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 2024-01-13.

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	12% 6% 3 6% 6% 4 6% 5% 6% 8% 39%	74730
	Night	12% 5% 2 5% 5% 3 4 4 6% 8% 46%	74979
Transparency	Day	40% 9% 12% 17% 16% 6%	74466
	Night	35% 8% 13% 18% 17% 8%	74764
Seeing	Day	13% 8% 20% 24% 23% 11%	13650
	Night	13% 6% 21% 30% 23% 6%	23430

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% 7% 2 5% 4 4 4 4 5% 6% 43%	1707
	11th to 20th	Day	16% 7% 4 5% 4 3 3 4 5% 6% 44%	1638
	21st to end	Day	21% 5% 2 5% 5% 2 4 4 4 5% 44%	1921
February	1st to 10th	Day	18% 8% 4 6% 5% 3 5% 4 4 6% 39%	1800
	11th to 20th	Day	18% 7% 2 5% 4 3 4 5% 4 6% 42%	1748
	21st to end	Day	18% 8% 3 7% 6% 3 5% 4 4 5% 38%	1480
March	1st to 10th	Day	17% 7% 3 6% 5% 3 5% 4 4 7% 41%	1916
	11th to 20th	Day	14% 7% 3 6% 6% 3 5% 5% 5% 7% 38%	1980
	21st to end	Day	12% 5% 2 5% 5% 3 5% 5% 5% 8% 46%	2263
April	1st to 10th	Day	12% 6% 3 5% 5% 3 6% 6% 6% 9% 40%	2160
	11th to 20th	Day	16% 6% 3 5% 5% 3 5% 6% 5% 7% 39%	2134
	21st to end	Day	9% 5% 2 5% 4 3 6% 5% 6% 9% 46%	2160
May	1st to 10th	Day	11% 5% 2 6% 6% 3 5% 4 6% 9% 44%	2160
	11th to 20th	Day	10% 5% 3 5% 6% 3 6% 6% 6% 8% 41%	2124
	21st to end	Day	7% 5% 3 5% 6% 4 7% 7% 7% 10% 39%	2337
June	1st to 10th	Day	9% 5% 3 7% 8% 4 8% 7% 7% 10% 31%	2180
	11th to 20th	Day	5% 5% 3 6% 9% 5% 8% 7% 8% 11% 34%	2340
	21st to end	Day	9% 6% 3 7% 8% 5% 9% 7% 8% 10% 27%	2340
July	1st to 10th	Day	10% 6% 4 7% 9% 5% 9% 8% 9% 9% 24%	2340
	11th to 20th	Day	8% 5% 4 7% 10% 6% 8% 8% 9% 10% 24%	2340
	21st to end	Day	9% 7% 4 7% 8% 5% 8% 6% 8% 11% 28%	2369
August	1st to 10th	Day	6% 5% 3 8% 7% 5% 8% 8% 8% 10% 32%	2148
	11th to 20th	Day	10% 6% 3 7% 8% 4 9% 6% 8% 10% 29%	2125
	21st to end	Day	11% 6% 2 7% 8% 5% 6% 7% 7% 9% 32%	2376
September	1st to 10th	Day	11% 6% 2 7% 8% 5% 6% 7% 7% 9% 33%	2160
	11th to 20th	Day	11% 6% 3 5% 7% 4 6% 5% 7% 8% 39%	2124
	21st to end	Day	9% 5% 2 5% 6% 4 5% 5% 5% 7% 48%	1980
October	1st to 10th	Day	6% 5% 2 4 5% 3 4 5% 6% 8% 53%	1866
	11th to 20th	Day	8% 4 3 5% 5% 3 4 4 4 8% 54%	1800
	21st to end	Day	11% 3 2 4 4 2 4 4 4 6% 56%	1980
November	1st to 10th	Day	9% 5% 2 4 5% 3 4 4 4 6% 54%	1858
	11th to 20th	Day	11% 5% 2 4 4 2 5% 5% 3 5% 54%	1869
	21st to end	Day	14% 6% 2 5% 5% 3 4 5% 5% 6% 45%	1710
December	1st to 10th	Day	17% 5% 2 4 5% 3 4 5% 5% 5% 44%	1710
	11th to 20th	Day	16% 8% 4 6% 6% 3 4 4 6% 5% 39%	2280
	21st to end	Day	18% 7% 3 5% 5% 4 5% 5% 5% 6% 38%	3307
January	1st to 10th	Night	18% 7% 2 4 4 2 3 4 4 6% 46%	2458
	11th to 20th	Night	17% 7% 2 4 4 2 4 4 4 6% 45%	2367
	21st to end	Night	20% 7% 3 5% 4 3 3 4 4 5% 44%	2430
February	1st to 10th	Night	19% 5% 3 4 5% 3 4 4 5% 6% 42%	2160
	11th to 20th	Night	19% 7% 3 5% 4 3 3 3 5% 6% 43%	2098
	21st to end	Night	21% 7% 2 6% 5% 3 4 5% 5% 6% 36%	1776
