Engle Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Engle Ridge Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	14% 11% 5% 10% 5% 5% 4 3 5% 6% 32%	130113
	Night	14% 14% 5% 12% 6% 5% 4 3 4 5% 27%	130955
Transparency	Day	52% 6% 10% 20% 10% 1	133593
	Night	59% 6% 10% 16% 8% 1	135549
Seeing	Day	19% 11% 26% 26% 13% 4	17378
	Night	20% 10% 28% 29% 11% 1	27358

By Month

Percentage Cloud Cover NE (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	12% 22% 12% 28% 7% 4 2 2 2 2 6%	4691
	11th to 20th	Day	10% 18% 8% 26% 14% 11% 4 2 2 2 3	4679
	21st to end	Day	11% 15% 8% 18% 10% 8% 8% 3 3 4 12%	5156
February	1st to 10th	Day	9% 18% 8% 16% 14% 7% 4 3 4 4 13%	1797
	11th to 20th	Day	12% 15% 8% 17% 11% 7% 5% 3 4 4 14%	1997
	21st to end	Day	12% 18% 7% 14% 8% 4 5% 3 5% 4 19%	1650
March	1st to 10th	Day	12% 11% 4 13% 6% 4 4 4 5% 5% 32%	4478
	11th to 20th	Day	11% 12% 6% 13% 5% 4 5% 4 6% 7% 26%	4842
	21st to end	Day	21% 12% 8% 14% 5% 4 3 3 3 4 22%	5679
April	1st to 10th	Day	17% 16% 4 9% 5% 5% 4 3 4 6% 29%	2400
	11th to 20th	Day	20% 13% 4 6% 3 4 4 3 4 6% 34%	2476
	21st to end	Day	15% 11% 4 9% 4 4 4 3 6% 7% 33%	2600
May	1st to 10th	Day	15% 10% 3 7% 4 3 4 3 5% 6% 42%	5888
	11th to 20th	Day	12% 9% 2 7% 4 4 4 3 5% 6% 43%	6076
	21st to end	Day	15% 8% 2 7% 4 4 4 3 5% 6% 43%	6949
June	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 6% 8% 43%	2845
	11th to 20th	Day	12% 7% 2 6% 5% 4 4 4 6% 9% 41%	3000
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 8% 41%	2828
July	1st to 10th	Day	16% 5% 2 5% 3 3 4 3 5% 9% 44%	6359
	11th to 20th	Day	11% 4 1 4 3 4 4 4 5% 8% 50%	6360
	21st to end	Day	10% 5% 2 5% 3 4 4 4 6% 10% 48%	6926
August	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 8% 41%	4375
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 47%	4174
	21st to end	Day	13% 6% 3 7% 5% 4 4 3 5% 6% 44%	4480
September	1st to 10th	Day	15% 5% 2 7% 5% 3 5% 4 6% 8% 42%	2372
	11th to 20th	Day	14% 5% 2 4 4 4 4 4 5% 8% 46%	2200
	21st to end	Day	16% 7% 3 7% 4 4 4 5% 5% 8% 37%	2200
October	1st to 10th	Day	18% 9% 3 7% 4 4 4 4 5% 6% 37%	2200
	11th to 20th	Day	24% 14% 3 8% 6% 5% 4 3 5% 5% 23%	1894
	21st to end	Day	25% 15% 4 10% 5% 5% 5% 4 5% 5% 18%	1980
November	1st to 10th	Day	17% 18% 6% 7% 5% 3 5% 3 5% 6% 26%	1852
	11th to 20th	Day	13% 24% 6% 11% 6% 5% 4 4 5% 5% 17%	1890
	21st to end	Day	17% 26% 9% 12% 5% 4 3 3 4 3 14%	1775
December	1st to 10th	Day	15% 28% 12% 16% 7% 5% 4 3 2 2 6%	2350
	11th to 20th	Day	11% 24% 10% 18% 9% 5% 6% 2 4 3 8%	2691
	21st to end	Day	19% 21% 9% 14% 6% 5% 5% 4 4 4 8%	4004
January	1st to 10th	Night	12% 20% 8% 30% 8% 4 3 2 2 3 8%	8233
	11th to 20th	Night	10% 20% 8% 21% 14% 9% 4 3 3 3 5%	8236
	21st to end	Night	9% 17% 9% 18% 12% 8% 6% 3 4 4 10%	9058
February	1st to 10th	Night	10% 20% 6% 16% 14% 8% 4 3 3 3 14%	2821
	11th to 20th	Night	11% 15% 7% 17% 11% 7% 4 3 4 4 17%	2399
