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JAT Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JAT 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	16% 5% 5% 5% 7% 38%	16465
	Night	18% 5% 5% 7% 39%	16468
Cloud Cover	Day	16% 5% 5% 5% 5% 5% 7% 38%	16475
	Night	17% 5% 5% 7% 38%	16488
Transparency	Day	5% 15% 26% 17%	13159
	Night	15% 26% 18%	13229
Seeing	Day	44% 13% 15% 9%	1129
	Night	46% 16% 17% 5%	5199

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	16% 6% 5% 5% 5% 5% 6% 38%	1025
	Night	20% 5% 5% 8% 37%	1673
February	Day	13% 6% 5% 6% 43%	1048
	Night	14% 6% 5% 5% 6% 5% 43%	1359
March	Day	15% 5% 5% 5% 5% 7% 39%	1328
	Night	18% 7% 5% 7% 37%	1351
April	Day	17% 5% 5% 5% 5% 6% 37%	1418
	Night	20% 5% 5% 5% 5% 6% 38%	1186
May	Day	14% 5% 5% 7% 5% 5% 7% 37%	1678
	Night	16% 5% 5% 5% 6% 40%	1034
June	Day	16% 5% 5% 6% 6% 6% 6% 10% 28%	1663
	Night	17% 5% 5% 5% 6% 8% 35%	956
July	Day	12% 5% 5% 7% 6% 6% 9% 36%	1715
	Night	9% 5% 7% 7% 7% 10% 36%	974
August	Day	14% 5% 5% 6% 6% 6% 40%	1516
	Night	16% 5% 6% 8% 39%	1197
September	Day	17% 7% 43%	1373
	Night	14% 5% 6% 47%	1231
October	Day	16% 5% 5% 6% 44%	1211
	Night	19% 5% 5% 45%	1483
November	Day	24% 7% 5% 39%	1273
	Night	23% 7% 6% 6% 32%	1898
December	Day	17% 6% 5% 5% 6% 39%	1217
	Night	20% 5% 5% 6% 6% 38%	2126

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	16% 6% 5% 5% 5% 6% 38%	1023
	Night	20% 5% 5% 8% 36%	1674
February	Day	13% 6% 5% 5% 5% 42%	1045
	Night	13% 6% 6% 6% 42%	1351
March	Day	15% 5% 5% 5% 6% 7% 39%	1349
	Night	17% 8% 6% 7% 36%	1375
April	Day	17% 5% 6% 5% 7% 36%	1417
	Night	20% 5% 5% 5% 7% 37%	1186
May	Day	14% 5% 5% 5% 7% 5% 5% 7% 36%	1672
	Night	16% 5% 5% 6% 7% 39%	1028
June	Day	16% 5% 6% 6% 6% 7% 9% 29%	1672
	Night	16% 5% 5% 5% 5% 6% 9% 35%	958
July	Day	11% 5% 5% 7% 6% 6% 9% 35%	1715
	Night	9% 5% 5% 7% 6% 8% 9% 36%	975
August	Day	13% 5% 5% 5% 7% 6% 40%	1511
	Night	15% 5% 6% 9% 39%	1199
September	Day	17% 8% 43%	1372
	Night	14% 5% 7% 46%	1232
October	Day	16% 5% 5% 7% 43%	1211
	Night	19% 5% 6% 45%	1483
November	Day	23% 7% 5% 39%	1271
	Night	23% 6% 6% 5% 5% 32%	1897
December	Day	17% 5% 5% 6% 40%	1217
	Night	20% 6% 6% 6% 37%	2130

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	20% 28% 14%	770
	Night	18% 34% 14%	1243
February	Day	5% 18% 32% 12%	847
	Night	5% 17% 31% 17%	1093
March	Day	16% 27% 18%	1043
	Night	17% 23% 17%	1057
April	Day	11% 20% 24% 6%	1102
	Night	14% 19% 23% 5%	919
May	Day	5% 14% 24% 19%	1334
	Night	5% 13% 22% 23%	818
June	Day	9% 21% 27% 8%	1264
	Night	8% 20% 29% 12%	709
July	Day	6% 13% 29% 17%	1108
	Night	12% 14% 26% 17%	646
August	Day	5% 10% 25% 19%	1339
	Night	14% 22% 21%	1053
September	Day	9% 23% 26%	1193
	Night	8% 26% 29%	1078
October	Day	12% 27% 18%	1104
	Night	11% 28% 20%	1339
November	Day	15% 26% 14%	1052
	Night	14% 25% 13%	1507
December	Day	5% 21% 26% 13%	1003
	Night	20% 24% 14%	1767

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	58% 18% 11%	491
February	Day	60% 24% 7%	65
	Night	56% 18% 13%	430
March	Day	59% 15% 16%	124
	Night	58% 14% 9%	495
April	Day	54% 12% 5% 9%	119
	Night	56% 16% 11%	473
May	Day	54% 9% 15% 5%	122
	Night	56% 15% 15%	293
June	Day	47% 15% 12% 9%	113
	Night	49% 14% 22%	228
July	Day	45% 5% 18% 19%	110
	Night	45% 13% 26% 9%	220
August	Day	28% 10% 19% 12% 14%	121
	Night	27% 10% 29% 16%	440
September	Day	27% 14% 17% 21% 6%	116
	Night	28% 13% 26% 18%	463
October	Day	28% 16% 20% 12%	121
	Night	31% 18% 24% 8%	487
November	Day	42% 15% 16%	118
	Night	43% 19% 17%	573
December	Night	49% 20% 10%	606

Page generated at 2009-12-30 05:20:36. Version ab11