March	1st to 10th	Night	14% 6% 2 5% 4 3 5% 4 5% 7% 46%	2058
	11th to 20th	Night	15% 8% 3 5% 5% 3 4 4 5% 9% 40%	1980
	21st to end	Night	12% 5% 2 4 4 2 3 4 4 7% 51%	2091
April	1st to 10th	Night	14% 5% 3 5% 5% 3 4 3 5% 7% 47%	1800
	11th to 20th	Night	14% 5% 2 6% 4 3 3 4 5% 7% 46%	1775
	21st to end	Night	8% 3 2 3 3 3 4 4 5% 7% 56%	1800
May	1st to 10th	Night	9% 4 2 4 4 3 4 4 6% 9% 53%	1800
	11th to 20th	Night	8% 5% 2 4 5% 3 4 5% 6% 9% 49%	1769
	21st to end	Night	6% 4 2 4 6% 3 5% 6% 8% 11% 47%	1938
June	1st to 10th	Night	6% 5% 4 4 6% 4 5% 7% 7% 10% 42%	1774
	11th to 20th	Night	2 3 2 4 6% 3 5% 7% 8% 11% 48%	1620
	21st to end	Night	6% 6% 3 6% 5% 4 6% 7% 8% 10% 40%	1620
July	1st to 10th	Night	5% 5% 3 6% 5% 4 6% 6% 9% 13% 39%	1620
	11th to 20th	Night	5% 4 2 5% 5% 3 9% 6% 10% 14% 37%	1620
	21st to end	Night	6% 4 3 5% 6% 3 6% 6% 8% 13% 40%	1944
August	1st to 10th	Night	4 3 2 3 5% 3 4 7% 9% 14% 47%	1790
	11th to 20th	Night	5% 4 1 6% 6% 4 6% 8% 8% 14% 38%	1776
	21st to end	Night	8% 4 2 5% 6% 4 7% 6% 7% 11% 40%	1980
September	1st to 10th	Night	7% 5% 3 6% 6% 4 5% 5% 8% 10% 42%	1800
	11th to 20th	Night	9% 4 2 5% 5% 3 5% 6% 6% 9% 44%	1818
	21st to end	Night	8% 5% 2 4 5% 3 4 4 6% 8% 50%	1980
October	1st to 10th	Night	7% 4 1 4 4 2 4 4 4 7% 60%	2088
	11th to 20th	Night	7% 5% 2 3 4 2 3 3 5% 7% 58%	2160
	21st to end	Night	12% 4 2 3 3 2 3 3 4 6% 57%	2376
November	1st to 10th	Night	10% 5% 2 4 4 2 3 3 4 7% 57%	2220
	11th to 20th	Night	13% 4 2 3 4 3 3 4 4 6% 54%	2304
	21st to end	Night	13% 7% 2 5% 4 3 4 4 4 6% 48%	2470
December	1st to 10th	Night	16% 7% 3 6% 4 3 3 3 4 6% 45%	2470
	11th to 20th	Night	14% 8% 3 6% 5% 3 4 4 5% 5% 42%	2905
	21st to end	Night	18% 9% 3 6% 5% 3 3 3 4 5% 41%	4344

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	42% 5% 7% 15% 22% 9%	1708
	11th to 20th	Day	41% 5% 6% 14% 25% 9%	1638
	21st to end	Day	43% 5% 6% 15% 20% 11%	1921
February	1st to 10th	Day	48% 4 8% 12% 18% 10%	1800
	11th to 20th	Day	43% 6% 7% 16% 18% 10%	1758
	21st to end	Day	48% 6% 9% 13% 16% 8%	1480
March	1st to 10th	Day	43% 6% 10% 17% 17% 7%	1916
	11th to 20th	Day	43% 9% 9% 12% 20% 7%	1980
	21st to end	Day	35% 7% 13% 18% 20% 6%	2263
April	1st to 10th	Day	39% 8% 11% 15% 18% 8%	2160
	11th to 20th	Day	41% 7% 8% 19% 16% 8%	2134
	21st to end	Day	33% 8% 12% 21% 22% 5%	2160
May	1st to 10th	Day	36% 7% 11% 21% 18% 7%	2160
	11th to 20th	Day	37% 9% 13% 22% 18% 2	2128
	21st to end	Day	35% 11% 17% 21% 14% 2	2337
June	1st to 10th	Day	42% 12% 15% 21% 9%	2180
	11th to 20th	Day	40% 11% 16% 20% 12% 1	2340
	21st to end	Day	46% 14% 17% 17% 6% 1	2340
July	1st to 10th	Day	48% 15% 19% 14% 4	2276
	11th to 20th	Day	46% 17% 20% 13% 3 1	2271
	21st to end	Day	45% 14% 18% 17% 5% 1	2237
August	1st to 10th	Day	40% 15% 22% 15% 8%	2136
	11th to 20th	Day	47% 13% 18% 15% 4 1	2125
	21st to end	Day	44% 15% 15% 14% 9% 2	2376
