Thomas Harriot Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas Harriot Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	126228
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	123479
Transparency	Day	25% 38% 8% 14% 12% 3	129821
	Night	25% 38% 6% 15% 12% 3	127956
Seeing	Day	17% 9% 26% 26% 16% 6%	16466
	Night	17% 9% 28% 31% 13% 2	26029

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% 4 1 4 3 3 3 3 4 4 57%	3850
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 4 5% 53%	4060
	21st to end	Day	13% 7% 3 6% 5% 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	13% 8% 3 6% 4 4 5% 4 6% 7% 40%	1800
	11th to 20th	Day	21% 6% 2 4 3 4 4 3 4 5% 45%	1877
	21st to end	Day	16% 8% 2 6% 4 5% 4 4 6% 5% 39%	1650
March	1st to 10th	Day	19% 7% 3 6% 4 3 4 3 5% 6% 39%	4818
	11th to 20th	Day	10% 6% 2 6% 3 3 3 3 5% 6% 52%	4980
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 6% 45%	5595
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 4 7% 50%	2400
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 6% 49%	2374
	21st to end	Day	13% 6% 2 5% 4 5% 4 4 5% 7% 44%	2400
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 10% 34%	5492
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 4 6% 9% 32%	6039
	21st to end	Day	15% 5% 2 5% 3 4 4 4 6% 9% 43%	6934
June	1st to 10th	Day	10% 4 3 5% 4 5% 6% 5% 6% 9% 43%	2800
	11th to 20th	Day	11% 6% 2 5% 5% 5% 5% 4 6% 9% 43%	2800
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 12% 41%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 12% 39%	6360
	11th to 20th	Day	10% 7% 3 9% 6% 6% 7% 5% 9% 11% 27%	5978
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 6% 9% 11% 36%	6424
August	1st to 10th	Day	14% 9% 3 8% 5% 5% 6% 6% 7% 10% 26%	4234
	11th to 20th	Day	10% 6% 3 8% 5% 6% 6% 6% 6% 10% 34%	3864
	21st to end	Day	8% 4 2 5% 3 4 4 4 7% 11% 47%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 8% 43%	2400
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 4 5% 8% 41%	2400
	21st to end	Day	20% 8% 2 6% 4 5% 4 3 5% 6% 36%	2124
October	1st to 10th	Day	15% 7% 2 5% 4 4 4 3 5% 6% 45%	2000
	11th to 20th	Day	15% 5% 2 6% 3 2 3 3 4 5% 52%	1980
	21st to end	Day	12% 4 2 6% 3 3 3 3 4 5% 53%	2090
November	1st to 10th	Day	13% 6% 2 5% 3 3 4 3 4 6% 52%	1882
	11th to 20th	Day	14% 7% 2 5% 4 2 3 3 5% 6% 51%	1800
	21st to end	Day	15% 4 1 5% 3 3 3 3 5% 6% 52%	1695
December	1st to 10th	Day	11% 5% 2 4 4 3 3 3 4 6% 54%	2600
	11th to 20th	Day	18% 7% 2 6% 3 3 2 2 4 5% 47%	2990
	21st to end	Day	13% 6% 2 5% 4 3 4 3 4 7% 49%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 5% 53%	6092
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 4 4 54%	5900
	21st to end	Night	17% 7% 2 5% 3 3 3 2 4 5% 50%	6490
February	1st to 10th	Night	17% 7% 3 5% 4 4 4 3 5% 6% 42%	2600
	11th to 20th	Night	21% 7% 2 4 4 3 3 4 5% 5% 41%	2408
