Table Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Table Mountain clear sky chart from 2005-11-06 to 2025-10-19.

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	7% 10% 3 7% 4 4 4 3 5% 6% 45%	130769
	Night	9% 11% 4 8% 5% 4 4 3 5% 6% 42%	127676
Transparency	Day	36% 7% 11% 24% 17% 6%	130495
	Night	38% 8% 12% 23% 15% 5%	127527
Seeing	Day	14% 19% 29% 25% 10% 2	15579
	Night	12% 15% 25% 29% 16% 2	24496

By Month

Percentage Cloud Cover NW (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% 23% 6% 11% 5% 5% 4 3 5% 5% 18%	3639
	11th to 20th	Day	8% 13% 5% 8% 5% 3 4 4 5% 6% 39%	3616
	21st to end	Day	15% 14% 4 8% 5% 4 4 3 5% 5% 33%	4004
February	1st to 10th	Day	12% 18% 5% 11% 7% 5% 5% 4 5% 5% 22%	1776
	11th to 20th	Day	10% 19% 6% 9% 5% 4 5% 3 5% 7% 27%	1755
	21st to end	Day	8% 17% 5% 12% 7% 6% 5% 5% 6% 6% 23%	1635
March	1st to 10th	Day	5% 13% 4 11% 7% 6% 6% 5% 7% 6% 30%	4518
	11th to 20th	Day	6% 16% 5% 11% 5% 4 4 3 5% 5% 37%	4907
	21st to end	Day	11% 18% 6% 11% 7% 6% 6% 4 6% 6% 19%	5824
April	1st to 10th	Day	10% 15% 5% 10% 6% 6% 7% 5% 6% 6% 24%	2400
	11th to 20th	Day	7% 15% 5% 8% 6% 5% 5% 4 6% 6% 34%	2372
	21st to end	Day	7% 11% 3 8% 5% 5% 5% 5% 7% 9% 35%	2631
May	1st to 10th	Day	6% 8% 3 7% 5% 4 4 3 4 5% 52%	6320
	11th to 20th	Day	7% 10% 3 8% 5% 4 5% 4 6% 7% 44%	6277
	21st to end	Day	8% 11% 3 7% 5% 4 4 3 5% 7% 42%	6903
June	1st to 10th	Day	10% 8% 3 7% 5% 5% 5% 4 6% 7% 40%	2800
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 8% 46%	2800
	21st to end	Day	6% 5% 2 4 3 3 4 4 6% 6% 57%	2800
July	1st to 10th	Day	1 2 1 3 1 2 3 2 3 4 78%	6320
	11th to 20th	Day	2 2 1 2 2 2 1 1 2 3 80%	6320
	21st to end	Day	2 3 1 3 2 2 2 2 4 6% 72%	6874
August	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 6% 68%	4541
	11th to 20th	Day	2 3 1 3 2 3 4 3 6% 8% 63%	4221
	21st to end	Day	7% 6% 3 6% 3 3 3 3 4 5% 58%	4344
September	1st to 10th	Day	4 3 2 4 3 3 4 3 5% 7% 64%	2400
	11th to 20th	Day	6% 5% 2 5% 5% 4 5% 4 5% 6% 53%	2400
	21st to end	Day	9% 6% 2 6% 4 3 4 3 6% 5% 50%	2149
October	1st to 10th	Day	6% 6% 2 5% 4 3 4 4 6% 9% 51%	3280
	11th to 20th	Day	11% 11% 4 8% 5% 5% 5% 3 6% 7% 36%	3242
	21st to end	Day	11% 11% 4 10% 5% 5% 5% 3 5% 6% 35%	3140
November	1st to 10th	Day	15% 16% 4 9% 6% 5% 4 5% 5% 6% 25%	1712
	11th to 20th	Day	13% 17% 6% 12% 6% 5% 4 4 5% 5% 23%	1600
	21st to end	Day	12% 16% 5% 9% 5% 5% 5% 5% 5% 7% 27%	1600
December	1st to 10th	Day	10% 12% 4 8% 5% 4 4 4 5% 6% 38%	2620
	11th to 20th	Day	13% 18% 6% 14% 6% 6% 5% 3 5% 6% 16%	3025
	21st to end	Day	13% 20% 7% 13% 8% 5% 6% 3 4 5% 17%	4004
January	1st to 10th	Night	12% 19% 7% 12% 6% 6% 5% 4 5% 5% 19%	6306
	11th to 20th	Night	9% 10% 5% 10% 5% 4 4 3 5% 9% 36%	6282
	21st to end	Night	13% 14% 4 9% 5% 4 4 3 4 5% 34%	6952
February	1st to 10th	Night	11% 17% 5% 10% 7% 5% 5% 4 6% 6% 24%	2620
	11th to 20th	Night	9% 16% 4 9% 5% 5% 5% 4 5% 6% 33%	2532
