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 2024-03-28.

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	16% 11% 5% 8% 6% 3 5% 5% 5% 7% 29%	78920
	Night	16% 12% 5% 9% 6% 3 5% 4 5% 6% 28%	81034
Transparency	Day	53% 6% 12% 18% 9% 1	78633
	Night	56% 6% 12% 17% 9% 1	80856
Seeing	Day	19% 11% 28% 24% 13% 5%	14166
	Night	20% 10% 29% 28% 11% 1	23598

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	16% 19% 9% 12% 9% 4 5% 3 3 4 15%	2404
	11th to 20th	Day	12% 20% 11% 17% 11% 5% 6% 4 3 4 8%	2820
	21st to end	Day	21% 22% 11% 12% 8% 4 4 3 3 4 9%	3124
February	1st to 10th	Day	16% 16% 5% 13% 11% 6% 6% 4 4 4 15%	1710
	11th to 20th	Day	14% 13% 9% 13% 12% 5% 6% 4 4 5% 17%	1664
	21st to end	Day	16% 14% 8% 12% 7% 3 5% 4 4 6% 22%	1570
March	1st to 10th	Day	15% 12% 6% 10% 7% 4 4 5% 5% 7% 25%	2024
	11th to 20th	Day	13% 13% 6% 9% 7% 3 5% 4 4 6% 29%	2090
	21st to end	Day	19% 11% 6% 9% 6% 3 4 4 6% 5% 29%	2255
April	1st to 10th	Day	17% 14% 6% 7% 5% 3 4 5% 6% 7% 28%	2042
	11th to 20th	Day	19% 10% 3 5% 4 3 3 4 5% 8% 34%	2307
	21st to end	Day	20% 10% 5% 6% 6% 3 3 4 5% 6% 32%	2340
May	1st to 10th	Day	18% 8% 3 6% 5% 3 4 5% 6% 9% 33%	2340
	11th to 20th	Day	18% 8% 4 5% 5% 3 4 5% 6% 6% 37%	2301
	21st to end	Day	13% 5% 2 5% 5% 3 5% 5% 6% 9% 41%	2725
June	1st to 10th	Day	13% 5% 3 6% 6% 3 5% 5% 7% 8% 39%	2538
	11th to 20th	Day	12% 7% 2 5% 6% 3 6% 5% 6% 8% 41%	2700
	21st to end	Day	13% 5% 3 5% 5% 3 5% 5% 6% 8% 41%	2700
July	1st to 10th	Day	9% 4 2 5% 5% 3 5% 5% 6% 9% 48%	2700
	11th to 20th	Day	11% 5% 2 6% 5% 3 5% 5% 6% 9% 41%	2434
	21st to end	Day	10% 6% 2 5% 6% 4 5% 5% 7% 9% 41%	2547
August	1st to 10th	Day	10% 5% 2 5% 6% 4 5% 6% 7% 11% 38%	2327
	11th to 20th	Day	11% 7% 2 5% 6% 3 6% 6% 7% 10% 37%	2303
	21st to end	Day	12% 6% 3 6% 5% 3 5% 4 6% 9% 41%	2552
September	1st to 10th	Day	12% 7% 2 6% 6% 4 6% 5% 6% 8% 39%	2104
	11th to 20th	Day	12% 5% 3 5% 5% 2 4 5% 6% 9% 44%	1980
	21st to end	Day	17% 7% 2 5% 5% 3 5% 5% 6% 8% 36%	1980
October	1st to 10th	Day	18% 7% 3 6% 5% 3 5% 5% 6% 7% 35%	1842
	11th to 20th	Day	21% 11% 4 8% 7% 4 5% 5% 6% 6% 24%	1800
	21st to end	Day	26% 12% 4 8% 7% 4 5% 5% 5% 5% 20%	1935
November	1st to 10th	Day	16% 14% 4 7% 7% 3 4 4 6% 7% 29%	1672
	11th to 20th	Day	17% 14% 4 9% 7% 4 6% 5% 6% 7% 22%	1558
	21st to end	Day	20% 18% 8% 10% 7% 3 5% 5% 4 5% 14%	1520
December	1st to 10th	Day	16% 19% 10% 14% 8% 4 4 4 5% 3 13%	1520
	11th to 20th	Day	19% 16% 9% 15% 8% 4 4 5% 4 3 13%	1906
	21st to end	Day	28% 19% 9% 10% 6% 3 4 4 4 5% 8%	2586
January	1st to 10th	Night	16% 18% 9% 16% 9% 4 4 4 3 4 11%	4550
	11th to 20th	Night	15% 16% 9% 16% 11% 4 5% 4 4 5% 10%	4121
	21st to end	Night	21% 17% 10% 14% 8% 3 5% 4 3 4 11%	4532
February	1st to 10th	Night	16% 16% 6% 11% 10% 5% 6% 5% 4 4 17%	2470
	11th to 20th	Night	14% 14% 7% 11% 9% 5% 5% 5% 5% 6% 18%	2399
