 8% 35%	1024
	Night	10% 9% 6% 8% 5% 6% 5% 7% 32%	1676
February	Day	8% 9% 5% 5% 6% 7% 5% 10% 32%	988
	Night	10% 11% 7% 8% 5% 5% 5% 6% 6% 29%	1407
March	Day	10% 10% 5% 5% 5% 6% 10% 33%	1351
	Night	10% 8% 5% 6% 6% 5% 5% 6% 8% 32%	1370
April	Day	15% 8% 5% 5% 5% 5% 5% 5% 7% 32%	1465
	Night	15% 8% 5% 6% 5% 5% 6% 33%	1138
May	Day	15% 5% 5% 5% 7% 42%	1708
	Night	16% 5% 5% 6% 5% 7% 37%	991
June	Day	9% 5% 5% 7% 7% 47%	1740
	Night	11% 5% 5% 6% 9% 45%	888
July	Day	6% 8% 61%	1757
	Night	6% 7% 60%	933
August	Day	5% 7% 61%	1584
	Night	5% 5% 7% 58%	1134
September	Day	12% 6% 56%	1360
	Night	14% 6% 54%	1242
October	Day	15% 11% 6% 43%	1213
	Night	18% 7% 6% 41%	1476
November	Day	11% 8% 5% 5% 6% 44%	1164
	Night	12% 6% 5% 5% 5% 7% 41%	2007
December	Day	8% 11% 7% 9% 5% 6% 5% 7% 29%	1217
	Night	8% 10% 5% 9% 6% 5% 5% 5% 35%	2129

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	7% 25% 23% 7%	822
	Night	7% 20% 20% 11%	1243
February	Day	8% 26% 30%	723
	Night	7% 22% 27% 6%	1012
March	Day	9% 24% 27% 6%	1045
	Night	10% 23% 27% 5%	1058
April	Day	10% 19% 22% 13%	1082
	Night	8% 21% 25% 11%	818
May	Day	6% 19% 32% 11%	1393
	Night	7% 23% 27% 12%	785
June	Day	7% 20% 22% 19% 7%	1480
	Night	7% 23% 21% 18%	746
July	Day	10% 26% 27% 17%	1314
	Night	9% 26% 28% 14%	708
August	Day	9% 24% 28% 16%	1502
	Night	5% 9% 25% 28% 13%	1051
September	Day	9% 22% 28% 12%	1237
	Night	10% 22% 28% 10%	1125
October	Day	5% 10% 25% 17%	1063
	Night	6% 13% 25% 15%	1289
November	Day	6% 17% 24% 20%	953
	Night	5% 17% 27% 16%	1653
December	Day	7% 21% 25% 5%	936
	Night	7% 17% 27% 8%	1708

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	12% 50% 21% 5%	554
February	Night	13% 37% 33% 7%	411
March	Day	13% 36% 33% 7%	103
	Night	7% 28% 38% 16%	467
April	Day	19% 42% 22%	110
	Night	6% 22% 44% 13%	329
May	Day	8% 27% 26% 20%	219
	Night	8% 22% 40% 17%	352
June	Day	6% 23% 36% 22%	230
	Night	21% 44% 20%	266
July	Day	26% 39% 25%	217
	Night	20% 46% 25%	295
August	Day	6% 35% 39% 11%	119
	Night	5% 19% 49% 19%	356
September	Day	10% 23% 38% 19%	100
	Night	13% 39% 27%	376
October	Night	8% 25% 33% 14%	469
November	Night	9% 34% 39% 5%	694
December	Night	11% 38% 32% 7%	741

Page generated at 2009-12-30 03:20:56. Version ab11