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 2024-01-15.

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	15% 11% 5% 8% 6% 3 5% 4 5% 6% 32%	74541
	Night	15% 13% 6% 9% 7% 3 4 4 4 6% 28%	75200
Transparency	Day	52% 7% 12% 19% 10% 1	74247
	Night	58% 7% 11% 16% 8% 1	75029
Seeing	Day	18% 10% 26% 27% 14% 4	14548
	Night	20% 9% 28% 30% 11% 1	24509

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% 20% 12% 15% 11% 6% 4 4 3 4 10%	1517
	11th to 20th	Day	15% 17% 10% 17% 13% 5% 5% 4 3 3 7%	1472
	21st to end	Day	13% 18% 11% 18% 11% 4 7% 4 3 3 8%	1584
February	1st to 10th	Day	10% 16% 9% 13% 17% 6% 6% 4 4 4 12%	1540
	11th to 20th	Day	12% 15% 8% 15% 13% 5% 6% 4 3 4 15%	1747
	21st to end	Day	13% 16% 8% 13% 9% 4 6% 4 5% 5% 18%	1480
March	1st to 10th	Day	13% 12% 4 13% 9% 4 5% 5% 5% 5% 27%	1798
	11th to 20th	Day	11% 11% 7% 9% 7% 4 5% 5% 5% 6% 32%	1926
	21st to end	Day	17% 13% 5% 6% 7% 3 4 4 4 7% 31%	2254
April	1st to 10th	Day	18% 14% 5% 7% 6% 4 4 4 4 6% 28%	2160
	11th to 20th	Day	20% 13% 4 5% 4 2 4 3 4 6% 34%	2225
	21st to end	Day	16% 11% 4 7% 5% 2 5% 4 6% 6% 33%	2340
May	1st to 10th	Day	15% 9% 3 6% 5% 3 4 4 5% 6% 39%	2340
	11th to 20th	Day	16% 8% 3 6% 4 3 4 5% 5% 6% 41%	2376
	21st to end	Day	11% 6% 2 5% 4 3 5% 4 6% 8% 44%	2725
June	1st to 10th	Day	14% 5% 2 4 4 3 5% 5% 6% 8% 44%	2560
	11th to 20th	Day	11% 6% 2 5% 5% 3 4 4 6% 9% 43%	2700
	21st to end	Day	13% 5% 3 5% 5% 3 4 5% 6% 9% 42%	2546
July	1st to 10th	Day	9% 3 1 4 4 2 4 5% 6% 9% 52%	2520
	11th to 20th	Day	12% 5% 2 4 5% 3 6% 6% 5% 9% 45%	2520
	21st to end	Day	10% 5% 2 4 4 2 5% 5% 5% 10% 47%	2743
August	1st to 10th	Day	10% 5% 2 5% 5% 3 5% 6% 7% 8% 44%	2409
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 5% 5% 7% 45%	2303
	21st to end	Day	14% 5% 3 5% 4 3 4 4 6% 7% 44%	2469
September	1st to 10th	Day	16% 5% 2 6% 5% 3 4 5% 5% 8% 42%	2136
	11th to 20th	Day	16% 5% 3 3 4 3 4 5% 5% 7% 46%	1980
	21st to end	Day	17% 6% 3 6% 5% 3 5% 6% 5% 8% 36%	1980
October	1st to 10th	Day	19% 9% 3 7% 6% 3 3 5% 5% 7% 34%	1980
	11th to 20th	Day	25% 15% 3 7% 8% 4 4 5% 4 5% 21%	1706
	21st to end	Day	26% 15% 5% 9% 6% 3 6% 5% 5% 5% 16%	1782
November	1st to 10th	Day	17% 16% 6% 7% 6% 2 5% 4 5% 6% 27%	1672
	11th to 20th	Day	13% 23% 7% 10% 7% 4 5% 5% 4 5% 17%	1710
	21st to end	Day	17% 24% 9% 11% 6% 3 4 4 4 4 14%	1608
December	1st to 10th	Day	15% 22% 13% 16% 9% 3 5% 3 3 2 10%	1330
	11th to 20th	Day	14% 20% 10% 13% 11% 4 5% 4 4 4 11%	1671
	21st to end	Day	20% 20% 11% 13% 8% 4 5% 5% 3 4 8%	2732
January	1st to 10th	Night	14% 17% 10% 17% 12% 5% 4 4 3 3 11%	2648
	11th to 20th	Night	15% 19% 9% 17% 10% 4 5% 3 4 3 11%	2577
	21st to end	Night	12% 19% 9% 15% 12% 6% 6% 4 3 4 9%	2772
February	1st to 10th	Night	11% 18% 8% 14% 16% 6% 6% 4 3 3 13%	2418
	11th to 20th	Night	12% 14% 8% 15% 12% 5% 5% 4 4 4 17%	2098
