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 2026-06-01.

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% 10% 4 8% 5% 5% 5% 4 5% 7% 33%	131901
	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 34%	131366
Transparency	Day	48% 6% 12% 21% 12% 1	135501
	Night	49% 5% 11% 21% 13% 1	135825
Seeing	Day	18% 10% 27% 24% 15% 7%	17613
	Night	19% 9% 28% 30% 13% 1	27465

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	16% 18% 8% 15% 6% 5% 4 3 4 4 17%	4691
	11th to 20th	Day	12% 15% 7% 13% 9% 7% 7% 4 3 3 20%	4724
	21st to end	Day	19% 15% 8% 13% 7% 5% 5% 4 4 4 16%	5780
February	1st to 10th	Day	17% 15% 7% 12% 7% 5% 3 4 4 5% 22%	1948
	11th to 20th	Day	15% 13% 5% 15% 7% 5% 4 3 4 5% 23%	2048
	21st to end	Day	15% 14% 6% 9% 5% 5% 4 3 5% 7% 28%	1730
March	1st to 10th	Day	18% 16% 7% 10% 4 4 3 3 3 5% 26%	4615
	11th to 20th	Day	12% 11% 3 7% 4 4 4 4 6% 7% 39%	5090
	21st to end	Day	18% 14% 5% 10% 6% 5% 4 3 5% 6% 26%	5586
April	1st to 10th	Day	15% 13% 3 8% 6% 5% 4 4 5% 6% 30%	2489
	11th to 20th	Day	17% 9% 3 7% 6% 4 4 3 5% 7% 35%	2583
	21st to end	Day	18% 8% 3 7% 6% 4 4 4 5% 8% 33%	2730
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 6% 28%	5992
	11th to 20th	Day	18% 7% 2 7% 4 5% 4 4 6% 8% 35%	5991
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 6% 9% 37%	6576
June	1st to 10th	Day	12% 7% 2 6% 4 4 6% 4 6% 8% 40%	2733
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 5% 7% 10% 40%	2800
	21st to end	Day	10% 5% 2 6% 5% 6% 5% 5% 7% 10% 40%	2800
July	1st to 10th	Day	10% 6% 2 7% 4 5% 5% 4 7% 10% 40%	6360
	11th to 20th	Day	9% 4 2 7% 5% 5% 6% 5% 7% 11% 39%	6360
	21st to end	Day	9% 6% 3 7% 5% 5% 6% 5% 8% 9% 37%	6452
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 6% 10% 44%	4234
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 44%	4174
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 7% 10% 47%	4422
September	1st to 10th	Day	12% 5% 2 4 4 4 4 3 5% 7% 49%	2287
	11th to 20th	Day	8% 5% 2 6% 4 5% 4 4 7% 8% 47%	2200
	21st to end	Day	17% 7% 2 5% 3 4 4 3 4 7% 44%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 5% 4 3 5% 7% 46%	2200
	11th to 20th	Day	15% 6% 2 7% 5% 5% 4 5% 5% 6% 42%	2010
	21st to end	Day	18% 10% 3 8% 6% 4 5% 3 5% 7% 31%	2018
November	1st to 10th	Day	10% 9% 3 6% 4 3 3 3 5% 6% 48%	1852
	11th to 20th	Day	17% 13% 3 8% 5% 4 4 3 4 6% 32%	1890
	21st to end	Day	20% 14% 5% 9% 5% 4 3 3 5% 5% 28%	1890
December	1st to 10th	Day	15% 15% 5% 12% 6% 4 4 3 4 5% 28%	3020
	11th to 20th	Day	16% 17% 8% 11% 5% 4 5% 3 4 5% 22%	3356
	21st to end	Day	24% 20% 8% 11% 6% 5% 3 3 3 4 13%	4070
January	1st to 10th	Night	15% 17% 7% 17% 6% 5% 4 3 4 4 19%	8232
	11th to 20th	Night	10% 18% 9% 16% 7% 6% 5% 4 4 4 18%	8248
	21st to end	Night	18% 13% 5% 9% 5% 5% 5% 4 5% 6% 24%	8401
February	1st to 10th	Night	19% 14% 5% 10% 7% 5% 4 3 4 4 26%	2671
	11th to 20th	Night	14% 14% 5% 10% 6% 4 5% 3 5% 5% 28%	2458
