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 2025-07-06.

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% 8% 3 6% 4 4 4 3 5% 7% 43%	120079
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 44%	118923
Transparency	Day	39% 6% 12% 24% 17% 3	123722
	Night	40% 5% 11% 24% 18% 3	123424
Seeing	Day	16% 11% 28% 27% 15% 4	16559
	Night	16% 10% 31% 30% 12% 1	26408

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	13% 13% 4 12% 5% 5% 4 3 5% 5% 32%	3755
	11th to 20th	Day	9% 10% 4 9% 4 7% 5% 3 4 5% 40%	4080
	21st to end	Day	14% 13% 6% 9% 4 3 4 3 4 5% 36%	4488
February	1st to 10th	Day	17% 11% 4 9% 5% 4 3 3 5% 6% 33%	1800
	11th to 20th	Day	13% 12% 3 7% 4 4 4 4 5% 5% 40%	1803
	21st to end	Day	15% 10% 4 7% 4 3 3 3 6% 5% 41%	1650
March	1st to 10th	Day	15% 10% 3 4 2 3 2 2 3 5% 50%	4882
	11th to 20th	Day	9% 7% 2 5% 4 3 4 3 5% 7% 51%	4980
	21st to end	Day	14% 12% 3 8% 4 5% 5% 4 6% 7% 32%	5543
April	1st to 10th	Day	14% 8% 3 8% 5% 4 4 2 5% 6% 43%	2400
	11th to 20th	Day	15% 10% 3 6% 4 3 4 3 6% 6% 40%	2374
	21st to end	Day	22% 6% 2 6% 4 5% 4 3 6% 8% 33%	2400
May	1st to 10th	Day	14% 8% 3 7% 5% 4 5% 4 7% 10% 34%	5743
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 5% 7% 40%	6127
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 7% 40%	6935
June	1st to 10th	Day	8% 4 2 5% 4 5% 5% 5% 7% 10% 46%	2800
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 10% 47%	2800
	21st to end	Day	6% 4 2 5% 5% 4 5% 5% 6% 9% 49%	2800
July	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 40%	4511
	11th to 20th	Day	5% 5% 2 5% 3 4 4 4 7% 10% 50%	4266
	21st to end	Day	7% 5% 2 6% 4 5% 6% 5% 7% 11% 42%	4465
August	1st to 10th	Day	4 4 2 4 4 5% 5% 5% 8% 10% 50%	4104
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 3 6% 8% 46%	3914
	21st to end	Day	5% 4 2 5% 3 4 4 4 6% 10% 53%	4180
September	1st to 10th	Day	10% 5% 2 5% 3 4 4 4 5% 8% 51%	2280
	11th to 20th	Day	9% 6% 2 4 3 3 4 4 5% 8% 52%	2280
	21st to end	Day	8% 5% 2 6% 3 4 5% 4 5% 7% 52%	1993
October	1st to 10th	Day	10% 5% 2 6% 3 4 3 2 5% 5% 55%	1900
	11th to 20th	Day	14% 7% 2 4 3 2 3 2 4 6% 53%	1900
	21st to end	Day	17% 9% 2 6% 3 3 3 2 3 5% 46%	2090
November	1st to 10th	Day	10% 8% 3 5% 3 2 3 3 4 6% 52%	1866
	11th to 20th	Day	14% 8% 2 6% 2 3 2 3 5% 6% 48%	1770
	21st to end	Day	18% 10% 3 5% 3 4 3 3 4 6% 42%	1600
December	1st to 10th	Day	7% 9% 3 7% 4 3 3 2 4 6% 51%	2610
	11th to 20th	Day	12% 9% 4 8% 4 4 4 3 4 6% 43%	3008
	21st to end	Day	22% 20% 5% 10% 5% 3 4 3 3 4 21%	3982
January	1st to 10th	Night	13% 11% 7% 14% 5% 4 4 3 3 4 31%	6192
	11th to 20th	Night	12% 9% 5% 13% 5% 5% 5% 4 4 4 36%	5900
	21st to end	Night	16% 13% 5% 7% 5% 4 3 3 4 4 37%	6490
February	1st to 10th	Night	15% 11% 4 7% 5% 4 5% 4 5% 6% 34%	2600
	11th to 20th	Night	15% 9% 3 6% 5% 3 3 4 5% 6% 40%	2483
	21st to end	Night	14% 10% 3 7% 3 3 4 4 5% 5% 42%	1980
