Salem
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Salem clear sky chart from 2005-11-06 to 2025-09-04.

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	15% 8% 3 7% 5% 4 4 3 5% 6% 39%	135371
	Night	18% 8% 3 7% 5% 4 4 3 5% 6% 37%	130694
Transparency	Day	45% 7% 13% 19% 13% 4	130156
	Night	49% 7% 13% 17% 11% 3	126908
Seeing	Day	19% 7% 17% 25% 22% 10%	15708
	Night	22% 6% 16% 33% 22% 2	24599

By Month

Percentage Cloud Cover SW (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	34% 14% 5% 12% 6% 5% 4 3 3 4 9%	3639
	11th to 20th	Day	27% 10% 3 10% 7% 5% 4 3 5% 5% 20%	3640
	21st to end	Day	22% 9% 4 10% 6% 5% 5% 3 4 5% 28%	4444
February	1st to 10th	Day	25% 14% 3 12% 6% 5% 6% 4 4 7% 14%	1800
	11th to 20th	Day	27% 12% 5% 10% 6% 5% 5% 3 4 5% 18%	1755
	21st to end	Day	25% 14% 5% 9% 6% 5% 4 4 4 5% 18%	1557
March	1st to 10th	Day	16% 12% 5% 11% 7% 6% 7% 5% 6% 7% 19%	4774
	11th to 20th	Day	24% 12% 4 9% 6% 5% 4 4 4 5% 25%	5000
	21st to end	Day	21% 11% 5% 11% 7% 7% 6% 5% 6% 6% 15%	5572
April	1st to 10th	Day	22% 12% 4 10% 7% 6% 5% 3 6% 7% 18%	2400
	11th to 20th	Day	20% 11% 3 8% 6% 5% 4 4 6% 6% 28%	2369
	21st to end	Day	18% 9% 3 9% 6% 5% 5% 5% 6% 7% 26%	2600
May	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 3 5% 6% 42%	6178
	11th to 20th	Day	12% 7% 3 8% 5% 5% 5% 4 5% 7% 39%	6277
	21st to end	Day	13% 6% 2 6% 5% 6% 6% 5% 6% 9% 36%	6902
June	1st to 10th	Day	17% 9% 3 7% 5% 4 4 3 5% 7% 37%	2800
	11th to 20th	Day	14% 8% 3 8% 5% 5% 7% 5% 6% 8% 31%	2800
	21st to end	Day	7% 5% 2 5% 4 4 4 4 5% 6% 53%	2800
July	1st to 10th	Day	4 2 1 2 2 2 2 2 3 4 75%	6319
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 80%	6320
	21st to end	Day	4 2 1 2 2 2 2 2 3 4 77%	6851
August	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 6% 66%	5777
	11th to 20th	Day	6% 4 2 4 4 4 4 3 6% 7% 57%	5796
	21st to end	Day	7% 5% 2 5% 3 4 4 4 5% 8% 54%	6162
September	1st to 10th	Day	5% 4 1 4 3 3 3 2 6% 6% 62%	2317
	11th to 20th	Day	10% 8% 3 6% 4 4 5% 4 5% 8% 44%	2270
	21st to end	Day	14% 7% 2 6% 3 4 4 4 5% 7% 45%	2005
October	1st to 10th	Day	13% 6% 3 6% 4 5% 4 3 5% 6% 46%	3180
	11th to 20th	Day	15% 7% 3 9% 6% 5% 6% 5% 6% 8% 29%	3161
	21st to end	Day	21% 10% 4 10% 6% 6% 4 4 5% 8% 22%	3400
November	1st to 10th	Day	26% 14% 5% 13% 6% 4 4 4 5% 5% 15%	1654
	11th to 20th	Day	26% 14% 5% 10% 7% 5% 5% 3 4 5% 17%	1600
	21st to end	Day	25% 12% 5% 11% 5% 5% 4 4 4 5% 20%	1600
December	1st to 10th	Day	20% 9% 4 9% 6% 5% 4 3 5% 6% 27%	2620
	11th to 20th	Day	28% 13% 5% 13% 7% 6% 6% 4 5% 5% 9%	3028
	21st to end	Day	32% 17% 6% 15% 7% 5% 4 3 3 3 7%	4004
January	1st to 10th	Night	32% 14% 5% 10% 6% 5% 4 2 4 4 13%	6306
	11th to 20th	Night	26% 12% 3 7% 6% 6% 5% 3 4 5% 22%	6320
	21st to end	Night	25% 9% 3 8% 5% 3 4 3 4 4 31%	6529
February	1st to 10th	Night	23% 13% 5% 9% 7% 5% 4 4 5% 6% 18%	2600
