St. Thomas
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Thomas 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	15% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	132560
	Night	15% 11% 4 9% 5% 5% 4 4 5% 6% 32%	129894
Transparency	Day	50% 6% 13% 19% 11% 1	136159
	Night	50% 6% 12% 19% 12% 1	134384
Seeing	Day	18% 11% 28% 24% 14% 5%	16823
	Night	19% 9% 29% 29% 12% 1	26023

By Month

Percentage Cloud Cover NE (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% 15% 6% 19% 6% 5% 5% 3 4 5% 17%	5063
	11th to 20th	Day	10% 22% 10% 20% 9% 7% 4 3 3 4 9%	5246
	21st to end	Day	16% 17% 7% 14% 7% 5% 5% 3 4 5% 18%	5780
February	1st to 10th	Day	16% 16% 4 13% 7% 7% 5% 3 5% 6% 19%	1890
	11th to 20th	Day	16% 13% 7% 13% 7% 6% 5% 3 4 5% 21%	1843
	21st to end	Day	16% 14% 4 12% 7% 5% 6% 4 5% 6% 20%	1730
March	1st to 10th	Day	18% 11% 7% 12% 6% 4 4 3 5% 5% 27%	4939
	11th to 20th	Day	12% 12% 4 10% 6% 4 4 3 5% 8% 31%	5090
	21st to end	Day	19% 13% 5% 9% 5% 5% 4 3 4 5% 30%	5586
April	1st to 10th	Day	17% 13% 5% 8% 5% 4 4 3 5% 7% 29%	2388
	11th to 20th	Day	17% 11% 3 7% 5% 4 4 4 5% 7% 32%	2674
	21st to end	Day	19% 10% 3 7% 4 5% 4 3 4 6% 35%	2730
May	1st to 10th	Day	20% 10% 3 8% 5% 4 4 4 5% 7% 32%	5901
	11th to 20th	Day	17% 9% 3 7% 4 4 4 4 6% 8% 34%	5861
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 7% 39%	6940
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 40%	2800
	11th to 20th	Day	10% 6% 3 6% 5% 5% 5% 5% 6% 10% 39%	2933
	21st to end	Day	11% 5% 2 5% 5% 5% 5% 4 8% 9% 42%	3000
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 9% 43%	6802
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 7% 9% 45%	5910
	21st to end	Day	8% 5% 2 5% 5% 4 5% 4 7% 8% 47%	6424
August	1st to 10th	Day	15% 6% 1 5% 5% 4 5% 4 6% 9% 39%	4234
	11th to 20th	Day	9% 7% 2 7% 5% 5% 6% 4 8% 10% 36%	4174
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 8% 43%	4609
September	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 6% 8% 44%	2345
	11th to 20th	Day	9% 5% 1 5% 4 4 4 4 6% 8% 48%	2200
	21st to end	Day	16% 7% 3 7% 4 5% 5% 4 7% 7% 36%	2200
October	1st to 10th	Day	15% 7% 3 6% 5% 3 4 3 6% 8% 39%	2040
	11th to 20th	Day	18% 9% 3 7% 5% 4 6% 3 6% 7% 32%	2000
	21st to end	Day	23% 11% 3 9% 5% 4 5% 3 5% 7% 26%	2174
November	1st to 10th	Day	14% 10% 3 7% 5% 4 5% 5% 5% 8% 33%	1852
	11th to 20th	Day	16% 13% 3 7% 4 4 4 4 6% 8% 29%	1728
	21st to end	Day	19% 16% 5% 9% 5% 5% 6% 4 5% 6% 19%	1680
December	1st to 10th	Day	18% 17% 5% 11% 6% 5% 5% 4 6% 7% 16%	2680
	11th to 20th	Day	22% 19% 6% 9% 5% 4 4 4 5% 5% 18%	3066
	21st to end	Day	27% 19% 6% 12% 5% 5% 4 3 4 5% 10%	4048
January	1st to 10th	Night	15% 14% 6% 16% 6% 5% 5% 4 5% 6% 18%	7814
	11th to 20th	Night	11% 18% 7% 16% 9% 6% 5% 3 5% 4 15%	7639
	21st to end	Night	16% 14% 7% 14% 6% 5% 5% 3 4 5% 22%	8401
February	1st to 10th	Night	16% 16% 5% 10% 6% 5% 4 4 6% 6% 23%	2730