September	1st to 10th	Day	42% 12% 17% 14% 12% 3	2160
	11th to 20th	Day	39% 11% 15% 15% 13% 7%	2124
	21st to end	Day	34% 8% 10% 21% 18% 8%	1980
October	1st to 10th	Day	27% 7% 12% 20% 21% 13%	1866
	11th to 20th	Day	28% 5% 8% 18% 27% 13%	1800
	21st to end	Day	29% 5% 7% 16% 26% 16%	1980
November	1st to 10th	Day	31% 5% 8% 18% 26% 11%	1858
	11th to 20th	Day	31% 6% 6% 17% 26% 13%	1869
	21st to end	Day	39% 6% 7% 16% 20% 11%	1710
December	1st to 10th	Day	41% 7% 9% 18% 18% 8%	1710
	11th to 20th	Day	45% 7% 7% 11% 23% 7%	2278
	21st to end	Day	45% 7% 10% 14% 21% 3	3307
January	1st to 10th	Night	39% 5% 9% 15% 23% 10%	2458
	11th to 20th	Night	40% 6% 7% 17% 21% 11%	2367
	21st to end	Night	44% 3 5% 13% 23% 11%	2430
February	1st to 10th	Night	42% 6% 8% 16% 18% 10%	2160
	11th to 20th	Night	43% 6% 6% 15% 20% 12%	2099
	21st to end	Night	47% 7% 9% 13% 13% 12%	1776
March	1st to 10th	Night	37% 7% 9% 16% 20% 10%	2058
	11th to 20th	Night	41% 7% 12% 15% 17% 7%	1980
	21st to end	Night	33% 7% 11% 20% 23% 7%	2091
April	1st to 10th	Night	38% 6% 11% 18% 17% 10%	1800
	11th to 20th	Night	38% 6% 11% 19% 17% 9%	1775
	21st to end	Night	26% 7% 12% 21% 26% 8%	1800
May	1st to 10th	Night	28% 7% 12% 26% 20% 7%	1800
	11th to 20th	Night	31% 8% 16% 22% 20% 3	1770
	21st to end	Night	27% 12% 18% 27% 14% 2	1939
June	1st to 10th	Night	33% 12% 19% 24% 11%	1774
	11th to 20th	Night	26% 12% 21% 27% 12% 2	1620
	21st to end	Night	36% 13% 18% 23% 9%	1620
July	1st to 10th	Night	34% 14% 28% 18% 6%	1575
	11th to 20th	Night	29% 21% 29% 17% 4 1	1566
	21st to end	Night	31% 15% 27% 21% 6%	1836
August	1st to 10th	Night	23% 15% 34% 21% 7% 1	1778
	11th to 20th	Night	32% 19% 28% 17% 4 1	1776
	21st to end	Night	36% 15% 22% 17% 8% 4	1980
September	1st to 10th	Night	34% 12% 19% 19% 10% 6%	1800
	11th to 20th	Night	33% 13% 18% 17% 12% 7%	1818
	21st to end	Night	31% 7% 14% 23% 15% 10%	1980
October	1st to 10th	Night	25% 5% 12% 22% 22% 15%	2088
	11th to 20th	Night	25% 7% 8% 21% 26% 14%	2160
	21st to end	Night	28% 5% 7% 17% 25% 17%	2376
November	1st to 10th	Night	28% 5% 10% 20% 24% 14%	2220
	11th to 20th	Night	31% 4 9% 16% 26% 15%	2304
	21st to end	Night	37% 5% 7% 18% 20% 12%	2470
December	1st to 10th	Night	41% 5% 8% 20% 19% 7%	2470
	11th to 20th	Night	43% 6% 8% 11% 18% 14%	2906
	21st to end	Night	45% 6% 8% 15% 19% 7%	4344

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% 14% 36% 17% 7% 1	270
	11th to 20th	Day	19% 21% 35% 18% 7%	246
	21st to end	Day	25% 21% 38% 12% 3	264
February	1st to 10th	Day	26% 20% 33% 16% 5%	240
	11th to 20th	Day	32% 18% 31% 16% 4	240
	21st to end	Day	18% 32% 37% 11% 3	198
March	1st to 10th	Day	18% 23% 38% 20% 2	240
	11th to 20th	Day	22% 25% 34% 16% 3	240
	21st to end	Day	16% 19% 34% 22% 8% 1	349
April	1st to 10th	Day	15% 16% 33% 25% 11% 1	420
	11th to 20th	Day	18% 18% 29% 21% 12% 2	412
