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 2024-11-10.

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% 2 7% 5% 5% 5% 4 6% 8% 39%	100355
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	98774
Transparency	Day	39% 9% 12% 17% 17% 7%	104050
	Night	35% 8% 13% 18% 17% 9%	103219
Seeing	Day	13% 9% 20% 24% 23% 11%	14790
	Night	13% 6% 21% 30% 23% 6%	24452

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 6% 4 4 4 3 5% 7% 41%	2847
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 4 54%	2850
	21st to end	Day	25% 7% 2 6% 4 3 4 3 4 5% 37%	3365
February	1st to 10th	Day	17% 9% 3 7% 4 5% 4 3 5% 5% 38%	1900
	11th to 20th	Day	18% 7% 2 6% 4 4 4 3 4 6% 42%	1848
	21st to end	Day	17% 8% 3 8% 5% 4 4 3 4 5% 39%	1570
March	1st to 10th	Day	25% 10% 3 7% 4 4 4 3 5% 7% 28%	3334
	11th to 20th	Day	16% 7% 3 8% 4 5% 5% 4 6% 8% 33%	3480
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 4 5% 6% 41%	3968
April	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	2280
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 5% 6% 7% 39%	2254
	21st to end	Day	9% 6% 1 5% 4 5% 5% 4 6% 8% 46%	2280
May	1st to 10th	Day	11% 6% 2 8% 6% 6% 6% 5% 7% 9% 34%	3765
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 4 5% 7% 41%	3764
	21st to end	Day	6% 4 2 5% 5% 5% 6% 5% 8% 10% 44%	4129
June	1st to 10th	Day	9% 6% 2 8% 7% 7% 7% 5% 7% 10% 31%	2302
	11th to 20th	Day	5% 6% 2 7% 8% 7% 7% 6% 8% 10% 34%	2470
	21st to end	Day	8% 7% 2 8% 7% 8% 8% 6% 9% 10% 28%	2470
July	1st to 10th	Day	7% 7% 3 9% 8% 8% 8% 6% 10% 11% 24%	4120
	11th to 20th	Day	6% 5% 2 9% 8% 9% 8% 7% 10% 11% 25%	4120
	21st to end	Day	9% 9% 3 11% 8% 7% 7% 6% 8% 11% 21%	4151
August	1st to 10th	Day	3 3 2 5% 4 5% 5% 4 7% 9% 54%	3785
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 5% 9% 12% 39%	3729
	21st to end	Day	10% 6% 2 9% 7% 7% 7% 5% 6% 7% 34%	4180
September	1st to 10th	Day	12% 7% 2 8% 7% 6% 6% 5% 7% 9% 32%	2280
	11th to 20th	Day	11% 6% 2 6% 6% 6% 6% 4 7% 9% 38%	2241
	21st to end	Day	9% 5% 2 5% 5% 4 6% 3 5% 7% 50%	2090
October	1st to 10th	Day	6% 5% 2 4 4 3 5% 4 6% 7% 54%	1969
	11th to 20th	Day	7% 4 2 5% 4 3 4 3 5% 8% 55%	1900
	21st to end	Day	10% 3 1 5% 3 3 4 3 4 6% 57%	2090
November	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 4 7% 52%	1948
	11th to 20th	Day	11% 5% 2 4 3 3 4 5% 3 5% 54%	1869
	21st to end	Day	14% 6% 2 6% 5% 4 4 4 5% 6% 45%	1710
December	1st to 10th	Day	17% 6% 2 5% 5% 4 4 4 5% 5% 44%	1710
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 3 6% 5% 39%	2280
	21st to end	Day	18% 8% 3 6% 4 5% 5% 4 5% 6% 38%	3307
January	1st to 10th	Night	18% 8% 2 4 3 3 4 3 4 6% 47%	4108
