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 2025-01-07.

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% 3 8% 5% 5% 5% 4 6% 7% 32%	105516
	Night	15% 11% 4 8% 5% 4 4 4 5% 6% 33%	103496
Transparency	Day	49% 7% 13% 19% 11% 1	109125
	Night	49% 6% 13% 19% 12% 1	107977
Seeing	Day	18% 10% 28% 25% 14% 5%	15108
	Night	19% 9% 29% 29% 13% 2	24314

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	17% 18% 5% 12% 6% 5% 4 3 4 4 21%	2774
	11th to 20th	Day	13% 20% 8% 18% 7% 5% 4 3 4 5% 13%	2840
	21st to end	Day	25% 21% 8% 11% 5% 4 4 2 3 4 14%	3124
February	1st to 10th	Day	17% 16% 4 12% 7% 7% 5% 3 5% 6% 18%	1710
	11th to 20th	Day	16% 13% 7% 13% 7% 6% 6% 3 4 5% 21%	1663
	21st to end	Day	17% 14% 4 11% 7% 4 6% 4 5% 6% 21%	1570
March	1st to 10th	Day	21% 12% 5% 10% 5% 4 4 3 5% 4 28%	3360
	11th to 20th	Day	12% 14% 4 10% 6% 5% 4 4 5% 7% 29%	3480
	21st to end	Day	18% 12% 4 9% 5% 5% 4 3 4 5% 32%	3815
April	1st to 10th	Day	16% 13% 4 8% 5% 4 4 3 6% 7% 30%	2160
	11th to 20th	Day	18% 11% 3 6% 5% 4 4 4 6% 7% 32%	2416
	21st to end	Day	20% 10% 3 7% 4 5% 4 3 4 6% 34%	2470
May	1st to 10th	Day	19% 8% 3 7% 5% 4 5% 4 5% 8% 33%	4108
	11th to 20th	Day	16% 9% 3 7% 4 5% 5% 4 7% 8% 32%	4081
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 8% 46%	4828
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	2660
	11th to 20th	Day	10% 6% 3 6% 5% 4 5% 5% 6% 10% 39%	2786
	21st to end	Day	11% 6% 2 5% 5% 5% 4 4 8% 9% 41%	2850
July	1st to 10th	Day	10% 5% 2 6% 4 5% 6% 5% 7% 9% 42%	4743
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 7% 10% 44%	4130
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	4466
August	1st to 10th	Day	15% 6% 1 5% 5% 4 5% 4 6% 9% 39%	4104
	11th to 20th	Day	9% 7% 2 7% 5% 5% 6% 4 8% 10% 35%	4044
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 9% 42%	4468
September	1st to 10th	Day	10% 6% 2 6% 6% 4 5% 4 6% 8% 43%	2228
	11th to 20th	Day	10% 6% 1 5% 4 5% 5% 4 7% 8% 47%	2090
	21st to end	Day	16% 8% 3 7% 4 5% 5% 4 7% 7% 36%	2090
October	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 4 6% 8% 37%	1938
	11th to 20th	Day	18% 9% 3 8% 5% 4 6% 3 6% 7% 31%	1900
	21st to end	Day	23% 11% 3 9% 5% 4 5% 4 5% 7% 25%	2066
November	1st to 10th	Day	14% 9% 3 7% 6% 4 5% 5% 5% 8% 33%	1762
	11th to 20th	Day	16% 13% 3 7% 4 4 4 4 6% 8% 29%	1646
	21st to end	Day	20% 16% 5% 9% 5% 5% 6% 4 5% 6% 19%	1600
December	1st to 10th	Day	18% 17% 5% 11% 6% 5% 5% 4 6% 7% 17%	2600
	11th to 20th	Day	22% 19% 6% 9% 5% 4 4 4 4 5% 18%	2986
	21st to end	Day	27% 20% 6% 11% 5% 5% 4 3 4 5% 10%	3960
January	1st to 10th	Night	17% 16% 7% 12% 5% 5% 5% 4 4 5% 20%	4287
	11th to 20th	Night	15% 18% 7% 14% 7% 6% 5% 4 5% 5% 13%	4120
	21st to end	Night	25% 17% 7% 13% 5% 4 4 3 4 5% 15%	4532
February	1st to 10th	Night	17% 16% 5% 10% 6% 5% 4 4 6% 6% 22%	2470
	11th to 20th	Night	16% 13% 5% 9% 6% 5% 4 4 6% 6% 26%	2403
