Thomas Conway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas Conway Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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% 8% 4 6% 6% 3 5% 5% 5% 7% 36%	74837
	Night	15% 9% 4 6% 5% 3 4 4 5% 7% 38%	74956
Transparency	Day	46% 6% 12% 21% 12% 2	74538
	Night	46% 5% 12% 21% 14% 2	74776
Seeing	Day	17% 10% 29% 25% 15% 6%	14584
	Night	18% 9% 30% 30% 12% 1	24553

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	17% 14% 6% 10% 5% 3 5% 5% 5% 5% 26%	1517
	11th to 20th	Day	15% 16% 7% 11% 8% 4 6% 5% 3 6% 20%	1472
	21st to end	Day	15% 17% 7% 11% 7% 2 4 4 4 6% 24%	1607
February	1st to 10th	Day	18% 13% 6% 8% 8% 4 5% 3 4 6% 25%	1721
	11th to 20th	Day	15% 11% 5% 6% 8% 3 5% 4 5% 6% 31%	1748
	21st to end	Day	18% 12% 7% 9% 7% 2 4 3 5% 6% 27%	1480
March	1st to 10th	Day	18% 13% 5% 8% 6% 3 5% 5% 4 5% 29%	1800
	11th to 20th	Day	15% 10% 4 7% 6% 3 5% 4 6% 6% 33%	1854
	21st to end	Day	22% 11% 5% 8% 6% 4 5% 3 4 6% 25%	2359
April	1st to 10th	Day	17% 10% 4 6% 5% 3 4 5% 5% 7% 33%	2160
	11th to 20th	Day	16% 9% 3 6% 5% 3 4 5% 5% 7% 37%	2134
	21st to end	Day	20% 7% 4 6% 5% 3 5% 4 5% 7% 32%	2281
May	1st to 10th	Day	18% 9% 3 6% 5% 4 5% 5% 6% 7% 31%	2340
	11th to 20th	Day	18% 7% 3 6% 6% 3 5% 4 5% 8% 35%	2340
	21st to end	Day	11% 6% 3 6% 6% 3 5% 6% 5% 9% 39%	2725
June	1st to 10th	Day	10% 5% 2 4 5% 3 5% 6% 6% 10% 43%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 4 4 5% 6% 8% 46%	2520
	21st to end	Day	10% 5% 3 6% 6% 4 6% 6% 7% 9% 39%	2520
July	1st to 10th	Day	8% 4 2 6% 5% 3 5% 6% 7% 10% 44%	2520
	11th to 20th	Day	9% 4 3 4 5% 3 6% 5% 6% 9% 45%	2520
	21st to end	Day	7% 4 2 5% 5% 4 5% 6% 7% 10% 44%	2743
August	1st to 10th	Day	7% 5% 3 6% 5% 3 5% 5% 7% 9% 46%	2395
	11th to 20th	Day	9% 4 2 6% 5% 4 6% 5% 6% 9% 44%	2166
	21st to end	Day	8% 5% 2 5% 4 3 5% 5% 6% 9% 48%	2376
September	1st to 10th	Day	11% 5% 2 5% 5% 4 4 5% 6% 8% 46%	2160
	11th to 20th	Day	12% 4 2 4 4 3 5% 5% 6% 7% 49%	2012
	21st to end	Day	10% 5% 3 5% 5% 3 5% 5% 6% 9% 45%	1980
October	1st to 10th	Day	12% 7% 2 5% 5% 3 4 4 6% 7% 45%	1980
	11th to 20th	Day	20% 7% 3 5% 6% 3 5% 5% 5% 6% 36%	1773
	21st to end	Day	20% 9% 3 7% 5% 4 4 4 5% 7% 31%	1782
November	1st to 10th	Day	13% 6% 3 5% 4 2 5% 3 6% 8% 45%	1672
	11th to 20th	Day	18% 8% 4 7% 5% 3 4 4 4 7% 36%	1710
	21st to end	Day	17% 11% 4 7% 5% 3 4 4 4 5% 36%	1710
December	1st to 10th	Day	16% 14% 3 9% 7% 4 5% 5% 5% 5% 28%	1539
	11th to 20th	Day	14% 12% 5% 11% 7% 3 5% 5% 5% 6% 28%	1918
	21st to end	Day	22% 22% 9% 10% 6% 3 3 4 3 4 13%	2783
January	1st to 10th	Night	16% 13% 5% 8% 6% 3 4 4 3 5% 31%	2648
	11th to 20th	Night	17% 14% 5% 10% 6% 3 5% 4 3 5% 26%	2577
	21st to end	Night	16% 14% 7% 10% 7% 3 4 3 5% 6% 26%	2745
February	1st to 10th	Night	18% 13% 6% 8% 7% 4 4 3 4 6% 28%	2237
	11th to 20th	Night	16% 10% 4 7% 6% 3 5% 4 5% 7% 34%	2098
