Columbia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Columbia clear sky chart from 2005-11-06 to 2025-08-05.

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	12% 7% 3 7% 5% 4 5% 4 6% 8% 40%	125825
	Night	13% 7% 3 7% 4 4 4 3 5% 7% 42%	122296
Transparency	Day	41% 4 11% 23% 17% 4	129528
	Night	40% 4 10% 22% 19% 5%	126664
Seeing	Day	16% 8% 22% 24% 19% 10%	16120
	Night	16% 7% 24% 31% 20% 3	25761

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	14% 12% 5% 10% 5% 3 4 3 4 6% 34%	3617
	11th to 20th	Day	15% 6% 3 12% 4 3 3 2 3 5% 43%	3839
	21st to end	Day	18% 9% 4 6% 3 3 4 4 5% 6% 39%	4972
February	1st to 10th	Day	18% 12% 3 7% 4 4 3 3 3 5% 37%	2000
	11th to 20th	Day	19% 11% 4 7% 5% 4 3 3 5% 4 35%	1948
	21st to end	Day	19% 9% 3 7% 4 4 4 4 3 6% 38%	1650
March	1st to 10th	Day	16% 11% 4 9% 5% 4 4 4 4 5% 35%	4480
	11th to 20th	Day	12% 7% 3 7% 4 3 4 3 4 6% 47%	4738
	21st to end	Day	11% 8% 2 7% 5% 4 5% 4 5% 7% 43%	5825
April	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 5% 6% 45%	2400
	11th to 20th	Day	12% 8% 3 6% 4 4 3 3 4 6% 45%	2374
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 40%	2405
May	1st to 10th	Day	18% 7% 2 7% 5% 5% 5% 5% 7% 9% 29%	5862
	11th to 20th	Day	14% 9% 4 10% 6% 5% 5% 4 6% 8% 30%	5861
	21st to end	Day	15% 8% 3 7% 5% 4 4 4 5% 7% 38%	6433
June	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 7% 10% 41%	2600
	11th to 20th	Day	5% 4 2 5% 6% 6% 6% 5% 7% 10% 44%	2790
	21st to end	Day	5% 5% 2 6% 6% 6% 6% 5% 9% 11% 41%	2800
July	1st to 10th	Day	5% 5% 2 7% 6% 5% 6% 5% 8% 11% 39%	6360
	11th to 20th	Day	7% 5% 2 7% 6% 6% 7% 6% 9% 12% 32%	6204
	21st to end	Day	10% 8% 3 8% 6% 5% 7% 5% 8% 11% 30%	6424
August	1st to 10th	Day	4 3 1 5% 4 4 5% 5% 7% 10% 51%	3870
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 9% 43%	3729
	21st to end	Day	6% 4 2 5% 4 4 5% 4 6% 7% 52%	4180
September	1st to 10th	Day	11% 6% 2 7% 4 5% 4 3 7% 8% 43%	2262
	11th to 20th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 6% 43%	2090
	21st to end	Day	11% 4 2 5% 4 3 4 3 6% 7% 50%	2090
October	1st to 10th	Day	10% 4 2 5% 4 3 4 3 3 6% 55%	2090
	11th to 20th	Day	10% 5% 2 4 3 3 4 3 5% 7% 54%	2038
	21st to end	Day	15% 5% 2 5% 4 4 4 4 4 6% 48%	1922
November	1st to 10th	Day	12% 6% 2 4 3 3 3 3 4 5% 55%	1762
	11th to 20th	Day	13% 7% 2 5% 4 3 4 3 5% 6% 49%	1800
	21st to end	Day	14% 7% 3 5% 3 4 4 3 6% 7% 44%	1800
December	1st to 10th	Day	14% 7% 2 7% 4 4 4 3 4 5% 45%	2930
	11th to 20th	Day	16% 11% 4 10% 5% 5% 4 3 3 4 34%	3373
	21st to end	Day	15% 10% 5% 8% 5% 4 5% 3 7% 7% 32%	4307
January	1st to 10th	Night	15% 11% 4 9% 4 4 4 3 4 6% 37%	6348
	11th to 20th	Night	16% 10% 4 8% 4 3 3 2 3 4 43%	6105
	21st to end	Night	18% 11% 4 6% 3 4 4 2 4 5% 40%	5984
February	1st to 10th	Night	19% 10% 4 8% 5% 4 4 3 5% 5% 35%	2400
	11th to 20th	Night	21% 11% 3 7% 5% 4 4 2 4 4 36%	2338
	21st to end	Night	22% 8% 3 7% 5% 4 3 3 5% 5% 36%	1980
