Tzec Maun Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tzec Maun Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	3 4 2 5% 4 4 4 4 6% 7% 56%	139511
	Night	2 3 2 5% 4 3 4 4 6% 8% 61%	140463
Transparency	Day	21% 8% 9% 16% 26% 20%	134600
	Night	17% 8% 9% 17% 27% 21%	136380
Seeing	Day	14% 19% 27% 23% 13% 3	16650
	Night	10% 16% 26% 29% 16% 3	26959

By Month

Percentage Cloud Cover SW (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	1 6% 3 9% 6% 6% 4 3 4 5% 53%	5318
	11th to 20th	Day	1 3 3 3 2 2 2 2 4 5% 74%	5340
	21st to end	Day	3 7% 4 6% 4 3 4 3 6% 6% 54%	5844
February	1st to 10th	Day	1 5% 2 5% 3 3 3 3 5% 7% 63%	2056
	11th to 20th	Day	2 6% 3 5% 4 3 3 3 5% 7% 60%	2049
	21st to end	Day	1 3 2 4 4 5% 3 3 3 6% 65%	1730
March	1st to 10th	Day	1 5% 2 4 5% 3 3 3 5% 6% 63%	4835
	11th to 20th	Day	1 3 1 5% 7% 4 4 3 5% 6% 60%	5110
	21st to end	Day	1 5% 2 5% 3 4 3 4 6% 9% 57%	5820
April	1st to 10th	Day	1 4 2 5% 3 3 3 3 6% 7% 64%	2520
	11th to 20th	Day	2 4 2 4 3 3 3 2 4 6% 68%	2491
	21st to end	Day	1 4 1 3 3 3 2 3 4 6% 71%	2520
May	1st to 10th	Day	1 2 1 4 3 3 3 3 4 5% 71%	5453
	11th to 20th	Day	1 2 1 3 2 3 4 4 6% 6% 68%	5405
	21st to end	Day	1 1 2 2 3 3 3 5% 6% 74%	6354
June	1st to 10th	Day	2 3 1 4 4 4 4 4 7% 9% 60%	2800
	11th to 20th	Day	2 2 1 2 2 3 3 4 6% 8% 67%	2800
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 5% 8% 8% 43%	2746
July	1st to 10th	Day	6% 4 2 7% 5% 6% 7% 6% 9% 9% 39%	5879
	11th to 20th	Day	3 3 2 6% 6% 6% 8% 7% 9% 11% 39%	5880
	21st to end	Day	7% 4 3 8% 8% 7% 9% 7% 10% 11% 26%	6368
August	1st to 10th	Day	5% 4 2 7% 5% 6% 7% 5% 8% 11% 40%	5348
	11th to 20th	Day	5% 5% 2 7% 6% 7% 8% 6% 8% 11% 35%	5366
	21st to end	Day	5% 5% 2 7% 6% 7% 7% 6% 9% 11% 36%	5984
September	1st to 10th	Day	9% 7% 3 9% 5% 4 6% 4 7% 8% 38%	2400
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 5% 7% 50%	2400
	21st to end	Day	4 3 2 5% 3 4 4 3 5% 6% 62%	2200
October	1st to 10th	Day	4 3 2 5% 3 3 2 3 3 5% 66%	3280
	11th to 20th	Day	4 3 1 4 3 4 4 3 5% 7% 64%	3250
	21st to end	Day	1 1 1 2 2 1 1 2 2 3 83%	3548
November	1st to 10th	Day	1 2 1 5% 3 2 2 2 5% 7% 70%	2059
	11th to 20th	Day	1 3 1 4 3 4 3 3 5% 7% 66%	1917
	21st to end	Day	2 4 2 5% 4 4 4 3 6% 8% 58%	1890
December	1st to 10th	Day	3 5% 1 4 2 2 3 3 5% 7% 64%	3030
	11th to 20th	Day	1 4 2 6% 4 4 3 2 5% 8% 62%	3172
	21st to end	Day	1 3 2 4 3 3 3 2 4 9% 67%	4349
January	1st to 10th	Night	1 5% 3 10% 6% 4 4 3 5% 6% 53%	7605
	11th to 20th	Night	2 1 3 3 2 2 2 4 5% 74%	7650
	21st to end	Night	2 6% 3 7% 3 3 4 5% 7% 8% 53%	8370
February	1st to 10th	Night	4 2 5% 3 4 4 4 5% 7% 62%	2561
	11th to 20th	Night	2 5% 2 5% 3 2 4 3 6% 7% 61%	2458
	21st to end	Night	1 3 1 5% 4 3 4 3 5% 8% 62%	2076
March	1st to 10th	Night	1 4 2 5% 5% 3 3 3 4 6% 63%	5252
	11th to 20th	Night	2 4 2 6% 8% 3 4 3 4 6% 59%	5110
