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 2024-04-10.

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	9% 10% 4 7% 6% 3 5% 5% 5% 6% 40%	83012
	Night	10% 11% 4 7% 6% 3 5% 5% 5% 7% 38%	85792
Transparency	Day	42% 6% 11% 23% 15% 4	82739
	Night	43% 6% 13% 22% 13% 3	85645
Seeing	Day	14% 18% 29% 26% 11% 2	13559
	Night	12% 15% 25% 30% 16% 3	22730

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	12% 23% 8% 12% 7% 4 4 4 4 5% 16%	2539
	11th to 20th	Day	12% 18% 7% 11% 6% 3 4 4 4 5% 24%	2516
	21st to end	Day	20% 17% 6% 9% 6% 4 4 4 5% 6% 18%	2794
February	1st to 10th	Day	13% 18% 5% 9% 8% 3 6% 5% 5% 6% 23%	1687
	11th to 20th	Day	11% 18% 6% 8% 6% 4 5% 4 5% 7% 27%	1665
	21st to end	Day	8% 16% 6% 11% 8% 5% 6% 6% 6% 6% 23%	1555
March	1st to 10th	Day	7% 13% 5% 9% 7% 4 6% 5% 6% 6% 30%	3138
	11th to 20th	Day	7% 11% 4 6% 5% 3 4 4 5% 5% 47%	3423
	21st to end	Day	11% 16% 6% 9% 7% 4 6% 5% 6% 6% 24%	4059
April	1st to 10th	Day	11% 14% 6% 10% 7% 4 8% 6% 6% 6% 24%	2270
	11th to 20th	Day	9% 14% 5% 7% 7% 3 6% 5% 6% 7% 31%	2132
	21st to end	Day	8% 11% 3 7% 6% 4 5% 6% 7% 9% 35%	2367
May	1st to 10th	Day	6% 7% 3 7% 7% 4 5% 5% 6% 8% 41%	2520
	11th to 20th	Day	11% 9% 3 6% 7% 4 6% 6% 7% 8% 33%	2477
	21st to end	Day	11% 9% 4 7% 7% 4 5% 6% 5% 7% 35%	2723
June	1st to 10th	Day	11% 7% 3 6% 6% 4 6% 5% 7% 7% 39%	2520
	11th to 20th	Day	6% 5% 2 5% 5% 3 6% 6% 7% 9% 45%	2520
	21st to end	Day	6% 4 2 4 4 2 4 5% 5% 7% 57%	2520
July	1st to 10th	Day	3 3 1 4 3 2 5% 4 4 6% 65%	2520
	11th to 20th	Day	4 3 2 2 3 2 3 3 4 5% 70%	2520
	21st to end	Day	2 1 1 2 2 2 2 2 3 4 78%	2741
August	1st to 10th	Day	3 2 1 3 3 1 3 4 4 6% 69%	2507
	11th to 20th	Day	3 3 1 2 2 1 3 3 5% 7% 71%	2337
	21st to end	Day	3 4 2 5% 4 3 4 4 4 7% 61%	2394
September	1st to 10th	Day	4 3 2 3 3 2 3 4 5% 6% 65%	2160
	11th to 20th	Day	7% 5% 3 5% 5% 3 5% 5% 4 6% 51%	2160
	21st to end	Day	10% 6% 2 5% 5% 3 4 4 6% 6% 49%	1936
October	1st to 10th	Day	10% 7% 2 5% 5% 3 5% 5% 5% 7% 46%	1800
	11th to 20th	Day	13% 9% 3 6% 5% 3 4 5% 6% 7% 39%	1800
	21st to end	Day	15% 12% 3 7% 5% 3 5% 4 5% 7% 34%	1808
November	1st to 10th	Day	15% 15% 4 8% 7% 4 5% 5% 4 6% 26%	1625
	11th to 20th	Day	13% 16% 7% 11% 6% 3 5% 4 5% 5% 24%	1520
	21st to end	Day	13% 15% 5% 8% 6% 3 5% 6% 5% 7% 27%	1520
December	1st to 10th	Day	13% 14% 6% 9% 7% 4 5% 4 4 5% 28%	1520
	11th to 20th	Day	13% 14% 5% 12% 8% 4 6% 6% 5% 6% 20%	1925
	21st to end	Day	12% 17% 6% 11% 10% 4 6% 4 4 5% 20%	2794
January	1st to 10th	Night	13% 17% 8% 12% 7% 4 6% 5% 5% 5% 17%	4406
	11th to 20th	Night	13% 14% 7% 12% 7% 3 4 4 5% 9% 23%	4382
	21st to end	Night	17% 17% 6% 9% 7% 4 5% 5% 5% 6% 19%	4862
February	1st to 10th	Night	12% 16% 6% 8% 8% 4 5% 5% 7% 6% 24%	2489
	11th to 20th	Night	10% 15% 4 7% 6% 4 5% 5% 5% 7% 34%	2402
	21st to end	Night	10% 16% 7% 9% 7% 4 5% 5% 5% 6% 25%	1899
