Hammond
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hammond clear sky chart from 2005-11-06 to 2025-12-03.

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	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	125895
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 45%	124891
Transparency	Day	38% 8% 13% 17% 16% 7%	129554
	Night	35% 8% 13% 18% 17% 9%	129307
Seeing	Day	13% 9% 20% 25% 23% 11%	16187
	Night	13% 7% 22% 30% 23% 6%	25831

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	17% 6% 2 6% 4 3 4 3 5% 6% 45%	4077
	11th to 20th	Day	13% 7% 2 6% 4 3 3 3 4 4 50%	4080
	21st to end	Day	23% 8% 2 7% 4 4 4 4 4 5% 35%	4822
February	1st to 10th	Day	17% 9% 3 7% 4 4 4 3 5% 5% 38%	2000
	11th to 20th	Day	18% 7% 2 6% 4 4 4 3 4 6% 42%	1948
	21st to end	Day	17% 8% 3 8% 5% 4 4 3 4 5% 39%	1650
March	1st to 10th	Day	20% 9% 3 7% 4 4 4 3 5% 7% 35%	4792
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 44%	4980
	21st to end	Day	14% 7% 3 8% 5% 5% 4 3 4 6% 41%	5686
April	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 4 6% 7% 39%	2400
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 5% 5% 7% 40%	2374
	21st to end	Day	9% 6% 1 5% 4 5% 5% 4 6% 9% 45%	2400
May	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 32%	5405
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	5404
	21st to end	Day	6% 5% 3 6% 5% 5% 6% 5% 8% 11% 41%	5932
June	1st to 10th	Day	9% 6% 2 8% 7% 7% 7% 5% 7% 10% 32%	2423
	11th to 20th	Day	5% 6% 2 7% 8% 7% 7% 6% 8% 10% 34%	2600
	21st to end	Day	8% 6% 2 8% 7% 8% 8% 6% 9% 10% 28%	2600
July	1st to 10th	Day	5% 5% 2 8% 7% 7% 8% 6% 10% 12% 28%	5900
	11th to 20th	Day	5% 4 2 8% 7% 8% 8% 7% 10% 12% 29%	5900
	21st to end	Day	7% 6% 3 9% 6% 7% 7% 6% 9% 13% 27%	5969
August	1st to 10th	Day	3 3 2 5% 4 5% 5% 4 7% 9% 53%	3905
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 5% 9% 12% 40%	3849
	21st to end	Day	10% 6% 2 9% 7% 7% 6% 5% 6% 7% 34%	4312
September	1st to 10th	Day	12% 6% 2 8% 6% 6% 6% 5% 7% 9% 33%	2400
	11th to 20th	Day	10% 6% 2 6% 6% 6% 6% 4 7% 9% 39%	2359
	21st to end	Day	9% 5% 2 5% 5% 4 6% 3 5% 7% 50%	2200
October	1st to 10th	Day	6% 5% 2 4 4 3 4 4 6% 7% 54%	2072
	11th to 20th	Day	7% 4 2 5% 4 3 4 3 5% 7% 57%	2000
	21st to end	Day	10% 3 2 5% 4 3 4 3 4 6% 57%	2200
November	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 4 7% 53%	2058
	11th to 20th	Day	11% 5% 2 5% 3 3 4 5% 4 6% 54%	2065
	21st to end	Day	13% 6% 2 6% 5% 4 4 3 6% 7% 45%	1890
December	1st to 10th	Day	18% 7% 3 6% 4 4 4 4 4 5% 41%	2948
	11th to 20th	Day	11% 6% 3 7% 4 4 4 4 5% 6% 47%	3570
	21st to end	Day	17% 8% 2 6% 4 4 4 3 4 5% 44%	4725
January	1st to 10th	Night	19% 7% 2 3 2 2 3 3 4 6% 49%	5888
	11th to 20th	Night	19% 8% 2 5% 3 2 3 2 4 5% 48%	5900
	21st to end	Night	24% 8% 2 5% 4 4 3 3 5% 6% 37%	6108
February	1st to 10th	Night	18% 6% 3 5% 4 3 4 4 5% 5% 42%	2400
