BRAC Ivy Hill Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BRAC Ivy Hill Site clear sky chart from 2005-11-06 to 2025-12-03.

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	14% 5% 2 6% 4 4 5% 4 6% 8% 43%	126309
	Night	15% 6% 2 5% 4 4 4 3 5% 7% 45%	124362
Transparency	Day	38% 6% 13% 22% 17% 4	129914
	Night	38% 5% 11% 22% 19% 5%	128826
Seeing	Day	18% 11% 24% 25% 16% 6%	16515
	Night	17% 8% 25% 32% 16% 2	26628

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	15% 5% 1 4 4 4 4 3 5% 5% 50%	4056
	11th to 20th	Day	14% 5% 1 5% 3 4 4 2 4 5% 54%	4060
	21st to end	Day	17% 6% 2 5% 3 4 4 3 4 5% 47%	4466
February	1st to 10th	Day	13% 7% 3 5% 5% 5% 5% 4 7% 7% 41%	1800
	11th to 20th	Day	20% 5% 1 5% 4 4 4 3 5% 6% 41%	1753
	21st to end	Day	19% 7% 3 6% 4 4 4 4 5% 6% 40%	1755
March	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 39%	4935
	11th to 20th	Day	11% 5% 2 5% 3 3 4 3 5% 7% 52%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 4 3 5% 6% 41%	5465
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 7% 49%	2313
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 5% 8% 47%	2374
	21st to end	Day	15% 6% 1 6% 5% 3 4 3 5% 7% 43%	2475
May	1st to 10th	Day	13% 5% 2 6% 5% 6% 7% 5% 8% 10% 34%	5862
	11th to 20th	Day	21% 7% 2 7% 5% 4 5% 4 6% 8% 32%	5861
	21st to end	Day	15% 4 2 5% 4 4 4 4 6% 9% 43%	6433
June	1st to 10th	Day	9% 4 1 5% 5% 5% 5% 5% 6% 10% 45%	2685
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 10% 41%	2789
	21st to end	Day	8% 4 2 5% 5% 5% 5% 5% 7% 11% 44%	2600
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 12% 40%	5900
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 6% 9% 13% 32%	5900
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 6% 8% 11% 34%	6424
August	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 11% 32%	4234
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 5% 7% 10% 40%	4146
	21st to end	Day	8% 4 2 6% 3 4 4 4 6% 10% 49%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 5% 7% 50%	2400
	11th to 20th	Day	14% 6% 3 5% 3 3 5% 4 5% 7% 45%	2235
	21st to end	Day	24% 6% 3 5% 4 4 3 3 5% 5% 36%	2175
October	1st to 10th	Day	16% 5% 2 5% 3 3 3 3 4 5% 53%	2000
	11th to 20th	Day	16% 4 1 4 3 2 3 2 4 6% 54%	2000
	21st to end	Day	14% 5% 2 5% 3 3 4 3 4 5% 53%	2200
November	1st to 10th	Day	12% 4 1 4 3 4 3 3 4 6% 56%	2057
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 5% 6% 51%	1981
	21st to end	Day	14% 5% 1 5% 3 3 4 3 5% 8% 49%	1680
December	1st to 10th	Day	14% 6% 2 5% 3 3 4 4 5% 6% 48%	2616
	11th to 20th	Day	22% 5% 2 3 3 2 3 3 5% 7% 46%	2990
	21st to end	Day	22% 6% 2 5% 3 4 4 3 5% 7% 37%	4397
January	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 5% 56%	5889
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 5% 56%	5900
	21st to end	Night	19% 6% 2 4 3 2 3 2 4 5% 50%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 6% 7% 44%	2600
