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 2024-11-04.

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	12% 7% 3 7% 5% 4 5% 4 6% 8% 41%	101175
	Night	13% 7% 3 6% 4 4 4 3 5% 7% 44%	97796
Transparency	Day	40% 5% 10% 23% 18% 4	104920
	Night	38% 5% 10% 23% 20% 5%	102190
Seeing	Day	16% 8% 21% 25% 20% 11%	15119
	Night	16% 7% 23% 31% 20% 3	24782

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 9% 6% 4 5% 4 5% 6% 32%	2527
	11th to 20th	Day	19% 8% 3 11% 4 3 4 3 4 5% 37%	2670
	21st to end	Day	22% 11% 5% 6% 3 3 3 3 4 5% 35%	3476
February	1st to 10th	Day	19% 11% 3 7% 4 4 3 3 3 5% 38%	1900
	11th to 20th	Day	19% 11% 3 7% 5% 4 3 3 5% 4 36%	1848
	21st to end	Day	20% 9% 3 7% 4 4 3 4 3 6% 36%	1570
March	1st to 10th	Day	20% 12% 4 7% 4 4 4 4 4 5% 31%	3120
	11th to 20th	Day	13% 7% 3 6% 4 3 4 3 4 7% 46%	3308
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 6% 7% 40%	4065
April	1st to 10th	Day	12% 6% 2 6% 5% 3 5% 4 5% 6% 45%	2280
	11th to 20th	Day	12% 8% 3 6% 4 4 3 3 4 6% 45%	2254
	21st to end	Day	11% 7% 3 7% 5% 4 5% 4 6% 8% 41%	2285
May	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 4 6% 8% 36%	4082
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 5% 8% 37%	4081
	21st to end	Day	10% 6% 3 7% 5% 5% 4 4 6% 9% 40%	4476
June	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 5% 7% 10% 40%	2470
	11th to 20th	Day	5% 4 2 5% 6% 5% 6% 5% 7% 10% 45%	2650
	21st to end	Day	5% 5% 2 6% 6% 6% 6% 5% 9% 10% 41%	2660
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 6% 5% 8% 10% 38%	4440
	11th to 20th	Day	8% 6% 2 7% 6% 6% 6% 6% 8% 11% 35%	4326
	21st to end	Day	14% 10% 3 10% 6% 6% 6% 5% 7% 8% 25%	4466
August	1st to 10th	Day	3 2 1 5% 4 4 5% 5% 7% 11% 52%	3821
	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% 5% 2 3 3 3 3 3 4 5% 57%	1699
	11th to 20th	Day	13% 7% 2 5% 4 3 4 3 5% 6% 50%	1710
	21st to end	Day	14% 7% 3 5% 3 4 4 3 6% 7% 44%	1710
December	1st to 10th	Day	21% 9% 3 7% 4 4 4 3 4 6% 36%	1710
	11th to 20th	Day	15% 9% 3 8% 5% 5% 4 4 4 5% 40%	2153
	21st to end	Day	14% 10% 5% 8% 5% 4 5% 3 6% 7% 33%	3017
January	1st to 10th	Night	17% 11% 3 6% 4 3 4 3 4 6% 38%	4428
	11th to 20th	Night	17% 10% 4 9% 5% 3 4 3 4 4 37%	4269
	21st to end	Night	21% 13% 4 6% 3 3 3 2 4 4 37%	4180
February	1st to 10th	Night	19% 9% 3 8% 5% 4 4 3 5% 5% 35%	2280
	11th to 20th	Night	20% 11% 3 6% 5% 4 4 3 4 4 37%	2218
	21st to end	Night	23% 8% 3 7% 5% 4 3 3 5% 5% 34%	1884
March	1st to 10th	Night	22% 9% 3 7% 4 4 3 3 4 6% 34%	3733
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 4 6% 48%	3651
	21st to end	Night	14% 8% 3 7% 5% 4 4 4 5% 5% 40%	3594
April	1st to 10th	Night	12% 6% 2 5% 4 4 3 3 6% 7% 48%	1900
	11th to 20th	Night	14% 6% 2 4 3 3 4 3 4 6% 49%	1875
	21st to end	Night	11% 6% 2 6% 5% 4 5% 3 6% 8% 45%	1894
May	1st to 10th	Night	13% 7% 3 7% 5% 4 4 4 6% 7% 41%	2813
	11th to 20th	Night	11% 9% 3 7% 4 4 4 3 5% 8% 42%	2812
	21st to end	Night	7% 6% 2 7% 5% 4 5% 4 7% 9% 43%	3079
June	1st to 10th	Night	8% 6% 2 6% 4 4 5% 4 7% 9% 45%	1710
	11th to 20th	Night	4 2 1 4 3 4 4 4 7% 10% 57%	1523
	21st to end	Night	4 4 1 4 4 3 6% 4 8% 11% 51%	1520
July	1st to 10th	Night	6% 4 3 6% 4 5% 6% 5% 9% 12% 40%	2530
	11th to 20th	Night	7% 5% 2 5% 5% 5% 5% 5% 9% 11% 41%	2623
	21st to end	Night	10% 9% 3 10% 6% 6% 6% 5% 7% 9% 28%	3087
August	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 8% 60%	3103
	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	10% 5% 2 5% 4 3 3 3 4 6% 53%	2452
	11th to 20th	Night	11% 6% 2 4 4 3 3 3 7% 8% 49%	2470
	21st to end	Night	15% 7% 3 6% 3 3 4 3 5% 5% 46%	2470
December	1st to 10th	Night	19% 9% 3 6% 4 5% 4 3 5% 6% 36%	2470
	11th to 20th	Night	15% 8% 2 6% 4 3 3 3 4 5% 47%	3042
	21st to end	Night	16% 10% 5% 9% 5% 5% 5% 3 6% 6% 30%	4632