	21st to end	Night	9% 17% 7% 10% 6% 5% 5% 4 6% 6% 26%	1995
March	1st to 10th	Night	6% 11% 4 10% 6% 6% 6% 5% 7% 7% 33%	5376
	11th to 20th	Night	6% 14% 5% 10% 4 4 4 3 5% 6% 39%	5091
	21st to end	Night	10% 14% 5% 11% 7% 6% 6% 4 7% 6% 24%	5150
April	1st to 10th	Night	8% 10% 4 7% 6% 6% 5% 6% 8% 8% 32%	2000
	11th to 20th	Night	7% 10% 3 8% 5% 4 4 3 6% 8% 42%	1979
	21st to end	Night	7% 10% 3 8% 4 5% 5% 4 5% 9% 40%	1765
May	1st to 10th	Night	5% 10% 3 6% 3 4 3 2 4 5% 55%	3640
	11th to 20th	Night	5% 10% 4 6% 4 4 4 4 5% 6% 49%	3616
	21st to end	Night	6% 10% 2 6% 5% 5% 5% 4 7% 9% 43%	3972
June	1st to 10th	Night	8% 6% 2 6% 5% 5% 4 5% 6% 6% 47%	1600
	11th to 20th	Night	6% 6% 2 7% 5% 4 4 4 5% 8% 49%	1600
	21st to end	Night	3 4 2 4 3 4 4 4 6% 7% 60%	1600
July	1st to 10th	Night	1 1 1 2 2 2 2 2 3 5% 78%	3640
	11th to 20th	Night	1 1 2 1 1 1 2 3 3 84%	3640
	21st to end	Night	1 2 1 3 2 2 2 2 3 5% 76%	3969
August	1st to 10th	Night	2 2 1 4 3 2 2 2 4 5% 74%	2609
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 7% 68%	2831
	21st to end	Night	6% 6% 2 4 3 3 3 3 5% 5% 61%	3575
September	1st to 10th	Night	4 3 2 3 3 3 3 2 5% 7% 65%	2000
	11th to 20th	Night	5% 5% 2 5% 4 3 4 4 6% 6% 55%	2000
	21st to end	Night	6% 6% 2 5% 4 4 3 3 6% 8% 54%	2248
October	1st to 10th	Night	6% 6% 2 5% 4 4 5% 4 6% 8% 50%	3920
	11th to 20th	Night	10% 9% 3 8% 4 4 5% 4 5% 7% 41%	3873
	21st to end	Night	13% 11% 3 8% 5% 4 4 3 5% 6% 37%	4418
November	1st to 10th	Night	17% 12% 4 8% 4 4 5% 4 6% 6% 28%	2585
	11th to 20th	Night	14% 15% 4 8% 6% 5% 4 4 5% 6% 28%	2800
	21st to end	Night	14% 13% 5% 9% 6% 5% 5% 4 6% 6% 28%	2800
December	1st to 10th	Night	8% 11% 4 8% 5% 4 4 3 4 6% 42%	4560
	11th to 20th	Night	18% 18% 5% 11% 7% 6% 5% 4 4 6% 16%	5180
	21st to end	Night	17% 17% 6% 12% 6% 5% 4 3 5% 6% 19%	6952

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	60% 11% 10% 13% 6%	3639
	11th to 20th	Day	44% 6% 12% 27% 11% 1	3640
	21st to end	Day	51% 6% 11% 17% 10% 5%	4004
February	1st to 10th	Day	59% 6% 14% 16% 4 1	1776
	11th to 20th	Day	55% 7% 12% 16% 8% 1	1764
	21st to end	Day	54% 10% 11% 16% 8% 1	1635
March	1st to 10th	Day	43% 12% 9% 15% 18% 3	4518
	11th to 20th	Day	41% 8% 11% 32% 8%	4907
	21st to end	Day	55% 13% 12% 15% 5%	5824
April	1st to 10th	Day	52% 9% 12% 19% 7%	2400
	11th to 20th	Day	46% 6% 12% 22% 12% 2	2372
	21st to end	Day	42% 7% 12% 26% 12% 2	2631
May	1st to 10th	Day	30% 7% 8% 25% 24% 6%	6320
	11th to 20th	Day	33% 10% 14% 27% 16%	6280
	21st to end	Day	36% 9% 17% 29% 8% 1	6904
June	1st to 10th	Day	39% 6% 10% 29% 13% 4	2800
	11th to 20th	Day	32% 5% 13% 30% 18% 2	2800
	21st to end	Day	25% 5% 13% 32% 22% 4	2800
July	1st to 10th	Day	12% 3 7% 23% 30% 25%	6251
	11th to 20th	Day	12% 2 5% 22% 31% 29%	6245
	21st to end	Day	14% 5% 5% 30% 32% 14%	6681
August	1st to 10th	Day	15% 4 8% 22% 33% 18%	4524
	11th to 20th	Day	16% 7% 12% 38% 21% 5%	4262
