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 2024-11-05.

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%	100907
	Night	13% 9% 3 6% 4 4 4 3 5% 7% 41%	98113
Transparency	Day	42% 7% 10% 21% 16% 4	104674
	Night	40% 6% 10% 21% 18% 5%	102471
Seeing	Day	20% 12% 24% 22% 16% 7%	14609
	Night	19% 11% 20% 28% 20% 4	24607

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	16% 13% 6% 7% 5% 5% 5% 4 5% 5% 30%	2836
	11th to 20th	Day	21% 8% 4 14% 4 3 3 2 4 5% 32%	2840
	21st to end	Day	21% 13% 5% 7% 3 3 2 3 4 6% 33%	3380
February	1st to 10th	Day	20% 12% 3 8% 3 3 4 3 4 5% 34%	1900
	11th to 20th	Day	22% 10% 4 7% 4 4 4 3 4 4 33%	1848
	21st to end	Day	21% 10% 4 7% 4 4 3 3 4 5% 35%	1570
March	1st to 10th	Day	22% 11% 3 7% 5% 4 4 3 5% 5% 29%	3120
	11th to 20th	Day	15% 8% 3 7% 4 4 3 3 4 5% 45%	3408
	21st to end	Day	15% 9% 3 8% 5% 4 4 4 5% 7% 36%	3895
April	1st to 10th	Day	14% 6% 3 6% 5% 5% 5% 4 5% 7% 42%	2280
	11th to 20th	Day	14% 8% 2 7% 5% 3 3 3 5% 7% 43%	2254
	21st to end	Day	12% 8% 2 5% 6% 5% 4 3 5% 8% 40%	2351
May	1st to 10th	Day	15% 8% 2 7% 5% 5% 5% 4 6% 7% 36%	4082
	11th to 20th	Day	16% 9% 4 8% 5% 5% 5% 4 6% 7% 32%	4081
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	4476
June	1st to 10th	Day	10% 7% 2 8% 6% 6% 6% 5% 6% 9% 35%	2470
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 9% 11% 40%	2470
	21st to end	Day	6% 6% 2 7% 5% 6% 6% 6% 9% 11% 36%	2470
July	1st to 10th	Day	7% 6% 2 8% 7% 7% 7% 5% 9% 11% 31%	4120
	11th to 20th	Day	10% 6% 2 7% 7% 8% 7% 5% 7% 10% 29%	4120
	21st to end	Day	19% 10% 3 9% 7% 7% 7% 5% 7% 8% 19%	4466
August	1st to 10th	Day	5% 4 2 5% 5% 5% 6% 5% 8% 11% 44%	3977
	11th to 20th	Day	7% 5% 2 6% 5% 7% 7% 6% 8% 11% 36%	3729
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 6% 7% 48%	4180
September	1st to 10th	Day	11% 6% 3 8% 6% 7% 5% 4 7% 8% 36%	2199
	11th to 20th	Day	11% 7% 3 8% 5% 4 6% 4 7% 8% 39%	2090
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 6% 7% 44%	2090
October	1st to 10th	Day	10% 6% 2 6% 4 4 3 3 4 5% 54%	2090
	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 3 4 5% 55%	1708
	11th to 20th	Day	12% 8% 3 6% 4 3 4 3 5% 5% 47%	1710
	21st to end	Day	16% 9% 2 5% 4 3 4 4 4 6% 42%	1710
December	1st to 10th	Day	20% 12% 3 8% 5% 3 3 4 5% 5% 33%	1710
	11th to 20th	Day	14% 10% 4 9% 6% 4 5% 4 5% 5% 36%	2153
	21st to end	Day	16% 12% 7% 8% 4 4 4 3 5% 7% 30%	3124
January	1st to 10th	Night	17% 15% 4 6% 4 3 3 3 4 6% 36%	4109
	11th to 20th	Night	20% 11% 5% 7% 5% 4 4 3 4 4 35%	4120
	21st to end	Night	20% 15% 5% 6% 4 3 3 2 3 4 35%	4272
February	1st to 10th	Night	18% 10% 4 8% 4 3 3 3 4 6% 37%	2280
	11th to 20th	Night	21% 11% 3 8% 4 4 4 3 3 3 35%	2218
	21st to end	Night	24% 11% 2 6% 4 4 4 3 4 5% 32%	1884
