Cordell-Lorenz Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cordell-Lorenz Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	126141
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 41%	123259
Transparency	Day	42% 7% 11% 21% 16% 4	129866
	Night	41% 6% 10% 20% 18% 5%	127591
Seeing	Day	20% 12% 24% 22% 15% 7%	15813
	Night	19% 11% 20% 27% 19% 4	25774

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	15% 12% 6% 9% 5% 4 3 3 4 5% 34%	4056
	11th to 20th	Day	17% 6% 4 16% 4 2 2 2 3 4 40%	4060
	21st to end	Day	19% 11% 5% 7% 4 3 3 3 5% 6% 35%	4834
February	1st to 10th	Day	20% 11% 4 8% 3 4 4 3 4 5% 34%	2000
	11th to 20th	Day	22% 11% 4 7% 5% 4 4 3 5% 4 32%	1948
	21st to end	Day	20% 9% 3 7% 4 4 3 3 4 5% 37%	1650
March	1st to 10th	Day	18% 11% 3 7% 5% 5% 4 3 5% 5% 32%	4480
	11th to 20th	Day	13% 9% 3 6% 4 4 3 4 4 5% 46%	4880
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 6% 40%	5571
April	1st to 10th	Day	14% 7% 3 5% 5% 5% 5% 4 5% 7% 41%	2400
	11th to 20th	Day	13% 8% 2 7% 4 3 3 3 5% 7% 44%	2374
	21st to end	Day	13% 8% 2 6% 6% 5% 4 4 5% 8% 39%	2475
May	1st to 10th	Day	18% 9% 3 7% 5% 5% 5% 4 7% 8% 30%	5862
	11th to 20th	Day	16% 10% 4 10% 6% 5% 5% 4 6% 7% 26%	5861
	21st to end	Day	17% 7% 3 8% 5% 4 4 4 5% 7% 36%	6433
June	1st to 10th	Day	9% 7% 3 8% 6% 6% 6% 5% 7% 10% 35%	2600
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 5% 9% 11% 39%	2600
	21st to end	Day	6% 5% 2 7% 6% 6% 6% 6% 9% 11% 36%	2600
July	1st to 10th	Day	7% 5% 2 7% 6% 7% 6% 5% 8% 11% 36%	5900
	11th to 20th	Day	9% 6% 2 7% 7% 7% 7% 6% 8% 11% 29%	5900
	21st to end	Day	14% 8% 3 8% 8% 7% 7% 5% 8% 10% 22%	6424
August	1st to 10th	Day	6% 4 2 6% 5% 6% 6% 5% 8% 11% 43%	4103
	11th to 20th	Day	7% 5% 2 6% 5% 7% 7% 6% 8% 11% 36%	3849
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 6% 7% 48%	4312
September	1st to 10th	Day	11% 6% 3 8% 5% 7% 5% 4 7% 8% 37%	2315
	11th to 20th	Day	11% 6% 2 7% 5% 4 6% 4 7% 8% 40%	2200
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 6% 7% 43%	2200
October	1st to 10th	Day	10% 5% 2 6% 4 4 3 3 4 5% 54%	2123
	11th to 20th	Day	11% 6% 2 4 4 4 3 3 4 6% 53%	2067
	21st to end	Day	13% 6% 2 5% 4 4 3 4 5% 6% 49%	1933
November	1st to 10th	Day	11% 6% 2 5% 4 3 3 4 4 5% 53%	1762
	11th to 20th	Day	12% 8% 3 6% 4 3 4 4 5% 5% 47%	1800
	21st to end	Day	16% 10% 2 5% 3 3 4 4 4 7% 43%	1800
December	1st to 10th	Day	17% 9% 2 6% 4 3 3 3 4 4 43%	2930
	11th to 20th	Day	16% 12% 6% 10% 6% 4 4 3 4 5% 30%	3373
	21st to end	Day	17% 12% 6% 8% 4 3 4 3 5% 7% 31%	4466
January	1st to 10th	Night	14% 15% 4 9% 4 3 2 3 4 5% 38%	5889
	11th to 20th	Night	18% 10% 5% 7% 4 3 3 2 3 3 41%	5900
	21st to end	Night	17% 12% 4 6% 4 3 3 2 3 5% 39%	6104
February	1st to 10th	Night	19% 10% 4 8% 4 3 3 3 4 6% 37%	2400
	11th to 20th	Night	22% 11% 3 9% 4 4 4 3 3 3 34%	2338
	21st to end	Night	23% 11% 2 6% 4 4 4 3 4 5% 34%	1980