	21st to end	Night	18% 13% 4 10% 6% 5% 6% 5% 6% 5% 23%	1884
March	1st to 10th	Night	18% 10% 4 9% 6% 4 4 3 4 6% 32%	3478
	11th to 20th	Night	12% 9% 3 8% 5% 6% 5% 4 5% 7% 37%	3479
	21st to end	Night	17% 10% 4 8% 4 4 4 4 4 4 38%	3817
April	1st to 10th	Night	18% 10% 3 6% 4 4 5% 3 5% 7% 34%	2019
	11th to 20th	Night	18% 7% 3 6% 5% 5% 4 3 5% 7% 36%	1729
	21st to end	Night	16% 8% 3 6% 6% 4 5% 4 5% 7% 36%	1710
May	1st to 10th	Night	20% 8% 3 5% 3 4 4 4 5% 7% 36%	2833
	11th to 20th	Night	11% 9% 2 8% 5% 5% 6% 4 6% 7% 36%	2812
	21st to end	Night	11% 5% 2 4 3 4 4 4 5% 7% 50%	2747
June	1st to 10th	Night	13% 5% 2 5% 5% 3 4 5% 6% 8% 44%	1520
	11th to 20th	Night	9% 5% 2 4 4 4 6% 5% 9% 9% 43%	1386
	21st to end	Night	11% 5% 2 5% 4 5% 6% 4 6% 7% 46%	1330
July	1st to 10th	Night	7% 4 2 5% 4 4 4 3 6% 11% 51%	2210
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 5% 8% 52%	2825
	21st to end	Night	6% 5% 1 5% 5% 5% 4 5% 7% 9% 49%	3087
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	2825
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 50%	2785
	21st to end	Night	8% 5% 3 6% 4 4 4 4 6% 9% 49%	3156
September	1st to 10th	Night	9% 7% 2 7% 5% 4 4 3 6% 6% 48%	1947
	11th to 20th	Night	9% 4 2 5% 3 3 4 4 4 7% 54%	2090
	21st to end	Night	16% 5% 3 6% 4 5% 5% 4 5% 6% 40%	2090
October	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 6% 44%	2242
	11th to 20th	Night	18% 10% 3 7% 4 3 4 3 5% 6% 37%	2280
	21st to end	Night	22% 9% 3 8% 5% 4 4 3 6% 6% 30%	2532
November	1st to 10th	Night	14% 9% 3 6% 5% 4 4 4 6% 7% 40%	2536
	11th to 20th	Night	17% 10% 3 6% 6% 5% 5% 5% 6% 6% 30%	2751
	21st to end	Night	22% 13% 4 8% 5% 5% 4 4 6% 6% 24%	2800
December	1st to 10th	Night	15% 16% 6% 12% 6% 6% 5% 4 6% 7% 17%	4580
	11th to 20th	Night	16% 22% 5% 10% 5% 5% 4 3 4 5% 21%	5208
	21st to end	Night	24% 18% 5% 11% 6% 4 4 3 4 5% 15%	6996

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	67% 7% 11% 10% 5%	2775
	11th to 20th	Day	72% 7% 11% 6% 3	2840
	21st to end	Day	76% 6% 8% 6% 3	3124
February	1st to 10th	Day	67% 5% 10% 12% 5% 1	1710
	11th to 20th	Day	65% 4 10% 14% 7%	1672
	21st to end	Day	62% 6% 12% 13% 6% 1	1570
March	1st to 10th	Day	60% 6% 9% 20% 5% 1	3363
	11th to 20th	Day	53% 5% 18% 17% 7% 1	3480
	21st to end	Day	53% 5% 13% 17% 11% 1	3815
April	1st to 10th	Day	54% 4 12% 19% 11% 1	2160
	11th to 20th	Day	50% 6% 11% 19% 13% 1	2416
	21st to end	Day	51% 5% 12% 18% 12% 2	2470
May	1st to 10th	Day	47% 7% 14% 19% 11% 2	4120
	11th to 20th	Day	47% 11% 13% 19% 8% 1	4085
	21st to end	Day	35% 7% 13% 21% 18% 5%	4828
June	1st to 10th	Day	40% 7% 13% 25% 13% 2	2660
	11th to 20th	Day	39% 7% 14% 22% 17% 2	2786
	21st to end	Day	37% 7% 14% 25% 15% 1	2850
July	1st to 10th	Day	37% 7% 14% 25% 15% 3	4669
	11th to 20th	Day	33% 6% 15% 31% 14% 1	4060
	21st to end	Day	35% 8% 17% 20% 19% 2	4324
August	1st to 10th	Day	40% 7% 15% 23% 13% 1	4091
