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 2026-10-29.

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	11% 8% 3 6% 4 4 4 3 5% 8% 43%	149594
	Night	12% 8% 3 7% 4 4 4 3 5% 7% 44%	142829
Transparency	Day	39% 6% 12% 25% 16% 2	153195
	Night	39% 5% 11% 25% 17% 3	147308
Seeing	Day	15% 11% 28% 27% 15% 4	18399
	Night	16% 10% 31% 30% 12% 1	28132

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	12% 13% 4 11% 4 5% 4 3 5% 5% 35%	4871
	11th to 20th	Day	7% 9% 3 9% 5% 7% 5% 4 5% 7% 42%	5310
	21st to end	Day	14% 13% 5% 9% 4 3 3 2 4 5% 37%	5812
February	1st to 10th	Day	16% 10% 4 9% 5% 4 3 3 5% 7% 34%	1890
	11th to 20th	Day	13% 11% 3 6% 4 4 4 4 5% 5% 40%	1896
	21st to end	Day	15% 9% 4 7% 4 3 4 3 6% 5% 41%	1730
March	1st to 10th	Day	16% 10% 3 4 2 3 2 2 3 5% 50%	4990
	11th to 20th	Day	9% 7% 2 5% 4 3 4 3 5% 7% 51%	5090
	21st to end	Day	14% 11% 3 8% 4 5% 5% 4 6% 7% 33%	5666
April	1st to 10th	Day	14% 8% 3 8% 5% 4 4 2 5% 6% 42%	2520
	11th to 20th	Day	15% 10% 3 6% 4 3 4 3 6% 6% 41%	2494
	21st to end	Day	21% 7% 2 6% 4 5% 4 3 6% 8% 33%	2520
May	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 4 7% 10% 36%	7456
	11th to 20th	Day	13% 7% 3 7% 5% 4 5% 3 5% 7% 41%	7976
	21st to end	Day	17% 9% 3 7% 5% 5% 4 3 5% 7% 35%	9047
June	1st to 10th	Day	8% 5% 2 5% 5% 5% 5% 5% 7% 10% 45%	2940
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 10% 46%	2940
	21st to end	Day	7% 4 2 5% 5% 4 5% 5% 6% 9% 48%	2940
July	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 6% 9% 46%	8280
	11th to 20th	Day	6% 6% 2 5% 4 5% 5% 4 7% 10% 46%	7966
	21st to end	Day	8% 5% 2 5% 4 5% 6% 5% 8% 11% 41%	8337
August	1st to 10th	Day	7% 4 2 5% 4 4 5% 5% 7% 9% 47%	6014
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	5761
	21st to end	Day	4 3 1 5% 3 4 4 4 6% 10% 56%	6080
September	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 5% 8% 52%	2520
	11th to 20th	Day	9% 6% 2 5% 3 3 4 4 5% 8% 51%	2520
	21st to end	Day	9% 6% 2 6% 3 4 5% 3 5% 7% 50%	2201
October	1st to 10th	Day	10% 5% 2 6% 3 3 3 2 5% 5% 56%	2100
	11th to 20th	Day	14% 7% 2 4 3 2 3 2 4 6% 53%	2100
	21st to end	Day	17% 9% 2 6% 3 3 3 2 4 5% 46%	2280
November	1st to 10th	Day	10% 8% 3 5% 3 2 3 3 4 6% 53%	1961
	11th to 20th	Day	14% 7% 2 6% 3 3 3 3 5% 6% 48%	1858
	21st to end	Day	18% 10% 3 6% 3 4 3 3 4 6% 41%	1680
December	1st to 10th	Day	8% 9% 3 7% 4 3 3 2 4 6% 50%	2690
	11th to 20th	Day	12% 9% 4 8% 4 4 4 3 4 6% 43%	3088
	21st to end	Day	21% 19% 5% 11% 5% 4 4 3 3 4 21%	4070
January	1st to 10th	Night	12% 11% 6% 12% 5% 3 4 3 3 4 35%	8034
	11th to 20th	Night	9% 8% 4 11% 5% 5% 5% 4 4 4 41%	7680
	21st to end	Night	14% 12% 4 7% 5% 4 4 3 4 5% 36%	8402
February	1st to 10th	Night	15% 11% 4 7% 5% 4 5% 4 5% 6% 35%	2730
	11th to 20th	Night	15% 10% 3 6% 5% 3 3 4 5% 6% 39%	2610
	21st to end	Night	14% 10% 3 7% 3 3 4 4 5% 5% 43%	2076
