Danville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Danville clear sky chart from 2005-11-06 to 2024-11-25.

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% 9% 3 7% 5% 4 4 4 6% 8% 37%	101339
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 39%	98168
Transparency	Day	44% 5% 12% 23% 14% 2	104945
	Night	45% 4 10% 22% 16% 2	102703
Seeing	Day	17% 9% 26% 25% 17% 6%	15736
	Night	18% 9% 28% 30% 14% 1	25565

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	15% 14% 10% 14% 4 4 4 3 5% 6% 21%	2527
	11th to 20th	Day	15% 11% 8% 14% 7% 6% 5% 7% 6% 5% 17%	2530
	21st to end	Day	25% 23% 5% 10% 4 3 3 2 4 5% 17%	2915
February	1st to 10th	Day	17% 12% 5% 11% 6% 4 4 3 4 5% 29%	1840
	11th to 20th	Day	15% 11% 3 9% 7% 6% 3 4 4 5% 31%	1848
	21st to end	Day	18% 14% 5% 8% 5% 4 3 2 4 6% 31%	1570
March	1st to 10th	Day	19% 19% 3 8% 5% 3 3 3 4 3 31%	3120
	11th to 20th	Day	11% 7% 4 7% 4 3 4 3 6% 9% 42%	3246
	21st to end	Day	15% 11% 5% 9% 6% 5% 5% 4 6% 7% 29%	4165
April	1st to 10th	Day	15% 10% 3 8% 5% 5% 3 4 6% 8% 35%	2280
	11th to 20th	Day	16% 9% 3 6% 5% 4 4 4 5% 6% 39%	2254
	21st to end	Day	21% 8% 3 7% 5% 4 3 3 5% 7% 35%	2365
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 7% 10% 28%	4082
	11th to 20th	Day	18% 8% 2 6% 4 4 4 3 5% 8% 37%	4081
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 6% 9% 44%	4824
June	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 5% 7% 8% 43%	2660
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 46%	2660
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 4 4 5% 6% 10% 38%	4440
	11th to 20th	Day	6% 5% 2 6% 5% 4 5% 5% 7% 11% 44%	4440
	21st to end	Day	10% 6% 2 5% 5% 5% 5% 5% 7% 11% 39%	4813
August	1st to 10th	Day	7% 4 2 5% 4 3 4 4 6% 10% 50%	4144
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 6% 8% 10% 41%	3729
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 10% 51%	4180
September	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 5% 7% 46%	2280
	11th to 20th	Day	9% 5% 1 5% 4 4 4 4 6% 7% 50%	2128
	21st to end	Day	12% 5% 2 6% 4 5% 4 3 6% 7% 46%	2090
October	1st to 10th	Day	12% 6% 2 4 4 4 4 4 5% 7% 48%	2090
	11th to 20th	Day	15% 7% 3 6% 4 4 5% 3 5% 6% 42%	1901
	21st to end	Day	18% 10% 2 8% 4 4 4 3 5% 6% 36%	1881
November	1st to 10th	Day	12% 7% 2 6% 4 3 4 4 4 6% 49%	1762
	11th to 20th	Day	18% 10% 4 7% 4 3 3 3 4 6% 39%	1800
	21st to end	Day	17% 11% 3 8% 5% 3 5% 3 4 6% 36%	1746
December	1st to 10th	Day	18% 13% 5% 10% 5% 5% 3 4 5% 5% 28%	1587
	11th to 20th	Day	14% 13% 5% 11% 5% 4 4 3 5% 5% 30%	1918
	21st to end	Day	24% 19% 7% 12% 6% 4 4 3 3 4 14%	2783
January	1st to 10th	Night	16% 16% 7% 10% 4 4 3 3 5% 4 27%	4428
	11th to 20th	Night	14% 20% 6% 12% 7% 6% 6% 4 4 3 20%	4440
	21st to end	Night	25% 16% 7% 8% 4 3 3 2 3 5% 23%	4747
February	1st to 10th	Night	19% 13% 4 8% 5% 5% 3 3 5% 6% 29%	2337
