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 2025-02-06.

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% 4 4 5% 7% 34%	108595
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 34%	109749
Transparency	Day	48% 6% 12% 21% 12% 1	112239
	Night	49% 5% 11% 21% 13% 2	114233
Seeing	Day	18% 9% 27% 24% 15% 7%	15816
	Night	19% 9% 28% 30% 13% 1	25729

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	14% 19% 9% 16% 7% 5% 3 3 3 4 18%	3617
	11th to 20th	Day	14% 17% 7% 13% 8% 5% 7% 5% 3 3 18%	3634
	21st to end	Day	22% 17% 9% 12% 6% 4 3 3 3 4 17%	4466
February	1st to 10th	Day	18% 14% 6% 11% 7% 5% 3 4 4 5% 22%	1807
	11th to 20th	Day	15% 13% 5% 15% 7% 5% 4 3 4 5% 24%	1848
	21st to end	Day	16% 14% 6% 9% 5% 4 3 3 5% 7% 27%	1570
March	1st to 10th	Day	21% 20% 5% 8% 5% 3 3 3 3 5% 23%	3141
	11th to 20th	Day	14% 10% 3 7% 4 5% 4 4 5% 7% 37%	3480
	21st to end	Day	20% 11% 5% 9% 5% 5% 4 3 5% 6% 27%	3815
April	1st to 10th	Day	14% 13% 3 9% 6% 5% 4 4 5% 6% 30%	2251
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 35%	2334
	21st to end	Day	19% 7% 3 7% 6% 4 5% 4 5% 7% 33%	2470
May	1st to 10th	Day	20% 9% 2 8% 5% 5% 5% 4 6% 7% 29%	4082
	11th to 20th	Day	19% 6% 2 7% 4 5% 4 4 6% 8% 36%	4081
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	4476
June	1st to 10th	Day	12% 7% 2 7% 4 4 6% 5% 6% 9% 39%	2596
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 7% 10% 41%	2660
	21st to end	Day	10% 5% 2 6% 6% 6% 5% 5% 7% 10% 39%	2660
July	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	7% 3 2 5% 4 4 5% 5% 6% 11% 46%	4440
	21st to end	Day	7% 6% 3 7% 6% 5% 6% 5% 8% 9% 39%	4480
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 4 6% 10% 43%	4104
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 44%	4044
	21st to end	Day	7% 5% 2 5% 4 4 4 4 7% 10% 47%	4287
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 7% 48%	2173
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 4 7% 8% 46%	2090
	21st to end	Day	17% 7% 2 5% 3 4 4 3 4 7% 44%	2090
October	1st to 10th	Day	12% 7% 3 7% 4 5% 4 3 5% 7% 45%	2090
	11th to 20th	Day	15% 6% 2 7% 5% 5% 4 5% 5% 6% 41%	1910
	21st to end	Day	18% 10% 3 8% 6% 4 5% 3 5% 7% 31%	1917
November	1st to 10th	Day	10% 9% 3 6% 4 3 2 3 5% 6% 49%	1762
	11th to 20th	Day	17% 13% 3 8% 5% 4 4 3 4 6% 32%	1800
	21st to end	Day	20% 14% 4 8% 5% 4 3 3 5% 5% 29%	1800
December	1st to 10th	Day	15% 15% 5% 11% 6% 3 4 3 4 5% 29%	2930
	11th to 20th	Day	16% 17% 8% 11% 5% 4 5% 3 4 5% 22%	3268
	21st to end	Day	24% 20% 8% 10% 6% 5% 3 3 3 4 13%	3982
January	1st to 10th	Night	16% 17% 7% 18% 6% 4 3 3 4 4 19%	6348
	11th to 20th	Night	12% 19% 9% 16% 7% 6% 5% 4 4 4 14%	6342
	21st to end	Night	22% 15% 6% 9% 4 4 4 4 4 4 24%	6490
February	1st to 10th	Night	20% 14% 5% 9% 6% 5% 4 3 4 4 25%	2483
	11th to 20th	Night	14% 14% 5% 10% 6% 4 5% 3 5% 6% 28%	2218