	11th to 20th	Night	21% 7% 2 5% 3 4 4 3 4 5% 42%	2533
	21st to end	Night	16% 6% 2 5% 5% 3 3 4 5% 6% 44%	1869
March	1st to 10th	Night	17% 6% 2 6% 4 3 4 3 5% 6% 44%	4918
	11th to 20th	Night	10% 3 1 4 3 3 4 4 6% 8% 54%	4980
	21st to end	Night	14% 6% 2 6% 4 3 4 4 5% 7% 44%	5467
April	1st to 10th	Night	15% 7% 1 5% 4 4 4 3 5% 7% 46%	2085
	11th to 20th	Night	12% 5% 1 5% 4 4 4 3 6% 6% 50%	1975
	21st to end	Night	14% 5% 2 6% 3 4 5% 3 6% 8% 44%	1941
May	1st to 10th	Night	16% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	4033
	11th to 20th	Night	22% 8% 3 7% 4 4 4 3 5% 7% 32%	4032
	21st to end	Night	16% 5% 2 5% 4 4 4 3 6% 9% 41%	4421
June	1st to 10th	Night	9% 4 1 6% 4 4 4 5% 7% 10% 44%	1700
	11th to 20th	Night	10% 5% 2 6% 6% 4 5% 3 7% 9% 43%	1600
	21st to end	Night	8% 4 2 5% 5% 5% 5% 4 7% 11% 45%	1795
July	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	4058
	11th to 20th	Night	11% 8% 3 8% 5% 5% 6% 5% 8% 11% 29%	4057
	21st to end	Night	10% 7% 3 8% 7% 6% 6% 5% 7% 9% 32%	4427
August	1st to 10th	Night	16% 7% 2 8% 5% 5% 6% 4 6% 10% 31%	2915
	11th to 20th	Night	9% 5% 2 7% 5% 6% 6% 5% 8% 8% 39%	2875
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 6% 9% 44%	3579
September	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 7% 47%	2000
	11th to 20th	Night	16% 6% 2 5% 4 4 4 4 5% 6% 44%	2160
	21st to end	Night	25% 7% 3 6% 4 4 4 3 4 5% 36%	2225
October	1st to 10th	Night	16% 6% 1 5% 3 3 3 3 3 6% 51%	2400
	11th to 20th	Night	15% 5% 1 4 3 4 4 2 4 5% 52%	2400
	21st to end	Night	14% 4 1 4 3 3 3 3 4 6% 55%	2640
November	1st to 10th	Night	13% 6% 3 5% 3 3 3 2 4 4 53%	2461
	11th to 20th	Night	13% 5% 2 4 4 4 3 3 4 6% 51%	2631
	21st to end	Night	13% 4 2 5% 4 4 3 3 5% 7% 50%	2938
December	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 4 6% 53%	4609
	11th to 20th	Night	21% 7% 2 5% 4 4 3 3 4 6% 41%	5205
	21st to end	Night	18% 5% 2 5% 4 4 4 4 6% 7% 42%	6554

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	34% 4 6% 23% 26% 7%	4057
	11th to 20th	Day	34% 4 6% 19% 27% 11%	4060
	21st to end	Day	41% 5% 6% 25% 20% 4	4466
February	1st to 10th	Day	40% 7% 11% 23% 16% 3	1800
	11th to 20th	Day	42% 5% 9% 21% 17% 5%	1762
	21st to end	Day	45% 6% 11% 16% 17% 6%	1755
March	1st to 10th	Day	44% 4 8% 19% 15% 9%	4935
	11th to 20th	Day	32% 5% 8% 22% 22% 10%	4980
	21st to end	Day	44% 5% 9% 18% 18% 6%	5465
April	1st to 10th	Day	35% 5% 11% 23% 18% 8%	2313
	11th to 20th	Day	35% 6% 12% 23% 19% 5%	2374
	21st to end	Day	41% 7% 11% 23% 14% 4	2475
May	1st to 10th	Day	42% 7% 15% 22% 11% 2	5897
	11th to 20th	Day	51% 7% 10% 20% 12% 1	5865
	21st to end	Day	37% 8% 15% 20% 18% 3	6447
June	1st to 10th	Day	33% 10% 17% 25% 15% 1	2685
	11th to 20th	Day	35% 11% 16% 21% 16% 1	2789
	21st to end	Day	33% 10% 16% 24% 15% 2	2600
July	1st to 10th	Day	36% 8% 21% 19% 12% 4	5835
	11th to 20th	Day	41% 9% 23% 19% 7%	5831
	21st to end	Day	42% 7% 17% 22% 11%	6240