	21st to end	Night	1 4 3 4 3 3 4 3 5% 7% 63%	5386
April	1st to 10th	Night	1 3 2 4 3 3 4 3 6% 8% 64%	2100
	11th to 20th	Night	2 4 1 3 3 3 3 3 5% 8% 66%	2077
	21st to end	Night	2 3 1 3 3 3 3 3 4 6% 70%	2100
May	1st to 10th	Night	2 3 1 3 3 2 3 3 5% 5% 70%	4550
	11th to 20th	Night	1 2 1 2 3 3 3 3 4 6% 72%	4510
	21st to end	Night	1 1 2 2 2 3 3 4 5% 78%	4520
June	1st to 10th	Night	2 3 2 4 4 4 4 5% 8% 8% 56%	1600
	11th to 20th	Night	2 2 1 2 2 3 3 3 5% 7% 68%	1600
	21st to end	Night	4 5% 3 6% 6% 6% 6% 6% 7% 8% 44%	1646
July	1st to 10th	Night	4 4 2 8% 5% 5% 5% 5% 7% 9% 47%	4100
	11th to 20th	Night	2 2 1 4 5% 5% 6% 5% 9% 11% 49%	4100
	21st to end	Night	4 4 1 5% 5% 5% 6% 7% 11% 15% 35%	4488
August	1st to 10th	Night	2 2 1 4 4 4 5% 5% 9% 14% 50%	4464
	11th to 20th	Night	3 3 2 5% 4 5% 5% 5% 10% 14% 45%	4483
	21st to end	Night	2 4 2 5% 4 5% 5% 5% 8% 12% 47%	4993
September	1st to 10th	Night	5% 4 2 6% 4 5% 5% 5% 7% 8% 48%	2000
	11th to 20th	Night	5% 3 1 5% 4 4 4 5% 6% 7% 56%	2000
	21st to end	Night	2 2 2 4 3 4 4 4 6% 6% 64%	2200
October	1st to 10th	Night	2 1 1 3 2 3 3 2 4 5% 74%	3905
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 6% 70%	3898
	21st to end	Night	2 2 1 3 2 2 2 2 3 6% 76%	4252
November	1st to 10th	Night	2 3 1 3 3 3 3 3 5% 6% 69%	2460
	11th to 20th	Night	1 3 1 3 3 3 3 4 5% 8% 67%	2678
	21st to end	Night	2 5% 2 5% 4 3 4 3 5% 7% 59%	2730
December	1st to 10th	Night	2 4 1 4 3 3 3 3 6% 7% 64%	4380
	11th to 20th	Night	1 2 1 5% 3 4 5% 4 5% 7% 63%	5250
	21st to end	Night	1 5% 2 5% 3 3 2 3 5% 8% 63%	6911

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	20% 12% 6% 12% 34% 16%	5321
	11th to 20th	Day	11% 5% 6% 13% 32% 33%	5340
	21st to end	Day	22% 9% 5% 11% 27% 27%	5844
February	1st to 10th	Day	19% 5% 7% 16% 28% 25%	2056
	11th to 20th	Day	21% 6% 6% 19% 26% 21%	2059
	21st to end	Day	17% 8% 4 10% 29% 31%	1730
March	1st to 10th	Day	19% 5% 5% 10% 26% 36%	4835
	11th to 20th	Day	14% 7% 10% 15% 28% 26%	5110
	21st to end	Day	17% 12% 6% 16% 32% 17%	5820
April	1st to 10th	Day	17% 6% 6% 17% 34% 20%	2520
	11th to 20th	Day	17% 4 4 13% 32% 29%	2494
	21st to end	Day	14% 5% 5% 16% 35% 24%	2520
May	1st to 10th	Day	13% 6% 6% 14% 36% 25%	5453
	11th to 20th	Day	13% 6% 7% 15% 30% 30%	5408
	21st to end	Day	9% 5% 7% 11% 22% 45%	6355
June	1st to 10th	Day	19% 7% 12% 20% 23% 19%	2800
	11th to 20th	Day	14% 5% 7% 16% 25% 34%	2800
	21st to end	Day	33% 10% 11% 17% 18% 11%	2746
July	1st to 10th	Day	33% 14% 16% 17% 13% 7%	5816
	11th to 20th	Day	30% 14% 19% 19% 15% 3	5811
	21st to end	Day	40% 18% 18% 15% 7% 1	6184
August	1st to 10th	Day	32% 12% 15% 29% 11% 2	3892
	11th to 20th	Day	33% 12% 16% 22% 16% 1	3882
	21st to end	Day	37% 15% 18% 22% 8% 1	4311
September	1st to 10th	Day	40% 9% 11% 16% 18% 7%	2400
	11th to 20th	Day	32% 8% 10% 19% 25% 7%	2400
