Purdue University
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Purdue University clear sky chart from 2005-11-06 to 2024-11-04.

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% 9% 3 7% 5% 4 4 4 6% 7% 37%	101113
	Night	15% 10% 3 7% 4 4 4 3 5% 6% 38%	97958
Transparency	Day	46% 3 12% 24% 14% 2	104723
	Night	46% 3 11% 23% 15% 2	102476
Seeing	Day	18% 10% 27% 24% 15% 6%	15671
	Night	19% 9% 28% 29% 13% 1	25484

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% 15% 9% 12% 6% 5% 3 4 5% 5% 20%	2527
	11th to 20th	Day	15% 13% 8% 15% 6% 5% 6% 7% 5% 4 16%	2530
	21st to end	Day	27% 22% 7% 9% 4 3 3 2 3 4 17%	2823
February	1st to 10th	Day	18% 13% 4 10% 8% 6% 4 3 4 5% 25%	1881
	11th to 20th	Day	16% 12% 3 10% 5% 4 4 3 5% 6% 32%	1848
	21st to end	Day	17% 15% 5% 9% 4 4 4 3 4 6% 29%	1570
March	1st to 10th	Day	20% 19% 4 7% 4 4 3 3 3 4 28%	3120
	11th to 20th	Day	12% 8% 3 7% 5% 4 4 3 5% 7% 42%	3305
	21st to end	Day	16% 12% 5% 8% 5% 5% 4 4 5% 7% 28%	4087
April	1st to 10th	Day	15% 12% 3 8% 4 4 4 4 5% 7% 35%	2280
	11th to 20th	Day	17% 9% 3 5% 5% 4 5% 3 5% 6% 38%	2254
	21st to end	Day	20% 9% 3 7% 4 4 3 3 6% 7% 34%	2413
May	1st to 10th	Day	18% 8% 3 8% 5% 6% 5% 4 6% 8% 29%	4082
	11th to 20th	Day	18% 7% 3 7% 4 4 4 3 6% 7% 37%	4081
	21st to end	Day	9% 6% 3 6% 4 4 5% 5% 6% 9% 43%	4757
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	2660
	11th to 20th	Day	9% 4 2 6% 5% 4 4 4 7% 8% 46%	2660
	21st to end	Day	11% 6% 2 7% 5% 5% 4 4 7% 9% 40%	2660
July	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 4 7% 10% 36%	4440
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 7% 11% 45%	4440
	21st to end	Day	10% 6% 2 7% 4 4 5% 4 7% 9% 42%	4813
August	1st to 10th	Day	8% 5% 1 5% 4 4 4 4 7% 10% 48%	4112
	11th to 20th	Day	9% 5% 2 5% 4 5% 5% 5% 7% 11% 42%	3800
	21st to end	Day	6% 5% 2 5% 4 4 4 4 7% 9% 51%	4180
September	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 3 6% 7% 47%	2280
	11th to 20th	Day	10% 5% 2 5% 3 4 5% 4 7% 7% 49%	2100
	21st to end	Day	13% 7% 2 5% 4 4 4 4 4 6% 46%	2090
October	1st to 10th	Day	13% 6% 1 5% 4 4 3 4 5% 8% 47%	2090
	11th to 20th	Day	16% 6% 2 7% 5% 4 4 4 5% 5% 42%	1904
	21st to end	Day	19% 9% 3 8% 4 4 5% 4 4 6% 34%	1881
November	1st to 10th	Day	13% 6% 3 4 3 4 3 3 5% 7% 50%	1699
	11th to 20th	Day	17% 12% 3 7% 4 3 3 3 4 5% 38%	1710
	21st to end	Day	17% 11% 3 8% 5% 4 3 4 5% 5% 35%	1710
December	1st to 10th	Day	18% 14% 5% 10% 6% 4 3 3 5% 5% 28%	1625
	11th to 20th	Day	13% 15% 6% 13% 5% 4 4 3 5% 5% 27%	1918
	21st to end	Day	24% 21% 8% 12% 5% 4 3 3 3 3 14%	2783
January	1st to 10th	Night	17% 18% 7% 11% 5% 4 3 2 4 4 24%	4428
	11th to 20th	Night	13% 20% 7% 15% 5% 5% 6% 3 4 4 18%	4440
	21st to end	Night	24% 18% 7% 9% 4 3 3 3 4 4 21%	4842
February	1st to 10th	Night	19% 14% 4 9% 6% 4 3 3 4 6% 27%	2297
	11th to 20th	Night	14% 12% 4 10% 5% 4 3 3 4 5% 35%	2218
