TBM Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the TBM Observatory clear sky chart from 2005-11-06 to 2024-11-01.

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	14% 6% 2 5% 4 4 4 4 6% 8% 43%	101170
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	97697
Transparency	Day	39% 6% 13% 25% 15% 3	100964
	Night	41% 5% 11% 24% 16% 3	97574
Seeing	Day	17% 10% 27% 25% 15% 6%	15043
	Night	17% 9% 29% 31% 13% 1	24425

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	16% 5% 2 5% 3 3 3 3 5% 6% 49%	2517
	11th to 20th	Day	16% 9% 3 8% 5% 4 4 4 4 5% 40%	2758
	21st to end	Day	19% 7% 3 6% 4 3 4 4 5% 5% 39%	3124
February	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 41%	1710
	11th to 20th	Day	18% 7% 2 4 4 4 3 3 5% 6% 44%	1781
	21st to end	Day	18% 8% 3 6% 4 4 4 2 5% 7% 39%	1570
March	1st to 10th	Day	23% 6% 2 5% 3 3 4 3 5% 7% 39%	3237
	11th to 20th	Day	10% 5% 2 4 4 4 4 3 5% 7% 52%	3480
	21st to end	Day	21% 6% 2 6% 4 3 3 3 4 5% 43%	3969
April	1st to 10th	Day	14% 7% 2 6% 4 5% 4 4 4 7% 44%	2280
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 5% 8% 48%	2254
	21st to end	Day	16% 6% 3 5% 3 3 3 3 6% 8% 44%	2280
May	1st to 10th	Day	24% 8% 3 6% 3 4 4 4 6% 8% 32%	3909
	11th to 20th	Day	21% 7% 2 7% 5% 5% 4 4 6% 8% 31%	4398
	21st to end	Day	11% 4 2 5% 4 4 4 4 6% 10% 46%	4823
June	1st to 10th	Day	10% 5% 1 6% 4 4 5% 4 7% 10% 43%	2660
	11th to 20th	Day	10% 4 2 5% 4 4 5% 4 7% 10% 44%	2660
	21st to end	Day	10% 4 2 4 3 3 5% 4 7% 11% 48%	2660
July	1st to 10th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	4440
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 5% 8% 11% 40%	4440
	21st to end	Day	9% 6% 3 7% 5% 6% 6% 5% 8% 10% 36%	4513
August	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 8% 11% 37%	4104
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 44%	3765
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 9% 49%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 5% 6% 46%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 50%	2280
	21st to end	Day	19% 9% 3 6% 3 3 5% 3 4 7% 38%	2038
October	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 4 6% 47%	1900
	11th to 20th	Day	16% 5% 1 4 3 3 3 2 4 7% 52%	1900
	21st to end	Day	14% 6% 2 5% 4 3 3 2 4 5% 51%	2090
November	1st to 10th	Day	13% 5% 1 5% 3 3 4 3 4 6% 53%	1681
	11th to 20th	Day	13% 7% 2 4 3 4 4 4 6% 6% 48%	1710
	21st to end	Day	14% 6% 2 5% 4 4 4 3 4 6% 48%	1577
December	1st to 10th	Day	16% 6% 3 5% 4 4 3 3 5% 7% 43%	1520
	11th to 20th	Day	16% 9% 3 5% 4 3 3 2 5% 5% 44%	1910
	21st to end	Day	14% 8% 2 7% 4 4 4 4 6% 6% 41%	2772
January	1st to 10th	Night	16% 8% 2 5% 4 4 4 3 4 6% 43%	4429
	11th to 20th	Night	18% 8% 3 5% 3 3 4 3 4 5% 44%	4186
	21st to end	Night	20% 9% 3 6% 4 3 3 3 4 5% 41%	4532
February	1st to 10th	Night	16% 7% 3 5% 5% 4 4 4 5% 6% 42%	2470
	11th to 20th	Night	19% 8% 3 6% 3 3 3 3 4 6% 42%	2284