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% 21% 14% 2	2528
	11th to 20th	Day	50% 4 7% 19% 19% 1	2670
	21st to end	Day	51% 4 7% 23% 15% 1	3476
February	1st to 10th	Day	50% 3 7% 19% 17% 3	1900
	11th to 20th	Day	51% 3 6% 20% 16% 4	1858
	21st to end	Day	50% 4 7% 16% 17% 6%	1570
March	1st to 10th	Day	55% 4 7% 12% 13% 8%	3120
	11th to 20th	Day	39% 5% 8% 25% 18% 6%	3308
	21st to end	Day	42% 6% 10% 25% 16% 2	4065
April	1st to 10th	Day	39% 5% 9% 23% 19% 4	2280
	11th to 20th	Day	41% 4 7% 20% 22% 6%	2254
	21st to end	Day	40% 5% 10% 26% 16% 2	2285
May	1st to 10th	Day	44% 9% 11% 22% 14% 2	4120
	11th to 20th	Day	44% 7% 12% 24% 13%	4085
	21st to end	Day	39% 7% 11% 23% 19% 1	4489
June	1st to 10th	Day	37% 7% 13% 24% 17% 2	2470
	11th to 20th	Day	31% 8% 16% 25% 19% 2	2650
	21st to end	Day	34% 8% 17% 20% 17% 4	2660
July	1st to 10th	Day	38% 6% 20% 27% 7% 2	4371
	11th to 20th	Day	41% 5% 17% 29% 7% 2	4251
	21st to end	Day	54% 7% 12% 19% 7% 1	4324
August	1st to 10th	Day	25% 6% 14% 33% 20% 1	3808
	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	30% 4 7% 23% 30% 6%	1699
	11th to 20th	Day	35% 3 6% 22% 26% 6%	1710
	21st to end	Day	39% 4 7% 24% 21% 5%	1710
December	1st to 10th	Day	51% 4 5% 21% 16% 3	1710
	11th to 20th	Day	48% 4 8% 20% 18% 2	2150
	21st to end	Day	49% 6% 11% 22% 10% 3	3017
January	1st to 10th	Night	47% 4 7% 23% 17% 2	4428
	11th to 20th	Night	51% 4 6% 17% 16% 5%	4269
	21st to end	Night	52% 3 6% 22% 16% 2	4180
February	1st to 10th	Night	51% 4 9% 17% 16% 2	2280
	11th to 20th	Night	52% 3 6% 18% 16% 5%	2219
	21st to end	Night	52% 3 7% 16% 17% 5%	1884
March	1st to 10th	Night	51% 5% 7% 13% 16% 7%	3733
	11th to 20th	Night	38% 5% 7% 23% 17% 10%	3651
	21st to end	Night	45% 5% 8% 22% 19% 1	3594
April	1st to 10th	Night	34% 5% 12% 24% 22% 4	1900
	11th to 20th	Night	36% 4 8% 20% 23% 9%	1875
	21st to end	Night	37% 5% 11% 26% 18% 2	1894
May	1st to 10th	Night	42% 8% 11% 23% 15% 2	2840
	11th to 20th	Night	40% 9% 14% 20% 17% 1	2813
	21st to end	Night	34% 9% 13% 25% 17% 1	3087
June	1st to 10th	Night	34% 7% 15% 24% 19% 3	1710
	11th to 20th	Night	22% 6% 18% 32% 20% 3	1523
	21st to end	Night	24% 8% 19% 28% 17% 4	1520
July	1st to 10th	Night	35% 6% 23% 26% 8% 2	2490
	11th to 20th	Night	33% 5% 19% 33% 7% 3	2575
	21st to end	Night	50% 8% 14% 20% 7% 1	2988
August	1st to 10th	Night	21% 5% 11% 38% 24% 1	3092
	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	33% 3 7% 23% 26% 8%	2452
	11th to 20th	Night	32% 4 7% 21% 27% 8%	2470
	21st to end	Night	41% 3 6% 23% 22% 5%	2470
December	1st to 10th	Night	48% 5% 8% 18% 17% 3	2470
	11th to 20th	Night	41% 3 6% 23% 24% 4	3044
	21st to end	Night	54% 4 8% 19% 13% 2	4632