	21st to end	Night	17% 7% 2 5% 3 4 3 2 4 6% 47%	1980
March	1st to 10th	Night	19% 7% 2 5% 4 3 3 3 4 5% 44%	5017
	11th to 20th	Night	13% 5% 3 5% 4 4 4 3 4 6% 49%	4980
	21st to end	Night	14% 7% 2 5% 4 4 4 3 4 6% 47%	5335
April	1st to 10th	Night	14% 7% 2 6% 4 5% 3 4 6% 7% 42%	2000
	11th to 20th	Night	12% 7% 2 4 3 4 3 3 5% 8% 48%	1975
	21st to end	Night	14% 6% 2 6% 4 3 4 4 6% 7% 45%	2000
May	1st to 10th	Night	15% 7% 2 8% 5% 6% 5% 4 7% 8% 32%	4444
	11th to 20th	Night	21% 8% 2 6% 5% 4 4 4 5% 7% 34%	3851
	21st to end	Night	16% 7% 3 6% 5% 4 3 3 6% 8% 39%	3920
June	1st to 10th	Night	11% 5% 2 5% 6% 5% 5% 5% 7% 9% 40%	1600
	11th to 20th	Night	10% 5% 2 7% 4 5% 7% 5% 6% 10% 40%	1600
	21st to end	Night	8% 6% 2 7% 4 6% 4 5% 6% 10% 43%	1600
July	1st to 10th	Night	9% 5% 2 6% 6% 6% 7% 4 8% 10% 37%	3598
	11th to 20th	Night	10% 8% 3 7% 5% 6% 6% 6% 9% 11% 29%	3961
	21st to end	Night	10% 7% 3 7% 5% 5% 6% 5% 7% 9% 36%	4427
August	1st to 10th	Night	17% 10% 3 8% 5% 5% 4 5% 7% 10% 29%	2915
	11th to 20th	Night	8% 7% 3 8% 5% 5% 5% 5% 6% 9% 40%	3149
	21st to end	Night	9% 5% 1 6% 4 4 4 4 7% 9% 48%	3586
September	1st to 10th	Night	12% 4 2 6% 4 5% 4 4 6% 8% 45%	2000
	11th to 20th	Night	12% 6% 2 7% 4 4 5% 3 5% 7% 45%	2000
	21st to end	Night	21% 7% 3 7% 4 4 4 4 5% 5% 36%	2270
October	1st to 10th	Night	14% 5% 2 6% 4 4 4 3 5% 6% 47%	2400
	11th to 20th	Night	15% 6% 2 5% 4 3 2 3 4 6% 50%	2377
	21st to end	Night	15% 4 2 4 4 4 3 2 4 5% 54%	2508
November	1st to 10th	Night	14% 6% 2 6% 3 3 4 3 4 5% 50%	2415
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 4 6% 49%	2600
	21st to end	Night	13% 4 1 5% 4 4 4 3 5% 6% 50%	2700
December	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 4 6% 55%	4580
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 4 5% 5% 39%	5205
	21st to end	Night	13% 6% 2 6% 4 3 4 3 5% 6% 48%	6996

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	23% 26% 1 19% 20% 11%	3851
	11th to 20th	Day	25% 26% 3 14% 25% 7%	4060
	21st to end	Day	32% 28% 5% 21% 11% 2	4466
February	1st to 10th	Day	17% 62% 4 11% 5% 1	1800
	11th to 20th	Day	19% 61% 2 8% 7% 3	1886
	21st to end	Day	17% 62% 5% 7% 7% 2	1650
March	1st to 10th	Day	37% 29% 5% 9% 15% 5%	4818
	11th to 20th	Day	23% 28% 6% 16% 20% 7%	4980
	21st to end	Day	28% 29% 10% 12% 16% 6%	5595
April	1st to 10th	Day	15% 62% 4 9% 8% 2	2400
	11th to 20th	Day	13% 62% 5% 11% 7% 2	2374
	21st to end	Day	15% 62% 7% 9% 6% 2	2400
May	1st to 10th	Day	35% 31% 13% 14% 7% 1	5524
	11th to 20th	Day	37% 30% 11% 15% 7%	6043
	21st to end	Day	28% 29% 10% 18% 13% 2	6949
June	1st to 10th	Day	15% 63% 6% 9% 7% 1	2800
	11th to 20th	Day	13% 62% 7% 11% 7%	2800
	21st to end	Day	12% 61% 7% 12% 8%	2800
July	1st to 10th	Day	29% 27% 17% 15% 9% 2	6290
	11th to 20th	Day	40% 28% 20% 9% 2	5907
	21st to end	Day	30% 27% 11% 18% 14% 1	6240
August	1st to 10th	Day	37% 35% 12% 14% 2	4221
	11th to 20th	Day	29% 36% 9% 18% 8%	3900