March	1st to 10th	Night	17% 11% 3 6% 3 3 3 3 3 4 45%	5124
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 5% 6% 51%	5090
	21st to end	Night	15% 11% 4 8% 5% 4 3 3 5% 6% 34%	5504
April	1st to 10th	Night	12% 8% 2 5% 4 4 4 3 4 6% 46%	2100
	11th to 20th	Night	15% 7% 3 6% 3 4 4 2 5% 6% 45%	2075
	21st to end	Night	17% 7% 2 6% 4 3 4 4 5% 7% 41%	2100
May	1st to 10th	Night	15% 9% 3 7% 5% 4 4 4 5% 8% 36%	5403
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 5% 8% 46%	4913
	21st to end	Night	16% 8% 3 7% 4 5% 4 4 5% 8% 37%	5139
June	1st to 10th	Night	7% 4 1 4 4 4 5% 3 6% 9% 53%	1680
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 6% 11% 49%	1680
	21st to end	Night	4 3 2 5% 4 4 4 4 6% 9% 56%	1680
July	1st to 10th	Night	8% 5% 2 6% 3 4 4 3 6% 10% 50%	4708
	11th to 20th	Night	5% 4 2 6% 4 4 4 4 6% 9% 52%	5003
	21st to end	Night	6% 4 2 6% 4 5% 4 5% 7% 10% 47%	5770
August	1st to 10th	Night	7% 3 1 3 3 3 3 3 5% 8% 59%	4156
	11th to 20th	Night	9% 5% 2 6% 4 4 4 3 6% 9% 48%	4295
	21st to end	Night	4 3 1 4 3 3 4 3 5% 9% 60%	5046
September	1st to 10th	Night	10% 4 1 4 3 3 4 3 5% 6% 57%	2100
	11th to 20th	Night	8% 4 2 5% 3 4 4 4 6% 8% 51%	2100
	21st to end	Night	9% 5% 2 6% 3 3 4 3 5% 6% 55%	2392
October	1st to 10th	Night	8% 5% 1 4 3 3 3 3 4 6% 60%	2520
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 5% 52%	2520
	21st to end	Night	16% 7% 2 5% 3 3 3 3 4 6% 47%	2737
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 6% 55%	2555
	11th to 20th	Night	12% 7% 3 6% 3 3 4 3 4 6% 49%	2758
	21st to end	Night	17% 9% 2 6% 3 3 4 3 4 5% 44%	2940
December	1st to 10th	Night	10% 8% 3 6% 4 4 4 3 5% 6% 48%	4720
	11th to 20th	Night	15% 10% 4 6% 3 3 4 3 4 6% 42%	5339
	21st to end	Night	22% 18% 5% 9% 4 3 3 3 3 5% 24%	7150

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% 4 11% 22% 11% 2	4872
	11th to 20th	Day	42% 4 11% 23% 15% 4	5310
	21st to end	Day	51% 3 11% 16% 16% 3	5812
February	1st to 10th	Day	51% 4 12% 21% 11% 1	1890
	11th to 20th	Day	45% 6% 11% 20% 14% 4	1905
	21st to end	Day	45% 5% 8% 24% 16% 3	1730
March	1st to 10th	Day	39% 3 7% 23% 24% 3	4990
	11th to 20th	Day	33% 4 9% 25% 24% 6%	5090
	21st to end	Day	48% 7% 15% 24% 6%	5668
April	1st to 10th	Day	43% 6% 10% 24% 15% 1	2520
	11th to 20th	Day	43% 6% 11% 23% 14% 3	2494
	21st to end	Day	48% 7% 12% 21% 12% 1	2520
May	1st to 10th	Day	43% 8% 14% 23% 12%	7493
	11th to 20th	Day	42% 6% 10% 24% 15% 2	7980
	21st to end	Day	47% 7% 9% 22% 14% 1	9061
June	1st to 10th	Day	32% 9% 15% 28% 14% 1	2940
	11th to 20th	Day	32% 8% 13% 26% 17% 3	2940
	21st to end	Day	31% 8% 16% 27% 15% 4	2940
July	1st to 10th	Day	34% 6% 14% 25% 19% 2	8211
	11th to 20th	Day	32% 7% 16% 26% 16% 3	7892
	21st to end	Day	34% 7% 22% 29% 7%	8155
August	1st to 10th	Day	31% 7% 14% 32% 16%	6000
	11th to 20th	Day	35% 7% 16% 30% 11% 1	5800