	21st to end	Night	20% 13% 4 10% 5% 4 4 3 5% 5% 27%	1884
March	1st to 10th	Night	19% 14% 4 7% 4 4 3 2 3 5% 34%	3711
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 6% 7% 45%	3480
	21st to end	Night	18% 10% 4 8% 5% 4 3 3 4 5% 37%	3817
April	1st to 10th	Night	16% 10% 2 6% 4 4 4 4 6% 7% 36%	1943
	11th to 20th	Night	18% 7% 3 6% 5% 4 4 3 5% 6% 37%	1793
	21st to end	Night	18% 9% 2 7% 5% 4 4 4 5% 6% 36%	1710
May	1st to 10th	Night	19% 9% 3 9% 5% 5% 5% 3 5% 7% 29%	2813
	11th to 20th	Night	20% 7% 3 4 4 3 4 3 5% 8% 37%	2812
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 6% 10% 44%	3079
June	1st to 10th	Night	9% 5% 2 7% 4 4 4 5% 6% 6% 48%	1579
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 4 6% 9% 47%	1520
	21st to end	Night	10% 4 3 6% 5% 4 4 4 7% 9% 44%	1520
July	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	2530
	11th to 20th	Night	7% 4 1 5% 4 4 4 4 6% 10% 52%	2530
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 7% 8% 43%	3069
August	1st to 10th	Night	12% 7% 2 7% 4 4 4 4 6% 9% 40%	2825
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 4 6% 7% 48%	2785
	21st to end	Night	6% 4 2 6% 4 4 4 3 6% 7% 51%	3366
September	1st to 10th	Night	12% 5% 2 4 4 3 4 4 4 7% 52%	1987
	11th to 20th	Night	9% 4 2 5% 4 2 3 3 5% 7% 55%	2090
	21st to end	Night	13% 6% 2 6% 5% 4 4 4 5% 5% 46%	2090
October	1st to 10th	Night	10% 6% 2 5% 4 3 4 3 5% 7% 50%	2090
	11th to 20th	Night	17% 9% 3 7% 4 4 3 3 4 6% 41%	2270
	21st to end	Night	20% 7% 2 6% 4 4 4 4 6% 8% 37%	2679
November	1st to 10th	Night	13% 10% 2 7% 4 4 3 2 5% 5% 45%	2536
	11th to 20th	Night	15% 12% 4 8% 5% 4 4 3 5% 7% 35%	2600
	21st to end	Night	18% 14% 4 8% 5% 4 4 4 4 6% 27%	2600
December	1st to 10th	Night	14% 12% 5% 9% 5% 5% 5% 4 6% 7% 26%	4250
	11th to 20th	Night	17% 18% 7% 12% 5% 4 3 3 4 5% 21%	4914
	21st to end	Night	25% 20% 6% 9% 5% 5% 4 3 4 5% 14%	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	71% 4 7% 14% 4	3618
	11th to 20th	Day	71% 4 9% 7% 9% 1	3634
	21st to end	Day	72% 3 8% 10% 7%	4466
February	1st to 10th	Day	64% 4 11% 13% 6% 1	1807
	11th to 20th	Day	63% 4 8% 17% 7%	1858
	21st to end	Day	58% 4 13% 16% 7% 2	1570
March	1st to 10th	Day	64% 4 9% 16% 6%	3141
	11th to 20th	Day	45% 6% 11% 27% 11% 1	3480
	21st to end	Day	56% 7% 15% 18% 4	3815
April	1st to 10th	Day	53% 6% 11% 18% 12% 1	2251
	11th to 20th	Day	48% 5% 11% 22% 12% 2	2334
	21st to end	Day	50% 6% 13% 19% 10% 2	2470
May	1st to 10th	Day	52% 8% 11% 20% 9% 1	4120
	11th to 20th	Day	44% 8% 13% 25% 8% 1	4085
	21st to end	Day	37% 9% 13% 21% 18% 2	4489
June	1st to 10th	Day	39% 7% 12% 24% 15% 3	2596
	11th to 20th	Day	35% 9% 13% 25% 16% 2	2660
	21st to end	Day	37% 9% 15% 20% 15% 4	2660
July	1st to 10th	Day	41% 7% 12% 21% 16% 2	4371
	11th to 20th	Day	30% 8% 14% 28% 19% 1	4365
	21st to end	Day	38% 8% 14% 19% 18% 3	4338
August	1st to 10th	Day	34% 8% 17% 25% 15% 1	4091