	11th to 20th	Night	17% 6% 1 4 3 3 3 2 4 5% 52%	4120
	21st to end	Night	27% 7% 2 4 2 3 3 2 4 6% 40%	4276
February	1st to 10th	Night	18% 6% 3 5% 4 3 4 4 5% 6% 41%	2280
	11th to 20th	Night	19% 8% 3 6% 3 4 3 3 4 6% 42%	2218
	21st to end	Night	21% 7% 2 7% 4 3 4 3 6% 6% 37%	1884
March	1st to 10th	Night	24% 11% 3 7% 4 4 4 3 4 5% 31%	3547
	11th to 20th	Night	16% 8% 2 7% 5% 4 5% 4 5% 8% 36%	3480
	21st to end	Night	15% 7% 2 5% 4 4 4 4 5% 7% 43%	3661
April	1st to 10th	Night	14% 5% 3 6% 5% 4 3 3 5% 6% 46%	1900
	11th to 20th	Night	14% 6% 2 7% 4 4 3 4 5% 6% 46%	1875
	21st to end	Night	9% 4 2 4 3 4 3 4 5% 7% 56%	1900
May	1st to 10th	Night	13% 7% 2 7% 4 4 3 3 5% 8% 43%	3130
	11th to 20th	Night	16% 6% 2 6% 4 4 4 3 5% 7% 42%	3129
	21st to end	Night	7% 4 2 5% 4 4 5% 5% 8% 12% 44%	3426
June	1st to 10th	Night	7% 6% 3 5% 5% 5% 4 6% 7% 10% 42%	1872
	11th to 20th	Night	3 4 2 5% 5% 4 5% 5% 7% 12% 48%	1710
	21st to end	Night	6% 6% 2 7% 5% 6% 5% 6% 8% 10% 40%	1710
July	1st to 10th	Night	5% 6% 2 8% 6% 5% 6% 7% 10% 14% 32%	2850
	11th to 20th	Night	3 3 2 6% 5% 6% 6% 7% 10% 14% 38%	2850
	21st to end	Night	6% 7% 2 8% 7% 7% 6% 5% 9% 13% 29%	3398
August	1st to 10th	Night	2 2 1 3 3 3 4 4 7% 13% 57%	3144
	11th to 20th	Night	3 3 1 6% 5% 5% 6% 6% 9% 13% 44%	3100
	21st to end	Night	5% 5% 2 7% 6% 6% 6% 4 7% 10% 41%	3476
September	1st to 10th	Night	8% 6% 3 7% 5% 5% 5% 4 8% 10% 40%	1900
	11th to 20th	Night	9% 4 2 6% 4 5% 6% 5% 7% 9% 43%	1919
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 8% 52%	2090
October	1st to 10th	Night	7% 4 1 5% 3 3 3 3 4 7% 60%	2205
	11th to 20th	Night	7% 5% 1 4 3 2 3 2 5% 6% 59%	2280
	21st to end	Night	12% 4 2 4 3 3 3 2 4 5% 58%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 4 7% 55%	2335
	11th to 20th	Night	13% 4 1 4 3 3 3 3 4 6% 54%	2304
	21st to end	Night	13% 7% 2 6% 3 4 4 3 4 6% 48%	2470
December	1st to 10th	Night	16% 7% 3 6% 4 4 3 2 5% 6% 45%	2470
	11th to 20th	Night	14% 8% 2 7% 4 4 4 3 5% 5% 42%	2905
	21st to end	Night	18% 10% 3 6% 4 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	41% 6% 7% 15% 24% 8%	2848
	11th to 20th	Day	33% 5% 5% 16% 32% 9%	2850
	21st to end	Day	49% 5% 5% 13% 19% 8%	3365
February	1st to 10th	Day	48% 4 8% 12% 17% 10%	1900
	11th to 20th	Day	42% 6% 7% 15% 19% 10%	1858
	21st to end	Day	47% 6% 9% 14% 17% 8%	1570
March	1st to 10th	Day	53% 7% 10% 14% 10% 4	3334
	11th to 20th	Day	44% 11% 9% 16% 16% 4	3480
	21st to end	Day	39% 9% 10% 14% 19% 8%	3968
April	1st to 10th	Day	40% 8% 11% 15% 17% 9%	2280
	11th to 20th	Day	41% 7% 9% 19% 15% 9%	2254
	21st to end	Day	33% 8% 12% 20% 21% 6%	2280
May	1st to 10th	Day	40% 13% 13% 18% 11% 4	3800