	21st to end	Night	17% 12% 6% 11% 9% 4 5% 4 5% 5% 22%	1884
March	1st to 10th	Night	15% 10% 6% 9% 7% 4 4 4 5% 6% 29%	2142
	11th to 20th	Night	13% 11% 5% 9% 6% 4 5% 4 5% 6% 33%	2089
	21st to end	Night	18% 11% 4 8% 6% 4 5% 4 5% 5% 32%	2256
April	1st to 10th	Night	19% 10% 4 6% 5% 4 4 4 6% 6% 32%	1918
	11th to 20th	Night	17% 9% 3 6% 6% 3 4 4 5% 7% 37%	1615
	21st to end	Night	16% 11% 4 7% 5% 2 5% 5% 5% 7% 34%	1620
May	1st to 10th	Night	18% 8% 3 6% 4 2 4 5% 6% 7% 37%	1620
	11th to 20th	Night	13% 7% 3 6% 6% 3 4 5% 6% 8% 39%	1591
	21st to end	Night	10% 4 2 4 4 2 5% 5% 6% 8% 50%	1550
June	1st to 10th	Night	12% 6% 3 5% 5% 3 5% 5% 5% 8% 44%	1417
	11th to 20th	Night	9% 5% 3 6% 5% 3 5% 5% 6% 11% 41%	1260
	21st to end	Night	10% 5% 2 6% 5% 3 5% 4 6% 7% 48%	1260
July	1st to 10th	Night	6% 4 2 4 3 3 3 5% 6% 9% 54%	1260
	11th to 20th	Night	8% 5% 3 4 4 2 5% 5% 5% 8% 50%	1518
	21st to end	Night	8% 5% 3 6% 5% 2 5% 6% 6% 8% 46%	1771
August	1st to 10th	Night	9% 4 2 6% 5% 3 5% 6% 7% 8% 46%	1611
	11th to 20th	Night	8% 5% 3 4 4 3 5% 5% 7% 8% 47%	1598
	21st to end	Night	11% 6% 2 3 4 3 5% 5% 4 8% 47%	1786
September	1st to 10th	Night	11% 5% 2 6% 5% 3 4 5% 5% 7% 45%	1854
	11th to 20th	Night	11% 5% 2 4 4 3 4 4 5% 7% 51%	1980
	21st to end	Night	16% 6% 3 7% 5% 3 4 4 5% 6% 42%	1980
October	1st to 10th	Night	15% 8% 3 7% 6% 3 4 4 5% 7% 38%	2116
	11th to 20th	Night	22% 10% 4 7% 5% 3 5% 4 5% 6% 29%	2160
	21st to end	Night	25% 11% 4 8% 6% 3 4 4 5% 6% 23%	2420
November	1st to 10th	Night	16% 11% 3 6% 5% 3 4 5% 5% 6% 34%	2406
	11th to 20th	Night	16% 17% 4 9% 6% 3 6% 5% 5% 7% 21%	2619
	21st to end	Night	21% 17% 6% 9% 8% 3 4 5% 5% 5% 18%	2660
December	1st to 10th	Night	19% 20% 10% 10% 8% 3 5% 3 4 4 12%	2660
	11th to 20th	Night	15% 21% 8% 13% 6% 4 5% 5% 5% 4 15%	3288
	21st to end	Night	27% 21% 8% 11% 6% 4 4 4 3 3 10%	5053

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	72% 6% 11% 9% 2	2405
	11th to 20th	Day	81% 6% 6% 4 2	2820
	21st to end	Day	80% 5% 6% 5% 3	3124
February	1st to 10th	Day	71% 6% 10% 10% 3	1710
	11th to 20th	Day	69% 5% 8% 12% 5%	1673
	21st to end	Day	64% 5% 12% 12% 6% 1	1570
March	1st to 10th	Day	60% 5% 12% 16% 6% 1	2027
	11th to 20th	Day	56% 5% 11% 17% 9% 2	2090
	21st to end	Day	58% 5% 9% 17% 9% 1	2255
April	1st to 10th	Day	54% 6% 12% 18% 9%	2042
	11th to 20th	Day	48% 5% 13% 19% 14% 1	2307
	21st to end	Day	52% 6% 12% 18% 11% 2	2340
May	1st to 10th	Day	48% 6% 12% 21% 12% 1	2340
	11th to 20th	Day	46% 6% 11% 21% 13% 2	2305
	21st to end	Day	39% 7% 14% 23% 15% 2	2725
June	1st to 10th	Day	40% 8% 13% 25% 13% 2	2538
	11th to 20th	Day	39% 8% 11% 23% 17% 2	2700
	21st to end	Day	39% 7% 12% 24% 16% 1	2700
July	1st to 10th	Day	33% 7% 12% 27% 19% 2	2626
	11th to 20th	Day	38% 7% 14% 26% 14% 1	2359
	21st to end	Day	37% 9% 13% 24% 15% 2	2405
August	1st to 10th	Day	37% 9% 16% 27% 10% 1	2314