	21st to end	Night	20% 12% 5% 7% 5% 3 4 3 5% 5% 32%	1776
March	1st to 10th	Night	16% 10% 5% 6% 6% 4 4 5% 4 5% 34%	2160
	11th to 20th	Night	14% 7% 3 6% 5% 3 4 5% 5% 8% 40%	2106
	21st to end	Night	20% 10% 4 6% 5% 3 4 5% 5% 7% 31%	2012
April	1st to 10th	Night	14% 7% 4 6% 6% 3 4 5% 5% 7% 39%	1800
	11th to 20th	Night	18% 8% 3 6% 5% 2 4 4 4 6% 39%	1775
	21st to end	Night	17% 7% 2 6% 5% 3 5% 5% 6% 6% 36%	1679
May	1st to 10th	Night	17% 7% 3 6% 5% 3 5% 5% 6% 8% 34%	1620
	11th to 20th	Night	17% 6% 3 5% 6% 3 5% 5% 5% 7% 39%	1551
	21st to end	Night	10% 4 3 6% 6% 4 6% 5% 7% 8% 41%	1551
June	1st to 10th	Night	9% 4 2 4 5% 3 4 5% 5% 8% 50%	1440
	11th to 20th	Night	8% 6% 3 5% 4 3 5% 5% 6% 9% 48%	1440
	21st to end	Night	10% 4 3 7% 6% 3 6% 6% 6% 6% 43%	1440
July	1st to 10th	Night	6% 5% 2 5% 5% 3 4 4 5% 12% 50%	1440
	11th to 20th	Night	8% 3 2 4 4 3 4 4 5% 8% 55%	1440
	21st to end	Night	4 4 3 4 5% 2 5% 5% 6% 8% 55%	1575
August	1st to 10th	Night	6% 3 1 4 5% 3 5% 5% 8% 11% 49%	1540
	11th to 20th	Night	7% 4 2 4 4 3 5% 5% 6% 9% 51%	1732
	21st to end	Night	8% 4 2 4 5% 3 4 4 5% 8% 53%	1980
September	1st to 10th	Night	11% 4 2 5% 5% 3 3 4 5% 6% 52%	1800
	11th to 20th	Night	9% 3 2 4 4 2 5% 5% 6% 8% 54%	1930
	21st to end	Night	9% 6% 2 4 5% 4 5% 4 5% 8% 48%	1980
October	1st to 10th	Night	11% 5% 2 5% 5% 3 4 4 5% 6% 51%	1980
	11th to 20th	Night	17% 8% 3 4 4 2 3 3 4 6% 45%	2185
	21st to end	Night	21% 9% 3 6% 5% 3 3 4 4 6% 38%	2574
November	1st to 10th	Night	13% 7% 3 5% 4 3 3 3 5% 7% 45%	2406
	11th to 20th	Night	14% 9% 3 7% 5% 3 5% 5% 5% 6% 37%	2470
	21st to end	Night	18% 9% 4 6% 6% 2 4 4 4 6% 37%	2470
December	1st to 10th	Night	16% 12% 4 8% 5% 3 5% 4 5% 6% 32%	2636
	11th to 20th	Night	16% 13% 4 7% 6% 4 5% 4 4 5% 32%	3279
	21st to end	Night	24% 18% 6% 10% 7% 3 4 4 4 5% 16%	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	58% 5% 11% 15% 10%	1518
	11th to 20th	Day	67% 6% 11% 12% 4	1472
	21st to end	Day	62% 4 11% 15% 7%	1607
February	1st to 10th	Day	61% 4 11% 16% 7% 1	1721
	11th to 20th	Day	53% 5% 10% 20% 9% 1	1758
	21st to end	Day	58% 4 11% 19% 6% 2	1480
March	1st to 10th	Day	56% 6% 10% 21% 6% 1	1800
	11th to 20th	Day	51% 5% 11% 21% 10% 1	1854
	21st to end	Day	61% 5% 10% 18% 5% 1	2359
April	1st to 10th	Day	50% 5% 11% 21% 12%	2160
	11th to 20th	Day	47% 5% 11% 22% 12% 3	2134
	21st to end	Day	51% 6% 12% 17% 12% 2	2281
May	1st to 10th	Day	51% 6% 12% 23% 8%	2340
	11th to 20th	Day	48% 6% 13% 20% 12% 1	2344
	21st to end	Day	41% 7% 12% 21% 16% 2	2725
June	1st to 10th	Day	36% 8% 12% 25% 17% 3	2520
	11th to 20th	Day	35% 7% 11% 23% 21% 3	2520
	21st to end	Day	40% 8% 16% 20% 15% 2	2520
July	1st to 10th	Day	35% 7% 12% 25% 17% 3	2451
	11th to 20th	Day	34% 8% 15% 26% 15% 2	2445
	21st to end	Day	34% 8% 16% 24% 15% 3	2591
August	1st to 10th	Day	33% 8% 16% 28% 14% 2	2380
