Elginfield Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Elginfield Observatory 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% 11% 4 10% 5% 5% 4 4 5% 7% 30%	126309
	Night	14% 12% 5% 11% 6% 5% 4 3 5% 6% 29%	122357
Transparency	Day	52% 5% 12% 19% 11% 1	129891
	Night	54% 5% 12% 18% 11% 1	126863
Seeing	Day	19% 12% 28% 24% 13% 5%	16158
	Night	20% 10% 29% 28% 11% 1	25326

By Month

Percentage Cloud Cover NE (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	12% 17% 8% 29% 6% 5% 4 2 2 4 11%	3452
	11th to 20th	Day	10% 21% 10% 24% 9% 8% 4 2 3 3 6%	4006
	21st to end	Day	17% 19% 8% 16% 7% 6% 6% 3 3 4 12%	4466
February	1st to 10th	Day	15% 17% 5% 15% 9% 7% 5% 3 4 4 15%	1800
	11th to 20th	Day	14% 15% 8% 14% 11% 6% 5% 2 4 5% 17%	1755
	21st to end	Day	15% 15% 7% 13% 6% 4 5% 3 5% 6% 21%	1650
March	1st to 10th	Day	16% 11% 7% 14% 7% 4 3 3 4 5% 25%	4793
	11th to 20th	Day	11% 14% 6% 10% 5% 4 4 4 5% 8% 29%	4980
	21st to end	Day	19% 13% 6% 12% 4 4 4 3 4 5% 27%	5465
April	1st to 10th	Day	17% 15% 5% 8% 4 3 4 4 6% 6% 28%	2270
	11th to 20th	Day	18% 11% 3 6% 3 3 4 4 6% 8% 34%	2567
	21st to end	Day	19% 11% 4 7% 5% 3 4 3 5% 6% 33%	2600
May	1st to 10th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 33%	5888
	11th to 20th	Day	16% 10% 3 7% 4 4 5% 4 7% 7% 32%	5861
	21st to end	Day	17% 9% 2 6% 4 4 4 3 5% 6% 40%	6949
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 7% 8% 38%	2820
	11th to 20th	Day	11% 7% 2 6% 5% 5% 5% 4 6% 9% 39%	3000
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 40%	3000
July	1st to 10th	Day	10% 6% 3 7% 5% 4 5% 5% 6% 9% 41%	6819
	11th to 20th	Day	9% 6% 2 6% 5% 5% 4 4 6% 8% 45%	6106
	21st to end	Day	8% 5% 2 5% 4 5% 4 4 6% 9% 46%	6424
August	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 10% 37%	4234
	11th to 20th	Day	11% 8% 2 7% 4 5% 6% 5% 8% 11% 34%	4174
	21st to end	Day	9% 6% 3 7% 5% 4 6% 4 7% 9% 40%	4624
September	1st to 10th	Day	11% 7% 2 6% 5% 5% 5% 4 6% 8% 41%	2256
	11th to 20th	Day	11% 6% 2 6% 4 3 4 4 6% 9% 45%	2090
	21st to end	Day	17% 7% 2 6% 4 5% 5% 4 7% 8% 34%	2090
October	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 6% 7% 35%	1944
	11th to 20th	Day	20% 11% 3 10% 6% 5% 4 3 6% 6% 25%	1900
	21st to end	Day	25% 13% 4 9% 5% 5% 5% 4 5% 6% 20%	2042
November	1st to 10th	Day	16% 15% 4 8% 6% 4 4 3 6% 6% 29%	1762
	11th to 20th	Day	16% 16% 4 9% 6% 5% 6% 4 5% 7% 21%	1640
	21st to end	Day	19% 20% 7% 11% 7% 5% 4 4 4 5% 13%	1600
December	1st to 10th	Day	18% 23% 10% 16% 6% 4 4 3 4 3 9%	2600
	11th to 20th	Day	21% 21% 7% 14% 6% 5% 4 3 4 3 13%	2986
	21st to end	Day	26% 21% 8% 12% 6% 4 3 3 4 4 8%	3696
January	1st to 10th	Night	13% 17% 8% 26% 7% 5% 4 2 4 4 11%	6498
	11th to 20th	Night	12% 16% 9% 22% 11% 6% 5% 3 4 4 8%	5860
	21st to end	Night	16% 16% 8% 16% 6% 5% 6% 3 4 5% 14%	6490
February	1st to 10th	Night	15% 18% 5% 13% 9% 5% 5% 4 5% 4 17%	2600