	11th to 20th	Day	32% 8% 13% 30% 15% 1	4083
	21st to end	Day	30% 7% 14% 32% 14% 3	4286
September	1st to 10th	Day	35% 6% 14% 23% 19% 4	2173
	11th to 20th	Day	34% 7% 11% 25% 22% 2	2090
	21st to end	Day	41% 6% 9% 24% 20% 1	2090
October	1st to 10th	Day	27% 4 12% 33% 23% 1	3480
	11th to 20th	Day	34% 5% 12% 25% 23% 1	3150
	21st to end	Day	43% 4 10% 24% 18% 1	3129
November	1st to 10th	Day	37% 4 12% 28% 18% 1	1762
	11th to 20th	Day	53% 5% 9% 20% 11% 2	1800
	21st to end	Day	58% 4 11% 18% 8% 1	1800
December	1st to 10th	Day	59% 5% 13% 20% 4	2930
	11th to 20th	Day	65% 4 12% 15% 4	3265
	21st to end	Day	76% 4 7% 10% 4	3982
January	1st to 10th	Night	68% 5% 7% 14% 5%	6348
	11th to 20th	Night	74% 4 7% 8% 7%	6342
	21st to end	Night	64% 4 13% 13% 6% 1	6490
February	1st to 10th	Night	63% 5% 10% 14% 7% 1	2483
	11th to 20th	Night	57% 5% 10% 17% 10% 1	2219
	21st to end	Night	59% 5% 9% 16% 9% 2	1884
March	1st to 10th	Night	54% 4 8% 24% 9% 1	3711
	11th to 20th	Night	37% 7% 12% 29% 14% 2	3480
	21st to end	Night	50% 6% 11% 25% 8% 1	3817
April	1st to 10th	Night	47% 6% 12% 20% 14% 1	1943
	11th to 20th	Night	47% 6% 10% 18% 17% 3	1793
	21st to end	Night	48% 5% 11% 20% 13% 3	1710
May	1st to 10th	Night	53% 8% 9% 19% 10% 2	2840
	11th to 20th	Night	43% 8% 13% 25% 10% 1	2813
	21st to end	Night	32% 9% 14% 24% 17% 2	3087
June	1st to 10th	Night	35% 6% 11% 26% 19% 3	1579
	11th to 20th	Night	32% 8% 13% 26% 18% 3	1520
	21st to end	Night	34% 8% 15% 24% 16% 3	1520
July	1st to 10th	Night	35% 6% 11% 26% 16% 5%	2490
	11th to 20th	Night	27% 7% 16% 29% 21% 1	2482
	21st to end	Night	36% 6% 15% 19% 18% 5%	2971
August	1st to 10th	Night	38% 7% 14% 25% 14% 1	2814
	11th to 20th	Night	33% 7% 11% 31% 17% 1	2812
	21st to end	Night	31% 7% 13% 27% 19% 3	3366
September	1st to 10th	Night	32% 6% 13% 25% 19% 6%	1987
	11th to 20th	Night	29% 6% 9% 27% 25% 3	2090
	21st to end	Night	37% 7% 9% 23% 20% 3	2090
October	1st to 10th	Night	23% 4 8% 31% 33% 2	3480
	11th to 20th	Night	35% 4 11% 25% 24% 1	3769
	21st to end	Night	34% 4 8% 31% 20% 3	4405
November	1st to 10th	Night	40% 5% 10% 25% 18% 2	2536
	11th to 20th	Night	49% 4 10% 21% 14% 3	2600
	21st to end	Night	58% 4 11% 19% 7%	2600
December	1st to 10th	Night	56% 5% 12% 21% 5%	4250
	11th to 20th	Night	66% 5% 12% 12% 6%	4916
	21st to end	Night	73% 5% 9% 10% 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% 36% 18% 5% 1	300
	11th to 20th	Day	25% 16% 41% 13% 4 1	300
	21st to end	Day	33% 12% 35% 15% 5% 1	330
February	1st to 10th	Day	28% 16% 38% 14% 4 1	285
	11th to 20th	Day	24% 19% 34% 18% 6%	270
	21st to end	Day	23% 18% 45% 12% 2	225
March	1st to 10th	Day	26% 17% 38% 15% 5%	278
	11th to 20th	Day	27% 13% 29% 23% 7% 1	460
	21st to end	Day	30% 13% 31% 19% 6%	506
April	1st to 10th	Day	19% 16% 31% 23% 8% 2	460
	11th to 20th	Day	23% 17% 27% 20% 10% 5%	452
	21st to end	Day	23% 12% 33% 23% 8% 2	460
