Griffith Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Griffith 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	6% 5% 66%	8693
	Night	7% 6% 58%	8471
Transparency	Day	11% 6% 20% 31% 29%	7459
	Night	15% 9% 19% 27% 28%	6925
Seeing	Day	5% 24% 44% 24%	651
	Night	14% 31% 42% 8%	2905

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	13% 5% 55%	548
	Night	14% 5% 5% 6% 51%	804
February	Day	24% 5% 7% 36%	561
	Night	23% 45%	684
March	Day	6% 5% 5% 58%	669
	Night	7% 6% 6% 6% 58%	692
April	Day	6% 5% 7% 62%	720
	Night	6% 8% 62%	600
May	Day	8% 67%	792
	Night	5% 9% 57%	572
June	Day	5% 76%	824
	Night	7% 5% 9% 52%	493
July	Day	5% 75%	811
	Night	5% 5% 5% 5% 8% 60%	539
August	Day	6% 75%	1114
	Night	6% 9% 58%	929
September	Day	77%	1008
	Night	6% 71%	937
October	Day	6% 5% 66%	641
	Night	9% 5% 5% 58%	722
November	Day	6% 5% 63%	468
	Night	5% 65%	698
December	Day	16% 5% 56%	537
	Night	14% 57%	801

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	11% 6% 30% 27% 23%	372
	Night	14% 5% 24% 30% 23%	555
February	Day	20% 5% 32% 18% 22%	302
	Night	12% 11% 28% 21% 26%	349
March	Day	16% 9% 20% 25% 28%	478
	Night	20% 11% 20% 19% 27%	504
April	Day	13% 9% 25% 30% 20%	568
	Night	11% 6% 30% 31% 20%	473
May	Day	14% 7% 27% 26% 22%	667
	Night	20% 7% 18% 28% 24%	440
June	Day	13% 22% 51% 8%	763
	Night	34% 5% 21% 34%	391
July	Day	10% 7% 18% 30% 31%	752
	Night	21% 15% 17% 22% 22%	473
August	Day	10% 6% 15% 30% 37%	1030
	Night	20% 14% 13% 28% 23%	790
September	Day	6% 8% 32% 47%	951
	Night	8% 11% 13% 28% 37%	873
October	Day	11% 6% 19% 27% 34%	805
	Night	11% 8% 19% 23% 37%	871
November	Day	11% 10% 28% 32% 16%	423
	Night	9% 11% 26% 28% 24%	633
December	Day	9% 9% 36% 44%	348
	Night	9% 5% 13% 28% 43%	573

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	Day	5% 36% 34% 18% 5%	38
	Night	17% 44% 31%	222
February	Day	26% 42% 28%	45
	Night	30% 38% 24%	177
March	Day	11% 30% 35% 22%	59
	Night	19% 40% 33%	236
April	Day	13% 45% 32% 8%	59
	Night	15% 38% 39%	230
May	Day	9% 27% 46% 16%	62
	Night	12% 37% 46%	197
June	Day	21% 46% 31%	60
	Night	6% 10% 25% 55%	134
July	Day	6% 56% 37%	62
	Night	5% 11% 63% 17%	167
August	Day	6% 57% 35%	92
	Night	8% 11% 57% 19%	371
September	Day	21% 47% 28%	90
	Night	8% 26% 48% 14%	359
October	Day	23% 44% 29%	84
	Night	10% 31% 47% 9%	356
November	Night	18% 47% 30%	242
December	Night	22% 41% 26% 5%	214

Page generated at 2008-08-27 04:51:42. Version ab11