Pettis Rd Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pettis Rd Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 12% 5% 10% 5% 5% 4 3 5% 6% 28%	131595
	Night	17% 14% 5% 11% 5% 4 4 3 4 6% 27%	130548
Transparency	Day	55% 6% 10% 18% 9% 1	135195
	Night	57% 5% 10% 17% 9% 1	135018
Seeing	Day	25% 17% 28% 18% 9% 3	16379
	Night	24% 15% 27% 23% 10% 1	26117

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	17% 21% 8% 28% 5% 3 3 2 3 3 7%	5039
	11th to 20th	Day	11% 22% 10% 22% 9% 8% 3 1 2 3 9%	5280
	21st to end	Day	18% 17% 7% 15% 10% 8% 4 3 3 3 12%	5780
February	1st to 10th	Day	16% 20% 6% 15% 8% 5% 3 3 2 4 17%	1890
	11th to 20th	Day	15% 16% 8% 17% 8% 4 3 3 5% 4 16%	1843
	21st to end	Day	17% 20% 4 13% 5% 5% 3 3 4 5% 21%	1698
March	1st to 10th	Day	18% 12% 8% 11% 5% 4 3 3 4 5% 27%	5045
	11th to 20th	Day	12% 15% 6% 10% 5% 4 3 2 4 5% 32%	5090
	21st to end	Day	21% 14% 5% 11% 5% 3 3 2 4 5% 27%	5586
April	1st to 10th	Day	18% 15% 3 8% 4 4 4 3 5% 7% 29%	2430
	11th to 20th	Day	20% 14% 3 6% 4 4 4 4 5% 5% 32%	2513
	21st to end	Day	21% 12% 4 7% 4 4 3 3 4 5% 34%	2600
May	1st to 10th	Day	22% 10% 3 8% 5% 5% 4 4 5% 6% 28%	5862
	11th to 20th	Day	21% 10% 3 7% 5% 4 4 4 6% 8% 28%	5901
	21st to end	Day	20% 8% 3 6% 4 4 5% 4 6% 7% 32%	6934
June	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 3 6% 8% 35%	2800
	11th to 20th	Day	16% 8% 2 8% 4 5% 5% 4 5% 8% 35%	2800
	21st to end	Day	16% 6% 2 6% 5% 5% 5% 4 6% 9% 37%	2800
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 6% 10% 40%	6360
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 9% 37%	5912
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 42%	6424
August	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 4 6% 9% 31%	4234
	11th to 20th	Day	15% 7% 3 8% 5% 6% 6% 5% 8% 8% 30%	4174
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 9% 37%	4486
September	1st to 10th	Day	17% 8% 2 6% 4 5% 4 3 5% 7% 40%	2400
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 3 6% 9% 42%	2214
	21st to end	Day	21% 8% 3 7% 4 4 4 4 5% 7% 32%	2195
October	1st to 10th	Day	16% 8% 2 6% 4 5% 5% 4 6% 7% 37%	2000
	11th to 20th	Day	21% 11% 3 8% 5% 4 4 3 5% 6% 31%	2000
	21st to end	Day	25% 13% 3 8% 4 4 4 4 5% 6% 24%	2200
November	1st to 10th	Day	14% 13% 3 7% 5% 5% 5% 3 6% 6% 33%	1931
	11th to 20th	Day	18% 20% 4 8% 5% 4 4 4 5% 5% 24%	1696
	21st to end	Day	20% 24% 6% 10% 5% 4 4 2 5% 5% 16%	1680
December	1st to 10th	Day	20% 24% 16% 14% 4 4 3 2 2 2 9%	2680
	11th to 20th	Day	22% 24% 7% 11% 7% 3 2 2 4 4 13%	3070
	21st to end	Day	26% 24% 7% 13% 6% 4 3 2 2 3 10%	4048
January	1st to 10th	Night	15% 22% 9% 28% 6% 3 2 2 2 3 8%	7836
	11th to 20th	Night	12% 20% 11% 22% 9% 6% 4 2 3 3 8%	7680
	21st to end	Night	18% 17% 8% 14% 8% 6% 5% 3 4 4 14%	8401
February	1st to 10th	Night	19% 18% 6% 13% 8% 5% 3 3 4 4 19%	2730
