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 2024-11-05.

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%	101018
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 43%	97964
Transparency	Day	22% 46% 7% 13% 10% 2	104655
	Night	22% 46% 6% 13% 10% 3	102467
Seeing	Day	17% 9% 26% 27% 16% 6%	15215
	Night	17% 8% 28% 32% 14% 2	24792

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	17% 5% 1 4 3 2 3 3 4 5% 53%	2686
	11th to 20th	Day	13% 7% 2 6% 4 3 4 3 5% 6% 47%	2840
	21st to end	Day	16% 9% 3 6% 5% 4 3 3 4 6% 40%	3124
February	1st to 10th	Day	12% 8% 3 6% 4 4 5% 4 6% 7% 40%	1710
	11th to 20th	Day	20% 6% 2 4 3 4 4 3 4 6% 46%	1780
	21st to end	Day	17% 8% 2 6% 4 5% 5% 5% 6% 6% 37%	1570
March	1st to 10th	Day	23% 9% 3 6% 4 3 3 3 5% 5% 37%	3360
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 5% 6% 55%	3480
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 6% 42%	3903
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 4 7% 51%	2280
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 3 5% 6% 49%	2254
	21st to end	Day	14% 6% 3 5% 4 5% 4 4 5% 7% 44%	2280
May	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 9% 32%	3824
	11th to 20th	Day	20% 6% 2 7% 4 4 5% 4 6% 8% 33%	4203
	21st to end	Day	10% 4 2 4 3 4 5% 4 6% 9% 48%	4823
June	1st to 10th	Day	10% 4 2 5% 4 5% 6% 5% 6% 10% 42%	2660
	11th to 20th	Day	10% 5% 2 5% 5% 5% 5% 5% 6% 10% 42%	2660
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 12% 40%	2660
July	1st to 10th	Day	5% 4 2 6% 6% 5% 7% 5% 9% 12% 39%	4440
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 10% 34%	4170
	21st to end	Day	9% 7% 3 7% 6% 6% 7% 6% 8% 10% 32%	4466
August	1st to 10th	Day	14% 9% 3 8% 5% 5% 7% 6% 8% 10% 26%	4104
	11th to 20th	Day	10% 6% 3 7% 5% 6% 6% 6% 6% 10% 35%	3743
	21st to end	Day	8% 4 2 5% 3 4 4 4 7% 11% 47%	4180
September	1st to 10th	Day	13% 6% 2 5% 5% 4 5% 4 6% 8% 42%	2280
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 4 5% 8% 41%	2280
	21st to end	Day	20% 7% 3 7% 4 5% 4 4 5% 6% 37%	2018
October	1st to 10th	Day	15% 7% 2 5% 4 4 4 3 5% 6% 45%	1900
	11th to 20th	Day	14% 5% 2 6% 3 2 3 3 4 5% 52%	1900
	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 2 3 3 4 6% 53%	1827
	11th to 20th	Day	13% 7% 2 5% 4 2 3 3 4 5% 51%	1710
	21st to end	Day	15% 4 1 5% 4 3 2 3 5% 6% 52%	1611
December	1st to 10th	Day	18% 6% 2 5% 4 3 4 3 6% 7% 43%	1520
	11th to 20th	Day	16% 6% 2 5% 3 2 2 2 4 5% 52%	1910
	21st to end	Day	16% 7% 2 5% 4 4 4 3 4 7% 44%	2772
January	1st to 10th	Night	13% 7% 3 6% 4 4 4 3 5% 5% 47%	4256
	11th to 20th	Night	16% 7% 3 7% 4 3 3 3 5% 5% 44%	4120
	21st to end	Night	20% 8% 2 6% 3 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	16% 7% 3 5% 4 4 4 3 5% 6% 43%	2470
	11th to 20th	Night	20% 8% 2 5% 4 3 3 4 5% 6% 41%	2285
	21st to end	Night	18% 7% 2 5% 3 3 3 2 4 6% 46%	1884