	11th to 20th	Day	40% 8% 16% 26% 9% 1	4083
	21st to end	Day	36% 7% 16% 28% 12% 1	4468
September	1st to 10th	Day	38% 8% 16% 24% 13% 1	2228
	11th to 20th	Day	33% 7% 14% 29% 16%	2090
	21st to end	Day	45% 7% 13% 23% 11% 1	2090
October	1st to 10th	Day	35% 7% 16% 23% 18% 1	3224
	11th to 20th	Day	41% 5% 14% 24% 16%	3140
	21st to end	Day	48% 7% 16% 23% 6%	3380
November	1st to 10th	Day	47% 8% 13% 22% 8% 2	1762
	11th to 20th	Day	52% 7% 16% 14% 11% 1	1646
	21st to end	Day	66% 6% 13% 13% 3	1600
December	1st to 10th	Day	66% 6% 15% 11% 3	2600
	11th to 20th	Day	67% 6% 13% 7% 7%	2986
	21st to end	Day	77% 7% 8% 6% 2	3960
January	1st to 10th	Night	65% 6% 14% 10% 4	4287
	11th to 20th	Night	72% 7% 10% 8% 3	4120
	21st to end	Night	71% 6% 11% 8% 3	4532
February	1st to 10th	Night	62% 7% 11% 14% 6%	2470
	11th to 20th	Night	59% 5% 12% 16% 7%	2405
	21st to end	Night	59% 7% 10% 17% 6% 1	1884
March	1st to 10th	Night	54% 5% 13% 21% 7%	3488
	11th to 20th	Night	43% 5% 19% 18% 13% 1	3480
	21st to end	Night	48% 5% 12% 19% 15% 2	3817
April	1st to 10th	Night	50% 5% 11% 19% 13% 2	2019
	11th to 20th	Night	47% 5% 10% 20% 16% 2	1729
	21st to end	Night	46% 5% 10% 20% 15% 3	1710
May	1st to 10th	Night	46% 6% 10% 19% 14% 5%	2840
	11th to 20th	Night	44% 10% 12% 22% 10% 2	2813
	21st to end	Night	31% 8% 12% 23% 24% 3	2748
June	1st to 10th	Night	36% 8% 10% 27% 15% 3	1520
	11th to 20th	Night	33% 9% 13% 24% 19% 2	1386
	21st to end	Night	36% 6% 12% 26% 19% 1	1330
July	1st to 10th	Night	29% 6% 11% 29% 18% 7%	2175
	11th to 20th	Night	30% 6% 13% 32% 18% 2	2773
	21st to end	Night	31% 7% 17% 22% 20% 4	2988
August	1st to 10th	Night	37% 6% 13% 26% 17% 2	2814
	11th to 20th	Night	34% 6% 15% 32% 13% 1	2812
	21st to end	Night	31% 6% 12% 32% 17% 1	3156
September	1st to 10th	Night	36% 6% 12% 27% 16% 2	1947
	11th to 20th	Night	30% 5% 12% 32% 19% 1	2090
	21st to end	Night	43% 6% 13% 25% 13% 1	2090
October	1st to 10th	Night	29% 5% 9% 30% 26% 2	3736
	11th to 20th	Night	43% 3 14% 22% 17%	3779
	21st to end	Night	40% 5% 17% 26% 12% 1	4168
November	1st to 10th	Night	42% 6% 12% 24% 14% 1	2536
	11th to 20th	Night	52% 6% 13% 18% 10% 1	2751
	21st to end	Night	59% 6% 16% 14% 4 1	2800
December	1st to 10th	Night	65% 7% 16% 9% 3	4580
	11th to 20th	Night	65% 5% 11% 11% 8%	5208
	21st to end	Night	71% 7% 11% 9% 3	6996

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	22% 19% 34% 17% 6% 1	255
	11th to 20th	Day	24% 14% 43% 13% 5% 2	270
	21st to end	Day	27% 13% 31% 24% 4 1	297
February	1st to 10th	Day	25% 16% 39% 16% 3	270
	11th to 20th	Day	23% 19% 37% 18% 2	270
	21st to end	Day	23% 25% 38% 13% 1	225
March	1st to 10th	Day	24% 11% 35% 22% 6% 1	410
	11th to 20th	Day	18% 17% 35% 24% 5% 1	460
	21st to end	Day	28% 15% 28% 21% 7% 1	506
April	1st to 10th	Day	19% 15% 33% 23% 8% 1	460
	11th to 20th	Day	24% 17% 27% 17% 10% 4	452
	21st to end	Day	23% 13% 31% 23% 8% 3	460
