Denholm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Denholm Observatory clear sky chart from 2005-11-06 to 2024-04-25.

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% 9% 4 7% 6% 4 5% 5% 5% 6% 33%	83166
	Night	17% 11% 4 8% 6% 3 5% 4 5% 6% 31%	85937
Transparency	Day	51% 6% 11% 20% 10% 1	82871
	Night	53% 6% 11% 19% 10% 1	85766
Seeing	Day	22% 15% 30% 20% 10% 3	13711
	Night	22% 12% 29% 26% 9% 1	22993

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	15% 15% 4 10% 9% 4 6% 5% 5% 5% 22%	2517
	11th to 20th	Day	13% 13% 6% 13% 8% 5% 6% 4 5% 6% 22%	2520
	21st to end	Day	14% 18% 7% 11% 9% 5% 5% 4 4 4 18%	2906
February	1st to 10th	Day	14% 14% 5% 10% 8% 5% 5% 5% 6% 5% 24%	1710
	11th to 20th	Day	12% 14% 6% 10% 7% 3 5% 4 4 6% 29%	1663
	21st to end	Day	18% 12% 4 8% 7% 3 6% 5% 5% 5% 27%	1566
March	1st to 10th	Day	21% 13% 4 8% 7% 3 5% 4 4 4 27%	3137
	11th to 20th	Day	12% 14% 5% 8% 8% 4 6% 5% 5% 6% 28%	3480
	21st to end	Day	13% 8% 3 6% 6% 3 4 4 5% 6% 43%	4021
April	1st to 10th	Day	23% 9% 3 6% 5% 3 4 4 4 6% 32%	2280
	11th to 20th	Day	23% 8% 3 6% 5% 3 3 4 4 5% 36%	2254
	21st to end	Day	22% 10% 3 6% 6% 3 4 5% 4 5% 32%	2335
May	1st to 10th	Day	19% 7% 3 7% 6% 3 5% 4 5% 5% 35%	2520
	11th to 20th	Day	15% 7% 4 6% 6% 3 5% 4 5% 6% 39%	2478
	21st to end	Day	16% 5% 3 5% 6% 4 5% 5% 6% 6% 39%	2725
June	1st to 10th	Day	20% 7% 4 6% 5% 4 6% 5% 5% 7% 30%	2520
	11th to 20th	Day	14% 6% 2 5% 5% 3 6% 6% 6% 8% 40%	2520
	21st to end	Day	17% 7% 3 5% 6% 4 5% 6% 7% 7% 32%	2520
July	1st to 10th	Day	11% 5% 2 5% 5% 3 6% 6% 7% 9% 40%	2520
	11th to 20th	Day	11% 6% 3 5% 5% 4 5% 5% 7% 9% 40%	2520
	21st to end	Day	11% 5% 2 5% 5% 3 6% 5% 7% 10% 40%	2744
August	1st to 10th	Day	10% 5% 2 6% 5% 4 6% 6% 6% 8% 40%	2458
	11th to 20th	Day	11% 5% 2 7% 6% 4 6% 6% 6% 8% 40%	2285
	21st to end	Day	13% 6% 3 6% 5% 3 5% 5% 6% 9% 40%	2376
September	1st to 10th	Day	13% 5% 3 5% 6% 3 5% 5% 5% 8% 42%	2160
	11th to 20th	Day	12% 5% 2 5% 5% 3 4 5% 6% 9% 45%	2160
	21st to end	Day	15% 6% 2 5% 5% 3 5% 6% 6% 7% 39%	1919
October	1st to 10th	Day	19% 7% 4 6% 5% 4 4 4 6% 7% 35%	1800
	11th to 20th	Day	22% 8% 3 6% 6% 3 6% 5% 5% 7% 29%	1800
	21st to end	Day	26% 8% 4 7% 6% 3 5% 4 4 6% 27%	1845
November	1st to 10th	Day	15% 8% 4 6% 6% 4 5% 5% 6% 7% 34%	1669
	11th to 20th	Day	19% 11% 4 7% 6% 4 6% 5% 5% 6% 28%	1520
	21st to end	Day	20% 12% 4 8% 8% 4 7% 6% 5% 5% 21%	1520
December	1st to 10th	Day	20% 18% 5% 9% 8% 4 5% 5% 5% 6% 16%	1520
	11th to 20th	Day	21% 13% 5% 8% 7% 3 5% 5% 4 6% 24%	1906
	21st to end	Day	28% 15% 7% 11% 6% 2 4 2 2 3 19%	2772
January	1st to 10th	Night	17% 16% 6% 12% 8% 3 4 4 4 6% 20%	4429
	11th to 20th	Night	12% 12% 6% 11% 10% 5% 6% 5% 5% 5% 23%	4440
	21st to end	Night	17% 20% 7% 9% 7% 5% 4 4 4 4 21%	4747
February	1st to 10th	Night	14% 14% 5% 10% 9% 4 6% 5% 5% 6% 23%	2470
	11th to 20th	Night	13% 12% 5% 11% 8% 3 4 4 5% 5% 29%	2403
