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 2025-08-05.

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 8% 5% 4 4 4 6% 7% 36%	125470
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 37%	122812
Transparency	Day	47% 3 11% 24% 13% 2	129035
	Night	48% 3 10% 23% 15% 2	127308
Seeing	Day	19% 10% 27% 24% 15% 6%	16704
	Night	19% 9% 28% 29% 13% 1	26500

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% 15% 8% 13% 7% 4 3 3 5% 6% 22%	3617
	11th to 20th	Day	12% 14% 9% 15% 7% 7% 5% 5% 4 4 19%	3620
	21st to end	Day	22% 19% 6% 10% 5% 3 3 3 3 4 23%	4050
February	1st to 10th	Day	18% 13% 5% 10% 8% 6% 4 4 4 5% 25%	1981
	11th to 20th	Day	16% 11% 4 10% 5% 4 4 3 5% 6% 31%	1948
	21st to end	Day	16% 14% 5% 8% 4 4 4 3 4 7% 30%	1650
March	1st to 10th	Day	20% 16% 5% 8% 4 3 3 2 3 4 34%	4480
	11th to 20th	Day	9% 10% 2 6% 4 4 4 3 5% 8% 44%	4721
	21st to end	Day	17% 13% 4 9% 5% 5% 4 4 5% 7% 28%	5861
April	1st to 10th	Day	16% 11% 3 8% 4 4 4 4 5% 7% 34%	2400
	11th to 20th	Day	17% 9% 3 5% 5% 4 5% 3 5% 6% 37%	2374
	21st to end	Day	20% 9% 3 7% 4 4 3 3 6% 7% 34%	2540
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 8% 30%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 4 5% 4 6% 7% 37%	5861
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 5% 6% 9% 37%	6840
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 6% 9% 43%	2800
	11th to 20th	Day	10% 5% 2 6% 5% 4 4 4 7% 8% 45%	2800
	21st to end	Day	10% 6% 2 7% 5% 5% 4 4 7% 9% 41%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 5% 7% 11% 39%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 4 6% 5% 7% 11% 38%	6360
	21st to end	Day	12% 7% 2 7% 4 4 5% 4 6% 9% 39%	6924
August	1st to 10th	Day	7% 5% 1 5% 4 4 4 4 7% 10% 49%	4164
	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% 2 5% 3 4 3 3 5% 7% 49%	1762
	11th to 20th	Day	17% 12% 3 7% 4 3 3 3 4 5% 38%	1800
	21st to end	Day	17% 11% 3 8% 5% 5% 4 4 5% 5% 34%	1800
December	1st to 10th	Day	12% 12% 4 8% 4 3 3 3 4 5% 42%	2780
	11th to 20th	Day	13% 16% 6% 13% 6% 4 6% 4 5% 5% 23%	3008
	21st to end	Day	28% 18% 7% 11% 5% 5% 3 3 4 4 12%	3982
January	1st to 10th	Night	16% 16% 8% 14% 5% 4 4 3 4 4 22%	6348
	11th to 20th	Night	13% 19% 7% 14% 7% 5% 6% 3 4 3 19%	6360
	21st to end	Night	19% 14% 6% 9% 6% 5% 3 3 3 4 28%	6926
February	1st to 10th	Night	19% 14% 5% 9% 6% 4 3 3 5% 6% 27%	2417
	11th to 20th	Night	15% 12% 5% 11% 5% 4 3 3 4 5% 34%	2338
	21st to end	Night	19% 12% 3 9% 5% 3 3 3 4 6% 32%	1980
March	1st to 10th	Night	18% 12% 4 6% 4 3 3 2 4 6% 38%	5373
	11th to 20th	Night	12% 7% 2 6% 5% 5% 5% 3 5% 7% 44%	5238
	21st to end	Night	19% 9% 4 8% 4 4 4 4 5% 6% 33%	5110
April	1st to 10th	Night	15% 10% 2 7% 4 3 4 4 6% 6% 39%	2000
	11th to 20th	Night	17% 9% 2 6% 4 4 5% 4 5% 7% 38%	1975
	21st to end	Night	17% 7% 3 7% 4 4 4 4 6% 8% 35%	1859
May	1st to 10th	Night	19% 10% 3 8% 5% 4 5% 3 5% 7% 31%	4033
	11th to 20th	Night	16% 7% 2 6% 3 4 5% 4 6% 9% 40%	4032
	21st to end	Night	14% 8% 2 7% 5% 5% 4 4 5% 7% 39%	4029
June	1st to 10th	Night	10% 5% 1 5% 4 4 4 3 6% 7% 50%	1600