	21st to end	Night	19% 13% 4 11% 5% 4 4 3 5% 5% 28%	2076
March	1st to 10th	Night	18% 13% 4 7% 4 4 3 2 3 5% 36%	5457
	11th to 20th	Night	11% 7% 3 7% 4 5% 4 3 6% 8% 42%	5090
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 4 6% 36%	5588
April	1st to 10th	Night	18% 10% 3 6% 4 4 4 4 5% 7% 35%	2147
	11th to 20th	Night	17% 7% 3 6% 5% 5% 5% 3 6% 7% 36%	1984
	21st to end	Night	17% 9% 3 7% 4 4 3 4 5% 6% 37%	1890
May	1st to 10th	Night	21% 10% 4 8% 5% 4 5% 3 5% 6% 29%	4123
	11th to 20th	Night	18% 7% 3 5% 4 4 4 3 6% 9% 37%	4122
	21st to end	Night	14% 6% 2 6% 4 4 5% 3 6% 9% 41%	4520
June	1st to 10th	Night	10% 6% 2 7% 4 4 4 5% 6% 6% 48%	1662
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 4 6% 9% 46%	1600
	21st to end	Night	10% 4 3 6% 5% 4 4 4 7% 9% 45%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	3618
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 10% 45%	3617
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 4 6% 8% 43%	4396
August	1st to 10th	Night	12% 6% 2 7% 4 4 4 4 6% 9% 41%	2915
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 6% 8% 47%	2875
	21st to end	Night	6% 4 2 6% 4 5% 4 3 6% 7% 51%	3472
September	1st to 10th	Night	11% 5% 2 4 4 3 4 4 4 7% 52%	2092
	11th to 20th	Night	9% 4 2 5% 3 3 3 3 5% 6% 55%	2200
	21st to end	Night	13% 6% 2 6% 5% 4 4 4 5% 6% 46%	2200
October	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 5% 7% 51%	2200
	11th to 20th	Night	17% 9% 3 7% 4 4 3 3 4 5% 41%	2390
	21st to end	Night	20% 7% 2 6% 4 4 4 4 6% 8% 37%	2820
November	1st to 10th	Night	12% 10% 2 7% 4 4 3 2 5% 5% 45%	2666
	11th to 20th	Night	15% 12% 4 8% 5% 4 4 3 5% 7% 35%	2730
	21st to end	Night	18% 14% 4 8% 5% 4 4 4 4 6% 27%	2730
December	1st to 10th	Night	14% 13% 5% 10% 5% 5% 5% 4 6% 7% 26%	4380
	11th to 20th	Night	17% 18% 8% 12% 5% 4 4 4 4 5% 21%	5046
	21st to end	Night	25% 20% 6% 9% 5% 5% 4 3 4 5% 14%	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	70% 4 10% 12% 3	4692
	11th to 20th	Day	70% 3 8% 8% 10% 1	4724
	21st to end	Day	71% 3 9% 11% 5%	5780
February	1st to 10th	Day	64% 4 10% 14% 6% 1	1948
	11th to 20th	Day	64% 4 8% 17% 7%	2058
	21st to end	Day	58% 4 14% 16% 7% 2	1730
March	1st to 10th	Day	61% 4 8% 19% 8%	4615
	11th to 20th	Day	43% 5% 11% 29% 11% 1	5090
	21st to end	Day	58% 6% 13% 16% 6% 1	5586
April	1st to 10th	Day	54% 6% 11% 17% 12%	2489
	11th to 20th	Day	48% 6% 11% 22% 12% 2	2583
	21st to end	Day	48% 5% 13% 20% 10% 2	2730
May	1st to 10th	Day	54% 7% 10% 16% 12%	6027
	11th to 20th	Day	45% 7% 13% 24% 11% 1	5995
	21st to end	Day	41% 7% 11% 23% 15% 2	6590
June	1st to 10th	Day	39% 7% 12% 24% 15% 3	2733
	11th to 20th	Day	37% 8% 13% 25% 16% 2	2800
	21st to end	Day	37% 9% 15% 20% 15% 4	2800
July	1st to 10th	Day	37% 7% 11% 27% 17% 2	6291
	11th to 20th	Day	36% 8% 16% 24% 15% 1	6285
	21st to end	Day	40% 8% 13% 21% 17% 2	6268