May	1st to 10th	Day	22% 13% 26% 24% 13% 3	463
	11th to 20th	Day	18% 8% 26% 26% 18% 5%	487
	21st to end	Day	13% 5% 24% 32% 18% 8%	529
June	1st to 10th	Day	13% 6% 32% 30% 15% 4	490
	11th to 20th	Day	10% 7% 25% 34% 18% 5%	616
	21st to end	Day	13% 5% 31% 31% 17% 4	680
July	1st to 10th	Day	9% 5% 20% 32% 24% 10%	671
	11th to 20th	Day	14% 4 24% 27% 20% 11%	493
	21st to end	Day	10% 3 18% 35% 26% 9%	532
August	1st to 10th	Day	10% 4 19% 29% 24% 14%	486
	11th to 20th	Day	10% 4 18% 29% 25% 13%	479
	21st to end	Day	12% 4 18% 29% 26% 11%	539
September	1st to 10th	Day	13% 3 22% 29% 22% 11%	490
	11th to 20th	Day	7% 3 20% 23% 31% 15%	490
	21st to end	Day	17% 8% 26% 25% 16% 8%	490
October	1st to 10th	Day	19% 9% 25% 22% 19% 7%	490
	11th to 20th	Day	18% 14% 33% 24% 9% 2	490
	21st to end	Day	27% 13% 32% 21% 6% 1	515
November	1st to 10th	Day	19% 20% 33% 18% 7% 3	300
	11th to 20th	Day	22% 22% 30% 18% 5% 3	223
	21st to end	Day	25% 17% 36% 20% 2	200
December	1st to 10th	Day	28% 17% 30% 19% 6% 2	200
	11th to 20th	Day	28% 21% 32% 15% 4	200
	21st to end	Day	28% 18% 34% 17% 2 1	220
January	1st to 10th	Night	23% 16% 37% 17% 6% 1	785
	11th to 20th	Night	30% 13% 31% 22% 4	760
	21st to end	Night	22% 10% 36% 25% 6%	836
February	1st to 10th	Night	25% 11% 37% 24% 3	760
	11th to 20th	Night	22% 14% 33% 26% 4 1	743
	21st to end	Night	24% 13% 38% 23% 3	628
March	1st to 10th	Night	20% 10% 36% 27% 7%	760
	11th to 20th	Night	19% 11% 38% 24% 7% 1	760
	21st to end	Night	23% 8% 32% 28% 9% 1	836
April	1st to 10th	Night	23% 11% 33% 28% 5%	760
	11th to 20th	Night	22% 9% 32% 25% 10% 1	605
	21st to end	Night	20% 6% 31% 34% 9%	570
May	1st to 10th	Night	21% 5% 26% 33% 14% 1	570
	11th to 20th	Night	15% 3 28% 39% 14% 2	561
	21st to end	Night	12% 3 24% 38% 21% 2	421
June	1st to 10th	Night	15% 2 25% 41% 16% 1	380
	11th to 20th	Night	11% 3 26% 43% 16%	380
	21st to end	Night	12% 3 24% 42% 17% 1	380
July	1st to 10th	Night	6% 1 20% 43% 26% 4	370
	11th to 20th	Night	12% 2 15% 39% 28% 4	552
	21st to end	Night	9% 1 16% 45% 26% 2	624
August	1st to 10th	Night	10% 2 15% 39% 31% 5%	567
	11th to 20th	Night	7% 2 13% 39% 34% 4	564
	21st to end	Night	11% 2 18% 39% 27% 4	646
September	1st to 10th	Night	11% 5% 19% 34% 26% 5%	760
	11th to 20th	Night	11% 3 16% 35% 31% 5%	760
	21st to end	Night	17% 5% 22% 33% 17% 6%	760
October	1st to 10th	Night	18% 5% 24% 32% 19% 3	760
	11th to 20th	Night	22% 11% 27% 26% 13% 2	760
	21st to end	Night	25% 9% 32% 27% 6%	836
November	1st to 10th	Night	18% 12% 28% 28% 13% 1	780
	11th to 20th	Night	23% 12% 37% 21% 6% 2	800
	21st to end	Night	28% 11% 31% 24% 6%	800
December	1st to 10th	Night	27% 15% 34% 18% 5% 1	800
	11th to 20th	Night	24% 17% 36% 18% 4	800
	21st to end	Night	30% 12% 34% 22% 3	880

Page generated at 2025-01-04 03:23:58. Version ac07

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