	11th to 20th	Day	35% 7% 14% 25% 17% 1	2166
	21st to end	Day	31% 7% 14% 25% 18% 5%	2376
September	1st to 10th	Day	36% 7% 12% 24% 16% 4	2160
	11th to 20th	Day	33% 6% 12% 25% 20% 4	2012
	21st to end	Day	35% 6% 11% 26% 20% 2	1980
October	1st to 10th	Day	38% 6% 11% 25% 17% 2	1980
	11th to 20th	Day	47% 5% 10% 22% 14% 1	1773
	21st to end	Day	53% 4 12% 21% 9% 1	1782
November	1st to 10th	Day	37% 4 16% 24% 16% 3	1672
	11th to 20th	Day	49% 4 13% 22% 11% 1	1710
	21st to end	Day	51% 5% 12% 21% 11%	1710
December	1st to 10th	Day	58% 5% 13% 16% 8%	1539
	11th to 20th	Day	57% 6% 10% 21% 5% 1	1915
	21st to end	Day	76% 4 9% 9% 3	2783
January	1st to 10th	Night	56% 4 10% 18% 10% 1	2648
	11th to 20th	Night	62% 4 14% 12% 7%	2577
	21st to end	Night	60% 4 11% 16% 8% 1	2745
February	1st to 10th	Night	59% 4 11% 16% 8% 2	2237
	11th to 20th	Night	50% 5% 12% 20% 12% 2	2099
	21st to end	Night	55% 5% 9% 18% 11% 3	1776
March	1st to 10th	Night	53% 5% 11% 20% 10% 1	2160
	11th to 20th	Night	42% 6% 13% 21% 15% 3	2106
	21st to end	Night	52% 6% 12% 20% 9% 1	2012
April	1st to 10th	Night	44% 5% 13% 23% 13% 2	1800
	11th to 20th	Night	47% 4 8% 21% 16% 4	1775
	21st to end	Night	47% 6% 11% 20% 13% 3	1679
May	1st to 10th	Night	47% 6% 12% 22% 12%	1620
	11th to 20th	Night	44% 7% 12% 20% 16% 1	1552
	21st to end	Night	40% 8% 13% 19% 16% 4	1552
June	1st to 10th	Night	32% 6% 12% 26% 21% 3	1440
	11th to 20th	Night	33% 5% 12% 23% 24% 3	1440
	21st to end	Night	38% 8% 14% 22% 14% 3	1440
July	1st to 10th	Night	30% 7% 15% 27% 17% 5%	1400
	11th to 20th	Night	28% 6% 13% 27% 23% 4	1392
	21st to end	Night	26% 6% 17% 28% 18% 5%	1487
August	1st to 10th	Night	28% 7% 16% 30% 17% 1	1531
	11th to 20th	Night	29% 7% 16% 25% 21% 2	1732
	21st to end	Night	29% 6% 11% 27% 21% 5%	1980
September	1st to 10th	Night	32% 6% 12% 24% 20% 6%	1800
	11th to 20th	Night	28% 6% 10% 27% 25% 4	1930
	21st to end	Night	35% 5% 10% 27% 20% 3	1980
October	1st to 10th	Night	36% 5% 10% 25% 22% 2	1980
	11th to 20th	Night	41% 4 10% 25% 19% 2	2185
	21st to end	Night	49% 4 10% 22% 13% 2	2574
November	1st to 10th	Night	39% 5% 10% 26% 18% 3	2406
	11th to 20th	Night	46% 5% 12% 21% 13% 2	2470
	21st to end	Night	50% 4 11% 24% 11%	2470
December	1st to 10th	Night	54% 4 12% 20% 9% 1	2636
	11th to 20th	Night	55% 4 11% 18% 10% 1	3281
	21st to end	Night	72% 4 9% 10% 4	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	23% 14% 34% 21% 7% 1	270
	11th to 20th	Day	24% 14% 39% 17% 5% 1	252
	21st to end	Day	20% 20% 34% 20% 6%	264
February	1st to 10th	Day	26% 19% 38% 15% 2	240
	11th to 20th	Day	27% 13% 39% 17% 3 1	240
	21st to end	Day	28% 16% 40% 13% 3	198
March	1st to 10th	Day	23% 21% 34% 20% 2	240
	11th to 20th	Day	23% 13% 37% 20% 7%	294
	21st to end	Day	29% 15% 29% 20% 6% 1	462
April	1st to 10th	Day	23% 12% 34% 22% 8% 1	420
	11th to 20th	Day	21% 16% 30% 18% 11% 4	412