	11th to 20th	Night	14% 10% 3 10% 6% 5% 3 3 5% 7% 35%	2218
	21st to end	Night	19% 10% 4 8% 4 4 4 3 4 6% 33%	1884
March	1st to 10th	Night	19% 14% 3 7% 4 4 3 3 4 5% 34%	3733
	11th to 20th	Night	11% 8% 3 6% 4 4 4 3 4 7% 46%	3714
	21st to end	Night	18% 9% 4 9% 5% 5% 4 3 5% 7% 32%	3480
April	1st to 10th	Night	12% 9% 3 7% 5% 4 4 3 5% 6% 41%	1900
	11th to 20th	Night	18% 9% 2 5% 3 3 3 4 5% 6% 41%	1875
	21st to end	Night	17% 7% 3 7% 5% 4 5% 5% 6% 6% 35%	1812
May	1st to 10th	Night	17% 10% 3 6% 4 4 5% 4 6% 8% 33%	2813
	11th to 20th	Night	18% 7% 2 7% 4 4 4 3 6% 8% 38%	2796
	21st to end	Night	9% 6% 2 7% 5% 5% 4 4 6% 9% 43%	2743
June	1st to 10th	Night	9% 5% 3 6% 4 4 4 3 5% 8% 49%	1520
	11th to 20th	Night	7% 6% 2 6% 4 4 4 4 5% 8% 50%	1520
	21st to end	Night	7% 5% 2 6% 6% 5% 5% 4 8% 10% 44%	1520
July	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 7% 9% 45%	2530
	11th to 20th	Night	5% 3 1 4 3 3 4 4 7% 11% 54%	2530
	21st to end	Night	7% 7% 3 7% 4 5% 5% 4 6% 10% 42%	2750
August	1st to 10th	Night	8% 4 2 5% 4 3 3 4 5% 8% 55%	2781
	11th to 20th	Night	6% 4 2 6% 5% 5% 5% 4 6% 8% 48%	3100
	21st to end	Night	6% 4 2 5% 4 4 4 3 5% 8% 56%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 6% 7% 52%	1900
	11th to 20th	Night	7% 4 2 5% 4 3 4 3 5% 9% 55%	2031
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 7% 48%	2090
October	1st to 10th	Night	10% 5% 1 5% 3 3 4 4 5% 6% 54%	2090
	11th to 20th	Night	15% 6% 1 4 4 4 3 3 4 6% 50%	2275
	21st to end	Night	17% 9% 3 6% 4 3 4 3 5% 6% 40%	2717
November	1st to 10th	Night	11% 7% 2 6% 3 4 4 3 4 6% 51%	2536
	11th to 20th	Night	13% 10% 3 7% 4 4 4 4 5% 6% 41%	2600
	21st to end	Night	19% 10% 3 8% 5% 4 3 3 5% 5% 35%	2529
December	1st to 10th	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	2590
	11th to 20th	Night	17% 11% 4 9% 5% 4 4 3 4 5% 33%	3279
	21st to end	Night	26% 18% 7% 10% 5% 4 4 3 3 4 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	64% 4 9% 16% 6%	2528
	11th to 20th	Day	67% 5% 11% 9% 7%	2530
	21st to end	Day	72% 3 8% 13% 4 1	2915
February	1st to 10th	Day	59% 4 10% 17% 9% 2	1840
	11th to 20th	Day	54% 4 12% 21% 9% 1	1858
	21st to end	Day	56% 4 10% 20% 8% 2	1570
March	1st to 10th	Day	59% 4 7% 17% 12% 1	3120
	11th to 20th	Day	39% 5% 13% 28% 15% 1	3246
	21st to end	Day	53% 6% 13% 22% 5% 1	4165
April	1st to 10th	Day	48% 6% 12% 19% 14% 1	2280
	11th to 20th	Day	45% 5% 11% 23% 12% 4	2254
	21st to end	Day	50% 5% 10% 21% 12% 2	2365
May	1st to 10th	Day	49% 8% 14% 22% 7%	4120
	11th to 20th	Day	46% 7% 11% 26% 9%	4085
	21st to end	Day	35% 6% 13% 19% 22% 4	4837
June	1st to 10th	Day	37% 7% 12% 25% 16% 3	2660
	11th to 20th	Day	33% 6% 10% 24% 22% 4	2660
	21st to end	Day	39% 7% 14% 20% 16% 4	2660
July	1st to 10th	Day	43% 5% 10% 27% 13% 2	4371
	11th to 20th	Day	33% 4 15% 30% 17% 1	4365
	21st to end	Day	39% 5% 17% 20% 16% 2	4662
August	1st to 10th	Day	30% 5% 17% 33% 15% 1	4129
