Hallam Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hallam Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	13% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	137031
	Night	14% 10% 4 8% 5% 5% 4 4 5% 7% 34%	135051
Transparency	Day	47% 4 12% 23% 13% 1	140628
	Night	49% 3 11% 22% 13% 1	139526
Seeing	Day	17% 9% 29% 25% 14% 5%	17327
	Night	19% 9% 28% 29% 13% 2	26949

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	13% 14% 7% 13% 6% 6% 5% 4 5% 6% 21%	4691
	11th to 20th	Day	12% 17% 6% 15% 9% 6% 5% 3 5% 6% 16%	5027
	21st to end	Day	18% 15% 6% 12% 6% 4 5% 4 5% 7% 18%	5780
February	1st to 10th	Day	15% 16% 3 11% 6% 5% 5% 3 5% 6% 25%	1890
	11th to 20th	Day	14% 13% 5% 11% 7% 5% 5% 4 5% 6% 25%	1947
	21st to end	Day	15% 13% 4 12% 5% 4 5% 5% 6% 6% 26%	1730
March	1st to 10th	Day	15% 12% 5% 10% 5% 4 5% 3 5% 5% 31%	4786
	11th to 20th	Day	12% 11% 3 9% 6% 5% 4 3 5% 7% 35%	5090
	21st to end	Day	17% 13% 4 9% 5% 5% 4 3 4 6% 29%	5586
April	1st to 10th	Day	15% 12% 3 8% 5% 3 4 4 6% 7% 32%	2374
	11th to 20th	Day	16% 9% 3 7% 4 5% 4 4 6% 7% 34%	2702
	21st to end	Day	17% 9% 3 7% 5% 5% 4 3 4 7% 36%	2730
May	1st to 10th	Day	18% 9% 3 7% 5% 4 4 4 5% 7% 34%	7616
	11th to 20th	Day	14% 7% 3 7% 5% 4 5% 4 6% 9% 36%	7626
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 8% 43%	8391
June	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 8% 9% 41%	2987
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	3076
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 8% 10% 41%	2942
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 10% 43%	6360
	11th to 20th	Day	7% 4 2 5% 4 4 5% 5% 7% 10% 46%	6222
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	6424
August	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 6% 9% 44%	4234
	11th to 20th	Day	9% 5% 3 7% 5% 6% 5% 5% 8% 11% 38%	4174
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 7% 9% 43%	4675
September	1st to 10th	Day	9% 6% 3 6% 5% 4 5% 4 6% 8% 44%	2248
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 7% 11% 47%	2200
	21st to end	Day	17% 6% 3 7% 5% 4 4 4 5% 7% 39%	2200
October	1st to 10th	Day	11% 5% 2 7% 4 5% 5% 5% 6% 7% 43%	2109
	11th to 20th	Day	17% 7% 2 7% 5% 4 5% 4 6% 8% 37%	2000
	21st to end	Day	19% 9% 3 7% 5% 4 5% 3 6% 7% 31%	2100
November	1st to 10th	Day	12% 9% 3 7% 5% 4 6% 5% 5% 7% 37%	1852
	11th to 20th	Day	15% 10% 3 8% 5% 5% 4 4 6% 7% 33%	1890
	21st to end	Day	17% 14% 4 9% 6% 5% 5% 4 6% 6% 24%	1700
December	1st to 10th	Day	15% 12% 5% 12% 6% 6% 5% 4 6% 6% 22%	2680
	11th to 20th	Day	20% 19% 5% 8% 4 5% 5% 4 6% 6% 19%	3070
	21st to end	Day	25% 18% 6% 13% 5% 4 4 3 4 4 13%	3922
January	1st to 10th	Night	14% 14% 5% 13% 6% 4 5% 4 5% 6% 24%	8232
	11th to 20th	Night	9% 15% 7% 13% 7% 6% 6% 4 6% 6% 21%	7940
	21st to end	Night	16% 14% 6% 9% 5% 4 4 3 5% 5% 27%	8401
February	1st to 10th	Night	16% 15% 4 10% 7% 5% 4 4 5% 4 25%	2730
