Have you ever run across a confusing weather chart while trying to figure out whether or not it'll snow tomorrow? Have you seen a weather chart that just looks like a bunch of squiggles that mean nothing to you? Do you have a general grasp of weather charts, and want to learn more? You've clicked into the right place, my friend!
I recently started a new diary series called State of the Skies, bringing the DailyKos community a small snapshot of the previous day's significant weather events. In this series, I'll explain and shine light on some major weather stories around the country and world. From time to time, I'll use weather maps of various detail to help aid in these explanations. I don't want to alienate those of you who have a hard time reading some (or all) weather maps, so keep reading for what I hope will be a thorough and helpful overview.
Fear not, your regularly scheduled State of the Skies will return tomorrow around 7-730 PM Eastern time.
The Basics -- Pressures
I won't patronize you -- I'm fairly sure that you all pretty much know what high and low pressures are. On a weather map, a high pressure is denoted by a blue H and a low pressure is denoted by a red L. High pressures help to cause sinking air, which generally leads to clear skies and boring weather. Low pressures cause lifting and rising air, which promotes clouds and precipitation.
The Basics II -- Fronts
If you live east of the Rocky Mountains in the US or Canada, you've experienced one of those rip roaring cold fronts that come through in the spring and fall. A front is the leading edge of an airmass (either hot or cold) that usually (but not always) moves counterclockwise around a low pressure system. There are 4 types of fronts:
-Cold Front: A cold front is the edge of cold air advancing into warm air.
-Warm Front: A warm front is the edge of warm air advancing into cold air.
-Stationary Front: The boundary between a warm airmass and a cold airmass that is either stationary or moves very little over time.
-Occluded Front: A cold front that catches up with a warm front. The cold air slices under the warm air, completely lifting the warm airmass off the ground. An occluded front forms when a low pressure is at its most mature stage, and will choke off and begin to kill the low (such violence).
On a surface chart, fronts are denoted by colored lines with spikes and domes, which point in the direction of the front's movement.
-A cold front is denoted by a blue line with spikes along its edge.
-A warm front is denoted by a red line with domes along its edge.
-A stationary front is denoted by a multicolored line, alternating blue and red along with alternating spikes and domes.
-An occluded front is denoted by a purple line with alternating spikes and domes along its edge.
The point at which the cold front, warm front, and occluded front meet is called the triple point. A new low pressure center is favored to form at the triple point.
The Basics III -- Contours
A contour is a line that links equal values together, with higher values to the right and lower values to the left. I know you've seen a contour map before. The one you're most familiar with is probably the temperature contour map:
There are all kinds of contour maps, but the major ones are as follows:
-pressure (isobars)
-heights (isoheights)
-temperature (isotherms)
-precipitation (isohyets)
-wind (isotachs)
...and tons more less-common types in meteorology. A general rule of contouring is that higher values fall to the right of the contour, and lower values fall to the left. This is true for most every type of map. Below is a diagram of a low pressure system, complete with isobars every 4 millibars. Higher millibars equal higher pressures, and lower are the opposite. Wind is directly related to pressure gradient, or the rate at which pressure changes over distance. If the pressure gradient is strong, the wind will blow fast. If the gradient is weak, the wind will blow lightly or not at all. The pressure gradient is shown on a contour map by the distance between contours. The closer the lines, the stronger the winds.
The Surface Map
Now that you have a general idea of pressure systems, frontal systems, and map contours, we can start talking about actual maps. Below is a surface chart. You're probably familiar with this if you've ever watched TV or read the newspaper or gone to weather.com. A surface chart, and this is a tricky one, shows weather occurring at the surface. Amazing, huh?
Here's a surface chart from Oct 26 2010, the day of the Chiclone.
The Chiclone was the perfect example of a pressure system, all 4 types of fronts, and pressure gradients/contours.
Upper-Air Weather Charts
Now, this is the fun part. Everything that happens at the surface (save for flushing ice cubes down the toilet to cause snow) is caused by stuff happening over our heads. At the upper levels, we use isoheights instead of isobars. At the surface, pressures are usually adjusted to reflect 0 feet, sea level (hence the term "sea level pressure"). Since everyone is allegedly at the same height, we can measure the pressure changes. At the upper levels, this is reversed. We use a constant pressure surface, and measure the height changes. With a high pressure system, there is higher pressure because there is more air above that particular location. If there's more air at that location, the height of all the pressures rises higher in the atmosphere. The opposite goes for low pressure systems. In the upper levels, a high height center indicates sinking air, which leads to clear and dry conditions. A low height center indicates rising air, which leads to moist and turbulent conditions.
For a more detailed explanation of upper-level heights, I encourage you to read this excellent PDF document from the University of Colorado (skip to page 5).
There are 5 major levels (called mandatory levels) of the atmosphere we use to understand what's going on and what's going to happen here on the ground:
-850 millibars (~1500 meters up): This level is predominately used to show warm air advection (warm air blowing towards cold air) or cold air advection (cold air blowing towards warm air). Both of these can be crucial in the development of severe or winter weather, as well as formation of general clouds and precipitation.
-700 millibars (~3000 meters up): This level is predominately used to show moist or dry air at upper levels, which can strongly influence the success or failure of storm systems and severe weather development.
-500 millibars (~5400 meters up): This level is predominately used to show troughs (area of lower heights) and ridges (area of higher heights). Troughs signal rising air which can lead to disturbed weather, and ridges signal sinking air which can lead to calm weather.
-300 millibars (200/250 in winter...heights vary between 9k-12k meters up): These three levels are used throughout the year (depending on season) to show the jet stream. The jet stream drives most of the upper level weather, and can strongly influence where surface low pressure systems develop.
The following map is a model depiction of the jet stream at the 250 millibar level from Monday morning around 700AM Eastern Time. Anywhere in blue denotes the jet stream, and the deeper blue/purple colors indicate stronger winds, making up the core of the jet stream (called a jetstreak). Generally, the bottom-left and top-right regions of these jetstreaks cause extreme lifting of air, and a surface low pressure system (or at least active weather) is favored to develop in these locations. At the time of this map's generation, there was very active weather in both the bottom-left and top-right regions of the jetstreak.
Click to enlarge
The data for these upper-air charts are taken from weather balloons, which ascend with a box full of weather equipment that measures the weather throughout a sliver of the atmosphere. The last time I counted, there were 69 weather balloon launch sites throughout the lower 48 United States, with hundreds more scattered across the globe. These balloons are launched every 12 hours, and the data collected from each launch is compiled to create all the upper-level charts and initiate the weather models to aid in forecasting.
I hope this explanation will help you in reading weather maps, now or in the future. If you have any questions, please don't hesitate to ask in the comments. Trust me, there are tons more weather geeks here than you could ever imagine, so I'm sure somebody can answer your question(s). :)
Links
Below is a set of links to help you find weather maps, as well as an explanation of each.
HPC Maps -- Current surface maps as well as 1-7 day surface weather forecasts from the HPC.
NWS DIFAX Maps -- Great for analyzing almost any map you can imagine
SUNY Albany DIFAX Maps -- Essentially the same maps from SUNY Albany, in case you can't get the NWS site to cooperate.
SPC Upper-Air Maps -- Unanalyzed upper-level maps from the SPC. Great if you want to practice contouring.
NCEP Model Runs -- GFS and NAM model runs. Great to help with forecasting.
Detailed Info on the Basics:
-Pressures
-Pressure Gradients
-Fronts
-Contours
Map Reading Help:
-Surface
-850 millibars
-700 millibars
-500 millibars
-300/200 millibars