	11th to 20th	Day	36% 5% 12% 30% 16% 1	3765
	21st to end	Day	30% 4 15% 31% 16% 4	4179
September	1st to 10th	Day	36% 6% 11% 24% 18% 4	2280
	11th to 20th	Day	33% 5% 10% 25% 22% 4	2128
	21st to end	Day	37% 4 10% 23% 23% 2	2090
October	1st to 10th	Day	25% 3 7% 28% 34% 2	3480
	11th to 20th	Day	33% 2 9% 25% 30% 1	3129
	21st to end	Day	44% 2 8% 23% 21% 1	3081
November	1st to 10th	Day	37% 4 14% 24% 17% 4	1762
	11th to 20th	Day	48% 4 9% 21% 13% 4	1800
	21st to end	Day	52% 4 10% 21% 13% 1	1746
December	1st to 10th	Day	58% 5% 10% 19% 8% 1	1587
	11th to 20th	Day	58% 4 8% 24% 6% 1	1915
	21st to end	Day	74% 5% 8% 9% 4	2783
January	1st to 10th	Night	59% 4 12% 16% 8%	4428
	11th to 20th	Night	70% 4 7% 13% 6%	4440
	21st to end	Night	66% 3 8% 14% 7% 1	4747
February	1st to 10th	Night	57% 4 12% 16% 9% 2	2337
	11th to 20th	Night	51% 3 10% 21% 12% 2	2219
	21st to end	Night	53% 4 10% 20% 11% 2	1884
March	1st to 10th	Night	52% 5% 9% 23% 10% 1	3733
	11th to 20th	Night	37% 5% 11% 25% 20% 2	3714
	21st to end	Night	51% 6% 12% 23% 7% 1	3480
April	1st to 10th	Night	43% 6% 10% 23% 16% 2	1900
	11th to 20th	Night	44% 5% 8% 23% 16% 4	1875
	21st to end	Night	48% 6% 10% 19% 12% 4	1812
May	1st to 10th	Night	48% 7% 12% 24% 9%	2840
	11th to 20th	Night	43% 7% 14% 25% 10% 1	2797
	21st to end	Night	36% 7% 13% 20% 20% 4	2751
June	1st to 10th	Night	34% 5% 10% 28% 20% 3	1520
	11th to 20th	Night	33% 6% 11% 25% 22% 4	1520
	21st to end	Night	34% 7% 15% 23% 15% 5%	1520
July	1st to 10th	Night	37% 6% 12% 27% 14% 5%	2490
	11th to 20th	Night	24% 4 14% 33% 24% 1	2482
	21st to end	Night	38% 4 15% 21% 17% 4	2662
August	1st to 10th	Night	29% 4 17% 33% 16% 1	2772
	11th to 20th	Night	33% 5% 14% 28% 19% 1	3130
	21st to end	Night	29% 4 12% 32% 19% 4	3476
September	1st to 10th	Night	32% 5% 10% 25% 20% 7%	1900
	11th to 20th	Night	28% 4 10% 25% 28% 5%	2031
	21st to end	Night	37% 4 9% 24% 22% 4	2090
October	1st to 10th	Night	21% 3 6% 30% 38% 2	3480
	11th to 20th	Night	29% 2 8% 26% 33% 2	3786
	21st to end	Night	38% 2 7% 23% 25% 5%	4467
November	1st to 10th	Night	36% 4 9% 27% 20% 4	2536
	11th to 20th	Night	44% 4 11% 22% 17% 3	2600
	21st to end	Night	51% 3 10% 23% 12% 1	2529
December	1st to 10th	Night	55% 4 11% 19% 10% 1	2590
	11th to 20th	Night	53% 4 11% 19% 12% 1	3281
	21st to end	Night	73% 4 7% 10% 6%	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	26% 15% 32% 21% 6%	270
	11th to 20th	Day	26% 17% 36% 17% 4 1	270
	21st to end	Day	27% 15% 34% 20% 4	297
February	1st to 10th	Day	28% 16% 34% 18% 3	270
	11th to 20th	Day	27% 18% 34% 14% 4 2	270
	21st to end	Day	30% 15% 40% 13% 2	225
March	1st to 10th	Day	21% 21% 37% 16% 4	270
	11th to 20th	Day	28% 16% 37% 16% 3	317
	21st to end	Day	32% 14% 29% 18% 7% 1	506
April	1st to 10th	Day	20% 17% 33% 22% 8% 2	460
	11th to 20th	Day	20% 19% 27% 19% 13% 2	452
	21st to end	Day	27% 13% 32% 18% 8% 2	460