March	1st to 10th	Night	23% 13% 4 9% 4 4 3 3 4 4 28%	3733
	11th to 20th	Night	12% 8% 2 6% 4 4 4 3 5% 6% 46%	3550
	21st to end	Night	14% 9% 3 8% 5% 5% 5% 3 4 5% 38%	3764
April	1st to 10th	Night	12% 7% 2 5% 4 4 5% 3 4 8% 45%	1900
	11th to 20th	Night	15% 7% 2 5% 2 3 4 3 6% 8% 46%	1875
	21st to end	Night	12% 7% 2 6% 4 3 4 4 6% 7% 45%	1829
May	1st to 10th	Night	14% 7% 3 6% 4 4 5% 3 6% 8% 40%	2813
	11th to 20th	Night	16% 9% 3 7% 4 4 4 4 6% 7% 35%	2812
	21st to end	Night	7% 6% 2 6% 5% 5% 5% 4 7% 10% 41%	3079
June	1st to 10th	Night	7% 7% 3 7% 5% 5% 5% 4 7% 10% 39%	1710
	11th to 20th	Night	4 2 1 4 4 4 5% 4 8% 12% 52%	1710
	21st to end	Night	6% 5% 1 5% 4 4 4 5% 7% 11% 47%	1710
July	1st to 10th	Night	8% 5% 2 8% 5% 6% 5% 5% 8% 10% 37%	2840
	11th to 20th	Night	9% 5% 2 6% 4 5% 4 4 7% 11% 43%	2840
	21st to end	Night	14% 10% 4 10% 6% 6% 6% 5% 7% 8% 25%	3087
August	1st to 10th	Night	4 3 2 4 3 4 4 4 8% 11% 53%	2947
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 7% 10% 47%	3100
	21st to end	Night	5% 4 2 4 3 3 4 4 7% 8% 55%	3476
September	1st to 10th	Night	9% 6% 2 5% 4 4 5% 4 6% 9% 45%	1960
	11th to 20th	Night	10% 7% 2 7% 5% 4 4 4 6% 7% 45%	2090
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 6% 52%	2090
October	1st to 10th	Night	10% 4 1 4 3 2 3 3 4 6% 59%	2090
	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	12% 6% 2 5% 4 3 3 3 5% 6% 51%	2470
	11th to 20th	Night	11% 9% 2 5% 4 3 2 3 6% 6% 49%	2470
	21st to end	Night	14% 11% 3 5% 4 4 4 3 5% 6% 42%	2470
December	1st to 10th	Night	20% 11% 5% 7% 4 4 4 3 4 5% 33%	2470
	11th to 20th	Night	16% 10% 4 7% 4 3 4 3 4 5% 40%	3042
	21st to end	Night	16% 14% 7% 8% 5% 5% 5% 4 5% 6% 27%	4532

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	52% 4 5% 18% 19% 2	2837
	11th to 20th	Day	54% 4 5% 14% 22% 2	2840
	21st to end	Day	52% 3 7% 21% 15% 2	3380
February	1st to 10th	Day	53% 4 5% 18% 17% 3	1900
	11th to 20th	Day	54% 4 5% 17% 15% 5%	1858
	21st to end	Day	52% 4 6% 14% 18% 5%	1570
March	1st to 10th	Day	56% 5% 7% 12% 13% 8%	3120
	11th to 20th	Day	41% 4 7% 24% 18% 7%	3408
	21st to end	Day	47% 6% 9% 22% 15% 2	3895
April	1st to 10th	Day	41% 5% 10% 22% 18% 4	2280
	11th to 20th	Day	41% 5% 7% 19% 24% 5%	2254
	21st to end	Day	41% 6% 9% 28% 15% 2	2351
May	1st to 10th	Day	43% 8% 11% 23% 12% 2	4120
	11th to 20th	Day	47% 8% 13% 23% 9%	4085
	21st to end	Day	40% 8% 10% 22% 18% 1	4489
June	1st to 10th	Day	43% 9% 14% 20% 13% 1	2470
	11th to 20th	Day	33% 9% 18% 24% 14% 2	2470
	21st to end	Day	37% 10% 18% 19% 13% 3	2470
July	1st to 10th	Day	40% 12% 20% 21% 6% 1	4055
	11th to 20th	Day	43% 12% 20% 19% 5% 2	4051
	21st to end	Day	60% 10% 11% 14% 5% 1	4324
August	1st to 10th	Day	29% 10% 14% 33% 13% 1	3964
	11th to 20th	Day	35% 11% 15% 25% 13% 1	3765