	11th to 20th	Day	38% 9% 11% 18% 22% 3	3768
	21st to end	Day	28% 12% 18% 30% 11% 1	4141
June	1st to 10th	Day	42% 12% 15% 21% 10% 1	2302
	11th to 20th	Day	40% 11% 16% 20% 13% 1	2470
	21st to end	Day	45% 14% 17% 17% 6% 1	2470
July	1st to 10th	Day	47% 14% 22% 15% 2	4056
	11th to 20th	Day	44% 16% 29% 8% 2 1	4051
	21st to end	Day	52% 13% 16% 14% 4	4019
August	1st to 10th	Day	24% 9% 14% 27% 25% 1	3773
	11th to 20th	Day	34% 11% 21% 25% 9% 1	3765
	21st to end	Day	46% 11% 13% 14% 14% 2	4179
September	1st to 10th	Day	43% 12% 16% 14% 12% 3	2280
	11th to 20th	Day	39% 10% 14% 16% 14% 6%	2241
	21st to end	Day	33% 7% 10% 22% 18% 9%	2090
October	1st to 10th	Day	25% 4 7% 12% 21% 30%	3268
	11th to 20th	Day	16% 3 5% 20% 26% 29%	3140
	21st to end	Day	20% 4 6% 12% 40% 18%	3428
November	1st to 10th	Day	32% 5% 9% 18% 25% 11%	1948
	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	37% 5% 8% 15% 25% 10%	4108
	11th to 20th	Night	34% 6% 6% 18% 26% 12%	4120
	21st to end	Night	46% 4 5% 16% 20% 9%	4276
February	1st to 10th	Night	42% 6% 8% 16% 18% 9%	2280
	11th to 20th	Night	43% 6% 5% 14% 20% 12%	2219
	21st to end	Night	46% 7% 9% 13% 14% 12%	1884
March	1st to 10th	Night	53% 8% 9% 12% 12% 6%	3547
	11th to 20th	Night	43% 10% 11% 15% 16% 6%	3480
	21st to end	Night	38% 9% 10% 19% 18% 6%	3661
April	1st to 10th	Night	38% 7% 10% 17% 17% 11%	1900
	11th to 20th	Night	38% 6% 10% 19% 17% 10%	1875
	21st to end	Night	26% 7% 12% 21% 25% 8%	1900
May	1st to 10th	Night	35% 11% 15% 20% 15% 4	3160
	11th to 20th	Night	39% 8% 12% 21% 18% 2	3130
	21st to end	Night	26% 12% 21% 27% 12% 2	3435
June	1st to 10th	Night	33% 12% 19% 24% 12%	1872
	11th to 20th	Night	26% 12% 21% 27% 13% 2	1710
	21st to end	Night	36% 13% 19% 23% 10%	1710
July	1st to 10th	Night	37% 15% 31% 14% 3	2805
	11th to 20th	Night	29% 19% 40% 10% 2	2796
	21st to end	Night	42% 18% 19% 17% 3	3290
August	1st to 10th	Night	16% 10% 22% 28% 24% 1	3132
	11th to 20th	Night	26% 14% 26% 26% 7% 1	3130
	21st to end	Night	38% 11% 17% 18% 12% 4	3476
September	1st to 10th	Night	36% 11% 19% 18% 10% 6%	1900
	11th to 20th	Night	33% 12% 17% 17% 13% 7%	1919
	21st to end	Night	29% 7% 14% 23% 16% 10%	2090
October	1st to 10th	Night	27% 4 8% 14% 21% 26%	3686
	11th to 20th	Night	16% 4 5% 19% 28% 28%	3779
	21st to end	Night	18% 4 6% 12% 36% 24%	4120
November	1st to 10th	Night	30% 5% 10% 20% 23% 13%	2335
	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% 22% 34% 18% 7%	270
	21st to end	Day	26% 21% 38% 11% 3	297
February	1st to 10th	Day	27% 20% 33% 15% 5%	270
	11th to 20th	Day	31% 18% 31% 16% 4	270
	21st to end	Day	17% 33% 37% 11% 3	225