March	1st to 10th	Night	18% 9% 3 8% 4 4 4 4 5% 7% 34%	5373
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 6% 49%	5221
	21st to end	Night	14% 8% 3 7% 4 4 4 4 5% 6% 42%	5146
April	1st to 10th	Night	12% 6% 2 5% 4 4 3 3 6% 7% 47%	2000
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 7% 49%	1975
	21st to end	Night	11% 6% 2 6% 5% 4 5% 3 6% 8% 44%	1993
May	1st to 10th	Night	13% 9% 3 8% 6% 5% 4 4 6% 7% 33%	4033
	11th to 20th	Night	13% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	4032
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 6% 8% 37%	4421
June	1st to 10th	Night	8% 6% 3 6% 4 4 5% 4 7% 9% 45%	1800
	11th to 20th	Night	4 3 1 5% 3 4 4 4 7% 10% 55%	1603
	21st to end	Night	4 3 1 4 4 3 6% 5% 8% 11% 51%	1600
July	1st to 10th	Night	6% 4 3 7% 4 5% 6% 5% 8% 11% 41%	3618
	11th to 20th	Night	5% 4 2 5% 5% 5% 6% 5% 9% 14% 38%	3749
	21st to end	Night	7% 7% 3 8% 5% 6% 6% 5% 9% 13% 31%	4427
August	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 8% 59%	3142
	11th to 20th	Night	3 3 2 5% 4 4 5% 5% 8% 11% 50%	3100
	21st to end	Night	5% 3 2 4 4 3 4 4 7% 9% 56%	3476
September	1st to 10th	Night	9% 5% 3 5% 4 4 4 4 6% 8% 48%	1900
	11th to 20th	Night	10% 5% 3 5% 4 4 4 4 5% 7% 48%	2087
	21st to end	Night	10% 4 2 5% 4 3 4 3 5% 7% 53%	2090
October	1st to 10th	Night	9% 4 1 4 3 3 2 3 4 6% 60%	2090
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 5% 6% 52%	2137
	21st to end	Night	14% 6% 2 6% 3 3 4 2 4 6% 51%	2674
November	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 4 7% 51%	2536
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 7% 8% 48%	2600
	21st to end	Night	15% 8% 3 6% 3 3 4 3 5% 5% 46%	2600
December	1st to 10th	Night	17% 10% 3 7% 3 4 3 3 4 5% 42%	4250
	11th to 20th	Night	19% 9% 3 7% 4 3 3 2 4 4 42%	4822
	21st to end	Night	16% 11% 5% 9% 5% 5% 5% 3 5% 6% 30%	6646

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% 3 9% 20% 13% 2	3618
	11th to 20th	Day	45% 3 5% 22% 25% 1	3839
	21st to end	Day	46% 3 6% 23% 18% 5%	4972
February	1st to 10th	Day	51% 3 7% 19% 16% 3	2000
	11th to 20th	Day	52% 3 6% 20% 15% 3	1958
	21st to end	Day	49% 3 7% 16% 18% 6%	1650
March	1st to 10th	Day	52% 3 5% 14% 17% 9%	4480
	11th to 20th	Day	40% 3 6% 22% 20% 9%	4738
	21st to end	Day	41% 4 9% 28% 17% 1	5827
April	1st to 10th	Day	39% 5% 10% 24% 19% 4	2400
	11th to 20th	Day	40% 4 7% 22% 22% 6%	2374
	21st to end	Day	41% 5% 10% 26% 16% 2	2405
May	1st to 10th	Day	49% 6% 14% 19% 10% 1	5897
	11th to 20th	Day	52% 5% 13% 20% 10%	5865
	21st to end	Day	45% 5% 9% 23% 17% 2	6447
June	1st to 10th	Day	37% 7% 13% 24% 17% 2	2600
	11th to 20th	Day	32% 7% 15% 24% 18% 2	2790
	21st to end	Day	33% 8% 17% 20% 17% 4	2800
July	1st to 10th	Day	38% 5% 18% 29% 10% 1	6291
	11th to 20th	Day	42% 4 21% 26% 5% 1	6129
	21st to end	Day	46% 6% 22% 21% 5%	6240
August	1st to 10th	Day	26% 6% 14% 33% 20% 1	3857