	11th to 20th	Night	10% 5% 3 6% 3 4 4 4 7% 8% 48%	1600
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 6% 9% 43%	1600
July	1st to 10th	Night	9% 5% 2 7% 4 5% 5% 4 6% 10% 44%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 7% 10% 47%	3617
	21st to end	Night	9% 6% 3 8% 5% 5% 5% 3 6% 9% 42%	3947
August	1st to 10th	Night	9% 4 2 5% 4 4 5% 3 7% 9% 50%	2851
	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	12% 7% 2 5% 4 4 3 3 5% 6% 49%	2536
	11th to 20th	Night	14% 10% 2 7% 4 4 3 4 5% 7% 39%	2600
	21st to end	Night	19% 12% 3 9% 5% 4 4 2 3 5% 33%	2600
December	1st to 10th	Night	12% 10% 3 7% 5% 4 3 3 5% 5% 42%	4400
	11th to 20th	Night	17% 14% 5% 9% 4 4 5% 3 4 5% 29%	5199
	21st to end	Night	26% 18% 6% 10% 5% 5% 4 3 5% 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	64% 1 8% 19% 6% 1	3618
	11th to 20th	Day	70% 1 9% 7% 9% 2	3620
	21st to end	Day	68% 1 9% 14% 7% 1	4050
February	1st to 10th	Day	63% 3 10% 15% 8% 1	1981
	11th to 20th	Day	54% 3 11% 22% 9% 1	1958
	21st to end	Day	56% 3 11% 20% 8% 2	1650
March	1st to 10th	Day	58% 2 5% 21% 12% 3	4480
	11th to 20th	Day	40% 2 10% 35% 12% 1	4721
	21st to end	Day	57% 2 13% 18% 8% 1	5863
April	1st to 10th	Day	51% 3 12% 20% 14%	2400
	11th to 20th	Day	48% 3 11% 25% 10% 3	2374
	21st to end	Day	49% 3 13% 21% 11% 2	2540
May	1st to 10th	Day	53% 3 12% 21% 10%	5897
	11th to 20th	Day	49% 3 12% 24% 12% 1	5865
	21st to end	Day	44% 3 11% 25% 15% 2	6854
June	1st to 10th	Day	37% 4 13% 26% 16% 3	2800
	11th to 20th	Day	35% 4 12% 26% 20% 4	2800
	21st to end	Day	39% 5% 15% 21% 17% 4	2800
July	1st to 10th	Day	40% 2 11% 29% 17% 2	6291
	11th to 20th	Day	40% 3 16% 27% 13% 1	6285
	21st to end	Day	42% 3 15% 24% 14% 2	6728
August	1st to 10th	Day	31% 5% 17% 31% 16% 1	4150
	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	36% 3 14% 26% 17% 3	1762
	11th to 20th	Day	50% 2 10% 23% 13% 3	1800
	21st to end	Day	53% 3 11% 23% 9% 1	1800
December	1st to 10th	Day	47% 1 12% 30% 10%	2780
	11th to 20th	Day	65% 2 10% 19% 4	3005
	21st to end	Day	78% 2 8% 9% 3	3982
January	1st to 10th	Night	67% 2 8% 16% 8%	6348
	11th to 20th	Night	71% 2 7% 10% 9% 2	6360
	21st to end	Night	62% 1 9% 18% 9% 1	6926
February	1st to 10th	Night	60% 2 11% 17% 8% 2	2417
	11th to 20th	Night	55% 2 10% 20% 11% 2	2339
	21st to end	Night	54% 3 10% 20% 10% 2	1980
March	1st to 10th	Night	50% 2 8% 25% 12% 3	5373
	11th to 20th	Night	42% 2 11% 28% 16% 2	5238
	21st to end	Night	53% 2 10% 25% 8% 1	5110
April	1st to 10th	Night	45% 4 12% 23% 15% 2	2000
	11th to 20th	Night	46% 4 9% 22% 15% 4	1975
	21st to end	Night	46% 5% 11% 22% 12% 4	1859
May	1st to 10th	Night	54% 3 10% 18% 13% 2	4060
	11th to 20th	Night	43% 4 12% 26% 13% 2	4033
	21st to end	Night	46% 3 11% 23% 15% 3	4037
June	1st to 10th	Night	33% 4 11% 27% 22% 4	1600
	11th to 20th	Night	34% 4 12% 23% 22% 4	1600
	21st to end	Night	36% 6% 14% 23% 16% 5%	1600
July	1st to 10th	Night	38% 3 11% 29% 15% 4	3580
	11th to 20th	Night	34% 3 15% 29% 19% 1	3572
	21st to end	Night	41% 3 15% 24% 14% 3	3832
August	1st to 10th	Night	32% 3 18% 30% 15% 2	2841