March	1st to 10th	Day	20% 23% 36% 19% 3	270
	11th to 20th	Day	22% 24% 33% 17% 3	270
	21st to end	Day	17% 21% 34% 21% 7% 1	387
April	1st to 10th	Day	17% 17% 32% 23% 10% 1	460
	11th to 20th	Day	18% 19% 29% 21% 11% 2	452
	21st to end	Day	10% 12% 31% 29% 16% 3	460
May	1st to 10th	Day	13% 9% 29% 28% 18% 3	463
	11th to 20th	Day	12% 7% 20% 32% 22% 8%	487
	21st to end	Day	7% 1 17% 36% 26% 13%	529
June	1st to 10th	Day	9% 3 14% 27% 33% 14%	512
	11th to 20th	Day	6% 2 13% 34% 33% 12%	680
	21st to end	Day	9% 3 11% 27% 31% 19%	680
July	1st to 10th	Day	8% 3 7% 22% 39% 21%	671
	11th to 20th	Day	6% 1 6% 21% 41% 25%	668
	21st to end	Day	10% 1 5% 16% 41% 26%	552
August	1st to 10th	Day	5% 1 4 21% 38% 31%	486
	11th to 20th	Day	6% 1 6% 16% 37% 33%	479
	21st to end	Day	11% 3 7% 23% 31% 25%	539
September	1st to 10th	Day	11% 3 7% 24% 36% 19%	490
	11th to 20th	Day	10% 3 11% 25% 35% 15%	490
	21st to end	Day	10% 3 11% 28% 36% 12%	490
October	1st to 10th	Day	7% 5% 22% 38% 21% 7%	369
	11th to 20th	Day	8% 6% 24% 35% 22% 6%	300
	21st to end	Day	16% 9% 19% 37% 14% 4	330
November	1st to 10th	Day	15% 6% 25% 38% 13% 2	297
	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% 11% 30% 31% 8%	950
	21st to end	Night	23% 12% 33% 27% 5%	893
February	1st to 10th	Night	20% 13% 34% 29% 4	760
	11th to 20th	Night	23% 12% 30% 28% 6%	743
	21st to end	Night	22% 15% 37% 24% 2	628
March	1st to 10th	Night	16% 12% 36% 29% 7%	652
	11th to 20th	Night	15% 8% 34% 33% 9%	570
	21st to end	Night	14% 12% 33% 30% 10%	627
April	1st to 10th	Night	16% 10% 30% 31% 12%	570
	11th to 20th	Night	16% 10% 28% 32% 13% 1	563
	21st to end	Night	9% 5% 30% 39% 16% 1	570
May	1st to 10th	Night	9% 6% 23% 44% 17% 1	570
	11th to 20th	Night	9% 5% 17% 41% 25% 2	561
	21st to end	Night	6% 2 16% 38% 31% 7%	617
June	1st to 10th	Night	8% 2 8% 36% 40% 6%	570
	11th to 20th	Night	4 1 7% 32% 45% 12%	570
	21st to end	Night	7% 1 6% 28% 40% 17%	570
July	1st to 10th	Night	4 2 4 19% 50% 22%	555
	11th to 20th	Night	4 1 2 15% 50% 29%	552
	21st to end	Night	4 1 5% 20% 50% 20%	624
August	1st to 10th	Night	4 1 2 17% 50% 27%	567
	11th to 20th	Night	4 1 3 16% 48% 28%	564
	21st to end	Night	9% 1 5% 23% 42% 20%	627
September	1st to 10th	Night	10% 2 7% 22% 44% 16%	570
	11th to 20th	Night	10% 3 7% 25% 45% 8%	589
	21st to end	Night	10% 1 12% 31% 42% 5%	760
October	1st to 10th	Night	8% 1 17% 38% 32% 3	760
	11th to 20th	Night	9% 4 20% 40% 24% 3	760
	21st to end	Night	13% 6% 22% 35% 22% 2	836
November	1st to 10th	Night	14% 6% 19% 38% 22% 2	779
	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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