	11th to 20th	Night	16% 13% 5% 9% 6% 5% 4 4 6% 6% 26%	2662
	21st to end	Night	17% 13% 4 10% 7% 5% 6% 5% 6% 5% 23%	2076
March	1st to 10th	Night	16% 10% 3 9% 6% 5% 4 4 5% 6% 32%	5119
	11th to 20th	Night	10% 8% 3 8% 5% 5% 4 4 5% 8% 38%	5089
	21st to end	Night	18% 10% 4 8% 4 4 4 3 4 4 36%	5588
April	1st to 10th	Night	19% 10% 3 7% 5% 4 5% 3 5% 7% 32%	2231
	11th to 20th	Night	17% 8% 3 6% 5% 5% 4 3 5% 7% 36%	1913
	21st to end	Night	15% 8% 2 6% 6% 4 5% 4 5% 7% 37%	1890
May	1st to 10th	Night	21% 9% 4 7% 4 4 4 3 4 6% 33%	4062
	11th to 20th	Night	14% 9% 3 8% 5% 5% 5% 4 5% 6% 37%	4032
	21st to end	Night	17% 6% 2 5% 3 4 4 4 5% 7% 43%	3935
June	1st to 10th	Night	13% 5% 2 5% 4 4 4 5% 6% 8% 45%	1600
	11th to 20th	Night	9% 5% 2 5% 4 4 6% 5% 9% 9% 43%	1459
	21st to end	Night	10% 5% 2 5% 4 5% 6% 4 5% 7% 47%	1400
July	1st to 10th	Night	7% 5% 2 6% 4 5% 5% 3 6% 10% 47%	3160
	11th to 20th	Night	9% 6% 2 6% 4 3 4 4 5% 8% 50%	4032
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 51%	4428
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	2915
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 50%	2875
	21st to end	Night	8% 4 3 6% 4 4 4 5% 6% 9% 49%	3256
September	1st to 10th	Night	8% 7% 2 7% 5% 4 4 3 6% 6% 48%	2050
	11th to 20th	Night	9% 4 2 5% 3 3 4 4 5% 6% 55%	2200
	21st to end	Night	16% 6% 3 6% 4 5% 5% 3 5% 6% 41%	2200
October	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 46%	2360
	11th to 20th	Night	18% 10% 3 7% 4 3 4 3 5% 6% 38%	2400
	21st to end	Night	21% 8% 3 8% 5% 4 4 3 6% 6% 32%	2665
November	1st to 10th	Night	14% 9% 3 6% 5% 4 4 4 6% 6% 40%	2666
	11th to 20th	Night	17% 10% 3 6% 6% 5% 5% 5% 6% 6% 30%	2889
	21st to end	Night	21% 13% 4 8% 5% 5% 4 4 5% 6% 24%	2940
December	1st to 10th	Night	14% 16% 6% 12% 6% 6% 5% 4 6% 7% 17%	4720
	11th to 20th	Night	16% 21% 5% 10% 5% 5% 4 3 4 5% 22%	5348
	21st to end	Night	24% 18% 5% 12% 6% 4 4 3 4 5% 15%	7150

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	69% 5% 12% 9% 4	5064
	11th to 20th	Day	79% 5% 7% 5% 3	5280
	21st to end	Day	69% 5% 10% 8% 7% 1	5780
February	1st to 10th	Day	67% 5% 10% 12% 5% 1	1890
	11th to 20th	Day	66% 4 10% 14% 6%	1852
	21st to end	Day	63% 6% 12% 13% 5% 1	1730
March	1st to 10th	Day	60% 5% 13% 16% 5%	4942
	11th to 20th	Day	50% 4 19% 19% 7% 1	5090
	21st to end	Day	57% 4 12% 17% 9% 1	5586
April	1st to 10th	Day	55% 4 11% 19% 10% 1	2388
	11th to 20th	Day	51% 5% 11% 19% 12% 1	2674
	21st to end	Day	50% 5% 12% 19% 12% 2	2730
May	1st to 10th	Day	50% 6% 12% 17% 10% 5%	5910
	11th to 20th	Day	47% 9% 12% 20% 11% 1	5865
	21st to end	Day	44% 6% 11% 19% 16% 5%	6940
June	1st to 10th	Day	41% 7% 12% 25% 13% 2	2800
	11th to 20th	Day	39% 7% 14% 22% 16% 2	2933
	21st to end	Day	36% 7% 15% 25% 16% 1	3000
July	1st to 10th	Day	35% 7% 14% 26% 16% 2	6729
	11th to 20th	Day	34% 6% 15% 29% 15% 2	5840
	21st to end	Day	32% 7% 15% 24% 20% 1	6240
