What makes a heat wave?

Question Answered by Nicole Desmet, Meteorologist, National Weather Service-Pocatello, Idaho

A heat wave is simply a period of prolonged period of very hot, uncomfortable temperatures. Typically a heat wave lasts two or more days. Several factors can influence how hot temperatures can get, where temperatures will be warmest and how long it will be that way. The elevation you live at will play a role in how hot temperatures can get; where hotter temperatures are noted at lower elevations and cooler temperatures are observed at higher elevations such at a mountain peak. This is owing to the difference in pressure observed between the two locations. The more pressure in the atmosphere typically produces a warmer temperature. Urbanization can add to excessive heat conditions and you will notice larger cities with hotter temperatures compared to their open air, green grassy area counterparts.

Wind is how air is transported about our planet. Therefore, it will take some sort of influence, whether that be a cold front or a low pressure system to “move” the heat wave out of the area.

During a heat wave, the air outside will likely feel warmer than it actually is. This is known as the Heat Index and it measures how hot it really feels when relative humidity is factored in with the actual air temperature. Use the below Heat Index chart to give you an idea on estimating the Heat Index, or ‘feels like’ value.

Heat Index
The Heat index in the summer is the opposite of wind chill in the winter. It is basically the “feels like” temperature.

Sweating is a process your body uses to cool down. Your body is always trying to maintain an even body temperature. Sweating reduces body heat through a process known as evaporative cooling.

Try this simple experiment.

  1. Wet the back of your hand.
  2. Blow gently across your hand. You should feel a cooling sensation.
  3. Blot your hand dry and use the opposite hand to feel the actual temperature of your skin. It will be cooler to the touch!

Does warm air hold more water?
Actually, air does not hold water. As air molecules are warmed by the Earth’s surface, the water molecules in the air are also warmed, giving them more energy. When water molecules have more energy they are less likely to condense back into liquid water. Therefore, warm air, which is filled with highly-energized water molecules, often contains more water molecules than cooler air, which is filled with water molecules that more easily condense and become liquid water.

Let’s model how heat affects the evaporation and condensation of water molecules.

Materials: friends, 4 dice for each person

  1. Divide the room into a liquid water area and a water vapor area.
  2. Each friend will play the role of a molecule of water. Have everyone stand in the liquid water area of the room.
  3. Give each friend 2 dice.
  4. All friends will roll their dice to see how much energy they have. If they get a sum of 11 or more, they have enough energy to go to the water vapor area. If they have a sum 10 or less, they should remain in the liquid water area. Which molecules left? The most energetic ones! So the average energy of the ones that stayed behind is less. That’s why evaporation cools things off.
  5. Have your friends shake their dice a few more times, so they can observe people switching from one side to another. How many molecules are leaving the liquid phase for the gas phase? How many are leaving the gas phase for the liquid phase? This is equilibrium.
  6. Now explain that you are “heating” the water and give each friend a third die. Predict what will happen if the molecules have more energy. Have your friends shake the dice. Just like before, a sum of 10 or less means the molecule goes to or stays in the liquid water phase, a sum of 11 or more means the molecule goes to or stays in the water vapor phase. As you play, did you notice that there are more molecules in the gas phase now. That means that more water molecules are energetic enough to evaporate.
  7. Give everyone a fourth dice. What will happen now?

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