Thomas S. Toth Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas S. Toth Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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% 9% 4 7% 6% 4 5% 5% 5% 7% 34%	75059
	Night	16% 10% 4 7% 6% 3 4 4 5% 6% 35%	75003
Transparency	Day	48% 6% 12% 20% 12% 2	74772
	Night	49% 6% 11% 20% 13% 2	74811
Seeing	Day	18% 9% 26% 24% 16% 7%	14415
	Night	19% 8% 28% 30% 14% 1	24352

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	18% 16% 6% 10% 9% 4 4 5% 5% 5% 19%	1517
	11th to 20th	Day	18% 17% 8% 11% 7% 4 5% 4 4 5% 17%	1521
	21st to end	Day	17% 18% 8% 10% 9% 4 4 4 4 6% 18%	1836
February	1st to 10th	Day	19% 14% 7% 11% 7% 4 4 4 4 5% 21%	1670
	11th to 20th	Day	16% 12% 6% 14% 8% 4 5% 4 4 5% 22%	1748
	21st to end	Day	17% 14% 7% 8% 6% 3 4 4 4 7% 27%	1480
March	1st to 10th	Day	19% 13% 6% 7% 6% 2 4 4 4 7% 29%	1807
	11th to 20th	Day	18% 11% 4 7% 6% 3 5% 4 5% 7% 29%	1980
	21st to end	Day	24% 11% 5% 7% 6% 4 4 4 4 7% 25%	2178
April	1st to 10th	Day	14% 11% 5% 7% 7% 3 5% 5% 5% 6% 31%	2132
	11th to 20th	Day	18% 8% 3 6% 6% 3 4 4 5% 8% 35%	2209
	21st to end	Day	20% 7% 3 6% 7% 3 5% 4 5% 7% 32%	2340
May	1st to 10th	Day	18% 8% 3 8% 7% 4 5% 5% 5% 6% 31%	2340
	11th to 20th	Day	19% 7% 3 7% 5% 4 5% 5% 5% 7% 34%	2301
	21st to end	Day	10% 5% 3 5% 6% 4 6% 6% 6% 9% 39%	2531
June	1st to 10th	Day	13% 5% 3 6% 4 3 6% 5% 7% 8% 40%	2459
	11th to 20th	Day	11% 5% 2 5% 5% 3 6% 6% 6% 10% 41%	2520
	21st to end	Day	11% 5% 2 5% 6% 4 6% 6% 7% 10% 39%	2520
July	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 7% 7% 10% 40%	2520
	11th to 20th	Day	8% 3 2 5% 5% 4 5% 6% 6% 11% 46%	2520
	21st to end	Day	9% 4 2 5% 5% 3 5% 6% 8% 10% 42%	2561
August	1st to 10th	Day	7% 4 2 4 4 4 5% 6% 6% 10% 46%	2327
	11th to 20th	Day	9% 5% 3 5% 6% 4 6% 7% 7% 9% 41%	2303
	21st to end	Day	9% 4 2 5% 5% 3 4 6% 7% 10% 44%	2441
September	1st to 10th	Day	13% 5% 3 4 5% 3 4 5% 5% 7% 47%	2059
	11th to 20th	Day	9% 5% 3 6% 5% 4 5% 5% 6% 8% 45%	1980
	21st to end	Day	16% 6% 2 4 4 3 4 4 4 7% 46%	1980
October	1st to 10th	Day	13% 7% 3 6% 6% 4 4 4 5% 7% 43%	1980
	11th to 20th	Day	16% 6% 2 6% 6% 4 4 6% 5% 7% 39%	1810
	21st to end	Day	19% 9% 3 7% 7% 3 5% 4 5% 7% 29%	1817
November	1st to 10th	Day	10% 8% 3 5% 5% 3 3 3 4 7% 49%	1672
	11th to 20th	Day	17% 12% 4 7% 5% 3 4 4 4 6% 33%	1710
	21st to end	Day	19% 12% 5% 7% 5% 3 3 4 5% 5% 30%	1710
December	1st to 10th	Day	18% 18% 8% 10% 7% 3 4 4 4 5% 21%	1710
	11th to 20th	Day	16% 15% 7% 11% 6% 4 5% 5% 4 6% 23%	2087
	21st to end	Day	21% 22% 10% 10% 7% 4 4 3 3 4 11%	2783
January	1st to 10th	Night	18% 14% 7% 11% 8% 4 4 4 4 3 23%	2648
	11th to 20th	Night	18% 17% 7% 10% 6% 3 4 4 4 5% 21%	2656
	21st to end	Night	18% 17% 7% 11% 6% 3 4 4 4 4 22%	2664
February	1st to 10th	Night	21% 14% 6% 8% 7% 4 5% 4 4 4 24%	2290
	11th to 20th	Night	15% 13% 6% 9% 7% 3 4 4 5% 6% 27%	2098
