No this is not an EKG, but rather a look at the dew point, temperature and heat index arrhythmia over time? ;)
The GFS BUFKIT OVERVIEW shows a lot of things and I'll point them out to you.
Let us start with the green dew point line on the bottom...
It shows dew point temperatures rising from the comfy 50s we have been enjoying into the uncomfortable 70s by the weekend.
The second line up which is the temperature line, shows a spike early on, them temperatures held back a bit over the weekend and then rising well into the middle and upper 90s next week. Yuck!
The line above that is the heat index or how hot it feels. At this time, despite the warm up next week, feels like temperatures won't be too extreme thanks to slightly lower dew point temperatures.
Since I brought up the heart, I wanted to share some information about how exerting your body on a hot and humid day can impact you:
How hot weather affects your physiology
When you exercise, your blood gets pumping to deliver adequate oxygen to your muscles. This leads to a rise in body temperature. When you're getting physical in above-average temperatures, this effect is more intense, because your heart sends large amounts of blood to the skin in an
attempt to cool it down and, in turn, leaves less blood in the muscles, further speeding the heart rate.
What's more, in humid weather, sweat is sometimes unable to do its job because it doesn't evaporate as efficiently as it does in a drier climate. This may drive body temperatures to dangerous extremes.
There are three basic levels of heat-related illness: heat cramps, heat exhaustion and heatstroke. You should be familiar with the signs of each of these conditions so you can identify them when you or someone else may be in danger of overheating.
Jill Gilardi Fox 6 Meteorologist
We are tracking a trough to the west and tropical moisture that will be on the increase as the weekend approaches. The two combined will result in the development of scattered showers and storms. The greatest chance for rain is on Saturday and then rain chances lower as we near Labor Day.
Drought conditions continue to expand and worsen across the state. A new area of moderate drought has developed across portions of Clay and Tallapoosa counties and a small portion of Coosa, Randolph and Chambers. Rainfall for the year is below by a little over two inches in Anniston, almost five inches in Birmingham and close to two inches in Tuscaloosa.
You may have to adjust your outdoor plans this upcoming Labor Day Weekend and or bring rain gear to any outdoor event, game etc. you are attending. The timing is poor but we need the rain and computer models indicate the possibility of some areas getting up to one inch by Sunday morning.
TIMING THE RAIN CHANCES:
Saturday morning: A few passing showers are possible, otherwise dry. Best chance will be along and to the west of the I-59/I-20 corridor.
Saturday afternoon: The steadiest zone of precipitation sets up across the northwest corner of the state and it could be quite a damp day across Marion county for example. East of the main rain axis, we'll see more scattered showers and storms developing and tracking from southwest to northeast.
Jill Gilardi Fox 6 Meteorologist
It's been nice while it has lasted...low humidity/dewpoint levels...but a subtle change begins today. Southeast winds (instead of out of the northeast) will slowly bring moisture out of the Gulf of Mexico our way.
Humidity/dewpoint levels are expected to rise going into Friday. That means the sticky "air you can wear" typical to the south in Summer will be back.
Meanwhile, an upper level trough is expected to move into the Ohio and Tennessee vallies out of the plains states. These two forces should combine to give us a decent chance of rainfall for Labor Day Weekend 2014.
The chance of rainfall and additional cloud cover should help to moderate our temps into the low 90s to upper 80s for the weekend and into next week. Variably cloudy skies along with scattered showers and storms should be with us Monday through next Friday.
Mickey Ferguson, FOX6 Weather
In my first blog on humidity, found http://birmingham.raycomweather.com/2014/08/humidity-part-1.html, I discussed relative humidity, dew point, mixing ratio, and other neat stuff about the amount of water vapor actually in the air (it may be more than you think). This blog will have some other neat facts about humidity and how it affects temperature, etc.
1. Warming/cooling outdoors
When water is evaporated (from liquid to vapor), that uses heat energy. When water is condensed (from vapor to liquid), that releases heat energy.
So, when the ground is very moist and the sun in shining, some of the sun energy must be used to evaporate water, while the rest of it goes into heating the ground (and then the air above the ground). This is one reason it did not get as hot last week as the computer models were saying, in my view. The ground was still moist enough from recent rains, the trees and grass somewhat moist and green, etc. Recall in 2007 when it reached 100 degrees day after day here in Alabama, we had been in a drought for weeks also. The ground was very dry, and almost all the sun energy went into heating the ground and the air, not into evaporating water.
The same goes for cooling at night, but slightly in a different way. On a clear night, once the temperature drops to the dewpoint, moisture begins to condense as dew and/or fog. That condensation releases heat, slowing the cooling of the ground and the air. The temperature can drop below where the dewpoint is at midnight, etc., because when moisture condenses out of the air, the air has less moisture and the dewpoint drops. But, the release of heat by dew and fog typically doesn't allow the temperature to fall very far below the dewpoint.