August	1st to 10th	Day	33% 8% 17% 25% 15% 1	4221
	11th to 20th	Day	32% 8% 13% 30% 15% 1	4213
	21st to end	Day	30% 7% 14% 32% 14% 3	4421
September	1st to 10th	Day	34% 6% 14% 24% 19% 4	2287
	11th to 20th	Day	33% 7% 10% 26% 23% 2	2200
	21st to end	Day	41% 5% 9% 23% 21% 1	2200
October	1st to 10th	Day	27% 4 12% 33% 23% 1	3590
	11th to 20th	Day	34% 5% 12% 26% 22% 1	3250
	21st to end	Day	43% 4 10% 24% 18% 1	3230
November	1st to 10th	Day	38% 4 12% 28% 17% 1	1852
	11th to 20th	Day	53% 5% 9% 20% 11% 2	1890
	21st to end	Day	58% 4 11% 18% 8% 1	1890
December	1st to 10th	Day	59% 5% 13% 19% 4	3020
	11th to 20th	Day	65% 4 12% 15% 4	3353
	21st to end	Day	76% 4 7% 10% 4	4070
January	1st to 10th	Night	68% 5% 8% 15% 4	8233
	11th to 20th	Night	70% 4 7% 10% 8% 1	8248
	21st to end	Night	60% 4 13% 16% 6% 1	8401
February	1st to 10th	Night	62% 5% 10% 15% 7% 1	2671
	11th to 20th	Night	57% 5% 10% 18% 9% 1	2459
	21st to end	Night	58% 5% 10% 17% 9% 1	2076
March	1st to 10th	Night	52% 3 7% 27% 9% 1	5457
	11th to 20th	Night	38% 6% 13% 29% 13% 1	5090
	21st to end	Night	49% 5% 10% 23% 12% 1	5588
April	1st to 10th	Night	48% 6% 12% 20% 14% 1	2147
	11th to 20th	Night	47% 6% 11% 17% 16% 3	1984
	21st to end	Night	47% 5% 11% 21% 13% 3	1890
May	1st to 10th	Night	56% 6% 8% 15% 12% 3	4150
	11th to 20th	Night	43% 8% 11% 23% 14% 1	4123
	21st to end	Night	39% 7% 13% 24% 15% 2	4528
June	1st to 10th	Night	35% 6% 11% 25% 20% 3	1662
	11th to 20th	Night	33% 8% 13% 25% 18% 3	1600
	21st to end	Night	34% 8% 15% 23% 17% 3	1600
July	1st to 10th	Night	33% 7% 12% 25% 18% 5%	3580
	11th to 20th	Night	33% 7% 16% 26% 17% 1	3572
	21st to end	Night	37% 6% 14% 20% 20% 3	4267
August	1st to 10th	Night	37% 7% 14% 25% 15% 1	2904
	11th to 20th	Night	33% 7% 11% 31% 17% 1	2902
	21st to end	Night	30% 7% 13% 28% 19% 3	3472
September	1st to 10th	Night	31% 6% 13% 26% 19% 5%	2092
	11th to 20th	Night	28% 6% 9% 28% 26% 3	2200
	21st to end	Night	37% 7% 9% 23% 21% 3	2200
October	1st to 10th	Night	23% 4 8% 30% 33% 2	3590
	11th to 20th	Night	35% 4 11% 25% 24% 1	3889
	21st to end	Night	34% 4 8% 31% 20% 3	4546
November	1st to 10th	Night	40% 5% 9% 26% 18% 2	2666
	11th to 20th	Night	49% 4 10% 21% 13% 2	2730
	21st to end	Night	59% 4 10% 19% 7%	2730
December	1st to 10th	Night	57% 5% 12% 21% 6%	4380
	11th to 20th	Night	65% 5% 12% 12% 6%	5048
	21st to end	Night	73% 5% 9% 10% 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% 36% 18% 5% 1	330
	11th to 20th	Day	25% 18% 40% 12% 4 1	330
	21st to end	Day	31% 14% 33% 16% 5% 1	363
February	1st to 10th	Day	25% 15% 39% 16% 4 1	330
	11th to 20th	Day	24% 19% 34% 18% 5%	330
	21st to end	Day	21% 18% 48% 11% 2	273
March	1st to 10th	Day	26% 17% 39% 14% 4	339
	11th to 20th	Day	28% 14% 29% 21% 7% 1	540
	21st to end	Day	29% 15% 32% 19% 5%	594
April	1st to 10th	Day	21% 16% 32% 21% 7% 2	540
	11th to 20th	Day	22% 18% 27% 19% 9% 4	532