	21st to end	Night	21% 13% 4 9% 6% 2 5% 4 4 5% 27%	1776
March	1st to 10th	Night	16% 10% 4 8% 7% 4 5% 3 4 5% 34%	2152
	11th to 20th	Night	15% 8% 3 6% 5% 3 4 4 6% 7% 38%	1980
	21st to end	Night	19% 11% 4 6% 6% 3 4 3 4 5% 34%	2178
April	1st to 10th	Night	16% 8% 3 5% 5% 3 5% 5% 6% 7% 37%	1841
	11th to 20th	Night	18% 7% 3 5% 6% 3 4 4 5% 6% 37%	1698
	21st to end	Night	18% 8% 3 6% 5% 4 4 5% 5% 7% 36%	1620
May	1st to 10th	Night	20% 9% 2 8% 6% 4 6% 5% 5% 6% 31%	1620
	11th to 20th	Night	21% 6% 3 4 5% 3 4 4 5% 9% 36%	1592
	21st to end	Night	8% 5% 2 5% 5% 3 5% 6% 6% 12% 43%	1744
June	1st to 10th	Night	9% 5% 2 6% 5% 3 4 6% 6% 6% 49%	1496
	11th to 20th	Night	9% 5% 2 4 5% 3 5% 5% 6% 8% 48%	1440
	21st to end	Night	11% 4 3 5% 5% 3 4 5% 6% 9% 44%	1440
July	1st to 10th	Night	9% 5% 2 4 6% 3 5% 5% 6% 8% 47%	1440
	11th to 20th	Night	7% 4 2 5% 5% 4 4 5% 6% 9% 50%	1440
	21st to end	Night	8% 4 2 4 5% 2 5% 5% 5% 8% 50%	1753
August	1st to 10th	Night	7% 4 2 5% 5% 4 5% 6% 6% 10% 47%	1611
	11th to 20th	Night	9% 4 2 5% 5% 3 5% 6% 6% 7% 48%	1598
	21st to end	Night	7% 4 2 5% 4 3 5% 4 6% 7% 53%	1912
September	1st to 10th	Night	12% 5% 2 4 4 2 4 5% 4 8% 50%	1882
	11th to 20th	Night	10% 4 2 5% 5% 2 3 4 5% 6% 53%	1980
	21st to end	Night	11% 5% 3 5% 5% 3 5% 4 5% 6% 48%	1980
October	1st to 10th	Night	11% 6% 2 5% 5% 3 4 4 5% 8% 49%	1980
	11th to 20th	Night	18% 9% 3 6% 5% 3 3 4 4 5% 40%	2150
	21st to end	Night	22% 7% 2 5% 5% 3 4 5% 5% 7% 36%	2538
November	1st to 10th	Night	13% 9% 3 5% 4 3 4 3 5% 5% 45%	2406
	11th to 20th	Night	15% 11% 4 7% 5% 3 4 3 5% 7% 35%	2470
	21st to end	Night	18% 13% 4 8% 6% 3 5% 5% 4 6% 28%	2470
December	1st to 10th	Night	19% 15% 7% 7% 6% 3 4 4 5% 6% 24%	2470
	11th to 20th	Night	17% 16% 7% 8% 6% 3 4 5% 4 5% 25%	3106
	21st to end	Night	25% 19% 8% 8% 7% 3 4 4 4 5% 14%	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	68% 5% 7% 15% 5%	1518
	11th to 20th	Day	71% 4 11% 10% 4	1521
	21st to end	Day	69% 4 9% 12% 5% 1	1836
February	1st to 10th	Day	67% 4 10% 13% 6% 1	1670
	11th to 20th	Day	65% 4 8% 15% 7%	1758
	21st to end	Day	59% 4 12% 16% 7% 2	1480
March	1st to 10th	Day	57% 5% 11% 20% 7% 1	1807
	11th to 20th	Day	54% 5% 12% 17% 10% 1	1980
	21st to end	Day	60% 5% 12% 18% 5%	2178
April	1st to 10th	Day	53% 6% 11% 18% 12% 1	2132
	11th to 20th	Day	48% 5% 11% 21% 13% 3	2209
	21st to end	Day	50% 5% 13% 19% 10% 2	2340
May	1st to 10th	Day	52% 7% 11% 18% 11% 1	2340
	11th to 20th	Day	50% 6% 11% 20% 11% 1	2305
	21st to end	Day	39% 8% 15% 23% 12% 2	2531
June	1st to 10th	Day	40% 7% 12% 24% 14% 3	2459
	11th to 20th	Day	36% 9% 13% 24% 16% 2	2520
	21st to end	Day	38% 9% 15% 20% 14% 4	2520
July	1st to 10th	Day	36% 10% 14% 21% 16% 3	2451
	11th to 20th	Day	31% 9% 15% 28% 16% 1	2445
	21st to end	Day	34% 9% 15% 25% 15% 3	2419
August	1st to 10th	Day	31% 9% 15% 28% 16% 1	2314