	21st to end	Night	20% 13% 3 9% 5% 3 3 3 4 6% 31%	1884
March	1st to 10th	Night	18% 13% 4 6% 4 4 3 3 4 6% 35%	3733
	11th to 20th	Night	11% 7% 3 6% 5% 4 4 3 5% 6% 46%	3654
	21st to end	Night	20% 9% 4 8% 4 4 4 4 5% 6% 33%	3572
April	1st to 10th	Night	14% 10% 2 7% 4 3 4 4 6% 6% 39%	1900
	11th to 20th	Night	18% 9% 2 5% 4 4 5% 4 5% 6% 38%	1875
	21st to end	Night	18% 7% 3 7% 5% 4 4 4 6% 7% 35%	1766
May	1st to 10th	Night	17% 9% 3 8% 5% 4 5% 4 6% 8% 31%	2813
	11th to 20th	Night	19% 7% 2 6% 3 4 5% 4 5% 7% 38%	2812
	21st to end	Night	10% 6% 2 6% 5% 5% 4 4 6% 8% 44%	2804
June	1st to 10th	Night	9% 5% 2 5% 4 5% 4 3 5% 7% 50%	1520
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 7% 7% 49%	1520
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 6% 9% 43%	1520
July	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 3 6% 10% 45%	2530
	11th to 20th	Night	5% 4 1 4 3 3 3 4 6% 9% 56%	2530
	21st to end	Night	7% 6% 3 7% 5% 5% 5% 3 6% 9% 44%	2750
August	1st to 10th	Night	10% 4 1 5% 4 4 5% 3 7% 9% 50%	2815
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 5% 9% 47%	3026
	21st to end	Night	7% 4 2 5% 4 3 3 3 5% 8% 55%	3476
September	1st to 10th	Night	11% 6% 2 5% 3 4 3 4 5% 6% 52%	1900
	11th to 20th	Night	8% 3 1 5% 3 3 4 3 5% 8% 56%	2061
	21st to end	Night	12% 6% 2 5% 4 5% 3 3 5% 6% 49%	2090
October	1st to 10th	Night	10% 5% 2 6% 3 4 4 3 5% 5% 54%	2090
	11th to 20th	Night	17% 5% 2 5% 3 4 3 3 4 6% 47%	2270
	21st to end	Night	19% 8% 2 7% 4 4 4 4 5% 7% 38%	2717
November	1st to 10th	Night	11% 7% 2 5% 4 4 3 3 5% 6% 50%	2452
	11th to 20th	Night	14% 10% 3 7% 4 3 3 3 6% 7% 39%	2470
	21st to end	Night	19% 12% 3 9% 5% 4 4 2 3 5% 34%	2470
December	1st to 10th	Night	16% 14% 5% 9% 6% 4 3 3 5% 5% 29%	2550
	11th to 20th	Night	16% 14% 4 8% 4 4 4 3 5% 5% 32%	3279
	21st to end	Night	24% 18% 7% 11% 5% 4 4 3 4 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	67% 2 8% 17% 6%	2528
	11th to 20th	Day	70% 2 10% 10% 7%	2530
	21st to end	Day	74% 2 8% 13% 4 1	2823
February	1st to 10th	Day	62% 3 10% 15% 9% 2	1881
	11th to 20th	Day	53% 3 11% 23% 9% 1	1858
	21st to end	Day	57% 3 11% 20% 7% 2	1570
March	1st to 10th	Day	61% 2 6% 19% 10% 1	3120
	11th to 20th	Day	42% 3 11% 29% 14% 1	3305
	21st to end	Day	56% 3 15% 22% 4	4087
April	1st to 10th	Day	50% 3 13% 20% 14%	2280
	11th to 20th	Day	47% 3 11% 25% 11% 3	2254
	21st to end	Day	50% 3 13% 20% 11% 2	2413
May	1st to 10th	Day	53% 3 13% 24% 7%	4120
	11th to 20th	Day	48% 3 13% 26% 9% 1	4085
	21st to end	Day	37% 4 12% 24% 21% 3	4770
June	1st to 10th	Day	37% 5% 13% 26% 16% 3	2660
	11th to 20th	Day	34% 5% 12% 26% 21% 4	2660
	21st to end	Day	40% 5% 15% 20% 16% 4	2660
July	1st to 10th	Day	44% 3 11% 25% 14% 2	4371
	11th to 20th	Day	33% 3 16% 30% 17% 1	4365
	21st to end	Day	39% 4 17% 20% 18% 3	4662
August	1st to 10th	Day	31% 5% 17% 31% 16% 1	4098