	21st to end	Day	32% 9% 11% 28% 19% 2	4179
September	1st to 10th	Day	42% 8% 12% 18% 15% 5%	2199
	11th to 20th	Day	41% 8% 11% 20% 19% 2	2090
	21st to end	Day	37% 6% 11% 22% 18% 6%	2090
October	1st to 10th	Day	24% 5% 6% 17% 35% 13%	3480
	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	33% 3 7% 21% 29% 7%	1708
	11th to 20th	Day	39% 3 5% 22% 23% 7%	1710
	21st to end	Day	42% 4 6% 21% 23% 4	1710
December	1st to 10th	Day	53% 4 5% 19% 15% 3	1710
	11th to 20th	Day	48% 5% 8% 15% 20% 4	2150
	21st to end	Day	52% 5% 9% 19% 13% 3	3124
January	1st to 10th	Night	50% 4 6% 18% 21% 2	4109
	11th to 20th	Night	53% 5% 4 15% 16% 7%	4120
	21st to end	Night	55% 3 5% 15% 19% 4	4272
February	1st to 10th	Night	50% 3 8% 18% 18% 3	2280
	11th to 20th	Night	55% 3 5% 16% 15% 6%	2219
	21st to end	Night	54% 5% 5% 15% 16% 5%	1884
March	1st to 10th	Night	58% 6% 7% 11% 14% 6%	3733
	11th to 20th	Night	39% 5% 7% 23% 17% 10%	3550
	21st to end	Night	48% 5% 7% 21% 17% 2	3764
April	1st to 10th	Night	38% 5% 11% 22% 21% 3	1900
	11th to 20th	Night	35% 5% 9% 20% 23% 8%	1875
	21st to end	Night	37% 5% 12% 27% 17% 2	1829
May	1st to 10th	Night	41% 8% 13% 23% 14% 1	2840
	11th to 20th	Night	44% 9% 13% 21% 12%	2813
	21st to end	Night	33% 10% 15% 26% 14% 2	3087
June	1st to 10th	Night	38% 8% 15% 23% 13% 2	1710
	11th to 20th	Night	22% 8% 20% 33% 16% 2	1710
	21st to end	Night	28% 9% 20% 25% 14% 4	1710
July	1st to 10th	Night	38% 11% 20% 22% 8% 1	2795
	11th to 20th	Night	32% 9% 25% 24% 7% 3	2786
	21st to end	Night	54% 10% 13% 16% 6% 1	2988
August	1st to 10th	Night	22% 9% 13% 39% 16% 1	2936
	11th to 20th	Night	28% 9% 22% 25% 14% 1	3130
	21st to end	Night	24% 8% 15% 29% 23% 2	3476
September	1st to 10th	Night	35% 6% 17% 20% 16% 6%	1960
	11th to 20th	Night	36% 8% 13% 20% 21% 2	2090
	21st to end	Night	32% 6% 10% 23% 22% 7%	2090
October	1st to 10th	Night	19% 5% 6% 24% 27% 19%	3480
	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	34% 5% 7% 21% 24% 9%	2470
	11th to 20th	Night	36% 3 6% 20% 25% 9%	2470
	21st to end	Night	44% 3 5% 19% 22% 6%	2470
December	1st to 10th	Night	53% 5% 6% 17% 16% 3	2470
	11th to 20th	Night	45% 5% 6% 15% 24% 5%	3044
	21st to end	Night	55% 5% 8% 18% 12% 2	4532

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	34% 21% 31% 11% 2	270
	11th to 20th	Day	40% 22% 23% 11% 4	270
	21st to end	Day	29% 30% 32% 7% 2	297
February	1st to 10th	Day	34% 23% 31% 11% 2	270
	11th to 20th	Day	47% 21% 25% 6% 1	270
	21st to end	Day	45% 23% 26% 4 1	225
March	1st to 10th	Day	33% 22% 34% 9% 2	270
	11th to 20th	Day	29% 24% 23% 18% 5% 1	414
	21st to end	Day	31% 19% 30% 12% 8%	506
April	1st to 10th	Day	26% 18% 28% 19% 9% 1	460
	11th to 20th	Day	25% 17% 29% 19% 9% 1	452