	11th to 20th	Night	15% 13% 4 10% 6% 5% 5% 4 5% 7% 25%	2558
	21st to end	Night	17% 14% 4 9% 4 4 5% 4 4 5% 29%	2076
March	1st to 10th	Night	16% 11% 3 8% 5% 4 4 3 5% 6% 34%	5272
	11th to 20th	Night	9% 8% 3 6% 5% 5% 5% 4 6% 8% 41%	5090
	21st to end	Night	15% 9% 3 8% 5% 4 4 4 5% 6% 37%	5588
April	1st to 10th	Night	17% 10% 3 7% 5% 4 3 4 5% 7% 34%	2263
	11th to 20th	Night	16% 8% 3 7% 4 4 4 4 6% 8% 35%	1867
	21st to end	Night	16% 8% 2 6% 4 3 4 4 5% 7% 40%	1890
May	1st to 10th	Night	20% 8% 3 8% 6% 5% 5% 4 5% 6% 30%	5237
	11th to 20th	Night	14% 8% 3 8% 4 5% 4 4 5% 7% 38%	5234
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	5761
June	1st to 10th	Night	11% 5% 2 6% 4 4 5% 3 6% 8% 46%	1622
	11th to 20th	Night	10% 5% 1 7% 5% 4 5% 5% 6% 9% 44%	1435
	21st to end	Night	9% 6% 2 5% 4 5% 4 4 5% 8% 48%	1451
July	1st to 10th	Night	7% 5% 1 5% 4 4 4 4 7% 11% 47%	3618
	11th to 20th	Night	9% 4 2 5% 4 5% 5% 4 6% 9% 47%	3744
	21st to end	Night	7% 4 2 6% 4 4 4 4 6% 9% 50%	4427
August	1st to 10th	Night	12% 6% 2 6% 3 4 4 3 5% 8% 47%	2915
	11th to 20th	Night	9% 6% 3 7% 4 4 5% 4 4 7% 48%	2875
	21st to end	Night	8% 4 2 5% 5% 5% 5% 4 7% 7% 48%	3223
September	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 7% 49%	2126
	11th to 20th	Night	9% 5% 2 5% 4 3 3 2 5% 8% 55%	2200
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 5% 7% 42%	2200
October	1st to 10th	Night	13% 5% 1 7% 4 4 3 3 4 7% 48%	2290
	11th to 20th	Night	17% 8% 2 8% 5% 4 4 4 4 7% 37%	2400
	21st to end	Night	19% 9% 3 7% 5% 4 5% 4 5% 6% 34%	2737
November	1st to 10th	Night	13% 9% 2 6% 5% 4 4 3 5% 6% 43%	2666
	11th to 20th	Night	16% 10% 3 7% 5% 4 5% 3 6% 8% 32%	2730
	21st to end	Night	19% 13% 4 9% 5% 4 4 4 5% 6% 26%	2917
December	1st to 10th	Night	15% 13% 5% 10% 6% 5% 5% 4 7% 7% 24%	4720
	11th to 20th	Night	16% 17% 6% 11% 5% 5% 4 4 5% 5% 23%	5345
	21st to end	Night	25% 19% 6% 11% 5% 4 4 3 4 5% 14%	7271

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	64% 3 12% 15% 5% 1	4692
	11th to 20th	Day	70% 3 7% 10% 10%	5027
	21st to end	Day	66% 3 11% 14% 7%	5780
February	1st to 10th	Day	61% 3 9% 18% 9%	1890
	11th to 20th	Day	61% 3 9% 18% 8% 1	1956
	21st to end	Day	57% 4 12% 18% 7% 2	1730
March	1st to 10th	Day	56% 3 13% 20% 8% 1	4786
	11th to 20th	Day	50% 2 15% 23% 9% 1	5090
	21st to end	Day	56% 3 11% 19% 10% 1	5586
April	1st to 10th	Day	51% 3 12% 20% 13% 1	2374
	11th to 20th	Day	49% 3 11% 22% 13% 2	2702
	21st to end	Day	49% 3 12% 22% 13% 2	2730
May	1st to 10th	Day	49% 4 10% 23% 11% 2	7651
	11th to 20th	Day	45% 4 14% 24% 11% 2	7630
	21st to end	Day	40% 3 10% 21% 21% 5%	8405
June	1st to 10th	Day	38% 3 12% 29% 16% 2	2987
	11th to 20th	Day	39% 3 15% 23% 18% 2	3076
	21st to end	Day	36% 5% 14% 26% 17% 2	2942
July	1st to 10th	Day	35% 5% 12% 28% 18% 2	6291
	11th to 20th	Day	31% 5% 15% 31% 17% 1	6147
	21st to end	Day	34% 5% 15% 22% 22% 2	6240