	21st to end	Day	22% 6% 10% 21% 30% 11%	2200
October	1st to 10th	Day	19% 6% 6% 11% 33% 25%	3280
	11th to 20th	Day	19% 7% 6% 20% 30% 19%	3250
	21st to end	Day	8% 3 4 15% 39% 32%	3548
November	1st to 10th	Day	14% 4 6% 20% 35% 21%	2059
	11th to 20th	Day	17% 6% 6% 18% 29% 24%	1917
	21st to end	Day	20% 7% 7% 15% 30% 20%	1890
December	1st to 10th	Day	18% 6% 6% 17% 33% 21%	3030
	11th to 20th	Day	20% 5% 6% 11% 36% 22%	3170
	21st to end	Day	13% 7% 5% 16% 33% 26%	4349
January	1st to 10th	Night	21% 10% 7% 13% 35% 14%	7605
	11th to 20th	Night	10% 6% 4 14% 33% 33%	7650
	21st to end	Night	21% 11% 6% 10% 24% 28%	8370
February	1st to 10th	Night	17% 7% 7% 16% 25% 28%	2561
	11th to 20th	Night	19% 7% 8% 16% 29% 22%	2458
	21st to end	Night	17% 8% 5% 13% 28% 29%	2076
March	1st to 10th	Night	14% 6% 5% 13% 32% 29%	5252
	11th to 20th	Night	17% 8% 11% 12% 29% 24%	5110
	21st to end	Night	12% 13% 6% 18% 34% 17%	5386
April	1st to 10th	Night	15% 6% 8% 17% 35% 19%	2100
	11th to 20th	Night	14% 8% 8% 14% 30% 26%	2077
	21st to end	Night	14% 5% 7% 19% 34% 22%	2100
May	1st to 10th	Night	14% 6% 6% 15% 33% 26%	4547
	11th to 20th	Night	11% 6% 7% 16% 37% 23%	4511
	21st to end	Night	9% 3 5% 11% 18% 54%	4521
June	1st to 10th	Night	21% 8% 10% 19% 21% 21%	1600
	11th to 20th	Night	15% 3 6% 17% 21% 38%	1600
	21st to end	Night	32% 9% 11% 19% 14% 14%	1646
July	1st to 10th	Night	27% 12% 17% 25% 14% 5%	4055
	11th to 20th	Night	22% 11% 24% 26% 15% 3	4046
	21st to end	Night	28% 16% 26% 20% 10% 1	4360
August	1st to 10th	Night	21% 9% 19% 32% 16% 2	3244
	11th to 20th	Night	21% 9% 16% 33% 20% 1	3243
	21st to end	Night	28% 12% 21% 26% 12% 1	3597
September	1st to 10th	Night	29% 12% 12% 17% 20% 10%	2000
	11th to 20th	Night	23% 10% 11% 20% 26% 11%	2000
	21st to end	Night	18% 7% 10% 22% 32% 11%	2200
October	1st to 10th	Night	13% 5% 6% 13% 34% 28%	3905
	11th to 20th	Night	14% 5% 8% 18% 34% 20%	3898
	21st to end	Night	10% 3 5% 17% 34% 33%	4252
November	1st to 10th	Night	14% 5% 6% 18% 33% 23%	2460
	11th to 20th	Night	13% 7% 7% 18% 31% 24%	2678
	21st to end	Night	22% 7% 6% 16% 30% 19%	2730
December	1st to 10th	Night	15% 8% 6% 16% 36% 19%	4380
	11th to 20th	Night	17% 7% 7% 14% 27% 29%	5251
	21st to end	Night	16% 7% 5% 17% 31% 25%	6911

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% 32% 23% 18% 2	330
	11th to 20th	Day	24% 25% 26% 20% 4 1	330
	21st to end	Day	18% 30% 28% 15% 8%	363
February	1st to 10th	Day	23% 26% 25% 19% 6% 1	330
	11th to 20th	Day	29% 31% 25% 12% 3	330
	21st to end	Day	31% 33% 22% 10% 3 1	273
March	1st to 10th	Day	22% 37% 28% 11% 3	435
	11th to 20th	Day	28% 29% 27% 10% 6% 1	540
	21st to end	Day	25% 32% 31% 7% 3 1	594
April	1st to 10th	Day	18% 34% 34% 11% 2 1	540
	11th to 20th	Day	22% 36% 31% 9% 2	532
	21st to end	Day	22% 33% 31% 11% 2	540
May	1st to 10th	Day	8% 34% 43% 12% 3	508