	21st to end	Day	24% 13% 31% 21% 8% 2	460
May	1st to 10th	Day	21% 14% 31% 19% 10% 4	463
	11th to 20th	Day	15% 10% 30% 24% 15% 7%	487
	21st to end	Day	14% 6% 22% 29% 16% 12%	529
June	1st to 10th	Day	17% 8% 21% 24% 18% 13%	490
	11th to 20th	Day	8% 5% 18% 30% 22% 19%	490
	21st to end	Day	12% 4 22% 27% 21% 15%	490
July	1st to 10th	Day	12% 4 15% 25% 27% 17%	485
	11th to 20th	Day	14% 3 13% 23% 27% 20%	484
	21st to end	Day	13% 5% 10% 26% 29% 18%	532
August	1st to 10th	Day	13% 4 14% 25% 30% 15%	486
	11th to 20th	Day	13% 3 16% 31% 29% 8%	479
	21st to end	Day	16% 3 9% 26% 32% 14%	539
September	1st to 10th	Day	11% 4 15% 30% 27% 12%	490
	11th to 20th	Day	14% 5% 16% 27% 28% 10%	490
	21st to end	Day	16% 7% 17% 28% 29% 4	490
October	1st to 10th	Day	12% 8% 24% 29% 20% 7%	490
	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	17% 12% 36% 26% 8% 1	282
	11th to 20th	Day	18% 20% 40% 21% 2	270
	21st to end	Day	24% 17% 37% 12% 9% 1	270
December	1st to 10th	Day	37% 16% 28% 13% 5%	270
	11th to 20th	Day	28% 23% 32% 11% 6%	270
	21st to end	Day	33% 20% 32% 12% 2	297
January	1st to 10th	Night	28% 21% 26% 20% 5%	760
	11th to 20th	Night	29% 21% 25% 19% 5%	760
	21st to end	Night	23% 18% 33% 23% 3	836
February	1st to 10th	Night	26% 17% 27% 26% 4	760
	11th to 20th	Night	30% 21% 27% 20% 3	743
	21st to end	Night	34% 18% 28% 18% 2	628
March	1st to 10th	Night	24% 18% 26% 27% 4	760
	11th to 20th	Night	21% 17% 28% 26% 7%	760
	21st to end	Night	22% 16% 26% 26% 10%	808
April	1st to 10th	Night	18% 12% 27% 31% 12% 1	570
	11th to 20th	Night	21% 11% 28% 26% 12% 1	563
	21st to end	Night	17% 9% 27% 32% 13% 2	570
May	1st to 10th	Night	15% 7% 26% 29% 20% 2	570
	11th to 20th	Night	15% 6% 19% 32% 22% 5%	561
	21st to end	Night	11% 2 14% 35% 32% 6%	617
June	1st to 10th	Night	12% 3 12% 34% 34% 5%	570
	11th to 20th	Night	5% 2 10% 34% 38% 12%	570
	21st to end	Night	8% 2 11% 31% 40% 8%	570
July	1st to 10th	Night	9% 3 8% 25% 43% 11%	555
	11th to 20th	Night	10% 2 6% 26% 40% 16%	552
	21st to end	Night	11% 2 6% 25% 45% 12%	624
August	1st to 10th	Night	8% 1 4 25% 48% 13%	567
	11th to 20th	Night	10% 2 7% 31% 41% 8%	564
	21st to end	Night	11% 3 5% 29% 44% 9%	627
September	1st to 10th	Night	11% 3 10% 29% 39% 9%	630
	11th to 20th	Night	14% 5% 13% 28% 34% 6%	760
	21st to end	Night	12% 4 13% 33% 33% 4	760
October	1st to 10th	Night	13% 6% 19% 28% 28% 7%	760
	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% 9% 22% 31% 19% 3	760
	11th to 20th	Night	17% 16% 26% 27% 13% 1	760
	21st to end	Night	23% 13% 22% 29% 12% 1	760
December	1st to 10th	Night	28% 16% 23% 27% 6%	760
	11th to 20th	Night	24% 15% 24% 28% 8% 1	760
	21st to end	Night	28% 17% 24% 25% 5%	836

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