St. Louis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Louis clear sky chart from 2005-11-06 to 2024-03-28.

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	13% 7% 3 6% 5% 3 5% 5% 5% 7% 41%	78827
	Night	14% 8% 3 6% 5% 3 4 4 5% 6% 42%	81214
Transparency	Day	40% 7% 12% 23% 15% 2	78535
	Night	41% 6% 11% 23% 16% 3	81027
Seeing	Day	14% 10% 25% 27% 18% 6%	14683
	Night	16% 8% 29% 32% 14% 1	24709

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	11% 10% 6% 9% 7% 4 5% 6% 5% 7% 30%	2527
	11th to 20th	Day	14% 8% 3 7% 8% 7% 6% 6% 5% 6% 30%	2699
	21st to end	Day	22% 16% 6% 7% 5% 2 3 3 3 4 28%	3135
February	1st to 10th	Day	16% 9% 5% 9% 4 3 5% 5% 5% 6% 33%	1710
	11th to 20th	Day	15% 7% 5% 6% 6% 3 5% 5% 4 7% 37%	1846
	21st to end	Day	15% 9% 3 7% 5% 3 4 3 5% 6% 40%	1570
March	1st to 10th	Day	14% 8% 4 6% 5% 3 5% 4 4 6% 41%	1907
	11th to 20th	Day	19% 9% 4 6% 5% 3 4 4 4 6% 37%	2090
	21st to end	Day	20% 10% 5% 7% 6% 3 5% 5% 5% 7% 28%	2383
April	1st to 10th	Day	11% 5% 3 6% 6% 3 5% 5% 7% 7% 43%	2160
	11th to 20th	Day	16% 8% 3 6% 5% 3 5% 4 4 5% 41%	2134
	21st to end	Day	17% 7% 3 6% 6% 3 5% 4 5% 6% 37%	2160
May	1st to 10th	Day	16% 6% 3 6% 5% 3 6% 6% 6% 7% 37%	2226
	11th to 20th	Day	16% 5% 3 6% 5% 3 4 5% 5% 8% 40%	2429
	21st to end	Day	10% 6% 3 6% 6% 4 6% 6% 8% 9% 37%	2725
June	1st to 10th	Day	8% 4 2 4 5% 4 5% 5% 7% 9% 47%	2520
	11th to 20th	Day	6% 4 2 4 5% 3 5% 5% 7% 9% 49%	2520
	21st to end	Day	4 3 2 5% 6% 3 5% 6% 8% 11% 47%	2520
July	1st to 10th	Day	9% 3 1 4 5% 4 6% 6% 8% 11% 43%	2520
	11th to 20th	Day	5% 3 2 4 4 4 4 6% 7% 11% 51%	2520
	21st to end	Day	8% 4 2 4 6% 3 6% 5% 6% 9% 48%	2631
August	1st to 10th	Day	7% 5% 2 5% 5% 4 6% 7% 7% 10% 43%	2306
	11th to 20th	Day	9% 4 2 4 6% 5% 6% 6% 7% 9% 42%	2125
	21st to end	Day	7% 5% 2 4 5% 3 5% 5% 6% 10% 48%	2376
September	1st to 10th	Day	13% 5% 2 4 4 3 3 4 5% 7% 49%	2160
	11th to 20th	Day	8% 5% 2 5% 4 3 4 4 5% 9% 50%	2125
	21st to end	Day	7% 5% 2 5% 4 3 5% 4 5% 8% 54%	1980
October	1st to 10th	Day	10% 4 2 4 5% 3 4 3 4 6% 55%	1814
	11th to 20th	Day	13% 6% 2 5% 4 2 3 3 4 7% 51%	1800
	21st to end	Day	19% 6% 3 4 4 3 3 3 4 5% 47%	1948
November	1st to 10th	Day	10% 5% 2 4 4 3 4 4 5% 7% 54%	1672
	11th to 20th	Day	15% 6% 3 5% 5% 2 4 3 5% 7% 46%	1710
	21st to end	Day	15% 9% 4 6% 5% 3 4 3 3 5% 41%	1658
December	1st to 10th	Day	14% 10% 3 7% 5% 3 4 4 5% 8% 37%	1520
	11th to 20th	Day	14% 10% 4 7% 5% 4 4 5% 4 7% 37%	1918
	21st to end	Day	21% 13% 5% 8% 6% 3 5% 3 4 5% 26%	2783
January	1st to 10th	Night	15% 9% 6% 8% 6% 3 4 4 5% 5% 35%	4428
	11th to 20th	Night	14% 10% 5% 10% 7% 5% 7% 4 4 5% 30%	4266
	21st to end	Night	24% 14% 5% 7% 5% 3 3 3 4 5% 26%	4532
February	1st to 10th	Night	16% 10% 4 7% 7% 4 3 4 4 6% 35%	2470
	11th to 20th	Night	14% 7% 3 7% 6% 3 4 4 4 6% 42%	2218
	21st to end	Night	15% 9% 4 7% 6% 3 4 3 5% 6% 39%	1884
