We covered a lot of ground the last couple of classes. We started Monday's class by reviewing the results of the lab experiment. First we established that the bottle was filled with "air" and that once the balloon was placed on the bottle that air could neither enter nor exit. When the bottle was placed in the hot water, the balloon expanded and rose. When the bottle was placed in the ice water, the balloon deflated and fell. What we learned is that when air is heated it expands, becomes less dense, and rises. Colder air contracts, becomes more dense, and sinks. |
So how does this relate to wind? The Sun heats up the earth, but it heats it up unevenly. Remember, we talked about this when we learned about temperature. The warm air rises. The cool air sinks and rushes in to replace it. The movement of the air in this cycle is what causes wind.
You can’t see air molecules, but they are there and they have weight. The weight of the air pushing down on the earth is what causes air pressure. Air pressure decreases as you go higher in elevation. So the air pressure at the top of a mountain is less than at the bottom. You’ve experienced air pressure changes if you’ve ever flown or driven up or down a mountain and your ears pop. |
High Pressure = Sinking Air = Fair Weather
Low Pressure = Rising Air = Bad Weather
Here's how all this information comes together...
We started Tuesday's class with a whole class experiment that showed how "convection" works. Imagine the red dye being warm air at the equator and the blue dye being cold air at the poles...
The Sun heats the Earth unevenly. There are hotter areas and colder areas. Here are some examples:
- Earth is a sphere, it is hotter at the equator than at the poles.
- The earth tilts, when it is summer in one hemisphere, it is winter in another.
- The earth rotates, we have day and night. It is warmer during the day and colder in the evening.
- During the day, land generally heats up faster than water. Think of a day at the beach and how refreshing it is to go in the water.
- During the evening, land generally loses heat faster than water.
- It is cooler at higher elevations. It is cooler at the top of a mountain than at the bottom.
Have you ever noticed there are breezes coming off the ocean in the afternoon and land breezes in the evening? That’s because during the day, the land heats up faster than the water. The warm air rises over the land and creates a low pressure area. The cooler air over the ocean causes a high pressure area. The wind blows from high to low pressure. At night the land loses heat faster than the ocean, so the roles reverse.
Now it will make more sense when you watch a weather report and you see H and L's on the map, like this one...
So, what are those lines of blue triangles and red semi-circles?
A warm front is indicated by a red line with red half circles. The line indicates the leading edge of the warm area of a low pressure system. The red half circles indicate the direction the warm front is moving.
A cold front is indicated by a blue line with blue triangles. The line indicates the leading edge of the cold area of a high pressure system. The blue triangles indicate the direction the cold front is moving.
Sometimes you’ll see a line with alternating red half circles and blue triangles. This indicates a stationary front. This means neither the warm nor the cold front is strong enough to replace the other.
Look out, where two fronts meet, frequently there is a change in the weather. Maybe a wind shift, temperature change, and/or precipitation.
A cold front is indicated by a blue line with blue triangles. The line indicates the leading edge of the cold area of a high pressure system. The blue triangles indicate the direction the cold front is moving.
Sometimes you’ll see a line with alternating red half circles and blue triangles. This indicates a stationary front. This means neither the warm nor the cold front is strong enough to replace the other.
Look out, where two fronts meet, frequently there is a change in the weather. Maybe a wind shift, temperature change, and/or precipitation.
We finished Tuesday's class by watching a video on the Jet Stream, as we hear a lot about how the jet stream effects our weather this time of year.