	11th to 20th	Night	15% 16% 5% 12% 8% 6% 5% 4 5% 6% 18%	2528
	21st to end	Night	16% 13% 6% 13% 8% 5% 5% 4 5% 5% 21%	1980
March	1st to 10th	Night	13% 12% 5% 9% 6% 5% 5% 4 5% 6% 31%	5046
	11th to 20th	Night	10% 10% 5% 9% 5% 5% 5% 4 6% 8% 33%	4979
	21st to end	Night	18% 12% 4 9% 5% 4 4 3 4 4 33%	5467
April	1st to 10th	Night	19% 12% 3 7% 5% 4 4 4 5% 5% 31%	2130
	11th to 20th	Night	17% 11% 3 7% 5% 4 4 3 6% 6% 35%	1797
	21st to end	Night	15% 11% 3 8% 5% 4 5% 5% 6% 7% 33%	1800
May	1st to 10th	Night	19% 9% 3 7% 4 4 4 3 5% 6% 36%	4053
	11th to 20th	Night	13% 10% 4 9% 5% 5% 5% 3 6% 7% 35%	4017
	21st to end	Night	18% 7% 2 4 3 3 3 3 5% 7% 44%	3926
June	1st to 10th	Night	12% 7% 2 5% 5% 4 5% 4 5% 7% 43%	1573
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 10% 40%	1400
	21st to end	Night	10% 5% 2 6% 4 5% 3 4 6% 7% 48%	1400
July	1st to 10th	Night	6% 5% 2 6% 4 4 5% 4 6% 10% 47%	3160
	11th to 20th	Night	10% 6% 2 5% 3 4 4 3 5% 8% 50%	3827
	21st to end	Night	8% 6% 2 5% 4 4 5% 4 6% 8% 49%	4428
August	1st to 10th	Night	11% 6% 2 7% 4 4 5% 4 6% 7% 44%	2915
	11th to 20th	Night	13% 8% 3 5% 4 3 4 3 6% 7% 45%	2875
	21st to end	Night	9% 6% 2 5% 4 4 5% 4 5% 8% 47%	3241
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 47%	1988
	11th to 20th	Night	10% 5% 2 5% 3 4 3 3 5% 7% 52%	2090
	21st to end	Night	16% 6% 3 8% 5% 5% 4 3 5% 6% 40%	2090
October	1st to 10th	Night	14% 8% 3 8% 5% 4 4 3 5% 6% 40%	2234
	11th to 20th	Night	21% 10% 3 7% 4 4 5% 3 5% 6% 31%	2280
	21st to end	Night	24% 11% 4 9% 5% 4 5% 3 5% 6% 25%	2555
November	1st to 10th	Night	16% 12% 3 7% 5% 4 3 4 5% 6% 34%	2536
	11th to 20th	Night	16% 18% 4 9% 6% 5% 6% 4 6% 6% 21%	2757
	21st to end	Night	21% 17% 6% 11% 7% 5% 4 4 5% 5% 17%	2800
December	1st to 10th	Night	12% 23% 10% 16% 7% 6% 4 3 5% 4 9%	4580
	11th to 20th	Night	14% 25% 8% 13% 7% 5% 4 3 4 3 14%	5208
	21st to end	Night	23% 20% 8% 11% 5% 4 4 3 4 4 13%	7249

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	78% 5% 8% 8% 2	3453
	11th to 20th	Day	85% 5% 5% 3 2	4040
	21st to end	Day	77% 4 8% 5% 6%	4466
February	1st to 10th	Day	70% 6% 10% 11% 3	1800
	11th to 20th	Day	70% 4 8% 12% 5%	1764
	21st to end	Day	65% 4 11% 12% 6% 1	1650
March	1st to 10th	Day	62% 3 13% 16% 4	4796
	11th to 20th	Day	53% 4 17% 18% 7% 1	4980
	21st to end	Day	60% 4 10% 16% 8% 1	5465
April	1st to 10th	Day	54% 6% 12% 18% 9% 1	2270
	11th to 20th	Day	48% 5% 13% 20% 13% 1	2567
	21st to end	Day	51% 6% 11% 19% 12% 2	2600
May	1st to 10th	Day	49% 5% 11% 19% 10% 5%	5897
	11th to 20th	Day	48% 7% 12% 19% 12% 2	5865
	21st to end	Day	45% 5% 10% 19% 17% 4	6949
June	1st to 10th	Day	40% 7% 13% 25% 13% 2	2820
	11th to 20th	Day	41% 7% 12% 23% 16% 2	3000
	21st to end	Day	40% 7% 13% 24% 16% 1	3000
July	1st to 10th	Day	40% 4 12% 25% 18% 2	6746
	11th to 20th	Day	37% 3 11% 31% 16% 2	6031
	21st to end	Day	35% 4 13% 25% 21% 1	6240