	21st to end	Night	15% 17% 5% 12% 7% 6% 4 3 4 5% 21%	1980
March	1st to 10th	Night	15% 12% 4 10% 7% 3 4 3 4 6% 34%	5362
	11th to 20th	Night	11% 14% 5% 9% 4 4 4 4 6% 7% 33%	5117
	21st to end	Night	20% 14% 5% 11% 5% 4 3 3 4 5% 27%	5294
April	1st to 10th	Night	19% 13% 4 9% 4 3 4 3 5% 6% 30%	2000
	11th to 20th	Night	20% 12% 3 7% 4 3 3 3 4 6% 34%	1872
	21st to end	Night	17% 11% 3 7% 4 3 4 4 5% 6% 37%	1800
May	1st to 10th	Night	15% 9% 3 5% 4 4 4 3 5% 7% 41%	4053
	11th to 20th	Night	14% 9% 2 6% 4 4 5% 4 5% 6% 40%	3825
	21st to end	Night	19% 8% 2 5% 3 3 3 3 5% 6% 42%	3949
June	1st to 10th	Night	13% 6% 2 7% 5% 4 5% 3 5% 6% 43%	1545
	11th to 20th	Night	11% 5% 3 6% 4 3 5% 4 7% 9% 42%	1400
	21st to end	Night	13% 6% 3 6% 4 4 4 3 5% 7% 44%	1560
July	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 7% 8% 43%	3620
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 6% 51%	3620
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 7% 51%	3948
August	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 4 5% 8% 41%	2777
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 7% 48%	2875
	21st to end	Night	14% 5% 2 6% 4 4 4 3 5% 7% 47%	3415
September	1st to 10th	Night	12% 7% 2 6% 3 4 3 3 5% 7% 48%	2007
	11th to 20th	Night	16% 5% 2 4 3 3 4 3 5% 8% 48%	2200
	21st to end	Night	19% 6% 3 6% 4 4 4 3 5% 6% 40%	2200
October	1st to 10th	Night	18% 9% 3 6% 4 3 4 3 4 5% 41%	2200
	11th to 20th	Night	23% 12% 3 9% 5% 4 5% 4 5% 6% 25%	2504
	21st to end	Night	25% 16% 4 9% 5% 4 4 3 4 5% 24%	2860
November	1st to 10th	Night	15% 16% 4 9% 5% 4 4 3 5% 6% 28%	2666
	11th to 20th	Night	14% 24% 6% 10% 6% 5% 4 3 4 5% 18%	2730
	21st to end	Night	17% 25% 8% 12% 5% 5% 4 3 3 4 14%	2836
December	1st to 10th	Night	12% 24% 13% 17% 7% 5% 4 3 4 3 9%	5060
	11th to 20th	Night	11% 24% 9% 19% 8% 4 4 3 4 4 9%	5736
	21st to end	Night	21% 20% 9% 15% 6% 5% 4 3 4 4 10%	7197

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	87% 4 5% 4 1	4692
	11th to 20th	Day	90% 3 3 3 1	4710
	21st to end	Day	76% 5% 8% 9% 2	5156
February	1st to 10th	Day	75% 6% 8% 9% 2	1797
	11th to 20th	Day	73% 6% 9% 8% 3	2007
	21st to end	Day	66% 7% 10% 14% 3	1650
March	1st to 10th	Day	53% 5% 15% 16% 9% 1	4480
	11th to 20th	Day	55% 6% 15% 17% 7% 1	4842
	21st to end	Day	66% 5% 9% 14% 5%	5679
April	1st to 10th	Day	57% 5% 9% 19% 9%	2400
	11th to 20th	Day	51% 5% 9% 21% 13% 1	2476
	21st to end	Day	49% 7% 12% 16% 14% 3	2600
May	1st to 10th	Day	42% 6% 9% 22% 18% 3	5897
	11th to 20th	Day	40% 6% 9% 25% 17% 2	6080
	21st to end	Day	40% 6% 9% 22% 20% 2	6949
June	1st to 10th	Day	37% 8% 11% 27% 16% 2	2845
	11th to 20th	Day	38% 7% 14% 23% 17% 1	3000
	21st to end	Day	39% 7% 14% 24% 14% 1	2828
July	1st to 10th	Day	37% 5% 14% 29% 14% 1	6291
	11th to 20th	Day	31% 6% 12% 36% 14% 1	6285
	21st to end	Day	31% 6% 13% 33% 16% 1	6728
August	1st to 10th	Day	36% 8% 13% 28% 13% 1	4360