	21st to end	Day	21% 11% 31% 24% 10% 2	540
May	1st to 10th	Day	25% 10% 30% 22% 9% 2	543
	11th to 20th	Day	23% 6% 25% 25% 13% 7%	567
	21st to end	Day	14% 5% 26% 27% 20% 8%	617
June	1st to 10th	Day	14% 5% 24% 32% 18% 6%	663
	11th to 20th	Day	13% 5% 26% 27% 22% 6%	730
	21st to end	Day	11% 7% 25% 30% 21% 7%	730
July	1st to 10th	Day	11% 3 18% 30% 21% 17%	721
	11th to 20th	Day	7% 3 23% 29% 23% 14%	718
	21st to end	Day	10% 3 17% 29% 26% 16%	591
August	1st to 10th	Day	9% 2 14% 27% 30% 17%	526
	11th to 20th	Day	12% 3 18% 27% 26% 14%	519
	21st to end	Day	9% 3 14% 28% 32% 14%	583
September	1st to 10th	Day	13% 4 16% 24% 29% 14%	530
	11th to 20th	Day	8% 2 14% 23% 32% 20%	530
	21st to end	Day	17% 9% 18% 25% 21% 10%	530
October	1st to 10th	Day	10% 7% 22% 31% 20% 10%	530
	11th to 20th	Day	16% 14% 29% 26% 13% 3	530
	21st to end	Day	23% 10% 37% 23% 4 2	401
November	1st to 10th	Day	15% 15% 32% 25% 9% 4	330
	11th to 20th	Day	24% 19% 28% 18% 11% 1	330
	21st to end	Day	29% 12% 39% 17% 3	330
December	1st to 10th	Day	21% 17% 40% 16% 6% 1	330
	11th to 20th	Day	24% 16% 38% 18% 4	330
	21st to end	Day	30% 16% 34% 17% 2	363
January	1st to 10th	Night	22% 14% 37% 21% 5%	1050
	11th to 20th	Night	25% 15% 38% 18% 4	1044
	21st to end	Night	24% 14% 35% 22% 4	924
February	1st to 10th	Night	26% 11% 33% 28% 3	840
	11th to 20th	Night	23% 13% 39% 24% 2	823
	21st to end	Night	25% 12% 39% 21% 3	692
March	1st to 10th	Night	23% 11% 33% 26% 6%	840
	11th to 20th	Night	18% 14% 34% 27% 6%	840
	21st to end	Night	23% 13% 30% 26% 7% 1	924
April	1st to 10th	Night	20% 14% 33% 27% 5% 1	677
	11th to 20th	Night	20% 11% 34% 26% 8% 1	623
	21st to end	Night	20% 7% 36% 26% 10% 1	630
May	1st to 10th	Night	23% 6% 32% 31% 6% 1	630
	11th to 20th	Night	22% 6% 28% 29% 15% 1	621
	21st to end	Night	11% 4 23% 39% 21% 3	683
June	1st to 10th	Night	10% 3 23% 44% 20%	600
	11th to 20th	Night	11% 2 21% 45% 21% 1	600
	21st to end	Night	10% 3 21% 39% 26% 1	600
July	1st to 10th	Night	10% 2 14% 40% 29% 4	585
	11th to 20th	Night	8% 2 16% 40% 30% 5%	582
	21st to end	Night	10% 2 11% 45% 30% 2	657
August	1st to 10th	Night	9% 1 10% 45% 31% 4	597
	11th to 20th	Night	10% 2 12% 39% 34% 3	594
	21st to end	Night	8% 2 10% 39% 35% 5%	660
September	1st to 10th	Night	12% 4 14% 34% 31% 4	692
	11th to 20th	Night	9% 4 11% 36% 35% 5%	800
	21st to end	Night	15% 5% 21% 33% 23% 3	800
October	1st to 10th	Night	12% 5% 24% 37% 19% 4	800
	11th to 20th	Night	22% 10% 28% 30% 11% 1	800
	21st to end	Night	24% 10% 30% 29% 7%	880
November	1st to 10th	Night	17% 9% 35% 25% 13% 1	820
	11th to 20th	Night	21% 15% 32% 24% 6% 1	840
	21st to end	Night	26% 11% 31% 27% 5% 1	840
December	1st to 10th	Night	25% 12% 35% 22% 4	840
	11th to 20th	Night	24% 12% 34% 24% 5%	882
	21st to end	Night	30% 14% 33% 20% 3	1155

Page generated at 2026-05-29 01:53:30. Version ac07

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