# Weather 101 Lesson 2 – Pressure, Temperature, and Moisture

By: Randy Bynon , 5:07 AM GMT on November 22, 2006

The podcast for this lesson can be found at

http://www.bynon.cc/blog/weather101_lesson_2.mp3

In our last lesson, I talked about how the earth is heated by the sun. But since the equator gets most of the heat and the poles get almost none, the heat has to spread poleward. To understand what happens when the heat tries to move poleward, you have to understand a few things about the atmosphere in order to know how it will behave.

There are three primary properties of our atmosphere that affect the weather. They are temperature, pressure, and water vapor. Each has a huge affect on the state of the atmosphere and on each other. Let’s look at these properties. Since we’re talking about the atmosphere, we’ll apply these definitions to the air around us.

PRESSURE

Pressure is probably the simplest property of the three to understand. The pressure of a parcel of air in our atmosphere is affected by a number of factors including altitude, temperature, and water content. Air has mass and is affected by gravity meaning it has weight. The atmosphere is like an ocean of air. The deeper you go into the ocean of air, the greater the pressure of the air around you. The higher the pressure of the air, the greater it‘s density. At sea level, the average atmospheric pressure is 1013.2mb or 29.92 inches of mercury.

TEMPERATURE

What is temperature? Most people would say it’s a measure of the amount of heat in the air (or any other substance for that matter). But in reality, the measure of heat is the calorie (and you thought that was the measure of fat around your waist). Temperature is actually the measure of the average energy in a substance. Physically, it’s a measure of the motion of the molecules in a substance. As molecules of air are heated, they speed up. As they speed up, they bounce off one another and they move farther apart. This results in a decrease in density. Less dense air is buoyant and tends to rise. But it also works the other way around. If you remove heat from molecules of air, they slow down and become more tightly packed. This results in an increase in the density of the air. Dense air tends to sink. So what affect does pressure have on temperature? It’s actually pretty easy to see in the atmosphere. As you rise up in the atmosphere, pressure decreases and therefore so does the density. This results in a lower temperature. As you increase pressure, molecules are forced closer together and begin to bounce off one another. This causes them to speed up resulting in an increase in temperature. Changing the temperature of the air by increasing or decreasing pressure rather than by adding or removing heat energy is referred to as adiabatic cooling or heating.

WATER VAPOR

Water vapor occurs naturally in our atmosphere. It is water molecules suspended in the air and invisible to the eye. Water evaporates from our oceans, lakes, rivers, and streams into the air as water vapor. Under the right conditions, the water vapor condenses into visible water droplets that we call clouds. If the condensation process continues, the droplets get bigger until they can no longer be suspended in the atmosphere and they fall out as rain or snow and are added once again to the oceans, lakes, rivers, and streams. This entire process is called the water cycle.

The temperature of the air determines how much water vapor can be evaporated into it. It acts much like a suspension in fluids. Did you ever do the experiment in school where you dissolved sugar into a beaker of water until no more would dissolve into it? Then you would suspend a string in the beaker and place the beaker in the freezer or refrigerator. As the water cooled, it was not able to support as much sugar in suspension. So the excess sugar crystallized on the string as rock candy. The air works in much the same way but it doesn’t make rock candy! It is able to hold more water vapor at higher temperatures than it can at lower temperatures. Many people think that relative humidity tells you how much water vapor is in the air. In reality, it doesn’t. To meteorologists, RH means very little. What it actually tells you is how close the air is to holding all the water vapor it can hold. If you evaporate enough water into 40 degree F air to bring it to 100% RH (i.e. you saturate the air so that it can’t hold anymore water), then warm the air to 60 degrees F without adding or removing water vapor, the RH will decrease to near 50%. This is because air at 60 degrees F is capable of holding nearly twice as much water vapor as air at 40 degrees F. So what happens if you saturate the air at 60 degrees F and then cool it to 40 degrees F? Well, if the air is saturated at 60F, the moment it cools below 60F, condensation will begin to take place until water droplets form and excess water will precipitate out as rain.