May	1st to 10th	Day	25% 10% 28% 24% 10% 3	463
	11th to 20th	Day	24% 5% 24% 25% 14% 8%	487
	21st to end	Day	11% 4 26% 28% 22% 9%	529
June	1st to 10th	Day	14% 5% 24% 32% 19% 6%	616
	11th to 20th	Day	13% 5% 26% 27% 23% 7%	680
	21st to end	Day	11% 8% 25% 29% 20% 7%	680
July	1st to 10th	Day	11% 3 18% 29% 20% 18%	671
	11th to 20th	Day	7% 3 23% 29% 23% 14%	668
	21st to end	Day	10% 2 17% 30% 25% 16%	546
August	1st to 10th	Day	10% 2 15% 27% 29% 17%	486
	11th to 20th	Day	13% 3 20% 27% 24% 14%	479
	21st to end	Day	9% 3 14% 27% 32% 15%	539
September	1st to 10th	Day	14% 4 15% 25% 29% 13%	490
	11th to 20th	Day	8% 3 15% 24% 32% 18%	490
	21st to end	Day	17% 10% 19% 25% 20% 9%	490
October	1st to 10th	Day	10% 7% 23% 31% 19% 10%	490
	11th to 20th	Day	16% 14% 30% 24% 12% 3	490
	21st to end	Day	24% 11% 37% 22% 4 2	366
November	1st to 10th	Day	14% 14% 33% 25% 10% 4	300
	11th to 20th	Day	23% 18% 28% 19% 12% 1	300
	21st to end	Day	29% 12% 39% 18% 2	300
December	1st to 10th	Day	22% 16% 40% 15% 6% 1	300
	11th to 20th	Day	23% 15% 39% 19% 4	300
	21st to end	Day	30% 16% 34% 18% 3	330
January	1st to 10th	Night	23% 15% 37% 21% 5%	1000
	11th to 20th	Night	26% 13% 37% 19% 4	995
	21st to end	Night	25% 13% 35% 23% 4	880
February	1st to 10th	Night	27% 11% 33% 26% 3	781
	11th to 20th	Night	23% 13% 39% 24% 2	743
	21st to end	Night	28% 11% 38% 20% 3	628
March	1st to 10th	Night	22% 11% 33% 27% 7%	760
	11th to 20th	Night	18% 13% 34% 28% 6% 1	760
	21st to end	Night	23% 12% 29% 27% 8% 1	836
April	1st to 10th	Night	19% 14% 33% 28% 5% 1	613
	11th to 20th	Night	21% 10% 33% 27% 8% 1	563
	21st to end	Night	22% 7% 36% 25% 9% 1	570
May	1st to 10th	Night	22% 5% 32% 32% 7% 1	570
	11th to 20th	Night	22% 5% 27% 30% 16% 1	561
	21st to end	Night	10% 4 23% 37% 23% 3	617
June	1st to 10th	Night	9% 2 23% 45% 21%	570
	11th to 20th	Night	10% 2 22% 45% 21% 1	570
	21st to end	Night	10% 3 21% 40% 25% 1	570
July	1st to 10th	Night	11% 2 14% 40% 28% 5%	555
	11th to 20th	Night	7% 2 16% 40% 30% 5%	552
	21st to end	Night	10% 1 11% 46% 30% 2	624
August	1st to 10th	Night	10% 1 10% 45% 31% 4	567
	11th to 20th	Night	10% 2 12% 40% 32% 4	564
	21st to end	Night	8% 2 11% 38% 36% 5%	627
September	1st to 10th	Night	13% 4 14% 34% 30% 5%	657
	11th to 20th	Night	10% 4 12% 36% 34% 5%	760
	21st to end	Night	15% 5% 23% 33% 22% 3	760
October	1st to 10th	Night	12% 5% 24% 36% 18% 4	760
	11th to 20th	Night	22% 10% 28% 29% 11% 1	760
	21st to end	Night	24% 9% 31% 28% 7%	836
November	1st to 10th	Night	18% 9% 33% 25% 13% 1	780
	11th to 20th	Night	21% 15% 32% 24% 7% 1	800
	21st to end	Night	26% 11% 31% 27% 5% 1	800
December	1st to 10th	Night	26% 13% 35% 22% 4 1	800
	11th to 20th	Night	25% 11% 34% 24% 6%	840
	21st to end	Night	30% 14% 33% 20% 3	1100

Page generated at 2025-02-03 03:05:55. Version ac07

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