March	1st to 10th	Night	16% 9% 4 5% 5% 3 4 4 4 5% 43%	2280
	11th to 20th	Night	18% 7% 3 7% 5% 2 3 4 5% 6% 39%	2098
	21st to end	Night	18% 10% 4 6% 6% 3 4 5% 5% 6% 33%	2126
April	1st to 10th	Night	12% 5% 3 5% 5% 3 4 4 4 8% 47%	1800
	11th to 20th	Night	14% 7% 3 4 4 3 4 5% 5% 7% 44%	1775
	21st to end	Night	19% 7% 3 6% 5% 2 4 4 4 6% 41%	1800
May	1st to 10th	Night	15% 5% 3 6% 5% 3 4 4 5% 7% 43%	1733
	11th to 20th	Night	15% 7% 2 5% 4 3 3 4 5% 7% 45%	1462
	21st to end	Night	8% 5% 3 5% 6% 4 5% 6% 7% 9% 41%	1551
June	1st to 10th	Night	7% 4 2 5% 5% 2 5% 4 6% 10% 49%	1440
	11th to 20th	Night	5% 4 1 4 4 2 5% 5% 5% 9% 55%	1440
	21st to end	Night	5% 3 2 4 5% 3 3 6% 7% 10% 54%	1440
July	1st to 10th	Night	8% 4 1 3 4 3 4 5% 7% 10% 51%	1440
	11th to 20th	Night	5% 3 2 5% 4 3 3 6% 6% 9% 55%	1440
	21st to end	Night	6% 4 3 4 5% 3 5% 5% 7% 9% 50%	1683
August	1st to 10th	Night	6% 4 1 4 4 2 5% 7% 7% 8% 51%	1632
	11th to 20th	Night	6% 4 1 6% 6% 3 5% 5% 6% 8% 50%	1776
	21st to end	Night	6% 3 2 4 5% 2 4 4 5% 7% 58%	1980
September	1st to 10th	Night	12% 5% 1 4 4 2 4 4 5% 7% 53%	1800
	11th to 20th	Night	8% 5% 2 5% 4 3 3 4 6% 8% 52%	1815
	21st to end	Night	9% 3 1 3 5% 3 4 5% 5% 6% 55%	1980
October	1st to 10th	Night	9% 4 2 4 4 2 4 3 5% 7% 56%	2142
	11th to 20th	Night	13% 6% 2 4 3 2 3 3 4 7% 52%	2160
	21st to end	Night	18% 7% 2 4 4 2 4 3 4 5% 46%	2407
November	1st to 10th	Night	11% 4 2 4 4 2 4 4 4 5% 57%	2406
	11th to 20th	Night	14% 5% 2 4 4 3 4 4 4 6% 49%	2470
	21st to end	Night	18% 8% 3 5% 3 2 3 3 5% 6% 44%	2517
December	1st to 10th	Night	16% 10% 4 6% 5% 3 4 5% 5% 6% 36%	2660
	11th to 20th	Night	17% 8% 4 6% 4 3 4 3 4 5% 42%	3279
	21st to end	Night	19% 16% 5% 9% 7% 3 5% 5% 5% 4 23%	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	48% 8% 12% 23% 8% 1	2528
	11th to 20th	Day	47% 7% 14% 20% 11%	2699
	21st to end	Day	60% 4 9% 16% 10% 1	3135
February	1st to 10th	Day	50% 6% 11% 22% 10% 1	1710
	11th to 20th	Day	47% 6% 10% 21% 14% 3	1856
	21st to end	Day	45% 6% 10% 20% 15% 4	1570
March	1st to 10th	Day	45% 5% 9% 24% 15% 2	1907
	11th to 20th	Day	48% 6% 11% 19% 13% 2	2090
	21st to end	Day	55% 8% 12% 17% 8% 1	2383
April	1st to 10th	Day	38% 7% 12% 26% 16% 1	2160
	11th to 20th	Day	45% 5% 11% 23% 13% 3	2134
	21st to end	Day	46% 6% 12% 23% 12% 1	2160
May	1st to 10th	Day	44% 8% 14% 23% 9% 1	2226
	11th to 20th	Day	41% 8% 13% 22% 14% 1	2433
	21st to end	Day	40% 10% 15% 21% 12% 2	2725
June	1st to 10th	Day	32% 8% 14% 26% 17% 3	2520
	11th to 20th	Day	29% 9% 13% 23% 23% 3	2520
	21st to end	Day	28% 10% 18% 23% 16% 5%	2520
July	1st to 10th	Day	32% 10% 17% 25% 14% 4	2451
	11th to 20th	Day	26% 9% 15% 27% 20% 3	2445
	21st to end	Day	32% 8% 14% 29% 16% 2	2485
August	1st to 10th	Day	32% 11% 14% 28% 16%	2292
	11th to 20th	Day	35% 9% 15% 26% 14% 1	2125
