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 2024-11-03.

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%	101253
	Night	12% 8% 3 6% 4 4 4 3 5% 6% 44%	97813
Transparency	Day	39% 7% 12% 25% 15% 2	101055
	Night	40% 6% 12% 24% 16% 3	97683
Seeing	Day	16% 10% 28% 28% 15% 4	15662
	Night	16% 10% 31% 30% 12% 1	25463

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% 5% 11% 4 4 4 3 5% 6% 33%	2623
	11th to 20th	Day	12% 12% 4 9% 5% 7% 5% 3 4 4 36%	2850
	21st to end	Day	16% 16% 7% 10% 4 4 4 3 4 6% 29%	3135
February	1st to 10th	Day	17% 11% 4 9% 5% 4 3 3 5% 6% 33%	1710
	11th to 20th	Day	13% 11% 3 6% 4 4 4 4 5% 6% 41%	1710
	21st to end	Day	16% 10% 4 7% 4 3 4 3 6% 5% 39%	1570
March	1st to 10th	Day	17% 11% 3 5% 3 3 2 2 4 6% 44%	3396
	11th to 20th	Day	12% 8% 2 5% 3 3 4 3 5% 7% 48%	3480
	21st to end	Day	17% 13% 4 8% 4 4 4 3 5% 6% 31%	3867
April	1st to 10th	Day	13% 8% 3 8% 5% 4 3 2 5% 6% 44%	2280
	11th to 20th	Day	15% 10% 3 5% 4 3 4 3 6% 6% 41%	2254
	21st to end	Day	22% 6% 2 6% 4 5% 4 3 6% 7% 34%	2280
May	1st to 10th	Day	14% 8% 3 7% 5% 4 5% 4 7% 10% 34%	4005
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 4 6% 39%	4263
	21st to end	Day	11% 7% 2 6% 5% 5% 4 4 6% 8% 44%	4824
June	1st to 10th	Day	7% 4 2 5% 4 5% 5% 5% 7% 10% 46%	2660
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 9% 48%	2660
	21st to end	Day	6% 4 2 5% 5% 4 5% 5% 6% 10% 48%	2660
July	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 40%	4440
	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% 2 5% 3 2 4 3 4 6% 53%	1791
	11th to 20th	Day	15% 7% 2 6% 2 3 3 3 5% 6% 48%	1682
	21st to end	Day	17% 9% 3 5% 3 4 3 3 4 6% 43%	1520
December	1st to 10th	Day	12% 11% 4 7% 3 3 3 2 5% 7% 42%	1520
	11th to 20th	Day	13% 10% 4 7% 4 4 4 3 4 6% 41%	1918
	21st to end	Day	19% 15% 5% 10% 5% 4 5% 3 4 5% 23%	2783
January	1st to 10th	Night	10% 12% 7% 12% 5% 4 4 3 4 5% 35%	4328
	11th to 20th	Night	13% 10% 5% 11% 5% 5% 6% 5% 5% 4 31%	4120
	21st to end	Night	20% 16% 6% 7% 5% 3 3 3 4 4 30%	4532
February	1st to 10th	Night	15% 11% 3 8% 5% 4 5% 4 5% 6% 34%	2470
	11th to 20th	Night	14% 9% 3 6% 5% 3 3 4 5% 6% 41%	2356
	21st to end	Night	15% 11% 3 7% 4 3 4 4 5% 5% 41%	1884
March	1st to 10th	Night	18% 10% 3 6% 3 3 3 3 3 4 45%	3484
	11th to 20th	Night	14% 7% 2 5% 2 3 3 3 4 6% 50%	3480
	21st to end	Night	18% 13% 5% 8% 6% 5% 3 3 5% 5% 30%	3763
April	1st to 10th	Night	11% 8% 2 5% 4 4 4 3 4 6% 47%	1900
	11th to 20th	Night	14% 8% 2 7% 3 3 4 2 5% 6% 46%	1875
	21st to end	Night	18% 7% 2 5% 4 3 4 4 5% 6% 41%	1900
May	1st to 10th	Night	15% 8% 2 6% 4 3 5% 4 6% 9% 38%	2895
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 9% 41%	2639
	21st to end	Night	10% 4 2 5% 4 4 4 4 6% 9% 48%	2743
June	1st to 10th	Night	6% 4 2 4 4 4 5% 4 6% 9% 53%	1520
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 6% 10% 49%	1520
	21st to end	Night	4 3 2 4 4 4 4 4 6% 9% 57%	1520
July	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 9% 45%	2530
	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% 57%	2330
	11th to 20th	Night	12% 7% 3 6% 3 3 4 3 4 6% 49%	2494
	21st to end	Night	17% 9% 2 6% 4 3 4 3 4 5% 45%	2660
December	1st to 10th	Night	13% 11% 3 7% 5% 4 4 3 5% 6% 39%	2660
	11th to 20th	Night	14% 11% 4 7% 4 3 4 3 4 6% 40%	3279
	21st to end	Night	19% 20% 6% 10% 4 4 3 2 3 5% 24%	4884

