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 2025-02-21.

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% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	108525
	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 34%	110104
Transparency	Day	48% 3 13% 22% 12% 1	112166
	Night	50% 3 11% 22% 13% 1	114602
Seeing	Day	17% 9% 29% 26% 14% 5%	15537
	Night	19% 9% 28% 29% 13% 2	25262

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	12% 15% 7% 14% 6% 5% 5% 4 5% 6% 22%	3617
	11th to 20th	Day	13% 16% 6% 16% 8% 6% 6% 3 5% 6% 15%	3859
	21st to end	Day	22% 16% 7% 13% 5% 4 4 3 5% 6% 16%	4466
February	1st to 10th	Day	15% 16% 3 11% 6% 5% 5% 3 5% 6% 25%	1800
	11th to 20th	Day	14% 13% 5% 11% 7% 5% 5% 4 5% 6% 25%	1852
	21st to end	Day	16% 13% 4 11% 5% 4 5% 5% 6% 6% 27%	1570
March	1st to 10th	Day	18% 13% 5% 9% 5% 4 5% 3 5% 5% 29%	3266
	11th to 20th	Day	13% 11% 4 9% 6% 5% 4 3 5% 7% 33%	3480
	21st to end	Day	17% 12% 5% 9% 6% 5% 4 3 4 6% 29%	3815
April	1st to 10th	Day	14% 12% 3 7% 5% 3 4 4 6% 7% 33%	2148
	11th to 20th	Day	17% 9% 3 7% 4 5% 4 4 6% 7% 34%	2442
	21st to end	Day	18% 9% 3 6% 6% 5% 4 3 4 7% 36%	2470
May	1st to 10th	Day	17% 8% 3 7% 5% 4 4 4 5% 7% 36%	4082
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 7% 9% 35%	4081
	21st to end	Day	9% 6% 2 6% 5% 4 4 4 5% 9% 46%	4476
June	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 4 8% 9% 40%	2703
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	2850
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 8% 10% 41%	2795
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	4440
	11th to 20th	Day	8% 4 2 5% 4 4 5% 5% 7% 11% 47%	4344
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	4466
August	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 6% 9% 43%	4104
	11th to 20th	Day	9% 5% 3 7% 5% 6% 5% 5% 8% 11% 37%	4044
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	4532
September	1st to 10th	Day	9% 6% 3 6% 5% 4 5% 4 6% 8% 44%	2136
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 11% 46%	2090
	21st to end	Day	17% 6% 3 7% 5% 4 4 4 5% 8% 38%	2090
October	1st to 10th	Day	11% 5% 2 7% 4 5% 5% 5% 6% 7% 42%	2004
	11th to 20th	Day	17% 7% 2 7% 5% 4 4 4 6% 8% 36%	1900
	21st to end	Day	19% 10% 3 7% 6% 4 5% 3 6% 7% 31%	1995
November	1st to 10th	Day	11% 9% 3 7% 6% 4 6% 5% 5% 7% 38%	1762
	11th to 20th	Day	15% 10% 3 8% 5% 5% 4 4 6% 7% 33%	1800
	21st to end	Day	17% 14% 4 9% 6% 5% 5% 4 6% 6% 25%	1619
December	1st to 10th	Day	15% 12% 4 12% 6% 6% 5% 4 6% 6% 23%	2600
	11th to 20th	Day	21% 19% 5% 8% 4 5% 5% 4 6% 6% 19%	2990
	21st to end	Day	26% 18% 6% 13% 5% 4 4 3 4 4 12%	3837
January	1st to 10th	Night	13% 13% 5% 13% 5% 4 5% 3 5% 6% 27%	6348
	11th to 20th	Night	11% 16% 7% 13% 7% 5% 5% 4 6% 6% 20%	6104
	21st to end	Night	20% 16% 7% 10% 4 4 4 3 4 4 25%	6490
February	1st to 10th	Night	17% 15% 4 11% 7% 5% 4 4 5% 4 24%	2600