An important property of water is that it is lighter than dry air and has a higher specific heat than dry air. What this means is that it takes more energy to warm up moist air than dry air. It also means that moist air contains more latent heat than dry air and is less dense. That becomes important in later lessons.

PUTTING IT ALL TOGETHER

One important thing to remember is that all these properties affect one another. In the next lesson we’ll see how these properties come into play as heat is moved from the equator to the poles in our atmosphere.

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##### 200. Randy Bynon , Dropsonde SysOp/AVAPS PM
10:49 PM GMT on December 01, 2006
 hurrycanegal, I am still there. But now I'm the Gulf Coast Winter Weather blog rather than the tropical weather blog. Figured I'd switch once the season was over.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 199. cgableshurrycanegal
10:25 PM GMT on December 01, 2006
 Hi Randy!How come they've moved you out of the upper echelon? :(::G:: We still find you anywhere!Thanks for confirming what I remembered about average bp in the tropics... don't know that FL qualifies...
Member Since: July 12, 2005 Posts: 24 Comments: 212
##### 198. Raysfan70
11:06 AM GMT on November 30, 2006
 Good Morning {{Randy}}!Make it a Great Thursday. :-)
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 197. EmmyRose
5:46 AM GMT on November 30, 2006
 We're going from 70 degrees to 40 degrees on Thursday with storms and big winds woohoo
Member Since: July 15, 2005 Posts: 347 Comments: 76406
##### 196. Raysfan70
8:52 PM GMT on November 29, 2006
 Good Afternoon {{Randy}}.I thought that when I got home today that I would be the last one to comment on a new blog. Don't work top hard.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 195. EmmyRose
1:11 PM GMT on November 29, 2006
 GOOD MORNING!
Member Since: July 15, 2005 Posts: 347 Comments: 76406
##### 194. Raysfan70
10:37 AM GMT on November 29, 2006
 Good Morning {{Randy}}.Have a Great Wednesday. :-)
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 193. EmmyRose
7:35 AM GMT on November 29, 2006
 Sweet Dreams!
Member Since: July 15, 2005 Posts: 347 Comments: 76406
##### 192. Raysfan70
7:32 PM GMT on November 28, 2006
 Good Afternoon {{Randy}}.Weather feels like a summer day. Cloudy with some rain on and off this morning.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 191. EmmyRose
3:10 PM GMT on November 28, 2006
 Morning O Great Teacher!!!!!!!!!!!!!
Member Since: July 15, 2005 Posts: 347 Comments: 76406
##### 190. Randy Bynon , Dropsonde SysOp/AVAPS PM
12:26 PM GMT on November 28, 2006
 Yup.. I'll try to get that up today!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 189. Raysfan70
11:49 AM GMT on November 28, 2006
 Time for a new Lesson. I'm ready.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 188. Randy Bynon , Dropsonde SysOp/AVAPS PM
11:28 AM GMT on November 28, 2006
 Good morning!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 187. Raysfan70
10:35 AM GMT on November 28, 2006
 Good Morning {{Randy}}.Make it a Great Tuesday. :-)
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 186. Raysfan70
8:03 PM GMT on November 27, 2006
 Good Afternoon {{Randy}}.Beautiful weather day here. Please don't let it change.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 185. Randy Bynon , Dropsonde SysOp/AVAPS PM
6:33 PM GMT on November 27, 2006
 I will be posting a discussion in the next day or so on the current weather and the forecast over the rest of this week.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 184. weathermantrey
4:46 PM GMT on November 27, 2006
 is there ever going to be in discussion on this blog about actual winter weather events? I was looking forward to hearing your thoughts on the system that could bring some wintry precip to northern mississippi, alabama, arkansas, and possibly northern louisiana this week.
##### 183. Raysfan70
11:53 AM GMT on November 27, 2006
 Good Morning {{Randy}}!Make it a Great WUBAday. :-)
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 182. Randy Bynon , Dropsonde SysOp/AVAPS PM
11:27 AM GMT on November 27, 2006
Member Since: July 17, 2001 Posts: 190 Comments: 2012
7:50 AM GMT on November 27, 2006