March	1st to 10th	Night	22% 13% 4 8% 4 4 3 3 4 5% 29%	5373
	11th to 20th	Night	13% 7% 2 5% 4 4 4 3 6% 6% 46%	5078
	21st to end	Night	15% 8% 3 8% 5% 5% 4 3 4 5% 41%	5386
April	1st to 10th	Night	12% 7% 2 5% 4 5% 5% 3 4 8% 45%	2000
	11th to 20th	Night	14% 7% 2 5% 2 3 3 3 6% 7% 47%	1975
	21st to end	Night	12% 7% 2 6% 4 3 4 4 6% 7% 44%	1925
May	1st to 10th	Night	15% 9% 3 8% 5% 5% 5% 3 6% 7% 32%	4033
	11th to 20th	Night	18% 11% 4 9% 6% 5% 4 4 6% 7% 27%	4032
	21st to end	Night	15% 7% 3 7% 4 4 5% 4 6% 9% 36%	4421
June	1st to 10th	Night	7% 7% 3 7% 5% 5% 5% 4 7% 10% 40%	1800
	11th to 20th	Night	5% 2 1 4 4 4 5% 4 8% 12% 51%	1800
	21st to end	Night	6% 5% 1 5% 4 4 4 5% 7% 11% 47%	1800
July	1st to 10th	Night	9% 6% 2 7% 4 5% 5% 5% 7% 10% 41%	4058
	11th to 20th	Night	8% 5% 2 7% 4 5% 5% 5% 8% 13% 39%	4057
	21st to end	Night	10% 7% 3 8% 5% 6% 5% 5% 8% 11% 32%	4427
August	1st to 10th	Night	4 3 2 4 3 4 5% 4 8% 11% 52%	3041
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 47%	3200
	21st to end	Night	5% 5% 2 5% 3 3 4 4 7% 8% 55%	3586
September	1st to 10th	Night	9% 6% 2 5% 4 4 5% 4 6% 9% 45%	2063
	11th to 20th	Night	9% 7% 2 7% 5% 4 4 4 6% 7% 46%	2200
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 6% 51%	2200
October	1st to 10th	Night	10% 4 1 4 3 2 3 3 4 6% 59%	2124
	11th to 20th	Night	13% 5% 2 6% 3 4 4 3 4 5% 51%	2111
	21st to end	Night	13% 8% 2 5% 3 3 3 3 4 6% 50%	2660
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 5% 5% 49%	2536
	11th to 20th	Night	12% 9% 2 5% 4 3 2 3 6% 6% 48%	2600
	21st to end	Night	14% 11% 3 5% 4 4 4 3 4 5% 43%	2600
December	1st to 10th	Night	19% 10% 4 6% 3 3 3 2 4 4 41%	4250
	11th to 20th	Night	19% 12% 4 8% 4 3 3 3 3 4 36%	4822
	21st to end	Night	15% 13% 7% 8% 4 4 4 3 4 6% 31%	6490

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	51% 5% 7% 18% 18% 2	4057
	11th to 20th	Day	48% 4 4 16% 27% 1	4060
	21st to end	Day	49% 4 6% 21% 16% 4	4834
February	1st to 10th	Day	52% 4 6% 18% 17% 3	2000
	11th to 20th	Day	55% 4 6% 17% 14% 4	1958
	21st to end	Day	51% 3 6% 15% 19% 6%	1650
March	1st to 10th	Day	52% 5% 6% 12% 16% 8%	4480
	11th to 20th	Day	39% 4 6% 19% 20% 11%	4880
	21st to end	Day	44% 5% 9% 26% 15% 1	5573
April	1st to 10th	Day	41% 6% 10% 22% 17% 4	2400
	11th to 20th	Day	40% 5% 7% 20% 24% 5%	2374
	21st to end	Day	43% 6% 9% 27% 14% 2	2475
May	1st to 10th	Day	48% 9% 12% 20% 9% 1	5897
	11th to 20th	Day	54% 9% 13% 18% 7%	5865
	21st to end	Day	46% 7% 8% 21% 15% 2	6447
June	1st to 10th	Day	42% 9% 14% 21% 13% 1	2600
	11th to 20th	Day	34% 9% 18% 23% 14% 2	2600
	21st to end	Day	37% 10% 19% 19% 13% 3	2600
July	1st to 10th	Day	37% 11% 18% 24% 9% 1	5835
	11th to 20th	Day	40% 15% 24% 17% 4 1	5831
	21st to end	Day	51% 13% 18% 15% 3	6240
August	1st to 10th	Day	30% 10% 15% 32% 13% 1	4090
	11th to 20th	Day	35% 11% 15% 25% 12% 1	3885