	11th to 20th	Day	35% 5% 14% 27% 17% 3	3765
	21st to end	Day	31% 4 11% 27% 25% 2	4179
September	1st to 10th	Day	38% 7% 11% 21% 18% 5%	2262
	11th to 20th	Day	40% 6% 9% 21% 20% 3	2090
	21st to end	Day	33% 5% 11% 23% 21% 6%	2090
October	1st to 10th	Day	23% 2 4 16% 43% 11%	3480
	11th to 20th	Day	22% 3 4 25% 30% 15%	3369
	21st to end	Day	30% 2 6% 23% 30% 8%	3147
November	1st to 10th	Day	33% 4 7% 22% 29% 6%	1762
	11th to 20th	Day	36% 3 6% 22% 26% 6%	1800
	21st to end	Day	39% 4 7% 25% 21% 5%	1800
December	1st to 10th	Day	43% 2 4 29% 21% 2	2930
	11th to 20th	Day	56% 3 6% 19% 14% 3	3370
	21st to end	Day	50% 4 11% 25% 8% 2	4307
January	1st to 10th	Night	50% 3 9% 20% 16% 2	6348
	11th to 20th	Night	48% 3 5% 18% 22% 4	6105
	21st to end	Night	49% 2 5% 21% 18% 5%	5984
February	1st to 10th	Night	52% 4 9% 17% 16% 2	2400
	11th to 20th	Night	53% 3 6% 18% 16% 4	2339
	21st to end	Night	50% 3 7% 17% 18% 5%	1980
March	1st to 10th	Night	50% 3 5% 16% 19% 7%	5373
	11th to 20th	Night	36% 3 6% 20% 20% 14%	5221
	21st to end	Night	44% 3 7% 24% 18% 3	5146
April	1st to 10th	Night	35% 5% 12% 23% 21% 4	2000
	11th to 20th	Night	36% 4 8% 21% 23% 8%	1975
	21st to end	Night	37% 5% 12% 26% 18% 2	1993
May	1st to 10th	Night	49% 6% 13% 19% 11% 1	4060
	11th to 20th	Night	50% 7% 13% 17% 12%	4033
	21st to end	Night	44% 7% 11% 23% 14% 1	4429
June	1st to 10th	Night	34% 7% 14% 24% 19% 2	1800
	11th to 20th	Night	23% 6% 17% 31% 19% 3	1603
	21st to end	Night	24% 8% 19% 28% 17% 4	1600
July	1st to 10th	Night	36% 5% 20% 27% 11% 1	3580
	11th to 20th	Night	34% 4 25% 29% 5% 2	3704
	21st to end	Night	43% 6% 25% 20% 5% 1	4297
August	1st to 10th	Night	22% 5% 11% 38% 24% 1	3131
	11th to 20th	Night	26% 6% 21% 29% 16% 2	3130
	21st to end	Night	24% 5% 12% 32% 24% 3	3476
September	1st to 10th	Night	34% 7% 11% 24% 19% 5%	1900
	11th to 20th	Night	34% 6% 13% 20% 26% 2	2087
	21st to end	Night	31% 5% 9% 24% 24% 7%	2090
October	1st to 10th	Night	22% 2 5% 18% 39% 13%	3480
	11th to 20th	Night	21% 2 4 21% 30% 21%	3545
	21st to end	Night	26% 2 5% 26% 29% 12%	4399
November	1st to 10th	Night	34% 3 7% 22% 25% 8%	2536
	11th to 20th	Night	33% 4 7% 21% 28% 7%	2600
	21st to end	Night	40% 3 6% 24% 22% 5%	2600
December	1st to 10th	Night	46% 3 6% 18% 25% 2	4250
	11th to 20th	Night	48% 2 4 23% 21% 2	4824
	21st to end	Night	55% 3 8% 22% 10% 2	6646

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	25% 17% 37% 16% 5% 1	300
	11th to 20th	Day	23% 20% 38% 14% 3 2	300
	21st to end	Day	20% 20% 40% 17% 4	330
February	1st to 10th	Day	28% 16% 34% 17% 4 1	300
	11th to 20th	Day	34% 17% 35% 11% 2 1	300
	21st to end	Day	32% 19% 35% 9% 4	249
March	1st to 10th	Day	26% 17% 34% 17% 5% 1	300
	11th to 20th	Day	22% 16% 36% 20% 5% 1	388
	21st to end	Day	23% 15% 32% 22% 7% 1	550
April	1st to 10th	Day	21% 14% 29% 25% 10% 2	500
	11th to 20th	Day	19% 13% 31% 24% 11% 2	492