	11th to 20th	Day	12% 24% 39% 18% 5% 1	528
	21st to end	Day	7% 23% 43% 21% 6% 1	663
June	1st to 10th	Day	4 10% 33% 35% 16% 2	730
	11th to 20th	Day	3 12% 34% 33% 16% 2	730
	21st to end	Day	7% 6% 26% 34% 24% 4	676
July	1st to 10th	Day	8% 4 20% 36% 27% 6%	526
	11th to 20th	Day	7% 1 12% 37% 34% 9%	524
	21st to end	Day	6% 2 10% 33% 39% 9%	575
August	1st to 10th	Day	6% 2 13% 35% 35% 10%	527
	11th to 20th	Day	9% 3 16% 35% 27% 8%	517
	21st to end	Day	9% 3 12% 37% 31% 7%	583
September	1st to 10th	Day	12% 5% 22% 38% 18% 4	530
	11th to 20th	Day	7% 7% 31% 35% 16% 4	530
	21st to end	Day	6% 11% 37% 28% 13% 5%	330
October	1st to 10th	Day	12% 14% 31% 28% 12% 4	330
	11th to 20th	Day	12% 16% 33% 25% 12% 2	330
	21st to end	Day	12% 26% 26% 20% 12% 4	363
November	1st to 10th	Day	15% 28% 29% 17% 8% 2	330
	11th to 20th	Day	16% 18% 31% 21% 13% 2	330
	21st to end	Day	22% 29% 24% 17% 7% 1	330
December	1st to 10th	Day	24% 26% 24% 20% 5% 1	330
	11th to 20th	Day	20% 28% 26% 19% 6% 1	330
	21st to end	Day	26% 28% 29% 14% 3	363
January	1st to 10th	Night	11% 22% 27% 26% 10% 3	840
	11th to 20th	Night	12% 21% 30% 25% 11% 2	840
	21st to end	Night	14% 21% 28% 24% 11% 1	924
February	1st to 10th	Night	11% 22% 33% 23% 10% 2	840
	11th to 20th	Night	17% 24% 34% 20% 4 1	824
	21st to end	Night	16% 31% 32% 17% 3 1	692
March	1st to 10th	Night	17% 26% 28% 22% 6% 1	840
	11th to 20th	Night	18% 20% 26% 26% 10% 1	840
	21st to end	Night	13% 26% 33% 21% 7%	829
April	1st to 10th	Night	12% 26% 38% 19% 6%	630
	11th to 20th	Night	14% 27% 31% 22% 5%	623
	21st to end	Night	13% 25% 33% 24% 5%	630
May	1st to 10th	Night	5% 23% 41% 25% 5%	603
	11th to 20th	Night	8% 17% 28% 34% 12% 1	591
	21st to end	Night	4 17% 36% 35% 7% 1	650
June	1st to 10th	Night	3 9% 28% 44% 15% 1	600
	11th to 20th	Night	4 7% 33% 40% 15% 2	600
	21st to end	Night	6% 4 21% 44% 24% 2	600
July	1st to 10th	Night	5% 3 16% 41% 32% 3	585
	11th to 20th	Night	4 1 11% 38% 40% 7%	582
	21st to end	Night	5% 2 11% 36% 36% 10%	657
August	1st to 10th	Night	4 2 6% 33% 42% 13%	597
	11th to 20th	Night	6% 2 11% 32% 39% 10%	594
	21st to end	Night	4 2 12% 35% 40% 7%	660
September	1st to 10th	Night	7% 3 14% 36% 34% 6%	600
	11th to 20th	Night	7% 4 16% 40% 30% 4	600
	21st to end	Night	4 6% 23% 41% 24% 3	600
October	1st to 10th	Night	3 11% 28% 37% 18% 3	785
	11th to 20th	Night	6% 11% 29% 35% 16% 3	800
	21st to end	Night	8% 16% 24% 27% 19% 6%	880
November	1st to 10th	Night	9% 18% 23% 29% 16% 5%	820
	11th to 20th	Night	9% 14% 25% 35% 14% 3	998
	21st to end	Night	13% 20% 25% 24% 14% 3	1050
December	1st to 10th	Night	15% 21% 24% 29% 11% 2	1050
	11th to 20th	Night	12% 23% 31% 25% 8% 1	1050
	21st to end	Night	16% 22% 29% 26% 7% 1	1055

Page generated at 2026-04-29 00:21:46. Version ac07

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