August	1st to 10th	Day	40% 7% 16% 23% 14% 1	4221
	11th to 20th	Day	40% 8% 15% 27% 9% 1	4213
	21st to end	Day	36% 7% 15% 29% 12% 1	4609
September	1st to 10th	Day	37% 7% 16% 25% 13% 1	2345
	11th to 20th	Day	32% 7% 14% 31% 16%	2200
	21st to end	Day	45% 7% 13% 23% 11% 1	2200
October	1st to 10th	Day	35% 7% 16% 24% 18% 1	3326
	11th to 20th	Day	40% 5% 14% 24% 17%	3240
	21st to end	Day	48% 6% 16% 22% 7%	3488
November	1st to 10th	Day	47% 8% 12% 22% 8% 2	1852
	11th to 20th	Day	52% 7% 16% 15% 10% 1	1728
	21st to end	Day	66% 6% 13% 13% 3	1680
December	1st to 10th	Day	66% 6% 15% 11% 3	2680
	11th to 20th	Day	67% 6% 13% 7% 7%	3066
	21st to end	Day	77% 7% 8% 6% 2	4048
January	1st to 10th	Night	65% 6% 14% 11% 4	7814
	11th to 20th	Night	71% 6% 9% 9% 5%	7680
	21st to end	Night	63% 5% 10% 12% 9%	8401
February	1st to 10th	Night	61% 6% 11% 16% 6%	2730
	11th to 20th	Night	58% 5% 13% 16% 7%	2664
	21st to end	Night	60% 7% 10% 16% 6% 1	2076
March	1st to 10th	Night	51% 5% 14% 18% 10% 1	5129
	11th to 20th	Night	42% 4 20% 21% 12% 1	5090
	21st to end	Night	51% 4 12% 17% 16% 2	5588
April	1st to 10th	Night	52% 5% 11% 18% 13% 2	2231
	11th to 20th	Night	47% 5% 10% 20% 16% 2	1913
	21st to end	Night	45% 5% 11% 20% 16% 3	1890
May	1st to 10th	Night	52% 5% 9% 14% 12% 8%	4069
	11th to 20th	Night	47% 8% 10% 22% 11% 3	4033
	21st to end	Night	39% 7% 12% 21% 19% 2	3936
June	1st to 10th	Night	36% 8% 11% 26% 15% 4	1600
	11th to 20th	Night	33% 9% 13% 25% 18% 2	1459
	21st to end	Night	36% 6% 12% 25% 20% 1	1400
July	1st to 10th	Night	31% 6% 13% 27% 17% 5%	3125
	11th to 20th	Night	32% 6% 12% 29% 18% 2	3980
	21st to end	Night	31% 6% 13% 21% 25% 4	4297
August	1st to 10th	Night	36% 6% 13% 25% 17% 2	2904
	11th to 20th	Night	34% 6% 14% 32% 13% 1	2902
	21st to end	Night	31% 6% 12% 33% 16% 1	3256
September	1st to 10th	Night	35% 6% 12% 29% 16% 2	2050
	11th to 20th	Night	29% 5% 12% 33% 19% 1	2200
	21st to end	Night	43% 6% 12% 25% 13% 1	2200
October	1st to 10th	Night	28% 5% 9% 30% 26% 2	3854
	11th to 20th	Night	42% 3 14% 22% 18%	3899
	21st to end	Night	40% 5% 17% 25% 12% 1	4301
November	1st to 10th	Night	42% 6% 13% 24% 14% 1	2666
	11th to 20th	Night	52% 6% 13% 19% 10% 1	2889
	21st to end	Night	60% 6% 16% 14% 4 1	2940
December	1st to 10th	Night	65% 7% 15% 10% 3	4720
	11th to 20th	Night	65% 5% 11% 11% 8%	5348
	21st to end	Night	71% 7% 11% 9% 3	7150

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	23% 18% 37% 15% 5% 1	294
	11th to 20th	Day	24% 18% 42% 11% 4 2	330
	21st to end	Day	27% 16% 31% 21% 4 1	363
February	1st to 10th	Day	24% 16% 38% 18% 3	330
	11th to 20th	Day	24% 20% 36% 17% 2	330
	21st to end	Day	21% 23% 41% 14% 1	273
March	1st to 10th	Day	25% 13% 36% 21% 5% 1	485
	11th to 20th	Day	19% 18% 35% 22% 5% 1	540
	21st to end	Day	28% 16% 29% 20% 6% 1	594
April	1st to 10th	Day	23% 16% 33% 21% 7% 1	540
	11th to 20th	Day	24% 18% 28% 17% 10% 3	532