	21st to end	Day	10% 11% 30% 30% 16% 2	420
May	1st to 10th	Day	13% 7% 28% 29% 19% 4	423
	11th to 20th	Day	11% 5% 20% 32% 23% 8%	447
	21st to end	Day	8% 1 14% 35% 28% 14%	485
June	1st to 10th	Day	9% 2 14% 27% 33% 15%	470
	11th to 20th	Day	6% 2 12% 34% 34% 12%	630
	21st to end	Day	10% 3 10% 27% 31% 19%	630
July	1st to 10th	Day	8% 3 7% 22% 39% 21%	621
	11th to 20th	Day	6% 1 5% 21% 42% 24%	618
	21st to end	Day	10% 1 6% 16% 40% 27%	507
August	1st to 10th	Day	5% 2 4 19% 37% 33%	446
	11th to 20th	Day	7% 1 6% 15% 38% 33%	439
	21st to end	Day	12% 3 7% 21% 32% 27%	495
September	1st to 10th	Day	9% 2 7% 24% 38% 20%	450
	11th to 20th	Day	10% 3 11% 26% 35% 16%	450
	21st to end	Day	11% 2 10% 28% 38% 12%	450
October	1st to 10th	Day	6% 5% 21% 39% 22% 7%	336
	11th to 20th	Day	8% 6% 22% 35% 23% 6%	270
	21st to end	Day	18% 10% 20% 36% 13% 3	297
November	1st to 10th	Day	15% 5% 24% 39% 14% 2	270
	11th to 20th	Day	14% 17% 30% 27% 12% 1	270
	21st to end	Day	12% 19% 36% 23% 9% 2	270
December	1st to 10th	Day	21% 10% 30% 25% 13% 1	270
	11th to 20th	Day	20% 19% 37% 19% 4	270
	21st to end	Day	27% 15% 33% 17% 8% 1	297
January	1st to 10th	Night	21% 10% 33% 29% 6% 1	950
	11th to 20th	Night	21% 10% 29% 32% 8%	911
	21st to end	Night	22% 12% 33% 28% 5%	846
February	1st to 10th	Night	20% 12% 33% 30% 5%	720
	11th to 20th	Night	23% 12% 31% 28% 7%	703
	21st to end	Night	23% 14% 37% 24% 2	592
March	1st to 10th	Night	15% 12% 37% 29% 8%	618
	11th to 20th	Night	16% 8% 33% 33% 10%	540
	21st to end	Night	13% 12% 32% 31% 11%	594
April	1st to 10th	Night	15% 10% 30% 32% 13%	540
	11th to 20th	Night	17% 9% 28% 32% 14% 1	533
	21st to end	Night	9% 4 30% 40% 16% 1	540
May	1st to 10th	Night	8% 6% 22% 45% 18% 1	540
	11th to 20th	Night	8% 4 18% 41% 26% 3	531
	21st to end	Night	6% 1 15% 38% 32% 8%	584
June	1st to 10th	Night	8% 2 8% 35% 41% 7%	540
	11th to 20th	Night	3 1 7% 32% 46% 12%	540
	21st to end	Night	8% 1 6% 28% 40% 17%	540
July	1st to 10th	Night	4 2 4 19% 50% 22%	525
	11th to 20th	Night	4 1 2 15% 49% 29%	522
	21st to end	Night	5% 1 5% 19% 49% 21%	591
August	1st to 10th	Night	4 1 2 16% 49% 28%	537
	11th to 20th	Night	4 1 2 16% 49% 28%	534
	21st to end	Night	9% 1 5% 23% 41% 21%	594
September	1st to 10th	Night	9% 1 6% 22% 45% 17%	540
	11th to 20th	Night	9% 3 7% 25% 47% 9%	558
	21st to end	Night	10% 1 11% 31% 42% 5%	720
October	1st to 10th	Night	7% 1 16% 39% 34% 4	720
	11th to 20th	Night	9% 4 20% 39% 25% 3	720
	21st to end	Night	14% 7% 23% 34% 21% 2	792
November	1st to 10th	Night	14% 5% 18% 39% 23% 2	740
	11th to 20th	Night	14% 7% 27% 34% 16% 1	760
	21st to end	Night	16% 7% 29% 33% 14% 1	760
December	1st to 10th	Night	17% 11% 25% 32% 15%	760
	11th to 20th	Night	18% 9% 33% 31% 10%	760
	21st to end	Night	26% 11% 29% 27% 7%	935

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