	21st to end	Night	16% 17% 6% 11% 8% 4 6% 3 4 5% 20%	1776
March	1st to 10th	Night	11% 12% 5% 9% 8% 3 5% 5% 4 6% 34%	2149
	11th to 20th	Night	12% 12% 5% 8% 6% 3 5% 4 5% 5% 35%	2033
	21st to end	Night	17% 10% 4 8% 6% 3 4 5% 5% 6% 33%	2116
April	1st to 10th	Night	19% 12% 4 8% 5% 3 4 4 5% 6% 29%	1800
	11th to 20th	Night	20% 11% 3 6% 5% 2 3 4 4 6% 35%	1683
	21st to end	Night	18% 9% 3 6% 5% 3 4 5% 5% 6% 37%	1620
May	1st to 10th	Night	16% 7% 4 5% 6% 4 5% 5% 6% 7% 35%	1620
	11th to 20th	Night	15% 10% 4 6% 5% 4 5% 5% 4 6% 36%	1515
	21st to end	Night	12% 6% 2 4 4 2 4 5% 6% 10% 44%	1551
June	1st to 10th	Night	13% 5% 2 5% 6% 3 5% 5% 5% 6% 44%	1391
	11th to 20th	Night	11% 5% 3 5% 4 3 4 6% 7% 10% 43%	1260
	21st to end	Night	11% 5% 3 5% 5% 3 4 4 5% 8% 47%	1403
July	1st to 10th	Night	9% 4 1 3 3 3 4 4 7% 10% 52%	1440
	11th to 20th	Night	8% 4 2 5% 5% 3 5% 5% 6% 8% 49%	1440
	21st to end	Night	10% 5% 2 4 5% 2 5% 4 5% 7% 50%	1575
August	1st to 10th	Night	8% 5% 2 5% 5% 3 6% 6% 6% 9% 46%	1528
	11th to 20th	Night	10% 4 2 5% 4 2 5% 5% 6% 8% 49%	1598
	21st to end	Night	15% 5% 2 6% 5% 3 4 4 4 7% 45%	1884
September	1st to 10th	Night	12% 7% 2 5% 5% 3 3 4 5% 7% 48%	1805
	11th to 20th	Night	17% 5% 2 4 4 3 3 4 4 7% 46%	1980
	21st to end	Night	19% 6% 3 5% 5% 4 4 3 4 7% 39%	1980
October	1st to 10th	Night	19% 9% 3 6% 5% 2 4 4 4 5% 39%	1980
	11th to 20th	Night	25% 11% 4 7% 6% 3 5% 4 5% 7% 24%	2253
	21st to end	Night	26% 16% 4 7% 6% 3 4 3 4 5% 22%	2574
November	1st to 10th	Night	15% 15% 5% 9% 6% 3 4 4 5% 6% 28%	2406
	11th to 20th	Night	16% 22% 6% 9% 7% 4 4 4 4 5% 18%	2470
	21st to end	Night	18% 22% 8% 11% 6% 4 4 4 4 5% 15%	2565
December	1st to 10th	Night	14% 23% 12% 16% 8% 4 4 3 4 3 10%	2850
	11th to 20th	Night	13% 21% 9% 18% 9% 3 5% 4 4 4 10%	3526
	21st to end	Night	23% 21% 10% 12% 7% 4 4 4 3 4 10%	4916

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	80% 6% 6% 5% 2	1518
	11th to 20th	Day	82% 6% 5% 5% 2	1472
	21st to end	Day	82% 5% 6% 6% 1	1584
February	1st to 10th	Day	77% 6% 8% 8% 1	1540
	11th to 20th	Day	73% 6% 9% 9% 3	1757
	21st to end	Day	68% 6% 9% 13% 3 1	1480
March	1st to 10th	Day	59% 7% 12% 16% 7% 1	1800
	11th to 20th	Day	53% 6% 12% 19% 7% 2	1926
	21st to end	Day	53% 7% 12% 20% 7% 1	2254
April	1st to 10th	Day	58% 5% 10% 18% 9%	2160
	11th to 20th	Day	52% 5% 10% 20% 13% 1	2225
	21st to end	Day	49% 7% 12% 14% 14% 3	2340
May	1st to 10th	Day	46% 5% 11% 23% 12% 3	2340
	11th to 20th	Day	43% 5% 13% 22% 15% 3	2380
	21st to end	Day	37% 8% 14% 25% 15% 1	2725
June	1st to 10th	Day	36% 8% 12% 27% 15% 2	2560
	11th to 20th	Day	37% 7% 14% 23% 18% 2	2700
	21st to end	Day	38% 8% 15% 24% 14% 1	2546
July	1st to 10th	Day	28% 6% 16% 30% 19% 2	2451
	11th to 20th	Day	34% 8% 13% 27% 16% 2	2445
	21st to end	Day	32% 7% 14% 28% 18% 1	2591
August	1st to 10th	Day	35% 9% 16% 28% 11% 1	2394