	21st to end	Day	28% 5% 8% 27% 29% 4	4343
September	1st to 10th	Day	20% 4 9% 31% 28% 8%	2400
	11th to 20th	Day	31% 5% 12% 25% 22% 6%	2400
	21st to end	Day	33% 4 15% 27% 18% 4	2149
October	1st to 10th	Day	27% 7% 12% 35% 18% 1	3280
	11th to 20th	Day	40% 10% 13% 23% 14%	3242
	21st to end	Day	39% 12% 10% 23% 11% 6%	3140
November	1st to 10th	Day	55% 7% 12% 17% 8% 1	1712
	11th to 20th	Day	58% 8% 13% 15% 5% 1	1600
	21st to end	Day	54% 6% 14% 17% 7% 2	1600
December	1st to 10th	Day	42% 8% 12% 21% 15% 1	2620
	11th to 20th	Day	65% 8% 13% 11% 3	3028
	21st to end	Day	66% 8% 9% 12% 5%	4004
January	1st to 10th	Night	60% 9% 10% 13% 7%	6306
	11th to 20th	Night	41% 7% 11% 25% 15% 1	6320
	21st to end	Night	51% 5% 12% 18% 9% 5%	6952
February	1st to 10th	Night	55% 8% 16% 16% 5% 1	2620
	11th to 20th	Night	48% 7% 14% 20% 8% 2	2534
	21st to end	Night	53% 9% 10% 17% 10%	1995
March	1st to 10th	Night	37% 13% 9% 19% 17% 4	5376
	11th to 20th	Night	35% 12% 14% 30% 9%	5092
	21st to end	Night	47% 12% 14% 22% 4	5150
April	1st to 10th	Night	44% 9% 16% 22% 9%	2000
	11th to 20th	Night	39% 5% 15% 24% 15% 2	1979
	21st to end	Night	38% 7% 12% 27% 14% 1	1765
May	1st to 10th	Night	26% 9% 7% 27% 26% 5%	3637
	11th to 20th	Night	28% 9% 14% 32% 16% 1	3616
	21st to end	Night	29% 10% 20% 32% 7% 1	3972
June	1st to 10th	Night	34% 7% 13% 27% 16% 3	1600
	11th to 20th	Night	31% 5% 16% 30% 16% 2	1600
	21st to end	Night	22% 5% 14% 34% 21% 5%	1600
July	1st to 10th	Night	10% 3 7% 26% 23% 32%	3600
	11th to 20th	Night	7% 2 5% 24% 30% 32%	3592
	21st to end	Night	10% 5% 6% 30% 30% 19%	3857
August	1st to 10th	Night	13% 3 6% 20% 38% 20%	2601
	11th to 20th	Night	15% 6% 14% 36% 22% 6%	2856
	21st to end	Night	20% 7% 11% 26% 29% 6%	3575
September	1st to 10th	Night	19% 3 11% 29% 29% 9%	2000
	11th to 20th	Night	27% 5% 10% 28% 22% 8%	2000
	21st to end	Night	27% 5% 14% 31% 17% 6%	2248
October	1st to 10th	Night	26% 7% 10% 36% 20% 1	3920
	11th to 20th	Night	36% 10% 11% 27% 15% 2	3873
	21st to end	Night	38% 11% 10% 24% 14% 3	4418
November	1st to 10th	Night	51% 7% 11% 19% 10% 1	2585
	11th to 20th	Night	51% 9% 14% 18% 7% 1	2800
	21st to end	Night	53% 6% 13% 20% 8% 1	2800
December	1st to 10th	Night	38% 8% 12% 23% 17% 2	4560
	11th to 20th	Night	63% 9% 13% 11% 3	5176
	21st to end	Night	60% 9% 12% 14% 6%	6952

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	18% 19% 27% 22% 14% 2	200
	11th to 20th	Day	22% 14% 28% 19% 14% 4	200
	21st to end	Day	17% 16% 25% 26% 12% 3	220
February	1st to 10th	Day	20% 28% 29% 18% 3 1	376
	11th to 20th	Day	22% 22% 35% 16% 5%	395
	21st to end	Day	19% 22% 33% 18% 6% 1	407
March	1st to 10th	Day	17% 26% 30% 20% 7% 1	500
	11th to 20th	Day	19% 24% 23% 24% 9% 1	500
	21st to end	Day	23% 25% 29% 18% 5%	550
April	1st to 10th	Day	25% 23% 27% 17% 7% 2	500
	11th to 20th	Day	18% 27% 29% 16% 7% 3	492
	21st to end	Day	16% 24% 31% 19% 8% 2	500