	11th to 20th	Day	38% 6% 11% 33% 12% 1	4213
	21st to end	Day	40% 6% 12% 30% 11% 1	4479
September	1st to 10th	Day	40% 7% 15% 24% 14% 1	2372
	11th to 20th	Day	35% 8% 15% 29% 13% 1	2200
	21st to end	Day	44% 7% 15% 23% 11%	2200
October	1st to 10th	Day	35% 5% 8% 33% 18% 1	3590
	11th to 20th	Day	56% 5% 9% 19% 11%	3045
	21st to end	Day	63% 6% 12% 16% 3	3180
November	1st to 10th	Day	58% 6% 10% 18% 7%	1852
	11th to 20th	Day	68% 7% 11% 10% 4	1890
	21st to end	Day	75% 5% 11% 8% 2	1775
December	1st to 10th	Day	85% 6% 4 4 1	2350
	11th to 20th	Day	82% 6% 7% 5% 1	2691
	21st to end	Day	77% 7% 6% 7% 2	4004
January	1st to 10th	Night	84% 4 6% 5% 1	8233
	11th to 20th	Night	86% 5% 5% 3 1	8280
	21st to end	Night	77% 6% 6% 8% 2	9058
February	1st to 10th	Night	76% 6% 6% 9% 3	2821
	11th to 20th	Night	69% 7% 9% 12% 3	2400
	21st to end	Night	64% 6% 12% 14% 4	1980
March	1st to 10th	Night	52% 5% 17% 18% 8%	5373
	11th to 20th	Night	49% 6% 18% 20% 7% 1	5118
	21st to end	Night	58% 7% 10% 16% 8%	5294
April	1st to 10th	Night	54% 6% 9% 20% 10% 1	2000
	11th to 20th	Night	52% 4 8% 20% 13% 2	1872
	21st to end	Night	47% 6% 10% 20% 15% 2	1800
May	1st to 10th	Night	41% 7% 12% 20% 18% 4	4060
	11th to 20th	Night	43% 7% 8% 23% 18% 1	3826
	21st to end	Night	43% 6% 12% 21% 14% 4	3950
June	1st to 10th	Night	40% 6% 10% 28% 15% 1	1545
	11th to 20th	Night	36% 9% 14% 23% 16% 2	1400
	21st to end	Night	39% 7% 12% 25% 15% 2	1560
July	1st to 10th	Night	38% 5% 13% 26% 15% 3	3580
	11th to 20th	Night	32% 6% 12% 30% 19% 2	3572
	21st to end	Night	32% 6% 15% 29% 17% 1	3832
August	1st to 10th	Night	40% 7% 15% 25% 12% 1	2768
	11th to 20th	Night	36% 5% 15% 29% 15% 1	2902
	21st to end	Night	36% 6% 14% 25% 18% 1	3415
September	1st to 10th	Night	36% 6% 14% 24% 18% 1	2007
	11th to 20th	Night	34% 7% 15% 29% 14%	2200
	21st to end	Night	45% 6% 14% 24% 10% 1	2200
October	1st to 10th	Night	42% 6% 9% 30% 13%	3590
	11th to 20th	Night	51% 5% 9% 22% 12%	4079
	21st to end	Night	56% 4 12% 21% 6%	4610
November	1st to 10th	Night	56% 8% 11% 18% 7% 1	2666
	11th to 20th	Night	68% 7% 10% 10% 5% 1	2730
	21st to end	Night	74% 6% 10% 8% 1	2836
December	1st to 10th	Night	80% 7% 5% 6% 1	5060
	11th to 20th	Night	79% 6% 7% 7% 1	5735
	21st to end	Night	78% 8% 7% 5% 3	7197

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	18% 19% 37% 18% 7% 2	330
	11th to 20th	Day	22% 16% 37% 22% 2 1	330
	21st to end	Day	23% 14% 33% 25% 6%	363
February	1st to 10th	Day	21% 23% 32% 18% 5% 1	330
	11th to 20th	Day	19% 20% 37% 19% 5%	315
	21st to end	Day	22% 22% 36% 16% 3	249
March	1st to 10th	Day	22% 19% 36% 20% 4	300
	11th to 20th	Day	21% 15% 28% 26% 10% 1	440
	21st to end	Day	28% 12% 27% 23% 10% 1	550
April	1st to 10th	Day	24% 13% 30% 24% 7% 1	500
	11th to 20th	Day	27% 18% 24% 18% 10% 3	492
	21st to end	Day	21% 12% 29% 25% 12% 2	500