	21st to end	Day	17% 36% 10% 22% 15% 1	4312
September	1st to 10th	Day	17% 58% 8% 10% 6% 1	2400
	11th to 20th	Day	20% 57% 5% 10% 7% 2	2400
	21st to end	Day	26% 57% 4 11% 3 1	2124
October	1st to 10th	Day	24% 35% 4 16% 18% 3	3260
	11th to 20th	Day	12% 35% 3 12% 30% 7%	3220
	21st to end	Day	9% 36% 5% 20% 18% 11%	3428
November	1st to 10th	Day	18% 56% 4 12% 9% 2	1882
	11th to 20th	Day	17% 56% 3 8% 12% 3	1800
	21st to end	Day	15% 56% 3 13% 11% 2	1695
December	1st to 10th	Day	22% 35% 4 20% 14% 5%	2600
	11th to 20th	Day	30% 31% 9% 13% 13% 4	2986
	21st to end	Day	26% 27% 10% 23% 14% 1	3960
January	1st to 10th	Night	27% 26% 2 14% 22% 9%	6092
	11th to 20th	Night	25% 26% 2 17% 25% 5%	5900
	21st to end	Night	28% 28% 4 21% 16% 3	6490
February	1st to 10th	Night	16% 62% 6% 10% 5% 1	2600
	11th to 20th	Night	19% 60% 3 8% 8% 2	2410
	21st to end	Night	16% 61% 3 10% 7% 4	1980
March	1st to 10th	Night	34% 29% 4 12% 14% 7%	5017
	11th to 20th	Night	25% 28% 5% 14% 19% 9%	4980
	21st to end	Night	28% 28% 6% 18% 14% 5%	5335
April	1st to 10th	Night	17% 62% 4 6% 9% 1	2000
	11th to 20th	Night	15% 63% 4 10% 7% 2	1975
	21st to end	Night	16% 62% 5% 9% 7% 1	2000
May	1st to 10th	Night	37% 31% 13% 13% 7%	4474
	11th to 20th	Night	38% 30% 8% 19% 5%	3852
	21st to end	Night	34% 30% 9% 17% 10%	3927
June	1st to 10th	Night	17% 63% 5% 8% 6% 1	1600
	11th to 20th	Night	14% 62% 7% 9% 7% 1	1600
	21st to end	Night	12% 61% 6% 11% 8% 1	1600
July	1st to 10th	Night	33% 29% 13% 14% 9% 1	3560
	11th to 20th	Night	39% 31% 18% 11% 1	3913
	21st to end	Night	35% 27% 8% 17% 12%	4297
August	1st to 10th	Night	38% 36% 11% 13% 2	2904
	11th to 20th	Night	25% 36% 13% 18% 8%	3179
	21st to end	Night	19% 35% 6% 19% 20% 1	3586
September	1st to 10th	Night	17% 59% 9% 9% 5% 1	2000
	11th to 20th	Night	18% 58% 4 10% 8% 1	2000
	21st to end	Night	26% 58% 5% 8% 3	2270
October	1st to 10th	Night	25% 35% 7% 14% 16% 3	3920
	11th to 20th	Night	13% 35% 3 14% 25% 10%	3876
	21st to end	Night	8% 36% 3 25% 19% 10%	4120
November	1st to 10th	Night	20% 55% 4 11% 8% 2	2415
	11th to 20th	Night	16% 56% 3 9% 9% 6%	2600
	21st to end	Night	14% 57% 4 12% 12% 2	2700
December	1st to 10th	Night	19% 35% 5% 20% 18% 4	4580
	11th to 20th	Night	35% 31% 7% 12% 9% 5%	5208
	21st to end	Night	28% 27% 9% 23% 12% 1	6996

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	23% 20% 38% 13% 5%	255
	11th to 20th	Day	18% 26% 40% 15% 2	300
	21st to end	Day	18% 18% 42% 19% 3	330
February	1st to 10th	Day	16% 21% 43% 16% 5%	300
	11th to 20th	Day	29% 19% 35% 15% 2	425
	21st to end	Day	22% 17% 39% 17% 4	414
March	1st to 10th	Day	20% 19% 36% 19% 5%	500
	11th to 20th	Day	23% 17% 34% 19% 6% 1	500
	21st to end	Day	19% 17% 36% 22% 5% 1	550
April	1st to 10th	Day	17% 21% 36% 17% 8% 1	500
	11th to 20th	Day	14% 14% 35% 24% 9% 3	492
	21st to end	Day	16% 10% 35% 28% 9% 2	500