	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	37% 3 9% 28% 20% 3	2536
	11th to 20th	Night	45% 3 10% 24% 16% 3	2600
	21st to end	Night	56% 2 8% 23% 10%	2600
December	1st to 10th	Night	45% 2 12% 29% 12% 1	4400
	11th to 20th	Night	59% 1 12% 17% 10% 1	5201
	21st to end	Night	74% 2 10% 10% 4	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	26% 16% 34% 18% 5% 1	300
	11th to 20th	Day	27% 19% 38% 12% 3 1	300
	21st to end	Day	30% 18% 34% 15% 2	330
February	1st to 10th	Day	27% 18% 36% 15% 3	300
	11th to 20th	Day	29% 20% 32% 12% 6% 1	300
	21st to end	Day	33% 18% 38% 10% 2	249
March	1st to 10th	Day	25% 20% 38% 14% 2	300
	11th to 20th	Day	33% 12% 35% 15% 5%	384
	21st to end	Day	33% 14% 31% 15% 6% 1	550
April	1st to 10th	Day	24% 17% 31% 21% 6% 1	500
	11th to 20th	Day	24% 16% 29% 17% 10% 3	492
	21st to end	Day	27% 16% 28% 20% 8% 2	500
May	1st to 10th	Day	22% 11% 33% 21% 10% 2	503
	11th to 20th	Day	24% 8% 26% 24% 13% 5%	527
	21st to end	Day	13% 7% 32% 26% 15% 8%	746
June	1st to 10th	Day	14% 6% 28% 29% 18% 5%	730
	11th to 20th	Day	13% 4 26% 33% 18% 5%	730
	21st to end	Day	12% 7% 28% 30% 17% 6%	730
July	1st to 10th	Day	13% 3 19% 26% 26% 13%	721
	11th to 20th	Day	10% 3 22% 32% 20% 13%	718
	21st to end	Day	11% 2 18% 32% 25% 11%	794
August	1st to 10th	Day	8% 3 15% 31% 28% 15%	510
	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	16% 14% 34% 22% 11% 3	300
	11th to 20th	Day	27% 17% 32% 15% 8% 1	300
	21st to end	Day	31% 12% 36% 18% 3	300
December	1st to 10th	Day	21% 22% 36% 14% 6% 1	300
	11th to 20th	Day	23% 14% 41% 15% 7%	300
	21st to end	Day	33% 15% 35% 15% 2	330
January	1st to 10th	Night	23% 15% 35% 23% 4	1000
	11th to 20th	Night	24% 17% 33% 22% 4	1000
	21st to end	Night	24% 13% 35% 25% 4	1100
February	1st to 10th	Night	29% 12% 34% 22% 2	817
	11th to 20th	Night	23% 15% 34% 23% 4	783
	21st to end	Night	28% 14% 35% 20% 3	660
March	1st to 10th	Night	22% 16% 32% 25% 5%	800
	11th to 20th	Night	19% 12% 38% 26% 5% 1	800
	21st to end	Night	26% 10% 35% 24% 5% 1	730
April	1st to 10th	Night	21% 12% 37% 24% 7%	600
	11th to 20th	Night	22% 11% 32% 25% 9%	593
	21st to end	Night	22% 9% 37% 25% 8% 1	600
May	1st to 10th	Night	20% 8% 36% 28% 9%	600
	11th to 20th	Night	20% 6% 26% 32% 15% 1	591
	21st to end	Night	12% 4 26% 38% 18% 2	650
June	1st to 10th	Night	12% 4 23% 41% 20%	600
	11th to 20th	Night	12% 3 23% 43% 19% 1	600
	21st to end	Night	11% 3 23% 39% 23% 1	600
July	1st to 10th	Night	10% 1 14% 41% 29% 5%	585
	11th to 20th	Night	8% 2 18% 39% 29% 4	582
	21st to end	Night	8% 1 15% 42% 32% 2	657
August	1st to 10th	Night	8% 2 12% 44% 30% 5%	579
	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	17% 8% 39% 26% 9% 1	780
	11th to 20th	Night	20% 16% 31% 26% 8%	800
	21st to end	Night	26% 14% 31% 23% 7%	800
December	1st to 10th	Night	23% 12% 36% 24% 4	885
	11th to 20th	Night	26% 11% 31% 24% 7%	1000
	21st to end	Night	31% 13% 31% 22% 3	1100

Page generated at 2025-08-02 01:32:24. Version ac07

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