August	1st to 10th	Day	36% 5% 15% 29% 14% 1	4221
	11th to 20th	Day	37% 6% 13% 34% 8% 1	4213
	21st to end	Day	35% 5% 16% 30% 13% 1	4674
September	1st to 10th	Day	37% 4 15% 27% 15% 1	2248
	11th to 20th	Day	32% 4 13% 34% 17% 1	2200
	21st to end	Day	44% 3 12% 27% 14% 1	2200
October	1st to 10th	Day	29% 5% 13% 32% 20% 1	3434
	11th to 20th	Day	37% 3 14% 27% 18%	3240
	21st to end	Day	43% 4 15% 23% 14% 1	3376
November	1st to 10th	Day	45% 4 11% 25% 12% 2	1852
	11th to 20th	Day	50% 3 15% 21% 10% 1	1890
	21st to end	Day	60% 3 12% 19% 6% 1	1700
December	1st to 10th	Day	60% 5% 16% 14% 4	2680
	11th to 20th	Day	67% 2 12% 14% 5%	3066
	21st to end	Day	76% 3 8% 9% 4	3922
January	1st to 10th	Night	61% 3 11% 17% 7% 1	8233
	11th to 20th	Night	63% 2 10% 16% 8% 1	7940
	21st to end	Night	60% 2 10% 18% 9%	8401
February	1st to 10th	Night	62% 3 10% 18% 6% 1	2730
	11th to 20th	Night	59% 3 9% 19% 9% 1	2560
	21st to end	Night	58% 2 11% 19% 9% 1	2076
March	1st to 10th	Night	52% 2 13% 21% 10% 1	5272
	11th to 20th	Night	42% 3 15% 25% 15% 1	5090
	21st to end	Night	49% 2 10% 20% 17% 1	5588
April	1st to 10th	Night	51% 2 10% 21% 13% 2	2263
	11th to 20th	Night	46% 3 11% 21% 17% 2	1867
	21st to end	Night	45% 3 10% 23% 16% 4	1890
May	1st to 10th	Night	54% 4 9% 15% 15% 4	5264
	11th to 20th	Night	47% 2 9% 26% 14% 2	5235
	21st to end	Night	41% 3 11% 22% 19% 4	5771
June	1st to 10th	Night	37% 2 10% 28% 20% 3	1622
	11th to 20th	Night	37% 4 14% 26% 16% 2	1435
	21st to end	Night	35% 3 13% 27% 22% 1	1451
July	1st to 10th	Night	30% 5% 15% 28% 18% 5%	3580
	11th to 20th	Night	33% 4 14% 29% 18% 2	3699
	21st to end	Night	32% 4 13% 22% 27% 2	4297
August	1st to 10th	Night	36% 4 13% 27% 17% 2	2904
	11th to 20th	Night	36% 5% 10% 34% 14% 2	2902
	21st to end	Night	33% 5% 13% 32% 16% 1	3223
September	1st to 10th	Night	34% 5% 12% 29% 18% 3	2126
	11th to 20th	Night	29% 3 10% 35% 20% 2	2200
	21st to end	Night	42% 3 12% 26% 16% 1	2200
October	1st to 10th	Night	26% 4 8% 33% 28% 1	3745
	11th to 20th	Night	43% 3 10% 24% 19% 1	3899
	21st to end	Night	41% 2 11% 29% 15% 1	4411
November	1st to 10th	Night	42% 3 11% 27% 16% 1	2666
	11th to 20th	Night	50% 2 12% 23% 12% 1	2730
	21st to end	Night	59% 2 13% 19% 6%	2917
December	1st to 10th	Night	58% 4 18% 16% 4	4720
	11th to 20th	Night	64% 2 12% 14% 7% 1	5348
	21st to end	Night	74% 3 10% 10% 4	7271

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% 16% 36% 18% 6% 1	330
	11th to 20th	Day	23% 18% 41% 13% 4 1	330
	21st to end	Day	26% 13% 33% 22% 6%	363
February	1st to 10th	Day	23% 16% 34% 21% 5% 1	330
	11th to 20th	Day	22% 16% 40% 18% 4	330
	21st to end	Day	22% 19% 42% 16% 1	273
March	1st to 10th	Day	25% 14% 39% 19% 3	419
	11th to 20th	Day	22% 15% 34% 21% 6% 1	540
	21st to end	Day	25% 14% 33% 23% 4 1	594
April	1st to 10th	Day	22% 13% 36% 22% 6% 1	540
	11th to 20th	Day	23% 14% 33% 17% 9% 4	532