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	24% 17% 38% 15% 5% 1	270
	11th to 20th	Day	26% 18% 36% 14% 4 2	270
	21st to end	Day	20% 20% 40% 17% 4	297
February	1st to 10th	Day	27% 16% 34% 17% 5% 1	270
	11th to 20th	Day	35% 15% 34% 12% 2 1	270
	21st to end	Day	35% 20% 34% 7% 4	225
March	1st to 10th	Day	25% 17% 34% 18% 5% 1	270
	11th to 20th	Day	22% 15% 36% 20% 6% 1	353
	21st to end	Day	23% 14% 32% 23% 8% 1	506
April	1st to 10th	Day	20% 13% 28% 26% 11% 2	460
	11th to 20th	Day	19% 12% 31% 25% 12% 2	452
	21st to end	Day	16% 10% 32% 26% 12% 4	460
May	1st to 10th	Day	17% 10% 25% 28% 15% 5%	463
	11th to 20th	Day	15% 6% 21% 28% 18% 11%	487
	21st to end	Day	11% 4 19% 29% 22% 16%	529
June	1st to 10th	Day	11% 6% 17% 23% 24% 20%	490
	11th to 20th	Day	6% 2 16% 27% 31% 18%	670
	21st to end	Day	7% 2 15% 32% 27% 16%	680
July	1st to 10th	Day	9% 3 10% 28% 30% 20%	671
	11th to 20th	Day	8% 3 7% 26% 31% 25%	606
	21st to end	Day	10% 2 7% 25% 31% 24%	532
August	1st to 10th	Day	8% 1 8% 24% 37% 22%	486
	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% 10% 27% 30% 12% 3	279
	11th to 20th	Day	15% 14% 34% 27% 10%	270
	21st to end	Day	20% 15% 28% 22% 11% 4	270
December	1st to 10th	Day	26% 16% 31% 21% 6% 1	270
	11th to 20th	Day	21% 14% 34% 20% 9% 1	270
	21st to end	Day	24% 18% 34% 19% 5%	297
January	1st to 10th	Night	22% 12% 34% 27% 5%	950
	11th to 20th	Night	24% 15% 33% 24% 4	857
	21st to end	Night	20% 10% 42% 25% 3	836
February	1st to 10th	Night	24% 11% 35% 25% 4	760
	11th to 20th	Night	26% 15% 33% 24% 3	743
	21st to end	Night	31% 13% 33% 22% 2	628
March	1st to 10th	Night	22% 12% 33% 29% 6%	760
	11th to 20th	Night	21% 11% 35% 25% 7% 1	760
	21st to end	Night	18% 8% 35% 30% 9%	703
April	1st to 10th	Night	15% 9% 32% 32% 10%	570
	11th to 20th	Night	17% 9% 31% 33% 9% 1	563
	21st to end	Night	16% 6% 33% 32% 12% 1	570
May	1st to 10th	Night	13% 4 29% 34% 18% 2	570
	11th to 20th	Night	11% 3 23% 36% 23% 4	561
	21st to end	Night	9% 2 14% 38% 32% 5%	617
June	1st to 10th	Night	11% 2 14% 37% 32% 5%	570
	11th to 20th	Night	5% 2 10% 37% 42% 5%	570
	21st to end	Night	5% 9% 38% 39% 8%	570
July	1st to 10th	Night	8% 3 6% 28% 44% 11%	555
	11th to 20th	Night	8% 1 6% 29% 44% 11%	552
	21st to end	Night	7% 1 6% 31% 45% 10%	624
August	1st to 10th	Night	7% 2 3 28% 49% 11%	567
	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	12% 6% 22% 39% 19% 2	755
	11th to 20th	Night	13% 9% 34% 32% 12% 1	760
	21st to end	Night	20% 8% 28% 31% 12% 1	760
December	1st to 10th	Night	24% 10% 29% 30% 7%	760
	11th to 20th	Night	21% 8% 31% 32% 8% 1	760
	21st to end	Night	25% 11% 31% 27% 5%	897

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