	11th to 20th	Night	21% 8% 3 6% 3 4 3 3 4 6% 41%	2338
	21st to end	Night	21% 8% 2 7% 4 3 4 3 6% 6% 37%	1980
March	1st to 10th	Night	18% 9% 3 7% 4 3 4 3 5% 6% 38%	5103
	11th to 20th	Night	15% 7% 2 6% 3 3 4 3 4 6% 47%	4980
	21st to end	Night	17% 6% 2 5% 4 3 4 4 5% 6% 44%	5241
April	1st to 10th	Night	15% 6% 3 6% 5% 4 3 3 5% 6% 45%	2000
	11th to 20th	Night	14% 5% 2 6% 4 3 3 3 5% 6% 47%	1975
	21st to end	Night	9% 4 2 5% 3 4 3 4 5% 7% 55%	2000
May	1st to 10th	Night	15% 8% 2 7% 5% 4 4 3 5% 8% 37%	4490
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 9% 43%	4489
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 7% 11% 42%	4922
June	1st to 10th	Night	6% 6% 3 5% 5% 5% 4 6% 7% 10% 43%	1970
	11th to 20th	Night	3 4 2 6% 5% 4 6% 5% 7% 11% 47%	1800
	21st to end	Night	5% 6% 2 7% 5% 6% 5% 6% 8% 11% 39%	1800
July	1st to 10th	Night	4 5% 2 7% 5% 5% 7% 7% 10% 14% 36%	4078
	11th to 20th	Night	3 3 2 6% 5% 5% 6% 6% 9% 13% 42%	4077
	21st to end	Night	4 6% 2 8% 6% 7% 7% 5% 9% 12% 34%	4879
August	1st to 10th	Night	2 2 1 3 3 3 4 4 8% 13% 56%	3244
	11th to 20th	Night	3 3 1 6% 5% 5% 6% 6% 9% 13% 44%	3200
	21st to end	Night	5% 5% 2 7% 6% 6% 6% 5% 7% 9% 41%	3586
September	1st to 10th	Night	8% 6% 2 7% 5% 5% 6% 4 8% 10% 41%	2000
	11th to 20th	Night	9% 4 2 6% 4 5% 6% 5% 7% 9% 44%	2020
	21st to end	Night	7% 5% 2 5% 4 4 4 3 6% 8% 52%	2200
October	1st to 10th	Night	7% 4 1 5% 3 3 4 3 4 7% 58%	2321
	11th to 20th	Night	7% 5% 1 4 3 3 3 2 5% 6% 61%	2400
	21st to end	Night	11% 4 1 4 2 3 3 2 4 6% 58%	2640
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 4 7% 55%	2460
	11th to 20th	Night	12% 5% 1 4 3 3 3 3 4 6% 55%	2548
	21st to end	Night	12% 7% 2 6% 3 4 3 3 5% 6% 48%	2730
December	1st to 10th	Night	17% 7% 3 6% 4 4 4 3 4 5% 43%	4277
	11th to 20th	Night	12% 8% 2 7% 4 4 4 3 5% 5% 45%	4615
	21st to end	Night	19% 9% 2 6% 4 3 3 2 3 4 44%	6232

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	39% 5% 6% 19% 26% 6%	4078
	11th to 20th	Day	38% 4 5% 14% 30% 10%	4080
	21st to end	Day	51% 5% 6% 12% 16% 11%	4822
February	1st to 10th	Day	47% 4 8% 12% 18% 10%	2000
	11th to 20th	Day	43% 6% 7% 15% 19% 10%	1958
	21st to end	Day	46% 6% 8% 14% 17% 8%	1650
March	1st to 10th	Day	48% 6% 8% 14% 14% 10%	4792
	11th to 20th	Day	37% 8% 8% 13% 17% 16%	4980
	21st to end	Day	43% 8% 9% 15% 17% 8%	5688
April	1st to 10th	Day	41% 8% 11% 15% 17% 10%	2400
	11th to 20th	Day	40% 7% 9% 18% 16% 9%	2374
	21st to end	Day	33% 8% 13% 21% 20% 5%	2400
May	1st to 10th	Day	45% 10% 13% 20% 10% 3	5440
	11th to 20th	Day	32% 8% 15% 23% 19% 2	5408
	21st to end	Day	31% 10% 18% 28% 12% 2	5945
June	1st to 10th	Day	41% 12% 15% 21% 10% 1	2423
	11th to 20th	Day	40% 11% 16% 19% 12% 1	2600
	21st to end	Day	44% 14% 18% 18% 6% 1	2600
July	1st to 10th	Day	41% 14% 26% 17% 3	5836
	11th to 20th	Day	40% 16% 34% 9% 1	5831
	21st to end	Day	44% 13% 19% 21% 3	5798
August	1st to 10th	Day	25% 10% 15% 26% 24% 1	3893