	11th to 20th	Night	17% 16% 6% 16% 8% 5% 4 3 4 4 17%	2663
	21st to end	Night	19% 15% 6% 15% 7% 4 4 3 4 4 21%	2106
March	1st to 10th	Night	20% 16% 6% 9% 7% 3 2 3 4 5% 27%	5028
	11th to 20th	Night	14% 14% 6% 7% 4 4 3 3 4 6% 35%	5090
	21st to end	Night	19% 13% 6% 9% 4 4 3 3 4 4 31%	5588
April	1st to 10th	Night	19% 15% 4 8% 4 4 5% 3 4 5% 27%	2189
	11th to 20th	Night	18% 12% 3 7% 5% 4 5% 4 5% 6% 32%	1987
	21st to end	Night	15% 11% 3 7% 5% 4 5% 3 6% 7% 33%	1800
May	1st to 10th	Night	27% 11% 3 6% 4 4 4 3 4 6% 27%	4033
	11th to 20th	Night	18% 11% 4 8% 4 4 3 3 5% 6% 34%	3991
	21st to end	Night	21% 8% 2 5% 4 3 3 4 5% 9% 37%	3920
June	1st to 10th	Night	11% 7% 2 6% 5% 4 5% 4 7% 7% 41%	1600
	11th to 20th	Night	12% 6% 3 8% 6% 4 4 4 6% 7% 41%	1600
	21st to end	Night	13% 6% 2 6% 4 5% 5% 4 6% 9% 41%	1600
July	1st to 10th	Night	13% 8% 3 7% 4 3 4 4 5% 9% 43%	3598
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 10% 40%	4027
	21st to end	Night	9% 6% 2 6% 5% 4 4 4 6% 8% 47%	4427
August	1st to 10th	Night	12% 8% 3 7% 5% 4 5% 3 6% 7% 39%	2915
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 6% 7% 39%	2875
	21st to end	Night	10% 5% 2 7% 4 5% 6% 4 6% 9% 43%	3378
September	1st to 10th	Night	13% 6% 3 6% 3 4 4 4 5% 6% 46%	2000
	11th to 20th	Night	11% 5% 2 5% 3 3 5% 4 5% 7% 51%	2182
	21st to end	Night	20% 7% 2 7% 5% 4 4 3 5% 7% 36%	2204
October	1st to 10th	Night	17% 7% 2 5% 3 4 5% 3 5% 7% 43%	2400
	11th to 20th	Night	20% 8% 3 7% 5% 4 4 4 6% 6% 34%	2400
	21st to end	Night	26% 12% 3 7% 3 4 4 3 4 7% 29%	2640
November	1st to 10th	Night	14% 13% 3 7% 4 4 4 3 6% 6% 35%	2584
	11th to 20th	Night	19% 17% 4 9% 4 4 5% 3 5% 6% 24%	2921
	21st to end	Night	21% 21% 5% 8% 5% 4 4 3 4 6% 20%	2940
December	1st to 10th	Night	17% 21% 13% 13% 5% 4 4 3 3 4 12%	4720
	11th to 20th	Night	20% 23% 9% 12% 5% 4 3 2 3 4 16%	5345
	21st to end	Night	23% 22% 7% 12% 6% 4 4 3 4 4 12%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	84% 3 6% 5% 2	5040
	11th to 20th	Day	84% 2 5% 6% 2	5280
	21st to end	Day	79% 3 6% 8% 3	5780
February	1st to 10th	Day	72% 3 7% 12% 6%	1890
	11th to 20th	Day	71% 4 7% 12% 6%	1852
	21st to end	Day	66% 4 10% 14% 5% 2	1698
March	1st to 10th	Day	58% 4 11% 20% 6%	5045
	11th to 20th	Day	55% 4 15% 19% 6% 1	5090
	21st to end	Day	60% 4 7% 18% 11% 1	5586
April	1st to 10th	Day	55% 6% 12% 15% 12% 1	2430
	11th to 20th	Day	54% 4 10% 19% 11% 2	2513
	21st to end	Day	54% 5% 9% 19% 12% 2	2600
May	1st to 10th	Day	55% 7% 8% 19% 8% 4	5897
	11th to 20th	Day	52% 8% 10% 20% 9% 1	5905
	21st to end	Day	48% 8% 11% 19% 11% 2	6949
June	1st to 10th	Day	46% 8% 11% 23% 12% 1	2800
	11th to 20th	Day	47% 7% 12% 20% 13% 1	2800
	21st to end	Day	42% 7% 12% 24% 14% 1	2800
July	1st to 10th	Day	37% 8% 12% 26% 15% 2	6290
	11th to 20th	Day	42% 8% 12% 24% 14% 1	5842
	21st to end	Day	39% 7% 10% 24% 19% 1	6240
August	1st to 10th	Day	48% 8% 12% 22% 9%	4221