May	1st to 10th	Day	22% 14% 25% 24% 12% 3	503
	11th to 20th	Day	17% 8% 27% 30% 14% 4	612
	21st to end	Day	15% 10% 26% 27% 18% 4	789
June	1st to 10th	Day	15% 9% 28% 29% 15% 3	730
	11th to 20th	Day	16% 9% 26% 32% 14% 4	730
	21st to end	Day	16% 7% 27% 31% 17% 3	730
July	1st to 10th	Day	11% 4 23% 35% 20% 8%	721
	11th to 20th	Day	12% 7% 24% 31% 19% 6%	718
	21st to end	Day	10% 4 25% 36% 19% 6%	794
August	1st to 10th	Day	11% 4 20% 31% 26% 9%	616
	11th to 20th	Day	13% 4 18% 31% 22% 12%	519
	21st to end	Day	16% 8% 18% 29% 21% 8%	583
September	1st to 10th	Day	20% 6% 20% 28% 18% 9%	530
	11th to 20th	Day	13% 5% 15% 31% 25% 11%	530
	21st to end	Day	19% 9% 18% 27% 20% 6%	530
October	1st to 10th	Day	20% 13% 24% 24% 15% 5%	530
	11th to 20th	Day	30% 12% 29% 21% 7%	409
	21st to end	Day	31% 13% 31% 17% 6% 2	363
November	1st to 10th	Day	28% 18% 27% 16% 9% 1	330
	11th to 20th	Day	23% 23% 32% 18% 5%	330
	21st to end	Day	31% 21% 31% 15% 2	293
December	1st to 10th	Day	32% 19% 25% 17% 5% 1	220
	11th to 20th	Day	25% 25% 32% 15% 3	220
	21st to end	Day	28% 20% 33% 16% 2 1	349
January	1st to 10th	Night	20% 15% 35% 23% 6% 1	1050
	11th to 20th	Night	23% 16% 32% 23% 5%	1050
	21st to end	Night	19% 12% 35% 27% 6% 1	1155
February	1st to 10th	Night	18% 10% 38% 27% 6% 1	931
	11th to 20th	Night	19% 14% 36% 25% 6%	804
	21st to end	Night	24% 13% 32% 26% 5%	660
March	1st to 10th	Night	16% 13% 33% 29% 10% 1	800
	11th to 20th	Night	18% 12% 34% 29% 6% 1	800
	21st to end	Night	22% 9% 29% 32% 8% 1	810
April	1st to 10th	Night	24% 11% 29% 29% 6% 1	600
	11th to 20th	Night	24% 11% 27% 28% 9% 1	593
	21st to end	Night	21% 8% 26% 34% 10% 1	600
May	1st to 10th	Night	18% 5% 29% 37% 10% 1	600
	11th to 20th	Night	19% 4 25% 37% 14% 1	591
	21st to end	Night	16% 6% 22% 37% 18% 1	650
June	1st to 10th	Night	16% 4 26% 38% 15% 1	545
	11th to 20th	Night	13% 5% 22% 44% 16%	400
	21st to end	Night	15% 4 23% 35% 21% 1	560
July	1st to 10th	Night	11% 3 15% 44% 24% 3	585
	11th to 20th	Night	11% 3 20% 38% 25% 3	582
	21st to end	Night	10% 4 18% 43% 23% 2	657
August	1st to 10th	Night	9% 4 16% 43% 27% 2	597
	11th to 20th	Night	12% 3 16% 37% 29% 4	594
	21st to end	Night	16% 5% 16% 40% 22% 2	660
September	1st to 10th	Night	14% 4 21% 37% 20% 4	607
	11th to 20th	Night	16% 4 18% 35% 23% 4	800
	21st to end	Night	22% 7% 19% 31% 19% 3	800
October	1st to 10th	Night	21% 8% 26% 29% 15% 1	800
	11th to 20th	Night	30% 11% 26% 26% 6% 1	800
	21st to end	Night	34% 11% 25% 24% 6% 1	880
November	1st to 10th	Night	27% 12% 30% 25% 6%	820
	11th to 20th	Night	26% 17% 33% 20% 4	840
	21st to end	Night	30% 14% 34% 20% 3	882
December	1st to 10th	Night	23% 17% 36% 20% 4	1050
	11th to 20th	Night	24% 16% 37% 19% 4 1	1050
	21st to end	Night	26% 15% 35% 21% 3	1155

Page generated at 2026-02-13 01:42:12. Version ac07

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