PHM Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PHM Planetarium 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	21% 7% 5% 6% 7% 30%	8735
	Night	23% 7% 5% 6% 6% 31%	8439
Transparency	Day	18% 18% 41% 18%	4147
	Night	18% 18% 41% 19%	4017
Seeing	Day	6% 24% 28% 27% 11%	551
	Night	10% 28% 36% 21%	2366

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	35% 13% 8% 9% 8%	489
	Night	38% 12% 8% 5% 15%	864
February	Day	27% 10% 9% 6% 5% 5% 19%	562
	Night	24% 8% 6% 6% 5% 5% 5% 5% 26%	692
March	Day	34% 12% 7% 5% 6% 13%	666
	Night	30% 10% 8% 6% 5% 21%	700
April	Day	17% 7% 5% 5% 5% 6% 38%	742
	Night	22% 5% 7% 5% 36%	578
May	Day	19% 5% 5% 7% 5% 6% 7% 31%	820
	Night	15% 5% 8% 7% 6% 9% 31%	532
June	Day	14% 5% 5% 7% 5% 5% 10% 34%	840
	Night	11% 5% 7% 6% 5% 8% 38%	476
July	Day	11% 7% 6% 6% 12% 37%	866
	Night	14% 5% 6% 7% 10% 40%	496
August	Day	16% 6% 5% 5% 7% 7% 33%	1170
	Night	10% 5% 5% 6% 5% 7% 43%	872
September	Day	13% 5% 5% 7% 48%	998
	Night	12% 6% 55%	944
October	Day	21% 5% 6% 6% 7% 30%	613
	Night	17% 6% 5% 5% 5% 5% 10% 34%	749
November	Day	42% 5% 5% 5% 5% 23%	473
	Night	41% 5% 6% 6% 21%	694
December	Day	39% 11% 5% 5% 19%	496
	Night	29% 16% 7% 5% 6% 17%	842

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	44% 20% 35%	88
	Night	29% 27% 33% 9%	199
February	Day	30% 26% 36% 6%	156
	Night	25% 20% 46% 7%	272
March	Day	37% 28% 33%	165
	Night	26% 23% 38% 12%	226
April	Day	17% 18% 47% 12%	388
	Night	17% 7% 57% 15%	289
May	Day	16% 25% 45% 12%	389
	Night	19% 24% 44% 11%	257
June	Day	18% 12% 49% 17%	458
	Night	22% 14% 39% 23%	267
July	Day	15% 15% 43% 17% 7%	505
	Night	17% 12% 42% 22% 5%	286
August	Day	18% 22% 37% 20%	577
	Night	19% 17% 37% 24%	522
September	Day	6% 13% 37% 36% 5%	634
	Night	7% 13% 39% 35% 5%	675
October	Day	19% 12% 48% 19%	462
	Night	15% 19% 44% 16%	538
November	Day	17% 29% 34% 18%	192
	Night	18% 26% 40% 13%	269
December	Day	29% 19% 33% 18%	133
	Night	33% 23% 32% 9%	217

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	24% 47% 19% 8%	183
February	Night	9% 42% 32% 13%	168
March	Day	9% 40% 31% 13%	22
	Night	19% 42% 28% 9%	163
April	Day	26% 42% 28%	50
	Night	9% 37% 40% 12%	141
May	Day	5% 46% 30% 15%	69
	Night	9% 26% 51% 12%	163
June	Day	6% 27% 33% 23% 7%	101
	Night	20% 50% 24%	162
July	Day	8% 25% 27% 25% 12%	108
	Night	14% 44% 34%	161
August	Day	7% 13% 16% 35% 27%	81
	Night	15% 34% 40% 6%	261
September	Day	5% 10% 29% 36% 18%	79
	Night	14% 34% 41% 6%	296
October	Day	9% 19% 21% 36% 12%	41
	Night	9% 23% 38% 27%	310
November	Night	20% 41% 34%	156
December	Night	22% 40% 32%	202

Page generated at 2008-08-27 07:51:41. Version ab11