2. Air conditioner performance
In the previous blog, I talked about how a humid house can have as much as 3 gallons or more of water vapor in it, and on a humid day I measured an air conditioner condense a liter of water out of the air in 7 minutes.
That condensation of water on your air conditioner coil releases heat, and reduces the cooling of your air conditioner. However, since a lot of that condensed water drops off your coil and runs outside in a condensate line, the air conditioner is reducing the humidity in your house.
Looking at some numbers, suppose the temperature starts out at 85 and the dewpoint starts out at 70 inside your 2,000 square foot house when you've been gone on vacation. You turn on your 3-ton air conditioner (that performs 36000 Btu/hr of cooling, or 38,000 kilojoules per hour). Suppose that your air conditioning coil removes 0.5 gallons (about 2 kg) per hour. The condensation of that water out of the air releases 4500 kilojoules of heat back into the air. So, the amount of cooling your air conditioner can do is 38,000 (total output) minus 4,500 (water condensation) kJ/hr. The removal of moisture reduces the efficiency of cooling of your air conditioner by about 10%. On a dry day, your air conditioner is more efficient at cooling the air.
One more note...have you ever noticed when it rains outside it suddenly gets cooler inside when the air conditioning is on? That is partially due to the fact that the outside coil of your air conditioner (where the heat is being released) gets wet. So, not only is blowing cooler air over the coil cooling it, but evaporation of water is cooling it, allowing for more cooling from your air conditoner.
3. Heat index
Humidity makes it feel hotter outside than it really is. This is because your body naturally cools itself by evaporation of sweat (that uses heat). But, if the humidity is higher, your sweat will not evaporate as efficiently, and it feels hotter. You also feel like you sweat more. Maybe you do a little bit, but even if you didn't the sweat wouldn't be evaporating as quickly and you'd feel more of it.
When it's hot and humid, people tend to drink more water, maybe because they notice their own sweat. When it's hot but very dry, you have to be extra careful not to get dehydrated, because you're staying cooler as the sweat evaporates but you don't notice all the water you're losing to sweat.
I hope some of the information in these two blogs has been helpful.
Dr. Tim Coleman
We began with cooler temps this morning...but are expected to end up a little warmer this afternoon than yesterday.
Under mostly sunny skies, highs today could make it in the mid-90s into west Alabama...near 93 into central Alabama...and close to 90-degrees into east Alabama.
Humidity/dewpoint levels should remain low today.
Temps remain warm until the end of the workweek with highs in the mid-90s and lows around 70.
Labor Day Weekend brings a decent chance of showers and thunderstorms. (Don't kill the messenger, please) Moisture levels should be high enough for us to see scattered showers and storms in the afternoon to evening hours. A shortwave trough moving into the Tennessee Valley will also help increase our rain potential. The additional cloud cover and rain will likely keep temps a little cooler this weekend, but the humidity/dewpoint levels will, of course, be high again.
Mickey Ferguson, FOX6 Weather
It feels much nicer outside today than it has! It's not as hot, and also importantly it's not as humid. On Sunday morning, in an area outdoors the size of a 2,000 square foot house, there were 3.2 gallons of water vapor in the air! Today, in that same volume, there's only about 2 gallons. People talk about humidity and moisture a lot. Weather people use terms like relative humidity, dew point, etc. But many people don't know what a given dewpoint or humidity means, in terms of the amount of water vapor actually in the air. You might be surprised just how much moisture is in the air (and in the soil), and how much it effects the high and low temperature and the performance of your air conditioner, and how big of a difference there is in actual humidity between summer and winter. Some are also surprised how much heat is released/absorbed when water condenses/evaporates.
This is the first of two blogs on humidity. Here, I'll explain dew point (and why we meteorologists love to use it), relative humidity, and the mixing ratio (the actual amount of water vapor in the air), and some other neat stuff. In blog 2, I'll discuss how humidity and moisture affect the warming/cooling of the air outside, the performance of your air conditioner, and how it feels to you outside (heat index).
1. Dew point and mixing ratio
The dewpoint is the temperature at where water would begin to condense if the air were cooled to that temperature. It is dependent on the amount of moisture in the air, not on the temperature (like relative humidity is). That's why we meteorologists like it so much. Below is a chart of mixing ratio (or the amount of water vapor in the air in grams of water vapor per kilogram of dry air) vs. dewpoint for standard atmospheric pressure.
Notice that at a dewpoint of 20 degrees (common in cold, dry air in winter), there are only 2.3 grams of water for every kilogram of dry air. You can show that means only 0.23% of the air is actually water vapor. Now look at a typical summertime dewpoint of 70 degrees. The mixing ratio then is 15.7 grams per kilogram, almost 7 times as much water vapor as at a dewpoint of 20 degrees, but even on a humid day like that, only 1.55% of the air is actually composed of water vapor. The rest of it is nitrogen, oxygen, carbon dioxide, etc.