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	50% 6% 10% 24% 10% 1	2624
	11th to 20th	Day	52% 6% 11% 20% 11%	2850
	21st to end	Day	57% 5% 10% 15% 12% 1	3135
February	1st to 10th	Day	51% 5% 12% 20% 12% 1	1710
	11th to 20th	Day	44% 6% 12% 21% 15% 3	1719
	21st to end	Day	47% 5% 9% 22% 14% 3	1570
March	1st to 10th	Day	43% 5% 9% 23% 18% 3	3396
	11th to 20th	Day	34% 6% 12% 27% 16% 4	3480
	21st to end	Day	51% 8% 17% 17% 6%	3867
April	1st to 10th	Day	41% 6% 11% 25% 15% 2	2280
	11th to 20th	Day	43% 6% 11% 24% 14% 3	2254
	21st to end	Day	48% 7% 12% 20% 13% 1	2280
May	1st to 10th	Day	43% 10% 14% 22% 11% 1	4042
	11th to 20th	Day	45% 8% 11% 24% 12%	4267
	21st to end	Day	35% 9% 11% 26% 18% 1	4837
June	1st to 10th	Day	30% 10% 15% 29% 14% 2	2660
	11th to 20th	Day	31% 8% 13% 26% 17% 4	2660
	21st to end	Day	29% 8% 16% 27% 15% 4	2660
July	1st to 10th	Day	40% 7% 12% 23% 15% 2	4371
	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	32% 5% 9% 24% 25% 5%	1900
	11th to 20th	Day	34% 4 9% 25% 24% 5%	1900
	21st to end	Day	42% 3 9% 24% 19% 3	2090
November	1st to 10th	Day	32% 5% 12% 23% 22% 5%	1791
	11th to 20th	Day	37% 4 9% 24% 22% 3	1682
	21st to end	Day	44% 4 10% 27% 14% 1	1520
December	1st to 10th	Day	42% 6% 12% 28% 12% 1	1520
	11th to 20th	Day	44% 6% 12% 25% 12% 1	1915
	21st to end	Day	61% 6% 10% 14% 8% 1	2783
January	1st to 10th	Night	52% 5% 10% 22% 10% 1	4328
	11th to 20th	Night	54% 5% 12% 19% 9% 1	4120
	21st to end	Night	58% 4 10% 19% 8% 1	4532
February	1st to 10th	Night	50% 6% 11% 19% 12% 2	2470
	11th to 20th	Night	44% 5% 11% 21% 17% 3	2357
	21st to end	Night	44% 5% 10% 20% 17% 4	1884
March	1st to 10th	Night	43% 6% 9% 22% 17% 3	3484
	11th to 20th	Night	34% 6% 9% 24% 23% 4	3480
	21st to end	Night	55% 7% 10% 22% 5% 1	3763
April	1st to 10th	Night	38% 5% 10% 26% 18% 4	1900
	11th to 20th	Night	39% 5% 9% 26% 16% 5%	1875
	21st to end	Night	43% 6% 10% 23% 18% 1	1900
May	1st to 10th	Night	41% 10% 16% 18% 14% 1	2923
	11th to 20th	Night	39% 9% 13% 27% 11% 1	2640
	21st to end	Night	31% 9% 13% 24% 22% 2	2751
June	1st to 10th	Night	25% 9% 15% 30% 17% 3	1520
	11th to 20th	Night	29% 8% 11% 26% 23% 4	1520
	21st to end	Night	23% 7% 17% 28% 19% 6%	1520
July	1st to 10th	Night	37% 6% 10% 29% 15% 3	2490
	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	26% 5% 9% 25% 29% 6%	2280
	11th to 20th	Night	34% 4 7% 27% 22% 6%	2280
	21st to end	Night	40% 4 9% 22% 22% 3	2508
November	1st to 10th	Night	29% 5% 12% 28% 20% 6%	2330
	11th to 20th	Night	37% 4 8% 26% 22% 4	2494
	21st to end	Night	43% 5% 10% 24% 17% 2	2660
December	1st to 10th	Night	45% 5% 12% 24% 13% 1	2660
	11th to 20th	Night	45% 5% 11% 26% 13% 1	3281
	21st to end	Night	64% 5% 9% 13% 8% 1	4884