	11th to 20th	Day	46% 9% 12% 25% 7% 1	4213
	21st to end	Day	39% 8% 13% 27% 12% 1	4486
September	1st to 10th	Day	43% 6% 12% 23% 15% 2	2400
	11th to 20th	Day	39% 7% 12% 26% 15% 1	2214
	21st to end	Day	50% 7% 11% 20% 11% 1	2195
October	1st to 10th	Day	36% 6% 14% 25% 18% 1	3260
	11th to 20th	Day	46% 6% 12% 20% 12% 4	3240
	21st to end	Day	46% 5% 14% 23% 11% 1	3538
November	1st to 10th	Day	50% 6% 11% 19% 13% 2	1931
	11th to 20th	Day	61% 4 11% 14% 7% 2	1696
	21st to end	Day	70% 4 9% 11% 4 1	1680
December	1st to 10th	Day	82% 3 7% 6% 1	2680
	11th to 20th	Day	75% 3 8% 12% 2	3066
	21st to end	Day	81% 3 8% 7% 1	4048
January	1st to 10th	Night	85% 3 5% 6% 2	7837
	11th to 20th	Night	83% 4 6% 6% 2	7680
	21st to end	Night	74% 3 8% 11% 4	8401
February	1st to 10th	Night	70% 4 8% 13% 5%	2730
	11th to 20th	Night	70% 4 8% 12% 5%	2665
	21st to end	Night	67% 4 8% 13% 8% 1	2106
March	1st to 10th	Night	61% 4 11% 18% 6% 1	5028
	11th to 20th	Night	51% 5% 13% 21% 9% 2	5090
	21st to end	Night	56% 4 8% 16% 14% 1	5588
April	1st to 10th	Night	58% 5% 10% 15% 11% 1	2189
	11th to 20th	Night	52% 6% 11% 18% 12% 2	1987
	21st to end	Night	48% 6% 12% 21% 11% 3	1800
May	1st to 10th	Night	58% 5% 8% 14% 10% 4	4060
	11th to 20th	Night	49% 7% 9% 22% 11% 1	3992
	21st to end	Night	44% 7% 12% 22% 13% 2	3927
June	1st to 10th	Night	39% 7% 11% 25% 15% 3	1600
	11th to 20th	Night	40% 7% 13% 22% 15% 2	1600
	21st to end	Night	39% 8% 14% 21% 17% 1	1600
July	1st to 10th	Night	39% 6% 10% 26% 18% 1	3560
	11th to 20th	Night	38% 8% 13% 27% 13% 1	3977
	21st to end	Night	34% 7% 12% 19% 27% 1	4297
August	1st to 10th	Night	43% 8% 11% 25% 12% 1	2904
	11th to 20th	Night	41% 8% 13% 30% 8%	2902
	21st to end	Night	36% 8% 15% 27% 12% 2	3378
September	1st to 10th	Night	35% 7% 13% 26% 16% 3	2000
	11th to 20th	Night	31% 6% 14% 27% 20% 1	2182
	21st to end	Night	47% 7% 11% 20% 15% 1	2204
October	1st to 10th	Night	37% 5% 10% 24% 23% 1	3920
	11th to 20th	Night	44% 6% 12% 18% 15% 5%	3899
	21st to end	Night	42% 4 14% 22% 16% 1	4252
November	1st to 10th	Night	47% 5% 13% 21% 11% 2	2584
	11th to 20th	Night	60% 5% 11% 15% 7% 3	2921
	21st to end	Night	66% 5% 11% 14% 3 1	2940
December	1st to 10th	Night	77% 5% 10% 7% 2	4720
	11th to 20th	Night	74% 3 8% 12% 2	5348
	21st to end	Night	75% 4 9% 10% 1	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	38% 26% 28% 6% 3	289
	11th to 20th	Day	39% 30% 23% 5% 2 1	330
	21st to end	Day	37% 30% 25% 7% 1	363
February	1st to 10th	Day	33% 26% 34% 7% 1	330
	11th to 20th	Day	39% 26% 27% 6% 1	330
	21st to end	Day	39% 26% 30% 5%	289
March	1st to 10th	Day	33% 24% 29% 13% 2	540
	11th to 20th	Day	33% 23% 30% 10% 2 1	540
	21st to end	Day	37% 22% 26% 12% 3 1	594
April	1st to 10th	Day	33% 21% 29% 13% 4 1	540
	11th to 20th	Day	32% 20% 26% 14% 7% 2	516
	21st to end	Day	29% 19% 32% 15% 4 1	500