	21st to end	Day	24% 6% 17% 35% 16% 3	6079
September	1st to 10th	Day	31% 6% 12% 26% 20% 5%	2520
	11th to 20th	Day	30% 6% 11% 28% 21% 4	2520
	21st to end	Day	33% 6% 10% 24% 23% 4	2201
October	1st to 10th	Day	20% 3 6% 20% 41% 11%	3360
	11th to 20th	Day	23% 2 6% 20% 38% 9%	3340
	21st to end	Day	37% 3 8% 24% 26% 2	3618
November	1st to 10th	Day	33% 5% 12% 24% 22% 5%	1961
	11th to 20th	Day	37% 4 9% 25% 21% 3	1858
	21st to end	Day	45% 4 10% 27% 14% 1	1680
December	1st to 10th	Day	36% 4 14% 30% 15% 1	2690
	11th to 20th	Day	43% 5% 12% 26% 13% 1	3085
	21st to end	Day	67% 5% 11% 11% 5%	4070
January	1st to 10th	Night	53% 4 12% 22% 9%	8035
	11th to 20th	Night	46% 3 8% 24% 15% 4	7680
	21st to end	Night	51% 4 12% 18% 14% 2	8402
February	1st to 10th	Night	49% 6% 11% 20% 12% 2	2730
	11th to 20th	Night	45% 5% 11% 21% 16% 2	2611
	21st to end	Night	42% 5% 9% 22% 18% 3	2076
March	1st to 10th	Night	44% 4 7% 21% 21% 3	5124
	11th to 20th	Night	34% 5% 7% 23% 26% 5%	5090
	21st to end	Night	50% 6% 11% 23% 9% 1	5504
April	1st to 10th	Night	39% 5% 10% 25% 18% 4	2100
	11th to 20th	Night	40% 5% 9% 25% 16% 5%	2075
	21st to end	Night	43% 6% 10% 23% 17% 1	2100
May	1st to 10th	Night	45% 8% 12% 21% 13% 1	5431
	11th to 20th	Night	36% 7% 11% 25% 18% 4	4914
	21st to end	Night	45% 8% 10% 20% 17% 1	5149
June	1st to 10th	Night	26% 8% 15% 30% 17% 3	1680
	11th to 20th	Night	29% 8% 11% 26% 23% 3	1680
	21st to end	Night	23% 7% 17% 28% 19% 6%	1680
July	1st to 10th	Night	30% 5% 14% 30% 19% 2	4670
	11th to 20th	Night	27% 7% 15% 30% 18% 3	4958
	21st to end	Night	30% 7% 20% 34% 8%	5641
August	1st to 10th	Night	23% 5% 17% 31% 22% 1	4146
	11th to 20th	Night	32% 6% 14% 32% 14% 2	4322
	21st to end	Night	20% 5% 16% 38% 17% 3	5046
September	1st to 10th	Night	27% 6% 11% 23% 26% 7%	2100
	11th to 20th	Night	30% 6% 11% 27% 21% 5%	2100
	21st to end	Night	31% 5% 8% 26% 26% 4	2392
October	1st to 10th	Night	17% 3 6% 21% 45% 8%	4040
	11th to 20th	Night	25% 3 6% 21% 32% 13%	4019
	21st to end	Night	36% 3 7% 26% 26% 2	4349
November	1st to 10th	Night	31% 5% 11% 28% 19% 6%	2555
	11th to 20th	Night	37% 4 9% 25% 22% 4	2758
	21st to end	Night	44% 4 10% 24% 16% 2	2940
December	1st to 10th	Night	38% 4 10% 34% 13% 1	4720
	11th to 20th	Night	44% 4 12% 24% 16% 1	5341
	21st to end	Night	64% 5% 12% 13% 6% 1	7150

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	21% 17% 38% 22% 2	330
	11th to 20th	Day	26% 21% 40% 11% 1	330
	21st to end	Day	18% 18% 47% 14% 3	363
February	1st to 10th	Day	25% 16% 42% 15% 2	330
	11th to 20th	Day	25% 23% 36% 12% 4	330
	21st to end	Day	24% 24% 35% 15% 2	273
March	1st to 10th	Day	20% 22% 43% 13% 2	330
	11th to 20th	Day	27% 17% 34% 20% 2	330
	21st to end	Day	30% 21% 33% 13% 3	395
April	1st to 10th	Day	24% 16% 36% 18% 5% 1	540
	11th to 20th	Day	22% 20% 33% 20% 5%	532
	21st to end	Day	24% 12% 35% 23% 5% 1	540