	11th to 20th	Day	35% 10% 15% 26% 13% 2	2303
	21st to end	Day	33% 8% 14% 23% 17% 5%	2441
September	1st to 10th	Day	36% 6% 14% 22% 18% 4	2059
	11th to 20th	Day	35% 7% 11% 25% 21% 2	1980
	21st to end	Day	39% 6% 10% 25% 20% 1	1980
October	1st to 10th	Day	41% 6% 11% 25% 15% 2	1980
	11th to 20th	Day	42% 6% 11% 22% 17% 2	1810
	21st to end	Day	54% 5% 12% 18% 10% 1	1817
November	1st to 10th	Day	37% 4 13% 27% 18% 1	1672
	11th to 20th	Day	53% 5% 9% 20% 11% 2	1710
	21st to end	Day	57% 5% 11% 18% 8% 1	1710
December	1st to 10th	Day	67% 5% 9% 13% 5%	1710
	11th to 20th	Day	63% 5% 10% 18% 4	2084
	21st to end	Day	78% 4 6% 7% 4	2783
January	1st to 10th	Night	66% 4 7% 15% 8%	2648
	11th to 20th	Night	66% 4 10% 15% 5%	2656
	21st to end	Night	66% 4 9% 14% 6%	2664
February	1st to 10th	Night	64% 5% 10% 14% 6% 1	2290
	11th to 20th	Night	59% 5% 10% 16% 9% 1	2099
	21st to end	Night	60% 5% 9% 16% 9% 1	1776
March	1st to 10th	Night	53% 5% 10% 21% 11% 1	2152
	11th to 20th	Night	45% 5% 11% 23% 13% 3	1980
	21st to end	Night	54% 4 9% 22% 10% 1	2178
April	1st to 10th	Night	46% 6% 13% 21% 14% 1	1841
	11th to 20th	Night	47% 5% 10% 18% 17% 3	1698
	21st to end	Night	48% 5% 12% 20% 13% 3	1620
May	1st to 10th	Night	53% 7% 9% 18% 10% 3	1620
	11th to 20th	Night	45% 6% 11% 21% 14% 2	1593
	21st to end	Night	34% 8% 16% 25% 15% 3	1745
June	1st to 10th	Night	35% 6% 11% 26% 19% 3	1496
	11th to 20th	Night	32% 8% 13% 26% 18% 3	1440
	21st to end	Night	35% 8% 15% 23% 16% 3	1440
July	1st to 10th	Night	35% 7% 12% 22% 19% 4	1400
	11th to 20th	Night	29% 7% 15% 29% 18% 1	1392
	21st to end	Night	29% 7% 15% 27% 17% 4	1655
August	1st to 10th	Night	30% 8% 14% 28% 19% 1	1600
	11th to 20th	Night	32% 8% 13% 27% 19% 1	1598
	21st to end	Night	29% 7% 14% 22% 22% 5%	1912
September	1st to 10th	Night	33% 6% 13% 25% 17% 6%	1882
	11th to 20th	Night	30% 7% 9% 27% 24% 2	1980
	21st to end	Night	35% 7% 10% 24% 21% 3	1980
October	1st to 10th	Night	33% 6% 12% 25% 22% 1	1980
	11th to 20th	Night	46% 5% 8% 24% 15% 1	2150
	21st to end	Night	47% 5% 10% 22% 13% 2	2538
November	1st to 10th	Night	40% 5% 10% 25% 19% 2	2406
	11th to 20th	Night	49% 4 10% 21% 14% 2	2470
	21st to end	Night	57% 4 11% 20% 8%	2470
December	1st to 10th	Night	61% 5% 10% 17% 7%	2470
	11th to 20th	Night	60% 6% 11% 17% 6% 1	3108
	21st to end	Night	73% 5% 10% 9% 3	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	22% 18% 35% 19% 5% 1	270
	11th to 20th	Day	27% 14% 41% 13% 4 1	270
	21st to end	Day	33% 12% 34% 15% 5% 1	282
February	1st to 10th	Day	31% 16% 38% 14% 1	240
	11th to 20th	Day	25% 19% 37% 15% 5%	240
	21st to end	Day	24% 19% 45% 10% 2	198
March	1st to 10th	Day	26% 17% 38% 14% 5%	247
	11th to 20th	Day	28% 11% 30% 23% 8% 1	420
	21st to end	Day	32% 13% 31% 18% 7%	462
April	1st to 10th	Day	20% 15% 30% 24% 9% 2	420
	11th to 20th	Day	23% 16% 26% 19% 10% 5%	412