	21st to end	Night	16% 12% 4 8% 7% 4 5% 4 6% 6% 28%	1886
March	1st to 10th	Night	21% 12% 4 7% 6% 3 4 4 3 5% 31%	3701
	11th to 20th	Night	16% 14% 5% 10% 7% 3 5% 4 5% 5% 26%	3479
	21st to end	Night	16% 8% 3 6% 5% 3 4 4 5% 6% 40%	3610
April	1st to 10th	Night	21% 9% 3 7% 6% 3 4 4 4 6% 33%	1900
	11th to 20th	Night	20% 9% 3 6% 6% 3 4 4 4 6% 36%	1875
	21st to end	Night	20% 9% 3 7% 6% 4 4 4 4 6% 33%	1738
May	1st to 10th	Night	21% 7% 3 5% 5% 3 4 5% 6% 7% 36%	1440
	11th to 20th	Night	17% 8% 3 5% 4 3 4 5% 5% 7% 40%	1415
	21st to end	Night	13% 6% 2 6% 5% 3 3 5% 5% 8% 45%	1550
June	1st to 10th	Night	15% 6% 3 7% 6% 3 5% 6% 7% 8% 34%	1440
	11th to 20th	Night	14% 5% 2 6% 5% 3 4 4 6% 8% 43%	1440
	21st to end	Night	17% 8% 3 5% 5% 3 5% 5% 6% 8% 38%	1440
July	1st to 10th	Night	10% 5% 2 6% 5% 3 5% 5% 5% 8% 46%	1440
	11th to 20th	Night	12% 5% 3 5% 5% 3 5% 5% 6% 9% 43%	1440
	21st to end	Night	9% 6% 2 4 6% 4 6% 5% 6% 9% 44%	1574
August	1st to 10th	Night	8% 6% 4 7% 6% 4 4 5% 6% 9% 43%	1476
	11th to 20th	Night	9% 5% 3 5% 6% 3 5% 5% 6% 9% 46%	1598
	21st to end	Night	11% 5% 3 6% 5% 3 5% 4 6% 8% 45%	1980
September	1st to 10th	Night	11% 5% 3 5% 6% 3 5% 5% 5% 6% 46%	1800
	11th to 20th	Night	15% 4 2 4 4 3 4 3 5% 7% 48%	1800
	21st to end	Night	17% 5% 2 6% 5% 2 4 4 5% 6% 43%	2037
October	1st to 10th	Night	17% 7% 3 5% 6% 3 5% 4 5% 6% 37%	2160
	11th to 20th	Night	22% 8% 4 5% 6% 4 5% 4 5% 7% 30%	2160
	21st to end	Night	23% 10% 4 6% 5% 4 5% 5% 4 6% 28%	2510
November	1st to 10th	Night	19% 10% 3 6% 5% 3 3 4 4 6% 35%	2407
	11th to 20th	Night	19% 10% 4 7% 6% 3 6% 5% 5% 6% 29%	2660
	21st to end	Night	22% 12% 4 9% 6% 4 5% 4 5% 7% 23%	2660
December	1st to 10th	Night	21% 16% 5% 9% 7% 4 6% 5% 5% 6% 17%	2660
	11th to 20th	Night	21% 14% 5% 9% 6% 3 4 4 3 4 26%	3288
	21st to end	Night	31% 17% 4 10% 5% 3 4 3 3 4 18%	4884

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	62% 9% 13% 10% 5% 1	2518
	11th to 20th	Day	63% 6% 11% 13% 7% 1	2520
	21st to end	Day	69% 6% 10% 9% 5% 1	2906
February	1st to 10th	Day	60% 6% 9% 18% 4 1	1710
	11th to 20th	Day	56% 5% 11% 17% 9% 1	1672
	21st to end	Day	58% 6% 10% 14% 10% 1	1566
March	1st to 10th	Day	59% 6% 9% 17% 9% 1	3140
	11th to 20th	Day	56% 5% 12% 15% 10% 2	3480
	21st to end	Day	42% 4 9% 23% 19% 3	4021
April	1st to 10th	Day	53% 4 7% 19% 13% 3	2280
	11th to 20th	Day	51% 4 7% 21% 15% 2	2254
	21st to end	Day	54% 5% 8% 14% 14% 4	2335
May	1st to 10th	Day	50% 5% 7% 19% 15% 4	2520
	11th to 20th	Day	45% 6% 8% 22% 17% 3	2482
	21st to end	Day	45% 7% 12% 21% 13% 3	2725
June	1st to 10th	Day	52% 7% 9% 18% 12% 1	2520
	11th to 20th	Day	41% 8% 11% 24% 13% 3	2520
	21st to end	Day	49% 8% 13% 20% 11% 1	2520
July	1st to 10th	Day	37% 8% 13% 28% 14% 1	2450
	11th to 20th	Day	37% 8% 15% 27% 11%	2445
	21st to end	Day	39% 8% 15% 27% 11%	2591
August	1st to 10th	Day	39% 9% 16% 25% 10%	2444