	21st to end	Day	23% 10% 34% 20% 10% 3	540
May	1st to 10th	Day	23% 11% 30% 23% 9% 3	543
	11th to 20th	Day	17% 8% 31% 25% 14% 5%	567
	21st to end	Day	15% 7% 25% 30% 17% 6%	617
June	1st to 10th	Day	12% 7% 30% 34% 14% 4	638
	11th to 20th	Day	12% 7% 31% 33% 15% 2	756
	21st to end	Day	12% 5% 31% 34% 14% 5%	730
July	1st to 10th	Day	11% 2 23% 31% 23% 10%	721
	11th to 20th	Day	11% 4 25% 31% 18% 10%	658
	21st to end	Day	10% 3 18% 33% 27% 9%	576
August	1st to 10th	Day	10% 3 16% 29% 27% 15%	526
	11th to 20th	Day	11% 3 15% 34% 24% 13%	519
	21st to end	Day	10% 2 18% 29% 27% 14%	583
September	1st to 10th	Day	11% 4 23% 28% 22% 13%	530
	11th to 20th	Day	8% 2 16% 27% 32% 15%	530
	21st to end	Day	20% 5% 21% 27% 18% 9%	530
October	1st to 10th	Day	13% 6% 25% 27% 21% 7%	530
	11th to 20th	Day	17% 11% 34% 26% 9% 2	530
	21st to end	Day	24% 14% 36% 19% 6% 1	483
November	1st to 10th	Day	15% 17% 33% 23% 9% 2	330
	11th to 20th	Day	18% 21% 32% 22% 5% 2	330
	21st to end	Day	25% 13% 42% 18% 2	230
December	1st to 10th	Day	24% 15% 35% 20% 7%	220
	11th to 20th	Day	26% 16% 36% 16% 6%	220
	21st to end	Day	28% 16% 36% 17% 4	309
January	1st to 10th	Night	23% 14% 36% 20% 6% 1	1050
	11th to 20th	Night	26% 16% 33% 21% 4	931
	21st to end	Night	22% 13% 34% 26% 6%	924
February	1st to 10th	Night	24% 13% 35% 24% 4	840
	11th to 20th	Night	23% 13% 34% 24% 6%	823
	21st to end	Night	24% 15% 38% 20% 2	692
March	1st to 10th	Night	22% 10% 36% 27% 5%	840
	11th to 20th	Night	18% 13% 38% 25% 5% 1	840
	21st to end	Night	22% 11% 33% 25% 9%	924
April	1st to 10th	Night	21% 14% 33% 28% 5%	793
	11th to 20th	Night	20% 11% 35% 26% 7% 1	623
	21st to end	Night	20% 7% 33% 29% 11%	630
May	1st to 10th	Night	21% 6% 30% 32% 9% 2	630
	11th to 20th	Night	20% 4 27% 33% 14% 1	621
	21st to end	Night	13% 3 24% 37% 21% 2	683
June	1st to 10th	Night	12% 3 26% 43% 15% 1	436
	11th to 20th	Night	12% 2 23% 45% 19%	410
	21st to end	Night	12% 4 21% 46% 16% 1	451
July	1st to 10th	Night	9% 2 12% 43% 30% 4	585
	11th to 20th	Night	11% 2 15% 40% 28% 4	582
	21st to end	Night	7% 2 15% 45% 28% 3	657
August	1st to 10th	Night	9% 2 12% 42% 29% 7%	597
	11th to 20th	Night	9% 1 14% 37% 34% 5%	594
	21st to end	Night	10% 2 13% 38% 32% 5%	660
September	1st to 10th	Night	11% 4 16% 33% 31% 6%	760
	11th to 20th	Night	11% 3 15% 33% 32% 7%	800
	21st to end	Night	17% 5% 19% 33% 21% 5%	800
October	1st to 10th	Night	15% 5% 24% 31% 22% 4	800
	11th to 20th	Night	20% 10% 30% 26% 12% 2	800
	21st to end	Night	23% 11% 31% 29% 6%	880
November	1st to 10th	Night	17% 12% 30% 28% 11% 1	820
	11th to 20th	Night	23% 13% 33% 23% 8% 1	840
	21st to end	Night	28% 12% 32% 21% 7%	840
December	1st to 10th	Night	25% 14% 34% 20% 7% 1	840
	11th to 20th	Night	24% 13% 38% 22% 4	840
	21st to end	Night	29% 12% 33% 22% 4	1113

Page generated at 2026-06-13 02:31:39. Version ac07

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