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What is Convection?

18 Oct 2017, 12:04 pm

Your weather word for the day is convection.

And I’m not talking about the setting on your oven!

Do You Know the Different Types of Thunderstorms?

A Little Background

We meteorologists use the word a lot.

In fact, my non-meteorologist boss thinks I use it too much. So it’s time to talk about it in more detail.

We use convection when we refer to a storm. Talking about development or strengthening of a storm. This can happen on a small scale, like a supercell thunderstorm. Or as a part of a much larger scale phenomena, like a hurricane.

It can also happen without the presence of a storm. This is called dry convection and is often the favorite topic of any glider pilot.

Heat Transfer

Coming from a background in thermodynamics before my career in weather, heat transfer is one of my favorite things to talk about. (Nerd alert)

There are a few types of heat transfer.

  • Conduction
  • Radiant
  • Convection

Each transfers heat from one object to another in a different way.

  1. Conduction – Heat transfer from direct contact. Like setting a pan on a stove top to heat it up. (Or as Marcus put it, air touching the ground)
  2. Radiation – (Not like the nuclear stuff we are all afraid of) But instead, the sun heating us from so far away because it’s that hot! Or like the radiator in a room that heats up and therefore heats up the whole room.
  3. Convection – Heat transfer through the movement of a liquid or gas. That is, the liquid or gas of a certain temperature moves to a new area, thus changing the temperature of the new area. This is the heat transfer we are most interested in.

Convection

Before we get into the meat and potatoes of this, let me just reiterate that convection greatly depends on the other two types of heat transfer in order to occur.

Differences in density.

  • Cold air is more dense than warm air
  • Dry air is more dense than humid air

Therefore to be at equilibrium, warm air wants to be on top of cold air and air with more water content wants to be on top of dry air.
And the atmosphere wants nothing more than to be at equilibrium.

In an effort to achieve equilibrium, air is constantly moving around the atmosphere. This movement gives us our convective heat transfer, and as we mentioned earlier there are two types.

  1. Convection – When warm, moist air near the surface rises to be above the heavier cool, dry air this is a form of heat transfer or convection. The rising motion typically cools the air. As the air cools, it reaches the dewpoint and all of the moisture in the air condenses– forming clouds. Depending on many factors, these clouds can form rain and even storms.
  2. Dry Convection – The type of convection not associated with storms is called dry convection. This occurs when warm air at the surface rises to be above the cooler air overhead. Because there is no moisture, this typically doesn’t have cloud cover associated with it.

There are many laws we could talk about now, but to keep it simple we’ll just say convection isn’t just the movement of warm air. It also includes the cold air sinking. Because if the warm air goes up, something needs to take its place. So the cold air sinks to the surface.

This happens horizontally due to heating differentials at the surface as well. But lets just focus on those storms.

Thunderstorms

In a meteorologist’s vocabulary, convection and thunderstorm are practically interchangeable. The upward vertical motion of the warm, moist air is what fuels thunderstorms and on a much larger scale– even hurricanes!

Convection is often associated with lightning production! As these air particles rise they create winds moving upward. Those winds lift the tiny frozen droplets of water as gravity tries to bring them crashing down to the earth’s surface. The particles moving past each other generate electricity and when the charge gets strong enough– a bolt of lightning!

Photo Credit: NOAA

For WeatherNation — Meteorologist Jeremy LaGoo

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