August	1st to 10th	Day	42% 6% 13% 23% 15% 1	4221
	11th to 20th	Day	42% 6% 15% 27% 10%	4213
	21st to end	Day	40% 4 12% 31% 12% 1	4624
September	1st to 10th	Day	41% 7% 13% 25% 12% 1	2256
	11th to 20th	Day	35% 7% 14% 29% 16%	2090
	21st to end	Day	46% 7% 13% 24% 10% 1	2090
October	1st to 10th	Day	42% 5% 12% 24% 16% 1	3230
	11th to 20th	Day	44% 4 13% 21% 17%	3140
	21st to end	Day	54% 4 17% 19% 6%	3344
November	1st to 10th	Day	55% 5% 12% 19% 8% 1	1762
	11th to 20th	Day	59% 6% 15% 11% 7% 1	1640
	21st to end	Day	72% 5% 11% 11% 2	1600
December	1st to 10th	Day	79% 5% 10% 5% 1	2600
	11th to 20th	Day	75% 6% 9% 7% 4	2986
	21st to end	Day	79% 5% 8% 7% 1	3696
January	1st to 10th	Night	76% 6% 8% 9% 2	6498
	11th to 20th	Night	79% 6% 7% 5% 2	5901
	21st to end	Night	71% 5% 10% 9% 6%	6490
February	1st to 10th	Night	68% 5% 11% 12% 4	2600
	11th to 20th	Night	64% 5% 12% 14% 5%	2530
	21st to end	Night	62% 5% 12% 14% 6%	1980
March	1st to 10th	Night	53% 6% 13% 18% 10% 1	5056
	11th to 20th	Night	46% 5% 19% 20% 9% 1	4980
	21st to end	Night	52% 6% 10% 18% 14% 1	5467
April	1st to 10th	Night	53% 5% 10% 19% 12% 1	2130
	11th to 20th	Night	48% 4 10% 20% 16% 2	1797
	21st to end	Night	48% 6% 12% 17% 15% 3	1800
May	1st to 10th	Night	49% 4 11% 16% 13% 8%	4060
	11th to 20th	Night	47% 6% 10% 22% 11% 3	4018
	21st to end	Night	40% 5% 12% 23% 17% 3	3927
June	1st to 10th	Night	38% 5% 13% 25% 17% 3	1573
	11th to 20th	Night	37% 6% 12% 26% 16% 3	1400
	21st to end	Night	35% 6% 13% 25% 19% 1	1400
July	1st to 10th	Night	34% 3 14% 26% 18% 5%	3125
	11th to 20th	Night	34% 4 12% 28% 20% 2	3779
	21st to end	Night	33% 4 12% 24% 23% 3	4297
August	1st to 10th	Night	38% 5% 14% 25% 18% 1	2904
	11th to 20th	Night	38% 5% 14% 28% 14% 1	2902
	21st to end	Night	35% 5% 12% 33% 15% 1	3241
September	1st to 10th	Night	35% 7% 13% 27% 16% 2	1988
	11th to 20th	Night	32% 5% 12% 31% 19% 1	2090
	21st to end	Night	45% 5% 13% 24% 13%	2090
October	1st to 10th	Night	35% 4 9% 27% 24% 1	3728
	11th to 20th	Night	45% 3 17% 19% 15% 1	3779
	21st to end	Night	48% 4 14% 24% 10%	4203
November	1st to 10th	Night	48% 5% 13% 23% 10% 1	2536
	11th to 20th	Night	59% 7% 14% 14% 6% 1	2757
	21st to end	Night	67% 7% 14% 9% 3	2800
December	1st to 10th	Night	77% 6% 10% 6% 2	4580
	11th to 20th	Night	73% 5% 9% 8% 4 1	5208
	21st to end	Night	75% 5% 10% 9% 2	7249

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	20% 25% 30% 19% 5% 2	263
	11th to 20th	Day	19% 19% 45% 13% 3 1	300
	21st to end	Day	28% 13% 37% 18% 4	330
February	1st to 10th	Day	23% 23% 37% 13% 3	300
	11th to 20th	Day	25% 23% 36% 15%	300
	21st to end	Day	27% 26% 35% 11% 2	249
March	1st to 10th	Day	25% 14% 40% 16% 6%	434
	11th to 20th	Day	23% 17% 31% 24% 4 1	500
	21st to end	Day	29% 15% 30% 20% 5% 1	550
April	1st to 10th	Day	23% 17% 33% 19% 6% 1	500
	11th to 20th	Day	28% 14% 27% 17% 10% 3	492
	21st to end	Day	24% 13% 32% 20% 9% 2	500