August	1st to 10th	Day	45% 7% 17% 24% 6%	4221
	11th to 20th	Day	38% 7% 16% 28% 10% 1	4185
	21st to end	Day	31% 6% 19% 27% 15% 1	4312
September	1st to 10th	Day	33% 9% 13% 19% 21% 4	2400
	11th to 20th	Day	39% 6% 11% 24% 18% 3	2235
	21st to end	Day	49% 7% 10% 16% 17% 2	2175
October	1st to 10th	Day	34% 3 8% 20% 30% 6%	3260
	11th to 20th	Day	25% 3 7% 23% 28% 14%	3240
	21st to end	Day	24% 3 12% 26% 24% 12%	3538
November	1st to 10th	Day	31% 4 10% 24% 26% 5%	2057
	11th to 20th	Day	34% 4 9% 24% 21% 8%	1981
	21st to end	Day	34% 5% 10% 24% 24% 3	1680
December	1st to 10th	Day	36% 5% 8% 23% 20% 7%	2616
	11th to 20th	Day	40% 3 13% 21% 19% 5%	2986
	21st to end	Day	45% 4 12% 23% 14% 1	4397
January	1st to 10th	Night	32% 3 6% 22% 26% 10%	5889
	11th to 20th	Night	32% 3 7% 19% 30% 8%	5900
	21st to end	Night	37% 4 7% 27% 19% 6%	6490
February	1st to 10th	Night	38% 6% 13% 23% 18% 2	2600
	11th to 20th	Night	44% 4 10% 19% 18% 6%	2535
	21st to end	Night	40% 6% 9% 22% 16% 7%	1869
March	1st to 10th	Night	40% 5% 6% 20% 19% 9%	4918
	11th to 20th	Night	27% 5% 9% 20% 30% 10%	4980
	21st to end	Night	40% 4 9% 22% 20% 6%	5467
April	1st to 10th	Night	39% 5% 12% 18% 21% 6%	2085
	11th to 20th	Night	34% 7% 11% 22% 21% 5%	1975
	21st to end	Night	38% 7% 12% 23% 17% 3	1941
May	1st to 10th	Night	45% 7% 16% 18% 11% 3	4060
	11th to 20th	Night	52% 6% 10% 19% 12% 1	4033
	21st to end	Night	39% 7% 16% 25% 11% 3	4429
June	1st to 10th	Night	33% 9% 16% 26% 15% 1	1700
	11th to 20th	Night	37% 7% 17% 20% 17% 2	1600
	21st to end	Night	32% 10% 17% 21% 17% 4	1795
July	1st to 10th	Night	42% 8% 18% 17% 13% 2	4015
	11th to 20th	Night	48% 8% 21% 16% 6% 1	4006
	21st to end	Night	48% 7% 15% 21% 11%	4297
August	1st to 10th	Night	49% 7% 17% 20% 7%	2904
	11th to 20th	Night	40% 7% 16% 26% 10% 1	2902
	21st to end	Night	35% 7% 14% 32% 10% 2	3579
September	1st to 10th	Night	36% 8% 14% 19% 18% 6%	2000
	11th to 20th	Night	40% 7% 10% 22% 19% 2	2160
	21st to end	Night	51% 5% 8% 17% 15% 3	2225
October	1st to 10th	Night	39% 3 9% 19% 24% 6%	3920
	11th to 20th	Night	27% 3 7% 21% 30% 12%	3899
	21st to end	Night	21% 3 8% 28% 28% 12%	4252
November	1st to 10th	Night	35% 4 9% 21% 25% 7%	2461
	11th to 20th	Night	34% 5% 8% 21% 24% 7%	2631
	21st to end	Night	34% 5% 9% 25% 22% 6%	2938
December	1st to 10th	Night	33% 3 7% 28% 24% 3	4609
	11th to 20th	Night	46% 3 10% 19% 15% 7%	5208
	21st to end	Night	40% 4 12% 25% 17% 2	6554

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	27% 28% 25% 14% 4 1	300
	11th to 20th	Day	30% 23% 33% 13% 1	300
	21st to end	Day	28% 23% 35% 11% 4	330
February	1st to 10th	Day	20% 22% 38% 14% 5%	300
	11th to 20th	Day	42% 20% 30% 6% 3	300
	21st to end	Day	28% 20% 31% 17% 4	369
March	1st to 10th	Day	25% 17% 32% 21% 5%	500
	11th to 20th	Day	21% 21% 33% 19% 6%	500
	21st to end	Day	23% 16% 35% 21% 5% 1	550
April	1st to 10th	Day	22% 23% 26% 22% 6% 1	500