	11th to 20th	Night	15% 14% 4 10% 6% 5% 5% 4 5% 7% 26%	2433
	21st to end	Night	18% 13% 4 9% 5% 4 5% 4 4 5% 30%	1885
March	1st to 10th	Night	17% 13% 4 8% 5% 4 4 3 4 5% 32%	3586
	11th to 20th	Night	10% 8% 3 6% 5% 5% 5% 4 6% 8% 40%	3480
	21st to end	Night	15% 8% 3 8% 5% 5% 4 4 5% 6% 37%	3817
April	1st to 10th	Night	16% 10% 3 7% 6% 4 3 4 5% 7% 36%	2047
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 6% 8% 35%	1687
	21st to end	Night	16% 8% 3 7% 5% 3 4 4 5% 6% 39%	1710
May	1st to 10th	Night	17% 7% 3 7% 6% 5% 4 4 5% 7% 34%	2813
	11th to 20th	Night	14% 7% 3 8% 5% 5% 5% 4 6% 8% 35%	2812
	21st to end	Night	9% 5% 2 6% 4 5% 4 3 5% 8% 49%	3079
June	1st to 10th	Night	11% 5% 2 6% 4 4 5% 3 6% 9% 45%	1468
	11th to 20th	Night	10% 5% 1 7% 5% 5% 5% 5% 6% 9% 44%	1330
	21st to end	Night	9% 6% 2 5% 4 4 4 4 6% 8% 48%	1378
July	1st to 10th	Night	8% 4 1 5% 3 4 3 4 7% 11% 50%	2530
	11th to 20th	Night	9% 4 1 4 4 4 4 4 6% 9% 51%	2618
	21st to end	Night	5% 4 1 5% 4 5% 5% 5% 8% 10% 47%	3087
August	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 8% 46%	2825
	11th to 20th	Night	9% 6% 3 7% 4 4 5% 3 5% 7% 48%	2785
	21st to end	Night	8% 4 2 5% 5% 5% 5% 4 7% 7% 48%	3124
September	1st to 10th	Night	9% 5% 2 6% 4 3 5% 4 6% 7% 48%	2019
	11th to 20th	Night	9% 5% 2 5% 4 3 3 3 6% 8% 53%	2090
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 5% 7% 41%	2090
October	1st to 10th	Night	13% 5% 2 7% 4 4 4 3 4 7% 47%	2175
	11th to 20th	Night	17% 8% 2 8% 5% 4 4 4 5% 7% 37%	2280
	21st to end	Night	19% 9% 3 7% 5% 4 4 4 5% 6% 33%	2600
November	1st to 10th	Night	13% 9% 2 6% 5% 4 4 3 5% 6% 43%	2536
	11th to 20th	Night	17% 10% 3 7% 5% 4 5% 3 6% 8% 32%	2600
	21st to end	Night	20% 13% 4 8% 5% 4 4 4 5% 6% 27%	2778
December	1st to 10th	Night	15% 12% 5% 10% 6% 5% 5% 4 7% 7% 24%	4580
	11th to 20th	Night	16% 18% 6% 11% 5% 5% 4 4 5% 5% 22%	5205
	21st to end	Night	25% 20% 6% 11% 5% 4 4 3 4 5% 14%	7115

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	63% 3 11% 15% 6% 2	3618
	11th to 20th	Day	71% 3 8% 9% 10%	3859
	21st to end	Day	70% 2 11% 10% 6%	4466
February	1st to 10th	Day	61% 3 10% 17% 9% 1	1800
	11th to 20th	Day	61% 3 9% 18% 8% 1	1861
	21st to end	Day	57% 4 12% 18% 7% 2	1570
March	1st to 10th	Day	59% 2 10% 23% 5% 1	3266
	11th to 20th	Day	53% 2 14% 21% 9% 1	3480
	21st to end	Day	58% 1 11% 19% 10% 1	3815
April	1st to 10th	Day	49% 4 13% 20% 14% 1	2148
	11th to 20th	Day	49% 3 11% 22% 13% 2	2442
	21st to end	Day	50% 2 12% 21% 13% 2	2470
May	1st to 10th	Day	48% 2 9% 26% 13% 2	4120
	11th to 20th	Day	48% 2 15% 24% 9% 2	4085
	21st to end	Day	37% 2 12% 21% 21% 7%	4489
June	1st to 10th	Day	39% 3 13% 29% 14% 2	2703
	11th to 20th	Day	38% 3 15% 23% 18% 2	2850
	21st to end	Day	36% 5% 14% 26% 17% 2	2795
July	1st to 10th	Day	39% 5% 12% 25% 18% 2	4371
	11th to 20th	Day	29% 5% 14% 34% 16% 2	4269
	21st to end	Day	35% 6% 16% 19% 23% 2	4324