	21st to end	Day	30% 7% 15% 24% 18% 5%	2376
September	1st to 10th	Day	34% 6% 11% 26% 18% 4	2160
	11th to 20th	Day	32% 5% 10% 25% 24% 3	2125
	21st to end	Day	29% 6% 9% 28% 24% 3	1980
October	1st to 10th	Day	31% 4 10% 26% 25% 4	1814
	11th to 20th	Day	35% 4 9% 25% 21% 5%	1800
	21st to end	Day	41% 4 8% 28% 16% 4	1948
November	1st to 10th	Day	30% 5% 13% 27% 19% 6%	1672
	11th to 20th	Day	38% 5% 10% 23% 19% 5%	1710
	21st to end	Day	46% 5% 9% 26% 14%	1658
December	1st to 10th	Day	45% 6% 13% 25% 11%	1520
	11th to 20th	Day	47% 6% 11% 22% 13% 2	1915
	21st to end	Day	59% 6% 11% 14% 8% 1	2783
January	1st to 10th	Night	49% 6% 12% 23% 9% 1	4428
	11th to 20th	Night	54% 6% 10% 20% 9% 1	4266
	21st to end	Night	59% 5% 10% 19% 7% 1	4532
February	1st to 10th	Night	51% 6% 10% 21% 12% 1	2470
	11th to 20th	Night	44% 5% 10% 21% 18% 2	2219
	21st to end	Night	46% 5% 11% 21% 13% 4	1884
March	1st to 10th	Night	44% 5% 9% 23% 16% 3	2280
	11th to 20th	Night	47% 5% 11% 21% 13% 3	2098
	21st to end	Night	52% 7% 12% 19% 9% 1	2126
April	1st to 10th	Night	36% 6% 11% 26% 18% 3	1800
	11th to 20th	Night	38% 6% 11% 23% 18% 3	1775
	21st to end	Night	45% 5% 9% 22% 17% 3	1800
May	1st to 10th	Night	41% 7% 12% 22% 16% 2	1733
	11th to 20th	Night	39% 8% 12% 23% 16% 2	1463
	21st to end	Night	37% 9% 14% 20% 16% 4	1552
June	1st to 10th	Night	30% 8% 14% 26% 19% 3	1440
	11th to 20th	Night	25% 7% 12% 27% 25% 4	1440
	21st to end	Night	24% 9% 15% 28% 17% 7%	1440
July	1st to 10th	Night	28% 9% 15% 28% 15% 5%	1400
	11th to 20th	Night	24% 7% 16% 28% 20% 5%	1392
	21st to end	Night	28% 8% 13% 30% 19% 2	1589
August	1st to 10th	Night	27% 9% 15% 31% 16% 2	1622
	11th to 20th	Night	29% 9% 14% 25% 21% 2	1776
	21st to end	Night	24% 7% 15% 26% 23% 5%	1980
September	1st to 10th	Night	31% 5% 11% 26% 20% 7%	1800
	11th to 20th	Night	30% 6% 10% 26% 24% 4	1815
	21st to end	Night	27% 8% 9% 23% 28% 5%	1980
October	1st to 10th	Night	28% 5% 10% 25% 26% 6%	2142
	11th to 20th	Night	33% 5% 8% 27% 21% 7%	2160
	21st to end	Night	40% 5% 10% 20% 21% 4	2407
November	1st to 10th	Night	31% 5% 11% 25% 22% 6%	2406
	11th to 20th	Night	36% 4 9% 25% 22% 4	2470
	21st to end	Night	42% 4 11% 24% 18% 2	2517
December	1st to 10th	Night	47% 5% 11% 24% 12% 1	2660
	11th to 20th	Night	44% 6% 10% 24% 15% 1	3281
	21st to end	Night	61% 7% 10% 14% 7% 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% 32% 19% 8%	270
	11th to 20th	Day	26% 16% 36% 20% 1	270
	21st to end	Day	19% 20% 41% 15% 5%	297
February	1st to 10th	Day	22% 17% 38% 19% 4	270
	11th to 20th	Day	22% 22% 31% 18% 5% 1	270
	21st to end	Day	23% 20% 36% 21% 1	225
March	1st to 10th	Day	19% 17% 47% 14% 3	270
	11th to 20th	Day	29% 17% 34% 14% 5% 1	270
	21st to end	Day	31% 15% 31% 18% 6%	413
April	1st to 10th	Day	15% 16% 37% 22% 10% 1	420
	11th to 20th	Day	19% 15% 30% 22% 13% 1	412