May	1st to 10th	Day	14% 12% 37% 24% 10% 2	503
	11th to 20th	Day	21% 6% 27% 27% 15% 4	527
	21st to end	Day	20% 4 22% 27% 20% 8%	573
June	1st to 10th	Day	12% 3 24% 30% 25% 6%	530
	11th to 20th	Day	14% 4 18% 33% 23% 9%	530
	21st to end	Day	10% 2 16% 38% 27% 7%	530
July	1st to 10th	Day	11% 1 12% 35% 31% 10%	525
	11th to 20th	Day	9% 1 11% 32% 34% 12%	524
	21st to end	Day	10% 2 15% 34% 29% 11%	576
August	1st to 10th	Day	15% 2 17% 30% 26% 10%	526
	11th to 20th	Day	14% 1 13% 32% 26% 12%	519
	21st to end	Day	8% 3 16% 31% 27% 15%	583
September	1st to 10th	Day	16% 2 18% 30% 21% 14%	530
	11th to 20th	Day	18% 3 16% 32% 20% 11%	530
	21st to end	Day	28% 5% 22% 27% 14% 5%	530
October	1st to 10th	Day	18% 4 21% 29% 19% 9%	530
	11th to 20th	Day	18% 10% 23% 27% 17% 5%	522
	21st to end	Day	14% 9% 29% 29% 14% 5%	539
November	1st to 10th	Day	14% 9% 27% 29% 16% 4	458
	11th to 20th	Day	19% 13% 32% 25% 10% 1	400
	21st to end	Day	18% 16% 32% 20% 13% 2	295
December	1st to 10th	Day	23% 13% 36% 20% 7% 2	200
	11th to 20th	Day	24% 10% 35% 22% 11%	200
	21st to end	Day	22% 12% 35% 17% 12% 1	220
January	1st to 10th	Night	19% 19% 36% 23% 3 1	800
	11th to 20th	Night	20% 16% 33% 25% 6%	800
	21st to end	Night	19% 12% 41% 26% 2	880
February	1st to 10th	Night	22% 12% 40% 23% 3	800
	11th to 20th	Night	26% 16% 39% 17% 2	783
	21st to end	Night	20% 16% 37% 24% 2	660
March	1st to 10th	Night	18% 15% 36% 27% 5%	800
	11th to 20th	Night	20% 14% 37% 23% 5% 1	800
	21st to end	Night	18% 12% 41% 24% 5%	880
April	1st to 10th	Night	17% 17% 39% 23% 4	800
	11th to 20th	Night	16% 9% 38% 27% 10% 1	791
	21st to end	Night	17% 8% 34% 34% 7%	800
May	1st to 10th	Night	17% 9% 35% 31% 7% 1	780
	11th to 20th	Night	16% 5% 29% 38% 12% 1	514
	21st to end	Night	16% 2 19% 42% 19% 2	433
June	1st to 10th	Night	12% 2 19% 42% 22% 3	400
	11th to 20th	Night	11% 2 18% 48% 19% 3	400
	21st to end	Night	9% 3 18% 46% 22% 3	400
July	1st to 10th	Night	11% 2 11% 48% 26% 2	390
	11th to 20th	Night	9% 1 11% 40% 34% 5%	541
	21st to end	Night	10% 2 16% 42% 27% 4	657
August	1st to 10th	Night	12% 1 12% 43% 29% 4	597
	11th to 20th	Night	11% 2 14% 40% 29% 5%	754
	21st to end	Night	11% 2 13% 36% 32% 6%	880
September	1st to 10th	Night	13% 2 14% 38% 28% 5%	800
	11th to 20th	Night	15% 3 16% 34% 28% 5%	800
	21st to end	Night	25% 4 17% 35% 17% 2	800
October	1st to 10th	Night	18% 4 20% 28% 24% 6%	800
	11th to 20th	Night	19% 7% 25% 32% 14% 2	793
	21st to end	Night	17% 8% 27% 31% 16% 2	836
November	1st to 10th	Night	17% 6% 26% 33% 15% 3	780
	11th to 20th	Night	17% 11% 33% 30% 9% 1	800
	21st to end	Night	14% 12% 31% 30% 12% 1	800
December	1st to 10th	Night	21% 12% 33% 25% 9% 1	800
	11th to 20th	Night	20% 9% 37% 29% 6%	800
	21st to end	Night	19% 11% 30% 33% 6%	880

Page generated at 2025-10-16 01:50:23. Version ac07

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