August	1st to 10th	Day	37% 5% 15% 29% 14% 1	4091
	11th to 20th	Day	37% 6% 13% 34% 9% 1	4083
	21st to end	Day	35% 5% 16% 29% 13% 1	4531
September	1st to 10th	Day	38% 4 16% 26% 15% 1	2136
	11th to 20th	Day	33% 4 13% 32% 18% 1	2090
	21st to end	Day	44% 3 12% 27% 13% 1	2090
October	1st to 10th	Day	30% 5% 13% 31% 19% 1	3329
	11th to 20th	Day	38% 3 14% 27% 18% 1	3140
	21st to end	Day	44% 4 15% 24% 13% 1	3271
November	1st to 10th	Day	45% 5% 12% 25% 12% 2	1762
	11th to 20th	Day	51% 2 15% 21% 10% 1	1800
	21st to end	Day	60% 3 12% 19% 6% 1	1619
December	1st to 10th	Day	60% 5% 17% 14% 4	2600
	11th to 20th	Day	67% 2 11% 14% 5%	2986
	21st to end	Day	76% 3 8% 9% 4	3837
January	1st to 10th	Night	59% 2 11% 20% 7% 1	6348
	11th to 20th	Night	65% 2 10% 15% 7% 1	6104
	21st to end	Night	65% 2 10% 13% 10%	6490
February	1st to 10th	Night	63% 3 10% 17% 6% 1	2600
	11th to 20th	Night	59% 3 9% 19% 9% 1	2435
	21st to end	Night	58% 2 10% 20% 9% 1	1885
March	1st to 10th	Night	56% 1 11% 24% 8%	3586
	11th to 20th	Night	43% 2 15% 20% 19% 1	3480
	21st to end	Night	49% 1 10% 23% 15% 1	3817
April	1st to 10th	Night	49% 2 10% 22% 14% 2	2047
	11th to 20th	Night	46% 3 11% 21% 17% 2	1687
	21st to end	Night	46% 3 10% 21% 17% 4	1710
May	1st to 10th	Night	51% 2 8% 20% 15% 3	2840
	11th to 20th	Night	49% 2 8% 27% 13% 1	2813
	21st to end	Night	35% 2 11% 24% 21% 6%	3087
June	1st to 10th	Night	37% 2 10% 30% 17% 3	1468
	11th to 20th	Night	37% 4 14% 26% 17% 3	1330
	21st to end	Night	35% 3 13% 27% 22% 1	1378
July	1st to 10th	Night	29% 3 12% 31% 19% 6%	2490
	11th to 20th	Night	30% 4 13% 33% 19% 2	2570
	21st to end	Night	30% 5% 17% 22% 23% 2	2988
August	1st to 10th	Night	37% 4 12% 28% 17% 2	2814
	11th to 20th	Night	36% 5% 10% 33% 14% 2	2812
	21st to end	Night	33% 5% 12% 32% 16% 1	3124
September	1st to 10th	Night	34% 5% 12% 28% 19% 3	2019
	11th to 20th	Night	31% 3 10% 33% 21% 2	2090
	21st to end	Night	43% 3 12% 26% 15% 1	2090
October	1st to 10th	Night	26% 4 8% 33% 28% 1	3630
	11th to 20th	Night	44% 3 11% 24% 18% 1	3779
	21st to end	Night	41% 2 11% 30% 15% 1	4274
November	1st to 10th	Night	43% 3 11% 27% 16% 1	2536
	11th to 20th	Night	51% 2 11% 23% 12% 1	2600
	21st to end	Night	59% 2 13% 19% 6%	2778
December	1st to 10th	Night	58% 4 19% 16% 4	4580
	11th to 20th	Night	65% 2 12% 14% 7% 1	5208
	21st to end	Night	74% 3 10% 10% 4	7115

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% 17% 34% 19% 6% 1	300
	11th to 20th	Day	22% 16% 43% 13% 4 1	300
	21st to end	Day	28% 11% 33% 23% 5%	330
February	1st to 10th	Day	24% 17% 34% 20% 4 1	300
	11th to 20th	Day	21% 17% 40% 18% 3	300
	21st to end	Day	23% 20% 40% 16% 1	225
March	1st to 10th	Day	26% 15% 36% 20% 4	351
	11th to 20th	Day	21% 14% 33% 23% 7% 1	460
	21st to end	Day	26% 13% 32% 24% 5% 1	506
April	1st to 10th	Day	19% 11% 37% 23% 7% 2	460
	11th to 20th	Day	25% 12% 32% 18% 9% 4	452