	11th to 20th	Day	40% 9% 15% 26% 9% 1	2285
	21st to end	Day	39% 8% 14% 27% 12% 1	2376
September	1st to 10th	Day	40% 6% 14% 28% 12%	2160
	11th to 20th	Day	36% 6% 13% 32% 12% 1	2160
	21st to end	Day	44% 7% 14% 27% 9%	1919
October	1st to 10th	Day	48% 7% 12% 22% 10% 1	1800
	11th to 20th	Day	55% 7% 15% 16% 7% 1	1800
	21st to end	Day	59% 5% 12% 15% 8% 1	1845
November	1st to 10th	Day	48% 6% 16% 22% 7% 1	1669
	11th to 20th	Day	57% 6% 13% 14% 9% 1	1520
	21st to end	Day	64% 7% 12% 10% 7% 1	1520
December	1st to 10th	Day	68% 6% 11% 10% 3 1	1520
	11th to 20th	Day	62% 6% 13% 14% 5%	1906
	21st to end	Day	72% 3 7% 15% 3	2772
January	1st to 10th	Night	65% 7% 14% 11% 3	4429
	11th to 20th	Night	62% 7% 14% 12% 5%	4440
	21st to end	Night	66% 6% 10% 11% 6% 1	4747
February	1st to 10th	Night	61% 6% 10% 15% 7% 1	2470
	11th to 20th	Night	57% 5% 10% 19% 8% 1	2405
	21st to end	Night	56% 5% 9% 19% 10% 1	1886
March	1st to 10th	Night	56% 6% 9% 19% 9% 2	3711
	11th to 20th	Night	58% 5% 13% 14% 8% 1	3480
	21st to end	Night	44% 4 9% 21% 18% 4	3610
April	1st to 10th	Night	53% 5% 8% 18% 14% 2	1900
	11th to 20th	Night	50% 5% 7% 21% 16% 1	1875
	21st to end	Night	51% 6% 8% 16% 16% 3	1738
May	1st to 10th	Night	47% 5% 7% 20% 15% 5%	1440
	11th to 20th	Night	43% 5% 8% 23% 18% 3	1416
	21st to end	Night	38% 6% 15% 20% 18% 3	1551
June	1st to 10th	Night	46% 8% 13% 22% 10% 1	1440
	11th to 20th	Night	40% 6% 10% 25% 17% 2	1440
	21st to end	Night	44% 8% 13% 20% 14% 1	1440
July	1st to 10th	Night	36% 6% 12% 27% 18% 2	1400
	11th to 20th	Night	37% 7% 12% 30% 13%	1392
	21st to end	Night	36% 8% 15% 26% 14%	1486
August	1st to 10th	Night	38% 9% 16% 24% 13% 1	1466
	11th to 20th	Night	35% 7% 17% 29% 12%	1598
	21st to end	Night	36% 8% 14% 29% 12%	1980
September	1st to 10th	Night	37% 7% 11% 29% 14% 1	1800
	11th to 20th	Night	35% 7% 13% 35% 9% 2	1800
	21st to end	Night	41% 6% 11% 31% 10%	2037
October	1st to 10th	Night	47% 7% 10% 22% 13% 1	2160
	11th to 20th	Night	53% 6% 11% 21% 8%	2160
	21st to end	Night	57% 6% 12% 16% 8%	2510
November	1st to 10th	Night	50% 5% 14% 21% 9% 1	2407
	11th to 20th	Night	54% 7% 11% 18% 9% 1	2660
	21st to end	Night	60% 7% 13% 15% 5%	2660
December	1st to 10th	Night	67% 7% 10% 12% 4	2660
	11th to 20th	Night	61% 6% 9% 16% 7% 1	3288
	21st to end	Night	72% 4 7% 12% 4	4884

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	22% 21% 34% 18% 4 1	180
	11th to 20th	Day	27% 21% 32% 17% 3	180
	21st to end	Day	20% 17% 37% 21% 4	238
February	1st to 10th	Day	28% 24% 34% 12% 2	270
	11th to 20th	Day	26% 30% 35% 9% 1	270
	21st to end	Day	31% 23% 29% 15% 2	378
March	1st to 10th	Day	27% 18% 33% 18% 4	460
	11th to 20th	Day	26% 20% 33% 17% 3 1	460
	21st to end	Day	31% 19% 28% 14% 7% 1	506
April	1st to 10th	Day	31% 19% 29% 14% 6% 1	460
	11th to 20th	Day	34% 16% 23% 15% 9% 2	452
	21st to end	Day	29% 20% 28% 14% 7% 2	437