	11th to 20th	Day	40% 9% 16% 23% 12% 1	2303
	21st to end	Day	40% 6% 13% 26% 13% 1	2552
September	1st to 10th	Day	42% 7% 14% 23% 12% 1	2104
	11th to 20th	Day	36% 7% 14% 29% 13%	1980
	21st to end	Day	45% 7% 14% 24% 10% 1	1980
October	1st to 10th	Day	48% 6% 13% 20% 12% 1	1842
	11th to 20th	Day	58% 7% 14% 15% 7%	1800
	21st to end	Day	65% 6% 12% 12% 5%	1935
November	1st to 10th	Day	55% 5% 12% 19% 8% 1	1672
	11th to 20th	Day	59% 6% 16% 11% 6% 1	1558
	21st to end	Day	72% 5% 11% 11% 2	1520
December	1st to 10th	Day	72% 8% 10% 8% 2	1520
	11th to 20th	Day	74% 7% 10% 8% 2	1906
	21st to end	Day	79% 5% 11% 4	2586
January	1st to 10th	Night	74% 7% 10% 7% 2	4550
	11th to 20th	Night	76% 8% 9% 5% 2	4121
	21st to end	Night	76% 5% 10% 7% 2 1	4532
February	1st to 10th	Night	68% 6% 10% 12% 3	2470
	11th to 20th	Night	64% 5% 12% 13% 5%	2401
	21st to end	Night	62% 6% 12% 14% 6%	1884
March	1st to 10th	Night	55% 6% 11% 19% 9% 1	2152
	11th to 20th	Night	51% 6% 12% 20% 10% 2	2090
	21st to end	Night	54% 5% 9% 19% 12% 2	2256
April	1st to 10th	Night	51% 5% 10% 20% 13% 1	1918
	11th to 20th	Night	47% 5% 10% 20% 17% 2	1615
	21st to end	Night	49% 6% 11% 17% 15% 3	1620
May	1st to 10th	Night	46% 6% 10% 23% 13% 3	1620
	11th to 20th	Night	42% 6% 11% 23% 14% 4	1592
	21st to end	Night	31% 8% 14% 27% 17% 3	1551
June	1st to 10th	Night	38% 5% 13% 26% 17% 2	1417
	11th to 20th	Night	36% 7% 12% 25% 17% 3	1260
	21st to end	Night	35% 6% 13% 26% 19% 1	1260
July	1st to 10th	Night	27% 6% 11% 29% 22% 5%	1225
	11th to 20th	Night	30% 8% 14% 28% 16% 3	1470
	21st to end	Night	32% 8% 16% 24% 16% 3	1672
August	1st to 10th	Night	33% 8% 16% 29% 14% 1	1600
	11th to 20th	Night	31% 8% 15% 30% 14% 2	1598
	21st to end	Night	33% 7% 13% 29% 16% 2	1786
September	1st to 10th	Night	37% 7% 13% 25% 15% 2	1854
	11th to 20th	Night	33% 5% 13% 31% 18% 1	1980
	21st to end	Night	44% 5% 13% 24% 13%	1980
October	1st to 10th	Night	45% 6% 11% 21% 15% 1	2116
	11th to 20th	Night	52% 5% 15% 19% 8% 1	2160
	21st to end	Night	60% 5% 11% 15% 8% 1	2420
November	1st to 10th	Night	48% 5% 14% 23% 10% 1	2406
	11th to 20th	Night	58% 7% 14% 14% 6% 1	2619
	21st to end	Night	66% 7% 15% 9% 3	2660
December	1st to 10th	Night	72% 8% 10% 8% 2	2660
	11th to 20th	Night	70% 5% 14% 9% 2	3288
	21st to end	Night	80% 5% 10% 5% 1	5053

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	21% 24% 28% 21% 5% 2	236
	11th to 20th	Day	20% 17% 46% 13% 3 1	270
	21st to end	Day	29% 12% 36% 19% 4	297
February	1st to 10th	Day	23% 22% 38% 13% 3	270
	11th to 20th	Day	24% 23% 36% 16%	270
	21st to end	Day	28% 28% 32% 11% 2	225
March	1st to 10th	Day	25% 14% 39% 16% 7%	397
	11th to 20th	Day	22% 15% 32% 24% 5% 1	460
	21st to end	Day	29% 15% 31% 19% 6% 1	490
April	1st to 10th	Day	21% 18% 33% 21% 7% 1	420
	11th to 20th	Day	28% 12% 27% 19% 10% 4	412
	21st to end	Day	25% 12% 32% 20% 9% 2	420