	21st to end	Night	18% 8% 1 5% 4 4 4 2 4 6% 44%	1884
March	1st to 10th	Night	22% 8% 3 5% 4 4 3 2 4 5% 39%	3611
	11th to 20th	Night	12% 5% 2 5% 4 4 5% 4 5% 8% 48%	3480
	21st to end	Night	16% 8% 2 6% 3 3 3 3 4 5% 46%	3661
April	1st to 10th	Night	15% 8% 2 5% 4 4 4 3 4 7% 43%	1900
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 6% 46%	1875
	21st to end	Night	18% 5% 2 5% 3 4 3 4 6% 8% 44%	1900
May	1st to 10th	Night	20% 8% 3 7% 5% 5% 4 4 6% 6% 32%	3001
	11th to 20th	Night	21% 8% 3 6% 4 4 5% 4 6% 7% 32%	2495
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	2732
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 4 6% 9% 44%	1520
	11th to 20th	Night	8% 5% 2 7% 6% 4 5% 4 6% 9% 42%	1520
	21st to end	Night	10% 5% 2 5% 5% 5% 6% 4 8% 10% 41%	1520
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 7% 4 8% 10% 41%	2520
	11th to 20th	Night	10% 6% 2 6% 4 5% 5% 4 7% 11% 39%	2520
	21st to end	Night	12% 8% 3 7% 5% 6% 5% 4 6% 8% 35%	3022
August	1st to 10th	Night	17% 6% 2 7% 5% 5% 5% 4 7% 9% 33%	2825
	11th to 20th	Night	11% 6% 3 8% 4 4 5% 4 7% 9% 37%	3032
	21st to end	Night	9% 5% 2 7% 4 4 5% 4 6% 8% 49%	3476
September	1st to 10th	Night	14% 5% 2 6% 4 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	9% 6% 2 6% 3 3 4 3 5% 7% 52%	1900
	21st to end	Night	18% 6% 3 6% 4 4 4 3 5% 7% 39%	2136
October	1st to 10th	Night	16% 8% 2 6% 4 3 4 3 4 5% 45%	2280
	11th to 20th	Night	14% 5% 2 5% 3 2 3 2 4 6% 52%	2280
	21st to end	Night	16% 5% 1 5% 3 3 4 3 4 6% 50%	2508
November	1st to 10th	Night	12% 5% 2 6% 3 3 3 3 5% 5% 52%	2401
	11th to 20th	Night	15% 7% 2 6% 4 3 4 2 4 5% 48%	2470
	21st to end	Night	12% 5% 1 4 5% 4 4 3 6% 6% 48%	2598
December	1st to 10th	Night	18% 7% 3 6% 3 3 3 4 6% 6% 40%	2660
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 5% 8% 42%	3285
	21st to end	Night	15% 7% 3 7% 4 4 5% 4 6% 7% 38%	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	36% 5% 8% 32% 15% 3	2518
	11th to 20th	Day	47% 4 10% 21% 14% 4	2758
	21st to end	Day	46% 5% 11% 24% 11% 2	3124
February	1st to 10th	Day	41% 6% 13% 25% 14% 2	1710
	11th to 20th	Day	41% 3 9% 27% 17% 2	1790
	21st to end	Day	46% 4 12% 20% 15% 4	1570
March	1st to 10th	Day	44% 5% 9% 26% 13% 4	3237
	11th to 20th	Day	31% 5% 8% 30% 22% 4	3480
	21st to end	Day	44% 5% 8% 20% 19% 4	3969
April	1st to 10th	Day	39% 6% 12% 21% 18% 5%	2280
	11th to 20th	Day	34% 6% 11% 24% 19% 6%	2254
	21st to end	Day	38% 5% 12% 21% 20% 4	2280
May	1st to 10th	Day	50% 7% 11% 20% 10% 2	3944
	11th to 20th	Day	48% 8% 14% 21% 8% 1	4402
	21st to end	Day	32% 8% 13% 27% 18% 2	4837
June	1st to 10th	Day	34% 9% 15% 25% 15% 2	2660
	11th to 20th	Day	33% 9% 14% 27% 16% 2	2660
	21st to end	Day	29% 9% 15% 26% 20% 2	2660
July	1st to 10th	Day	34% 7% 23% 19% 13% 5%	4370
	11th to 20th	Day	36% 6% 19% 28% 9% 2	4365
	21st to end	Day	41% 7% 14% 21% 15% 1	4369
August	1st to 10th	Day	41% 6% 18% 27% 8%	4091