May	1st to 10th	Day	30% 19% 27% 17% 5% 2	503
	11th to 20th	Day	26% 15% 28% 21% 8% 2	527
	21st to end	Day	20% 12% 29% 23% 12% 5%	573
June	1st to 10th	Day	19% 15% 31% 24% 8% 3	530
	11th to 20th	Day	21% 13% 27% 25% 12% 2	530
	21st to end	Day	17% 13% 32% 25% 10% 3	530
July	1st to 10th	Day	12% 9% 25% 28% 20% 6%	525
	11th to 20th	Day	14% 9% 27% 31% 13% 7%	524
	21st to end	Day	13% 8% 31% 28% 15% 5%	576
August	1st to 10th	Day	12% 8% 27% 26% 19% 7%	526
	11th to 20th	Day	16% 8% 24% 28% 14% 9%	519
	21st to end	Day	11% 10% 26% 31% 17% 5%	583
September	1st to 10th	Day	17% 11% 24% 25% 17% 6%	530
	11th to 20th	Day	10% 6% 22% 29% 23% 8%	530
	21st to end	Day	24% 12% 29% 18% 13% 4	530
October	1st to 10th	Day	18% 13% 27% 23% 16% 3	530
	11th to 20th	Day	26% 17% 35% 14% 6% 2	530
	21st to end	Day	34% 18% 29% 15% 4	583
November	1st to 10th	Day	25% 22% 32% 14% 6% 1	409
	11th to 20th	Day	38% 21% 28% 10% 3	228
	21st to end	Day	34% 24% 34% 9%	220
December	1st to 10th	Day	39% 29% 25% 4 3 1	220
	11th to 20th	Day	41% 20% 30% 8% 1	220
	21st to end	Day	37% 25% 29% 9% 1	242
January	1st to 10th	Night	32% 24% 32% 11% 2	840
	11th to 20th	Night	34% 24% 28% 12% 2	840
	21st to end	Night	29% 23% 33% 13% 2	924
February	1st to 10th	Night	32% 21% 28% 18% 1	840
	11th to 20th	Night	30% 23% 32% 13% 1	823
	21st to end	Night	30% 22% 33% 14% 1	692
March	1st to 10th	Night	28% 18% 35% 17% 3	840
	11th to 20th	Night	29% 20% 27% 19% 4	840
	21st to end	Night	28% 18% 31% 18% 4 1	924
April	1st to 10th	Night	30% 17% 30% 19% 5%	840
	11th to 20th	Night	27% 16% 29% 20% 7% 1	767
	21st to end	Night	22% 11% 34% 29% 4 1	600
May	1st to 10th	Night	23% 12% 26% 30% 10%	600
	11th to 20th	Night	20% 10% 28% 32% 9% 1	576
	21st to end	Night	15% 6% 29% 29% 19% 2	433
June	1st to 10th	Night	11% 6% 32% 37% 14% 1	400
	11th to 20th	Night	14% 8% 24% 41% 14%	400
	21st to end	Night	11% 5% 29% 37% 17% 1	400
July	1st to 10th	Night	8% 4 22% 42% 20% 4	390
	11th to 20th	Night	13% 3 20% 35% 25% 4	566
	21st to end	Night	13% 4 14% 43% 26% 1	657
August	1st to 10th	Night	11% 3 20% 38% 26% 3	597
	11th to 20th	Night	14% 3 14% 38% 28% 4	594
	21st to end	Night	14% 4 22% 31% 25% 4	750
September	1st to 10th	Night	15% 6% 21% 30% 25% 3	800
	11th to 20th	Night	12% 5% 17% 36% 27% 4	800
	21st to end	Night	22% 10% 23% 26% 16% 3	800
October	1st to 10th	Night	18% 9% 27% 26% 18% 2	800
	11th to 20th	Night	23% 14% 29% 25% 8% 1	800
	21st to end	Night	32% 14% 30% 21% 3	880
November	1st to 10th	Night	22% 16% 29% 25% 8%	820
	11th to 20th	Night	30% 19% 28% 17% 5% 1	840
	21st to end	Night	34% 17% 31% 15% 3	840
December	1st to 10th	Night	34% 22% 28% 13% 4	840
	11th to 20th	Night	32% 26% 26% 13% 3	840
	21st to end	Night	33% 21% 30% 15% 1	924

Page generated at 2026-04-14 02:44:29. Version ac07

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