	11th to 20th	Day	36% 8% 13% 29% 13% 2	2303
	21st to end	Day	39% 6% 14% 24% 15% 1	2469
September	1st to 10th	Day	40% 6% 14% 24% 15% 1	2136
	11th to 20th	Day	36% 8% 15% 29% 11% 1	1980
	21st to end	Day	44% 8% 16% 23% 9%	1980
October	1st to 10th	Day	50% 7% 11% 23% 10% 1	1980
	11th to 20th	Day	66% 7% 9% 14% 4	1706
	21st to end	Day	69% 8% 12% 9% 3	1782
November	1st to 10th	Day	57% 7% 10% 18% 7%	1672
	11th to 20th	Day	69% 7% 11% 9% 4	1710
	21st to end	Day	74% 6% 12% 8% 1	1608
December	1st to 10th	Day	82% 6% 6% 5% 1	1330
	11th to 20th	Day	78% 7% 7% 6% 1	1671
	21st to end	Day	80% 9% 6% 4 2	2732
January	1st to 10th	Night	80% 6% 7% 6% 2	2648
	11th to 20th	Night	78% 7% 6% 6% 2	2577
	21st to end	Night	80% 6% 6% 6% 1	2772
February	1st to 10th	Night	78% 6% 6% 8% 3	2418
	11th to 20th	Night	70% 6% 9% 12% 3	2099
	21st to end	Night	67% 6% 10% 13% 4	1776
March	1st to 10th	Night	51% 8% 13% 20% 8%	2160
	11th to 20th	Night	52% 5% 11% 19% 11% 1	2034
	21st to end	Night	52% 6% 11% 20% 10% 1	2116
April	1st to 10th	Night	55% 6% 9% 19% 10%	1800
	11th to 20th	Night	51% 4 9% 20% 13% 2	1683
	21st to end	Night	46% 6% 10% 20% 14% 2	1620
May	1st to 10th	Night	47% 6% 12% 20% 11% 4	1620
	11th to 20th	Night	49% 5% 9% 19% 14% 3	1516
	21st to end	Night	35% 7% 16% 25% 14% 2	1552
June	1st to 10th	Night	39% 7% 10% 28% 15% 1	1391
	11th to 20th	Night	34% 9% 14% 24% 16% 2	1260
	21st to end	Night	36% 7% 13% 26% 16% 2	1403
July	1st to 10th	Night	26% 7% 15% 30% 19% 3	1400
	11th to 20th	Night	31% 7% 15% 25% 20% 1	1392
	21st to end	Night	32% 7% 15% 29% 16% 1	1487
August	1st to 10th	Night	33% 8% 18% 29% 11% 2	1519
	11th to 20th	Night	32% 8% 17% 28% 15% 1	1598
	21st to end	Night	39% 7% 14% 24% 16% 1	1884
September	1st to 10th	Night	36% 6% 13% 23% 19% 1	1805
	11th to 20th	Night	36% 7% 16% 29% 11%	1980
	21st to end	Night	46% 6% 15% 25% 7%	1980
October	1st to 10th	Night	48% 7% 9% 23% 13% 1	1980
	11th to 20th	Night	60% 8% 11% 16% 5%	2253
	21st to end	Night	66% 6% 10% 13% 5%	2574
November	1st to 10th	Night	56% 8% 11% 18% 7% 1	2406
	11th to 20th	Night	68% 7% 10% 9% 4 1	2470
	21st to end	Night	72% 7% 11% 8% 1	2565
December	1st to 10th	Night	79% 8% 6% 7% 1	2850
	11th to 20th	Night	78% 8% 8% 4 2	3525
	21st to end	Night	78% 10% 7% 3 1	4916

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	17% 16% 37% 20% 8% 2	270
	11th to 20th	Day	21% 13% 36% 25% 3 1	252
	21st to end	Day	23% 11% 33% 27% 6%	264
February	1st to 10th	Day	22% 20% 36% 18% 3	240
	11th to 20th	Day	18% 21% 37% 21% 4	240
	21st to end	Day	22% 24% 37% 14% 3	198
March	1st to 10th	Day	22% 17% 37% 20% 5%	240
	11th to 20th	Day	20% 10% 29% 29% 11% 1	366
	21st to end	Day	27% 10% 27% 24% 11% 1	462
April	1st to 10th	Day	24% 12% 30% 26% 8% 1	420
	11th to 20th	Day	26% 17% 24% 19% 11% 3	412
	21st to end	Day	21% 11% 28% 25% 13% 2	420