	11th to 20th	Day	36% 5% 13% 31% 14% 1	3802
	21st to end	Day	32% 5% 13% 29% 18% 3	4180
September	1st to 10th	Day	37% 6% 12% 21% 19% 4	2280
	11th to 20th	Day	34% 5% 11% 25% 23% 3	2280
	21st to end	Day	47% 5% 8% 19% 18% 3	2038
October	1st to 10th	Day	39% 5% 10% 24% 19% 3	1900
	11th to 20th	Day	34% 4 9% 28% 23% 4	1900
	21st to end	Day	36% 4 8% 28% 20% 4	2090
November	1st to 10th	Day	32% 5% 12% 29% 20% 3	1681
	11th to 20th	Day	35% 5% 11% 23% 22% 3	1710
	21st to end	Day	38% 4 9% 29% 17% 3	1577
December	1st to 10th	Day	41% 6% 14% 25% 12% 2	1520
	11th to 20th	Day	42% 4 9% 31% 11% 3	1906
	21st to end	Day	42% 5% 16% 23% 13%	2772
January	1st to 10th	Night	42% 4 9% 26% 15% 3	4429
	11th to 20th	Night	43% 4 10% 25% 16% 2	4186
	21st to end	Night	46% 4 7% 27% 15% 1	4532
February	1st to 10th	Night	42% 5% 13% 23% 15% 2	2470
	11th to 20th	Night	44% 3 8% 22% 19% 4	2286
	21st to end	Night	43% 4 10% 21% 17% 5%	1884
March	1st to 10th	Night	48% 4 8% 25% 14% 2	3611
	11th to 20th	Night	33% 6% 10% 28% 19% 4	3480
	21st to end	Night	42% 4 8% 20% 21% 5%	3661
April	1st to 10th	Night	42% 5% 9% 19% 20% 5%	1900
	11th to 20th	Night	38% 5% 10% 22% 19% 6%	1875
	21st to end	Night	39% 6% 10% 23% 18% 5%	1900
May	1st to 10th	Night	50% 7% 10% 21% 10% 2	3031
	11th to 20th	Night	51% 8% 13% 18% 10% 1	2496
	21st to end	Night	35% 8% 16% 21% 15% 4	2739
June	1st to 10th	Night	36% 7% 13% 24% 19% 1	1520
	11th to 20th	Night	36% 8% 14% 25% 15% 2	1520
	21st to end	Night	35% 8% 14% 23% 16% 3	1520
July	1st to 10th	Night	37% 10% 21% 17% 12% 2	2480
	11th to 20th	Night	39% 5% 21% 25% 9% 1	2472
	21st to end	Night	46% 6% 13% 21% 14%	2926
August	1st to 10th	Night	47% 6% 18% 20% 9%	2814
	11th to 20th	Night	42% 6% 12% 29% 12% 1	3061
	21st to end	Night	33% 5% 10% 28% 21% 2	3476
September	1st to 10th	Night	38% 7% 11% 21% 18% 5%	1900
	11th to 20th	Night	31% 7% 8% 26% 23% 4	1900
	21st to end	Night	45% 6% 8% 20% 18% 3	2136
October	1st to 10th	Night	42% 5% 9% 22% 19% 3	2280
	11th to 20th	Night	34% 4 11% 26% 21% 4	2280
	21st to end	Night	36% 4 9% 27% 21% 4	2508
November	1st to 10th	Night	33% 5% 10% 27% 19% 6%	2401
	11th to 20th	Night	40% 4 9% 21% 21% 6%	2470
	21st to end	Night	35% 5% 12% 26% 19% 3	2598
December	1st to 10th	Night	43% 5% 12% 24% 13% 2	2660
	11th to 20th	Night	41% 4 11% 22% 18% 4	3288
	21st to end	Night	43% 7% 14% 24% 11% 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% 20% 36% 17% 6% 1	180
	11th to 20th	Day	29% 21% 31% 15% 5%	249
	21st to end	Day	24% 16% 42% 13% 4 1	297
February	1st to 10th	Day	24% 20% 34% 14% 7%	270
	11th to 20th	Day	26% 19% 40% 12% 2	389
	21st to end	Day	28% 18% 33% 18% 4	382
March	1st to 10th	Day	22% 20% 32% 21% 5%	460
	11th to 20th	Day	21% 19% 33% 19% 5% 2	460
	21st to end	Day	21% 17% 33% 21% 7% 1	506
April	1st to 10th	Day	22% 16% 35% 19% 6% 2	460
	11th to 20th	Day	17% 15% 30% 22% 13% 3	452