	21st to end	Day	24% 11% 33% 23% 7% 3	540
May	1st to 10th	Day	23% 13% 27% 22% 12% 3	507
	11th to 20th	Day	18% 8% 27% 25% 17% 5%	527
	21st to end	Day	14% 6% 25% 30% 17% 8%	573
June	1st to 10th	Day	14% 7% 33% 28% 14% 4	530
	11th to 20th	Day	10% 8% 27% 33% 17% 5%	663
	21st to end	Day	12% 5% 31% 31% 17% 3	730
July	1st to 10th	Day	10% 6% 21% 31% 22% 10%	721
	11th to 20th	Day	13% 5% 24% 27% 20% 11%	533
	21st to end	Day	10% 3 19% 35% 25% 9%	576
August	1st to 10th	Day	10% 3 18% 31% 24% 13%	526
	11th to 20th	Day	10% 4 18% 29% 25% 13%	519
	21st to end	Day	11% 4 19% 29% 26% 10%	583
September	1st to 10th	Day	13% 3 22% 27% 22% 12%	530
	11th to 20th	Day	7% 3 19% 23% 33% 16%	530
	21st to end	Day	17% 8% 25% 25% 16% 8%	530
October	1st to 10th	Day	18% 9% 25% 22% 20% 6%	530
	11th to 20th	Day	18% 13% 35% 23% 8% 2	530
	21st to end	Day	27% 14% 33% 20% 6% 1	557
November	1st to 10th	Day	21% 20% 33% 18% 7% 2	330
	11th to 20th	Day	24% 22% 29% 17% 5% 2	245
	21st to end	Day	25% 19% 34% 20% 2	220
December	1st to 10th	Day	28% 17% 30% 18% 5% 2	220
	11th to 20th	Day	28% 22% 31% 14% 5%	220
	21st to end	Day	29% 17% 34% 17% 2 1	242
January	1st to 10th	Night	23% 17% 36% 17% 5% 1	840
	11th to 20th	Night	29% 16% 32% 20% 3	840
	21st to end	Night	21% 13% 37% 23% 6%	924
February	1st to 10th	Night	24% 13% 36% 24% 3	840
	11th to 20th	Night	22% 15% 33% 25% 4	823
	21st to end	Night	23% 14% 38% 22% 3	692
March	1st to 10th	Night	20% 11% 35% 27% 6%	840
	11th to 20th	Night	20% 11% 38% 23% 6% 1	840
	21st to end	Night	23% 10% 32% 26% 8% 1	924
April	1st to 10th	Night	24% 12% 32% 27% 5%	840
	11th to 20th	Night	20% 11% 33% 25% 9% 1	669
	21st to end	Night	20% 7% 31% 33% 8%	630
May	1st to 10th	Night	21% 6% 26% 33% 13% 1	603
	11th to 20th	Night	14% 3 29% 38% 14% 2	591
	21st to end	Night	14% 3 25% 37% 20% 2	443
June	1st to 10th	Night	15% 3 26% 41% 15% 1	400
	11th to 20th	Night	12% 3 26% 44% 16%	400
	21st to end	Night	12% 4 25% 42% 18% 1	400
July	1st to 10th	Night	5% 1 20% 44% 26% 4	390
	11th to 20th	Night	12% 3 15% 39% 28% 4	582
	21st to end	Night	10% 2 15% 46% 26% 2	657
August	1st to 10th	Night	9% 2 14% 39% 31% 4	597
	11th to 20th	Night	7% 2 14% 39% 34% 4	594
	21st to end	Night	10% 2 18% 39% 27% 4	680
September	1st to 10th	Night	11% 5% 19% 34% 26% 5%	800
	11th to 20th	Night	10% 3 15% 35% 32% 5%	800
	21st to end	Night	17% 5% 22% 34% 17% 6%	800
October	1st to 10th	Night	17% 5% 24% 32% 19% 3	800
	11th to 20th	Night	22% 11% 28% 26% 12% 2	800
	21st to end	Night	24% 10% 32% 28% 6%	880
November	1st to 10th	Night	18% 12% 28% 29% 13% 1	820
	11th to 20th	Night	23% 13% 36% 21% 5% 2	840
	21st to end	Night	29% 12% 31% 23% 6%	840
December	1st to 10th	Night	27% 15% 34% 18% 5% 1	840
	11th to 20th	Night	24% 17% 37% 18% 4	840
	21st to end	Night	29% 13% 34% 21% 3	924

Page generated at 2026-04-29 02:21:40. Version ac07

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