 Have a great Monday!
Member Since: April 29, 2006 Posts: 10 Comments: 7042
##### 179. Raysfan70
12:44 AM GMT on November 27, 2006
 Good Evening {{Randy}}!Hope that you and Your Family had a Great Holiday Weekend.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 177. Raysfan70
3:15 PM GMT on November 26, 2006
 Yeap football. It's Sunday.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 176. Randy Bynon , Dropsonde SysOp/AVAPS PM
3:11 PM GMT on November 26, 2006
 Good morning {{{{ Rays }}}}!!!Are there football games on today?? ;-)
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 175. Raysfan70
11:26 AM GMT on November 26, 2006
 Good Morning {{Randy}}!Have a Great Football Sunday. :-)
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 174. EmmyRose
5:09 PM GMT on November 25, 2006
 EMMY SHIVERING UP NORTH - SELF PORTRAIT!
Member Since: July 15, 2005 Posts: 347 Comments: 76406
##### 173. seflagamma
4:42 PM GMT on November 25, 2006
 Randy,Good Saturday morning to you!!!!!Have a little time to blog this morning and hoping to make it to a few of my friend's blogs as I can before the busy stuff starts!Have a wonderful day!
Member Since: August 29, 2005 Posts: 305 Comments: 41026
##### 172. Randy Bynon , Dropsonde SysOp/AVAPS PM
3:40 PM GMT on November 25, 2006
 Good morning bug!!! And you to Rays!! Thanks! It looks like the weather should be beautiful here today! Sunny with highs in the low 70s!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 171. Raysfan70
12:35 PM GMT on November 25, 2006
 Good Morning {{Randy}}.Have a Great Saturday. :-)Warm Sunshine to you my friend.Picture Courtesy of Mobal.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 170. palmettobug53
12:10 PM GMT on November 25, 2006
 Morning, Randy!
Member Since: October 7, 2005 Posts: 245 Comments: 26014
##### 169. EmmyRose
4:40 AM GMT on November 25, 2006
 GOOD MORNING GOOD EVENING THE DAY AFTER THANKSGIVING
Member Since: July 15, 2005 Posts: 347 Comments: 76406
##### 168. Randy Bynon , Dropsonde SysOp/AVAPS PM
8:51 PM GMT on November 24, 2006
 If that turns out to be the case cyclone, I look forward to being proved wrong! Once again, good luck!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 167. cyclonebuster
8:47 PM GMT on November 24, 2006
 Thats ok Randy. Once it is all out there, and figured out. They will change your mind once you see how good they are for the Earth!
Member Since: January 2, 2006 Posts: 133 Comments: 20680
##### 166. Randy Bynon , Dropsonde SysOp/AVAPS PM
8:43 PM GMT on November 24, 2006
 No hard feelings on my end cyclone. But sorry... I don't believe in your idea. I don't think it'll work and even if I did, I think it'll do more harm than good. As such, I'd rather not contribute to your cause.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 165. Randy Bynon , Dropsonde SysOp/AVAPS PM
8:41 PM GMT on November 24, 2006
 bigtrucker, that's a good question and one that I'll cover in more detail in a future lesson but it does warrant some detail now too. This is something of a review of basic physical science but it is indeed VERY important in tropical weather as well as mid-latitude severe thunderstorm development. I'll limit this discussion to tropical weather for the moment.When a tropical system begins to develop, water evaporates from the ocean surface. The warmer the air over the ocean surface, the more water vapor it can hold. That's why warm water temps have such a huge effect on the ability of tropical storm to develop. The water temps, to a large degree, moderate the air temperature above the water. When water evaporates, it takes heat from it's surroundings, in this case, the ocean, thus cooling the water surface temps. We call this evaporative cooling. It's the same effect you feel when you get out of the shower. Even though the air temperature in the bathroom may be quite comfortable, the water evaporating off your skin is actually taking heat energy away from your skin, making it feel cooler. So we have this water vapor loose in the air with excess heat energy. Because it has that heat energy and because, as we discussed in the lesson, moist air is less dense than dry air, this air has tendancy to rise. Under ideal circumstances, it'll have the help of some low level