McDonald Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McDonald Observatory clear sky chart from 2003-08-01 to 2005-11-07.

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% 6% 7% 47%	8651
	Night	9% 6% 5% 6% 7% 49%	8518
Transparency	Day	11% 10% 26% 30% 21%	6081
	Night	12% 11% 24% 29% 22%	6079
Seeing	Day	12% 19% 40% 25%	475
	Night	11% 20% 40% 26%	2752

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	12% 6% 6% 8% 45%	611
	Night	10% 5% 6% 5% 8% 6% 44%	742
February	Day	19% 7% 7% 5% 6% 36%	561
	Night	23% 5% 41%	684
March	Day	10% 5% 5% 63%	676
	Night	7% 6% 6% 58%	686
April	Day	11% 5% 5% 6% 58%	720
	Night	10% 6% 5% 5% 53%	600
May	Day	6% 6% 6% 5% 8% 52%	762
	Night	7% 7% 10% 51%	602
June	Day	8% 6% 6% 7% 51%	780
	Night	7% 6% 7% 8% 50%	537
July	Day	13% 8% 7% 8% 9% 34%	734
	Night	11% 10% 7% 7% 11% 36%	618
August	Day	9% 5% 6% 8% 8% 9% 7% 32%	1115
	Night	5% 9% 8% 10% 12% 37%	928
September	Day	12% 6% 7% 5% 9% 40%	1011
	Night	12% 5% 7% 7% 44%	934
October	Day	7% 5% 6% 5% 6% 8% 47%	644
	Night	7% 5% 5% 6% 5% 8% 46%	719
November	Day	10% 58%	482
	Night	11% 5% 55%	682
December	Day	5% 5% 7% 68%	555
	Night	69%	786

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	16% 8% 38% 29% 6%	393
	Night	15% 13% 29% 32% 8%	480
February	Day	8% 13% 29% 34% 14%	281
	Night	7% 10% 27% 37% 16%	343
March	Day	9% 6% 30% 34% 19%	533
	Night	6% 5% 26% 40% 20%	502
April	Day	8% 7% 21% 24% 37%	534
	Night	7% 5% 18% 33% 34%	403
May	Day	9% 10% 21% 27% 30%	561
	Night	14% 12% 25% 20% 27%	433
June	Day	12% 17% 27% 36%	543
	Night	18% 7% 18% 23% 31%	377
July	Day	20% 12% 21% 28% 16%	419
	Night	21% 14% 22% 27% 14%	383
August	Day	20% 21% 34% 17% 5%	621
	Night	25% 24% 34% 12%	612
September	Day	10% 8% 35% 31% 13%	638
	Night	14% 14% 24% 29% 17%	601
October	Day	8% 15% 30% 37% 8%	680
	Night	8% 14% 21% 33% 21%	746
November	Day	9% 6% 13% 34% 35%	402
	Night	7% 7% 23% 31% 30%	559
December	Day	6% 23% 35% 30%	476
	Night	15% 30% 47%	640

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	14% 27% 45% 12%	234
February	Night	21% 32% 40% 5%	192
March	Day	31% 28% 37%	32
	Night	19% 26% 41% 12%	216
April	Day	38% 43% 14%	57
	Night	13% 47% 35%	168
May	Day	11% 27% 48% 12%	77
	Night	17% 58% 17%	179
June	Day	17% 35% 39%	120
	Night	13% 45% 37%	180
July	Day	6% 10% 55% 26%	60
	Night	8% 46% 42%	174
August	Day	9% 47% 36%	88
	Night	33% 52% 7%	271
September	Day	12% 9% 41% 36%	41
	Night	7% 12% 33% 43%	260
October	Night	8% 16% 42% 31%	319
November	Night	19% 27% 30% 22%	272
December	Night	18% 21% 36% 20%	287

Page generated at 2008-09-30 04:41:03. Version ab11