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% 18% 36% 22% 2	270
	11th to 20th	Day	32% 18% 38% 12% 1	270
	21st to end	Day	19% 20% 45% 13% 3	297
February	1st to 10th	Day	27% 17% 39% 14% 2	270
	11th to 20th	Day	24% 23% 35% 13% 5%	270
	21st to end	Day	28% 23% 35% 13% 1	225
March	1st to 10th	Day	18% 22% 45% 14% 1	270
	11th to 20th	Day	27% 14% 35% 21% 2	270
	21st to end	Day	35% 21% 29% 12% 3	325
April	1st to 10th	Day	22% 16% 37% 19% 5% 1	460
	11th to 20th	Day	24% 17% 32% 21% 6%	452
	21st to end	Day	25% 13% 33% 22% 5% 1	460
May	1st to 10th	Day	18% 14% 36% 24% 7% 1	601
	11th to 20th	Day	19% 12% 25% 29% 13% 2	675
	21st to end	Day	15% 8% 31% 26% 15% 5%	734
June	1st to 10th	Day	7% 4 25% 36% 21% 7%	680
	11th to 20th	Day	8% 3 24% 41% 21% 3	680
	21st to end	Day	6% 6% 33% 36% 16% 3	680
July	1st to 10th	Day	11% 3 15% 32% 28% 11%	671
	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	13% 13% 38% 24% 9% 3	276
	11th to 20th	Day	21% 15% 34% 21% 7% 2	270
	21st to end	Day	26% 15% 30% 25% 4 1	270
December	1st to 10th	Day	17% 20% 34% 19% 7% 3	270
	11th to 20th	Day	24% 14% 39% 19% 4 1	270
	21st to end	Day	24% 25% 35% 15% 1	297
January	1st to 10th	Night	16% 17% 39% 23% 5%	950
	11th to 20th	Night	20% 19% 36% 20% 4 1	950
	21st to end	Night	19% 14% 42% 24% 2	1045
February	1st to 10th	Night	20% 15% 39% 22% 3	950
	11th to 20th	Night	20% 14% 40% 22% 3	882
	21st to end	Night	21% 19% 38% 19% 3	628
March	1st to 10th	Night	19% 15% 40% 23% 3	613
	11th to 20th	Night	22% 13% 33% 26% 5%	570
	21st to end	Night	27% 14% 34% 21% 4	627
April	1st to 10th	Night	16% 16% 38% 25% 4	570
	11th to 20th	Night	18% 15% 34% 26% 7%	563
	21st to end	Night	19% 11% 37% 27% 5%	570
May	1st to 10th	Night	16% 7% 35% 35% 7%	570
	11th to 20th	Night	16% 7% 23% 39% 13% 1	561
	21st to end	Night	12% 5% 29% 34% 18% 2	617
June	1st to 10th	Night	8% 2 21% 41% 25% 2	570
	11th to 20th	Night	5% 2 26% 43% 23% 1	570
	21st to end	Night	3 4 29% 47% 18% 1	570
July	1st to 10th	Night	11% 2 12% 38% 33% 4	555
	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% 11% 37% 31% 9%	749
	11th to 20th	Night	16% 14% 38% 26% 6% 1	784
	21st to end	Night	22% 12% 37% 24% 5%	950
December	1st to 10th	Night	20% 14% 37% 24% 5%	950
	11th to 20th	Night	23% 12% 35% 25% 4	950
	21st to end	Night	24% 17% 35% 23% 2	1045

Page generated at 2024-10-31 02:39:27. Version ac07

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