March	1st to 10th	Night	16% 11% 3 6% 3 3 3 3 3 4 45%	5012
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 5% 6% 51%	4980
	21st to end	Night	15% 11% 4 8% 5% 5% 3 3 5% 6% 34%	5385
April	1st to 10th	Night	11% 8% 2 5% 4 4 5% 3 4 6% 47%	2000
	11th to 20th	Night	15% 7% 2 6% 3 3 4 2 5% 6% 46%	1975
	21st to end	Night	17% 7% 2 6% 4 3 4 4 5% 7% 41%	2000
May	1st to 10th	Night	17% 9% 3 6% 4 4 4 4 5% 8% 36%	4151
	11th to 20th	Night	12% 7% 3 6% 4 4 4 4 5% 8% 44%	3785
	21st to end	Night	15% 6% 2 6% 4 4 3 4 5% 8% 43%	3942
June	1st to 10th	Night	7% 4 2 4 4 4 5% 4 6% 9% 53%	1600
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 6% 10% 49%	1600
	21st to end	Night	3 3 2 4 4 4 4 4 6% 9% 57%	1600
July	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 9% 45%	2570
	11th to 20th	Night	3 3 1 4 3 3 3 3 6% 10% 60%	2686
	21st to end	Night	6% 4 2 6% 4 5% 4 4 7% 9% 48%	3098
August	1st to 10th	Night	4 2 1 3 3 3 3 4 5% 9% 63%	2836
	11th to 20th	Night	11% 6% 2 6% 3 4 3 3 6% 8% 49%	2919
	21st to end	Night	4 3 2 4 3 3 4 4 5% 9% 58%	3476
September	1st to 10th	Night	10% 4 1 4 3 3 4 3 4 6% 58%	1900
	11th to 20th	Night	9% 4 2 4 3 4 4 4 5% 7% 53%	1900
	21st to end	Night	8% 5% 2 5% 3 3 4 3 5% 6% 56%	2164
October	1st to 10th	Night	8% 5% 1 4 3 3 4 3 5% 7% 59%	2280
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 6% 53%	2280
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 5% 47%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 4 6% 56%	2430
	11th to 20th	Night	12% 7% 3 6% 4 3 4 3 4 6% 49%	2626
	21st to end	Night	17% 9% 2 6% 3 3 4 3 4 5% 45%	2800
December	1st to 10th	Night	10% 8% 3 6% 4 4 4 2 5% 6% 49%	4580
	11th to 20th	Night	15% 10% 4 6% 3 3 4 3 4 6% 42%	5199
	21st to end	Night	23% 18% 6% 9% 4 3 3 3 3 5% 23%	6996

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	53% 5% 10% 20% 11% 2	3756
	11th to 20th	Day	46% 5% 10% 21% 14% 4	4080
	21st to end	Day	51% 4 10% 14% 18% 3	4488
February	1st to 10th	Day	52% 4 12% 20% 11% 1	1800
	11th to 20th	Day	45% 6% 11% 20% 15% 3	1812
	21st to end	Day	45% 4 9% 23% 16% 3	1650
March	1st to 10th	Day	39% 4 7% 24% 24% 3	4882
	11th to 20th	Day	33% 5% 9% 24% 24% 6%	4980
	21st to end	Day	48% 7% 16% 23% 6%	5545
April	1st to 10th	Day	43% 6% 10% 25% 15% 2	2400
	11th to 20th	Day	43% 6% 11% 23% 14% 3	2374
	21st to end	Day	48% 7% 12% 20% 12% 1	2400
May	1st to 10th	Day	43% 9% 14% 22% 12%	5780
	11th to 20th	Day	43% 7% 10% 24% 13% 2	6131
	21st to end	Day	42% 7% 9% 25% 16% 1	6949
June	1st to 10th	Day	31% 9% 15% 29% 14% 1	2800
	11th to 20th	Day	32% 8% 13% 26% 17% 3	2800
	21st to end	Day	30% 8% 16% 27% 15% 4	2800
July	1st to 10th	Day	39% 7% 12% 24% 16% 2	4442
	11th to 20th	Day	28% 7% 14% 32% 18% 1	4192
	21st to end	Day	32% 9% 19% 28% 11% 1	4325
August	1st to 10th	Day	26% 8% 16% 35% 14% 1	4090
	11th to 20th	Day	37% 6% 13% 28% 16% 1	3953