March	1st to 10th	Night	25% 8% 3 6% 4 3 3 3 5% 5% 36%	3489
	11th to 20th	Night	15% 5% 2 5% 3 3 4 3 5% 6% 50%	3480
	21st to end	Night	17% 8% 2 5% 4 3 3 2 3 4 48%	3727
April	1st to 10th	Night	14% 7% 2 6% 4 5% 3 3 6% 7% 43%	1900
	11th to 20th	Night	12% 7% 2 4 4 4 3 3 5% 7% 49%	1875
	21st to end	Night	14% 6% 2 6% 4 3 4 4 6% 7% 45%	1900
May	1st to 10th	Night	18% 7% 2 8% 5% 5% 5% 4 6% 6% 34%	3098
	11th to 20th	Night	21% 7% 2 6% 4 4 4 4 6% 7% 36%	2687
	21st to end	Night	12% 5% 2 5% 5% 4 3 4 6% 9% 46%	2732
June	1st to 10th	Night	11% 5% 1 5% 6% 5% 5% 5% 8% 9% 40%	1520
	11th to 20th	Night	10% 5% 2 6% 4 4 7% 5% 6% 10% 41%	1520
	21st to end	Night	8% 6% 2 7% 4 6% 4 5% 6% 9% 42%	1520
July	1st to 10th	Night	8% 4 2 6% 6% 6% 7% 5% 8% 10% 39%	2520
	11th to 20th	Night	11% 6% 2 6% 5% 5% 6% 5% 8% 11% 35%	2772
	21st to end	Night	13% 9% 4 8% 6% 6% 6% 4 7% 7% 30%	3087
August	1st to 10th	Night	17% 9% 3 8% 4 5% 4 5% 7% 10% 29%	2825
	11th to 20th	Night	8% 6% 3 8% 5% 5% 5% 5% 6% 9% 41%	3051
	21st to end	Night	9% 5% 1 6% 4 4 4 4 6% 9% 48%	3476
September	1st to 10th	Night	12% 4 2 6% 4 5% 4 4 6% 8% 45%	1900
	11th to 20th	Night	12% 6% 3 7% 4 4 4 3 5% 7% 45%	1900
	21st to end	Night	21% 6% 3 7% 4 4 4 4 5% 5% 37%	2156
October	1st to 10th	Night	15% 5% 2 6% 4 4 4 3 5% 6% 46%	2280
	11th to 20th	Night	14% 6% 2 5% 4 3 2 3 4 6% 51%	2280
	21st to end	Night	15% 4 2 4 4 4 3 2 4 5% 54%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 4 5% 50%	2350
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 4 6% 49%	2470
	21st to end	Night	13% 4 1 5% 4 4 4 3 5% 6% 50%	2565
December	1st to 10th	Night	17% 7% 3 6% 4 4 4 3 4 5% 42%	2660
	11th to 20th	Night	17% 6% 2 4 4 3 3 3 5% 6% 46%	3285
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 5% 44%	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	22% 37% 2 21% 16% 3	2687
	11th to 20th	Day	24% 37% 4 13% 18% 4	2840
	21st to end	Day	31% 40% 5% 13% 10% 1	3124
February	1st to 10th	Day	15% 65% 4 10% 5% 1	1710
	11th to 20th	Day	16% 64% 2 8% 7% 3	1789
	21st to end	Day	17% 65% 5% 6% 5% 2	1570
March	1st to 10th	Day	36% 41% 6% 5% 8% 3	3360
	11th to 20th	Day	14% 40% 4 16% 19% 6%	3480
	21st to end	Day	26% 41% 8% 10% 12% 3	3903
April	1st to 10th	Day	13% 66% 3 9% 8% 2	2280
	11th to 20th	Day	12% 66% 5% 9% 6% 2	2254
	21st to end	Day	14% 65% 6% 9% 5% 2	2280
May	1st to 10th	Day	31% 43% 9% 13% 3 1	3859
	11th to 20th	Day	31% 42% 8% 14% 5%	4207
	21st to end	Day	17% 42% 10% 16% 13% 2	4837
June	1st to 10th	Day	13% 66% 6% 8% 6% 1	2660
	11th to 20th	Day	11% 65% 6% 10% 6%	2660
	21st to end	Day	11% 64% 7% 11% 7%	2660
July	1st to 10th	Day	25% 38% 17% 11% 7% 2	4370
	11th to 20th	Day	32% 37% 18% 11% 2	4099
	21st to end	Day	32% 38% 8% 11% 10% 1	4324
August	1st to 10th	Day	37% 36% 12% 13% 2	4091
	11th to 20th	Day	28% 37% 8% 19% 8%	3779