May	1st to 10th	Day	15% 19% 34% 26% 5% 1	503
	11th to 20th	Day	21% 15% 30% 22% 10% 2	528
	21st to end	Day	17% 16% 26% 27% 12% 2	573
June	1st to 10th	Day	13% 23% 31% 23% 9% 2	530
	11th to 20th	Day	13% 23% 36% 20% 7% 1	530
	21st to end	Day	9% 18% 30% 29% 12% 2	530
July	1st to 10th	Day	6% 15% 34% 36% 8% 1	526
	11th to 20th	Day	5% 17% 35% 30% 11% 3	524
	21st to end	Day	3 11% 33% 35% 13% 4	575
August	1st to 10th	Day	6% 12% 29% 33% 17% 4	527
	11th to 20th	Day	3 12% 26% 38% 15% 5%	517
	21st to end	Day	7% 15% 29% 30% 16% 4	583
September	1st to 10th	Day	6% 12% 32% 32% 13% 5%	530
	11th to 20th	Day	10% 15% 32% 29% 12% 2	530
	21st to end	Day	13% 20% 28% 26% 11% 2	530
October	1st to 10th	Day	16% 17% 27% 25% 12% 2	530
	11th to 20th	Day	20% 17% 25% 23% 11% 3	523
	21st to end	Day	20% 20% 25% 24% 8% 3	357
November	1st to 10th	Day	19% 24% 24% 27% 6%	273
	11th to 20th	Day	21% 22% 26% 16% 11% 6%	200
	21st to end	Day	13% 18% 27% 27% 13% 4	200
December	1st to 10th	Day	17% 21% 19% 20% 16% 9%	200
	11th to 20th	Day	19% 17% 27% 27% 10% 2	200
	21st to end	Day	21% 20% 20% 27% 10% 2	220
January	1st to 10th	Night	19% 21% 22% 25% 12% 2	800
	11th to 20th	Night	17% 16% 23% 28% 15% 2	800
	21st to end	Night	16% 14% 20% 31% 16% 3	880
February	1st to 10th	Night	16% 18% 28% 25% 13% 1	800
	11th to 20th	Night	15% 17% 24% 29% 13% 2	784
	21st to end	Night	17% 20% 23% 26% 12% 2	660
March	1st to 10th	Night	14% 17% 24% 30% 13% 1	800
	11th to 20th	Night	13% 18% 24% 29% 16% 2	800
	21st to end	Night	14% 21% 28% 25% 10% 1	880
April	1st to 10th	Night	14% 16% 28% 28% 12% 2	800
	11th to 20th	Night	11% 20% 30% 24% 12% 3	791
	21st to end	Night	10% 17% 33% 25% 13% 2	565
May	1st to 10th	Night	9% 19% 28% 30% 13% 2	400
	11th to 20th	Night	12% 12% 24% 30% 19% 3	394
	21st to end	Night	9% 10% 25% 36% 16% 3	433
June	1st to 10th	Night	9% 11% 33% 31% 15% 1	400
	11th to 20th	Night	9% 18% 35% 28% 10% 1	400
	21st to end	Night	5% 12% 34% 28% 20% 2	400
July	1st to 10th	Night	3 10% 29% 40% 18% 1	390
	11th to 20th	Night	1 8% 37% 36% 16% 1	388
	21st to end	Night	2 8% 29% 33% 27% 1	438
August	1st to 10th	Night	2 6% 24% 38% 28% 3	398
	11th to 20th	Night	4 4 22% 32% 31% 7%	547
	21st to end	Night	5% 11% 24% 32% 24% 4	860
September	1st to 10th	Night	5% 8% 24% 32% 28% 4	800
	11th to 20th	Night	7% 9% 20% 30% 27% 7%	800
	21st to end	Night	9% 11% 22% 33% 21% 5%	800
October	1st to 10th	Night	12% 10% 22% 33% 21% 2	800
	11th to 20th	Night	12% 15% 21% 28% 19% 5%	792
	21st to end	Night	17% 16% 24% 27% 13% 3	836
November	1st to 10th	Night	17% 15% 23% 34% 10% 1	780
	11th to 20th	Night	17% 20% 25% 24% 12% 3	800
	21st to end	Night	14% 18% 24% 27% 15% 3	800
December	1st to 10th	Night	16% 14% 24% 29% 15% 3	800
	11th to 20th	Night	18% 15% 26% 30% 10%	800
	21st to end	Night	17% 22% 23% 31% 6% 1	880

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