	11th to 20th	Day	33% 11% 22% 25% 9% 1	3885
	21st to end	Day	45% 12% 13% 15% 14% 2	4311
September	1st to 10th	Day	42% 11% 17% 15% 12% 3	2400
	11th to 20th	Day	38% 10% 14% 17% 14% 6%	2359
	21st to end	Day	33% 7% 11% 22% 19% 9%	2200
October	1st to 10th	Day	25% 4 7% 12% 22% 29%	3371
	11th to 20th	Day	16% 3 5% 20% 26% 29%	3240
	21st to end	Day	20% 4 6% 12% 40% 17%	3538
November	1st to 10th	Day	31% 5% 9% 19% 25% 12%	2058
	11th to 20th	Day	30% 6% 6% 17% 27% 13%	2065
	21st to end	Day	38% 6% 8% 17% 21% 11%	1890
December	1st to 10th	Day	45% 5% 7% 14% 23% 6%	2948
	11th to 20th	Day	36% 6% 10% 16% 23% 9%	3568
	21st to end	Day	42% 6% 7% 17% 23% 4	4725
January	1st to 10th	Night	37% 4 7% 20% 25% 8%	5888
	11th to 20th	Night	39% 4 5% 15% 26% 10%	5900
	21st to end	Night	49% 4 6% 13% 18% 11%	6108
February	1st to 10th	Night	42% 6% 8% 16% 19% 10%	2400
	11th to 20th	Night	45% 5% 5% 14% 19% 11%	2339
	21st to end	Night	46% 6% 9% 13% 14% 12%	1980
March	1st to 10th	Night	45% 7% 9% 11% 18% 10%	5103
	11th to 20th	Night	36% 7% 9% 14% 19% 15%	4980
	21st to end	Night	39% 7% 9% 20% 16% 8%	5241
April	1st to 10th	Night	39% 7% 10% 17% 16% 11%	2000
	11th to 20th	Night	37% 6% 10% 19% 18% 11%	1975
	21st to end	Night	27% 7% 12% 22% 24% 8%	2000
May	1st to 10th	Night	42% 9% 14% 19% 13% 3	4520
	11th to 20th	Night	37% 7% 16% 24% 15% 1	4490
	21st to end	Night	30% 10% 20% 29% 10% 1	4931
June	1st to 10th	Night	33% 12% 19% 24% 12%	1970
	11th to 20th	Night	27% 12% 21% 26% 12% 2	1800
	21st to end	Night	36% 12% 20% 22% 9%	1800
July	1st to 10th	Night	34% 14% 30% 17% 4	4035
	11th to 20th	Night	29% 17% 43% 10% 1	4026
	21st to end	Night	38% 17% 19% 23% 3	4737
August	1st to 10th	Night	16% 10% 22% 28% 23% 1	3232
	11th to 20th	Night	26% 14% 26% 26% 7%	3230
	21st to end	Night	38% 11% 18% 18% 11% 4	3586
September	1st to 10th	Night	35% 11% 19% 18% 10% 5%	2000
	11th to 20th	Night	32% 12% 18% 18% 13% 7%	2020
	21st to end	Night	29% 7% 15% 23% 16% 10%	2200
October	1st to 10th	Night	28% 4 8% 14% 20% 26%	3802
	11th to 20th	Night	16% 4 5% 19% 28% 28%	3899
	21st to end	Night	19% 4 6% 12% 36% 24%	4252
November	1st to 10th	Night	30% 5% 10% 19% 23% 13%	2460
	11th to 20th	Night	30% 4 9% 16% 26% 15%	2548
	21st to end	Night	36% 5% 8% 18% 21% 12%	2730
December	1st to 10th	Night	45% 4 5% 16% 24% 6%	4277
	11th to 20th	Night	38% 6% 9% 16% 19% 11%	4616
	21st to end	Night	44% 5% 7% 15% 22% 7%	6232

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	24% 15% 37% 16% 6% 1	300
	11th to 20th	Day	19% 23% 34% 17% 6%	300
	21st to end	Day	26% 20% 38% 13% 3	330
February	1st to 10th	Day	26% 18% 35% 15% 6%	300
	11th to 20th	Day	31% 21% 30% 15% 3	300
	21st to end	Day	17% 31% 37% 11% 4	249
March	1st to 10th	Day	21% 23% 35% 18% 2	300
	11th to 20th	Day	24% 25% 31% 18% 2	300
	21st to end	Day	17% 20% 34% 21% 7%	425
April	1st to 10th	Day	19% 17% 32% 22% 10% 1	500