	21st to end	Day	17% 37% 10% 22% 14% 1	4180
September	1st to 10th	Day	16% 61% 8% 8% 6% 1	2280
	11th to 20th	Day	17% 60% 5% 9% 6% 2	2280
	21st to end	Day	24% 60% 3 10% 3 1	2018
October	1st to 10th	Day	24% 36% 4 15% 17% 3	3160
	11th to 20th	Day	11% 36% 3 12% 30% 7%	3140
	21st to end	Day	9% 36% 5% 20% 18% 11%	3428
November	1st to 10th	Day	17% 57% 4 11% 9% 2	1827
	11th to 20th	Day	15% 59% 3 7% 12% 3	1710
	21st to end	Day	14% 59% 3 13% 10% 2	1611
December	1st to 10th	Day	21% 60% 3 8% 4 3	1520
	11th to 20th	Day	17% 47% 3 16% 11% 6%	1906
	21st to end	Day	27% 38% 6% 20% 10% 1	2772
January	1st to 10th	Night	27% 37% 3 15% 14% 4	4256
	11th to 20th	Night	27% 37% 3 14% 16% 3	4120
	21st to end	Night	30% 40% 3 13% 11% 4	4532
February	1st to 10th	Night	14% 65% 6% 10% 5% 1	2470
	11th to 20th	Night	17% 63% 2 8% 7% 2	2287
	21st to end	Night	15% 64% 3 9% 6% 2	1884
March	1st to 10th	Night	37% 42% 4 10% 7% 1	3489
	11th to 20th	Night	18% 40% 3 13% 19% 7%	3480
	21st to end	Night	24% 40% 3 17% 13% 3	3727
April	1st to 10th	Night	15% 66% 4 6% 9% 1	1900
	11th to 20th	Night	13% 66% 4 9% 6% 1	1875
	21st to end	Night	15% 65% 5% 9% 6% 1	1900
May	1st to 10th	Night	33% 42% 9% 11% 5%	3128
	11th to 20th	Night	29% 42% 7% 18% 3	2688
	21st to end	Night	21% 43% 11% 14% 11%	2739
June	1st to 10th	Night	15% 66% 5% 8% 5% 1	1520
	11th to 20th	Night	12% 66% 7% 9% 7% 1	1520
	21st to end	Night	12% 65% 5% 9% 8% 1	1520
July	1st to 10th	Night	27% 42% 12% 12% 6% 1	2480
	11th to 20th	Night	30% 38% 15% 15% 2	2721
	21st to end	Night	37% 38% 6% 10% 9%	2988
August	1st to 10th	Night	38% 37% 11% 12% 2	2814
	11th to 20th	Night	24% 37% 13% 18% 8%	3081
	21st to end	Night	18% 36% 6% 19% 20% 1	3476
September	1st to 10th	Night	15% 62% 9% 8% 4 1	1900
	11th to 20th	Night	17% 61% 4 10% 7% 2	1900
	21st to end	Night	23% 61% 4 9% 3	2156
October	1st to 10th	Night	25% 36% 7% 14% 15% 3	3800
	11th to 20th	Night	12% 36% 3 14% 25% 10%	3779
	21st to end	Night	8% 36% 3 25% 19% 10%	4120
November	1st to 10th	Night	20% 57% 3 10% 8% 2	2350
	11th to 20th	Night	15% 59% 3 8% 10% 6%	2470
	21st to end	Night	13% 60% 4 11% 11% 2	2565
December	1st to 10th	Night	22% 60% 4 9% 3 2	2660
	11th to 20th	Night	20% 49% 4 12% 11% 5%	3288
	21st to end	Night	26% 39% 7% 17% 11% 1	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	23% 17% 39% 15% 6%	229
	11th to 20th	Day	19% 25% 39% 15% 2	270
	21st to end	Day	19% 18% 41% 20% 3	297
February	1st to 10th	Day	15% 20% 43% 17% 6%	270
	11th to 20th	Day	27% 18% 36% 16% 2	388
	21st to end	Day	24% 17% 38% 17% 4 1	382
March	1st to 10th	Day	20% 18% 37% 19% 5%	460
	11th to 20th	Day	22% 18% 34% 18% 7% 1	460
	21st to end	Day	20% 16% 36% 23% 6% 1	506
April	1st to 10th	Day	17% 20% 36% 18% 8% 1	460
	11th to 20th	Day	13% 15% 34% 25% 10% 4	452
	21st to end	Day	17% 10% 36% 27% 8% 2	460
May	1st to 10th	Day	14% 12% 35% 26% 11% 2	463