March	1st to 10th	Night	8% 12% 5% 8% 7% 4 5% 6% 7% 8% 30%	3736
	11th to 20th	Night	6% 10% 4 7% 4 2 4 3 4 6% 49%	3555
	21st to end	Night	7% 13% 6% 8% 8% 4 6% 6% 7% 7% 28%	3593
April	1st to 10th	Night	8% 10% 4 7% 7% 5% 6% 7% 8% 8% 32%	1900
	11th to 20th	Night	8% 8% 4 7% 7% 3 4 4 6% 8% 41%	1779
	21st to end	Night	7% 9% 4 7% 5% 4 5% 4 5% 9% 40%	1589
May	1st to 10th	Night	6% 7% 4 4 5% 3 4 5% 6% 7% 49%	1440
	11th to 20th	Night	8% 6% 3 5% 6% 3 6% 5% 6% 7% 44%	1416
	21st to end	Night	6% 6% 3 7% 6% 4 7% 6% 7% 8% 40%	1552
June	1st to 10th	Night	9% 6% 2 5% 6% 4 4 6% 6% 7% 46%	1440
	11th to 20th	Night	7% 6% 2 7% 5% 3 5% 5% 5% 8% 48%	1440
	21st to end	Night	3 3 2 4 4 3 4 5% 5% 7% 61%	1440
July	1st to 10th	Night	3 3 1 3 2 2 3 4 4 8% 67%	1440
	11th to 20th	Night	1 2 1 3 2 1 3 3 3 6% 76%	1440
	21st to end	Night	1 1 1 1 1 2 3 3 4 83%	1576
August	1st to 10th	Night	2 2 1 3 3 2 2 3 4 6% 72%	1432
	11th to 20th	Night	2 2 1 1 2 1 3 4 4 6% 73%	1559
	21st to end	Night	3 2 1 3 3 2 4 4 5% 7% 66%	1962
September	1st to 10th	Night	4 2 2 2 3 2 3 3 5% 7% 66%	1800
	11th to 20th	Night	6% 5% 2 5% 5% 3 4 4 6% 6% 54%	1800
	21st to end	Night	6% 5% 2 4 5% 3 4 4 6% 8% 54%	2022
October	1st to 10th	Night	11% 7% 3 4 4 2 4 4 5% 6% 50%	2160
	11th to 20th	Night	11% 9% 2 6% 6% 3 5% 6% 5% 6% 43%	2160
	21st to end	Night	14% 10% 3 6% 6% 3 4 4 6% 7% 37%	2547
November	1st to 10th	Night	18% 12% 4 7% 5% 3 5% 6% 6% 6% 29%	2452
	11th to 20th	Night	14% 14% 4 8% 6% 4 5% 5% 5% 7% 29%	2660
	21st to end	Night	14% 12% 5% 8% 7% 4 5% 5% 5% 7% 28%	2660
December	1st to 10th	Night	13% 14% 7% 8% 6% 3 5% 5% 5% 7% 28%	2660
	11th to 20th	Night	15% 16% 5% 9% 7% 4 5% 6% 4 7% 21%	3280
	21st to end	Night	14% 14% 6% 11% 9% 4 5% 4 5% 6% 21%	4862

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	64% 11% 10% 11% 4 1	2539
	11th to 20th	Day	60% 6% 14% 16% 4 1	2540
	21st to end	Day	67% 4 14% 10% 4	2794
February	1st to 10th	Day	59% 6% 14% 16% 4 1	1687
	11th to 20th	Day	56% 6% 12% 17% 8% 1	1674
	21st to end	Day	55% 9% 11% 16% 8% 1	1555
March	1st to 10th	Day	45% 11% 8% 17% 16% 3	3138
	11th to 20th	Day	35% 7% 10% 41% 7%	3423
	21st to end	Day	55% 9% 12% 18% 6%	4059
April	1st to 10th	Day	54% 8% 12% 19% 7%	2270
	11th to 20th	Day	49% 6% 12% 20% 11% 2	2132
	21st to end	Day	42% 7% 12% 24% 13% 2	2367
May	1st to 10th	Day	38% 6% 14% 29% 13% 2	2520
	11th to 20th	Day	46% 6% 11% 26% 11%	2480
	21st to end	Day	45% 6% 13% 26% 8% 1	2724
June	1st to 10th	Day	41% 6% 10% 28% 12% 3	2520
	11th to 20th	Day	33% 5% 14% 31% 16% 2	2520
	21st to end	Day	25% 4 13% 31% 23% 4	2520
July	1st to 10th	Day	20% 4 8% 25% 29% 13%	2451
	11th to 20th	Day	18% 3 7% 21% 34% 17%	2445
	21st to end	Day	12% 2 3 23% 40% 19%	2589
August	1st to 10th	Day	17% 3 8% 27% 32% 13%	2490
	11th to 20th	Day	15% 3 9% 30% 35% 8%	2337
	21st to end	Day	22% 4 7% 32% 29% 6%	2394
September	1st to 10th	Day	20% 3 9% 31% 28% 9%	2160