	21st to end	Day	25% 12% 31% 21% 9% 2	420
May	1st to 10th	Day	26% 11% 26% 24% 9% 3	423
	11th to 20th	Day	24% 5% 25% 25% 14% 7%	447
	21st to end	Day	12% 4 25% 27% 22% 10%	485
June	1st to 10th	Day	14% 4 24% 33% 19% 6%	569
	11th to 20th	Day	13% 5% 26% 25% 23% 7%	630
	21st to end	Day	11% 7% 25% 29% 20% 7%	630
July	1st to 10th	Day	11% 3 17% 29% 21% 19%	621
	11th to 20th	Day	7% 3 22% 29% 23% 15%	618
	21st to end	Day	10% 2 16% 29% 25% 17%	502
August	1st to 10th	Day	10% 2 12% 27% 31% 18%	446
	11th to 20th	Day	13% 3 19% 25% 25% 15%	439
	21st to end	Day	9% 3 14% 28% 30% 15%	495
September	1st to 10th	Day	15% 4 15% 24% 29% 13%	450
	11th to 20th	Day	9% 3 16% 24% 30% 19%	450
	21st to end	Day	16% 9% 20% 25% 21% 10%	450
October	1st to 10th	Day	11% 7% 23% 30% 18% 10%	450
	11th to 20th	Day	16% 15% 31% 25% 10% 2	450
	21st to end	Day	26% 11% 37% 22% 4 1	332
November	1st to 10th	Day	13% 14% 33% 26% 10% 4	270
	11th to 20th	Day	23% 18% 27% 19% 13% 1	270
	21st to end	Day	26% 11% 40% 19% 3	270
December	1st to 10th	Day	22% 17% 39% 14% 7% 1	270
	11th to 20th	Day	22% 14% 39% 20% 4	270
	21st to end	Day	29% 18% 33% 18% 3	297
January	1st to 10th	Night	23% 14% 37% 21% 6%	950
	11th to 20th	Night	27% 13% 36% 19% 5%	946
	21st to end	Night	25% 13% 34% 24% 4	820
February	1st to 10th	Night	28% 10% 33% 25% 3	720
	11th to 20th	Night	24% 13% 37% 24% 2	703
	21st to end	Night	28% 11% 38% 20% 3	592
March	1st to 10th	Night	22% 11% 34% 27% 7%	720
	11th to 20th	Night	19% 13% 33% 29% 6% 1	720
	21st to end	Night	23% 12% 29% 27% 8% 1	792
April	1st to 10th	Night	18% 13% 34% 28% 5% 1	581
	11th to 20th	Night	21% 10% 33% 27% 9% 1	533
	21st to end	Night	22% 7% 35% 25% 10% 1	540
May	1st to 10th	Night	23% 5% 32% 31% 7% 1	540
	11th to 20th	Night	23% 4 26% 30% 15% 1	531
	21st to end	Night	10% 4 23% 37% 24% 3	584
June	1st to 10th	Night	9% 2 22% 44% 21%	540
	11th to 20th	Night	10% 2 21% 44% 21% 1	540
	21st to end	Night	10% 3 19% 40% 26% 1	540
July	1st to 10th	Night	11% 2 14% 39% 30% 5%	525
	11th to 20th	Night	7% 1 16% 40% 30% 5%	522
	21st to end	Night	10% 2 11% 45% 30% 2	591
August	1st to 10th	Night	9% 1 9% 45% 32% 4	537
	11th to 20th	Night	10% 2 11% 39% 34% 4	534
	21st to end	Night	8% 2 11% 39% 35% 5%	594
September	1st to 10th	Night	13% 4 14% 33% 31% 5%	622
	11th to 20th	Night	10% 4 12% 35% 33% 5%	720
	21st to end	Night	13% 5% 23% 33% 23% 3	720
October	1st to 10th	Night	12% 5% 25% 36% 18% 4	720
	11th to 20th	Night	23% 10% 28% 29% 9% 1	720
	21st to end	Night	26% 10% 30% 28% 7%	792
November	1st to 10th	Night	18% 8% 34% 25% 14% 1	740
	11th to 20th	Night	21% 15% 32% 24% 7% 1	760
	21st to end	Night	25% 11% 31% 27% 5% 1	760
December	1st to 10th	Night	26% 13% 35% 22% 4 1	760
	11th to 20th	Night	25% 11% 34% 24% 6%	798
	21st to end	Night	29% 13% 34% 20% 3	1045

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