converging air associated with a tropical wave. As the air rises, it's pressure decreases and it cools. At some point, it will cool close enough to it's dewpoint temperature that the water vapor in the air will condense. The process of condensing water is just the opposite as the process of evaporating it. When condensation takes place, the excess heat that the water molecules absorbed during evaporation is released into the air. We call this the latent heat of condensation. This causes the air to become even warmer and less dense, thus rising even faster. The rising air continues to cool and condense releasing stil more heat. Meanwhile, evaporation is still taking place at the surface adding more latent heat to the process. This whole process is indeed the heat engine that feeds a tropical system and, as we'll see later, helps maintain the heat budget of the planet.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 163. cyclonebuster
8:31 PM GMT on November 24, 2006
 I hope I didn't make you mad in any way! Anyways, If I need help with them would you help me with them to figure some things out with them, perhaps??
Member Since: January 2, 2006 Posts: 133 Comments: 20680
##### 162. Randy Bynon , Dropsonde SysOp/AVAPS PM
8:25 PM GMT on November 24, 2006
 Thanks cyclone! And good luck with the idea!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 161. cyclonebuster
8:13 PM GMT on November 24, 2006
 Hey, Randy Hugh Willoughby @ FIU already figured out ome of those numbers you mentioned for the idea!He came up with the actual size of them, and how many would be needed. Although, I feel they would work in the gulfstream because of the current migrating to North on the ~100+ miles per day.They only cool to 70 degrees and sea life thrives between 70 and 85 degrees in the stream. Over a nine day period they cool an area of 1000 x 40 miles. It would take longer than that for the water to warm back up.I agree this is not a debate, but thanks for what you think!
Member Since: January 2, 2006 Posts: 133 Comments: 20680
##### 160. Raysfan70
7:59 PM GMT on November 24, 2006
 Good Afternoon {{Randy}}!Thank You for finally explaining that to CB.
Member Since: July 28, 2005 Posts: 138 Comments: 57354
##### 159. bigtrucker
7:59 PM GMT on November 24, 2006
 Randy thanks for the lesson. one thing that confuses me about the water vapor lyes within the what, if any, does latent heat do to fuel storms. does it even have an effect on anything?
Member Since: January 9, 2006 Posts: 80 Comments: 6119
##### 157. EmmyRose
5:17 PM GMT on November 24, 2006
 ((((((((((((GOOD MORNING)))))))))) from the North - am beginning to thaw today - 50 degrees once the fog goes away...or maybe it's my mind that's in a fog LOLI can feel my fingers once again so all is good in happy Emmy landRandy hope you had a wonderful gobble daythe sweet potatoes and pecan pie were the hit of the season LOL dang...what do they do the rest of the year for eats? LOL have a good one y'all!!!!!!!!!!!!!!!!!!!!!
Member Since: July 15, 2005 Posts: 347 Comments: 76406
##### 156. Randy Bynon , Dropsonde SysOp/AVAPS PM
2:40 PM GMT on November 24, 2006
 Thanks code! Have a great weekend!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 155. Randy Bynon , Dropsonde SysOp/AVAPS PM
2:39 PM GMT on November 24, 2006
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 154. code1
2:39 PM GMT on November 24, 2006
 Another good one Randy!! Thanks. Have a great weekend.
Member Since: September 18, 2005 Posts: 66 Comments: 13872
##### 152. Randy Bynon , Dropsonde SysOp/AVAPS PM
2:25 PM GMT on November 24, 2006
 Good morning {{{{ Rays }}}}}!
Member Since: July 17, 2001 Posts: 190 Comments: 2012
##### 151. cyclonebuster
1:41 PM GMT on November 24, 2006
 Good Morning, Randy....Hope your Thanksgiving was a good one!
Member Since: January 2, 2006 Posts: 133 Comments: 20680
##### 150. cyclonebuster
1:40 PM GMT on November 24, 2006
 Good morning Randy,hope your thanksgiving was a good one! There how's that?
Member Since: January 2, 2006 Posts: 133 Comments: 20680

Viewing: 200 - 150

I was an AF aviation weather forecaster for 12 years, then 15 years as a dropsonde systems operator with the AF Reserve Hurricane Hunters.

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