Low pressure

[Guest Stratus_Fractus]

Over the last day, a strong low pressure system moved in over the Northeast. Here's the last 36 hours of METAR reports for Toronto:CYYZ 092100Z 11004KT 3/4SM -SN OVC002 M00/M00 A2929 RMK NS8 /S02/ SLP926CYYZ 092025Z 09005KT 3/4SM -SN OVC002 RMK NS8CYYZ 092000Z 11006KT 1SM -SN FEW002 OVC006 00/M00 A2929 RMK SF1NS7 SLP925CYYZ 091953Z 11004KT 1SM -SN FEW002 OVC006 RMK SF2NS6CYYZ 091947Z 11004KT 1 1/4SM -SN OVC002 RMK NS8CYYZ 091933Z 13004KT 1SM -SN SCT003 OVC008 RMK SF3NS5CYYZ 091915Z 14006KT 3/4SM -SN OVC002 RMK NS8CYYZ 091900Z 14005KT 1 3/4SM -SN OVC002 00/M00 A2930 RMK NS8 SLP926CYYZ 091848Z 15005KT 1 1/4SM -SN OVC002 RMK NS8CYYZ 091835Z 16005KT 3/4SM -SN OVC003 RMK NS8CYYZ 091822Z 16004KT 1SM -SN FEW003 OVC012 RMK SF2NS6CYYZ 091815Z 14005KT 2SM -SN OVC003 RMK NS8CYYZ 091808Z 15004KT 4SM -SN OVC004 RMK NS8CYYZ 091800Z 10003KT 4SM -SN OVC013 02/01 A2929 RMK NS8 SLP924CYYZ 091754Z 09004KT 4SM -SN OVC013 RMK NS8CYYZ 091722Z 09005KT 7SM -SN BKN030 OVC040 RMK SF5SC3CYYZ 091700Z 36003KT 12SM BKN025 OVC042 02/00 A2930 RMK SF6SC2 SLP928CYYZ 091623Z 30005KT 12SM FEW040 BKN065 RMK SC2SC6CYYZ 091600Z 27004KT 12SM -RA OVC035 03/00 A2932 RMK SC8 SLP934CYYZ 091518Z 24005KT 10SM -RA FEW018 OVC023 RMK SF2SC6CYYZ 091500Z 27003KT 10SM OVC019 02/M00 A2931 RMK SC8 SLP933CYYZ 091400Z 28003KT 15SM BKN033 OVC045 03/M01 A2929 RMK SC5SC3 SLP924CYYZ 091300Z 30005KT 12SM BKN025 OVC045 03/M01 A2925 RMK SC6SC2 SLP909CYYZ 091200Z 30009KT 15SM FEW028 OVC046 03/M00 A2921 RMK CF1SC8 SLP896CYYZ 091100Z 31018KT 15SM SCT026 BKN043 03/M00 A2915 RMK SC3SC4 SLP876CYYZ 091000Z 31015G21KT 15SM BKN025 BKN060 03/00 A2910 RMK SC6SC2 SLP860CYYZ 090900Z 30023G30KT 15SM SCT030 BKN061 04/M00 A2907 RMK CF3SC4 SLP847CYYZ 090800Z 30026G38KT 15SM BKN038 BKN062 04/M00 A2902 RMK SC7SC1 SLP830CYYZ 090700Z 31029G38KT 15SM FEW030 BKN046 04/M01 A2898 RMK CF2SC6 SLP816CYYZ 090600Z 30024G33KT 15SM FEW032 BKN045 05/M00 A2892 RMK SC2SC3 SLP799CYYZ 090500Z 28018G24KT 15SM FEW057 03/M00 A2892 RMK SC2 SLP797CYYZ 090400Z 27014KT 15SM BKN028 BKN059 03/00 A2893 RMK SC6SC2 SLP803CYYZ 090300Z 21007KT 15SM BKN020 OVC052 02/00 A2896 RMK ST7SC1 PRESFR SLP811CYYZ 090200Z 18005KT 15SM OVC020 02/00 A2902 RMK ST8 PRESFR SLP833CYYZ 090100Z 14005KT 15SM BKN021 OVC150 02/00 A2909 RMK SC7AC1 SLP856CYYZ 090000Z 15005KT 15SM FEW030 FEW170 BKN200 02/M00 A2914 RMK SC1AC2CC5 SLP873CYYZ 082300Z 21004KT 15SM OVC030 03/M00 A2919 RMK SC8 SLP892CYYZ 082200Z 21005KT 15SM BKN030 OVC110 03/M01 A2923 RMK SC6AS2 SLP904CYYZ 082100Z 24009KT 15SM BKN035 OVC090 03/M01 A2928 RMK SC7AS1 SLP922CYYZ 082000Z 25009G16KT 15SM BKN040 OVC110 03/M02 A2931 RMK SC6AS2 SLP930CYYZ 081900Z 27010KT 15SM BKN040 OVC100 03/M02 A2933 RMK SC5AS3 SLP938CYYZ 081800Z 30009KT 15SM BKN042 OVC270 04/M02 A2935 RMK SC7CI1 SLP945CYYZ 081700Z 30015G20KT 15SM BKN040 OVC050 04/M03 A2937 RMK SC6SC2 SLP954CYYZ 081600Z 29013KT 15SM BKN040 BKN270 03/M02 A2939 RMK SC5CI1 SLP959CYYZ 081500Z 29009KT 15SM FEW030 BKN043 BKN270 03/M02 A2938 RMK CF1SC7CI1 SLP956CYYZ 081400Z 30012KT 15SM FEW030 SCT040 BKN270 03/M01 A2937 RMK CF1SC3CI2 SLP951CYYZ 081300Z 33015G25KT 15SM BKN030 03/M01 A2934 RMK SC7 SLP941CYYZ 081200Z 31014KT 10SM FEW030 OVC038 03/00 A2933 RMK SF2SC6 SLP937CYYZ 081100Z 29013KT 7SM BKN032 02/00 A2928 RMK SC6 SLP922Pressure dropped to a low of 28.92" Hg. Are they any special procedures that go along with low pressure? ATC and ATIS kept repeating the pressure a few times, advising caution when setting altimeters.Just some trivia, the highest recorded pressure was around 32 inches, lowest was close to 27 inches! :-eek

[KevinAu 441]

The main thing is that it renders the lowest flight levels unuseable. ATC will not assign altitudes such as FL180. Since flight levels are flown with reference to an altimeter setting of 29.92, when the actual pressure is less, you will actually be at a lower true altitude. This is bad since that would mean you can have two aicraft, one at a local altitmeter setting of 28.92 flying at 17000' while another aircraft at FL180, occupy the same space.

[Guest Stratus_Fractus]

Thanks for the explanation Kevin, makes sense.