	11th to 20th	Day	21% 6% 25% 29% 15% 3	487
	21st to end	Day	19% 3 21% 28% 21% 8%	529
June	1st to 10th	Day	12% 3 22% 31% 25% 6%	490
	11th to 20th	Day	13% 4 18% 33% 23% 10%	490
	21st to end	Day	10% 3 17% 38% 24% 7%	490
July	1st to 10th	Day	10% 1 11% 34% 33% 11%	485
	11th to 20th	Day	9% 1 11% 32% 35% 12%	484
	21st to end	Day	10% 2 15% 35% 28% 10%	532
August	1st to 10th	Day	14% 2 16% 31% 26% 10%	486
	11th to 20th	Day	14% 1 14% 31% 27% 13%	479
	21st to end	Day	8% 3 14% 30% 29% 16%	539
September	1st to 10th	Day	14% 2 17% 30% 22% 15%	490
	11th to 20th	Day	16% 3 15% 33% 21% 12%	490
	21st to end	Day	28% 5% 22% 27% 14% 5%	490
October	1st to 10th	Day	19% 3 21% 29% 19% 8%	490
	11th to 20th	Day	16% 9% 23% 28% 18% 5%	490
	21st to end	Day	14% 9% 29% 29% 14% 5%	539
November	1st to 10th	Day	15% 9% 27% 29% 15% 5%	439
	11th to 20th	Day	18% 13% 31% 25% 11% 1	370
	21st to end	Day	19% 14% 31% 21% 14% 2	271
December	1st to 10th	Day	24% 11% 37% 19% 8% 2	180
	11th to 20th	Day	22% 10% 34% 23% 11%	180
	21st to end	Day	23% 11% 34% 18% 13% 1	198
January	1st to 10th	Night	18% 18% 36% 24% 3 1	760
	11th to 20th	Night	21% 16% 32% 25% 6%	760
	21st to end	Night	19% 12% 40% 27% 2	836
February	1st to 10th	Night	21% 12% 40% 24% 3	760
	11th to 20th	Night	24% 17% 39% 18% 2	743
	21st to end	Night	21% 16% 36% 24% 3	628
March	1st to 10th	Night	17% 14% 36% 27% 5%	760
	11th to 20th	Night	20% 14% 38% 22% 5% 1	760
	21st to end	Night	18% 12% 40% 25% 5%	836
April	1st to 10th	Night	17% 16% 39% 23% 4	760
	11th to 20th	Night	16% 8% 38% 27% 10% 1	751
	21st to end	Night	17% 8% 34% 34% 7%	760
May	1st to 10th	Night	17% 10% 34% 31% 7% 1	741
	11th to 20th	Night	16% 5% 29% 38% 13% 1	488
	21st to end	Night	15% 2 18% 43% 20% 2	411
June	1st to 10th	Night	12% 2 19% 42% 22% 3	380
	11th to 20th	Night	10% 2 17% 49% 19% 3	380
	21st to end	Night	9% 3 19% 47% 20% 3	380
July	1st to 10th	Night	10% 2 11% 48% 27% 2	370
	11th to 20th	Night	9% 1 12% 41% 34% 4	513
	21st to end	Night	10% 2 17% 42% 26% 3	624
August	1st to 10th	Night	10% 1 12% 43% 29% 4	567
	11th to 20th	Night	11% 2 14% 40% 28% 5%	716
	21st to end	Night	11% 2 12% 36% 33% 6%	836
September	1st to 10th	Night	13% 2 13% 38% 29% 5%	760
	11th to 20th	Night	15% 3 15% 34% 28% 5%	760
	21st to end	Night	24% 4 17% 35% 18% 2	760
October	1st to 10th	Night	18% 4 20% 28% 24% 5%	760
	11th to 20th	Night	18% 7% 25% 33% 15% 2	760
	21st to end	Night	17% 8% 27% 31% 16% 2	836
November	1st to 10th	Night	18% 6% 26% 33% 15% 3	760
	11th to 20th	Night	16% 11% 33% 30% 9% 1	760
	21st to end	Night	14% 12% 30% 30% 13% 1	760
December	1st to 10th	Night	21% 12% 32% 25% 9% 1	760
	11th to 20th	Night	19% 8% 38% 29% 6%	760
	21st to end	Night	19% 11% 30% 33% 6%	836

Page generated at 2024-11-03 02:11:16. Version ac07

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