	11th to 20th	Day	32% 4 12% 23% 23% 6%	2160
	21st to end	Day	34% 4 15% 25% 18% 4	1936
October	1st to 10th	Day	37% 6% 10% 32% 13% 2	1800
	11th to 20th	Day	42% 5% 14% 27% 12% 1	1800
	21st to end	Day	48% 5% 11% 22% 13% 1	1808
November	1st to 10th	Day	56% 7% 11% 17% 8% 1	1625
	11th to 20th	Day	58% 8% 13% 15% 5% 1	1520
	21st to end	Day	54% 5% 14% 17% 8% 2	1520
December	1st to 10th	Day	57% 6% 12% 18% 7%	1520
	11th to 20th	Day	63% 6% 16% 11% 4	1928
	21st to end	Day	68% 5% 9% 12% 6%	2794
January	1st to 10th	Night	62% 9% 12% 11% 5% 1	4406
	11th to 20th	Night	54% 8% 11% 19% 7% 1	4420
	21st to end	Night	64% 5% 16% 11% 3 1	4862
February	1st to 10th	Night	56% 7% 16% 16% 5% 1	2489
	11th to 20th	Night	49% 6% 15% 20% 8% 2	2404
	21st to end	Night	55% 9% 10% 17% 9%	1899
March	1st to 10th	Night	43% 11% 9% 20% 14% 4	3736
	11th to 20th	Night	30% 9% 14% 39% 8%	3556
	21st to end	Night	48% 8% 15% 25% 4	3593
April	1st to 10th	Night	45% 8% 16% 22% 9%	1900
	11th to 20th	Night	40% 5% 15% 23% 15% 2	1779
	21st to end	Night	40% 7% 12% 25% 14% 1	1589
May	1st to 10th	Night	33% 5% 13% 33% 15% 2	1440
	11th to 20th	Night	37% 4 14% 33% 12% 1	1416
	21st to end	Night	37% 7% 16% 29% 11% 1	1552
June	1st to 10th	Night	35% 7% 14% 27% 15% 2	1440
	11th to 20th	Night	32% 5% 17% 31% 13% 2	1440
	21st to end	Night	22% 3 14% 35% 20% 5%	1440
July	1st to 10th	Night	17% 4 12% 28% 27% 12%	1400
	11th to 20th	Night	12% 2 10% 28% 30% 18%	1392
	21st to end	Night	7% 3 5% 24% 40% 21%	1488
August	1st to 10th	Night	13% 3 8% 27% 32% 16%	1424
	11th to 20th	Night	13% 3 10% 30% 33% 11%	1559
	21st to end	Night	17% 4 10% 30% 32% 7%	1962
September	1st to 10th	Night	19% 3 11% 29% 29% 9%	1800
	11th to 20th	Night	28% 5% 10% 25% 23% 9%	1800
	21st to end	Night	28% 4 13% 31% 16% 7%	2022
October	1st to 10th	Night	34% 3 10% 31% 20% 1	2160
	11th to 20th	Night	40% 6% 10% 28% 14% 3	2160
	21st to end	Night	44% 6% 10% 24% 16%	2547
November	1st to 10th	Night	52% 7% 11% 20% 10% 1	2452
	11th to 20th	Night	51% 8% 14% 19% 7% 1	2660
	21st to end	Night	53% 5% 13% 19% 9% 1	2660
December	1st to 10th	Night	54% 7% 13% 20% 6%	2660
	11th to 20th	Night	62% 6% 17% 11% 4	3276
	21st to end	Night	63% 5% 11% 13% 8%	4862

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% 18% 26% 22% 16% 2	180
	11th to 20th	Day	24% 12% 27% 18% 14% 4	180
	21st to end	Day	19% 15% 25% 26% 12% 3	198
February	1st to 10th	Day	20% 27% 29% 18% 3 1	347
	11th to 20th	Day	22% 21% 34% 16% 5%	365
	21st to end	Day	18% 22% 33% 20% 6% 1	375
March	1st to 10th	Day	17% 26% 30% 20% 7% 1	460
	11th to 20th	Day	17% 23% 23% 26% 10% 1	460
	21st to end	Day	20% 25% 30% 20% 6%	506
April	1st to 10th	Day	25% 22% 27% 17% 7% 2	457
	11th to 20th	Day	18% 27% 28% 17% 7% 4	412
	21st to end	Day	16% 24% 32% 18% 8% 1	420
May	1st to 10th	Day	13% 18% 35% 26% 6% 1	423