	21st to end	Day	17% 10% 31% 26% 12% 4	500
May	1st to 10th	Day	18% 11% 25% 27% 14% 5%	503
	11th to 20th	Day	16% 7% 23% 27% 17% 10%	527
	21st to end	Day	13% 4 20% 28% 20% 14%	573
June	1st to 10th	Day	10% 5% 17% 24% 25% 19%	530
	11th to 20th	Day	7% 2 17% 26% 31% 18%	720
	21st to end	Day	7% 2 14% 31% 28% 18%	730
July	1st to 10th	Day	9% 3 10% 28% 31% 20%	721
	11th to 20th	Day	8% 2 7% 25% 32% 25%	653
	21st to end	Day	9% 2 7% 24% 32% 25%	576
August	1st to 10th	Day	10% 1 8% 24% 36% 21%	502
	11th to 20th	Day	10% 2 9% 32% 33% 15%	479
	21st to end	Day	13% 3 6% 17% 38% 23%	539
September	1st to 10th	Day	12% 2 10% 28% 33% 16%	490
	11th to 20th	Day	15% 2 10% 24% 32% 18%	490
	21st to end	Day	17% 5% 12% 28% 27% 10%	490
October	1st to 10th	Day	14% 2 18% 28% 26% 12%	490
	11th to 20th	Day	13% 11% 29% 29% 15% 4	438
	21st to end	Day	22% 9% 27% 28% 12% 2	330
November	1st to 10th	Day	18% 11% 28% 29% 11% 3	300
	11th to 20th	Day	16% 15% 32% 26% 9%	300
	21st to end	Day	20% 14% 30% 21% 12% 4	300
December	1st to 10th	Day	24% 15% 34% 21% 5% 1	300
	11th to 20th	Day	24% 14% 33% 20% 8% 1	300
	21st to end	Day	24% 18% 33% 19% 6%	330
January	1st to 10th	Night	22% 12% 33% 28% 4	1000
	11th to 20th	Night	24% 14% 34% 24% 4	902
	21st to end	Night	20% 10% 42% 26% 3	880
February	1st to 10th	Night	24% 11% 35% 26% 4	800
	11th to 20th	Night	27% 15% 33% 23% 3	783
	21st to end	Night	29% 13% 33% 23% 2	660
March	1st to 10th	Night	22% 12% 32% 28% 5%	800
	11th to 20th	Night	20% 12% 35% 25% 7% 1	800
	21st to end	Night	18% 8% 35% 30% 8%	740
April	1st to 10th	Night	16% 10% 32% 32% 10%	600
	11th to 20th	Night	16% 9% 32% 34% 9% 1	593
	21st to end	Night	15% 6% 33% 32% 13% 1	600
May	1st to 10th	Night	14% 4 30% 34% 17% 2	600
	11th to 20th	Night	12% 3 24% 35% 22% 4	591
	21st to end	Night	10% 2 15% 38% 30% 5%	650
June	1st to 10th	Night	11% 2 14% 38% 31% 5%	600
	11th to 20th	Night	5% 2 10% 37% 41% 5%	600
	21st to end	Night	5% 9% 37% 41% 9%	600
July	1st to 10th	Night	9% 2 6% 27% 45% 11%	585
	11th to 20th	Night	8% 1 6% 28% 45% 11%	582
	21st to end	Night	7% 1 6% 29% 47% 10%	657
August	1st to 10th	Night	7% 2 3 29% 48% 11%	579
	11th to 20th	Night	6% 2 6% 40% 42% 5%	564
	21st to end	Night	9% 2 4 29% 46% 10%	627
September	1st to 10th	Night	10% 2 8% 33% 41% 6%	570
	11th to 20th	Night	12% 2 10% 31% 40% 5%	757
	21st to end	Night	12% 3 14% 30% 36% 5%	760
October	1st to 10th	Night	11% 3 15% 35% 31% 6%	760
	11th to 20th	Night	15% 6% 23% 35% 19% 2	760
	21st to end	Night	17% 5% 26% 32% 18% 2	836
November	1st to 10th	Night	13% 6% 22% 38% 18% 2	780
	11th to 20th	Night	14% 8% 33% 32% 12% 1	800
	21st to end	Night	20% 8% 28% 31% 12% 1	800
December	1st to 10th	Night	24% 10% 29% 30% 7%	800
	11th to 20th	Night	22% 8% 31% 31% 8% 1	800
	21st to end	Night	26% 11% 31% 27% 5%	945

Page generated at 2025-08-02 00:32:26. Version ac07

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