Strathmore
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Strathmore clear sky chart from 2005-11-06 to 2024-09-29.

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	9% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	101552
	Night	7% 9% 4 9% 6% 5% 5% 4 6% 8% 37%	96757
Transparency	Day	41% 7% 15% 25% 11% 2	101296
	Night	43% 7% 15% 23% 11% 2	96660
Seeing	Day	12% 7% 25% 31% 19% 6%	15290
	Night	11% 6% 29% 36% 16% 2	24307

By Month

Percentage Cloud Cover NW (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	4 9% 6% 11% 5% 6% 6% 4 8% 9% 31%	2219
	11th to 20th	Day	4 9% 4 13% 6% 4 7% 7% 8% 8% 31%	2203
	21st to end	Day	5% 8% 4 10% 6% 5% 5% 5% 7% 8% 38%	2794
February	1st to 10th	Day	5% 13% 5% 12% 8% 7% 6% 4 6% 7% 28%	1535
	11th to 20th	Day	6% 13% 4 11% 6% 5% 6% 5% 7% 6% 29%	1665
	21st to end	Day	4 13% 5% 12% 7% 7% 6% 4 7% 7% 28%	1503
March	1st to 10th	Day	3 12% 5% 13% 7% 5% 7% 4 5% 6% 31%	3138
	11th to 20th	Day	7% 9% 2 6% 4 4 5% 3 5% 9% 45%	3483
	21st to end	Day	10% 21% 6% 13% 5% 4 5% 3 4 6% 24%	3841
April	1st to 10th	Day	9% 13% 5% 9% 6% 5% 5% 5% 7% 7% 29%	2351
	11th to 20th	Day	11% 12% 4 9% 5% 5% 5% 4 6% 8% 31%	2440
	21st to end	Day	13% 11% 4 8% 6% 5% 4 4 6% 8% 31%	2470
May	1st to 10th	Day	19% 13% 4 8% 5% 5% 4 3 5% 6% 29%	4335
	11th to 20th	Day	10% 7% 3 9% 6% 6% 6% 5% 8% 9% 32%	4694
	21st to end	Day	15% 8% 3 9% 6% 6% 6% 5% 6% 7% 27%	5161
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 6% 5% 7% 9% 32%	2850
	11th to 20th	Day	18% 9% 3 6% 6% 5% 5% 5% 6% 8% 29%	2850
	21st to end	Day	8% 5% 2 5% 5% 5% 6% 4 8% 10% 43%	2850
July	1st to 10th	Day	7% 3 1 5% 4 4 4 4 7% 8% 52%	4740
	11th to 20th	Day	6% 3 1 4 3 3 4 4 7% 10% 54%	4740
	21st to end	Day	3 2 1 3 3 3 4 3 6% 9% 61%	5062
August	1st to 10th	Day	8% 6% 2 6% 5% 4 6% 5% 7% 9% 44%	4401
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 11% 46%	4033
	21st to end	Day	10% 4 1 4 2 4 4 4 5% 7% 54%	4521
September	1st to 10th	Day	10% 5% 3 6% 3 3 3 2 5% 7% 52%	2365
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 44%	2090
	21st to end	Day	10% 8% 2 6% 5% 4 4 3 6% 8% 46%	2069
October	1st to 10th	Day	14% 9% 2 6% 5% 4 5% 3 5% 6% 41%	1886
	11th to 20th	Day	10% 8% 3 8% 5% 5% 5% 3 5% 7% 42%	1689
	21st to end	Day	8% 10% 3 8% 5% 5% 5% 6% 6% 6% 37%	1782
November	1st to 10th	Day	10% 15% 5% 9% 5% 4 4 4 6% 8% 29%	1531
	11th to 20th	Day	7% 12% 3 7% 7% 6% 5% 4 5% 6% 38%	1477