	21st to end	Day	32% 9% 11% 28% 19% 2	4311
September	1st to 10th	Day	42% 8% 12% 19% 15% 4	2315
	11th to 20th	Day	39% 8% 11% 21% 19% 2	2200
	21st to end	Day	38% 7% 10% 21% 18% 6%	2200
October	1st to 10th	Day	23% 5% 6% 18% 35% 13%	3513
	11th to 20th	Day	22% 3 6% 20% 31% 18%	3411
	21st to end	Day	26% 5% 8% 24% 26% 11%	3157
November	1st to 10th	Day	35% 4 7% 20% 28% 7%	1762
	11th to 20th	Day	39% 3 5% 22% 24% 7%	1800
	21st to end	Day	42% 4 6% 22% 22% 4	1800
December	1st to 10th	Day	44% 4 4 21% 23% 4	2930
	11th to 20th	Day	56% 5% 6% 17% 14% 3	3370
	21st to end	Day	51% 5% 9% 21% 11% 2	4466
January	1st to 10th	Night	49% 3 8% 18% 20% 2	5889
	11th to 20th	Night	48% 4 4 16% 22% 6%	5900
	21st to end	Night	49% 3 4 17% 20% 7%	6104
February	1st to 10th	Night	50% 3 8% 18% 17% 3	2400
	11th to 20th	Night	56% 3 5% 16% 15% 6%	2339
	21st to end	Night	52% 5% 5% 15% 17% 5%	1980
March	1st to 10th	Night	56% 5% 5% 13% 17% 5%	5373
	11th to 20th	Night	37% 5% 6% 19% 19% 14%	5078
	21st to end	Night	47% 4 7% 23% 16% 3	5386
April	1st to 10th	Night	38% 5% 11% 22% 20% 3	2000
	11th to 20th	Night	34% 5% 9% 21% 23% 8%	1975
	21st to end	Night	38% 5% 12% 26% 16% 2	1925
May	1st to 10th	Night	49% 9% 14% 18% 10% 1	4060
	11th to 20th	Night	54% 9% 13% 16% 8%	4033
	21st to end	Night	42% 9% 13% 23% 12% 1	4429
June	1st to 10th	Night	38% 8% 16% 23% 14% 2	1800
	11th to 20th	Night	24% 8% 19% 32% 15% 2	1800
	21st to end	Night	28% 10% 19% 25% 14% 4	1800
July	1st to 10th	Night	35% 10% 19% 23% 10% 2	4015
	11th to 20th	Night	32% 12% 29% 20% 5% 2	4006
	21st to end	Night	42% 12% 23% 18% 4 1	4297
August	1st to 10th	Night	22% 9% 13% 38% 16% 1	3030
	11th to 20th	Night	28% 9% 22% 25% 14% 1	3230
	21st to end	Night	24% 8% 15% 28% 22% 2	3586
September	1st to 10th	Night	35% 6% 17% 20% 16% 6%	2063
	11th to 20th	Night	35% 8% 13% 21% 21% 2	2200
	21st to end	Night	33% 6% 10% 22% 22% 7%	2200
October	1st to 10th	Night	19% 5% 6% 24% 27% 19%	3514
	11th to 20th	Night	22% 3 4 19% 31% 20%	3506
	21st to end	Night	24% 3 6% 26% 28% 12%	4373
November	1st to 10th	Night	35% 6% 7% 21% 24% 8%	2536
	11th to 20th	Night	36% 3 6% 20% 25% 9%	2600
	21st to end	Night	44% 3 5% 20% 22% 6%	2600
December	1st to 10th	Night	47% 4 4 16% 25% 3	4250
	11th to 20th	Night	52% 4 4 17% 21% 3	4824
	21st to end	Night	53% 5% 8% 21% 11% 2	6490

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	36% 22% 31% 10% 2	300
	11th to 20th	Day	39% 24% 23% 10% 3	300
	21st to end	Day	28% 32% 31% 8% 2	330
February	1st to 10th	Day	32% 22% 33% 11% 2	300
	11th to 20th	Day	49% 21% 24% 5% 1	300
	21st to end	Day	42% 23% 28% 6% 1	249
March	1st to 10th	Day	35% 23% 32% 9% 2	300
	11th to 20th	Day	30% 25% 23% 17% 4 1	452
	21st to end	Day	30% 19% 31% 12% 7%	550
April	1st to 10th	Day	26% 19% 28% 18% 8% 1	500
	11th to 20th	Day	24% 18% 30% 18% 8% 1	492
	21st to end	Day	24% 13% 32% 22% 7% 2	500
