At the surface of the earth, weather maps depict lines of equal pressure (reduced to sea level).
At higher altitudes, the convention is to show lines of equal height at a given pressure.
There is no practical difference. When we speak of the height of the 500 millibar surface (around 18,000 feet in elevation above sea level), it is equivalent to talking about pressure as far as the gradients (changes with distance) are concerned. And it is the pressure gradient that creates wind.
Relationship Between Surface Temperature and Height of a Pressure Surface
Cold air is denser than warm. So, other things being equal, the height at 500 millibars is greater over warm air than cold. Think of it this way: If you have an air mass at a certain temperature, what happens if the temperature rises? The air expands and becomes less dense. As the air expands, it lifts everything above it, and the height of the 500 millibar pressure level rises. The opposite is true for cold air.
This winter has been characterized by two areas of anomalous temperature: the northeast Pacific Ocean and the area near the North Pole. Each has contributed to the historic cold in the northern plains, midwest, and northeast of the United States.
Pacific Ocean Water Temperatures
The Gulf of Alaska and surrounding waters have consistently averaged many degrees above the seasonal normals this winter.
Air temperatures tend to be very close to water temperatures over the ocean. The result has been an anomalously warm air mass in the eastern Pacific for the entire winter.
The warm air mass has created a ridge in the jet stream flow (the 500 millibar level can be taken as the heart of the jet stream). Ridges tend to create troughs about 2,000 miles downstream.
Think of tossing a stone into a pond. Waves ripple outward with a characteristic wavelength. The long waves in the jet stream tend to move very slowly, so a pattern, especially one anchored by a temperature anomaly, can persist for a long time.
Temperatures in the Arctic
The North Pole has been the poster child for global warming. In the winter of 2013-14 the surface temperature north of the 80th parallel has been above the long-term average every day, with readings ten or more degrees above normal on more than half the days.
The 500 millibar pattern shows heights well above normal over the north pole. The corollary has been a polar vortex displaced to the south.
The Double Whammy
The combination of the ridge in the eastern Pacific over the warm water, and high pressure over the warm North Pole which depressed the polar vortex south, have combined to create a cold flow from Canada into the central United States, often extending into the deep south and the Atlantic coastal states. These parts of the United States have experienced a bitterly cold winter as a result.
The California Drought
Though the extreme cold in the northern plains and upper midwest have gotten more attention, the drought in California, part of the same weather pattern, will have more long-lasting effects.
Because of excessive and extravagant use of water by citizens and farmers, California would be in water deficit even with normal rainfall. Since California abuts an ocean of considerable volume, it seems reasonable that the ultimate solution will be desalination.