	21st to end	Day	4 12% 4 9% 5% 5% 5% 5% 8% 9% 34%	1330
December	1st to 10th	Day	6% 15% 5% 10% 5% 6% 6% 4 5% 6% 31%	1330
	11th to 20th	Day	4 12% 5% 12% 7% 6% 6% 5% 6% 8% 31%	1682
	21st to end	Day	4 8% 4 8% 5% 4 5% 4 6% 8% 44%	2442
January	1st to 10th	Night	4 13% 5% 12% 8% 6% 5% 5% 7% 10% 27%	4726
	11th to 20th	Night	3 9% 6% 13% 6% 6% 7% 5% 6% 7% 33%	4699
	21st to end	Night	4 10% 4 11% 7% 5% 4 4 6% 8% 38%	4890
February	1st to 10th	Night	5% 11% 4 14% 7% 6% 6% 5% 6% 7% 30%	2644
	11th to 20th	Night	5% 14% 5% 10% 6% 5% 5% 5% 7% 7% 30%	2402
	21st to end	Night	4 11% 5% 11% 7% 6% 6% 4 7% 8% 32%	1948
March	1st to 10th	Night	3 10% 5% 14% 9% 7% 5% 5% 7% 8% 26%	3736
	11th to 20th	Night	8% 8% 2 6% 5% 4 5% 4 6% 8% 43%	3495
	21st to end	Night	9% 21% 7% 13% 7% 4 5% 4 5% 6% 18%	3809
April	1st to 10th	Night	9% 13% 4 9% 6% 6% 5% 4 7% 9% 29%	1829
	11th to 20th	Night	9% 12% 5% 9% 6% 6% 5% 4 6% 7% 31%	1691
	21st to end	Night	11% 11% 4 7% 6% 5% 5% 4 8% 8% 31%	1710
May	1st to 10th	Night	19% 11% 4 8% 5% 5% 5% 4 5% 5% 30%	2628
	11th to 20th	Night	9% 7% 3 8% 5% 6% 6% 5% 7% 11% 33%	2199
	21st to end	Night	15% 9% 3 10% 7% 6% 6% 5% 6% 7% 26%	2414
June	1st to 10th	Night	14% 9% 2 8% 6% 4 6% 5% 8% 7% 31%	1330
	11th to 20th	Night	15% 8% 4 7% 5% 5% 6% 6% 7% 8% 30%	1330
	21st to end	Night	8% 6% 2 5% 5% 4 3 6% 8% 8% 44%	1330
July	1st to 10th	Night	6% 4 2 6% 5% 5% 6% 5% 6% 8% 47%	2220
	11th to 20th	Night	4 3 2 5% 4 4 5% 5% 9% 12% 47%	2220
	21st to end	Night	4 3 1 5% 3 4 5% 4 8% 8% 56%	2475
August	1st to 10th	Night	8% 7% 2 6% 5% 5% 5% 5% 7% 9% 43%	2529
	11th to 20th	Night	6% 5% 2 7% 5% 5% 5% 4 8% 12% 42%	2806
	21st to end	Night	9% 3 1 4 4 3 5% 4 6% 9% 53%	3146
September	1st to 10th	Night	9% 6% 3 5% 4 4 5% 4 5% 7% 49%	1811
	11th to 20th	Night	13% 7% 2 5% 4 4 3 3 5% 7% 47%	2090
	21st to end	Night	9% 5% 2 5% 3 4 5% 5% 6% 8% 48%	2079
October	1st to 10th	Night	11% 8% 3 6% 5% 4 4 4 6% 8% 41%	2074
	11th to 20th	Night	9% 8% 3 6% 5% 4 5% 4 7% 8% 41%	2268
	21st to end	Night	7% 11% 3 8% 5% 5% 5% 5% 7% 7% 37%	2574
November	1st to 10th	Night	9% 13% 4 8% 5% 6% 5% 4 5% 8% 34%	2529
	11th to 20th	Night	5% 11% 3 8% 7% 6% 5% 5% 6% 8% 37%	2702
	21st to end	Night	5% 12% 4 8% 6% 5% 5% 4 7% 9% 36%	2850
December	1st to 10th	Night	5% 13% 5% 10% 7% 6% 5% 4 6% 6% 33%	2850
	11th to 20th	Night	3 10% 5% 12% 7% 6% 6% 4 6% 9% 32%	3510
	21st to end	Night	4 9% 4 7% 5% 6% 5% 4 6% 7% 44%	5214