	11th to 20th	Day	34% 3 13% 33% 15% 1	3838
	21st to end	Day	30% 3 14% 30% 18% 4	4179
September	1st to 10th	Day	36% 4 12% 25% 18% 4	2280
	11th to 20th	Day	33% 4 11% 26% 22% 4	2100
	21st to end	Day	39% 3 9% 25% 21% 2	2090
October	1st to 10th	Day	26% 2 8% 31% 31% 2	3480
	11th to 20th	Day	35% 2 9% 26% 28% 1	3133
	21st to end	Day	46% 2 7% 27% 17% 2	3081
November	1st to 10th	Day	35% 3 15% 27% 17% 4	1699
	11th to 20th	Day	49% 2 10% 23% 12% 2	1710
	21st to end	Day	51% 3 11% 23% 10% 1	1710
December	1st to 10th	Day	60% 2 11% 19% 8%	1625
	11th to 20th	Day	60% 3 7% 25% 4 1	1915
	21st to end	Day	76% 3 8% 9% 4	2783
January	1st to 10th	Night	65% 2 10% 16% 6%	4428
	11th to 20th	Night	71% 2 8% 13% 6%	4440
	21st to end	Night	68% 2 8% 14% 6%	4842
February	1st to 10th	Night	59% 3 11% 17% 8% 2	2297
	11th to 20th	Night	53% 3 10% 21% 12% 1	2219
	21st to end	Night	56% 3 10% 20% 9% 2	1884
March	1st to 10th	Night	52% 2 10% 24% 9% 2	3733
	11th to 20th	Night	39% 4 10% 27% 18% 2	3654
	21st to end	Night	53% 3 13% 24% 6% 1	3572
April	1st to 10th	Night	44% 4 12% 24% 15% 2	1900
	11th to 20th	Night	46% 4 9% 22% 15% 4	1875
	21st to end	Night	47% 5% 11% 21% 13% 4	1766
May	1st to 10th	Night	51% 4 12% 23% 10% 1	2840
	11th to 20th	Night	45% 5% 14% 26% 9% 1	2813
	21st to end	Night	38% 5% 11% 23% 20% 4	2812
June	1st to 10th	Night	33% 5% 11% 27% 21% 3	1520
	11th to 20th	Night	33% 4 13% 24% 23% 4	1520
	21st to end	Night	37% 6% 14% 23% 15% 5%	1520
July	1st to 10th	Night	37% 3 13% 27% 13% 6%	2490
	11th to 20th	Night	26% 2 14% 32% 25% 1	2482
	21st to end	Night	38% 3 15% 22% 17% 4	2662
August	1st to 10th	Night	32% 3 18% 30% 14% 2	2805
	11th to 20th	Night	35% 3 13% 29% 19% 1	3054
	21st to end	Night	29% 3 11% 32% 21% 4	3476
September	1st to 10th	Night	33% 3 12% 26% 19% 7%	1900
	11th to 20th	Night	27% 4 9% 30% 26% 4	2061
	21st to end	Night	37% 4 9% 25% 22% 3	2090
October	1st to 10th	Night	21% 2 6% 33% 36% 2	3480
	11th to 20th	Night	32% 2 8% 24% 33% 2	3767
	21st to end	Night	37% 2 8% 25% 24% 3	4467
November	1st to 10th	Night	36% 3 9% 28% 21% 3	2452
	11th to 20th	Night	44% 3 10% 24% 16% 3	2470
	21st to end	Night	54% 2 9% 24% 10%	2470
December	1st to 10th	Night	56% 3 10% 20% 9% 1	2550
	11th to 20th	Night	55% 2 10% 20% 11% 1	3281
	21st to end	Night	73% 3 9% 10% 5%	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% 16% 33% 19% 6% 1	270
	11th to 20th	Day	27% 16% 39% 13% 3 1	270
	21st to end	Day	31% 15% 34% 17% 3	297
February	1st to 10th	Day	27% 17% 36% 16% 4	270
	11th to 20th	Day	28% 19% 33% 13% 7% 1	270
	21st to end	Day	33% 16% 37% 12% 2	225
March	1st to 10th	Day	24% 21% 37% 16% 2	270
	11th to 20th	Day	32% 11% 37% 14% 5%	350
	21st to end	Day	33% 13% 31% 16% 6% 1	506
April	1st to 10th	Day	22% 17% 32% 21% 7% 2	460
	11th to 20th	Day	23% 16% 30% 17% 11% 3	452
	21st to end	Day	27% 17% 27% 19% 8% 2	460