May	1st to 10th	Day	18% 14% 36% 25% 7% 1	695
	11th to 20th	Day	18% 13% 27% 28% 13% 2	775
	21st to end	Day	16% 8% 29% 27% 15% 5%	844
June	1st to 10th	Day	8% 5% 25% 35% 21% 7%	780
	11th to 20th	Day	9% 4 24% 40% 20% 3	780
	21st to end	Day	8% 6% 31% 36% 16% 4	780
July	1st to 10th	Day	11% 3 14% 32% 29% 11%	771
	11th to 20th	Day	7% 4 19% 36% 26% 8%	768
	21st to end	Day	9% 3 16% 36% 31% 5%	849
August	1st to 10th	Day	6% 3 17% 42% 24% 7%	776
	11th to 20th	Day	9% 3 16% 37% 27% 8%	682
	21st to end	Day	8% 2 14% 36% 31% 10%	627
September	1st to 10th	Day	11% 3 16% 32% 30% 7%	570
	11th to 20th	Day	7% 4 18% 32% 27% 13%	570
	21st to end	Day	11% 9% 23% 29% 21% 7%	389
October	1st to 10th	Day	11% 6% 31% 28% 19% 5%	360
	11th to 20th	Day	16% 19% 30% 24% 10% 1	360
	21st to end	Day	29% 18% 26% 22% 4 1	387
November	1st to 10th	Day	13% 14% 41% 23% 7% 2	330
	11th to 20th	Day	20% 15% 35% 21% 7% 2	330
	21st to end	Day	26% 17% 28% 24% 5% 1	330
December	1st to 10th	Day	16% 20% 37% 19% 6% 2	330
	11th to 20th	Day	22% 15% 40% 18% 4 1	330
	21st to end	Day	25% 23% 35% 16% 1	363
January	1st to 10th	Night	16% 17% 39% 23% 5%	1050
	11th to 20th	Night	19% 19% 37% 21% 4	1050
	21st to end	Night	18% 14% 43% 24% 2	1155
February	1st to 10th	Night	19% 15% 40% 23% 3	1050
	11th to 20th	Night	21% 14% 40% 22% 3	976
	21st to end	Night	20% 18% 40% 20% 2	692
March	1st to 10th	Night	20% 16% 39% 22% 3	677
	11th to 20th	Night	22% 13% 33% 26% 5%	630
	21st to end	Night	26% 14% 36% 20% 3	693
April	1st to 10th	Night	17% 16% 38% 25% 4	630
	11th to 20th	Night	18% 15% 35% 25% 6%	623
	21st to end	Night	19% 11% 37% 28% 5%	630
May	1st to 10th	Night	16% 7% 36% 34% 7%	630
	11th to 20th	Night	16% 8% 25% 38% 13% 1	621
	21st to end	Night	13% 5% 28% 35% 17% 2	683
June	1st to 10th	Night	9% 3 22% 41% 25% 2	630
	11th to 20th	Night	5% 3 26% 43% 22% 1	630
	21st to end	Night	3 3 29% 46% 18% 1	630
July	1st to 10th	Night	10% 2 11% 37% 35% 4	615
	11th to 20th	Night	6% 2 14% 42% 34% 2	612
	21st to end	Night	7% 1 16% 44% 30% 2	690
August	1st to 10th	Night	7% 2 13% 46% 29% 2	627
	11th to 20th	Night	6% 3 12% 44% 32% 3	624
	21st to end	Night	6% 1 13% 40% 37% 3	693
September	1st to 10th	Night	11% 2 17% 39% 28% 4	630
	11th to 20th	Night	10% 3 17% 38% 27% 5%	630
	21st to end	Night	11% 4 24% 36% 21% 4	745
October	1st to 10th	Night	10% 4 27% 36% 20% 3	840
	11th to 20th	Night	15% 10% 34% 29% 10%	840
	21st to end	Night	21% 12% 32% 29% 7%	913
November	1st to 10th	Night	12% 10% 38% 31% 9%	820
	11th to 20th	Night	16% 13% 39% 26% 6% 1	868
	21st to end	Night	23% 12% 35% 25% 5%	1050
December	1st to 10th	Night	19% 13% 38% 25% 5%	1050
	11th to 20th	Night	22% 12% 36% 24% 4	1050
	21st to end	Night	24% 16% 35% 24% 2	1155

Page generated at 2026-10-26 03:37:51. Version ac07

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