May	1st to 10th	Day	21% 16% 30% 19% 9% 4	503
	11th to 20th	Day	15% 13% 29% 23% 13% 6%	527
	21st to end	Day	17% 7% 23% 27% 15% 11%	573
June	1st to 10th	Day	16% 8% 22% 24% 18% 12%	530
	11th to 20th	Day	9% 5% 18% 31% 21% 17%	530
	21st to end	Day	11% 4 21% 27% 22% 15%	530
July	1st to 10th	Day	11% 4 15% 28% 26% 16%	525
	11th to 20th	Day	14% 3 12% 23% 28% 20%	524
	21st to end	Day	12% 4 11% 26% 31% 16%	576
August	1st to 10th	Day	12% 6% 14% 25% 29% 14%	526
	11th to 20th	Day	12% 3 15% 31% 29% 9%	519
	21st to end	Day	15% 3 10% 26% 32% 13%	583
September	1st to 10th	Day	11% 5% 16% 30% 26% 12%	530
	11th to 20th	Day	13% 6% 15% 28% 28% 10%	530
	21st to end	Day	15% 7% 17% 28% 28% 4	530
October	1st to 10th	Day	12% 8% 23% 31% 20% 6%	502
	11th to 20th	Day	15% 12% 30% 28% 12% 3	490
	21st to end	Day	21% 10% 31% 25% 12% 1	382
November	1st to 10th	Day	18% 13% 35% 24% 8% 1	300
	11th to 20th	Day	19% 20% 38% 20% 2	300
	21st to end	Day	26% 18% 35% 13% 8% 1	300
December	1st to 10th	Day	35% 18% 30% 12% 4	300
	11th to 20th	Day	30% 22% 30% 12% 6%	300
	21st to end	Day	32% 20% 32% 12% 3	330
January	1st to 10th	Night	28% 21% 26% 20% 5%	800
	11th to 20th	Night	29% 21% 26% 19% 5%	800
	21st to end	Night	23% 17% 34% 23% 2	880
February	1st to 10th	Night	27% 16% 27% 26% 4	800
	11th to 20th	Night	31% 21% 26% 19% 2	783
	21st to end	Night	32% 18% 28% 19% 2	660
March	1st to 10th	Night	25% 18% 27% 27% 4	800
	11th to 20th	Night	22% 17% 29% 26% 7%	800
	21st to end	Night	22% 16% 27% 25% 9%	850
April	1st to 10th	Night	18% 13% 27% 30% 11% 1	600
	11th to 20th	Night	20% 11% 30% 27% 11% 1	593
	21st to end	Night	17% 9% 28% 32% 13% 2	600
May	1st to 10th	Night	16% 8% 27% 29% 19% 2	600
	11th to 20th	Night	16% 7% 20% 31% 21% 5%	591
	21st to end	Night	13% 3 15% 34% 30% 6%	650
June	1st to 10th	Night	12% 3 13% 34% 34% 5%	600
	11th to 20th	Night	6% 2 10% 33% 37% 12%	600
	21st to end	Night	8% 2 10% 30% 42% 9%	600
July	1st to 10th	Night	10% 2 8% 25% 44% 11%	585
	11th to 20th	Night	10% 2 6% 24% 41% 16%	582
	21st to end	Night	11% 2 6% 24% 45% 13%	657
August	1st to 10th	Night	8% 2 6% 25% 47% 12%	597
	11th to 20th	Night	9% 3 7% 30% 41% 9%	594
	21st to end	Night	11% 3 6% 30% 42% 8%	660
September	1st to 10th	Night	11% 3 10% 29% 39% 8%	663
	11th to 20th	Night	13% 5% 13% 28% 35% 6%	800
	21st to end	Night	12% 5% 13% 33% 33% 4	800
October	1st to 10th	Night	12% 5% 18% 29% 28% 7%	773
	11th to 20th	Night	16% 10% 23% 30% 20% 1	760
	21st to end	Night	19% 9% 20% 35% 15% 2	836
November	1st to 10th	Night	17% 10% 22% 31% 18% 3	780
	11th to 20th	Night	18% 16% 26% 26% 13% 1	800
	21st to end	Night	24% 14% 21% 29% 11% 1	800
December	1st to 10th	Night	28% 16% 24% 27% 6%	800
	11th to 20th	Night	25% 15% 25% 28% 8% 1	800
	21st to end	Night	28% 17% 24% 26% 5%	880

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