Unit 1 Section 3 Air Pressure

What is Air Pressure? How do you measure atmospheric pressure?

A Look at Air Pressure

If you really want to know how the weather will be tomorrow keep an eye on the barometer. The barometer is an instrument used to measure air pressure. Changing air pressure is the single best clue to forecasting tomorrows weather.

Altitude and Pressure What do you think happens to air molecules as height increases? If you have ever spent time at high altitudes (over 2400 meters, or 8000 feet) you know that breathing becomes more difficult the higher you go. Some climbers even succumb to acute altitude illness that can cause death. Reduced atmospheric pressure (less molecules) at high altitudes creates a lack of oxygen in the body.

At lower altitudes, gravity pulls air molecules towards the Earth's surface, creating denser air (more molecules). We may be unaware of the pressure that air exerts but it is a critical and essential component of living on Earth's surface. Without atmospheric pressure liquid water would not exist (it would boil away), nor would there be any weather as we know it.

Pressure vs. Altitude

Atmospheric Pressure On average, air exerts a constant pressure of 14.7 lbs./sq. in. at sea level. This is equivalent to a column of mercury inches in height or 1013 millibars. One inch of mercury = 33.8 millibars inches of mercury = 33.8 millibars of pressure

Barometers Measuring High and Low Pressure

Mercury vs. Aneroid Barometer A single sheet of paper at sea level when laid flat experiences about 1,400 pounds of atmospheric pressure! Barometric pressure is measured in units called millibars. Most barometers use mercury in a glass column, like a thermometer, to measure the change in air pressure.

Why Does Warm Air Rise? The Sun's heat is the driving force behind weather. It causes warm air masses to form. As we have seen, Earth's atmosphere is 78% nitrogen, with oxygen and twenty other gases making up the remaining 21%. These gaseous molecules are in constant rapid motion (kinetic energy), colliding with each other to create what we call air pressure. Why does warm air rise? As a parcel of air is heated near the earth’s surface, the air molecules move faster (increased KE). The molecules bump into each other more often, which spreads them apart (larger volume). As the volume of the parcel increases, the density of the air decreases and the parcel of warm air rises.

Air Temperature and Atmospheric Pressure The role air pressure plays in influencing weather is one of the most important of any of the key weather elements. Warm air masses rise creating or leaving low-pressure at the surface. Rising air also cools and condenses to form clouds. Low pressure is therefore associated with cloudy skies and wet weather. Once cooled, the heavier air sinks downwards creating high pressure at the earth’s surface. High pressure is associated with clear skies and sunny conditions. Since the atmosphere seeks to create equilibrium, air moves from high to low pressure.

Temperature, Air Pressure and Wind

Air Temperature and Atmospheric Pressure Summary Warm Air rises leaving less molecules at the earth surface creating low pressure. The warm air rises, cools, condenses and forms clouds. Low Pressure is associated with clouds. Cools Air sinks creating more molecules at the earth’s surface generating high pressure. High Pressure is associated with clear skies.

Question: What does nature hate? Answer: A vacuum Nature hates an imbalance. When the pressure in one location is different than the pressure in another location, nature will attempt to balance the pressure between these two locations. In nature, things always move from high to low. In nature, molecules will move from areas of high concentration (high pressure) to areas of low concentration (low pressure). High to Low is the way to flow!

Changes in Pressure- Formation of Wind The change in pressure from areas of highs to lows which triggers the movement of air is called the pressure gradient or pressure force. The greater the difference between the high and low pressure areas, the greater the pressure gradient force and the stronger the winds. Air at the ground flows towards lower pressure trying to equalize itself.

Temperature and Pressure

Sample Questions 1. Pressure is always changing. One reason is the uneven heating of the earth’s surface cause air masses to move. 2. When air is heated at the earth’s surface it volume (increases/decreases) resulting in a(n) (increase/ decrease) in pressure. 3. We call this (increase/decrease) in pressure (high/low) pressure. 4. A second reason pressure is always changing is humidity. Water Vapor weighs less than dry air and when water vapor is present, the air is less massive meaning is exerts (more/less pressure). 5. At sea level, atmospheric pressure has a value of ___76_ centimeters, or ___29.92__ inches or mercury. These values are equivalent to ___1013_____ millibars of pressure.

For a parcel of rising air heated at the earth’s surface: volume of the parcel is (increasing / decreasing) temperature of the parcel is (increasing / decreasing) density of the parcel is (increasing / decreasing) pressure within the parcel is (increasing / decreasing) As the parcel rises, what type of pressure is formed at the earth’s surface? (low / high) As the parcel rises, what type of pressure formed above it? (low / high)

Pressure Maps “Isobar Maps” Isobar maps are essential for the identification of areas of high and low pressure. These areas are the result of uneven heating of the earth’s surface. Winds direction is determined by locations of high and low pressure. Wind Speed is determined by the differences in pressure between a high and low and the distance between the high and low.

We will use our understanding of topographic maps to understand isobar (pressure) maps

Introduction to Topographic maps Differences in elevation are shown on topographic maps by means of contour lines, which are lines connecting points of equal elevation. A mountainous area will require many more contours than a flat area of little relief.

Contour lines never cross or intersect one another (but may merge to form a single contour line where there is a vertical cliff). The closer the contour lines are together, the steeper the gradient. Contour lines for most areas should be smooth, curving lines. Every contour line closes on itself.

Isobars lines of constant pressure A line drawn on a weather map connecting points of equal pressure is called an isobar. The isobars are measured in millibars.

The diagram below depicts a pair of isobars. At every point along the top isobar, the pressure is 996 mb and at every point along the bottom isobar the pressure is 1000 mb.

Any point in between these two isobars will have a pressure somewhere between 996 mb and 1000 mb. Point A, for example, has a pressure of 998 mb and is therefore located between the 996 mb isobar and the 1000 mb isobar.

Sea level pressure reports are available every hour, which means that maps of isobars are likewise available every hour. The solid blue contours are isobars and the numbers along particular contours indicate the pressure value of the isobar.