	21st to end	Day	23% 10% 35% 18% 10% 3	460
May	1st to 10th	Day	24% 10% 29% 24% 10% 4	463
	11th to 20th	Day	19% 7% 29% 26% 15% 6%	487
	21st to end	Day	14% 6% 22% 32% 18% 7%	529
June	1st to 10th	Day	12% 7% 30% 33% 14% 4	552
	11th to 20th	Day	12% 6% 30% 34% 16% 2	680
	21st to end	Day	12% 5% 31% 32% 14% 5%	680
July	1st to 10th	Day	11% 2 23% 31% 23% 10%	671
	11th to 20th	Day	12% 4 25% 31% 18% 10%	611
	21st to end	Day	10% 3 18% 33% 27% 10%	532
August	1st to 10th	Day	11% 4 17% 27% 26% 15%	486
	11th to 20th	Day	11% 3 16% 33% 24% 13%	479
	21st to end	Day	11% 2 17% 29% 27% 14%	539
September	1st to 10th	Day	11% 3 23% 29% 22% 12%	490
	11th to 20th	Day	9% 2 17% 28% 32% 13%	490
	21st to end	Day	20% 6% 21% 27% 17% 9%	490
October	1st to 10th	Day	14% 7% 26% 27% 20% 7%	490
	11th to 20th	Day	18% 12% 33% 25% 10% 2	490
	21st to end	Day	25% 13% 35% 19% 6% 1	444
November	1st to 10th	Day	13% 17% 34% 23% 10% 3	300
	11th to 20th	Day	18% 20% 32% 23% 6% 2	300
	21st to end	Day	25% 12% 43% 18% 1	209
December	1st to 10th	Day	23% 15% 35% 21% 7% 1	200
	11th to 20th	Day	27% 15% 37% 17% 6%	200
	21st to end	Day	27% 17% 35% 17% 4	281
January	1st to 10th	Night	23% 14% 37% 20% 6% 1	1000
	11th to 20th	Night	27% 15% 33% 21% 4	887
	21st to end	Night	23% 12% 33% 26% 6%	880
February	1st to 10th	Night	25% 13% 35% 24% 3	800
	11th to 20th	Night	23% 13% 33% 25% 6%	783
	21st to end	Night	25% 16% 37% 21% 2	629
March	1st to 10th	Night	22% 10% 35% 28% 6%	760
	11th to 20th	Night	17% 13% 38% 26% 6% 1	760
	21st to end	Night	23% 10% 32% 26% 10%	836
April	1st to 10th	Night	19% 13% 33% 29% 5%	717
	11th to 20th	Night	20% 10% 34% 27% 7% 1	563
	21st to end	Night	19% 6% 34% 29% 11%	570
May	1st to 10th	Night	20% 6% 29% 34% 10% 2	570
	11th to 20th	Night	20% 4 25% 34% 15% 1	561
	21st to end	Night	12% 3 23% 37% 23% 2	617
June	1st to 10th	Night	13% 3 27% 42% 15% 1	394
	11th to 20th	Night	12% 1 22% 45% 19%	380
	21st to end	Night	12% 4 21% 46% 15% 1	428
July	1st to 10th	Night	9% 2 13% 42% 30% 5%	555
	11th to 20th	Night	11% 2 15% 40% 29% 4	552
	21st to end	Night	6% 2 15% 46% 27% 3	624
August	1st to 10th	Night	9% 2 13% 42% 28% 7%	567
	11th to 20th	Night	9% 14% 37% 34% 5%	564
	21st to end	Night	10% 2 14% 38% 32% 5%	627
September	1st to 10th	Night	11% 4 16% 33% 31% 5%	722
	11th to 20th	Night	11% 3 16% 33% 30% 7%	760
	21st to end	Night	17% 5% 20% 32% 21% 5%	760
October	1st to 10th	Night	16% 5% 25% 30% 21% 4	760
	11th to 20th	Night	21% 10% 29% 25% 13% 2	760
	21st to end	Night	24% 11% 30% 28% 6%	836
November	1st to 10th	Night	18% 11% 29% 29% 12% 2	780
	11th to 20th	Night	24% 12% 33% 23% 7% 1	800
	21st to end	Night	28% 12% 31% 21% 8%	800
December	1st to 10th	Night	25% 14% 34% 20% 7% 1	800
	11th to 20th	Night	25% 12% 37% 22% 4	800
	21st to end	Night	29% 11% 33% 22% 4	1060

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