	21st to end	Day	26% 6% 18% 32% 13% 4	4179
September	1st to 10th	Day	31% 6% 12% 25% 21% 6%	2280
	11th to 20th	Day	30% 6% 11% 27% 22% 4	2280
	21st to end	Day	31% 6% 10% 25% 23% 4	1993
October	1st to 10th	Day	19% 3 6% 20% 41% 11%	3160
	11th to 20th	Day	22% 2 6% 21% 38% 10%	3140
	21st to end	Day	36% 3 8% 24% 27% 2	3428
November	1st to 10th	Day	33% 5% 12% 23% 22% 5%	1866
	11th to 20th	Day	38% 4 9% 24% 22% 4	1770
	21st to end	Day	44% 4 10% 27% 14% 1	1600
December	1st to 10th	Day	35% 4 14% 30% 16% 1	2610
	11th to 20th	Day	43% 5% 12% 26% 13% 1	3005
	21st to end	Day	67% 5% 11% 11% 5%	3982
January	1st to 10th	Night	58% 4 9% 20% 9%	6192
	11th to 20th	Night	52% 4 10% 20% 12% 3	5900
	21st to end	Night	51% 4 10% 18% 16% 2	6490
February	1st to 10th	Night	50% 6% 11% 19% 12% 2	2600
	11th to 20th	Night	45% 5% 11% 20% 16% 3	2484
	21st to end	Night	43% 5% 9% 22% 17% 3	1980
March	1st to 10th	Night	44% 4 7% 21% 22% 3	5012
	11th to 20th	Night	33% 5% 7% 23% 26% 5%	4980
	21st to end	Night	50% 6% 11% 23% 9% 1	5385
April	1st to 10th	Night	38% 5% 10% 25% 18% 4	2000
	11th to 20th	Night	39% 5% 9% 26% 16% 5%	1975
	21st to end	Night	43% 6% 10% 23% 17% 1	2000
May	1st to 10th	Night	45% 8% 14% 18% 14% 1	4179
	11th to 20th	Night	38% 8% 11% 25% 14% 4	3786
	21st to end	Night	38% 8% 11% 23% 19% 1	3950
June	1st to 10th	Night	26% 8% 15% 31% 17% 3	1600
	11th to 20th	Night	29% 8% 11% 26% 23% 4	1600
	21st to end	Night	22% 7% 17% 28% 19% 6%	1600
July	1st to 10th	Night	37% 6% 10% 29% 16% 3	2530
	11th to 20th	Night	19% 6% 14% 38% 20% 2	2638
	21st to end	Night	30% 7% 20% 30% 13% 1	2999
August	1st to 10th	Night	18% 6% 20% 34% 21% 1	2826
	11th to 20th	Night	34% 6% 12% 27% 18% 3	2946
	21st to end	Night	22% 6% 18% 34% 16% 4	3476
September	1st to 10th	Night	26% 6% 12% 23% 26% 7%	1900
	11th to 20th	Night	29% 6% 11% 25% 23% 6%	1900
	21st to end	Night	29% 5% 8% 28% 27% 4	2164
October	1st to 10th	Night	17% 3 6% 21% 45% 8%	3800
	11th to 20th	Night	24% 3 6% 22% 32% 14%	3779
	21st to end	Night	36% 3 7% 26% 26% 2	4120
November	1st to 10th	Night	30% 5% 12% 28% 20% 6%	2430
	11th to 20th	Night	37% 4 8% 25% 22% 4	2626
	21st to end	Night	43% 5% 10% 24% 17% 2	2800
December	1st to 10th	Night	37% 4 10% 35% 13% 1	4580
	11th to 20th	Night	44% 4 12% 24% 16% 1	5201
	21st to end	Night	65% 5% 12% 12% 5% 1	6996

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	22% 17% 38% 21% 2	300
	11th to 20th	Day	29% 19% 40% 12% 1	300
	21st to end	Day	19% 19% 45% 14% 3	330
February	1st to 10th	Day	27% 16% 41% 13% 2	300
	11th to 20th	Day	26% 23% 35% 12% 4	300
	21st to end	Day	26% 23% 35% 15% 2	249
March	1st to 10th	Day	18% 22% 44% 14% 2	300
	11th to 20th	Day	26% 16% 34% 20% 3	300
	21st to end	Day	33% 21% 31% 13% 3	360
April	1st to 10th	Day	23% 17% 35% 18% 6% 1	500
	11th to 20th	Day	24% 18% 32% 20% 6%	492
	21st to end	Day	25% 12% 33% 24% 5% 1	500