	11th to 20th	Day	20% 15% 26% 24% 11% 3	492
	21st to end	Day	21% 11% 31% 22% 13% 2	500
May	1st to 10th	Day	20% 13% 33% 22% 10% 3	503
	11th to 20th	Day	20% 11% 24% 26% 15% 4	527
	21st to end	Day	19% 5% 22% 28% 19% 7%	573
June	1st to 10th	Day	10% 4 30% 31% 18% 7%	530
	11th to 20th	Day	11% 5% 18% 33% 20% 11%	530
	21st to end	Day	9% 3 23% 31% 24% 10%	530
July	1st to 10th	Day	10% 3 15% 28% 30% 13%	525
	11th to 20th	Day	6% 2 12% 35% 30% 15%	524
	21st to end	Day	10% 2 16% 31% 28% 13%	576
August	1st to 10th	Day	16% 3 18% 26% 25% 12%	526
	11th to 20th	Day	11% 3 15% 29% 28% 14%	519
	21st to end	Day	10% 3 10% 29% 27% 20%	583
September	1st to 10th	Day	12% 5% 14% 25% 29% 16%	530
	11th to 20th	Day	16% 5% 12% 27% 27% 13%	530
	21st to end	Day	29% 8% 17% 26% 17% 4	530
October	1st to 10th	Day	20% 5% 17% 31% 19% 8%	530
	11th to 20th	Day	15% 12% 24% 26% 20% 4	530
	21st to end	Day	16% 12% 27% 30% 11% 3	583
November	1st to 10th	Day	18% 9% 29% 30% 12% 2	535
	11th to 20th	Day	19% 16% 29% 24% 11% 1	421
	21st to end	Day	19% 21% 29% 20% 9% 2	220
December	1st to 10th	Day	24% 16% 36% 18% 7%	204
	11th to 20th	Day	25% 14% 27% 26% 7% 2	200
	21st to end	Day	27% 13% 32% 20% 7% 1	315
January	1st to 10th	Night	20% 16% 36% 25% 3	800
	11th to 20th	Night	18% 17% 33% 30% 2	800
	21st to end	Night	19% 12% 41% 25% 3	880
February	1st to 10th	Night	18% 13% 40% 27% 2	800
	11th to 20th	Night	27% 16% 37% 20% 1	783
	21st to end	Night	18% 15% 38% 26% 2	660
March	1st to 10th	Night	16% 15% 35% 29% 5%	800
	11th to 20th	Night	22% 13% 32% 26% 6% 1	800
	21st to end	Night	18% 12% 36% 29% 5%	880
April	1st to 10th	Night	18% 16% 34% 27% 6%	800
	11th to 20th	Night	15% 12% 31% 31% 11% 1	791
	21st to end	Night	17% 6% 31% 37% 9%	741
May	1st to 10th	Night	15% 9% 29% 33% 14% 1	600
	11th to 20th	Night	20% 6% 21% 36% 16% 2	591
	21st to end	Night	18% 2 12% 34% 30% 4	650
June	1st to 10th	Night	10% 2 15% 44% 27% 2	500
	11th to 20th	Night	12% 3 15% 39% 28% 4	400
	21st to end	Night	10% 2 10% 43% 31% 4	595
July	1st to 10th	Night	13% 1 10% 34% 37% 5%	585
	11th to 20th	Night	7% 1 6% 33% 47% 6%	582
	21st to end	Night	9% 1 8% 42% 35% 5%	657
August	1st to 10th	Night	14% 3 8% 38% 32% 5%	597
	11th to 20th	Night	14% 2 9% 36% 36% 4	594
	21st to end	Night	11% 2 10% 35% 37% 5%	873
September	1st to 10th	Night	14% 2 10% 34% 35% 6%	800
	11th to 20th	Night	16% 2 12% 33% 32% 5%	800
	21st to end	Night	27% 4 15% 33% 20% 1	800
October	1st to 10th	Night	19% 5% 16% 29% 27% 4	800
	11th to 20th	Night	16% 8% 24% 35% 17% 2	800
	21st to end	Night	16% 7% 25% 37% 13% 2	880
November	1st to 10th	Night	16% 7% 27% 35% 13% 2	820
	11th to 20th	Night	17% 10% 29% 33% 11%	840
	21st to end	Night	15% 10% 30% 34% 10% 1	840
December	1st to 10th	Night	21% 11% 34% 28% 5%	809
	11th to 20th	Night	18% 9% 36% 32% 5%	800
	21st to end	Night	22% 13% 29% 32% 4	880

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