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Prairie Grass Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prairie Grass Observatory 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	17% 8% 5% 5% 5% 5% 6% 33%	16488
	Night	18% 9% 6% 6% 34%	16448
Cloud Cover	Day	18% 8% 5% 5% 5% 5% 6% 33%	16502
	Night	18% 8% 6% 6% 34%	16464
Transparency	Day	6% 15% 23% 14%	12476
	Night	5% 14% 23% 16%	12524
Seeing	Day	6% 22% 29% 22% 8%	1016
	Night	11% 29% 31% 12%	5078

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	22% 12% 8% 8% 5% 21%	985
	Night	23% 15% 6% 6% 24%	1715
February	Day	17% 12% 7% 7% 6% 7% 22%	1091
	Night	19% 11% 6% 7% 5% 30%	1315
March	Day	22% 11% 6% 7% 5% 5% 5% 24%	1310
	Night	21% 9% 5% 5% 6% 30%	1372
April	Day	20% 7% 6% 39%	1448
	Night	19% 8% 7% 37%	1156
May	Day	18% 6% 6% 5% 6% 35%	1631
	Night	17% 5% 5% 6% 6% 5% 7% 34%	1078
June	Day	13% 6% 6% 6% 5% 5% 8% 35%	1661
	Night	10% 5% 5% 5% 5% 5% 7% 42%	958
July	Day	11% 5% 6% 5% 5% 11% 39%	1714
	Night	11% 6% 9% 42%	978
August	Day	10% 5% 5% 5% 6% 7% 42%	1533
	Night	8% 5% 5% 7% 49%	1184
September	Day	15% 6% 5% 44%	1343
	Night	13% 5% 5% 47%	1257
October	Day	20% 9% 5% 5% 6% 7% 31%	1220
	Night	20% 6% 6% 7% 39%	1473
November	Day	22% 8% 5% 6% 33%	1299
	Night	19% 8% 5% 7% 35%	1872
December	Day	24% 15% 5% 9% 5% 5% 5% 17%	1253
	Night	26% 15% 7% 8% 6% 5% 16%	2090

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	22% 13% 7% 8% 6% 21%	984
	Night	24% 14% 6% 7% 24%	1714
February	Day	17% 12% 7% 8% 6% 6% 5% 22%	1089
	Night	19% 11% 6% 7% 5% 5% 30%	1306
March	Day	23% 11% 6% 6% 5% 5% 5% 24%	1331
	Night	22% 8% 5% 5% 5% 5% 30%	1397
April	Day	21% 7% 6% 39%	1447
	Night	19% 8% 7% 38%	1156
May	Day	18% 6% 6% 6% 36%	1626
	Night	17% 5% 6% 6% 5% 5% 7% 35%	1071
June	Day	13% 7% 6% 5% 5% 5% 8% 36%	1671
	Night	10% 5% 5% 6% 5% 7% 42%	959
July	Day	11% 5% 7% 5% 5% 10% 40%	1714
	Night	11% 5% 6% 9% 43%	979
August	Day	10% 5% 5% 5% 5% 6% 7% 42%	1528
	Night	8% 5% 5% 7% 49%	1185
September	Day	16% 6% 5% 5% 44%	1342
	Night	13% 5% 6% 47%	1258
October	Day	20% 8% 5% 6% 7% 31%	1220
	Night	20% 6% 5% 6% 39%	1473
November	Day	21% 9% 5% 6% 34%	1297
	Night	19% 8% 7% 36%	1872
December	Day	24% 15% 5% 8% 6% 5% 6% 17%	1253
	Night	26% 15% 7% 7% 6% 5% 17%	2094

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	6% 15% 23% 5%	638
	Night	14% 24% 10%	1108
February	Day	5% 19% 19% 5%	757
	Night	25% 21% 5%	965
March	Day	7% 21% 21% 6%	887
	Night	5% 18% 27% 10%	978
April	Day	18% 26% 16%	1127
	Night	14% 24% 18% 5%	893
May	Day	7% 12% 23% 20%	1215
	Night	7% 14% 23% 17%	805
June	Day	7% 17% 23% 17%	1277
	Night	9% 15% 26% 19%	767
July	Day	7% 18% 24% 16%	1121
	Night	6% 17% 25% 20%	660
August	Day	5% 10% 26% 22%	1317
	Night	6% 9% 26% 28%	1042
September	Day	6% 9% 22% 21% 8%	1160
	Night	9% 22% 24% 9%	1086
October	Day	6% 12% 22% 14%	1065
	Night	12% 24% 18%	1276
November	Day	5% 15% 23% 14%	991
	Night	5% 14% 24% 19%	1461
December	Day	6% 14% 16%	921
	Night	5% 13% 16%	1483

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	21% 38% 24%	526
February	Night	13% 42% 24%	396
March	Day	11% 27% 27% 11%	54
	Night	17% 38% 25% 6%	426
April	Day	8% 29% 28% 12%	108
	Night	13% 33% 29% 8%	336
May	Day	6% 24% 26% 26% 6%	142
	Night	7% 24% 36% 20%	347
June	Day	7% 30% 29% 17% 6%	221
	Night	20% 48% 17%	332
July	Day	5% 21% 30% 23% 10%	209
	Night	12% 48% 22%	318
August	Day	12% 30% 34% 18%	116
	Night	14% 45% 26% 7%	349
September	Day	6% 9% 27% 30% 14%	114
	Night	18% 33% 27% 7%	406
October	Day	21% 30% 23%	52
	Night	12% 28% 27% 12%	457
November	Night	14% 32% 29% 8%	534
December	Night	14% 33% 21%	651

Page generated at 2009-12-30 05:20:22. Version ab11