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	47% 7% 14% 22% 9% 1	2219
	11th to 20th	Day	47% 7% 11% 16% 16% 4	2224
	21st to end	Day	41% 7% 15% 29% 8% 1	2794
February	1st to 10th	Day	53% 7% 14% 15% 9% 2	1535
	11th to 20th	Day	51% 7% 14% 19% 8% 1	1674
	21st to end	Day	52% 6% 12% 17% 12% 1	1503
March	1st to 10th	Day	50% 7% 13% 18% 12% 1	3138
	11th to 20th	Day	35% 7% 18% 26% 13% 1	3484
	21st to end	Day	62% 6% 12% 15% 5%	3841
April	1st to 10th	Day	51% 8% 14% 18% 8% 1	2351
	11th to 20th	Day	49% 6% 12% 22% 10% 1	2440
	21st to end	Day	50% 6% 13% 21% 10% 1	2470
May	1st to 10th	Day	55% 6% 12% 22% 5%	4335
	11th to 20th	Day	42% 9% 13% 25% 10%	4698
	21st to end	Day	51% 9% 13% 20% 6% 1	5162
June	1st to 10th	Day	46% 8% 15% 21% 9% 1	2850
	11th to 20th	Day	50% 8% 14% 19% 7% 2	2850
	21st to end	Day	33% 8% 20% 28% 9% 2	2850
July	1st to 10th	Day	27% 6% 19% 29% 16% 3	4666
	11th to 20th	Day	25% 6% 19% 36% 13% 1	4659
	21st to end	Day	19% 5% 13% 30% 24% 8%	4902
August	1st to 10th	Day	36% 6% 15% 34% 8% 1	4384
	11th to 20th	Day	32% 6% 23% 31% 8% 1	4072
	21st to end	Day	29% 5% 19% 34% 13% 1	4520
September	1st to 10th	Day	33% 5% 15% 27% 15% 6%	2365
	11th to 20th	Day	40% 5% 14% 26% 12% 2	2090
	21st to end	Day	36% 6% 15% 28% 12% 4	2069
October	1st to 10th	Day	43% 6% 14% 26% 9% 1	1886
	11th to 20th	Day	39% 8% 14% 27% 10% 2	1689
	21st to end	Day	44% 8% 13% 23% 11% 1	1782
November	1st to 10th	Day	53% 6% 11% 20% 8% 2	1531
	11th to 20th	Day	47% 5% 14% 24% 9% 1	1477
	21st to end	Day	42% 8% 16% 23% 8% 2	1330
December	1st to 10th	Day	53% 5% 12% 17% 10% 3	1330
	11th to 20th	Day	49% 7% 14% 23% 6% 1	1684
	21st to end	Day	35% 7% 11% 24% 18% 5%	2442
January	1st to 10th	Night	51% 7% 13% 21% 6% 2	4726
	11th to 20th	Night	49% 6% 11% 16% 15% 3	4740
	21st to end	Night	43% 6% 14% 25% 11% 1	4890
February	1st to 10th	Night	50% 6% 12% 19% 11% 1	2644
	11th to 20th	Night	50% 6% 13% 20% 11% 1	2403
	21st to end	Night	46% 7% 12% 22% 11% 2	1948
March	1st to 10th	Night	50% 8% 12% 21% 8% 1	3736
	11th to 20th	Night	36% 9% 18% 24% 12% 1	3495
	21st to end	Night	65% 6% 12% 12% 4	3809
April	1st to 10th	Night	49% 7% 13% 19% 11%	1829
	11th to 20th	Night	49% 8% 12% 21% 9% 1	1691
	21st to end	Night	46% 8% 13% 22% 9% 2	1710
May	1st to 10th	Night	55% 5% 12% 21% 6% 1	2628
	11th to 20th	Night	40% 10% 13% 27% 10% 1	2199
	21st to end	Night	53% 8% 16% 17% 5% 1	2414
June	1st to 10th	Night	47% 9% 13% 20% 9% 1	1330
	11th to 20th	Night	48% 9% 17% 17% 7% 2	1330
	21st to end	Night	34% 9% 21% 27% 9% 1	1330
July	1st to 10th	Night	33% 6% 18% 24% 18% 1	2185
	11th to 20th	Night	26% 7% 18% 37% 8% 4	2178
	21st to end	Night	23% 7% 12% 30% 22% 6%	2397
August	1st to 10th	Night	36% 6% 15% 30% 13% 1	2521
	11th to 20th	Night	33% 6% 23% 28% 9% 1	2833
	21st to end	Night	28% 5% 19% 34% 12% 3	3146
September	1st to 10th	Night	33% 6% 14% 26% 15% 6%	1811
	11th to 20th	Night	37% 5% 15% 26% 13% 4	2090
	21st to end	Night	32% 7% 17% 27% 15% 4	2079
October	1st to 10th	Night	40% 6% 14% 25% 13% 2	2074
	11th to 20th	Night	38% 8% 15% 24% 12% 3	2268
	21st to end	Night	43% 8% 14% 25% 10% 1	2574
November	1st to 10th	Night	49% 6% 12% 21% 10% 2	2529
	11th to 20th	Night	44% 6% 16% 21% 13% 1	2702
	21st to end	Night	43% 7% 18% 21% 9% 2	2850
December	1st to 10th	Night	50% 5% 13% 20% 9% 3	2850
	11th to 20th	Night	46% 8% 19% 22% 6% 1	3507
	21st to end	Night	38% 5% 12% 26% 15% 4	5214