May	1st to 10th	Day	25% 14% 32% 20% 7% 3	423
	11th to 20th	Day	19% 18% 32% 20% 8% 3	447
	21st to end	Day	19% 14% 32% 22% 12% 1	485
June	1st to 10th	Day	24% 12% 31% 21% 9% 2	450
	11th to 20th	Day	16% 14% 30% 25% 12% 3	450
	21st to end	Day	18% 14% 30% 24% 11% 4	450
July	1st to 10th	Day	11% 7% 27% 35% 16% 3	445
	11th to 20th	Day	13% 8% 30% 30% 15% 5%	444
	21st to end	Day	14% 8% 30% 28% 15% 4	488
August	1st to 10th	Day	13% 8% 28% 28% 19% 4	446
	11th to 20th	Day	12% 5% 26% 29% 19% 9%	439
	21st to end	Day	15% 11% 27% 23% 14% 9%	495
September	1st to 10th	Day	15% 12% 29% 19% 17% 9%	450
	11th to 20th	Day	12% 9% 28% 24% 19% 9%	450
	21st to end	Day	17% 12% 29% 21% 16% 5%	450
October	1st to 10th	Day	24% 13% 26% 19% 13% 5%	450
	11th to 20th	Day	26% 20% 27% 19% 6% 1	450
	21st to end	Day	29% 19% 26% 20% 6%	427
November	1st to 10th	Day	24% 19% 30% 18% 8% 1	363
	11th to 20th	Day	28% 19% 33% 12% 6% 1	180
	21st to end	Day	28% 19% 35% 12% 4 1	180
December	1st to 10th	Day	32% 13% 38% 15% 2 1	180
	11th to 20th	Day	27% 27% 32% 10% 3	180
	21st to end	Day	32% 20% 32% 12% 3 1	198
January	1st to 10th	Night	27% 15% 34% 20% 4	760
	11th to 20th	Night	27% 18% 28% 21% 6% 1	760
	21st to end	Night	19% 12% 37% 26% 6%	836
February	1st to 10th	Night	22% 17% 35% 22% 4	760
	11th to 20th	Night	22% 17% 39% 19% 4	743
	21st to end	Night	25% 18% 29% 24% 4	628
March	1st to 10th	Night	24% 14% 33% 25% 3	760
	11th to 20th	Night	23% 19% 32% 21% 6%	760
	21st to end	Night	24% 16% 27% 25% 7%	836
April	1st to 10th	Night	27% 13% 31% 24% 5%	760
	11th to 20th	Night	24% 19% 27% 22% 8%	751
	21st to end	Night	25% 12% 29% 26% 7% 1	628
May	1st to 10th	Night	24% 8% 28% 30% 10% 1	360
	11th to 20th	Night	21% 12% 28% 30% 9%	354
	21st to end	Night	14% 6% 38% 28% 13%	389
June	1st to 10th	Night	18% 6% 32% 35% 10%	360
	11th to 20th	Night	17% 8% 27% 33% 14% 1	360
	21st to end	Night	19% 6% 29% 31% 14% 1	360
July	1st to 10th	Night	9% 5% 28% 43% 15% 1	350
	11th to 20th	Night	13% 5% 24% 43% 15% 1	348
	21st to end	Night	10% 5% 24% 45% 15% 2	394
August	1st to 10th	Night	10% 4 22% 42% 22%	402
	11th to 20th	Night	11% 2 19% 39% 26% 2	534
	21st to end	Night	12% 7% 25% 35% 19% 2	792
September	1st to 10th	Night	13% 5% 30% 31% 18% 2	720
	11th to 20th	Night	16% 7% 21% 32% 20% 4	720
	21st to end	Night	18% 9% 25% 26% 20% 3	720
October	1st to 10th	Night	20% 13% 25% 26% 15% 2	720
	11th to 20th	Night	25% 12% 30% 24% 8% 1	720
	21st to end	Night	27% 15% 26% 26% 6%	792
November	1st to 10th	Night	27% 12% 27% 24% 9%	740
	11th to 20th	Night	23% 17% 32% 18% 9% 1	760
	21st to end	Night	30% 15% 26% 23% 6%	760
December	1st to 10th	Night	30% 15% 29% 21% 4	760
	11th to 20th	Night	28% 16% 29% 21% 5% 1	760
	21st to end	Night	33% 15% 29% 19% 4	836

Page generated at 2024-04-22 09:25:42. Version ac07

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