May	1st to 10th	Day	23% 14% 24% 23% 13% 3	423
	11th to 20th	Day	17% 7% 29% 29% 13% 4	522
	21st to end	Day	13% 9% 27% 28% 18% 5%	679
June	1st to 10th	Day	15% 7% 28% 30% 17% 4	630
	11th to 20th	Day	15% 7% 25% 34% 15% 4	630
	21st to end	Day	13% 5% 25% 33% 19% 3	630
July	1st to 10th	Day	10% 3 20% 35% 23% 9%	621
	11th to 20th	Day	12% 6% 22% 33% 21% 7%	618
	21st to end	Day	10% 4 23% 37% 20% 6%	684
August	1st to 10th	Day	12% 3 18% 30% 27% 10%	528
	11th to 20th	Day	13% 4 16% 31% 23% 13%	439
	21st to end	Day	16% 7% 17% 29% 23% 8%	495
September	1st to 10th	Day	19% 5% 20% 28% 19% 9%	450
	11th to 20th	Day	14% 6% 17% 31% 23% 10%	450
	21st to end	Day	19% 10% 17% 25% 21% 7%	450
October	1st to 10th	Day	22% 11% 22% 24% 15% 5%	450
	11th to 20th	Day	33% 12% 26% 21% 8% 1	342
	21st to end	Day	33% 12% 30% 18% 6% 2	297
November	1st to 10th	Day	25% 18% 27% 18% 11% 1	270
	11th to 20th	Day	19% 23% 36% 18% 5%	270
	21st to end	Day	28% 21% 33% 16% 3	241
December	1st to 10th	Day	31% 17% 28% 17% 6% 1	180
	11th to 20th	Day	23% 24% 32% 17% 3 1	180
	21st to end	Day	24% 21% 34% 18% 2 1	285
January	1st to 10th	Night	20% 14% 34% 24% 7% 1	950
	11th to 20th	Night	24% 14% 31% 24% 6% 1	921
	21st to end	Night	20% 10% 34% 27% 7% 1	990
February	1st to 10th	Night	17% 11% 39% 27% 5% 1	798
	11th to 20th	Night	19% 12% 37% 26% 6%	703
	21st to end	Night	25% 13% 32% 25% 5%	592
March	1st to 10th	Night	15% 11% 33% 29% 11% 1	720
	11th to 20th	Night	18% 11% 34% 30% 7% 1	720
	21st to end	Night	21% 8% 29% 33% 8% 1	730
April	1st to 10th	Night	24% 10% 29% 29% 7% 1	540
	11th to 20th	Night	23% 10% 28% 28% 10% 1	533
	21st to end	Night	21% 7% 26% 35% 11% 1	540
May	1st to 10th	Night	19% 4 29% 36% 11% 1	540
	11th to 20th	Night	19% 4 26% 36% 14% 1	531
	21st to end	Night	14% 5% 22% 39% 19% 1	584
June	1st to 10th	Night	15% 3 25% 39% 16% 1	491
	11th to 20th	Night	12% 4 21% 45% 18%	360
	21st to end	Night	13% 3 22% 37% 23% 1	503
July	1st to 10th	Night	10% 2 14% 45% 26% 3	525
	11th to 20th	Night	10% 2 20% 38% 26% 3	522
	21st to end	Night	11% 3 18% 42% 24% 2	591
August	1st to 10th	Night	9% 3 15% 44% 27% 2	537
	11th to 20th	Night	11% 3 16% 37% 30% 4	534
	21st to end	Night	16% 4 17% 39% 22% 2	594
September	1st to 10th	Night	14% 3 21% 37% 21% 4	545
	11th to 20th	Night	17% 5% 19% 33% 22% 4	720
	21st to end	Night	23% 7% 18% 31% 18% 3	720
October	1st to 10th	Night	22% 8% 23% 30% 17% 1	720
	11th to 20th	Night	31% 10% 26% 27% 6% 1	720
	21st to end	Night	35% 12% 24% 22% 6% 1	792
November	1st to 10th	Night	25% 11% 30% 27% 6%	740
	11th to 20th	Night	25% 17% 33% 21% 4	760
	21st to end	Night	29% 13% 34% 21% 3	798
December	1st to 10th	Night	23% 17% 36% 19% 4	950
	11th to 20th	Night	23% 15% 38% 19% 4 1	950
	21st to end	Night	26% 13% 36% 22% 2	1045

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