	11th to 20th	Night	26% 10% 4 9% 5% 4 4 3 5% 4 25%	2532
	21st to end	Night	23% 14% 3 8% 6% 4 4 3 5% 6% 23%	2071
March	1st to 10th	Night	16% 11% 4 10% 7% 7% 7% 4 6% 6% 21%	5117
	11th to 20th	Night	20% 11% 4 8% 6% 6% 4 3 5% 5% 30%	5000
	21st to end	Night	21% 11% 4 11% 8% 6% 6% 4 6% 7% 17%	5402
April	1st to 10th	Night	24% 11% 4 9% 5% 5% 4 3 5% 6% 24%	2000
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 3 6% 7% 34%	1976
	21st to end	Night	19% 8% 3 9% 5% 4 5% 4 6% 7% 32%	1800
May	1st to 10th	Night	15% 6% 2 5% 4 3 3 3 4 6% 48%	3787
	11th to 20th	Night	12% 6% 3 6% 4 4 4 3 5% 8% 47%	3616
	21st to end	Night	12% 7% 3 5% 5% 4 4 4 6% 8% 42%	3972
June	1st to 10th	Night	15% 8% 2 7% 5% 5% 5% 4 6% 6% 38%	1600
	11th to 20th	Night	13% 7% 3 7% 4 5% 4 3 5% 7% 42%	1600
	21st to end	Night	5% 5% 2 5% 4 4 3 3 4 5% 59%	1600
July	1st to 10th	Night	4 2 2 1 2 2 2 3 4 78%	3640
	11th to 20th	Night	2 2 1 2 1 2 1 1 2 3 82%	3640
	21st to end	Night	3 2 1 2 2 2 2 2 4 5% 77%	3984
August	1st to 10th	Night	4 3 1 4 2 3 3 3 5% 5% 67%	4035
	11th to 20th	Night	11% 4 2 4 3 4 3 3 4 6% 57%	4052
	21st to end	Night	8% 4 2 5% 3 3 3 3 4 5% 60%	4825
September	1st to 10th	Night	5% 4 1 4 2 2 3 2 4 7% 67%	1930
	11th to 20th	Night	9% 5% 2 7% 5% 4 4 3 4 6% 51%	1909
	21st to end	Night	11% 5% 1 5% 4 3 4 3 4 7% 53%	2175
October	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 4 7% 50%	3800
	11th to 20th	Night	15% 5% 3 6% 5% 5% 5% 4 6% 8% 37%	3777
	21st to end	Night	20% 8% 4 9% 6% 5% 5% 3 6% 6% 28%	4167
November	1st to 10th	Night	26% 9% 3 9% 6% 4 5% 4 6% 5% 23%	2644
	11th to 20th	Night	29% 11% 3 9% 6% 4 4 3 4 4 21%	2800
	21st to end	Night	22% 10% 3 10% 6% 4 4 4 4 5% 27%	2800
December	1st to 10th	Night	18% 9% 3 7% 5% 6% 6% 5% 6% 7% 30%	4560
	11th to 20th	Night	33% 13% 5% 10% 6% 5% 5% 3 4 4 11%	5176
	21st to end	Night	29% 13% 5% 12% 7% 6% 4 3 4 4 12%	6952

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	79% 6% 6% 5% 3	3639
	11th to 20th	Day	68% 5% 6% 13% 7% 1	3640
	21st to end	Day	57% 7% 7% 11% 15% 3	4444
February	1st to 10th	Day	67% 10% 10% 9% 4	1800
	11th to 20th	Day	68% 9% 8% 9% 4 2	1764
	21st to end	Day	68% 7% 10% 8% 6% 2	1557
March	1st to 10th	Day	59% 9% 15% 14% 4	4774
	11th to 20th	Day	59% 8% 8% 17% 7% 1	5000
	21st to end	Day	66% 9% 14% 8% 3	5572
April	1st to 10th	Day	65% 7% 13% 12% 3	2400
	11th to 20th	Day	55% 8% 12% 14% 8% 2	2372
	21st to end	Day	55% 8% 15% 13% 7% 1	2600
May	1st to 10th	Day	42% 6% 12% 23% 16%	6178
	11th to 20th	Day	42% 8% 18% 23% 8% 1	6280
	21st to end	Day	42% 11% 25% 19% 4	6904
June	1st to 10th	Day	48% 6% 18% 20% 7% 1	2800
	11th to 20th	Day	48% 8% 20% 17% 7%	2800
	21st to end	Day	30% 7% 18% 30% 11% 4	2800
July	1st to 10th	Day	13% 4 10% 31% 20% 22%	6251
	11th to 20th	Day	11% 2 7% 20% 40% 19%	6245
	21st to end	Day	12% 3 16% 36% 28% 5%	6652
August	1st to 10th	Day	17% 4 12% 29% 27% 11%	4206