May	1st to 10th	Day	23% 12% 27% 26% 9% 3	503
	11th to 20th	Day	18% 9% 28% 24% 15% 6%	527
	21st to end	Day	17% 7% 24% 28% 17% 7%	573
June	1st to 10th	Day	12% 7% 35% 28% 13% 4	550
	11th to 20th	Day	13% 9% 27% 35% 13% 3	730
	21st to end	Day	14% 8% 28% 33% 13% 4	730
July	1st to 10th	Day	11% 6% 23% 29% 23% 8%	721
	11th to 20th	Day	15% 6% 26% 27% 19% 7%	609
	21st to end	Day	9% 5% 22% 33% 23% 9%	576
August	1st to 10th	Day	12% 4 17% 29% 25% 12%	526
	11th to 20th	Day	13% 3 20% 28% 21% 14%	519
	21st to end	Day	13% 5% 21% 29% 22% 11%	583
September	1st to 10th	Day	13% 5% 22% 28% 21% 11%	502
	11th to 20th	Day	9% 5% 19% 25% 29% 13%	490
	21st to end	Day	18% 9% 25% 27% 12% 9%	490
October	1st to 10th	Day	18% 12% 26% 22% 17% 5%	490
	11th to 20th	Day	23% 14% 32% 21% 8% 2	490
	21st to end	Day	28% 17% 32% 18% 4	491
November	1st to 10th	Day	25% 20% 28% 17% 8% 2	300
	11th to 20th	Day	27% 17% 36% 13% 5% 2	220
	21st to end	Day	28% 18% 38% 14% 4	200
December	1st to 10th	Day	29% 19% 32% 16% 4 1	200
	11th to 20th	Day	28% 24% 29% 16% 4	200
	21st to end	Day	30% 23% 31% 13% 3	220
January	1st to 10th	Night	24% 17% 38% 16% 5% 1	999
	11th to 20th	Night	28% 15% 32% 20% 4	801
	21st to end	Night	21% 13% 37% 23% 6%	880
February	1st to 10th	Night	25% 14% 36% 22% 3	800
	11th to 20th	Night	24% 14% 33% 24% 5%	783
	21st to end	Night	24% 16% 36% 21% 3	660
March	1st to 10th	Night	19% 14% 36% 25% 6%	800
	11th to 20th	Night	18% 16% 36% 22% 7% 1	800
	21st to end	Night	25% 11% 29% 26% 8% 1	880
April	1st to 10th	Night	23% 15% 30% 27% 5%	800
	11th to 20th	Night	22% 11% 33% 23% 10% 1	613
	21st to end	Night	19% 11% 34% 29% 7% 1	600
May	1st to 10th	Night	22% 6% 31% 30% 11% 1	600
	11th to 20th	Night	16% 5% 30% 35% 13% 2	589
	21st to end	Night	14% 3 29% 38% 16% 1	433
June	1st to 10th	Night	15% 4 30% 38% 13% 1	400
	11th to 20th	Night	12% 3 25% 47% 13% 1	400
	21st to end	Night	11% 5% 22% 45% 16% 1	400
July	1st to 10th	Night	4 2 21% 49% 23% 1	390
	11th to 20th	Night	10% 3 19% 41% 24% 2	566
	21st to end	Night	9% 4 15% 48% 23% 1	657
August	1st to 10th	Night	9% 2 13% 43% 31% 3	597
	11th to 20th	Night	10% 3 13% 40% 30% 4	594
	21st to end	Night	12% 3 16% 39% 27% 3	665
September	1st to 10th	Night	12% 5% 21% 33% 24% 5%	773
	11th to 20th	Night	12% 3 17% 36% 28% 4	760
	21st to end	Night	18% 7% 21% 32% 17% 5%	760
October	1st to 10th	Night	19% 7% 26% 29% 18% 2	760
	11th to 20th	Night	26% 12% 25% 26% 10% 1	760
	21st to end	Night	28% 12% 31% 25% 4	836
November	1st to 10th	Night	22% 12% 28% 29% 8% 1	780
	11th to 20th	Night	25% 14% 34% 21% 6% 1	800
	21st to end	Night	29% 12% 31% 23% 5%	800
December	1st to 10th	Night	28% 18% 32% 18% 4	800
	11th to 20th	Night	24% 19% 37% 17% 3	800
	21st to end	Night	30% 13% 36% 18% 2	990

Page generated at 2025-09-01 01:51:03. Version ac07

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