May	1st to 10th	Day	22% 11% 34% 21% 10% 2	463
	11th to 20th	Day	24% 7% 26% 24% 14% 5%	487
	21st to end	Day	12% 6% 31% 26% 16% 9%	693
June	1st to 10th	Day	13% 6% 28% 30% 19% 5%	680
	11th to 20th	Day	12% 4 26% 33% 19% 6%	680
	21st to end	Day	12% 7% 30% 29% 16% 7%	680
July	1st to 10th	Day	13% 3 19% 26% 25% 14%	671
	11th to 20th	Day	9% 3 22% 32% 21% 13%	668
	21st to end	Day	11% 2 18% 32% 25% 12%	739
August	1st to 10th	Day	8% 3 15% 30% 29% 15%	494
	11th to 20th	Day	12% 4 20% 28% 26% 11%	479
	21st to end	Day	8% 3 15% 31% 31% 12%	539
September	1st to 10th	Day	13% 6% 14% 32% 26% 8%	490
	11th to 20th	Day	10% 2 14% 29% 29% 15%	490
	21st to end	Day	16% 11% 21% 20% 24% 8%	490
October	1st to 10th	Day	13% 7% 23% 32% 19% 6%	490
	11th to 20th	Day	15% 20% 27% 25% 11% 1	352
	21st to end	Day	31% 11% 39% 16% 3	330
November	1st to 10th	Day	14% 13% 35% 22% 13% 3	279
	11th to 20th	Day	27% 15% 31% 16% 9% 1	270
	21st to end	Day	29% 11% 36% 20% 3	270
December	1st to 10th	Day	21% 23% 34% 14% 7% 1	270
	11th to 20th	Day	22% 14% 40% 16% 7%	270
	21st to end	Day	30% 16% 36% 15% 2	297
January	1st to 10th	Night	23% 15% 35% 23% 4	950
	11th to 20th	Night	25% 15% 33% 22% 5%	950
	21st to end	Night	24% 12% 34% 26% 4	1045
February	1st to 10th	Night	29% 11% 35% 22% 3	777
	11th to 20th	Night	22% 14% 35% 24% 5%	743
	21st to end	Night	29% 14% 34% 21% 3	628
March	1st to 10th	Night	22% 16% 32% 26% 5%	760
	11th to 20th	Night	19% 12% 38% 26% 5% 1	760
	21st to end	Night	26% 9% 35% 24% 5% 1	694
April	1st to 10th	Night	20% 12% 37% 23% 7%	570
	11th to 20th	Night	22% 11% 32% 25% 10%	563
	21st to end	Night	22% 9% 37% 24% 8% 1	570
May	1st to 10th	Night	20% 7% 35% 29% 9%	570
	11th to 20th	Night	20% 4 25% 33% 16% 1	561
	21st to end	Night	12% 3 26% 39% 18% 2	617
June	1st to 10th	Night	11% 4 24% 42% 20%	570
	11th to 20th	Night	11% 3 24% 43% 19% 1	570
	21st to end	Night	11% 3 23% 40% 23%	570
July	1st to 10th	Night	9% 1 14% 41% 29% 5%	555
	11th to 20th	Night	8% 3 17% 38% 30% 4	552
	21st to end	Night	8% 1 15% 42% 32% 2	624
August	1st to 10th	Night	8% 2 11% 44% 30% 5%	567
	11th to 20th	Night	10% 3 11% 42% 29% 5%	564
	21st to end	Night	8% 2 14% 33% 39% 4	627
September	1st to 10th	Night	14% 2 14% 38% 28% 4	570
	11th to 20th	Night	9% 3 16% 36% 32% 4	731
	21st to end	Night	15% 5% 19% 32% 26% 2	760
October	1st to 10th	Night	12% 4 25% 36% 19% 3	760
	11th to 20th	Night	21% 10% 27% 28% 13%	760
	21st to end	Night	24% 13% 33% 22% 7%	836
November	1st to 10th	Night	15% 9% 39% 26% 9% 1	755
	11th to 20th	Night	19% 16% 30% 27% 8%	760
	21st to end	Night	25% 13% 31% 23% 8%	760
December	1st to 10th	Night	23% 12% 36% 25% 5%	840
	11th to 20th	Night	25% 11% 31% 25% 8%	950
	21st to end	Night	30% 13% 31% 23% 3	1045

Page generated at 2024-11-01 22:47:32. Version ac07

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