	11th to 20th	Day	23% 6% 20% 32% 15% 4	4195
	21st to end	Day	26% 6% 13% 25% 22% 7%	4444
September	1st to 10th	Day	20% 6% 13% 30% 20% 11%	2317
	11th to 20th	Day	39% 8% 14% 20% 14% 6%	2270
	21st to end	Day	39% 6% 15% 20% 14% 5%	2005
October	1st to 10th	Day	38% 6% 9% 23% 21% 3	3180
	11th to 20th	Day	49% 8% 18% 14% 11% 1	3161
	21st to end	Day	59% 7% 9% 15% 9% 1	3400
November	1st to 10th	Day	69% 8% 9% 10% 4	1654
	11th to 20th	Day	69% 7% 9% 8% 5% 2	1600
	21st to end	Day	66% 8% 8% 8% 11%	1600
December	1st to 10th	Day	58% 5% 11% 14% 11% 1	2620
	11th to 20th	Day	74% 9% 9% 7% 1	3028
	21st to end	Day	79% 9% 6% 4 2	4004
January	1st to 10th	Night	75% 6% 9% 5% 4 1	6306
	11th to 20th	Night	65% 6% 8% 14% 6% 2	6320
	21st to end	Night	56% 6% 6% 12% 16% 3	6529
February	1st to 10th	Night	65% 10% 10% 10% 4 1	2600
	11th to 20th	Night	61% 7% 12% 11% 6% 2	2534
	21st to end	Night	62% 8% 11% 11% 5% 2	2071
March	1st to 10th	Night	60% 8% 13% 14% 5%	5117
	11th to 20th	Night	55% 9% 12% 17% 7% 1	5000
	21st to end	Night	65% 9% 16% 9% 1	5402
April	1st to 10th	Night	61% 7% 14% 14% 5%	2000
	11th to 20th	Night	50% 7% 18% 15% 7% 3	1976
	21st to end	Night	53% 6% 16% 16% 7% 2	1800
May	1st to 10th	Night	37% 5% 15% 29% 13% 1	3784
	11th to 20th	Night	38% 5% 21% 26% 8% 1	3616
	21st to end	Night	38% 11% 29% 19% 3	3972
June	1st to 10th	Night	45% 7% 18% 24% 6% 1	1600
	11th to 20th	Night	43% 6% 21% 23% 7%	1600
	21st to end	Night	28% 5% 19% 31% 13% 4	1600
July	1st to 10th	Night	13% 4 10% 28% 25% 20%	3600
	11th to 20th	Night	10% 2 9% 16% 43% 20%	3592
	21st to end	Night	12% 3 20% 34% 26% 6%	3871
August	1st to 10th	Night	13% 3 11% 37% 28% 8%	2929
	11th to 20th	Night	26% 6% 21% 31% 11% 5%	2929
	21st to end	Night	25% 5% 13% 29% 21% 7%	3470
September	1st to 10th	Night	19% 6% 14% 30% 20% 11%	1930
	11th to 20th	Night	33% 8% 15% 23% 13% 9%	1909
	21st to end	Night	31% 7% 16% 23% 16% 8%	2175
October	1st to 10th	Night	34% 6% 9% 29% 20% 2	3800
	11th to 20th	Night	42% 7% 21% 17% 11% 2	3777
	21st to end	Night	55% 7% 12% 14% 10% 1	4167
November	1st to 10th	Night	60% 8% 11% 14% 6%	2644
	11th to 20th	Night	66% 7% 10% 8% 8% 2	2800
	21st to end	Night	59% 9% 9% 10% 12% 2	2800
December	1st to 10th	Night	52% 7% 11% 18% 12% 1	4560
	11th to 20th	Night	74% 8% 9% 7% 2	5176
	21st to end	Night	71% 10% 12% 5% 2	6952

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	44% 8% 16% 19% 12% 3	200
	11th to 20th	Day	36% 5% 20% 18% 14% 8%	200
	21st to end	Day	29% 6% 15% 24% 16% 10%	413
February	1st to 10th	Day	31% 11% 19% 18% 16% 6%	400
	11th to 20th	Day	31% 8% 27% 19% 10% 5%	395
	21st to end	Day	33% 12% 16% 23% 13% 3	369
March	1st to 10th	Day	30% 10% 21% 23% 13% 2	500
	11th to 20th	Day	28% 12% 17% 20% 18% 5%	500
	21st to end	Day	29% 11% 23% 21% 13% 2	550
April	1st to 10th	Day	28% 14% 18% 23% 11% 6%	500
	11th to 20th	Day	26% 9% 23% 24% 12% 6%	492