May	1st to 10th	Day	17% 14% 35% 25% 8% 1	648
	11th to 20th	Day	18% 13% 25% 28% 14% 2	725
	21st to end	Day	16% 9% 30% 26% 15% 5%	789
June	1st to 10th	Day	8% 4 25% 35% 21% 7%	730
	11th to 20th	Day	9% 3 23% 41% 21% 3	730
	21st to end	Day	6% 6% 31% 36% 17% 4	730
July	1st to 10th	Day	11% 3 14% 32% 29% 11%	697
	11th to 20th	Day	7% 4 19% 36% 27% 7%	668
	21st to end	Day	9% 3 16% 36% 31% 5%	739
August	1st to 10th	Day	6% 3 17% 42% 25% 7%	676
	11th to 20th	Day	10% 3 18% 37% 26% 7%	590
	21st to end	Day	9% 2 13% 35% 30% 11%	539
September	1st to 10th	Day	12% 3 17% 32% 28% 8%	490
	11th to 20th	Day	8% 4 18% 33% 26% 12%	490
	21st to end	Day	10% 10% 25% 29% 20% 6%	327
October	1st to 10th	Day	12% 7% 33% 26% 17% 5%	300
	11th to 20th	Day	18% 19% 31% 23% 8% 1	300
	21st to end	Day	29% 18% 27% 22% 3 1	330
November	1st to 10th	Day	14% 13% 38% 24% 8% 3	300
	11th to 20th	Day	21% 15% 35% 20% 7% 2	300
	21st to end	Day	25% 16% 30% 24% 4 1	300
December	1st to 10th	Day	16% 20% 37% 19% 6% 3	300
	11th to 20th	Day	23% 16% 39% 18% 4 1	300
	21st to end	Day	26% 23% 34% 15% 1	330
January	1st to 10th	Night	17% 17% 38% 23% 5%	1000
	11th to 20th	Night	19% 19% 36% 21% 4 1	1000
	21st to end	Night	18% 13% 43% 24% 2	1100
February	1st to 10th	Night	20% 15% 40% 22% 3	1000
	11th to 20th	Night	21% 15% 39% 22% 3	929
	21st to end	Night	20% 18% 39% 20% 3	660
March	1st to 10th	Night	19% 16% 39% 23% 3	645
	11th to 20th	Night	22% 13% 33% 26% 5%	600
	21st to end	Night	27% 15% 35% 21% 3	660
April	1st to 10th	Night	17% 16% 39% 25% 4	600
	11th to 20th	Night	18% 15% 34% 26% 7%	593
	21st to end	Night	20% 11% 37% 28% 5%	600
May	1st to 10th	Night	16% 7% 35% 34% 7%	600
	11th to 20th	Night	16% 8% 24% 39% 13% 1	591
	21st to end	Night	12% 5% 29% 34% 17% 2	650
June	1st to 10th	Night	9% 3 21% 41% 25% 2	600
	11th to 20th	Night	5% 3 25% 43% 23% 1	600
	21st to end	Night	3 3 28% 46% 19% 1	600
July	1st to 10th	Night	11% 2 12% 37% 34% 4	570
	11th to 20th	Night	5% 1 15% 44% 32% 2	552
	21st to end	Night	7% 1 17% 42% 30% 2	624
August	1st to 10th	Night	7% 2 13% 46% 30% 2	567
	11th to 20th	Night	7% 2 13% 45% 30% 3	564
	21st to end	Night	7% 1 14% 38% 37% 3	627
September	1st to 10th	Night	11% 2 17% 38% 27% 4	570
	11th to 20th	Night	11% 3 17% 38% 26% 4	570
	21st to end	Night	10% 5% 26% 36% 19% 4	674
October	1st to 10th	Night	11% 5% 28% 33% 20% 3	760
	11th to 20th	Night	16% 10% 36% 28% 9%	760
	21st to end	Night	21% 12% 32% 28% 6%	836
November	1st to 10th	Night	12% 10% 37% 31% 9%	780
	11th to 20th	Night	16% 14% 38% 26% 6% 1	826
	21st to end	Night	22% 12% 36% 24% 5%	1000
December	1st to 10th	Night	19% 14% 37% 24% 5%	1000
	11th to 20th	Night	23% 12% 35% 25% 4	1000
	21st to end	Night	24% 16% 34% 23% 2	1100

Page generated at 2025-07-03 00:30:14. Version ac07

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