	21st to end	Day	23% 15% 32% 20% 7% 2	420
May	1st to 10th	Day	23% 12% 30% 21% 11% 3	423
	11th to 20th	Day	22% 6% 33% 22% 13% 4	486
	21st to end	Day	13% 8% 29% 24% 17% 8%	679
June	1st to 10th	Day	12% 6% 29% 29% 18% 6%	630
	11th to 20th	Day	11% 5% 29% 30% 19% 5%	630
	21st to end	Day	14% 7% 30% 30% 13% 6%	630
July	1st to 10th	Day	9% 4 20% 28% 22% 16%	621
	11th to 20th	Day	10% 4 22% 33% 21% 11%	618
	21st to end	Day	8% 3 25% 32% 25% 7%	684
August	1st to 10th	Day	8% 3 17% 29% 29% 14%	514
	11th to 20th	Day	10% 4 18% 28% 27% 13%	439
	21st to end	Day	10% 3 17% 34% 25% 12%	495
September	1st to 10th	Day	12% 5% 20% 31% 24% 8%	450
	11th to 20th	Day	11% 4 17% 28% 27% 13%	450
	21st to end	Day	10% 9% 27% 24% 22% 9%	450
October	1st to 10th	Day	15% 5% 31% 29% 17% 4	450
	11th to 20th	Day	20% 22% 28% 23% 7%	279
	21st to end	Day	32% 13% 34% 18% 3 1	297
November	1st to 10th	Day	14% 13% 37% 23% 9% 4	270
	11th to 20th	Day	21% 15% 37% 16% 9% 1	270
	21st to end	Day	25% 12% 39% 19% 4	270
December	1st to 10th	Day	20% 19% 37% 18% 7%	270
	11th to 20th	Day	20% 17% 37% 20% 6%	270
	21st to end	Day	28% 19% 36% 14% 3	297
January	1st to 10th	Night	21% 15% 37% 22% 4	950
	11th to 20th	Night	22% 15% 36% 21% 5%	921
	21st to end	Night	22% 13% 35% 25% 4	990
February	1st to 10th	Night	25% 13% 38% 21% 3	797
	11th to 20th	Night	21% 15% 35% 26% 4	703
	21st to end	Night	26% 13% 38% 21% 3	592
March	1st to 10th	Night	21% 13% 37% 24% 5%	720
	11th to 20th	Night	18% 10% 40% 25% 6% 1	720
	21st to end	Night	23% 12% 35% 22% 8%	626
April	1st to 10th	Night	17% 13% 39% 24% 6% 1	540
	11th to 20th	Night	20% 9% 36% 26% 8% 1	533
	21st to end	Night	22% 8% 33% 29% 8% 1	540
May	1st to 10th	Night	20% 7% 34% 30% 9% 1	540
	11th to 20th	Night	19% 4 28% 32% 16% 1	531
	21st to end	Night	11% 3 25% 42% 17% 1	584
June	1st to 10th	Night	10% 3 23% 48% 16% 1	540
	11th to 20th	Night	10% 4 30% 38% 17% 1	540
	21st to end	Night	10% 6% 24% 37% 21% 1	540
July	1st to 10th	Night	9% 2 15% 41% 29% 4	525
	11th to 20th	Night	8% 2 19% 39% 28% 3	522
	21st to end	Night	5% 2 15% 47% 30% 1	591
August	1st to 10th	Night	7% 1 10% 46% 31% 5%	537
	11th to 20th	Night	7% 2 13% 42% 31% 5%	534
	21st to end	Night	9% 2 16% 39% 32% 3	594
September	1st to 10th	Night	12% 4 15% 38% 27% 4	540
	11th to 20th	Night	9% 4 17% 38% 28% 4	670
	21st to end	Night	11% 5% 24% 32% 25% 3	720
October	1st to 10th	Night	14% 5% 25% 36% 19% 1	720
	11th to 20th	Night	20% 13% 30% 28% 9%	720
	21st to end	Night	26% 13% 30% 24% 6%	792
November	1st to 10th	Night	16% 11% 37% 25% 10% 1	740
	11th to 20th	Night	19% 16% 33% 25% 6%	760
	21st to end	Night	22% 12% 36% 24% 6%	760
December	1st to 10th	Night	23% 13% 35% 24% 4	926
	11th to 20th	Night	25% 12% 32% 26% 5%	950
	21st to end	Night	29% 14% 32% 22% 2	1045

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