	21st to end	Day	21% 9% 22% 24% 18% 6%	500
May	1st to 10th	Day	16% 9% 20% 27% 20% 8%	503
	11th to 20th	Day	20% 8% 18% 23% 20% 11%	528
	21st to end	Day	15% 5% 16% 25% 25% 14%	573
June	1st to 10th	Day	18% 6% 15% 28% 22% 11%	530
	11th to 20th	Day	14% 8% 22% 29% 21% 6%	530
	21st to end	Day	6% 5% 14% 30% 35% 10%	530
July	1st to 10th	Day	5% 2 13% 31% 34% 14%	526
	11th to 20th	Day	4 3 12% 35% 33% 13%	524
	21st to end	Day	3 2 9% 28% 40% 18%	575
August	1st to 10th	Day	3 3 8% 27% 38% 21%	527
	11th to 20th	Day	3 3 8% 31% 33% 22%	517
	21st to end	Day	5% 4 14% 31% 31% 16%	583
September	1st to 10th	Day	7% 2 18% 28% 33% 13%	503
	11th to 20th	Day	12% 6% 13% 26% 30% 13%	490
	21st to end	Day	15% 4 18% 30% 23% 9%	490
October	1st to 10th	Day	17% 6% 18% 27% 21% 11%	490
	11th to 20th	Day	20% 6% 15% 25% 22% 10%	490
	21st to end	Day	28% 8% 25% 21% 11% 8%	515
November	1st to 10th	Day	31% 9% 21% 20% 12% 6%	245
	11th to 20th	Day	32% 9% 21% 16% 16% 9%	200
	21st to end	Day	31% 6% 19% 17% 18% 10%	200
December	1st to 10th	Day	37% 5% 13% 18% 16% 12%	200
	11th to 20th	Day	36% 10% 20% 23% 10% 3	200
	21st to end	Day	37% 10% 19% 21% 10% 3	220
January	1st to 10th	Night	37% 9% 17% 21% 14% 2	800
	11th to 20th	Night	34% 7% 14% 24% 19% 2	800
	21st to end	Night	28% 6% 14% 29% 20% 3	880
February	1st to 10th	Night	25% 10% 18% 28% 18% 2	800
	11th to 20th	Night	29% 7% 20% 26% 18% 1	784
	21st to end	Night	31% 9% 17% 32% 12%	660
March	1st to 10th	Night	27% 8% 19% 30% 16% 1	800
	11th to 20th	Night	23% 8% 19% 32% 16% 1	800
	21st to end	Night	27% 8% 21% 29% 14% 1	880
April	1st to 10th	Night	26% 9% 17% 28% 19% 1	800
	11th to 20th	Night	22% 7% 21% 31% 18% 1	788
	21st to end	Night	22% 5% 17% 37% 18% 1	600
May	1st to 10th	Night	14% 5% 19% 42% 19% 1	469
	11th to 20th	Night	14% 4 17% 39% 24% 2	394
	21st to end	Night	15% 6% 15% 37% 26% 2	433
June	1st to 10th	Night	16% 6% 13% 40% 24% 1	400
	11th to 20th	Night	13% 5% 20% 44% 17% 1	400
	21st to end	Night	8% 2 20% 42% 28% 1	400
July	1st to 10th	Night	5% 3 8% 52% 31% 1	390
	11th to 20th	Night	4 1 16% 48% 29% 1	388
	21st to end	Night	3 2 8% 49% 37% 2	440
August	1st to 10th	Night	4 2 7% 39% 41% 7%	597
	11th to 20th	Night	5% 2 5% 37% 47% 4	594
	21st to end	Night	7% 2 10% 45% 33% 3	794
September	1st to 10th	Night	5% 2 15% 40% 36% 2	772
	11th to 20th	Night	10% 3 10% 34% 37% 5%	760
	21st to end	Night	12% 4 13% 39% 29% 3	760
October	1st to 10th	Night	15% 4 17% 34% 27% 4	760
	11th to 20th	Night	18% 5% 11% 34% 26% 5%	760
	21st to end	Night	25% 6% 18% 30% 17% 4	836
November	1st to 10th	Night	29% 6% 16% 31% 17% 1	780
	11th to 20th	Night	33% 9% 18% 25% 13% 2	800
	21st to end	Night	26% 6% 17% 31% 18% 3	800
December	1st to 10th	Night	30% 7% 12% 30% 18% 4	800
	11th to 20th	Night	35% 8% 17% 25% 14% 1	800
	21st to end	Night	34% 8% 20% 26% 11% 1	880

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