Question Answered by Kurt Buffalo, Meteorologist, National Weather Service-Pocatello
We often hear about monsoon moisture or monsoon storms during the summer months. So what exactly is the monsoon, and what causes it? The term “monsoon” refers to a seasonal wind shift, which brings a change in moisture and precipitation patterns. There are several different monsoon patterns around the world. The one that affects us locally in Idaho is called the North American monsoon. The North American monsoon season runs from mid-June through late September, but is typically most pronounced during July and August. The effects of the North American monsoon are strongest over the southwest United States and northwest Mexico, but locations farther north through the Great Basin and into Idaho and neighboring areas are also affected.
The monsoon pattern develops during the summer months in response to changing pressure patterns. During the early summer months, intense heating of the deserts across the southwest U.S. and northwest Mexico leads to the formation of a low pressure system in lower levels of the atmosphere. This seasonal change in the pressure pattern causes winds to reverse their direction and bring moisture inland, as they blow from high pressure over the water towards low pressure over land.’
Meanwhile higher up in the atmosphere, a strong ridge of high pressure moves over the Southern Rockies or Southern Plains. The clockwise circulation around this high pressure ridge draws moisture at mid and upper levels of the atmosphere into portions of the western United States from the Gulf of Mexico, Gulf of California, and eastern Pacific Ocean. This increased moisture at various levels of the atmosphere provides the fuel needed for thunderstorms to develop. As the strong summer sun heats the Earth’s surface, the warm moist air rises, eventually growing into clouds and thunderstorms. These thunderstorms are often referred to as monsoon storms, as they result from the increased moisture drawn into the region by the change in seasonal wind flow patterns known as the monsoon.
The strength and position of the upper-level ridge of high pressure changes throughout the monsoon season and plays an important role in the location and coverage of thunderstorms. Idaho is on the northern reach of the monsoon influence, so it typically takes a stronger surge of monsoon moisture for us to be affected. Some summers bring a few strong monsoon surges, while other years we have none. It really depends on the strength and location of the ridge of high pressure. If we get a strong moisture surge, it can bring thunderstorms with heavy rain and flash flooding. On the other hand, if we find ourselves on the fringe of a weaker monsoon surge, we can get thunderstorms producing gusty winds and little rainfall, leading to wildfire starts.
During the summer months, sunlight heats the surfaces of both lands and oceans, but land temperatures rise more quickly. As the land’s surface becomes warmer, the air above it expands and an area of low pressure develops. Meanwhile, the ocean remains at a lower temperature than the land and so the air above it retains a higher pressure. Since winds flow from areas of low to high pressure, this deficit in pressure over the continent causes winds to blow in an ocean-to-land circulation.
Let’s create a sea breeze.
Materials: 2 baking dishes, sand (to fill one dish), pot, stove, incense stick, lighter, adult helper, ice water
- Put the sand in a pot and, with the help of an adult, heat it on the stove until it is very hot to touch.
- Place the sand in one of the baking dishes.
- Fill the other dish with ice water.
- Put the dishes next to each other. Make sure there is no wind to disturb your experiment.
- With the help of an adult, light the incense stick. Once it makes a lot of smoke, hold it in between the sand and ice water dishes. Observe the movement of the smoke. Where did the smoke go?
You heated the sand on the stove like the sun heats the sand at the beach. When the sand is much warmer than the water you should have seen the smoke of the incense stick steadily flow from the water to the sand dish.