	11th to 20th	Day	21% 12% 30% 23% 12% 2	448
	21st to end	Day	15% 15% 26% 29% 13% 2	485
June	1st to 10th	Day	14% 22% 30% 24% 9% 2	450
	11th to 20th	Day	12% 23% 37% 20% 6% 1	450
	21st to end	Day	8% 16% 30% 30% 14% 2	450
July	1st to 10th	Day	6% 12% 35% 38% 8% 1	446
	11th to 20th	Day	4 15% 36% 30% 11% 3	444
	21st to end	Day	2 10% 32% 37% 14% 4	487
August	1st to 10th	Day	6% 11% 28% 34% 17% 3	447
	11th to 20th	Day	3 8% 26% 40% 17% 5%	437
	21st to end	Day	5% 15% 30% 30% 16% 4	495
September	1st to 10th	Day	7% 13% 34% 30% 13% 4	450
	11th to 20th	Day	10% 14% 30% 31% 12% 2	450
	21st to end	Day	12% 19% 27% 28% 12% 2	450
October	1st to 10th	Day	16% 16% 26% 26% 14% 2	450
	11th to 20th	Day	17% 17% 25% 25% 13% 3	450
	21st to end	Day	20% 20% 23% 26% 8% 3	323
November	1st to 10th	Day	16% 24% 25% 28% 7%	246
	11th to 20th	Day	17% 23% 26% 17% 12% 6%	180
	21st to end	Day	13% 18% 24% 27% 13% 4	180
December	1st to 10th	Day	17% 22% 20% 20% 13% 8%	180
	11th to 20th	Day	17% 16% 26% 28% 11% 2	180
	21st to end	Day	18% 19% 21% 30% 11% 2	198
January	1st to 10th	Night	19% 21% 20% 25% 13% 2	760
	11th to 20th	Night	17% 17% 20% 28% 15% 2	760
	21st to end	Night	17% 14% 19% 31% 16% 3	836
February	1st to 10th	Night	16% 17% 27% 25% 14% 1	760
	11th to 20th	Night	15% 17% 23% 29% 14% 2	744
	21st to end	Night	17% 20% 22% 27% 13% 2	628
March	1st to 10th	Night	14% 18% 23% 30% 14% 1	760
	11th to 20th	Night	11% 17% 24% 30% 16% 2	760
	21st to end	Night	12% 21% 29% 26% 11% 1	836
April	1st to 10th	Night	14% 16% 28% 27% 13% 2	760
	11th to 20th	Night	11% 20% 29% 24% 12% 3	711
	21st to end	Night	10% 17% 34% 24% 12% 2	509
May	1st to 10th	Night	9% 18% 29% 28% 14% 2	360
	11th to 20th	Night	12% 11% 22% 32% 20% 4	354
	21st to end	Night	10% 9% 26% 36% 17% 3	389
June	1st to 10th	Night	10% 10% 32% 31% 15% 1	360
	11th to 20th	Night	9% 16% 35% 28% 11% 1	360
	21st to end	Night	4 10% 34% 29% 21% 2	360
July	1st to 10th	Night	4 8% 29% 41% 18% 1	350
	11th to 20th	Night	1 8% 37% 36% 17% 1	348
	21st to end	Night	1 8% 29% 32% 28% 1	394
August	1st to 10th	Night	2 6% 22% 39% 28% 3	358
	11th to 20th	Night	4 3 21% 33% 32% 7%	491
	21st to end	Night	4 11% 25% 33% 23% 4	774
September	1st to 10th	Night	5% 8% 25% 32% 25% 4	720
	11th to 20th	Night	6% 9% 20% 29% 28% 8%	720
	21st to end	Night	8% 10% 22% 34% 22% 5%	720
October	1st to 10th	Night	12% 9% 21% 32% 23% 3	720
	11th to 20th	Night	11% 14% 20% 28% 21% 5%	720
	21st to end	Night	17% 16% 23% 28% 14% 4	792
November	1st to 10th	Night	16% 15% 24% 34% 11% 1	740
	11th to 20th	Night	16% 19% 25% 25% 13% 3	760
	21st to end	Night	14% 18% 23% 27% 15% 3	760
December	1st to 10th	Night	16% 14% 24% 29% 14% 3	760
	11th to 20th	Night	18% 14% 26% 31% 10%	760
	21st to end	Night	17% 21% 23% 32% 7% 1	836

Page generated at 2024-04-07 06:28:16. Version ac07

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