	11th to 20th	Day	17% 20% 30% 22% 11% 2	492
	21st to end	Day	10% 11% 31% 29% 16% 3	500
May	1st to 10th	Day	14% 9% 28% 28% 17% 3	503
	11th to 20th	Day	11% 7% 23% 31% 21% 7%	527
	21st to end	Day	8% 1 19% 36% 25% 12%	573
June	1st to 10th	Day	8% 3 15% 28% 32% 14%	553
	11th to 20th	Day	6% 3 12% 34% 32% 12%	730
	21st to end	Day	9% 3 10% 27% 31% 20%	730
July	1st to 10th	Day	7% 3 7% 23% 39% 20%	721
	11th to 20th	Day	6% 1 6% 22% 41% 25%	718
	21st to end	Day	9% 1 5% 17% 41% 26%	597
August	1st to 10th	Day	4 2 4 21% 40% 30%	526
	11th to 20th	Day	6% 1 6% 16% 38% 34%	519
	21st to end	Day	11% 3 7% 23% 31% 25%	583
September	1st to 10th	Day	10% 3 8% 25% 36% 18%	530
	11th to 20th	Day	10% 3 11% 25% 36% 15%	530
	21st to end	Day	10% 3 12% 29% 36% 11%	530
October	1st to 10th	Day	6% 5% 23% 38% 21% 7%	402
	11th to 20th	Day	7% 6% 25% 34% 22% 6%	330
	21st to end	Day	17% 9% 21% 37% 13% 4	363
November	1st to 10th	Day	14% 8% 27% 36% 14% 2	330
	11th to 20th	Day	13% 16% 29% 28% 12% 2	330
	21st to end	Day	12% 18% 37% 22% 8% 2	330
December	1st to 10th	Day	23% 10% 31% 23% 12% 1	306
	11th to 20th	Day	19% 19% 37% 20% 5% 1	300
	21st to end	Day	25% 15% 34% 17% 8% 1	330
January	1st to 10th	Night	21% 10% 34% 29% 6% 1	1000
	11th to 20th	Night	22% 11% 29% 30% 7%	1000
	21st to end	Night	23% 12% 33% 27% 5%	939
February	1st to 10th	Night	19% 12% 34% 29% 5%	800
	11th to 20th	Night	25% 13% 30% 27% 6%	783
	21st to end	Night	23% 14% 37% 23% 2	660
March	1st to 10th	Night	16% 13% 36% 28% 7%	686
	11th to 20th	Night	15% 9% 35% 32% 9%	600
	21st to end	Night	14% 12% 34% 31% 10%	660
April	1st to 10th	Night	18% 10% 29% 31% 12%	600
	11th to 20th	Night	16% 10% 28% 32% 13% 1	593
	21st to end	Night	10% 5% 29% 40% 16% 1	600
May	1st to 10th	Night	10% 6% 23% 44% 16% 1	600
	11th to 20th	Night	9% 6% 19% 40% 24% 3	591
	21st to end	Night	7% 2 15% 38% 31% 7%	650
June	1st to 10th	Night	8% 2 9% 37% 38% 6%	600
	11th to 20th	Night	3 1 7% 31% 46% 12%	600
	21st to end	Night	7% 1 6% 28% 41% 17%	600
July	1st to 10th	Night	4 2 4 19% 51% 21%	585
	11th to 20th	Night	4 1 2 14% 50% 29%	582
	21st to end	Night	4 1 4 20% 49% 20%	657
August	1st to 10th	Night	3 1 2 18% 50% 26%	597
	11th to 20th	Night	4 1 3 16% 47% 28%	594
	21st to end	Night	9% 1 5% 22% 42% 20%	660
September	1st to 10th	Night	10% 2 8% 24% 43% 15%	600
	11th to 20th	Night	10% 3 7% 25% 46% 8%	620
	21st to end	Night	9% 2 12% 32% 41% 5%	800
October	1st to 10th	Night	8% 1 18% 38% 31% 3	800
	11th to 20th	Night	8% 4 20% 40% 25% 3	800
	21st to end	Night	13% 6% 22% 36% 21% 2	880
November	1st to 10th	Night	13% 6% 19% 38% 22% 2	820
	11th to 20th	Night	14% 6% 28% 35% 16% 1	840
	21st to end	Night	15% 7% 30% 32% 14% 1	840
December	1st to 10th	Night	18% 11% 25% 31% 14%	809
	11th to 20th	Night	17% 9% 33% 31% 10%	800
	21st to end	Night	26% 11% 29% 27% 7%	985

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