May	1st to 10th	Day	21% 10% 31% 24% 10% 3	463
	11th to 20th	Day	23% 7% 25% 26% 13% 6%	487
	21st to end	Day	12% 7% 29% 25% 18% 9%	734
June	1st to 10th	Day	12% 7% 26% 30% 21% 5%	680
	11th to 20th	Day	10% 5% 26% 31% 22% 6%	680
	21st to end	Day	12% 7% 28% 28% 19% 6%	680
July	1st to 10th	Day	12% 3 15% 29% 26% 14%	671
	11th to 20th	Day	8% 4 18% 31% 26% 12%	668
	21st to end	Day	10% 2 17% 32% 29% 10%	739
August	1st to 10th	Day	8% 2 16% 31% 29% 15%	532
	11th to 20th	Day	10% 4 16% 30% 30% 11%	479
	21st to end	Day	8% 3 15% 27% 35% 14%	539
September	1st to 10th	Day	13% 3 15% 30% 30% 9%	490
	11th to 20th	Day	8% 3 13% 30% 29% 16%	490
	21st to end	Day	14% 9% 23% 20% 22% 12%	490
October	1st to 10th	Day	12% 5% 26% 32% 18% 8%	490
	11th to 20th	Day	13% 18% 31% 26% 9% 2	308
	21st to end	Day	29% 14% 33% 19% 5%	330
November	1st to 10th	Day	13% 13% 35% 23% 12% 4	300
	11th to 20th	Day	25% 16% 33% 16% 9% 1	300
	21st to end	Day	24% 10% 38% 22% 6%	282
December	1st to 10th	Day	19% 17% 37% 17% 8% 1	270
	11th to 20th	Day	20% 13% 42% 17% 7% 1	270
	21st to end	Day	28% 17% 32% 22% 2	297
January	1st to 10th	Night	22% 14% 36% 23% 4	950
	11th to 20th	Night	24% 15% 35% 22% 5%	950
	21st to end	Night	24% 12% 35% 26% 3	1045
February	1st to 10th	Night	27% 11% 35% 24% 3	817
	11th to 20th	Night	21% 15% 35% 26% 3	743
	21st to end	Night	27% 13% 36% 21% 3	628
March	1st to 10th	Night	23% 14% 34% 25% 4	760
	11th to 20th	Night	19% 12% 38% 25% 6% 1	760
	21st to end	Night	24% 8% 37% 24% 6% 1	641
April	1st to 10th	Night	17% 14% 35% 27% 6%	570
	11th to 20th	Night	22% 12% 30% 26% 10%	563
	21st to end	Night	20% 10% 35% 27% 8% 1	570
May	1st to 10th	Night	20% 7% 33% 32% 8%	570
	11th to 20th	Night	19% 6% 26% 30% 17% 2	561
	21st to end	Night	12% 5% 26% 36% 20% 2	617
June	1st to 10th	Night	11% 3 24% 43% 19%	570
	11th to 20th	Night	11% 2 22% 44% 19% 1	570
	21st to end	Night	8% 3 23% 40% 24% 1	570
July	1st to 10th	Night	11% 2 14% 37% 30% 6%	555
	11th to 20th	Night	5% 3 17% 41% 31% 4	552
	21st to end	Night	8% 2 12% 43% 33% 2	624
August	1st to 10th	Night	8% 2 10% 43% 32% 5%	567
	11th to 20th	Night	7% 3 12% 42% 32% 4	564
	21st to end	Night	8% 1 14% 32% 42% 3	627
September	1st to 10th	Night	15% 3 14% 35% 30% 3	570
	11th to 20th	Night	8% 3 14% 39% 32% 4	701
	21st to end	Night	13% 6% 20% 30% 28% 2	760
October	1st to 10th	Night	13% 5% 22% 38% 20% 3	760
	11th to 20th	Night	20% 10% 29% 27% 14%	760
	21st to end	Night	21% 14% 34% 25% 6%	836
November	1st to 10th	Night	16% 9% 38% 25% 11% 1	780
	11th to 20th	Night	20% 14% 31% 27% 8% 1	800
	21st to end	Night	25% 11% 31% 26% 7%	779
December	1st to 10th	Night	23% 11% 36% 24% 5%	880
	11th to 20th	Night	25% 9% 33% 27% 6%	950
	21st to end	Night	31% 13% 30% 22% 4	1045

Page generated at 2024-11-22 07:02:24. Version ac07

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