May	1st to 10th	Day	23% 11% 25% 27% 10% 4	423
	11th to 20th	Day	19% 9% 28% 24% 14% 6%	447
	21st to end	Day	14% 7% 23% 29% 18% 8%	485
June	1st to 10th	Day	13% 5% 34% 29% 13% 5%	468
	11th to 20th	Day	14% 9% 26% 34% 14% 4	630
	21st to end	Day	14% 8% 27% 33% 14% 4	630
July	1st to 10th	Day	10% 6% 21% 30% 25% 9%	621
	11th to 20th	Day	16% 6% 24% 28% 20% 7%	521
	21st to end	Day	9% 5% 22% 34% 21% 9%	488
August	1st to 10th	Day	12% 3 17% 28% 26% 14%	446
	11th to 20th	Day	13% 3 19% 27% 23% 14%	439
	21st to end	Day	14% 6% 21% 27% 22% 11%	495
September	1st to 10th	Day	13% 5% 22% 28% 21% 11%	450
	11th to 20th	Day	10% 6% 20% 25% 27% 12%	450
	21st to end	Day	18% 10% 24% 26% 13% 10%	450
October	1st to 10th	Day	19% 11% 25% 22% 18% 5%	450
	11th to 20th	Day	24% 15% 33% 20% 6% 1	450
	21st to end	Day	29% 16% 33% 17% 4	450
November	1st to 10th	Day	24% 19% 28% 18% 9% 2	270
	11th to 20th	Day	27% 17% 35% 14% 6% 2	198
	21st to end	Day	28% 18% 37% 14% 3	180
December	1st to 10th	Day	29% 19% 33% 14% 4 1	180
	11th to 20th	Day	26% 25% 30% 15% 4	180
	21st to end	Day	30% 24% 31% 13% 2	198
January	1st to 10th	Night	24% 16% 38% 16% 5% 1	950
	11th to 20th	Night	29% 15% 32% 21% 4	761
	21st to end	Night	22% 11% 36% 24% 7%	836
February	1st to 10th	Night	25% 14% 36% 22% 3	760
	11th to 20th	Night	23% 14% 32% 25% 5%	743
	21st to end	Night	25% 16% 35% 21% 3	628
March	1st to 10th	Night	19% 13% 36% 25% 7%	760
	11th to 20th	Night	18% 16% 35% 23% 7% 1	760
	21st to end	Night	25% 11% 28% 27% 8% 1	821
April	1st to 10th	Night	23% 14% 29% 28% 5%	720
	11th to 20th	Night	21% 11% 33% 23% 11% 1	551
	21st to end	Night	20% 10% 32% 30% 8% 1	540
May	1st to 10th	Night	22% 6% 30% 30% 12% 1	540
	11th to 20th	Night	17% 4 30% 35% 12% 2	530
	21st to end	Night	12% 3 29% 39% 17% 1	389
June	1st to 10th	Night	15% 4 29% 38% 14% 1	360
	11th to 20th	Night	12% 3 25% 46% 13% 1	360
	21st to end	Night	11% 5% 20% 47% 16% 1	360
July	1st to 10th	Night	4 2 20% 48% 25% 1	350
	11th to 20th	Night	10% 3 20% 40% 24% 2	507
	21st to end	Night	9% 4 16% 47% 23% 1	591
August	1st to 10th	Night	9% 2 12% 44% 31% 2	537
	11th to 20th	Night	9% 3 13% 39% 31% 4	534
	21st to end	Night	13% 3 17% 37% 27% 3	598
September	1st to 10th	Night	12% 5% 21% 34% 23% 5%	720
	11th to 20th	Night	13% 4 18% 35% 27% 4	720
	21st to end	Night	18% 7% 22% 32% 17% 5%	720
October	1st to 10th	Night	19% 7% 26% 29% 18% 2	720
	11th to 20th	Night	26% 13% 26% 26% 9% 1	720
	21st to end	Night	29% 13% 31% 24% 4	792
November	1st to 10th	Night	21% 12% 28% 30% 9% 1	740
	11th to 20th	Night	25% 14% 34% 20% 6% 1	760
	21st to end	Night	29% 12% 31% 24% 5%	760
December	1st to 10th	Night	29% 17% 31% 18% 4	760
	11th to 20th	Night	24% 19% 37% 16% 3	760
	21st to end	Night	30% 14% 36% 17% 2	940

Page generated at 2024-03-24 23:37:02. Version ac07

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