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	6% 3 30% 34% 17% 10%	180
	11th to 20th	Day	7% 6% 29% 32% 20% 6%	180
	21st to end	Day	8% 7% 28% 37% 17% 4	198
February	1st to 10th	Day	13% 8% 25% 37% 12% 5%	187
	11th to 20th	Day	9% 7% 35% 32% 14% 3	270
	21st to end	Day	7% 7% 30% 36% 16% 4	315
March	1st to 10th	Day	10% 10% 31% 32% 13% 5%	460
	11th to 20th	Day	9% 10% 30% 33% 14% 5%	460
	21st to end	Day	12% 11% 34% 22% 16% 6%	506
April	1st to 10th	Day	13% 10% 32% 28% 14% 2	460
	11th to 20th	Day	12% 9% 29% 27% 18% 5%	452
	21st to end	Day	15% 14% 27% 25% 16% 4	460
May	1st to 10th	Day	18% 9% 22% 24% 21% 6%	529
	11th to 20th	Day	14% 6% 20% 32% 23% 5%	675
	21st to end	Day	16% 7% 20% 28% 23% 8%	734
June	1st to 10th	Day	14% 8% 24% 31% 19% 4	680
	11th to 20th	Day	17% 8% 28% 30% 15% 2	680
	21st to end	Day	10% 4 23% 33% 23% 8%	680
July	1st to 10th	Day	10% 4 17% 35% 26% 9%	671
	11th to 20th	Day	10% 4 18% 35% 25% 8%	668
	21st to end	Day	5% 4 20% 33% 27% 10%	699
August	1st to 10th	Day	8% 4 22% 34% 22% 9%	487
	11th to 20th	Day	8% 1 19% 34% 29% 8%	477
	21st to end	Day	10% 6% 19% 30% 21% 12%	539
September	1st to 10th	Day	12% 6% 23% 31% 21% 6%	490
	11th to 20th	Day	18% 6% 20% 29% 21% 6%	490
	21st to end	Day	12% 12% 30% 28% 13% 5%	483
October	1st to 10th	Day	18% 8% 19% 28% 20% 7%	356
	11th to 20th	Day	10% 7% 34% 25% 18% 6%	270
	21st to end	Day	14% 13% 32% 28% 10% 2	297
November	1st to 10th	Day	16% 11% 30% 29% 12% 3	270
	11th to 20th	Day	15% 8% 37% 26% 13% 1	249
	21st to end	Day	5% 7% 36% 30% 12% 10%	180
December	1st to 10th	Day	5% 9% 37% 32% 13% 4	180
	11th to 20th	Day	6% 12% 34% 34% 13% 1	180
	21st to end	Day	8% 4 24% 32% 27% 4	198
January	1st to 10th	Night	8% 7% 34% 37% 12% 2	950
	11th to 20th	Night	9% 6% 36% 35% 13% 1	950
	21st to end	Night	11% 8% 33% 33% 13% 2	851
February	1st to 10th	Night	9% 8% 30% 34% 17% 3	760
	11th to 20th	Night	11% 6% 32% 37% 14% 2	744
	21st to end	Night	7% 4 30% 35% 20% 5%	628
March	1st to 10th	Night	9% 4 31% 36% 17% 3	760
	11th to 20th	Night	9% 7% 35% 35% 13% 2	760
	21st to end	Night	14% 6% 28% 34% 15% 2	836
April	1st to 10th	Night	13% 7% 29% 35% 15% 1	689
	11th to 20th	Night	15% 6% 31% 35% 13%	563
	21st to end	Night	15% 7% 31% 37% 10%	570
May	1st to 10th	Night	17% 7% 26% 34% 16% 1	508
	11th to 20th	Night	12% 3 25% 38% 20% 1	374
	21st to end	Night	18% 6% 23% 35% 17% 1	411
June	1st to 10th	Night	17% 7% 24% 37% 13% 2	380
	11th to 20th	Night	14% 6% 31% 33% 15% 1	380
	21st to end	Night	11% 3 23% 39% 23% 1	380
July	1st to 10th	Night	10% 4 13% 45% 27% 1	370
	11th to 20th	Night	8% 3 20% 44% 23% 2	368
	21st to end	Night	4 3 22% 43% 26% 2	416
August	1st to 10th	Night	9% 3 16% 44% 25% 2	378
	11th to 20th	Night	8% 4 18% 47% 22% 1	559
	21st to end	Night	10% 4 22% 39% 25% 1	627
September	1st to 10th	Night	11% 3 22% 40% 22% 2	638
	11th to 20th	Night	16% 4 18% 38% 23% 1	760
	21st to end	Night	10% 5% 30% 38% 16% 2	756
October	1st to 10th	Night	15% 4 24% 37% 17% 2	720
	11th to 20th	Night	13% 5% 28% 36% 14% 3	720
	21st to end	Night	10% 7% 34% 35% 13% 1	792
November	1st to 10th	Night	15% 7% 34% 33% 10% 1	864
	11th to 20th	Night	9% 8% 41% 30% 10% 2	950
	21st to end	Night	10% 5% 36% 35% 12% 2	950
December	1st to 10th	Night	10% 7% 31% 34% 16% 2	950
	11th to 20th	Night	7% 6% 37% 37% 11% 2	950
	21st to end	Night	12% 4 30% 36% 15% 2	1045

Page generated at 2024-09-26 07:52:20. Version ac07

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