	21st to end	Day	22% 12% 30% 24% 9% 1	460
May	1st to 10th	Day	22% 14% 31% 21% 8% 4	463
	11th to 20th	Day	16% 9% 33% 26% 12% 2	487
	21st to end	Day	16% 6% 21% 28% 19% 10%	529
June	1st to 10th	Day	12% 5% 28% 27% 20% 8%	490
	11th to 20th	Day	14% 6% 21% 30% 21% 8%	490
	21st to end	Day	10% 3 23% 35% 21% 7%	490
July	1st to 10th	Day	9% 3 16% 35% 24% 13%	485
	11th to 20th	Day	5% 1 16% 32% 31% 14%	484
	21st to end	Day	10% 2 18% 31% 30% 8%	532
August	1st to 10th	Day	12% 2 17% 30% 27% 12%	486
	11th to 20th	Day	11% 2 17% 29% 28% 11%	479
	21st to end	Day	9% 4 17% 32% 21% 17%	539
September	1st to 10th	Day	14% 5% 20% 24% 21% 15%	490
	11th to 20th	Day	11% 3 23% 30% 21% 12%	490
	21st to end	Day	25% 8% 24% 26% 13% 5%	490
October	1st to 10th	Day	19% 8% 22% 26% 16% 9%	490
	11th to 20th	Day	16% 12% 26% 24% 16% 5%	490
	21st to end	Day	17% 10% 33% 24% 12% 3	539
November	1st to 10th	Day	17% 10% 30% 25% 15% 4	370
	11th to 20th	Day	16% 18% 30% 24% 12% 1	370
	21st to end	Day	20% 14% 30% 24% 10% 2	237
December	1st to 10th	Day	18% 20% 38% 16% 7% 2	180
	11th to 20th	Day	24% 24% 29% 16% 7%	180
	21st to end	Day	25% 15% 28% 22% 9% 1	198
January	1st to 10th	Night	21% 17% 38% 22% 3	760
	11th to 20th	Night	21% 13% 34% 27% 4	760
	21st to end	Night	18% 16% 40% 24% 2	836
February	1st to 10th	Night	20% 12% 37% 28% 3	760
	11th to 20th	Night	24% 19% 39% 18% 1	743
	21st to end	Night	23% 16% 36% 22% 4	628
March	1st to 10th	Night	18% 15% 39% 23% 5%	760
	11th to 20th	Night	20% 16% 36% 22% 6% 1	760
	21st to end	Night	19% 12% 38% 27% 5%	836
April	1st to 10th	Night	20% 15% 39% 20% 6%	760
	11th to 20th	Night	15% 11% 36% 31% 7%	751
	21st to end	Night	20% 9% 33% 30% 8%	760
May	1st to 10th	Night	18% 9% 31% 31% 10%	696
	11th to 20th	Night	16% 6% 32% 35% 10% 1	374
	21st to end	Night	12% 3 23% 41% 20% 1	411
June	1st to 10th	Night	13% 4 24% 40% 18% 1	380
	11th to 20th	Night	9% 3 23% 46% 18% 1	380
	21st to end	Night	10% 2 22% 46% 18% 1	380
July	1st to 10th	Night	11% 2 17% 41% 28% 2	370
	11th to 20th	Night	8% 1 15% 43% 31% 2	368
	21st to end	Night	8% 1 14% 46% 28% 3	585
August	1st to 10th	Night	10% 1 14% 43% 30% 2	567
	11th to 20th	Night	12% 3 16% 36% 30% 3	709
	21st to end	Night	10% 2 14% 36% 32% 5%	836
September	1st to 10th	Night	16% 1 15% 38% 26% 4	760
	11th to 20th	Night	12% 2 17% 35% 30% 4	760
	21st to end	Night	20% 4 22% 33% 19% 2	760
October	1st to 10th	Night	21% 5% 18% 32% 19% 4	760
	11th to 20th	Night	17% 8% 26% 35% 15% 1	760
	21st to end	Night	18% 7% 32% 30% 13% 1	836
November	1st to 10th	Night	15% 6% 33% 31% 13% 2	743
	11th to 20th	Night	17% 13% 31% 30% 8% 1	760
	21st to end	Night	15% 15% 32% 29% 9% 1	760
December	1st to 10th	Night	22% 14% 34% 20% 9% 1	760
	11th to 20th	Night	19% 12% 37% 25% 7%	760
	21st to end	Night	19% 12% 35% 31% 4	836

Page generated at 2024-10-29 11:20:46. Version ac07

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