2. Temperature and saturated mixing ratio
Warmer air can hold more water vapor than cooler air at the same pressure. Maybe it's because the molecules are more spread out in warm air, making more room for water. The amount of water vapor the atmosphere can hold at a given temperature is known as the saturation mixing ratio. It is shown below.
The graph is the same as the one of mixing ratio vs. dewpoint! That will make sense when we talk about relative humidity. But, notice that hot air at 90 degrees can hold up to 30.9 grams of water per kg air, while air at 50 degrees can only hold 7.6 g/kg. It's a big difference.
3. Relative humidity
Meteorologists don't like to deal with relative humidity as much, because it depends on temperature and water vapor. The relative humidity is the mixing ratio (based on dewpoint) divided by the saturation mixing ratio (based on temperature).
For example, suppose on an early morning, the temperature is 55 degrees, and the dewpoint is 53 degrees. The mixing ratio at a dewpoint of 53 F is 8.5 g/kg, but the saturation mixing ratio at 55 F is 9.2 g/kg (both can be determined from either chart above since they are the same), meaning the relative humidity is 8.5/9.2 = 92%. Sounds pretty humid. But, with no change in the actual amount of moisture, the temperature warms to 70, where it can hold up to 15.7 g/kg (sat. mix. ratio). Without any change in the amount of moisture in the air, the relative humidity is then 8.5/15.7 = 54%.
So, the relative humidity may fluctuate a lot during the day, even if the actual humidity does not. That does not mean relative humidity is not useful. It affects how the air feels, and it also affects things like mold growth, water evaporation potential, etc. It is just not the absolute measure that dewpoint is.
4. Neat stuff
Knowing about mixing ratio, etc., helps us figure out a lot about the amount of water vapor, that could condense and fall out as rain, in the air. One can sum up the mixing ratio over a square foot of ground all the way up to the top of the atmosphere and determine how much precipitable water is available to rain out onto the ground in a thunderstorm, etc. On days with deep tropical moisture, the precipitable water may be 2.5". In a cold dry air mass in winter, it may only be 0.2".
I had to put a new pipe on my condensation line that runs out from my air conditioner coil recently. I was fascinated at the amount of water that was pouring out. All that water was being condensed as the humid air inside the house passed across the 45 degree coil and some chunks of air became saturated. It only took 7 minutes for 1 liter of water to come out. That's about 2 gallons per hour!
How much water vapor is in your house in the summer? If one left the air conditioner off for a while and the dewpoint got up to 70 degrees (similar to outside), we can figure it out. For a 2,000 square foot house with 9' ceilings, that's about 558 cubic meters. Air has a density close to 1.19 kg per cubic meter at comfortable room temp, so we'll assume that. Doing the math, that's 664 kg of air in your house (or 1,460 pounds). At a dewpoint of 70, the mixing ratio is 15.7 g/kg, and doing some more math, that shows about 10 kg of water vapor in your house (about 22 pounds, or 2.75 gallons). Fortunately, a lot of that water vapor is condensed out of the air in the air conditioner and goes outside in the condensation pipe, keeping the dewpoint lower than outside and the humidity reasonable if the air conditioner is working properly. Still, 2.75 gallons is a lot of water to have in the air in one house!
One final thing. Can you have 95 degrees with 95% humidity? People say that a lot, but it doesn't really happen around here. The world record dewpoint of 95 F occurred in Saudi Arabia, so maybe there in a few instances. In BHM, the average dewpoint in the summer, based on records going back to 1948, is 68. Even on very humid days the dewpoint is usually only 75 or 76. The highest dewpoint ever recorded at the BHM airport was 83 F on July 12, 1989, after a very rainy June (lots of water probably evaporating out of the ground). So in BHM, even if we had our all-time record dewpoint of 83 F, at a temperature of 95 F the relative humidity would only be 68%.
More on humidity in part 2 later this week.
Dr. Tim Coleman
I've got to give a big shout out to some of my friends in Tuscaloosa and Pickens counties! On Monday I visited the 6th graders at Sipsey Valley Middle School. In one of the pictures you can see weather watcher Ronald Hughes from Coker. He lives near the school and was sharing information about how he monitors the weather and reports to our weather team. The teachers also presented another yummy cake! Afterwards the Storm Tracker was back on the road to Reform in Pickens County for a severe weather safety presentation. I want to thank our friends at the Turtle Creek Community Center for the invite to Reform. I drove through Reform quite often when I was studying Meteorology at Mississippi State.
Fox6 Meteorologist Wes Wyatt
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