	21st to end	Day	21% 14% 34% 20% 10% 1	420
May	1st to 10th	Day	17% 14% 31% 27% 9% 2	423
	11th to 20th	Day	17% 10% 27% 26% 15% 4	575
	21st to end	Day	12% 4 28% 33% 16% 7%	679
June	1st to 10th	Day	10% 5% 20% 30% 25% 10%	630
	11th to 20th	Day	8% 4 19% 37% 24% 8%	630
	21st to end	Day	3 6% 26% 36% 23% 7%	630
July	1st to 10th	Day	10% 2 13% 27% 31% 18%	621
	11th to 20th	Day	5% 3 14% 38% 31% 10%	618
	21st to end	Day	8% 1 17% 34% 29% 11%	684
August	1st to 10th	Day	7% 2 13% 36% 30% 12%	604
	11th to 20th	Day	10% 3 13% 29% 33% 12%	439
	21st to end	Day	9% 1 14% 26% 38% 13%	495
September	1st to 10th	Day	13% 4 14% 27% 32% 10%	450
	11th to 20th	Day	8% 3 11% 31% 31% 16%	450
	21st to end	Day	8% 9% 19% 29% 25% 10%	450
October	1st to 10th	Day	13% 8% 22% 36% 17% 5%	284
	11th to 20th	Day	14% 16% 34% 26% 10% 1	270
	21st to end	Day	28% 19% 24% 20% 9%	297
November	1st to 10th	Day	11% 10% 35% 24% 15% 5%	270
	11th to 20th	Day	19% 17% 31% 20% 11% 1	270
	21st to end	Day	25% 13% 28% 25% 9% 1	270
December	1st to 10th	Day	13% 18% 41% 16% 8% 4	270
	11th to 20th	Day	22% 13% 34% 25% 4 1	270
	21st to end	Day	24% 20% 38% 14% 3 1	297
January	1st to 10th	Night	17% 13% 39% 26% 5%	950
	11th to 20th	Night	21% 14% 38% 22% 5%	950
	21st to end	Night	18% 13% 39% 28% 2	1045
February	1st to 10th	Night	22% 13% 37% 25% 3	950
	11th to 20th	Night	19% 14% 38% 27% 1	743
	21st to end	Night	21% 14% 39% 24% 2	628
March	1st to 10th	Night	20% 13% 38% 25% 4	760
	11th to 20th	Night	22% 10% 35% 30% 4	578
	21st to end	Night	23% 11% 32% 28% 6%	615
April	1st to 10th	Night	16% 10% 39% 27% 7%	540
	11th to 20th	Night	15% 10% 34% 29% 11% 1	533
	21st to end	Night	21% 7% 38% 26% 7% 1	540
May	1st to 10th	Night	17% 6% 31% 35% 10% 1	540
	11th to 20th	Night	17% 5% 25% 35% 17% 1	531
	21st to end	Night	10% 3 22% 40% 21% 4	584
June	1st to 10th	Night	8% 2 19% 42% 28% 1	540
	11th to 20th	Night	6% 1 19% 49% 23% 2	540
	21st to end	Night	6% 2 22% 42% 26% 2	540
July	1st to 10th	Night	9% 1 9% 38% 37% 6%	525
	11th to 20th	Night	5% 1 12% 44% 34% 3	522
	21st to end	Night	6% 2 14% 40% 35% 3	591
August	1st to 10th	Night	7% 2 10% 39% 38% 3	537
	11th to 20th	Night	8% 10% 41% 36% 3	534
	21st to end	Night	7% 1 11% 37% 38% 6%	594
September	1st to 10th	Night	13% 2 11% 38% 31% 5%	540
	11th to 20th	Night	10% 2 14% 33% 35% 5%	555
	21st to end	Night	9% 5% 23% 32% 27% 4	720
October	1st to 10th	Night	10% 3 27% 33% 24% 4	720
	11th to 20th	Night	16% 10% 28% 33% 13% 1	720
	21st to end	Night	23% 9% 33% 29% 7%	792
November	1st to 10th	Night	13% 7% 36% 32% 11% 1	740
	11th to 20th	Night	17% 11% 33% 30% 8% 1	760
	21st to end	Night	24% 8% 36% 26% 7%	807
December	1st to 10th	Night	20% 11% 35% 28% 5% 1	950
	11th to 20th	Night	23% 9% 29% 33% 6%	950
	21st to end	Night	24% 14% 35% 24% 4	1045

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